Geohistory analysis of the Santa Maria basin, California, and its relationship to tectonic evolution of the continental margin

DOE Office of Scientific and Technical Information (OSTI.GOV)

McCrory, P.A.; Arends, R.G.; Ingle, J.C. Jr.

1991-02-01

The Santa Maria basin of central California is a geologically complex area located along the tectonically active California continental margin. The record of Cenozoic tectonism preserved in Santa Maria strata provides an opportunity to compare the evolution of the region with plate tectonic models for Cenozoic interactions along the margin. Geohistory analysis of Neogene Santa Maria basin strata provides important constraints for hypotheses of the tectonic evolution of the central California margin during its transition from a convergent to a transform plate boundary. Preliminary analyses suggest that the tectonic evolution of the Santa Maria area was dominated by coupling betweenmore » adjacent oceanic plates and the continental margin. This coupling is reflected in the timing of major hiatuses within the basin sedimentary sequence and margin subsidence and uplift which occurred during periods of tectonic plate adjustment. Stratigraphic evidence indicates that the Santa Maria basin originated on the continental shelf in early Miocene time. A component of margin subsidence is postulated to have been caused by cessation of spreading on adjacent offshore microplates approximately 19-18 ma. A sharp reduction in rate of tectonic subsidence in middle Miocene time, observed in the Santa Maria basin both onshore and offshore, was coeval with rotation of crustal blocks as major shearing shifts shoreward. Tectonic uplift of two eastern sites, offshore Point Arguello and near Point Sal, in the late Miocene may have been related to a change to transpressional motion between the Pacific and North American plates, as well as to rotation of the western Transverse Ranges in a restraining geometry.« less

Plate Kinematic model of the NW Indian Ocean and derived regional stress history of the East African Margin

NASA Astrophysics Data System (ADS)

Tuck-Martin, Amy; Adam, Jürgen; Eagles, Graeme

2015-04-01

Starting with the break up of Gondwana, the northwest Indian Ocean and its continental margins in Madagascar, East Africa and western India formed by divergence of the African and Indian plates and were shaped by a complicated sequence of plate boundary relocations, ridge propagation events, and the independent movement of the Seychelles microplate. As a result, attempts to reconcile the different plate-tectonic components and processes into a coherent kinematic model have so far been unsatisfactory. A new high-resolution plate kinematic model has been produced in an attempt to solve these problems, using seafloor spreading data and rotation parameters generated by a mixture of visual fitting of magnetic isochron data and iterative joint inversion of magnetic isochron and fracture zone data. Using plate motion vectors and plate boundary geometries derived from this model, the first-order regional stress pattern was modelled for distinct phases of margin formation. The stress pattern is correlated with the tectono-stratigraphic history of related sedimentary basins. The plate kinematic model identifies three phases of spreading, from the Jurassic to the Paleogene, which resulted in the formation of three main oceanic basins. Prior to these phases, intracontinental 'Karoo' rifting episodes in the late Carboniferous to late Triassic had failed to break up Gondwana, but initiated the formation of sedimentary basins along the East African and West Madagascan margins. At the start of the first phase of spreading (183 to 133 Ma) predominantly NW - SE extension caused continental rifting that separated Madagascar/India/Antarctica from Africa. Maximum horizontal stresses trended perpendicular to the local plate-kinematic vector, and parallel to the rift axes. During and after continental break-up and subsequent spreading, the regional stress regime changed drastically. The extensional stress regime became restricted to the active spreading ridges that in turn adopted trends normal to the plate divergence vector. Away from the active ridges, compressional horizontal stresses caused by ridge-push forces were transmitted through the subsiding oceanic lithosphere, with an SH max orientation parallel to plate divergence vectors. These changes are documented by the lower Bajocian continental breakup unconformity, which can be traced throughout East African basins. At 133 Ma, the plate boundary moved from north to south of Madagascar, incorporating it into the African plate and initiating its separation from Antarctica. The orientation of the plate divergence vector however did not change markedly. The second phase (89 - 61 Ma) led to the separation of India from Madagascar, initiating a new and dramatic change in stress orientation from N-S to ENE-WSW. This led to renewed tectonic activity in the sedimentary basins of western Madagascar. In the third phase (61 Ma to present) asymmetric spreading of the Carlsberg Ridge separated India from the Seychelles and the Mascarene Plateau via the southward propagation of the Carlsberg Ridge to form the Central Indian Ridge. The anti-clockwise rotation of the independent Seychelles microplate between chrons 28n (64.13 Ma) and 26n (58.38 Ma) and the opening of the short-lived Laxmi Basin (67 Ma to abandonment within chron 28n (64.13 - 63.10 Ma)) have been further constrained by the new plate kinematic model. Along the East African margin, SH max remained in a NE - SW orientation and the sedimentary basins experienced continued thick, deep water sediment deposition. Contemporaneously, in the sedimentary basins along East African passive margin, ridge-push related maximum horizontal stresses became progressively outweighed by local gravity-driven NE-SW maximum horizontal stresses trending parallel to the margin. These stress regimes are caused by sediment loading and extensional collapse of thick sediment wedges, predominantly controlled by margin geometry. Our study successfully integrates an interpretation of paleo-stress regimes constrained by the new high resolution plate kinematic and basin history to produce a margin scale tectono-stratigraphic framework that highlights the important interplay of plate boundary forces and basin formation events along the East African margin.

3-D subduction dynamics in the western Pacific: Mantle pressure, plate kinematics, and dynamic topography.

NASA Astrophysics Data System (ADS)

Holt, A. F.; Royden, L.; Becker, T. W.; Faccenna, C.

2017-12-01

While it is well established that the slab pull of negatively buoyant oceanic plates is the primary driving force of plate tectonics, the dynamic "details" of subduction have proved difficult to pin down. We use the Philippine Sea Plate region of the western Pacific as a site to explore links between kinematic observables (e.g. topography and plate motions) and the dynamics of the subduction system (e.g. mantle flow, mantle pressure). To first order, the Philippine Sea Plate can be considered to be the central plate of a double slab system containing two slabs that dip in the same direction, to the west. This subduction configuration presents the opportunity to explore subduction dynamics in a setting where two closely spaced slabs interact via subduction-induced mantle flow and stresses transmitted through the intervening plate. We use a 3-D numerical approach (e.g. Holt et al., 2017), augmented by semi-analytical models (e.g. Jagoutz et al., 2017), to develop relationships between dynamic processes and kinematic properties, including plate velocities, lithospheric stress state, slab dip angles, and topography. When combined with subduction zone observables, this allows us to isolate the first order dynamic processes that are in operation in the Philippine Sea Plate region. Our results suggest that positive pressure build-up occurs in the asthenosphere between the two slabs (Izu-Bonin-Mariana and Ryukyu-Nankai), and that this is responsible for producing much of the observed kinematic variability in the region, including the steep dip of the Pacific slab at the Izu-Bonin-Mariana trench, as compared to the flat dip of the Pacific slab north of Japan. We then extend our understanding of the role of asthenospheric pressure to examine the forces responsible for the plate kinematics and dynamic topography of the entire Western Pacific subduction margin(s). References:Holt, A. F., Royden, L. H., Becker, T. W., 2017. Geophys. J. Int., 209, 250-265Jagoutz, O., Royden, L., Holt, A. F., Becker, T. W., 2015. Nature Geo., 8, doi:10.1038/ngeo2418

High-resolution reconstructions of Pacific-North America plate motion: 20 Ma to present

NASA Astrophysics Data System (ADS)

DeMets, C.; Merkouriev, S.

2016-11-01

We present new rotations that describe the relative positions and velocities of the Pacific and North America plates at 22 times during the past 19.7 Myr, offering ≈1-Myr temporal resolution for studies of the geotectonic evolution of western North America and other plate boundary locations. Derived from ≈18 000 magnetic reversal, fracture zone and transform fault identifications from the Pacific-Antarctic-Nubia-North America plate circuit and the velocities of 935 GPS sites on the Pacific and North America plates, the new rotations and GPS-derived angular velocity indicate that the rate of motion between the two plates increased by ≈70 per cent from 19.7 to 9±1 Ma, but changed by less than 2 per cent since 8 Ma and even less since 4.2 Ma. The rotations further suggest that the relative plate direction has rotated clockwise for most of the past 20 Myr, with a possible hiatus from 9 to 5 Ma. This conflicts with previously reported evidence for a significant clockwise change in the plate direction at ≈8-6 Ma. Our new rotations indicate that Pacific plate motion became obliquely convergent with respect to the San Andreas Fault of central California at 5.2-4.2 Ma, in agreement with geological evidence for a Pliocene onset of folding and faulting in central California. Our reconstruction of the northern Gulf of California at 6.3 Ma differs by only 15-30 km from structurally derived reconstructions after including 3-4 km Myr-1 of geodetically measured slip between the Baja California Peninsula and Pacific plate. This implies an approximate 15-30 km upper bound for plate non-rigidity integrated around the global circuit at 6.3 Ma. A much larger 200±54 km discrepancy between our reconstruction of the northern Gulf of California at 12 Ma and that estimated from structural and marine geophysical observations suggests that faults in northwestern Mexico or possibly west of the Baja California Peninsula accommodated large amounts of obliquely divergent dextral shear from 12-6.3 Ma. Pacific-North America plate motion since 16 Myr estimated with our new rotations agrees well with structurally summed deformation along two transects of western North America between the Colorado Plateau and western California, with a difference as small as 40 km out of 760 km of margin-parallel motion. A strong resemblance between a 20-Myr-to-present flow line reconstructed with our new rotations and the traces of the 700-km-long Queen Charlotte Fault and continental slope west of Canada suggests that the plate margin geometry was influenced by the passage of the Pacific plate and Yakutat block. The new rotations also suggest that (1) oblique convergence west of Canada initiated at 12-11 Ma, 5-8 Myr earlier than previously estimated, (2) no significant margin-normal shortening has occurred in areas of Canada located east of the Haida Gwaii archipelago since 20 Ma and (3) Pacific plate underthrusting of Haida Gwaii has accommodated the margin-normal component of plate motion since 12-11 Ma. Our rotations suggest an ≈70 per cent increase in the rate that the Pacific plate has been consumed by subduction beneath the Aleutian arc since 19.7 Ma, with still-unknown consequences for the rate of arc magmatism.

Tectonics of the Scotia-Antarctica plate boundary constrained from seismic and seismological data

NASA Astrophysics Data System (ADS)

Civile, D.; Lodolo, E.; Vuan, A.; Loreto, M. F.

2012-07-01

The plate boundary between the Scotia and Antarctic plates runs along the broadly E-W trending South Scotia Ridge. It is a mainly transcurrent margin that juxtaposes thinned continental and transitional crust elements with restricted oceanic basins and deep troughs. Seismic profiles and regional-scale seismological constraints are used to define the peculiarities of the crustal structures in and around the southern Scotia Sea, and focal solutions from recent earthquakes help to understand the present-day geodynamic setting. The northern edge of the western South Scotia Ridge is marked by a sub-vertical, left-lateral master fault. Locally, a narrow wedge of accreted sediments is present at the base of the slope. This segment represents the boundary between the Scotia plate and the independent South Shetland continental block. Along the northern margin of the South Orkney microcontinent, the largest fragment of the South Scotia Ridge, an accretionary prism is present at the base of the slope, which was possibly created by the eastward drift of the South Orkney microcontinent and the consequent subduction of the transitional crust present to the north. East of the South Orkney microcontinent, the physiography and structure of the plate boundary are less constrained. Here the tectonic regime exhibits mainly strike-slip behavior with some grade of extensional component, and the plate boundary is segmented by a series of NNW-SSE trending release zones which favored the fragmentation and dispersion of the crustal blocks. Seismic data have also identified, along the north-western edge of the South Scotia Ridge, an elevated region - the Ona Platform - which can be considered, along with the Terror Rise, as the conjugate margin of the Tierra del Fuego, before the Drake Passage opening. We propose here an evolutionary sketch for the plate boundary (from the Late Oligocene to the present) encompassing the segment from the Elephant Island platform to the Herdman Bank.

Plate tectonic evolution of the southern margin of Eurasia in the Mesozoic and Cenozoic

NASA Astrophysics Data System (ADS)

Golonka, J.

2004-03-01

Thirteen time interval maps were constructed, which depict the Triassic to Neogene plate tectonic configuration, paleogeography and general lithofacies of the southern margin of Eurasia. The aim of this paper is to provide an outline of the geodynamic evolution and position of the major tectonic elements of the area within a global framework. The Hercynian Orogeny was completed by the collision of Gondwana and Laurussia, whereas the Tethys Ocean formed the embayment between the Eurasian and Gondwanian branches of Pangea. During Late Triassic-Early Jurassic times, several microplates were sutured to the Eurasian margin, closing the Paleotethys Ocean. A Jurassic-Cretaceous north-dipping subduction boundary was developed along this new continental margin south of the Pontides, Transcaucasus and Iranian plates. The subduction zone trench-pulling effect caused rifting, creating the back-arc basin of the Greater Caucasus-proto South Caspian Sea, which achieved its maximum width during the Late Cretaceous. In the western Tethys, separation of Eurasia from Gondwana resulted in the formation of the Ligurian-Penninic-Pieniny-Magura Ocean (Alpine Tethys) as an extension of Middle Atlantic system and a part of the Pangean breakup tectonic system. During Late Jurassic-Early Cretaceous times, the Outer Carpathian rift developed. The opening of the western Black Sea occurred by rifting and drifting of the western-central Pontides away from the Moesian and Scythian platforms of Eurasia during the Early Cretaceous-Cenomanian. The latest Cretaceous-Paleogene was the time of the closure of the Ligurian-Pieniny Ocean. Adria-Alcapa terranes continued their northward movement during Eocene-Early Miocene times. Their oblique collision with the North European plate led to the development of the accretionary wedge of the Outer Carpathians and its foreland basin. The formation of the West Carpathian thrusts was completed by the Miocene. The thrust front was still propagating eastwards in the eastern Carpathians. During the Late Cretaceous, the Lesser Caucasus, Sanandaj-Sirjan and Makran plates were sutured to the Iranian-Afghanistan plates in the Caucasus-Caspian Sea area. A north-dipping subduction zone jumped during Paleogene to the Scythian-Turan Platform. The Shatski terrane moved northward, closing the Greater Caucasus Basin and opening the eastern Black Sea. The South Caspian underwent reorganization during Oligocene-Neogene times. The southwestern part of the South Caspian Basin was reopened, while the northwestern part was gradually reduced in size. The collision of India and the Lut plate with Eurasia caused the deformation of Central Asia and created a system of NW-SE wrench faults. The remnants of Jurassic-Cretaceous back-arc systems, oceanic and attenuated crust, as well as Tertiary oceanic and attenuated crust were locked between adjacent continental plates and orogenic systems.

Tectonics and Current Plate Motions of Northern Vancouver Island and the Adjacent Mainland

NASA Astrophysics Data System (ADS)

Jiang, Y.; Leonard, L. J.; Henton, J.; Hyndman, R. D.

2016-12-01

Northern Vancouver Island comprises a complex transition zone along the western margin of the North America plate, between the subducting Juan de Fuca plate to the south and the transcurrent Queen Charlotte Fault to the north off Haida Gwaii. The tectonic history and seismic potential for this region are unclear. Here we present current plate motions for northern Vancouver Island and the adjacent mainland, determined from continuous and campaign GPS measurements processed in a consistent manner. Immediately to the north of the mid-Vancouver Island Nootka Fault Zone, the northern limit of Juan de Fuca plate subduction, GPS velocity vectors show slower Explorer plate subduction than the Juan de Fuca Plate. Off northernmost Vancouver Island, the Winona Block is possibly converging at a slow rate that decreases northward to zero. We find a constant northward margin-parallel translation of up to 5 mm/year from northern Vancouver Island extending to Alaska. The southern limit of this translation coincides with areas of high heat flow that may reflect extension and the northern limit of episodic tremor and slip (ETS) on the Cascadia megathrust. The origin of the northward translation is poorly understood. We find a mainland coastal shear zone extends as far south as northern Vancouver Island where the offshore plate boundary is likely subduction. The pattern of the observed coastal shear cannot reflect interseismic locking on a major offshore transcurrent fault. The geodetically determined mainland coastal zone velocities decrease landward from 5 to 0 mm/yr across a region where no active faults have been identified and there is very little current seismicity. In Haida Gwaii, oblique convergence is apparent in the GPS data, consistent with partitioning between margin-parallel and margin-perpendicular strain. After removing the margin parallel translation from the data, we determine an average maximum locking depth of 15 km for the Queen Charlotte transcurrent fault, consistent with seismicity and seismic structure data.

Basin formation and Neogene sedimentation in a backarc setting, Halmahera, eastern Indonesia

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hall, R.; Nichols, G.J.

1991-03-01

It has been proposed that basins in backarc setting form in association with subduction by thinning of continental crust, backarc spreading in oceanic crust, compression, or trapping of pieces of oceanic plate behind an arc. The Halmahera basin in eastern Indonesia developed in a backarc setting but does not fall into these categories; it formed by subsidence of thickened crust made up of imbricated Mesozoic-Paleogene arc and ophiolite rocks. Halmahera lies at the western edge of the Philippine Sea Plate in a complex zone of convergence between the Eurasian margin, the oceanic plates of the West Pacific, and the Australian/Indianmore » Plate to the south. The basement is an imbricated complex of Mesozoic to Paleogene ophiolite, arc, and arc-related rocks. During the Miocene this basement complex formed an area of thickened crust upon which carbonate reef and reef-associated sediments were deposited. The authors interpret this shallow marine region to be similar to many of the oceanic plateaus and ridges found within the Philippine Sea Plate today. In the Late Miocene, convergence between the Philippine Sea Plate and the Eurasian margin resulted in the formation of the Halmahera Trench to the west of this region of thickened crust. Subduction of the Molucca Sea Plate caused the development of a volcanic island arc. Subsidence in the backarc area produced a broad sedimentary basin filled by clastics eroded from the arc and from uplifted basement and cover rocks. The basin was asymmetric with the thickest sedimentary fill on the western side, against the volcanic arc. The Halmahera basin was modified in the Plio-Pleistocene by east-west compression as the Molucca Sea Plate was eliminated by subduction.« less

Global tectonic studies: Hotspots and anomalous topography

NASA Technical Reports Server (NTRS)

Burke, K.; Kidd, W. S. F.; Delong, S.; Thiessen, R. L.; Carosella, R.; Mcgetchin, T. R.

1979-01-01

Volcanic activity on Earth and its secular variations are compared with that on other terrestrial planets. Activity at divergent, transform, and convergent plate margins is described with particular emphasis on hot spots and flood basalts. The timing and causing of uplifting above 500 meters, which in not associated with either plate boundaries or the normal nonplate margin edges of continents is considered with particular focus on the Guyana Highlands in southern Venezuela and western British Guiana, and the Brazilian Highlands in the central, eastern, and southern parts of the country. The mode and mechanism of plateau uplifting and the re-elevation of old mountain belts and subsidence of intra-continental basins are also discussed.

India plate angular velocity and contemporary deformation rates from continuous GPS measurements from 1996 to 2015.

PubMed

Jade, Sridevi; Shrungeshwara, T S; Kumar, Kireet; Choudhury, Pallabee; Dumka, Rakesh K; Bhu, Harsh

2017-09-12

We estimate a new angular velocity for the India plate and contemporary deformation rates in the plate interior and along its seismically active margins from Global Positioning System (GPS) measurements from 1996 to 2015 at 70 continuous and 3 episodic stations. A new India-ITRF2008 angular velocity is estimated from 30 GPS sites, which include stations from western and eastern regions of the plate interior that were unrepresented or only sparsely sampled in previous studies. Our newly estimated India-ITRF2008 Euler pole is located significantly closer to the plate with ~3% higher angular velocity than all previous estimates and thus predicts more rapid variations in rates and directions along the plate boundaries. The 30 India plate GPS site velocities are well fit by the new angular velocity, with north and east RMS misfits of only 0.8 and 0.9 mm/yr, respectively. India fixed velocities suggest an approximate of 1-2 mm/yr intra-plate deformation that might be concentrated along regional dislocations, faults in Peninsular India, Kachchh and Indo-Gangetic plain. Relative to our newly-defined India plate frame of reference, the newly estimated velocities for 43 other GPS sites along the plate margins give insights into active deformation along India's seismically active northern and eastern boundaries.

Permian-Triassic thermal anomaly of the active margin of South America as a result of plate kinematics reorganization

NASA Astrophysics Data System (ADS)

Riel, Nicolas; Jaillard, Etienne; Guillot, Stéphane; Martelat, Jean-Emmanuel; Braun, Jean

2013-04-01

From Permian to Triassic times, tectonic plate reorganization provoked Pangaea breakup, counterclockwise rotation of Gondwana, closing of the Paleo-Tethys Ocean and opening of the Neo-Tethys oceanic realm. Meanwhile, the switch from arc volcanism to widespread S-type magmatism along the western South American active margin around 275-265 Ma is symptomatic of the onset of a large-scale Permian-Triassic thermal anomaly (PTTA)affecting the whole margin. Here we report metamorphic and U-Pb geochronological results from the El Oro metamorphic complex in the forearc zone of southwestern Ecuador, which recorded the last step, at 230-225 Ma, of the PTTA. The change in the drift direction of Gondwana from north to east at ca. 270 Ma was related to plate reorganization and provoked the verticalization of the subducted Panthalassa slab. As the slab verticalized, strong heat advection produced a high heat flow beneath the active margin inducing the development of a huge thermal anomaly responsible for the PTTA, which lasted 30 Ma. This voluminous magmatic activity culminated at the Permian-Triassic boundary, and may have contributed to the degradation of life conditions on the Earth surface.

Inherited segmentation of the Iberian-African margins and tectonic reconstruction of a diffuse plate boundary.

NASA Astrophysics Data System (ADS)

Fernàndez, Manel; Torne, Montserrat; Vergés, Jaume; Casciello, Emilio

2016-04-01

Diffuse plate-boundary regions are characterized by non-well defined contacts between tectonic plates thus making difficult their reconstruction through time. The Western Mediterranean is one of these regions, where the convergence between the African and Iberian plates since Late Cretaceous resulted in the Betic-Rif arcuate orogen, the Gulf of Cadiz imbricate wedge, and the Alboran back-arc basin. Whereas the Iberia-Africa plate boundary is well defined west to the Gorringe Bank and along the Gloria Fault, it becomes much more diffuse eastwards with seismicity spreading over both the south-Iberian and north-African margins. Gravity data, when filtered for short wavelengths, show conspicuous positive Bouguer anomalies associated with the Gorringe Bank, the Gulf of Cadiz High and the Ronda/Beni-Bousera peridotitic massifs reflecting an inherited Jurassic margin segmentation. The subsequent Alpine convergence between Africa and Iberia reactivated these domains, producing crustal-scale thrusting in the Atlantic segments and eventually subduction in the proto-Mediterranean segments. The Jurassic segmentation of the Iberia-Africa margins substantiates the double-polarity subduction model proposed for the region characterized by a change from SE-dipping polarity in the Gorringe, Gulf of Cadiz and Betic-Rif domains, to NW-dipping polarity in the proto-Algerian domain. Therefore, the Algerian and Tyrrhenian basins in the east and the Alboran basin in the west are the result of SSE-E and NW-W retreating slabs of oceanic and/or hyper-extended Tethyan domains, respectively.

Seismic stratigraphy of the Mianwali and Bannu depressions, north-western Indus foreland basin

NASA Astrophysics Data System (ADS)

Farid, Asam; Khalid, Perveiz; Ali, Muhammad Y.; Iqbal, Muhammad Asim; Jadoon, Khan Zaib

2017-11-01

Regional seismic reflection profiles, deep exploratory wells, and outcrop data have been used to study the structure and stratigraphic architecture of the Mianwali and Bannu depressions, north-western Indus foreland basin. Synthetic seismograms have been used to identify and tie the seismic horizons to the well data. Nine mappable seismic sequences are identified within the passive and active margin sediments. In general, the Mianwali and Bannu depressions deepens towards north due to the flexure generated by the loading and southward shifting of the thrust sheets of the North-western Himalayan Fold and Thrust Belt. The seismic profiles show a classic wedge shaped foreland basin with a prominent angular unconformity which clearly differentiates the active and passive margin sediments. The onlap patterns in the Late Cretaceous sediments suggest the initial onset of foreland basin formation when the Indian Plate collided with Eurasian Plate. As the collision progressed, the lithospheric flexure caused an uplift along the flexural bulge which resulted in onlaps within the Paleocene and Eocene sequences. The tectonic activity reached to its maximum during Oligocene with the formation of a prominent unconformity, which caused extensive erosion that increases towards the flexural bulge.

Juan de Fuca slab geometry and its relation to Wadati-Benioff zone seismicity

USGS Publications Warehouse

McCrory, Patricia A.; Blair, J. Luke; Waldhause, Felix; Oppenheimer, David H.

2012-01-01

A new model of the subducted Juan de Fuca plate beneath western North America allows first-order correlations between the occurrence of Wadati-Benioff zone earthquakes and slab geometry, temperature, and hydration state. The geo-referenced 3D model, constructed from weighted control points, integrates depth information from earthquake locations and regional seismic velocity studies. We use the model to separate earthquakes that occur in the Cascadia forearc from those that occur within the underlying Juan de Fuca plate and thereby reveal previously obscured details regarding the spatial distribution of earthquakes. Seismicity within the slab is most prevalent where the slab is warped beneath northwestern California and western Washington suggesting that slab flexure, in addition to expected metamorphic dehydration processes, promotes earthquake occurrence within the subducted oceanic plate. Earthquake patterns beneath western Vancouver Island are consistent with slab dehydration processes. Conversely, the lack of slab earthquakes beneath western Oregon is consistent with an anhydrous slab. Double-differenced relocated seismicity resolves a double seismic zone within the slab beneath northwestern California that strongly constrains the location of the plate interface and delineates a cluster of seismicity 10 km above the surface that includes the 1992 M7.1 Mendocino earthquake. We infer that this earthquake ruptured a surface within the Cascadia accretionary margin above the Juan de Fuca plate. We further speculate that this earthquake is associated with a detached fragment of former Farallon plate. Other subsurface tectonic elements within the forearc may have the potential to generate similar damaging earthquakes.

Phanerozoic tectonic evolution of the Circum-North Pacific

USGS Publications Warehouse

Nokleberg, Warren J.; Parfenov, Leonid M.; Monger, James W.H.; Norton, Ian O.; Khanchuk, Alexander I.; Stone, David B.; Scotese, Christopher R.; Scholl, David W.; Fujita, Kazuya

2000-01-01

The Phanerozoic tectonic evolution of the Circum-North Pacific is recorded mainly in the orogenic collages of the Circum-North Pacific mountain belts that separate the North Pacific from the eastern part of the North Asian Craton and the western part of the North American Craton. These collages consist of tectonostratigraphic terranes that are composed of fragments of igneous arcs, accretionary-wedge and subduction-zone complexes, passive continental margins, and cratons; they are overlapped by continental-margin-arc and sedimentary-basin assemblages. The geologic history of the terranes and overlap assemblages is highly complex because of postaccretionary dismemberment and translation during strike-slip faulting that occurred subparallel to continental margins.We analyze the complex tectonics of this region by the following steps. (1) We assign tectonic environments for the orogenic collages from regional compilation and synthesis of stratigraphic and faunal data. The types of tectonic environments include cratonal, passive continental margin, metamorphosed continental margin, continental-margin arc, island arc, oceanic crust, seamount, ophiolite, accretionary wedge, subduction zone, turbidite basin, and metamorphic. (2) We make correlations between terranes. (3) We group coeval terranes into a single tectonic origin, for example, a single island arc or subduction zone. (4) We group igneous-arc and subduction- zone terranes, which are interpreted as being tectonically linked, into coeval, curvilinear arc/subduction-zone complexes. (5) We interpret the original positions of terranes, using geologic, faunal, and paleomagnetic data. (6) We construct the paths of tectonic migration. Six processes overlapping in time were responsible for most of the complexities of the collage of terranes and overlap assemblages around the Circum-North Pacific, as follows. (1) During the Late Proterozoic, Late Devonian, and Early Carboniferous, major periods of rifting occurred along the ancestral margins of present-day Northeast Asia and northwestern North America. The rifting resulted in the fragmentation of each continent and the formation of cratonal and passive continental-margin terranes that eventually migrated and accreted to other sites along the evolving margins of the original or adjacent continents. (2) From about the Late Triassic through the mid-Cretaceous, a succession of island arcs and tectonically paired subduction zones formed near the continental margins. (3) From about mainly the mid-Cretaceous through the present, a succession of igneous arcs and tectonically paired subduction zones formed along the continental margins. (4) From about the Jurassic to the present, oblique convergence and rotations caused orogenparallel sinistral and then dextral displacements within the upper-plate margins of cratons that have become Northeast Asia and North America. The oblique convergences and rotations resulted in the fragmentation, displacement, and duplication of formerly more nearly continuous arcs, subduction zones, and passive continental margins. These fragments were subsequently accreted along the expanding continental margins. (5) From the Early Jurassic through Tertiary, movement of the upper continental plates toward subduction zones resulted in strong plate coupling and accretion of the former island arcs and subduction zones to the continental margins. Accretions were accompanied and followed by crustal thickening, anatexis, metamorphism, and uplift. The accretions resulted in substantial growth of the North Asian and North American Continents. (6) During the middle and late Cenozoic, oblique to orthogonal convergence of the Pacifi c plate with present-day Alaska and Northeast Asia resulted in formation of the modern-day ring of volcanoes around the Circum-North Pacific. Oblique convergence between the Pacific plate and Alaska also resulted in major dextral-slip faulting in interior and southern Alaska and along the western p

Active faults and minor plates in NE Asia

NASA Astrophysics Data System (ADS)

Kozhurin, Andrey I.; Zelenin, Egor A.

2014-05-01

Stated nearly 40 yr ago the uncertainty with plate boundaries location in NE Asia (Chapman, Solomon, 1976) still remains unresolved. Based on the prepositions that a plate boundary must, first, reveal itself in linear sets of active structures, and, second, be continuous and closed, we have undertaken interpretation of medium-resolution KH-9 Hexagon satellite imageries, mostly in stereoscopic regime, for nearly the entire region of NE Asia. Main findings are as follows. There are two major active fault zones in the region north of the Bering Sea. One of them, the Khatyrka-Vyvenka zone, stretches NE to ENE skirting the Bering Sea from the Kamchatka isthmus to the Navarin Cape. Judging by the kinematics of the Olyutorsky 2006 earthquake fault, the fault zones move both right-laterally and reversely. The second active fault zone, the Lankovaya-Omolon zone, starts close to the NE margin of the Okhotsk Sea and extends NE up to nearly the margin of the Chukcha Sea. The fault zone is mostly right-lateral, with topographically expressed cumulative horizontal offsets amounting to 2.5-2.6 km. There may be a third NE-SW zone between the major two coinciding with the Penzhina Range as several active faults found in the southern termination of the Range indicate. The two active fault zones divide the NE Asia area into two large domains, which both could be parts of the Bering Sea plate internally broken and with uncertain western limit. Another variant implies the Khatyrka-Vyvenka zone as the Bering Sea plate northern limit, and the Lankovaya-Omolon zone as separating an additional minor plate from the North-American plate. The choice is actually not crucial, and more important is that both variants leave the question of where the Bering Sea plate boundary is in Alaska. The Lankovaya-Omolon zone stretches just across the proposed northern boundary of the Okhorsk Sea plate. NW of the zone, there is a prominent left-lateral Ulakhan fault, which is commonly interpreted to be a portion of the plate northern boundary. With this, we have discovered no active faults or fault zones of the Ulakhan fault strike, which could be the portion of the boundary between the Lankovaya-Omolon zone and either the western margin of the Komandor basin or the westernmost Aleutians. We conclude that there is a certain disagreement between active faulting pattern and plate models for NE Asia, relating to the extent of the plates and missing portions of the plate boundaries. The research was supported by grant # 110500136-a from the Russian Foundation for Basic Research.

Abbot Ice Shelf, the Amundsen Sea Continental Margin and the Southern Boundary of the Bellingshausen Plate Seaward of West Antarctica

NASA Astrophysics Data System (ADS)

Cochran, J. R.; Tinto, K. J.; Bell, R. E.

2014-12-01

The Abbot Ice Shelf extends 450 km along the coast of West Antarctica between 103°W and 89°W and straddles the boundary between the Bellingshausen Sea continental margin, which overlies a former subduction zone, and Amundsen Sea rifted continental margin. Inversion of NASA Operation IceBridge airborne gravity data for sub-ice bathymetry shows that the western part of the ice shelf, as well as Cosgrove Ice Shelf to the south, are underlain by a series of east-west trending rift basins. The eastern boundary of the rifted terrain coincides with the eastern boundary of rifting between Antarctica and Zealandia and the rifts formed during the early stages of this rifting. Extension in these rifts is minor as rifting quickly jumped north of Thurston Island. The southern boundary of the Cosgrove Rift is aligned with the southern boundary of a sedimentary basin under the Amundsen Embayment continental shelf to the west, also formed by Antarctica-Zealandia rifting. The shelf basin has an extension factor, β, of 1.5 - 1.7 with 80 -100 km of extension occurring in an area now ~250 km wide. Following this extension early in the rifting process, rifting centered to the north of the present shelf edge and proceeded to continental rupture. Since then, the Amundsen Embayment continental shelf has been tectonically quiescent and has primarily been shaped though subsidence, sedimentation and the passage of the West Antarctic Ice Sheet back and forth across it. The former Bellingshausen Plate was located seaward of the Amundsen Sea margin prior to its incorporation into the Antarctic Plate at ~62 Ma. During the latter part of its existence, Bellingshausen plate motion had a clockwise rotational component relative to Antarctica producing convergence between the Bellingshausen and Antarctic plates east of 102°W. Seismic reflection and gravity data show that this convergence is expressed by an area of intensely deformed sediments beneath the continental slope from 102°W to 95°W and by incipient subduction beneath the Bellingshausen Gravity Anomaly on the western edge of a salient of the Antarctic plate near 94°W. West of 102°W, relative motion was extensional and occurred in a diffuse zone occupied by the Marie Byrd Seamounts that are dated at 65-56 Ma and extend 800 km along the continental margin near the base of the continental rise.

Seismicity of the Earth 1900-2012 Philippine Sea plate and vicinity

USGS Publications Warehouse

Smoczyk, Gregory M.; Hayes, Gavin P.; Hamburger, Michael W.; Benz, Harley M.; Villaseñor, Antonio; Furlong, Kevin P.

2013-01-01

The complex tectonics surrounding the Philippine Islands are dominated by the interactions of the Pacific, Sunda, and Eurasia plates with the Philippine Sea plate (PSP). The latter is unique because it is almost exclusively surrounded by zones of plate convergence. At its eastern and southeastern edges, the Pacific plate is subducted beneath the PSP at the Izu-Bonin, Mariana, and Yap trenches. Here, the subduction zone exhibits high rates of seismic activity to depths of over 600 km, though no great earthquakes (M>8.0) have been observed, likely because of weak coupling along the plate interface. In the northeast, the PSP subducts beneath Japan and the eastern margin of the Eurasia plate at the Nankai and Ryukyu trenches, extending westward to Taiwan. The Nankai portion of this subduction zone has hosted some of the largest earthquakes along the margins of the PSP, including a pair of Mw8.1 megathrust events in 1944 and 1946. Along its western margin, the convergence of the PSP and the Sunda plate is responsible for a broad and active plate boundary system extending along both sides of the Philippine Islands chain. The region is characterized by opposite-facing subduction systems on the east and west sides of the islands, and the archipelago is cut by a major transform structure: the Philippine Fault. Subduction of the Philippine Sea plate occurs at the eastern margin of the islands along the Philippine Trench and its northern extension, the East Luzon Trough. On the west side of Luzon, the Sunda Plate subducts eastward along a series of trenches, including the Manila Trench in the north, the smaller Negros Trench in the central Philippines, and the Sulu and Cotabato trenches in the south. Twentieth and early twentyfirst century seismic activity along the boundaries of the Philippine Sea plate has produced seven great (M>8.0) earthquakes and 250 large (M>7) events. Among the most destructive events were the 1923 Kanto, the 1948 Fukui, and the 1995 Kobe, Japan, earthquakes; the 1935 and the 1999 Chi-Chi, Taiwan, earthquakes; and the 1976 M7.6 Moro Gulf and 1990 M7.6 Luzon, Philippines, earthquakes.

Mantle-derived peridotites in southwestern Oregon: relation to plate tectonics.

PubMed

Medaris, L G; Dott, R H

1970-09-04

A group of peridotites in southwestern Oregon contains high-pressure mineral assemblages reflecting recrystallization at high temperatures (1100 degrees to 1200 degrees C) over a range of pressure decreasing from 19 to 5 kilobars. It is proposed that the peridotites represent upper-mantle material brought from depth along the ancestral Gorda-Juan de Fuca ridge system, transported eastward by the spreading Gorda lithosphere plate, and then emplaced by thrust-faulting in the western margin of the Cordillera during late Mesozoic time.

Seismicity of the Earth 1900-2007, Caribbean Plate and Vicinity

USGS Publications Warehouse

Benz, Harley M.; Tarr, Arthur C.; Hayes, Gavin P.; Villaseñor, Antonio; Furlong, Kevin P.; Dart, Richard L.; Rhea, Susan

2010-01-01

The depth profile panels on this map portray earthquakes that extend from the Middle America Trench axis in the west to depths as great as 300 km beneath Guatemala, and from the Lesser Antilles Trench axis in the east to depths of approximately 200 km beneath Guadeloupe and the northeast Caribbean. In contrast, seismicity along the segments of the Caribbean plate margins from Guatemala to Hispaniola and from Trinidad to western Venezuela is indicative of transform fault tectonics.

Origin of the oceanic basalt basement of the Solomon Islands arc and its relationship to the Ontong Java Plateau-insights from Cenozoic plate motion models

USGS Publications Warehouse

Wells, R.E.

1989-01-01

Cenozoic global plate motion models based on a hotspot reference frame may provide a useful framework for analyzing the tectonic evolution of the Solomon Islands convergent margin. A postulated late Miocene collision of the Ontong Java Plateau (OJP) with a NE-facing arc is consistent with the predicted path of the OJP across the Pacific Basin and its Miocene arrival at the trench. Late-stage igneous activity (65-30 Ma) predicted for the OJP as it rode over the Samoan hotspot occurred in correlative stratigraphic sections on Malaita, the supposed accreted flake of OJP in the Solomon Islands arc. Convergence similar to the present velocities between Australia and the Pacific plates was characteristic of the last 43 million years. Prior to 43 Ma Pacific-Australia plate motions were divergent, seemingly at odds with geologic evidence for early Tertiary convergence, particularly in Papua New Guinea. A postulated South Pacific plate may have existed between Australia and the Pacific plate and would have allowed implied northward subduction along the northeastern Australia plate boundary that lasted into the early Eocene. Subsequent reorganization of plate motions in the middle Eocene correlates with middle Eocene marginal basin formation along ridges oblique to the main plate boundary. Cessation of spreading on the Pacific-South Pacific Ridge and its subsequent subduction beneath Asia followed the change in Pacific plate motion at 43 Ma. A trapped remnant of the extinct, NW-trending ridge may still lie beneath the western Philippine Sea. The terminal deformation, metamorphism and ophiolite obduction in the Eocene orogen of the southwest Pacific also correlates with the major change in Pacific plate motion at 43 Ma and the subsequent compression of the dying Eocene arc against outlying continental and oceanic crustal blocks of the Australian plate. The Solomon Islands oceanic basement may represent juxtaposition of oceanic plateaus of the Australian plate beneath overthrust, dismembered ophiolite derived from adjacent marginal basin crust. ?? 1989.

Pn tomography of South China Sea, Taiwan Island, Philippine archipelago, and adjacent regions

NASA Astrophysics Data System (ADS)

Li, Xibing; Song, Xiaodong; Li, Jiangtao

2017-02-01

The South China Sea (SCS) and its surrounding areas are geologically highly heterogeneous from the interactions of multiple plates in Southeast Asia (Eurasian plate, Indian-Australian plate, Philippine Sea plate, and Pacific plate). To understand the tectonics at depth, here we combined bulletin and handpicked data to conduct Pn tomography of the region. The results show distinct features that are correlated with the complex geology at surface, suggesting a lithosphere-scale tectonics of the region. Low Pn velocities are found along a belt of the western Pacific transpressional system from the Okinawa Trough and eastern East China Sea, across central and eastern Taiwan orogeny, to the island arcs of the Luzon Strait and the entire Philippine Islands, as well as under the Palawan Island and part of the continental margin north of the Pearl River Basin. High velocities are found under Ryukyu subduction zone, part of the Philippine subduction zone, part of the Eurasian subduction beneath the southwestern Taiwan, and the continent-ocean boundary between the south China and the SCS basin. The Taiwan Strait, the Mainland SE coast, and the main SCS basin sea are relatively uniform with average Pn values. Crustal thicknesses show large variations in the study region but also coherency with tectonic elements. The Pn pattern in Taiwan shows linear trends of surface geology and suggests strongly lithosphere-scale deformation of the young Taiwan orogenic belt marked by the deformation boundary under the Western Foothill and the Western Coastal Plain at depth, and the crustal thickness shows a complex pattern from the transpressional collision. Our observations are consistent with rifting and extension in the northern margin of the SCS but are not consistent with mantle upwelling as a mechanism for the opening and the subsequent closing of the SCS. The Philippine island arc is affected by volcanisms from both the Asian and Philippine Sea subductions in the south but mainly from the Asian subduction in the north and under the Luzon Strait.

Basement thrust sheets in the Clearwater orogenic zone, central Idaho and western Montana

NASA Astrophysics Data System (ADS)

Skipp, Betty

1987-03-01

The Clearwater orogenic zone in central Idaho and western Montana contains at least two major northeast-directed Cordilleran thrust plates of Early Proterozoic metasedimentary and metaigneous rocks that overrode previously folded Middle Proterozoic rocks of the Belt basin in Cretaceous time. The northeastward migration of the resultant thickened wedge of crustal material combined with Cretaceous subduction along the western continental margin produced a younger northern Bitterroot lobe of the Idaho batholith relative to an older southern Atlanta lobe. Eocene extensional unroofing and erosion of the Bitterroot lobe has exposed the roots of the thick Cordilleran thrust sheets.

Fault evolution in the Potiguar rift termination, equatorial margin of Brazil

NASA Astrophysics Data System (ADS)

de Castro, D. L.; Bezerra, F. H. R.

2015-02-01

The transform shearing between South American and African plates in the Cretaceous generated a series of sedimentary basins on both plate margins. In this study, we use gravity, aeromagnetic, and resistivity surveys to identify architecture of fault systems and to analyze the evolution of the eastern equatorial margin of Brazil. Our study area is the southern onshore termination of the Potiguar rift, which is an aborted NE-trending rift arm developed during the breakup of Pangea. The basin is located along the NNE margin of South America that faces the main transform zone that separates the North and the South Atlantic. The Potiguar rift is a Neocomian structure located at the intersection of the equatorial and western South Atlantic and is composed of a series of NE-trending horsts and grabens. This study reveals new grabens in the Potiguar rift and indicates that stretching in the southern rift termination created a WNW-trending, 10 km wide, and ~ 40 km long right-lateral strike-slip fault zone. This zone encompasses at least eight depocenters, which are bounded by a left-stepping, en echelon system of NW-SE- to NS-striking normal faults. These depocenters form grabens up to 1200 m deep with a rhomb-shaped geometry, which are filled with rift sedimentary units and capped by postrift sedimentary sequences. The evolution of the rift termination is consistent with the right-lateral shearing of the equatorial margin in the Cretaceous and occurs not only at the rift termination but also as isolated structures away from the main rift. This study indicates that the strike-slip shearing between two plates propagated to the interior of one of these plates, where faults with similar orientation, kinematics, geometry, and timing of the major transform are observed. These faults also influence rift geometry.

Global tectonic reconstructions with continuously deforming and evolving rigid plates

NASA Astrophysics Data System (ADS)

Gurnis, Michael; Yang, Ting; Cannon, John; Turner, Mark; Williams, Simon; Flament, Nicolas; Müller, R. Dietmar

2018-07-01

Traditional plate reconstruction methodologies do not allow for plate deformation to be considered. Here we present software to construct and visualize global tectonic reconstructions with deforming plates within the context of rigid plates. Both deforming and rigid plates are defined by continuously evolving polygons. The deforming regions are tessellated with triangular meshes such that either strain rate or cumulative strain can be followed. The finite strain history, crustal thickness and stretching factor of points within the deformation zones are tracked as Lagrangian points. Integrating these tools within the interactive platform GPlates enables specialized users to build and refine deforming plate models and integrate them with other models in time and space. We demonstrate the integrated platform with regional reconstructions of Cenozoic western North America, the Mesozoic South American Atlantic margin, and Cenozoic southeast Asia, embedded within global reconstructions, using different data and reconstruction strategies.

Subducting plate geology in three great earthquake ruptures of the western Alaska margin, Kodiak to Unimak

USGS Publications Warehouse

von Huene, Roland E.; Miller, John J.; Weinrebe, Wilhelm

2012-01-01

Three destructive earthquakes along the Alaska subduction zone sourced transoceanic tsunamis during the past 70 years. Since it is reasoned that past rupture areas might again source tsunamis in the future, we studied potential asperities and barriers in the subduction zone by examining Quaternary Gulf of Alaska plate history, geophysical data, and morphology. We relate the aftershock areas to subducting lower plate relief and dissimilar materials in the seismogenic zone in the 1964 Kodiak and adjacent 1938 Semidi Islands earthquake segments. In the 1946 Unimak earthquake segment, the exposed lower plate seafloor lacks major relief that might organize great earthquake rupture. However, the upper plate contains a deep transverse-trending basin and basement ridges associated with the Eocene continental Alaska convergent margin transition to the Aleutian island arc. These upper plate features are sufficiently large to have affected rupture propagation. In addition, massive slope failure in the Unimak area may explain the local 42-m-high 1946 tsunami runup. Although Quaternary geologic and tectonic processes included accretion to form a frontal prism, the study of seismic images, samples, and continental slope physiography shows a previous history of tectonic erosion. Implied asperities and barriers in the seismogenic zone could organize future great earthquake rupture.

Geomorphology and Neogene tectonic evolution of the Palomares continental margin (Western Mediterranean)

NASA Astrophysics Data System (ADS)

Gómez de la Peña, Laura; Gràcia, Eulàlia; Muñoz, Araceli; Acosta, Juan; Gómez-Ballesteros, María; R. Ranero, César; Uchupi, Elazar

2016-10-01

The Palomares continental margin is located in the southeastern part of Spain. The margin main structure was formed during Miocene times, and it is currently part of the wide deformation zone characterizing the region between the Iberian and African plates, where no well-defined plate boundary occurs. The convergence between these two plates is here accommodated by several structures, including the left lateral strike-slip Palomares Fault. The region is characterized by sparse, low to moderate magnitude (Mw < 5.2) shallow instrumental earthquakes, although large historical events have also occurred. To understand the recent tectonic history of the margin we analyze new high-resolution multibeam bathymetry data and re-processed three multichannel seismic reflection profiles crossing the main structures. The analysis of seafloor morphology and associated subsurface structure provides new insights of the active tectonic features of the area. In contrast to other segments of the southeastern Iberian margin, the Palomares margin contains numerous large and comparatively closely spaced canyons with heads that reach near the coast. The margin relief is also characterized by the presence of three prominent igneous submarine ridges that include the Aguilas, Abubacer and Maimonides highs. Erosive processes evidenced by a number of scars, slope failures, gullies and canyon incisions shape the present-day relief of the Palomares margin. Seismic images reveal the deep structure distinguishing between Miocene structures related to the formation of the margin and currently active features, some of which may reactivate inherited structures. The structure of the margin started with an extensional phase accompanied by volcanic accretion during the Serravallian, followed by a compressional pulse that started during the Latemost Tortonian. Nowadays, tectonic activity offshore is subdued and limited to few, minor faults, in comparison with the activity recorded onshore. The deep Algero-Balearic Basin is affected by surficial processes, associated to halokinesis of Messinian evaporites.

From transpressional to transtensional tectonics in Northern Central America controlled by Cocos - Caribbean subduction coupling change

NASA Astrophysics Data System (ADS)

Alonso-Henar, Jorge; Alvarez-Gomez, José Antonio; Jesús Martinez-Diaz, José

2017-04-01

The Central American Volcanic Arc (CAVA) is located at the western margin of the Caribbean plate, over the Chortís Block, spanning from Guatemala to Costa Rica. The CAVA is associated to the subduction of the Cocos plate under the Caribbean plate at the Middle America Trench. Our study is focused in the Salvadorian CAVA segment, which is tectonically characterized by the presence of the El Salvador Fault Zone (ESFZ), part of the western boundary of a major block forming the Caribbean plate (the Chortis Block). The structural evolution of the western boundary of the Chortis Block, particularly in the CAVA crossing El Salvador remains unknown. We have done a kinematic analysis from seismic and fault slip data and combined our results with a review of regional previous studies. This approach allowed us to constrain the tectonic evolution and the forces that control the deformation in northern Central America. Along the active volcanic arc we identified active transtensional deformation. On the other hand, we have identified two deformation phases in the back arc region: A first one of transpressional wrenching close to simple shearing (Miocene); and a second one characterized by almost E-W extension. Our results reveal a change from transpressional to transtensional shearing coeval with a migration of the volcanism towards the trench in Late Miocene times. This strain change could be related with a coupled to decoupled transition on the Cocos - Caribbean subduction interface, which could be related to a slab roll-back of the Cocos Plate beneath the Chortis Block. The combination of different degrees of coupling on the subduction interface, together with a constant relative eastward drift of the Caribbean Plate, control the deformation style along the western boundary of the Chortis Block.

Petrology and age of volcanic-arc rocks from the continental margin of the Bering Sea: implications for Early Eocene relocation of plate boundaries

USGS Publications Warehouse

Davis, A.S.; Pickthorn, L.-B.G.; Vallier, T.L.; Marlow, M. S.

1989-01-01

Eocene volcanic flow and dike rocks from the Beringian margin have arc characteristics, implying a convergent history for this region during the early Tertiary. Chemical and mineralogical compositions are similar to those of modern Aleutian-arc lavas. They also resemble volcanic-arc compositions from western mainland Alaska, although greater chemical diversity and a stronger continental influence are observed in the Alaskan mainland rocks. Early Eocene ages of 54.4-50.2 Ma for the Beringian samples are well constrained by conventional K-Ar ages of nine plagioclase separates and by concordant 40Ar/39Ar incremental heating and total-fusion experiments. A concordant U-Pb zircon age of 53 Ma for the quartz-diorite dike is in good agreement with the K-Ar data. Plate motion studies of the North Pacific Ocean indicate more northerly directed subduction prior to the Tertiary and a continuous belt of arc-type volcanism extending from Siberia, along the Beringian margin, into mainland Alaska. Around 56 Ma (chron 25-24), subduction changed to a more westerly direction and subduction-related volcanism ceased for most of mainland Alaska. The increasingly oblique angle of convergence should have ended subduction along the Beringian margin as well. However, consistent ages of 54-50 Ma indicate a final pulse in arc-type magmatism during this period of plate adjustment. -from Authors

The Rovuma Transform Margin: the enigmatic continent-ocean boundary of East Africa

NASA Astrophysics Data System (ADS)

Phethean, Jordan; Kalnins, Lara; van Hunen, Jeroen; McCaffrey, Ken; Davies, Richard

2017-04-01

The N-S trending Davie Fracture Zone (DFZ) is often assumed to form the continent-ocean transform margin (COTM) of the Western Somali Basin. However, multiple plate tectonic reconstructions favour a pre-breakup location for Madagascar that crosses the DFZ, incompatible with its interpretation as the COTM (e.g., Lottes & Rowley, 1990; Reeves, 2014; Phethean et al., 2016). For the first time, we have identified classic COTM features in seismic reflection data from the Southern Rovuma Basin, to the west and inboard of the DFZ. These suggest a NNW trend to the margin, consistent with the tectonic reconstructions. 2D gravity models, with the seabed and top basement constrained by seismic data, are used to investigate the Moho structure across the Rovuma margin and are best fit using steep 'transform style' geometries, confirming the nature of the margin. We thus model generic COTM geometries elsewhere along the East African and Madagascan transform margins to locate best-fitting positions for these conjugate COTMs. This analysis confirms that the COTMs follow a NNW trend along the Rovuma Basin and Southern Madagascar, respectively, and allows a restoration of the conjugate COTMs. This restoration is used alongside geological maps and satellite imagery from Madagascar and East Africa to refine early plate motions and further constrain the precise origin of Madagascar within Gondwana. Our refined plate tectonic model independently predicts major observations made from seismic reflection and gravity data across the basin, including: regions of major transpression/transtension along the DFZ, merging of fracture zones to form the DFZ, oceanic crust on either side of the DFZ and within the Tanzania coastal basin, and the location of an abandoned MOR within the Tanzania coastal basin. We believe that this study finally provides conclusive evidence that Madagascar originated from within the Tanzania Coastal Basin, inboard of the DFZ, after some 30 years of debate regarding this matter. Lottes, A.L., Rowley, D.B., 1990. Reconstruction of the Laurasian and Gondwanan segments of Permian Pangea. Geol. Soc. London Mem., 12, 383-395. Reeves, C., 2014. The position of Madagascar within Gondwana and its movements during Gondwana dispersal. J. Afr. Earth Sci., 94, 45-57. Phethean, J.J.J., Kalnins, L.M., van Hunen, J.,Biffi, P.G., Davies, R.J., McCaffrey, K.J.W., 2016. Madagascar's escape from Africa: A high-resolution plate reconstruction for the Western Somali Basin and implications for supercontinent dispersal. Geochem. Geophys. Geosyst., 17, doi:10.1002/2016GC006624.

Playing jigsaw with large igneous provinces - a plate-tectonic reconstruction of Ontong Java Nui

NASA Astrophysics Data System (ADS)

Hochmuth, Katharina; Gohl, Karsten; Uenzelmann-Neben, Gabriele; Werner, Reinhard

2015-04-01

Ontong Java Nui is a Cretaceous large igneous province (LIP), which was rifted apart into various smaller plateaus shortly after its emplacement around 125 Ma in the central Pacific. It incorporated the Ontong Java Plateau, the Hikurangi Plateau and the Manihiki Plateau as well as multiple smaller fragments, which have been subducted. Its size has been estimated to be approximately 0.8% of the Earth's surface. A volcanic edifice of this size has potentially had a great impact on the environment such as its CO2 release. The break-up of the "Super"-LIP is poorly constrained, because the break-up and subsequent seafloor spreading occurred within the Cretaceous Quiet Period. The Manihiki Plateau is presumably the centerpiece of this "Super"-LIP and shows by its margins and internal fragmentation that its tectonic and volcanic activity is related to the break-up of Ontong Java Nui. By incorporating two new seismic refraction/wide-angle reflection lines across two of the main sub-plateaus of the Manihiki Plateau, we can classify the break-up modes of the individual margins of the Manihiki Plateau. The Western Plateaus experienced crustal stretching due to the westward motion of the Ontong Java Plateau. The High Plateau shows sharp strike-slip movements at its eastern boundary towards an earlier part of Ontong Java Nui, which is has been subducted, and a rifted margin with a strong volcanic overprint at its southern edges towards the Hikurangi Plateau. These observations allow us a re-examination of the conjugate margins of the Hikurangi Plateau and the Ontong Java Plateau. The repositioning of the different plateaus leads to the conclusion that Ontong Java Nui was larger (~1.2% of the Earth's surface at emplacement) than previously anticipated. We use these finding to improve the plate tectonic reconstruction of the Cretaceous Pacific and to illuminate the role of the LIPs within the plate tectonic circuit in the western and central Pacific.

Constraints Imposed by Rift Inheritance on the Compressional Reactivation of a Hyperextended Margin: Mapping Rift Domains in the North Iberian Margin and in the Cantabrian Mountains

NASA Astrophysics Data System (ADS)

Cadenas, P.; Fernández-Viejo, G.; Pulgar, J. A.; Tugend, J.; Manatschal, G.; Minshull, T. A.

2018-03-01

The Alpine Pyrenean-Cantabrian orogen developed along the plate boundary between Iberia and Europe, involving the inversion of Mesozoic hyperextended basins along the southern Biscay margin. Thus, this margin represents a natural laboratory to analyze the control of structural rift inheritance on the compressional reactivation of a continental margin. With the aim to identify former rift domains and investigate their role during the subsequent compression, we performed a structural analysis of the central and western North Iberian margin, based on the interpretation of seismic reflection profiles and local constraints from drill-hole data. Seismic interpretations and published seismic velocity models enabled the development of crustal thickness maps that helped to constrain further the offshore and onshore segmentation. Based on all these constraints, we present a rift domain map across the central and western North Iberian margin, as far as the adjacent western Cantabrian Mountains. Furthermore, we provide a first-order description of the margin segmentation resulting from its polyphase tectonic evolution. The most striking result is the presence of a hyperthinned domain (e.g., Asturian Basin) along the central continental platform that is bounded to the north by the Le Danois High, interpreted as a rift-related continental block separating two distinctive hyperextended domains. From the analysis of the rift domain map and the distribution of reactivation structures, we conclude that the landward limit of the necking domain and the hyperextended domains, respectively, guide and localize the compressional overprint. The Le Danois block acted as a local buttress, conditioning the inversion of the Asturian Basin.

A Plate Tectonic Model for the Neoproterozoic with Evolving Plate Boundaries

NASA Astrophysics Data System (ADS)

Merdith, Andrew; Collins, Alan; Williams, Simon; Pisarevsky, Sergei; Müller, Dietmar

2017-04-01

The Neoproterozoic was dominated by the formation of the supercontinent Rodinia, its break-up and the subsequent amalgamation of Gondwana, during which, the planet experienced large climatic variations and the emergence of complex life. Here we present a topological plate model of the Neoproterozoic based on a synthesis of available geological and palaeomagnetic data. Subduction zones, which are well preserved in the geological record, are used as a proxy for convergent margins; evidence for mid-ocean ridges and transform motion is less clearly preserved, though passive margins are used as a proxy for spreading centres, and evidence for strike-slip motions are used to model transform boundaries. We find that the model presented here only predicts 70% of the total length of subduction active today, though it models similar lengths of both transform and divergent boundaries, suggesting that we have produced a conservative model and are probably underestimating the amount of subduction. Where evidence for convergent, divergent or transform motion is not preserved, we interpret the locations of plate boundaries based on the relative motions of cratonic crust as suggested through either palaeomagnetic data or the geological record. Using GPlates, we tie these boundaries together to generate a plate model that depicts the motion of tectonic plates through the Neoproterozoic. We omit India and South China from Rodinia completely, due to long-lived subduction preserved on margins of India and conflicting palaeomagnetic data for the Cryogenian, but tie them together due to similar Tonian aged accretionary patterns along their respective (present-day) north-western and northern margins, such that these two cratons act as a "lonely wanderer" for much of the Neoproterozoic, and form their own tectonic plate. We also introduce a Tonian-Cryogenian aged rotation of the Congo-São Francisco Craton relative to Rodinia to better fit palaeomagnetic data and account for thick passive margin sediments along its southern margin during the Tonian. The model depicts a sequential breakup of Rodinia, with Australia-Antarctica rifting first ( 800 Ma), Congo-São Francisco (and the Sahara Metacraton) second ( 750 Ma) and Kalahari third (700 Ma). Amazonia and West Africa rift later with the opening of the Iapetus Ocean from 600 Ma. We expect that this global model will assist in the development of future regional models for the Neoproterozoic, and that the production of this full-plate topological reconstruction will facilitate the investigation of controls on other earth systems, such as the possible role of volcanism on initiation of the Cryogenian, or the nature of mantle convection in the Neoproterozoic.

Seismicity and deep structure of the Indo-Burman plate margin

NASA Astrophysics Data System (ADS)

Vaněk, J.; Hanuš, V.; Sitaram, M. V. D.

Two differently inclined segments of the Wadati-Benioff zone beneath the Chin Hills and Naga Hills segments of the Indo-Burman Ranges were verified on the basis of the geometrical analysis of distribution of 566 earthquakes. The Wadati-Benioff zone and young calc-alkaline volcanism point to the existence of a Mio-Pliocene subduction with the trench at the western boundary of the Oligocene Indo-Burman orogenic belt. A system of ten seismically active fracture zones was delineated in the adjacent Indian and Burman plates, the tectonic pattern of which represents the eastern manifestation of the continental collision of the Indian and Eurasian plates. The position of historical disastrous earthquakes confirms the reality of this pattern.

Crustal structure of the Gulf of Aden southern margin: Evidence from receiver functions on Socotra Island (Yemen)

NASA Astrophysics Data System (ADS)

Ahmed, Abdulhakim; Leroy, Sylvie; Keir, Derek; Korostelev, Félicie; Khanbari, Khaled; Rolandone, Frédérique; Stuart, Graham; Obrebski, Mathias

2014-12-01

Breakup of continents in magma-poor setting occurs primarily by faulting and plate thinning. Spatial and temporal variations in these processes can be influenced by the pre-rift basement structure as well as by early syn-rift segmentation of the rift. In order to better understand crustal deformation and influence of pre-rift architecture on breakup we use receiver functions from teleseismic recordings from Socotra which is part of the subaerial Oligo-Miocene age southern margin of the Gulf of Aden. We determine variations in crustal thickness and elastic properties, from which we interpret the degree of extension related thinning and crustal composition. Our computed receiver functions show an average crustal thickness of ~ 28 km for central Socotra, which decreases westward along the margin to an average of ~ 21 km. In addition, the crust thins with proximity to the continent-ocean transition to ~ 16 km in the northwest. Assuming an initial pre-rift crustal thickness of 35 km (undeformed Arabian plate), we estimate a stretching factor in the range of ~ 2.1-2.4 beneath Socotra. Our results show considerable differences between the crustal structure of Socotra's eastern and western sides on either side of the Hadibo transfer zone; the east displays a clear intracrustal conversion phase and thick crust when compared with the western part. The majority of measurements across Socotra show Vp/Vs ratios of between 1.70 and 1.77 and are broadly consistent with the Vp/Vs values expected from the granitic and carbonate rock type exposed at the surface. Our results strongly suggest that intrusion of mafic rock is absent or minimal, providing evidence that mechanical thinning accommodated the majority of crustal extension. From our observations we interpret that the western part of Socotra corresponds to the necking zone of a classic magma-poor continental margin, while the eastern part corresponds to the proximal domain.

Active Deformation along the Southern End of the Tosco-Abreojos Fault System: New Insights from Multibeam Swath Bathymetry

NASA Astrophysics Data System (ADS)

Michaud, François; Calmus, Thierry; Ratzov, Gueorgui; Royer, Jean-Yves; Sosson, Marc; Bigot-Cormier, Florence; Bandy, William; Mortera Gutiérrez, Carlos

2011-08-01

The relative motion of the Pacific plate with respect to the North America plate is partitioned between transcurrent faults located along the western margin of Baja California and transform faults and spreading ridges in the Gulf of California. However, the amount of right lateral offset along the Baja California western margin is still debated. We revisited multibeam swath bathymetry data along the southern end of the Tosco-Abreojos fault system. In this area the depths are less than 1,000 m and allow a finer gridding at 60 m cell spacing. This improved resolution unveils several transcurrent right lateral faults offsetting the seafloor and canyons, which can be used as markers to quantify local offsets. The seafloor of the southern end of the Tosco-Abreojos fault system (south of 24°N) displays NW-SE elongated bathymetric highs and lows, suggesting a transtensional tectonic regime associated with the formation of pull-apart basins. In such an active tectonic context, submarine canyon networks are unstable. Using the deformation rate inferred from kinematic predictions and pull-apart geometry, we suggest a minimum age for the reorganization of the canyon network.

Geometry of the neoproterozoic and paleozoic rift margin of western Laurentia: Implications for mineral deposit settings

USGS Publications Warehouse

Lund, K.

2008-01-01

The U.S. and Canadian Cordilleran miogeocline evolved during several phases of Cryogenian-Devonian intracontinental rifting that formed the western mangin of Laurentia. Recent field and dating studies across central Idaho and northern Nevada result in identification of two segments of the rift margin. Resulting interpretations of rift geometry in the northern U.S. Cordillera are compatible with interpretations of northwest- striking asymmetric extensional segments subdivided by northeast-striking transform and transfer segments. The new interpretation permits integration of miogeoclinal segments along the length of the western North American Cordillera. For the U.S. Cordillera, miogeoclinal segments include the St. Mary-Moyie transform, eastern Washington- eastern Idaho upper-plate margin, Snake River transfer, Nevada-Utah lower-plate margin, and Mina transfer. The rift is orthogonal to most older basement domains, but the location of the transform-transfer zones suggests control of them by basement domain boundaries. The zigzag geometry of reentrants and promontories along the rift is paralleled by salients and recesses in younger thrust belts and by segmentation of younger extensional domains. Likewise, transform transfer zones localized subsequent transcurrent structures and igneous activity. Sediment-hosted mineral deposits trace the same zigzag geometry along the margin. Sedimentary exhalative (sedex) Zn-Pb-Ag ??Au and barite mineral deposits formed in continental-slope rocks during the Late Devonian-Mississippian and to a lesser degree, during the Cambrian-Early Ordovician. Such deposits formed during episodes of renewed extension along miogeoclinal segments. Carbonate-hosted Mississippi Valley- type (MVT) Zn-Pb deposits formed in structurally reactivated continental shelf rocks during the Late Devonian-Mississippian and Mesozoic due to reactivation of preexisting structures. The distribution and abundance of sedex and MVT deposits are controlled by the polarity and kinematics of the rift segment. Locally, discrete mineral belts parallel secondary structures such as rotated crustal blocks at depth that produced sedimentary subbasins and conduits for hydrothermal fluids. Where the miogeocline was overprinted by Mesozoic and Cenozoic deformation and magmatism, igneous rock-related mineral deposits are common. ??2008 Geological Society of America.

Divergent plate motion drives rapid exhumation of (ultra)high pressure rocks

NASA Astrophysics Data System (ADS)

Liao, Jie; Malusà, Marco G.; Zhao, Liang; Baldwin, Suzanne L.; Fitzgerald, Paul G.; Gerya, Taras

2018-06-01

Exhumation of (ultra)high pressure [(U)HP] rocks by upper-plate divergent motion above an unbroken slab, first proposed in the Western Alps, has never been tested by numerical methods. We present 2D thermo-mechanical models incorporating subduction of a thinned continental margin beneath either a continental or oceanic upper plate, followed by upper-plate divergent motion away from the lower plate. Results demonstrate how divergent plate motion may trigger rapid exhumation of large volumes of (U)HP rocks directly to the Earth's surface, without the need for significant overburden removal by erosion. Model exhumation paths are fully consistent with natural examples for a wide range of upper-plate divergence rates. Exhumation rates are systematically higher than the divergent rate imposed to the upper plate, and the modeled size of exhumed (U)HP domes is invariant for different rates of upper-plate divergence. Major variations are instead predicted at depth for differing model scenarios, as larger amounts of divergent motion may allow mantle-wedge exhumation to shallow depth under the exhuming domes. The transient temperature increase, due to ascent of mantle-wedge material in the subduction channel, has a limited effect on exhumed continental (U)HP rocks already at the surface. We test two examples, the Cenozoic (U)HP terranes of the Western Alps (continental upper plate) and eastern Papua New Guinea (oceanic upper plate). The good fit between model predictions and the geologic record in these terranes encourages the application of these models globally to pre-Cenozoic (U)HP terranes where the geologic record of exhumation is only partly preserved.

Plume-stagnant slab-lithosphere interactions: Origin of the late Cenozoic intra-plate basalts on the East Eurasia margin

NASA Astrophysics Data System (ADS)

Kimura, Jun-Ichi; Sakuyama, Tetsuya; Miyazaki, Takashi; Vaglarov, Bogdan S.; Fukao, Yoshio; Stern, Robert J.

2018-02-01

Intra-plate basalts of 35-0 Ma in East Eurasia formed in a broad backarc region above the stagnant Pacific Plate slab in the mantle transition zone. These basalts show regional-scale variations in Nd-Hf isotopes. The basalts with the most radiogenic Nd-Hf center on the Shandong Peninsula with intermediate Nd-Hf at Hainan and Datong. The least radiogenic basalts occur in the perimeters underlain by the thick continental lithosphere. Shandong basalts possess isotopic signatures of the young igneous oceanic crust of the subducted Pacific Plate. Hainan and Datong basalts have isotopic signatures of recycled subduction materials with billions of years of storage in the mantle. The perimeter basalts have isotopic signatures similar to pyroxenite xenoliths from the subcontinental lithospheric mantle beneath East Eurasia. Hainan basalts exhibit the highest mantle potential temperature (Tp), while the Shandong basalts have the lowest Tp. We infer that a deep high-Tp plume interacted with the subducted Pacific Plate slab in the mantle transition zone to form a local low-Tp plume by entraining colder igneous oceanic lithosphere. We infer that the subducted Izanagi Plate slab, once a part of the Pacific Plate mosaic, broke off from the Pacific Plate slab at 35 Ma to sink into the lower mantle. The sinking Izanagi slab triggered the plume that interacted with the stagnant Pacific slab and caused subcontinental lithospheric melting. This coincided with formation of the western Pacific backarc marginal basins due to Pacific Plate slab rollback and stagnation.

Collision processes at the northern margin of the Black Sea

NASA Astrophysics Data System (ADS)

Gobarenko, V. S.; Murovskaya, A. V.; Yegorova, T. P.; Sheremet, E. E.

2016-07-01

Extended along the Crimea-Caucasus coast of the Black Sea, the Crimean Seismic Zone (CSZ) is an evidence of active tectonic processes at the junction of the Scythian Plate and Black Sea Microplate. A relocation procedure applied to weak earthquakes (mb ≤ 3) recorded by ten local stations during 1970-2013 helped to determine more accurately the parameters of hypocenters in the CSZ. The Kerch-Taman, Sudak, Yuzhnoberezhnaya (South Coast), and Sevastopol subzones have also been recognized. Generalization of the focal mechanisms of 31 strong earthquakes during 1927-2013 has demonstrated the predominance of reverse and reverse-normal-faulting deformation regimes. This ongoing tectonic process occurs under the settings of compression and transpression. The earthquake foci with strike-slip component mechanisms concentrate in the west of the CSZ. Comparison of deformation modes in the western and eastern Crimean Mountains according to tectonophysical data has demonstrated that the western part is dominated by strike-slip and normal- faulting, while in the eastern part, reverse-fault and strike-slip deformation regimes prevail. Comparison of the seismicity and gravity field and modes of deformation suggests underthusting of the East Black Sea Microplate with thin suboceanic crust under the Scythian Plate. In the Yuzhnoberezhnaya Subzone, this process is complicated by the East Black Sea Microplate frontal part wedging into the marginal part of the Scythian Plate crust. The indentation mechanism explains the strong gravity anomaly in the Crimean Mountains and their uplift.

Cenozoic deformation from the Yakutat-North American collision to the eastern margin of the Northern Canadian Cordillera

NASA Astrophysics Data System (ADS)

Enkelmann, E.

2017-12-01

The western margin of the Northern Cordillera of North America is dominated by transform motion of the Yakutat microplate along the Fairweather fault system. In southeast Alaska the transform boundary changes to convergence and the oblique collision of the buoyant Yakutat microplate formed the St. Elias Mountains. One of the outstanding questions in understanding the St. Elias orogeny is how stress from the plate boundary has been transferred inboard and distributed strain in the North American plate. The timing, amount, and spatial pattern of deformation and rock exhumation have been studied using multiple thermochronology methods. Together the data reveal that Late Cenozoic deformation inboard of the Fairweather Fault and the colliding Yakutat plate corner at the St. Elias syntaxis was spatially very limited, resulting in rock exhumation within a <30 km-wide corridor north and northeast of the plate boundary. The data from this inboard region, located in Yukon and northern British Columbia, record Late Cretaceous-Early Eocene cooling associated with Cordilleran deformation, and Paleocene-Eocene cooling due to spreading-ridge subduction. In contrast, the region west of the St. Elias syntaxis is dominated by convergence, which resulted in significant Cenozoic deformation in southeastern and southern Alaska. In the St. Elias orogen itself, most of the Late Cenozoic deformation and exhumation occurs within the Yakutat microplate and its Cenozoic sedimentary cover that composes the fold-thrust belt. The efficient interaction between tectonic uplift and glacial erosion resulted in rapid exhumation (>1 km/Myr) and extreme rates (4 km/Myr) that are localized at the syntaxis region and have shifted southward over the past 10 Myr. Far-field deformation reaches more than 500 km to the northwest of the convergent margin and caused mountain building in south-central Alaska. Deformation to the northeast is unclear. New thermochronology data from the eastern margin of the Northern Canadian Cordillera (Northwest Territory) reveal exhumation during the Oligocene to early Miocene. At this time, transform motion was already dominating the plate margin in the west. The post-Cordilleran deformation at the eastern front may thus be related to mantle convection and/or stresses associated with the North Atlantic opening.

Are diamond-bearing Cretaceous kimberlites related to shallow-angle subduction beneath western North America?

NASA Astrophysics Data System (ADS)

Currie, C. A.; Beaumont, C.

2009-05-01

The origin of deep-seated magmatism (in particular, kimberlites and lamproites) within continental plate interiors remains enigmatic in the context of plate tectonic theory. One hypothesis proposes a relationship between kimberlite occurrence and lithospheric subduction, such that a subducting plate releases fluids below a continental craton, triggering melting of the deep lithosphere and magmatism (Sharp, 1974; McCandless, 1999). This study provides a quantitative evaluation of this hypothesis, focusing on the Late Cretaceous- Eocene (105-50 Ma) kimberlites and lamproites of western North America. These magmas were emplaced along a corridor of Archean and Proterozoic lithosphere, 1000-1500 km inboard of the plate margin separating the subducting Farallon Plate and continental North America Plate. Kimberlite-lamproite magmatism coincides with tectonic events, including the Laramide orogeny, shut-down of the Sierra Nevada arc, and eastward migration of volcanism, that are commonly attributed to a change in Farallon Plate geometry to a shallow-angle trajectory (<25° dip). Thermal-mechanical numerical models demonstrate that rapid Cretaceous plate convergence rates and enhanced westward velocity of North America result in shallow-angle subduction that places the Farallon Plate beneath the western edge of the cratonic interior of North America. This geometry is consistent with the observed continental dynamic subsidence that lead to the development of the Western Interior Seaway. The models also show that the subducting plate has a cool thermal structure, and subducted hydrous minerals (serpentine, phengite and phlogopite) remain stable to more than 1200 km from the trench, where they may break down and release fluids that infiltrate the overlying craton lithosphere. This is supported by geochemical studies that indicate metasomatism of the Colorado Plateau and Wyoming craton mantle lithosphere by an aqueous fluid and/or silicate melt with a subduction signature. Through Cretaceous shallow-angle subduction, the Farallon Plate was in a position to mechanically and chemically interact with North American craton lithosphere at the time of kimberlite-lamproite magmatism, making the subduction hypothesis a viable mechanism for the genesis of these magmas. REFERENCES: McCandless, T.E., Proceedings of the 7th International Kimberlite Conference, v.2, pp.545-549, 1999; Sharp, W.E., Earth Planet. Sci. Lett., v.21, pp.351-354, 1974.

The Ionian and Alfeo-Etna fault zones: New segments of an evolving plate boundary in the central Mediterranean Sea?

NASA Astrophysics Data System (ADS)

Polonia, A.; Torelli, L.; Artoni, A.; Carlini, M.; Faccenna, C.; Ferranti, L.; Gasperini, L.; Govers, R.; Klaeschen, D.; Monaco, C.; Neri, G.; Nijholt, N.; Orecchio, B.; Wortel, R.

2016-04-01

The Calabrian Arc is a narrow subduction-rollback system resulting from Africa/Eurasia plate convergence. While crustal shortening is taken up in the accretionary wedge, transtensive deformation accounts for margin segmentation along transverse lithospheric faults. One of these structures is the NNW-SSE transtensive fault system connecting the Alfeo seamount and the Etna volcano (Alfeo-Etna Fault, AEF). A second, NW-SE crustal discontinuity, the Ionian Fault (IF), separates two lobes of the CA subduction complex (Western and Eastern Lobes) and impinges on the Sicilian coasts south of the Messina Straits. Analysis of multichannel seismic reflection profiles shows that: 1) the IF and the AEF are transfer crustal tectonic features bounding a complex deformation zone, which produces the downthrown of the Western lobe along a set of transtensive fault strands; 2) during Pleistocene times, transtensive faulting reactivated structural boundaries inherited from the Mesozoic Tethyan domain which acted as thrust faults during the Messinian and Pliocene; and 3) the IF and the AEF, and locally the Malta escarpment, accommodate a recent tectonic event coeval and possibly linked to the Mt. Etna formation. Regional geodynamic models show that, whereas AEF and IF are neighboring fault systems, their individual roles are different. Faulting primarily resulting from the ESE retreat of the Ionian slab is expressed in the northwestern part of the IF. The AEF, on the other hand, is part of the overall dextral shear deformation, resulting from differences in Africa-Eurasia motion between the western and eastern sectors of the Tyrrhenian margin of northern Sicily, and accommodating diverging motions in the adjacent compartments, which results in rifting processes within the Western Lobe of the Calabrian Arc accretionary wedge. As such, it is primarily associated with Africa-Eurasia relative motion.

Tracking the India-Arabia Transform Plate Boundary during Paleogene Times.

NASA Astrophysics Data System (ADS)

Rodriguez, M.; Huchon, P.; Chamot-Rooke, N. R. A.; Fournier, M.; Delescluse, M.

2014-12-01

The Zagros and Himalaya mountain belts are the most prominent reliefs built by continental collision. They respectively result from Arabia and India collision with Eurasia. Convergence motions at mountain belts induced most of plate reorganization events in the Indian Ocean during the Cenozoic. Although critical for paleogeographic reconstructions, the way relative motion between Arabia and India was accommodated prior to the formation of the Sheba ridge in the Gulf of Aden remains poorly understood. The India-Arabia plate-boundary belongs to the category of long-lived (~90-Ma) oceanic transform faults, thus providing a good case study to investigate the role of major kinematic events over the structural evolution of a long-lived transform system. A seismic dataset crossing the Owen Fracture Zone, the Owen Basin, and the Oman Margin was acquired to track the past locations of the India-Arabia plate boundary. We highlight the composite age of the Owen Basin basement, made of Paleocene oceanic crust drilled on its eastern part, and composed of pre-Maastrichtian continental crust overlaid by Early Paleocene ophiolites on its western side. A major transform fault system crossing the Owen Basin juxtaposed these two slivers of lithosphere of different ages, and controlled the uplift of marginal ridges along the Oman Margin. This transform system deactivated ~40 Ma ago, coeval with the onset of ultra-slow spreading at the Carlsberg Ridge. The transform boundary then jumped to the edge of the present-day Owen Ridge during the Late Eocene-Oligocene period, before seafloor spreading began at the Sheba Ridge. This migration of the plate boundary involved the transfer of a part of the Indian oceanic lithosphere accreted at the Carlsberg Ridge to the Arabian plate. The episode of plate transfer at the India-Arabia plate boundary during the Late Eocene-Oligocene interval is synchronous with a global plate reorganization event corresponding to geological events at the Zagros and Himalaya belts. The Owen Ridge uplifted later, in Late Miocene times, and is unrelated to any major migration of the India-Arabia boundary.

Foreland sedimentary record of Andean mountain building during advancing and retreating subduction

NASA Astrophysics Data System (ADS)

Horton, Brian K.

2016-04-01

As in many ocean-continent (Andean-type) convergent margins, the South American foreland has long-lived (>50-100 Myr) sedimentary records spanning not only protracted crustal shortening, but also periods of neutral to extensional stress conditions. A regional synthesis of Andean basin histories is complemented by new results from the Mesozoic Neuquén basin system and succeeding Cenozoic foreland system of west-central Argentina (34-36°S) showing (1) a Late Cretaceous shift from backarc extension to retroarc contraction and (2) an anomalous mid-Cenozoic (~40-20 Ma) phase of sustained nondeposition. New detrital zircon U-Pb geochronological results from Jurassic through Neogene clastic deposits constrain exhumation of the evolving Andean magmatic arc, retroarc thrust belt, foreland basement uplifts, and distal eastern craton. Abrupt changes in sediment provenance and distal-to-proximal depositional conditions can be reconciled with a complex Mesozoic-Cenozoic history of extension, post-extensional thermal subsidence, punctuated tectonic inversion involving thick- and thin-skinned shortening, alternating phases of erosion and rapid accumulation, and overlapping igneous activity. U-Pb age distributions define the depositional ages of several Cenozoic stratigraphic units and reveal a major late middle Eocene-earliest Miocene (~40-20 Ma) hiatus in the Malargüe foreland basin. This boundary marks an abrupt shift in depositional conditions and sediment sources, from Paleocene-middle Eocene distal fluviolacustrine deposition of sediments from far western volcanic sources (Andean magmatic arc) and subordinate eastern cratonic basement (Permian-Triassic Choiyoi igneous complex) to Miocene-Quaternary proximal fluvial and alluvial-fan deposition of sediments recycled from emerging western sources (Malargüe fold-thrust belt) of Mesozoic basin fill originally derived from basement and magmatic arc sources. Neogene eastward advance of the fold-thrust belt involved thick-skinned basement inversion with geometrically and kinematically linked thin-skinned thrust structures at shallower levels in the eastern foreland, including well-dated late Miocene growth strata. The mid-Cenozoic hiatus potentially signifies nondeposition during passage of a flexural forebulge or nondeposition during neutral to extensional conditions possibly driven by a transient retreating-slab configuration along the western margin of South America. Similar long-lived stratigraphic gaps are commonly observed in other foreland records of continental convergent margins. It is proposed that Andean orogenesis along the South American convergent margin has long been sensitive to variations in subduction dynamics throughout Mesozoic-Cenozoic time, such that shifts in relative convergence and degree of mechanical coupling along the subduction interface (i.e., transitions between advancing versus retreating modes of subduction) have governed fluctuating contractional, extensional, and neutral conditions. Unclear is whether these various modes affected the entire convergent margin simultaneously due to continental-scale changes (e.g., temporal shifts in plate convergence, absolute motion of upper plate, or mantle wedge circulation) or whether parts of the margin behaved independently due to smaller-scale fluctuations (e.g., spatial variations in the age of the subducted plate, buoyant asperities in the downgoing slab, or asthenospheric anomalies).

Plate convergence and deformation, North Luzon Ridge, Philippines

NASA Astrophysics Data System (ADS)

Lewis, Stephen D.; Hayes, Dennis E.

1989-10-01

Marine geophysical and earthquake seismology data indicate that the North Luzon Ridge, a volcano-capped bathymetrie ridge system that extends between Luzon and Taiwan, is presently undergoing deformation in response to the relative motion between the Asian and Philippine Sea plates. Plate motion models predict convergence along the western side of the Philippine Sea plate, from Japan in the north to Indonesia in the south, and most of this plate margin is defined by active subduction zones. However, the western boundary of the Philippine Sea plate adjacent to the North Luzon Ridge shows no evidence of an active WNW-dipping subduction zone; this is in marked contrast to the presence of both the Philippine Trench/East Luzon Trough subduction zones to the south and the Ryukyu Trench subduction zone to the north. Crustal shortening, in response to ongoing plate convergence in the North Luzon Ridge region, apparently takes place through a complex pattern of strike-slip and thrust faulting, rather than by the typical subduction of oceanic lithosphere along a discreet zone. The curvilinear bathymetrie trends within the North Luzon Ridge represent the traces of active faults. The distribution of these faults, mapped by both multichannel and single-channel seismic reflection methods and earthquake seismicity patterns and focal mechanism solutions, suggest that right-lateral, oblique-slip faulting occurs along NE-trending faults, and left-lateral, oblique-slip faulting takes place on N- and NNW-trending faults. The relative plate convergence accommodated by the deformation of the North Luzon Ridge will probably be taken up in the future by the northward-propagating East Luzon Trough subduction zone.

The reactivation of the SW Iberian passive margin: a brief review

NASA Astrophysics Data System (ADS)

Duarte, Joao; Rosas, Filipe; Terrinha, Pedro; Schellart, Wouter; Almeida, Pedro; Gutscher, Marc-André; Riel, Nicolas; Ribeiro, António

2016-04-01

On the morning of the 1st of November of 1755 a major earthquake struck offshore the Southwest Iberian margin. This was the strongest earthquake ever felt in Western Europe. The shake, fire and tsunami devastated Lisbon, was felt as far as Finland and had a profound impact on the thinkers of that time, in particular on the Enlightenment philosophers such as Voltaire, Rousseau and Kant. The Great Lisbon Earthquake is considered by many as the event that marks the birth of modern geosciences; and made of this region one of the most well studied areas in the world. After the 1755 earthquake, Kant and others authors wrote several treaties dealing with the causes and dynamics of earthquakes and tsunamis and were close to identify some key elements of what we now call plate tectonics. More than two hundred years later, in the year of 1969, the region was struck by another major earthquake. This was precisely during the period in which the theory of plate tectonics was being built. Geoscientists like Fukao (1973), Purdy (1975) and Mackenzie (1977) immediately focused their attention in the area. They suggested that these events were related with "transient" subduction of Africa below Iberia, along the East-West Azores-Gibraltar plate boundary. Several years later, Ribeiro (1989) suggested that instead of Africa being subducted below Iberia, it was the West Iberian passive margin that was being reactivated, a process that may, in time, lead to the formation of a new subduction zone. In the turning of the millennium, a subducting slab was imaged bellow the Gibraltar Straits, a remanent of the Western Mediterranean arc system that according to Gutscher et al. (2002) was related with ongoing subduction. Recently, it was proposed that a causal link between the Gibraltar subduction system and the reactivation of the SW Iberian margin might exist. In addition, the large-scale Africa-Eurasia convergence is inducing compressive stresses along the West Iberian margin. The margin reactivation is expressed by the presence of several active lithospheric-scale thrust faults. In this communication, we will highlight the main moments of the journey that lead to the understanding that the Southwest Iberian is in fact being reactivated. We will present some of the data and ideas that were gathered over the years, including the most recent findings. Finally, we will see that despite the numerous endeavours and the substantial improvements in our tectonic knowledge of the region there are still many enigmas waiting to be resolved. Publication supported by project FCT UID/GEO/50019/2013 - Instituto Dom Luiz

Seismicity of the Earth 1900-2013, seismotectonics of South America (Nazca Plate Region)

USGS Publications Warehouse

Hayes, Gavin P.; Smoczyk, Gregory M.; Benz, Harley M.; Furlong, Kevin P.; Villaseñor, Antonio

2015-01-01

The South American arc extends over 7,000 kilometers (km), from the Chilean margin triple junction offshore of southern Chile, to its intersection with the Panama fracture zone, offshore of the southern coast of Panama in Central America. It marks the plate boundary between the subducting Nazca plate and the South America plate, where the oceanic crust and lithosphere of the Nazca plate begin their descent into the mantle beneath South America. The convergence associated with this subduction process is responsible for the uplift of the Andes Mountains, and for the active volcanic chain present along much of this deformation front. Relative to a fixed South America plate, the Nazca plate moves slightly north of eastwards at a rate varying from approximately 80 millimeters/year (mm/yr) in the south, to approximately 65 mm/yr in the north. Although the rate of subduction varies little along the entire arc, there are complex changes in the geologic processes along the subduction zone that dramatically influence volcanic activity, crustal deformation, earthquake generation and occurrence all along the western edge of South America.

Impact of Vishnu Fracture Zone on Tectono-Stratigraphy of Kerala Deepwater Basin, India

NASA Astrophysics Data System (ADS)

Bastia, R.; Krishna, K. S.; Nathaniel, D. M.; Tenepalli, S.

2008-12-01

Integration of regional seismic data extending from coast to deep water with the gravity-magnetics reveals the expression and evolution of ridge systems and fracture zones in Indian Ocean. Kerala deepwater basin, situated in the south-western tip of India, is bounded by two prominent north-south oriented ocean fracture zones viz., Vishnu (west) and Indrani (east) of the Indian Ocean. Vishnu Fracture Zone (VFZ), which extends from the Kerala shelf southward to the Carlsberg-Ridge, over a length of more than 2500 km, has a strong bearing on the sedimentation as well as structural fabric of the basin. VFZ is identified as the transform plate margin formed during Late-Cretaceous-Tertiary separation of Seychelles from India. Represented by a highly deformed structural fabric, VFZ forms an abrupt boundary between ocean floors of about 65 MY in the west and 140 MY in the east, implying a great scope for sedimentary pile on this very older ocean floor. Armed with this premise of an older sedimentary pile towards east of VFZ, congenial for petroleum hunt, the implemented modern long offset seismic program with an objective to enhance sub-basalt (Deccan) imagery, gravity-magnetic modelling and plate-tectonic reconstructions unraveled huge Mesozoic Basin, unheard earlier. Multi-episodic rifting in western continental margin of India starting during Mid Jurassic Karoo rift along the western Madagascar, Kerala deepwater basin, and western Antarctica and conjugate margins of Africa forms the main corridor for sedimentation. Subsequent Late Cretaceous dextral oblique extension of Madagascar rift reactivated pre-existing structural framework creating major accommodation zones along the southern tip of India. Followed by separation of Seychelles during KT boundary led to the formation of VFZ (an oceanic fracture zone) forming a transform boundary between newly formed Tertiary oceanic crust to the west and older basin to the east. The pulses of right-lateral movement were associated with various degrees of transpression, transtension, uplift and erosion. This activity continued in stages until Mid.Miocene, subsequent to phase of India- Seychelles separation. As a result, Mesozoic stratigraphy was inverted along VFZ's eastern border, folded in the basin centers and finally shifted the Tertiary depo-center towards east of VFZ. Plate tectonic reconstruction of Late Jurassic to Early Cretaceous demonstrates that the basin as situated in the north-east part of Proto-Mozambique Ocean, with Antarctica as the major provenance of sediment supply under favorable conditions for organic enrichment of sediments.

Rheology and strength of the Eurasian continental lithosphere in the foreland of the Taiwan collision belt: Constraints from seismicity, flexure, and structural styles

NASA Astrophysics Data System (ADS)

Mouthereau, FréDéRic; Petit, Carole

2003-11-01

Deformation in western Taiwan is characterized by variable depth-frequency distribution of crustal earthquakes which are closely connected with along-strike variations of tectonic styles (thin or thick skinned) around the Peikang High, a major inherited feature of the Chinese margin. To fit the calculated high crustal geotherm and the observed distribution of the crustal seismic activity, a Qz-diorite and granulite composition for the upper and the lower crust is proposed. We then model the plate flexure, through Te estimates, using brittle-elastic-ductile plate rheology. Flexure modeling shows that the best fit combination of Te-boundary condition is for thrust loads acting at the belt front. The calculated Te vary in the range of ˜15-20 km. These values are primarily a reflection of the thermal state of the rifted Chinese margin inherited from the Oligocene spreading in the South China Sea. However, other mechanical properties such as the degree of crust/mantle coupling and the thickness of the mechanically competent crust and mantle are considered. South of the Peikang High, flexure modeling reveals lower Te associated with thinner mechanically strong layers. Variable stress/strain distribution associated with a higher degree of crust/mantle decoupling is examined to explain plate weakening. We first show that plate curvature cannot easily explain strength reduction and observed seismic activity. Additional plate-boundary forces arising from the strong coupling induced by more frontal subduction of a buoyant crustal asperity, i.e., the Peikang High, with the overriding plate are required. Favorably oriented inherited features in the adjacent Tainan basin produce acceleration of strain rates in the upper crust and hence facilitate the crust/mantle decoupling as attested by high seismic activity and thick-skinned deformation. The relative weakening of the lower crust and mantle then leads to weaken the lithosphere. By contrast, to the north, more oblique collision and the lack of inherited features keep the lithosphere stronger. This study suggests that when the Eurasian plate enters the Taiwan collision, tectonic inheritance of the continental margin exerts a strong control on the plate deformation by modifying its strength.

DOE Office of Scientific and Technical Information (OSTI.GOV)

James, K.H.

A prolific hydrocarbon province extends across the northern margin of South America from Colombia to east of Trinidad. Two key components are a world-class source rock, formed on a regional Late Cretaceous passive margin, and a complex tectonic setting in which a variety of structural and stratigraphic traps, reservoirs, seals and hydrocarbon kitchens have evolved through time. Convergence between the Farallon and Caribbean plates with South America culminated in the late Cretaceous-early Palaeogene with emplacement of Colombia`s Central Cordillera in the west and a nappe-foreland basin system in the north. Regional hydrocarbon generation probably occurred below associated basins. Subsequent obliquemore » convergence between the Caribbean and South America, partitioned into strike-slip and compressional strain, generated an eastward migrating and ongoing uplift-foredeep (kitchen) system from central Venezuela to Trinidad. Similarly, oblique interaction of western Colombia with the Nazca Plate caused segmentation of the earlier orogen, northward extrusion of elements such as the Maracaibo Block, and eastward migration of uplift progressively dividing earlier kitchens into localized foredeeps.« less

Controls on continental strain partitioning above an oblique subduction zone, Northern Andes

NASA Astrophysics Data System (ADS)

Schütt, Jorina M.; Whipp, David M., Jr.

2016-04-01

Strain partitioning is a common process at obliquely convergent plate margins dividing oblique convergence into margin-normal slip on the plate-bounding fault and horizontal shearing on a strike-slip system parallel to the subduction margin. In subduction zones, strain partitioning in the upper continental plate is mainly controlled by the shear forces acting on the plate interface and the strength of the continental crust. The plate interface forces are influenced by the subducting plate dip angle and the obliquity angle between the normal to the plate margin and the convergence velocity vector, and the crustal strength of the continent is strongly affected by the presence or absence of a volcanic arc, with the presence of the volcanic arcs being common at steep subduction zones. Along the ˜7000 km western margin of South America the convergence obliquity, subduction dip angles and presence of a volcanic arc all vary, but strain partitioning is only observed along parts of it. This raises the questions, to what extent do subduction zone characteristics control strain partitioning in the overriding continental plate, and which factors have the largest influence? We address these questions using lithospheric-scale 3D numerical geodynamic experiments to investigate the influence of subduction dip angle, convergence obliquity, and weaknesses in the crust owing to the volcanic arc on strain partitioning behavior. We base the model design on the Northern Volcanic Zone of the Andes (5° N - 2° S), characterized by steep subduction (˜ 35°), a convergence obliquity between 31° -45° and extensive arc volcanism, and where strain partitioning is observed. The numerical modelling software (DOUAR) solves the Stokes flow and heat transfer equations for a viscous-plastic creeping flow to calculate velocity fields, thermal evolution, rock uplift and strain rates in a 1600 km x 1600 km box with depth 160 km. Subduction geometry and material properties are based on a simplified, generic subduction zone similar to the northern Andes. The upper surface is initially defined to resemble the Andes, but is free to deform during the experiments. We consider two main model designs, one with and one without a volcanic arc (weak continental zone). A relatively high angle of convergence obliquity is predicted to favor strain partitioning, but preliminary model results show no strain partitioning for a uniform continental crustal strength with a friction angle of Φ = 15° . However, strain partitioning does occur when including a weak zone in the continental crust resulting from arc volcanic activity with Φ = 5° . This results in margin-parallel northeastward translation of a continental sliver at 3.2 cm/year. The presence of the sliver agrees well with observations of a continental sliver identified by GPS measurements in the Northern Volcanic Zone with a translation velocity of about 1 cm/year, though the GPS-derived velocity may not be representative of the long-term rate of translation depending on whether the observation period includes one or more seismic cycles. Regardless, the observed behavior is consistent with the observed earthquake focal mechanisms and GPS measurements, suggesting significant northeastward transport of Andean crust along the margin of the northern Andes.

Crustal deformation evidences for viscous coupling and fragmented lithosphere at the Nubia-Iberia plate boundary (Western Mediterranean)

NASA Astrophysics Data System (ADS)

Palano, Mimmo; González, Pablo J.; Fernández, José

2016-04-01

A spatially dense crustal velocity field, based on up to 15 years of GNSS observations at more than 380 sites and extensively covering the Iberian Peninsula and Northern Africa, allow us to provide new insights into two main tectonic processes currently occurring in this area. We detected a slow large-scale clockwise rotation of the Iberian Peninsula with respect to a local pole located closely to the northwestern sector of the Pyrenean mountain range (Palano et al., 2015). Although this crustal deformation pattern could suggest a rigid rotating lithosphere block, this model would predict significant shortening along the Western (off-shore Lisbon) and North Iberian margin which cannot totally ruled out but currently is not clearly observed. Conversely, we favour the interpretation that this pattern reflects the quasi-continuous straining of the ductile lithosphere in some sectors of South and Western Iberia in response to viscous coupling of the NW Nubia and Iberian plate boundary in the Gulf of Cádiz. Furthermore, the western Mediterranean basin appears fragmented into independent crustal tectonic blocks, which delimited by inherited lithospheric shear structures and trapped within the Nubia-Eurasia collision, are currently accommodating most of the plate convergence rate. Among these blocks, an (oceanic-like western) Algerian one is currently transferring a significant fraction of the Nubia-Eurasia convergence rate into the Eastern Betics (SE Iberia) and likely causing the eastward motion of the Baleares Promontory. Most of the observed crustal ground deformation can be attributed to processes driven by spatially variable lithospheric plate forces imposed along the Nubia-Eurasia convergence boundary. Nevertheless, the observed deformation field infers a very low convergence rates as observed also at the eastern side of the western Mediterranean, along the Calabro Peloritan Arc, by space geodesy (e.g. Palano, 2015). References Palano M. (2015). On the present-day crustal stress, strain-rate fields and mantle anisotropy pattern of Italy. Geophysical Journal International, 200 (2), 969-985, doi:10.1093/gji/ggu451. Palano M., González P. J., Fernández J. (2015). The diffuse plate boundary of Nubia and Iberia in the Western Mediterranean: crustal deformation evidence for viscous coupling and fragmented lithosphere. Earth and Planetary Science Letters, 430, 439-447, doi:10.1016/j.epsl.2015.08.040.

African hot spot volcanism: small-scale convection in the upper mantle beneath cratons.

PubMed

King, S D; Ritsema, J

2000-11-10

Numerical models demonstrate that small-scale convection develops in the upper mantle beneath the transition of thick cratonic lithosphere and thin oceanic lithosphere. These models explain the location and geochemical characteristics of intraplate volcanos on the African and South American plates. They also explain the presence of relatively high seismic shear wave velocities (cold downwellings) in the mantle transition zone beneath the western margin of African cratons and the eastern margin of South American cratons. Small-scale, edge-driven convection is an alternative to plumes for explaining intraplate African and South American hot spot volcanism, and small-scale convection is consistent with mantle downwellings beneath the African and South American lithosphere.

Introducing tectonically and thermo-mechanically realistic lithosphere in the models of plume head -lithosphere interactions (PLI) including intra-continental plate boundaries.

NASA Astrophysics Data System (ADS)

Guillou-Frottier, L.; Burov, E.; Cloetingh, S.

2007-12-01

Plume-Lithosphere Interactions (PLI) in continets have complex topographic and magmatic signatures and are often identified near boundaries between younger plates (e.g., orogenic) and older stable plates (e.g., cratons), which represent important geometrical, thermal and rheological barriers that interact with the emplacement of the plume head (e.g., Archean West Africa, East Africa, Pannonian - Carpathian system). The observable PLI signatures are conditioned by plume dynamics but also by complex rheology and structure of continental lithosphere. We address this problem by considering a new free-surface thermo-mechanical numerical model of PLI with two stratified elasto-viscous-plastic (EVP) continental plates of contrasting age, thickness and structure. The results show that: (1) surface deformation is poly-harmonic and contains smaller wavelengths (50-500 km) than that associated with the plume head (>1000 km). (2) below intra-plate boundaries, plume head flattening is asymmetric, it is blocked from one side by the cold vertical boundary of the older plate, which leads to mechanical decoupling of crust from mantle lithosphere, and to localized faulting at the cratonic margin; (2) the return flow from the plume head results in sub-vertical down-thrusting (delamination) of the lithosphere at the margin, producing sharp vertical cold boundary down to the 400 km depth; (3) plume head flattening and migration towards the younger plate results in concurrent surface extension above the centre of the plume and in compression (pushing), down-thrusting and magmatic events at the cratonic margin (down-thrusting is also produced at the opposite border of the younger plate); these processes may result in continental growth at the "craton side"; (4) topographic signatures of PLI show basin-scale uplifts and subsidences preferentially located at cratonic margins. Negative Rayleigh-Taylor instabilities in the lithosphere above the plume head provide a mechanism for crustal delamination. In case of several cratonic blocks, the combined effect of subsidence and lithospheric thinning at cratons edges, while plume head material is being stocked in between the cratons, favours major magmatic events at cratonic margins. Numerous field evidence (West Africa, Western Australia) underline the trapping effect of cratonic margins for formation of (e.g.) orogenic gold deposits, which require particular extreme P-T conditions. Location of gemstones deposits is also associated with cratonic margins, as demonstrated by the Tanzanian Ruby belt. Their formation depend on particularly fast isothermal deepening processes, which can be reproduced by slab-like instabilities induced by plume head-cratonic margin interaction. On the other hand, absence of magmatic events should not be interpreted as evidence for the absence of plume: at surface, these events may not necessary have unambiguous deep geochemical signatures, as the hot source plume material stalls below Moho and forms a long-lasting (10 to 100 Myr) sub-Moho reservoir. This should induce strong crustal melting that may overprint deeper signatures since crustal melts are generated at much lower temperatures than mantle, and produce light low-viscous rapidly ascending magmas. Drip-like down- sagging of the lithospheric mantle and metamorphic lower crustal material inside the plume head may contaminate the latter and also alter the geochemical signature of related magmas.

Plate tectonics 2.5 billion years ago: evidence at kolar, South India.

PubMed

Krogstad, E J; Balakrishnan, S; Mukhopadhyay, D K; Rajamani, V; Hanson, G N

1989-03-10

The Archean Kolar Schist Belt, south India, is a suture zone where two gneiss terranes and at least two amphibolite terranes with distinct histories were accrted. Amphibolites from the eastern and western sides of the schist belt have distinct incompatible element and isotopic characteristics sugesting that their volcanic protoliths were derived from dint mantle sources. The amphibolite and gneiss terranes were juxtaposed by horizontal compression and shearing between 2530 and 2420 million years ago (Ma) along a zone marked by the Kolar Schist Belt. This history of accretion of discrete crustal terranes resembles those of Phanerozoic convergent margins and thus suggests that plate tectonics operated on Earth by 2500 Ma.

Plate tectonics 2.5 billion years ago - Evidence at Kolar, south India

NASA Technical Reports Server (NTRS)

Krogstad, E. J.; Hanson, G. N.; Balakrishnan, S.; Rajamani, V.; Mukhopadhyay, D. K.

1989-01-01

The Archean Kolar Schist Belt, south India, is a suture zone where two gneiss terranes and at least two amphibolite terranes with distinct histories were accreted. Amphibolites from the eastern and western sides of the schist belt have distinct incompatible element and isotopic characteristics suggesting that their volcanic protoliths were derived from different mantle sources. The amphibolite and gneiss terranes were juxtaposed by horizontal compression and shearing between 2530 and 2420 million years ago (Ma) along a zone marked by the Kolar Schist Belt. This history of accretion of discrete crustal terranes resembles those of Phanerozoic convergent margins and thus suggests that plate tectonics operated on earth by 2500 Ma.

Mechanical deformation model of the western United States instantaneous strain-rate field

USGS Publications Warehouse

Pollitz, F.F.; Vergnolle, M.

2006-01-01

We present a relationship between the long-term fault slip rates and instantaneous velocities as measured by Global Positioning System (GPS) or other geodetic measurements over a short time span. The main elements are the secularly increasing forces imposed by the bounding Pacific and Juan de Fuca (JdF) plates on the North American plate, viscoelastic relaxation following selected large earthquakes occurring on faults that are locked during their respective interseismic periods, and steady slip along creeping portions of faults in the context of a thin-plate system. In detail, the physical model allows separate treatments of faults with known geometry and slip history, faults with incomplete characterization (i.e. fault geometry but not necessarily slip history is available), creeping faults, and dislocation sources distributed between the faults. We model the western United States strain-rate field, derived from 746 GPS velocity vectors, in order to test the importance of the relaxation from historic events and characterize the tectonic forces imposed by the bounding Pacific and JdF plates. Relaxation following major earthquakes (M ??? 8.0) strongly shapes the present strain-rate field over most of the plate boundary zone. Equally important are lateral shear transmitted across the Pacific-North America plate boundary along ???1000 km of the continental shelf, downdip forces distributed along the Cascadia subduction interface, and distributed slip in the lower lithosphere. Post-earthquake relaxation and tectonic forcing, combined with distributed deep slip, constructively interfere near the western margin of the plate boundary zone, producing locally large strain accumulation along the San Andreas fault (SAF) system. However, they destructively interfere further into the plate interior, resulting in smaller and more variable strain accumulation patterns in the eastern part of the plate boundary zone. Much of the right-lateral strain accumulation along the SAF system is systematically underpredicted by models which account only for relaxation from known large earthquakes. This strongly suggests that in addition to viscoelastic-cycle effects, steady deep slip in the lower lithosphere is needed to explain the observed strain-rate field. ?? 2006 The Authors Journal compilation ?? 2006 RAS.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Trevena, A.S.; Varga, R.J.; Collins, I.D.

Salin basin of central Myanmar is a tertiary fore-arc basin that extends over 10,000 mi{sup 2} and contains 30,000+ ft of siliciclastic rocks. In the western Salin basin, Tertiary deltaic and fluvial formations contain thousands of feet of lithic sandstones that alternate with transgressive shallow marine shales. Facies and paleocurrent studies indicate deposition by north-to-south prograding tidal deltas and associated fluvial systems in a semi-restricted basin. Presence of serpentinite and volcanic clasts in Tertiary sandstones may imply that the basin was bounded to the east by the volcanic arc and to the west by a fore-arc accretionary ridge throughout muchmore » of the Cenozoic. Salin basin is currently defined by a regional north/south-trending syncline with uplifts along the eastern and western margins. Elongate folds along the eastern basin margin verge to the east and lie above the reverse faults that dip west; much of Myanmar's present hydrocarbon production is from these structures. Analogous structures occur along the western margin, but verge to the west and are associated with numerous hydrocarbon seeps and hand-dug wells. These basin-bounding structures are the result of fault-propagation folding. In the western Salin basin, major detachments occur within the shaly Tabyin and Laungshe formations. Fault ramps propagated through steep forelimbs on the western sides of the folds, resulting in highly asymmetric footwall synclines. Stratigraphic and apatite fission track data are consistent with dominantly Plio-Pleistocene uplift, with limited uplift beginning approximately 10 Ma. Paleostress analysis of fault/slickenside data indicates that fold and thrust structures formed during regional east/west compression and are not related in any simple way to regional transpression as suggested by plate kinematics.« less

Late Pleistocene and Holocene uplift history of Cyprus: implications for active tectonics along the southern margin of the Anatolian microplate

USGS Publications Warehouse

Harrison, R.W.; Tsiolakis, E.; Stone, B.D.; Lord, A.; McGeehin, J.P.; Mahan, S.A.; Chirico, P.

2013-01-01

The nature of the southern margin of the Anatolian microplate during the Neogene is complex, controversial and fundamental in understanding active plate-margin tectonics and natural hazards in the Eastern Mediterranean region. Our investigation provides new insights into the Late Pleistocene uplift history of Cyprus and the Troodos Ophiolite. We provide isotopic (14C) and radiogenic (luminescence) dates of outcropping marine sediments in eastern Cyprus that identify periods of deposition during marine isotope stages (MIS) 3, 4, 5 and 6. Past sea-levels indicated by these deposits are c. 95±25 m higher in elevation than estimates of worldwide eustatic sea-level. An uplift rate of c. 1.8 mm/year and possibly as much as c. 4.1 mm/year in the past c. 26–40 ka is indicated. Holocene marine deposits also occur at elevations higher than those expected for past SL and suggest uplift rates of c. 1.2–2.1 mm/year. MIS-3 marine deposits that crop out in southern and western Cyprus indicate uniform island-wide uplift. We propose a model of tectonic wedging at a plate-bounding restraining bend as a mechanism for Late Pleistocene to Holocene uplift of Cyprus; uplift is accommodated by deformation and seismicity along the margins of the Troodos Ophiolite and re-activation of its low-angle, basal shear zone.

Investigating the 3-D Subduction Initiation Processes at Transform Faults and Passive Margins

NASA Astrophysics Data System (ADS)

Peng, H.; Leng, W.

2017-12-01

Studying the processes of subduction initiation is a key for understanding the Wilson cycle and improving the theory of plate tectonics. Previous studies investigated subduction initiation with geological synthesis and geodynamic modeling methods, discovering that subduction intends to initiate at the transform faults close to oceanic arcs, and that its evolutionary processes and surface volcanic expressions are controlled by plate strength. However, these studies are mainly conducted with 2-D models, which cannot deal with lateral heterogeneities of crustal thickness and strength along the plate interfaces. Here we extend the 2-D model to a 3-D parallel subduction model with high computational efficiency. With the new model, we study the dynamic controlling factors, morphology evolutionary processes and surface expressions for subduction initiation with lateral heterogeneities of material properties along transform faults and passive margins. We find that lateral lithospheric heterogeneities control the starting point of the subduction initiation along the newly formed trenches and the propagation speed for the trench formation. New subduction tends to firstly initiate at the property changing point along the transform faults or passive margins. Such finds may be applied to explain the formation process of the Izu-Bonin-Mariana (IBM) subduction zone in the western Pacific and the Scotia subduction zone at the south end of the South America. Our results enhance our understanding for the formation of new trenches and help to provide geodynamic modeling explanations for the observed remnant slabs in the upper mantle and the surface volcanic expressions.

Evidence for an east-west regional gravity trend in northern Tunisia: Insight into the structural evolution of northern Tunisian Atlas

NASA Astrophysics Data System (ADS)

Jallouli, Chokri; Mogren, Saad; Mickus, Kevin; Turki, Mohamed Moncef

2013-11-01

The Atlas orogeny in northern Algeria and Tunisia led to the destruction of Tethys oceanic lithosphere and cumulated in a collision of microplates rifted off the European margin with the North African continental margin. The location of the boundary between African plate and Kabylian microplate is expressed in northern Algeria by a crustal wedge with double vergence of thrust sheets, whereas in northern Tunisia the geologic environment is more complex and the location of the plate boundary is ambiguous. In this study, we analyzed gravity data to constrain the crustal structure along the northern margin of Tunisia. The analysis includes a separation of regional and residual gravity anomalies and the application of gradient operators to locate density contrast boundaries. The horizontal gradient magnitude and directional gradient highlight a prominent regional E-W gravity gradient in the northern Tunisian Atlas interpreted as a deep fault (active since at least the Early Mesozoic) having a variable kinematic activity depending on the tectonic regime in the region. The main E-W gravity gradient separates two blocks having different gravitational and seismic responses. The southern block has numerous gravity lineaments trending in different directions implying several density variations within the crust, whereas the northern block shows a long-wavelength negative gravity anomaly with a few lineaments. Taking into account the geologic context of the Western Mediterranean region, we consider the E-W prominent feature as the boundary between African plate and Kabylian microplate in northern Tunisia that rifted off Europe. This hypothesis fits most previous geological and geophysical studies and has an important impact on the petroleum and mineral resource prospection as these two blocks were separated by an ocean and they did not belong to the same margin.

Tethyan, Mediterranean, and Pacific analogues for the Neoproterozoic Paleozoic birth and development of peri-Gondwanan terranes and their transfer to Laurentia and Laurussia

NASA Astrophysics Data System (ADS)

Keppie, J. Duncan; Nance, R. Damian; Murphy, J. Brendan; Dostal, J.

2003-04-01

Modern Tethyan, Mediterranean, and Pacific analogues are considered for several Appalachian, Caledonian, and Variscan terranes (Carolina, West and East Avalonia, Oaxaquia, Chortis, Maya, Suwannee, and Cadomia) that originated along the northern margin of Neoproterozoic Gondwana. These terranes record a protracted geological history that includes: (1) ˜1 Ga (Carolina, Avalonia, Oaxaquia, Chortis, and Suwannee) or ˜2 Ga (Cadomia) basement; (2) 750-600 Ma arc magmatism that diachronously switched to rift magmatism between 590 and 540 Ma, accompanied by development of rift basins and core complexes, in the absence of collisional orogenesis; (3) latest Neoproterozoic-Cambrian separation of Avalonia and Carolina from Gondwana leading to faunal endemism and the development of bordering passive margins; (4) Ordovician transport of Avalonia and Carolina across Iapetus terminating in Late Ordovician-Early Silurian accretion to the eastern Laurentian margin followed by dispersion along this margin; (5) Siluro-Devonian transfer of Cadomia across the Rheic Ocean; and (6) Permo-Carboniferous transfer of Oaxaquia, Chortis, Maya, and Suwannee during the amalgamation of Pangea. Three potential models are provided by more recent tectonic analogues: (1) an "accordion" model based on the orthogonal opening and closing of Alpine Tethys and the Mediterranean; (2) a "bulldozer" model based on forward-modelling of Australia during which oceanic plateaus are dispersed along the Australian plate margin; and (3) a "Baja" model based on the Pacific margin of North America where the diachronous replacement of subduction by transform faulting as a result of ridge-trench collision has been followed by rifting and the transfer of Baja California to the Pacific Plate. Future transport and accretion along the western Laurentian margin may mimic that of Baja British Columbia. Present geological data for Avalonia and Carolina favour a transition from a "Baja" model to a "bulldozer" model. By analogy with the eastern Pacific, we name the oceanic plates off northern Gondwana: Merlin (≡Farallon), Morgana (≡Pacific), and Mordred (≡Kula). If Neoproterozoic subduction was towards Gondwana, application of this combined model requires a total rotation of East Avalonia and Carolina through 180° either during separation (using a western Transverse Ranges model), during accretion (using a Baja British Columbia "train wreck" model), or during dispersion (using an Australia "bulldozer" model). On the other hand, Siluro-Devonian orthogonal transfer ("accordion" model) from northern Africa to southern Laurussia followed by a Carboniferous "Baja" model appears to best fit the existing data for Cadomia. Finally, Oaxaquia, Chortis, Maya, and Suwannee appear to have been transported along the margin of Gondwana until it collided with southern Laurentia on whose margin they were stranded following the breakup of Pangea. Forward modeling of a closing Mediterranean followed by breakup on the African margin may provide a modern analogue. These actualistic models differ in their dictates on the initial distribution of the peri-Gondwanan terranes and can be tested by comparing features of the modern analogues with their ancient tectonic counterparts.

Rayleigh phase velocities in the upper mantle of the Pacific-North American plate boundary in southern California

NASA Astrophysics Data System (ADS)

Escobar, L.; Weeraratne, D. S.; Kohler, M. D.

2013-05-01

The Pacific-North America plate boundary, located in Southern California, presents an opportunity to study a unique tectonic process that has been shaping the plate tectonic setting of the western North American and Mexican Pacific margin since the Miocene. This is one of the few locations where the interaction between a migrating oceanic spreading center and a subduction zone can be studied. The rapid subduction of the Farallon plate outpaced the spreading rate of the East Pacific Rise rift system causing it to be subducted beneath southern California and northern Mexico 30 Ma years ago. The details of microplate capture, reorganization, and lithospheric deformation on both the Pacific and North American side of this boundary is not well understood, but may have important implications for fault activity, stresses, and earthquake hazard analysis both onshore and offshore. We use Rayleigh waves recorded by an array of 34 ocean bottom seismometers deployed offshore southern California for a 12 month duration from August 2010 to 2011. Our array recorded teleseismic earthquakes at distances ranging from 30° to 120° with good signal-to-noise ratios for magnitudes Mw ≥ 5.9. The events exhibit good azimuthal distribution and enable us to solve simultaneously for Rayleigh wave phase velocities and azimuthal anisotropy. Fewer events occur at NE back-azimuths due to the lack of seismicity in central North America. We consider seismic periods between 18 - 90 seconds. The inversion technique considers non-great circle path propagation by representing the arriving wave field as two interfering plane waves. This takes advantage of statistical averaging of a large number of paths that travel offshore southern California and northern Mexico allowing for improved resolution and parameterization of lateral seismic velocity variations at lithospheric and sublithospheric depths. We present phase velocity results for periods sampling mantle structure down to 150 km depth along the west coast margin. With this study, we seek to understand the strength and deformation of the Pacific oceanic lithosphere resulting from plate convergence and subduction beneath Southern California 30 Ma as well as translational stresses present today. We also test for predictions of several geodynamic models which describe the kinematic mantle flow that accompanies plate motion within this area including passive mantle drag due to Pacific plate motion and toroidal flow in the western U.S. region that may extend offshore.

Boninite and boninite-series volcanics in northern Zambales ophiolite: doubly vergent subduction initiation along Philippine Sea plate margins

NASA Astrophysics Data System (ADS)

Perez, Americus; Umino, Susumu; Yumul, Graciano P., Jr.; Ishizuka, Osamu

2018-06-01

A key component of subduction initiation rock suites is boninite, a high-magnesium andesite that is uniquely predominant in western Pacific forearc terranes and in select Tethyan ophiolites such as Oman and Troodos. We report, for the first time, the discovery of low-calcium, high-silica boninite in the middle Eocene Zambales ophiolite (Luzon Island, Philippines). Olivine-orthopyroxene microphyric high-silica boninite, olivine-clinopyroxene-phyric low-silica boninite and boninitic basalt occur as lapilli fall deposits and pillow lava flows in the upper volcanic unit of the juvenile arc section (Barlo locality, Acoje Block) of the Zambales ophiolite. This upper volcanic unit overlies a lower volcanic unit consisting of basaltic andesite, andesite to dacitic lavas and explosive eruptive material (subaqueous pahoehoe and lobate sheet flows, agglutinate and spatter deposits) forming a low-silica boninite series. The overall volcanic stratigraphy of the extrusive sequence at Barlo resembles holes U1439 and U1442 drilled by IODP Expedition 352 in the Izu-Ogasawara (Bonin) trench slope. The presence of depleted proto-arc basalts in the Coto Block (45 Ma) (Geary et al., 1989), boninite and boninite series volcanics in Barlo (Acoje Block (44 Ma)) and simultaneous and post-boninite moderate-Fe arc tholeiites in Sual and Subic areas of the Acoje Block (44-43 Ma) indicate that the observed subduction initiation stratigraphy in the Izu-Ogasawara-Mariana forearc is also present in the Zambales ophiolite. Paleolatitudes derived from tilt-corrected sites in the Acoje Block place the juvenile arc of northern Zambales ophiolite in the western margin of the Philippine Sea plate. In this scenario, the origin of Philippine Sea plate boninites (IBM and Zambales) would be in a doubly vergent subduction initiation setting.

Late Cretaceous through Cenozoic strike-slip tectonics of southwestern Alaska

USGS Publications Warehouse

Miller, M.L.; Bradley, D.C.; Bundtzen, T.K.; McClelland, W.

2002-01-01

New geologic mapping and geochronology show that margin-parallel strike-slip faults on the western limb of the southern Alaska orocline have experienced multiple episodes of dextral motion since ~100 Ma. These faults are on the upper plate of a subduction zone ~350-450 km inboard of the paleotrench. In southwestern Alaska, dextral displacement is 134 km on the Denali fault, at least 88-94 km on the Iditarod-Nixon Fork fault, and perhaps tens of kilometers on the Dishna River fault. The strike-slip regime coincided with Late Cretaceous sedimentation and then folding in the Kuskokwim basin, and with episodes of magmatism and mineralization at ~70, ~60, and ~30 Ma. No single driving mechanism can explain all of the ~95 million-year history of strike-slip faulting. Since ~40 Ma, the observed dextral sense of strike slip has run contrary to the sense of subduction obliquity. This may be explained by northward motion of the Pacific plate driving continental margin slivers into and/or around the oroclinal bend. From 44 to 66 Ma, oroclinal rotation, perhaps involving large-scale flexural slip, may have been accompanied by westward escape of crustal blocks along strike-slip faults. However, reconstructions of this period involve unproven assumptions about the identity of the subducting plate, the position of subducting ridges, and the exact timing of oroclinal bending, thus obscuring the driving mechanisms of strike slip. Prior to 66 Ma, oblique subduction is the most plausible driving mechanism for dextral strike slip. Cumulative displacement on all faults of the western limb of the orocline is at least 400 km, about half that on the eastern limb; this discrepancy might be explained by a combination of thrusting and unrecognized strike-slip faulting.

Contrasting sedimentary processes along a convergent margin: the Lesser Antilles arc system

NASA Astrophysics Data System (ADS)

Picard, Michel; Schneider, Jean-Luc; Boudon, Georges

2006-12-01

Sedimentation processes occurring in an active convergent setting are well illustrated in the Lesser Antilles island arc. The margin is related to westward subduction of the North and/or the South America plates beneath the Caribbean plate. From east to west, the arc can be subdivided into several tectono-sedimentary depositional domains: the accretionary prism, the fore-arc basin, the arc platform and inter-arc basin, and the Grenada back-arc basin. The Grenada back-arc basin, the fore-arc basin (Tobago Trough) and the accretionary prism on the east side of the volcanic arc constitute traps for particles derived from the arc platform and the South American continent. The arc is volcanically active, and provides large volumes of volcaniclastic sediments which accumulate mainly in the Grenada basin by volcaniclastic gravity flows (volcanic debris avalanches, debris flows, turbiditic flows) and minor amounts by fallout. By contrast, the eastern side of the margin is fed by ash fallout and minor volcaniclastic turbidites. In this area, the dominant component of the sediments is pelagic in origin, or derived from South America (siliciclastic turbidites). Insular shelves are the locations of carbonate sedimentation, such as large platforms which develop in the Limestone Caribbees in the northern part of the margin. Reworking of carbonate material by turbidity currents also delivers lesser amounts to eastern basins of the margin. This contrasting sedimentation on both sides of the arc platform along the margin is controlled by several interacting factors including basin morphology, volcanic productivity, wind and deep-sea current patterns, and sea-level changes. Basin morphology appears to be the most dominant factor. The western slopes of the arc platform are steeper than the eastern ones, thus favouring gravity flow processes.

Probabilistic Seismic Risk Model for Western Balkans

NASA Astrophysics Data System (ADS)

Stejskal, Vladimir; Lorenzo, Francisco; Pousse, Guillaume; Radovanovic, Slavica; Pekevski, Lazo; Dojcinovski, Dragi; Lokin, Petar; Petronijevic, Mira; Sipka, Vesna

2010-05-01

A probabilistic seismic risk model for insurance and reinsurance purposes is presented for an area of Western Balkans, covering former Yugoslavia and Albania. This territory experienced many severe earthquakes during past centuries producing significant damage to many population centres in the region. The highest hazard is related to external Dinarides, namely to the collision zone of the Adriatic plate. The model is based on a unified catalogue for the region and a seismic source model consisting of more than 30 zones covering all the three main structural units - Southern Alps, Dinarides and the south-western margin of the Pannonian Basin. A probabilistic methodology using Monte Carlo simulation was applied to generate the hazard component of the model. Unique set of damage functions based on both loss experience and engineering assessments is used to convert the modelled ground motion severity into the monetary loss.

Geological history of the Cretaceous ophiolitic complexes of northwestern South America (Colombian Andes)

NASA Astrophysics Data System (ADS)

Bourgois, Jacques; Toussaint, Jean-François; Gonzalez, Humberto; Azema, Jacques; Calle, Bernardo; Desmet, Alain; Murcia, Luis A.; Acevedo, Alvaro P.; Parra, Eduardo; Tournon, Jean

1987-12-01

The Western Cordillera of Colombia was formed by intense alpine-type nappe-forming folding and thrusting. The Cretaceous (80-120 Ma B.P.) tholeiitic material of the Western Cordilleran nappes has been obducted onto the Paleozoic and Precambrian polymetamorphic micaschists and gneiss of the Central Cordillera. Near Yarumal, the Antioquia batholith (60-80 Ma B.P.) intrudes both obducted Cretaceous oceanic material and the polymetamorphic basement rock of the Central Cordillera. Therefore, nappe emplacement and obduction onto the Central Cordillera occurred during Late Senonian to Early Paleocene. The nappes travelled from northwest to southeast so that the highest unit, the Rio Calima nappe therefore has the most northwestern source, whereas the lowest units originated from a more southeastward direction. Sedimentological analysis of the volcanoclastic and sandy turbidite material from each unit suggests a marginal marine environment. During Cretaceous times the opening of this marginal sea, from now on called the "Colombia marginal basin", probably originated by detachment of a block from the South American continent related to the Farallon-South America plate convergence. In the Popayan area (southern Colombia), the Central Cordilleran basement exhibits glaucophane schist facies metamorphism. This high pressure low temperature metamorphism is of Early Cretaceous (125 Ma B.P.) age and is related to an undated metaophiolitic complex. The ophiolitic material originating from the Western Cordilleran is thrust over both the blueschist belt and the metaophiolitic complex. These data suggest that the "Occidente Colombiano" suffered at least two phases of ophiolitic obduction during Mesozoic time.

Tectonostratigraphy of the Passive Continental Margin Offshore Indus Pakistan

NASA Astrophysics Data System (ADS)

Aslam, K.; Khan, M.; Liu, Y.; Farid, A.

2017-12-01

The tectonic evolution and structural complexities are poorly understood in the passive continental margin of the Offshore Indus of Pakistan. In the present study, an attempt has been made to interpret the structural trends and seismic stratigraphic framework in relation to the tectonics of the region. Seismic reflection data revealed tectonically controlled, distinct episodes of normal faulting representing rifting at different ages and transpression in the Late Eocene time. This transpression has resulted in the reactivation of the Pre-Cambrian basement structures. The movement of these basement structures has considerably affected the younger sedimentary succession resulting in push up structures resembling anticlines. The structural growth of the push-up structures was computed. The most remarkable tectonic setting in the region is represented by the normal faulting and by the basement uplift which divides the rifting and transpression stages. Ten mappable seismic sequences have been identified on the seismic records. A Jurassic aged paleo-shelf has also been identified on all regional seismic profiles which is indicative of Indian-African Plates separation during the Jurassic time. Furthermore, the backstripping technique was applied which has been proved to be a powerful technique to quantify subsidence/uplift history of rift-type passive continental margins. The back strip curves suggest that transition from an extensional rifted margin to transpression occurred during Eocene time (50-30 Ma). The backstripping curves show uplift had happened in the area. We infer that the uplift has occurred due to the movement of basement structures by the transpression movements of Arabian and Indian Plates. The present study suggests that the structural styles and stratigraphy of the Offshore Indus Pakistan were significantly affected by the tectonic activities during the separation of Gondwanaland in the Mesozoic and northward movement of the Indian Plate, post-rifting, and sedimentations along its western margin during the Middle Cenozoic. The present comprehensive interpretation can help in understanding the structural complexities and stratigraphy associated with tectonics in other parts of the passive continental margins worldwide dominated by rifting and drifting tectonics.

Geological and geodynamic investigations of Alaskan tectonics: Responses in the ancient and modern geologic records to oblique plate convergence

NASA Astrophysics Data System (ADS)

Kalbas, James L.

Stratigraphic, structural, and geophysical modeling studies focusing on both the Mesozoic and modern development of southern Alaska aid in understanding the nature of tectonic responses to oblique plate convergence. Analyses of the Lower to Upper (?) Cretaceous Kahiltna assemblage of the western Alaska Range and the Upper Cretaceous Kuskokwim Group of the northern Kuskokwim Mountains provide a stratigraphic record of orogenic growth in southwestern Alaska. The Kahiltna assemblage records dominantly west-directed gravity-flow transport of sediment to the axis of an obliquely closing basin that made up the suture zone between the allochthonous Wrangellia composite terrane and the North American pericratonic margin. Stratigraphic, compositional, and geochronologic analyses suggest that submarine-fan systems of the Kahiltna basin were fed from the subearial suture zone and contain detrital grains derived from both allochthonous and pericratonic sources, thereby implying a relatively close proximity of the island-arc terrane to the North American margin by late Early Cretaceous time. In contrast, Upper Cretaceous strata exposed immediately west of the Kahiltna assemblage record marine deposition during a period of transition from island arc accretion to strike-slip tectonics. The new stratigraphic model presented here recognizes diverse bathyal- to shelfal-marine depositional systems within the Kuskokwim Group that represent distinctive regional sediment entry points to the basin. Collectively, these strata suggest that the Kuskokwim Group represents the waning stages of marine deposition in a long-lived intra-oceanic and continental margin basin. Geodynamic studies focus on the mechanics of contemporary fault systems in southern Alaska inboard of the collisional Yakutat microplate. Finite-element analyses predict that a poorly understood Holocene strike-slip fault in the St. Elias Mountains transfers shear from the Queen Charlotte fault northward to the Denali fault, thereby forming a continuous transform system that accommodates right-lateral motion of the Pacific plate and Yakutat microplate relative to the stable North American craton. Although the best-fit model implies some component of anelastic deformation in the vicinity of the St. Elias Mountains and the western Alaska Range, results imply overall block-like behavior throughout the area of interest.

Origin of back-arc basins and effects of western Pacific subduction systems on eastern China geology

NASA Astrophysics Data System (ADS)

Niu, Y.

2013-12-01

Assuming that subduction initiation is a consequence of lateral compositional buoyancy contrast within the lithosphere [1], and recognizing that subduction initiation within normal oceanic lithosphere is unlikely [1], we can assert that passive continental margins that are locations of the largest compositional buoyancy contrast within the lithosphere are the loci of future subduction zones [1]. We hypothesize that western Pacific back-arc basins were developed as and evolved from rifting at passive continental margins in response to initiation and continuation of subduction zones. This hypothesis can be tested by demonstrating that intra-oceanic island arcs must have basement of continental origin. The geology of the Islands of Japan supports this. The highly depleted forearc peridotites (sub-continental lithosphere material) from Tonga and Mariana offer independent lines of evidence for the hypothesis [1]. The origin and evolution of the Okinawa Trough (back-arc basin) and Ryukyu Arc/Trench systems represents the modern example of subduction initiation and back-arc basin formation along a (Chinese) continental margin. The observation why back-arc basins exit behind some subduction zones (e.g., western Pacific) but not others (e.g., in South America) depends on how the overlying plate responds to subduction, slab-rollback and trench retreat. In the western Pacific, trench retreat towards east results in the development of extension in the upper Eurasian plate and formation of back-arc basins. In the case of South America, where no back-arc basins form because trench retreat related extension is focused at the 'weakest' South Mid-Atlantic Ridge. It is thus conceptually correct that the South Atlantic is equivalent to a huge 'back-arc basin' although its origin may be different. Given the negative Clayperon slope of the Perovskite-ringwoodite phase transition at the 660 km mantle seismic discontinuity (660-D), slab penetration across the 660-D is difficult and trench retreat in the western Pacific readily result in the horizontal stagnation of the Pacific plate in the transition zone beneath eastern Asian continent [2]. Dehydration of this slab supplies water, which rises and results in 'basal hydration weakening' of the eastern China lithosphere and its thinning by converting it into weak material of asthenospheric property [3]. We note the proposal that multiple subduction zones with more water (i.e., subduction of the South China Block beneath the North China Craton, NCC; subduction of the Siberian/Mongolian block beneath the NCC) all contribute to the lithosphere thinning beneath the NCC [4]. However, 'South China-NCC' and 'Siberian/Mongolian-NCC' represent two collisional tectonics involving no trench retreat, causing no transition-zone slab stagnation, supplying no water, and thus contributing little to lithosphere thinning beneath the NCC. Furthermore, lithosphere thinning happened to the entire eastern China, not just limited to the NCC, emphasizing the effects of the western Pacific subduction system on eastern China geology. References: [1] Niu et al., 2003, Journal of Petrology, 44, 851-866. [2] Kárason & van der Hilst, R., 2000, Geophysical Monograph, 121, 277-288. [3] Niu, 2005, Geological Journal of China Universities, 11, 9-46. [4] Windley et al., 2010, American Journal of Science, 310, 1250-1293.

Two-phase opening of Andaman Sea: a new seismotectonic insight

NASA Astrophysics Data System (ADS)

Khan, P. K.; Chakraborty, Partha Pratim

2005-01-01

High-resolution reconstruction of Benioff zone depth-dip angle trajectory for Burma-Java subduction margin between 2° and 17°N Lat. reveals two major episodes of plate geometry change expressed as abrupt deviation in subduction angle. Estimation of effective rate of subduction in different time slices (and then length of subducted slab) allowed drawing of isochrones in Ma interval through these trajectories for the time period 5-12 Ma. With these isochrones, the deformation events on the subducting Indian plate are constrained in time as of 4-5 and 11 Ma old. This well-constrained time connotation offered scope for the correlation of slab deformation events with the well-established two-phase opening history of the Andaman Sea. While the 11 Ma event recorded from southern part of the study area is correlated with early stretching and rifting phase, the 4-5 Ma event is interpreted as major forcing behind the spreading phase of the Andaman Sea. Systematic spatio-temporal evaluation of Indian plate obliquity on the Andaman Sea evolution shows its definite control on the early rifting phase, initiated towards south near northwest Sumatra. The much young spreading phase recorded towards north of 7° Lat. is possibly the result of late Miocene-Pliocene trench retreat and follow-up transcurrent movement (along Sagaing and Sumatran fault system) with NW-SE pull-apart extension. Nonconformity between plate shape and subduction margin geometry is interpreted as the causative force behind Mid-Miocene intraplate extension and tearing. Enhanced stretching in the overriding plate consequently caused active forearc subsidence, recorded all along this plate margin. Initial phase of the Andaman Sea opening presumably remains concealed in this early-middle Miocene forearc subsidence history. The late Miocene-Pliocene pull-apart opening and spreading was possibly initiated near the western part of the Mergui-Sumatra region and propagated northward in subsequent period. A temporary halt in rifting at this pull-apart stage and northeastward veering of the Andaman Sea Ridge (ASR) are related with uplifting of oceanic crust in post-middle Miocene time in form of Alcock and Sewell seamounts, lying symmetrically north and south of this spreading ridge.

Anatomy of the western Java plate interface from depth-migrated seismic images

NASA Astrophysics Data System (ADS)

Kopp, H.; Hindle, D.; Klaeschen, D.; Oncken, O.; Reichert, C.; Scholl, D.

2009-11-01

Newly pre-stack depth-migrated seismic images resolve the structural details of the western Java forearc and plate interface. The structural segmentation of the forearc into discrete mechanical domains correlates with distinct deformation styles. Approximately 2/3 of the trench sediment fill is detached and incorporated into frontal prism imbricates, while the floor sequence is underthrust beneath the décollement. Western Java, however, differs markedly from margins such as Nankai or Barbados, where a uniform, continuous décollement reflector has been imaged. In our study area, the plate interface reveals a spatially irregular, nonlinear pattern characterized by the morphological relief of subducted seamounts and thicker than average patches of underthrust sediment. The underthrust sediment is associated with a low velocity zone as determined from wide-angle data. Active underplating is not resolved, but likely contributes to the uplift of the large bivergent wedge that constitutes the forearc high. Our profile is located 100 km west of the 2006 Java tsunami earthquake. The heterogeneous décollement zone regulates the friction behavior of the shallow subduction environment where the earthquake occurred. The alternating pattern of enhanced frictional contact zones associated with oceanic basement relief and weak material patches of underthrust sediment influences seismic coupling and possibly contributed to the heterogeneous slip distribution. Our seismic images resolve a steeply dipping splay fault, which originates at the décollement and terminates at the sea floor and which potentially contributes to tsunami generation during co-seismic activity.

Anatomy of the western Java plate interface from depth-migrated seismic images

USGS Publications Warehouse

Kopp, H.; Hindle, D.; Klaeschen, D.; Oncken, O.; Reichert, C.; Scholl, D.

2009-01-01

Newly pre-stack depth-migrated seismic images resolve the structural details of the western Java forearc and plate interface. The structural segmentation of the forearc into discrete mechanical domains correlates with distinct deformation styles. Approximately 2/3 of the trench sediment fill is detached and incorporated into frontal prism imbricates, while the floor sequence is underthrust beneath the d??collement. Western Java, however, differs markedly from margins such as Nankai or Barbados, where a uniform, continuous d??collement reflector has been imaged. In our study area, the plate interface reveals a spatially irregular, nonlinear pattern characterized by the morphological relief of subducted seamounts and thicker than average patches of underthrust sediment. The underthrust sediment is associated with a low velocity zone as determined from wide-angle data. Active underplating is not resolved, but likely contributes to the uplift of the large bivergent wedge that constitutes the forearc high. Our profile is located 100 km west of the 2006 Java tsunami earthquake. The heterogeneous d??collement zone regulates the friction behavior of the shallow subduction environment where the earthquake occurred. The alternating pattern of enhanced frictional contact zones associated with oceanic basement relief and weak material patches of underthrust sediment influences seismic coupling and possibly contributed to the heterogeneous slip distribution. Our seismic images resolve a steeply dipping splay fault, which originates at the d??collement and terminates at the sea floor and which potentially contributes to tsunami generation during co-seismic activity. ?? 2009 Elsevier B.V.

Seismicity of the Earth 1900–2010 Middle East and vicinity

USGS Publications Warehouse

Jenkins, Jennifer; Turner, Bethan; Turner, Rebecca; Hayes, Gavin P.; Davies, Sian; Dart, Richard L.; Tarr, Arthur C.; Villaseñor, Antonio; Benz, Harley M.

2013-01-01

No fewer than four major tectonic plates (Arabia, Eurasia, India, and Africa) and one smaller tectonic block (Anatolia) are responsible for seismicity and tectonics in the Middle East and surrounding region. Geologic development of the region is a consequence of a number of first-order plate tectonic processes that include subduction, large-scale transform faulting, compressional mountain building, and crustal extension. In the east, tectonics are dominated by the collision of the India plate with Eurasia, driving the uplift of the Himalaya, Karakorum, Pamir and Hindu Kush mountain ranges. Beneath the Pamir‒Hindu Kush Mountains of northern Afghanistan, earthquakes occur to depths as great as 200 km as a result of remnant lithospheric subduction. Along the western margin of the India plate, relative motions between India and Eurasia are accommodated by strike-slip, reverse, and oblique-slip faulting, resulting in the complex Sulaiman Range fold and thrust belt, and the major translational Chaman Fault in Afghanistan. Off the south coasts of Pakistan and Iran, the Makran trench is the surface expression of active subduction of the Arabia plate beneath Eurasia. Northwest of this subduction zone, collision between the two plates forms the approximately 1,500-km-long fold and thrust belts of the Zagros Mountains, which cross the whole of western Iran and extend into northeastern Iraq. Tectonics in the eastern Mediterranean region are dominated by complex interactions between the Africa, Arabia, and Eurasia plates, and the Anatolia block. Dominant structures in this region include: the Red Sea Rift, the spreading center between the Africa and Arabia plates; the Dead Sea Transform, a major strike-slip fault, also accommodating Africa-Arabia relative motions; the North Anatolia Fault, a right-lateral strike-slip structure in northern Turkey accommodating much of the translational motion of the Anatolia block westwards with respect to Eurasia and Africa; and the Cyprian Arc, a convergent boundary between the Africa plate to the south, and Anatolia Block to the north.

Earthquake stress drops, ambient tectonic stresses and stresses that drive plate motions

USGS Publications Warehouse

Hanks, T.C.

1977-01-01

A variety of geophysical observations suggests that the upper portion of the lithosphere, herein referred to as the elastic plate, has long-term material properties and frictional strength significantly greater than the lower lithosphere. If the average frictional stress along the non-ridge margin of the elastic plate is of the order of a kilobar, as suggested by the many observations of the frictional strength of rocks at mid-crustal conditions of pressure and temperature, the only viable mechanism for driving the motion of the elastic plate is a basal shear stress of several tens of bars. Kilobars of tectonic stress are then an ambient, steady condition of the earth's crust and uppermost mantle. The approximate equality of the basal shear stress and the average crustal earthquake stress drop, the localization of strain release for major plate margin earthquakes, and the rough equivalence of plate margin slip rates and gross plate motion rates suggest that the stress drops of major plate margin earthquakes are controlled by the elastic release of the basal shear stress in the vicinity of the plate margin, despite the existence of kilobars of tectonic stress existing across vertical planes parallel to the plate margin. If the stress differences available to be released at the time of faulting are distributed in a random, white fasbion with a mean-square value determined by the average earthquake stress drop, the frequency of occurrence of constant stress drop earthquakes will be proportional to reciprocal faulting area, in accordance with empirically known frequency of occurrence statistics. ?? 1977 Birkha??user Verlag.

Topographic form of the Coast Ranges of the Cascadia Margin in relation ot coastal uplift rates and plate subduction

NASA Technical Reports Server (NTRS)

Kelsey, Harvey M.; Engebretson, David C.; Mitchell, Clifton E.; Ticknor, Robert L.

1994-01-01

The Coast Ranges of the Cascadia margin are overriding the subducted Juan de Fuca/Gorda plate. We investigate the extent to which the latitudinal change in attributes related to the subduction process. These attributes include the varibale age of the subducted slab that underlies the Coast Ranges and average vertical crustal velocities of the western margin of the Coast Rnages for two markedly different time periods, the last 45 years and the last 100 kyr. These vertical crustal velocities are computed from the resurveying of highway bech marks and from the present elevation of shore platforms that have been uplifted in the late Quaternary, respectively. Topogarphy of the Coast Ranges is in part a function of the age and bouyancy of the underlying subducted plate. This is evident in the fact that the two highest topographic elements of the Coast Rnages, the Klamath Mountains and the Olympic Mountains, are underlain by youngest subducted oceanic crust. The subducted Blanco Fracture Zone in southernmost Oregon is responsible for an age discontinuity of subducted crust under the Klamath Mountains. The norhtern terminus of hte topographically higher Klamaths is offset to the north relative to the position of the underlying Blanco Fracture Zone, teh offset being in the direction of migration of the farcture zone, as dictated by relative plate motions. Vertical crustal velocities at the coast, derived from becnh mark surveys, are as much as an order of magnitude greater than vertical crustal velocities derived from uplifted shore platforms. This uplift rate discrepancy indicates that strain is accumulating on the plate margin, to be released during the next interplate earthquake. In a latitudinal sense, average Coast Rnage topography is relatively high where bench mark-derived, short-term vertical crustal velocities are highest. Becuase the shore platform vertical crustal velocities reflect longer-term, premanent uplift, we infer that a small percentage of the interseismic strain that accumulates as rapid short-term uplift is not recovered by subduction earthquakes but rather contributes to rock uplift of the Coast Ranges. The conjecture that permanent rock uplift is related to interseismic uplift is consistent with the observation that those segments of the subduction zone subject to greater interseismic uplift rates are at approximately the same latitudes as those segments of the Coast Ranges that have higher magnitudes of rock uplift over the long term.

Tsujal Marine Survey: Crustal Characterization of the Rivera Plate-Jalisco Block Boundary and its Implications for Seismic and Tsunami Hazard Assessment

NASA Astrophysics Data System (ADS)

Bartolome, R.; Danobeitia, J.; Barba, D. C., Sr.; Nunez-Cornu, F. J.; Cameselle, A. L.; Estrada, F.; Prada, M.; Bandy, W. L.

2014-12-01

During the spring of 2014, a team of Spanish and Mexican scientists explored the western margin of Mexico in the frame of the TSUJAL project. The two main objectives were to characterize the nature and structure of the lithosphere and to identify potential sources triggering earthquakes and tsunamis at the contact between Rivera plate-Jalisco block with the North American Plate. With these purposes a set of marine geophysical data were acquired aboard the RRS James Cook. This work is focus in the southern part of the TSUJAL survey, where we obtain seismic images from the oceanic domain up to the continental shelf. Thus, more than 800 km of MCS data, divided in 7 profiles, have been acquired with a 6km long streamer and using an air-gun sources ranging from 5800 c.i. to 3540 c.i. Furthermore, a wide-angle seismic profile of 190 km length was recorded in 16 OBS deployed perpendicular to the coast of Manzanillo. Gravity and magnetic, multibeam bathymetry and sub-bottom profiler data were recorded simultaneously with seismic data in the offshore area. Preliminary stacked MCS seismic sections reveal the crustal structure in the different domains of the Mexican margin. The contact between the Rivera and NA Plates is observed as a strong reflection at 6 s two way travel time (TWTT), in a parallel offshore profile (TS01), south of Manzanillo. This contact is also identified in a perpendicular profile, TS02, along a section of more than 100 km in length crossing the Rivera transform zone, and the plate boundary between Cocos and Rivera Plates. Northwards, offshore Pto. Vallarta, the MCS data reveals high amplitude reflections at around 7-8.5 s TWTT, roughly 2.5-3.5 s TWTT below the seafloor, that conspicuously define the subduction plane (TS06b). These strong reflections which we interpret as the Moho discontinuity define the starting bending of subduction of Rivera Plate. Another clear pattern observed within the first second of the MCS data shows evidences of a bottom simulating reflector (BSR) along the continental margin, particularly strong offshore Pto. Vallarta. The integration of all these acquired geophysical information will allow obtaining a comprehensive image of the lithosphere that will be valuable for the seismic and tsunamigenic hazard assessment.

Oceanic Remnants In The Caribbean Plate: Origin And Loss Of Related LIPs.

NASA Astrophysics Data System (ADS)

Giunta, G.

2005-12-01

The modern Caribbean Plate is an independent lithospheric entity, occupying more than 4 Mkm2 and consisting of the remnants of little deformed Cretaceous oceanic plateau of the Colombia and Venezuela Basins (almost 1 Mkm2) and the Palaeozoic-Mesozoic Chortis continental block (about 700,000 km2), both bounded by deformed marginal belts. The northern (Guatemala and Greater Antilles) and the southern (northern Venezuela) plate margins are marked by collisional zones, whereas the western (Central America Isthmus) and the eastern (Lesser Antilles) margins are represented by convergent boundaries and their magmatic arcs, all involving ophiolitic terranes. The evolutionary history of the Caribbean Plate since the Jurassic-Early Cretaceous encompasses plume, accretionary, and collisional tectonics, the evidence of which has been recorded in the oceanic remnants of lost LIPs, as revealed in: i) the MORB to OIB thickened crust of the oceanic plateau, including its un-deformed or little deformed main portion, and scattered deformed tectonic units; ii) ophiolitic tectonic units of MORB affinity and the rock blocks in ophiolitic melanges; iii) intra-oceanic, supra subduction magmatic sequences with IAT and CA affinities. The Mesozoic oceanic LIPs, from which the remnants of the Caribbean Plate have been derived, have been poorly preserved during various episodes of the intra-oceanic convergence, either those related to the original proto-Caribbean oceanic realm or those connected with two eo-Caribbean stages of subduction. The trapped oceanic plateau of the Colombia and Venezuela Basins is likely to be an unknown portion of a bigger crustal element of a LIP, similar to the Ontong-Java plateau. The Jurassic-Early Cretaceous proto-Caribbean oceanic domain consists of oceanic crust generated at multiple spreading centres; during the Cretaceous, part of this crust was thickened to form an oceanic plateau with MORB and OIB affinities. At the same time, both South and North American continental margins, inferred to be close to the oceanic realm, were affected by rifting and within-plate tholeiitic magmatism (WPT); this interpretation supports a near mid-America original location of the "proto-Caribbean" LIP. The MORB magmatic sections and rock blocks in the ophiolitic melanges are interpreted as exhumed tectonic sheets of the normal proto-Caribbean oceanic lithosphere, or part of a back-arc crust, both deformed in the eo-Caribbean stages. The SSZ complexes, considered as Cordilleran-type deformed ophiolites, were derived from a LIP that experienced two superimposed eo-Caribbean stages of intra-oceanic subduction. The older (Mid-Cretaceous) stage involved the eastward subduction of the un-thickened proto-Caribbean lithosphere, resulting in IAT and CA magmatism accompanied by HP-LT metamorphism and melange formation. The second, Late Cretaceous stage involved a westward dipping intra-oceanic subduction, which generated tonalitic arc magmatism. The eastward wedging of the Caribbean Plateau between the North and South American plates progressively trapped remnants of the Colombia and Venezuela Basins between the Atlantic and Pacific subduction zones and their new volcanic arcs (Aves-Lesser Antilles and Central American Isthmus). Unlike the proto-Caribbean, it appears that this LIP did not involve the main continental margins, even though the northern and southern Caribbean borders experienced different evolutionary paths. It was largely lost by superimposed accretionary and collisional events producing the marginal belts of the Caribbean Plate; its evolution has been dominated by a strongly oblique tectonic regime, constraining seafloor spreading, subduction, crustal exhumation, emplacement, and dismembering processes.

One microplate - three orogens: Alps, Dinarides, Apennines and the role of the Adriatic plate

NASA Astrophysics Data System (ADS)

Ustaszewski, Kamil; Le Breton, Eline; Balling, Philipp; Handy, Mark R.; Molli, Giancarlo; Tomljenović, Bruno

2017-04-01

The motion of the Adriatic microplate with respect to the Eurasian and African plates is responsible for the Mesozoic to present tectonic evolution of the Alps, Carpathians, the Dinarides and Hellenides as well as the Apennines. The classical approach for reconstructing plate motions is to assume that tectonic plates are rigid, then apply Euler's theorem to describe their rotation on an ideally spherical Earth by stepwise restorations of magnetic anomalies and fracture zones in oceanic basins. However, this approach is inadequate for reconstructing the motion of Mediterranean microplates like Adria, which, at present, is surrounded by convergent margins and whose oceanic portions have by now been entirely subducted. Most constraints on the motion of the Adriatic microplate come either from palaeomagnetics or from shortening estimates in the Alps, i.e., its northern margin. This approach renders plate tectonic reconstructions prone to numerous errors, yielding inadmissible misfits in the Ionian Sea between southern Italy and northern Greece. At the same time, Adria's western and eastern margins in the Apennines and in the Dinarides have hitherto not been appropriately considered for improving constraints on the motion of Adria. This presentation presents new results of ongoing collaborative research that aims at improving the relative motion path for the Adriatic microplate for the Cenozoic by additionally quantifying and restoring the amount of shortening and extension in a set of geophysical-geological transects from the Tyrrhenian Sea, the Apennines and the Dinarides. Already now, our approach yields an improved motion path for the Adriatic microplate for the last 20 Ma, which minimizes misfits in previous reconstructions. The currently largest challenge in our reconstructions is to reconcile amount and age of shortening in the Dinarides fold-and-thrust belt. For one thing, we see good agreement between the cross-sectional length of subducted material (c. 135 km, estimated from p-wave tomographic models) and shortening in the external carbonate platform of the Dinarides thrust belt (c. 127 km, from balanced cross sections). However, most of the thrust belt shortening is of Palaeogene age, which is difficult to bring into agreement with the fact that most of the subduction observed in tomographic models is most likely of Neogene age. This suggests that a substantial amount of Neogene crustal shortening must have been accommodated in the internal parts of the Dinarides fold-and-thrust belt rather than along its front. More field studies are therefore badly needed to obtain a better understanding of the timing of individual faults and their role during the Neogene evolution of the NE margin of the Adriatic plate.

A true polar wander model for Neoproterozoic plate motions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ripperdan, R.L.

1992-01-01

Recent paleogeographic reconstructions for the interval 750--500 Ma (Neoproterozoic to Late Cambrian) require rapid rates of plate motion and/or rotation around an equatorial Euler pole to accommodate reconstructions for the Early Paleozoic. Motions of this magnitude appear to be very uncommon during the Phanerozoic. A model for plate motions based on the hypothesis that discrete intervals of rapid true polar wander (RTPW) occurred during the Neoproterozoic can account for the paleogeographic changes with minimum amounts of plate motion. The model uses the paleogeographic reconstructions of Hoffman (1991). The following constraints were applied during derivation of the model: (1) relative motionsmore » between major continental units were restricted to be combinations of great circle or small circle translations with Euler poles of rotation = spin axis; (2) maximum rates of relative translational plate motion were 0.2 m/yr. Based on these constraints, two separate sets of synthetic plate motion trajectories were determined. The sequence of events in both can be summarized as: (1) A rapid true polar wander event of ca 90[degree] rafting a supercontinent to the spin axis; (2) breakup of the polar supercontinent into two fragments, one with the Congo, West Africa, Amazonia, and Baltica cratons, the other with the Laurentia, East Gondwana, and Kalahari cratons; (3) great circle motion of the blocks towards the equator; (4) small circle motion leading to amalgamation of Gondwana and separation of Laurentia and Baltica. In alternative 1, rifting initiates between East Antarctica and Laurentia and one episode of RTPW is required. Alternative 2 requires two episodes of RTPW; and that rifting occurred first along the eastern margin and later along the western margin of Laurentia. Synthetic plate motion trajectories are compared to existing paleomagnetic and geological data, and implications of the model for paleoclimatic changes during the Neoproterozoic are discussed.« less

Inverse methods-based estimation of plate coupling in a plate motion model governed by mantle flow

NASA Astrophysics Data System (ADS)

Ratnaswamy, V.; Stadler, G.; Gurnis, M.

2013-12-01

Plate motion is primarily controlled by buoyancy (slab pull) which occurs at convergent plate margins where oceanic plates undergo deformation near the seismogenic zone. Yielding within subducting plates, lateral variations in viscosity, and the strength of seismic coupling between plate margins likely have an important control on plate motion. Here, we wish to infer the inter-plate coupling for different subduction zones, and develop a method for inferring it as a PDE-constrained optimization problem, where the cost functional is the misfit in plate velocities and is constrained by the nonlinear Stokes equation. The inverse models have well resolved slabs, plates, and plate margins in addition to a power law rheology with yielding in the upper mantle. Additionally, a Newton method is used to solve the nonlinear Stokes equation with viscosity bounds. We infer plate boundary strength using an inexact Gauss-Newton method with line search for backtracking. Each inverse model is applied to two simple 2-D scenarios (each with three subduction zones), one with back-arc spreading and one without. For each case we examine the sensitivity of the inversion to the amount of surface velocity used: 1) full surface velocity data and 2) surface velocity data simplified using a single scalar average (2-D equivalent to an Euler pole) for each plate. We can recover plate boundary strength in each case, even in the presence of highly nonlinear flow with extreme variations in viscosity. Additionally, we ascribe an uncertainty in each plate's velocity and perform an uncertainty quantification (UQ) through the Hessian of the misfit in plate velocities. We find that as plate boundaries become strongly coupled, the uncertainty in the inferred plate boundary strength decreases. For very weak, uncoupled subduction zones, the uncertainty of inferred plate margin strength increases since there is little sensitivity between plate margin strength and plate velocity. This result is significant because it implies we can infer which plate boundaries are more coupled (seismically) for a realistic dynamic model of plates and mantle flow.

2D/3D Numerical Models of the Taiwan Orogen: Oblique Arc-Continent Collision overlying Orthogonal Subduction Systems

NASA Astrophysics Data System (ADS)

Kanda, R. V.; Suppe, J.; Wu, J. E.

2013-12-01

Recent plate-tectonic reconstructions based on mapping of subducted slabs imaged by state-of-the-art tomographic models, and constrained by paleomagnetic data demonstrate that the Philippine Sea Plate (PSP) was originally part of the Sunda Plate (SP). These reconstructions show that the PSP has moved northward with Australia across 25° of latitude since the early Eocene (~ 43 Ma). Most of this motion of the PSP was accommodated on the north and east by overriding a southward subducting East Asian Sea (EAS) ocean basin that was contiguous with the present-day Eurasian Plate (EP). On the western margin of the PSP, this northward advance was accommodated by a N-S transform system. Ages of the Luzon volcanic arc suggest that by early Miocene (~ 15-20 Ma), the EP seafloor west of this transform started subducting eastwards, and highly obliquely, underneath a NNW moving PSP that was detached from the SP. Further, by late Miocene (~10 Ma), northward subduction of the PSP along the present Ryukyu Trench began as a result of arc-continent collision of the PSP along the Eurasian continental margin and flipping of subduction polarity due to slab break-off of the south-subducting EAS. A significant rotation of the PSP-EP convergence to the present more northwesterly direction occurred only over the last ~2 Ma. This present-day juxtaposition of orthogonal subduction polarities beneath Taiwan can be understood in terms of a margin-parallel lithospheric STEP fault, that accomplishes the progressive SW extension of the Ryukyu Trench (RT), and also marks the northern limit of the EP subduction. The torn edge of the Eurasian lithosphere is imaged tomographically. Further support for this tearing comes from our newly developed multi-resolution stress maps based on focal-mechanism inversions and the seismicity distribution. Our inferred stress orientations indicate orthogonal contact between the subducting PSP and the Eurasian lithospheres, resulting in present-day E-W strike-parallel compression and horizontal flexure in the PSP above 100 km depth. Here, we present first-order 2.5D/3D lithospheric scale models of the Taiwan orogen resulting from the progressive deformation of the Eurasian margin and based on the above plate motion history. These models are also constrained by large-scale geologic and slab structure as well as 3D geophysical data: focal-mechanism based stress orientations and geodetic strain-rates. We use a particle-tracer based 3D Lagrangian-Eulerian code, SULEC, that can model the evolution of finite plastic and viscoelastic deformation. Our hierarchical modeling approach involves first using intuition building 2D models having simplified versions of the above spatio-temporal constraints, before considering more complex 3D setups. For simplicity, we start our models from the time of initiation of PSP subduction along the RT (~ 10 Ma), and pre-existing slabs in the upper-mantle. Our models address: (a) the timing of subduction flipping from southwards to northwards at the Ruykyu Trench; (b) the tearing of the EP lithosphere as a STEP fault; (c) the mechanism(s) by which the subducting PSP 'slid' under the EP continental margin as far north as Shanghai; and (d) the role of pre-existing subducting slabs along the PSP's western and eastern edges on the recent sudden change to northwesterly convergence.

Fast Episodes of West-Mediterranean-Tyrrhenian Oceanic Opening and Revisited Relations with Tectonic Setting

PubMed Central

Savelli, Carlo

2015-01-01

Extension and calc-alkaline volcanism of the submerged orogen of alpine age (OAA) initiated in Early Oligocene (~33/32 Ma) and reached the stage of oceanic opening in Early-Miocene (Burdigalian), Late-Miocene and Late-Pliocene. In the Burdigalian (~20–16 Ma) period of widespread volcanism of calcalkaline type on the margins of oceanic domain, seafloor spreading originated the deep basins of north Algeria (western part of OAA) and Sardinia/Provence (European margin). Conversely, when conjugate margins’ volcanism has been absent or scarce seafloor spreading formed the plains Vavilov (7.5–6.3 Ma) and Marsili (1.87–1.67 Ma) within OAA eastern part (Tyrrhenian Sea). The contrast between occurrence and lack of margin’s igneous activity probably implies the diversity of the geotectonic setting at the times of oceanization. It appears that the Burdigalian calcalkaline volcanism on the continental margins developed in the absence of subduction. The WNW-directed subduction of African plate probably commenced at ~16/15 Ma (waning Burdigalian seafloor spreading) after ~18/16 Ma of rifting. Space-time features indicate that calcalkaline volcanism is not linked only to subduction. From this view, temporal gap would exist between the steep subduction beneath the Apennines and the previous, flat-type plunge of European plate with opposite direction producing the OAA accretion and double vergence. PMID:26391973

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bird, P.R.; Johns, C.C.; Clark-Lowes, D.D.

Western Turkey consists of a number of tectonic terranes joined together by a network of suture zones. The terranes originated as microcontinental plates that rifted away from the continental margins forming the northern and southern boundaries of the Tethyan sea. These micro-continents were united by a series of collisions beginning in the Late Triassic and ending in the Miocene, with the final closure of the Tethyan sea. The sedimentary cover of the microcontinents consists of Paleozoic and Mesozoic passive margin and rift basin sequences containing numerous potential source and reservoir intervals. Most of these sequences show affinities with Gondwanaland, withmore » the notable exception of the Istanbul nappe, which is strongly Laurasian in character. Forearc basin sequences were also deposited on the margins of the microcontinents during early Tertiary plate convergence. Ensuing continental collisions resulted in compressional deformation of sedimentary cover sequences. The intensity of deformation ranged from basin inversion producing numerous potential hydrocarbon traps, to large-scale overthrusting. Following continental suturing, continued compression in eastern Turkey has been accommodated since the Miocene by westward escape of continental lithosphere between the North and South Anatolian transform faults. Neotectonic pull-apart basins formed in response to these movements, accumulating large thicknesses of Miocene-Pliocene carbonates and clastic sediments. Potential reservoirs in the Neotectonic basins may be sourced either in situ or from underlying Paleozoic and Mesozoic source rocks that remain within the hydrocarbon generating window today.« less

Early Neogene unroofing of the Sierra Nevada de Santa Marta along the Bucaramanga -Santa Marta Fault

NASA Astrophysics Data System (ADS)

Piraquive Bermúdez, Alejandro; Pinzón, Edna; Bernet, Matthias; Kammer, Andreas; Von Quadt, Albrecht; Sarmiento, Gustavo

2016-04-01

Plate interaction between Caribbean and Nazca plates with Southamerica gave rise to an intricate pattern of tectonic blocks in the Northandean realm. Among these microblocks the Sierra Nevada de Santa Marta (SNSM) represents a fault-bounded triangular massif composed of a representative crustal section of the Northandean margin, in which a Precambrian to Late Paleozoic metamorphic belt is overlain by a Triassic to Jurassic magmatic arc and collateral volcanic suites. Its western border fault belongs to the composite Bucaramanga - Santa Marta fault with a combined left lateral-normal displacement. SE of Santa Marta it exposes remnants of an Oligocene marginal basin, which attests to a first Cenoizoic activation of this crustal-scale lineament. The basin fill consists of a sequence of coarse-grained cobble-pebble conglomerates > 1000 m thick that unconformably overlay the Triassic-Jurassic magmatic arc. Its lower sequence is composed of interbedded siltstones; topwards the sequence becomes dominated by coarser fractions. These sedimentary sequences yields valuable information about exhumation and coeval sedimentation processes that affected the massif's western border since the Upper Eocene. In order to analyse uplifting processes associated with tectonics during early Neogene we performed detrital zircon U-Pb geochronology, detrital thermochronology of zircon and apatites coupled with the description of a stratigraphic section and its facies composition. We compared samples from the Aracataca basin with analog sequences found at an equivalent basin at the Oca Fault at the northern margin of the SNSM. Our results show that sediments of both basins were sourced from Precambrian gneisses, along with Mesozoic acid to intermediate plutons; sedimentation started in the Upper Eocene-Oligocene according to palynomorphs, subsequently in the Upper Oligocene a completion of Jurassic to Cretaceous sources was followed by an increase of Precambrian input that became the dominant source for sediments, this shift in provenance is related to an increase in exhumation and erosion rates. The instauration of such a highly erosive regime since the Upper Oligocene attests how the Santa Marta massif was subject to uplifting and erosion, our data shows how in the Upper Oligocene an exhaustion of Cretaceous to Permian sources was followed by an increase in Neo-Proterozoic to Meso-Proterozoic input that is related to the unroofing of the basement rocks, this accelerated exhumation is directly related to the reactivation of the Orihueca Fault as a NW verging thrust at the interior of the massif coeval with Bucaramanga-Santa Marta Fault trans-tensional tectonics in response to the fragmentation of the Farallon plate into the Nazca an Cocos Plates.

Remagnetization and northward coastwise transport of Franciscan Complex Rocks, northern California: A reinterpretation of the paleomagnetic data

NASA Astrophysics Data System (ADS)

Hagstrum, Jonathan T.

1990-10-01

Previous paleomagnetic studies of accreted oceanic rocks within the Franciscan Complex of northern California have concluded that these rocks originated far to the south of their present positions with respect to the North American continent. Based on positive "fold" tests, the characteristic remanent magnetizations were inferred to predate accretion-related deformation and metamorphism, and to have been acquired during or soon after deposition of these rocks. Thus, the paleomagnetic data were thought to provide direct information on ancient oceanic plate motions. However, the plate motions implied by some of these paleomagnetic data are problematic (e.g., exceptional plate velocities), and uniform-polarity magnetizations in almost all of these rocks indicate the possibility of remagnetization. Recent work on oceanic rocks in similar subduction complexes of Japan and Mexico have shown that they were most likely chemically remagnetized during accretion prior to disruption of the original stratigraphic sequences. Modern analogs indicate that the oceanic rocks in Mexico were probably remagnetized while still part of a shallow-dipping subducting slab (<10°) at the base of an accretionary prism. Assuming these rocks were near horizontal at the time of remagnetization, paleolatitudes at which these rocks were subducted and subsequent arc-parallel displacements along the western margin of North America can be inferred. In this paper, Franciscan rocks in northern California are reinterpreted as also having been remagnetized prior to accretion-related deformation. This scenario satisfies both geologic and paleomagnetic constraints for these rocks, and resolves conflicts between data indicating both remagnetization and tectonic displacement. Transport of the Laytonville Limestone from the southern hemisphere is not required. Paleolatitudes of subduction and remagnetization in the northern hemisphere (12° to 33°) appear to be inversely proportional to age of accretion (middle Cretaceous to Oligocene) for the Franciscan rocks. Subsequent northward diplacements (800 to 3700 km) and clockwise rotations (56° and 154°) of these rocks inferred from the paleomagnetic data are consistent with potential displacements along the western margin of North America during late Mesozoic and Cenozoic time calculated using examples of modern subduction zones and current plate reconstruction models.

Active Tectonics of the Far North Pacific Observed with GPS

NASA Astrophysics Data System (ADS)

Elliott, J.; Freymueller, J. T.; Jiang, Y.; Leonard, L. J.; Hyndman, R. D.; Mazzotti, S.

2017-12-01

The idea that the tectonics of the northeastern Pacific is defined by relatively discrete deformation along the boundary between the Pacific and North American plates has given way to a more complex picture of broad plate boundary zones and distributed deformation. This is due in large part to the Plate Boundary Observatory and several focused GPS studies, which have greatly increased the density of high-quality GPS data throughout the region. We will present an updated GPS velocity field in a consistent reference frame as well as a new, integrated block model that sheds light on regional tectonics and provides improved estimates of motion along faults and their potential seismic hazard. Crustal motions in southern Alaska are strongly influenced by the collision and flat-slab subduction of the Yakutat block along the central Gulf of Alaska margin. In the area nearest to the collisional front, small blocks showing evidence of internal deformation are required. East of the front, block motions show clockwise rotation into the Canadian Cordillera while west of the front there are counterclockwise rotations that extend along the Alaska forearc, suggesting crustal extrusion. Farther from the convergent margin, the crust appears to move as rigid blocks, with uniform motion over large areas. In western Alaska, block motions show a southwesterly rotation into the Bering Sea. Arctic Alaska displays southeasterly motions that gradually transition into easterly motion in Canada. Much of the southeastern Alaska panhandle and coastal British Columbia exhibit northwesterly motions. Although the relative plate motions are mainly accommodated along major faults systems, including the Fairweather-Queen Charlotte transform system, the St. Elias fold-and-thrust belt, the Denali-Totschunda system, and the Alaska-Aleutian subduction zone, a number of other faults accommodate lesser but still significant amounts of motion in the model. These faults include the eastern Denali/Duke River system, the Castle Mountain fault, the western Denali fault, the Kaltag fault, and the Kobuk fault. Based on the expanded GPS data set, locked or partially locked sections of the Alaska subduction zone may extend as far north and east as the eastern Alaska Range.

Deep structure of the western part of the Central Caucasus from geophysical data

NASA Astrophysics Data System (ADS)

Shempelev, A. G.; Zaalishvili, V. B.; Kukhmazov, S. U.

2017-09-01

The paper presents new data on seismotectonic studies along the Adygei profile in the western part of the Central Caucasus and provides an overview of deep geophysical studies of the Greater Caucasus. For the first time, comprehensive geophysical characteristics of a crustal section of the Greater Caucasus across an orogenic structure (along the Adygei profile) have been obtained with a uniform step of observations. Based on factual data obtained by such methods as converted waves from distant earthquakes, magnetotelluric sounding, and gravimagnetic surveys, sinking of the marginal part of the southern microplate into the mantle is verified. It is noted that the contemporary Alpine structure of the Greater Caucasus formed during gentle thrusting of the Earth's crust (Scythian Plate) from the north on the consolidated crust of the southern microplate.

Age of Izu-Bonin-Mariana arc basement

NASA Astrophysics Data System (ADS)

Ishizuka, Osamu; Hickey-Vargas, Rosemary; Arculus, Richard J.; Yogodzinski, Gene M.; Savov, Ivan P.; Kusano, Yuki; McCarthy, Anders; Brandl, Philipp A.; Sudo, Masafumi

2018-01-01

Documenting the early tectonic and magmatic evolution of the Izu-Bonin-Mariana (IBM) arc system in the Western Pacific is critical for understanding the process and cause of subduction initiation along the current convergent margin between the Pacific and Philippine Sea plates. Forearc igneous sections provide firm evidence for seafloor spreading at the time of subduction initiation (52 Ma) and production of "forearc basalt". Ocean floor drilling (International Ocean Discovery Program Expedition 351) recovered basement-forming, low-Ti tholeiitic basalt crust formed shortly after subduction initiation but distal from the convergent margin (nominally reararc) of the future IBM arc (Amami Sankaku Basin: ASB). Radiometric dating of this basement gives an age range (49.3-46.8 Ma with a weighted average of 48.7 Ma) that overlaps that of basalt in the present-day IBM forearc, but up to 3.3 m.y. younger than the onset of forearc basalt activity. Similarity in age range and geochemical character between the reararc and forearc basalts implies that the ocean crust newly formed by seafloor spreading during subduction initiation extends from fore- to reararc of the present-day IBM arc. Given the age difference between the oldest forearc basalt and the ASB crust, asymmetric spreading caused by ridge migration might have taken place. This scenario for the formation of the ASB implies that the Mesozoic remnant arc terrane of the Daito Ridges comprised the overriding plate at subduction initiation. The juxtaposition of a relatively buoyant remnant arc terrane adjacent to an oceanic plate was more favourable for subduction initiation than would have been the case if both downgoing and overriding plates had been oceanic.

LITHOPROBE East onshore-offshore seismic refraction survey -constraints on interpretation of reflection data in the Newfoundland Appalachians

USGS Publications Warehouse

Marillier, F.; Hall, J.; Hughes, S.; Louden, K.; Reid, I.; Roberts, B.; Clowes, R.; Cote, T.; Fowler, J.; Guest, S.; Lu, H.; Luetgert, J.; Quinlan, G.; Spencer, C.; Wright, J.

1994-01-01

Combined onshore-offshore seismic refraction/ wide-angle reflection data have been acquired across Newfoundland, eastern Canada, to investigate the structural architecture of the northern Appalachians, particularly of distinct crustal zones recognized from earlier LITHOPROBE vertical incidence studies. A western crustal unit, correlated with the Grenville province of the Laurentian plate margin thins from 44 to 40 km and a portion of the lower crust becomes highly reflective with velocities of 7.2 km/s. In central Newfoundland, beneath the central mobile belt, the crust thins to 35 km or less and is marked by average continental velocities, not exceeding 7.0 km/s in the lower crust. Further east, in a crustal unit underlying the Avalon zone and associated with the Gondwanan plate margin, the crust is 40 km thick, and has velocities of 6.8 km/s in the lower crust. Explanations for the thin crust beneath the central mobile belt include (1) post-orogenic isostatic readjustment associated with a density in the mantle which is lower beneath this part of the orogen than beneath the margin, (2) mechanical thinning at the base of the crust during orogenic collapse perhaps caused by delamination, and (3) transformation by phase change of a gabbroic lower crust to eclogite which seismologically would be difficult to distinguish from mantle. Except for a single profile in western Newfoundland, velocities in the crust are of typical continental affinity with lower-crustal velocities less than 7.0 km/s. This indicates that there was no significant magmatic underplating under the Newfoundland Appalachians during Mesozoic rifting of the Atlantic Ocean as proposed elsewhere for the New England Appalachians. A mid-crustal velocity discontinuity observed in the Newfoundland region does not coincide with any consistent reflection pattern on vertical incidence profiles. However, we suggest that localized velocity heterogeneities at mid-crustal depths correspond to organized vertical incidence reflections. ?? 1994.

Architecture of the Distal Piedmont-Ligurian Rifted Margin in NW Italy: Hints for a Flip of the Rift System Polarity

NASA Astrophysics Data System (ADS)

Decarlis, Alessandro; Beltrando, Marco; Manatschal, Gianreto; Ferrando, Simona; Carosi, Rodolfo

2017-11-01

The Alpine Tethys rifted margins were generated by a Mesozoic polyphase magma-poor rifting leading to the opening of the Piedmont-Ligurian "Ocean." This latter developed through different phases of rifting that terminated with the exhumation of subcontinental mantle along an extensional detachment system. At the onset of simple shear detachment faulting, two margin types were generated: an upper and a lower plate corresponding to the hanging wall and footwall of the final detachment system, respectively. The two margin architectures were markedly different and characterized by a specific asymmetry. In this study the detailed analysis of the Adriatic margin, exposed in the Serie dei Laghi, Ivrea-Verbano, and Canavese Zone, enabled to recognize the diagnostic elements of an upper plate rifted margin. This thesis contrasts with the classic interpretation of the Southalpine units, previously compared with the adjacent fossil margin preserved in the Austroalpine nappes and considered as part of a lower plate. The proposed scenario suggests the segmentation and flip of the Alpine rifting system along strike and the passage from a lower to an upper plate. Following this interpretation, the European and Southern Adria margins are coevally developed upper plate margins, respectively resting NE and SW of a major transform zone that accommodates a flip in the polarity of the rift system. This new explanation has important implications for the study of the pre-Alpine rift-related structures, for the comprehension of their role during the reactivation of the margin and for the paleogeographic evolution of the Alpine orogen.

The tectonographic development of Patagonia and its relevance to hydrocarbon exploration

DOE Office of Scientific and Technical Information (OSTI.GOV)

Light, M.P.R.; Urien, C.M.; Maslanyj, M.P.

1993-02-01

Patagonia accreted successively from the southwest onto the southern margin of the Proterozoic Plata Craton and Brazilian Guapore Shield between the Late Proterozoic and Early Devonian. The thrust-like stacking of terranes onto the southern termination of the Pelotas Terrane is considered to have developed a pervasive northwest to north-trending fabric. During the Permo-Triassic the northwest to north-trending fabric of the Patagonian Plate was re-activated by dextral strike-slip movement causing extension. The deformation was caused by oblique subduction and accretion of the madre Dos Dios to Pichidangui Terranes along its western margin. To the northeast the more competent shield underwent compressionmore » (Ventania-Gond-wanide Folding) and extension occurred parallel to the axis of the embryo South Atlantic, where a shallow sea transgressed. Ridge on its western side, now preserved on the facing shelf margins of Argentina and Namibia. In the Late Triassic-Lower Jurassic, the Malvinas and Microplate was situated south of the Transkei (South Africa) and an intracratonic basin separated it from two sutures formed at the margin of the Argentine Shelf and along the axis of the West Malvinas Basin. Subduction/arc activity on the west flank of this intracratonic basin, in association with trench pull is believed to have initiated Late Triassic-Early Jurassic strike slip extension and volcanicity in Patagonia. This exploited the pervasive northwest and north-trending Paleozoic fabric. By the Mid-Jurassic the Malvinas Microplate had docked with the eastern margin of the Patagonian Shelf and was undergoing clockwise rotation between the Malvinas-Agulhas and Burwood Bank-Scotia Ridge dextral strike-slip systems. Rifting had now progressed southwestwards to the Pacific and north eastwards to the Colorado and Outeniqua Basins.« less

A revised subduction inception model to explain the Late Cretaceous, doubly vergent orogen in the pre-collisional western Tethys: evidences from the Northern Apennine

NASA Astrophysics Data System (ADS)

Meneghini, Francesca; Marroni, Michele; Pandolfi, Luca

2017-04-01

Orogenic processes are widely demonstrated to be strongly controlled by inherited structures. The paleogeography of the converging margins, and the tectonic processes responsible for their configuration, will influence the location of subduction initiation, the distribution of deformation between upper and lower plate, the shape of the accretionary prism and of the subsequent orogeny, through controlling the development of single or doubly-vergent orogens, and, as a corollary, the modality of exhumation of metamorphosed units. The "alpine age" collisional belts of the Mediterranean area are characterized by tangled architectures derived from the overlapping of several deformation events related to a multiphase, long history that comprises not only the collision of continental margins, but that can be regarded as an heritage of both the rifting-related configuration of the continental margins, and the subduction-related structures. The Northern Apennines is a segment of these collisional belts that originated by the Late Cretaceous-Middle Eocene closure of the northern branch of the western Tethys, and the subsequent Late Eocene-Early Oligocene continental collision between the Europe and Adria plates. Due to a different configuration of the paired Adria and Europe continental margins, inherited from a rifting phase dominated by asymmetric, simple-shear kinematics, the Northern Apennines expose a complex groups of units, referred to as Ligurian Units, that record the incorporation into the subduction factory of either fragments of the Ligure-Piemontese oceanic domain (i.e. Internal Ligurian Units), and various portions of the thinned Adria margin (i.e. External Ligurian Units), describable as an Ocean-Continent Transition Zone (OCTZ). The structural relationships between these groups of Units are crucial for the definition of the pre-collisional evolution of the belt and have been the subject of big debates in the literature, together with the location and orientation of subduction initiation. We have reviewed the ages and characteristics of the tectono-metamorphic events recorded in both the External and Internal Ligurian Units. Deformation and metamorphism in the External Ligurian Units pre-dates the subduction-related metamorphism recorded in the ocean-derived Internal Ligurian Units. We thus propose that closure of the Ligure-Piemontese branch of the western Tethys occurred through a subduction that nucleated inside the OCTZ of Adria, instead of localizing at the boundary between the oceanic basin and the Adria margin, and developed a doubly-vergent prism fed firstly by both continental extensional allochthons and ocean-derived rocks from the OCTZ, and only after by rocks and sediments from the oceanic realm. We believe that this revised location of the inception of subduction, and the subsequent pre-collisional architecture, considered as inherited from the rifting and the oceanic opening phases, allow reconciling most of the controversies on the geodynamic evolution of the Apenninic orogeny, prior to collision.

Deformation during terrane accretion in the Saint Elias orogen, Alaska

USGS Publications Warehouse

Bruhn, R.L.; Pavlis, T.L.; Plafker, G.; Serpa, L.

2004-01-01

The Saint Elias orogen of southern Alaska and adjacent Canada is a complex belt of mountains formed by collision and accretion of the Yakutat terrane into the transition zone from transform faulting to subduction in the northeast Pacific. The orogen is an active analog for tectonic processes that formed much of the North American Cordillera, and is also an important site to study (1) the relationships between climate and tectonics, and (2) structures that generate large- to great-magnitude earthquakes. The Yakutat terrane is a fragment of the North American plate margin that is partly subducted beneath and partly accreted to the continental margin of southern Alaska. Interaction between the Yakutat terrane and the North American and Pacific plates causes significant differences in the style of deformation within the terrane. Deformation in the eastern part of the terrane is caused by strike-slip faulting along the Fairweather transform fault and by reverse faulting beneath the coastal mountains, but there is little deformation immediately offshore. The central part of the orogen is marked by thrusting of the Yakutat terrane beneath the North American plate along the Chugach-Saint Elias fault and development of a wide, thin-skinned fold-and-thrust belt. Strike-slip faulting in this segment may he localized in the hanging wall of the Chugach-Saint Elias fault, or dissipated by thrust faulting beneath a north-northeast-trending belt of active deformation that cuts obliquely across the eastern end of the fold-and-thrust belt. Superimposed folds with complex shapes and plunging hinge lines accommodate horizontal shortening and extension in the western part of the orogen, where the sedimentary cover of the Yakutat terrane is accreted into the upper plate of the Aleutian subduction zone. These three structural segments are separated by transverse tectonic boundaries that cut across the Yakutat terrane and also coincide with the courses of piedmont glaciers that flow from the topographic backbone of the Saint Elias Mountains onto the coastal plain. The Malaspina fault-Pamplona structural zone separates the eastern and central parts of the orogen and is marked by reverse faulting and folding. Onshore, most of this boundary is buried beneath the western or "Agassiz" lobe of the Malaspina piedmont glacier. The boundary between the central fold-and-thrust belt and western zone of superimposed folding lies beneath the middle and lower course of the Bering piedmont glacier. ?? 2004 Geological Society of America.

Linking Observations of Dynamic Topography from Oceanic and Continental Realms around Australia

NASA Astrophysics Data System (ADS)

Czarnota, K.; Hoggard, M. J.; White, N.; Winterbourne, J.

2012-04-01

In the last decade, there has been growing interest in predicting the spatial and temporal evolution of dynamic topography (i.e. the surface manifestation of mantle convection). By directly measuring Neogene and Quaternary dynamic topography around Australia's passive margins we assess the veracity of these predictions and the interplay between mantle convection and plate motion. We mapped the present dynamic topography by carefully measuring residual topography of oceanic lithosphere adjacent to passive margins. This map provides a reference with respect to which the relative record of vertical motions, preserved within the stratigraphic architecture of the margins, can be interpreted. We carefully constrained the temporal record of vertical motions along Australia's Northwest Shelf by backstripping Neogene carbonate clinoform rollover trajectories in order to minimise paleobathymetric errors. Elsewhere, we compile temporal constraints from published literature. Three principal insights emerge from our analysis. First, the present-day drawn-down residual topography of Australia, cannot be approximated by a regional tilt down towards the northeast, as previously hypothesised. The south-western and south-eastern corners of Australia are at negligible to slightly positive residual topography which slopes down towards Australia's northern margin and the Great Australian Bight. Secondly, the record of passive margin subsidence suggests drawdown across northern Australia commenced synchronously at 8±2 Ma. The amplitude of this synchronous drawdown corresponds to the amplitude of oceanic residual topography, indicating northern Australia was at an unperturbed dynamic elevation until drawdown commenced. The synchronicity of this subsidence suggests that the Australian plate has not been affected by a southward propagating wave of drawdown, despite Australia's rapid northward motion towards the subduction realm in south-east Asia. In contrast, it appears the mantle anomaly responsible for this drawdown is a relatively young, long-wavelength feature. Thirdly, there is an apparent mismatch between the current drawdown of oceanic lithosphere observed along Australia's southern margin and the onshore record of Cenozoic uplift. This disparity we attribute to the region undergoing recent uplift from a position of dynamic drawdown.

Subduction zone and crustal dynamics of western Washington; a tectonic model for earthquake hazards evaluation

USGS Publications Warehouse

Stanley, Dal; Villaseñor, Antonio; Benz, Harley

1999-01-01

The Cascadia subduction zone is extremely complex in the western Washington region, involving local deformation of the subducting Juan de Fuca plate and complicated block structures in the crust. It has been postulated that the Cascadia subduction zone could be the source for a large thrust earthquake, possibly as large as M9.0. Large intraplate earthquakes from within the subducting Juan de Fuca plate beneath the Puget Sound region have accounted for most of the energy release in this century and future such large earthquakes are expected. Added to these possible hazards is clear evidence for strong crustal deformation events in the Puget Sound region near faults such as the Seattle fault, which passes through the southern Seattle metropolitan area. In order to understand the nature of these individual earthquake sources and their possible interrelationship, we have conducted an extensive seismotectonic study of the region. We have employed P-wave velocity models developed using local earthquake tomography as a key tool in this research. Other information utilized includes geological, paleoseismic, gravity, magnetic, magnetotelluric, deformation, seismicity, focal mechanism and geodetic data. Neotectonic concepts were tested and augmented through use of anelastic (creep) deformation models based on thin-plate, finite-element techniques developed by Peter Bird, UCLA. These programs model anelastic strain rate, stress, and velocity fields for given rheological parameters, variable crust and lithosphere thicknesses, heat flow, and elevation. Known faults in western Washington and the main Cascadia subduction thrust were incorporated in the modeling process. Significant results from the velocity models include delineation of a previously studied arch in the subducting Juan de Fuca plate. The axis of the arch is oriented in the direction of current subduction and asymmetrically deformed due to the effects of a northern buttress mapped in the velocity models. This buttress occurs under the North Cascades region of Washington and under southern Vancouver Island. We find that regional faults zones such as the Devils Mt. and Darrington zones follow the margin of this buttress and the Olympic-Wallowa lineament forms its southern boundary east of the Puget Lowland. Thick, high-velocity, lower-crustal rocks are interpreted to be a mafic/ultramafic wedge occuring just above the subduction thrust. This mafic wedge appears to be jointly deformed with the arch, suggesting strong coupling between the subducting plate and upper plate crust in the Puget Sound region at depths >30 km. Such tectonic coupling is possible if brittle-ductile transition temperatures for mafic/ultramafic rocks on both sides of the thrust are assumed. The deformation models show that dominant north-south compression in the coast ranges of Washington and Oregon is controlled by a highly mafic crust and low heat flow, allowing efficient transmission of margin-parallel shear from Pacific plate interaction with North America. Complex stress patterns which curve around the Puget Sound region require a concentration of northwest-directed shear in the North Cascades of Washington. The preferred model shows that greatest horizontal shortening occurs across the Devils Mt. fault zone and the east end of the Seattle fault.

Anomalous Late Jurassic motion of the Pacific Plate with implications for true polar wander

NASA Astrophysics Data System (ADS)

Fu, R. R.; Kent, D.

2017-12-01

True polar wander, or TPW, is the rotation of the entire mantle-crust system that results in simultaneous change in latitude and orientation for all lithospheric plates. One of the most recent candidate TPW events consists of a 30˚ rotation during Late Jurassic time (160 - 145 Ma). However, existing paleomagnetic documentation of this event derives exclusively from continental studies. Because all major landmasses except China were connected directly or via spreading centers in the Late Jurassic, the velocities of these continents were mutually constrained and their motion as a group over the underlying mantle would be indistinguishable from TPW using only continental data. On the other hand, plates of the Pacific Basin constituted a kinematically independent domain, interfacing with continents at subduction zones and slip-strike boundaries. Coherent motion of both Pacific Basin and continental plates would therefore indicate uniform motion of virtually the entire lithosphere, providing a means to distinguish TPW from continental drift. We performed thermal demagnetization on remaining samples from Ocean Drilling Program (ODP) Site 801B, which were cored from the oldest sampled oceanic crust in the Western Pacific, to determine its change in paleolatitude during the Late Jurassic and Early Cretaceous (167 - 134 Ma). We find that the Pacific Plate likely underwent a steady southward drift during this time period, consistent with previous results from magnetic anomalies, except for an episode of northward motion between Oxfordian and Tithonian time (161 - 147 Ma). Although the amplitude of this northward shift is subject to significant uncertainty due to the sparse recovery of core samples, the trajectory of the Pacific Plate is most simply explained by TPW in the 160 - 145 Ma interval as inferred from continental data. Furthermore, such an interpretation is consistent with the sense of shear inferred at the Farallon-North American Plate boundary, whereas uniform motion of the Pacific Plate without TPW contradicts inferred relative motions. The Late Jurassic motion of the Pacific Plate therefore provides support for the occurrence of TPW. Candidate drivers for such an event include subducting slabs at the western margin of North America and the Mongol-Okhotsk Ocean and mantle plumes associated with the Paraná LIP.

Preliminary assessment of a Cretaceous-Paleogene Atlantic passive margin, Serrania del Interior and Central Ranges, Venezuela/Trinidad

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pindell, J.L.; Drake, C.L.; Pitman, W.C.

1991-03-01

For several decades, Cretaceous arc collision was assumed along northern Venezuela based on isotopic ages of metamorphic minerals. From subsidence histories in Venezuelan/Trinidadian basins, however, it is now clear that the Cretaceous metamorphic rocks were emplaced southeastward as allochthons above an autochthonous suite of rocks in the Cenozoic, and that the pre-Cenozoic autochthonous rocks represent a Mesozoic passive margin. The passive margin rocks have been metamorphosed separately during overthrusting by the allochthons in central Venezuela, but they are uplifted but not significantly metamorphosed in Eastern Venezuela and Trinidad. There, in the Serrania del Interior and Central Ranges of Venezuela/Trinidad, Mesozoic-Paleogenemore » passive margin sequences were uplifted in Neogene time, when the Caribbean Plate arrived from the west and transpressionally inverted the passive margin. Thus, this portion of South America's Atlantic margin subsided thermally without tectonism from Jurassic to Eocene time, and these sections comprise the only Mesozoic-Cenozoic truly passive Atlantic margin in the Western Hemisphere that is now exposed for direct study. Direct assessments of sedimentological, depositional and faunal features indicative of, and changes in, water depth for Cretaceous and Paleogene time may be made here relative to a thermally subsiding passive margin without the complications of tectonism. Work is underway, and preliminary assessments presented here suggest that sea level changes of Cretaceous-Paleogene time are not as pronounced as the frequent large and rapid sea level falls and rises that are promoted by some.« less

Block versus continuum deformation in the Western United States

USGS Publications Warehouse

King, G.; Oppenheimer, D.; Amelung, F.

1994-01-01

The relative role of block versus continuum deformation of continental lithosphere is a current subject of debate. Continuous deformation is suggested by distributed seismicity at continental plate margins and by cumulative seismic moment sums which yield slip estimates that are less than estimates from plate motion studies. In contrast, block models are favored by geologic studies of displacement in places like Asia. A problem in this debate is a lack of data from which unequivocal conclusions may be reached. In this paper we apply the techniques of study used in regions such as the Alpine-Himalayan belt to an area with a wealth of instrumental data-the Western United States. By comparing plate rates to seismic moment release rates and assuming a typical seismogenic layer thickness of 15 km it appears that since 1850 about 60% of the Pacific-North America motion across the plate boundary in California and Nevada has occurred seismically and 40% aseismically. The San Francisco Bay area shows similar partitioning between seismic and aseismic deformation, and it can be shown that within the seismogenic depth range aseismic deformation is concentrated near the surface and at depth. In some cases this deformation can be located on creeping surface faults, but elsewhere it is spread over a several kilometer wide zone adjacent to the fault. These superficial creeping deformation zones may be responsible for the palaeomagnetic rotations that have been ascribed elsewhere to the surface expression of continuum deformation in the lithosphere. Our results support the dominant role of non-continuum deformation processes with the implication that deformation localization by strain softening must occur in the lower crust and probably the upper mantle. Our conclusions apply only to the regions where the data are good, and even within the Western United States (i.e., the Basin and Range) deformation styles remain poorly resolved. Nonetheless, we maintain that block motion is the deformation style of choice for those continental regions where the data are best. ?? 1994.

Seismicity of the Earth 1900–2010 Himalaya and vicinity

USGS Publications Warehouse

Turner, Bethan; Jenkins, Jennifer; Turner, Rebecca; Parker, Amy; Sinclair, Alison; Davies, Sian; Hayes, Gavin P.; Villaseñor, Antonio; Dart, Rirchard L.; Tarr, Arthur C.; Furlong, Kevin P.; Benz, Harley M.

2013-01-01

Seismicity in the Himalaya region predominantly results from the collision of the India and Eurasia continental plates, which are converging at a relative rate of 40–50 mm/yr. Northward underthrusting of India beneath Eurasia generates numerous earthquakes and consequently makes this area one of the most seismically hazardous regions on Earth. The surface expression of the plate boundary is marked by the foothills of the north-south trending Sulaiman Range in the west, the Indo-Burmese Arc in the east, and the east-west trending Himalaya Front in the north of India. Along the western margin of the India plate, relative motions between India and Eurasia are accommodated by strike-slip, reverse, and oblique-slip faulting resulting in the complex Sulaiman Range fold and thrust belt, and the major translational Chaman Fault in Afghanistan. Beneath the Pamir‒Hindu Kush Mountains of northern Afghanistan, earthquakes occur to depths as great as 200 km as a result of remnant lithospheric subduction. Further north again, the Tian Shan is a seismically active intra-continental mountain belt defined by a series of east-west trending thrust faults thought to be related to the broad footprint of the India-Eurasia collision. Tectonics in northern India are dominated by motion along the Main Frontal Thrust and associated thrust faults of the India-Eurasia plate boundary, which have resulted in a series of large and devastating earthquakes in (and prior to) the 20th century. The Tibetan Plateau to the north of the main plate boundary is a broad region of uplift associated with the India-Eurasia collision, and is cut by a series of generally east-west trending strike-slip faults. These include the Kunlun, Haiyuan, and the Altyn Tagh faults, all of which are left-lateral structures, and the Kara-Koram right-lateral fault. Throughout the plateau, thrust faults accommodate the north-south compressional component of crustal shortening associated with the ongoing collision of India and Eurasia, while strike-slip and normal faults accommodate east-west extension. To the east, The Longmen Shan thrust belt marks the eastern margin of the Tibetan Plateau separating the complex tectonics of the plateau region from the relatively undeformed Sichuan Basin. Further south, the left-lateral Xiangshuihe-Xiaojiiang, right-lateral Red River and right-lateral Sagaing strike-slip fault systems accommodate deformation along the eastern margin of the India plate. Deep earthquakes have also occurred in the Indo-Burmese Arc region, thought to be an expression of eastward-directed subduction of the India plate, though whether subduction is ongoing is still debated.

New age constraints on the palaeoenvironmental evolution of the late Paleozoic back-arc basin along the western Gondwana margin of southern Peru

NASA Astrophysics Data System (ADS)

Boekhout, F.; Reitsma, M. J.; Spikings, R.; Rodriguez, R.; Ulianov, A.; Gerdes, A.; Schaltegger, U.

2018-03-01

The tectonic evolution of the western Gondwana margin during Pangaea amalgation is recorded in variations in the Permo-Carboniferous back-arc basin sedimentation of Peru. This study provides the first radiometric age constraints on the volcanic and sedimentary sequences of south-central eastern Peru up to the western-most tip of Bolivia, and now permits the correlation of lateral facies variations to the late Paleozoic pre-Andean orogenic cycle. The two phases of Gondwanide magmatism and metamorphism at c. 315 Ma and c. 260 Ma are reflected in two major changes in this sedimentary environment. Our detrital U-Pb zircon ages demonstrate that the timing of Ambo Formation deposition corroborates the Late Mississipian age estimates. The transition from the Ambo to the Tarma Formation around the Middle Pennsylvanian Early Gondwanide Orogeny (c. 315 Ma) represents a relative deepening of the basin. Throughout the shallow marine deposits of the Tarma Formation evidence for contemporaneous volcanism becomes gradually more pronounced and culminates around 312 - 309 Ma. Continuous basin subsidence resulted in a buildup of platform carbonates of the Copacabana Formation. Our data highlights the presence of a previously unrecognized phase of deposition of mainly fluvial sandstones and localized volcanism (281-270 Ma), which we named ´Oqoruro Formation'. This sedimentary succession was previously miss-assigned to the so-called Mitu Group, which has recently been dated to start deposition in the Middle Triassic (∼245-240 Ma). The emersion of this marine basin coincides with the onset of a major plutonic pulse related to the Late Gondwanide Orogeny (c. 260). Exhumation lead to the consequent retreat of the epeiric sea to the present-day sub-Andean region, and the coeval accumulation of the fluvial Oqoruro Formation in south eastern Peru. These late Paleozoic palaeoenvironmental changes in the back-arc basins along the western Gondwana margin of southern reflect changes in tectonic plate reorganization in a long-lived Paleozoic accretionary orogeny.

Madagascar's Escape from Africa: New Constraints and Understanding for Plate Tectonic Reconstructions

NASA Astrophysics Data System (ADS)

Phethean, J. J. J.; Davies, R. J.; Van Hunen, J.; Kalnins, L. M.; McCaffrey, K. J. W.

2015-12-01

We present a new plate tectonic reconstruction for the drift of Madagascar away from East Africa using the new Sandwell and Smith gravity dataset (V23.1). Detailed interpretation of free-air and Bouguer anomalies, together with gravity gradients, has allowed interpretation of the extinct mid ocean ridge and associated fracture zone lineaments from the Western Somali Basin. Combined with temporal constraints from previous ocean magnetic anomaly interpretations, this analysis produces a reconstruction that supports Reeves' (2014) tight fit of Gondwana fragments. Furthermore, it sheds light on the nature of the Davie Fracture Zone (DFZ) and the position of the continent-ocean boundary (COB) in the region. The model predicts that the COB lies along the Rovuma Basin; and that offshore Tanzania is most likely a segmented and obliquely rifted margin, not a transform continental margin along the DFZ as previously thought. This places the COB up to several hundred kilometres farther inboard than previous interpretations, which is supported by new seismic evidence of oceanic crust inboard of the DFZ. We show the DFZ to be a major ocean-ocean fracture zone formed by the coalescence of several smaller fracture zones during a change in plate motions as Madagascar escaped from Africa. This new geodynamical understanding has important implications for petroleum industry activities in East Africa, as the expected heat flow varies dramatically between oceanic and continental crust. Reeves, C., 2014. The position of Madagascar within Gondwana and its movements during Gondwana dispersal. J. Afr. Earth. Sci. 94, 45-57.

The Oligo-/Miocene Qom Formation (Iran): evidence for an early Burdigalian restriction of the Tethyan Seaway and closure of its Iranian gateways

NASA Astrophysics Data System (ADS)

Reuter, M.; Piller, W. E.; Harzhauser, M.; Mandic, O.; Berning, B.; Rögl, F.; Kroh, A.; Aubry, M.-P.; Wielandt-Schuster, U.; Hamedani, A.

2009-04-01

In the central Iranian Esfahan-Sirjan and Qom basins sedimentation of the Oligo-/Miocene Qom Formation took place on extensive mixed carbonate-siliciclastic ramps. During this time, both basins were positioned at the Eurasian margin of the Tethyan Seaway, which connected the western and eastern regions of the Tethys Ocean at least until the late Burdigalian. During the so-called Terminal Tethyan Event the Tethyan Seaway was then closed due to the collision of the African/Arabian and Iranian/Eurasian plates. Facies analysis of the sedimentary record of both basins indicates paleoenvironments ranging from terrestrial to open marine settings, including mangrove, restricted inner shelf lagoon, seagrass meadow, reefal, and deeper offshore environments. Recognition of eight depositional sequences and elaboration of an integrated biostratigraphic framework (calcareous nannoplankton, planktic and larger benthic foraminifers, gastropods, and pectinids) allow us to construct a basin-spanning stratigraphy. The assignment of the recognized sea-level lowstands to the Ru 3 to Bur 3 lowstands of the global sea-level curve enables a comparison with time-equivalent sections from the Zagros Basin, which was part of the African/Arabian Plate on the opposing southern margin of the Tethyan Seaway. The so calibrated sections display restrictions of the Tethyan Seaway and interruption of the south Iranian gateways between the Qom Basin and the Proto-Indopacific in relation to ongoing plate collision during the early Burdigalian.

Topographic asymmetry of the South Atlantic from global models of mantle flow and lithospheric stretching

NASA Astrophysics Data System (ADS)

Flament, Nicolas; Gurnis, Michael; Williams, Simon; Seton, Maria; Skogseid, Jakob; Heine, Christian; Dietmar Müller, R.

2014-02-01

The relief of the South Atlantic is characterized by elevated passive continental margins along southern Africa and eastern Brazil, and by the bathymetric asymmetry of the southern oceanic basin where the western flank is much deeper than the eastern flank. We investigate the origin of these topographic features in the present and over time since the Jurassic with a model of global mantle flow and lithospheric deformation. The model progressively assimilates plate kinematics, plate boundaries and lithospheric age derived from global tectonic reconstructions with deforming plates, and predicts the evolution of mantle temperature, continental crustal thickness, long-wavelength dynamic topography, and isostatic topography. Mantle viscosity and the kinematics of the opening of the South Atlantic are adjustable parameters in thirteen model cases. Model predictions are compared to observables both for the present-day and in the past. Present-day predictions are compared to topography, mantle tomography, and an estimate of residual topography. Predictions for the past are compared to tectonic subsidence from backstripped borehole data along the South American passive margin, and to dynamic uplift as constrained by thermochronology in southern Africa. Comparison between model predictions and observations suggests that the first-order features of the topography of the South Atlantic are due to long-wavelength dynamic topography, rather than to asthenospheric processes. The uplift of southern Africa is best reproduced with a lower mantle that is at least 40 times more viscous than the upper mantle.

Topographic asymmetry of the South Atlantic from global models of mantle flow and lithospheric stretching

NASA Astrophysics Data System (ADS)

Flament, Nicolas; Gurnis, Michael; Williams, Simon; Seton, Maria; Skogseid, Jakob; Heine, Christian; Müller, Dietmar

2014-05-01

The relief of the South Atlantic is characterized by elevated passive continental margins along southern Africa and eastern Brazil, and by the bathymetric asymmetry of the southern oceanic basin where the western flank is much deeper than the eastern flank. We investigate the origin of these topographic features in the present and over time since the Jurassic with a model of global mantle flow and lithospheric deformation. The model progressively assimilates plate kinematics, plate boundaries and lithospheric age derived from global tectonic reconstructions with deforming plates, and predicts the evolution of mantle temperature, continental crustal thickness, long-wavelength dynamic topography, and isostatic topography. Mantle viscosity and the kinematics of the opening of the South Atlantic are adjustable parameters in multiple model cases. Model predictions are compared to observables both for the present-day and in the past. Present-day predictions are compared to topography, mantle tomography, and an estimate of residual topography. Predictions for the past are compared to tectonic subsidence from backstripped borehole data along the South American passive margin, and to dynamic uplift as constrained by thermochronology in southern Africa. Comparison between model predictions and observations suggests that the first-order features of the topography of the South Atlantic are due to long-wavelength dynamic topography, rather than to asthenospheric processes. We find the uplift of southern Africa to be best reproduced with a lower mantle that is at least 40 times more viscous than the upper mantle.

Detrital zircon geochronology support for Baja-BC hypothesis or Why zircons in the Nanaimo Basin, British Columbia are not from the Rocky Mountains.

NASA Astrophysics Data System (ADS)

Guest, B.; Matthews, W.; Hubbard, S. M.; Coutts, D. S.; Bain, H.

2016-12-01

The development of Cordilleran orogen of western North American is disputed despite a century of study. Paleomagnetic observations require large-scale dextral displacement of crustal fragments along the western margin of North America, from low latitudes to moderate latitudes during the Cretaceous-Paleogene. A lack of corroborating geological evidence for large-scale displacements has prevented the widespread integration of paleomagnetic data into contemporary tectonic models for the margin. Here we investigate the Cretaceous paleogeographic position of the Baja-BC block, a crustal fragment consisting of the Alexander and Wrangel terranes, using detrital zircons from the Nanaimo Basin of Vancouver Island, British Columbia. We compare 4310 detrital zircon U/Pb analyses from 16 samples to potential source areas in western North America to test hypothesized northern and southern paleogeographic positions. Our detrital zircon data suggest that sediment in the Nanaimo Basin derives from the Mojave-Sonoran Region of southwestern North America, supporting a southerly late Cretaceous paleogeographic position. We present a speculative Cretaceous to Paleogene paleogeographic reconstruction for the southwestern United States and northern Mexico that accommodates the presence, and northward transport, of the Baja-BC block. We propose that the Western Coast Mountains Batholith and the Nanaimo Basin represent the missing segment of the Mesozoic magmatic arc and associated forearc regions, between the Sierra Nevada and Peninsular Ranges Batholiths. This segment was translated northward following capture by the Kula plate. As such, we reconcile the paleomagnetic data for the Baja-BC block with the geology of the southwestern United States. Our model, albeit speculative, is compatible with the large-scale tectonic and magmatic processes that affected western North America in the Late Cretaceous and Paleogene.

Petrogenesis and tectonic association of rift-related basic Panjal dykes from the northern Indian plate, North-Western Pakistan: evidence of high-Ti basalts analogous to dykes from Tibet

NASA Astrophysics Data System (ADS)

Sajid, Muhammad; Andersen, Jens; Arif, Mohammad

2017-10-01

Rift related magmatism during Permian time in the northern margin of Indian plate is represented by basic dykes in several Himalayan terranes including north western Pakistan. The field relations, mineralogy and whole rock geochemistry of these basic dykes reveal significant textural, mineralogical and chemical variation between two major types (a) dolerite and (b) amphibolite. Intra-plate tectonic settings for both rock types have been interpreted on the basis of low Zr/Nb ratios (< 10), K/Ba ratios (20-40) and Hf-Ta-Th and FeO-MgO-Al2O3 discrimination diagrams. The compositional zoning in plagioclase and clinopyroxene, variation in olivine compositions and major elements oxide trends indicate a vital role of fractional crystallization in the evolution of dolerites, which also show depletion in rare earth elements (REEs) and other incompatible elements compared to the amphibolites. The equilibrium partial melting models from primitive mantle using Dy/Yb, La/Yb, Sm/Yb and La/Sm ratios show that amphibolite formed by smaller degrees (< 5%) of partial melting than the dolerites (< 10%). The trace elements ratios suggest the origination of dolerites from the subcontinental lithospheric mantle with some crustal contamination. This is consistent with a petrogenetic relationship with Panjal trap magmatism, reported from Kashmir and other parts of north western India. The amphibolites, in contrast, show affinity towards Ocean Island basalts (OIB) with a relatively deep asthenospheric mantle source and minimal crustal contribution and are geochemically similar to the High-Ti mafic dykes of southern Qiangtang, Tibet. These similarities combined with Permian tectonic restoration of Gondwana indicate the coeval origin for both dykes from distinct mantle source during continental rifting related to formation of the Neotethys Ocean.

Petrogenesis and tectonic association of rift-related basic Panjal dykes from the northern Indian plate, North-Western Pakistan: evidence of high-Ti basalts analogous to dykes from Tibet

NASA Astrophysics Data System (ADS)

Sajid, Muhammad; Andersen, Jens; Arif, Mohammad

2018-06-01

Rift related magmatism during Permian time in the northern margin of Indian plate is represented by basic dykes in several Himalayan terranes including north western Pakistan. The field relations, mineralogy and whole rock geochemistry of these basic dykes reveal significant textural, mineralogical and chemical variation between two major types (a) dolerite and (b) amphibolite. Intra-plate tectonic settings for both rock types have been interpreted on the basis of low Zr/Nb ratios (< 10), K/Ba ratios (20-40) and Hf-Ta-Th and FeO-MgO-Al2O3 discrimination diagrams. The compositional zoning in plagioclase and clinopyroxene, variation in olivine compositions and major elements oxide trends indicate a vital role of fractional crystallization in the evolution of dolerites, which also show depletion in rare earth elements (REEs) and other incompatible elements compared to the amphibolites. The equilibrium partial melting models from primitive mantle using Dy/Yb, La/Yb, Sm/Yb and La/Sm ratios show that amphibolite formed by smaller degrees (< 5%) of partial melting than the dolerites (< 10%). The trace elements ratios suggest the origination of dolerites from the subcontinental lithospheric mantle with some crustal contamination. This is consistent with a petrogenetic relationship with Panjal trap magmatism, reported from Kashmir and other parts of north western India. The amphibolites, in contrast, show affinity towards Ocean Island basalts (OIB) with a relatively deep asthenospheric mantle source and minimal crustal contribution and are geochemically similar to the High-Ti mafic dykes of southern Qiangtang, Tibet. These similarities combined with Permian tectonic restoration of Gondwana indicate the coeval origin for both dykes from distinct mantle source during continental rifting related to formation of the Neotethys Ocean.

Magmatism evolution on the last Neoproterozoic development stage of the western Siberian active continental margin

NASA Astrophysics Data System (ADS)

Vernikovskaya, Antonina E.; Vernikovsky, Valery A.; Matushkin, Nikolay Yu.; Kadilnikov, Pavel I.; Romanova, Irina V.

2017-04-01

Rocks from active continental margin complexes are characterized by a wide variety of chemical compositions from depleted in alkali to alkali differentiates. When addressing issues of geodynamic settings in which such rocks form, it is important to understand the evolution of the host tectonic structure, as well as the chemical affiliation of the various rocks composing it. The Yenisey Ridge orogen located in the south-western framing of Siberia is one of the more studied regions with a long history of Neoproterozoic magmatic events. This orogen was formed during the collision of the Central Angara terrane with Siberia, which took place 761-718 Ma. Subsequent subduction-related events in the orogen have been recorded in the coeval magmatism (711-629 Ma) of two complexes: one is the active continental margin complex (Nb enriched igneous rocks - gabbroids, trachybasalts, A-type granites and carbonatites, including contact metasomatites zones with Nb mineralization), and the other one is an island arc complex (differentiated series volcanics, gabbroids and plagiogranites). The rocks of these complexes are respectively located in two suture zones: the Tatarka-Ishimba zone that formed due to the collision mentioned above, and the Yenisei suture marking the subduction zone [Vernikovsky et al., 2003; 2008]. The final Neoproterozoic stage in the evolution of the active margin of Siberia is manifested as adakite-gabbro-anorthosite magmatism in the 576-546 Ma interval. Our results indicate a genetic relationship between the adakites and their host NEB-type metabasites of the Zimovey massif. These Neoproterozoic adakites could have formed in a setting of transform-strike-slip drift of lithospheric plates after the subduction stopped, both from a crustal and mantle-crustal source, similarly to the Cenozoic magmatic complexes of the transform margin in the eastern framing of Eurasia [Khanchuk et al., 2016]. Vernikovsky V.A., Vernikovskaya A.E., Kotov A.B., Sal'nikova E.B., Kovach V.P. Neoproterozoic accretionary and collisional events on the western margin of the Siberian craton: new geological and geochronological evidence from the Yenisey Ridge // Tectonophysics, 2003, V. 375, P. 147-168. Vernikovsky V.A., Vernikovskaya A.E., Sal'nikova E.B., Berezhnaya N.G., Larionov A.N., Kotov A.B., Kovach V.P., Vernikovskaya I.V., Matushkin N.Yu., Yasenev A.M. Late Riphean alkaline magmatism in the western margin of the Siberian Craton: A result of continental rifting or accretionary events? // Doklady Earth Sciences, 2008, V. 419, Iss. 1, P. 226-230. Khanchuk A.I., Kemkin I.V., Kruk N.N. The Sikhote-Alin orogenic belt, Russian South East: Terranes and the formation of continental lithosphere based on geological and isotopic Data // Journal of Asian Earth Sciences, 2016, V. 120, P. 117-138.

Late Cenozoic deformation of the Eurasian and Burma Plates due to subduction of the Indian Plate beneath SE Tibetan Plateau and Myanmar

NASA Astrophysics Data System (ADS)

Ni, J.; Holt, W. E.; Flesch, L. M.; Sandvol, E. A.; Hearn, T. M.; Schmerr, N. C.

2015-12-01

The late Cenozoic tectonics of the southeastern Tibetan Plateau and surrounding regions needs to be evaluated within the context of a larger dynamic system related to the subduction of the Indian oceanic lithosphere beneath Myanmar and Yunnan. The details of the geodynamic processes involve mantle flows associated with rollback and tears (which probably occur both laterally and horizontally) of the Indian slab with consequent effects on the geology of the overriding plate. These effects include: 1) volcanism in Tongchong, Yunnan Province, 2) clockwise rotational deformation of the overriding plate and 3) Burma Plate capture. In this talk we will present the strain rate throughout the region with a moment tensor summation of earthquake data. The deformation of SE Tibet, Yunnan and western Sichuan constitutes a distributed N-S oriented dextral shear zone with clockwise rotations up to 1.7° per million years. It is the clockwise vorticity relative to south China that accounts for the relative northward motion of India at a rate of 38±12 mm/yr at the Himalayas. Relative to south China, there is no southeastward extrusion of crustal material beyond the eastern margin of the Tibetan Plateau. Studies on earthquake seismic moment data, fault-slip data, and GPS measurements all show a clockwise rotational motion of SE Tibet, Yunnan, western Sichuan and eastern Myanmar around the EHS. The mirror image of this situation that is occurring today is the counterclockwise rotation of Anatolia, which is associated with the rollback of the Hellenic and Cyprian Trenches. In this talk we will also discuss the extreme oblique convergence between Indian and Burma plates and one of the effects of the oblique subducation is the transfer of right-lateral strike-slip faulting to the Indo Burma Range, one of the largest GeoPRISMs on Earth.

Kimberlites in western Liberia - An overview of the geological setting in a plate tectonic framework

NASA Astrophysics Data System (ADS)

Haggerty, S. E.

1982-12-01

Evidence which includes Landsat images is presented for prolonged periods of tectonism, marginal to and extending within the intracratonic region of the West African platform. Also found are indications of intermittent, or perhaps even sustained activity, dating back to more than three billion years. The petrology and mineral chemistry of kimberlites, and their associated nodule suites in the present region, are broadly similar to those from kimberlite localities throughout the African continent, and should therefore be considered as part of a major province. Attention is drawn to the lineament control of kimberlites, and the coincidence of these lineaments with the basement fabric and with faults. The proposed interpretation for the distribution of West African kimberlites is in essential agreement with the intraplate and intracratonic model of Dawson (1970) and Sykes (1978), which calls upon the reactivation of paleofaults and sutures during plate tectonism.

Giardia muris and Giardia duodenalis groups: ultrastructural differences between the trophozoites.

PubMed

Sogayar, M I; Gregório, E A

1989-01-01

Trophozoites of the Giardia muris group from hamsters, domestic rats and mice and of the Giardia duodenalis group from hamsters and domestic rats were examined by transmission electron microscopy. The basic ultrastructure of the trophozoites was similar. Differences were shown in the morphology of the ventrolateral flange of the trophozoites of Giardia muris and Giardia duodenalis groups. Marginal plates are less developed in the species of the Giardia duodenalis group. In this group, the distal extremity of the lateral flange is short and thick and the marginal plate does not penetrate into the distal extremity of the flange. In the Giardia muris group, the ventro-lateral flange is well developed and narrow and the marginal plate penetrates the distal extremity of the flange. The osmiophilic lamella, which accompanies the dorsal surface of the marginal plate is seen only in the Giardia muris group.

Geophysical constraints on geodynamic processes at convergent margins: A global perspective

NASA Astrophysics Data System (ADS)

Artemieva, Irina; Thybo, Hans; Shulgin, Alexey

2016-04-01

Convergent margins, being the boundaries between colliding lithospheric plates, form the most disastrous areas in the world due to intensive, strong seismicity and volcanism. We review global geophysical data in order to illustrate the effects of the plate tectonic processes at convergent margins on the crustal and upper mantle structure, seismicity, and geometry of subducting slab. We present global maps of free-air and Bouguer gravity anomalies, heat flow, seismicity, seismic Vs anomalies in the upper mantle, and plate convergence rate, as well as 20 profiles across different convergent margins. A global analysis of these data for three types of convergent margins, formed by ocean-ocean, ocean-continent, and continent-continent collisions, allows us to recognize the following patterns. (1) Plate convergence rate depends on the type of convergent margins and it is significantly larger when, at least, one of the plates is oceanic. However, the oldest oceanic plate in the Pacific ocean has the smallest convergence rate. (2) The presence of an oceanic plate is, in general, required for generation of high-magnitude (M N 8.0) earthquakes and for generating intermediate and deep seismicity along the convergent margins. When oceanic slabs subduct beneath a continent, a gap in the seismogenic zone exists at depths between ca. 250 km and 500 km. Given that the seismogenic zone terminates at ca. 200 km depth in case of continent-continent collision, we propose oceanic origin of subducting slabs beneath the Zagros, the Pamir, and the Vrancea zone. (3) Dip angle of the subducting slab in continent-ocean collision does not correlate neither with the age of subducting oceanic slab, nor with the convergence rate. For ocean-ocean subduction, clear trends are recognized: steeply dipping slabs are characteristic of young subducting plates and of oceanic plates with high convergence rate, with slab rotation towards a near-vertical dip angle at depths below ca. 500 km at very high convergence rate. (4) Local isostasy is not satisfied at the convergent margins as evidenced by strong free air gravity anomalies of positive and negative signs. However, near-isostatic equilibrium may exist in broad zones of distributed deformation such as Tibet. (5) No systematic patterns are recognized in heat flow data due to strong heterogeneity of measured values which are strongly affected by hydrothermal circulation, magmatic activity, crustal faulting, horizontal heat transfer, and also due to low number of heat flow measurements across many margins. (6) Low upper mantle Vs seismic velocities beneath the convergent margins are restricted to the upper 150 km and may be related to mantle wedge melting which is confined to shallow mantle levels. Artemieva, I.M., Thybo, H., and Shulgin, A., 2015. Geophysical constraints on geodynamic processes at convergent margins: A global perspective. Gondwana Research, http://dx.doi.org/10.1016/j.gr.2015.06.010

From continental to oceanic rifting in the Gulf of California

NASA Astrophysics Data System (ADS)

Ferrari, Luca; Bonini, Marco; Martín, Arturo

2017-11-01

The continental margin of northwestern Mexico is the youngest example of the transition from a convergent plate boundary to an oblique divergent margin that formed the Gulf of California rift. Subduction of the Farallon oceanic plate during the Cenozoic progressively brought the East Pacific Rise (EPR) toward the North America trench. In this process increasingly younger and buoyant oceanic lithosphere entered the subduction zone until subduction ended just before most of the EPR could collide with the North America continental lithosphere. The EPR segments bounding the unsubducted parts of the Farallón plate remnants (Guadalupe and Magdalena microplates) also ceased spreading (Lonsdale, 1991) and a belt of the North American plate (California and Baja California Peninsula) became coupled with the Pacific Plate and started moving northwestward forming the modern Gulf of California oblique rift (Nicholson et al., 1994; Bohannon and Parsons, 1995). The timing of the change from plate convergence to oblique divergence off western Mexico has been constrained at the middle Miocene (15-12.5 Ma) by ocean floor morphology and magnetic anomalies as well as plate tectonic reconstructions (Atwater and Severinghaus, 1989; Stock and Hodges, 1989; Lonsdale, 1991), although the onset of transtensional deformation and the amount of right lateral displacement within the Gulf region are still being studied (Oskin et al., 2001; Fletcher et al., 2007; Bennett and Oskin, 2014). Other aspects of the formation of the Gulf of California remain not well understood. At present the Gulf of California straddles the transition from continental transtension in the north to oceanic spreading in the south. Seismic reflection-refraction data indicate asymmetric continent-ocean transition across conjugate margins of rift segments (González-Fernández et al., 2005; Lizarralde et al., 2007; Miller and Lizarralde, 2013; Martín-Barajas et al., 2013). The asymmetry may be related to crustal heterogeneities and thus early evidence of extension may provide useful information about the thermal conditions of the crust over a broader region encompassing the effects of coeval subduction and crustal stretching. On the other hand, onshore and offshore geologic studies have shown that lithospheric extension associated with a wide rift mode was already ongoing during the final stage of subduction of the Farallon plate and its remnants in the early to middle Miocene times (Ferrari et al., 2013; Murray et al., 2013; Bryan et al., 2014; Duque-Trujillo et al., 2014, 2015). More broadly, the complexity in the present rift architecture and Plio-Quaternary magmatism is related to the pre-middle Miocene geodynamic history that accompanied the removal of the slab since the Eocene (Ferrari et al., 2017).

Structure and Stratigraphy of the Rift Basins in the Northern Gulf of California: Results from Analysis of Seismic Reflection and Borehole Data.

NASA Astrophysics Data System (ADS)

Martín, A.; González, M.; Helenes, J.; García, J.; Aragón, M.; Carreño, A.

2008-12-01

The northern Gulf of California contains two parallel, north-south trending rift basin systems separated by a basement-high. The interpretation of several exploration wells, and ~4500 km of seismic reflection data from PEMEX (Mexican national oil company) indicate that the tectonically active basins to the west (Wagner- Consag and Upper Delfin basins) may have initiated synchronously with the now abandoned Tiburón- Tepoca-Altar basins to the east in the Sonora margin. In both basin systems the lower sequence (A) is marine mudstone-siltstone, has parallel reflectors and a largely uniform thickness that reaches up to1.5 km, and gradually pinches out toward the lateral margins. This suggests that the unit was deposited prior to their segmentation by transtensional faulting. Marine microfossils from borehole samples from sequence A in the Tiburón and Consag basins indicates middle Miocene (>11.2 Ma) proto-Gulf conditions. Sequence B conformably overlies sequence A, and is characterized by up to 2 km growth strata with a fanning geometry that show a clear genetic relationship to the major transtensional faults that control the segmentation of the two basin systems. Sequence C in the Tiburón and Tepoca basins is comparatively thin (<800 m) and includes several unconformities, but is much less affected by faulting. In contrast, sequence C in the active Wagner, Consag and Upper Delfin basin is a much thicker (up to 2 km) growth sequence with abundant volcanic intrusions. Marked variations in sequence C in the different basin systems clearly demonstrate a major westward shift of deformation and subsidence at this time. The modern depocenter in Wagner-Consag basins is controlled by the Consag and Wagner faults, which trend parallel to the north ~20 km apart, and show opposite normal offset. These two faults merge at an oblique angle (70°-50°, respectively) into the Cerro Prieto transform fault to the north and likely accommodate an important amount of dextral shear. To the south the Consag and Wagner faults connect with a diffuse zone of deformation defined by a series of NE trending faults with moderate normal displacement in the Upper Delfin basin. These NE-trending faults intersect the northern strand of the Ballenas transform fault along the Baja California margin, whereas the eastern end of the NE-trending faults is poorly defined along the western flank of the central antiform. In summary, sequence A was likely deposited across most of the northern gulf in the late Miocene, sequence B marks the onset of two discrete transtensional basin systems controlled by both low and high-angle faults in late Miocene-Pliocene time, and sequence C marks the regional migration of plate- margin shearing to its present location in the western gulf. Thermal effects associated with abundant volcanism and sedimentation along the western margin of the gulf likely controlled the asymmetric partitioning plate margin and shearing during the most recent phase of oblique rifting.

Mechanistic solutions to the opening of the Gulf of Mexico

USGS Publications Warehouse

Schouten, Hans; Klitgord, Kim D.

1994-01-01

Two mechanistic models-which are unlike the traditional plate-tectonic landfill models used for most proposed Pangea reconstructions of the Yucatán block-relate the Mesozoic opening of the Gulf of Mexico directly to the movement of the North and South American plates: (1) a previous piggyback model in which Yucatán moves with South America out of the western gulf and (2) a new edge-driven model in which the motion of the Yucatán block is caused by forces applied to its margins by the movement of the North and South American plates. In the second model, Yucatán moves out of the northern Gulf of Mexico as a gear or roller bearing. On the basis of magnetic edge anomalies around the gulf, this edge-driven model predicts that from the Bathonian to Tithonian (~170 to ~50 Ma), Yucatán was rotated ~60° counterclockwise as a rigid block between North and South America with rift propagation and extension occurring simultaneously in the Gulf of Mexico and Yucatán Basin.

Mesozoic carbonate-siliciclastic platform to basin systems of a South Tethyan margin (Egypt, East Mediterranean)

NASA Astrophysics Data System (ADS)

Tassy, Aurélie; Crouzy, Emmanuel; Gorini, Christian; Rubino, Jean-Loup

2015-04-01

The Mesozoïc Egyptian margin is the south margin of a remnant of the Neo-Tethys Ocean, at the African northern plate boundary. East Mediterranean basin developed during the late Triassic-Early Jurassic rifting with a NW-SE opening direction (Frizon de Lamotte et al., 2011). During Mesozoïc, Egypt margin was a transform margin with a NW-SE orientation of transform faults. In the Eastern Mediterranean basin, Mesozoïc margins are characterized by mixed carbonate-siliciclastics platforms where subsidence and eustacy are the main parameters controlling the facies distribution and geometries of the platform-to-basin transition. Geometries and facies on the platform-slope-basin system, today well constrained on the Levant area, where still poorly known on the Egyptian margin. Geometries and stratigraphic architecture of the Egyptian margin are revealed, thanks to a regional seismic and well data-base provided by an industrial-academic group (GRI, Total). The objective is to understand the sismostratigraphic architecture of the platform-slope-basin system in a key area from Western Desert to Nile delta and Levant margin. Mapping of the top Jurassic and top Cretaceous show seismic geomorphology of the margin, with the cartography of the hinge line from Western Desert to Sinaï. During the Jurassic, carbonate platform show a prograding profile and a distally thickening of the external platform, non-abrupt slope profiles, and palaeovalleys incisions. Since the Cretaceous, the aggrading and retrograding mixed carbonate-siliciclastic platform show an alternation of steep NW-SE oblique segments and distally steepened segments. These structures of the platform edge are strongly controlled by the inherited tethyan transform directions. Along the hinge line, embayments are interpreted as megaslides. The basin infilling is characterised by an alternation of chaotic seismic facies and high amplitude reflectors onlaping the paleoslopes. MTC deposits can mobilize thick sedimentary series (up to 3500 m) as a mixed combination of debris flows, internal preserved blocks, and/or compressively-deformed distal allochthonous masses. Transported material have proceeded from the dismantling of the Mesozoic mixed carbonate-siliciclastic platform. They can spread down slope over areas as large as 70000 of km2. According to stratigraphic correlations with global sea-level positions, platform instability would have been triggered by the gravitational collapse of the carbonate-siliciclastic platform under its own weight after successive subaerial exposures which were able to generate karstification processes. Seismic interpretation is constrained by a detailed assessment of the Egyptian margin paleogeography supported by wells. This margin segment is briefly compared to the outcropping Apulian margin in Italy.

Subduction erosion off central Java: transition from accretion to erosion manifested by wide-angle seismic studies

NASA Astrophysics Data System (ADS)

Wittwer, A.; Flueh, E.; Rabbel, W.; Wagner, D.

2006-12-01

In this study, offshore wide-angle data acquired by ocean bottom instruments of a combined onshore- offshore investigation of the tectonic framework of central Java will be presented. The joint interdisciplinary project MERAMEX (Merapi Amphibious Experiment) was carried out in order to characterize the subduction of the Indo-Australian plate beneath Eurasia. The interpretation of three wide-angle data profiles, modelled with forward raytracing, indicates that the subduction of the Roo Rise with its thickened oceanic crust strongly influences the subduction zone. The dip angle of the downgoing oceanic plate is 10° and its crustal thickness increases to the east from 8 km to 9 km between both dip profiles off central Java. Large scale forearc uplift is manifested in isolated forearc highs, reaching water depths of only 1000 m compared to 2000 m water depth off western Java, and results from oceanic basement relief subduction. A broad band of seamounts trends E-W at approximately 10°S. Its incipient subduction off central Java causes frontal erosion of the margin here and leads to mass wasting due to oversteepening of the upper trench wall. A suite of wide-angle profiles off southern Sumatra to central Java indicates a clear change in the tectonic environment between longitude 108°E and 109°E. The well-developed accretionary wedge off southern Sumatra and western Java changes into a small frontal prism with steep slope angles of the upper plate off central Java.

New constraints on Neogene counter-clockwise rotation of Adria relative to Europe

NASA Astrophysics Data System (ADS)

Le Breton, Eline; Handy, Mark R.; Molli, Giancarlo; Ustaszewski, Kamil

2017-04-01

The Adriatic microplate (Adria) is a key player in the geodynamics of Alpine-Mediterranean belts because of its location between two converging plates, Europe and Africa. Most of Adria has been subducted and is presently surrounded by deformed margins comprising the Alps, Apennines, Dinarides and the Calabrian Arc. The Alps-Apennines and Alps-Dinarides junctions are marked by switches in subduction polarity, with Adria being the indenting upper plate in the Alps and the lower plate in the Apennines and Dinarides. Reconstructing Neogene motion and rotation of Adria is therefore key to understanding how such contrasting orogenic styles develop within a similar convergent tectonic regime. We propose a new kinematic reconstruction that balances shortening and extension in the northern Apennines; it reveals that Adria rotated counter-clockwise as it subducted beneath the European Plate to the west and to the east, while indenting the Alps to the north. Syn-collisional back-arc extension in the Liguro-Provençal and northern Tyrrhenian basins exceeds collisional shortening in the northern Apennines, indicating that after 20 Ma Adria and Europe diverged. When combined with existing estimates of Neogene shortening in the Western and Eastern Alps, this overall divergence in the Apennines constrains Adria to have moved to the NW while rotating counter-clockwise relative to Europe. We furthermore consider the length of the present Adriatic slab (135 km) imaged by P-wave tomography in the southern Dinarides to represent the maximum convergence since late Paleogene slab-breakoff, constraining Adria to have rotated 6.5˚ counter-clockwise about an axis in northwestern Italy. Thus, the best fit of available structural data from the Apennines, Alps and Dinarides constrains Adria to have moved 113 km to the NW (azimuth 325˚ ) while rotating 6.5˚ counter-clockwise relative to Europe since 20 Ma. Our model predicts some 80-100 km of Neogene extension between Adria and Africa, most likely accommodated along a NW-SE striking rift system on the African margin and by transtension along NW-SE striking transform faults in the Ionian Sea. We propose that this Neogene motion of Adria resulted from a combination of Africa pushing from the south, the Adriatic-Hellenic slab pulling to the northeast and crustal wedging in the Western Alps, which acted as a pivot and stopped further northwestward motion of Adria.

Passive margins getting squeezed in the mantle convection vice

NASA Astrophysics Data System (ADS)

Husson, Laurent; Yamato, Philippe; Becker, Thorsten; Pedoja, Kevin

2013-04-01

Quaternary coastal geomorphology reveals that passive margins underwent wholesale uplift at least during the glacial cycle. In addition, these not-so-passive margins often exhibit long term exhumation and tectonic inversion, which suggest that compression and tectonic shortening could be the mechanism that triggers their overall uplift. We speculate that the compression in the lithosphere gradually increased during the Cenozoic. The many mountain belts at active margins that accompany this event readily witness this increase. Less clear is how that compression increase affects passive margins. In order to address this issue, we design minimalist 2D viscous models to quantify the impact of plate collision on the stress regime. In these models, a sluggish plate is disposed on a less viscous mantle. It is driven by a "mantle conveyor belt" alternatively excited by lateral shear stresses that represent a downwelling on one side, an upwelling on the other side, or both simultaneously. The lateral edges of the plate are either free or fixed, respectively representing the cases of free convergence and collision. In practice, it dramatically changes the upper boundary condition for mantle circulation and subsequently, for the stress field. The flow pattern transiently evolves almost between two end-members, starting from a situation close to a Couette flow to a pattern that looks like a Poiseuille flow with an almost null velocity at the surface (though in the models, the horizontal velocity at the surface is not strictly null, as the lithosphere deforms). In the second case, the lithosphere is highly stressed horizontally and deforms. For an equivalent bulk driving force, compression increases drastically at passive margins if upwellings are active because they push plates towards the collision. Conversely, if only downwellings are activated, compression occurs on one half of the plate and extension on the other half, because only the downwelling is pulling the plate. Thus, active upwellings underneath oceanic plates are required to explain compression at passive margins. This conclusion is corroborated by "real-Earth" 3D spherical models, wherein the flow is alternatively driven by density anomalies inferred from seismic tomography -and therefore include both downwellings at subduction zones and upwellings above the superswells- and density anomalies that correspond to subducting slabs only. While the second scenario mostly compresses the active margins of upper plates and leave other areas at rest, the first scenario efficiently compresses passive margins where the geological record reveals their uplift, exhumation, and tectonic inversion.

Influence of obliquely subducting slab on Pacific-North America shear motion inferred from seismic anisotropy along the Queen Charlotte margin

NASA Astrophysics Data System (ADS)

Cao, L.; Kao, H.; Wang, K.; Wang, Z.

2016-12-01

Haida Gwaii is located along the transpressive Queen Charlotte margin between the Pacific (PA) and North America (NA) plates. The highly oblique relative plate motion is partitioned, with the strike-slip component accommodated by the Queen Charlotte Fault (QCF) and the convergent component by a thrust fault offshore. To understand how the presence of a obliquely subducting slab influences shear deformation of the plate boundary, we investigate mantle anisotropy by analyzing shear-wave splitting of teleseismic SKS phases recorded at 17 seismic stations in and around Haida Gwaii. We used the MFAST program to determine the polarization direction of the fast wave (φ) and the delay time (δt) between the fast and slow phases. The fast directions derived from stations on Haida Gwaii and two stations to the north on the Alaska Panhandle are predominantly margin-parallel (NNW). However, away from the plate boundary, the fast direction transitions to WSW-trending, very oblique or perpendicular to the plate boundary. Because the average delay time of 0.6-2.45 s is much larger than values based on an associated local S phase splitting analysis in the same study area, it is reasonable to infer that most of the anisotropy from our SKS analysis originates from the upper mantle and is associated with lattice-preferred orientation of anisotropic minerals. The margin-parallel fast direction within about 100 km of the QCF (average φ = -40º and δt = 1.2 s) is likely induced by the PA-NA shear motion. The roughly margin-normal fast directions farther away, although more scatterd, are consistent with that previously observed in the NA continent and are attributed to the absolute motion of the NA plate. However, the transition between the two regimes based on our SKS analysis appears to be gradual, suggesting that the plate boundary shear influences a much broader region at mantle depths than would be inferred from the surface trace of the QCF. We think this is due to the presence of a subducted portion of the Pacific plate. Because the slab travels mostly in the strike direction, it is expected to induce margin-parallel shear deformation of the mantle material. This result has importance implications to the geodynamics of transpressive plate margins.

Basement thrust sheets in the Clearwater orogenic zone, central Idaho and western Montana ( USA).

USGS Publications Warehouse

Skipp, B.

1987-01-01

The Clearwater orogenic zone in central Idaho and W Montana contains at least 2 major NE-directed Cordilleran thrust plates of Early Proterozoic metasedimentary and metaigneous rocks that overrode previously folded Middle Proterozoic rocks of the Belt basin in Cretaceous time. The northeastward migration of the resultant thickened wedge of crustal material combined with Cretaceous subduction along the W continental margin produced a younger N Bitterroot lobe of the Idaho batholith relative to an older S Atlanta lobe. Eocene extensional unroofing and erosion of the Bitterroot lobe has exposed the roots of the thick Cordilleran thrust sheets.-Author

Anomalous Accretionary Margin Topography Formed By Repeated Earthquakes

NASA Astrophysics Data System (ADS)

Furlong, Kevin P.

2014-05-01

It has long been recognized that accretionary margins of major subduction zones undergo substantial deformation. However even with the large amounts of shortening accommodated within the margin, for most subduction zones, there is an extended submarine portion to the accretionary, highly-deformed upper-plate between the trench and the coast. This is a vexing situation since this submarine section typically overlies the actual locked or coupled patch of the plate interface. The result of this is added difficulty in directly observing processes related to the plate interface coupling - such processes as micro-seismicity and the actual patterns of plate coupling. There are a few locations globally in which there are sub-aerially exposed terranes that lie closer to the trench and overlie the inferred coupled or seismogenic portion of the plate interface. Such regions have taken on significance in subduction zone studies as they provide locations to observe the plate interface coupling effects in the near-field. In particular the Pacific coast of Costa Rica provides such a location, and there has been substantial geologic, geophysical, and geodetic research exploiting the positions of these near-trench peninsulas (Nicoya, Osa, and Burica). These sites provide near-field access to plate-interface processes, but whether they represent typical subduction zone behavior remains an open question as the deformational processes or inherited structures that have produced this anomalous topography are not well constrained. Simply put, if the existence of these sub-aerial, near-trench terranes is a result of anomalous behavior on the plate interface (as has been suggested), then their utility in providing high-fidelity near-field insight into the plate interface properties and processes is substantially reduced. Here we propose a new mechanism that could be responsible for the formation of both the Nicoya and Osa Peninsulas in the past, and is currently producing a third peninsula - the Burica Peninsula at the intersection of the Panama fracture zone and the margin. Specifically we propose that the anomalous topography along the Pacific coast of Costa Rica has been produced by repeated, great subduction earthquakes that have ruptured across the boundary separating the Cocos and Nazca plates - the subducted continuation of the Panama fracture zone. The pattern of upper-plate shortening generated by such a process (documented in the 2007 Mw 8.1 Solomon Islands earthquake, which produced co-seismic localized uplift above the subducted transform plate boundary) convolved with the migration history of the Panama triple junction (PTJ) is proposed as the mechanism to produce substantial along-margin, long-lived accretionary margin topography. Specifically we argue that repeated great subduction earthquakes that rupture across fundamental plate boundary structures can produce substantial, long-lived upper plate deformation above the inter-seismically coupled plate interface.

The Caribbean-Colombian cretaceous igneous province: The internal anatomy of an oceanic plateau

NASA Astrophysics Data System (ADS)

Kerr, Andrew C.; Tarney, John; Marriner, Giselle F.; Nivia, Alvaro; Saunders, Andrew D.

The Late Cretaceous Caribbean—Colombian igneous province is one of the world's best-exposed examples of a plume-derived oceanic plateau. The buoyancy of the plateau (resulting from residual heat and thick crust) kept it from being totally subducted as it moved eastward with the Farallon Plate from its site of generation in the eastern Pacific and encountered a destructive plate margin. In effect, the plateau makes up much of the Caribbean Plate; it is well exposed around its margins, but more so in accreted terranes in western Colombia (including the well-known Gorgona komatiites and Bolívar mafic/ultramafic cumulates). Compositionally, the lavas of the plateau form three groups: (a) basalts, picrites, and komatiites with light-rare-earth-element (LREE)-depleted chondrite-normalised patterns; (b) basalts with LREE-enriched patterns; and (c) basalts with essentially flat REE patterns (the most dominant type) similar to many of the basalts from the Ontong Java Plateau. These three types demonstrate the heterogeneous nature of the mantle plume source region. The picrites and the komatiites seem to lie nearer the base of the plateau than the more homogeneous basalts; thus, the more MgO-rich melts may have been erupted before large magma chambers had a chance to develop. A reconstructed crustal cross section through the plateau consists of dunitic and pyroxenitic cumulates near the base which are overlain by layered olivine-rich gabbros and more isotropic gabbros. The lowermost eruptive sequence comprises compositionally heterogeneous picrites/komatiites overlain by more homogeneous pillow basalts. Spectacular hornblende-plagioclase veins cut the Bolívar assemblage and these may represent local partial melts of the plateau's base as it was thrusted onto the continent. Subduction-related batholiths and extrusive rocks found around the margin of the province are of two distinct ages; one suite represents pre-plateau collision-related volcanism whereas the other suite, slightly younger than the plateau, may be associated with obduction.

Accretionary orogens through Earth history

USGS Publications Warehouse

Cawood, Peter A.; Kroner, A.; Collins, W.J.; Kusky, T.M.; Mooney, W.D.; Windley, B.F.

2009-01-01

Accretionary orogens form at intraoceanic and continental margin convergent plate boundaries. They include the supra-subduction zone forearc, magmatic arc and back-arc components. Accretionary orogens can be grouped into retreating and advancing types, based on their kinematic framework and resulting geological character. Retreating orogens (e.g. modern western Pacific) are undergoing long-term extension in response to the site of subduction of the lower plate retreating with respect to the overriding plate and are characterized by back-arc basins. Advancing orogens (e.g. Andes) develop in an environment in which the overriding plate is advancing towards the downgoing plate, resulting in the development of foreland fold and thrust belts and crustal thickening. Cratonization of accretionary orogens occurs during continuing plate convergence and requires transient coupling across the plate boundary with strain concentrated in zones of mechanical and thermal weakening such as the magmatic arc and back-arc region. Potential driving mechanisms for coupling include accretion of buoyant lithosphere (terrane accretion), flat-slab subduction, and rapid absolute upper plate motion overriding the downgoing plate. Accretionary orogens have been active throughout Earth history, extending back until at least 3.2 Ga, and potentially earlier, and provide an important constraint on the initiation of horizontal motion of lithospheric plates on Earth. They have been responsible for major growth of the continental lithosphere through the addition of juvenile magmatic products but are also major sites of consumption and reworking of continental crust through time, through sediment subduction and subduction erosion. It is probable that the rates of crustal growth and destruction are roughly equal, implying that net growth since the Archaean is effectively zero. ?? The Geological Society of London 2009.

Cretaceous to present kinematics of the Indian, African and Seychelles plates

NASA Astrophysics Data System (ADS)

Eagles, Graeme; Hoang, Ha H.

2014-01-01

An iterative inverse model of seafloor spreading data from the Mascarene and Madagascar basins and the flanks of the Carlsberg Ridge describes a continuous history of Indian-African Plate divergence since 84 Ma. Visual-fit modelling of conjugate magnetic anomaly data from near the Seychelles platform and Laxmi Ridge documents rapid rotation of a Seychelles Plate about a nearby Euler pole in Palaeocene times. As the Euler pole migrated during this rotation, the Amirante Trench on the western side of the plate accommodated first convergence and later divergence with the African Plate. The unusual present-day morphology of the Amirante Trench and neighbouring Amirante Banks can be related to crustal thickening by thrusting and folding during the convergent phase and the subsequent development of a spreading centre with a median valley during the divergent phase. The model fits FZ trends in the north Arabian and east Somali basins, suggesting that they formed in India-Africa Plate divergence. Seafloor fabric in and between the basins shows that they initially hosted a segmented spreading ridge that accommodated slow plate divergence until 71-69 Ma, and that upon arrival of the Deccan-Réunion plume and an increase to faster plate divergence rates in the period 69-65 Ma, segments of the ridge lengthened and propagated. Ridge propagation into the Indian continental margin led first to the formation of the Laxmi Basin, which accompanied extensive volcanism onshore at the Deccan Traps and offshore at the Saurashtra High and Somnath Ridge. A second propagation episode initiated the ancestral Carlsberg Ridge at which Seychelles-India and India-Africa Plate motions were accommodated. With the completion of this propagation, the plate boundaries in the Mascarene Basin were abandoned. Seafloor spreading between this time and the present has been accommodated solely at the Carlsberg Ridge.

Geomorphic Evolution and Slip rate Measurements of the Noushki Segment , Chaman Fault Zone, Pakistan

NASA Astrophysics Data System (ADS)

Abubakar, Y.; Khan, S. D.; Owen, L. A.; Khan, A.

2012-12-01

The Nushki segment of the Chaman fault system is unique in its nature as it records both the imprints of oblique convergence along the western Indian Plate boundary as well as the deformation along the Makran subduction zone. The left-lateral Chaman transform zone has evolved from a subduction zone along the Arabian-Eurasian collision complex to a strike-slip fault system since the collision of the Indian Plate with the Eurasia. The geodetically and geologically constrained displacement rates along the Chaman fault varies from about 18 mm/yr to about 35 mm/yr respectively throughout its total length of ~ 860 km. Two major hypothesis has been proposed by workers for these variations; i) Variations in rates of elastic strain accumulation along the plate boundary and, ii) strain partitioning along the plate boundary. Morphotectonic analysis is a very useful tool in investigations of spatial variations in tectonic activities both regionally and locally. This work uses morphotectonic analysis to investigate the degree of variations in active tectonic deformation, which can be directly related to elastic strain accumulation and other kinematics in the western boundary of the plate margin. Geomorphic mapping was carried out using remotely sensed data. ASTER and RADAR data were used in establishing Quaternary stratigraphy and measurement of geomorphic indices such as stream length gradient index, valley floor width to height ratio and, river/stream longitudinal profile within the study area. High resolution satellite images (e.g., IKONOS imagery) and 30m ASTER DEMs were employed to measure displacement recorded by landforms along individual strands of the fault. Results from geomorphic analysis shows three distinct levels of tectonic deformation. Areas showing high levels of tectonic deformation are characterized by displaced fan surfaces, deflected streams and beheaded streams. Terrestrial Cosmogenic nuclide surface exposure dating of the displaced landforms is being carried out to calculate slip-rates. Slip-rates estimation along this segment of this plate boundary will help in understanding of tectonic evolution of this plate boundary and seismic activity in the region.

Gravity-Driven Deposits in an Active Margin (Ionian Sea) Over the Last 330,000 Years

NASA Astrophysics Data System (ADS)

Köng, Eléonore; Zaragosi, Sébastien; Schneider, Jean-Luc; Garlan, Thierry; Bachèlery, Patrick; Sabine, Marjolaine; San Pedro, Laurine

2017-11-01

In the Ionian Sea, the subduction of the Nubia plate underneath the Eurasia plate leads to an important sediment remobilization on the Calabrian Arc and the Mediterranean Ridge. These events are often associated with earthquakes and tsunamis. In this study, we analyze gravity-driven deposits in order to establish their recurrence time on the Calabrian Arc and the western Mediterranean Ridge. Four gravity cores collected on ridges and slope basins of accretionary prisms record turbidites, megaturbidites, slumping and micro-faults over the last 330,000 years. These turbidites were dated by correlation with a hemipelagic core with a multi-proxy approach: radiometric dating, δ18O, b* colour curve, sapropels and tephrochronology. The origin of the gravity-driven deposits was studied with a sedimentary approach: grain-size, lithology, thin section, geochemistry of volcanic glass. The results suggest three periods of presence/absence of gravity-driven deposits: a first on the western lobe of the Calabrian Arc between 330,000 and 250,000 years, a second between 120,000 years and present day on the eastern lobe of the Calabrian Arc and over the last 60,000 years on the western lobe, and a third on the Mediterranean Ridge over the last 37,000 years. Return times for gravity-driven deposits are around 1,000 years during the most important record periods. The turbidite activity also highlights the presence of volcaniclastic turbidites that seems to be link to the Etna changing morphology over the last 320,000 years.

Ancient plate kinematics derived from the deformation pattern of continental crust: Paleo- and Neo-Tethys opening coeval with prolonged Gondwana-Laurussia convergence

NASA Astrophysics Data System (ADS)

Kroner, Uwe; Roscher, Marco; Romer, Rolf L.

2016-06-01

The formation and destruction of supercontinents requires prolonged convergent tectonics between particular plates, followed by intra-continental extension during subsequent breakup stages. A specific feature of the Late Paleozoic supercontinent Pangea is the prolonged and diachronous formation of the collisional belts of the Rheic suture zone coeval with recurrent continental breakup and subsequent formation of the mid-ocean ridge systems of the Paleo- and Neo-Tethys oceans at the Devonian and Permian margins of the Gondwana plate, respectively. To decide whether these processes are causally related or not, it is necessary to accurately reconstruct the plate motion of Gondwana relative to Laurussia. Here we propose that the strain pattern preserved in the continental crust can be used for the reconstruction of ancient plate kinematics. We present Euler pole locations for the three fundamental stages of the Late Paleozoic assembly of Pangea and closure of the Rheic Ocean: (I) Early Devonian (ca. 400 Ma) collisional tectonics affected Gondwana at the Armorican Spur north of western Africa and at the promontory of the South China block/Australia of eastern Gondwana, resulting in the Variscan and the Qinling orogenies, respectively. The Euler pole of the rotational axis between Gondwana and Laurussia is positioned east of Gondwana close to Australia. (II) Continued subduction of the western Rheic Ocean initiates the clockwise rotation of Gondwana that is responsible for the separation of the South China block from Gondwana and the opening of Paleo-Tethys during the Late Devonian. The position of the rotational axis north of Africa reveals a shift of the Euler pole to the west. (III) The terminal closure of the Rheic Ocean resulted in the final tectonics of the Alleghanides, the Mauritanides and the Ouachita-Sonora-Marathon belt, occurred after the cessation of the Variscan orogeny in Central Europe, and is coeval with the formation of the Central European Extensional Province and the opening of Neo-Tethys at ca. 300 Ma. The Euler pole for the final closure of the Rheic Ocean is positioned near Oslo (Laurussia). Thus, the concomitant formation of convergent and divergent plate boundaries during the assembly of Pangea is due to the relocation of the particular rotational axis. From a geodynamic point of view, coupled collisional (western Pangea) and extensional tectonics (eastern Pangea) due to plate tectonic reorganization is fully explained by slab pull and ridge push forces.

Tomotectonic constraints on the assembly of Jura-cretaceous western North America, and a southwest Pacific analogue

NASA Astrophysics Data System (ADS)

Mihalynuk, M. G.; Sigloch, K.

2017-12-01

We integrate the topology of subducted slabs imaged beneath North America by seismic tomography, with surviving Atlantic and Pacific seafloor isochrons, to reveal evolving arc, trench, and plate geometries back to the breakup of Pangea. Land geological evidence is used to independently validate or reject the inferred paleogeographies. Such tomotectonic analysis offers a methodology for resolving fundamental questions about the assembly of North America. For example: When, where and how did superterranes collide with North America? Did an Andean-type margin exist along western North America for all times between Cenozoic Cascade arc formation and the Jurassic, when an arc was rooted in continental crust of southwest USA? Deep mantle and seafloor isochron evidence show that TWO massive arc complexes originated in the seas west of Pangea as it started to fragment ( 190-170 Ma), a time when eastward subduction beneath the continental margin arc was shutting down. Most recognizable, and 2000-4000 km off the west coast of Pangea, is a >10,000 km long, east-pointing chevron of slab walls in the lower mantle, with its apex near present-day Nova Scotia. Formerly considered Farallon slab, its location and geometry are not consistent with continent-hugging, Jurassic to Recent Farallon subduction. Instead, a second massive and more westerly slab wall, 4000 - 6000 km west of Pangea, must have intercepted and consumed all northern Farallon lithosphere, and continues to do so beneath the Cascades. Both slabs initiated intraoceanic. Their arcs did not contribute to an Andean-style margin until they were diachronously overridden by North America, beginning 155 Ma. Implied is a continent-spanning suture between these two arc complexes and North America. Land geological evidence of this suture is an Alaska to Mexico track of at least 12 Jura-Cretaceous basins that collapsed between the Insular and Intermontane microcontinents, over half of which contain relicts of mantle. A conceptual framework for the Early Cretaceous North American suturing is found in the southwest Pacific. An analogous arrangement of plates and diachronous suture is forming today as Australia overrides arcs to its north.

Seismic images of the sliver strike-slip fault and back thrust in the Andaman-Nicobar region

NASA Astrophysics Data System (ADS)

Singh, Satish C.; Moeremans, Raphaele; McArdle, Jo; Johansen, Kjell

2013-10-01

sliver strike-slip Great Sumatra Fault (GSF) traverses mainland Sumatra from the Sunda Strait in the southeast to Banda Aceh in the northwest, and defines the present day plate boundary between the Sunda Plate in the north and the Burmese Sliver Plate in the south. It has been well studied on mainland Sumatra but poorly north of Banda Aceh in the Andaman Sea. Here we present deep seismic reflection images along the northward extension of the GSF over 700 km until it joins the Andaman Sea Spreading Centre, and we interpret these images in the light of earthquake, gravity, and bathymetry data. We find that the GSF has two strands between Banda Aceh and Nicobar Island: a transpression in the south and a deep narrow active rift system in the north, dotted with volcanoes in the center, suggesting that the volcanic arc is coincident with rifting. Farther north of Nicobar Island, an active strike-slip fault, the Andaman-Nicobar Fault, cuts through a rifted deep basin until its intersection with the Andaman Sea Spreading Centre. The volcanic arc lies just east of the rift basin. The western margin of this basin seems to be a rifted continental margin, tilted westward, and flooring the Andaman-Nicobar fore-arc basin. The Andaman-Nicobar fore-arc basin is bounded in the west by back thrusts similar to the West Andaman and Mentawai faults. The cluster of seismicity after the 2004 great Andaman-Sumatra earthquake just north of Nicobar Island coincides with the intersection of two strike-slip fault systems.

Post-Neogene tectonism along the Aravalli Range, Rajasthan, India

NASA Astrophysics Data System (ADS)

Sen, Deepawati; Sen, Saurindranath

1983-03-01

The Aravalli Range runs southwest from Delhi for a distance of about 700 km. Its western margin is well defined, but the eastern margin is diffuse. Five geomorphic provinces are recognized in the study area: the western piedmont plains; the ridge and valley province which in the Central Aravallis occurs at two different heights separated by a fault scarp; the plateau province demarcated from the former by a fault scarp, confined to the Southern Aravallis, and occurring for a short stretch at two heights across another fault scarp; the BGC rolling plains east of the Range; and the BGC uplands south of the above. The scarps coincide with Precambrian faults. A series of rapids and water-falls, together with deeply entrenched river courses across the scarps and the youthful aspects of the escarpments with no projecting spurs, or straight river courses along their feet, all point unmistakably to a recent or post-Neogene vertical uplift along pre-existing faults. Presence of knickpoints at a constant distance from the Range in all west-flowing rivers, the ubiquitous terraces, and river courses entrenched within their own flood-plain deposits of thick gritty to conglomeratic sand, are indicative of a constant disturbance with a gradual rise of the Range east of the knickpoint, wherefrom the coarse materials were carried by the fast west-flowing streams. There is a differential uplift across the plateau scarp together with a right-lateral offset. This epeirogenic tectonism is ascribed to the collision of the Eurasian and the subducting Indian plates and to a locking of their continental crusts. By early Pleistocene, with the MBT gradually dying off, continued plate movement caused a flexural bending of the plate by a moment generated at the back, and a possible delinking of the continental crust along the zone of subduction. The felexural bending ripped open the Precambrian regional faults. The differential uplift and the difference in the distances of the nodes on two sides of the major reactivated fault were possibly caused by a difference in the values of the flexural rigidity and the foundation modulus owing to a slight compositional difference of the constiuent rocks in the two sectors.

Rollback of an intraoceanic subduction system and termination against a continental margin

NASA Astrophysics Data System (ADS)

Campbell, S. M.; Simmons, N. A.; Moucha, R.

2017-12-01

The Southeast Indian Slab (SEIS) seismic anomaly has been suggested to represent a Tethyan intraoceanic subduction system which operated during the Jurassic until its termination at or near the margin of East Gondwana (Simmons et al., 2015). As plate reconstructions suggest the downgoing plate remained coupled to the continental margin, this long-lived system likely experienced a significant amount of slab rollback and trench migration (up to 6000 km). Using a 2D thermomechanical numerical code that includes the effects of phase transitions, we test this interpretation by modeling the long-term subduction, transition zone stagnation, and rollback of an intraoceanic subduction system in which the downgoing plate remains coupled to a continental margin. In addition, we also investigate the termination style of such a system, with a particular focus on the potential for some continental subduction beneath an overriding oceanic plate. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-735738

Collisional emplacement history of the Naga-Andaman ophiolites and the position of the eastern Indian suture

NASA Astrophysics Data System (ADS)

Acharyya, S. K.

2007-02-01

Dismembered late Mesozoic ophiolites occur in two parallel belts along the eastern margin of the Indian Plate. The Eastern Belt, closely following the magmatic arc of the Central Burma Basin, coincides with a zone of high gravity. It is considered to mark a zone of steeply dipping mafic-ultramafic rocks and continental metamorphic rocks, which are the locus of two closely juxtaposed sutures. In contrast, the Western Belt, which follows the eastern margin of the Indo-Burma Range and the Andaman outer-island-arc, broadly follows a zone of negative gravity anomalies. Here the ophiolites occur mainly as rootless subhorizontal bodies overlying Eocene-Oligocene flyschoid sediments. Two sets of ophiolites that were accreted during the Early Cretaceous and mid-Eocene are juxtaposed in this belt. These are inferred to be westward propagated nappes from the Eastern Belt, emplaced during the late Oligocene collision between the Burmese and Indo-Burma-Andaman microcontinents. Ophiolite occurrences in the Andaman Islands belong to the Western Belt and are generally interpreted as upthrust oceanic crust, accreted due to prolonged subduction activity to the west of the island arc. This phase of subduction began only in the late Miocene and thus could not have produced the ophiolitic rocks, which were accreted in the late Early Eocene.

Structure and rheology of the lithosphere below southeastern margin of India and Sri Lanka, and its conjugate segment of the east Antarctica: Implications on early breakup history and margin formation

NASA Astrophysics Data System (ADS)

Rao Gangumalla, Srinivasa; Radhakrishna, Munukutla

2014-05-01

The eastern continental margin of India has evolved as a consequence of rifting and breakup between India and east Antarctica during the early Cretaceous. Plate reconstruction models for the breakup of eastern Gondwanaland by many earlier workers have unambiguously placed the southeast margin of Sri Lanka and India together as a conjugate segment with the east Antarctica margin that extends from Gunnerus Ridge in the west to western Enderby basin in the east. In this study, we present results of integrated analysis of gravity, geoid, magnetic and seismic data from these two conjugate portions in order to examine the lithosphere structure and early seafloor spreading, style of breakup, continent-ocean boundary (COB) and rheological characteristics at these margins. The interpreted COB lies at a distance of 55-140 km on the side of southeast margin of Sri Lanka and India, whereas, it lies at a distance of 190-550 km on the side of east Antarctica margin. The seismic profiles and the constrained potential field models across these two segments do not show the existence of seaward dipping reflector sequences or magmatic underplating suggesting that these segments have not encountered major magmatic activity. While, significant crustal thinning/stretching is observed at the east Antarctic margin, the Cauvery offshore had experienced limited stretching with faulted Moho interface. Further, the conspicuous residual geoid low in the Cauvery offshore basin is inferred to be due to a continental crustal block. The modelled Lithosphere-Astenosphere Boundary (LAB) in these two margins is located around 110-120 km depth with slightly thicker lithosphere at the east Antarctica margin. In addition, the interpretation of magnetic anomalies provided structure of the oceanic crust generated through seafloor spreading processes with age and magnetization data constrained from the identified magnetic anomalies in the respective margins. Using the Bouguer coherence method, we computed spatial variations in effective elastic thickness (Te) at these margin segments. The estimated Te values at the Indian margin ranges between 5-8 km in the southeast of Sri Lanka to around 10-12 km in the Cauvery offshore which decrease further north to < 5 km in the Cauvery-Palar basin. Along the east Antarctic margin, the Te values ranges between 5-10 km in the Gunnerus ridge region, 35-40 km in the western Enderby basin which decrease further towards the central Enderby basin up to 20 km. In this study, the above results have been analyzed in terms of early breakup mechanism and subsequent evolution of these two conjugate segments.

Bermuda and Appalachian-Labrador rises: Common non-hotspot processes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Vogt, P.R.

1991-01-01

Other than the Corner Rise-New England seamounts and associated White Mountains, most postbreakup intraplate igneous activity and topographic uplift in the western North Atlantic and eastern North America do not readily conform to simple hotspot models. For examples, the Bermuda Rise trends normal to its predicted hotspot trace. On continental crust, Cretaceous-Eocene igneous activity is scattered along a northeast-trending belt {approximately}500-1,000 km west of and paralleling the continent-ocean boundary. Corresponding activity in the western Atlantic generated seamounts preferentially clustered in a belt {approximately}1,000 km east of the boundary. The Eocene volcanism on Bermuda is paired with coeval magmatism of themore » Shenandoah igneous province, and both magmatic belts are associated with northeast-trending topographic bulges - the Appalachian-Labrador Rise to the west and the Bermuda Rise (Eocene ) to the east. The above observations suggest the existence of paired asthenosphere upwelling, paralleling and controlled by the deep thermal contrast across the northeast-trending continental margin. Such convection geometry, apparently fixed to the North American plate rather than to hotspots, is consistent with recent convection models by B. Hager. The additional importance of plate-kinematic reorganizations (causing midplate stress enhancement) is suggested by episodic igneous activity ca. 90-100 Ma and 40-45 Ma.« less

Epithermal gold-siver deposits in the western United States: time-space products of evolving plutonic, volcanic and tectonic environments

USGS Publications Warehouse

Berger, Byron R.; Bonham, Harold F.

1990-01-01

The western United States has been the locus of considerable subaerial volcanic and plutonic igneous activity since the mid-Mesozoic. After the destruction of the Jurassic-Cretaceous magmatic arc-trench system, subduction was re-established in the Late Mesozoic with low-angle underthrusting of the oceanic plate beneath western North America. This resulted in crustal shortening during the Late Cretaceous to Early Tertiary and removal of the mantle lithosphere west of the Rocky Mountains. Commencing in the Eocene, flat subduction ceased, the volcanic arc began to re-establish itself along the continental margin, and the hingeline along the steepening subducting plate migrated from east to west. The crust east of the migrating hingeline was exposed to hot asthenosphere, and widespread tectonics and volcanic activity resulted. Hydrothermal activity accompanied the volcanism resulting in numerous epithermal gold-silver deposits. The temporal and spatial distributions of epithermal deposits in the region are therefore systematic and can be subdivided into discrete time intervals which are related to widespread changes in magmatic activity. Time intervals selected for discussion are Pre-Cenozoic, 66-55 Ma, 54-43 Ma, 42-34 Ma, 33-24 Ma, 23-17 Ma, and <17 Ma. Many of these intervals contain both sedimentary-rock and two varieties of volcanic-rock hosted deposits (adularia-sericite and alunite-kaolinite ± pyrophyllite). Continental rifting is important to the formation of deposits, and, within any given region, it is at the initiation of deep rifting that alunite-kaolinite ± pyrophyllite type epithermal deposits are formed. Adularia-sericite type deposits are most common, being related to all compositions and styles of volcanic activity. Therefore, the volcano-tectonic context of the western United States provides a unified framework in which to understand and explore for epithermal type deposits.

Electrical conductivity of the crust in central Baja California, México, based on magnetotelluric observations

NASA Astrophysics Data System (ADS)

Romo, J. M.; Gómez-Treviño, E.; Flores-Luna, C.; García-Abdeslem, J.

2017-12-01

Crustal and sub-crustal structure of northwestern Mexico (peninsular California) resulted from major accretion episodes occurred during the long-lived subduction of the Farallon plate beneath the North American plate, since late Jurassic time. A magnetotelluric profile across central Baja California reveals several electrical conductivity anomalies probably associated to the crustal boundaries of distinct Mezosoic terranes juxtaposed in the current peninsular crust. It is known that electrical conductivity is significantly increased by the pervasive presence of conductive minerals generated during metamorphic processes in highly sheared zones. We interpret a striking sub-horizontal conductivity anomaly reveled in the model as explained by the presence of high-salinity fluids released after dehydration of the subducted Magdalena microplate (Farallon plate?). The presence of fluids at the base of the peninsular crust may produce a zone of weakness, which supports the idea that Baja California lithosphere has not been entirely coupled to the Pacific plate. In addition, crustal thickness is estimated in our model in about 35 km beneath the western Peninsular Ranges batholith (PRB) and 20 km beneath the eastern PRB. This crustal thickness is in good agreement with independent estimations of a thinner crust in the Gulf of California margin and a thicker crust along the axial PRB.

Observations at convergent margins concerning sediment subduction, subduction erosion, and the growth of continental crust

USGS Publications Warehouse

von Huene, Roland E.; Scholl, D. W.

1991-01-01

At ocean margins where two plates converge, the oceanic plate sinks or is subducted beneath an upper one topped by a layer of terrestrial crust. This crust is constructed of continental or island arc material. The subduction process either builds juvenile masses of terrestrial crust through arc volcanism or new areas of crust through the piling up of accretionary masses (prisms) of sedimentary deposits and fragments of thicker crustal bodies scraped off the subducting lower plate. At convergent margins, terrestrial material can also bypass the accretionary prism as a result of sediment subduction, and terrestrial matter can be removed from the upper plate by processes of subduction erosion. Sediment subduction occurs where sediment remains attached to the subducting oceanic plate and underthrusts the seaward position of the upper plate's resistive buttress (backstop) of consolidated sediment and rock. Sediment subduction occurs at two types of convergent margins: type 1 margins where accretionary prisms form and type 2 margins where little net accretion takes place. At type 2 margins (???19,000 km in global length), effectively all incoming sediment is subducted beneath the massif of basement or framework rocks forming the landward trench slope. At accreting or type 1 margins, sediment subduction begins at the seaward position of an active buttress of consolidated accretionary material that accumulated in front of a starting or core buttress of framework rocks. Where small-to-mediumsized prisms have formed (???16,300 km), approximately 20% of the incoming sediment is skimmed off a detachment surface or decollement and frontally accreted to the active buttress. The remaining 80% subducts beneath the buttress and may either underplate older parts of the frontal body or bypass the prism entirely and underthrust the leading edge of the margin's rock framework. At margins bordered by large prisms (???8,200 km), roughly 70% of the incoming trench floor section is subducted beneath the frontal accretionary body and its active buttress. In rounded figures the contemporary rate of solid-volume sediment subduction at convergent ocean margins (???43,500 km) is calculated to be 1.5 km3/yr. Correcting type 1 margins for high rates of terrigenous seafloor sedimentation during the past 30 m.y. or so sets the long-term rate of sediment subduction at 1.0 km3/yr. The bulk of the subducted material is derived directly or indirectly from continental denudation. Interstitial water currently expulsed from accreted and deeply subducted sediment and recycled to the ocean basins is estimated at 0.9 km3/yr. The thinning and truncation caused by subduction erosion of the margin's framework rock and overlying sedimentary deposits have been demonstrated at many convergent margins but only off northern Japan, central Peru, and northern Chile has sufficient information been collected to determine average or long-term rates, which range from 25 to 50 km3/m.y. per kilometer of margin. A conservative long-term rate applicable to many sectors of convergent margins is 30 km3/km/m.y. If applied to the length of type 2 margins, subduction erosion removes and transports approximately 0.6 km3/yr of upper plate material to greater depths. At various places, subduction erosion also affects sectors of type 1 margins bordered by small- to medium-sized accretionary prisms (for example, Japan and Peru), thus increasing the global rate by possibly 0.5 km3/yr to a total of 1.1 km3/yr. Little information is available to assess subduction erosion at margins bordered by large accretionary prisms. Mass balance calculations allow assessments to be made of the amount of subducted sediment that bypasses the prism and underthrusts the margin's rock framework. This subcrustally subducted sediment is estimated at 0.7 km3/yr. Combined with the range of terrestrial matter removed from the margin's rock framework by subduction erosion, the global volume of subcrustally subducted materia

Earth's glacial record and its tectonic setting

NASA Astrophysics Data System (ADS)

Eyles, N.

1993-09-01

Glaciations have occurred episodically at different time intervals and for different durations in Earth's history. Ice covers have formed in a wide range of plate tectonic and structural settings but the bulk of Earth's glacial record can be shown to have been deposited and preserved in basins within extensional settings. In such basins, source area uplift and basin subsidence fulfill the tectonic preconditions for the initiation of glaciation and the accomodation and preservation of glaciclastic sediments. Tectonic setting, in particular subsidence rates, also dictates the type of glaciclastic facies and facies successions that are deposited. Many pre-Pleistocene glaciated basins commonly contain well-defined tectonostratigraphic successions recording the interplay of tectonics and sedimentation; traditional climatostratigraphic approaches involving interpretation in terms of either ice advance/retreat cycles or glacio-eustatic sea-level change require revision. The direct record of continental glaciation in Earth history, in the form of classically-recognised continental glacial landforms and "tillites", is meagre; it is probable that more than 95% of the volume of preserved "glacial" strata are glacially-influenced marine deposits that record delivery of large amounts of glaciclastic sediment to offshore basins. This flux has been partially or completely reworked by "normal" sedimentary processes such that the record of glaciation and climate change is recorded in marine successions and is difficult to decipher. The dominant "glacial" facies in the rock record are subaqueous debris flow diamictites and turbidites recording the selective preservation of poorly-sorted glaciclastic sediment deposited in deep water basins by sediment gravity flows. However, these facies are also typical of many non-glacial settings, especially volcanically-influenced environments; numerous Archean and Proterozoic diamictites, described in the older literature as tillites, have no clearly established glacial parentage. The same remarks apply to many successions of laminated and thin-bedded facies interpreted as "varvites". Despite suggestions of much lower values of solar luminosity (the weak young sun hypothesis), the stratigraphic record of Archean glaciations is not extensive and may be the result of non-preservation. However, the effects of very different Archean global tectonic regimes and much higher geothermal heat flows, combined with a Venus-like atmosphere warmed by elevated levels of CO 2, cannot be ruled out. The oldest unambiguous glacial succession in Earth history appears to be the Early Proterozoic Gowganda Formation of the Huronian Supergroup in Ontario; the age of this event is not well-constrained but glaciation coincided with regional rifting, and may be causally related to, oxygenation of Earth's atmosphere just after 2300 Ma. New evidence that oxygenation is tectonically, not biologically driven, stresses the intimate relationship between plate tectonics, evolution of the atmosphere and glaciation. Global geochemical controls, such as elevated atmospheric CO 2 levels, may be responsible for a long mid-Proterozoic non-glacial interval after 2000 Ma that was terminated by the Late Proterozoic glaciations just after 800 Ma. A persistent theme in both Late Proterozoic and Phanerozoic glaciations is the adiabatic effect of tectonic uplift, either along collisional margins or as a result of passive margin uplifts in areas of extended crust, as the trigger for glaciation; the process is reinforced by global geochemical feedback, principally the drawdown of atmospheric CO 2 and Milankovitch "astronomical" forcing but these are unlikely, by themselves, to inititiate glaciation. The same remarks apply to late Cenozoic glaciations. Late Proterozoic glacially-influenced strata occur on all seven continents and fall into two tectonostratigraphic types. In the first category are thick sucessions of turbidites and mass flows deposited along active, compressional plate margins recording a protracted and complex phase of supercontinent assembly between 800 and 550 Ma. Local cordilleran glaciations of volcanic peaks is indicated. Many deposits are preserved within mobile belts that record the subduction of interior oceans now preserved as "welds" between different cratons. Discrimination between glacially-influenced and non-glacial, volcaniclastic mass flow successions continues to be problematic. The second tectonostratigraphic category of Late Proterozoic glacial strata includes successions of glacially-influenced, mostly marine strata deposited along rifted, extensional plate margins. The oldest (Sturtian) glaciclastic sediments result from the break-out of Laurentia from the Late Proterozoic supercontinent starting around 750 Ma along its "palaeo-Pacific" margin with a later (Marinoan) phase of rifting at about 650 Ma. "Passive margin" uplifts and the generation of "adiabatic" ice covers on uplifted crustal blocks triggered widespread glaciation along the "palaeo-Pacific" margin of North America and in Australia. A major phase of rifting along the opposite ("palaeo-Atlantic") margin of Laurentia occurred after 650 Ma and is similarly recorded by glaciclastic strata in basins preserved around the margins of the present day North Atlantic Ocean. Glaciation of the west African platform after 650 Ma is closely related to collision of the West African and Guyanan cratons and uplift of the orogenic belt; the same process, involving uplift around the northern and western margins of the Afro-Arabian platform subsequently triggered Late Ordovician glaciation at about 440 Ma when the south polar region lay over North Africa. Early Silurian glaciation in Bolivia and Brazil was followed by a non-glacial episode and renewed Late Devonian glaciation of northern Brazil and Bolivia. The latter event may have resulted from rotation of Gondwana under the South Pole combined with active orogenesis along the western margin of the supercontinent. Hercynian uplift along the western margin of South America caused by the collision and docking of "Chilinia" at about 350 Ma (Late Tournasian—Early Visean) was the starting point of a long Late Palaeozoic glacial record that terminated at about 255 Ma (Kungurian-Kazanian) in western Australia. The arrival of large landmasses at high latitude may have been an important precondition for ice growth. Strong Namurian uplift around virtually the entire palaeo-Pacific rim of Gondwana culminated in glaciation of the interior of the supercontinent during the latest Westphalian (c. 300 Ma). There is a clear picture of plate margin compression and propagation of "far field" stresses to the plate interior allowing preservation of glacially-influenced strata in newly-rifted intracratonic basins. Many basins show a "steer's head" style of infill architecture recording successive phases of subsidence and overstepping of younger strata during basin subsidence and expansion. Exploration for oil and gas in Gondwanan glaciated basins is currently a major stimulus to understanding the relationship between tectonics and sedimentation. Warm Mesozoic palaeoclimates do not rule out the existence of restricted ice covers in the interiors of continental landmasses at high palaeolatitudes (e.g. Siberia, Antarctica) but there is as yet, no direct geological record of their existence. The most likely record of glaciers is contained in Late Jurassic and early Cretaceous strata. In any event, these ice masses are unlikely to have had any marked effect on global sea levels and alternative explanations should perhaps be sought for 4th order, so-called "glacio-eustatic" changes in sea level, inferred from Triassic, Jurassic and Cretaceous strata. The growth of extensive Northern Hemisphere ice sheets in Plio-Pleistocene time (c. 2.5 Ma) was the culmination of a long global climatic deterioration that began sometime after 60 Ma during the late Tertiary. Tectonic uplift of areas such as the Tibetan Plateau and plate tectonic reorganizations have been identified as first-order controls. Initiation of the East Antarctic ice sheet, at about 36 Ma, is the result of the progressive thermal isolation of the continent combined with uplift along the Transantarctic Mountains. In the Northern Hemisphere, the upwarping of extensive passive margin plateaux around the margins of the newly-rifted North Atlantic may have amplified global climatic changes and set the scene for the growth of continental ice sheets after 2.5 Ma. Ice sheet growth and decay was driven by complexly interrelated changes in ocean circulation, Milankovitch orbital forcing and global geochemical cycles. It is arguable whether continental glaciations of the Northern Hemisphere, and the evolution of hominids, would have occurred without the necessary precondition of tectonic uplift.

Seismicity of the Earth 1900-2007, Japan and Vicinity

USGS Publications Warehouse

Rhea, Susan; Tarr, Arthur C.; Hayes, Gavin P.; Villaseñor, Antonio; Benz, Harley

2010-01-01

This map shows details of Japan and vicinity not visible in an earlier publication, U.S. Geological Survey Scientific Investigations Map 3064. Japan and its island possessions lie across four major tectonic plates: Pacific plate, North America plate; Eurasia plate; and Philippine Sea plate. The Pacific plate is subducted into the mantle, beneath Hokkaido and northern Honshu, along the eastern margin of the Okhotsk microplate, a proposed subdivision of the North America plate (Bird, 2003). Farther south, the pacific plate is subducted beneath volcanic islands along the eastern margin of the Philippine Sea plate. This 2,200 km-long zone of subduction of the Pacific plate is responsible for the creation of the deep offshore Ogasawara and Japan trenches as well as parallel chains of islands and volcanoes, typical of the Circumpacific island arcs. Similarly, the Philippine Sea plate is itself subducting under the Eurasia plate along a zone, extending from Taiwan to southern Honshu, that comprises the Ryuku Islands and the Nansei-Shonto trench.

Seismic tomographic constraints on plate-tectonic reconstructions of Nazca subduction under South America since late Cretaceous (˜80 Ma)

NASA Astrophysics Data System (ADS)

Chen, Y. W.; Wu, J.; Suppe, J.

2017-12-01

Global seismic tomography has provided new and increasingly higher resolution constraints on subducted lithospheric remnants in terms of their position, depth, and volumes. In this study we aim to link tomographic slab anomalies in the mantle under South America to Andean geology using methods to unfold (i.e. structurally restore) slabs back to earth surface and input them to globally consistent plate reconstructions (Wu et al., 2016). The Andean margin of South America has long been interpreted as a classic example of a continuous subduction system since early Jurassic or later. However, significant gaps in Andean plate tectonic reconstructions exist due to missing or incomplete geology from extensive Nazca-South America plate convergence (i.e. >5000 km since 80 Ma). We mapped and unfolded the Nazca slab from global seismic tomography to produce a quantitative plate reconstruction of the Andes back to the late Cretaceous 80 Ma. Our plate model predicts the latest phase of Nazca subduction began in the late Cretaceous subduction after a 100 to 80 Ma plate reorganization, which is supported by Andean geology that indicates a margin-wide compressional event at the mid-late Cretaceous (Tunik et al., 2010). Our Andean plate tectonic reconstructions predict the Andean margin experienced periods of strike-slip/transtensional and even divergent plate tectonics between 80 to 55 Ma. This prediction is roughly consistent with the arc magmatism from northern Chile between 20 to 36°S that resumed at 80 Ma after a magmatic gap. Our model indicates the Andean margin only became fully convergent after 55 Ma. We provide additional constraints on pre-subduction Nazca plate paleogeography by extracting P-wave velocity perturbations within our mapped slab surfaces following Wu et al. (2016). We identified localized slow anomalies within our mapped Nazca slab that apparently show the size and position of the subducted Nazca ridge, Carnegie ridge and the hypothesized Inca plateau within the Nazca slab. These intra-slab velocity anomalies provide the most complete tomographic evidence to date in support the classic, but still controversial hypothesis of subducted, relatively buoyant oceanic lithosphere features along the Andean margin.

Fuel cell separator plate with bellows-type sealing flanges

DOEpatents

Louis, G.A.

1984-05-29

A fuel cell separator includes a rectangular flat plate having two unitary upper sealing flanges respectively comprising opposite marginal edges of the plate folded upwardly and back on themselves and two lower sealing flanges respectively comprising the other two marginal edges of the plate folded downwardly and back on themselves. Each of the sealing flanges includes a flat wall spaced from the plate and substantially parallel thereto and two accordion-pleated side walls, one of which interconnects the flat wall with the plate and the other of which steps just short of the plate, these side walls affording resilient compressibility to the sealing flange in a direction generally normal to the plane of the plate. Four corner members close the ends of the sealing flanges. An additional resiliently compressible reinforcing member may be inserted in the passages formed by each of the sealing flanges with the plate.

Fuel cell separator plate with bellows-type sealing flanges

DOEpatents

Louis, George A.

1986-08-05

A fuel cell separator includes a rectangular flat plate having two unitary upper sealing flanges respectively comprising opposite marginal edges of the plate folded upwardly and back on themselves and two lower sealing flanges respectively comprising the other two marginal edges of the plate folded downwardly and back on themselves. Each of the sealing flanges includes a flat wall spaced from the plate and substantially parallel thereto and two accordion-pleated side walls, one of which interconnects the flat wall with the plate and the other of which stops just short of the plate, these side walls affording resilient compressibility to the sealing flange in a directiongenerally normal to the plane of the plate. Four corner members close the ends of the sealing flanges. An additional resiliently compressible reinforcing member may be inserted in the passages formed by each of the sealing flanges with the plate.

Discovery of a Devonian mafic magmatism on the western border of the Murzuq basin (Saharan metacraton): Paleomagnetic dating and geodynamical implications

NASA Astrophysics Data System (ADS)

Derder, M. E. M.; Maouche, S.; Liégeois, J. P.; Henry, B.; Amenna, M.; Ouabadi, A.; Bellon, H.; Bruguier, O.; Bayou, B.; Bestandji, R.; Nouar, O.; Bouabdallah, H.; Ayache, M.; Beddiaf, M.

2016-03-01

Intraplate deformation is most often linked to major stress applied on plate margins. When such intraplate events are accompanied by magmatism, the use of several dating methods integrated within a multidisciplinary approach can bring constraints on the age, nature and source mobilized for generating the magma and in turn on the nature of the intraplate deformation. This study focuses on the large gabbro Arrikine sill (35 km in extension) emplaced within the Silurian sediments of the western margin of the Murzuq cratonic basin in southeastern Algeria. Its emplacement is dated during the early Devonian (415-400 Ma) through the determination of a reliable paleomagnetic pole by comparison with the Gondwana Apparent Polar Wander Path (APWP). This age can be correlated with deep phreatic eruptions before Pragian time thought to be at the origin of sand injections and associated circular structures in Algeria and Libya. For the sill, the K-Ar age of 325.6 ± 7.7 Ma is related to a K-rich aplitic phase that has K-enriched by more than 20% the Devonian gabbro. Laser-ICP-MS U-Pb method dates only inherited zircons mostly at c. 2030 Ma with additional ages at c. 2700 Ma and younger ones in the 766-598 Ma age range. The Arrikine sill is a high-Ti alkaline gabbro having the geochemical composition of a hawaiite akin to several intraplate continental and oceanic provinces, including the contemporaneous Aïr ring complexes province in Niger, but also to the Mauna Loa volcano in Hawaii. This peculiar composition akin to that of the contemporaneous Aïr province is in agreement with a lower Devonian age for the Arrikine sill. The lower Devonian Arrikine sill emplacement is related to a "Caledonian" transtensive reactivation of the western metacratonic boundary of the Murzuq craton. This event also generated in the Saharan platform the so-called "Caledonian unconformity" of regional extension, the Aïr ring complexes and magmatic rocks that produced sand injections. It could be related to rifting of the Hun terranes that occurred at the plate margin to the north (Stampfli and Borel, 2002, Blackey, 2008 and references therein). The mid-Carboniferous (c. 326 Ma) reactivation corresponds to Variscan compression on NW Africa generating aplitic fluids, but also to the major "Hercynian unconformity" of regional extension. The generation of the Arrikine magma is attributed to partial melting through adiabatic pressure release of uprising asthenosphere along tectonically reactivated mega-shear zones, here bordering the relictual Murzuq craton enclosed in the Saharan metacraton.

Structure and evolution of the eastern Gulf of Aden conjugate margins from seismic reflection data

NASA Astrophysics Data System (ADS)

d'Acremont, Elia; Leroy, Sylvie; Beslier, Marie-Odile; Bellahsen, Nicolas; Fournier, Marc; Robin, Cécile; Maia, Marcia; Gente, Pascal

2005-03-01

The Gulf of Aden is a young and narrow oceanic basin formed in Oligo-Miocene time between the rifted margins of the Arabian and Somalian plates. Its mean orientation, N75°E, strikes obliquely (50°) to the N25°E opening direction. The western conjugate margins are masked by Oligo-Miocene lavas from the Afar Plume. This paper concerns the eastern margins, where the 19-35 Ma breakup structures are well exposed onshore and within the sediment-starved marine shelf. Those passive margins, about 200 km distant, are non-volcanic. Offshore, during the Encens-Sheba cruise we gathered swath bathymetry, single-channel seismic reflection, gravity and magnetism data, in order to compare the structure of the two conjugate margins and to reconstruct the evolution of the thinned continental crust from rifting to the onset of oceanic spreading. Between the Alula-Fartak and Socotra major fracture zones, two accommodation zones trending N25°E separate the margins into three N110°E-trending segments. The margins are asymmetric: offshore, the northern margin is narrower and steeper than the southern one. Including the onshore domain, the southern rifted margin is about twice the breadth of the northern one. We relate this asymmetry to inherited Jurassic/Cretaceous rifts. The rifting obliquity also influenced the syn-rift structural pattern responsible for the normal faults trending from N70°E to N110°E. The N110°E fault pattern could be explained by the decrease of the influence of rift obliquity towards the central rift, and/or by structural inheritance. The transition between the thinned continental crust and the oceanic crust is characterized by a 40 km wide zone. Our data suggest that its basement is made up of thinned continental crust along the southern margin and of thinned continental crust or exhumed mantle, more or less intruded by magmatic rocks, along the northern margin.

Balancing shortening and extension around the Adriatic Plate to constrain its independent motion and driving forces since Late Cretaceous time.

NASA Astrophysics Data System (ADS)

Le Breton, E.; Handy, M.; Ustaszewski, K. M.

2015-12-01

The Adriatic microplate (Adria) is a key player in the geodynamics of the Western Mediterranean area because it separates two major plates, Africa and Europe, that have been converging since Late Cretaceous time. Today, Adria comprises only continental lithosphere and is surrounded by zones of distributed deformation along convergent boundaries (Alps, Apennines, Calabrian Arc, Dinarides-Hellenides,) and back-arc basins (Liguro-Provencal, Tyrrhenian). For a long time, Adria was thought to be a promontory of Africa and thus to have moved coherently with Africa. However, recent re-evaluation of geological and geophysical data from the Alps yields an independent motion path for Adria that features a significant change in the direction and rate of its motion relative to both Africa and Europe since late Cretaceous time. To evaluate this, we first compare existing plate reconstructions of the Western Mediterranean to develop a best-fit model for the motion of Africa, Iberia and the Corsica-Sardinia block relative to Europe. We then use two motion models for Adria in which Adria moved either coherently or independently of Africa since late Cretaceous time. The model for independent Adria motion is further constrained by new estimates of extension and shortening in the Western Mediterranean and Northern Apennines based on field observations and recently published Moho depth maps, seismic profiles along the Gulf of Lion - Sardinian passive margins and the Northern Apennines. Initial results suggest that Miocene extension and opening of the Liguro-Provencal basin exceeds Miocene-to-Recent shortening related to roll-back subduction in the Northern Apennines; we attribute this to counter-clockwise rotation of the Adriatic plate with respect to Europe. Combined with the previously published estimates of shortening in the Alps, this counter-clockwise motion is predicted to have produced significantly less post-Paleogene, orogen-normal shortening in the Dinarides than previously thought. This modified motion path for Adria raises the question of what forces drive the motion of Adria; so far, the most likely explanation invokes a combination of trench suction and slab pull along the northern borders of Adria in Late Cretaceous-Paleogene time, transitional to Africa push since Early Miocene time.

Edge-driven microplate kinematics

USGS Publications Warehouse

Schouten, Hans; Klitgord, Kim D.; Gallo, David G.

1993-01-01

It is known from plate tectonic reconstructions that oceanic microplates undergo rapid rotation about a vertical axis and that the instantaneous rotation axes describing the microplate's motion relative to the bounding major plates are frequently located close to its margins with those plates, close to the tips of propagating rifts. We propose a class of edge-driven block models to illustrate how slip across the microplate margins, block rotation, and propagation of rifting may be related to the relative motion of the plates on either side. An important feature of these edge-driven models is that the instantaneous rotation axes are always located on the margins between block and two bounding plates. According to those models the pseudofaults or traces of disrupted seafloor resulting from the propagation of rifting between microplate and major plates may be used independently to approximately trace the continuous kinematic evolution of the microplate back in time. Pseudofault geometries and matching rotations of the Easter microplate show that for most of its 5 m.y. history, block rotation could be driven by the drag of the Nazca and Pacific plates on the microplate's edges rather than by a shear flow of mantle underneath.

Cenozoic forearc tectonics in northeastern Japan: Relationships between outer forearc subsidence and plate boundary kinematics

NASA Astrophysics Data System (ADS)

Regalla, Christine

Here we investigate the relationships between outer forearc subsidence, the timing and kinematics of upper plate deformation and plate convergence rate in Northeast Japan to evaluate the role of plate boundary dynamics in driving forearc subsidence. The Northeastern Japan margin is one of the first non-accretionary subduction zones where regional forearc subsidence was argued to reflect tectonic erosion of large volumes of upper crustal rocks. However, we propose that a significant component of forearc subsidence could be the result of dynamic changes in plate boundary geometry. We provide new constraints on the timing and kinematics of deformation along inner forearc faults, new analyses of the evolution of outer forearc tectonic subsidence, and updated calculations of plate convergence rate. These data collectively reveal a temporal correlation between the onset of regional forearc subsidence, the initiation of upper plate extension, and an acceleration in local plate convergence rate. A similar analysis of the kinematic evolution of the Tonga, Izu-Bonin, and Mariana subduction zones indicates that the temporal correlations observed in Japan are also characteristic of these three non-accretionary margins. Comparison of these data with published geodynamic models suggests that forearc subsidence is the result of temporal variability in slab geometry due to changes in slab buoyancy and plate convergence rate. These observations suggest that a significant component of forearc subsidence at these four margins is not the product of tectonic erosion, but instead reflects changes in plate boundary dynamics driven by variable plate kinematics.

Deformation of the Western Caribbean: Insights from Block and Geodynamic Models of Geodetic, Seismic and Geologic Data

NASA Astrophysics Data System (ADS)

La Femina, P. C.; Geirsson, H.; Kobayashi, D.

2012-12-01

Cocos - Caribbean convergence along the Middle America Trench, including subduction of the Cocos Ridge and seamount domain, and Nazca - Caribbean oblique convergence along the South Panama Deformed Belt have resulted in complex plate boundary zone deformation since Miocene - Pliocene time. Plate boundary evolution and upper plate deformation in the western Caribbean is well studied and indicates, 1) Quaternary migration of the volcanic arc toward the back-arc northwest of the Cocos Ridge; 2) Pleistocene to present northwestward motion of the Central American Fore Arc (CAFA); 3) Quaternary to present deformation within the Central Costa Rica Deformed Belt; 4) Miocene-Pliocene cessation of volcanism and uplift of the Cordillera de Talamanca inboard the ridge; 5) Quaternary to present shortening across the fore-arc Fila Costeña fold and thrust belt and back-arc North Panama Deformed Belt (NPDB); 6) Quaternary to present outer fore-arc uplift above the seamount domain (Nicoya Peninsula), and above (Osa Peninsula) and flanking (Burica Peninsula) the ridge; and 7) Quaternary to present faulting along the Sona-Azuero and Canal Discontinuity fault systems. We investigate the dynamic effects of Cocos and Nazca convergence along the entire Central American margin, and the implications on western Caribbean plate boundary evolution with a new GPS derived three-dimensional (horizontal and vertical) velocity field and kinematic block and geodynamic models. Specifically, we test the hypotheses that the Cocos Ridge is the main driver for upper plate deformation and that there is an independent Panama block. Our model results provide new Euler vectors for the CAFA and Panama block, rates of relative plate and block motions in the region, and constraints on interseismic coupling along the Middle America Trench and other major block bounding fault systems. These results are compared to existing geophysical and geologic data for the region and add insights into the rates of deformation across the regions listed above. We demonstrate that Cocos Ridge collision drives northwest-directed motion of the CAFA and the northeast-directed motion of the Panama region. The Panama region is driven into the Caribbean across the NPDB and into the Choco and North Andes blocks of northwestern South America, which are also converging with the Panama region, pushing it toward the west-northwest. Motion of the Panama region can be fit by an Euler vector suggesting that it is a rigid block, however, this is not in agreement with Quaternary faulting across the isthmus.

Ridge-trench collision in Archean and Post-Archean crustal growth: Evidence from southern Chile

NASA Technical Reports Server (NTRS)

Nelson, E. P.; Forsythe, R. D.

1988-01-01

The growth of continental crust at convergent plate margins involves both continuous and episodic processes. Ridge-trench collision is one episodic process that can cause significant magmatic and tectonic effects on convergent plate margins. Because the sites of ridge collision (ridge-trench triple junctions) generally migrate along convergent plate boundaries, the effects of ridge collision will be highly diachronous in Andean-type orogenic belts and may not be adequately recognized in the geologic record. The Chile margin triple junction (CMTJ, 46 deg S), where the actively spreading Chile rise is colliding with the sediment-filled Peru-Chile trench, is geometrically and kinematically the simplest modern example of ridge collision. The south Chile margin illustrates the importance of the ridge-collision tectonic setting in crustal evolution at convergent margins. Similarities between ridge-collision features in southern Chile and features of Archean greenstone belts raise the question of the importance of ridge collision in Archean crustal growth. Archean plate tectonic processes were probably different than today; these differences may have affected the nature and importance of ridge collision during Archean crustal growth. In conclusion, it is suggested that smaller plates, greater ridge length, and/or faster spreading all point to the likelihood that ridge collision played a greater role in crustal growth and development of the greenstone-granite terranes during the Archean. However, the effects of modern ridge collision, and the processes involved, are not well enough known to develop specific models for the Archean ridge collison.

Evolution of passive continental margins and initiation of subduction zones

NASA Astrophysics Data System (ADS)

Cloetingh, S. A. P. L.; Wortel, M. J. R.; Vlaar, N. J.

1982-05-01

Although the initiation of subduction is a key element in plate tectonic schemes for evolution of lithospheric plates, the underlying mechanisms are not well understood. Plate rupture is an important aspect of the process of creating a new subduction zone, as stresses of the order of kilobars are required to fracture oceanic lithosphere1. Therefore initiation of subduction could take place preferentially at pre-existing weakness zones or in regions where the lithosphere is prestressed. As such, transform faults2,3 and passive margins4,5 where the lithosphere is downflexed under the influence of sediment loading have been suggested. From a model study of passive margin evolution we found that ageing of passive margins alone does not make them more suitable sites for initiation of subduction. However, extensive sediment loading on young lithosphere might be an effective mechanism for closure of small ocean basins.

Seismicity and Seismic Hazard along the Western part of the Eurasia-Nubia plate boundary

NASA Astrophysics Data System (ADS)

Bezzeghoud, Mourad; Fontiela, João; Ferrão, Celia; Borges, José Fernando; Caldeira, Bento; Dib, Assia; Ousadou, Farida

2016-04-01

The seismic phenomenon is the most damaging natural hazard known in the Mediterranean area. The western part of the Eurasia-Nubia plate boundary extends from the Azores to the Mediterranean region. The oceanic part of the plate boundary is well delimited from the Azores Islands, along the Azores-Gibraltar fault to approximately 12°W (west of the Strait of Gibraltar). From 12°W to 3.5°E, including the Iberia-Nubia region and extending to the western part of Algeria, the boundary is more diffuse and forms a wider area of deformation. The boundary between the Iberia and Nubia plates is the most complex part of the margin. This region corresponds to the transition from an oceanic boundary to a continental boundary, where Iberia and Nubia collide. Although most earthquakes along this plate boundary are shallow and generally have magnitudes less than 5.5, there have been several high-magnitude events. Many devastating earthquakes, some of them tsunami-triggering, inflicted heavy loss and considerable economic damage to the region. From 1920 to present, three earthquakes with magnitudes of about 8.0 (Mw 8.2, 25 November 1941; Ms 8.0, 25 February 1969; and Mw 7.9, 26 May 1975) occurred in the oceanic region, and four earthquakes with magnitudes of about 7.0 (Mw 7.1, 8 May 1939, Santa Maria Island and Mw 7.1, January 1980, Terceira and Graciosa Islands, both in the Azores; Ms 7.1, 20 May 1931, Azores-Gibraltar fracture zone; and Mw 7.3, 10 October 1980, El Asnam, Algeria) occurred along the western part of the Eurasia-Nubia plate boundary. In general, large earthquakes (M ≥7) occur within the oceanic region, with the exception of the El Asnam (Algeria) earthquakes. Some of these events caused extensive damage. The 1755 Lisbon earthquake (˜Mw 9) on the Portugal Atlantic margin, about 200 km W-SW of Cape St. Vincent, was followed by a tsunami and fires that caused the near-total destruction of Lisbon and adjacent areas. Estimates of the death toll in Lisbon alone (~70,000) make it one of the deadliest earthquakes in history. Measured in lives lost, the 1926, 1980 and 1998 Azores earthquakes (Portugal), the 1954 and 1980 El Asnam earthquakes (North Algeria), the 1994 and 2004 Alhoceima earthquakes (North Morocco), and the 2003 Boumerdes earthquakes (North Algeria) were the worst earthquakes in the past 120 years in the study area. Hence, this region has experienced many large and damaging earthquakes. The city of Cairo (Egypt) was struck in October 1992 by an Mw 5.8 magnitude earthquake, which caused large damage. In 1935, the Syrte region in Libya experienced an M6.9 earthquake with severe damage. Generally, North Africa has experienced moderate earthquakes. However, the region remains vulnerable due to the shallow seismicity, the poor mechanical properties of its soil and local site conditions, and the consequent strength of the ground shaking. Knowing the behaviour of a seismogenic area, particularly the fault zone, will lead us to better assess the hazard and risk in and around large urban areas. In order to mitigate the destructive impact of the earthquakes, the regional seismic hazard in North Africa is assessed using different approaches (ex. deterministic and probabilistic) using historical and instrumental seismicity, earthquake sources, seismotectonic zonation, structural models and attenuation laws. As a result, reliable seismic hazard maps are produced in terms of maximum displacement and in terms of maximum intensity map. This research is funded by the Fundação para a Ciência e a Tecnologia (FCT, Portugal) under the project ICT-UID/GEO/04683/2013. This study also was conducted within the scope of the MEDYNA FP7-PEOPLE-2013-IRSES project, WP-1: Present-day Kinematics and seismic hazards, funded by the Seventh Framework European Programme.

Integrating surface and mantle constraints for palaeo-ocean evolution: a tour of the Arctic and adjacent regions (Arne Richter Award for Outstanding Young Scientists Lecture)

NASA Astrophysics Data System (ADS)

Shephard, Grace E.

2016-04-01

Plate tectonic reconstructions heavily rely on absolute motions derived from hotspot trails or palaeomagnetic data and ocean-floor magnetic anomaies and fracture-zone geometries to constrain the detailed history of ocean basins. However, as oceanic lithosphere is progressively recycled into the mantle, kinematic data regarding the history of these now extinct-oceans is lost. In order to better understand their evolution, novel workflows, which integrate a wide range of complementary yet independent geological and geophysical datasets from both the surface and deep mantle, must be utilised. In particular, the emergence of time-dependent, semi or self-consistent geodynamic models of ever-increasing temporal and spatial resolution are revealing some critical constraints on the evolution and fate of oceanic slabs. The tectonic evolution of the circum-Arctic is no exception; since the breakup of Pangea, this enigmatic region has seen major plate reorganizations and the opening and closure of several ocean basins. At the surface, a myriad of potential kinematic scenarios including polarity, timing, geometry and location of subduction have emerged, including for systems along continental margins and intra-oceanic settings. Furthermore, recent work has reignited a debate about the origins of 'anchor' slabs, such as the Farallon and Mongol-Okhotsk slabs, which have been used to refine absolute plate motions. Moving to the mantle, seismic tomography models reveal a region peppered with inferred slabs, however assumptions about their affinities and subduction location, timing, geometry and polarity are often made in isolation. Here, by integrating regional plate reconstructions with insights from seismic tomography, satellite derived gravity gradients, slab sinking rates and geochemistry, I explore some Mesozoic examples from the palaeo-Arctic, northern Panthalassa and western margin of North America, including evidence for a discrete and previously undescribed slab under present-day Greenland. While regional in focus, the methods and insights described have global applications and illustrate the power of an integrated approach.

Large-scale fault interactions at the termination of a subduction margin

NASA Astrophysics Data System (ADS)

Mouslopoulou, V.; Nicol, A., , Prof; Moreno, M.; Oncken, O.; Begg, J.; Kufner, S. K.

2017-12-01

Active subduction margins terminate against, and transfer their slip onto, plate-boundary transform faults. The manner in which plate motion is accommodated and partitioned across such kinematic transitions from thrust to strike-slip faulting over earthquake timescales, is poorly documented. The 2016 November 14th, Mw 7.8 Kaikoura Earthquake provides a rare snapshot of how seismic-slip may be accommodated at the tip of an active subduction margin. Analysis of uplift data collected using a range of techniques (field measurements, GPS, LiDAR) and published mapping coupled with 3D dislocation modelling indicates that earthquake-slip ruptured multiple faults with various orientations and slip mechanisms. Modelled and measured uplift patterns indicate that slip on the plate-interface was minor. Instead, a large offshore thrust fault, modelled to splay-off the plate-interface and to extend to the seafloor up to 15 km east of the South Island, appears to have released subduction-related strain and to have facilitated slip on numerous, strike-slip and oblique-slip faults on its hanging-wall. The Kaikoura earthquake suggests that these large splay-thrust faults provide a key mechanism in the transfer of plate motion at the termination of a subduction margin and represent an important seismic hazard.

Arabian Plate Deformation: The role of inherited structures in the localization of strain in the Red Sea extensional system

NASA Astrophysics Data System (ADS)

Aldaajani, T.; Furlong, K.; Malservisi, R.

2017-12-01

The Red Sea rift structural architecture changes dramatically along strike from narrow localized spreading (with creation of new oceanic crust) in the south to asymmetrical diffuse extension north of 21 ° latitude. The region of diffuse extension falls within a triangle that is bounded to the east by the Sarhan graben, (a Cenozoic failed rift), to the west by the northern Red Sea Rift, and to the south by the Makkah-Madinah-Nafud (MMN) volcanic line. Geological observations appear to show that tectonic stresses acting on inherited structures within the NW Arabian margin are associated with the region of diffuse extension. In contrast, in the southern Red Sea, a single strong block within the SW Arabian margin led to localize the extension there. Using current velocities from more than 30 GNSS stations distributed within the Arabian plate, we are able to map its rigidity and the distribution of strain along the plate margin. The data show that the transition between the two styles of extension within the Red Sea (crustal accretion vs crustal extension) corresponds with a transition between rigid behavior and diffuse extension within the Arabian Plate. This suggests that the preexisting structures within the Arabian plate play a significant role in the style of extension along the Red Sea margin.

To accrete or not accrete, that is the question

USGS Publications Warehouse

von Huene, Roland E.

1986-01-01

Along modern convergent margins tectonic processes span a spectrum from accretion to erosion. The process of accretion is generally recognized because it leaves a geologic record, whereas the process of erosion is generally hypothetical because it produces a geologic hiatus. Major conditions that determine the dominance of accretion or erosion at modern convergent margins are: 1) rate and direction of plate convergence, 2) sediment supply and type in the trench, and 3) topography of the subducting ocean floor. Most change in structure has been ascribed to plate motion, but both erosion and accretion are observed along the same convergence margin. Thus sediment supply and topography are probably of equivalent importance to plate motion because both erosion and accretion are observed under constant conditions of plate convergence. The dominance of accretion or erosion at a margin varies with the thickness of trench sediment. In a sediment flooded trench, the proportions of subducted and accreted sediment are commonly established by the position of a decollement along a weak horizon in the sediment section. Thus, the vertical variation of sediment strength and the distribution of horizontal stress are important factors. Once deformation begins, the original sediment strength is decreased by sediment remolding and where sediment thickens rapidly, increases in pore fluid pressure can be pronounced. In sediment-starved trenches, where the relief of the subducting ocean floor is not smoothed over, the front of the margin must respond to the topography subducted as well as that accreted. The hypothesized erosion by the drag of positive features against the underside of the upper plate (a high stress environment) may alternate with erosion due to the collapse of a margin front into voids such as graben (a low stress environment). ?? 1986 Ferdinand Enke Verlag Stuttgart.

A source-sink model of the generation of plate tectonics from non-Newtonian mantle flow

NASA Technical Reports Server (NTRS)

Bercovici, David

1995-01-01

A model of mantle convection which generates plate tectonics requires strain rate- or stress-dependent rheology in order to produce strong platelike flows with weak margins as well as strike-slip deformation and plate spin (i.e., toroidal motion). Here, we employ a simple model of source-sink driven surface flow to determine the form of such a rheology that is appropriate for Earth's present-day plate motions. In this model, lithospheric motion is treated as shallow layer flow driven by sources and sinks which correspond to spreading centers and subduction zones, respectively. Two plate motion models are used to derive the source sink field. As originally implied in the simpler Cartesian version of this model, the classical power law rheologies do not generate platelike flows as well as the hypothetical Whitehead-Gans stick-slip rheology (which incorporates a simple self-lubrication mechanism). None of the fluid rheologies examined, however, produce more than approximately 60% of the original maximum shear. For either plate model, the viscosity fields produced by the power law rheologies are diffuse, and the viscosity lows over strike-slip shear zones or pseudo-margins are not as small as over the prescribed convergent-divergent margins. In contrast, the stick-slip rheology generates very platelike viscosity fields, with sharp gradients at the plate boundaries, and margins with almost uniformly low viscosity. Power law rheologies with high viscosity contrasts, however, lead to almost equally favorable comparisons, though these also yield the least platelike viscosity fields. This implies that the magnitude of toroidal flow and platelike strength distributions are not necessarily related and thus may present independent constraints on the determination of a self-consistent plate-mantle rheology.

A source-sink model of the generation of plate tectonics from non-Newtonian mantle flow

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bercovici, D.

1995-02-01

A model of mantle convection which generates plate tectonics requires strain rate- or stress-dependent rheology in order to produce strong platelike flows with weak margins as well as strike-slip deformation and plate spin (i.e., toroidal motion). Here, we employ a simple model of source-sink driven surface flow to determine the form of such a rheology that is appropriate for Earth`s present-day plate motions. In this model, lithospheric motion is treated as shallow layer flow driven by sources and sinks which correspond to spreading centers and subduction zones, respectively. Two plate motion models are used to derive the source sink field.more » As originally implied in the simpler Cartesian version of this model, the classical power law rheologies do not generate platelike flows as well as the hypothetical Whitehead-Gans stick-slip rheology (which incorporates a simple self-lubrication mechanism). None of the fluid rheologies examined, however, produce more than approximately 60% of the original maximum shear. For either plate model, the viscosity fields produced by the power law rheologies are diffuse, and the viscosity lows over strike-slip shear zones or pseudo-margins are not as small as over the prescribed convergent-divergent margins. In contrast, the stick-slip rheology generates very platelike viscosity fields, with sharp gradients at the plate boundaries, and margins with almost uniformly low viscosity. Power law rheologies with high viscosity contrasts, however, lead to almost equally favorable comparisons, though these also yield the least platelike viscosity fields. This implies that the magnitude of toroidal flow and platelike strength distributions are not necessarily related and thus may present independent constraints on the determination of a self-consistent plate-mantle rheology.« less

Active tectonics of Peru: Heterogeneous interseismic coupling along the Nazca megathrust, rigid motion of the Peruvian Sliver, and Subandean shortening accommodation

NASA Astrophysics Data System (ADS)

Villegas-Lanza, J. C.; Chlieh, M.; Cavalié, O.; Tavera, H.; Baby, P.; Chire-Chira, J.; Nocquet, J.-M.

2016-10-01

Over 100 GPS sites measured in 2008-2013 in Peru provide new insights into the present-day crustal deformation of the 2200 km long Peruvian margin. This margin is squeezed between the eastward subduction of the oceanic Nazca Plate at the South America trench axis and the westward continental subduction of the South American Plate beneath the Eastern Cordillera and Subandean orogenic wedge. Continental active faults and GPS data reveal the rigid motion of a Peruvian Forearc Sliver that extends from the oceanic trench axis to the Western-Eastern Cordilleras boundary and moves southeastward at 4-5 mm/yr relative to a stable South America reference frame. GPS data indicate that the Subandean shortening increases southward by 2 to 4 mm/yr. In a Peruvian Sliver reference frame, the residual GPS data indicate that the interseismic coupling along the Nazca megathrust is highly heterogeneous. Coupling in northern Peru is shallow and coincides with the site of previous moderate-sized and shallow tsunami-earthquakes. Deep coupling occurs in central and southern Peru, where repeated large and great megathrust earthquakes have occurred. The strong correlation between highly coupled areas and large ruptures suggests that seismic asperities are persistent features of the megathrust. Creeping segments appear at the extremities of great ruptures and where oceanic fracture zones and ridges enter the subduction zone, suggesting that these subducting structures play a major role in the seismic segmentation of the Peruvian margin. In central Peru, we estimate a recurrence time of 305 ± 40 years to reproduce the great 1746 Mw 8.8 Lima-Callao earthquake.

Lower plate deformation structures along the Costa Rica erosive plate boundary - results from IODP Expedition 344 (CRISP 2)

NASA Astrophysics Data System (ADS)

Brandstätter, Jennifer; Kurz, Walter; Micheuz, Peter; Krenn, Kurt

2015-04-01

The primary objective of Integrated Ocean Drilling Program (IODP) Expedition 344 offshore the Osa Peninsula in Costa Rica was to sample and quantify the material entering the seismogenic zone of the Costa Rican erosive subduction margin. Fundamental to this objective is an understanding of the nature of both the subducting Cocos plate crust and of the overriding Caribbean plate. The subducting Cocos plate is investigated trying to define its hydrologic system and thermal state. The forearc structures recorded by the sediment deposited on the forearc, instead, document periods of uplift and subsidence and provide important information about the process of tectonic erosion that characterizes the Costa Rica margin. Offshore the western margin of Costa Rica, the oceanic Cocos plate subducts under the Caribbean plate, forming the southern end of the Middle America Trench. Subduction parameters including the age, convergence rate, azimuth, obliquity, morphology, and slab dip all vary along strike. The age of the Cocos plate at the Middle America Trench decreases from 24 Ma offshore the Nicoya Peninsula to 15 Ma offshore the Osa Peninsula. Subduction rates vary from 70 mm/y offshore Guatemala to 90 mm/y offshore southern Costa Rica. Convergence obliquity across the trench varies from offshore Nicaragua, where it is as much as 25° oblique, to nearly orthogonal southeast of the Nicoya Peninsula. Passage of the Cocos plate over the Galapagos hotspot created the aseismic Cocos Ridge, an overthickened welt of oceanic crust. This ridge is ~25 km thick, greater than three times normal oceanic crustal thickness. During IODP Expedition 344, the incoming Cocos plate was drilled at sites U1381 and U1414. Site U1381 is located ~4.5 km seaward of the deformation front offshore the Osa Peninsula and Caño Island. It is located on a local basement high. Basement relief often focuses fluid flow, so data from this site are likely to document the vigor of fluid flow in this area. Site U1414 is located ~1 km seaward of the deformation front offshore the Osa Peninsula and Caño Island. Primary science goals at Site U1414 included characterization of the alteration state of the magmatic basement. Brittle structures within the incoming plate (sites U1380, U1414) are mineralized extensional fractures and shear fractures. The shear fractures mainly show a normal component of shear. Within the sedimentary sequence both types of fractures dip steeply (vertical to subvertical) and strike NNE-SSW. Deformation bands trend roughly ENE-WSW, sub-parallel to the trend of the Cocos ridge. Structures in the Cocos Ridge basalt mainly comprise mineralized veins at various orientations. A preferred orientation of strike directions was not observed. Some veins show straight boundaries, others are characterized by an irregular geometry characterized by brecciated wall rock clasts embedded within vein precipitates. The vein mineralization was analysed in detail by RAMAN spectroscopy. Precipitation conditions and fluid chemistry were analysed by fluid inclusions entrapped within vein minerals. Vein mineralizations mainly consist of carbonate (fibrous aragonite, calcite), chalcedony, and quartz. Vein mineralization is mainly characterized by zoned antitaxial growth of carbonate fibres including a suture along the central vein domains. Quartz is often characterized by fibre growth of crystals perpendicular to the vein boundaries, too. These zoned veins additinally have wall rock alteration seams consisting of clay minerals. The precipitation sequence basically indicates that fluid chemistry evolved from an CO2-rich towards a SiO2- rich fluid.

Architecture of ductile-type, hyper-extended passive margins: Geological constraints from the inverted Cretaceous basin of the North-Pyrenean Zone ('Chaînons Béarnais', Western Pyrenees)

NASA Astrophysics Data System (ADS)

Corre, Benjamin; Lagabrielle, Yves; Labaume, Pierre; Lahfid, Abdeltif; Boulvais, Philippe; Bergamini, Geraldine; Fourcade, Serge; Clerc, Camille

2017-04-01

Sub-continental lithospheric mantle rocks are exhumed at the foot of magma-poor distal passive margins as a response to extreme stretching of the continental crust during plate separation. Remnants of the Northern Iberian paleo-passive margin are now exposed in the North-Pyrenean Zone (NPZ) and represent field analogues to study the processes of continental crust thinning and subcontinental mantle exhumation. The NPZ results from the inversion of basins opened between the Iberia and Europa plates during Albo-Cenomanian times. In the western NPZ, the 'Chaînons Béarnais' ranges display a fold-and-thrust structure involving the Mesozoic sedimentary cover, decoupled from its continental basement and associated with peridotite bodies in tectonic contact with Palaeozoic basement lenses of small size. Continental extension developed under hot thermal conditions, as demonstrated by the syn-metamorphic Cretaceous ductile deformation affecting both the crustal basement and the allochthonous Mesozoic cover. In this study, we present structural and geochemical data providing constraints to reconstruct the evolution of the northern Iberia paleo-margin. Field work confirms that the pre-rift Mesozoic cover is intimately associated to mantle rocks and to thin tectonic lenses of crustal basement. It also shows that the pre-rift cover was detached from its bedrock at the Keuper evaporites level and was welded to mantle rocks during their exhumation at the foot of the hyper-extended margin. The crust/mantle detachment fault is a major shear zone characterized by anastomosed shear bands defining a plurimetric phacoidal fabric at the top of the serpentinized mantle. The detachment is marked by a layer of metasomatic rocks, locally 20 meters thick, made of talc-chlorite-pyrite-rich rocks that developped under greenschist facies conditions. Raman Spectroscopy on Carbonaceous Materials (RSCM), performed on the Mesozoic cover reveal that the entire sedimentary pile underwent temperatures ranging between 200°C and 480°C. We show that: (i) at the site of mantle rocks exhumation, the boudinaged pre-rift sediments have undergone drastic syn-metamorphic thinning with the genesis of a S0/S1 foliation and, (ii) the Paleozoic basement has been ductilely deformed, into thin tectonic lenses that remained welded to the exhumed mantle rocks. Therefore the overall crustal rheology appears dominated by shallow levels having a ductile behavior. This rheology is related to the presence of a thick pre- and syn-rift decoupled cover acting as an efficient thermal blanket. This new geological data set highlights important characteristics of ductile-type hyper-extended passive margin that cannot be obtained from the study of seismic lines. Finally, we stress that studying field analogues represents a major tool to better understand the mechanisms of extreme crustal thinning associated with mantle exhumation and their structural inheritance during tectonic inversion.

Toward Implementing Long-term Slip History and Paleoseismicity Into Active Fault Databases to Compute Effective Recurrence Models

NASA Astrophysics Data System (ADS)

Fitzenz, D. D.; Jalobeanu, A.; Ferry, M. A.

2011-12-01

The first year of data from the Cascadia Initiative ocean-bottom seismograph deployment has provided a unique opportunity to image the structure of a plate from formation at the spreading center to subduction beneath the continental margin. However, traditional Rayleigh wave tomography of the Juan de Fuca plate using teleseismic sources is unusually difficult, because the region contains a large velocity heterogeneity at the ocean-continent margin; the azimuthal range of sources is limited, with most earthquakes lying in narrow azimuthal ranges to the northwest along the Aleutian and western Pacific trenches or to the southeast along the Middle and South American trenches; the orientation of many of the focal mechanisms leads to nodes in Rayleigh wave excitation towards the Juan de Fuca region; and the great circle paths from most sources to the receivers travel great distances close to ocean/continent boundaries or trenches and island arcs, producing complex waveforms. Nevertheless, we construct an initial tomographic image of the Juan de Fuca plate by subdividing the area into regions with relatively uniform wavefield composition when necessary; by using the two-plane-wave representation of the wavefield within the subregions; and by removing noise from the vertical component of the Rayleigh wave signals using information from the horizontal and pressure records. If the seismometer is slightly tilted, some of the often large horizontal noise contaminates the vertical component, and when water (gravity) waves penetrate to the seafloor, the associated pressure variations cause vertical displacements. By removing these two sources of noise, we are able to construct Rayleigh wave phase velocity maps in the period range 20 to 125 s, yielding excellent control on lithospheric mantle structure.

Connecting the Bird's Head to the Bird's Body - Cenozoic arc magmatism extends along the length of New Guinea.

NASA Astrophysics Data System (ADS)

Webb, Max; White, Lloyd; Jost, Benjamin

2017-04-01

New Guinea has a long, complicated history of arc magmatism. The present day shape of the island (resembling that of a bird in flight) formed as a result of oblique convergence of the Pacific and Caroline/Philippine plates with the northward moving Australian plate. This convergence resulted in multiple collisions of island arcs with continental crust, representing a modern day analogue to ancient accretionary orogens. This complex geological history has formed four major tectonic belts; accreted Palaeogene island arcs, the New Guinea Mobile Belt, the New Guinea Fold Belt and a stable platform. These tectonic belts are drawn across most of New Guinea in major review papers. However, these tectonic belts are not generally considered to extend through to New Guinea's western most peninsula (the Bird's Head). We present new field evidence, together with new U-Pb zircon geochronology and geochemical analyses from rocks collected within the Bird's Head. These document Middle to Late Miocene intermediate to felsic volcanic rocks and associated granitoid intrusives that formed along an active continental margin. These are effectively the equivalent of the Maramuni arc and Freida River Complex in eastern New Guinea. Several, broadly Eocene island arcs composed of dominantly mafic volcanic rocks are also found in the Bird's Head. These island arcs accreted along the Bird's Head sometime after their initial formation, possibly coinciding with Middle to Late Miocene active continental margin magmatism and we consider them to be equivalents of the Cyclops Mountains arc in Central New Guinea. This work demonstrates that New Guinea's east-west terranes are more extensive than previously thought. This potentially has implications for locating future ore deposits and understanding the relative position of the Bird's Head with respect to the rest of New Guinea in major plate reconstructions.

Geochemical evidence of mantle reservoir evolution during progressive rifting along the western Afar margin

NASA Astrophysics Data System (ADS)

Rooney, Tyrone O.; Mohr, Paul; Dosso, Laure; Hall, Chris

2013-02-01

The Afar triple junction, where the Red Sea, Gulf of Aden and African Rift System extension zones converge, is a pivotal domain for the study of continental-to-oceanic rift evolution. The western margin of Afar forms the southernmost sector of the western margin of the Red Sea rift where that margin enters the Ethiopian flood basalt province. Tectonism and volcanism at the triple junction had commenced by ˜31 Ma with crustal fissuring, diking and voluminous eruption of the Ethiopian-Yemen flood basalt pile. The dikes which fed the Oligocene-Quaternary lava sequence covering the western Afar rift margin provide an opportunity to probe the geochemical reservoirs associated with the evolution of a still active continental margin. 40Ar/39Ar geochronology reveals that the western Afar margin dikes span the entire history of rift evolution from the initial Oligocene flood basalt event to the development of focused zones of intrusion in rift marginal basins. Major element, trace element and isotopic (Sr-Nd-Pb-Hf) data demonstrate temporal geochemical heterogeneities resulting from variable contributions from the Afar plume, depleted asthenospheric mantle, and African lithosphere. The various dikes erupted between 31 Ma and 22 Ma all share isotopic signatures attesting to a contribution from the Afar plume, indicating this initial period in the evolution of the Afar margin was one of magma-assisted weakening of the lithosphere. From 22 Ma to 12 Ma, however, diffuse diking during continued evolution of the rift margin facilitated ascent of magmas in which depleted mantle and lithospheric sources predominated, though contributions from the Afar plume persisted. After 10 Ma, magmatic intrusion migrated eastwards towards the Afar rift floor, with an increasing fraction of the magmas derived from depleted mantle with less of a lithospheric signature. The dikes of the western Afar margin reveal that magma generation processes during the evolution of this continental rift margin are increasingly dominated by shallow decompressional melting of the ambient asthenosphere, the composition of which may in part be controlled by preferential channeling of plume material along the developing neo-oceanic axes of extension.

Nurture Versus Nature: Accounting for the Differences Between the Taiwan and Timor active arc-continent collisions

NASA Astrophysics Data System (ADS)

Harris, R. A.

2011-12-01

The active Banda arc/continent collision of the Timor region provides many important contrasts to what is observed in Taiwan, which is mostly a function of differences in the nature of the subducting plate. One of the most important differences is the thermal state of the respective continental margins: 30 Ma China passive margin versus 160 Ma NW Australian continental margin. The subduction of the cold and strong NW Australian passive margin beneath the Banda trench provides many new constraints for resolving longstanding issues about the formative stages of collision and accretion of continental crust. Some of these issues include evidence for slab rollback and subduction erosion, deep continental subduction, emplacement or demise of forearc basement, relative amounts of uplift from crustal vs. lithospheric processes, influence of inherited structure, partitioning of strain away from the thrust front, extent of mélange development, metamorphic conditions and exhumation mechanisms, continental contamination and accretion of volcanic arcs, does the slab tear, and does subduction polarity reverse? Most of these issues link to the profound control of lower plate crustal heterogeneity, thermal state and inherited structure. The thermomechanical characteristics of subducting an old continental margin allow for extensive underthrusting of lower plate cover units beneath the forearc and emplacement and uplift of extensive nappes of forearc basement. It also promotes subduction of continental crust to deep enough levels to experience high pressure metamorphism (not found in Taiwan) and extensive contamination of the volcanic arc. Seismic tomography confirms subduction of continental lithosphere beneath the Banda Arc to at least 400 km with no evidence for slab tear. Slab rollback during this process results in massive subduction erosion and extension of the upper plate. Other differences in the nature of the subducting plates in Taiwan in Timor are differences in the lateral continuity of the continental margins. The northern Australian continental margin is highly irregular with many rift basins subducting parallel to their axes. This feature gives rise to irregularities in the uplift pattern of the collision and its continental margin parallel structural grain. Another major difference between Taiwan and Timor is the mechanical stratigraphy entering the trench. The Australian continental margin bears a carbonate rich pre and post rift sequence that is separated by a 1000 m thick, over pressured mudstone unit that acts as major detachment and promotes extensive mud diapirism. The post breakup Australian Passive Margin Sequence is incorporated into the orogenic wedge by frontal accretion and forms a classic imbricate thrust stack near the front of the Banda forearc. The pre breakup Gondwana Sequence below the detachment continues at least to depth of 30 km in the subduction channel beneath the Banda forearc upper plate and stacks up into a duplex zone that forms structural culminations throughout Timor. The upper plate of both collisions is similar in nature but is deformed in different ways due to the strong influence of the lower plate. However, both have extensive subduction erosion and demise of the forearc and systematic accretion of the arc.

Mapping rift domains within an inverted hyperextended rift system: The role of rift inheritance in controlling the present-day structure of the North Iberian margin (Bay of Biscay)

NASA Astrophysics Data System (ADS)

Cadenas, Patricia; Fernández-Viejo, Gabriela; Álvarez-Pulgar, Javier; Tugend, Julie; Manatschal, Gianreto; Minshull, Tim

2017-04-01

This study presents a new rift domain map in the central and western North Iberian margin, in the southern Bay of Biscay. This margin was structured during polyphase Triassic to Lower Cretaceous rifting events which led to hyperextension and exhumation and the formation of oceanic crust during a short-lived seafloor spreading period. Extension was halted due to the Alpine convergence between the Iberian and the European plates which led to the formation of the Cantabrian-Pyrenean orogen during the Cenozoic. In the Bay of Biscay, while the northern Biscay margin was slightly inverted, the North Iberian margin, which is at present-day part of the western branch of the Alpine belt together with the Cantabrian Mountains, exhibits several degrees of compressional reactivation. This makes this area a natural laboratory to study the influence of rift inheritance into the inversion of a passive margin. Relying on the interpretation of geological and geophysical data and the integration of wide-angle results, we have mapped five rift domains, corresponding to the proximal, necking, hyperthinned, exhumed mantle, and oceanic domains. One of the main outcomes of this work is the identification of the Asturian Basin as part of a hyperthinned domain bounded to the north by the Le Danois basement high. We interpret Le Danois High as a rift-related crustal block inherited from the margin structure. Our results suggest that the inherited rift architecture controlled the subsequent compressional reactivation. The hyperextended domains within the abyssal plain focused most of the compression resulting in the development of an accretionary wedge and the underthrusting of part of these distal domains beneath the margin. The presence of the Le Danois continental block added complexity, conditioning the inversion undergone by the Asturian Basin. This residual block of less thinned continental crust acted as a local buttress hampering further compressional reactivation within the platform and the inner basin, which were only slightly inverted and uplifted passively due to the underthrusting of the hyperextended domains beneath Le Danois High. The new inverted rift domain map adds some constraints to support kinematic reconstructions and confine palinspatic restorations of the inverted rifted margin. Furthermore, it provides more insights to comprehend the strain partitioning within the Bay of Biscay-Pyrenean inverted hyperextended rift and the broad structural variability observed in such a reduced area, arising from the strong segmentation and the obliquity between the NW-SE and WNW-ESE trending rift structures and the E-W compressional front.

Pacific-North America plate boundary reorganization in response to a change in relative plate motion: Offshore Canada

NASA Astrophysics Data System (ADS)

Rohr, K. M. M.; Tryon, A. J.

2010-06-01

The transition from subduction in Cascadia to the transform Queen Charlotte fault along western Canada is often drawn as a subduction zone, yet recent studies of GPS and earthquake data from northern Vancouver Island are not consistent with that model. In this paper we synthesize seismic reflection and gravity interpretations with microseismicity data in order to test models of (1) microplate subduction and (2) reorganization of the preexisting strike-slip plate boundary. We focus on the critical region of outer Queen Charlotte Sound and the adjacent offshore. On much of the continental shelf, several million years of subsidence above thin crust are a counterindicator for subduction. An undated episode of compression uplifted the southernmost shelf, but subsidence patterns offshore show that recent subduction is unlikely to be responsible. Previously unremarked near-vertical faults and a mix of extensional and compressional faults offshore indicate that strike-slip faulting has been a significant mode of deformation. Seismicity in the last 18 years is dominantly strike-slip and shows large amounts of moment release on the Revere-Dellwood fault and its overlap with the Queen Charlotte fault. The relative plate motion between the Pacific and North American plates rotated clockwise ˜6 Ma and appears to have triggered formation of an evolving array of structures. We suggest that the paleo-Queen Charlotte fault which had defined this continental margin retreated northward as offshore distributed shear and the newly formed Revere Dellwood fault propagated to the northwest.

Florida: A Jurassic transform plate boundary

USGS Publications Warehouse

Klitgord, Kim D.; Popenoe, Peter; Schouten, Hans

1984-01-01

Magnetic, gravity, seismic, and deep drill hole data integrated with plate tectonic reconstructions substantiate the existence of a transform plate boundary across southern Florida during the Jurassic. On the basis of this integrated suite of data the pre-Cretaceous Florida-Bahamas region can be divided into the pre-Jurassic North American plate, Jurassic marginal rift basins, and a broad Jurassic transform zone including stranded blocks of pre-Mesozoic continental crust. Major tectonic units include the Suwannee basin in northern Florida containing Paleozoic sedimentary rocks, a central Florida basement complex of Paleozoic age crystalline rock, the west Florida platform composed of stranded blocks of continental crust, the south Georgia rift containing Triassic sedimentary rocks which overlie block-faulted Suwannee basin sedimentary rocks, the Late Triassic-Jurassic age Apalachicola rift basin, and the Jurassic age south Florida, Bahamas, and Blake Plateau marginal rift basins. The major tectonic units are bounded by basement hinge zones and fracture zones (FZ). The basement hinge zone represents the block-faulted edge of the North American plate, separating Paleozoic and older crustal rocks from Jurassic rifted crust beneath the marginal basins. Fracture zones separate Mesozoic marginal sedimentary basins and include the Blake Spur FZ, Jacksonville FZ, Bahamas FZ, and Cuba FZ, bounding the Blake Plateau, Bahamas, south Florida, and southeastern Gulf of Mexico basins. The Bahamas FZ is the most important of all these features because its northwest extension coincides with the Gulf basin marginal fault zone, forming the southern edge of the North American plate during the Jurassic. The limited space between the North American and the South American/African plates requires that the Jurassic transform zone, connecting the Central Atlantic and the Gulf of Mexico spreading systems, was located between the Bahamas and Cuba FZ's in the region of southern Florida. Our plate reconstructions combined with chronostratigraphic and lithostratigraphic information for the Gulf of Mexico, southern Florida, and the Bahamas indicate that the gulf was sealed off from the Atlantic waters until Callovian time by an elevated Florida-Bahamas region. Restricted influx of waters started in Callovian as a plate reorganization, and increased plate separation between North America and South America/Africa produced waterways into the Gulf of Mexico from the Pacific and possibly from the Atlantic.

Finite-frequency P-wave tomography of the Western Canada Sedimentary Basin: Implications for the lithospheric evolution in Western Laurentia

NASA Astrophysics Data System (ADS)

Chen, Yunfeng; Gu, Yu Jeffrey; Hung, Shu-Huei

2017-02-01

The lithosphere beneath the Western Canada Sedimentary Basin has potentially undergone Precambrian subduction and collisional orogenesis, resulting in a complex network of crustal domains. To improve the understanding of its evolutionary history, we combine data from the USArray and three regional networks to invert for P-wave velocities of the upper mantle using finite-frequency tomography. Our model reveals distinct, vertically continuous high (> 1%) velocity perturbations at depths above 200 km beneath the Precambrian Buffalo Head Terrane, Hearne craton and Medicine Hat Block, which sharply contrasts with those beneath the Canadian Rockies (<- 1%) at comparable depths. The P velocity increases from - 0.5% above 70 km depth to 1.5% at 330 km depth beneath southern Alberta, which provides compelling evidence for a deep, structurally complex Hearne craton. In comparison, the lithosphere is substantially thinner beneath the adjacent Buffalo Head Terrane (160 km) and Medicine Hat Block (200 km). These findings are consistent with earlier theories of tectonic assembly in this region, which featured distinct Archean and Proterozoic plate convergences between the Hearne craton and its neighboring domains. The highly variable, bimodally distributed craton thicknesses may also reflect different lithospheric destruction processes beneath the western margin of Laurentia.

Rigidity and definition of Caribbean plate motion from COCONet and campaign GPS observations

NASA Astrophysics Data System (ADS)

Mattioli, G. S.; Miller, J. A.; DeMets, C.; Jansma, P. E.

2015-12-01

The kinematic model of the Caribbean plate presented by DeMets et al. (2007) is based on velocities from 6 continuous and 14 campaign GPS sites. COCONet is a multi-hazard GPS-Met observatory, which extends the existing infrastructure of the PBO in North America into the Caribbean basin. In 2010, UNAVCO in collaboration with UCAR, was funded by NSF to design, build, and initially maintain a network of 50 new cGPS/Met sites and include data from another 50 existing sites in the Caribbean region. The COCONet siting plan is for 46 new stations, 21 refurbished stations, and 77 existing stations across 26 nations in the Caribbean region. Data from all COCONet sites flow into the UNAVCO archive and are processed by the PBO analysis centers and are also processed independently by the UTA Geodesy Lab using GIPSY-OASISII (v.6.3) using an APP strategy and final, precise orbits, clocks, and EOP from JPL in the IGS08r frame. We present a refined estimate of Caribbean plate motion by evaluating data from an expanded number of stations with an improved spatial distribution. In order to better constrain the eastern margin of the plate near the Lesser Antilles subduction interface, campaign GPS observations have been collected on the island of Dominica over the last decade. These are combined with additional campaign observations from the western Caribbean, specifically from Honduras and Nicaragua. We have analyzed a total of 117 sites from the Caribbean region, including campaign data and the data from the cGPS stations that comprise COCONet. An updated velocity field for the Caribbean plate is presented and an inversion of the velocities for 24 sites yields a plate angular velocity that differs from previously published models. Our best fitting inversion to GPS velocities from these 24 sites suggests that 2-plate model for the Caribbean is required to fit the GPS observations, which implies that the Caribbean is undergoing modest (1-3 mm/yr) deformation within its interior. Some sites in the western Caribbean included in our analysis may be biased by small, but significant coseismic deformation, which has not been removed from the site velocities used in our inversion to define Caribbean motion and rigidity. Scenarios for possible east-west deformation accommodated across the Lower Nicaraguan Rise and Beata Ridge will be presented.

The Nova-Canton Trough and the Late Cretaceous evolution of the central Pacific

NASA Astrophysics Data System (ADS)

Joseph, Devorah; Taylor, Brain; Shor, Alexander N.; Yamazaki, Toshitsugu

Free-air gravity anomalies derived from satellite altimetry data show that the major Pacific fracture zones, from the Pau to Marquesas, are co-polar about an Euler pole located at 150.5°W, 34.6°S for the period preceding chron 33 and including a large portion of the Cretaceous Normal Superchron. They also show continuity of the Clipperton Fracture Zone through the Line Islands to the Nova-Canton ridge and trough; this Canton-Clipperton trend is co-polar to the same pole. Sidescan-sonar and bathymetry data in the Nova-Canton Trough region reveal N140°E-striking abyssal hill topography south of the N70°E-striking structures of the Nova-Canton Trough and crustal fabric striking normal to the trough (N160°E) to the north. We conclude that the Nova-Canton Trough is the Middle Cretaceous extension of the Clipperton Fracture Zone. We propose that the anomalous depths (7000-8400 m) of the trough between 167°30'-168°30'W are the result of a complex plate reorganization. Conjugate magnetic anomaly lineations M1-M3 in the Phoenix lineations between the Central Pacific Fracture Zone and the Phoenix Fracture Zone and the absence of lineations younger than anomaly M3 west of the Phoenix Fracture Zone suggest that spreading may have gradually ceased along the Pacific-Phoenix system from west to east. We infer that the remaining active segment of the Pacific-Phoenix spreading system after anomaly M1 time was the easternmost section of the Phoenix lineations. At ˜M0 time, the Pacific-Phoenix spreading axis stretched from lineated bathymetric depressions lying between 180°W and the Phoenix Islands to ˜168°W and included the western deep of the Nova-Canton Trough. We hypothesize that accretion terminated on the Pacific-Phoenix spreading axis shortly after M0 time and that the absence of an M0 isochron in the region between the eastern Phoenix lineations and the Nova-Canton Trough, or along the Nova-Canton Trough itself, may be due to a decrease in spreading rate prior to termination. We concur with previous hypotheses that portions of the Phoenix plate were trapped on the Pacific plate by a ridge jump south to the nascent Manihiki Plateau; some portions were overprinted by the Aptian volcanism that formed the Manihiki Plateau and Robbie Ridge. Pacific-Farallon spreading south of the Nova-Canton Trough jumped westwards, initiating transcurrent motion along the easternmost section of the failed ˜M0 spreading axis (the western deep of the Nova-Canton Trough) which subsequently became the western end of the Clipperton (Pacific-Farallon) transform. In our reconstruction, the northeast and southeast margins of the Manihiki Plateau are rifted margins that form the western limit of Pacific-Farallon spreading between the Clipperton, Galapagos, and Marquesas fracture zones.

Provenance of Cretaceous-Pliocene Clastic Sediments in the Tachira Saddle, Western Venezuela, and Implications for Sediment Dispersal Patterns in the Northern Andes

NASA Astrophysics Data System (ADS)

Gomez, Ali Ricardo

Northwestern South America is highly deformed due to the transpressive plate boundary associated with complex interactions between the Caribbean plate, the South American plate, the Nazca plate and the Panama arc. Previous studies suggest that the Cenozoic uplift of the Merida Andes and Eastern Cordillera of Colombia affected sediment dispersal patterns in the region, shifting from a Paleocene foreland basin configuration to the modern isolated basins. Well-exposed Cretaceous to Pliocene strata in the Tachira Saddle provides a unique opportunity to test proposed sediment dispersal patterns in the region. U-Pb detrital zircon geochronology and supplementary XRD heavy mineral data are used together to document the provenance of the Tachira Saddle sediments and refine the sediment dispersal patterns in the region. Results from the U-Pb detrital zircon geochronology show that there are six age groups recorded in these samples. Two groups are related to the Precambrian Guyana shield terranes and Putumayo basement in the Eastern Cordillera, and four groups are related to different magmatic episodes occurring during the Andean orogenic process. The transition between the Cretaceous passive margin and the Paleocene foreland basin and the initial uplift of the Eastern Cordillera and the uplift of the Merida Andes by the Early Miocene were also recorded in the Tachira saddle detrital zircon signature.

Reconstructing the role of South China in Pangea and earlier supercontinents

NASA Astrophysics Data System (ADS)

Cawood, Peter; Zhao, Guochun; Yao, Jinlong; Wang, Wei; Xu, Yajun; Wang, Yuejun

2017-04-01

The history of the South China Craton and the constituent Yangtze and Cathaysia blocks is directly linked to Earth's Phanerozoic and Precambrian record of supercontinent assembly and dispersal. Exposed Archean rocks are limited to isolated fragments in the Yangtze Block and preserve a record of Meso- to Neo-Archean igneous activity, sedimentation and metamorphism associated with a period of global craton formation and stabilization that corresponds with assembly of the Kenor supercontinent/supercraton. However, there is insufficient data to link its history with other similar aged cratons. The tectonostratigraphic record in South China in the Paleoproterozoic, corresponding with assembly of Nuna, suggests that rock units in the Yangtze Block were spatially linked with northwestern Laurentia and possibly Siberia, whereas Cathaysia was joined to northern India. From the formation of Rodinia at the end of the Mesoproterozoic through to that of Pangea in the mid-Paleozoic, Cathaysia remained joined to northern India. Early Neoproterozoic supra-subduction zone magmatic arc-back arc assemblages ranging in age from 1000 Ma to 810 Ma occur within Cathaysia, along its northwestern margin, and along the southeastern margin of the Yangtze Block. These rocks provide a record of convergent plate interaction along the periphery of Rodinia, which continued along the western margin of the Yangtze Block until around 700 Ma and correlates with similar along strike subduction zone magmatism in northwest India, Seychelles and Madagascar. During final assembly of Gondwana in the early Paleozoic suturing of India-South China with the Western Australia-Mawson blocks along the Kuunga Orogen resulted in the accretion of the Sanya Block of Hainan Island with the rest of Cathaysia. The accretion of Laurussia to Gondwana in the mid-Paleozoic to form Pangea corresponds with the initiation of lithospheric extension along the northern margin of Gondwana and the separation of a number of continental blocks, including South China, which then drifted northward across the Paleo-Tethys to collide with the Asian segment of Pangea in the Permo-Triassic.

When Boundary Layers Collide: Plumes v. Subduction Zones

NASA Astrophysics Data System (ADS)

Moresi, L. N.; Betts, P. G.; Miller, M. S.; Willis, D.; O'Driscoll, L.

2014-12-01

Many subduction zones retreat while hotspots remain sufficiently stable in the mantle to provide an approximate reference frame. As a consequence, the mantle can be thought of as an unusual convecting system which self-organises to promote frequent collisions of downgoing material with upwellings. We present three 3D numerical models of subduction where buoyant material from a plume head and an associated ocean-island chain or plateau produce flat slab subduction and deformation of the over-riding plate. We observe transient instabilities of the convergent margin including: contorted trench geometry; trench migration parallel with the plate margin; folding of the subducting slab and orocline development at the convergent margin; and transfer of the plateau to the overriding plate. The presence of plume material beneath the oceanic plateau causes flat subduction above the plume, resulting in a "bowed" shaped subducting slab. In the absence of a plateau at the surface, the slab can remain uncoupled from the over-riding plate during very shallow subduction and hence there is very little shortening at the surface or advance of the plate boundary. In plateau-only models, plateau accretion at the edge of the overriding plate results in trench migration around the edge of the plateau before subduction re-establishes directly behind the trailing edge of the plateau. The plateau shortens during accretion and some plateau material subducts. In a plateau-plus-plume model, accretion is associated with rapid trench advance as the flat slab drives the plateau into the margin. This indentation stops once a new convergent boundary forms close to the original trench location. A slab window formed beneath the accreted plateau allows plume material to flow from beneath the subducting plate to the underside of the overriding plate. In all of these models the subduction zone maintains a relatively stable configuration away from the buoyancy anomalies within the downgoing plate. The models provide a dynamic context for plateau and plume accretion in accretionary orogenic systems.

Passive recording of an active transform, an example from the Levant continental margin and the Dead Sea Fault

NASA Astrophysics Data System (ADS)

Lang, Guy; Lazar, Michael; Schattner, Uri

2017-04-01

Transform faults accommodate lateral motion between two adjacent plates. Records of plate motion and consequent boundary development on land is, at times, scarce and limited to structures along the fault axis. Investigation of a passive continental margin adjacent to the plate boundary might broaden the scope and provide estimates for its structural development. To examine this hypothesis, we analyzed depth and time migrated 3D seismic data together with four boreholes located along the southern Levant continental margin, ca. 100 Km from the continental Dead Sea fault (DSF). The analysis focus on the Plio-Pleistocene sequence, a key period in the development of the DSF. It includes formation of structural maps, stacking pattern investigation and calculation of sedimentation rates based on decompacted 3D depth data. These, in turn, enabled the reconstruction of margin development. This includes Messinian-earliest Zanclean NNE-SSW sinistral strike-slip faulting followed by Zanclean-Late Gelasian syn-depositional folding striking in the same direction. Abrupt change is marked by the Top Gelasian surface that shows indications of regional mass slumping. Successive Mid-Late Pleistocene progradation marks a basinward shift of the depocenter. Progradation controls margin sedimentation rates during the mid-late Pleistocene. These were found to increase throughout the whole Plio-Pleistocene, in contrast to reported sediment discharge from the Nile, which was shown to decrease after the Gelasian. Correlations to onshore findings, suggest that the continental margin records strain localization on the DSF during the Pliocene-Gelasian. This trend peaked at 1.8 Ma when short wavelength strain ceased along the margin, and differential subsidence commenced basinwards. This is attributed to consequent deepening of the DSF plate boundary.

Tectonics of Precambrian basement along the Pacific margin of Antarctica and relation to western North America

DOE Office of Scientific and Technical Information (OSTI.GOV)

Goodge, J.W.; Hansen, V.L.; Walker, N.W.

1993-02-01

High-grade metamorphic rocks of the Precambrian Nimrod Group (NG) constitute one of few cratonal basement exposures in the Transantarctic Mountains. These rocks represent an outlier of the East Antarctic craton, evolved as part of Gondwana and pre-Gondwana (Rodinia) supercontinents. Despite pervasive, high-strain ductile deformation at T [>=] 650 C, they preserve petrologic and geochronologic evidence of an earlier history. Sm-Nd model ages from several NG lithologies, including that of a [approximately]1.7 Ga orthogneiss, range from about 2.7--2.9 Ga; these ages reflect both sedimentary and magmatic derivation from Archean crust. Individual detrital zircon U-Pb ages (about 1.7--2.6 Ga) from NG quartzitesmore » indicate clastic input from Archean to Paleoproterozoic source terrains. The Sm-Nd and U-Pb ages are reminiscent of both the Yavapai-Mazatzal (1.6--1.8 Ga) and Wyoming (> 2.5 Ga) provinces in western North America. U-Pb ages from syn-tectonic metaigneous and pelitic NG tectonites indicate that this basement complex was re-worked by the major ductile deformation in latest neoproterozoic to Early Cambrian time. Supracrustal assemblages that lie outboard of the Nimrod craton include Neoproterozoic graywacke, impure carbonate, and minor mafic volcanics (Beardmore Group), and Cambrian to Lower Ordovician carbonate and siliciclastic rocks (Byrd Group). Neoproterozoic ([approximately]750 Ma) rifting along the proto-Pacific margin of East Antarctica is reflected by deposition of Beardmore turbidites and coeval mafic magmatism. Latest Neoproterozoic to early Paleozoic orogenesis occurred along a left-oblique convergent plate margin of East Antarctica is reflected by deposition of Beardmore turbidites and coeval mafic magmatism.« less

Ridge Flank Flux as a Potential Source for the North Pacific Silica Plume

NASA Astrophysics Data System (ADS)

Johnson, H. P.; Hautala, S. L.; Bjorklund, T. A.

2005-12-01

The North Pacific silica plume is a global scale anomaly, extending from the North American continental margin to west of the Hawaii-Emperor seamount chain. Inventory of the plume at depths between 2000 and 3000 meters indicates that it contains 164 Teramols of dissolved silica, and is maintained by a horizontal flux of approximately 1.5 Tmols/year from the Eastern Pacific. The source region of this silica plume has been previously reported to be Cascadia Basin in the NE Pacific. However, simple box models based both on new hydrostations and compilations of archive data indicate that only a third of the dissolved silica that enters the larger North Pacific plume originates locally within the Cascadia/Gorda Basin. As it encounters the North American continental margin, the eastward-flowing deep Pacific bottom water is forced into `a U-turn' by seafloor topography. A portion of the bottom water is elevated from 4000 to 2300 meter depths by the high geothermal heat flow during rapid passage through Cascadia/Gorda Basin, and subsequently flows westward as the North Pacific mid-water plume. The plume water also absorbs an estimated 0.47 Tmol/year of locally derived silica during its passage adjacent to the continental margin. However, the Pacific bottom water is already relatively enriched in dissolved silica when it passes the Gorda Ridge/Mendocino junction, and the remaining 1 Tmol/year of silica must be acquired during near-bottom transit from the Western Pacific, over the portion of the easternmost Pacific plate where basement is younger than 65 Ma. Global compilations based on heat flow data argue that the upper crustal section of the young, eastern Pacific plate is an enormous aquifer, with active hydrothermal circulation and presumably diffuse venting into the bottom water. The suggestion that the large-scale flux of silica-rich hydrothermal fluid from the young eastern portion of the Pacific plate contributes to the North Pacific silica plume is a consequence of that interpretation, but is only a plausible and still untested hypothesis. If correct, however, it implies that the ridge flanks of the eastern Pacific Ocean are a global-scale source of a critically important nutrient.

Crustal Structure across Rivera Plate and Jalisco Block (MEXICO): TsuJal Project

NASA Astrophysics Data System (ADS)

Nuñez-Cornu, F. J.; Nunez, D.; Barba, D. C., Sr.; Trejo, E.; Escalona, F.; Danobeitia, J.; Gutierrez Pena, Q. J.

2015-12-01

Located on the western margin of Mexico, the collision zone between Rivera, Cocos and North American plates is a complex tectonic collage with high seismic hazards and potential tsunamigenic sources. During the spring of 2014, within the framework of TSUJAL project, Spanish and Mexican scientists investigated this region with the main objective of defining the crustal architecture of this active margin and recognizing potential structural sources that can trigger earthquakes and tsunamis at the convergence between Rivera plate-Jalisco block with the North American Plate. To achieve these goals, a wide-ranging of geophysical data was acquired in this region both offshore and onshore. In this paper, we present the preliminary results obtained from this project about bathymetric, structural geology and wide-angle seismic data of the southern coast of Bahía de Banderas. A crustal P-wave velocity model for the southern coast of Bahía de Banderas was obtained using WAS data recorded by OBS and land seismic stations for more than 150 km across Rivera Plate and Jalisco Block. The thickness of the slab in this area is about 10 km and presents a dip angle about 8º. Continental crustal thickness below Puerto Vallarta is about 20 km, no evidence of continental Moho was found in this study. This model support that due to the convergence of Rivera Plate against Jalisco Block, the region of Bahía de Banderas is under strong crustal stresses that generate structural lineaments and have the same trends offshore and inland. Most of the seismicity reported can be associated to the main structural lineaments. The Banderas Canyon apparently is in an opening process from west to east, which seems to continue through the Rio Pitillal river valley. There is no seismic or morphological evidence to consider that the Banderas Canyon is a continuation of Vallarta Graben.South of María Cleofas Island, the SC marks the limit between RP and JB, possibly being the result of the RP against JB push, and where it is established the beginning of current subduction process with seismic activity associated. If a subduction type earthquake occurs in the SC, which is 100 km length, the associated magnitude will be about 7.5 and could be tsunamigenic. In the studied area, no clear subduction features (trench, accretionary prism) are observed.

Cenozoic intracontinental deformation of the Kopeh Dagh Belt, Northeastern Iran

NASA Astrophysics Data System (ADS)

Chu, Yang; Wan, Bo; Chen, Ling; Talebian, Morteza

2016-04-01

Compressional intracontinental orogens represent large tectonic zones far from plate boundaries. Since intracontinental mountain belts cannot be framed in the conventional plate tectonics theory, several hypotheses have been proposed to account for the formations of these mountain belts. The far-field effect of collision/subduction at plate margins is now well accepted for the origin and evolution of the intracontinental crust thickening, as exemplified by the Miocene tectonics of central Asia. In northern Iran, the Binalud-Alborz mountain belt witnessed the Triassic tectonothermal events (Cimmerian orogeny), which are interpreted as the result of the Paleotethys Ocean closure between the Eurasia and Central Iran blocks. The Kopeh Dagh Belt, located to the north of the Binalud-Alborz Belt, has experienced two significant tectonic phases: (1) Jurassic to Eocene rifting with more than 7 km of sediments; and (2) Late Eocene-Early Oligocene to Quaternary continuous compression. Due to the high seismicity, deformation associated with earthquakes has received more and more attention; however, the deformation pattern and architecture of this range remain poorly understood. Detailed field observations on the Cenozoic deformation indicate that the Kopeh Dagh Belt can be divided into a western zone and an eastern zone, separated by a series of dextral strike-slip faults, i.e. the Bakharden-Quchan Fault System. The eastern zone characterized by km-scale box-fold structures, associated with southwest-dipping reverse faults and top-to-the NE kinematics. In contrast, the western zone shows top-to-the SW kinematics, and the deformation intensifies from NE to SW. In the northern part of this zone, large-scale asymmetrical anticlines exhibit SW-directed vergence with subordinate thrusts and folds, whereas symmetrical anticlines are observed in the southern part. In regard to its tectonic feature, the Kopeh Dagh Belt is a typical Cenozoic intracontinental belt without ophiolites or arc magmatism. During the Jurassic to Eocene rifting, this belt acted as the southern boundary of the Amu Darya Basin with normal faulting, which is also widespread in the South Caspian Sea and the Black Sea. Moreover, such an extended area became a relatively weak zone within the Eurasian Plate, and could be easily reworked. Because of the collision in the Zagros Belt, the intracontinental compression commenced as early as Late Eocene to Early Oligocene, which is interpreted as tectonic inversion along this weak zone. The western zone of the Kopeh Dagh Belt was also affected by southerly indentation/extrusion of the South Caspian block since middle Miocene, possibly resulting in the different deformation patterns between the western and eastern zones.

Global Geomorphology

NASA Technical Reports Server (NTRS)

Douglas, I.

1985-01-01

Any global view of landforms must include an evaluation of the link between plate tectonics and geomorphology. To explain the broad features of the continents and ocean floors, a basic distinction between the tectogene and cratogene part of the Earth's surface must be made. The tectogene areas are those that are dominated by crustal movements, earthquakes and volcanicity at the present time and are essentially those of the great mountain belts and mid ocean ridges. Cratogene areas comprise the plate interiors, especially the old lands of Gondwanaland and Laurasia. Fundamental as this division between plate margin areas and plate interiors is, it cannot be said to be a simple case of a distinction between tectonically active and stable areas. Indeed, in terms of megageomorphology, former plate margins and tectonic activity up to 600 million years ago have to be considered.

3D dynamics of crustal deformation driven by oblique subduction: Northern and Central Andes

NASA Astrophysics Data System (ADS)

Schütt, Jorina M.; Whipp, David M., Jr.

2017-04-01

The geometry and relative motion of colliding plates will affect how and where they deform. In oblique subduction systems, factors such as the dip angle of the subducting plate and the convergence obliquity, as well as the presence of weak zones in the overriding plate, all influence how oblique convergence is partitioned onto various fault systems in the overriding plate. The partitioning of strain into margin-normal slip on the plate-bounding fault and horizontal shearing on a strike-slip system parallel to the margin is mainly controlled by the margin-parallel shear forces acting on the plate interface and the strength of the continental crust. While these plate interface forces are influenced by the dip angle of the subducting plate (i.e., the length of plate interface in the frictional domain) and the obliquity angle between the normal to the plate margin and the plate convergence vector, the strength of the continental crust in the upper plate is strongly affected by the presence or absence of weak zones such as regions of arc volcanism, pre-existing fault systems, or boundaries of stronger crustal blocks. In order to investigate which of these factors are most important in controlling how the overriding continental plate deforms, we compare results of lithospheric-scale 3D numerical geodynamic experiments from two regions in the north-central Andes: the Northern Volcanic Zone (NVZ; 5°N - 3°S) and adjacent Peruvian Flat Slab Segment (PFSS; 3°S -14°S). The NVZ is characterized by a 35° subduction dip angle with an obliquity angle of about 40°, extensive volcanism and significant strain partitioning in the continental crust. In contrast, the PFSS is characterized by flat subduction (the slab flattens beneath the continent at around 100 km depth for several hundred kilometers), an obliquity angle of about 20°, no volcanism and minimal strain partitioning. The plate geometry and convergence obliquity for these regions are incorporated in 3D (1600 x 1600 x 160 km) numerical experiments of oceanic subduction beneath a continent, focusing on the conditions under which strain partitioning occurs in the continental plate. In addition to different slab geometries and obliquity angles, we consider the effect of a continental crustal of uniform strength (friction angle Φ=15^°) versus one including a weak zone in the continental crust (Φ=4^°) that runs parallel to the margin. Results of our experiments show that the obliquity angle has the largest effect on initiating strain partitioning, as expected based on strain partitioning theory, but strain partitioning is clearly enhanced by the presence of a continental weakness. Margin-parallel mass transport velocities in the continental sliver are similar to the values observed in the NVZ (about 1 cm/year) in models with a continental weakness and twice as high as those without. In addition, a shallower subduction angle results in formation of a wider continental sliver. Based upon our results, the lack of strain partitioning observed in the PFSS results from both a low convergence obliquity and lack of a weak zone in the continent, even though the shallow subduction should make strain partitioning more favorable.

Results of a magnetotelluric traverse across western Oregon: crustal resistivity structure and the subduction of the Juan de Fuca plate

NASA Astrophysics Data System (ADS)

Waff, Harve S.; Rygh, John T.; Livelybrooks, Dean W.; Clingman, William W.

1988-02-01

As part of project EMSLAB, we have collected and analysed wideband magnetotelluric data along an east-west transect in western Oregon. Preliminary modelling of the data using one-dimensional inversions based upon rotationally-invariant earth response functions was followed by finite-element two-dimensional modelling. The models produced indicate the presence of an electrical conductor beneath the Oregon Coast Range dipping eastward at 12-18° from a depth of 23-32 km. We believe that this conductor includes the thrust surface of the subducting Juan de Fuca plate and/or adjacent water-saturated rocks. Its high conductance (about 200 S) is thought to be due to one or more of the following mechanisms: (1) sediments subducted atop and with the Juan de Fuca plate, (2) saline fluids produced by dehydration of the former, or (3) seawater contained within subducted oceanic basalts. There is a distinct possibility that the high conductivity is due primarily to the presence of subducted sediments, in contrast with the notion that the subduction of young, buoyant lithosphere retards sediment subduction at this convergent margin. The conductive layer is overlain by relatively resistive rocks presumed to be accreted oceanic lithosphere. Model-determined resistivities for the upper part of the Coast Range section are in good agreement with deep well-log data. A strong electrical contrast appears in the determinant phase pseudosection between the Coast Range and the Willamette Valley suggesting a structural boundary between the two provinces. A surficial conductor is present in the valley to depths of 1-2 km and is due to alluvial fill. Induction arrow data show the geomagnetic coast effect and a smaller effect by the Willamette Valley alluvial fill.

The building of a biodiversity hotspot across a land-bridge in the Mediterranean

PubMed Central

Molina-Venegas, Rafael; Aparicio, Abelardo; Lavergne, Sébastien; Arroyo, Juan

2015-01-01

Many of the macroevolutionary processes that have shaped present-day phylogenetic patterns were caused by geological events such as plate tectonics and temporary land-bridges. The study of spatial patterns of phylogenetic diversity can provide insights into these past events. Here we focus on a western Mediterranean biodiversity hotspot located in the southern Iberian Peninsula and northwest Africa, two regions that are separated by the Strait of Gibraltar. We explore the spatial structure of the phylogenetic relationships within and across large-scale plant assemblages. Significant turnover in terminal lineages tends to occur between landmasses, whereas turnover in deep lineages tends to occur within landmasses. Plant assemblages in the western ecoregions of this hotspot tend to be phylogenetically overdispersed but are phylogenetically clustered on its eastern margins. We discuss our results in the light of potential scenarios of niche evolution (or conservatism) and lineage diversification. The significant turnover between landmasses suggests a common scenario of allopatric speciation that could have been facilitated by the intermittent joining of the two continents. This may have constituted an important stimulus for diversification and the emergence of this western Mediterranean biodiversity hotspot. PMID:26246551

The building of a biodiversity hotspot across a land-bridge in the Mediterranean.

PubMed

Molina-Venegas, Rafael; Aparicio, Abelardo; Lavergne, Sébastien; Arroyo, Juan

2015-08-22

Many of the macroevolutionary processes that have shaped present-day phylogenetic patterns were caused by geological events such as plate tectonics and temporary land-bridges. The study of spatial patterns of phylogenetic diversity can provide insights into these past events. Here we focus on a western Mediterranean biodiversity hotspot located in the southern Iberian Peninsula and northwest Africa, two regions that are separated by the Strait of Gibraltar. We explore the spatial structure of the phylogenetic relationships within and across large-scale plant assemblages. Significant turnover in terminal lineages tends to occur between landmasses, whereas turnover in deep lineages tends to occur within landmasses. Plant assemblages in the western ecoregions of this hotspot tend to be phylogenetically overdispersed but are phylogenetically clustered on its eastern margins. We discuss our results in the light of potential scenarios of niche evolution (or conservatism) and lineage diversification. The significant turnover between landmasses suggests a common scenario of allopatric speciation that could have been facilitated by the intermittent joining of the two continents. This may have constituted an important stimulus for diversification and the emergence of this western Mediterranean biodiversity hotspot. © 2015 The Author(s).

Structural controls on the hydrogeology of the Costa Rica subduction thrust NW of the Osa Peninisula (Invited)

NASA Astrophysics Data System (ADS)

Bangs, N. L.; McIntosh, K. D.; Silver, E. A.; Kluesner, J.; Ranero, C. R.

2013-12-01

Three-dimensional seismic reflection data from the Costa Rica margin NW of the Osa peninsula have enabled us to map the subduction megathrust from the trench to ~12 km subseafloor beneath the shelf. The subduction thrust has a large, abrupt downdip transition in seismic reflection amplitude from very high to low amplitude 6 km subseafloor beneath the upper slope. This transition broadly corresponds with an increase in concentration of microseismic earthquakes potentially due to a significant increase in plate coupling (Bangs et al., 2012, AGU Fall Meeting, T13A-2587), thus linking seismic reflection amplitude to fluid content and mechanical coupling along the fault. A detailed look at the overriding plate reflectivity shows numerous high-amplitude, continuous seismic reflections through the upper plate, many of which are clearly reversed-polarity from the seafloor reflection and are thus likely active fluid conduits through the overriding margin wedge, the slope cover sediment, and the seafloor. Broadly, the structural grain of the margin wedge trends E-W and dips landward across the lower slope and onto the shelf, presumably due to stress imparted by subducting ridges. However, directly above the abrupt high-to-low plate-boundary reflection amplitude transition, structures within the overlying margin wedge reverse dip, steepen, and change strike to an ESE direction. Within this zone we interpret a set of parallel reflections with small offsets and reverse-polarity as high-angle reverse faults that act as fluid conduits leading directly into shallow fluid migration systems described by Bangs et al., 2012 (AGU Fall Meeting, T13A-2587) and Kluesner et al. [this meeting]. The coincidence between the plate-boundary reflection amplitude patterns and the change in structure implies that the fluid migration pathways that drain the plate interface are locally disrupted by overriding plate structure in two possible ways: 1) by focusing up dip fluid migration along the plate interface into a thinner but richer fluid zone along the subduction thrust, or 2) by creating a more direct, nearly vertical route along high-angle reverse faults through the overlying margin wedge to the seafloor (possibly shortened by a factor of two) and draining deeper portions of the plate-boundary more efficiently.

Opal-CT in chert beneath the toe of the Tohoku margin and its influence on the seismic aseismic transition in subduction zones

NASA Astrophysics Data System (ADS)

Kameda, Jun; Okamoto, Atsushi; Sato, Kiminori; Fujimoto, Koichiro; Yamaguchi, Asuka; Kimura, Gaku

2017-01-01

Thick accumulation of chert is a ubiquitous feature of old oceanic plates at convergent margins. In this study, we investigate chert fragments recovered by the Integrated Ocean Drilling Program expedition 343 at the Japan Trench where the 2011 Tohoku-Oki earthquake (Mw 9.0) occurred. This sample provides a unique opportunity to investigate in situ chert diagenesis at an active subduction margin and its influence on the kinematics of megathrust faulting. Our mineralogical analyses revealed that the chert is characterized by hydrous opal-CT and may therefore be highly deformable via pressure solution creep and readily accommodate shear strain between the converging plates at driving stresses of kilopascal order. As chert diagenesis advances, any further deformation requires stresses of >100 MPa, given the increasing transport distances for solutes as represented in cherts on land. The chert diagenesis is thus related to the mechanical transition from a weakly to strongly coupled plate interface at this margin.

Intraplate mountain building in response to continent continent collision—the Ancestral Rocky Mountains (North America) and inferences drawn from the Tien Shan (Central Asia)

NASA Astrophysics Data System (ADS)

Dickerson, Patricia Wood

2003-04-01

The intraplate Ancestral Rocky Mountains of western North America extend from British Columbia, Canada, to Chihuahua, Mexico, and formed during Early Carboniferous through Early Permian time in response to continent-continent collision of Laurentia with Gondwana—the conjoined masses of Africa and South America, including Yucatán and Florida. Uplifts and flanking basins also formed within the Laurentian Midcontinent. On the Gondwanan continent, well inboard from the marginal fold belts, a counterpart structural array developed during the same period. Intraplate deformation began when full collisional plate coupling had been achieved along the continental margin; the intervening ocean had been closed and subduction had ceased—that is, the distinction between upper versus lower plates became moot. Ancestral Rockies deformation was not accompanied by volcanism. Basement shear zones that formed during Mesoproterozoic rifting of Laurentia were reactivated and exerted significant control on the locations, orientations, and modes of displacement on late Paleozoic faults. Ancestral Rocky Mountain uplifts extend as far south as Chihuahua and west Texas (28° to 33°N, 102° to 109°W) and include the Florida-Moyotes, Placer de Guadalupe-Carrizalillo, Ojinaga-Tascotal and Hueco Mountain blocks, as well as the Diablo and Central Basin Platforms. All are cored with Laurentian Proterozoic crystalline basement rocks and host correlative Paleozoic stratigraphic successions. Pre-late Paleozoic deformational, thermal, and metamorphic histories are similar as well. Southern Ancestral Rocky Mountain structures terminate along a line that trends approximately N 40°E (present coordinates), a common orientation for Mesoproterozoic extensional structures throughout southern to central North America. Continuing Tien Shan intraplate deformation (Central Asia) has created an analogous array of uplifts and basins in response to the collision of India with Eurasia, beginning in late Miocene time when full coupling of the colliding plates had occurred. As in the Laurentia-Gondwana case, structures of similar magnitude and spacing to those in Eurasia have developed in the Indian plate. Within the present orogen two ancient suture zones have been reactivated—the early Paleozoic Terskey zone and the late Paleozoic Turkestan suture between the Siberian and East Gondwanan cratons. Inverted Proterozoic to early Paleozoic rift structures and passive-margin deposits are exposed north of the Terskey zone. In the Alay and Tarim complexes, Vendian to mid-Carboniferous passive-margin strata and the subjacent Proterozoic crystalline basement have been uplifted. Data on Tien Shan uplifts, basins, structural arrays, and deformation rates guide paleotectonic interpretations of ancient intraplate mountain belts. Similarly, exhumed deep crustal shear zones in the Ancestral Rockies offer insight into partitioning and reorientation of strain during contemporary intraplate deformation.

High risk of tsunami in the northern Caribbean

NASA Astrophysics Data System (ADS)

Grindlay, Nancy R.; Hearne, Meghan; Mann, Paul

The magnitude Mw = 9.3 Sumatra earthquake of 26 December 2004 claimed the lives of an estimated 300,000 people living in coastal areas of seven different countries around the Indian Ocean. This event raised the question of whether similar far-traveled tsunamis generated by plate boundary faulting could affect the estimated 150 million people living in coastal areas of the United States, including Alaska, Hawaii, Puerto Rico, and the U.S. Virgin Islands.Aside from the Pacific plate margin of North America, the North America-Caribbean plate boundary is the closest (˜2000 km) active plate boundary to coastal areas in the Gulf of Mexico and the U.S. Atlantic seaboard. Researchers also have proposed that other possible tsunami-generating sources that could affect coastal areas of the United States include slumping of the shelf margin along the Virginia-North Carolina margin [Driscoll et al., 2000] and slumping of volcanic edifices in the Canary Islands [Ward and Day, 2001].

The Edges of the Ocean: An Introduction.

ERIC Educational Resources Information Center

Burke, Kevin

1979-01-01

Introduces a series of related articles on the study of ocean/continent boundaries (margins) within the framework of plate tectonics. Topics discussed include: early attempts to interpret ocean/continent boundaries, Atlantic-type margins, Pacific-type margins, the edges of ancient oceans, and future challenges in the study of continental margins.…

Permian-Triassic Tethyan realm reorganization: Implications for the outward Pangea margin

NASA Astrophysics Data System (ADS)

Riel, Nicolas; Jaillard, Etienne; Martelat, Jean-Emmanuel; Guillot, Stéphane; Braun, Jean

2018-01-01

We present a new conceptual model to explain the first order Permian-Triassic evolution of the whole > 30 000 km long Pangea margin facing the Panthalassa ocean. Compilation of available geological, geochemical, geochronogical and paleomagnetic data all along this system allowed us to distinguish three part of the margin: western Laurentia, western Gondwana and eastern Gondwana. These segments record distinct tectonic and magmatic events, which all occur synchronously along the whole margin and correlate well with the main geodynamic events of this period, i.e. subduction of the Paleotethys mid-ocean ridge at 310-280 Ma, opening of the Neotethys at 280-260 Ma, counterclockwise rotation of Pangea at 260-230 Ma and closure of the Paleotethys at 230-220 Ma. Between 260 and 230 Ma, the reorganization of the Tethyan realm triggered the up to 35° rotation of Pangea around an Euler pole located in northernmost South America. This implied both an increase and a decrease of the convergence rate between the margin and the Panthalassa ocean, north and south of the Euler pole, respectively. Thus, the Permian-Triassic Pangean margin was marked: in western Laurentia by marginal sea closure, in western Gondwana by widespread bimodal magmatic and volcanic activity, in eastern Gondwana by transpressive orogenic phase. Therefore, we propose that the Permian-Triassic evolution of the outward margin of Pangea was controlled by the Tethyan realm reorganization.

Reconstructing the Alps-Carpathians-Dinarides as a key to understanding switches in subduction polarity, slab gaps and surface motion

NASA Astrophysics Data System (ADS)

Handy, Mark R.; Ustaszewski, Kamil; Kissling, Eduard

2015-01-01

Palinspastic map reconstructions and plate motion studies reveal that switches in subduction polarity and the opening of slab gaps beneath the Alps and Dinarides were triggered by slab tearing and involved widespread intracrustal and crust-mantle decoupling during Adria-Europe collision. In particular, the switch from south-directed European subduction to north-directed "wrong-way" Adriatic subduction beneath the Eastern Alps was preconditioned by two slab-tearing events that were continuous in Cenozoic time: (1) late Eocene to early Oligocene rupturing of the oppositely dipping European and Adriatic slabs; these ruptures nucleated along a trench-trench transfer fault connecting the Alps and Dinarides; (2) Oligocene to Miocene steepening and tearing of the remaining European slab under the Eastern Alps and western Carpathians, while subduction of European lithosphere continued beneath the Western and Central Alps. Following the first event, post-late Eocene NW motion of the Adriatic Plate with respect to Europe opened a gap along the Alps-Dinarides transfer fault which was filled with upwelling asthenosphere. The resulting thermal erosion of the lithosphere led to the present slab gap beneath the northern Dinarides. This upwelling also weakened the upper plate of the easternmost part of the Alpine orogen and induced widespread crust-mantle decoupling, thus facilitating Pannonian extension and roll-back subduction of the Carpathian oceanic embayment. The second slab-tearing event triggered uplift and peneplainization in the Eastern Alps while opening a second slab gap, still present between the Eastern and Central Alps, that was partly filled by northward counterclockwise subduction of previously unsubducted Adriatic continental lithosphere. In Miocene time, Adriatic subduction thus jumped westward from the Dinarides into the heart of the Alpine orogen, where northward indentation and wedging of Adriatic crust led to rapid exhumation and orogen-parallel escape of decoupled Eastern Alpine crust toward the Pannonian Basin. The plate reconstructions presented here suggest that Miocene subduction and indentation of Adriatic lithosphere in the Eastern Alps were driven primarily by the northward push of the African Plate and possibly enhanced by neutral buoyancy of the slab itself, which included dense lower crust of the Adriatic continental margin.

Cenozoic Tectonic Evolution of Northeast China and Surrounding Areas Reproduced by Slab Subduction Models

NASA Astrophysics Data System (ADS)

Yang, T.; Moresi, L. N.; Zhao, D.; Sandiford, D.

2017-12-01

Northeast China lies at the continental margin of the western Pacific subduction zone where the Pacific Plate subducts beneath the Eurasia Plate along the Kuril-Japan trench during the Cenozoic, after the consumption of the Izanagi Plate. The Izanagi Plate and the Izanagi-Pacific mid-ocean ridge recycled to the mantle beneath Eurasia before the early Cenozoic. Plate reconstructions suggest that (1) age of the incoming Pacific Plate at the trench increases with time; (2) convergence rate between the Pacific and Eurasia Plates increased rapidly from the late Eocene to the early Miocene. Northeast China and surrounding areas suffered widespread extension and magmatism during the Cenozoic, culminating in the opening of the Japan Sea and the rifting of the Baikal Rift Zone. The Japan Sea opened during the early Miocene and kept spreading until the late Miocene, since when compression tectonics gradually prevailed. The Baikal Rift Zone underwent slow extension in the Cenozoic but its extension rate has increased rapidly since the late Miocene. We investigate the Cenozoic tectonic evolution of Northeast China and surrounding areas with geodynamic models. Our study suggests that the rapid aging of the incoming Pacific Plate at the subduction zone leads to the increase of plate convergence and trench motion rates, and explains the observed sequence of regional tectonic events. Our geodynamic model, which reproduces the Cenozoic regional tectonic events, predicts slab morphology and stress state consistent with seismic observations, including over 1000 km of slab stagnant in the transition zone, and the along-dip principal compressional stress direction. Our model requires a value of the 660 km phase transition Clapeyron slope of -2.5 MPa/K to reproduce the stagnant slab and tectonic events in the study region. This suggests that the Pacific slab is hydrated in the transition zone, explaining geochemical characteristics of some regional Cenozoic igneous rocks which were suggested to originate from a hydrous mantle transition zone.

Radiographic Outcomes of Dorsal Distraction Distal Radius Plating for Fractures With Dorsal Marginal Impaction.

PubMed

Huish, Eric G; Coury, John G; Ibrahim, Mohamed A; Trzeciak, Marc A

2017-04-01

The purpose of this study is to compare radiographic outcomes of patients treated with dorsal spanning plates with previously reported normal values of radiographic distal radius anatomy and compare the results with prior publications for both external fixation and internal fixation with volar locked plates. Patients with complex distal radius fractures including dorsal marginal impaction pattern necessitating dorsal distraction plating at the discretion of the senior authors (M.A.T. and M.A.I.) from May 30, 2013, to December 29, 2015, were identified and included in the study. Retrospective chart and radiograph review was performed on 19 patients, 11 male and 8 female, with mean age of 47.83 years (22-82). No patients were excluded from the study. All fractures united prior to plate removal. The average time the plate was in place was 80.5 days (49-129). Follow-up radiographs showed average radial inclination of 20.5° (13.2°-25.5°), radial height of 10.7 mm (7.5-14 mm), ulnar variance of -0.3 mm (-2.1 to 3.1 mm), and volar tilt of 7.9° (-3° to 15°). One patient had intra-articular step-off greater than 2 mm. Dorsal distraction plating of complex distal radius fractures yields good radiographic results with minimal complications. In cases of complex distal radius fractures including dorsal marginal impaction where volar plating is not considered adequate, a dorsal distraction plate should be considered as an alternative to external fixation due to reduced risk for infection and better control of volar tilt.

Tectonic Configuration of the Western Arabian Continental Margin, Southern Red Sea

NASA Astrophysics Data System (ADS)

Bohannon, Robert G.

1986-08-01

The young continental margin of the western Arabian Peninsula is uplifted 3.5 to 4 km and is well exposed. Rift-related extensional deformation is confined to a zone 150 km wide inland of the present coastline at 17 to 18° N and its intensity increases gradually from east to west. Extension is negligible near the crest of the Arabian escarpment, but it reaches a value of 8 to 10% in the western Asir, a highly dissected mountainous region west of the escarpment. There is an abrupt increase in extensional deformation in the foothills and pediment west of the Asir (about 40 km inland of the shoreline) where rocks in the upper plate of a system of low-angle normal faults with west dips are extended by 60 to 110%. The faults were active 23 to 29 Ma ago and the uplift occurred after 25 Ma ago. Tertiary mafic dike swarms and plutons of gabbro and granophyre 20 to 23 Ma old are concentrated in the foothills and pediment as well. The chemistry of the dikes suggests (1) fractionation at 10 to 20 kbar, (2) a rapid rise through the upper mantle and lower crust, and (3) differentiation and cooling at 1 Atm to 5 kbar. Structural relations between dikes, faults and dipping beds indicate that the mechanical extension and intrusional expansion were partly coeval, but that most of the extension preceded the expansion. A tectonic reconstruction of pre-Red Sea Afro/Arabia suggests that the early rift was narrow with intense extension confined to an axial belt 20 to 40 km wide. Steep Moho slopes probably developed during rift formation as indicated by published gravity data, two published seismic interpretations and the surface geology.

Jurassic subduction initiation in the western and central Neo-Tethys and the origin of the Balkan ophiolites

NASA Astrophysics Data System (ADS)

Van Hinsbergen, D. J. J.; Maffione, M.

2017-12-01

Jurassic subduction initiation in the Neo-Tethys Ocean was the first, critical step of a long tectonic process that eventually led to the collision of the Adria-Africa and Eurasia plates and the formation of a 6000 km long Alpine orogenic belt spanning from the Balkan Peninsula to Iran. Investigating the process of subduction initiation in the Neo-Tethys during the Jurassic is crucial to (i) reconstruct the complex geological evolution of this orogen from its initial stages, and (ii) shed new lights over the enigmatic kinematics and driving mechanisms of subduction initiation. Records of the initial closure of the Neo-Tethys are today preserved in a fragmented belt of Middle Jurassic ophiolites (170-160 Ma) distributed above the Alpine orogen. In particular, the well-preserved and extensively studied ophiolites of the Balkan Peninsula offer a unique chance to study the mechanisms leading to the closure of the western domain of the Neo-Tethys. Here we provide the first quantitative constraints on the geometry of the Jurassic Neo-Tethyan subduction system using a net tectonic rotation analysis based on paleomagnetic and structural geological data from the sheeted dyke complexes of various ophiolites of Serbia (Maljen, Ibar) and Greece (Othris, Pindos, Vourinos, Guevgueli). Our results show that closure of the western Neo-Tethys was accommodated by two subduction zones, one intra-oceanic, formed at the N-S trending Neo-Tethyan ridge, the other initiated at the European passive margin and curving southward from a N-S to a NW-SE direction following the shape of the passive margin. We propose that these two subduction zones formed upon propagation of subduction(s) initiated in the central Neo-Tethys (modern Turkey) in the late Early Jurassic ( 185-180 Ma).

Big mantle wedge, anisotropy, slabs and earthquakes beneath the Japan Sea

NASA Astrophysics Data System (ADS)

Zhao, Dapeng

2017-09-01

The Japan Sea is a part of the western Pacific trench-arc-backarc system and has a complex bathymetry and intense seismic activities in the crust and upper mantle. Local seismic tomography revealed strong lateral heterogeneities in the crust and uppermost mantle beneath the eastern margin of the Japan Sea, which was determined using P and S wave arrival times of suboceanic earthquakes relocated precisely with sP depth phases. Ambient-noise tomography revealed a thin crust and a thin lithosphere beneath the Japan Sea and significant low-velocity (low-V) anomalies in the shallow mantle beneath the western and eastern margins of the Japan Sea. Observations with ocean-bottom seismometers and electromagnetometers revealed low-V and high-conductivity anomalies at depths of 200-300 km in the big mantle wedge (BMW) above the subducting Pacific slab, and the anomalies are connected with the low-V zone in the normal mantle wedge beneath NE Japan, suggesting that both shallow and deep slab dehydrations occur and contribute to the arc and back-arc magmatism. The Pacific slab has a simple geometry beneath the Japan Sea, and earthquakes occur actively in the slab down to a depth of ∼600 km beneath the NE Asian margin. Teleseismic P and S wave tomography has revealed that the Philippine Sea plate has subducted aseismically down to the mantle transition zone (MTZ, 410-660 km) depths beneath the southern Japan Sea and the Tsushima Strait, and a slab window is revealed within the aseismic Philippine Sea slab. Seismic anisotropy tomography revealed a NW-SE fast-velocity direction in the BMW, which reflects corner flows induced by the fast deep subduction of the Pacific slab. Large deep earthquakes (M > 7.0; depth > 500 km) occur frequently beneath the Japan Sea western margin, which may be related to the formation of the Changbai and Ulleung intraplate volcanoes. A metastable olivine wedge is revealed within the cold core of the Pacific slab at the MTZ depth, which may be related to the deep seismicity. However, many of these results are still preliminary, due to the lack of seismic stations in the Japan Sea. The key to resolving these critical geoscientific issues is seismic instrumentation in the Japan Sea, for which international cooperation of geoscience communities in the East Asian countries is necessary.

Seismic structure of western Mediterranean back-arc basins and rifted margins - constraints from the Algerian-Balearic and Tyrrhenian Basins

NASA Astrophysics Data System (ADS)

Grevemeyer, Ingo; Ranero, Cesar; Sallares, Valenti; Prada, Manel; Booth-Rea, Guillermo; Gallart, Josep; Zitellini, Nevio

2017-04-01

The Western Mediterranean Sea is a natural laboratory to study the processes of continental extension, rifting and back-arc spreading in a convergent setting caused by rollback of fragmented subducting oceanic slabs during the latest phase of consumption of the Tethys ocean, leading to rapid extension in areas characterized by a constant convergence of the African and European Plates since Cretaceous time. Opening of the Algerian-Balearic Basin was governed by a southward and westward retreating slab 21 to 18 Myr and 18 to15 Myr ago, respectively. Opening of the Tyrrhenian Basin was controlled by the retreating Calabrian slab 6 to 2 Myr ago. Yet, little is known about the structure of the rifted margins, back-arc extension and spreading. Here we present results from three onshore/offshore seismic refraction and wide-angle lines and two offshore lines sampling passive continental margins of southeastern Spain and to the south of the Balearic promontory and the structure of the Tyrrhenian Basin to the north of Sicily. Seismic refraction and wide-angle data were acquired in the Algerian-Balearc Basin during a cruise of the German research vessel Meteor in September of 2006 and in the Tyrrhenian Sea aboard the Spanish research vessel Sarmiento de Gamboa in July of 2015. All profiles sampled both continental crust of the margins surrounding the basins and extend roughly 100 km into the Algerian-Balearic and the Tyrrhenian Basins, yielding constraints on the nature of the crust covering the seafloor in the basins and adjacent margins. Crust in the Algerian-Balearic basin is roughly 5-6 km thick and the seismic velocity structure mimics normal oceanic crust with the exception that lower crustal velocity is <6.8 km/s, clearly slower than lower crust sampled in the Pacific Basin. The seismic Moho in the Algerian-Balearic Basin occurs at 11 km below sea level, reaching >24 km under SE Spain and the Balearic Islands, displaying typical features and structure of continental crust. Offshore Sicily, continental crust reaches 22 km. However, the Tyrrhenian Basin indicates a lithosphere with velocities increasing continuously from 3 km/s to 7.5 km/s, mimicking features attributed to un-roofed and hence serpentinized mantle. Therefore, even though the opening of both basins was controlled by slab rollback, the resulting structures of the basins indicate striking differences. It is interesting to note that the continent/ocean transition zone of the margins did not show any evidence for high velocity lower crustal rocks, in contrast to what has been sampled in Western Pacific arc/back-arc systems.

Identification of hyper-extended crust east of Davie Ridge in the Mozambique Channel

NASA Astrophysics Data System (ADS)

Klimke, Jennifer; Franke, Dieter

2015-04-01

Davie Ridge is a ~1200 km wide, N-S trending bathymetrical high in the Mozambique Channel. Today, it is widely accepted that Davie Ridge is located along a fossil transform fault that was active during the Middle Jurassic and Early Cretaceous (~165-120 Ma). This transform fault results from the breakup of Gondwana, when Madagascar (together with India and Antarctica) drifted from its northerly position in the Gondwana Supercontinent (adjacent to the coasts of Tanzania, Somalia and Kenya) to its present position (e.g. Coffin and Rabinowitz, 1987; Rabinowitz et al., 1983; Segoufin and Patriat, 1980). The southward motion of Madagascar relative to Africa is constrained by the interpretation of magnetic anomalies in the Western Somali Basin, located north of Madagascar (e.g. Rabinowitz et al., 1983). According to Bird (2001), sheared margins share typical characteristics and a common evolution: 1. The transition from continental to oceanic crust is relatively abrupt (~ 50-80 km). 2. Along the continental side of the margin, complex rift basins form that display a wide range of faults. 3. Prominent marginal ridges form along the sheared margin that probably originate from the propagation of the oceanic spreading center along the plate boundary (Bird, 2001). In February and March 2014, a dense geophysical dataset (multichannel seismic, magnetics, gravimetry and bathymetry) with a total of 4300 profile km along the sheared margin was acquired with the R/V Sonne by the Federal Institute for Geosciences and Natural Resources (BGR). A special objective of the project, amongst others, is the characterization and interpretation of the continent-ocean transition seaward of Davie Ridge in the Mozambique Channel. Seismic profiles located east of Davie Ridge in the Western Somali Basin reveal a wide sequence of half-grabens bounded by listric normal faults. We tentatively suggest that this crust is of continental origin and results from rifting between Africa and Madagascar during the breakup of Gondwana. This implies that the continent-ocean transition is located at least ~ 150 km east of Davie Ridge. References Bird, D., 2001. Shear margins: Continent-ocean transform and fracture zone boundaries. The Leading Edge, 150-159. Coffin, M. F., und Rabinowitz, P. D., 1987. Reconstruction of Madagascar and Africa: Evidence from the Davie Fracture Zone and Western Somali Basin. Journal of Geophysical Research: Solid Earth, vol. 92, no. B9, 9385-9406. Rabinowitz, P.D., Coffin, M.F. and Falvey, D.A., 1983. The separation of Madagascar and Africa. Science 220, 67-69. Segoufin, J., und Patriat, P., 1980. Existence d'anomalies mesozoiques dans le bassin de Somalie. Implications pour les relations Afrique-Antarctique-Madagascar: C.R. Acad. Sci. Paris, v. 291, p. 85-88.

The role of tectonic inheritance in the morphostructural evolution of the Galicia continental margin and adjacent abyssal plains from digital bathymetric model (DBM) analysis (NW Spain)

NASA Astrophysics Data System (ADS)

Maestro, A.; Jané, G.; Llave, E.; López-Martínez, J.; Bohoyo, F.; Druet, M.

2018-06-01

The identification of recent major tectonic structures in the Galicia continental margin and adjacent abyssal plains was carried out by means of a quantitative analysis of the linear structures having bathymetric expression on the seabed. It was possible to identify about 5800 lineaments throughout the entire study area, of approximately 271,500 km2. Most lineaments are located in the Charcot and Coruña highs, in the western sector of the Galicia Bank, in the area of the Marginal Platforms and in the northern sector of the margin. Analysis of the lineament orientations shows a predominant NE-SW direction and three relative maximum directions: NW-SE, E-W and N-S. The total length of the lineaments identified is over 44,000 km, with a mode around 5000 m and an average length of about 7800 m. In light of different tectonic studies undertaken in the northwestern margin of the Iberian Peninsula, we establish that the lineaments obtained from analysis of the digital bathymetric model of the Galicia continental margin and adjacent abyssal plains would correspond to fracture systems. In general, the orientation of lineaments corresponds to main faults, tectonic structures following the directions of ancient faults that resulted from late stages of the Variscan orogeny and Mesozoic extension phases related to Triassic rifting and Upper Jurassic to Early Cretaceous opening of the North Atlantic Ocean. The N-S convergence between Eurasian and African plates since Palaeogene times until the Miocene, and NW-SE convergence from Neogene to present, reactivated the Variscan and Mesozoic fault systems and related physiography.

Paleogene volcanism in Central Afghanistan: Possible far-field effect of the India-Eurasia collision

NASA Astrophysics Data System (ADS)

Motuza, Gediminas; Šliaupa, Saulius

2017-10-01

A volcanic-sedimentary succession of Paleogene age is exposed in isolated patches at the southern margin of the Tajik block in the Ghor province of Central Afghanistan. The volcanic rocks range from basalts and andesites to dacites, including adakites. They are intercalated with sedimentary rocks deposited in shallow marine environments, dated biostratigraphically as Paleocene-Eocene. This age corresponds to the age of the Asyābēd andesites located in the western Ghor province estimated by the 40Ar/39Ar method as 54 Ma. The magmatism post-dates the Cimmerian collision between the Tajik block (including the Band-e-Bayan block) and the Farah Rod block located to the south. While the investigated volcanic rocks apparently bear geochemical signatures typical to an active continental margin environment, it is presumed that the magmatism was related to rifting processes most likely initiated by far-field tectonics caused by the terminal collision of the Indian plate with Eurasia (Najman et al., 2017). This event led to the dextral movement of the Farah Rod block, particularly along Hari Rod (Herat) fault system, resulting in the development of a transtensional regime in the proximal southern margin of the Tajik block and giving rise to a rift basin where marine sediments were interbedded with pillow lavas intruded by sheeted dyke series.

Accretion in the wake of terrane collision: The Neogene accretionary wedge off Kenai Peninsula, Alaska

USGS Publications Warehouse

Fruehn, J.; von Huene, Roland E.; Fisher, M.A.

1999-01-01

Subduction accretion and repeated terrane collision shaped the Alaskan convergent margin. The Yakutat Terrane is currently colliding with the continental margin below the central Gulf of Alaska. During the Neogene the terrane's western part was subducted after which a sediment wedge accreted along the northeast Aleutian Trench. This wedge incorporates sediment eroded from the continental margin and marine sediments carried into the subduction zone on the Pacific plate. Prestack depth migration was performed on six seismic reflection lines to resolve the structure within this accretionary wedge and its backstop. The lateral extent of the structures is constrained by high-resolution swath bathymetry and seismic lines collected along strike. Accretionary structure consists of variably sized thrust slices that were deformed against a backstop during frontal accretion and underplating. Toward the northeast the lower slope steepens, the wedge narrows, and the accreted volume decreases notwith-standing a doubling of sediments thickness in the trench. In the northeasternmost transect, near the area where the terrane's trailing edge subducts, no frontal accretion is observed and the slope is eroded. The structures imaged along the seismic lines discussed here most likely result from progressive evolution from erosion to accretion, as the trailing edge of the Yakutat Terrane is subducting.

A review of the tectonic evolution of the Northern Pacific and adjacent Cordilleran Orogen

NASA Astrophysics Data System (ADS)

Jakob, Johannes; Gaina, Carmen; Johnston, Stephen T.

2014-05-01

Numerous plate kinematic models for the North Pacific realm have been developed since the advent of plate tectonics in the early seventies (e.g Atwater (1970), Mammerickx and Sharman (1988)). Although published kinematic models are consistent with the broad scale features of the North Pacific, the link between plate motions and the evolution of the North American Cordillera remains poorly understood. Part of the problem lies in conflicting interpretations of geological versus paleomagnetic data sets, with the result being a lack of consensus regarding: the paleolocation of key geological units; the paleogeography of terrane formation and amalgamation; the motion, boundaries and even existence of oceanic plates; and the character (e.g. trend of subduction) and position of plate boundaries within the northern Pacific basin. Remnants of the Farallon and Kula plates, and some short-lived microplates, demonstrate the complicated tectonic evolution of the oceanic realm west of the North American margin (e.g. Rea and Dixon (1983); McCrory and Wilson (2013); Shephard et al. (2013)). The creation and destruction of major tectonic plates and microplates has presumably left a record in the Cordilleran orogen of western North America. However, working backward from the geological relationships to plate reconstructions remains difficult. Here we investigate the relationship between the plate motions of the Pacific Ocean and the terrane movements in the North American Cordillera by revising the marine magnetic and gravity anomalies of the northern Pacific. In particular, we reevaluate plate boundaries at times of major changes in plate geometry of the Pacific, Kula, Chinook and Farallon plates from C34n onward. Our focus is also on the plate geometries of the Resurrection, Eshamy and Siletz-Crescent plates during the time between anomaly C26 and C12, and the links between plate interactions and on-shore tectonic events recorded in the geological record of Vancouver Island, including the accretion of the Pacific Rim and Crescent terranes to Wrangellia between C25 and C18. References: Atwater, T. (1970). Implications of plate tectonics for the Cenozoic tectonic evolution of western North America. Geological Society of America Bulletin, 81, 3513-3536. McCrory, P. a., & Wilson, D. S. (2013). A kinematic model for the formation of the Siletz-Crescent forearc terrane by capture of coherent fragments of the Farallon and Resurrection plates. Tectonics, 32, 1-19. doi:10.1002/tect.20045 Rea, D. K., & Dixon, J. M. (1983). Late Cretaceous and Paleogene tectonic evolution of the North Pacific Ocean. Earth and Planetary Science Letters, 65, 145-166. Shephard, G. E., Müller, R. D., & Seton, M. (2013). The tectonic evolution of the Arctic since Pangea breakup: Integrating constraints from surface geology and geophysics with mantle structure. Earth-Science Reviews, 124, 148-183. doi:10.1016/j.earscirev.2013.05.012 Mammerickx, J., & Sharman, G. F. (1988). Tectonic evolution of the North Pacific during the Cretaceous quiet period. Journal of Geophysical Research, 93(B4), 3009-3024. doi:10.1029/JB093iB04p03009

The Sunda-Banda Arc Transition: New Insights From Marine Wide-Angle Seismic Data

NASA Astrophysics Data System (ADS)

Planert, L.; Shulgin, A.; Kopp, H.; Mueller, C.; Flueh, E.; Lueschen, E.; Engels, M.; Dayuf Jusuf, M.

2007-12-01

End of 2006, RV SONNE cruise SO190 SINDBAD (Seismic and Geoacoustic Investigations along the Sunda- Banda Arc Transition) went south of the Indonesian archipelago to acquire various geophysical datasets between 112 °E and 122 °E. The main goal of the project is to investigate the modifications of the lower plate (variability in the plate roughness, transition from oceanic to continental lower plate) and their effects on the tectonics of the upper plate (development of an outer high and forearc basin, accretionary and erosive processes). The tectonic style changes in neighboring margin segments from an oceanic plate-island arc subduction along the eastern Sunda margin to a continental plate-island arc collision along the Banda margin. Moreover, the character of the incoming oceanic plate varies from the rough topography in the area where the Roo Rise is subducting off eastern Java, to the smooth oceanic seafloor of the Argo- Abyssal Plain subducting off Bali, Lombok, and Sumbawa. In order to cover the entire variations of the lower plate, seven seismic refraction profiles were conducted along four major north-south oriented corridors of the margin, at 113 °E, 116 °E, 119 °E, and 121 °E, as well as three profiles running perpendicular to the major corridors. A total of 239 ocean bottom hydrophone and seismometer deployments were successfully recovered. Shooting was conducted along 1020 nm of seismic profiles using a G-gun cluster of 64 l. Here, we present velocity models obtained by applying a tomographic approach which jointly inverts for refracted and reflected phases. Additional geometry and velocity information for the uppermost layers, obtained by prestack depth migration of multichannel seismic reflection data (see poster of Mueller et al. in this session), is incorporated into our models and held fixed during the iterations. geomar.de/index.php?id=sindbad

Rigidity and definition of Caribbean plate motion from COCONet and campaign GPS observations

NASA Astrophysics Data System (ADS)

Mattioli, Glen; Miller, Jamie; DeMets, Charles; Jansma, Pamela

2014-05-01

The currently accepted kinematic model of the Caribbean plate presented by DeMets et al. (2007) is based on velocities from 6 continuous and 14 campaign GPS sites. COCONet is a multi-hazard GPS-Met observatory, which extends the existing infrastructure of the Plate Boundary Observatory in North America into the Caribbean basin. In 2010, UNAVCO in collaboration with UCAR, was funded by NSF to design, build, and initially maintain a network of 50 new cGPS/Met sites and include data from another 50 existing sites in the Caribbean region. The current COCONet siting plan calls for 46 new stations, 21 refurbished stations, and 77 existing stations across 26 nations in the Caribbean region. Data from all COCONet sites flow into the UNAVCO archive and are processed by the PBO analysis centers and are also processed independently by the UTA Geodesy Lab using GIPSY-OASISII (v.6.2) using an absolute point positioning strategy and final, precise orbits, clocks, and Earth orientation parameters from JPL in the IGS08 frame. We present here our refined estimate of Caribbean plate motion by evaluating data from an expanded number of stations with an improved spatial distribution. In order to better constrain the eastern margin of the plate near the Lesser Antilles subduction interface, campaign GPS observations have been collected on the island of Dominica over the last decade. These are combined with additional campaign observations from the western Caribbean, specifically from Honduras and Nicaragua. We have analyzed a total of 117 sites from the Caribbean region, including campaign data and the data from the cGPS stations that comprise COCONet. An updated velocity field for the Caribbean plate is presented and an inversion of the velocities for 24 sites yields a plate angular velocity that differs from previously published models. Our best fitting inversion to GPS velocities from these 24 sites suggests that 2-plate model for the Caribbean is required to fit the GPS observations, which implies that the Caribbean is undergoing modest (1-3 mm/yr) deformation within its interior. Some sites in the western Caribbean included in our analysis may be biased by small, but significant coseismic deformation, which has not been removed from the site velocities used in our inversion to define Caribbean motion and rigidity. Scenarios for possible east-west deformation accommodated across the Lower Nicaraguan Rise and Beata Ridge will be presented.

Living on the Edge? Clumped Isotope and Oxygen Isotope Record of Early Cascade Topography (Eocene Chumstick Basin, WA, USA)

NASA Astrophysics Data System (ADS)

Methner, K.; Mulch, A.; Fiebig, J.; Wacker, U.; Umhoefer, P. J.; Chamberlain, C. P.

2014-12-01

The topographic evolution of the world's major orogens exerts a strong impact on atmospheric circulation and precipitation patterns and is a key element in reconstructing the interactions among tectonics, climate, and Earth surface processes. Using carbonate stable and clumped isotope data together with low-temperature thermochronology information from the terrestrial Eocene Chumstick Basin (Central Washington; USA), today located to the East of the Washington Cascades, we investigate the E-W extent of the western North American plateau region and the evolution of Cascade topography. Oxygen isotope measurements of Eocene (51 to 37 Ma) pedogenic carbonate concretions and calcic horizons yield low δ18Ocarbonate values of +9 to +13 ‰ (SMOW) despite the proximity of the Eocene Chumstick Basin to the Pacific moisture source and paleofloral data that indicate moderate elevations and montane rain forest conditions during a warm and rather wet, seasonal climate. This either suggests that 51-37 Ma ago Cascade-like topography characterized the western edge of the North American-Pacific plate margin to the West of the Chumstick Basin or that the δ18Ocarbonate data were variably reset or only formed during burial and diagenesis. Clumped isotope (Δ47) thermometry of pedogenic carbonate and carbonate concretions (n=11 samples) indicates spatially variable burial temperatures of 80 to 120 °C that correlate with vitrinite reflectance data in these sediments. In concordance with changes in depositional environment the youngest (<40 Ma) Chumstick sediments experienced a lesser degree of post-depositional burial and heating (ca. 70 - 80 °C) compared to the older Chumstick series (80 - 120 °C). Calculated δ18O values of the circulating fluids in the Chumstick basin sediments range from -6 ‰ (T ~100 °C at ca. 40-30 Ma) to -9 ‰ (T ~75 °C at ca. 25-15 Ma). These values suggest a low-altitude meteoric fluid source and as a consequence only moderate Cascade topography during the Eo-/Oligocene. The combined stable isotope and clumped isotope data, therefore support models of late Miocene Cascade uplift and provide insight into the topographic configuration of the western North American plate margin prior to the late Miocene.

Pacific tectonics: Eastern-Pacific "stationarity" of EPR and causative association with Equator

NASA Astrophysics Data System (ADS)

Bostrom, R. C.

2003-04-01

The fundamentals of present-day Pacific tectonics are observed to be: its N/S mirror-symmetry about the Equator, displayed by the major transforms; its E/W asymmetry, represented by the western motion of the world's largest plate, originating in the eastern Equatorial Pacific; and correspondingly, development of the globally most voluminous subduction, at the western Pacific margin. The configuration seen at present is maintained at a fundamental level. The maximum in convective upwelling develops as coalescing plumes in the Galapagos region in the eastern Pacific. This has been found (Lonsdale 1988; McGuire and Hilde 2002; Chen and Lin 2002) to produce steady westward propagation of the Nazca/Cocos axis. Continually renewed, it determines the orientation and locus of a quasi-stationary EPR, centered on the Equator. Magnetic dating of boundaries in satellite gravity images records the Cenozoic history of the EPR, namely re-orientation in consequence of slow counter-clockwise re-orientation of the Equator. Relative to the present, during Maastrichtian times both Equator and plate motion were aligned WNW, recorded paleomagnetically and by features in the western, older part of the Pacific crust. Material subducted at that time accumulated principally beneath the SE Asia margin. Its slow heating is believed to play a role in the deep-seated activity and back-arc spreading associated with latter-day convergence in that region. The mechanism primarily responsible for the Pacific regime may be that mantle convection is not immune, as is generally tacitly supposed, to the minute westward tilt (c. 0.36°) under which it takes place. The latter, now astronomically quantifiable without tidal identification, represents the attraction component of water and solid-Earth masses which averaged over unit day lags the direction of purely geocentric g. Under gravity minutely E/W asymmetric, convection as always promoting the most efficient dissipative configuration, favors disproportionately large surface-west displacement, maximum at the contemporary Equator (=west limb of EPR upwelling). Some conclusions are a), that it is no longer adequate to model global convection assuming that angular momentum is conserved internally; in reality a considerable part is exported, here measured by expansion of the lunar orbit; and b), that the tectonics of an Earth simultaneously under vigorous convection and in asynchronous rotation relative to the mass center of Kuiper's Earth-Moon double planet, differs fundamentally from the regime developing within a fictitious isolated planet.

Spatial variations in the nature of the oceanic plate in the northwestern Pacific margin

NASA Astrophysics Data System (ADS)

Fujie, G.; Kodaira, S.; Shirai, T.; Dannowski, A.; Thorwart, M.; Grevemeyer, I.; Morgan, J. P.; Miura, S.

2016-12-01

Subduction of the oceanic plate plays an important role in the various processes in subduction zones, including arc magmatism and generation of earthquakes. Thus the nature of the incoming plate, such as its relief, thermal state, lithology, and the water content, are considered to shape these subduction zone processes.In 2014 and 2015, to reveal the nature of the incoming plate in the ocean-ward area of the 2011 M9 Tohoku earthquake, we conducted wide-angle seismic surveys in the trench-outer rise region of the Japan Trench. We designed a 600 km long seismic survey line perpendicular to the trench axis and deployed 88 OBSs at intervals of 6 km and shot a tuned airgun array of R/V Kairei.We have applied a traveltime inversion to model the P-wave velocity (Vp) structure. The resulting Vp model shows that Vp within the oceanic crust and the topmost mantle decreases in the vicinity of the trench axis probably due to the plate bending. In addition, we observed low Vp at the top of the oceanic crust in the area of petit spot volcanos. The low Vp area may be related to magma intrusions because we observed several structural interfaces in the shallow area.We found two structural features that we did not anticipate. First, crustal thickness abruptly changes at around the center of our survey line ( 300-km east from the trench axis); crust thickness is 7-km in the west and 6-km in the east. Second, mantle Vp shows significant variations along the survey line, 7.5 km/s in the bend-fault area (western area), 8.0 km/s around the center, 8.5 km/s in the eastern area. Based on the shear wave splitting observed in our data set, we infer that high mantle Vp in the eastern area is related with the changes in the orientation of the mantle anisotropy. Since we do not see any remarkable topographic features indicating the off-ridge activities, we consider that these observed structural features are related with the activities near the ancient spreading ridge when the oceanic plate formed, indicating that the oceanic plate in the NW Pacific margin, the input to the northeastern Japanese island arc, is more complicated here than we previously thought.In this presentation, we will show an overview of the Vp model along the whole profile and detailed seismic structure beneath the petit-spot area derived by the P-to-S converted waves.

Investigation of Collisional Styles of the Caribbean Large Igneous Province (CLIP) vs. Normal Oceanic Crust from Seismic Reflection Profiles

NASA Astrophysics Data System (ADS)

Mataracioglu, M.; Magnani, M.; DeShon, H. R.; Cox, R. T.

2011-12-01

The Caribbean plate subducts beneath the North American and the South American plates at the Muertos Trough and the South Caribbean Deformed Belt (SCDB), respectively. During subduction, large amount of crustal material may enter the subduction zone with the subducting plate or may be incorporated into the accretionary prism. To investigate the changes in collisional style and structures associated with subduction of the Caribbean Large Igneous Province (CLIP) versus normal oceanic crust, we interpret seismic reflection profiles collected around the northern and southern margins of the Venezuelan Basin. We refine the extent of the CLIP in the central and eastern Caribbean by identifying the structural differences at the top of the acoustic basement (Horizon B") on a dataset of 150 multi-channel seismic time stack and migrated marine reflection profiles acquired in eight cruises from 1975 to 2004, as well as some selected Integrated Ocean Drilling Program (IODP) drilling data. We will also attempt to determine whether sedimentary material enters the trench and is recycled back into the mantle, and therefore characterize the northern and southern subduction zones as accretionary or erosive. Our preliminary results show that the CLIP extends spatially to most of the Venezuelan Basin, the western part of the Columbian Basin, and a part of the Beata Ridge, but that it does not extend as far south as suggested by previous interpretations. Furthermore, some portions of the CLIP at the northern and southern boundaries subduct beneath the North and the South American plates at the Muertos Trough and the SCDB, respectively. The change in nature of the subducting plate (CLIP or normal oceanic crust) causes variations in the collisional style (i.e., accretionary versus erosive) and in structures at the accretionary wedge and on the downgoing plate.

Plate motion changes drive Eastern Indian Ocean microcontinent formation

NASA Astrophysics Data System (ADS)

Whittaker, J. M.; Williams, S.; Halpin, J.; Wild, T.; Stilwell, J.; Jourdan, F.; Daczko, N. R.

2016-12-01

The roles of plate tectonic or mantle dynamic forces in rupturing continental lithosphere remain controversial. Particularly enigmatic is the rifting of microcontinents from mature continental rifted margin - several well-studied microcontinent calving events coincide in space and time with mantle plume activity, but the significance of plumes in driving microcontinent formation remains controversial, and a role for plate-driven processes has also been suggested. In 2011, our team discovered two new microcontinents in the eastern Indian Ocean, the Batavia and Gulden Draak microcontinents. These microcontinents are unique as they are the only surviving remnants of the now-destroyed or highly deformed Greater Indian margin and provide us with an opportunity to test existing models of microcontinent formation against new observations. Here, we explore models for microcontinent formation using our new data from the Eastern Indian Ocean in a plate tectonic reconstruction framework. We use Argon dating and paleontology results to constrain calving from greater India at 101-104 Ma. This region had been proximal to the active Kerguelen plume for 30 Myrs but we demonstrate that calving did not correspond with a burst of volcanic activity. Rather, it is likely that plume-related thermal weakening of the Indian passive margin preconditioned it for microcontinent formation but calving was triggered by changes in plate tectonic boundary forces. Changes in the relative motions between Indian and Australia led to increasing compressive forces along the long-offset Wallaby-Zenith Fracture Zone, which was eventually abandoned during the jump of the spreading ridge into the Indian continental margin.

Paleogeography, Paleo-drainage Systems, and Tectonic Reconstructions of Eocene Northern South America Constrained by U-Pb Detrital Zircon Geochronology

NASA Astrophysics Data System (ADS)

Xie, X.; Mann, P.; Escalona, A.

2008-12-01

Thick, Eocene to Miocene clastic sedimentary basins are widespread across on- and offshore northern South America and have been identified using seismic reflection data in offshore basins of the Leeward Antilles, the Lesser Antilles arc and forearc, and the Barbados accretionary prism. Several 3 to12-km-thick Paleogene depocenters occur in shelf to deep basinal settings along the offshore margins of Venezuela, Trinidad and Tobago, and Barbados. Previous studies proposed that the proto-Orinoco River has been the single fluvial source for these distal, continentally-derived sandstone units along northern Venezuela as part of the early Eocene to Miocene, proto-Maracaibo fluvial-deltaic system that emanated from the northern Andes of western Venezuela and Colombia. Those distal sandstones were displaced eastward with the movement of the Caribbean plate by several hundred kilometers and are now found in basins and islands of the southeastern Caribbean region. We collected nine Eocene age sandstone samples from well cores and outcrops along the northern South America margin, including Lake Maracaibo, Trinidad and Tobago, and Barbados Island. In total, 945 single detrital zircon grains were analyzed using LA-ICP-MS. The objective is to reconstruct the paleogeography, paleo-drainage system, and tectonic history during Eocene time. New data show that the Eocene Misoa Formation of Lake Maracaibo was characterized by a mixture of Precambrian, Paleozoic, and Mesozoic ages matching age provinces from eastern Cordillera and the Guayana Shield, which is consistent with previous proto-Orinoco River model flowing from the western Amazonian region of Colombia and Brazil through the Maracaibo basin into the area of western Falcon basin. However, coeval Eocene samples from Barbados and Trinidad show a much different age population dominated by Precambrian matching the eastern part of the Guyana shield to the south, which suggests that the western onland system and eastern offshore units belong to different systems. We postulate that a series of smaller, north-flowing drainages provided a line of sediment source dispersal of Eocene sandstone from the north central and eastern edge of the Guyana shield onto the Eocene passive margin that extended from central Venezuela to Trinidad instead of being tectonically transported to their present locations suggested by earlier studies.

Sternal gland structures in males of bean flower thrips, Megalurothrips sjostedti, and Poinsettia thrips, Echinothrips americanus, in comparison with those of western flower thrips, Frankliniella occidentalis (Thysanoptera: Thripidae).

PubMed

Krueger, Stephanie; Subramanian, Sevgan; Niassy, Saliou; Moritz, Gerald B

2015-09-01

Sternal pores are important features for identification of male thrips, especially within the subfamily Thripinae. They vary in shape, size and distribution even between species of one genus. Their functional role is speculated to be that of sex- and/or aggregation pheromone production. Yet, sexual aggregations are not reported in Echinothrips americanus, known to have sternal pores, while we observed aggregations in Megalurothrips sjostedti, previously reported to lack them. We examined the sternal glands and pores of the thripine species E. americanus and M. sjostedti males, in comparison with those of Frankliniella occidentalis using light microscopy, as well as scanning and transmission electron microscopy. Pore plates of F. occidentalis were ellipsoid and medial on sternites III-VII, while in E. americanus they were distributed as multiple micro pore plates on sternites III-VIII. In M. sjostedti they appeared as an extremely small pore in front of the posterior margin of each of sternites IV-VII. Pore plate and pore plate area were distributed similarly on sternites III-VII in F. occidentalis. However, in E. americanus the total pore plate area increased significantly from sternites III to VIII. Ultrastructure of cells associated with sternal glands showed typical characteristics of gland cells that differ in size, shape and number. The function of sternal glands is further discussed on the basis of morphological comparisons with other thrips species. Copyright © 2015 Elsevier Ltd. All rights reserved.

Tectonostratigraphic Evolution of the Levant domain since Late Palaeozoic: a Review

NASA Astrophysics Data System (ADS)

Barrier, Eric

2015-04-01

During the last 270 my, the evolution of the African/Arabian platform and margins in Levant and surroundings is controlled by a succession of regional tectonic events, starting with a rifting period in the late Paleozoic, and ending with the ongoing Arabia-Eurasia collision. The main rifting period initiated in the mid-late Permian and lasted until the early-Jurassic, as a consequence of the Pangea break up. During this period the Anatolian blocks are still attached to southern Pangea, but some of the Palmyra-Levant and East Mediterranean basins were initiating. From the Mid-Late Permian to the Early Triassic the sedimentation is clastic-dominated in the continental platforms and basins. In the Early Mesozoic, with the initiation and development of the Levant and East Mediterranean basins, the sedimentation changed from clastic to carbonate deposition. Widespread Triassic to Liassic sediments accumulated in subsiding basins (Levant, Palmyride, Sinjar) and margins (East Mediterranean Basin). The rifting aborted in the Palmyride Trough and Levant Basin in the early Jurassic, while the East Mediterranean Basin (Mesogea) the oceanic accretion probably developed during the mid-Jurassic. Then, a 60 My-long cycle lasted from the late Jurassic to the Turonian, mainly characterized by the thermal subsidence of main the basins and margins. Only the early Cretaceous is marked by an extensional tectonic event, associated with magmatism, widespread all around the East Mediterranean Basin. This event, together with the early Cretaceous eustatic regressions, originated a major stratigraphic gap with emersions at the top-Jurassic - Neocomian period, and the deposition of thick clastic sequences in grabens. The following Cenomanian - Early Turonian interval is a major transgressive period characterized by the extension of the carbonate platforms on the African platform, and subsidence of the margins. The Senonian is characterized by an increase in water depth, mainly resulting from the opening of NW- to WNW-oriented major Senonian grabens (e.g. the Sirt, Azraq and Euphrates grabens). The main pulse of rifting is Campanian in age. In the northeastern African plate this extensional tectonics is coeval with the obduction of the Neo-Tethyan ophiolites onto the Northern Arabian platform where thick flysch sequences deposited. Within the upper-most Maastrichtian to Paleocene times, some of the basins and margins were inverted, resulting in unconformities in some of the Mesozoic basins. A 1600 km long right lateral strike-slip zone developed in the southern Mesogean margin (Cyrenaica, northern Egypt, Negev). In the Eocene-Oligocene period a sub-meridian extension prevailed in the Levant area pre-dating the Arabia-Anatolia collision. Chalky deposits are widespread in the western Arabian platform, significantly thickening and deepening westward toward the Levant Basin. The Neogene period is dominated by compressive deformations following the closure of Eastern Mesogea, and related to the Arabia/Anatolia collision that initiated at the Oligocene-Miocene boundary. This period is marked by the inversion of the Mesozoic basins in the western Arabian plate (Afrin, Palmyrides, Sinjar) Finally, in the Late Miocene, a regional strike-slip fault system developed, including the Levant Fault, and the eastern and north Anatolian faults in Anatolia.

Genetic links among fluid cycling, vein formation, regional deformation, and plutonism in the Juneau gold belt, southeastern Alaska

USGS Publications Warehouse

Miller, Lance D.; Goldfarb, Richard J.; Gehrels, George E.; Snee, Lawrence W.

1994-01-01

Gold-bearing quartz vein systems in the Juneau gold belt formed within a 160-km- long by 5- to 8-km-wide zone along the western margin of the Coast Mountains, Alaska. Vein systems are spatially associated with shear zones adjacent to terrane-bounding, mid-Cretaceous thrust faults. Analysis of vein orientations and sense of shear data define a stress configuration with greatest and least principal axes oriented subhorizontally with northeast-southwest trends and subverticaly, respectively. This local stress configuration is compatible with the far-field plate configuration during Eocene time. Isotopic ages of vein formation indicate that fluid cycling occurred between 56.5 and ≥52.8 Ma, and are consistent with a genetic link between veining and a change in plate motion in early Eocene time. Veining was also synchronous with the latter stages of rapid exhumation and voluminous plutonism immediately inboard of the gold belt. We propose a model in which interacting tectonic events facilitated fault-valve action and vein development along now-exhumed shear zones.

Deciphering the 3-D distribution of fluid along the shallow Hikurangi subduction zone using P- and S-wave attenuation

NASA Astrophysics Data System (ADS)

Eberhart-Phillips, Donna; Bannister, Stephen; Reyners, Martin

2017-11-01

We use local earthquake velocity spectra to solve for the 3-D distribution of P- and S-wave attenuation in the shallow Hikurangi subduction zone in the North Island of New Zealand to gain insight into how fluids control both the distribution of slip rate deficit and slow-slip events at the shallow plate interface. Qs/Qp gives us information on the 3-D distribution of fluid saturation, which we can compare with the previously determined 3-D distribution of Vp/Vs, which gives information on pore fluid pressure. The Hikurangi margin is unusual, in that a large igneous province (the Hikurangi Plateau) is being subducted. This plateau has had two episodes of subduction-first at 105-100 Ma during north-south convergence with Gondwana, and currently during east-west convergence between the Pacific and Australian plates. We find that in the southern part of the subduction zone, where there is a large deficit in slip rate at the plate interface, the plate interface region is only moderately fluid-rich because the underlying plateau had already had an episode of dehydration during Gondwana subduction. But fluid pressure is relatively high, due to an impermeable terrane in the upper plate trapping fluids below the plate interface. The central part of the margin, where the slip rate deficit is very low, is the most fluid-rich part of the shallow subduction zone. We attribute this to an excess of fluid from the subducted plateau. Our results suggest this part of the plateau has unusually high fracture permeability, on account of it having had two episodes of bending-first at the Gondwana trench and now at the Hikurangi Trough. Qs/Qp is consistent with fluids migrating across the plate interface in this region, leaving it drained and producing high fluid pressure in the overlying plate. The northern part of the margin is a region of heterogeneous deficit in slip rate. Here the Hikurangi Plateau is subducting for the first time, so there is less fluid available from its dehydration than in the central region. Fluid pressure in the overlying plate is high, but Qs/Qp indicates that it is not uniformly fluid-rich. This heterogeneity is consistent with the rough topography of the plateau, including seamounts which entrain fluid-rich sediments. Deep slow-slip events in the southern part of the margin occur where the Moho of the overlying plate meets the plate interface, as typically seen in other deep slow-slip events worldwide. But in the central and northern parts of the margin, the locations of shallow slow-slip events appear to be controlled by a shallow brittle-viscous transition within the fluid-rich upper plate. There is also evidence that a major fault zone in the overlying plate might bleed off some of the high fluid pressure promoting slow-slip events.

An unrecognized major collision of the Okhotomorsk Block with East Asia during the Late Cretaceous, constraints on the plate reorganization of the Northwest Pacific

NASA Astrophysics Data System (ADS)

Yang, Yong-Tai

2013-11-01

Interactions at plate boundaries induce stresses that constitute critical controls on the structural evolution of intraplate regions. However, the traditional tectonic model for the East Asian margin during the Mesozoic, invoking successive episodes of paleo-Pacific oceanic subduction, does not provide an adequate context for important Late Cretaceous dynamics across East Asia, including: continental-scale orogenic processes, significant sinistral strike-slip faulting, and several others. The integration of numerous documented field relations requires a new tectonic model, as proposed here. The Okhotomorsk continental block, currently residing below the Okhotsk Sea in Northeast Asia, was located in the interior of the Izanagi Plate before the Late Cretaceous. It moved northwestward with the Izanagi Plate and collided with the South China Block at about 100 Ma. The indentation of the Okhotomorsk Block within East Asia resulted in the formation of a sinistral strike-slip fault system in South China, formation of a dextral strike-slip fault system in North China, and regional northwest-southeast shortening and orogenic uplift in East Asia. Northeast-striking mountain belts over 500 km wide extended from Southeast China to Southwest Japan and South Korea. The peak metamorphism at about 89 Ma of the Sanbagawa high-pressure metamorphic belt in Southwest Japan was probably related to the continental subduction of the Okhotomorsk Block beneath the East Asian margin. Subsequently, the north-northwestward change of motion direction of the Izanagi Plate led to the northward movement of the Okhotomorsk Block along the East Asian margin, forming a significant sinistral continental transform boundary similar to the San Andreas fault system in California. Sanbagawa metamorphic rocks in Southwest Japan were rapidly exhumed through the several-kilometer wide ductile shear zone at the lower crust and upper mantle level. Accretionary complexes successively accumulated along the East Asian margin during the Jurassic-Early Cretaceous were subdivided into narrow and subparallel belts by the upper crustal strike-slip fault system. The departure of the Okhotomorsk Block from the northeast-striking Asian margin resulted in the occurrence of an extensional setting and formation of a wide magmatic belt to the west of the margin. In the Campanian, the block collided with the Siberian margin, in Northeast Asia. At about 77 Ma, a new oceanic subduction occurred to the south of the Okhotomorsk Block, ending its long-distance northward motion. Based on the new tectonic model, the abundant Late Archean to Early Proterozoic detrital zircons in the Cretaceous sandstones in Kamchatka, Southwest Japan, and Taiwan are interpreted to have been sourced from the Okhotomorsk Block basement which possibly formed during the Late Archean and Early Proterozoic. The new model suggests a rapidly northward-moving Okhotomorsk Block at an average speed of 22.5 cm/yr during 89-77 Ma. It is hypothesized that the Okhotomorsk-East Asia collision during 100-89 Ma slowed down the northwestward motion of the Izanagi Plate, while slab pull forces produced from the subducting Izanagi Plate beneath the Siberian margin redirected the plate from northwestward to north-northwestward motion at about 90-89 Ma.

Structure and petroleum potential of the continental margin between Cross Sound and Icy Bay, northern Gulf of Alaska

USGS Publications Warehouse

Bruns, T.R.

1982-01-01

Major structural features of the Yakutat segment, the segment of the continental margin between Cross Sound and Icy Bay, northern Gulf of Alaska, are delineated by multichannel seismic reflection data. A large structural high is centered on Fairweather Ground and lies generally at the edge of the shelf from Cross Sound to west of the Alsek Valley. A basement uplift, the Dangerous River zone, along which the seismic acoustic basement shallows by up to two kilometers, extends north from the western edge of Fairweather Ground towards the mouth of the Dangerous River. The Dangerous River zone separates the Yakutat segment into two distinct subbasins. The eastern subbasin has a maximum sediment thickness of about 4 km, and the axis of the basin is near and parallel to the coast. Strata in this basin are largely of late Cenozoic age (Neogene and Quaternary) and approximately correlate with the onshore Yakataga Formation. The western subbasin has a maximum of at least 9 km of sediment, comprised of a thick (greater than 4.5 km) Paleogene section overlain by late Cenozoic strata. The Paleogene section is truncated along the Dangerous River zone by a combination of erosion, faulting, and onlap onto the acoustic basement. Within the western subbasin, the late Cenozoic basin axis is near and parallel to the coast, but the Paleogene basin axis appears to trend in a northwest direction diagonally across the shelf. Sedimentary strata throughout the Yakutat shelf show regional subsidence and only minor deformation except in the vicinity of the Fairweather Ground structural high, near and along the Dangerous River zone, and at the shoreline near Lituya Bay. Seismic data across the continental slope and adjacent deep ocean show truncation at the continental slope of Paleogene strata, the presence of a thick (to 6 km) undeformed or mildly deformed abyssal sedimentary section at the base of the slope that in part onlaps the slope, and a relatively narrow zone along the slope or at the base of the slope where faulting may have occurred. Observed deformation at the base of the slope is primarily related to the late Cenozoic uplift of Fairweather Ground, and to Quaternary folding perpendicular to the Pacific-North America relative convergence vector. No accretionary section or major deformation is observed along the continental slope. The absence of these features suggests that no major subduction of the Pacific plate beneath the Yakutat margin has occurred during the late Cenozoic. However, transform faulting along the base of the slope has occurred, because probable Oligocene oceanic basement is juxtaposed against Mesozoic and Paleogene sedimentary strata of the Yakutat slope. This juxtaposition most likely occurred during late Oligocene and Miocene time. During much of the late Cenozoic, and especially during Pliocene-Pleistocene time, the Yakutat segment has apparently been moving northward with the Pacific plate. Dredge samples from the continental slope recovered potential hydrocarbon source and reservoir rocks from the Paleogene sedimentary sequence. Most of the organic matter from these samples is immature to marginally mature. Lopatin calculations suggest that rocks beneath the shelf are likely to be thermally mature at a depth of 4 to 5 km and deeper. In general, the strata at these depths are largely of Paleogene age. Thus, the Paleogene strata may have significant resource potential if source and reservoir rocks similar to those dredged at the slope are present below the shelf. The Paleogene strata are contained primarily within the western subbasin; strata in the east subbasin appear to have little resource potential. Structural traps are apparently present in parts of the basin near and along the Dangerous River zone. These traps are in an updip position from potentially mature strata of the western subbasin, and may hold commercial accumulations of hydrocarbons, if sufficient hydrocarbon generation and migration has occurred

Motion of the Rivera plate since 10 Ma relative to the Pacific and North American plates and the mantle

NASA Astrophysics Data System (ADS)

DeMets, Charles; Traylen, Stephen

2000-03-01

To better understand the influence of Rivera plate kinematics on the geodynamic evolution of western Mexico, we use more than 1400 crossings of seafloor spreading magnetic lineations along the Pacific-Rivera rise and northern Mathematician ridge to solve for rotations of the Rivera plate relative to the underlying mantle and the Pacific and North American plates at 14 times since 9.9 Ma. Our comparison of magnetic anomaly crossings from the undeformed Pacific plate to their counterparts on the Rivera plate indicates that significant areas of the Rivera plate have deformed since 9.9 Ma. Dextral shear along the southern edge of the plate from 3.3-2.2 Ma during a regional plate boundary reorganization deformed the Rivera plate farther into its interior than previously recognized. In addition, seafloor located north of two rupture zones within the Rivera plate sutured to North America after 1.5 Ma. Anomaly crossings from these two deformed regions thus cannot be used to reconstruct motion of the Rivera plate. Finite rotations that best reconstruct Pacific plate anomaly crossings onto their undeformed counterparts on the Rivera plate yield stage spreading rates that decrease gradually by 10% between 10 and 3.6 Ma, decrease rapidly by 20% after ˜3.6 Ma, and recover after 1 Ma. The slowdown in Pacific-Rivera seafloor spreading at 3.6 Ma coincided with the onset of dextral shear across the then-incipient southern boundary of the Rivera plate with the Pacific plate. The available evidence indicates that the Rivera plate has been an independent microplate since at least 10 Ma, contrary to published assertions that it fragmented from the Cocos plate at ˜5 Ma. Motion of the Rivera plate relative to North America has changed significantly since 10 Ma, in concert with significant changes in Pacific-Rivera motion. A significant and robust feature of Rivera-North America motion not previously recognized is the cessation of margin-normal convergence and thus subduction from 2.6 to 1.0 Ma along the entire plate boundary, followed by a resumption of trench-normal subduction along the southern half of the Rivera-North America plate boundary after 1.0 Ma. Motion of the Rivera plate relative to the underlying mantle since 10 Ma has oscillated between periods of landward motion and seaward motion. The evidence suggests that the torque exerted by slab pull on this young and hot oceanic plate is either minimal or is effectively counterbalanced by forces that resist its motion.

The Maule, 2010, earthquake - geophysical and kinematic observations of the South American margin prior to the earthquake (Invited)

NASA Astrophysics Data System (ADS)

Oncken, O.; Haberland, C. A.; Moreno, M.; Melnick, D.; Tilmann, F.; Tipteq Research Groups

2010-12-01

Accumulation of deformation at convergent plate margins is recently identified to be highly discontinuous and transient in nature: silent slip events, non-volcanic tremors, afterslip, fault coupling and complex response patterns of the upper plate during a single event as well as across several seismic cycles have all been observed in various settings and combinations. Segments of convergent plate margins with high recurrence rates and at different stages of the rupture cycle like the Chilean margin offer an exceptional opportunity to study these features and their interaction resolving behaviour during the seismic cycle and over repeated cycles. A past (TIPTEQ) and several active international initiatives (Integrated Plate Boundary Observatory Chile; IPOC-network.org) address these goals with research groups from IPG Paris, Seismological Survey of Chile, Free University Berlin, Potsdam University, Hamburg University, IFM-GEOMAR Kiel, GFZ Potsdam, and Caltech (USA) employing an integrated plate boundary observatory and associated projects. Results from these studies allow us to define the preseismic state - with respect to the Maule eartghquake - of the margin system at the south Central Chilean convergent margin. Here, two major seismic events have occurred in adjoining segments (Valdivia 1960, Mw = 9.5; Maule 2010, Mw = 8.8) yielding observations from critical time windows of the seismic cycle in the same region. Seismic imaging and seismological data have allowed us to relocate major rupture hypocentres and to locate the geometry and properties of the seismogenic zone. The reflection seismic data exhibit well defined changes of reflectivity and Vp/Vs ratio along the plate interface that can be correlated with different parts of the coupling zone and its hanging wall as well as with changes during the seismic cycle. Observations suggest an important role of the hydraulic system, and of lateral variation of locking degree on subsequent rupture and aftershock distribution as evidenced by the recent Maule earthquake. Moreover, the latter coseismic rupture pattern was foreseeable from its pre-seismic locking pattern as derived by inversion of GPS observations during the previous decade. Neogene surface deformation at the Chilean coast related to these locking properties has been complex exhibiting tectonically uplifting areas along the coast driven by interseismically active reverse faulting. In addition, we observe coseismically subsiding domains along other parts of the coast - mostly above fully locked patches. Finally, we note that the characteristic peninsulas along the South American margin constitute stable rupture boundaries and appear to have done so for a protracted time as evidenced by their long-term uplift history since at least the Late Pliocene. This suggests barriers to rupture being related to anomalous properties of the plate interface affecting the mode of strain accumulation and plate interface rupture - like e.g. velocity strengthening in contrast to the weakening property of most of the remaining domains.

Lithospheric structure of the southeastern margin of the Tibetan Plateau from Rayleigh wave tomography

NASA Astrophysics Data System (ADS)

Fu, Yuanyuan V.; Gao, Yuan; Li, Aibing; Li, Lun; Chen, Anguo

2017-06-01

Lithospheric shear wave velocity beneath the southeastern margin of the Tibetan Plateau is obtained from Rayleigh wave tomography using earthquake data recorded by the temporary ChinArray and permanent China Digital Seismic Array. Fundamental mode Rayleigh wave phase velocities at periods of 20-100 s are determined and used to construct the 3-D shear wave velocity model. Low-velocity anomalies appear along or close to the major faults in the middle crust and become a broad zone in the lower crust, suggesting block extrusion in the shallow crust and diffuse deformation in the lower crust, both of which play important roles in accommodating the collision between the Indian and Eurasian plates. A vertical low-velocity column beneath the Tengchong Volcano is observed, which could be caused by upwelling of warm mantle due to the lithosphere extension in the Thailand rift basin to the south or by fluid-induced partial melting due to the subduction of the Burma slab. The western Yangtze Craton is characterized by low velocity in the crust and uppermost mantle above the fast mantle lithosphere, indicating possible thermal erosion at the western craton edge resulted from the extrusion of the Tibetan Plateau. A low-velocity zone is imaged at the depths of 70-150 km beneath the eastern part of the Yangtze Craton, which could be caused by small-scale mantle convection associated with the subduction of the Burma microplate and/or the opening of the South China Sea.

Stratigraphic and Paleomagnetic Comparisons of Mesoproterozoic Strata and Sills from the Belt Basin, NW Montana, USA, and NW Anabar Shield, Russia: Testing a Precambrian Plate Reconstruction

NASA Astrophysics Data System (ADS)

Sears, J. W.; Pavlov, V.; Veselovskiy, R.; Khudoley, A.

2008-12-01

Mesoproterozoic sedimentary strata and mafic sills overlie Archean and Paleoproterozoic basement rocks with profound unconformity in NW Montana and along the NW margin of the Anabar Shield in northern Siberia. The two localities plot adjacent to one another on a Precambrian plate reconstruction proposed by Sears and Price (2003) that places the NE margin of the Siberian craton against the SW margin of the North American craton. The plate reconstruction predicts that these strata occupied contiguous parts of an intracratonic basin prior to late Neoproterozoic breakup of Rodinia. Here we show that the Mesoproterozoic stratigraphic sequences, sedimentary structures, and lithologies of the NW Anabar margin closely match the Neihart, Chamberlain, and Newland formations of the Little Belt Mountains of Montana. They may predate opening of the Belt Supergroup rift basin at ca. 1500 Ma, when a major mafic magmatic episode occurred in both regions. Preliminary paleomagnetic data from the Siberian section will be compared with the Laurentian APWP to evaluate the reconstruction.

Review of the subgenus Xizicus (Xizicus) Gorochov, 1993 (Orthoptera: Tettigoniidae: Meconematinae) from China.

PubMed

Feng, Jiyuan; Shi, Fuming; Mao, Shaoli

2017-03-23

Gorochov (1993) erected the subgenus Xizicus (Xizicus), with the type species Xizicus (Xizicus) fascipes (Bey-Bienko, 1955). This subgenus is distinguished from the other subgenera of Xizicus by the following characters: male tenth abdominal tergite with a pair of contiguous posterior processes in the middle; subgenital plate simple, styli slender, located on the apices or subapices of lateral margins; posterior margin of female subgenital plate rounded.

A description of Pseudechiniscus xiai sp. nov., with a key to genus Pseudechiniscus in China.

PubMed

Wang, Lizhi; Xue, Jing; Li, Xiaochen

2018-03-01

The new species from the Liupan Mountains in China, Pseudechiniscus xiai sp. nov., differs from other Pseudechiniscus species by the following characters: absence of striae between dots which institute cuticular sculpture, projections absent on caudal margin of pseudosegmental plate, notches absent on caudal margin of terminal plate, dots on ventral side of body forms a reticular patched design. A diagnostic key to the Chinese Pseudechiniscus species is provided.

Geologic framework of the Aleutian arc, Alaska

USGS Publications Warehouse

Vallier, Tracy L.; Scholl, David W.; Fisher, Michael A.; Bruns, Terry R.; Wilson, Frederic H.; von Huene, Roland E.; Stevenson, Andrew J.

1994-01-01

The Aleutian arc is the arcuate arrangement of mountain ranges and flanking submerged margins that forms the northern rim of the Pacific Basin from the Kamchatka Peninsula (Russia) eastward more than 3,000 km to Cooke Inlet (Fig. 1). It consists of two very different segments that meet near Unimak Pass: the Aleutian Ridge segment to the west and the Alaska Peninsula-the Kodiak Island segment to the east. The Aleutian Ridge segment is a massive, mostly submerged cordillera that includes both the islands and the submerged pedestal from which they protrude. The Alaska Peninsula-Kodiak Island segment is composed of the Alaska Peninsula, its adjacent islands, and their continental and insular margins. The Bering Sea margin north of the Alaska Peninsula consists mostly of a wide continental shelf, some of which is underlain by rocks correlative with those on the Alaska Peninsula.There is no pre-Eocene record in rocks of the Aleutian Ridge segment, whereas rare fragments of Paleozoic rocks and extensive outcrops of Mesozoic rocks occur on the Alaska Peninsula. Since the late Eocene, and possibly since the early Eocene, the two segments have evolved somewhat similarly. Major plutonic and volcanic episodes, however, are not synchronous. Furthermore, uplift of the Alaska Peninsula-Kodiak Island segment in late Cenozoic time was more extensive than uplift of the Aleutian Ridge segment. It is probable that tectonic regimes along the Aleutian arc varied during the Tertiary in response to such factors as the directions and rates of convergence, to bathymetry and age of the subducting Pacific Plate, and to the volume of sediment in the Aleutian Trench.The Pacific and North American lithospheric plates converge along the inner wall of the Aleutian trench at about 85 to 90 mm/yr. Convergence is nearly at right angles along the Alaska Peninsula, but because of the arcuate shape of the Aleutian Ridge relative to the location of the plates' poles of rotation, the angle of convergence lessens to the west (Minster and Jordan, 1978). Along the central Aleutian Ridge, underthrusting is about 30° from normal to the volcanic axis. Motion between plates is approximately parallel along the western Aleutian Ridge.In this paper we briefly describe and interpret the Cenozoic evolution of the Aleutian arc by focusing on the onshore and offshore geologic frameworks in four of its sectors, two sectors each from the Aleutian Ridge and Alaska Peninsula-Kodiak Island segments (Fig. 1). We compare the geologic evolution of the segments and comment on the implications of some new, previously unpublished data.

Eastern Indian Ocean microcontinent formation driven by plate motion changes

NASA Astrophysics Data System (ADS)

Whittaker, J. M.; Williams, S. E.; Halpin, J. A.; Wild, T. J.; Stilwell, J. D.; Jourdan, F.; Daczko, N. R.

2016-11-01

The roles of plate tectonic or mantle dynamic forces in rupturing continental lithosphere remain controversial. Particularly enigmatic is the rifting of microcontinents from mature continental rifted margins, with plume-driven thermal weakening commonly inferred to facilitate calving. However, a role for plate tectonic reorganisations has also been suggested. Here, we show that a combination of plate tectonic reorganisation and plume-driven thermal weakening were required to calve the Batavia and Gulden Draak microcontinents in the Cretaceous Indian Ocean. We reconstruct the evolution of these two microcontinents using constraints from new paleontological samples, 40Ar/39Ar ages, and geophysical data. Calving from India occurred at 101-104 Ma, coinciding with the onset of a dramatic change in Indian plate motion. Critically, Kerguelen plume volcanism does not appear to have directly triggered calving. Rather, it is likely that plume-related thermal weakening of the Indian passive margin preconditioned it for microcontinent formation but calving was triggered by changes in plate tectonic boundary forces.

Cordilleran Intermontane thermotectonic history and implications for neotectonic structure and petroleum systems, British Columbia, Canada

NASA Astrophysics Data System (ADS)

Majorowicz, Jacek; Osadetz, Kirk

2008-04-01

Heat flow increases northward along Intermontane Belt in the western Canadian Cordillera, as shown by geothermal differences between Bowser and Nechako sedimentary basins, where geothermal gradients and heat flows are ˜30 mK/m and ˜90 mW/m2 compared to ˜32 mK/m and 70 -80 mW/m2, respectively. Sparse temperature profile data from these two sedimenatary basins are consistent with an isostatic model of elevation and crustal parameters, which indicate that Bowser basin heat flow should be ˜20 mW/m2 greater than Nechako basin heat flow. Paleothermometric indicators record a significant northward increasing Eocene or older erosional denudation, up to ˜7 km. None of the heat generation, tectonic reorganization at the plate margin, or erosional denudation produce thermal effects of the type or magnitude that explain the north-south heat flow differences between Nechako and Bowser basins. The more southerly Nechako basin, where heat flow is lower, has lower mean elevation, is less deeply eroded, and lies opposite the active plate margin. In contrast, Bowser basin, where heat flow is higher, has higher mean elevation, is more deeply eroded, and sits opposite a transform margin that succeeded the active margin ˜40 Ma. Differences between Bowser and Nechako basins contrast with the tectonic history and erosion impacts on thermal state. Tectonic history and eroded sedimentary thickness suggest that Bowser basin lithosphere is cooling and contracting relative to Nechako basin lithosphere. This effect has reduced Bowser basin heat flow by ˜10-20 mW/m2 since ˜40 Ma. Neither can heat generation differences explain the northerly increasing Intermontane Belt heat flow. A lack of extensional structures in the Bowser basin precludes basin and range-like extension. Therefore, another, yet an unspecified mechanism perhaps associated with the Northern Cordilleran Volcanic Province, contributes additional heat. Bowser basin’s paleogeothermal gradients were higher, ˜36 mK/m, before the Eocene and this might affect petroleum and metallogenic systems.

Investigating Transition Zone Thickness Variation under the Arabian Plate: Evidence Lacking for Deep Mantle Upwellings

NASA Astrophysics Data System (ADS)

Juliá, J.; Tang, Z.; Mai, P. M.; Zahran, H.

2014-12-01

Cenozoic volcanic outcrops in Arabia - locally known as harrats - span more than 2000 km along the western half of the Arabian plate, from eastern Yemen to southern Syria. The magmatism is bimodal in character, with older volcanics (30 to 20 My) being tholeiitic-to-transitional and paralleling the Red Sea margin, and younger volcanics (12 Ma to Recent) being transitional-to-strongly-alkalic and aligning in a more north-south direction. The bimodal character has been attributed to a two-stage rifting process along the Red Sea, where the old volcanics would have produced from shallow sources related to an initial passive rifting stage, and young volcanics would have originated from one or more deep-seated mantle plumes driving present active rifting. Early models suggested the harrats would have resulted from either lateral flow from the Afar plume in Ethiopia, or more locally from a separate mantle plume directly located under the shield. Most recently, tomographic images of the Arabian mantle have suggested the northern harrats could be resulting from flow originating at a deep plume under Jordan. In this work, we investigate the location of deep mantle plumes under the Arabian plate by mapping transition zone thickness with teleseismic receiver functions. The transition zone is bounded by seismic discontinuities, nominally at 410 and 660 km depth, originating from phase transitions in the olivine-normative component of the mantle. The precise depth of the discontinuities is strongly dependent on temperature and, due to the opposing signs of the corresponding Clapeyron slopes, positive temperature anomalies are expected to result in thinning of the transition zone. Our dataset consists of ~5000 low-frequency (fc < 0.25 Hz) receiver function waveforms obtained at ~110 broadband stations belonging to a number of permanent and temporary seismic networks in the region. The receiver functions were migrated to depth and stacked along a ~2000 km long record section displaying P-to-S conversions at seismic discontinuities under Western Arabia. Our results display a normal to thicker-than-average transition zone under the study area, suggesting thermal perturbations of the transition zone due to deep mantle upwellings under the western shield and/or Jordan are unlikely.

A quantitative analysis of transtensional margin width

NASA Astrophysics Data System (ADS)

Jeanniot, Ludovic; Buiter, Susanne J. H.

2018-06-01

Continental rifted margins show variations between a few hundred to almost a thousand kilometres in their conjugated widths from the relatively undisturbed continent to the oceanic crust. Analogue and numerical modelling results suggest that the conjugated width of rifted margins may have a relationship to their obliquity of divergence, with narrower margins occurring for higher obliquity. We here test this prediction by analysing the obliquity and rift width for 26 segments of transtensional conjugate rifted margins in the Atlantic and Indian Oceans. We use the plate reconstruction software GPlates (http://www.gplates.org) for different plate rotation models to estimate the direction and magnitude of rifting from the initial phases of continental rifting until breakup. Our rift width corresponds to the distance between the onshore maximum topography and the last identified continental crust. We find a weak positive correlation between the obliquity of rifting and rift width. Highly oblique margins are narrower than orthogonal margins, as expected from analogue and numerical models. We find no relationships between rift obliquities and rift duration nor the presence or absence of Large Igneous Provinces (LIPs).

Probing the structure of the sub-Salinia mantle lithosphere using spinel lherzolite xenoliths from Crystal Knob, Santa Lucia Range, California

NASA Astrophysics Data System (ADS)

Quinn, D. P.; Saleeby, J.; Ducea, M. N.; Luffi, P. I.

2013-12-01

We present the first petrogenetic analysis of a suite of peridotite xenoliths from the Crystal Knob volcanic neck in the Santa Lucia Range, California. The neck was erupted during the Plio-Pleistocene through the Salinia terrane, a fragment of the Late Cretaceous southern Sierra-northwest Mojave supra-subduction core complex that was displaced ~310 km in the late Cenozoic along the dextral San Andreas fault. The marginal tectonic setting makes these xenoliths ideal for testing different models of upper-mantle evolution along the western North American plate boundary. Possible scenarios include the early Cenozoic underplating of Farallon-plate mantle lithosphere nappes (Luffi et al., 2009), Neogene slab window opening (Atwater and Stock, 1998), and the partial subduction and stalling of the Monterey microplate (Pisker et al., 2012). The xenoliths from Crystal Knob are spinel lherzolites, which sample the mantle lithosphere underlying Salinia, and dunite cumulates apparently related to the olivine-basalt host. Initial study is focused on the spinel lherzolites: these display an allotriomorphic granular texture with anisotropy largely absent. However, several samples exhibit a weak shape-preferred orientation in elongate spinels. Within each xenolith, the silicate phases are in Fe-Mg equilibrium; between samples, Mg# [molar Mg/(Mg+Fe)*100] ranges from 87 to 91. Spinels have Cr# [molar Cr/(Cr+Al)*100] ranging from 10 to 27. Clinopyroxene Rb-Sr and Sm-Nd radiogenic isotope data show that the lherzolites are depleted in large-ion lithophile (LIL) elements, with uniform enrichment in 143Nd (ɛNd from +10.3 to +11.0) and depletion in 87Sr (87/86Sr of .702). This data rules out origin in the continental lithosphere, such as that observed in xenoliths from above the relict subduction interface found at at Dish Hill and Cima Dome in the Mojave (Luffi et al., 2009). The Mesozoic mantle wedge, which is sampled by xenoliths from beneath the southern Sierra Nevada batholith (Ducea and Saleeby, 1998), is also ruled out as a source locale. The isotopic data are consistent with oceanic mantle originating from either the Farallon plate (underplated during Paleocene shallow subduction) or the Monterey plate (partially subducted during the Miocene). Ascended asthenosphere, presumably of slab-window origin, is also a possible source. Pyroxene Ca-Mg exchange geothermometry is in progress and will enable thermal modeling and comparisons with contemporary heat flow data. These results, along with trace-element analysis of clinopyroxene crystals, will be used to distinguish between the possible sources of LIL-depleted mantle in the sub-Salinia mantle lithosphere. The full petrogenetic survey of these xenoliths adds a distal constraint to the makeup of the mantle lithosphere beneath the western North American margin.

Tomographic Imaging of the Cascadia Subduction Zone and Juan de Fuca Plate System: Improved Methods Eliminate Artifacts and Reveal New Structures

NASA Astrophysics Data System (ADS)

Bodmer, M.; Toomey, D. R.; Hooft, E. E. E.; Bezada, M.; Schmandt, B.; Byrnes, J. S.

2017-12-01

Amphibious studies of subduction zones promise advances in understanding links between incoming plate structure, the subducting slab, and the upper mantle beneath the slab. However, joint onshore/offshore imaging is challenging due to contrasts between continental and oceanic structure. We present P-wave teleseismic tomography results for the Cascadia subduction zone (CSZ) that utilize existing western US datasets, amphibious seismic data from the Cascadia Initiative, and tomographic algorithms that permit 3D starting models, nonlinear ray tracing, and finite frequency kernels. Relative delay times show systematic onshore/offshore trends, which we attribute to structure in the upper 50 km. Shore-crossing CSZ seismic refraction models predict relative delays >1s, with equal contributions from elevation and crustal thickness. We use synthetic data to test methods of accounting for such shallow structure. Synthetic tests using only station static terms produce margin-wide, sub-slab low-velocity artifacts. Using a more realistic a priori 3D model for the upper 50 km better reproduces known input structures. To invert the observed delays, we use data-constrained starting models of the CSZ. Our preferred models utilize regional surface wave studies to construct a starting model, directly account for elevation, and use 3D nonlinear ray tracing. We image well-documented CSZ features, including the subducted slab down to 350 km, along strike slab variations below 150 km, and deep slab fragmentation. Inclusion of offshore data improves resolution of the sub-slab mantle, where we resolve localized low-velocity anomalies near the edges of the CSZ (beneath the Klamath and Olympic mountains). Our new imaging and resolution tests indicate that previously reported margin-wide, sub-slab low-velocity asthenospheric anomalies are an imaging artifact. Offshore, we observe low-velocity anomalies beneath the Gorda plate consistent with regional deformation and broad upwelling resulting from plate stagnation. At the Juan de Fuca Ridge we observe asymmetric low-velocity anomalies consistent with dynamic upwelling. Our results agree with recent offshore tomography studies using S wave data; however, differences in the recovered relative amplitudes are likely due to anisotropy, which we are exploring.

Crustal and upper mantle structure of the north-east of Egypt and the Afro-Arabian plate boundary region from Rayleigh-wave analysis

NASA Astrophysics Data System (ADS)

Corchete, V.; Chourak, M.; Hussein, H. M.; Atiya, K.; Timoulali, Y.

2017-05-01

The crustal and mantle structure of the north-eastern part of Egypt and the surrounding area is shown by means of S-velocity maps for depths ranging from zero to 45 km, determined by the regionalization and inversion of Rayleigh-wave dispersion. This analysis shows several types of crust with an average S-velocity ranging from 2.5 to 3.9 km/s. The values of S-velocity range from 2.5 km/s at the surface to 3.4 km/s at 10 km depth for the Sinai Peninsula, Gulf of Aqaba, Gulf of Suez, Red Sea, Dead Sea, western part of Dead sea and Arabian Plate. In the lower crust, the values of the S-velocity reach 4.0 km/s. In the uppermost mantle, the S-velocities range from 4.4 to 4.7 km/s. The crustal thickness ranges from the oceanic thin crust (around 15-20 km of thickness), for Red Sea and the extended continental margins, to 35-45 km of thickness for the Arabian plate. A gradual increasing crustal thickness is observed from north-east to south-west. While the Moho is located at 30-35 km of depth under the Sinai Peninsula, Gulf of Aqaba, Dead Sea Fault (DSF) and Dead Sea, a thinner crust (20-25 km of thickness) is found at the east of DSF and under the northern and the southern part of the Gulf of Suez. The crustal thickness varies within Sinai from the southern edge to the north, which provided an evidence for the presence of an Early Mesozoic passive margin with thinned continental crust in the north of Sinai. The change of crustal structure between the Gulf of Aqaba and the Gulf of Suez is due to the different tectonic and geodynamic processes affecting Sinai. In general, our results are consistent with surface geology and the Moho depth inferred from reflection and refraction data, receiver function, surface-wave analysis and P-S tomography. The strong variations in the base of the Moho reflect the complex evolution of the African and Arabian plate boundary region.

The Two Subduction Zones of the Southern Caribbean: Lithosphere Tearing and Continental Margin Recycling in the East, Flat Slab Subduction and Laramide-Style Uplifts in the West

NASA Astrophysics Data System (ADS)

Levander, A.; Bezada, M. J.; Niu, F.; Schmitz, M.

2015-12-01

The southern Caribbean plate boundary is a complex strike-slip fault system bounded by oppositely vergent subduction zones, the Antilles subduction zone in the east, and a currently locked Caribbean-South American subduction zone in the west (Bilham and Mencin, 2013). Finite-frequency teleseismic P-wave tomography images both the Atlanic (ATL) and the Caribbean (CAR) plates subducting steeply in opposite directions to transition zone depths under northern South America. Ps receiver functions show a depressed 660 discontinuity and thickened transition zone associated with each subducting plate. In the east the oceanic (ATL) part of the South American (SA) plate subducts westward beneath the CAR, initiating the El Pilar-San Sebastian strike slip system, a subduction-transform edge propagator (STEP) fault (Govers and Wortel, 2005). The point at which the ATL tears away from SA as it descends into the mantle is evidenced by the Paria cluster seismicity at depths of 60-110 km (Russo et al, 1993). Body wave tomography and lithosphere-asthenosphere boundary (LAB) thickness determined from Sp and Ps receiver functions and Rayleigh waves suggest that the descending ATL also viscously removes the bottom third to half of the SA continental margin lithospheric mantle as it descends. This has left thinned continental lithosphere under northern SA in the wake of the eastward migrating Antilles subduction zone. The thinned lithosphere occupies ~70% of the length of the El Pilar-San Sebastian fault system, from ~64oW to ~69oW, and extends inland several hundred kilometers. In northwestern SA the CAR subducts east-southeast at low angle under northern Colombia and western Venezuela. The subducting CAR is at least 200 km wide, extending from northernmost Colombia as far south as the Bucaramanga nest seismicity. The CAR descends steeply under Lake Maracaibo and the Merida Andes. This flat slab is associated with three Neogene basement cored, Laramide-style uplifts: the Santa Marta block, the Perija Range, and the Merida Andes (Kellogg and Bonini, 1982). The steep descent of the CAR under Maracaibo implies that the CAR plate is torn somewhere between the Merida Andes and the Caribbean Sea, where it forms the ocean floor. An upcoming broadband seismic experiment will examine the CAR flat slab and the suspected slab tear in detail.

Multi-phase structural and tectonic evolution of the Andaman Sea Region

NASA Astrophysics Data System (ADS)

Masterton, Sheona; Hill, Catherine; Sagi, David Adam; Webb, Peter; Sevastjanova, Inga

2017-04-01

We present a new regional tectonic interpretation for Myanmar and the Andaman Sea, built within the framework of global plate motions. In our model the Present Day Andaman Sea region has been subjected to multiple phases of extension, culminating in its mid-Miocene to Present Day opening as a rhomboidal pull-apart basin. The Andaman Sea region is historically thought to have developed as a consequence of back-arc opening associated with plate convergence at the Andaman-Nicobar subduction system. We have undertaken detailed structural interpretation of potential field, Landsat and SRTM data, supported by 2-D crustal models of the Andaman Sea. From this analysis we identified several major north-south striking faults and a series of northeast-southwest striking structures across the region. We have also mapped the extent of the Andaman-Nicobar Accretionary Prism, a fore arc trough and volcanic arc, which we associate with a phase of traditional trench-parallel back-arc extension from the Paleocene to the middle Miocene. A regional tectonic event occurred during the middle Miocene that caused the cessation of back-arc extension in the Present Day Andaman Sea and an eastward shift in the locus of arc-related volcanism. At that time, N-S striking faults onshore and offshore Myanmar were reactivated with widespread right-lateral motion. This motion, accompanied by extension along new NE-SW striking faults, facilitated the opening of the Central Andaman Basin as a pull-apart basin (rhombochasm) in which a strike-slip tectonic regime has a greater impact on the mode of opening than the subduction process. The integration of our plate model solution within a global framework allows identification of major plate reorganisation events and their impact on a regional scale. We therefore attribute the onset of pull-apart opening in the Andaman Sea to ongoing clockwise rotation of the western Sundaland margin throughout the late Paleogene and early Miocene, possibly driven by the opening of the South China Sea to the east. Consequently, the obliquity of plate convergence along this margin increased, ultimately resulting in a change from minor strain partitioning to hyper oblique convergence and full strain partitioning by the mid-Miocene. Investigation into the effects of slab-steepening and dynamic subsidence in the Indochina region could be used as further tests of our proposed tectonic evolution of the Andaman Sea.

Link between the northward extension of Great Sumatra Fault and continental rifting in the Andaman Sea: new results from seismic reflection studies

NASA Astrophysics Data System (ADS)

Singh, S. C.; Moeremans, R. E.; McArdle, J.; Johansen, K.

2012-12-01

The Great Sumatra Fault (GSF) traverses the main land Sumatra from Sunda Strait in the southeast to Banda Aceh in the northwest for about 1900 km, and defines the present day plate boundary between the Sunda Plate in the north and Burmese Sliver Plate in the south. It is formed due to the oblique subduction of the Indo-Australian Plate beneath the Sunda Plate. It has been well studied on land but is poorly studied north of Banda Aceh in the Andaman Sea. Its study is further complicated by the presence of volcanic arc in its vicinity and its interaction with the West Andaman Fault (WAF) further north. Here we present deep seismic reflection images along the northward extension of the GSF over 700 km until it joins the Andaman Spreading Centre and interpret these images in the light of earthquake, gravity and bathymetry data. We find that the GSF has two strands between Banda Aceh and Nicobar Island: a transpression in the south and a deep narrow active rift basin in the north dotted with volcanoes in the center, suggesting that the volcanic arc is coincident with the rifting. Further north of Nicobar Island, an active strike-slip fault cuts through a deep rifted basin until its intersection with Andaman Sea Spreading Centre. The volcanic arc lies just east of the basin. The western margin of this basin seems to be a rifted continental margin, tilted westward flooring the Andaman-Nicobar forearc basin, which was once a part of Malaya Peninsula, suggesting that a significant parts of the Andaman-Nicobar forearc system is underlain by the Sunda continental crust. The Andaman-Nicobar forearc basin is bounded in the west by backthrusts, similar to the West Andaman and Mentawai faults bounding the Aceh and Mentawai forearc basins in the south. The cluster of seismicity after the 2004 great Andaman-Sumatra earthquake just north of Nicobar Island coincides with the intersection of two NW-SE and N-S trending strike-slip fault systems. Some of hypocentre of these earthquakes lie in the mantle down to 30 km depth, which along with the presence of volcanic arc just 15 km east of these faults, suggest that there is no generic link between the strike-slip fault and volcanic arc.

Proto-South China Sea plate tectonics using subducted slab constraints from tomography

NASA Astrophysics Data System (ADS)

Wu, J.; Suppe, J.

2017-12-01

The geology of the South China Sea and surrounding margins is intimately tied to the subduction of its predecessor, the proto-South China Sea. However, published plate reconstructions have shown highly variable sizes and locations for the proto-South China Sea. Despite these differences, most studies agree that the proto-South China Sea was subducted southwards under north Borneo in the Cenozoic. Here we present new details on proto-South China Sea paleogeography using mapped and unfolded slabs from seismic tomography following Wu et al. (2016). We show that most of the proto-South China Sea lithosphere lies directly under the South China Sea and is represented by an extensive (>1000 km N-S) swath of detached, sub-horizontal slab at 450 to 700 km depth, here called the `northern Proto-South China Sea'. Furthermore, slab unfolding shows that prior to subduction at the Manila Trench, the eastern limit of the South China Sea, lay directly above the edge of the `northern Proto-South China Sea', both extending 400 to 500 km to the east of the present Manila trench. These observations show that the South China Sea opened directly above a northward-subducting `northern Proto-South China Sea', which runs counter to most proto-South China Sea plate models. We present a slab-constrained plate reconstruction that shows the proto-South China Sea was consumed by double-sided subduction to both the north and south, as follows: [1] The `northern proto-South China Sea' subducted in the Oligocene-Miocene under the Dangerous Grounds and southward expanding South China Sea by in-place 'self subduction' similar to the western Mediterranean basins. [2] Limited southward subduction of the proto-South China Sea under Borneo occurred pre-Oligocene, represented by the 'southern Proto-South China Sea' slab at 800 to 900 km depths. Our plate reconstruction implies the southern South China Sea was an active margin during South China Sea opening and had a south-facing arc and a north-dipping proto-South China Sea slab. A southern South China Sea arc was likely underthrusted under northern Borneo after 15 Ma. Our model also predicts the formation of a mantle wedge under the South China Sea during seafloor spreading. This suggests that South China Sea opening was accompanied by a more vigorous asthenospheric circulation than previously recognized.

Continental underplating after slab break-off

NASA Astrophysics Data System (ADS)

Magni, V.; Allen, M. B.; van Hunen, J.; Bouilhol, P.

2017-09-01

We present three-dimensional numerical models to investigate the dynamics of continental collision, and in particular what happens to the subducted continental lithosphere after oceanic slab break-off. We find that in some scenarios the subducting continental lithosphere underthrusts the overriding plate not immediately after it enters the trench, but after oceanic slab break-off. In this case, the continental plate first subducts with a steep angle and then, after the slab breaks off at depth, it rises back towards the surface and flattens below the overriding plate, forming a thick horizontal layer of continental crust that extends for about 200 km beyond the suture. This type of behaviour depends on the width of the oceanic plate marginal to the collision zone: wide oceanic margins promote continental underplating and marginal back-arc basins; narrow margins do not show such underplating unless a far field force is applied. Our models show that, as the subducted continental lithosphere rises, the mantle wedge progressively migrates away from the suture and the continental crust heats up, reaching temperatures >900 °C. This heating might lead to crustal melting, and resultant magmatism. We observe a sharp peak in the overriding plate rock uplift right after the occurrence of slab break-off. Afterwards, during underplating, the maximum rock uplift is smaller, but the affected area is much wider (up to 350 km). These results can be used to explain the dynamics that led to the present-day crustal configuration of the India-Eurasia collision zone and its consequences for the regional tectonic and magmatic evolution.

The Geodetic Signature of the Earthquake Cycle at Subduction Zones: Model Constraints on the Deep Processes

NASA Astrophysics Data System (ADS)

Govers, R.; Furlong, K. P.; van de Wiel, L.; Herman, M. W.; Broerse, T.

2018-03-01

Recent megathrust events in Tohoku (Japan), Maule (Chile), and Sumatra (Indonesia) were well recorded. Much has been learned about the dominant physical processes in megathrust zones: (partial) locking of the plate interface, detailed coseismic slip, relocking, afterslip, viscoelastic mantle relaxation, and interseismic loading. These and older observations show complex spatial and temporal patterns in crustal deformation and displacement, and significant differences among different margins. A key question is whether these differences reflect variations in the underlying processes, like differences in locking, or the margin geometry, or whether they are a consequence of the stage in the earthquake cycle of the margin. Quantitative models can connect these plate boundary processes to surficial and far-field observations. We use relatively simple, cyclic geodynamic models to isolate the first-order geodetic signature of the megathrust cycle. Coseismic and subsequent slip on the subduction interface is dynamically (and consistently) driven. A review of global preseismic, coseismic, and postseismic geodetic observations, and of their fit to the model predictions, indicates that similar physical processes are active at different margins. Most of the observed variability between the individual margins appears to be controlled by their different stages in the earthquake cycle. The modeling results also provide a possible explanation for observations of tensile faulting aftershocks and tensile cracking of the overriding plate, which are puzzling in the context of convergence/compression. From the inversion of our synthetic GNSS velocities we find that geodetic observations may incorrectly suggest weak locking of some margins, for example, the west Aleutian margin.

Effective strength of incoming sediments and its implications for plate boundary propagation: Nankai and Costa Rica as type examples of accreting vs. erosive convergent margins

NASA Astrophysics Data System (ADS)

Kopf, Achim

2013-11-01

The location of the seaward tip of a subduction thrust controls material transfer at convergent plate margins, and hence global mass balances. At approximately half of those margins, the material of the subducting plate is completely underthrust so that no accretion or even subduction erosion takes place. Along the remaining margins, material is scraped off the subducting plate and added to the upper plate by frontal accretion. We here examine the physical properties of subducting sediments off Costa Rica and Nankai, type examples for an erosional and an accretionary margin, to investigate which parameters control the level where the frontal thrust cuts into the incoming sediment pile. A series of rotary-shear experiments to measure the frictional strength of the various lithologies entering the two subduction zones were carried out. Results include the following findings: (1) At Costa Rica, clay-rich strata at the top of the incoming succession have the lowest strength (μres = 0.19) while underlying calcareous ooze, chalk and diatomite are strong (up to μres = 0.43; μpeak = 0.56). Hence the entire sediment package is underthrust. (2) Off Japan, clay-rich deposits within the lower Shikoku Basin inventory are weakest (μres = 0.13-0.19) and favour the frontal proto-thrust to migrate into one particular horizon between sandy, competent turbidites below and ash-bearing mud above. (3) Taking in situ data and earlier geotechnical testing into account, it is suggested that mineralogical composition rather than pore-pressure defines the position of the frontal thrust, which locates in the weakest, clay mineral-rich (up to 85 wt.%) materials. (4) Smectite, the dominant clay mineral phase at either margin, shows rate strengthening and stable sliding in the frontal 50 km of the subduction thrust (0.0001-0.1 mm/s, 0.5-25 MPa effective normal stress). (5) Progressive illitization of smectite cannot explain seismogenesis, because illite-rich samples also show velocity strengthening at the conditions tested.

Post-Jurassic tectonic evolution of Southeast Asia

NASA Astrophysics Data System (ADS)

Zahirovic, Sabin; Seton, Maria; Dietmar Müller, R.; Flament, Nicolas

2014-05-01

The accretionary growth of Asia, linked to long-term convergence between Eurasia, Gondwana-derived blocks and the Pacific, resulted in a mosaic of terranes for which conflicting tectonic interpretations exist. Here, we propose solutions to a number of controversies related to the evolution of Sundaland through a synthesis of published geological data and plate reconstructions that reconcile both geological and geophysical constraints with plate driving forces. We propose that West Sulawesi, East Java and easternmost Borneo rifted from northern Gondwana in the latest Jurassic, collided with an intra-oceanic arc at ~115 Ma and subsequently sutured to Sundaland by 80 Ma. Although recent models argue that the Southwest Borneo core accreted to Sundaland at this time, we use volcanic and biogeographic constraints to show that the core of Borneo was on the Asian margin since at least the mid Jurassic. This northward transfer of Gondwana-derived continental fragments required a convergent plate boundary in the easternmost Tethys that we propose gave rise to the Philippine Archipelago based on the formation of latest Jurassic-Early Cretaceous supra-subduction zone ophiolites on Halmahera, Obi Island and Luzon. The Late Cretaceous marks the shift from Andean-style subduction to back-arc opening on the east Asian margin. Arc volcanism along South China ceased by ~60 Ma due to the rollback of the Izanagi slab, leading to the oceanward migration of the volcanic arc and the opening of the Proto South China Sea (PSCS). We use the Apennines-Tyrrhenian system in the Mediterranean as an analogue to model this back-arc. Continued rollback detaches South Palawan, Mindoro and the Semitau continental blocks from the stable east Asian margin and transfers them onto Sundaland in the Eocene to produce the Sarawak Orogeny. The extrusion of Indochina and subduction polarity reversal along northern Borneo opens the South China Sea and transfers the Dangerous Grounds-Reed Bank southward to terminate PSCS south-dipping subduction and culminates in the Sarawak Orogeny on Borneo and ophiolite obduction on Palawan. We account for the regional plate reorganizations related to the initiation of Pacific subduction along the Izu-Bonin-Mariana Arc, the extrusion tectonics resulting from the India-Eurasia collision, and the shift from basin extension to inversion on Sundaland as an indicator of collision between the Australian continent and the active Asian margin. We generate continuously closing and evolving plate boundaries, seafloor age-grids and global plate velocity fields using the open-source and cross-platform GPlates plate reconstruction software. We link our plate motions to numerical mantle flow models in order to predict mantle structure at present-day that can be qualitatively compared to P- and S- wave seismic tomography models. This method allows us to analyse the evolution of the mantle related to Tethyan and Pacific subduction and to test alternative plate reconstructions. This iterative approach can be used to improve plate reconstructions in the absence of preserved seafloor and conjugate passive margins of continental blocks, which may have been destroyed or highly deformed by multiple episodes of accretion along the Asian margins.

Subduction history of the Paleo-Pacific plate beneath the Eurasian continent: Evidence from Mesozoic igneous rocks and accretionary complex in NE Asia

NASA Astrophysics Data System (ADS)

Xu, W.

2015-12-01

Mesozoic magmatisms in NE China can be subdivided into seven stages, i.e., Late Triassic, Early Jurassic, Middle Jurassic, Late Jurassic, early Early Cretaceous, late Early Cretaceous, and Late Cretaceous. Late Triassic magmatisms consist of calc-alkaline igneous rocks in the Erguna Massif, and bimodal igneous rocks in eastern margin of Eurasian continent. The former reveals southward subduction of the Mongol-Okhotsk oceanic plate, the latter reveals an extensional environment (Xu et al., 2013). Early Jurassic magmatisms are composed of calc-alkaline igneous rocks in the eastern margin of the Eurasian continent and the Erguna Massif, revealing westward subduction of the Paleo-pacific plate and southward subduction of the Mongol-Okhotsk oceanic plate (Tang et al., 2015), respectively. Middle Jurassic magmatism only occur in the Great Xing'an Range and the northern margin of the NCC, and consists of adakitic rocks that formed in crustal thickening, reflecting the closure of the Mongol-Okhotsk ocean (Li et al., 2015). Late Jurassic and early Early Cretaceous magmatisms only occur to the west of the Songliao Basin, and consist of trackyandesite and A-type of rhyolites, revealing an extensional environment related to delamination of thickened crust. The late Early Cretaceous magmatisms are widespread in NE China, and consist of calc-alkaline volcanics in eastern margin and bimodal volcanics in intracontinent, revealing westward subduction of the Paleo-pacific plate. Late Cretaceous magmatisms mainly occur to the east of the Songliao Basin, and consist of calc-alkaline volcanics in eastern margin and alkaline basalts in intracontinent (Xu et al., 2013), revealing westward subduction of the Paleo-pacific plate. The Heilongjiang complex with Early Jurassic deformation, together with Jurassic Khabarovsk complex in Russia Far East and Mino-Tamba complex in Japan, reveal Early Jurassic accretionary history. Additionally, the Raohe complex with the age of ca. 169 Ma was intruded by the 110-130 Ma massive granitoids, suggesting late Early Cretaceous accretionary event. From late Early Cretaceous to Late Cretaceous, the spatial extent of magmatisms was reduced from west to east, revealing roll-back of subducted slab. This research was financially supported by the NSFC (41330206).

Comparison of Subsidence Rates for Conjugate Margins of the Equatorial and Northern South Atlantic Ocean as A First-Order Constraint on Symmetry of Underlying, Early Rift Structures

NASA Astrophysics Data System (ADS)

Zavala, O.

2017-12-01

We compared subsidence histories from wells into Cretaceous-Cenozoic conjugate margins in the Equatorial and northern South Atlantic as a first-order constraint on whether rifting occurred in a symmetrical, pure shear mode, or whether rifting occurred in an asymmetrical, simple shear mode. For the pure shear mode of rifting, the prediction is for longterm subsidence on both conjugate margins to be similar and reflective of underlying, rift symmetry; for the simple shear mode of rifting, the prediction is that subsidence above the more thinned and wider, lower plate margin is greater than subsidence above the less thinned and more narrow, upper plate margin. A major caveat of this approach is that subsidence variations can be affected by other external factors that include increased sedimentation related to local deltas and structural or hotspot-related uplifts of coastal areas. In the northern Equatorial Atlantic, the longterm subsidence rate for the Guyana basin of northeastern South America of 18.52 m/Ma is less that of the Senegal area of west Africa of 54 m/Ma suggestive of an upper plate to the west and lower plate to the east. Moving southwards, the Potiguar basin of northern Brazil of 23 m/Ma is roughly the same as the Keta-Togo-Benin-Cote d'Ivoire basins of west Africa (21 m/Ma) and suggestive of an underlying rift symmetry. The Bahia Norte-Reconcavo-Sergipe-Alogoas basins of Brazil are less (28 m/Ma) than the Gabon basin (57 m/Ma) of west Africa suggesitive of an lower plate to the east and an upper plate to the west. The Bahia Sul-Espirito Santo basins of Brazil are less (20 m/Ma) than the Lower Congo basin (45 m/Ma) although the latter area includes the localized influence of the Congo delta. We compare additional evidence such as seismic reflection and refraction data and gravity modeling to the predictions of the subsidence values.

Seismic Microzonation of the City of Cali (Western Colombia)

NASA Astrophysics Data System (ADS)

Dimate, C.; Romero, J.; Ojeda, A.; Garcia, J.; Alvarado, C.

2007-05-01

The city of Cali is located in the western margin of the Cauca Valley in the flat area between the Western and Central cordilleras of the Colombian Andes, at 70 km east of the Eastern Pacific Subduction Zone. Even though present seismic activity associated with nearest faults is low, historical records demonstrate that earthquakes have caused damage in the city going up to intensity VIII (EMS). Those earthquakes have had origin on diverse sources: the intermediate-depth Benioff zone, near and far continental crustal faults and the Pacific Subduction Zone. Taking into account the location of the city and the seismologic history of the region, neotectonic and seismological studies extending over a region of about 120000 km2 were required to compute seismic hazard. Construction of the geotechnical model of the city included detailed geological mapping, geophysical profiling, single station ambient vibration essays and the deployment of a 12-stations accelerographic network. Geotechnical properties of the soils were determined by mechanical perforations, CPTU (piezocone) and CPT (static penetration) essays, flat plate dilatometer (DMT) tests and down-hole essays which were complemented in the Lab by analysis of consolidation and static and cyclic three-axial essays. As a result, ten geotechnical zones were outlined and characterized. Finally, expected ground motions were calculated at 39 sites in the city using numerical modeling methods.

Lithospheric Response of the Anatolian Plateau in the Realm of the Black Sea and the Eastern Mediterranean

NASA Astrophysics Data System (ADS)

Ergun, Mustafa

2016-04-01

The Eastern Mediterranean and the Middle East make up the southern boundary of the Tethys Ocean for the last 200 Ma by the disintegration of the Pangaea and closure of the Tethys Ocean. It covers the structures: Hellenic and Cyprus arcs; Eastern Anatolian Fault Zone; Bitlis Suture Zone and Zagros Mountains. The northern boundary of the Tethys Ocean is made up the Black Sea and the Caspian Sea, and it extends up to Po valley towards the west (Pontides, Caucasus). Between these two zones the Alp-Himalayan orogenic belt is situated where the Balkan, Anatolia and the Iran plateaus are placed as the remnants of the lost Ocean of the Tethys. The active tectonics of the eastern Mediterranean is the consequences of the convergence between the Africa, Arabian plates in the south and the Eurasian plate in the north. These plates act as converging jaws of vise forming a crustal mosaic in between. The active crustal deformation pattern reveals two N-S trending maximum compression or crustal shortening syntaxes': (i) the eastern Black Sea and the Arabian plate, (ii) the western Black Sea and the Isparta Angle. The transition in young mountain belts, from ocean crust through the agglomeration of arc systems with long histories of oceanic closures, to a continental hinterland is well exemplified by the plate margin in the eastern Mediterranean. The boundary between the African plate and the Aegean/Anatolian microplate is in the process of transition from subduction to collision along the Cyprus Arc. Since the Black Sea has oceanic lithosphere, it is actually a separate plate. However it can be considered as a block, because the Black Sea is a trapped oceanic basin that cannot move freely within the Eurasian Plate. Lying towards the northern margin of orogenic belts related to the closure of the Tethys Ocean, it is generally considered to be a result of back-arc extension associated with the northward subduction of the Tethyan plate to the south. Interface oceanic lithosphere at the leading edge of the northward moving African Plate in the eastern Mediterranean Sea and the deforming Aegean-Anatolian Plate continental lithosphere forms the northward dipping Hellenic and Cyprean subduction zones in the south. Since there is a velocity differential between the northward motion of African and Arabian Plates (10 mm/yr and 18 mm/yr, respectively), this difference is accommodated along the sinistral strike-slip Dead Sea Fault that forms the plate boundary between the African and the Arabian Plates. Continental crust forms from structurally thickened remnants of oceanic crust and overlying sediments, which are then invaded by arc magmatism. Understanding this process is a first order problem of lithospheric dynamics. The transition in young mountain belts, from ocean crust through the agglomeration of arc systems with long histories of oceanic closures, to a continental hinterland is well exemplified by the plate margin in the eastern Mediterranean. Mountains are subject to erosion, which can disturb isostatic compensation. If the eroded mountains are no longer high enough to justify their deep root-zones, the topography is isostatically overcompensated. Similarly, the buoyancy forces that result from overcompensation of mountainous topography cause vertical uplift. The Eastern Mediterranean Basin, having 100 milligal gravity values lower than other isostatically compensated oceans, it is in general overcompensated. Normally the Eastern Mediterranean Basin should rise under its present isostatic condition. It is known, however, that the Eastern Mediterranean Basin with its thick sediment-filled basins is actually sinking. Anatolia, having 100 milligals gravity values higher than other isostatically compensated zones of the world, is in general undercompensated. Normal isostatic conditions require that Anatolia should sink. It is known, however, that Anatolia, with the exception of local grabens, is rising. While the Black Sea, having 100-milligal lower gravity value than other isostatically compensated oceans, it is in general overcompensated and The Black Sea basin with very thick sedimentary cover (more than 12-14 km thick) is actually sinking.

Plate Margin Deformation and Active Tectonics Along the Northern Edge of the Yakutat Terrane in the Saint Elias Orogen, Alaska and Yukon, Canada

NASA Technical Reports Server (NTRS)

Bruhn, Ronald L.; Sauber, Jeanne; Cotton, Michele M.; Pavlis, Terry L.; Burgess, Evan; Ruppert, Natalia; Forster, Richard R.

2012-01-01

The northwest directed motion of the Pacific plate is accompanied by migration and collision of the Yakutat terrane into the cusp of southern Alaska. The nature and magnitude of accretion and translation on upper crustal faults and folds is poorly constrained, however, due to pervasive glaciation. In this study we used high-resolution topography, geodetic imaging, seismic, and geologic data to advance understanding of the transition from strike-slip motion on the Fairweather fault to plate margin deformation on the Bagley fault, which cuts through the upper plate of the collisional suture above the subduction megathrust. The Fairweather fault terminates by oblique-extensional splay faulting within a structural syntaxis, allowing rapid tectonic upwelling of rocks driven by thrust faulting and crustal contraction. Plate motion is partly transferred from the Fairweather to the Bagley fault, which extends 125 km farther west as a dextral shear zone that is partly reactivated by reverse faulting. The Bagley fault dips steeply through the upper plate to intersect the subduction megathrust at depth, forming a narrow fault-bounded crustal sliver in the obliquely convergent plate margin. Since . 20 Ma the Bagley fault has accommodated more than 50 km of dextral shearing and several kilometers of reverse motion along its southern flank during terrane accretion. The fault is considered capable of generating earthquakes because it is linked to faults that generated large historic earthquakes, suitably oriented for reactivation in the contemporary stress field, and locally marked by seismicity. The fault may generate earthquakes of Mw <= 7.5.

Shear deformation in the northeastern margin of the Izu collision zone, central Japan, inferred from GPS observations

NASA Astrophysics Data System (ADS)

Doke, R.; Harada, M.; Miyaoka, K.; Satomura, M.

2016-12-01

The Izu collision zone, which is characterized by the collision between the Izu-Bonin arc (Izu Peninsula) and the Honshu arc (the main island of Japan), is located in the northernmost part of the Philippine Sea (PHS) plate. Particularly in the northeastern margin of the zone, numerous large earthquakes have occurred. To clarify the convergent tectonics of the zone related to the occurrence of these earthquakes, in this study, we performed Global Positioning System (GPS) observations and analysis around the Izu collision zone. Based on the results of mapping the steady state of the GPS velocity and strain rate fields, we verified that there has been wide shear deformation in the northeastern part of the Izu collision zone, which agrees with the maximum shear directions in the left-lateral slip of the active faults in the study area. Based on the relative motion between the western Izu Peninsula and the eastern subducting forearc, the shear zone can be considered as a transition zone affected by both collision and subduction. The Higashi-Izu Monogenic Volcano Group, which is located in the southern part of the shear deformation zone, may have formed as a result of the steady motion of the subducting PHS plate and the collision of the Izu Peninsula with the Honshu arc. The seismic activities in the Tanzawa Mountains, which is located in the northern part of the shear deformation zone, and the eastern part of the Izu Peninsula may be related to the shear deformation zone, because the temporal patterns of the seismic activity in both areas are correlated.

Longitude through time

NASA Astrophysics Data System (ADS)

Torsvik, T. H.; Steinberger, B.; Cocks, R. L.

2007-12-01

Earth scientists today have had no objective method of calculating what the palaeolongitudes of tectonic plates and other geological units were in the long eons prior to the oldest known hotspot trails, which are only of Cretaceous age (ca. 130 Ma). Before this time, palaeomagnetism is the only method by which to position plates quantitatively on the globe. Palaeomagnetic studies only directly yield latitudes and plate rotations, but the longitude uncertainty can be minimized by selecting an appropriate reference plate: if one can determine which plate has moved least, then it should be used as the reference plate. Africa has been nearly surrounded by mid- ocean ridges since the break-up of Pangea, and thus the ridge push forces should have roughly cancelled each other out. Moving hotspot-based plate motion models show minimal longitudinal motion for Africa (<10 degrees) for the past 130 million years, confirming the lack of significant longitudinal motion inferred from consideration of the plate driving forces. It is uncertain whether the 'zero-longitude' assumption about Africa holds before Pangea's break-up, but in the absence of better reference points, we have regarded zero longitudinal average motion for Africa as the best assumption. With this approach we have been able to demonstrate that virtually all Large Igneous Province (LIPs) for the last 300 million years project radially onto the edges of the African and Pacific Large Low Shear Velocity Province (LLSVPs) near the core-mantle-boundary (CMB). The LIPs must for this reason be derived from mantle plumes, and CMB heterogeneities must have remained quite stationary since the formation of Pangea. LIPs have erupted since Archean times and there is no reason to preclude that they were all derived from LLSVPs in the deep mantle. That inspired us to consider whether older LIP events would yield similar results. We attempt to reconstruct Gondwana in longitude in Cambrian times based on the substantial Antrim plateau volcanics (Australia), a LIP of ca. 510 Ma age along the Gondwanan margin. If the LIP was formed at the margin of the Africa or Pacific LLSVPs and they have remained the same throughout Earth's history there are six possible marginal sites on the CMB from which to choose, but three sites that do not position the long- lived subduction margin of Gondwana (e.g. South America, East Antarctica and East Australia) above regions of high seismic velocity (the subduction graveyards) can be eliminated. If, as recently postulated, there have only been one LLSVP (or upwelling zone) in Pre-Pangean time (Pacific LLSVP) that reduces longitude choices to two possible marginal sites on the CMB.

Thin and layered subcontinental crust of the great Basin western north America inherited from Paleozoic marginal ocean basins?

USGS Publications Warehouse

Churkin, M.; McKee, E.H.

1974-01-01

The seismic profile of the crust of the northern part of the Basin and Range province by its thinness and layering is intermediate between typical continental and oceanic crust and resembles that of marginal ocean basins, especially those with thick sedimentary fill. The geologic history of the Great Basin indicates that it was the site of a succession of marginal ocean basins opening and closing behind volcanic arcs during much of Paleozoic time. A long process of sedimentation and deformation followed throughout the Mesozoic modifying, but possibly not completely transforming the originally oceanic crust to continental crust. In the Cenozoic, after at least 40 m.y. of quiescence and stable conditions, substantial crustal and upper-mantle changes are recorded by elevation of the entire region in isostatic equilibrium, crustal extension resulting in Basin and Range faulting, extensive volcanism, high heat flow and a low-velocity mantle. These phenomena, apparently the result of plate tectonics, are superimposed on the inherited subcontinental crust that developed from an oceanic origin in Paleozoic time and possibly retained some of its thin and layered characteristics. The present anomalous crust in the Great Basin represents an accretion of oceanic geosynclinal material to a Precambrian continental nucleus apparently as an intermediate step in the process of conversion of oceanic crust into a stable continental landmass or craton. ?? 1974.

Deposition of Franciscan Complex cherts along the paleoequator and accretion to the American margin at tropical paleolatitudes

USGS Publications Warehouse

Hagstrum, J.T.; Murchey, B.L.

1993-01-01

Red radiolarian cherts from three localities within the Franciscan subduction complex of northern California contain three components of remanent magnetization which are best isolated by progressive thermal demagnetization. The available paleomagnetic, biostratigraphic, and geochemical data indicate deposition of these cherts along the paleoequator (0??-2??N or S paleolatitude) between Pliensbachian and Oxfordian time as the oceanic plate moved eastward, relative to North America, beneath the equatorial zone of high biologic productivity. The chert sequences were subsequently accreted to the American continental margin. Plate reconstruction models for the Farallon plate corrobotate low-paleolatitude trajectories from ridge crest to subduction zone, and they imply subsequent northward translation of the Franciscan Complex by strike-slip faulting related to relative motions between the Farallon, Kula, Pacific, and North American plates. -from Authors

Structure and degree of magmatism of North and South Atlantic rifted margins

NASA Astrophysics Data System (ADS)

Faleide, Jan Inge; Breivik, Asbjørn J.; Blaich, Olav A.; Tsikalas, Filippos; Planke, Sverre; Mansour Abdelmalak, Mohamed; Mjelde, Rolf; Myklebust, Reidun

2014-05-01

The structure and evolution of conjugate rifted margins in the South and North Atlantic have been studied mainly based on seismic reflection and refraction profiles, complemented by potential field data and plate reconstructions. All margins exhibit distinct along-margin structural and magmatic changes reflecting both structural inheritance extending back to a complex pre-breakup geological history and the final breakup processes. The sedimentary basins at the conjugate margins developed as a result of multiple phases of rifting, associated with complex time-dependent thermal structure of the lithosphere. A series of conjugate crustal transects reveal tectonomagmatic asymmetry, both along-strike and across the conjugate margin systems. The continent-ocean transitional domain along the magma-dominated margin segments is characterized by a large volume of flood basalts and high-velocity/high-density lower crust emplaced during and after continental breakup. Both the volume and duration of excess magmatism varies. The extrusive and intrusive complexes make it difficult to pin down a COB to be used in plate reconstructions. The continent-ocean transition is usually well defined as a rapid increase of P-wave velocities at mid- to lower crustal levels. The transition is further constrained by comparing the mean P-wave velocity to the thickness of the crystalline crust. By this comparison we can also address the magmatic processes associated with breakup, whether they are convection dominated or temperature dominated. In the NE Atlantic there is a strong correlation between magma productivity and early plate spreading rate, suggesting a common cause. A model for the breakup-related magmatism should be able to explain this correlation, but also the magma production peak at breakup, the along-margin magmatic segmentation, and the active mantle upwelling. It is likely that mantle plumes (Iceland in the NE Atlantic, Tristan da Cunha in the South Atlantic) may have influenced the volume of magmatism but they did not necessarily alter the process of rifted margin formation, implying that parts of the margins may have much in common with more magma-poor margins. Conjugate margin segments from the North and South Atlantic will be compared and discussed with particular focus on the tectonomagmatic processes associated with continental breakup.

Mantle flow and deforming continents, insights from the Tethys realm

NASA Astrophysics Data System (ADS)

Jolivet, Laurent; Faccenna, Claudio; Becker, Thorsten; Tesauro, Magdala

2017-04-01

Continent deformation is partly a consequence of plate motion along plate boundaries. Whether underlying asthenospheric flow can also deform continents through basal shear or push on topographic irregularities of the base of the lithosphere is an open question. Eurasia has been extending at different scales since 50 Ma, from the Mediterranean back-arc domains to extension of Asia between the India-Asia collision zone and the Pacific subduction zones. While compression at plate margins, in subduction or collision zones can propagate far within continents, the mechanism explaining extension distributed over thousands of kilometres is unclear. We use trajectories of continental plates and continental fragments since 50 Ma in different kinematic frames and compare them with various proxies of asthenospheric flow such as seismic anisotropy at various depths. These trajectories partly fit sub-lithospheric seismic anisotropy with two main circulations, one carrying Africa and Eurasia away from the large low velocity anomaly (LLSVP) underlying South and West Africa and one carrying the Pacific plate away from the LLSVP underlying the southern Pacific. Under eastern Eurasia the flow converges with the Pacific flow and distributed extension affects eastern Asia all the way to Western Pacific back-arc basins. We speculate that the flow carrying India northward and Eurasia eastward has invaded the Pacific domain and caused this widely distributed extension that interferes with the strike-slip faults issued from the Himalaya-Tibet collision zone. This model is in line with earlier propositions based on geochemical proxies. We discuss this model and compare it to other widely distributed extensional deformation episodes such as the Early Cretaceous extension of Africa and lastly propose a scheme of large-scale continental deformation in relation to underlying mantle convection at different scales.

Mantle flow and deforming continents, the Tethys realm

NASA Astrophysics Data System (ADS)

Jolivet, L.; Faccenna, C.; Becker, T. W.

2016-12-01

Continent deformation is partly a consequence of plate motion along plate boundaries. Whether underlying asthenospheric flow can also deform continents through basal shear or push on topographic irregularities of the base of the lithosphere is an open question. Eurasia has been extending at different scales since 50 Ma, from the Mediterranean back-arc domains to extension of Asia between the India-Asia collision zone and the Pacific subduction zones. While compression at plate margins, in subduction or collision zones can propagate far within continents, the mechanism explaining extension distributed over thousands of kilometres is unclear. We use trajectories of continental plates and continental fragments since 50 Myrs in different kinematic frames and compare them with various proxies of asthenospheric flow such as seismic anisotropy at various depths. These trajectories partly fit sub-lithospheric seismic anisotropy with two main circulations, one carrying Africa and Eurasia away from the large low velocity anomaly (LLSVP) underlying South and West Africa and one carrying the Pacific plate away from the LLSVP underlying the southern Pacific. Under eastern Eurasia the flow converges with the Pacific flow and distributed extension affects eastern Asia all the way to Western Pacific back-arc basins. We speculate that the flow carrying India northward and Eurasia eastward has invaded the Pacific domain and caused this widely distributed extension that interferes with the strike-slip faults issued from the Himalaya-Tibet collision zone. This model is in line with earlier propositions based on geochemical proxies. We discuss this model and compare it to other widely distributed extensional deformation episodes such as the Early Cretaceous extension of Africa and finally propose a scheme of large-scale continental deformation in relation to underlying mantle convection at different scales.

Tectonic and thermal history of the western Serrania del Interior foreland fold and thrust belt and Guarico Basin, north central Venezuela: Implications of new apatite fission track analysis and seismic interpretation

NASA Astrophysics Data System (ADS)

Perez de Armas, Jaime Gonzalo

Structural analysis, interpretation of seismic reflection lines, and apatite fission-track analysis in the Western Serrania del Interior fold and thrust belt and in the Guarico basin of north-central Venezuela indicate that the area underwent Mesozoic and Tertiary-to-Recent deformation. Mesozoic deformation, related to the breakup of Pangea, resulted in the formation of the Espino graben in the southernmost portion of the Guarico basin and in the formation of the Proto-Caribbean lithosphere between the diverging North and South American plates. The northern margin of Venezuela became a northward facing passive margin. Minor normal faults formed in the Guarico basin. The most intense deformation took place in the Neogene when the Leeward Antilles volcanic island arc collided obliquely with South America. The inception of the basal foredeep unconformity in the Late Eocene-Early Oligocene marks the formation of a perisutural basin on top of a buried graben system. It is coeval with minor extension and possible reactivation of Cretaceous normal faults in the Guarico basin. It marks the deepening of the foredeep. Cooling ages derived from apatite fission-tracks suggest that the obduction of the fold and thrust belt in the study area occurred in the Late Oligocene through the Middle Miocene. Field data and seismic interpretations suggest also that contractional deformation began during the Neogene, and specifically during the Miocene. The most surprising results of the detrital apatite fission-track study are the ages acquired in the sedimentary rocks of the easternmost part of the study area in the foreland fold and thrust belt. They indicate an Eocene thermal event. This event may be related to the Eocene NW-SE convergence of the North and South American plates that must have caused the Proto-Caribbean lithosphere to be shortened. This event is not related to the collision of the arc with South America, as the arc was far to the west during the Eocene.

Advancing Understanding of Earthquakes by Drilling an Eroding Convergent Margin

NASA Astrophysics Data System (ADS)

von Huene, R.; Vannucchi, P.; Ranero, C. R.

2010-12-01

A program of IODP with great societal relevance is sampling and instrumenting the seismogenic zone. The zone generates great earthquakes that trigger tsunamis, and submarine slides thereby endangering coastal communities containing over sixty percent of the earth’s population. To asses and mitigate this endangerment it is urgent to advance understanding of fault dynamics that allows more timely anticipation of hazardous seismicity. Seismogenesis on accreting and eroding convergent plate boundaries apparently differ because of dissimilar materials along the interplate fault. As the history of instrumentally recorded earthquakes expands the difference becomes clearer. The more homogeneous clay, silt and sand subducted at accreting margins is associated with great earthquakes (M 9) whereas the fragmented upper plate rock that can dominate subducted material along an eroding margin plate interface is associated with many tsunamigenic earthquakes (Bilek, 2010). Few areas have been identified where the seismogenic zone can be reached with scientific drilling. In IODP accreting margins are studied on the NanTroSeize drill transect off Japan where the ultimate drilling of the seismogenic interface may occur by the end of IODP. The eroding Costa Rica margin will be studied in CRISP where a drill program will begin in 2011. The Costa Rican geophysical site survey will be complete with acquisition and processing of 3D seismic data in 2011 but the entire drilling will not be accomplished in IODP. It is appropriate that the accreting margin study be accomplished soon considering the indications of a pending great earthquake that will affect a country that has devoted enormous resources to IODP. However, understanding the erosional end-member is scientifically as important to an understanding of fault mechanics. Transoceanic tsunamis affect the entire Pacific rim where most subduction zones are eroding margins. The Costa Rican subduction zone is less complex operationally and perhaps geologically than the Nankai margin. The developing Central American countries do not have the resources to contribute to IODP but this should not deter acquiring the scientific insights proposed in CRISP considering the broader scientific benefits. Such benefits include the first sampling and instrumentation of an actively eroding plate interface and drilling near or into an earthquake asperity. Drilling an eroding margin should significantly advance understanding of subduction zone fault mechanisms and help improve assessment of future hazardous earthquakes and tsunamis.

Late Mesozoic- Cenozoic plate boundaries in the North Atlantic – Arctic: Quantitative reconstructions using Hellinger criterion in GPlates

NASA Astrophysics Data System (ADS)

Gaina, Carmen; Watson, Robin; Cirbus, Juraj

2015-04-01

Cretaceous extension that resulted in the formation of several sedimentary basins along the North American and western and southwestern Greenland margin was followed by seafloor spreading in the Labrador Sea and Baffin Bay. Controversy regarding the timing of the oldest oceanic crust in these basins spanned more than 25 years and it is still not resolved due to the complexity of the margins and non-uniqueness of potential field data interpretation. Here we revisit the geophysical data (in particular the magnetic and gravity data) available for the Labrador Sea and Baffin Bay in order to identify the age of oceanic crust and infer new parameters that can be used for quantitative kinematic reconstructions. We identify chrons 20 to 29 for the central part of the basin. For the crust formed near the extinct spreading ridge we have modelled chrons 19 to 15 assuming an ultraslow spreading rate. Oceanic crust older than chron 29 is uncertain and may be part of a transitional crust that possibly contains other type of crust or exhumed mantle. The new magnetic anomaly identifications were inverted using the Hellinger (1981) criterion of fit. In this method the magnetic data are regarded as points on two conjugate isochrons consisting of great circle segments. This method has been extensively used for kinematic reconstructions since Royer and Chang (1991) first implemented it for quantitative plate tectonics, and is now available as a new interactive tool in the open-source software GPlates (www.gplates.org). The GPlates Hellinger tool lets the user interactively generate a best-fit rotation pole to a series of segmented magnetic picks. The fitting and determination of uncertainties are based on the FORTRAN program hellinger1 (Chang, 1988; Hellinger, 1981; Hanna and Chang, 1990); Royer and Chang, 1991). Input data can be viewed and adjusted both tabularly and graphically, and the best fit can be viewed and tested on the GPlates globe. The new set of rotations and their uncertainties are combined with a regional model and used to infer the plate boundaries during the formation of Labrador Sea and Baffin Bay. Challenges for establishing the continuation of these plate boundaries the Arctic domain are also discussed. References Chang, T. (1988), Estimating the relative rotation of two tectonic plates from boundary crossings, J. Am. Stat. Assoc., 83, 1178-1183. Hellinger, S. J. (1981), The uncertainties of finite rotations in plate tectonics, J Geophys Res, 86, 9312-9318. Hanna, M.S and T. Chang (1990), On graphically representing the confidence region for an unknown rotation in three dimensions. Computers & Geosciences 16 (2), 163-194. Royer, J. Y., and T. Chang (1991), Evidence for Relative Motions between the Indian and Australian Plates during the Last 20 My from Plate Tectonic Reconstructions - Implications for the Deformation of the Indo-Australian Plate, J Geophys Res, 96(B7), 11779-11802.

Post-early cretaceous landform evolution along the western margin of the banca~nnia trough, western nsw

USGS Publications Warehouse

Gibson, D.L.

2000-01-01

Previously undated post-Devonian sediments outcropping north of Fowlers Gap station near the western margin of the Bancannia Trough are shown by plant macro- and microfossil determinations to be of Early Cretaceous (most likely Neocomian and/or Aptian) age, and thus part of the Eromanga Basin. They are assigned to the previously defined Telephone Creek Formation. Study of the structural configuration of this unit and the unconformably underlying Devonian rocks suggests that the gross landscape architecture of the area results from post-Early Cretaceous monoclinal folding along blind faults at the western margin of the trough, combined with the effects of differential erosion. This study shows that, while landscape evolution in the area has been dynamic, the major changes that have occurred are on a geological rather than human timescale.

The Red Sea and Gulf of Aden Basins

NASA Astrophysics Data System (ADS)

Bosworth, William; Huchon, Philippe; McClay, Ken

2005-10-01

We here summarize the evolution of the greater Red Sea-Gulf of Aden rift system, which includes the Gulfs of Suez and Aqaba, the Red Sea and Gulf of Aden marine basins and their continental margins, and the Afar region. Plume related basaltic trap volcanism began in Ethiopia, NE Sudan (Derudeb), and SW Yemen at ˜31 Ma, followed by rhyolitic volcanism at ˜30 Ma. Volcanism thereafter spread northward to Harrats Sirat, Hadan, Ishara-Khirsat, and Ar Rahat in western Saudi Arabia. This early magmatism occurred without significant extension, and continued to ˜25 Ma. Much of the Red Sea and Gulf of Aden region was at or near sea level at this time. Starting between ˜29.9 and 28.7 Ma, marine syn-tectonic sediments were deposited on continental crust in the central Gulf of Aden. At the same time the Horn of Africa became emergent. By ˜27.5-23.8 Ma a small rift basin was forming in the Eritrean Red Sea. At approximately the same time (˜25 Ma), extension and rifting commenced within Afar itself. At ˜24 Ma, a new phase of volcanism, principally basaltic dikes but also layered gabbro and granophyre bodies, appeared nearly synchronously throughout the entire Red Sea, from Afar and Yemen to northern Egypt. This second phase of magmatism was accompanied in the Red Sea by strong rift-normal extension and deposition of syn-tectonic sediments, mostly of marine and marginal marine affinity. Sedimentary facies were laterally heterogeneous, being comprised of inter-fingering siliciclastics, evaporite, and carbonate. Throughout the Red Sea, the principal phase of rift shoulder uplift and rapid syn-rift subsidence followed shortly thereafter at ˜20 Ma. Water depths increased dramatically and sedimentation changed to predominantly Globigerina-rich marl and deepwater limestone. Within a few million years of its initiation in the mid-Oligocene the Gulf of Aden continental rift linked the Owen fracture zone (oceanic crust) with the Afar plume. The principal driving force for extension was slab-pull beneath the Urumieh-Doktar arc on the north side of the narrowing Neotethys. Drag of Arabia by the northward-moving Indian plate across the partially locked northern Owen fracture zone and the position of the Carlsberg oceanic ridge probably also influenced the geometry of the Aden rift. The trigger for the onset of rifting, though, was the impingement of the Afar plume at ˜31 Ma. The Red Sea propagated away from the plume head, perpendicular to the extensional stresses then operating in Arabia, and arrived at the bend in the African-Levant margin, which itself may have been a stress concentration ripe for rifting. The local geometry of the early Red Sea rift was strongly influenced by pre-existing basement structures, and as a consequence followed a complex path from Afar to Suez. Each segment of the rift was initially an asymmetric half graben, with well-defined accommodation zones between sub-basins. In the Gulf of Aden, the positions of accommodation zones were strongly influenced by older Mesozoic rift basins. Early rift structures can be restored to their original contiguous geometries along both the Red Sea and Gulf of Aden conjugate margins. In both basins, present-day shorelines restore to a separation of 40-60 km along most of their lengths. The initial rift basins were 60-80 km in width. Oceanic spreading initiated on the Sheba Ridge east of the Alula-Fartaq fracture zone at ˜19-18 Ma. After stalling at this fracture zone, the ridge probably propagated west into the central Gulf of Aden by ˜16 Ma. This matches the observed termination of syn-tectonic deposition along the onshore Aden margins at approximately the same time. At ˜14 Ma, a transform boundary cut through Sinai and the Levant continental margin, linking the northern Red Sea with the Bitlis-Zagros convergence zone. This corresponded with collision of Arabia and Eurasia, which resulted in a new plate geometry with different boundary forces. Red Sea extension changed from rift normal (N60°E) to highly oblique and parallel to the Aqaba-Levant transform (N15°E). North of Suez in Egypt the rift system became emergent, perhaps due to minor compression of the Sinai sub-plate, and the marine connection to the Mediterranean Sea became restricted but not terminated. Red Sea sedimentation changed from predominantly open marine to evaporitic, although deep water persisted in many regions. A third phase of magmatism commenced, locally in Ethiopia but predominantly in western Saudi Arabia and extending north to Harrat Ash Shama and Jebel Druse in Jordan, Lebanon, and Syria. At ˜10 Ma, the Sheba Ridge rapidly propagated west over 400 km from the central Gulf of Aden to the Shukra al Sheik discontinuity. Oceanic spreading followed in the south-central Red Sea at ˜5 Ma. This corresponded in time to an important unconformity throughout the Red Sea basin and along the margins of the Gulf of Aden, coeval with the Messinian unconformity of the Mediterranean basin. A major phase of pull-apart basin development also occurred along the Aqaba-Levant transform. In the early Pliocene the influx of marine waters through Bab al Mandeb increased and Red Sea sedimentation thereafter returned to predominantly open marine conditions. By ˜3-2 Ma, oceanic spreading moved west of the Shukra al Sheik discontinuity, and the entire Gulf of Aden was an oceanic rift. During the last ˜1 My, the southern Red Sea plate boundary linked to the Aden spreading center through the Gulf of Zula, Danakil Depression, and Gulf of Tadjoura. Presently, the Red Sea spreading center appears to be propagating toward the northern Red Sea to link with the Aqaba-Levant transform. Alkali basaltic volcanism continues within the Younger Harrats of western Saudi Arabia and Yemen and offshore southern Red Sea islands. Most of the Arabian plate is now experiencing N-S upper crustal compression, whereas the maximum horizontal stress is oriented E-W in NE Africa. Arabia and Africa, now on separate plates, are therefore completely decoupled in terms of regional, far-field stresses.

Subduction-driven recycling of continental margin lithosphere.

PubMed

Levander, A; Bezada, M J; Niu, F; Humphreys, E D; Palomeras, I; Thurner, S M; Masy, J; Schmitz, M; Gallart, J; Carbonell, R; Miller, M S

2014-11-13

Whereas subduction recycling of oceanic lithosphere is one of the central themes of plate tectonics, the recycling of continental lithosphere appears to be far more complicated and less well understood. Delamination and convective downwelling are two widely recognized processes invoked to explain the removal of lithospheric mantle under or adjacent to orogenic belts. Here we relate oceanic plate subduction to removal of adjacent continental lithosphere in certain plate tectonic settings. We have developed teleseismic body wave images from dense broadband seismic experiments that show higher than expected volumes of anomalously fast mantle associated with the subducted Atlantic slab under northeastern South America and the Alboran slab beneath the Gibraltar arc region; the anomalies are under, and are aligned with, the continental margins at depths greater than 200 kilometres. Rayleigh wave analysis finds that the lithospheric mantle under the continental margins is significantly thinner than expected, and that thin lithosphere extends from the orogens adjacent to the subduction zones inland to the edges of nearby cratonic cores. Taking these data together, here we describe a process that can lead to the loss of continental lithosphere adjacent to a subduction zone. Subducting oceanic plates can viscously entrain and remove the bottom of the continental thermal boundary layer lithosphere from adjacent continental margins. This drives surface tectonics and pre-conditions the margins for further deformation by creating topography along the lithosphere-asthenosphere boundary. This can lead to development of secondary downwellings under the continental interior, probably under both South America and the Gibraltar arc, and to delamination of the entire lithospheric mantle, as around the Gibraltar arc. This process reconciles numerous, sometimes mutually exclusive, geodynamic models proposed to explain the complex oceanic-continental tectonics of these subduction zones.

Evidence of displacement-driven maturation along the San Cristobal Trough transform plate boundary

NASA Astrophysics Data System (ADS)

Neely, James S.; Furlong, Kevin P.

2018-03-01

The San Cristobal Trough (SCT), formed by the tearing of the Australia plate as it subducts under the Pacific plate near the Solomon Islands, provides an opportunity to study the transform boundary development process. Recent seismicity (2013-2016) along the 280 km long SCT, known as a Subduction-Transform Edge Propagator (STEP) fault, highlights the tearing process and ongoing development of the plate boundary. The region's earthquakes reveal two key characteristics. First, earthquakes at the western terminus of the SCT, which we interpret to indicate the Australia plate tearing, display disparate fault geometries. These events demonstrate that plate tearing is accommodated via multiple intersecting planes rather than a single through-going fault. Second, the SCT hosts sequences of Mw ∼7 strike-slip earthquakes that migrate westward through a rapid succession of events. Sequences in 1993 and 2015 both began along the eastern SCT and propagated west, but neither progression ruptured into or nucleated a large earthquake within the region near the tear. Utilizing b-value and Coulomb Failure Stress analyses, we examine these along-strike variations in the SCT's seismicity. b-Values are highest along the youngest, western end of the SCT and decrease with increasing distance from the tear. This trend may reflect increasing strain localization with increasing displacement. Coulomb Failure Stress analyses indicate that the stress conditions were conducive to continued western propagation of the 1993 and 2015 sequences suggesting that the unruptured western SCT may have fault geometries or properties that inhibit continued rupture. Our results indicate a displacement-driven fault maturation process. The multi-plane Australia plate tearing likely creates a western SCT with diffuse strain accommodated along a network of disorganized faults. After ∼90 km of cumulative displacement (∼900,000 yr of plate motion), strain localizes and faults align, allowing the SCT to host large earthquakes.

New Evidence for Quaternary Strain Partitioning Along the Queen Charlotte Fault System, Southeastern Alaska

NASA Astrophysics Data System (ADS)

Walton, M. A. L.; Miller, N. C.; Brothers, D. S.; Kluesner, J.; Haeussler, P. J.; Conrad, J. E.; Andrews, B. D.; Ten Brink, U. S.

2017-12-01

The Queen Charlotte Fault (QCF) is a fast-moving ( 53 mm/yr) transform plate boundary fault separating the Pacific Plate from the North American Plate along western Canada and southeastern Alaska. New high-resolution bathymetric data along the fault show that the QCF main trace accommodates nearly all strike-slip plate motion along a single narrow deformation zone, though questions remain about how and where smaller amounts of oblique convergence are accommodated along-strike. Obliquity and convergence rates are highest in the south, where the 2012 Haida Gwaii, British Columbia MW 7.8 thrust earthquake was likely caused by Pacific underthrusting. In the north, where obliquity is lower, aftershocks from the 2013 Craig, Alaska MW 7.5 strike-slip earthquake also indicate active convergent deformation on the Pacific (west) side of the plate boundary. Off-fault structures previously mapped in legacy crustal-scale seismic profiles may therefore be accommodating part of the lesser amounts of Quaternary convergence north of Haida Gwaii. Between 2015 and 2017, the USGS acquired more than 8,000 line-km of offshore high-resolution multichannel seismic (MCS) data along the QCF to better understand plate boundary deformation. The new MCS data show evidence for Quaternary deformation associated with a series of elongate ridges located within 30 km of the QCF main trace on the Pacific side. These ridges are anticlinal structures flanked by growth faults, with recent deformation and active fluid flow characterized by seafloor scarps and seabed gas seeps at ridge crests. Structural and morphological evidence for contractional deformation decreases northward along the fault, consistent with a decrease in Pacific-North America obliquity along the plate boundary. Preliminary interpretations suggest that plate boundary transpression may be partitioned into distinctive structural domains, in which convergent stress is accommodated by margin-parallel thrust faulting, folding, and ridge formation within the Pacific Plate, with strike-slip faulting localized to the primary trace of the QCF. Contractional structures may be occupying zones of pre-existing crustal weakness and/or re-activated fabrics in the oceanic crust, possibly explaining strain partitioning behavior in areas with a low convergence angle (<15°).

Fractures, not Plumes, Have Controlled Major Seamount Volcanism in the Pacific over 170 Million Years

NASA Astrophysics Data System (ADS)

Natland, J. H.; Winterer, E. L.

2003-12-01

The distribution of guyots and atolls and large volcanic islands on the Pacific plate can be used to outline the likely connection between stresses acting on the plate and the gradual development of large, linear volcanic chains over the past 170 Ma. We construe three general periods with different stress regimes in the history of the Pacific plate. 1) During the Jurassic and Early Cretaceous, the Pacific plate was surrounded by ridge segments and there were no major stress alignments within it. Within-plate volcanism thus assumed the scattered arrangement for the condition of no tectonic stress (1), and the large Magellan and Wake seamount clusters formed. Near the eastern boundaries of the plate, complex and shifting patterns of ridge reorganization dictated formation of very long, splayed, near-axis ridges such as Horizon Guyot and Necker Ridge. 2) At about 90 Ma, the growing middle-aged Pacific plate achieved its first persistent stress regime with the formation of subduction boundaries along its western or northwestern margin. The plate was no longer static but began to move over the asthenosphere and into the mantle. Subduction boundaries and the overall direction of subduction are uncertain, but this imparted a general yet not fully stable component of tension across the plate, producing the NNW Gilbert-Marshall, Line and Emperor Seamount ridges, generally orthogonal to the overall direction of least principal stress. The Line Island seamount chain, being near ridge axes, sustained a variable stress regime. It thus has no age progression of rocks dated between 70-90 Ma (2), great width, and a dual orientations of ridges. 3) By 47 Ma, nearly half of the boundaries of the Pacific plate now were trenches spanning from the Aleutians to New Zealand. In addition, northward migration of the Indian plate and Australia caught a major portion of the westerly moving Pacific plate between the northeast corner of the Tonga Trench and the Aleutians. The plate could not shift laterally in response to whatever was occurring along its eastern spreading boundaries. A very consistent and strong stress regime therefore developed across the Pacific plate with a NNE direction of least principal stress. The change in stress orientation may have taken up to 10 million years, during an interval marked by little or no volcanic productivity at the western end of the Hawaiian chain. Since that time, the predominant alignment of both linear island chains and Puka Puka-type ridges, from the Kodiak-Bowie chain in the Gulf of Alaska to the Louisville Ridge south of the Antarctic convergence, has been orthogonal to this direction. Development of large-volume persistent chains and shorter small-volume chains indicates patterns of differential stress in the plate, variable fertility and geochemistry of the asthenosphere and/or shallow convective overturn of the asthenosphere rather than the action of mantle plumes of different sizes and depths of origin. Tapping of enriched mantle by widespread volcano clusters during the Mesozoic suggests the presence of a shallow asthenospheric source layer rather than multiple narrow conduits. (1) Hieronymus, C.F., and Bercovici, D. 2000. Earth Planet. Sci. Lett. 181, 539-554. (2) Davis, A.S., Gray, L.B., Clague, D.A., and Hein, J.R., 2002 Geochem. Geophys. Geosyst. 3: 10.1029/2001GC0000190, 1-28.

Seismicity During Continental Breakup in the Red Sea Rift of Northern Afar

NASA Astrophysics Data System (ADS)

Illsley-Kemp, Finnigan; Keir, Derek; Bull, Jonathan M.; Gernon, Thomas M.; Ebinger, Cynthia; Ayele, Atalay; Hammond, James O. S.; Kendall, J.-Michael; Goitom, Berhe; Belachew, Manahloh

2018-03-01

Continental rifting is a fundamental component of plate tectonics. Recent studies have highlighted the importance of magmatic activity in accommodating extension during late-stage rifting, yet the mechanisms by which crustal thinning occurs are less clear. The Red Sea rift in Northern Afar presents an opportunity to study the final stages of continental rifting as these active processes are exposed subaerially. Between February 2011 and February 2013 two seismic networks were installed in Ethiopia and Eritrea. We locate 4,951 earthquakes, classify them by frequency content, and calculate 31 focal mechanisms. Results show that seismicity is focused at the rift axis and the western marginal graben. Rift axis seismicity accounts for ˜64% of the seismic moment release and exhibits a swarm-like behavior. In contrast, seismicity at the marginal graben is characterized by high-frequency earthquakes that occur at a constant rate. Results suggest that the rift axis remains the primary locus of seismicity. Low-frequency earthquakes, indicative of magmatic activity, highlight the presence of a magma complex ˜12 km beneath Alu-Dalafilla at the rift axis. Seismicity at the marginal graben predominantly occurs on westward dipping, antithetic faults. Focal mechanisms show that this seismicity is accommodating E-W extension. We suggest that the seismic activity at the marginal graben is either caused by upper crustal faulting accommodating enhanced crustal thinning beneath Northern Afar or as a result of flexural faulting between the rift and plateau. This seismicity is occurring in conjunction with magmatic extension at the rift axis, which accommodates the majority of long-term extension.

The Dauki Thrust Fault and the Shillong Anticline: An incipient plate boundary in NE India?

NASA Astrophysics Data System (ADS)

Ferguson, E. K.; Seeber, L.; Steckler, M. S.; Akhter, S. H.; Mondal, D.; Lenhart, A.

2012-12-01

The Shillong Massif is a regional contractional structure developing across the Assam sliver of the Indian plate near the Eastern Syntaxis between the Himalaya and Burma arcs. Faulting associated with the Shillong Massif is a major source of earthquake hazard. The massif is a composite basement-cored asymmetric anticline and is 100km wide, >350km long and 1.8km high. The high relief southern limb preserves a Cretaceous-Paleocene passive margin sequence despite extreme rainfall while the gentler northern limb is devoid of sedimentary cover. This asymmetry suggests southward growth of the structure. The Dauki fault along the south limb builds this relief. From the south-verging structure, we infer a regional deeply-rooted north-dipping blind thrust fault. It strikes E-W and obliquely intersects the NE-SW margin of India, thus displaying three segments: Western, within continental India; Central, along the former passive margin; and Eastern, overridden by the west-verging Burma accretion system. We present findings from recent geologic fieldwork on the western and central segments. The broadly warped erosional surface of the massif defines a single anticline in the central segment, east of the intersection with the hinge zone of the continental margin buried by the Ganges-Brahmaputra Delta. The south limb of the anticline forms a steep topographic front, but is even steeper structurally as defined by the Cretaceous-Eocene cover. Below it, Sylhet Trap Basalts intrude and cover Precambrian basement. Dikes, presumably parallel to the rifted margin, are also parallel to the front, suggesting thrust reactivation of rift-related faults. Less competent Neogene clastics are preserved only near the base of the mountain front. Drag folds in these rocks suggest north-vergence and a roof thrust above a blind thrust wedge floored by the Dauki thrust fault. West of the hinge zone, the contractional structure penetrates the Indian continent and bifurcates. After branching into the Dapsi Fault, the Dauki Fault continues westward as the erosion-deposition boundary combined with a belt of N-S shortening. The Dapsi thrust fault strikes WNW across the Shillong massif and dips NNE. It is mostly blind below a topographically expressed fold involving basement and passive-margin cover. Recent fieldwork has shown that the fault is better exposed in the west, where eventually Archean basement juxtaposes folded and steeply dipping fluvial sediment. Both Dauki and Dapsi faults probably continue beyond the Brahmaputra River, where extreme fluvial processes mask them. The area between the two faults is a gentle southward monocline with little or no shortening. Thus uplift of this area stems from slip on the Dauki thrust fault, not from pervasive shortening. The Burma foldbelt overrides the Shillong Plateau and is warped but continuous across the eastern segment of the Dauki fault. The Haflong-Naga thrust front north of the Dauki merges with the fold-thrust belt in the Sylhet basin to the south, despite >150km of differential advance due to much greater advance of the accretionary prism in the basin. Where the Dauki and Haflong-Naga thrusts cross, the thrust fronts are nearly parallel and opposite vergence. We trace a Dauki-related topographic front eastward across the Burma Range. This and other evidence suggest that the Dauki Fault continues below the foldbelt.

Chile's seismogenic coupling zones - geophysical and neotectonic observations from the South American subduction zone prior to the Maule 2010 earthquake

NASA Astrophysics Data System (ADS)

Oncken With Tipteq, Onno; Ipoc Research Groups

2010-05-01

Accumulation of deformation at convergent plate margins is recently identified to be highly discontinuous and transient in nature: silent slip events, non-volcanic tremors, afterslip, fault coupling and complex response patterns of the upper plate during a single event as well as across several seismic cycles have all been observed in various settings and combinations. Segments of convergent plate margins with high recurrence rates and at different stages of the rupture cycle like the Chilean margin offer an exceptional opportunity to study these features and their interaction resolving behaviour during the seismic cycle and over repeated cycles. A past (TIPTEQ) and an active international initiative (IPOC; Integrated Plate Boundary Observatory Chile) address these goals with research groups from IPG Paris, Seismological Survey of Chile, Free University Berlin, Potsdam University, Hamburg University, IFM-GEOMAR Kiel, and GFZ Potsdam employing an integrated plate boundary observatory and associated projects. We focus on the south Central Chilean convergent margin and the North Chilean margin as natural laboratories embracing the recent Maule 2010 megathrust event. Here, major recent seismic events have occurred (south Central Chile: 1960, Mw = 9.5; 2010, Mw = 8.8; North Chile: 1995, Mw = 8; 2001, Mw = 8.7; 2007, Mw: 7.8) or are expected in the very near future (Iquique, last ruptured 1877, Mw = 8.8) allowing observation at critical time windows of the seismic cycle. Seismic imaging and seismological data have allowed us to relocate major rupture hypocentres and to locate the geometry of the locked zone and the degree of locking in both areas. The reflection seismic data exhibit well defined changes of reflectivity and Vp/Vs ratio along the plate interface that can be correlated with different parts of the coupling zone as well as with changes during the seismic cycle. Observations suggest an important role of the hydraulic system, an inference that is strongly supported from recent findings along the exhumed, fossil seismogenic coupling zone of the European Alps. The data provide additional evidence that the degree of interseismic locking is closely mirrored by subsequent megathrust failure as evidenced by the slip and aftershock pattern of the Maule 2010 earthquake. Neogene surface deformation in Chile has been complex exhibiting tectonically uplifting areas along the coast driven by interseismically active reverse faulting. In addition, we observe coseismically subsiding domains along other parts of the coast. Moreover, the coseismic and interseismic vertical displacement identified is not coincident with long-term vertical motion that probably is superseded by slow basal underplating or tectonic erosion occurring at the downdip parts of the seismogenic zone causing discontinuous uplift. Analogue and numerical modelling lend additional support to the kinematic patterns linking slip at the seismogenic coupling zone and upper plate response. Finally we note that the characteristic peninsulas along the South American margin constitute stable rupture boundaries/barriers and appear to have done so for a protracted time as evidenced by their long-term uplift history since at least the Late Pliocene that points to anomalous properties of the plate interface affecting the mode of strain accumulation and plate interface rupture.

The 2008 Chiltan Earthquake Sequence: Implications for strain partitioning along the western Indian plate margin

NASA Astrophysics Data System (ADS)

Szeliga, W. M.; Mohammad Kakar, D.; Bilham, R.; Molnar, P.

2009-12-01

In October 2008 two Mw 6.4 earthquakes preceded a two-month-long aftershock sequence in the Chiltan region of N. Baluchistan 50 km northeast of Quetta. InSAR data combined with teleseimic body wave modeling and campaign GPS data indicate that the two mainshocks occurred at 10-13 km depth in a fold-and-thrust belt on segments of a northwest trending dextral fault whose NW extension would intersect the Chaman fault close to a 20 degree restraining bend on the fault. Although no surface rupture occurred and the trend of the subsurface fault is oblique to the surface fold axes, a line of diffuse deeper seismicity (20-40 km) can be discerned in the underlying Indian plate that approximately follows the outline of the Sulaiman Lobe. We surmise that a 300 km x 200 km fragment of the NW Indian plate, corresponding roughly to the area and location of the Katawaz basin has fragmented, and according to GPS velocities may be moving at 15-20 mm/yr to the SE towards the Indian plate. The SE edge of this zone appears to terminate some 100 km north of the SE foothills of the Sulaiman Lobe. We assume its NW edge is bounded by the Chaman fault. We infer that the motion of the Katawaz block is caused by slip partitioning of the southward 31 mm/yr motion of Asia towards India into pure sinistral slip of between 25--29 mm/year on the Chaman fault north of Chaman and 10--20 mm/yr of SE convergence of the Katawaz block

The Fairway-Aotea Basin and the New Caledonia Trough, witnesses of the Pacific-Australian plate boundary evolution : from mid-Cretaceous cessation of subduction to Eocene subduction renewal

NASA Astrophysics Data System (ADS)

Collot, J.; Geli, L. B.; Lafoy, Y.; Sutherland, R.; Herzer, R. H.; Roest, W. R.

2009-12-01

The geodynamical history of the SW Pacific is controlled since the Mesozoic by the evolution of peri-Pacific subduction zones, in a trench retreat by slab roll-back process, which successively occurred along the Eastern Gondwana margin. In this context, most basins which formed after 45 Ma reached a stage of seafloor spreading, have recorded the inversions of the earth's magnetic field and present typical oceanic crust morphologies. By contrast, the New Caledonia and Fairway basins, which are narrower and present thick sedimentary covers have a less known and more controversial origin. Based on a regional geological synthesis and on interpretation of multichannel seismic reflection and refraction data, combined with drill hole data off New Zealand and a compilation of regional potential data, we distinguish 2 phases of the evolution of the Fairway-Aotea Basin (FAB) and the New Caledonia Trough (NCT), which reflect the evolution of the Gondwana-Pacific plate boundary: Phase 1: Mid Cretaceous formation of the FAB in a continental intra- or back- arc position of the Pacific-Gondwana subduction system. The formation of this shallow basin reflects the onset of continental breakup of the Eastern Gondwana margin during Cenomanian which was most probably caused by a dynamic change of the subduction zone through a « verticalization » of the slab. This event may be the result of the 99 Ma kinematic plate reorganization which probably led to subduction cessation along the Gondwana-Pacific plate boundary. A tectonic escape mechanism, in relation with the locking of the subduction zone by the Hikurangi Plateau, could also be responsible of the trench retreat leading to backarc extension. Phase 2: Regional Eocene-Oligocene uplift followed by rapid subsidence (3-4 km) of the system « Lord Howe Rise - FAB - Norfolk Ridge ». The structural style of this deformation leads us to suggest that detachment of the lower crust is the cause of subsidence. We therefore propose a model in which the system, initially shallow during Cretaceous (phase 1), would have greatly subsided during Eocene-Oligocene, giving birth to the NCT, as the renewal of the Australia-Pacific convergent plate boundary took place. This renewal of convergence at 45 Ma would have driven the lithosphere of the system to thicken (uplift), leading to a root instability and to its detachment in the mantle (subsidence). Superposed on these two main phases, some local effects, controlled by the geometry of the plate boundary, also appear. Particularly, latest late Eocene local deformation of the Northern NCB is documented, synchronously with the New Caledonian obduction. This asymmetrical deformation which lasted less than a few million years led to the uplift of the Fairway Ridge and the subsidence of the Eastern margin of the basin along NC’s western coast (10 km vertical amplitude). We suggest that as the oceanic crust of the South Loyalty Basin was being obducted onto the Norfolk Ridge at 37 Ma, the NCB subsided under the effect of the overloading and underthrusted to accommodate the compressional deformation as a foreland flexural basin.

Pleistocene vertical motions of the Costa Rican outer forearc from subducting topography and a migrating fracture zone triple junction

USGS Publications Warehouse

Edwards, Joel H.; Kluesner, Jared W.; Silver, Eli A.; Bangs, Nathan L.

2018-01-01

Understanding the links between subducting slabs and upper-plate deformation is a longstanding goal in the field of tectonics. New 3D seismic sequence stratigraphy, mapped within the Costa Rica Seismogenesis Project (CRISP) seismic-reflection volume offshore southern Costa Rica, spatiotemporally constrains several Pleistocene outer forearc processes and provides clearer connections to subducting plate dynamics. Three significant shelf and/or slope erosional events at ca. 2.5–2.3 Ma, 1.95–1.78 Ma, and 1.78–1.19 Ma, each with notable differences in spatial extent, volume removed, and subsequent margin response, caused abrupt shifts in sedimentation patterns and rates. These shifts, coupled with observed deformation, suggest three primary mechanisms for Pleistocene shelf and slope vertical motions: (1) regional subaerial erosion and rapid subsidence linked to the southeastward Panama Fracture Zone triple-junction migration, with associated abrupt bathymetric variations and plate kinematic changes; (2) transient, kilometer-scale uplift and subsidence due to inferred subducting plate topography; and (3) progressive outer wedge shortening accommodated by landward- and seaward-dipping thrust faults and fold development due to the impinging Cocos Ridge. Furthermore, we find that the present-day wedge geometry (to within ∼3 km along strike) has been maintained through the Pleistocene, in contrast to modeled landward margin retreat. We also observe that deformation, i.e., extension and shortening, is decoupled from net margin subsidence. Our findings do not require basal erosion, and they suggest that the vertical motions of the Costa Rican outer forearc are not the result of a particular continuous process, but rather are a summation of plate to plate changes (e.g., passage of a fracture zone triple junction) and episodic events (e.g., subducting plate topography).

Kinematic evidence for the effect of changing plate boundary conditions on the tectonics of the northern U.S. Rockies

NASA Astrophysics Data System (ADS)

Schmeelk, Dylan; Bendick, Rebecca; Stickney, Michael; Bomberger, Cody

2017-06-01

We derive surface velocities from GPS sites in the interior Northwest U.S. relative to a fixed North American reference frame to investigate surface tectonic kinematics from the Snake River Plain (SRP) to the Canadian border. The Centennial Tectonic Belt (CTB) on the northern margin of the SRP exhibits west directed extensional velocity gradients and strain distributions similar to the main Basin and Range Province (BRP) suggesting that the CTB is part of the BRP. North of the CTB, however, the vergence of velocities relative to North America switches from westward to eastward along with a concomitant rotation of the principal stress axes based on available seismic focal mechanisms, revealing paired extension in the northern Rockies and shortening across the Rocky Mountain Front. This change in orientation of surface velocities suggests that the change in the boundary conditions on the western margin of North America influences the direction of gravitational collapse of Laramide thickened crust. Throughout the study region, fault slip rate estimates calculated from the new geodetic velocity field are consistently larger than previously reported fault slip rates determined from limited geomorphic and paleoseismic studies.

Permian-Carboniferous arc magmatism in southern Mexico: U-Pb dating, trace element and Hf isotopic evidence on zircons of earliest subduction beneath the western margin of Gondwana

NASA Astrophysics Data System (ADS)

Ortega-Obregón, C.; Solari, L.; Gómez-Tuena, A.; Elías-Herrera, M.; Ortega-Gutiérrez, F.; Macías-Romo, C.

2014-07-01

Undeformed felsic to mafic igneous rocks, dated by U-Pb zircon geochronology between 311 and 255 Ma, intrude different units of the Oaxacan and Acatlán metamorphic complexes in southwestern Mexico. Rare earth element concentrations on zircons from most of these magmatic rocks have a typical igneous character, with fractionated heavy rare earths and negative Eu anomalies. Only inherited Precambrian zircons are depleted in heavy rare earth elements, which suggest contemporaneous crystallization in equilibrium with metamorphic garnet during granulite facies metamorphism. Hf isotopic signatures are, however, different among these magmatic units. For example, zircons from two of these magmatic units (Cuanana pluton and Honduras batholith) have positive ɛHf values (+3.8-+8.5) and depleted mantle model ages (using a mean crustal value of 176Lu/177Hf = 0.015) ( T DMC) ranging between 756 and 1,057 Ma, whereas zircons from the rest of the magmatic units (Etla granite, Zaniza batholith, Carbonera stock and Sosola rhyolite) have negative ɛHf values (-1 to -14) and model ages between 1,330 and 2,160 Ma. This suggests either recycling of different crustal sources or, more likely, different extents of crustal contamination of arc-related mafic magmas in which the Oaxacan Complex acted as the main contaminant. These plutons thus represent the magmatic expression of the initial stages of eastward subduction of the Pacific plate beneath the western margin of Gondwana, and confirm the existence of a Late Carboniferous-Permian magmatic arc that extended from southern North America to Central America.

Geological history of the west Libyan offshore and adjoining regions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Benniran, M.M.; Taleb, T.M.; McCrossan, R.G.

1988-08-01

The continental margin of the African plate north of Libya is separated from the Saharan platform to the south by a major Variscan fault system running along the coastline. The structural evolution of three sedimentary basins within the margin is discussed. The Jeffara basin, onshore western Libya-southern Tunisia, formed as a right-lateral pull-part late in the Variscan event. When the strike-slip motion ceased in the Late Permian, the basin continued to subside thermally. The Sabratah (Tripolitanian) basin, offshore western Libya-southern Tunisia, and the Benghazi basin in the Sirte rise were both formed as left-lateral pull-aparts in the Late Triassic-Early Jurassic.more » From the Middle Jurassic to the present they have subsided thermally. Onshore the lower Mesozoic is characterized by continental and nearshore clastics, separated by an evaporite sequence of Late Triassic-Early Jurassic age. Offshore this sequence is thought to grade northward into open marine carbonates. Uplift along the edge of the Saharan platform during the Early Cretaceous sourced coarse clastics, which grade northward into a thick sequence of shallow-water carbonates. Throughout the Late Cretaceous and early Tertiary, high-energy carbonates were deposited around the flanks of the Sabratah basin, grading into deeper-water, fine-grained clastics and carbonates toward the center of the basin. The late Tertiary succession is dominated by clastics derived from the growing Tellian Atlas to the northwest. During the Mesozoic and Tertiary a thick sequence of carbonates was deposited on the Pelagian platform to the north of the Sabratah basin. Periodically the platform was exposed subaerially.« less

Crustal structure and tectonic deformation of the southern Ecuadorian margin

NASA Astrophysics Data System (ADS)

Calahorrano, Alcinoe; Collot, Jean-Yves; Sage, Françoise; Ranero, César R.

2010-05-01

Multichannel seismic lines acquired during the SISTEUR cruise (2000) provide new constraints on the structure and deformation of the subduction zone at the southern Ecuadorian margin, from the deformation front to the continental shelf of the Gulf of Guayaquil. The pre-stack depth migrated images allows to characterise the main structures of the downgoing and overriding plates and to map the margin stratigraphy in order to propose a chronology of the deformation, by means of integrating commercial well data and industry seismic lines located in the gulf area. The 100-km-long seismic lines show the oceanic Nazca plate underthrusting the South American plate, as well as the subduction channel and inter-plate contact from the deformation front to about 90 km landward and ~20 km depth. Based on seismic structure we identify four upper-plate units, consisting of basement and overlaying sedimentary sequences A, B and C. The sedimentary cover varies along the margin, being few hundreds of meters thick in the lower and middle slope, and ~2-3 km thick in the upper slope. Exceptionally, a ~10-km -thick basin, here named Banco Peru basin, is located on the upper slope at the southernmost part of the gulf. This basin seems to be the first evidence of the Gulf of Guayaquil opening resulting from the NE escaping of the North Andean Block. Below the continental shelf, thick sedimentary basins of ~6 to 8 km occupy most of the gulf area. Tectonic deformation across most of the upper-plate is dominated by extensional regime, locally disturbed by diapirism. Compression evidences are restricted to the deformation front and surrounding areas. Well data calibrating the seismic profiles indicate that an important portion of the total thickness of the sedimentary coverage of the overriding plate are Miocene or older. The data indicate the extensional deformation resulting from the NE motion of the North Andean Block and the opening of the Gulf of Guayaquil, evolves progressively in age from the southern edge of the gulf near Banco Peru, where main subsidence seems to be Miocene or older, toward the northern limit, where high subsidence rates are early Pleistocene.

Lower plate serpentinite diapirism in the Calabrian Arc subduction complex.

PubMed

Polonia, A; Torelli, L; Gasperini, L; Cocchi, L; Muccini, F; Bonatti, E; Hensen, C; Schmidt, M; Romano, S; Artoni, A; Carlini, M

2017-12-19

Mantle-derived serpentinites have been detected at magma-poor rifted margins and above subduction zones, where they are usually produced by fluids released from the slab to the mantle wedge. Here we show evidence of a new class of serpentinite diapirs within the external subduction system of the Calabrian Arc, derived directly from the lower plate. Mantle serpentinites rise through lithospheric faults caused by incipient rifting and the collapse of the accretionary wedge. Mantle-derived diapirism is not linked directly to subduction processes. The serpentinites, formed probably during Mesozoic Tethyan rifting, were carried below the subduction system by plate convergence; lithospheric faults driving margin segmentation act as windows through which inherited serpentinites rise to the sub-seafloor. The discovery of deep-seated seismogenic features coupled with inherited lower plate serpentinite diapirs, provides constraints on mechanisms exposing altered products of mantle peridotite at the seafloor long time after their formation.

Right-lateral shear across Iran and kinematic change in the Arabia-Eurasia collision zone

NASA Astrophysics Data System (ADS)

Allen, M. B.; Kheirkhah, M.; Emami, M.

2009-04-01

New offset determinations for right-lateral strike-slip faults in Iran redefine the kinematics of the Arabia-Eurasia collision. A series of right-lateral strike-slip faults is present across Iran between 48° and 57° E. Fault strikes vary between NW-SE and NNW-SSE. Individual faults west of ~53° E were active in the late Tertiary, but have limited evidence of activity. Faults east of ~53° E are seismically active and/or have geomorphic evidence for Holocene slip. None of the faults affects the GPS-derived regional velocity field, indicating active slip rates are ≤2 mm/yr. We estimate overall slip on these faults from offset geological and geomorphic markers, based on observations from satellite imagery, digital topography, geology maps and our own fieldwork observations, and combine these results with published estimates for fault slip in the east of the study area. Total offset of the Takab, Soltanieh, Indes, Bid Hand, Qom, Kashan, Deh Shir, Anar, Daviran, Kuh Banan and Dehu faults is at least 270 km and possibly higher. Other faults (e.g. Rafsanjan) have unknown amounts of right-lateral slip. Collectively, these faults are inferred to have accommodated part of the Arabia-Eurasia convergence by two mechanisms: (1) anti-clockwise, vertical axis rotations; (2) strain partitioning with coeval NE-SW crustal thickening in the Turkish-Iranian plateau to produce ~350 km of north-south plate convergence. The strike-slip faulting across Iran requires along-strike lengthening of the deformation zone. This was possible until the Pliocene, when the Afghan crust collided with the western margin of the Indian plate, thereby sealing off a free face at the eastern side of the Arabia-Eurasia collision zone. Continuing Arabia-Eurasia plate convergence had to be accommodated in new ways and new areas, leading to the present pattern of faulting from eastern Iran to western Turkey.

Late Paleozoic orogeny in Alaska's Farewell terrane

USGS Publications Warehouse

Bradley, D.C.; Dumoulin, Julie A.; Layer, P.; Sunderlin, D.; Roeske, S.; McClelland, B.; Harris, A.G.; Abbott, G.; Bundtzen, T.; Kusky, T.

2003-01-01

Evidence is presented for a previously unrecognized late Paleozoic orogeny in two parts of Alaska's Farewell terrane, an event that has not entered into published scenarios for the assembly of Alaska. The Farewell terrane was long regarded as a piece of the early Paleozoic passive margin of western Canada, but is now thought, instead, to have lain between the Siberian and Laurentian (North American) cratons during the early Paleozoic. Evidence for a late Paleozoic orogeny comes from two belts located 100-200 km apart. In the northern belt, metamorphic rocks dated at 284-285 Ma (three 40Ar/39Ar white-mica plateau ages) provide the main evidence for orogeny. The metamorphic rocks are interpreted as part of the hinterland of a late Paleozoic mountain belt, which we name the Browns Fork orogen. In the southern belt, thick accumulations of Pennsylvanian-Permian conglomerate and sandstone provide the main evidence for orogeny. These strata are interpreted as the eroded and deformed remnants of a late Paleozoic foreland basin, which we name the Dall Basin. We suggest that the Browns Fork orogen and Dall Basin comprise a matched pair formed during collision between the Farewell terrane and rocks to the west. The colliding object is largely buried beneath Late Cretaceous flysch to the west of the Farewell terrane, but may have included parts of the so-called Innoko terrane. The late Paleozoic convergent plate boundary represented by the Browns Fork orogen likely connected with other zones of plate convergence now located in Russia, elsewhere in Alaska, and in western Canada. Published by Elsevier B.V.

Lithospheric structure of Iberia and Morocco using finite-frequency Rayleigh wave tomography from earthquakes and seismic ambient noise

NASA Astrophysics Data System (ADS)

Palomeras, I.; Villaseñor, A.; Thurner, S.; Levander, A.; Gallart, J.; Harnafi, M.

2017-05-01

We present a new 3-D shear velocity model of the western Mediterranean from the Pyrenees, Spain, to the Atlas Mountains, Morocco, and the estimated crustal and lithospheric thickness. The velocity model shows different crustal and lithospheric velocities for the Variscan provinces, those which have been affected by Alpine deformation, and those which are actively deforming. The Iberian Massif has detectable differences in crustal thickness that can be related to the evolution of the Variscan orogen in Iberia. Areas affected by Alpine deformation have generally lower velocities in the upper and lower crust than the Iberian Massif. Beneath the Gibraltar Strait and surrounding areas, the crustal thickness is greater than 50 km, below which a high-velocity anomaly (>4.5 km/s) is mapped to depths greater than 200 km. We identify this as a subducted remnant of the NeoTethys plate referred to as the Alboran and western Mediterranean slab. Beneath the adjacent Betic and Rif Mountains, the Alboran slab is still attached to the base of the crust, depressing it, and ultimately delaminating the lower crust and mantle lithosphere as the slab sinks. Under the adjacent continents, the Alboran slab is surrounded by low upper mantle shear wave velocities (Vs < 4.3) that we interpret as asthenosphere that has replaced the continental margin lithosphere which was viscously removed by Alboran plate subduction. The southernmost part of the model features an anomalously thin lithosphere beneath the Atlas Mountains that could be related to lateral flow induced by the Alboran slab.

Interpretation of shallow crustal structure of the Imperial Valley, California, from seismic reflection profiles

DOE Office of Scientific and Technical Information (OSTI.GOV)

Severson, L.K.

1987-05-01

Eight seismic reflection profiles (285 km total length) from the Imperial Valley, California, were provided to CALCRUST for reprocessing and interpretation. Two profiles were located along the western margin of the valley, five profiles were situated along the eastern margin and one traversed the deepest portion of the basin. These data reveal that the central basin contains a wedge of highly faulted sediments that thins to the east. Most of the faulting is strike-slip but there is evidence for block rotations on the scale of 5 to 10 kilometers within the Brawley Seismic Zone. These lines provide insight into themore » nature of the east and west edges of the Imperial Valley. The basement at the northwestern margin of the valley, to the north of the Superstition Hills, has been normal-faulted and blocks of basement material have ''calved'' into the trough. A blanket of sediments has been deposited on this margin. To the south of the Superstition Hills and Superstition Mountain, the top of the basement is a detachment surface that dips gently into the basin. This margin is also covered by a thick sequence sediments. The basement of the eastern margin consists of metamorphic rocks of the upper plate of the Chocolate Mountain Thrust system underlain by the Orocopia Schist. These rocks dip to the southeast and extend westward to the Sand Hills Fault but do not appear to cross it. Thus, the Sand Hills Fault is interpreted to be the southern extension of the San Andreas Fault. North of the Sand Hills Fault the East Highline Canal seismicity lineament is associated with a strike-slip fault and is probably linked to the Sand Hills Fault. Six geothermal areas crossed by these lines, in agreement with previous studies of geothermal reservoirs, are associated with ''faded'' zones, Bouguer gravity and heat flow maxima, and with higher seismic velocities than surrounding terranes.« less

Mesozoic architecture of a tract of the European-Iberian continental margin: Insights from preserved submarine palaeotopography in the Longobucco Basin (Calabria, Southern Italy)

NASA Astrophysics Data System (ADS)

Santantonio, Massimo; Fabbi, Simone; Aldega, Luca

2016-01-01

The sedimentary successions exposed in northeast Calabria document the Jurassic-Early Cretaceous tectonic-sedimentary evolution of a former segment of the European-Iberian continental margin. They are juxtaposed today to units representing the deformation of the African and Adriatic plates margins as a product of Apenninic crustal shortening. A complex pattern of unconformities reveals a multi-stage tectonic evolution during the Early Jurassic, which affected the facies and geometries of siliciclastic and carbonate successions deposited in syn- and post-rift environments ranging from fluvial to deep marine. Late Sinemurian/Early Pliensbachian normal faulting resulted in exposure of the Hercynian basement at the sea-floor, which was onlapped by marine basin-fill units. Shallow-water carbonate aprons and reefs developed in response to the production of new accommodation space, fringing the newborn islands which represent structural highs made of Paleozoic crystalline and metamorphic rock. Their drowning and fragmentation in the Toarcian led to the development of thin caps of Rosso Ammonitico facies. Coeval to these deposits, a thick (> 1 km) hemipelagic/siliciclastic succession was sedimented in neighboring hanging wall basins, which would ultimately merge with the structural high successions. Footwall blocks of the Early Jurassic rift, made of Paleozoic basement and basin-margin border faults with their onlapping basin-fill formations, are found today at the hanging wall of Miocene thrusts, overlying younger (Middle/Late Jurassic to Late Paleogene) folded basinal sediments. This paper makes use of selected case examples to describe the richly diverse set of features, ranging from paleontology to sedimentology, to structural geology, which are associated with the field identification of basin-margin unconformities. Our data provide key constraints for restoring the pre-orogenic architecture of a continental margin facing a branch of the Liguria-Piedmont ocean in the Western Tethys, and for estimating displacements and slip rates along synsedimentary faults.

New records of marginal locations for American pika (Ochotona princeps) in the Western Great Basin

Treesearch

Constance I. Millar; Robert D. Westfall; Diane L. Delany

2013-01-01

We describe 46 new site records documenting occupancy by American pika (Ochotona princeps) at 21 locations from 8 mountain regions in the western Great Basin, California, and Nevada. These locations comprise a subset of sites selected from regional surveys to represent marginal, isolated, or otherwise atypical pika locations, and to provide...

New insights into Late Quaternary slip rate of the thrust fault zone, northern margin of the Qilian Shan, NE Tibet

NASA Astrophysics Data System (ADS)

Hai-bo, Y.; Yang, X., Sr.; LI, A.; Huang, X.; Huang, W.

2017-12-01

The India-Eurasian plate collision caused widespread Cenozoic crustal deformation within the Tibetan Plateau and on its margins. Ongoing post-collisional convergence formed multi-row NWW-trending folded mountain ranges and basins pattern in the northeastern Tibet. Late Quaternary tectonic deformation and quantitative slip rate estimates around the Qilian Shan and the Hexi corridor foreland basin are critical to understanding crustal deformation process of the Tibetan plateau and assessing regional seismic hazards. The Fodongmo-Hongyazi fault (FHF) is a major thrust at the Northeastern Tibet, bounding the Qilian Shan. It is accommodating the crustal shortening across this region and has produced strong historical earthquake. Until now the slip rate has been poorly constrained limiting our understanding of its role in the accommodation of deformation across this region. In this work, faulted terraces at the Hongshuiba River and Fengle River sites on the western and middle segments of the FHF were mapped with satellite imagery and field observations. Chronological constraints are placed on the ages of displaced river terraces at these sites using terrestrial cosmogenic nuclide (TCN) exposure dating. These ages combined with offsets measured from SPOT 6 DEM's yield average vertical slip rates of 1.3±0.1mm/yr for the western segment since 207 ka and 0.9±0.1 mm/yr since 46 ka for the middle segment. These data suggest that the FHF accommodates 15-20% of the total shortening across the Qilian Shan (5.5-7 mm/yr). In addition, comparisons of our data with published slip rates along the Northern Qilian Thrust Fault Zone show that the fastest tectonic uplift occurs along the western portion of the Northern Qilian Shan. This is consistent with estimates deduced from geomorphology. The western portion of the Qilian Shan is mainly controlled by compressional deformation produced by the northward movement of the Northeastern Tibetan Plateau, while the eastern Qilian Shan is mainly controlled by the eastward extrusion of material along the left-lateral Haiyuan strike-slip Fault.

Paleogeographic changes across the Pennsylvanian-Permian boundary within the Mid-Continent (USA) inferred from detrital zircon geochronology of continental deposits

NASA Astrophysics Data System (ADS)

Soreghan, M. J.; Soreghan, G. S.

2017-12-01

The Permo-Pennsylvanian was characterized by intense orogenesis associated with Pangaean assembly, and profound climate shifts as earth transitioned from full icehouse conditions in the Pennsylvanian-early Permian to collapse into greenhouse conditions by latest Permian time. The modern U.S. Midcontinent was part of equatorial western Pangaea (North America) sandwiched between a continental-scale orogenic zone to the east and south (Appalachian-Ouachita-Marathon orogenic belt) and a series of basement-cored, intra-plate uplifts along western Pangaea (Ancestral Rocky Mountains). Here, we present a compilation of detrital zircon geochronology data from the Permo-Pennsylvanian of the Midcontinent as well as coeval strata of the east and west to explore sediment dispersal and potential tectonic and climatic influences on these provenance signatures. Zircon provenance data come from mostly eolian and fluvial silt- and sandstones of Early Pennsylvanian through Mid Permian age, although some data include marine sandstones. Our new data were acquired by LA-ICPMS at the University of Arizona Laserchron, and predominant age groups include >2500 Ma (Archean), 1600-1800 Ma (Yavapai-Matzatzal), 1300-900 Ma (Grenville), 790-570 Ma (Neoproterozoic), and 480-360 (E-M Paleozoic). However, the relative distributions of these populations exhibit distinctive temporal differences, especially across the Pennsylvanian-Permian boundary, but also spatially in comparison to published data from the Appalachian-Ouachita-Marathon basin, Ancestral Rocky Mountain basins, and the western Pangaean margin. Although the Central Pangaean Mountains, and in particular the Grenville-age basement rocks, were a dominant source of sediment to the Midcontinent, the data suggest an abrupt introduction of Neoproterozoic zircons in the early Permian. This signature also appears within the Ancestral Rocky Mountains region, but is rare along the western margin and the Appalachian basin in the early Permian. This suggests that dispersal remained segmented in the early Permian and not dominated by a simple east-west integrated paleodisperal system. Temporal change in paleoclimatic conditions across the boundary also likely complicated these provenance signatures through the Permian.

Air and shipborne magnetic surveys of the Antarctic into the 21st century

NASA Astrophysics Data System (ADS)

Golynsky, A.; Bell, R.; Blankenship, D.; Damaske, D.; Ferraccioli, F.; Finn, C.; Golynsky, D.; Ivanov, S.; Jokat, W.; Masolov, V.; Riedel, S.; von Frese, R.; Young, D.

2013-02-01

The Antarctic geomagnetics' community remains very active in crustal anomaly mapping. More than 1.5 million line-km of new air- and shipborne data have been acquired over the past decade by the international community in Antarctica. These new data together with surveys that previously were not in the public domain significantly upgrade the ADMAP compilation. Aeromagnetic flights over East Antarctica have been concentrated in the Transantarctic Mountains, the Prince Charles Mountains - Lambert Glacier area, and western Dronning Maud Land (DML) — Coats Land. Additionally, surveys were conducted over Lake Vostok and the western part of Marie Byrd Land by the US Support Office for Aerogeophysical Research projects and over the Amundsen Sea Embayment during the austral summer of 2004/2005 by a collaborative US/UK aerogeophysical campaign. New aeromagnetic data over the Gamburtsev Subglacial Mountains (120,000 line-km), acquired within the IPY Antarctica's Gamburtsev Province project reveal fundamental geologic features beneath the East Antarctic Ice sheet critical to understanding Precambrian continental growth processes. Roughly 100,000 line-km of magnetic data obtained within the International Collaboration for Exploration of the Cryosphere through Aerogeophysical Profiling promises to shed light on subglacial lithology and identify crustal boundaries for the central Antarctic Plate. Since the 1996/97 season, the Alfred Wegener Institute has collected 90,000 km of aeromagnetic data along a 1200 km long segment of the East Antarctic coast over western DML. Recent cruises by Australian, German, Japanese, Russian, British, and American researchers have contributed to long-standing studies of the Antarctic continental margin. Along the continental margin of East Antarctica west of Maud Rise to the George V Coast of Victoria Land, the Russian Polar Marine Geological Research Expedition and Geoscience Australia obtained 80,000 and 20,000 line-km, respectively, of integrated seismic, gravity and magnetic data. Additionally, US expeditions collected 128,000 line-km of shipborne magnetic data in the Ross Sea sector.

Tectonic stratification and seismicity of the accretionary prism of the Azerbaijani part of Greater Caucasus

NASA Astrophysics Data System (ADS)

Alizade, Akif; Kangarli, Talat; Aliyev, Fuad

2013-04-01

The Greater Caucasus has formed during last stage of the tectogenesis in a geodynamic condition of the lateral compression, peculiar to the zone pseudo-subduction interaction zone between Northern and Southern Caucasian continental microplates. Its present day structure formed as a result of horizontal movements of the different phases and sub-phases of Alpine tectogenesis (from late Cimmerian to Valakhian), and is generally regarded as zone where, along Zangi deformation, the insular arc formations of the Northern edge of South Caucasian microplate thrust under the Meso-Cenozoic substantial complex contained in the facials of marginal sea of Greater Caucasus. The last, in its turn, has been pushed beneath the North-Caucasus continental margin of the Scythian plate along Main Caucasus Thrust fault. Data collected from the territory of Azerbaijan and its' sector of the Caspian area stands for pseudo-subduction interaction of microplates which resulted in the tectonic stratification of the continental slope of Alpine formations, marginal sea and insular arc into different scale plates of south vergent combined into napping complexes. In the orogeny's present structure, tectonically stratified Alpine substantial complex of the marginal sea of Greater Caucasus bordered by Main Caucasus and Zangi thrusts, is represented by allochthonous south vergent accretionary prism in the front of first deformation with its' root buried under the southern border of Scythian plate. Allocated beneath mentioned prism, the autochthonous bedding is presented by Meso-Cenosoic complex of the Northern flank of the South-Caucasian miroplate, which is in its' turn crushed and lensed into southward shifted tectonic microplates gently overlapping the northern flank of Kura flexure along Ganykh-Ayrichay-Alyat thrust. Data of real-time GPS measurement of regional geodynamics indicates that pseudo-subduction of South Caucasian microplate under the North Caucasian microplate still continues during present stage of alpine tectogenesis. Among others, ongoing pseudo-subduction is indicated by data of regional seismicity which is irregularly distributed by depth (foci levels 2-6; 8-12; 17-22; 25-45 km). Horizontal and vertical seismic zoning is explained by Earth crust's block divisibility and tectonic stratification, within the structure of which the earthquake focuses are mainly confined to the crossing nodes of differently oriented ruptures, or to the planes of deep tectonic disruptions and lateral displacements along unstable contacts of the substantial complexes with various degree of competence. At present stage of tectogenesis, seismically most active are the structures of the northern flank of South Caucasian microplate, controlled by Ganyx-Ayrichay-Alyat deep thrust with "General Caucasus" spread in the west, and sub-meridian right-lateral strike slip zone of the Western Caspian fault in the east of Azerbaijani part of Greater Caucasus.

A comparison of Holocene fluctuations of the eastern and western margins of the Greenland Ice Sheet

NASA Astrophysics Data System (ADS)

Levy, L.; Kelly, M. A.; Lowell, T. V.; Hall, B. L.; Applegate, P. J.; Howley, J.; Axford, Y.

2013-12-01

Determining how the Greenland Ice Sheet (GrIS) responded to past temperature fluctuations is important for assessing its future stability in a changing climate. We present a record of the Holocene extents of the western GrIS margin near Kangerlussuaq (67.0°N, 50.7°W) and compare this with the past fluctuations of Bregne ice cap (71°N, 25.6° W), a small ice cap in the Scoresby Sund region 90 km from the eastern GrIS margin, to examine the mechanisms that influenced past ice margin fluctuations. The past extents of the Bregne ice cap are a proxy for the climatic conditions that influenced the nearby GrIS margin. We used glacial geomorphic mapping, 10Be dating of boulders and bedrock, and sediment cores from proglacial and non-glacial lakes. In western Greenland, 10Be ages on the Keglen moraines, 13 km west of the current GrIS margin and the Ørkendalen moraines, ≤2 km west of the current ice margin date to 7.3 × 0.1 ka (n=6) and 6.8 × 0.3 ka (n=9), respectively. Fresh moraines, ≤50 m from the current ice margin date to AD 1830-1950 and are likely associated with advances during the Little Ice Age (LIA). In some areas, the LIA moraines lie stratigraphically above the Ørkendalen moraines, indicating the GrIS was inboard of the Ørkendalen limit from 6.8 ka to the 20th century. In eastern Greenland, 10Be ages show that Bregne ice cap retreated within its late Holocene limit by 10.7 ka. A lack of clastic sediment in a proglacial lake suggests the ice cap was smaller or completely absent from ~10-2.6 ka. A snowline analysis indicates that temperatures ~0.5°C warmer than present would render the entire ice cap into an ablation zone. Glacial silts in the proglacial lake at ~2.6 and ~1.9 cal kyr BP to present indicate advances of Bregne ice cap. Fresh moraines ≤200 m of Bregne ice cap were deposited ≤2.6 cal kyr BP and mark the largest advance of the Holocene. Both the western GrIS margin and Bregne ice cap were influenced by Northern Hemisphere summer insolation during the Holocene. The western GrIS margin retreated significantly and Bregne ice cap likely disappeared during the warm early to middle Holocene. 10Be ages (10.7 ka) outboard of the late Holocene moraines at Bregne ice cap compared to those outside of the LIA moraines near Kangerlussuaq (6.8 ka) differ by ~4 kyr. This disparity in ages may have been caused by a large late Holocene advance in eastern Greenland, or perhaps the western GrIS margin retreated farther inland during the middle Holocene. Decreasing Northern Hemisphere summer insolation during the late Holocene, combined with a strong, cold East Greenland Current near Scoresby Sund may have influenced a significant ice cap advance. The temporal pattern of the responses of the eastern and western ice margins to Holocene climate changes may be indicative of how the GrIS will respond to future changes.

Contrast of lithospheric dynamics across the southern and eastern margins of the Tibetan Plateau: a numerical study

NASA Astrophysics Data System (ADS)

Sun, Yujun; Fan, Taoyuan; Wu, Zhonghai

2018-05-01

Both of the southern and eastern margins of the Tibetan Plateau are bounded by the cratonic blocks (Indian plate and Sichuan basin). However, there are many differences in tectonic deformation, lithospheric structure and surface heat flow between these two margins. What dynamics cause these differences? With the constraints of the lithospheric structure and surface heat flow across the southern and eastern margins of Tibetan Plateau, we constructed 2-D thermal-mechanical finite-element models to investigate the dynamics across these two margins. The results show that the delamination of mantle lithosphere beneath the Lhasa terrane in Oligocene and the rheological contrast between the Indian and Tibetan crust are the two main factors that control the subduction of the Indian plate. The dynamics across the eastern margin of the Tibetan Plateau are different from the southern margin. During the lateral expansion of the Tibetan Plateau, pure shear thickening is the main deformation characteristic for the Songpan-Ganzi lithosphere. This thickening results in the reduction of geothermal gradient and surface heat flow. From this study, it can be seen that the delamination of the mantle lithosphere and the rheological contrast between the Tibetan Plateau and its bounding blocks are the two main factors that control the lithospheric deformation and surface heat flow.

The intertropical convergence zone modulates intense hurricane strikes on the western North Atlantic margin

NASA Astrophysics Data System (ADS)

van Hengstum, Peter J.; Donnelly, Jeffrey P.; Fall, Patricia L.; Toomey, Michael R.; Albury, Nancy A.; Kakuk, Brian

2016-02-01

Most Atlantic hurricanes form in the Main Development Region between 9°N to 20°N along the northern edge of the Intertropical Convergence Zone (ITCZ). Previous research has suggested that meridional shifts in the ITCZ position on geologic timescales can modulate hurricane activity, but continuous and long-term storm records are needed from multiple sites to assess this hypothesis. Here we present a 3000 year record of intense hurricane strikes in the northern Bahamas (Abaco Island) based on overwash deposits in a coastal sinkhole, which indicates that the ITCZ has likely helped modulate intense hurricane strikes on the western North Atlantic margin on millennial to centennial-scales. The new reconstruction closely matches a previous reconstruction from Puerto Rico, and documents a period of elevated intense hurricane activity on the western North Atlantic margin from 2500 to 1000 years ago when paleo precipitation proxies suggest that the ITCZ occupied a more northern position. Considering that anthropogenic warming is predicted to be focused in the northern hemisphere in the coming century, these results provide a prehistoric analog that an attendant northern ITCZ shift in the future may again return the western North Atlantic margin to an active hurricane interval.

The intertropical convergence zone modulates intense hurricane strikes on the western North Atlantic margin

PubMed Central

van Hengstum, Peter J.; Donnelly, Jeffrey P.; Fall, Patricia L.; Toomey, Michael R.; Albury, Nancy A.; Kakuk, Brian

2016-01-01

Most Atlantic hurricanes form in the Main Development Region between 9°N to 20°N along the northern edge of the Intertropical Convergence Zone (ITCZ). Previous research has suggested that meridional shifts in the ITCZ position on geologic timescales can modulate hurricane activity, but continuous and long-term storm records are needed from multiple sites to assess this hypothesis. Here we present a 3000 year record of intense hurricane strikes in the northern Bahamas (Abaco Island) based on overwash deposits in a coastal sinkhole, which indicates that the ITCZ has likely helped modulate intense hurricane strikes on the western North Atlantic margin on millennial to centennial-scales. The new reconstruction closely matches a previous reconstruction from Puerto Rico, and documents a period of elevated intense hurricane activity on the western North Atlantic margin from 2500 to 1000 years ago when paleo precipitation proxies suggest that the ITCZ occupied a more northern position. Considering that anthropogenic warming is predicted to be focused in the northern hemisphere in the coming century, these results provide a prehistoric analog that an attendant northern ITCZ shift in the future may again return the western North Atlantic margin to an active hurricane interval. PMID:26906670

The intertropical convergence zone modulates intense hurricane strikes on the western North Atlantic margin

USGS Publications Warehouse

van Hengstrum, Peter J.; Donnelly, Jeffrey P.; Fall, Patricia L.; Toomey, Michael; Albury, Nancy A.; Kakuk, Brian

2016-01-01

Most Atlantic hurricanes form in the Main Development Region between 9°N to 20°N along the northern edge of the Intertropical Convergence Zone (ITCZ). Previous research has suggested that meridional shifts in the ITCZ position on geologic timescales can modulate hurricane activity, but continuous and long-term storm records are needed from multiple sites to assess this hypothesis. Here we present a 3000 year record of intense hurricane strikes in the northern Bahamas (Abaco Island) based on overwash deposits in a coastal sinkhole, which indicates that the ITCZ has likely helped modulate intense hurricane strikes on the western North Atlantic margin on millennial to centennial-scales. The new reconstruction closely matches a previous reconstruction from Puerto Rico, and documents a period of elevated intense hurricane activity on the western North Atlantic margin from 2500 to 1000 years ago when paleo precipitation proxies suggest that the ITCZ occupied a more northern position. Considering that anthropogenic warming is predicted to be focused in the northern hemisphere in the coming century, these results provide a prehistoric analog that an attendant northern ITCZ shift in the future may again return the western North Atlantic margin to an active hurricane interval.

The intertropical convergence zone modulates intense hurricane strikes on the western North Atlantic margin.

PubMed

van Hengstum, Peter J; Donnelly, Jeffrey P; Fall, Patricia L; Toomey, Michael R; Albury, Nancy A; Kakuk, Brian

2016-02-24

Most Atlantic hurricanes form in the Main Development Region between 9°N to 20°N along the northern edge of the Intertropical Convergence Zone (ITCZ). Previous research has suggested that meridional shifts in the ITCZ position on geologic timescales can modulate hurricane activity, but continuous and long-term storm records are needed from multiple sites to assess this hypothesis. Here we present a 3000 year record of intense hurricane strikes in the northern Bahamas (Abaco Island) based on overwash deposits in a coastal sinkhole, which indicates that the ITCZ has likely helped modulate intense hurricane strikes on the western North Atlantic margin on millennial to centennial-scales. The new reconstruction closely matches a previous reconstruction from Puerto Rico, and documents a period of elevated intense hurricane activity on the western North Atlantic margin from 2500 to 1000 years ago when paleo precipitation proxies suggest that the ITCZ occupied a more northern position. Considering that anthropogenic warming is predicted to be focused in the northern hemisphere in the coming century, these results provide a prehistoric analog that an attendant northern ITCZ shift in the future may again return the western North Atlantic margin to an active hurricane interval.

Coordinated Dispersal and Pre-Isthmian Assembly of the Central American Ichthyofauna

PubMed Central

Tagliacollo, Victor A.; Duke-Sylvester, Scott M.; Matamoros, Wilfredo A.; Chakrabarty, Prosanta

2017-01-01

Abstract We document patterns of coordinated dispersal over evolutionary time frames in heroine cichlids and poeciliine live-bearers, the two most species-rich clades of freshwater fishes in the Caribbean basin. Observed dispersal rate (DO) values were estimated from time-calibrated molecular phylogenies in Lagrange+, a modified version of the ML-based parametric biogeographic program Lagrange. DO is measured in units of “wallaces” (wa) as the number of biogeographic range-expansion events per million years. DO estimates were generated on a dynamic paleogeographic landscape of five areas over three time intervals from Upper Cretaceous to Recent. Expected dispersal rate (DE) values were generated from alternative paleogeographic models, with dispersal rates proportional to target area and source-river discharge volume, and inversely proportional to paleogeographic distance. Correlations between DO and DE were used to assess the relative contributions of these three biogeographic parameters. DO estimates imply a persistent dispersal corridor across the Eastern (Antillean) margin of the Caribbean plate, under the influence of prevailing and perennial riverine discharge vectors such as the Proto–Orinoco–Amazon river. Ancestral area estimation places the earliest colonizations of the Greater Antilles and Central America during the Paleocene–Eocene (ca. 58–45 Ma), potentially during the existence of an incomplete Paleogene Arc (∼59 Ma) or Lesser Antilles Arc (∼45 Ma), but predating the GAARlandia land bridge (∼34–33 Ma). Paleogeographic distance is the single best predictor of DO. The Western (Central American) plate margin did not serve as a dispersal corridor until the Late Neogene (12–0 Ma), and contributed relatively little to the formation of modern distributions. PMID:26370565

Two new species of the genus Haplotropis Saussure, 1888 (Orthoptera, Acridoidea, Pamphagidae) from China.

PubMed

Ye, Bao-Hua; Yin, Zhan; Li, Xin-Jiang

2016-06-30

Two new species of the genus Haplotropis Saussure, 1888 from China are described in this paper. The new species Haplotropis xiai sp. nov. is similar to Haplotropis brunneriana Saussure, 1888, but differs from latter by frontal ridge of male widened at median ocellus; tegmina narrower, cover 2/5 tympanum; cercus of male apical half part gently tapering; lower margin of epiphallus with high projection in the middle; anterior margin of pronotum in female with distinct acute angular in middle; length of subgenital plate shorter than width in female. The Haplotropis zhuoluensis sp. nov. is similar to Haplotropis xiai sp. nov., but differs from latter by anterior margin of pronotum reaching hind margin of eyes; length of temina is 1.6 times in male and 1.3 times in female of width; length of interspace shorter than narrowest in mesosternum of male; ancorae of epiphallus oblique inward distinctly, lower margin with high projection in the middle; length of subgenital plate longer than width in female. Type specimens are deposited in the College of Life Sciences, Hebei University, Baoding, China.

Diffuse Extension of the Southern Mariana Margin: Implications for Subduction Zone Infancy and Plate Tectonics

NASA Astrophysics Data System (ADS)

Martinez, F.; Stern, R. J.; Kelley, K. A.; Ohara, Y.; Sleeper, J. D.; Ribeiro, J. M.; Brounce, M. N.

2017-12-01

Opening of the southern Mariana margin takes place in contrasting modes: Extension normal to the trench forms crust that is passively accreted to a rigid Philippine Sea plate and forms along focused and broad accretion axes. Extension also occurs parallel to the trench and has split apart an Eocene-Miocene forearc terrain accreting new crust diffusely over a 150-200 km wide zone forming a pervasive volcano-tectonic fabric oriented at high angles to the trench and the backarc spreading center. Earthquake seismicity indicates that the forearc extension is active over this broad area and basement samples date young although waning volcanic activity. Diffuse formation of new oceanic crust and lithosphere is unusual; in most oceanic settings extension rapidly focuses to narrow plate boundary zones—a defining feature of plate tectonics. Diffuse crustal accretion has been inferred to occur during subduction zone infancy, however. We hypothesize that, in a near-trench extensional setting, the continual addition of water from the subducting slab creates a weak overriding hydrous lithosphere that deforms broadly. This process counteracts mantle dehydration and strengthening proposed to occur at mid-ocean ridges that may help to focus deformation and melt delivery to narrow plate boundary zones. The observations from the southern Mariana margin suggest that where lithosphere is weakened by high water content narrow seafloor spreading centers cannot form. These conditions likely prevail during subduction zone infancy, explaining the diffuse contemporaneous volcanism inferred in this setting.

The Caribbean-South American plate boundary at 65°W: Results from wide-angle seismic data

NASA Astrophysics Data System (ADS)

Bezada, M. J.; Magnani, M. B.; Zelt, C. A.; Schmitz, M.; Levander, A.

2010-08-01

We present the results of the analysis of new wide-angle seismic data across the Caribbean-South American plate boundary in eastern Venezuela at about 65°W. The ˜500 km long profile crosses the boundary in one of the few regions dominated by extensional structures, as most of the southeastern Caribbean margin is characterized by the presence of fold and thrust belts. A combination of first-arrival traveltime inversion and simultaneous inversion of PmP and Pn arrivals was used to develop a P wave velocity model of the crust and the uppermost mantle. At the main strike-slip fault system, we image the Cariaco Trough, a major pull-apart basin along the plate boundary. The crust under the Southern Caribbean Deformed Belt exhibits a thickness of ˜15 km, suggesting that the Caribbean Large Igneous Province extends to this part of the Caribbean plate. The velocity structures of basement highs and offshore sedimentary basins imaged by the profile are comparable to those of features found in other parts of the margin, suggesting similarities in their tectonic history. We do not image an abrupt change in Moho depth or velocity structure across the main strike-slip system, as has been observed elsewhere along the margin. It is possible that a terrane of Caribbean island arc origin was accreted to South America at this site and was subsequently bisected by the strike-slip fault system. The crust under the continental portion of the profile is thinner than observed elsewhere along the margin, possibly as a result of thinning during Jurassic rifting.

Spectral, mineralogical, and geochemical variations across Home Plate, Gusev Crater, Mars indicate high and low temperature alteration

USGS Publications Warehouse

Schmidt, M.E.; Farrand, W. H.; Johnson, J. R.; Schroder, C.; Hurowitz, J.A.; McCoy, T.J.; Ruff, S.W.; Arvidson, R. E.; Des Marais, D.J.; Lewis, K.W.; Ming, D. W.; Squyres, S. W.; De Souza, P.A.

2009-01-01

Over the last ~ 3??years in Gusev Crater, Mars, the Spirit rover observed coherent variations in color, mineralogy, and geochemistry across Home Plate, an ~ 80??m-diameter outcrop of basaltic tephra. Observations of Home Plate from orbit and from the summit of Husband Hill reveal clear differences in visible/near-infrared (VNIR) colors between its eastern and western regions that are consistent with mineralogical compositions indicated by M??ssbauer spectrometer (MB) and by Miniature Thermal Emission Spectrometer (Mini-TES). Pyroxene and magnetite dominate the east side, while olivine, nanophase Fe oxide (npOx) and glass are more abundant on the western side. Alpha Particle X-Ray Spectrometer (APXS) observations reveal that eastern Home Plate has higher Si/Mg, Al, Zn, Ni, and K, while Cl and Br are higher in the west. We propose that these variations are the result of two distinct alteration regimes that may or may not be temporally related: a localized, higher temperature recrystallization and alteration of the east side of Home Plate and lower temperature alteration of the western side that produced npOx.

Neogene rotations and quasicontinuous deformation of the Pacific Northwest continental margin

USGS Publications Warehouse

England, Philip; Wells, Ray E.

1991-01-01

Paleomagnetically determined rotations about vertical axes of 15 to 12 Ma flows of the Miocene Columbia River Basalt Group of Oregon and Washington decrease smoothly with distance from the plate margin, consistent with a simple physical model for continental deformation that assumes the lithosphere behaves as a thin layer of fluid. The average rate of northward translation of the continental margin since 15 Ma calculated from the rotations, using this model, is about 15 mm/yr, which suggests that much of the tangential motion between the Juan de Fuca and North American plates since middle Miocene time has been taken up by deformation of North America. The fluid-like character of the large-scale deformation implies that the brittle upper crust follows the motions of the deeper parts of the lithosphere.

Uplift along passive continental margins, changes in plate motion and mantle convection

NASA Astrophysics Data System (ADS)

Japsen, Peter; Green, Paul F.; Chalmers, James A.; Bonow, Johan M.

2014-05-01

The origin of the forces that produce elevated, passive continental margins (EPCMs) is a hot topic in geoscience. It is, however, a new aspect in the debate that episodes of uplift coincide with changes in plate motion. This has been revealed, primarily, by studies of the burial, uplift and exhumation history of EPCMs based on integration on stratigraphic landscape analysis, low-temperature thermochronology and evidence from the geological record (Green et al., 2013). In the Campanian, Eocene and Miocene, uplift and erosion affected the margins of Brazil and Africa (Japsen et al., 2012b). The uplift phases in Brazil coincided with main phases of Andean orogeny which were periods of relatively rapid convergence at the Andean margin of South America (Cobbold et al., 2001). Because Campanian uplift in Brazil coincides, not only with rapid convergence at the Andean margin of South America, but also with a decline in Atlantic spreading rate, Japsen et al. (2012b) suggested that all these uplift events have a common cause, which is lateral resistance to plate motion. Because the uplift phases are common to margins of diverging plates, it was also suggested that the driving forces can transmit across the spreading axis; probably at great depth, e.g. in the asthenosphere. Late Eocene, Late Miocene and Pliocene uplift and erosion shaped the elevated margin of southern East Greenland (Bonow et al., in review; Japsen et al., in review). These regional uplift phases are synchronous with phases in West Greenland, overlap in time with similar events in North America and Europe and also correlate with changes in plate motion. The much higher elevation of East Greenland compared to West Greenland suggests dynamic support in the east from the Iceland plume. Japsen et al. (2012a) pointed out that EPCMs are typically located above thick crust/lithosphere that is closely juxtaposed to thinner crust/lithosphere. The presence of mountains along the Atlantic margin of Brazil and in East and West Greenland, close to where continental crust starts to thin towards oceanic crust, illustrates the common association between EPCMs and the edges of cratons. These observations indicate that the elevation of EPCMs may be due to processes operating where there is a rapid change in crustal/lithosphere thickness. Vertical motion of EPCMs may thus be related to lithosphere-scale folding caused by compressive stresses at the edge of a craton (e.g. Cloetingh et al., 2008). The compression may be derived either from orogenies elsewhere on a plate or from differential drag at the base of the lithosphere by horizontal asthenospheric flow (Green et al., 2013). Bonow, Japsen, Nielsen. Global Planet. Change in review. Cloetingh, Beekman, Ziegler, van Wees, Sokoutis, 2008. Geol. Soc. Spec. Publ. (London) 306. Cobbold, Meisling, Mount, 2001. AAPG Bull. 85. Green, Lidmar-Bergström, Japsen, Bonow, Chalmers, 2013. GEUS Bull. 2013/30. Japsen, Chalmers, Green, Bonow 2012a, Global Planet. Change 90-91. Japsen, Bonow, Green, Cobbold, Chiossi, Lilletveit, Magnavita, Pedreira, 2012b. GSA Bull. 124. Japsen, Green, Bonow, Nielsen. Global Planet. Change in review.

Pb-Sr-Nd-O isotopic characterization of Mesozoic rocks throughout the northern end of the Peninsular Ranges batholith: Isotopic evidence for the magmatic evolution of oceanic arc–continental margin accretion during the Late Cretaceous of southern California

USGS Publications Warehouse

Kistler, Ronald W.; Wooden, Joseph L.; Premo, Wayne R.; Morton, Douglas M.

2014-01-01

Within the duration of the U.S. Geological Survey (USGS)–based Southern California Areal Mapping Project (SCAMP), many samples from the northern Peninsular Ranges batholith were studied for their whole-rock radioisotopic systematics (rubidium-strontium [Rb-Sr], uranium-thorium-lead [U-Th-Pb], and samarium-neodymium [Sm-Nd]), as well as oxygen (O), a stable isotope. The results of three main studies are presented separately, but here we combine them (>400 analyses) to produce a very complete Pb-Sr-Nd-O isotopic profile of an arc-continent collisional zone—perhaps the most complete in the world. In addition, because many of these samples have U-Pb zircon as well as argon mineral age determinations, we have good control of the timing for Pb-Sr-Nd-O isotopic variations.The ages and isotopic variations help to delineate at least four zones across the batholith from west to east—an older western zone (126–108 Ma), a transitional zone (111–93 Ma), an eastern zone (94–91 Ma), and a much younger allochthonous thrust sheet (ca. 84 Ma), which is the upper plate of the Eastern Peninsular Ranges mylonite zone. Average initial 87Sr/86 Sr (Sri), initial 206Pb/204Pb (206 Pbi), initial 208Pb/204Pb (average 208Pbi), initial epsilon Nd (average εNdi), and δ18O signatures range from 0.704, 18.787, 38.445, +3.1, and 4.0‰–9.0‰, respectively, in the westernmost zone, to 0.7071, 19.199, 38.777, −5, and 9‰–12‰, respectively, in the easternmost zone. The older western zone is therefore the more chemically and isotopically juvenile, characterized mostly by values that are slightly displaced from a mantle array at ca. 115 Ma, and similar to some modern island-arc signatures. In contrast, the isotopic signatures in the eastern zones indicate significant amounts of crustal involvement in the magmatic plumbing of those plutons. These isotopic signatures confirm previously published results that interpreted the Peninsular Ranges batholith as a progressively contaminated magmatic arc. The Peninsular Ranges batholith magmatic arc was initially an oceanic arc built on Panthalassan lithosphere that eventually evolved into a continental margin magmatic arc collision zone, eventually overriding North American cratonic lithosphere. Our Pb-Sr-Nd data further suggest that the western arc rocks represent a nearshore or inboard oceanic arc, as they exhibit isotopic signatures that are more enriched than typical mid-ocean-ridge basalt (MORB). Isotopic signatures from the central zone are transitional and indicate that enriched crustal magma sources were becoming involved in the northern Peninsular Ranges batholith magmatic plumbing. As the oceanic arc–continental margin collision progressed, a mixture of oceanic mantle and continental magmatic sources transpired. Magmatic production in the northern Peninsular Ranges batholith moved eastward and continued to tap enriched crustal magmatic sources. Similar modeling has been previously proposed for two other western margin magmatic arcs, the Sierra Nevada batholith of central California and the Idaho batholith.Calculated initial Nd signatures at ca. 100 Ma for Permian–Jurassic and Proterozoic basement rocks from the nearby San Gabriel Mountains and possible source areas along the southwestern Laurentian margin of southern California, southwestern Arizona, and northern Sonora strongly suggest their involvement with deep crustal magma mixing beneath the eastern zones of the Peninsular Ranges batholith, as well as farther east in continental lithospheric zones.Last, several samples from the allochthonous, easternmost upper-plate zone, which are considerably younger (ca. 84 Ma) than any of the rocks from the northern Peninsular Ranges batholith proper, have even more enriched average Sri, 206Pbi, 208Pbi, and εNdisignatures of 0.7079, 19.344, 38.881, and −6.6, respectively, indicative of the most-evolved magma sources in the northern Peninsular Ranges batholith and similar to radioisotopic values for rocks from the nearby Transverse Ranges, suggesting a genetic connection between the two.

Fuel cell separator with compressible sealing flanges

DOEpatents

Mientek, A.P.

1984-03-30

A separator for separating adjacent fuel cells in a stack of such cells includes a flat, rectangular, gas-impermeable plate disposed between adjacent cells and having two opposite side margins thereof folded back over one side of the plate to form two first seal flanges and having the other side margins thereof folded back over the opposite side of the plate to form two second seal flanges, each of the seal flanges cooperating with the plate to define a channel in which is disposed a resiliently compressible stack of thin metal sheets. The two first seal flanges cooperate with the electrolyte matrix of one of the cells to form a gas-impermeable seal between an electrode of the one cell and one of two reactant gas manifolds. The second seal flanges cooperate with the electrolyte matrix of the other cell for forming a gas-impermeable seal between an electrode of the other cell and the other of the two reactant gas manifolds. The seal flanges cooperate with the associated compressible stacks of sheets for maintaining a spacing between the plate and the electrolyte matrices while accommodating variation of that spacing.

Fuel cell separator with compressible sealing flanges

DOEpatents

Mientek, Anthony P.

1985-04-30

A separator for separating adjacent fuel cells in a stack of such cells includes a flat, rectangular, gas-impermeable plate disposed between adjacent cells and having two opposite side margins thereof folded back over one side of the plate to form two first seal flanges and having the other side margins thereof folded back over the opposite side of the plate to form two second seal flanges, each of the seal flanges cooperating with the plate to define a channel in which is disposed a resiliently compressible stack of thin metal sheets. The two first seal flanges cooperate with the electrolyte matrix of one of the cells to form a gas-impermeable seal between an electrode of the one cell and one of two reactant gas manifolds. The second seal flanges cooperate with the electrolyte matrix of the other cell for forming a gas-impermeable seal between an electrode of the other cell and the other of the two reactant gas manifolds. The seal flanges cooperate with the associated compressible stacks of sheets for maintaining a spacing between the plate and the electrolyte matrices while accommodating variation of that spacing.

Seismicity of the Earth 1900-2013 offshore British Columbia-southeastern Alaska and vicinity

USGS Publications Warehouse

Hayes, Gavin P.; Smoczyk, Gregory M.; Ooms, Jonathan G.; McNamara, Daniel E.; Furlong, Kevin P.; Benz, Harley M.; Villaseñor, Antonio

2014-01-01

The tectonics of the Pacific margin of North America between Vancouver Island and south-central Alaska are dominated by the northwest motion of the Pacific plate with respect to the North America plate at a velocity of approximately 50 mm/yr. In the south of this mapped region, convergence between the northern extent of the Juan de Fuca plate (also known as the Explorer microplate) and North America plate dominate. North from the Explorer, Pacific, and North America plate triple junction, Pacific:North America motion is accommodated along the ~650-km-long Queen Charlotte fault system. Offshore of Haida Gwaii and to the southwest, the obliquity of the Pacific:North America plate motion vector creates a transpressional regime, and a complex mixture of strike-slip and convergent (underthrusting) tectonics. North of the Haida Gwaii islands, plate motion is roughly parallel to the plate boundary, resulting in almost pure dextral strike-slip motion along the Queen Charlotte fault. To the north, the Queen Charlotte fault splits into multiple structures, continuing offshore of southwestern Alaska as the Fairweather fault, and branching east into the Chatham Strait and Denali faults through the interior of Alaska. The plate boundary north and west of the Fairweather fault ultimately continues as the Alaska-Aleutians subduction zone, where Pacific plate lithosphere subducts beneath the North America plate at the Aleutians Trench. The transition is complex, and involves intraplate structures such as the Transition fault. The Pacific margin offshore British Columbia is one of the most active seismic zones in North America and has hosted a number of large earthquakes historically.

Constraints for timing of extensional tectonics in the western margin of the Red Sea in Eritrea

NASA Astrophysics Data System (ADS)

Ghebreab, Woldai; Carter, Andrew; Hurford, Anthony J.; Talbot, Christopher J.

2002-06-01

Recent work on asthenosphere-lithosphere coupling reinforces past observations that active and passive rifting models do not adequately describe real rifts. There remains insufficient knowledge of fundamental controls on rift architecture. In the actively extending Red Sea margin of eastern Eritrea, which lies at the Red Sea/Danakil-Gulf of Aden and the East African rift triple junction zone, the geometry and kinematics of extension are complex and poorly defined due to large data gaps. Extension and sea-floor spreading in both the Red Sea and Gulf of Aden have influenced the Neogene tectonic development of Eritrea but many of the structures have Pan-African origins and do not follow normal plate opening geometries. To constrain the rifting history in eastern Eritrea, apatite fission-track thermochronologic data were measured for 22 Pan-African rock samples. Results identify late Oligocene-early Miocene cooling coincident with extension and erosion along the conjugate margin in Yemen. A younger age group, confined to Mt Ghedem, relates to an episode of fault reactivation and dyke injection that began ˜10 Ma coincident with rotation of the nearby Danakil block. Initially this was driven by onset of sea-floor spreading in the Gulf of Aden and later, in the Pliocene, aided by northward rifting in the Afar depression concomitant with spreading in the Red Sea. These different processes highlight the complex linkage between different extensional events and rift architecture.

Onshore and offshore apatite fission-track dating from the southern Gulf of California: Insights into the time-space evolution of the rifting

NASA Astrophysics Data System (ADS)

Balestrieri, Maria Laura; Ferrari, Luca; Bonini, Marco; Duque-Trujillo, Jose; Cerca, Mariano; Moratti, Giovanna; Corti, Giacomo

2017-11-01

We present the results of a apatite fission-track (AFT) study on intrusive rocks in the southern Gulf of California, sampled along the eastern margin of Baja California Sur (western rift margin), as well as from islands and submerged rifted blocks within the Gulf of California, and from the conjugate Mexican margin (Nayarit state). For most of the samples U-Pb zircon and 40Ar-39Ar mineral ages were already available (Duque-Trujillo et al., 2015). Coupled with the new AFT data these ages provide a more complete information on cooling after emplacement. Our samples span a wide range of ages between 5.5 ± 1.1 and 73.7 ± 5.8 Ma, and show a general spatial distribution, with late Miocene AFT ages (about 6 Ma) aligned roughly NW-SE along a narrow offshore belt, parallel to Baja California Peninsula, separating older ages on both sides. This pattern suggests that in Late Miocene, deformation due to plate transtension focused at the eastern rheological boundary of the Baja California block. Some Early Miocene AFT ages onshore Baja California could be related to plutons emplaced at shallow depths and thermal resetting associated with the onset of volcanism at 19 Ma in this part of the Peninsula. On the other hand, an early extensional event similar to that documented in the eastern Gulf cannot be ruled out in the westernmost Baja California.

New insights into North America-Pacific Plate boundary deformation from Lake Tahoe, Salton Sea and southern Baja California

NASA Astrophysics Data System (ADS)

Brothers, Daniel Stephen

Five studies along the Pacific-North America (PA-NA) plate boundary offer new insights into continental margin processes, the development of the PA-NA tectonic margin and regional earthquake hazards. This research is based on the collection and analysis of several new marine geophysical and geological datasets. Two studies used seismic CHIRP surveys and sediment coring in Fallen Leaf Lake (FLL) and Lake Tahoe to constrain tectonic and geomorphic processes in the lakes, but also the slip-rate and earthquake history along the West Tahoe-Dollar Point Fault. CHIRP profiles image vertically offset and folded strata that record deformation associated with the most recent event (MRE). Radiocarbon dating of organic material extracted from piston cores constrain the age of the MRE to be between 4.1--4.5 k.y. B.P. Offset of Tioga aged glacial deposits yield a slip rate of 0.4--0.8 mm/yr. An ancillary study in FLL determined that submerged, in situ pine trees that date to between 900-1250 AD are related to a medieval megadrought in the Lake Tahoe Basin. The timing and severity of this event match medieval megadroughts observed in the western United States and in Europe. CHIRP profiles acquired in the Salton Sea, California provide new insights into the processes that control pull-apart basin development and earthquake hazards along the southernmost San Andreas Fault. Differential subsidence (>10 mm/yr) in the southern sea suggests the existence of northwest-dipping basin-bounding faults near the southern shoreline. In contrast to previous models, the rapid subsidence and fault architecture observed in the southern part of the sea are consistent with experimental models for pull-apart basins. Geophysical surveys imaged more than 15 ˜N15°E oriented faults, some of which have produced up to 10 events in the last 2-3 kyr. Potentially 2 of the last 5 events on the southern San Andreas Fault (SAF) were synchronous with rupture on offshore faults, but it appears that ruptures on three offshore faults are synchronous with Colorado River diversions into the basin. The final study was used coincident wide-angle seismic refraction and multichannel seismic reflection surveys that spanned the width of the of the southern Baja California (BC) Peninsula. The data provide insight into the spatial and temporal evolution of the BC microplate capture by the Pacific Plate. Seismic reflection profiles constrain the upper crustal structure and deformation history along fault zone on the western Baja margin and in the Gulf of California. Stratal divergence in two transtensional basins along the Magdalena Shelf records the onset of extension across the Tosco-Abreojos and Santa Margarita faults. We define an upper bound of 12 Ma on the age of the pre-rift sediments and an age of ˜8 Ma for the onset of extension. Tomographic imaging reveals a very heterogeneous upper crust and a narrow, high velocity zone that extends ˜40 km east of the paleotrench and is interpreted to be remnant oceanic crust.

Segmented Subduction Across the Juan De Fuca Plate: Challenges in Imaging with an Amphibious Array

NASA Astrophysics Data System (ADS)

Hawley, W. B.; Allen, R. M.

2014-12-01

The Cascadia Initiative (CI) is an amphibious array spanning the Juan de Fuca plate from formation at the ridge to the destruction of the slab in the mantle beneath western North America. This ambitions project has occupied over 300 onshore and offshore sites, providing an unprecedented opportunity to understand the dynamics of oceanic plates. The CI project is now in its fourth and final year of deployment. Here we present constraints on the structure of the Juan de Fuca plate and its interaction with western North America. We identify segmentation along the Cascadia subduction zone that can be traced back onto the Juan de Fuca plate prior to subduction. These results give insight into the life cycle of oceanic plates, from their creation at a mid-ocean ridge to their subduction and subsequent recycling into the mantle.

Actively dewatering fluid-rich zones along the Costa Rica plate boundary fault

NASA Astrophysics Data System (ADS)

Bangs, N. L.; McIntosh, K. D.; Silver, E. A.; Kluesner, J. W.; Ranero, C. R.; von Huene, R.

2012-12-01

New 3D seismic reflection data reveal distinct evidence for active dewatering above a 12 km wide segment of the plate boundary fault within the Costa Rica subduction zone NW of the Osa Peninsula. In the spring of 2011 we acquired a 11 x 55 km 3D seismic reflection data set on the R/V Langseth using four 6,000 m streamers and two 3,300 in3 airgun arrays to examine the structure of the Costa Rica margin from the trench into the seismogenic zone. We can trace the plate-boundary interface from the trench across our entire survey to where the plate-boundary thrust lies > 10 km beneath the margin shelf. Approximately 20 km landward of the trench beneath the mid slope and at the updip edge of the seismogenic zone, a 12 km wide zone of the plate-boundary interface has a distinctly higher-amplitude seismic reflection than deeper or shallower segments of the fault. Directly above and potentially directly connected with this zone are high-amplitude, reversed-polarity fault-plane reflections that extend through the margin wedge and into overlying slope sediment cover. Within the slope cover, high-amplitude reversed-polarity reflections are common within the network of closely-spaced nearly vertical normal faults and several broadly spaced, more gently dipping thrust faults. These faults appear to be directing fluids vertically toward the seafloor, where numerous seafloor fluid flow indicators, such as pockmarks, mounds and ridges, and slope failure features, are distinct in multibeam and backscatter images. There are distinctly fewer seafloor and subsurface fluid flow indicators both updip and downdip of this zone. We believe these fluids come from a 12 km wide fluid-rich segment of the plate-boundary interface that is likely overpressured and has relatively low shear stress.

Opening of the Central Atlantic Ocean: Implications for Geometric Rifting and Asymmetric Initial Seafloor Spreading after Continental Breakup

NASA Astrophysics Data System (ADS)

Klingelhoefer, F.; Biari, Y.; Sahabi, M.; Funck, T.; Benabdellouahed, M.; Schnabel, M.; Reichert, C. J.; Gutscher, M. A.; Bronner, A.; Austin, J. A., Jr.

2017-12-01

The structure of conjugate passive margins provides information about rifting styles, the initial phases of the opening of an ocean and the formation of its associated sedimentary basins. The study of the deep structure of conjugate passive continental margins combined with precise plate kinematic reconstructions can provide constraints on the mechanisms of rifting and formation of initial oceanic crust. In this study the Central Atlantic conjugate margins are compared, based on compilation of wide-angle seismic profiles from the NW-Africa Nova Scotian and US passive margins. Plate cinematic reconstructions were used to place the profiles in the position at opening and at the M25 magnetic anomaly. The patterns of volcanism, crustal thickness, geometry, and seismic velocities in the transition zone. suggest symmetric rifting followed by asymmetric oceanic crustal accretion. Conjugate profiles in the southern Central Atlantic image differences in the continental crustal thickness. While profiles on the eastern US margin are characterized by thick layers of magmatic underplating, no such underplate was imaged along the NW-African continental margin. It has been proposed that these volcanic products form part of the CAMP (Central Atlantic Magmatic Province). In the north, two wide-angle seismic profiles acquired in exactly conjugate positions show that the crustal geometry of the unthinned continental crust and the necking zone are nearly symmetric. A region including seismic velocities too high to be explained by either continental or oceanic crust is imaged along the Nova Scotia margin off Eastern Canada, corresponding on the African side to an oceanic crust with slightly elevated velocities. These might result from asymmetric spreading creating seafloor by faulting the existing lithosphere on the Canadian side and the emplacement of magmatic oceanic crust including pockets of serpentinite on the Moroccan margin. A slightly elevated crustal thickness along the African margin can be explained by the influence of the Canary hotspot between 60 and 30 Ma in the study region. After isochron M25, a large-scale plate reorganization may then have led to an increase in spreading velocity and the production of a more typical but thin magmatic crust on both sides.

Effects of Cocos Ridge Collision on the Western Caribbean: Is there a Panama Block?

NASA Astrophysics Data System (ADS)

Kobayashi, D.; La Femina, P. C.; Geirsson, H.; Chichaco, E.; Abrego M, A. A.; Fisher, D. M.; Camacho, E. I.

2011-12-01

It has been recognized that the subduction and collision of the Cocos Ridge, a 2 km high aseismic ridge standing on >20 km thick oceanic crust of the Cocos plate, drives upper plate deformation in southern Central America. Recent studies of Global Positioning System (GPS) derived horizontal velocities relative to the Caribbean Plate showed a radial pattern centered on the Cocos Ridge axis where Cocos-Caribbean convergence is orthogonal, and margin-parallel velocities to the northwest. Models of the full three-dimensional GPS velocity field and earthquake slip vectors demonstrate low mechanical coupling along the Middle America subduction zone in Nicaragua and El Salvador, and a broad zone of high coupling beneath the Osa Peninsula, where the Cocos Ridge intersects the margin. These results suggest that Cocos Ridge collision may be the main driver for trench-parallel motion of the fore arc to the northwest and for uplift and shortening of the outer fore arc in southern Central America, whereby thickened and hence buoyant Cocos Ridge crust acts as an indenter causing the tectonic escape of the fore arc. These studies, however, were not able to constrain well the pattern of surface deformation east-southeast of the ridge axis due to a lack of GPS stations, and Cocos Ridge collision may be responsible for the kinematics and deformation of the proposed Panama block. Recent reinforcement of the GPS network in southeastern Costa Rica and Panama has increased the spatial and temporal resolution of the network and made it possible to further investigate surface deformation of southern Central America and the Panama block. We present a new regional surface velocity field for Central America from geodetic GPS data collected at 11 recently-installed and 178 existing episodic, semi-continuous, and continuous GPS sites in Nicaragua, Costa Rica, and Panama. We investigate the effects of Cocos Ridge collision on the Panama block through kinematic block modeling. Published earthquake relocation and geologic data are used to define block boundaries and fault geometries. We invert the three-dimensional GPS velocity vectors and earthquake slip vectors to estimate the magnitude and spatial distribution of interplate mechanical coupling on active plate and block boundaries around the Panama block; the Middle America Trench - South Panama Deformed Belt, the Central Costa Rican Deformed Belt, and the North Panama Deformed Belt in particular, and the rates of relative plate motion between the Panama block and the adjacent Cocos, Nazca, and Caribbean plates. This study tests whether the Panama block responds to the ridge collision as a rigid tectonic block or as a deforming zone consisting of multiple blocks.

3D lithospheric mapping of the Iberian Peninsula and surrounding Atlantic and Mediterranean margins from 3D joint inversion of potential field and elevation data.

NASA Astrophysics Data System (ADS)

Torne, Montserrat; Zeyen, Hermann; Jimenez-Munt, Ivone; Fernandez, Manel; Vergés, Jaume

2017-04-01

We investigate the lithospheric density structure of the Iberian Peninsula and the surrounding Atlantic and Mediterranean margins from a 3D joint inversion of free-air, geoid and elevation data, based on a Bayesian approach. In addition, the crustal structure has been further constrained by incorporating about 750 Moho values from DSS investigations and RF analysis covering the entire region. Our preliminary results shows a significant lithospheric deformation along the plate boundaries, the Bay of Biscay-Pyrenees to the North and the Azores-Gibraltar to the south, where the CMB and LAB are located at depths more than 45 and 150 km, respectively. Noteworthy is the arcuate lithospheric thickening located at the westernmost end of the Gibraltar Arc system showing the presence of the NW-to-Westward retreated Gibraltar Arc slab that has given rise to the formation of the Betics-Rif Alpine belt system and the back arc Alboran basin. To the west, the stable-slightly deformed Iberian massif shows a quasi-flat CMB and LAB topography (30 to 32 km and about 110 km, respectively). The crust and mantle lithosphere thin towards the Mediterranean and Atlantic margins, with the exception of its northern margin where lithospheric thickening extends offshore to the Gulf of Biscay. In the western Mediterranean the SE-Neogene slab retreat has resulted in a significant thinning of the crust and mantle lithosphere. Thin lithosphere is also observed in the Tagus-Horseshoe abyssal plain region where the LAB shallows to less than 90 km. This work has been funded by the Spanish projects MITE (CGL2014-59516-P) and WEME-CSIC project 201330E11.

Depth to the Juan De Fuca slab beneath the Cascadia subduction margin - a 3-D model for sorting earthquakes

USGS Publications Warehouse

McCrory, Patricia A.; Blair, J. Luke; Oppenheimer, David H.; Walter, Stephen R.

2004-01-01

We present an updated model of the Juan de Fuca slab beneath southern British Columbia, Washington, Oregon, and northern California, and use this model to separate earthquakes occurring above and below the slab surface. The model is based on depth contours previously published by Fluck and others (1997). Our model attempts to rectify a number of shortcomings in the original model and update it with new work. The most significant improvements include (1) a gridded slab surface in geo-referenced (ArcGIS) format, (2) continuation of the slab surface to its full northern and southern edges, (3) extension of the slab surface from 50-km depth down to 110-km beneath the Cascade arc volcanoes, and (4) revision of the slab shape based on new seismic-reflection and seismic-refraction studies. We have used this surface to sort earthquakes and present some general observations and interpretations of seismicity patterns revealed by our analysis. For example, deep earthquakes within the Juan de Fuca Plate beneath western Washington define a linear trend that may mark a tear within the subducting plate Also earthquakes associated with the northern stands of the San Andreas Fault abruptly terminate at the inferred southern boundary of the Juan de Fuca slab. In addition, we provide files of earthquakes above and below the slab surface and a 3-D animation or fly-through showing a shaded-relief map with plate boundaries, the slab surface, and hypocenters for use as a visualization tool.

The mantle flow field beneath western North America.

PubMed

Silver, P G; Holt, W E

2002-02-08

Although motions at the surface of tectonic plates are well determined, the accompanying horizontal mantle flow is not. We have combined observations of surface deformation and upper mantle seismic anisotropy to estimate this flow field for western North America. We find that the mantle velocity is 5.5 +/- 1.5 centimeters per year due east in a hot spot reference frame, nearly opposite to the direction of North American plate motion (west-southwest). The flow is only weakly coupled to the motion of the surface plate, producing a small drag force. This flow field is probably due to heterogeneity in mantle density associated with the former Farallon oceanic plate beneath North America.

Marginal inherited structures impact on the oblique convergent N American Plate/ Central Caribbean plate-boundary in the Northern Caribbean. The tectonic evolution since Miocene times based on Haiti data acquired onshore and offshore since 2012- a step toward an ADP Drilling Proposal (Haiti-DRILL).

NASA Astrophysics Data System (ADS)

Ellouz, N.; Hamon, Y.; Deschamps, R.; Battani, A.; Wessels, R.; Boisson, D.; Prepetit, C.; Momplaisir, R.

2017-12-01

Since Early Paleogene times, the North Caribbean plate is colliding obliquely with the south continental part of the old N. American Margins, which is represented by various segments from West to East, inherited from Jurassic times. Location, amount of displacement, rotation and the structural deformation of these margin segments, resulting from the dislocation of the continental N American margin, are not clearly yet established. At present, the plate limits are marked either by two left lateral faults west and inside Haiti (OSF in the North and EPGF in the South), oblique collision front (further west in Cuba), oblique subducted segments (to the East, Porto-Rico). From our recent works operated both offshore (Haiti-SIS and Haiti-BGF surveys 2012-2015) and onshore (field campaigns 2013-2017) in Haitian zone, the position of the present-day and paleo major limits have been redefined. These paleolimits have been reconstructed up to early Miocene times, based on: restoration of regional structural cross-sections, sedimentology and on paleoenvironement studies. In a preliminary way, we analyzed the complexity of the tectonic heritage with possible nature, heterogeneity of the crustal fragments and associated margins close to Haiti (age, structure, environment, location of the dislocated blocks through times) which profoundly impact the partitioning of the deformation along this complex transformed margin. The change in the structure wavelength, decollement level variations are primary constraints in the restoration of the main units and do impose a deep connection along specific segments either related to strike-slip or to splay faults. The asymmetry on the repartition of the fault activity tend to prove that the past motion related to "EPGF transfer zone" is mainly partitioned in Haiti to the North of the present-day EPGF position. At present, these results are still coherent with the distribution of the aftershoks registered after 2010, and with the present-day seismicity during the last years.

Crustal seismic velocity structure from Eratosthenes Seamount to Hecataeus Rise across the Cyprus Arc, eastern Mediterranean

NASA Astrophysics Data System (ADS)

Welford, J. Kim; Hall, Jeremy; Hübscher, Christian; Reiche, Sönke; Louden, Keith

2015-02-01

Wide-angle reflection/refraction seismic profiles were recorded across the Cyprus Arc, the plate boundary between the African Plate and the Aegean-Anatolian microplate, from the Eratosthenes Seamount to the Hecataeus Rise immediately south of Cyprus. The resultant models were able to resolve detail of significant lateral velocity variations, though the deepest crust and Moho are not well resolved from the seismic data alone. Conclusions from the modelling suggest that (i) Eratosthenes Seamount consists of continental crust but exhibits a laterally variable velocity structure with a thicker middle crust and thinner lower crust to the northeast; (ii) the Hecataeus Rise has a thick sedimentary rock cover on an indeterminate crust (likely continental) and the crust is significantly thinner than Eratosthenes Seamount based on gravity modelling; (iii) high velocity basement blocks, coincident with highs in the magnetic field, occur in the deep water between Eratosthenes and Hecataeus, and are separated and bounded by deep low-velocity troughs and (iv) one of the high velocity blocks runs parallel to the Cyprus Arc, while the other two appear linked based on the magnetic data and run NW-SE, parallel to the margin of the Hecataeus Rise. The high velocity block beneath the edge of Eratosthenes Seamount is interpreted as an older magmatic intrusion while the linked high velocity blocks along Hecataeus Rise are interpreted as deformed remnant Tethyan oceanic crust or mafic intrusives from the NNW-SSE oriented transform margin marking the northern boundary of Eratosthenes Seamount. Eratosthenes Seamount, the northwestern limit of rifted continental crust from the Levant Margin, is part of a jagged rifted margin transected by transform faults on the northern edge of the lower African Plate that is being obliquely subducted under the Aegean-Anatolian upper plate. The thicker crust of Eratosthenes Seamount may be acting as an asperity on the subducting slab, locally locking up subduction of the Cyprus Arc on its northern margin, while deformed Tethyan oceanic crust remains trapped between its northeastern margin and the Hecataeus Rise.

Midterm Follow-up of Treating Volar Marginal Rim Fractures with Variable Angle Lcp Volar Rim Distal Radius Plates.

PubMed

Goorens, Chul Ki; Geeurickx, Stijn; Wernaers, Pascal; Staelens, Barbara; Scheerlinck, Thierry; Goubau, Jean

2017-06-01

Specific treatment of the volar marginal rim fragment of distal radius fractures avoids occurance of volar radiocarpal dislocation. Although several fixation systems are available to capture this fragment, adequately maintaining internal fixation is difficult. We present our experience of the first 10 cases using the 2.4 mm variable angle LCP volar rim distal radius plate (Depuy Synthes®, West Chester, US), a low-profile volar rim-contouring plate designed for distal plate positioning and stable buttressing of the volar marginal fragment. Follow-up patient satisfaction, range of motion, grips strength, functional scoring with the QuickDASH and residual pain with a numeric rating scale were assessed. Radiological evaluation consisted in evaluating fracture consolidation, ulnar variance, volar angulation and maintenance of the volar rim fixation. The female to male ratio was 5:5 and the mean age was 52.2 (range, 17-80) years. The mean follow-up period was 11 (range, 5-19) months postoperatively. Patient satisfaction was high. The mean total flexion/extension range was 144° (range, 100-180°) compared to the contralateral uninjured side 160° (range, 95-180°). The mean total pronation/supination range was 153° (range, 140-180°) compared to the contralateral uninjured side 170° (range, 155-180°). Mean grip strength was 14 kg (range, 9-22), compared to the contralateral uninjured side 20 kg (range, 12-25 kg). Mean pre-injury level activity QuickDASH was 23 (range, 0-34.1), while post-recovery QuickDASH was 25 (range 0-43.2). Residual pain was 1.5 on the visual numerical pain rating scale. Radiological evaluation revealed in all cases fracture consolidation, satisfactory reconstruction of ulnar variance, volar angulation and volar rim. We encountered no flexor tendon complications, although plate removal was systematically performed after fracture consolidation. The 2.4 mm variable angle LCP volar rim distal radius plates is a valid treatment option for treating the volar marginal fragment in distal radius fractures.

An Anisotropic Contrast in the Lithosphere Across the Central San Andreas Fault

NASA Astrophysics Data System (ADS)

Jiang, Chengxin; Schmandt, Brandon; Clayton, Robert W.

2018-05-01

Seismic anisotropy of the lithosphere and asthenosphere was investigated with a dense broadband seismic transect nearly orthogonal to the central San Andreas fault (SAF). A contrast in SK(K)S splitting was found across the SAF, with a clockwise rotation of the fast orientation 26° closer to the strike of the SAF and greater delay times for stations located within 35 km to the east. Dense seismograph spacing requires heterogeneous anisotropy east of the SAF in the uppermost mantle or crust. Based on existing station coverage, such a contrast in splitting orientations across the SAF may be unusual along strike and its location coincides with the high-velocity Isabella anomaly in the upper mantle. If the Isabella anomaly is a fossil slab fragment translating with the Pacific plate, the anomalous splitting east of the SAF could indicate a zone of margin-parallel shear beneath the western edge of North America.

Brittle deformation along the Gulf of Alaska margin in response to Paleocene-Eocene triple junction migration: in Sisson

USGS Publications Warehouse

Haeussler, Peter J.; Bradley, Dwight C.; Goldfarb, Richard J.

2003-01-01

A spreading center was subducted diachronously along a 2200 km segment of what is now the Gulf of Alaska margin between 61 and 50 Ma, and left in its wake near-trench intrusions and high-T, low-P metamorphic rocks. Gold-quartz veins and dikes, linked to ridge subduction by geochronological and relative timing evidence, provide a record of brittle deformation during and after passage of the ridge. The gold-quartz veins are typically hosted by faults, and their regional extent indicates there was widespread deformation of the forearc above the slab window at the time of ridge subduction. Considerable variability in the strain pattern was associated with the slab window and the trailing plate. A diffuse network of dextral, sinistral, and normal faults hosted small lode-gold deposits (<50,000 oz) in south-central Alaska, whereas crustal-scale dextral faults in southeastern Alaska are spatially associated with large gold deposits (up to 800,000 oz).We interpret the gold-quartz veins as having formed above an eastward-migrating slab window, where the forearc crust responded to the diminishing influence of the forward subducting plate, the increasing influence of the trailing plate, and the thermal pulse and decreased basal friction from the slab window. In addition, extensional deformation of the forearc resulted from the diverging motions of the two oceanic plates at the margins of the slab window. Factors that complicate interpretations of fault kinematics and near-trench dike orientations include a change in plate motions at ca. 52 Ma, northward translation of the accretionary complex, oroclinal bending of the south-central Alaska margin, and subduction of transform segments. We find the pattern of syn-ridge subduction faulting in southern Alaska is remarkably similar to brittle faults near the Chile triple junction and to earthquake focal mechanisms in the Woodlark basin - the two modern sites of ridge subduction. Therefore, extensional and strike-slip deformation above slab windows may be a common occurrence.

Farallon slab detachment and deformation of the Magdalena Shelf, southern Baja California

USGS Publications Warehouse

Brothers, Daniel S.; Harding, Alistair J.; Gonzalez-Fernandez, Antonio; Holbrook, W.S. Steven; Kent, Graham M.; Driscoll, Neal W.; Fletcher, John M.; Lizarralde, Daniel; Umhoefer, Paul J.; Axen, Gary

2012-01-01

Subduction of the Farallon plate beneath northwestern Mexico stalled by ~12 Ma when the Pacific-Farallon spreading-ridge approached the subduction zone. Coupling between remnant slab and the overriding North American plate played an important role in the capture of the Baja California (BC) microplate by the Pacific Plate. Active-source seismic reflection and wide-angle seismic refraction profiles across southwestern BC (~24.5°N) are used to image the extent of remnant slab and study its impact on the overriding plate. We infer that the hot, buoyant slab detached ~40 km landward of the fossil trench. Isostatic rebound following slab detachment uplifted the margin and exposed the Magdalena Shelf to wave-base erosion. Subsequent cooling, subsidence and transtensional opening along the shelf (starting ~8 Ma) starved the fossil trench of terrigenous sediment input. Slab detachment and the resultant rebound of the margin provide a mechanism for rapid uplift and exhumation of forearc subduction complexes.

Correlations between Crustal Structure and Slip on the Cascadia Megathrust (Invited)

NASA Astrophysics Data System (ADS)

Trehu, A. M.

2013-12-01

A number of active-source seismic imaging experiments of the Cascadia forearc margin have been conducted over the past three decades. Seismic P-wave velocity models derived from these experiments, when combined with geodetic, potential field, morphological and other data, reveal structures in both the upper and lower plate that can be correlated with current microseismic activity, geodetic signals indicating interplate locking, and apparent segmentation of past large plate boundary earthquakes as determined from onshore and offshore paleoseismic data. These data are being interpreted to construct maps of the apparent seismic velocity structure averaged over several km above and below the expected plate boundary and extending from the region characterized by episodic tremor and slip up dip to the deformation front. Preliminary results for the recent CIET, COAST and Ridge-to-Trench experiments that support, challenge or extend an evolving working model for structural constraints on plate boundary deformation in Cascadia will also be discussed. Other co-PIs who have planned and executed the CIET, COAST and Ridge-to-Trench experiments are listed below with the lead PI for each group listed first. CIET (Cascadia Initiative Science Team): Doug Toomey, Emilie Hooft (both at Un. of Oregon); Bob Dziak (Oregon State Un. NOAA); William Wilcock (Un. Washington); Susan Schwartz (UC Santa Cruz); John Collins, Jeff McGuire (WHOI); Maya Tolstoy (LDEO); Richard Allen (UC Berkeley) COAST (Cascadia Open-Access Seismic Transects): Steve Holbrook (Un. Wyoming); Graham Kent (Un. Nevada Reno); Katie Keranen (Un. Oklahoma); Paul Johnson (Un. Washington); Jackie Caplan-Auerbach (Western Washington Un.); Harold Tobin (Un. Wisconson) Ridge-to-Trench: Suzanne Carbotte, Helene Carton, Geoff Abers (all at LDEO); Pablo Canales (WHOI); Mladen Nedimovic (Dalhousie Un.)

Investigations into the Fish Lake Valley Fault Zone (FLVFZ) and its interactions with normal faulting within Eureka and Deep Springs Valleys

NASA Astrophysics Data System (ADS)

Lawson, M. J.; Rhodes, E.; Yin, A.

2016-12-01

In most textbooks, the San Andreas Fault is stated to be the plate boundary between the North American and the Pacific plates, as plate tectonics assumes that boundaries are essentially discrete. In the Western United States this is not the case, as up to 25% of relative plate motion is accommodated on other structures within the Walker Lane Shear Zone (WLSZ) in a diffuse 100 km margin (Faulds et al., 2005; Oldow et al., 2001). Fish Lake Valley Fault Zone (FLVFZ), situated at the northern border of Death Valley National Park, is the northern continuation of the Furnace Creek Fault Zone (FCFZ), and is an important transfer structure within the Walker Lane Shear Zone. Though the FLVFZ has a long term rate (since 10 Ma) of 5 mm/yr (Reheis and Sawyer, 1997), it has a highly variable slip rate. In the middle Pleistocene, the rate has a maximum of up to 11 mm/yr which would accommodate nearly the entirety of slip within the Walker Lane, and yet this rate decreases significantly ( 2.5 to 3 mm/yr) by the late Pleistocene due to unknown causes (Frankel et al. 2007). This variation in slip rate has been proposed by previous workers to be due to strain transience, an increase in the overall strain rate, or due to other unknown structures (Lee et al., 2009). Currently, we are investigating the cause of this variation, and the possibility of the transfer of slip to faults south of the FLVFZ on oblique normal faults within Eureka and Deep Springs Valleys. Preliminary data will be shown utilizing scarp transects, geomorphic scarp modeling, and Optically Stimulated Luminescence (OSL) dating techniques.

Eocene extension in Idaho generated massive sediment floods into Franciscan trench and into Tyee, Great Valley, and Green River basins

USGS Publications Warehouse

Dumitru, Trevor A.; Ernst, W.G.; Wright, James E.; Wooden, Joseph L.; Wells, Ray E.; Farmer, Lucia P.; Kent, Adam J.R.; Graham, Stephan A.

2013-01-01

The Franciscan Complex accretionary prism was assembled during an ∼165-m.y.-long period of subduction of Pacific Ocean plates beneath the western margin of the North American plate. In such fossil subduction complexes, it is generally difficult to reconstruct details of the accretion of continent-derived sediments and to evaluate the factors that controlled accretion. New detrital zircon U-Pb ages indicate that much of the major Coastal belt subunit of the Franciscan Complex represents a massive, relatively brief, surge of near-trench deposition and accretion during Eocene time (ca. 53–49 Ma). Sediments were sourced mainly from the distant Idaho Batholith region rather than the nearby Sierra Nevada. Idaho detritus also fed the Great Valley forearc basin of California (ca. 53–37 Ma), the Tyee forearc basin of coastal Oregon (49 to ca. 36 Ma), and the greater Green River lake basin of Wyoming (50–47 Ma). Plutonism in the Idaho Batholith spanned 98–53 Ma in a contractional setting; it was abruptly superseded by major extension in the Bitterroot, Anaconda, Clearwater, and Priest River metamorphic core complexes (53–40 Ma) and by major volcanism in the Challis volcanic field (51–43 Ma). This extensional tectonism apparently deformed and uplifted a broad region, shedding voluminous sediments toward depocenters to the west and southeast. In the Franciscan Coastal belt, the major increase in sediment input apparently triggered a pulse of massive accretion, a pulse ultimately controlled by continental tectonism far within the interior of the North American plate, rather than by some tectonic event along the plate boundary itself.

Lithospheric Structure and Isostasy of Central Andes: Implication for plate Coupling

NASA Astrophysics Data System (ADS)

Mahatsente, R.; Rutledge, S.

2017-12-01

A significant section of the Peru-Chile convergent zone is building up stresses. The interseismic coupling in northern and southern Peru is significantly high indicating, elastic energy accumulation since the 1746 and 1868 earthquakes of magnitude 8.6 and 8.8 , respectively. Similar seismic patterns have also been observed in Central Chile. The plate interface beneath Central Chile is highly coupled, and the narrow zones of low coupling separate seismic gaps. The reasons for the seismic gaps and plate coupling are yet unknown, but the configuration of the slab is thought to be the main factor. Here, we assessed the locking mechanism and isostatic state of the Central Andes based on gravity models of the crust and upper mantle structure. The density models are based on satellite gravity data and are constrained by velocity models and earthquake hypocenters. The gravity models indicate a high-density batholithic structure in the fore-arc, overlying the subducting Nazca plate. This high-density body pushes downward on the slab, causing the slab to lock with the overlying continental plate. The increased compressive stress closer to the trench, due to the increased contact area between the subducting and overriding plates, may have increased the plate coupling in the Central Andes. Thus, trench parallel crustal thickness and density variations along the Central Andes and buoyancy force on the subducting Nazca plate may control plate coupling and asperity generation. The western part of the Central Andes may be undercompensated. There is a residual topography of 800 m in the western part of the Central Andes that cannot be explained by the observed crustal thicknesses. Thus, part of the observed topography in the western part of the Central Andes may be dynamically supported by mantle wedge flow below the overriding plate.

Crustal underthrusting in the Crimea - Northern Black Sea area

NASA Astrophysics Data System (ADS)

Yegorova, Tamara; Gobarenko, Valentina; Murovskaya, Anna; Sheremet, Yevgeniya

2016-04-01

The southern Crimean Mountains and the Greater Caucasus form a fold and thrust belt located on the northern margin of the Black Sea, south of Precambrian East European Craton. It is limited to the south by the Main Caucasus Thrust that runs along the whole of the northern margin of the Black Sea and is related to a zone of present day seismicity along the southern Crimea-Caucasus coast of the Sea (Crimean Seismogenic Zone). Strong seismic activity in the region indicates active on-going tectonic processes caused by collision of Eurasian and Arabian plates. In the vicinity of the seismogenic zone there is a transition from thick continental crust on the north to thin suboceanic one on the south in the sea. However, type and structural relations between them are known poorly. To understand better geodynamic processes, there were collected data on the earthquakes that were analyzed together with focal mechanisms of strong earthquakes, new results of geological structural analysis and paleostress reconstructions by kinematic method. These allowed drawing the following conclusions. Seismic activity in the study region, evidenced of active tectonic processes under compression and transpression at the transition from the southern margin of the East European Craton (Scythian Platform) to the Black Sea, is confirmed by predominance of reverse mechanisms among 31 focal mechanisms. In the seismogenic zone, much of which is located along the continental slope, there are three subzones (from east to west): 1) Kerch-Taman one dipping northwards at angle 30 degrees to the depth of 90 km; 2) South-Coast subzone gently dipping to the southeast at angle of 18 degrees with foci depth range 10-45 km, and 3) orthogonal to the latter and confining it from the west the Sevastopol one, characterized by scattered seismicity. The earthquake foci are located in the gradient zone that separates intense Crimea gravity high and positive anomaly of Northern Caucasus from negative gravity field of the Black Sea. The north-south tight band of the South-Coast subzone epicenters relates with highest gravity gradient offshore and is traced northward onshore on transition zone between the Western and Eastern Crimean Mountains. In the eastern part the reverse and strike-slip faulting prevail, while in the western part - the strike-slip and normal faults occur. The Kerch-Taman subzone is characterized by underthrusting the East Black Sea microplate with thin suboceanic (or strongly extended continental) crust below the Scythian Platform with thick continental crust. In the South-Coast subzone this process is complicated by wedging the frontal part of suboceanic crust into the mid-crust of Crimea. Sevastopol branch of the earthquakes is interpreted as zone of strike-slip deformations. The wedging of the East Black Sea microplate into the Scythian Plate crust in Crimea causes intense gravity anomaly of Crimean Mountains and strong present-day uplift of the latter. The analogue of described wedging mechanism seems to be geodynamic processes in the Ivrea area in Western Alps, ccharacterized by similar crust structure and intense gravity anomaly.

Petrology, thermobarometry and geochronology of Yelapa Complex: Implications in the tectonic history of the basement of Puerto Vallarta Batholith, Mexico

NASA Astrophysics Data System (ADS)

Gutiérrez Aguilar, F.; Schaaf, P. E. G.; Hernandez-Trevino, T.; Solis-Pichardo, G.; Vite-Sánchez, O.

2017-12-01

The Yelapa Complex (YC) is localizated in the north, central and western of Cabo Corrientes in Jalisco, México. Is constituted by metasedimentary, metaigneous and migmatites which are intruded by deformed plutons. The YC are part of the Puerto Vallarta Batholith a body of 9000 km2 exposed at the mid-western part of the Mexican Pacific margin. The para-gneis of YC in the region of Chimo, present a mineral assemblage of Sil + Bt + Pl ± Grt ± Fsp ± Ilm. The orto-gneis in the región of Cabo Corrientes are constituted by Pl + Amp ± Qz ± Ap ± Zrn. Phase equilibria modelling of two paragneis yield peak conditions of 7-8 kbar and 650-700ºC. The patterns of REE of the studied rocks suggest: 1) Enrichment of LREE and flat patterns in HREE with respect to chondrite and; 2) Negative Eu anomaly in all samples analyzed suggesting plagioclase fractionation. On the other hand, the study of individual zircons using LA-ICP-MS from 3 para-gneis and 1 orto-gneis yield following information: 1) A máximum depositional age of 223 Ma, which also show abundant zircón populations with ages between 241-273 Ma for para-gneis and 2) The protolith age crystallization of 127 Ma for orto-gneis. The results along with new Sr-Nd isotopic data from whole rock and Rb-Sr in micas, suggest a tectonic evolution for the Yelapa Complex as a transition from a passive continental margin regime ( 223-273 Ma) to a continental arc setting ( 127). Thus, regional metamorphism and multiple magmatic episodes were associated to the convergence of the Farallon and North America plates during the Middle Jurassic to the Late Cretaceous.

Intracontinental Deformation in the NW Iranian Plateau and Comparisons with the Northern Margin of the Tibetan Plateau

NASA Astrophysics Data System (ADS)

Chen, L.; Jiang, M.; Talebian, M.; Wan, B.; Ai, Y.; Ghods, A.; Sobouti, F.; Xiao, W.; Zhu, R.

2017-12-01

This study investigates the intracontinental deformation and its relationship with the structure of the crust and uppermost mantle in the NW Iranian plateau by combining new seismic and geological observations, to understand how this part of the plateau deformed to accommodate the Arabia-Eurasia plate collision and how the property of the lithosphere controls the deformation pattern. In contrast to the adjacent Anatolian block that exhibits westward large-scale extrusion, the northwesternmost part of the Iranian plateau shows dispersed intracontinental deformations with the development of numerous small-scale and discontinuous right-lateral strike-slip faults. The dispersed surface structures and deformation pattern correspond well to the active volcanism and seismically slow crust and uppermost mantle, and hence a weak lithosphere of the area. Further to the southeast are the western part of the Alborz Mountains and the southern Caspian Sea, both of which are characterized by stronger and more rigid lithosphere with relatively fast crust and uppermost mantle and absence of Quaternary volcanoes. A sharp Moho offset of 18 km has been imaged at the border of the Alborz and southern Caspian Sea using teleseismic receiver function data from a dense seismic array deployed under a collaborative project named "China-Iran Geological and Geophysical Survey in the Iranian Plateau (CIGSIP)". The sharp Moho offset and the minor undulations of the Moho on both sides indicate insignificant intracrustal deformation but mainly relative crustal movements between the Alborz Mountains and southern Caspian Sea, a behavior consistent with the relatively rigid nature of the lithosphere. Similar Moho offsets and lithospheric structures have been reported at the borders between the Kunlun Mountains and Qaidam or Tarim Basins in the northern margin of the Tibetan plateau, suggesting the occurrence of relative crustal movements with the effects of rigid continental lithosphere in the region. The new observations in the NW Iranian plateau combined with those in the Tibetan plateau thus provide solid evidence that intracontinental deformation is primarily controlled by the structure and properties of the continental lithosphere that may or may not have been severely altered by the collisional processes at plate margins.

Tectonic escape of the Caribbean plate since the Paleocene: a consequence of the Chicxulub meteor impact?

NASA Astrophysics Data System (ADS)

Rangin, C.; Martinez-Reyes, J.; Crespy, A.; Zitter, T. A. C.

2012-04-01

The debate for Pacific exotic origin versus in situ inter American plate Atlantic origin of the Caribbean plate is active in the scientific community since decades. Independently of the origin of this plate, its fast motion towards the east at a present rate of 2cm/yr is accepted to have been initiated during the early-most Cenozoic. The Paleocene is a key period in the global evolution of Central America mainly marked also by the Chicxulub multiring meteor impact in Yucatan. We question here the genetic relationship between this impact event and the incipient tectonic escape of the Caribbean plate. The mostly recent published models suggest this impact has affected the whole crust down to the Moho, the upper mantle being rapidly and considerably uplifted. The crust was then fragmented 600km at least from the point of impact, and large circular depressions were rapidly filled by clastic sediments from Cantarell to Western Cuba via Chiapas and Belize. North of the impact, the whole Gulf of Mexico was affected by mass gravity sliding, initiated also during the Paleocene in Texas, remaining active in this basin up to present time. South of the impact, in the Caribbean plate, the Yucatan basin was rapidly opened, indicating a fast escape of the crustal material towards the unique free boundary, the paleo-Antilles subduction zone. Shear waves velocity data below the Caribbean plate suggest this crustal tectonic escape was enhanced by the fast eastward flowing mantle supporting a fragmented and stretched crust. The proposed model suggests Chicxulub impact (but also the hypothetic Beata impact) have fragmented brittle crust, then easily drifted towards the east. This could explain the Paleogene evolution of the Caribbean plate largely stretched during its early evolution. Geologically, this evolution could explain the absence of evident Paleogene oblique subduction along the Caribbean plate northern and southern margins, marked only by Mid Cretaceous dragged volcanic complexes, but also the relatively recent motion along the Cayman Fault zone (Miocene instead of Eocene). These results are part of a cooperative research-industry programm conducted by CEREGE/EGERIE, Aix-en-Provence and GeoAzur, Nice, with Frontier Basin study group, TOTAL S.A., Paris.

Control of the Lithospheric Mantle on intracontinental Deformation: Revival of Eastern U.S. Tectonism

NASA Astrophysics Data System (ADS)

Biryol, C. B.; Wagner, L. S.; Fischer, K. M.; Hawman, R. B.

2016-12-01

The present tectonic configuration of the southeastern United States is a product of earlier episodes of arc accretion, continental collision and breakup. This region is located in the interior of the North American Plate, some 1500 km away from closest active plate margin. However, there is ongoing tectonism across the area with multiple zones of seismicity, rejuvenation of the Appalachians of North Carolina, Virginia, and Pennsylvania, and Cenozoic intraplate volcanism. The mechanisms controlling this activity and the modern-day state of stress remain enigmatic. Two factors often regarded as major contributors are plate strength and preexisting inherited structures. Recent improvements in broadband seismic data coverage in the region associated with the South Eastern Suture of the Appalachian Margin Experiment (SESAME) and EarthScope Transportable Array make it possible to obtain detailed information on the structure of the lithosphere in the region. Here we present new tomographic images of the upper mantle beneath the Southeastern United States, revealing large-scale structural variations in the upper mantle. Our results indicate fast seismic velocity patterns that can be interpreted as ongoing lithospheric foundering. We observe an agreement between the locations of these upper mantle anomalies and the location of major zones of tectonism, volcanism and seismicity, providing a viable explanation for modern-day activity in this plate interior setting long after it became a passive margin. Based on distinct variations in the geometry and thickness of the lithospheric mantle and foundered lithosphere, we propose that piecemeal delamination has occurred beneath the region throughout the Cenozoic, removing a significant amount of reworked/deformed mantle lithosphere. Ongoing lithospheric foundering beneath the eastern margin of stable North America explains significant variations in thickness of lithospheric mantle across the former Grenville deformation front.

Synchronous changes in the rift-margin San Jose Island basin and initiation of the Alarcón spreading ridge: implications for rift to drift transition in the Gulf of California

NASA Astrophysics Data System (ADS)

Umhoefer, P. J.; Sutherland, F.; Kent, G.; Harding, A.; Lizarralde, D.; Fletcher, J.; Holbrook, W.; Axen, G.; González-Fernández, A.

2004-12-01

The rift to drift hypothesis is widely cited, but it well known in detail. The low sedimentation rate and recent rifting of the Gulf of California provides insight into the rift-to-drift process. Lizarralde et al. (2007) showed that the style of rifting, based on crustal structure, varies significantly between the central and southern Gulf of California, and this combined with the analysis of sedimentary basins shows the small-scale (~15 km) complexities of the rift-to-drift transition. The shut off of rifting on the eastern side of the plate boundary occurred at ca. 2 - 3 Ma (Aragon-Arreola etal, 2005, Aragon-Arreola & Martin-Barajas, 2007; our unpublished data). Many studies have shown that the western side of the Gulf is still active despite sea-floor spreading occurring on the Alarcón and other short spreading centers since 2 - 3 Ma. At the mouth of the Gulf, magnetic anomalies on the eastern side of the Alarcón rise show that it appears to have changed to seafloor spreading as early as 3.7 Ma. But comparatively, on the eastern side, magnetic anomalies do not indicate the formation of new oceanic crust until 2.5 Ma, so spreading was first fully established at 2.5 Ma. The San Jose Island basin (Umhoefer et al., 2007) began at approximately 4- 6 Ma; the basin had its most rapid subsidence, with faulting accompanying marine sedimentation, from 3.6 ± 0.5 Ma (Ar tuff age) to 2.5-2.4 Ma (forams). Basin margin faulting died and moved east (offshore) shortly after 2.5-2.4 Ma. Late Quaternary marine terraces suggest that faulting rates slowed by 1-2 orders of magnitude since the fault reorganization at 2.5 Ma. These observations suggest that the rift - drift transition started, but is not yet finished, on the western side of the Gulf of California, with low rates of faulting (<1? mm/yr) continuing on the continental margin for reasons that are not well understood. Our work highlights the importance of combining onshore field and MSC data and analyzing entire conjugate rifted margins to accurately assess rifting processes.

Synchronous changes in the rift-margin San Jose Island basin and initiation of the Alarcón spreading ridge: implications for rift to drift transition in the Gulf of California

NASA Astrophysics Data System (ADS)

Umhoefer, P. J.; Sutherland, F.; Kent, G.; Harding, A.; Lizarralde, D.; Fletcher, J.; Holbrook, W.; Axen, G.; González-Fernández, A.

2007-12-01

The rift to drift hypothesis is widely cited, but it well known in detail. The low sedimentation rate and recent rifting of the Gulf of California provides insight into the rift-to-drift process. Lizarralde et al. (2007) showed that the style of rifting, based on crustal structure, varies significantly between the central and southern Gulf of California, and this combined with the analysis of sedimentary basins shows the small-scale (~15 km) complexities of the rift-to-drift transition. The shut off of rifting on the eastern side of the plate boundary occurred at ca. 2 - 3 Ma (Aragon-Arreola etal, 2005, Aragon-Arreola & Martin-Barajas, 2007; our unpublished data). Many studies have shown that the western side of the Gulf is still active despite sea-floor spreading occurring on the Alarcón and other short spreading centers since 2 - 3 Ma. At the mouth of the Gulf, magnetic anomalies on the eastern side of the Alarcón rise show that it appears to have changed to seafloor spreading as early as 3.7 Ma. But comparatively, on the eastern side, magnetic anomalies do not indicate the formation of new oceanic crust until 2.5 Ma, so spreading was first fully established at 2.5 Ma. The San Jose Island basin (Umhoefer et al., 2007) began at approximately 4- 6 Ma; the basin had its most rapid subsidence, with faulting accompanying marine sedimentation, from 3.6 ± 0.5 Ma (Ar tuff age) to 2.5-2.4 Ma (forams). Basin margin faulting died and moved east (offshore) shortly after 2.5-2.4 Ma. Late Quaternary marine terraces suggest that faulting rates slowed by 1-2 orders of magnitude since the fault reorganization at 2.5 Ma. These observations suggest that the rift - drift transition started, but is not yet finished, on the western side of the Gulf of California, with low rates of faulting (<1? mm/yr) continuing on the continental margin for reasons that are not well understood. Our work highlights the importance of combining onshore field and MSC data and analyzing entire conjugate rifted margins to accurately assess rifting processes.

Devonian brachiopods of southwesternmost laurentia: Biogeographic affinities and tectonic significance

USGS Publications Warehouse

Boucot, A.J.; Poole, F.G.; Amaya-Martinez, R.; Harris, A.G.; Sandberg, C.A.; Page, W.R.

2008-01-01

Three brachiopod faunas discussed herein record different depositional and tectonic settings along the southwestern margin of Laurentia (North America) during Devonian time. Depositional settings include inner continental shelf (Cerros de Los Murcielagos), medial continental shelf (Rancho Placeritos), and offshelf continental rise (Rancho Los Chinos). Ages of Devonian brachiopod faunas include middle Early (Pragian) at Rancho Placeritos in west-central Sonora, late Middle (Givetian) at Cerros de Los Murcielagos in northwestern Sonora, and late Late (Famennian) at Rancho Los Chinos in central Sonora. The brachiopods of these three faunas, as well as the gastropod Orecopia, are easily recognized in outcrop and thus are useful for local and regional correlations. Pragian brachiopods dominated by Acrospirifer and Meristella in the "San Miguel Formation" at Rancho Placeritos represent the widespread Appohimchi Subprovince of eastern and southern Laurentia. Conodonts of the early to middle Pragian sulcatus to kindlei Zones associated with the brachiopods confirm the ages indicated by the brachiopod fauna and provide additional information on the depositional setting of the Devonian strata. Biostratigraphic distribution of the Appohimchi brachiopod fauna indicates continuous Early Devonian shelf deposition along the entire southern margin of Laurentia. The largely emergent southwest-trending Transcontinental arch apparently formed a barrier preventing migration and mixing of many genera and species of brachiopods from the southern shelf of Laurentia in northern Mexico to the western shelf (Cordilleran mio-geocline) in the western United States. Middle Devonian Stringocephalus brachiopods and Late Devonian Orecopia gastropods in the "Los Murcielagos Formation" in northwest Sonora represent the southwest-ernmost occurrence of these genera in North America and date the host rocks as Givetian and Frasnian, respectively. Rhynchonelloid brachiopods (Dzieduszyckia sonora) and associated worm tubes in the Los Pozos Formation of the Sonora allochthon in central Sonora are also found in strati-form-barite facies in the upper Upper Devonian (Famennian) part of the Slaven Chert in the Roberts Mountains allochthon (upper plate) of central and western Nevada. Although these brachiopods and worm tubes occur in similar depositional settings along the margin of Laurentia in Mexico, they occur in allochthons that exhibit different tectonic styles and times of emplacement. Thus, the allochthons containing the brachiopods and worm tubes in Sonora and Nevada are parts of separate orogenic belts and have different geographic settings and tectonic histories. Devonian facies belts and faunas in northern Mexico indicate a continuous continental shelf along the entire southern margin of Laurentia. These data, in addition to the continuity of the late Paleozoic Ouachita-Marathon-Sonora orogen across northern Mexico, contradict the early Late Jurassic Mojave-Sonora megashear as a viable hypothesis for large-magnitude offset (600-1100 km) of Proterozoic through Middle Jurassic rocks from California to Sonora. ?? 2008 The Geological Society of America.

Cretaceous to Recent Asymetrical Subsidence of South American and West African Conjugate Margins

NASA Astrophysics Data System (ADS)

Kenning, J.; Mann, P.

2017-12-01

Two divergent interpretations have been proposed for South American rifted-passive margins: the "mirror hypothesis" proposes that the rifted margins form symmetrically from pure shear of the lithosphere while upper-plate-lower plate models propose that the rifted margins form asymmetrically by simple shear. Models based on seismic reflection and refraction imaging and comparison of conjugate, rifted margins generally invoke a hybrid stretching process involving elements of both end member processes along with the effects of mantle plumes active during the rift and passive margin phases. We use subsidence histories of 14, 1-7 km-deep exploration wells located on South American and West African conjugate pairs now separated by the South Atlantic Ocean, applying long-term subsidence to reveal the symmetry or asymmetry of the underlying, conjugate, rift processes. Conjugate pairs characterize the rifted margin over a distance of 3500 km and include: Colorado-South Orange, Punta Del Este-North Orange, South Pelotas-Lüderitz and the North Pelotas-Walvis Basins. Of the four conjugate pairs, more rapid subsidence on the South American plate is consistently observed with greater initial rift and syn-rift subsidence rates of >60m/Ma (compared to <15 m/Ma) between approximately 145-115 Ma. High rates of tectonically-induced subsidence >100 m/Ma are observed offshore South Africa between approximately 120-80 Ma, compatible with onset of the post-rift thermal sag phase. During this period the majority of burial is completed and rates remain low at <10 m/Ma during most of the late Cretaceous and Cenozoic. The conjugate margin of Argentina/Uruguay displays more gradual subsidence throughout the Cretaceous, consistently averaging a moderate 15-30m/Ma. By the end of this stage there is a subsequent increase to 25-60 m/Ma within the last 20 Ma, interpreted to reflect lithospheric loading due to increased sedimentation rates during the Cenozoic. This increase in subsidence rate is not seen in the African conjugate section where the majority of sediments bypassed the highly aggraded Cretaceous shelf. Initially greater on the Brazilian margin compared to Namibia, here both margins exhibit moderate-steep subsidence curves until 65-55 Ma where there is reduced subsidence during much of the Late Cretaceous until 20 Ma.

Paleoseismicity and neotectonics of the Aleutian Subduction Zone—An overview

NASA Astrophysics Data System (ADS)

Carver, Gary; Plafker, George

The Aleutian subduction zone is one of the most seismically active plate boundaries and the source of several of the world's largest historic earthquakes. The structural architecture of the subduction zone varies considerably along its length. At the eastern end is a tectonically complex collision zone where the allochthonous Yakutat terrane is moving northwest into mainland Alaska. West of the collision zone a shallow-dipping subducted plate beneath a wide forearc, nearly orthogonal convergence, and a continental-type subduction regime characterizes the eastern part of the subduction zone. In the central part of the subduction zone, convergence becomes increasingly right oblique and the forearc is divided into a series of large clockwise-rotated fault-bounded blocks. Highly oblique convergence and island arc tectonics characterize the western part of the subduction zone. At the extreme western end of the arc, the relative plate motion is nearly pure strike-slip. A series of great subduction earthquakes ruptured most of the 4000-km length of the subduction zone during a period of several decades in the mid 1900s. The majority of these earthquakes broke multiple segments as defined by the large-scale structure of the overriding plate margin and patterns of historic seismicity. Several of these earthquakes generated Pacific-wide tsunamis and significant damage in the southwestern and south-central regions of Alaska. Characterization of previous subduction earthquakes is important in assessing future seismic and tsunami hazards. However, at present such information is available only for the eastern part of the subduction zone. The 1964 Alaska earthquake (M 9.2) ruptured about ˜950 km of the plate boundary that encompassed the Kodiak and Prince William Sound (PWS) segments. Within this region, nine paleosubduction earthquakes in the past ˜5000 years are recognized on the basis of geologic evidence of sudden land level change and, at some sites, coeval tsunami deposits. Carbon 14-based chronologies indicate recurrence intervals between median calibrated ages for these paleoearthquakes range from 333 to 875 years. The most recent occurred about 489 years ago and broke only the Kodiak segment. During the previous three cycles, both the Kodiak and PWS segments were involved in either multiple-segment ruptures or closely timed pairs of single segment ruptures. Evidence for the earlier paleosubduction earthquakes has been found only at sites in the PWS segment. Thus, future work on the paleoseismicity of other segments would by particular valuable in defining the seismic behavior of the subduction zone.

Gunbarrel mafic magmatic event: A key 780 Ma time marker for Rodinia plate reconstructions

USGS Publications Warehouse

Harlan, S.S.; Heaman, L.; LeCheminant, A.N.; Premo, W.R.

2003-01-01

Precise U-Pb baddeleyite dating of mafic igneous rocks provides evidence for a widespread and synchronous magmatic event that extended for >2400 km along the western margin of the Neoproterozoic Laurentian craton. U-Pb baddeleyite analyses for eight intrusions from seven localities ranging from the northern Canadian Shield to northwestern Wyoming-southwestern Montana are statistically indistinguishable and yield a composite U-Pb concordia age for this event of 780.3 ?? 1.4 Ma (95% confidence level). This 780 Ma event is herein termed the Gunbarrel magmatic event. The mafic magmatism of the Gunbarrel event represents the largest mafic dike swarm yet identified along the Neoproterozoic margin of Laurentia. The origin of the mafic magmatism is not clear, but may be related to mantle-plume activity or upwelling asthenosphere leading to crustal extension accompanying initial breakup of the supercontinent Rodinia and development of the proto-Pacific Ocean. The mafic magmatism of the Gunbarrel magmatic event at 780 Ma predates the voluminous magmatism of the 723 Ma Franklin igneous event of the northwestern Canadian Shield by ???60 m.y. The precise dating of the extensive Neoproterozoic Gunbarrel and Franklin magmatic events provides unique time markers that can ultimately be used for robust testing of Neoproterozoic continental reconstructions.

High-resolution multi-channel seismic images of the Queen Charlotte Fault system offshore southeastern Alaska

NASA Astrophysics Data System (ADS)

Miller, N. C.; Brothers, D. S.; Kluesner, J.; Balster-Gee, A.; Ten Brink, U. S.; Andrews, B. D.; Haeussler, P. J.; Watt, J. T.; Dartnell, P.; East, A. E.

2016-12-01

We present high-resolution multi-channel seismic (MCS) images of fault structure and sedimentary stratigraphy along the southeastern Alaska margin, where the northern Queen Charlotte Fault (QCF) cuts the shelf-edge and slope. The QCF is a dominantly strike slip system that forms the boundary between the Pacific (PA) and North American (NA) plates offshore western Canada and southeastern Alaska. The data were collected using a 64 channel, 200 m digital streamer and a 0.75-3 kJ sparker source aboard the R/V Norseman in August 2016. The survey was designed to cross a seafloor fault trace recently imaged by multibeam sonar (see adjacent poster by Brothers et al.) and to extend the subsurface information landward and seaward from the fault. Analysis of these MCS and multibeam data focus on addressing key questions that have significant implications for the kinematic and geodynamic history of the fault, including: Is the imaged surface fault in multibeam sonar the only recently-active fault trace? What is the shallow fault zone width and structure, is the internal structure of the recently-discovered pull-apart basin a dynamically developing structure? How does sediment thickness vary along the margin and how does this variation affect the fault expression? Can previous glacial sequences be identified in the stratigraphy?

The Red Sea structural architecture assessment based on yield strength spatial variations and Arabian margin preexisting structures

NASA Astrophysics Data System (ADS)

Alotaibi, T.; Furlong, K. P.

2016-12-01

Rift initiation and localization might reflect spatial changes in the lithospheric yield strength. However, this does not appear to be the case in the Red Sea extensional system where fission track analysis shows no significant changes in the geothermal gradient prior to the Red Sea rift onset. In contrast, though the whole Red Sea rift initiated 25 Ma ago, its extensional architecture changes dramatically along strike from narrow localized spreading in the south to asymmetrical diffuse extension north of 21° latitude. This onset of diffuse extension has been recorded in the north-western Arabian margin as old as 33 Ma. Such diversity in the extensional style might reflect along strike yield strength variations as a consequence of the geological setting in the Arabian margin. The north-western Arabian basin, which is part of the Arabian margin, bounded by Qiba high from the east, the Arabian shield from the south and the west and Syrian plateau from the north. The basin accommodates part of the Red Sea diffuse extension and has a preexisting structural architecture represented in the Cenozoic failed rift that called Sarhan graben. Our goal is to analyze the current lithospheric yield strength spatial variations along the Red Sea rift and emphasize their relationship with the Arabian margin structural architecture. We hypothesize that the north-western Arabian margin's lithospheric weakness and structural diversity are playing an important role in producing region of diffuse extension by their interaction with the forces applied by far field stresses represented by the New Tethys slab pull. On the other hand, the south-western Arabian margin interacts with the far field stresses as a single strong block in which led to localize the extension in the southern Red Sea. Our work may improve the scientific community understanding for how rifts initiate and evolve over time.

Crustal Structure at the North Eastern Tip of Rivera Plate, Nayarit- Marias Islands Region: Scenarios and Tectonic Implications. Tsujal Project

NASA Astrophysics Data System (ADS)

Danobeitia, J.; Bartolome, R.; Barba, D. C., Sr.; Nunez-Cornu, F. J.; Bandy, W. L.; Prada, M.; Cameselle, A. L.; Nunez, D.; Espindola, J. M.; Estrada, F.; Zamora, A.; Gomez, A.; Ortiz, M.

2014-12-01

A primarily analysis of marine geophysical data acquired aboard the RRS James Cook in the framework of the project "Characterization of seismic hazard and tsunami associated with cortical contact structure Rivera Jalisco Block Plate (TSUJAL)" is presented. This survey was held in the region of Nayarit-Tres Marias Islands between February and March 2014. The examination of data recorded by 16 OBS 's, deployed along 4 wide angle seismic profiles is presented, using an airgun-array seismic source of 6800 c.i., which allows sampling the crustal structure to the Moho. The profiles are located along the margin off the Marias Islands: a profile of over 200 km NNW-SSE direction and parallel to the western flank of the Islas Marias Islands and three orthogonal thereto. These perpendicular sections sample the lithosphere from the north of Maria Madre Island with a profile of 100 km length, across Maria Magdalena and Mari Cleofas Islands, with a profile of 50 km long, till south of Maria Cleofas with a profile of 100 km long. Coincident multichannel seismic profiles with refraction ones are also surveyed, although shooting with a source of 3,540 c. i., and acquired with a digital "streamer" of 6.0 km long. Simultaneously, multibeam, parametric and potential field data were recorded during seismic acquisition A first analysis shows an anomalously thickened crust in the western flank of the Marias Islands, as indicated by relatively short pre-critical distances of 30-35 km. While the moderate dip of 7 ° of the subduction of the Pacific oceanic plate favors somehow this effect, the existence of a remnant crustal fragment is also likely. Moreover, the images provided by the parametric sounding show abundant mass wasting deposits suggesting of recent active tectonics, possibly generated by earthquakes with moderate magnitude as those reported in the Marias Islands. This set of geophysical data, not only provide valuable information for the seismogenic characterization and associated hazard with Rivera-Plate/Jalisco block, but it will also contribute to decipher the complex interplay between the accretion of the East Pacific Rise, the Magdalena Crest and the opening of the Gulf of California in the northeastern part of the diffuse Middle America Trench.

Deep Stucture of the Northwestern Atlantic Moroccan Margin Studied by OBS and Deep Multichannel Seismic Reflection.

NASA Astrophysics Data System (ADS)

MALOD, J. A.; Réhault, J.; Sahabi, M.; Géli, L.; Matias, L.; Diaz, J.; Zitellini, N.

2001-12-01

The Northwestern Atlantic Moroccan margin, a conjugate of the New Scotland margin, is one of the oldest passive margin of the world. Continental break up occurred at early Liassic time and the deep margin is characterized by a large salt basin. A good knowledge of this basin is of major interest to improve the initial reconstruction between Africa, North America and Iberia (Eurasia). It is also a good opportunity to study a mature passive margin and model its structure and evolution.Moreover, there is a need to assess the geological hazards linked to the neotectonic activity within the Africa-Eurasia plate boundary. These topics have been adressed during the SISMAR cruise carried out from April 9th to May 4th 2001.During this cruise, 3667 km of multichannel seismic reflection (360 channels, 4500 m long streamer, 4800 ci array of air guns) were recorded together with refraction records by means of 48 OBH/OBS drops. Simultaneously, some of the marine profiles have been extended onshore with 16 portable seismic land stations. We present the initial results of this study. Off El Jadida, the Moho and structures within the thinned continental crust are well imaged on both the reflection and refraction records. In the northern area, off Casablanca, we follow the deepening of the moroccan margin beneath the up to 9 sec (twtt) allochtonous series forming a prism at the front the Rif-Betic chain. Sismar cruise has been also the opportunity to record long seismic profiles making the junction between the Portuguese margin and the Moroccan one, and crossing the Iberian-African plate boundary. This allows to observe the continuity of the sedimentary sequence after the end of the large inter-plate motion in Early Cretaceous. In addition to the authors, SISMAR Group includes: AMRHAR Mostafa, BERMUDEZ VASQUEZ Antoni, CAMURRI Francesca, CONTRUCCI Isabelle, CORELA Carlos, DIAZ Jordi, DORVAL Philippe, EL ARCHI Abdelkrim, EL ATTARI Ahmed, GONZALEZ Raquel, HARMEGNIES Francois, JAFFAL Mohamed, KLINGELÖFER Fraucke, LANDURÉ Jean Yves, LEGALL Bernard, MAILLARD-LENOIR Agnès, MARTIN Christophe, MEHDI Khalid, MERCIER Eric, MOULIN Maryline, OUAJHAIN Brahim, PERROT Julie, ROLET Joël, RUELLAN Etienne, TEIXIRA Fernando, TERRINHA Pedro, ZOURARAH Bendehhou.

Basins in ARC-continental collisions

USGS Publications Warehouse

Draut, Amy E.; Clift, Peter D.; Busby, Cathy; Azor, Antonio

2012-01-01

Arc-continent collisions occur commonly in the plate-tectonic cycle and result in rapidly formed and rapidly collapsing orogens, often spanning just 5-15 My. Growth of continental masses through arc-continent collision is widely thought to be a major process governing the structural and geochemical evolution of the continental crust over geologic time. Collisions of intra-oceanic arcs with passive continental margins (a situation in which the arc, on the upper plate, faces the continent) involve a substantially different geometry than collisions of intra-oceanic arcs with active continental margins (a situation requiring more than one convergence zone and in which the arc, on the lower plate, backs into the continent), with variable preservation potential for basins in each case. Substantial differences also occur between trench and forearc evolution in tectonically erosive versus tectonically accreting margins, both before and after collision. We examine the evolution of trenches, trench-slope basins, forearc basins, intra-arc basins, and backarc basins during arc-continent collision. The preservation potential of trench-slope basins is low; in collision they are rapidly uplifted and eroded, and at erosive margins they are progressively destroyed by subduction erosion. Post-collisional preservation of trench sediment and trench-slope basins is biased toward margins that were tectonically accreting for a substantial length of time before collision. Forearc basins in erosive margins are usually floored by strong lithosphere and may survive collision with a passive margin, sometimes continuing sedimentation throughout collision and orogeny. The low flexural rigidity of intra-arc basins makes them deep and, if preserved, potentially long records of arc and collisional tectonism. Backarc basins, in contrast, are typically subducted and their sediment either lost or preserved only as fragments in melange sequences. A substantial proportion of the sediment derived from collisional orogenesis ends up in the foreland basin that forms as a result of collision, and may be preserved largely undeformed. Compared to continent-continent collisional foreland basins, arc-continent collisional foreland basins are short-lived and may undergo partial inversion after collision as a new, active continental margin forms outboard of the collision zone and the orogen whose load forms the basin collapses in extension.

Stress Orientations and Strain Rates in the Upper Plate of a `Locked' subduction zone, at southernmost North Island, New Zealand

NASA Astrophysics Data System (ADS)

Evanzia, D. A. D.; Lamb, S. H.; Savage, M. K.

2017-12-01

The southern North Island, New Zealand is located at the southern Hikurangi Margin, where the Pacific Plate is obliquely subducting westward underneath the Australian Plate. The orientations of the principle stresses in the overriding plate are determined from microseismic focal mechanisms detected and located using the temporary SAHKE and permanent GeoNet seismic array operating during 2009-2010. The microseismic earthquakes are located with the NonLinLoc method, using a New Zealand specific 3D velocity model; only those earthquakes located above the modelled subduction plate interface are used. Strain rate parameters calculations are calculated using cGPS velocities from 56 stations located from the central North Island to the northernmost South Island, New Zealand. In the region west of the Tararua-range-bounding Wairarapa fault (the Western region), the orientations of stresses indicate a normal regime (S1: vertical; S2 & S3: horizontal), with SHmax trending ENE. In the Central Basin region (east of the Wairarapa fault) the orientations of the stresses indicate a reverse regime (S3: vertical; S1 & S2: horizontal), with SHmax orientated NW. The low seismicity rates in the Eastern region make the results unreliable. There is a distinct difference between the strain rate and vorticity on either side the Wairarapa fault. Strain rate and vorticity rates increase west and decreased east of the Wairarapa; this correlates well with the pattern of observed seismicity. The southern North Island is predominately contracting, except for a region on the West coast, where some expansion is occurs. This pattern of expansion in the West and contraction in the center of the study area, calculated from cGPS, is similar the stress inversion results calculated from focal mechanisms. These similarities suggest that the present stress and strain rates are collinear, as occurs in isotropic media.

Structural geology and stress history of the Platanares geothermal site, Honduras: implications on the tectonics of the northwestern Caribbean plate boundary

NASA Astrophysics Data System (ADS)

Aldrich, M. J.; Adams, Andrew I.; Escobar, Carlos

1991-03-01

The structural geology of the Platanares geothermal site in western Honduras, located about 25 km south of the northern boundary of the Caribbean plate, is the result of post Early Miocene extensional deformation. Normal faults, many with listric geometries, are numerous throughout the area. Strike-slip faulting has mostly occurred on reactived normal faults. Analysis of the fault slip data shows an older minimum principal stress, σ 3, oriented approximately N-S and a contemporary σ 3 tensional and oriented ENE-WSW. The analysis suggests that σ 3 has rotated clockwise since the Early Miocene although some of the change in orientation of σ 3 might reflect counterclockwise rotation of the crust about a vertical axis. The σ 1 and σ 2 stress axes apparently switched recently, with the σ 3 axis remaining unchanged. These results are consistent with a tectonic model in which the east-drifting Caribbean plate is pinned against North America by the subducting Cocos plate (Malfait and Dinkleman, 1972) and the northern and southern margins of the Caribbean plate are broad, mobile zones that are undergoing counterclockwise and clockwise rotations respectively (Gose, 1985). The majority of the hot springs at Platanares lie along Quebrada del Agua Caliente. Fractures control the movement of the geothermal waters. Hot springs occur along joints and faults and, in places, hot water flows laterally along bedding planes. If the fractures also control the movement of water at depth then the source reservoir of the geothermal waters may be located northeast of the principal hot spring areas along the quebrada since the majority of the faults dip in that direction. However, if the fault that seems to have controlled the development of Quebrada del Agua Caliente is vertical as inferred then the main reservoir may lie directly beneath this drainage.

Seismicity of the Earth 1900-2010 eastern margin of the Australia plate

USGS Publications Warehouse

Benz, Harley M.; Herman, Matthew; Tarr, Arthur C.; Hayes, Gavin P.; Furlong, Kevin P.; Villaseñor, Antonio; Dart, Richard L.; Rhea, Susan

2011-01-01

The eastern margin of the Australia plate is one of the most seismically active areas of the world due to high rates of convergence between the Australia and Pacific plates. In the region of New Zealand, the 3,000 km long Australia-Pacific plate boundary extends from south of Macquarie Island to the southern Kermadec Island chain. It includes an oceanic transform (the Macquarie Ridge), two oppositely verging subduction zones (Puysegur and Hikurangi), and a transpressive continental transform, the Alpine Fault through South Island, New Zealand. Since 1900, there have been 15 M7.5+ earthquakes recorded near New Zealand. Nine of these, and the four largest, occurred along or near the Macquarie Ridge, including the 1989 M8.2 event on the ridge itself, and the 2004 M8.1 event 200 km to the west of the plate boundary, reflecting intraplate deformation. The largest recorded earthquake in New Zealand itself was the 1931 M7.8 Hawke's Bay earthquake, which killed 256 people. The last M7.5+ earthquake along the Alpine Fault was 170 years ago; studies of the faults' strain accumulation suggest that similar events are likely to occur again.

The thermal state of the Arabian plate derived from heat flow measurements in Oman and Yemen

NASA Astrophysics Data System (ADS)

Rolandone, Frederique; Lucazeau, Francis; Leroy, Sylvie; Mareschal, Jean-Claude; Jorand, Rachel; Goutorbe, Bruno; Bouquerel, Hélène

2013-04-01

The dynamics of the Afar plume and the rifting of the Red Sea and the Gulf of Aden affect the present-day thermal regime of the Arabian plate. However, the Arabian plate is a Precambrian shield covered on its eastern part by a Phanerozoic platform and its thermal regime, before the plume and rifting activities, should be similar to that of other Precambrian shields with a thick and stable lithosphere. The first heat flow measurements in the shield, in Saudi Arabia, yielded low values (35-44 mW/m2), similar to the typical shields values. Recent heat flow measurements in Jordan indicate higher values (56-66 mW/m2). As part of the YOCMAL project (YOung Conjugate MArgins Laboratory), we have conducted heat flow measurements in southern and northern Oman to obtain 10 new heat flux values in the eastern Arabian plate. We also derived 20 heat flux values in Yemen and Oman by processing thermal data from oil exploration wells. The surface heat flux in these different locations is uniformly low (45 mW/m2). The heat production in samples from the Dhofar and Socotra Precambrian basement is also low (0.7 µW/m3). Differences in heat flow between the eastern (60 mW/m2) and the western (45 mW/m2) parts of Arabia reflect differences in crustal heat production as well as a higher mantle heat flux in the west. We have calculated a steady state geotherm for the Arabian platform that intersects the isentropic temperature profile at a depth of about 150 km, consistent with the seismic observations. Seismic tomography studies of the mantle beneath Arabia also show this east-west contrast. Seismic studies have shown that the lithosphere is rather thin, 100 km or less below the shield and 150 km below the platform. The lithospheric thickness for the Arabian plate is 150 km, and the progressive thinning near the Red Sea, caused by the thermal erosion of the plume material, is too recent to be detected at the surface. The Afar plume mostly affects the base of the Arabian lithosphere along the Red Sea and the western part of the Gulf of Aden. The extent of this effect is explained by channeling of the asthenospheric magma by the rift. The subdued penetration into the Gulf of Aden is probably due to the important segmentation of the rift. The continental domain is not affected by rifting in the Gulf of Aden. The main thermal effect of the Arabian plate is probably the channeling of the Afar plume to the North.

From rifting to orogeny; using sediments to unlock the secrets of the Greater Caucasus

NASA Astrophysics Data System (ADS)

Vincent, Stephen; Guo, Li; Lavrishchev, Vladimir; Maynard, James; Harland, Melise

2017-04-01

The western Greater Caucasus formed by the tectonic inversion of the western strand of the Greater Caucasus Basin, a Mesozoic rift that opened at the southern margin of Laurasia. Facies analysis has identified fault-bounded regions of basinal, turbiditic and hemipelagic sediments. These are flanked by areas of marginal, shallow marine sediments to the north and south. Subsidence analysis derived from lithology, thickness and palaeowater depth data indicates that the main phase of rifting occurred during the Aalenian to Bajocian synchronous with that in the eastern Alborz and, possibly, the South Caspian Basin. Secondary episodes of subsidence during the late Tithonian to Berriasian and Hauterivian to early Aptian are tentatively linked to initial rifting within the western, and possibly eastern, Black Sea, and during the late Campanian to Danian to the opening of the eastern Black Sea. Initial uplift, subaerial exposure and sediment derivation from the western Greater Caucasus occurred at the Eocene-Oligocene transition. Oligocene and younger sediments on the southern margin of the former basin were derived from the inverting basin and uplifted parts of its northern margin, indicating that the western Greater Caucasus Basin had closed by this time. The previous rift flanks were converted to flexural basins that accumulated thick, typically hemipelagic and turbiditic sediments in the early, underfilled, stage of their development. A predominance of pollen representing a montane forest environment (dominated by Pinacean pollen) within these sediments suggests that the uplifting Caucasian hinterland had a paleoaltitude of around 2 km from Early Oligocene time. The closure of the western Greater Caucasus Basin and significant uplift of the range at c. 34 Ma is earlier than stated in many studies and needs to be incorporated into geodynamic models for the Arabia-Eurasia region.

African humid periods triggered the reactivation of a large river system in Western Sahara.

PubMed

Skonieczny, C; Paillou, P; Bory, A; Bayon, G; Biscara, L; Crosta, X; Eynaud, F; Malaizé, B; Revel, M; Aleman, N; Barusseau, J-P; Vernet, R; Lopez, S; Grousset, F

2015-11-10

The Sahara experienced several humid episodes during the late Quaternary, associated with the development of vast fluvial networks and enhanced freshwater delivery to the surrounding ocean margins. In particular, marine sediment records off Western Sahara indicate deposition of river-borne material at those times, implying sustained fluvial discharges along the West African margin. Today, however, no major river exists in this area; therefore, the origin of these sediments remains unclear. Here, using orbital radar satellite imagery, we present geomorphological data that reveal the existence of a large buried paleodrainage network on the Mauritanian coast. On the basis of evidence from the literature, we propose that reactivation of this major paleoriver during past humid periods contributed to the delivery of sediments to the Tropical Atlantic margin. This finding provides new insights for the interpretation of terrigenous sediment records off Western Africa, with important implications for our understanding of the paleohydrological history of the Sahara.

Phanerozoic geological evolution of Northern and Central Africa: An overview

NASA Astrophysics Data System (ADS)

Guiraud, R.; Bosworth, W.; Thierry, J.; Delplanque, A.

2005-10-01

The principal paleogeographic characteristics of North and Central Africa during the Paleozoic were the permanency of large exposed lands over central Africa, surrounded by northerly and northwesterly dipping pediplanes episodically flooded by epicontinental seas related to the Paleotethys Ocean. The intra-continental Congo-Zaire Basin was also a long-lived feature, as well as the Somali Basin from Late Carboniferous times, in conjunction with the development of the Karoo basins of southern Africa. This configuration, in combination with eustatic sea-level fluctuations, had a strong influence on facies distributions. Significant transgressions occurred during the Early Cambrian, Tremadocian, Llandovery, Middle to Late Devonian, Early Carboniferous, and Moscovian. The Paleozoic tectonic history shows an alternation of long periods of predominantly gentle basin subsidence and short periods of gentle folding and occasionally basin inversion. Some local rift basins developed episodically, located mainly along the northern African-Arabian plate margin and near the West African Craton/Pan-African Belt suture. Several arches or spurs, mainly N-S to NE-SW trending and inherited from late Pan-African fault swarms, played an important role. The Nubia Province was the site of numerous alkaline anorogenic intrusions, starting in Ordovician times, and subsequently formed a large swell. Paleozoic compressional events occurred in the latest Early Cambrian ("Iskelian"), Medial Ordovician to earliest Silurian ("pre-Caradoc" and "Taconian"), the end Silurian ("Early Acadian" or "Ardennian"), mid-Devonian ("Mid-Acadian"), the end Devonian ("Late Acadian" or "Bretonnian"), the earliest Serpukhovian ("Sudetic"), and the latest Carboniferous-earliest Permian ("Alleghanian" or "Asturian"). The strongest deformations, including folding, thrusting, and active strike-slip faulting, were registered in Northwestern Africa during the last stage of the Pan-African Belt development around the West African Craton (end Early Cambrian) and during the polyphased Hercynian-Variscan Orogeny that extended the final closure of the Paleotethys Ocean and resulted in the formation of the Maghrebian and Mauritanides belts. Only gentle deformation affected central and northeastern African during the Paleozoic, the latter remaining a passive margin of the Paleotethys Ocean up to the Early Permian when the development of the Neotethys initiated along the Eastern Mediterranean Basins. The Mesozoic-Cenozoic sedimentary sequence similarly consists of a succession of eustatically and tectonically controlled depositional cycles. Through time, progressive southwards shift of the basin margins occurred, related to the opening of the Neotethys Ocean and to the transgressions resulting from warming of the global climate and associated rise of the global sea level. The Guinean-Nigerian Shield, the Hoggar, Tibesti-Central Cyrenaica, Nubia, western Saudi Arabia, Central African Republic, and other long-lived arches delimited the principal basins. The main tectonic events were the polyphased extension, inversion, and folding of the northern African-Arabian shelf margin resulting in the development of the Alpine Maghrebian and Syrian Arc belts, rifting and drifting along the Central Atlantic, Somali Basins, and Gulf of Aden-Red Sea domains, inversion of the Murzuq-Djado Basin, and rifting and partial inversion along the Central African Rift System. Two major compressional events occurred in the Late Santonian and early Late Eocene. The former entailed folding and strike-slip faulting along the northeastern African-northern Arabian margin (Syrian Arc) and the Central African Fold Belt System (from Benue to Ogaden), and thrusting in Oman. The latter ("Pyrenean-Atlasic") resulted in folding, thrusting, and local metamorphism of the northern African-Arabian plate margin, and rejuvenation of intra-plate fault zones. Minor or more localized compressional deformations took place in the end Cretaceous, the Burdigalian, the Tortonian and Early Quaternary. Recent tectonic activity is mainly concentrated along the Maghrebian Alpine Belt, the offshore Nile Delta, the Red Sea-East African Rifts Province, the Aqaba-Dead Sea-Bekaa sinistral strike-slip fault zone, and some major intra-plate fault zones including the Guinean-Nubian, Aswa, and central Sinai lineaments. Large, long-lived magmatic provinces developed in the Egypt-Sudan confines (Nubia), in the Hoggar-Air massifs, along the Cameroon Line and Nigerian Jos Plateau, and along the Levant margin, resulting in uplifts that influenced the paleogeography. Extensive tholeiitic basaltic magmatism at ˜200 Ma preceded continental break-up in the Central Atlantic domain, while extensive alkaline to transitional basaltic magmatism accompanied the Oligocene to Recent rifting along the Red Sea-Gulf of Aden-East African rift province.

Resolving the Detailed Spatiotemporal Slip Evolution of Deep Tremor in Western Japan

NASA Astrophysics Data System (ADS)

Ohta, Kazuaki; Ide, Satoshi

2017-12-01

We study the detailed spatiotemporal behavior of deep tremor in western Japan through the development and application of a new slip inversion method. Although many studies now recognize tremor as shear slip along the plate interface manifested in low-frequency earthquake (LFE) swarms, a conventional slip inversion analysis is not available for tremor due to insufficient knowledge of source locations and Green's functions. Here we introduce synthetic template waveforms, which are typical tremor waveforms obtained by stacking LFE seismograms at arranged points along the plate interface. Using these synthetic template waveforms as substitutes for Green's functions, we invert the continuous tremor waveforms using an iterative deconvolution approach with Bayesian constraints. We apply this method to two tremor burst episodes in western and central Shikoku, Japan. The estimated slip distribution from a 12 day tremor burst episode in western Shikoku is heterogeneous, with several patchy areas of slip along the plate interface where rapid moment releases with durations of <100 s regularly occur. We attribute these heterogeneous spatiotemporal slip patterns to heterogeneous material properties along the plate interface. For central Shikoku, where we focus on a tremor burst episode that occurred coincidentally with a very low frequency earthquake (VLF), we observe that the source size of the VLF is much larger than that estimated from tremor activity in western Shikoku. These differences in the size of the slip region may dictate the visibility of VLF signals in observed seismograms, which has implications for the mechanics of slow earthquakes and subduction zone processes.

Phenotypic and genetic characterization of western prairie clover collections from the western USA

USDA-ARS?s Scientific Manuscript database

Few North American legumes are available for rangeland revegetation in the semi-arid western USA. Western prairie clover (Dalea ornata [Douglas] Eaton & Wright) is a perennial legume with desirable characteristics and is distributed in the Great Basin, Snake River Basin, and southern Columbia Plate...

A reworked Lake Zone margin: Chronological and geochemical constraints from the Ordovician arc-related basement of the Hovd Zone (western Mongolia)

NASA Astrophysics Data System (ADS)

Soejono, Igor; Buriánek, David; Janoušek, Vojtěch; Svojtka, Martin; Čáp, Pavel; Erban, Vojtěch; Ganpurev, Nyamtsetseg

2017-12-01

The primary relationships and character of the boundaries between principal lithotectonic domains in the Mongolian tract of the Central Asian Orogenic Belt (CAOB) are still poorly constrained. This brings much uncertainty in understanding of the orogeny configuration and the complete accretionary history. The plutonic Khuurai Tsenkher Gol Complex and the mainly metasedimentary Bij Group represent associated medium- to high-grade basement complexes exposed in the Hovd Zone close to its boundary with the Lake Zone in western Mongolia. The Khuurai Tsenkher Gol Complex is composed of variously deformed acid to basic magmatic rocks intimately associated with the metamorphosed sedimentary and volcanic rocks of the Bij Group. Results of our field work, new U-Pb zircon ages and whole-rock geochemical data suggest an existence of two separate magmatic events within the evolution of the Khuurai Tsenkher Gol Complex. Early to Mid-Ordovician (476 ± 5 Ma and 467 ± 4 Ma protoliths) normal- to high-K calc-alkaline orthogneisses, metadiorites and metagabbros predominate over Mid-Silurian (430 ± 3 Ma) tholeiitic-mildly alkaline quartz monzodiorites. Whereas the geochemical signature of the former suite unequivocally demonstrates its magmatic-arc origin, that of the latter quartz monzodiorite suggests an intra-plate setting. As shown by Sr-Nd isotopic data, the older arc-related magmas were derived from depleted mantle and/or were generated by partial melting of juvenile metabasic crust. Detrital zircon age populations of the metasedimentary rocks together with geochemical signatures of the associated amphibolites imply that the Bij Group was a volcano-sedimentary sequence, formed probably in the associated fore-arc wedge basin. Moreover, our data argue for an identical provenance of the Altai and Hovd domains, overall westward sediment transport during the Early Palaeozoic and the east-dipping subduction polarity. The obvious similarities of the Khuurai Tsenkher Gol Complex (Hovd Zone) with the neighbouring Togtokhinshil Complex (Lake Zone) suggest that both magmatic complexes originally belonged to the same magmatic arc, related to the Palaeo-Asian subduction system. The geodynamic cause of the later, within-plate magmatic pulse is unclear, but was probably still related to the effects of retreating subduction (slab window/ocean ridge subduction or back-arc lithosphere thinning). The Khuurai Tsenkher Gol Complex was subsequently separated from the western margin of the Lake Zone and imbricated into the Hovd Zone mélange. It is proposed that the Lake/Hovd zones boundary in the study area represents a younger deformation zone rather than a true terrane boundary/suture. This could be a general feature of the suture zones within this part of the CAOB.

The dynamics of plate tectonics and mantle flow: from local to global scales.

PubMed

Stadler, Georg; Gurnis, Michael; Burstedde, Carsten; Wilcox, Lucas C; Alisic, Laura; Ghattas, Omar

2010-08-27

Plate tectonics is regulated by driving and resisting forces concentrated at plate boundaries, but observationally constrained high-resolution models of global mantle flow remain a computational challenge. We capitalized on advances in adaptive mesh refinement algorithms on parallel computers to simulate global mantle flow by incorporating plate motions, with individual plate margins resolved down to a scale of 1 kilometer. Back-arc extension and slab rollback are emergent consequences of slab descent in the upper mantle. Cold thermal anomalies within the lower mantle couple into oceanic plates through narrow high-viscosity slabs, altering the velocity of oceanic plates. Viscous dissipation within the bending lithosphere at trenches amounts to approximately 5 to 20% of the total dissipation through the entire lithosphere and mantle.

Active shortening of the Cascadia forearc and implications for seismic hazards of the Puget Lowland

USGS Publications Warehouse

Johnson, S.Y.; Blakely, R.J.; Stephenson, W.J.; Dadisman, S.V.; Fisher, M.A.

2004-01-01

Margin-parallel shortening of the Cascadia forearc is a consequence of oblique subduction of the Juan de Fuca plate beneath North America. Strike-slip, thrust, and oblique crustal faults beneath the densely populated Puget Lowland accommodate much of this north-south compression, resulting in large crustal earthquakes. To better understand this forearc deformation and improve earthquake hazard, assessment, we here use seismic reflection surveys, coastal exposures of Pleistocene strata, potential-field data, and airborne laser swath mapping to document and interpret a significant structural boundary near the City of Tacoma. This boundary is a complex structural zone characterized by two distinct segments. The northwest trending, eastern segment, extending from Tacoma to Carr Inlet, is formed by the broad (??? 11.5 km), southwest dipping (??? 11??-2??) Rosedale monocline. This monocline raises Crescent Formation basement about 2.5 km, resulting in a moderate gravity gradient. We interpret the Rosedale monocline as a fault-bend fold, forming above a deep thrust fault. Within the Rosedale monocline, inferred Quaternary strata thin northward and form a growth triangle that is 4.1 to 6.6 km wide at its base, suggesting ??? 2-3 mm/yr of slip on the underlying thrust. The western section of the >40-km-long, north dipping Tacoma fault, extending from Hood Canal to Carr Inlet, forms the western segment of the Tacoma basin margin. Structural relief on this portion of the basin margin may be several kilometers, resulting in steep gravity and aeromagnetic anomalies. Quaternary structural relief along the Tacoma fault is as much as 350-400 m, indicating a minimum slip rate of about 0.2 mm/yr. The inferred eastern section of the Tacoma fault (east of Carr Inlet) crosses the southern part of the Seattle uplift, has variable geometry along strike, and diminished structural relief. The Tacoma fault is regarded as a north dipping backthrust to the Seattle fault, so that slip on a master thrust fault at depth could result in movement on the Seattle fault, the Tacoma fault, or both.

Structure of the Red Dog District, western Brooks Range, Alaska

USGS Publications Warehouse

de Vera, Jean-Pierre P.; McClay, K. R.

2004-01-01

The Red Dog district of the western Brooks Range of northern Alaska, which includes the sediment-hosted Zn-Pb-Ag ± Ba deposits at Red Dog, Su-Lik, and Anarraaq, contains one of the world's largest reserves of zinc. This paper presents a new model for the structural development of the area and shows that understanding the structure is crucial for future exploration efforts and new mineral discoveries in the district. In the Red Dog district, a telescoped Late Devonian through Jurassic continental passive margin is exposed in a series of subhorizontally stacked, internally imbricated, and regionally folded thrust sheets. These sheets were emplaced during the Middle Jurassic to Late Cretaceous Brookian orogeny and subsequently were uplifted by late tectonic activity in the Tertiary. The thrust sheet stack comprises seven tectonostratigraphically distinct allochthonous sheets, three of which have been subject to regional and detailed structural analysis. The lowermost of these is the Endicott Mountains allochthon, which is overlain by the structurally higher Picnic Creek and Kelly River allochthons. Each individual allochthon is itself internally imbricated into a series of tectonostratigraphically coherent and distinct thrust plates and subplates. This structural style gives rise to duplex development and imbrication at a range of scales, from a few meters to tens of kilometers. The variable mechanical properties of the lithologic units of the ancient passive margin resulted in changes in structural styles and scales of structures across allochthon boundaries. Structural mapping and analysis of the district indicate a dominant northwest to west-northwest direction of regional tectonic transport. Local north to north-northeast transport of thrust sheets is interpreted to reflect the influence of underlying lateral and/or oblique ramps, which may have been controlled by inherited basin margin structures. Some thrust-sheet stacking patterns suggest out-of-sequence thrusting. The west-northwest-east-southeast-trending Wrench Creek and Sivukat Mountain faults were previously interpreted to be strike-slip faults, but this study shows that they are Tertiary (Eocene?) late extensional faults with little or no lateral displacement.

Regional uplift episodes along the NE Atlantic margin constrained by stratigraphic and thermochronologic data

NASA Astrophysics Data System (ADS)

Holford, S. P.; Green, P. F.; Hillis, R. R.; Duddy, I. R.; Turner, J. P.; Stoker, M. S.

2008-12-01

The magma-rich NE Atlantic passive margin provides a superb natural laboratory for studying vertical motions associated with continental rifting and the rift-drift transition. Here we present an extensive apatite fission-track analysis (AFTA) database from the British Isles which we combine with a detailed stratigraphic framework for the Cretaceous-Cenozoic sedimentary record of the NE Atlantic margin to constrain the uplift history along and inboard of this margin during the past 120 Myr. We show that the British Isles experienced a series of uplift episodes which began between 120 and 115 Ma, 65 and 55 Ma, 40 and 25 Ma and 20 and 15 Ma, respectively. Each episode is of regional extent (~100,000 sq km) and represents a major period of exhumation involving removal of up to 1 km or more of section. These uplift episodes can be correlated with a number of major tectonic unconformities recognised within the sedimentary succession of the NE Atlantic margin, suggesting that the margin was also affected by these uplift episodes. Anomalous syn- and post-rift uplift along this margin have been interpreted in terms of permanent and/or transient movements controlled by the Iceland plume, but neither the timing nor distribution of the uplift episodes, with the exception of the 65 to 55 Ma episode, supports a first-order control by plume activity on vertical motions. Each uplift episode correlates closely with key deformation events at adjacent plate boundaries, suggesting a causative link, and we examine the ways in which plate boundary forces can account for the observed uplift episodes. Similar km-scale uplift events are revealed by thermochronological studies in other magma-rich and magma-poor continental margins, e.g. SE Australia, South Africa, Brazil. The low angle unconformities which result from these regional episodes of km-scale burial and subsequent uplift are often incorrectly interpreted as representing periods of non-deposition and tectonic stability. Similar considerations have also led to an erroneous view of the post-rift stability of many continental margins. Our results indicate that km-scale regional uplift has affected many regions previously interpreted as areas of long-term stability, and that plate boundary deformation exerts the primary control on such episodes.

Comparison of geoelectrical/tectonic models for suture zones in the western U.S.A. and eastern Europe: are black shales a possible source of high conductivities?

USGS Publications Warehouse

Stanley, W.D.

1989-01-01

Large-scale geoelectrical anomalies have been mapped with geomagnetic depth sounding (GDS) and magnetotelluric (MT) surveys in the Carpathian Mountains region. These anomalies are associated with the zone of closure between stable Europe and a complex of microplates in front of the converging African plate. The zone of closure, or suture zone, is largely occupied by an extensive deformed flysch belt. The models derived to fit the observed geoelectrical data are useful in the study of other suture zones, and Carpathian structures have been compared with areas currently being studied in the western Cordillera of the U.S.A. Models derived for a smaller-scale suture zone mapped in western Washington State have features that are similar to the Carpathian models. The geoelectrical models for both the Carpathian and Washington anomalies require dipping conductive slabs of 1-5 ?? m material that extends to depths > 20 km. In both instances there is evidence that these materials may merge with lower crustal-mantle conductors along the down-dip margins of the slab. The main conductive units are interpreted to be sedimentary rocks that have been partially subducted due to collisional processes. Heat flow is low in both regions and it is difficult to explain fully the deep conduction mechanisms; however, evidence suggests that the conduction at depth may include electronic conduction in sulfide mineral or carbon films as well as ionic conduction in fluids or partial melt. ?? 1989.

Consequences of Chixculub Impact for the Tectonic and Geodynamic Evolution of the Gulf of Mexico North Carribean Region

NASA Astrophysics Data System (ADS)

Rangin, C.; Crespy, A.; Martinez-Reyes, J.

2013-05-01

The debate for Pacific exotic origin versus in situ inter American plate Atlantic origin of the Caribbean plate is active in the scientific community since decades. Independently of the origin of this plate, its fast motion towards the east at a present rate of 2cm/yr is accepted to have been initiated during the early-most Cenozoic. The Paleocene is a key period in the global evolution of Central America mainly marked also by the Chicxulub multiring meteor impact in Yucatan. We question here the genetic relationship between this impact event and the incipient tectonic escape of the Caribbean plate. The mostly recent published models suggest this impact has affected the whole crust down to the Moho, the upper mantle being rapidly and considerably uplifted. The crust was then fragmented 600km at least from the point of impact, and large circular depressions were rapidly filled by clastic sediments from Cantarell to Western Cuba via Chiapas and Belize. North of the impact, the whole Gulf of Mexico was affected by mass gravity sliding, initiated also during the Paleocene in Texas, remaining active in this basin up to present time. South of the impact, in the Caribbean plate, the Yucatan basin was rapidly opened, indicating a fast escape of the crustal material towards the unique free boundary, the paleo-Antilles subduction zone. Shear waves velocity data below the Caribbean plate suggest this crustal tectonic escape was enhanced by the fast eastward flowing mantle supporting a fragmented and stretched crust. The proposed model suggests Chicxulub impact (but also the hypothetic Beata impact) have fragmented brittle crust, then easily drifted towards the east. This could explain the Paleogene evolution of the Caribbean plate largely stretched during its early evolution. Geologically, this evolution could explain the absence of evident Paleogene oblique subduction along the Caribbean plate northern and southern margins, marked only by Mid Cretaceous dragged volcanic complexes, but also the relatively recent motion along the Cayman Fault zone (Miocene instead of Eocene). These results are part of a cooperative research-industry programm conducted by CEREGE/EGERIE, Aix-en-Provence and GeoAzur, Nice, with Frontier Basin study group TOTAL S.A., Paris.

Passive margins getting squeezed in the mantle convection vice

NASA Astrophysics Data System (ADS)

Yamato, Philippe; Husson, Laurent; Becker, Thorsten W.; Pedoja, Kevin

2013-12-01

margins often exhibit uplift, exhumation, and tectonic inversion. We speculate that the compression in the lithosphere gradually increased during the Cenozoic, as seen in the number of mountain belts found at active margins during that period. Less clear is how that compression increase affects passive margins. In order to address this issue, we design a 2-D viscous numerical model wherein a lithospheric plate rests above a weaker mantle. It is driven by a mantle conveyor belt, alternatively excited by a lateral downwelling on one side, an upwelling on the other side, or both simultaneously. The lateral edges of the plate are either free or fixed, representing the cases of free convergence, and collision (or slab anchoring), respectively. This distinction changes the upper mechanical boundary condition for mantle circulation and thus, the stress field. Between these two regimes, the flow pattern transiently evolves from a free-slip convection mode toward a no-slip boundary condition above the upper mantle. In the second case, the lithosphere is highly stressed horizontally and deforms. For a constant total driving force, compression increases drastically at passive margins if upwellings are active. Conversely, if downwellings alone are activated, compression occurs at short distances from the trench and extension prevails elsewhere. These results are supported by Earth-like models that reveal the same pattern, where active upwellings are required to excite passive margins compression. Our results substantiate the idea that compression at passive margins is in response to the underlying mantle flow that is increasingly resisted by the Cenozoic collisions.

Grooved Armor Plate

DTIC Science & Technology

1935-05-16

that many of the olateo tested do sux^ ass the reouired limita by a considerable margin. Therefore, the efficiency of these grooved plate...Plpte Thicknesn B>lll<tle .Llji^f - ./.. s... a’ ^ea:: of All RtSUltS !’ Teen of Rlgbeot lOf of All Reoultr. s/i«- 1830 an« 7/16" ?180

A new view for the geodynamics of Ecuador: Implication in seismogenic source definition and seismic hazard assessment

NASA Astrophysics Data System (ADS)

Yepes, Hugo; Audin, Laurence; Alvarado, Alexandra; Beauval, Céline; Aguilar, Jorge; Font, Yvonne; Cotton, Fabrice

2016-05-01

A new view of Ecuador's complex geodynamics has been developed in the course of modeling seismic source zones for probabilistic seismic hazard analysis. This study focuses on two aspects of the plates' interaction at a continental scale: (a) age-related differences in rheology between Farallon and Nazca plates—marked by the Grijalva rifted margin and its inland projection—as they subduct underneath central Ecuador, and (b) the rapidly changing convergence obliquity resulting from the convex shape of the South American northwestern continental margin. Both conditions satisfactorily explain several characteristics of the observed seismicity and of the interseismic coupling. Intermediate-depth seismicity reveals a severe flexure in the Farallon slab as it dips and contorts at depth, originating the El Puyo seismic cluster. The two slabs position and geometry below continental Ecuador also correlate with surface expressions observable in the local and regional geology and tectonics. The interseismic coupling is weak and shallow south of the Grijalva rifted margin and increases northward, with a heterogeneous pattern locally associated to the Carnegie ridge subduction. High convergence obliquity is responsible for the North Andean Block northeastward movement along localized fault systems. The Cosanga and Pallatanga fault segments of the North Andean Block-South American boundary concentrate most of the seismic moment release in continental Ecuador. Other inner block faults located along the western border of the inter-Andean Depression also show a high rate of moderate-size earthquake production. Finally, a total of 19 seismic source zones were modeled in accordance with the proposed geodynamic and neotectonic scheme.

Asthenospheric outflow from the shrinking Philippine Sea Plate: Evidence from Hf-Nd isotopes of southern Mariana lavas

NASA Astrophysics Data System (ADS)

Ribeiro, Julia M.; Stern, Robert J.; Martinez, Fernando; Woodhead, Jon; Chen, Min; Ohara, Yasuhiko

2017-11-01

At subduction zones, sinking of the downgoing lithosphere is thought to enable a return flow of asthenospheric mantle around the slab edges, so that the asthenosphere from underneath the slab invades the ambient mantle flowing underneath the volcanic arc and the backarc basin. For instance at the northern end of the Lau Basin, trench retreat and slab rollback enable toroidal return flow of Samoan mantle beneath a transform margin to provide a supply of fresh, undepleted Indian mantle that feeds the backarc spreading center. Questions, however, arise about the sense of mantle flow when plate kinematics predict that the trench is advancing, as seen in the Mariana convergent margin. Does the mantle flow in or does it escape outward through slab tears or gaps? Here, we address the origin and sense of asthenospheric mantle flow supplying the southern Mariana convergent margin, a region of strong extension occurring above the subducting Pacific plate. Does the asthenosphere flow northward, from underneath the Pacific plate and Caroline hotspot through a slab tear or gap, or does it flow outward from the Mariana Trough, which possesses a characteristic Indian Ocean isotopic signature? To address these questions, we integrate geodetic data along with new Hf-Nd isotopic data for fresh basaltic lavas from three tectonic provinces in the southernmost Marianas: the Fina Nagu volcanic complex, the Malaguana-Gadao backarc spreading ridge and the SE Mariana forearc rift. Our results indicate that Indian mantle flows outward and likely escapes through slab tears or gaps to accommodate shrinking of the Philippine Sea plate. We thus predict that asthenospheric flow around the Pacific slab at the southern Mariana Trench is opposite to that predicted by most subduction-driven mantle flow models.

Right-lateral shear across Iran and kinematic change in the Arabia-Eurasia collision zone

NASA Astrophysics Data System (ADS)

Allen, Mark B.; Kheirkhah, Monireh; Emami, Mohammad H.; Jones, Stuart J.

2011-02-01

New offset determinations for right-lateral strike-slip faults in Iran revise the kinematics of the Arabia-Eurasia collision, by indicating along-strike lengthening of the collision zone before a change to the present kinematic regime at ˜5 Ma. A series of right-lateral strike-slip faults is present across the Turkish-Iranian plateau between 48°E and 57°E. Fault strikes vary between NW-SE and NNW-SSE. Several of the faults are seismically active and/or have geomorphic evidence for Holocene slip. None of the faults affects the GPS-derived regional velocity field, indicating active slip rates are ≤2 mm yr-1. We estimate total offsets for these faults from displaced geological and geomorphic markers, based on observations from satellite imagery, digital topography, geology maps and our own fieldwork observations, and combine these results with published estimates for fault displacement. Total right-lateral offset of the Dehu, Anar, Deh Shir, Kashan, Ab-Shirin-Shurab, Kousht Nousrat, Qom, Bid Hand, Indes, Soltanieh and Takab faults is ˜250 km. Other faults (North Zanjan, Saveh, Jorjafk, Rafsanjan, Kuh Banan and Behabad) have unknown or highly uncertain amounts of slip. Collectively, these faults are inferred to have accommodated part of the Arabia-Eurasia convergence. Three roles are possible, which are not mutually exclusive: (1) shortening via anticlockwise, vertical axis rotations; (2) northward movement of Iranian crust with respect to stable Afghanistan to the east; (3) combination with coeval NW-SE thrusts in the Turkish-Iranian plateau, to produce north-south plate convergence (`strain partitioning'). This strike-slip faulting across Iran requires along-strike lengthening of the collision zone. This was possible until the Pliocene (≤ 5 Ma), when the Afghan crust collided with the western margin of the Indian plate, thereby sealing off a free face at the eastern side of the Arabia-Eurasia collision zone. Continuing Arabia-Eurasia plate convergence had to be accommodated in new ways and new areas, leading to the present pattern of faulting from eastern Iran to western Turkey, and involving the westward transport (`escape') of Anatolia and the concentration of thrusting in the Zagros and Alborz mountains.

A possible connection between post-subduction arc magmatism and adakite-NEB rock association in Baja California, Mexico

NASA Astrophysics Data System (ADS)

Castillo, P. R.

2007-05-01

Late Miocene to Recent arc-related magmatism occurs in Baja California, Mexico despite the cessation of plate subduction along its western margin at ~12.5 Ma. It includes calcalkaline and K-rich andesites, tholeiitic basalts and basaltic andesites, alkalic basalts similar to many ocean island basalts (OIB), magnesian and basaltic andesites with adakitic affinity (bajaiites), adakites, and Nb-enriched basalts (NEB). A popular model for the close spatial and temporal association of adakite (plus bajaiite) and NEB in Baja California is these are due to melting of the subducted Farallon/Cocos plate, which in turn is caused by the influx of hot asthenospheric mantle through a window created in the subducted slab directly beneath the Baja California peninsula [e.g., Benoit, M. et. al. (2002) J. Geol. 110, 627-648; Calmus, T. et al. (2003) Lithos 66, 77-105]. Here I propose an alternative model for the cause of post-subduction magmatism in Baja California in particular and origin of adakite-NEB rock association in general. The complicated tectonic configuration of the subducting Farallon/Cocos plate and westward motion of the North American continent caused western Mexico to override the hot, upwelling Pacific mantle that was decoupled from the spreading centers abandoned west of Baja California. The upwelling asthenosphere is best manifested east of the peninsula, beneath the Gulf of California, and is most probably due to a tear or window in the subducted slab there. The upwelling asthenosphere is compositionally heterogeneous and sends materials westward into the mantle wedge beneath the peninsula. These materials provide sources for post-subduction tholeiitic and alkalic magmas. Portions of tholeiitic magmas directly erupted at the surface produce tholeiitic lavas, but some get ponded beneath the crust. Re-melting and/or high-pressure fractional crystallization of the ponded tholeiitic magmas generate adakitic rocks. Alkalic magmas directly erupted at the surface produce OIB-like lavas but those that get contaminated during transit produce NEB. The influx of asthenosphere also provides thermal energy to melt the upper portion of the mantle wedge - producing calc- alkaline lavas, and the amphibolitized deeper portion of the wedge - producing bajaiites, after the cessation of subduction in Baja California.

Continental Break-up Above A Mantle Plume: Opening of The Southern Red Sea

NASA Astrophysics Data System (ADS)

Ebinger, C.; Eagles, G.; Elders, C.; Gloaguen, R.; McClay, K.; Tiberi, C.; Wolfenden, E.

Initial rifting in the Red Sea occurred concurrent with, or soon after flood basaltic mag- matism at~31 Ma in the Ethiopia-Yemen plume province. Yet, the development of the ca. 400 km-wide extensional province of the southern Red Sea between 31 Ma and the onset of seafloor spreading at ~4 Ma has been poorly understood, in large part owing to inaccessibility in the Afar depression. The Afar depression is a diffuse extensional province marking a triple point zone between plate boundaries in the Red Sea (Arabia Nubia), the Gulf of Aden (Arabia Somalia); and the Main Ethiopian Rift (Somalia Nu- bia). Complicating this setting, the Danakil horst is a microplate lying between oceanic provinces in the southernmost Red Sea and incipient seafloor spreading in the northern Afar depression. We have integrated exploration seismic, gravity, well, and magnetic data from offshore regions with remote sensing, geological and geophysical data from Ethiopia, Eritrea, and Yemen to evaluate models for continental break-up above mantle plumes. Plate kinematic reconstructions using a pole of rotation within the error ellipse of the Chu and Gordon (1999) pole predict real features in remote sensing and gravity data; these reconstructions provide a general framework for our interpretations. Field and geochronology studies along the western margin of Afar show a southward prop- agation of rifting since about 25 Ma when extension commenced offshore Red Sea and in Yemen. We also see an eastward migration of strain from the western border fault to narrow zones of primarily basaltic magmatism since mid-Miocene time. These magmatic sequences, where not onlapped by Pliocene-Recent sedimentary strata, dip steeply seaward and define a regional eastward flexure into transitional oceanic crust, as suggested by gravity models constrained by existing seismic data. Our synthesis suggests that the southern Afar depression, assumed to be most proximal to the plume, was the site of incipient seafloor spreading in Miocene time, but that this has ceased or stalled during plate reorganisation as the Aden rift propagated into Afar to make the Danakil a microplate.

Upper plate contraction north of the migrating Mendocino triple junction northern California: Implications for partitioning of strain

USGS Publications Warehouse

McCrory, P.A.

2000-01-01

Geologic measurement of permanent contraction across the Cascadia subduction margin constrains one component of the tectonic deformation along the convergent plate boundary, the component critical for the seismic hazard assessment of crustal faults. A comprehensive survey of active faults in onshore subduction margin rocks at the southern end of the Cascadia subduction zone indicates that these thrust faults accommodate ??10 mm/yr of convergence oriented 020??-045??. Seismotectonic models of subduction zones typically assign this upper plate strain to the estimate of aseismic slip on the megathrust. Geodetic models include this permanent crustal strain within estimates of elastic strain accumulation on the megathrust. Both types of models underestimate the seismic hazard associated with crustal faults. Subtracting the observed contraction from the plate convergence rate (40-50 mm/yr; directed 040??-055??) leaves 30-40 mm/yr of convergence to be partitioned between slip on the megathrust, contraction within the southern Juan de Fuca plate, and crustal contraction outside the subduction complex rocks. This simple estimate of slip partitioning neglects the discrepancy between the plate convergence and contraction directions in the vicinity of the Mendocino triple junction. The San Andreas and Cascadia limbs of the Mendocino triple junction are not collinear. The eastern edge of the broad San Andreas boundary is ??85 km east of the Cascadia subduction boundary, and across this zone the Pacific plate converges directly with the North America plate. The skewed orientation of crustal structures just north of the leading edge of the Pacific plate suggests that they are deforming in a hybrid stress field resulting from both Juan de Fuca-North America motion and Pacific-North America motion. The composite convergence direction (50 mm/yr: directed 023??) is consistent with the compressive stress axis (020??) inferred from focal mechanisms of crustal earthquakes in the Humboldt region. Deformation in such a hybrid stress field implies that the crustal faults are being loaded from two major tectonic sources. The slip on crustal faults north of the Mendocino triple junction may consume 4-5 mm/yr of Pacific-Humboldt convergence. The remaining 17-18 mm/yr of convergence may be consumed as distributed shortening expressed in the high rates of uplift in the Cape Mendocino region or as northward translation of the continental margin, north of the triple junction.

Mantle convection with plates and mobile, faulted plate margins.

PubMed

Zhong, S; Gurnis, M

1995-02-10

A finite-element formulation of faults has been incorporated into time-dependent models of mantle convection with realistic rheology, continents, and phase changes. Realistic tectonic plates naturally form with self-consistent coupling between plate and mantle dynamics. After the initiation of subduction, trenches rapidly roll back with subducted slabs temporarily laid out along the base of the transition zone. After the slabs have penetrated into the lower mantle, the velocity of trench migration decreases markedly. The inhibition of slab penetration into the lower mantle by the 670-kilometer phase change is greatly reduced in these models as compared to models without tectonic plates.

Tectono-sedimentary evolution of the eastern Gulf of Aden conjugate passive margins: Narrowness and asymmetry in oblique rifting context

NASA Astrophysics Data System (ADS)

Nonn, Chloé; Leroy, Sylvie; Khanbari, Khaled; Ahmed, Abdulhakim

2017-11-01

Here, we focus on the yet unexplored eastern Gulf of Aden, on Socotra Island (Yemen), Southeastern Oman and offshore conjugate passive margins between the Socotra-Hadbeen (SHFZ) and the eastern Gulf of Aden fracture zones. Our interpretation leads to onshore-offshore stratigraphic correlation between the passive margins. We present a new map reflecting the boundaries between the crustal domains (proximal, necking, hyper-extended, exhumed mantle, proto-oceanic and oceanic domains) and structures using bathymetry, magnetic surveys and seismic reflection data. The most striking result is that the magma-poor conjugate margins exhibit asymmetrical architecture since the thinning phase (Upper Rupelian-Burdigalian). Their necking domains are sharp ( 40-10 km wide) and their hyper-extended domains are narrow and asymmetric ( 10-40 km wide on the Socotra margin and 50-80 km wide on the Omani margin). We suggest that this asymmetry is related to the migration of the rift center producing significant lower crustal flow and sequential faulting in the hyper-extended domain. Throughout the Oligo-Miocene rifting, far-field forces dominate and the deformation is accommodated along EW to N110°E northward-dipping low angle normal faults. Convection in the mantle near the SHFZ may be responsible of change in fault dip polarity in the Omani hyper-extended domain. We show the existence of a northward-dipping detachment fault formed at the beginning of the exhumation phase (Burdigalien). It separates the northern upper plate (Oman) from southern lower plate (Socotra Island) and may have generated rift-induced decompression melting and volcanism affecting the upper plate. We highlight multiple generations of detachment faults exhuming serpentinized subcontinental mantle in the ocean-continent transition. Associated to significant decompression melting, final detachment fault may have triggered the formation of a proto-oceanic crust at 17.6 Ma and induced late volcanism up to 10 Ma. Finally, the setting up of a steady-state oceanic spreading center occurs at 17 Ma.

The northern Lesser Antilles oblique subduction zone: new insight about the upper plate deformation, 3D slab geometry and interplate coupling.

NASA Astrophysics Data System (ADS)

Marcaillou, B.; Laurencin, M.; Graindorge, D.; Klingelhoefer, F.

2017-12-01

In subduction zones, the 3D geometry of the plate interface is thought to be a key parameter for the control of margin tectonic deformation, interplate coupling and seismogenic behavior. In the northern Caribbean subduction, precisely between the Virgin Islands and northern Lesser Antilles, these subjects remain controversial or unresolved. During the ANTITHESIS cruises (2013-2016), we recorded wide-angle seismic, multichannel reflection seismic and bathymetric data along this zone in order to constrain the nature and the geometry of the subducting and upper plate. This experiment results in the following conclusions: 1) The Anegada Passage is a 450-km long structure accross the forearc related to the extension due to the collision with the Bahamas platform. 2) More recently, the tectonic partitioning due to the plate convergence obliquity re-activated the Anegada Passage in the left-lateral strike-slip system. The partitioning also generated the left-lateral strike-slip Bunce Fault, separating the accretionary prism from the forearc. 3) Offshore of the Virgin Islands margin, the subducting plate shows normal faults parallel to the ancient spreading center that correspond to the primary fabric of the oceanic crust. In contrast, offshore of Barbuda Island, the oceanic crust fabric is unresolved (fracture zone?, exhumed mantle? ). 4) In the direction of the plate convergence vector, the slab deepening angle decreases northward. It results in a shallower slab beneath the Virgin Islands Platform compared to the St Martin-Barbuda forearc. In the past, the collision of the Bahamas platform likely changed the geodynamic settings of the northeastern corner of the Caribbean subduction zone and we present a revised geodynamic history of the region. Currently, various features are likely to control the 3D geometry of the slab: the margin convexity, the convergence obliquity, the heterogeneity of the primary fabric of the oceanic crust and the Bahamas docking. We suggest that the slab deepening angle lower beneath the Virgin Islands segment than beneath the St Martin-Barbuda segment possibly generates a northward increasing interplate coupling. As a result, it possibly favors an increase in the seismic activity and the tectonic partitioning beneath the Virgin Islands margin contrary to the St Martin-Barbuda segment.

The Lithospheric Geoid as a Constraint on Plate Dynamics

NASA Astrophysics Data System (ADS)

Richardson, R. M.; Coblentz, D. D.

2015-12-01

100 years after Wegener's pioneering work there is still considerable debate about the dynamics of present-day plate motions. A better understanding of present-day dynamics is key to a better understanding of the supercontinent cycle. The Earth's gravity field is one of the primary data sets to help constrain horizontal density contrasts, and hence plate dynamic forces. Previous work has shown that the global average for the geoid step up from old oceanic lithosphere across passive continental margins to stable continental lithosphere is about 6-9m, and the global average for the geoid anomaly associated with cooling oceanic lithosphere (the so-called "ridge push") is 10-12m. The ridge geoid anomaly corresponds to a net force of ~3x1012N/m (averaged over the thickness of the lithosphere) due to 'ridge push.' However, for individual continental margins and mid-ocean ridge systems, there is considerable variation in the geoid step and geoid anomaly and consequently the associated forces contributing to the stress field. We explore the variation in geoid step across passive continental margins looking for correlations with age of continental breakup (and hence place within the supercontinent cycle), hot spot tracks, continental plate velocities, long-wavelength geoid energy (that may be masking signal), and small scale convection. For mid-ocean ridges, we explore variations in geoid anomaly looking for correlations with plate spreading rates, hot spot tracks, long-wavelength geoid energy (that may be masking signal), and small scale convection. We use a band-pass spherical harmonic filter on the full geoid (e.g., EGM2008-WGS84, complete to spherical harmonic degree and order 2159) between orders 6 and 80. The evaluation of the role of spatial variations in the geoid gradient for cooling oceanic lithosphere and across the continental margin in the dynamics of the intraplate stress field requires high spatial resolution modeling. We perform a high resolution finite element analysis (~35,000 elements for a spatial resolution of approximately 50 km) for the North American plate, where previous lower resolution modeling has shown the importance of the lithospheric cooling (ridge push) force to model the broad scale stress patterns observed from the middle of the continent to the Mid-Atlantic ridge.

U-Pb and Hf isotope analysis of detrital zircons from Mesozoic strata of the Gravina belt, southeast Alaska

NASA Astrophysics Data System (ADS)

Yokelson, Intan; Gehrels, George E.; Pecha, Mark; Giesler, Dominique; White, Chelsi; McClelland, William C.

2015-10-01

The Gravina belt consists of Upper Jurassic through Lower Cretaceous marine clastic strata and mafic-intermediate volcanic rocks that occur along the western flank of the Coast Mountains in southeast Alaska and coastal British Columbia. This report presents U-Pb ages and Hf isotope determinations of detrital zircons that have been recovered from samples collected from various stratigraphic levels and from along the length of the belt. The results support previous interpretations that strata in the western portion of the Gravina belt accumulated along the inboard margin of the Alexander-Wrangellia terrane and in a back-arc position with respect to the western Coast Mountains batholith. Our results are also consistent with previous suggestions that eastern strata accumulated along the western margin of the inboard Stikine, Yukon-Tanana, and Taku terranes and in a fore-arc position with respect to the eastern Coast Mountains batholith. The history of juxtaposition of western and eastern assemblages is obscured by subsequent plutonism, deformation, and metamorphism within the Coast Mountains orogen, but may have occurred along an Early Cretaceous sinistral transform system. Our results are inconsistent with models in which an east-facing subduction zone existed along the inboard margin of the Alexander-Wrangellia terrane during Late Jurassic-Early Cretaceous time.

How do long-offset oceanic transforms adapt to plate motion changes? The example of the Western Pacific-Antarctic plate boundary

NASA Astrophysics Data System (ADS)

Lodolo, Emanuele; Coren, Franco; Ben-Avraham, Zvi

2013-03-01

Oceanic transform faults respond to changes in the direction of relative plate motion. Studies have shown that short-offset transforms generally adjust with slight bends near the ridge axis, while long-offset ones have a remarkably different behavior. The western Pacific-Antarctic plate boundary highlights these differences. A set of previously unpublished seismic profiles, in combination with magnetic anomaly identifications, shows how across a former, ~1250 km long transform (the Emerald Fracture Zone), plate motion changes have produced a complex geometric readjustment. Three distinct sections are recognized along this plate boundary: an eastern section, characterized by parallel, multiple fault strand lineaments; a central section, shallower than the rest of the ridge system, overprinted by a mantle plume track; and a western section, organized in a cascade of short spreading axes/transform lineaments. This configuration was produced by changes that occurred since 30 Ma in the Australia-Pacific relative plate motion, combined with a gradual clockwise change in Pacific-Antarctic plate motion. These events caused extension along the former Emerald Fracture Zone, originally linking the Pacific-Antarctic spreading ridge system with the Southeast Indian ridge. Then an intra-transform propagating ridge started to develop in response to a ~6 Ma change in the Pacific-Antarctic spreading direction. The close proximity of the Euler poles of rotation amplified the effects of the geometric readjustments that occurred along the transform system. This analysis shows that when a long-offset transform older than 20 Ma is pulled apart by changes in spreading velocity vectors, it responds with the development of multiple discrete, parallel fault strands, whereas in younger lithosphere, locally modified by thermal anisotropies, tensional stresses generate an array of spreading axes offset by closely spaced transforms.

Tectonic activity evolution of the Scotia-Antarctic Plate boundary from mass transport deposit analysis

NASA Astrophysics Data System (ADS)

Pérez, Lara F.; Bohoyo, Fernando; Hernández-Molina, F. Javier; Casas, David; Galindo-Zaldívar, Jesús; Ruano, Patricia; Maldonado, Andrés.

2016-04-01

The spatial distribution and temporal occurrence of mass transport deposits (MTDs) in the sedimentary infill of basins and submerged banks near the Scotia-Antarctic plate boundary allowed us to decode the evolution of the tectonic activity of the relevant structures in the region from the Oligocene to present day. The 1020 MTDs identified in the available data set of multichannel seismic reflection profiles in the region are subdivided according to the geographic and chronological distributions of these features. Their spatial distribution reveals a preferential location along the eastern margins of the eastern basins. This reflects local deformation due to the evolution of the Scotia-Antarctic transcurrent plate boundary and the impact of oceanic spreading along the East Scotia Ridge (ESR). The vertical distribution of the MTDs in the sedimentary record evidences intensified regional tectonic deformation from the middle Miocene to Quaternary. Intensified deformation started at about 15 Ma, when the ESR progressively replaces the West Scotia Ridge (WSR) as the main oceanic spreading center in the Scotia Sea. Coevally with the WSR demise at about 6.5 Ma, increased spreading rates of the ESR and numerous MTDs were formed. The high frequency of MTDs during the Pliocene, mainly along the western basins, is also related to greater tectonic activity due to uplift of the Shackleton Fracture Zone by tectonic inversion and extinction of the Antarctic-Phoenix Ridge and involved changes at late Pliocene. The presence of MTDs in the southern Scotia Sea basins is a relevant indicator of the interplay between sedimentary instability and regional tectonics.

Numerical Models of Alaskan Tectonics: A Review and Looking Ahead to a New Era of Research

NASA Astrophysics Data System (ADS)

Jadamec, M. A.; Freymueller, J. T.

2015-12-01

The Pacific-North American plate boundary in Alaska is in the current scientific spotlight, as a highlighted tectonic region for integrated scientific investigation. It is timely, therefore, to step back and examine the previous numerical modeling studies of Alaska. Reviewing the numerical models is valuable, as geodynamic modeling can be a predictive tool that can guide and target field studies, both geologic and geophysical. This review presents a comparison of the previous numerical modeling studies of the Alaska-Aleutian subduction zone, including the mainland and extending into northwestern Canada. By distinguishing between the model set-up, governing equations, and underlying assumptions, non-modelers can more easily understand under what context the modeling predictions can be interpreted. Several key features in the Alaska tectonic setting appear in all the models to have a first order effect on the resulting deformation, such as the plate margin geometry and Denali fault. In addition, there are aspects of the tectonic setting that lead to very different results depending how they are implemented into the models. For example, models which fix the slab velocity to surface plate motions predict lower mantle flow rates than models that allow the slab to steepen. Despite the previous modeling studies, many unanswered questions remain, including the formation of the Wrangell volcanics, the driver for motion in western and interior Alaska, and the timing and nature of slab deformation. A synthesis of this kind will be of value to geologists, geodeticists, seismologists, volcanologists, sedimentologists, geochemists, as well as geodynamicists.

Neotectonic Geomorphology of the Owen Stanley Oblique-slip Fault System, Eastern Papua New Guinea

NASA Astrophysics Data System (ADS)

Watson, L.; Mann, P.; Taylor, F.

2003-12-01

Previous GPS studies have shown that the Australia-Woodlark plate boundary bisects the Papuan Peninsula of Papua New Guinea and that interplate motion along the boundary varies from about 19 mm/yr of orthogonal opening in the area of the western Woodlark spreading center and D'Entrecasteaux Islands, to about 12 mm/yr of highly oblique opening in the central part of the peninsula, to about 10 mm/yr of transpressional motion on the western part of the peninsula. We have compiled a GIS database for the peninsula that includes a digital elevation model, geologic map, LANDSAT and radar imagery, and earthquake focal mechanisms. This combined data set demonstrates the regional importance of the 600-km-long Owen Stanley fault system (OSFS) in accommodating interplate motion and controlling the geomorphology and geologic exposures of the peninsula. The OSFS originated as a NE-dipping, reactivated Oligocene-Early Miocene age ophiolitic suture zone between an Australian continental margin and the Melanesian arc system. Pliocene to recent motion on the plate boundary has reactivated motion on the former NE-dipping thrust fault either as a NE-dipping normal fault in the eastern area or as a more vertical strike-slip fault in the western area. The broadly arcuate shape of the OSFS is probably an inherited feature from the original thrust fault. Faults in the eastern area (east of 148° E) exhibit characteristics expected for normal and oblique slip faults including: discontinuous fault traces bounding an upthrown highland block and a downthrown coastal plain or submarine block, transfer faults parallel to the opening direction, scarps facing to both the northeast and southwest, and spatial association with recent volcanism. Faults in the western area (west of 148° E) exibit characteristics expected for left-lateral strike-slip faults including: linear and continuous fault trace commonly confined to a deep, intermontane valley and sinistral offsets and deflections of rivers and streams by 0.5 to 1.2 km. The northern edge of the OSFS merges with the Ramu-Markham strike-slip fault near Lae. SW tilting of the footwall block (Papuan Peninsula) is responsible for the asymmetrical topographic profile of the peninsula and drowned topography along the southern coast of the peninsula.

BOLIVAR: the Caribbean-South America plate boundary between 60W and 71W as imaged by seismic reflection data

NASA Astrophysics Data System (ADS)

Magnani, M.; Mann, P.; Clark, S. A.; Escalona, A.; Zelt, C. A.; Christeson, G. L.; Levander, A.

2007-12-01

We present the results of ~6000km of marine multi-channel seismic (MCS) reflection data collected offshore Venezuela as part of the Broadband Ocean Land Investigation of Venezuela and the Antilles arc Region project (BOLIVAR). The imaged area spans almost 12 degrees of longitude and 5 degrees of latitude and encompasses the diffuse plate boundary between South America (SA) and the SE Caribbean plate (CAR). This plate boundary has been evolving for at least the past 55My when the volcanic island arc that borders the CAR plate started colliding obliquely with the SA continent: the collision front has migrated from west to east. BOLIVAR MCS data show that the crustal architecture of the present plate boundary is dominated by the eastward motion of the Caribbean plate with respect to SA and is characterized by a complex combination of convergent and strike-slip tectonics. To the north, the reflection data image the South Caribbean Deformed Belt (SCDB) and the structures related to the thrusting of the CAR plate under the Leeward Antilles volcanic arc region. The data show that the CAR underthrusting continues as far east as the southern edge of the Aves ridge and detailed stratigraphic dating of the Venezuela basin and trench deposits suggests that the collision began in the Paleogene. The amount of shortening along the SCDB decreases toward the east, in part due to the geometry of plate motion vectors and in part as a result of the NNE escape of the Maracaibo block in western Venezuela. South of the SCDB the MCS profiles cross the Leeward Antilles island arc and Cenozoic sedimentary basins, revealing a complex history of Paleogene-Neogene multiphase extension, compression, and tectonic inversion, as well as the influence of the tectonic activity along the right-lateral El Pilar - San Sebastian fault system. East of the Bonaire basin the MCS data image the southern end of the Aves Ridge abandoned volcanic island arc and the southwestern termination of the Grenada basin, characterized here by middle Miocene inverted structures, likely related to the WNW-ESE transpression between CAR and SA. The easternmost MCS profile crosses the ongoing arc-continent collision of the Lesser Antilles arc with SA and the backarc (Grenada Basin) and forearc (Tobago Basin) basins as well as the suture between the Caribbean arc and the passive margin of the continental SA plate near eastern Trinidad.

The STAT7 Code for Statistical Propagation of Uncertainties In Steady-State Thermal Hydraulics Analysis of Plate-Fueled Reactors

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dunn, Floyd E.; Hu, Lin-wen; Wilson, Erik

The STAT code was written to automate many of the steady-state thermal hydraulic safety calculations for the MIT research reactor, both for conversion of the reactor from high enrichment uranium fuel to low enrichment uranium fuel and for future fuel re-loads after the conversion. A Monte-Carlo statistical propagation approach is used to treat uncertainties in important parameters in the analysis. These safety calculations are ultimately intended to protect against high fuel plate temperatures due to critical heat flux or departure from nucleate boiling or onset of flow instability; but additional margin is obtained by basing the limiting safety settings onmore » avoiding onset of nucleate boiling. STAT7 can simultaneously analyze all of the axial nodes of all of the fuel plates and all of the coolant channels for one stripe of a fuel element. The stripes run the length of the fuel, from the bottom to the top. Power splits are calculated for each axial node of each plate to determine how much of the power goes out each face of the plate. By running STAT7 multiple times, full core analysis has been performed by analyzing the margin to ONB for each axial node of each stripe of each plate of each element in the core.« less

The Cordon del Portillo Permian magmatism, Mendoza, Argentina, plutonic and volcanic sequences at the western margin of Gondwana

NASA Astrophysics Data System (ADS)

Gregori, Daniel; Benedini, Leonardo

2013-03-01

The Cerro Punta Blanca, Cerro Bayo and Cerro Punta Negra stocks, parts of the Cordillera Frontal Composite Batholith, cropping out in the Cordón del Portillo, records the Gondwana magmatic development of the Cordillera Frontal of Mendoza, in western Argentina. In this area, the San Rafael Orogenic phase, that represents the closure of the Late Carboniferous-Early Permian marine basins, begins at 284 Ma, and ceased before 276 Ma. The Cerro Punta Blanca, Cerro Bayo and Cerro Punta Negra stocks represent a post-orogenic magmatism and are equivalents to the Choiyoi Group. The Gondwana magmatic activity in the Cordón del Portillo area can be divided into two stages. The Cerro Punta Blanca stock (c.a. 276 Ma) represents an early post-orogenic, subduction-related magmatism similar to the basic-intermediate section of the Choiyoi Group (c.a. 277 Ma). The late post-orogenic second event was recorded by the Cerro Bayo (262 Ma) and Cerro Punta Negra stocks which represent a transition between subduction-related and intra-plate magmatism. This event represents the intrusive counterpart of the acidic facies of the upper section of the Choiyoi Group (c.a. 273 Ma). This extensional condition continued during the Triassic when the Cacheuta basin developed.

Reconciling the geological history of western Turkey with plate circuits and mantle tomography

NASA Astrophysics Data System (ADS)

Kaymakci, N.; van Hinsbergen, D. J.; Spakman, W.; Torsvik, T. H.

2010-12-01

We place the geological history since Cretaceous times in western Turkey in a context of convergence, subduction, collision and slab break-off. To this end, we compare the west Anatolian geological history with amounts of Africa-Europe convergence calculated from the Atlantic plate circuit, and the seismic tomography images of the west Anatolian mantle structure. Western Turkish geology reflects the convergence between the Sakarya continent (here treated as Eurasia) in the north and Africa in the south, with the Anatolide-Tauride Block (ATB) between two strands of the Neotethyan ocean. Convergence between the Sakarya and the ATB started at least ~95-90Myr ago, marked by ages of metamorphic soles of ophiolites that form the highest structural unit below Sakarya. These are underlain by high-pressure, low-temperature metamorphic rocks of the Tavsanli and Afyon zones, and the Ören Unit, which in turn are underlain by the Menderes Massif derived from the ATB. Underthrusting of the ATB below Sakarya was since ~50Ma, associated with high-temperature metamorphism and widespread granitic magmatism. Thrusting in the Menderes Massif continued until 35 Ma, after which there is no record of accretion in western Turkey. Plate circuits show that since 90 Ma, ~1400 km of Africa-Europe convergence occurred, of which ~700 km since 50 Ma and ~450 km since 35Ma. Seismic tomography shows that the African slab under western Turkey is decoupled from the African Plate. This detached slab is a single, coherent body, representing the lithosphere consumed since 90 Ma. There was no subduction re-initiation after slab break-off. ATB collision with Europe therefore did not immediately lead to slab break-off but instead to delamination of subducting lithospheric mantle from accreting ATB crust, while staying attached to the African Plate. This led to asthenospheric inflow below the ATB crust, high-temperature metamorphism and felsic magmatism. Slab break-off in western Turkey probably occurred ~15 Myr ago, after which overriding plate compression and rotation accommodated ongoing Africa-Europe convergence. Slab break-off was accommodated along a vertical NE trending subduction transform edge propagator (STEP) fault zone, accelerating southwestward slab retreat of the Aegean slab. The SE Aegean slab edge may have existed already since early Miocene times or before, but started to rapidly roll back along the southeastern Aegean STEP in middle Miocene times, penetrating the Aegean region in the Pliocene.

Breakup of Pangaea and plate kinematics of the central Atlantic and Atlas regions

NASA Astrophysics Data System (ADS)

Schettino, Antonio; Turco, Eugenio

2009-08-01

A new central Pangaea fit (type A) is proposed for the late Ladinian (230 Ma), together with a plate motions model for the subsequent phases of rifting, continental breakup and initial spreading in the central Atlantic. This model is based on: (1) a reinterpretation of the process of formation of the East Coast Magnetic Anomaly along the eastern margin of North America and the corresponding magnetic anomalies at the conjugate margins of northwest Africa and the Moroccan Meseta; (2) an analysis of major rifting events in the central Atlantic, Atlas and central Mediterranean and (3) a crustal balancing of the stretched margins of North America, Moroccan Meseta and northwest Africa. The process of fragmentation of central Pangaea can be described by three major phases spanning the time interval from the late Ladinian (230 Ma) to the Tithonian (147.7 Ma). During the first phase, from the late Ladinian (230 Ma) to the latest Rhaetian (200 Ma), rifting proceeded along the eastern margin of North America, the northwest African margin and the High, Saharan and Tunisian Atlas, determining the formation of a separate Moroccan microplate at the interface between Gondwana and Laurasia. During the second phase, from the latest Rhaetian (200 Ma) to the late Pliensbachian (185 Ma), oceanic crust started forming between the East Coast and Blake Spur magnetic anomalies, whereas the Morrocan Meseta simply continued to rift away from North America. During this time interval, the Atlas rift reached its maximum extent. Finally, the third phase, encompassing the time interval from the late Pliensbachian (185 Ma) to chron M21 (147.7 Ma), was triggered by the northward jump of the main plate boundary connecting the central Atlantic with the Tethys area. Therefore, as soon as rifting in the Atlas zone ceased, plate motion started along complex fault systems between Morocco and Iberia, whereas a rift/drift transition occurred in the northern segment of the central Atlantic, between Morocco and the conjugate margin of Nova Scotia. The inversion of the Atlas rift and the subsequent formation of the Atlas mountain belt occurred during the Oligocene-early Miocene time interval. In the central Atlantic, this event was associated with higher spreading rates of the ridge segments north of the Atlantis FZ. An estimate of 170 km of dextral offset of Morocco relative to northwest Africa, in the central Atlantic, is required by an analysis of marine magnetic anomalies. Five plate tectonic reconstructions and a computer animation are proposed to illustrate the late Triassic and Jurassic process of breakup of Pangaea in the central Atlantic and Atlas regions.

The dynamics of continental breakup-related magmatism on the Norwegian volcanic margin

NASA Astrophysics Data System (ADS)

Breivik, A. J.; Faleide, J. I.; Mjelde, R.

2007-12-01

The Vøring margin off mid-Norway was initiated during the earliest Eocene (~54 Ma), and large volumes of magmatic rocks were emplaced during and after continental breakup. In 2003, an ocean bottom seismometer survey was acquired on the Norwegian margin to constrain continental breakup and early seafloor spreading processes. The profile P-wave model described here crosses the northern part of the Vøring Plateau. Maximum igneous crustal thickness was found to be 18 km, decreasing to ~6.5 km over ~6 M.y. after continental breakup. Both the volume and the duration of excess magmatism after breakup is about twice of what is observed off the Møre Margin south of the Jan Mayen Fracture Zone, which offsets the margin segments by ~170 km. A similar reduction in magmatism occurs to the north over an along-margin distance of ~100 km to the Lofoten margin, but without a margin offset. There is a strong correlation between magma productivity and early plate spreading rate, which are highest just after breakup, falling with time. This is seen both at the Møre and the Vøring margin segments, suggesting a common cause. A model for the breakup- related magmatism should be able to (1) explain this correlation, (2) the magma production peak at breakup, and (3) the magmatic segmentation. Proposed end-member hypotheses are elevated upper-mantle temperatures caused by a hot mantle plume, or edge-driven small-scale convection fluxing mantle rocks through the melt zone. Both the average P-wave velocity and the major-element data at the Vøring margin indicate a low degree of melting consistent with convection. However, small scale convection does not easily explain the issues listed above. An elaboration of the mantle plume model by N. Sleep, in which buoyant plume material fills the rift-topography at the base of the lithosphere, can explain these: When the continents break apart, the buoyant plume-material flows up into the rift zone, causing excess magmatism by both elevated temperature and excess flux, and magmatism dies off as this rift-restricted material is spent. The buoyancy of the plume-material also elevates the plate boundaries and enhances plate spreading forces initially. The rapid drop in magma productivity to the north correlates with the northern boundary of the wide and deep Cretaceous Vøring Basin, thus less plume material was accommodated off Lofoten. This model predicts that the magma segmentation will show little variation in the geochemical signature.

Initiation of the Pyrenean plate boundary fault and its effect on the near- and far-field deformation of the European plate

NASA Astrophysics Data System (ADS)

Dielforder, Armin; Frasca, Gianluca; Ford, Mary

2017-04-01

The European plate was affected by contractional deformation events in Late Cretaceous time. This is recorded by inception of thrusting and foreland basin subsidence in the Pyrenean realm, and inversion of Mesozoic rift systems in the interior of the European plate. It is widely accepted that the plate-wide deformation resulted from the onset of NE-directed convergence of Africa-Iberia relative to Europe, and a strong mechanical coupling of the plates, which allowed the transfer of stresses far into Europe. Geological data from both the Pyrenean orogen and the interior of the European plate indicate, however, that these conditions persisted only for 15-20 Myr and that Europe experienced a plate-wide stress relaxation during Paleocene time. Although a slow down in plate convergence between Africa and Europe and North Atlantic continental rifting were proposed as potential causes for the stress relaxation, the subject has remained controversial. In particular, none of the mechanisms seem to be suitable to explain the required changes in the mechanical coupling of Iberian and European plates and the associated stress transfer. Here we propose a new model for the Upper Cretaceous to Paleocene tectonic evolution of the European plate, which takes the temporal evolution of the Pyrenean plate boundary fault into account. Based on plate reconstructions, geological field-data, and restored cross-sections we argue that the plate boundary fault initiated during the Upper Cretaceous within the exhumed mantle domain situated between the rifted margins of the Iberian and European plates. At the transition from the Late Cretaceous to Paleocene, the mantle domain was closed and the rifted margins collided. This evolution was associated with a substantial change in the fault rheology leading to an overall decrease in the plate coupling force. During Paleocene time, the plate coupling force was efficiently balanced by the gravitational push of the European plate, leading to a near neutral stress state in the upper plate and the observed plate-wide stress relaxation in Europe. This study is part of the Orogen research program and conducted in close collaboration with the BRGM (Bureau de Recherches Géologiques et Minières), the CNRS (Centre National de la Recherche Scientifique), and Total.

Satellite-Based Investigations of the Transition from an Oceanic to Continental Transform Margin

NASA Technical Reports Server (NTRS)

Miller, M. Meghan

1998-01-01

Detailed characterization of neotectonics evolution of the Valle de San Felipe and Arroyo Grande regions in northern Baja California. Reoccupied GEOMEX GPS sites, and occupied a regional GPS (Global Positioning System) network. The Baja California peninsula in Mexico offers a unique setting for studying the kinematic evolution of a complex, active strike-slip/rift plate boundary. We are currently conducting remote sensing, geologic, and geodetic studies of this boundary. The combined data sets will yield instantaneous and time integrated views of its evolution. This proposal solicits renewed funding from NASA to support remote sensing and geologic studies. During the late Cenozoic, Baja California has been the locus of changing fault geometry that has accommodated components of the relative motion between the North America and Pacific plates. Contemporary slip between the two plates occurs in a broad zone that encompasses much of southern California and the Baja California Peninsula. The transfer of slip across this zone in southern California is relatively well understood. South of the border, the geometry and role of specific faults and structural provinces in transferring plate margin deformation across the peninsula is enigmatic. Results We use Landsat Thematic Mapper imagery of the Baja California Peninsula to identify recent and active faults, and then conduct field studies that characterize the temporal and spatial structural evolution of the plate margin. These data address questions concerning the neotectonic development of the Gulf of California, the Baja California Peninsula, and their role in evolution of the post-Miocene Pacific - North American plate boundary. Moreover, these studies provide constraints on the geometry of active faults, allowing more exact understanding of the results of ongoing NASA-supported geodetic experiments. In addition, anticipated publication of the TM scenes will provide a widely available geological data base for relatively little-known peninsula California. Achievements include development of an ArcInfo data base of Landsat and SPOT imagery, detailed field studies of Neogene structures in northeastern Baja California, and new constraint on Pacific - North America plate motion at Baja California latitudes. These results are reported in maps, manuscripts and data products which are published or near completion.

Morphology and tectonics of the Andaman Forearc, northeastern Indian Ocean

NASA Astrophysics Data System (ADS)

Cochran, James R.

2010-08-01

The Andaman Sea has developed as the result of highly oblique subduction at the western Sunda Trench, leading to partitioning of convergence into trench-perpendicular and trench-parallel components and the formation of a northward-moving sliver plate to accommodate the trench parallel motion. The Andaman forearc contains structures resulting from both components of motion. The main elements of the forearc are the accretionary prism and outerarc ridge, a series of forearc basins and major N-S faults. The accretionary prism is an imbricate stack of fault slices and folds consisting of ophiolites and sediments scrapped off the subducting Indian Plate. The western, outer slope of the accretionary prism is very steep, rising to depths of 1500-2000 m within a distance of 30 km. There is a difference in the short wavelength morphology between the western and eastern portions of the accretionary prism. The outer portion consists of a series of faulted anticlines and synclines with amplitudes of a few 100 to ~1000 m and widths of 5-15 km resulting from ongoing deformation of the sediments. The inner portion is smoother with lower slopes and forms a strong backstop. The width of the deforming portion of the accretionary prism narrows from 80 to 100 km in the south to about 40 km between 10°N and 11° 30'N. It remains at about 40 km to ~14°40'N. North of there, the inner trench wall becomes a single steep slope up to the Myanmar shelf. The eastern edge of the outerarc ridge is fault bounded and, north of the Nicobar Islands, a forearc basin is located immediately to the east. A deep gravity low with very steep gradients lies directly over the forearc basin. The West Andaman Fault (WAF) and/or the Seulimeum strand of the Sumatra Fault System form the boundary between the Burma and Sunda plates south of the Andaman spreading centre. The WAF is the most prominent morphologic feature of the Andaman Sea and divides the sea into a shallow forearc and a deeper backarc region. The Diligent Fault runs through the forearc basin east of Little Andaman Island. Although it has the general appearance of a normal fault, multichannel seismic data show that it is a compressional feature that probably resulted from deformation of the hanging wall of the Eastern Margin Fault. This could occur if the forearc basins were formed by subduction erosion of the underlying crust rather than by east-west extension.

Surficial sediments on the western Canadian continental shelf

NASA Astrophysics Data System (ADS)

Bornhold, Brian D.; Barrie, J. Vaughn

1991-08-01

The active continental margin off western Canada is characterized in the south by convergence between the Explorer and Juan de Fuca Plates and the America Plate, and in the north by transform fault motion between the Pacific and America Plates along the Queen Charlotte Fault. Except in some of the deepest troughs and basins shelf sediments are dominated by immature lithic arenites reflecting this tectonic setting. The Vancouver Island shelf is from 5 to 75 km wide and displays complex topography on the inner shelf and a relatively featureless mid- and outer shelf. An exception is the area off southwestern Vancouver Island where large basins bounded by morainal deposits extend more than two-thirds of the distance across the shelf. The shelf edge varies from 180 to 225 m depth and is indented by numerous canyons. Nearshore sediments consist mainly of gravels and boulders and become finer offshore such that muds are slowly accumulating in depths greater than 100 m. Off northwestern Vancouver Island calcareous sediments are abundant with carbonate values often exceeding 75%. High wave and current energies and efficient sediment trapping in coastal fiords have resulted in low rates of accumulation. Olive, glauconitic, Holocene muds and muddy sands are generally less than 0.3 m thick and accumulate only on the outer shelf over an extensive stiff, gray, glaciomarine sandy mud. Queen Charlotte Sound exhibits three broad, shallow, glacially scoured troughs, filled mainly with clayey silts and fine sands and separated by sand and gravel covered banks. Hecate Strait, between the Queen Charlotte Islands and the mainland, consists of a prominent southward-opening trough along the east side of the Strait bounded by Dogfish and Laskeek Banks on the west adjacent to Graham and Moresby Islands, respectively. The trough below 200 m is filled by silts. The banks are covered by discontinuous sands and gravels of variable calcareous content. The bank edges often display megaripples and sand waves. Shore-parallel and oblique sand and gravel ridges occur along northernmost Dogfish Bank. Except for the inshore area (less than 100 m) off northern Graham Island little is known of the sediment distribution in Dixon Entrance. Off northwestern Graham Island the near shore zone is dominated by bedrock and discontinuous sands and gravels made up of up to 90% skeletal carbonate. Off northeastern Graham Island sandy sediments predominate in the nearshore. No studies have been undertaken on the narrow shelf off the western Queen Charlotte Islands.

Late Quaternary Normal Faulting and Hanging Wall Basin Evolution of the Southwestern Rift Margin from Gravity and Geology, B.C.S., MX and Exploring the Influence of Text-Figure Format on Introductory Geology Learning

ERIC Educational Resources Information Center

Busch, Melanie M. D.

2011-01-01

An array of north-striking, left-stepping, active normal faults is situated along the southwestern margin of the Gulf of California. This normal fault system is the marginal fault system of the oblique-divergent plate boundary within the Gulf of California. To better understand the role of upper-crustal processes during development of an obliquely…

75 FR 1031 - Certain Hot-Rolled Carbon Steel Flat Products from India: Notice of Preliminary Results of...

Federal Register 2010, 2011, 2012, 2013, 2014

2010-01-08

... measuring at least 10 times the thickness. Universal mill plate (i.e., flat-rolled products rolled on four... determinations. If the Department chooses as facts available a calculated dumping margin from the investigation... questionnaire. See Certain Cut-to-Length Carbon-Quality Steel Plate Products from the Republic of Korea: Final...

Subduction and exhumation of a continental margin in the Scandinavian Caledonides: Insights from ultrahigh pressure metamorphism, late orogenic basins and 3D numerical modelling

NASA Astrophysics Data System (ADS)

Cuthbert, Simon

2017-04-01

The Scandinavian Caledonides (SC) represents a plate collision zone of Himalayan style and scale. Three fundamental characteristics of this orogen are: (1) early foreland-directed, tectonic transport and stacking of nappes; (2) late, wholesale reversal of tectonic transport; (3) ultrahigh pressure metamorphism of felsic crust derived from the underthrusting plate at several levels in the orogenic wedge and below the main thrust surface, indicating subduction of continental crust into the mantle. The significance of this for crustal evolution is the profound remodeling of continental crust, direct geochemical interaction of such crust and the mantle and the opening of accommodation space trapping large volumes of clastic detritus within the orogen. The orogenic wedge of the SC was derived from the upper crust of the Baltica continental margin (a hyper-extended passive margin), plus terranes derived from an assemblage of outboard arcs and intra-oceanic basins and, at the highest structural level, elements of the Laurentian margin. Nappe emplacement was driven by Scandian ( 430Ma) collision of Baltica with Laurentia, but emerging Middle Ordovician ages for diamond-facies metamorphism for the most outboard (or rifted) elements of Baltica suggest prior collision with an arc or microcontinent. Nappes derived from Baltica continental crust were subducted, in some cases to depths sufficient to form diamond. These then detached from the upper part of the down-going plate along major thrust faults, at which time they ceased to descend and possibly rose along the subduction channel. Subduction of the remaining continental margin continued below these nappes, possibly driven by slab-pull of the previously subducted Iapetus oceanic lithosphere and metamorphic densification of subducted felsic continental margin. 3D numerical modelling based upon a Caledonide-like plate scenario shows that if a continental corner or promontory enters the subduction zone, the continental margin descends to greater depths than for a simple orthogonal collision and its modelled thermal evolution is consistent with UHP metamorphic assemblages recorded in the southern part of the SC. Furthermore, a tear initiates at the promontary tip along the ocean-continent junction and propagates rapidly along the orogen. The buoyant upthrust of the subducted margin can then lead to reversal of the motion vector of the entire subducting continent, which withdraws the subducted lithospheric margin out of the subduction channel ("eduction"). Because of the diachroneity of slab failure, the continent also rotates, which causes the eduction vector to change azimuth over time. These model behaviours are consistent with the late orogenic structural evolution of the southern SC. However, during the final exhumation stage the crust may not have acted entirely coherently, as some eduction models propose: There is evidence that some inboard Baltica crust experienced late, shallow subduction before detaching as giant "flakes" that carried the orogenic wedge piggyback, forelandwards. Eduction and flake-tectonics could have operated coevally; the model system does not preclude this. Finally, the traction of a large educting (or extruding) mass of continental margin against the overlying orogenic wedge may have stretched and ruptured the wedge, resulting in opening of the late-orogenic Old Red Sandstone molasse basins.

African humid periods triggered the reactivation of a large river system in Western Sahara

PubMed Central

Skonieczny, C.; Paillou, P.; Bory, A.; Bayon, G.; Biscara, L.; Crosta, X.; Eynaud, F.; Malaizé, B.; Revel, M.; Aleman, N.; Barusseau, J. -P.; Vernet, R.; Lopez, S.; Grousset, F.

2015-01-01

The Sahara experienced several humid episodes during the late Quaternary, associated with the development of vast fluvial networks and enhanced freshwater delivery to the surrounding ocean margins. In particular, marine sediment records off Western Sahara indicate deposition of river-borne material at those times, implying sustained fluvial discharges along the West African margin. Today, however, no major river exists in this area; therefore, the origin of these sediments remains unclear. Here, using orbital radar satellite imagery, we present geomorphological data that reveal the existence of a large buried paleodrainage network on the Mauritanian coast. On the basis of evidence from the literature, we propose that reactivation of this major paleoriver during past humid periods contributed to the delivery of sediments to the Tropical Atlantic margin. This finding provides new insights for the interpretation of terrigenous sediment records off Western Africa, with important implications for our understanding of the paleohydrological history of the Sahara. PMID:26556052

Tectonics and distribution of gold deposits in China - An overview

USGS Publications Warehouse

Zhou, T.; Goldfarb, R.J.; Phillips, G.N.

2002-01-01

Gold exploration in China has expanded rapidly during the last two decades since a modern approach to economic development has become a national priority. China currently produces 180 tonnes (t) of gold annually, which is still significantly less than South Africa, USA, and Australia. However, China is now recognized as possessing significant gold resources in a wide range of mineral deposit types. Present estimates of gold resources in China exceed 4,500 t, which comprise 60% in gold-only deposits, more than 25% in base metal-rich skarn, porphyry, and vein deposits, and more than 10% in placer accumulations. The major gold provinces in China formed during the main episodes of Phanerozoic tectonism. Such tectonism involved interaction of China's three major Precambrian cratons, North China, Tarim, and Yangtze (or South China when combined with Cathysia block), with the Angara (or Siberian), Kazakhstan-Kyrgyzstan, and Indian cratons. Resulting collisions included deformation of accreted oceanic sequences between the cratonic blocks. The most important ore-forming orogenies were (1) the late Paleozoic Variscan (405-270 Ma), which led to amalgamation of the Angara, North China and Yangtze cratons, (2) the Indosinian (270-208 Ma), which led to the collision of North China and South China cratons, (3) the Yanshanian (208-90 Ma), which was largely influenced by the subduction of the Izanagi-Pacific plates beneath eastern China, and (4) the Himalayan (<90 Ma) indentation of the Indian continent into Eurasia. No important Precambrian gold systems are recognized in China, mainly because of reworking of exposed Precambrian rocks by these younger orogenies, but there are a few Caledonian (600-405 Ma) gold-bearing system in northern Xinjiang. Most of China's orogenic, epithermal, and Carlinlike gold deposits are in the reworkerd margins of major cratonic blocks and in metasedimentary rock-dominated fold belts adjacent to these margins. Accordingly, the major gold provinces are present along the northern, southeastern and southern margins of the North China craton, along the southwestern and northwestern margins of the Yangtze craton, in the Tianshan and Altayshan orogenic belts in northern Xinjiang, and throughout the southeastern China fold belt. Gold-placer deposits derived from these primary deposits are concentrated in the northernmost part of northeastern China and along the northerwestern margin of the Yangtze craton. The major provinces with significant gold in porphyry-related copper systems and base metal skarns are present in the Yangtze River area along the northeastern and southeastern margin of the Yangtze craton, in the fold belt in southwestern China, and scattered through northern China. Three-quarters of the Chinese gold-only deposits occur within the North China craton margins. Half are located in the uplifted Precambrian metamorphie rocks and most of the remainder are hosted in the Phanerozoic granitoids that intruded the reworked Precambrian terranes. The abundance of granite-hosted gold contrasts the North China craton with other Precambrian cratons, such as those in Western Australia, central Canada, and Zimbabwe, where gold is mainly hosted in the Archean greenstone belts. This difference may be explained by the multiple episodes of Phanerozoic tectonism along the North China craton margins resulting from the collision of the Angara, North China, and South China cratons, and from subduction of the Izanagi-Pacific oceanic plates underneath the eastern China continent.

Does permanent extensional deformation in lower forearc slopes indicate shallow plate-boundary rupture?

NASA Astrophysics Data System (ADS)

Geersen, J.; Ranero, C. R.; Kopp, H.; Behrmann, J. H.; Lange, D.; Klaucke, I.; Barrientos, S.; Diaz-Naveas, J.; Barckhausen, U.; Reichert, C.

2018-05-01

Seismic rupture of the shallow plate-boundary can result in large tsunamis with tragic socio-economic consequences, as exemplified by the 2011 Tohoku-Oki earthquake. To better understand the processes involved in shallow earthquake rupture in seismic gaps (where megathrust earthquakes are expected), and investigate the tsunami hazard, it is important to assess whether the region experienced shallow earthquake rupture in the past. However, there are currently no established methods to elucidate whether a margin segment has repeatedly experienced shallow earthquake rupture, with the exception of mechanical studies on subducted fault-rocks. Here we combine new swath bathymetric data, unpublished seismic reflection images, and inter-seismic seismicity to evaluate if the pattern of permanent deformation in the marine forearc of the Northern Chile seismic gap allows inferences on past earthquake behavior. While the tectonic configuration of the middle and upper slope remains similar over hundreds of kilometers along the North Chilean margin, we document permanent extensional deformation of the lower slope localized to the region 20.8°S-22°S. Critical taper analyses, the comparison of permanent deformation to inter-seismic seismicity and plate-coupling models, as well as recent observations from other subduction-zones, including the area that ruptured during the 2011 Tohoku-Oki earthquake, suggest that the normal faults at the lower slope may have resulted from shallow, possibly near-trench breaking earthquake ruptures in the past. In the adjacent margin segments, the 1995 Antofagasta, 2007 Tocopilla, and 2014 Iquique earthquakes were limited to the middle and upper-slope and the terrestrial forearc, and so are upper-plate normal faults. Our findings suggest a seismo-tectonic segmentation of the North Chilean margin that seems to be stable over multiple earthquake cycles. If our interpretations are correct, they indicate a high tsunami hazard posed by the yet un-ruptured southern segment of the seismic gap.

Trace-element geochemistry of metabasaltic rocks from the Yukon-Tanana Upland and implications for the origin of tectonic assemblages in east-central Alaska

USGS Publications Warehouse

Dusel-Bacon, C.; Cooper, K.M.

1999-01-01

We present major- and trace- element geochemical data for 27 amphibolites and six greenstones from three structural packages in the Yukon-Tanana Upland of east-central Alaska: the Lake George assemblage (LG) of Devono-Mississippian augen gneiss, quartz-mica schist, quartzite, and amphibolite; the Taylor Mountain assemblage (TM) of mafic schist and gneiss, marble, quartzite, and metachert; and the Seventymile terrane of greenstone, serpentinized peridotite, and Mississippian to Late Triassic metasedimentary rocks. Most LG amphibolites have relatively high Nb, TiO2, Zr, and light rare earth element contents, indicative of an alkalic to tholeiitic, within-plate basalt origin. The within-plate affinities of the LG amphibolites suggest that their basaltic parent magmas developed in an extensional setting and support a correlation of these metamorphosed continental-margin rocks with less metamorphosed counterparts across the Tintina fault in the Selwyn Basin of the Canadian Cordillera. TM amphibolites have a tholeiitic or calc-alkalic composition, low normalized abundances of Nb and Ta relative to Th and La, and Ti/V values of <20, all indicative of a volcanic-arc origin. Limited results from Seventymile greenstones indicate a tholeiitic or calc-alkalic composition and intermediate to high Ti/V values (27-48), consistent with either a within-plate or an ocean-floor basalt origin. Y-La-Nb proportions in both TM and Seventymile metabasalts indicate the proximity of the arc and marginal basin to continental crust. The arc geochemistry of TM amphibolites is consistent with a model in which the TM assemblage includes arc rocks generated above a west-dipping subduction zone outboard of the North American continental margin in mid-Paleozoic through Triassic time. The ocean-floor or within-plate basalt geochemistry of the Seventymile greenstones supports the correlation of the Seventymile terrane with the Slide Mountain terrane in Canada and the hypothesis that these oceanic rocks originated in a basin between the continental margin and an arc to the west.

Distribution of long-lived radioactive iodine isotope (I-129) in pore waters from the gas hydrate fields on the continental margins: Indication for methane source of gas hydrate deposits

NASA Astrophysics Data System (ADS)

Tomaru, H.; Lu, Z.; Fehn, U.

2011-12-01

Because iodine has a strong association with organic matters in marine environments, pore waters in high methane potential region, in particular gas hydrate occurrences on the continental margins, are enriched significantly in iodine compared with seawater. Natural iodine system is composed of stable and radioactive species, I-129 (half-life of 15.7 Myr) has been used for estimating the age of source formations both for methane and iodine, because iodine can be liberated into pore water during the degradation of organic matter to methane in deep sediments. Here we present I-129 age data in pore waters collected from variety of gas hydrate occurrences on the continental margins. The I-129 ages in pore waters from these locations are significantly older than those of host sediments, indicating long-term transport and accumulation from deep/old sediments. The I-129 ages in the Japan Sea and Okhotsk Sea along the plate boundary between the North American and Amurian Plates correspond to the ages of initial spreading of these marginal seas, pointing to the massive deposition of organic matter for methane generation in deep sediments within limited periods. On the Pacific side of these areas, organic matter-rich back stop is responsible for methane in deep-seated gas hydrate deposits along the Nankai Trough. Deep coaly sequences responsible for deep conventional natural gas deposits are also responsible for overlying gas hydrate deposits off Shimokita Peninsula, NE Japan. Those in the Gulf of Mexico are correlative to the ages of sediments where the top of salt diapirs intrude. Marine sediments on the Pacific Plate subducting beneath the Australian Plate are likely responsible for the methane and iodine in the Hikurangi Trough, New Zealand. These ages reflect well the regional geological settings responsible for generation, transport, and accumulation of methane, I-129 is a key to understand the geological history of gas hydrate deposition.

Passive margins getting squeezed in the mantle convection vice

NASA Astrophysics Data System (ADS)

Yamato, Philippe; Husson, Laurent; Becker, Thorsten W.; Pedoja, Kevin

2014-05-01

Passive margins often exhibit uplift, exhumation and tectonic inversion. We speculate that the compression in the lithosphere gradually increased during the Cenozoic. In the same time, the many mountain belts at active margins that accompany this event seem readily witness this increase. However, how that compression increase affects passive margins remains unclear. In order to address this issue, we design a 2D viscous numerical model wherein a lithospheric plate rests above a weaker mantle. It is driven by a mantle conveyor belt, alternatively excited by a lateral downwelling on one side, an upwelling on the other side, or both simultaneously. The lateral edges of the plate are either free or fixed, representing the cases of free convergence, and collision or slab anchoring, respectively. This distinction changes the upper boundary condition for mantle circulation and, as a consequence, the stress field. Our results show that between these two regimes, the flow pattern transiently evolves from a free-slip convection mode towards a no-slip boundary condition above the upper mantle. In the second case, the lithosphere is highly stressed horizontally and deforms. For an equivalent bulk driving force, compression increases drastically at passive margins provided that upwellings are active. Conversely, if downwellings alone are activated, compression occurs at short distances from the trench and extension prevails elsewhere. These results are supported by Earth-like 3D spherical models that reveal the same pattern, where active upwellings are required to excite passive margins compression. These results support the idea that compression at passive margins, is the response to the underlying mantle flow, that is increasingly resisted by the Cenozoic collisions.

A Three-Component Seismic Refraction Survey in Northwestern Arizona

DTIC Science & Technology

1989-01-31

the Pacific Plate-North American Plate boundary. The area of investigation (see Figure 1) stretches 900 kin across the Pacific Ocean , southern...50 32. Speed, R.C. (1979) Collided Paleozoic Microplate in the Western United States, J. of Geol. 87:279-292. 33. Schweichert. R.A. and Cowan, D.S...72:1-50 32. Speed, R.C. (1979) Coilided Paleozoic Microplate in the Western United States, J. of Geol. 87:279-292. 33. Schweichert, R.A. and Cowan, D.S

New constraints on the crustal structure in the eastern part of northern Baffin Bay

NASA Astrophysics Data System (ADS)

Reichert, C. J.; Damm, V.; Altenbernd, T.; Berglar, K.; Block, M.; Ehrhardt, A.; Schnabel, M.

2010-12-01

The northern Baffin Bay is a key area for testing plate kinematic models for the Paleocene-Eocene motion of Greenland relative to North America and to decipher the evolution of the thick sedimentary basins in this area. In summer 2010, a multidisciplinary marine geoscientific expedition focusing on the Greenland part of northern Baffin Bay was performed under the direction of the Federal Institute for Geosciences and Natural Resources Hannover, Germany in cooperation with the Alfred-Wegener Institute Bremerhaven. Using 70 days ship time onboard the German R/V Polarstern a comprehensive data set was acquired along profiles extending from the deep oceanic basin in the central part of North Baffin Bay onto the Greenland continental margin in an area which was bordered by the Kane Basin in the North and Disco Island in the South. By means of multi-channel seismic, wide angle seismic, gravimetric and magnetic methods the structural inventory of the crust in the NW Baffin Bay was investigated. Additionally, heat flow data and sediment cores were collected at selected positions along lines across the Greenland continental margin. The cores were extracted for geochemical and geomicrobiological analysis to be used for basin modeling and studying the hydrocarbon potential. Aeromagnetic data was acquired covering part of the marine survey area to investigate magnetic signatures of the oceanic crust and the continental margin. In our presentation we will give an overview of the first results of the expedition with special focus on multi-channel seismic data. With a total length of 3500 km, the initial interpretation of multi-channel seismic data shows that the West Greenland margin is a typical passive continental margin with large rotated basement blocks, listric faults facing mainly seaward, and deep syn-rift-basins in between. The most prominent reflector under the shelf and the slope probably indicates the transition from rifting to drifting and therefore the beginning of seafloor spreading in the Baffin Bay. This is suggested by erosion on top of basement blocks, subsidence along the slope area, and termination of the prominent reflector in the area of the ocean-continent boundary. The syn-rift sediments were deposited in two single phases, which could be imaged along several sections of the newly acquired seismic lines. The Quaternary and late Pliocene glacial deposits are characterized by prograding sequences on the western shelf and the upper slope. Some lines show that the NNW striking Melville Ridge is a compression structure generated by thrusting of the Melville graben sedimentary fill on its western edge. We interpret the compression as a result of strike slip faulting in conjunction with the northward movement of Greenland in the second drift phase starting in the Eocene. At some segments of the crustal margin the opening of the Baffin Bay might be associated with volcanic activity.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Everett, J.R.; Russell, O.R.; Staskowski, R.J.

Analysis of 38 contiguous Landsat Multispectral Scanner scenes acquired over Myanmar (Burma) reveals numerous large-scale features associated with margins of the Burman plate, previously unidentified northeast-southwest-trending discontinuities, important extensions of previously mapped fault trends, and numerous structural features that appear favorable for petroleum exploration. A mosaic of these scenes at 1:1,000,000 scale shows a large number of tectonic elements and their spatial relationships. Within the area of investigation are portions of the Indian, Burman, Lhasa, and Shan-Thai plates, and perhaps other, smaller plates. The Himalayan front and Indo-Burman Ranges manifest effects of current and recently past plate movement. The complexitymore » of the kinematic history accounts for the diversity of structural features in the area. The last major event in this long and violent saga, which began in middle Miocene (approximately 11 Ma) time and continues to the present, is the recent change from a collisional to a right-lateral strike-slip transform margin between the Indian and Burman plates. The complexity of the structures visible is the product of multiple plate collisions, rotation of the Indian plate and parts of the Asian plate, and long-continued convergence that changed velocity and direction tbrough time. The most obvious evidence of this complexity, which is immediately apparent on geologic maps or the Landsat mosaic of the region, is the almost right-angle relationship of the folds of the Indo-Burman Ranges and the frontal thrusts and suture zones of the Himalaya. These two sets of compressive features imply maximum compressive stress axes that lie at right angles to each other. The implications are either that the orientation of the stress field changes rapidly over a short distance or that the stress field has changed through time. Both occurrences seem to be true.« less

3-D imaging of two episodes of Hikurangi Plateau subduction in the southern South Island of New Zealand

NASA Astrophysics Data System (ADS)

Eberhart-Phillips, D. M.; Reyners, M.; Upton, P.; Gubbins, D.

2016-12-01

The Hikurangi Plateau (originally part of the Ontong Java large igneous province) has been subducted beneath New Zealand twice - firstly at ca. 108-105 Ma during north-south convergence with Gondwana, and currently during east-west convergence between the Pacific and Australian plates. We have investigated the southern limit of this subducted plateau by supplementing the sparse GeoNet permanent seismograph network in the southeastern South Island with a forty-station broadband portable seismograph network. We have then used local earthquake tomography to determine detailed 3-D Vp and Vp/Vs structure in the southern South Island. We track a region of Vp 8.5 km/s (which has previously been associated with an eclogite layer at the base of the Hikurangi Plateau from studies in the North Island) across most of the southwestern South Island. Its southeastern edge runs diagonally from near Christchurch to northern Fiordland. It dips both to the northwest and the southwest, and impacts the subducted Australian plate in northern Fiordland, where it currently bends the subducted Australian slab to vertical. The plateau and its leading oceanic crust are distinguished by low Vp/Vs, consistent with extensive dehydration of the thick (ca. 35 km), buoyant plateau during ca. 500 km of flat subduction at the Gondwana margin. The plateau is also revealed by dipping zones of relocated earthquakes. The backstop of Gondwana subduction appears to be the ophiolitic Maitai terrane, which extends through the crust and forms the trenchward boundary of the ca. 60-65 km-thick Median Batholith. We image the low Vp crustal root associated with orogeny at the Gondwana margin in the southeastern South Island, as well as the crustal root resulting from the current convergent episode in the western South Island. The shapes of both crustal roots are controlled by the Hikurangi Plateau.

The Rae craton of Laurentia/Nuna: a tectonically unique entity providing critical insights into the concept of Precambrian supercontinental cyclicity

NASA Astrophysics Data System (ADS)

Bethune, K. M.

2015-12-01

Forming the nucleus of Laurentia/Nuna, the Rae craton contains rocks and structures ranging from Paleo/Mesoarchean to Mesoproterozoic in age and has long been known for a high degree of tectonic complexity. Recent work strongly supports the notion that the Rae developed independently from the Hearne; however, while the Hearne appears to have been affiliated with the Superior craton and related blocks of 'Superia', the genealogy of Rae is far less clear. A diagnostic feature of the Rae, setting it apart from both Hearne and Slave, is the high degree of late Neoarchean to early Paleoproterozoic reworking. Indeed, following a widespread 2.62-2.58 Ga granite bloom, the margins of Rae were subjected to seemingly continuous tectonism, with 2.55-2.50 Ga MacQuoid orogenesis in the east superseded by 2.50 to 2.28 Ga Arrowsmith orogenesis in the west. A recent wide-ranging survey of Hf isotopic ratios in detrital and magmatic zircons across Rae has demonstrated significant juvenile, subduction-related crustal production in this period. Following break-up at ca. 2.1 Ga, the Rae later became a tectonic aggregation point as the western and eastern margins transitioned back to convergent plate boundaries (Thelon-Taltson and Snowbird orogens) marking onset of the 2.0-1.8 Ga assembly of Nuna. The distinctive features of Rae, including orogenic imprints of MacQuoid and Arrowsmith vintage have now been identified in about two dozen cratonic blocks world-wide, substantiating the idea that the Rae cratonic family spawned from an independent earliest Paleoproterozoic landmass before its incorportation in Nuna. While critical tests remain to be made, including more reliable ground-truthing of proposed global correlations, these relationships strongly support the notion of supercontinental cyclicity in the Precambrian, including the Archean. They also challenge the idea of a globally quiescent period in the early Paleoproterozoic (2.45-2.2 Ga) in which plate tectonics slowed or shut down.

Growth and gravitational collapse of a mountain front of the Eastern Cordillera of Colombia

NASA Astrophysics Data System (ADS)

Kammer, Andreas; Montana, Jorge; Piraquive, Alejandro

2016-04-01

The Eastern Cordillera of Colombia is bracketed between the moderately east-dipping flank of the Central Cordillera on its western and the gently bent Guayana shield on its eastern side. It evolved as a response to a considerable displacement transfer from the Nazca to the Southamerican plate since the Oligocene break-up of the Farallon plate. One of its distinctive traits refers to its significant shortening by penetrative strain at lower and folding at higher structural levels, approximating a wholesale pure-shear in analogy to a vice model or a crustal welt sandwiched between rigid buttresses. This contrasting behavior may be explained by the spatial coincidence between Neogene mountain belt and a forebulge that shaped the foreland trough during a Cretaceous subduction cycle and was very effective in localizing a weakening of the backarc region comprised between two basin margin faults. In this paper we examine a two-phase evolution of the Eastern mountain front. Up to the late Miocene deformation was restrained by the inherited eastern basin margin fault and as the cordilleran crust extruded, a deformation front with an amplitude similar the present structural relief of up to 10.000 m may have built up. In the Pliocene convergence changed from a roughly strike-perpendicular to an oblique E-W direction and caused N-S trending faults to branch off from the deformation front. This shortening was partly driven by a gravitational collapse of the Miocene deformation front, that became fragmented by normal faults and extruded E on newly formed Pliocene thrust faults. Normal faults display displacements of up to 3000 m and channelized hydrothermal fluids, leading to the formation of widely distributed fault breccias and giving rise to a prolific Emerald mineralization. In terms of wedge dynamics, the Pliocene breaching of the early formed deformation front helped to establish a critical taper.

Radial Viscous Fingering and its Surface Expression due to Convective Upwelling Beneath North Atlantic Ocean

NASA Astrophysics Data System (ADS)

White, N. J.; Schoonman, C. M.

2016-12-01

The Icelandic mantle plume has had a significant influence on the geologic and oceanographic evolution of the North Atlantic Ocean during Cenozoic times. Full-waveform tomographic imaging of this region show that the planform of this plume has a complex irregular shape with significant shear wave velocity anomalies lying beneath the lithospheric plates between 100 and 200 km depth. The planform of these anomalies suggests that five or more horizontal fingers extend radially beneath the fringing continental margins. The best-imaged fingers lie beneath the British Isles and beneath western Norway where significant crustal isostatic departures have been measured. Here, we propose that these radial fingers are generated by a phenomenon known as the Saffman-Taylor instability. Experimental and theoretical analyses show that radial, miscible viscous fingering occurs when a less viscous fluid is injected into a more viscous fluid. The wavelength and number of fingers are controlled by the mobility (i.e. the ratio of viscosities), by the Peclet number (i.e. the ratio of advective and diffusive processes), and by the thickness of the horizontal layer into which fluid is injected. We have combined shear wave velocity estimates with residual depth measurements around the Atlantic margins to calculate the planform distribution of temperature and viscosity within an asthenospheric layer beneath the lithospheric plates. Our calculations suggest that the mobility is 20-50, that the Peclet number is O(104, and that the asthenospheric channel is 150 ± 50 km thick. The existence and form of viscous fingering is consistent with experimental observations and with linear stability analysis. A useful rule of thumb is that the wavelength of viscous fingering is 5 ± 1 times the thickness of the horizontal layer. Our proposal support the notion that dynamic topography of the Earth's surface can be influenced by rapid horizontal flow within spatially evolving asthenospheric fingers.

Radial, Viscous, Saffman-Taylor Fingering of Hot Asthenosphere associated with the Icelandic plume beneath the North Atlantic Ocean

NASA Astrophysics Data System (ADS)

White, Nicky; Schoonman, Charlotte

2017-04-01

The Icelandic plume has had a significant influence upon the geologic and oceanographic evolution of the North Atlantic Ocean throughout Cenozoic times. Published full-waveform earthquake tomographic imaging of this region shows that the planform of this plume has a complex irregular shape with significant shear wave velocity anomalies lying beneath the lithospheric plate at depths of between 100 and 200 km. The planform of these anomalies suggests that five or more horizontal fingers extend radially beneath the fringing continental margins. The best-resolved of these fingers lie beneath the British Isles and beneath western Norway where significant crustal isostatic departures have been measured. Here, we propose that these radial fingers are generated by a well-known fluid dynamical phenomenon known as the Saffman-Taylor instability. Experimental and theoretical analyses show that radial, miscible viscous fingering occurs when a less viscous fluid is injected into a more viscous fluid. The wavelength and number of fingers are controlled by the mobility (i.e. the ratio of viscosities), by the Peclet number (i.e. the ratio of advective and diffusive processes), and by the thickness of the horizontal layer into which fluid is injected. We have combined shear wave velocity estimates with residual depth measurements around the Atlantic margins to calculate the planform distribution of temperature and viscosity within an asthenospheric layer beneath the lithospheric plates. Our calculations suggest that the mobility is 20-50, that the Peclet number is O(10000), and that the asthenospheric channel is 150 +/- 50 km thick. The existence and form of viscous fingering is consistent with experimental observations and with linear stability analysis. A useful rule of thumb is that the wavelength of viscous fingering is 5 +/- 1 times the thickness of the horizontal layer. Our proposal support the notion that dynamic topography of the Earth's surface can be generated and maintained by rapid horizontal flow within spatially evolving asthenospheric fingers.

Comparison of Tarim and central Asian FSU basins, I: Phanerozoic paleogeography

DOE Office of Scientific and Technical Information (OSTI.GOV)

Heubeck, C.; Shangyou N.

1996-01-01

Large amounts of previously unpublished data on the petroleum geology of the FSU's Central Asian Republics and of China's Tarim region have found their way into the western public domain in the past few years. These data provide for the first time the opportunity to merge detailed stratigraphic, tectonic, and paleogeographic studies done during the past decades on both sides of the FSU-Chinese border and to place the results in a plate-tectonic and palinspastically restored reference frame. Major tectonic events affecting the active post-Silurian south-facing margin of Asia between the Caspian Sea and Tarim include (1) the collapse of themore » Kazakhstan arc fragments (ca. 400-300 Ma); (2) collision of YiIi with Tarim (ca. 375 Ma); (3) consolidation of the Turan Platform from pre-existing basement blocks (ca. 280-220 Ma), (4) collision of Tarim/Yili with the Kazakhstan arcs (ca. 260 Ma); (5) stabilization of a south-facing Triassic active margin (ca. 250 - 200 Ma); (6) accretion of Cimmeria (ca. 200 Ma) and associated reactivation events in Turan, Syr-Darja, and Tarim; (7) reactivation and modification of intracontinental structures during the collision of central Asia with India (ca. 55 Ma to present) and with the Arabian platform (ca. 25 Ma). Periodic large-scale flooding of denuded continental platforms (Turan, Tadjik) during sea-level highstands is recorded in the Jurassic, Mid-Late Cretaceous, and the Early Tertiary, resulting in extensive tracts of restricted marine sedimentary systems and marine incursions deep into central Asia (SW Tarim, Kuche Depression, Fergana, Turgay). Mesozoic-Cenozoic source rocks are sensitive to rapid lateral facies changes, and understanding their distribution requires detailed stratigraphic analysis. The attempted synthesis of data from China and the FSU with plate-tectonic concepts allows the transfer and testing of play concepts and hydrocarbons systems across the FSU-Chinese border.« less

Comparison of Tarim and central Asian FSU basins, I: Phanerozoic paleogeography

DOE Office of Scientific and Technical Information (OSTI.GOV)

Heubeck, C.; Shangyou N.

1996-12-31

Large amounts of previously unpublished data on the petroleum geology of the FSU`s Central Asian Republics and of China`s Tarim region have found their way into the western public domain in the past few years. These data provide for the first time the opportunity to merge detailed stratigraphic, tectonic, and paleogeographic studies done during the past decades on both sides of the FSU-Chinese border and to place the results in a plate-tectonic and palinspastically restored reference frame. Major tectonic events affecting the active post-Silurian south-facing margin of Asia between the Caspian Sea and Tarim include (1) the collapse of themore » Kazakhstan arc fragments (ca. 400-300 Ma); (2) collision of YiIi with Tarim (ca. 375 Ma); (3) consolidation of the Turan Platform from pre-existing basement blocks (ca. 280-220 Ma), (4) collision of Tarim/Yili with the Kazakhstan arcs (ca. 260 Ma); (5) stabilization of a south-facing Triassic active margin (ca. 250 - 200 Ma); (6) accretion of Cimmeria (ca. 200 Ma) and associated reactivation events in Turan, Syr-Darja, and Tarim; (7) reactivation and modification of intracontinental structures during the collision of central Asia with India (ca. 55 Ma to present) and with the Arabian platform (ca. 25 Ma). Periodic large-scale flooding of denuded continental platforms (Turan, Tadjik) during sea-level highstands is recorded in the Jurassic, Mid-Late Cretaceous, and the Early Tertiary, resulting in extensive tracts of restricted marine sedimentary systems and marine incursions deep into central Asia (SW Tarim, Kuche Depression, Fergana, Turgay). Mesozoic-Cenozoic source rocks are sensitive to rapid lateral facies changes, and understanding their distribution requires detailed stratigraphic analysis. The attempted synthesis of data from China and the FSU with plate-tectonic concepts allows the transfer and testing of play concepts and hydrocarbons systems across the FSU-Chinese border.« less

Identifying coseismic subsidence in tidal-wetland stratigraphic sequences at the Cascadia subduction zone of western North America

USGS Publications Warehouse

Nelson, Alan R.; Shennan, Ian; Long, Antony J.

1996-01-01

Tidal-wetland stratigraphy reveals that great plate boundary earthquakes have caused hundreds of kilometers of coast to subside at the Cascadia subduction zone. However, determining earthquake recurrence intervals and mapping the coastal extent of past great earthquake ruptures in this region are complicated by the effects of many sedimentologic, hydrographic, and oceanographic processes that occur on the coasts of tectonically passive as well as active continental margins. Tidal-wetland stratigraphy at many Cascadia estuaries differs little from that at similar sites on passive-margin coasts where stratigraphic sequences form through nonseismic processes unrelated to coseismic land level changes. Methods developed through study of similar stratigraphic sequences in Europe provide a framework for investigating the Cascadia estuarine record. Five kinds of criteria must be evaluated when inferring regional coastal subsidence due to great plate boundary earthquakes: the suddenness and amount of submergence, the lateral extent of submerged tidal-wetland soils, the coincidence of submergence with tsunami deposits, and the degree of synchroneity of submergence events at widely spaced sites. Evaluation of such criteria at the Cascadia subduction zone indicates regional coastal subsidence during at least two great earthquakes. Evidence for a coseismic origin remains equivocal, however, for the many peat-mud contacts in Cascadia stratigraphic sequences that lack (1) contrasts in lithology or fossils indicative of more than half a meter of submergence, (2) well-studied tsunami deposits, or (3) precise ages needed for regional correlation. Paleoecologic studies of fossil assemblages are particularly important in estimating the size of sudden sea level changes recorded by abrupt peat-mud contacts and in helping to distinguish erosional and gradually formed contacts from coseismic contacts. Reconstruction of a history of great earthquakes for the Cascadia subduction zone will require rigorous application of the above criteria and many detailed investigations.

Structure and Geochemistry of the Continental-Oceanic Crust Boundary of the Red Sea and the Rifted Margin of Western Arabia

NASA Astrophysics Data System (ADS)

Dilek, Y.; Furnes, H.; Schoenberg, R.

2009-12-01

The continental-oceanic crust boundary and an incipient oceanic crust of the Red Sea opening are exposed within the Arabian plate along a narrow zone of the Tihama Asir coastal plain in SW Saudi Arabia. Dike swarms, layered gabbros, granophyres and basalts of the 22 Ma Tihama Asir (TA) continental margin ophiolite represent products of magmatic differentiation formed during the initial stages of rifting between the African and Arabian plates. Nearly 4-km-wide zone of NW-trending sheeted dikes are the first products of mafic magmatism associated with incipient oceanic crust formation following the initial continental breakup. Gabbro intrusions are composed of cpx-ol-gabbro, cpx-gabbro, and norite/troctolite, and are crosscut by fine-grained basaltic dikes. Granophyre bodies intrude the sheeted dike swarms and are locally intrusive into the gabbros. Regional Bouger gravity anomalies suggest that the Miocene mafic crust represented by the TA complex extends westward beneath the coastal plain sedimentary rocks and the main trough of the Red Sea. The TA complex marks an incipient Red Sea oceanic crust that was accreted to the NE side of the newly formed continental rift in the earliest stages of seafloor spreading. Its basaltic to trachyandesitic lavas and dikes straddle the subalkaline-mildly alkaline boundary. Incompatible trace element relationships (e.g. Zr-Ti, Zr-P) indicate two distinct populations. The REE concentrations show an overall enrichment compared to N-MORB; light REEs are enriched over the heavy ones ((La/Yb)n > 1), pointing to an E-MORB influence. Nd-isotope data show ɛNd values ranging from +4 to +8, supporting an E-MORB melt source. The relatively large variations in ɛNd values also suggest various degrees of involvement of continental crust during ascent and emplacement, or by mixing of another mantle source.

On the initiation of subduction zones

NASA Astrophysics Data System (ADS)

Cloetingh, Sierd; Wortel, Rinus; Vlaar, N. J.

1989-03-01

Analysis of the relation between intraplate stress fields and lithospheric rheology leads to greater insight into the role that initiation of subduction plays in the tectonic evolution of the lithosphere. Numerical model studies show that if after a short evolution of a passive margin (time span a few tens of million years) subduction has not yet started, continued aging of the passive margin alone does not result in conditions more favorable for transformation into an active margin. Although much geological evidence is available in supporting the key role small ocean basins play in orogeny and ophiolite emplacement, evolutionary frameworks of the Wilson cycle usually are cast in terms of opening and closing of wide ocean basins. We propose a more limited role for large oceans in the Wilson cycle concept. In general, initiation of subduction at passive margins requires the action of external plate-tectonic forces, which will be most effective for young passive margins prestressed by thick sedimentary loads. It is not clear how major subduction zones (such as those presently ringing the Pacific Basin) form but it is unlikely they form merely by aging of oceanic lithosphere. Conditions likely to exist in very young oceanic regions are quite favorable for the development of subduction zones, which might explain the lack of preservation of back-arc basins and marginal seas. Plate reorganizations probably occur predominantly by the formation of new spreading ridges, because stress relaxation in the lithosphere takes place much more efficiently through this process than through the formation of new subduction zones.

Collision of the Tacheng block with the Mayile-Barleik-Tangbale accretionary complex in Western Junggar, NW China: Implication for Early-Middle Paleozoic architecture of the western Altaids

NASA Astrophysics Data System (ADS)

Zhang, Ji'en; Xiao, Wenjiao; Luo, Jun; Chen, Yichao; Windley, Brian F.; Song, Dongfang; Han, Chunming; Safonova, Inna

2018-06-01

Western Junggar in NW China, located to the southeast of the Boshchekul-Chingiz (BC) Range and to the north of the Chu-Balkhash-Yili microcontinent (CBY), played a key role in the architectural development of the western Altaids. However, the mutual tectonic relationships have been poorly constrained. In this paper, we present detailed mapping, field structural geology, and geochemical data from the Barleik-Mayile-Tangbale Complex (BMTC) in Western Junggar. The Complex is divisible into Zones I, II and III, which are mainly composed of Cambrian-Silurian rocks. Zone I contains pillow lava, siliceous shale, chert, coral-bearing limestone, sandstone and purple mudstone. Zone II consists of basaltic lava, siliceous shale, chert, sandstone and mudstone. Zone III is characterized by basalt, chert, sandstone and mudstone. These rocks represent imbricated ocean plate stratigraphy, which have been either tectonically juxtaposed by thrusting or form a mélange with a block-in-matrix structure. All these relationships suggest that the BMTC is an Early-Middle Paleozoic accretionary complex in the eastern extension of the BC Range. These Early Paleozoic oceanic rocks were thrust onto Silurian sediments forming imbricate thrust stacks that are unconformably overlain by Devonian limestone, conglomerate and sandstone containing fossils of brachiopoda, crinoidea, bryozoa, and plant stems and leaves. The tectonic vergence of overturned folds in cherts, drag-related curved cleavages and σ-type structures on the main thrust surface suggests top-to-the-NW transport. Moreover, the positive εNd(t) values of volcanic rocks from the Tacan-1 drill-core, and the positive εHf(t) values and post-Cambrian ages of detrital zircons from Silurian and Devonian strata to the south of the Tacheng block indicate that its basement is a depleted and juvenile lithosphere. And there was a radial outward transition from coral-bearing shallow marine (shelf) to deep ocean (pelagic) environments, and from OIB/E-MORB to N-MORB lava geochemistry away from the Tacheng block. Comparisons with published data suggest that these positive isotopic values, stratigraphic, structural and geochemical relationships can be best understood as an analogue of the relationships between the Ontong Java oceanic plateau and the Pacific oceanic crust. Therefore we propose that the basement of the Tacheng block was an Early Paleozoic oceanic plateau. The southern part of the Tacheng block was an accretionary complex and the northern part was an oceanic basin in the Early Paleozoic, the configuration of which is similar to that of the present Ontong Java oceanic plateau situated on the Pacific oceanic crust, and its accretion into the Solomon accretionary complex. The presence of Ordovician SSZ-type ophiolites, early Paleozoic blueschist and Silurian SSZ-type intrusions in the BMTC, and Early-Middle Paleozoic continental arc-related intrusive rocks in the northern margin of the CBY provide further corroboration of a former subduction zone between the southern West Junggar and the northern margin of the CBY. Furthermore, consideration of the fact that the Kokchetav-North Tianshan range was collaged to the southern margin of the CBY in the Ordovician-Devonian indicates that both ranges were amalgamated synchronously with the CBY constructing the Early-Middle Paleozoic architecture of western Altaids.

Mesozoic evolution of the Amu Darya basin

NASA Astrophysics Data System (ADS)

Brunet, Marie-Françoise; Ershov, Andrey; Korotaev, Maxim; Mordvintsev, Dmitriy; Barrier, Eric; Sidorova, Irina

2014-05-01

This study, granted by the Darius Programme, aims at proposing a model of tectono-stratigraphic evolution of the Amu Darya basin since the Late Palaeozoic and to understand the relationship with the nearby basins. The Amu Darya basin, as its close eastern neighbour, the Afghan-Tajik basin, lies on the Turan platform, after the closure of the Turkestan Ocean during the Late Paleozoic. These two basins, spread on mainly lowlands of Turkmenistan, southwest Uzbekistan, Tajikistan, and northern Afghanistan, are separated from one another by the South-Western Gissar meganticline, where series of the northern Amu Darya margin are outcropping. The evolution is closely controlled by several periods of crustal thinning (post-collision rifting and back-arc extension), with some marine incursions, coming in between accretions of continental blocks and collisions that succeeded from the Late Triassic-Early Jurassic (Eo-Cimmerian orogeny) to the Cenozoic times. These orogenies controlled the deposition of thick clastics sequences, and the collision of the Indian Plate with Eurasia strongly deformed the sedimentary cover of the Afghan-Tajik basin. The more than 7 km thick Meso-Cenozoic sedimentary succession of the Amu Darya basin, lies on a complex system of rifts and blocks. Their orientation and age (late Permian, Triassic?) are not well known because of deep burial. The north-eastern margin, with the Bukhara (upper margin) and Chardzhou steps, is NW oriented, parallel to the Paleozoic Turkestan suture. The orientation bends to W-E, in the part of the Gissar situated to the North of the Afghan-Tajik basin. This EW trending orientation prevails also in the south(-eastern) margin of the basin (series of North Afghanistan highs) and in the Murgab depression, the south-eastern deepest portion of the Amu Darya basin. It is in this area and in the eastern part of the Amu Darya basin that the Jurassic as well as the lower Cretaceous sediments are the thickest. The south-western part of the basin is occupied by the Pre-Kopet Dagh Cenozoic foreland basin NW oriented, possibly underlain by an earlier extensional trough. The main elements of the sedimentary pile, which can be partly observed in the South-Western Gissar are: Lower to Middle Jurassic continental to paralic clastic rocks; upper Middle to Upper Jurassic marine carbonate then thick Tithonian evaporite rocks, sealing the reservoirs in the Jurassic carbonates; continental Neocomian clastic rocks and red beds, Aptian to Paleogene marine carbonate and clastic rocks. To reconstruct the geodynamic evolution of the Amu Darya Basin, we analysed the subsidence by backstripping of some wells/pseudo-wells and of three cross-sections with some examples of thermal modelling on the periods of maturation of the potential source rocks. The crustal thinning events take place in the Permo-Triassic? (depending on the age of the rifts underlying the basin), in Early-Middle Jurassic and during the Early Cretaceous, resulting in increases of the tectonic subsidence rates.

Lithospheric strength variations as a control on new plate boundaries: examples from the northern Red Sea region

NASA Astrophysics Data System (ADS)

Steckler, Michael S.; ten Brink, Uri S.

1986-08-01

The complex plate boundary between Arabia and Africa at the northern end of the Red Sea includes the Gulf of Suez rift and the Gulf of Aqaba—Dead Sea transform. Geologic evidence indicates that during the earliest phase of rifting the Red Sea propagated NNW towards the Mediterranean Sea creating the Gulf of Suez. Subsequently, the majority of the relative movement between the plates shifted eastward to the Dead Sea transform. We propose that an increase in the strength of the lithosphere across the Mediterranean continental margin acted as a barrier to the propagation of the rift. A new plate boundary, the Dead Sea transform formed along a zone of minimum strength. We present an analysis of lithospheric strength variations across the Mediterranean continental margin. The main factors controlling these variations are the geotherm, crustal thickness and composition, and sediment thickness. The analysis predicts a characteristic strength profile at continental margins which consists of a marked increase in strength seaward of the hinge zone and a strength minimum landward of the hinge zone. This strength profile also favors the creation of thin continental slivers such as the Levant west of the Dead Sea transform and the continental promontory containing Socotra Island at the mouth of the Gulf of Aden. Calculations of strength variations based on changes of crustal thickness, geotherm and sediment thickness can be extended to other geologic settings as well. They can explain the location of rerifting events at intracratonic basins, of backarc basins and of major continental strike-slip zones.

Testing Spatial Correlation of Subduction Interplate Coupling and Forearc Morpho-Tectonics

NASA Technical Reports Server (NTRS)

Goldfinger, Chris; Meigs, Andrew; Meigs, Andrew; Kaye, Grant D.; VanLaningham, Sam

2005-01-01

Subduction zones that are capable of generating great (Mw greater than 8) earthquakes appear to have a common assemblage of forearc morphologic elements. Although details vary, each have (from the trench landward), an accretionary prism, outer arc high, outer forearc basin, an inner forean: basin, and volcanic arc. This pattern is common in spite of great variation in forearc architecture. Because interseismic strain is known to be associated with a locked seismogenic plate interface, we infer that this common forearc morphology is related, in an unknown way, to the process of interseismic Strain accumulation and release in great earthquakes. To date, however, no clear relationship between the subduction process and the common elements of upper plate form has emerged. Whereas certain elements of the system, i.e. the outer arc high, are reasonably well- understood in a structural context, there is little understanding of the structural or topographic evolution of the other key elements like the inner arc and inner forearc basin, particularly with respect to the coupled zone of earthquake generation. This project developed a model of the seismologic, topographic, and uplift/denudation linkages between forearc topography and the subduction system by: 1) comparing geophysical, geodetic, and topographic data from subduction margins that generate large earthquakes; 2) using existing GPS, seismicity, and other data to model the relationship between seismic cycles involving a locked interface and upper-plate topographic development; and 3) using new GPS data and a range-scale topographic, uplift, and denudation analysis of the presently aseismic Cascadia margin to constrain topographic/plate coupling relationships at this poorly understood margin.

A harbinger of plate tectonics: a commentary on Bullard, Everett and Smith (1965) 'The fit of the continents around the Atlantic'.

PubMed

Dewey, John F

2015-04-13

In the 1960s, geology was transformed by the paradigm of plate tectonics. The 1965 paper of Bullard, Everett and Smith was a linking transition between the theories of continental drift and plate tectonics. They showed, conclusively, that the continents around the Atlantic were once contiguous and that the Atlantic Ocean had grown at rates of a few centimetres per year since the Early Jurassic, about 160 Ma. They achieved fits of the continental margins at the 500 fathom line (approx. 900 m), not the shorelines, by minimizing misfits between conjugate margins and finding axes, poles and angles of rotation, using Euler's theorem, that defined the unique single finite difference rotation that carried congruent continents from contiguity to their present positions, recognizing that the real motion may have been more complex around a number of finite motion poles. Critically, they were concerned only with kinematic reality and were not restricted by considerations of the mechanism by which continents split and oceans grow. Many of the defining features of plate tectonics were explicit or implicit in their reconstructions, such as the torsional rigidity of continents, Euler's theorem, closure of the Tethyan ocean(s), major continental margin shear zones, the rapid rotation of small continental blocks (Iberia) around nearby poles, the consequent opening of small wedge-shaped oceans (Bay of Biscay), and misfit overlaps (deltas and volcanic piles) and underlaps (stretched continental edges). This commentary was written to celebrate the 350th anniversary of the journal Philosophical Transactions of the Royal Society.

A harbinger of plate tectonics: a commentary on Bullard, Everett and Smith (1965) ‘The fit of the continents around the Atlantic’

PubMed Central

Dewey, John F.

2015-01-01

In the 1960s, geology was transformed by the paradigm of plate tectonics. The 1965 paper of Bullard, Everett and Smith was a linking transition between the theories of continental drift and plate tectonics. They showed, conclusively, that the continents around the Atlantic were once contiguous and that the Atlantic Ocean had grown at rates of a few centimetres per year since the Early Jurassic, about 160 Ma. They achieved fits of the continental margins at the 500 fathom line (approx. 900 m), not the shorelines, by minimizing misfits between conjugate margins and finding axes, poles and angles of rotation, using Euler's theorem, that defined the unique single finite difference rotation that carried congruent continents from contiguity to their present positions, recognizing that the real motion may have been more complex around a number of finite motion poles. Critically, they were concerned only with kinematic reality and were not restricted by considerations of the mechanism by which continents split and oceans grow. Many of the defining features of plate tectonics were explicit or implicit in their reconstructions, such as the torsional rigidity of continents, Euler's theorem, closure of the Tethyan ocean(s), major continental margin shear zones, the rapid rotation of small continental blocks (Iberia) around nearby poles, the consequent opening of small wedge-shaped oceans (Bay of Biscay), and misfit overlaps (deltas and volcanic piles) and underlaps (stretched continental edges). This commentary was written to celebrate the 350th anniversary of the journal Philosophical Transactions of the Royal Society. PMID:25750142

High-pressure/low-temperature metamorphism in the collision zone between the Chilenia and Cuyania microcontinents (western Precordillera, Argentina)

NASA Astrophysics Data System (ADS)

Boedo, F. L.; Willner, A. P.; Vujovich, G. I.; Massonne, H.-J.

2016-12-01

In central-western Argentina, an Early Paleozoic belt including mafic-ultramafic bodies, marine metasedimentary rocks and high-pressure rocks occur along the western margin of the Precordillera and in the Frontal Cordillera. First pressure-temperature estimates are presented here for low-grade rocks of the southern sector of this belt based on two metasedimentary and one metabasaltic sample from the Peñasco Formation. Peak metamorphic conditions resulted within the range of 345-395 °C and 7.0-9.3 kbar within the high-pressure greenschist facies. The corresponding low metamorphic gradient of 13 °C/km is comparable with subduction related geothermal gradients. Comparison between these results and data from other localities of the same collision zone (Guarguaraz and Colohuincul complexes) confirms a collision between Chilenia and the composite margin of western Gondwana and suggests a stronger crustal thickening in the south of the belt, causing exhumation of more deeply buried sequences. During the Early Paleozoic a long-lived marine sedimentation coupled with the intrusion of MORB-like basalts occurred along a stable margin before the collision event. This contrasts with the almost contemporaneous sedimentation registered during accretion in accretionary prism settings and additionally proves the development of a collision zone along western Precordillera and the eastern Frontal Cordillera as well as the existence of Chilenia as a separate microcontinent.

Forearc kinematics in obliquely convergent margins: Examples from Nicaragua and the northern Lesser Antilles

NASA Astrophysics Data System (ADS)

Turner, Henry L., III

In this study, I use surface velocities derived from GPS geodesy, elastic half-space dislocation models, and modeled Coulomb stress changes to investigate deformation in the over-riding plate at obliquely convergent margins at the leading and trailing edges of the Caribbean plate. The two principal study areas are western Nicaragua, where the Cocos plate subducts beneath the Caribbean plate, and the northern Lesser Antilles, where the North American plate subducts beneath the Caribbean plate. In Nicaragua, plate convergence is rapid at 84 mm yr1 with a small angle of obliquity of 10° along a slightly concave portion of the Middle America Trench. GPS velocities for the period from 2000 to 2004 from sites located in the Nicaraguan forearc confirmed forearc sliver motion on the order of ˜14 mm yr1 in close agreement with the value predicted by DeMets (2001). These results are presented here in Chapter 3 and were reported in Geophysical Research Letters (Turner et al., 2007). GPS observations made on sites located in the interior and on the eastern coast of Nicaragua during the same time period were combined with new data from eastern Honduras to help better constrain estimates of rigid Caribbean plate motion (DeMets et al., 2007). Slip approaching the plate convergence rate along the Nicaraguan and El Salvadoran sections of the Middle America Trench was quantitatively demonstrated by finite element modeling of this section of the plate interface using GPS velocities from our Nicaraguan network together with velocities from El Salvador and Honduras as model constraints (Correa-Mora, 2009). The MW 6.9 earthquake that ruptured the seismogenic zone offshore Nicaragua on October 9, 2004 resulted in coseismic displacements and post-seismic motion at GPS sites in the central part of the Nicaraguan forearc that currently prevent extension of interseismic time-series in this region. An elastic half-space dislocation model was used to estimate coseismic displacements at these sites and to qualitatively examine the observed post-seismic motion. Coseismic and post-seismic motion in this portion of the forearc indicate that long-term motion of the forearc across the earthquake cycle may proceed in a zig-zag pattern, which may contribute to east-west extension as observed in the Managua graben. Sites to the northwest and southeast were not substantially effected by the earthquake, and longer duration time-series (˜7 yrs) from these areas support the earlier estimates of forearc sliver motion. Results from our analysis of Nicaraguan GPS time-series from 2004--2008 and our earthquake modeling efforts are discussed in Chapter 4 and will form the basis of an article to be submitted for publication. In the northern Lesser Antilles, plate convergence is slow at ˜2 cm yr1, and obliquity varies substantially along the convex Lesser Antilles Subduction Zone. In chapter 5, I present GPS velocities derived from a decade of observations on sites in the northern Lesser Antilles and Virgin Islands. The velocities support forearc sliver motion on the order of ˜2--3 mm yr1 consistent with the lower value estimated by Lopez et al. (2006), indicating convergence in the northern region is only partially partitioned. GPS velocities in the northern Lesser Antilles show considerable variation between islands, suggesting possible independent block motion and internal deformation within the forearc region, however, velocity uncertainties for some sites remain high and may reflect the low signal-to-noise ratio of our residual velocities for the region. The lack of a substantial arc-normal component of shortening is similar to that seen in Nicaragua and may indicate a small amount of locking along the subduction interface with forearc sliver motion being driven from a more fully locked region south of Guadeloupe where the Barracuda and Tiburon aseismic ridges impinge on the subduction zone. (Abstract shortened by UMI.)

The Cascadia Subduction Zone: two contrasting models of lithospheric structure

USGS Publications Warehouse

Romanyuk, T.V.; Blakely, R.; Mooney, W.D.

1998-01-01

The Pacific margin of North America is one of the most complicated regions in the world in terms of its structure and present day geodynamic regime. The aim of this work is to develop a better understanding of lithospheric structure of the Pacific Northwest, in particular the Cascadia subduction zone of Southwest Canada and Northwest USA. The goal is to compare and contrast the lithospheric density structure along two profiles across the subduction zone and to interpet the differences in terms of active processes. The subduction of the Juan de Fuca plate beneath North America changes markedly along the length of the subduction zone, notably in the angle of subduction, distribution of earthquakes and volcanism, goelogic and seismic structure of the upper plate, and regional horizontal stress. To investigate these characteristics, we conducted detailed density modeling of the crust and mantle along two transects across the Cascadia subduction zone. One crosses Vancouver Island and the Canadian margin, the other crosses the margin of central Oregon.

Les xénolites ultramafiques du volcanisme alcalin quaternaire d'Oranie (Tell, Algérie occidentale), témoins d'une lithosphère cisaillée et enrichieUltramafic xenoliths from Quaternary alkali volcanism from Oranie (Tell, western Algeria): witnesses of a sheared and enriched lithosphere

NASA Astrophysics Data System (ADS)

Zerka, Mohamed; Cottin, Jean-Yves; Grégoire, Michel; Lorand, Jean-Pierre; Megartsi, M'Hamed; Midoun, Mohamed

Numerous ultramafic xenoliths occur within the Aı̈n-Temouchent volcanic complex (Northwestern Oranie, Algeria). Most of them are type I mantle tectonites (lherzolites and harzburgites) and composite xenoliths (harzburgite/clinopyroxenite) are rare. Only a few samples of spinel lherzolites display relatively fertile compositions when the major part of type I xenoliths have refractory major element compositions but enriched LREE contents showing that they have been affected by mantle metasomatism. The composite xenoliths are witnesses of reactions of alkaline magmas with the upper mantle. An asthenospheric rising, in relation with the large strike slip fault affecting the North African plate margin at Trias time is proposed as a possible geodynamical setting. To cite this article: M. Zerka et al., C. R. Geoscience 334 (2002) 387-394.

Superposed orogenic collision and core-complex formation at the present contact between the Dinarides and the Pannonian basin: The Bukulja and Cer Mountains in central and western Serbia

NASA Astrophysics Data System (ADS)

Matenco, Liviu; Toljic, Marinko; Ducea, Mihai; Stojadinovic, Uros

2010-05-01

Formation of large extensional detachments during orogenic collapse can follow inherited weakness zones such as major asymmetries given by pre-existing subduction zones active during mountain building processes. This is valid in particular in low-topography foreland coupling orogens of Mediterranean type where large amounts of deformation is concentrated in their lower plates, favoring weakness zones activated during a subsequent phase of extensional collapse. One good place to study the orogenic collapse post-dating major collision is the NE margin of the Dinarides in central and western Serbia, where Cretaceous-Eocene shortening and collision was recorded in the Alpine Tethys Sava zone between the European-derived Dacia and Tisza mega-units and the lower Adriatic plate. This is the same place where the Pannonian basin formed as a Miocene back-arc basin in response to a different subduction and roll-back taking place along the external Carpathians. A lineament of Paleogene and Miocene plutons is observed at the northern and eastern margin of the Dinarides, interpreted to be the product of both syn- to post-orogenic subduction magmatism and of decompressional melting during the Pannonian extension. Two of these plutons, Cer and Bukulja, located in western and respectively central Serbia, are intruded in the Jadar-Kopaonik composite thrust sheet, part of the lower Adriatic plate, near the contact with the main suture formed during the Cretaceous-Eocene subduction of the Sava zone. The Lower Miocene age (19-17Ma) Bukulja intrusion is a S-type granite with rare aplitic veins (Cvetkovic et al., 2007). The Cer intrusive complex is a S type two mica granite of around 16Ma in age with an older I-type quartz monzonite component (Koroneos et al. in press). Both granitoids are intruded into the Jadar-Kopaonik metamorphic series, which are in direct contact along the northern, eastern and southern flank with non-metamorphosed, mainly clastic sediments of Cretaceous-Miocene in age and, in the case of Bukulja, with serpentinized ophiolites. The metamorphic sequences are generally characterized by a Paleozoic age meta-sedimentary basement and a meta-sedimentary and meta-volcanic sequence. In the case of Bukulja, a succession of contrasting metamorphosed lithologies has been observed such as sandstones, black limestones, shallow water white limestones, basic volcanic sequences, deep nodular limestones and turbiditic sequences. The lower part of the sequence represents a metamorphosed Triassic sequence similar to what has been defined as the Kopaonik and Studenica series in southern Serbia. This part of the sequence is characterized by at least 3 successive stages of folding, asymmetric folds with WSW-ward vergence and NNE-SSW upright folds being affected by vertical flattening folds associated with extension (see also Marovic et al., 2007). The upper part of the sequence, which is the only part outcropping along the eastern flank of the Cer granitoid, is made up by metamorphosed distal turbidites which have been palinologically dated in Bukulja as Upper Cretaceous in age. This is the metamorphosed equivalent of the Upper Cretaceous - Eocene "flysch"-type of deposits commonly observed elsewhere in the main Sava subduction zone. These rocks are overprinted with a pervasive and strong extensional milonitic foliation indicating top-100 movement of the hanging-wall and are in direct contact with non-metamorphosed, but similar Upper Cretaceous distal turbidites. This suggests a large-scale tectonic omission along the eastern flanks of the Bukulja and Cer detachment. In the case of Bukulja, the extension was associated with the formation of the Early Miocene Morava basin in the detachment hanging-wall, which is an endemic lacustrine precursor of the much larger Middle-Late Miocene Pannonian basin. These finding points towards a bi-modal evolution of the internal Dinarides in central and western Serbia near the present-day contact with the Pannonian basin. An Upper Cretaceous-Eocene phase of top-WSW shortening and metamorphism in the Sava zone and its subducting lower Adriatic plate was subsequently followed by massive core-complex exhumation and top-E directed extension during initiation of the Carpathians back-arc extension. Interestingly, the newly defined extensional detachments accompanying the Pannonian extension closely follow the pre-existing subduction zone and its associated duplications in the lower orogenic plate. This conclusion is compatible with observations in other areas of the Dinarides, such as the Prosara-Motajica in Bosnia/Croatia or Kopaonik-Studenica in southern Serbia (Schefer et al., 2008; Ustaszewski et al., 2009).

Geophysical evidence for a transform margin in Northwestern Algeria: possible vestige of a Subduction-Transform Edge Propagator

NASA Astrophysics Data System (ADS)

Badji, R.; Charvis, P.; Bracene, R.; Galve, A.; Badsi, M.; Ribodetti, A.; Benaissa, Z.; Klingelhoefer, F.; Medaouri, M.; Beslier, M.

2013-12-01

This work is part of the Algerian-French SPIRAL program (Sismique Profonde et Investigation Régionale du Nord de l'Algérie) which provides unprecedented images of the deep structure of the western Algerian Margin based on several wide-angle and multichannel seismic data shot across the Algerian Margin. One of the different hypotheses for the opening of the western Mediterranean Sea, we are testing is that the western part of the Algerian margin was possibly part of the southern edge of the Alboran continental block during its westward migration related to the rollback of the Betic-Rif-Alboran subduction zone. A tomographic inversion of the first arrival traveltimes along a 100-km long wide-angle seismic profile shot over 40 Ocean Bottom Seismometers, across the Margin offshore Mostaganem (Northwestern Algerian Margin) was conducted. The final model reveals striking feature in the deep structure of the margin from north to south: 1- the oceanic crust is as thin as 4-km, with velocities ranging from 5.0 to 7.1 km/s, covered by a 3.3 km thick sedimentary pile (seismic velocities from 1.5 to 5.0 km/s) characterized by an intense diapiric activity of the Messinian salt layer. 2- a sharp transition zone, less than 10 km wide, with seismic velocities intermediate between oceanic seismic velocities (observed northward) and continental seismic velocities (observed southward). This zone coincides with narrow and elongated pull apart basins imaged by multichannel seismic data. No evidence of volcanism nor of exhumed serpentinized upper mantle as described along many extensional continental margins are observed along this segment of the margin. 3- a thinned continental crust coincident with a rapid variation of the Moho depth imaged from 12 to ~20 km with a dip up to 50%. The seafloor bathymetry is showing a steep continental slope (>20%). Either normal or inverse faults are observed along MCS lines shot in the dip direction but they do not present large vertical displacement and could be related primarily to strike slip motion. These results support the hypothesis, that the margin offshore Mostaganem is not an extensional margin but rather a transform margin. There is little evidence of tectonic inversion as described eastward along the Kabylian Margin. Possibly strike slip motion affected the thinned continental crust and the transition zone suggesting that this margin is a vestige of the Subduction-Transform Edge Propagator (STEP) related to the westward migration of the Alboran block.

Tectonic lineaments in the cenozoic volcanics of southern Guatemala: Evidence for a broad continental plate boundary zone

NASA Technical Reports Server (NTRS)

Baltuck, M.; Dixon, T. H.

1984-01-01

The northern Caribbean plate boundary has been undergoing left lateral strike slip motion since middle Tertiary time. The western part of the boundary occurs in a complex tectonic zone in the continental crust of Guatemala and southernmost Mexico, along the Chixoy-Polochic, Motogua and possibly Jocotan-Chamelecon faults. Prominent lineaments visible in radar imagery in the Neogene volcanic belt of southern Guatemala and western El Salvador were mapped and interpreted to suggest southwest extensions of this already broad plate boundary zone. Because these extensions can be traced beneath Quaternary volcanic cover, it is thought that this newly mapped fault zone is active and is accommodating some of the strain related to motion between the North American and Caribbean plates. Onshore exposures of the Motoqua-Polochic fault systems are characterized by abundant, tectonically emplaced ultramafic rocks. A similar mode of emplacement for these off shore ultramafics, is suggested.

Plate tectonic reconstruction of South and East Asia since 43 Ma using seismic tomographic constraints: role of the subducted ';East Asia Sea' (Invited)

NASA Astrophysics Data System (ADS)

Wu, J. E.; Suppe, J.; Renqi, L.; Kanda, R. V.

2013-12-01

Lithosphere that subducts at convergent plate boundaries provides a potentially decipherable plate tectonic record. In this study we use global seismic tomography to map subducted slabs in the upper and lower mantle under South and East Asia to constrain plate reconstructions. The mapped slabs include the Pacific, the Indian Ocean and Banda Sea, the Molucca Sea, Celebes Sea, the Philippine Sea and Eurasia, New Guinea and other lower mantle detached slabs. The mapped slabs were restored to the earth surface and used with Gplates software to constrain a globally-consistent, fully animated plate reconstruction of South and East Asia. Three principal slab elements dominate possible plate reconstructions: [1] The mapped Pacific slabs near the Izu-Bonin and the Marianas trenches form a subvertical slab curtain or wall extending down to 1500 km in the lower mantle. The ';slab curtain' geometry and restored slabs lengths indicate that the Pacific subduction zone has remained fixed within +/- 250 km of its present position since ~43 Ma. In contrast, the Tonga Pacific slab curtain records at least 1000 km trench rollback associated with expansion of back-arc basins. [2] West of the Pacific slab curtain, a set of flat slabs exist in the lower mantle and record a major 8000km by 2500-3000km ocean that existed at ~43 Ma. This now-subducted ocean, which we call the ';East Asian Sea', existed between the Ryukyu Asian margin and the Lord Howe hotspot, present-day eastern Australia, and fills a major gap in Cenozoic plate reconstructions between Indo-Australia, the Pacific Ocean and Asia. [3] An observed ';picture puzzle' fit between the restored edges of the Philippine Sea, Molucca Sea and Indian Ocean slabs suggests that the Philippine Sea was once part of a larger Indo-Australian Ocean. Previous models of Philippine Sea plate motions are in conflict with the location of the East Asian Sea lithosphere. Using the mapped slab constraints, we propose the following 43 Ma to 0 plate tectonic reconstruction. At ~43 Ma a major plate reorganization occurred in South and East Asia marked by Indian Ocean Wharton ridge extinction, initiation of Pacific Ocean WNW motions and the rapid northward motion of the Australian plate. The Philippine Sea and Molucca Sea were clustered at the northern margin of Australia, northwest of New Guinea. During the mid-Cenozoic these plates moved NNE with Australia, accommodated by N-S transforms at the eastern margin of Sundaland. The East Asian Sea was subducted under the northward-moving Philippine Sea and Australia plates, and the expanding Melanesian and Shikoku-Parece Vela backarc basins. At ~20 to 25 Ma the Philippine Sea and Molucca Sea were fragmented from Indo-Australia and began to have a westward component of motion due to partial Pacific capture. Around 1-2 Ma the Philippine Sea was more fully captured by the Pacific and now has rapid Pacific-like northwestward motions.

Heat transfer with very high free stream turbulence

NASA Technical Reports Server (NTRS)

Moffat, Robert J.; Maciejewski, Paul K.

1985-01-01

Stanton numbers as much as 350 percent above the accepted correlations for flat plate turbulent boundary layer heat transfer have been found in experiments on a low velocity air flow with very high turbulence (up to 50 percent). These effects are far larger that have been previously reported and the data do not correlate as well in boundary layer coordinates (Stanton number and Reynolds number) as they do in simpler coordinates: h vs. X. The very high relative turbulence levels were achieved by placing the test plate in different positions in the margin of a large diameter free jet. The large increases may be due to organized structures of large scale which are present in the marginal flowfield around a free jet.

Tomographic Signatures of Ridge Subduction Along Western North America: Implications for Northern Cordillera Plate Tectonics Since the Cretaceous

NASA Astrophysics Data System (ADS)

Lin, Y. A.; Wu, J.

2017-12-01

A number of northern Cordillera plate reconstructions have predicted subduction of the Kula-Farallon ridge and possibly other ridges along western North America after the late Mesozoic. The timing and location of these predicted ridge subduction events have been controversial, with implications for rapid northward terrane motions (i.e. the Baja-British Columbia hypothesis). In contrast, Sigloch and Mihalynuk (2013) proposed an archipelago-style plate model that placed one or several Jurassic-Cretaceous ocean basins between the Farallon plate and western North America, which apparently would preclude any sustained Kula-Farallon ridge subduction along western North America. In this study we test the viability of these models by mapping and unfolding subducted slabs from MITP08 tomography (Li et al., 2008) between Alaska and California within the upper 1500 km mantle. Our aim was to locate significant slab gaps that might be related to ancient ridge subduction `slab windows'. Tomographic velocities were extracted and displayed on our mapped slabs following the methods of Wu et al. (2016) to assist with the identification of slab gaps or windows. Near Alaska, we mapped the Aleutian slab and a detached slab that was previously identified as the `K slab' by Sigloch and Mihalynuk (2013). When unfolded these slabs apparently account for Pacific-Kula convergence towards Alaska since the late Cretaceous. We did not find evidence for a ridge subduction-related slab gap under the Alaskan region. Between northern Canada to California, we mapped the Juan de Fuca slab and several detached slabs at 1000 to 1500 km depths that were previously identified by Sigloch and Mihalynuk (2013). The velocity perturbations within our mapped slabs revealed slower P-wavespeed `slab gaps' under southernmost Alaska, Yukon, and British Columbia between the mapped Kula and Juan de Fuca plate. We did not find evidence of the hypothesized Resurrection plate. We compare our mapped slab gaps to predicted slab window geometries from previous studies and discuss their implications for plate tectonic reconstructions of the northern Cordillera and surrounding area.

A dipping, thick Farallon slab below central United States

NASA Astrophysics Data System (ADS)

Sun, D.; Gurnis, M.; Saleeby, J.; Helmberger, D. V.

2015-12-01

It has been hypothesized that much of the Laramide orogeny was caused by dynamic effects induced by an extensive flat slab during a period of plateau subduction. A particularly thick block containing the Shatsky Rise conjugate, now in the mid-mantle, left a distinctive deformation footprint from southern California to Denver, Colorado. Thus mid-mantle, relic slabs can provide fundamental information about past subduction and the history of plate tectonics if properly imaged. Here we find clear evidence for a northeastward dipping (35° dip), slab-like, but fat (up to 400-500 km thick) seismic anomaly within the top of the lower mantle below the central United States. Using a deep focus earthquake below Spain with direct seismic paths that propagate along the top and bottom of the anomaly, we find that the observed, stacked seismic waveforms recorded with the dense USArray show multi-pathing indicative of sharp top and bottom surfaces. Plate tectonic reconstructions in which the slab is migrated back in time suggest strong coupling of the slab to North America. In combination with the reconstructions, we interpret the structure as arising from eastward dipping Farallon subduction at the western margin of North America during the Cretaceous, in contrast with recent interpretations. The slab could have been fattened through a combination of pure shear thickening during flat-slab subduction and a folding instability during penetration into the lower mantle.

Features of the Caucasian segment of the Alpine-Himalayan-Indonesian Convergence Zone

NASA Astrophysics Data System (ADS)

Sharkov, E.

2012-04-01

The Caucasus Mountain System is a part of the Cenozoic Alpine-Himalayan-Indonesian Convergence Zone (AHICZ) which lasted throughout Eurasia from Western Mediterranean to Western Pacific. This belt has been formed after closure of the Mesozoic Tethys and is marked by mountains building processes, appearance of riftogenic structures, numerous late Cenozoic basaltic plateaus, and chain of often within-continental andesite-latite volcanic arcs, which trace suture zones of the continental plates collision. Caucasus Mountains are located in eastern part of the proper Alpine Zone in zone of Arabian-Eurasian syntaxis and appeared as a result of submeridional pressure which generated by oncoming moving of these plates. The Great Caucasus is represent the south border of the Eurasian plate, uplifted along the Main Caucasian Fault (Thrust). The latter is a part of super-regional deep-seated fault ranged from the Kopetdag through Caspian Sea, Caucasus and Crimea; very likely, that its further continuation is Tornquist-Teisseyre Zone. This superfault separates areas of Alpine convergence from Eurasian plate sensu stricto. The Caucasus occurred between Black and Caspian seas with passive margins and oceanic crust, covered by sediments of 10-15 km thick. Depressions of the seas form large "downfall", or caldrons which cut off pre-Pliocene structures of Caucasus and Kopetdag. These seas are, probably, small remnants of the Tethys which gradually shallowing in the Miocene (Zonenshain, Le Pichon, 1986). New essential deepening of the Black Sea and South-Caspian deep began in the Pliocene- Quaternary; it occurred simultaneously with uprising of Crimea and Caucasus, which were not marked in relief before (Grachev, 2000). Large positive isostatic anomaly beneath the Trans-Caucasian Transverse Uplift (TCTU) of the Great Caucasus and Lesser Caucasus, which stretch out to Arabian plate, occurred between "subsides" Black and especially Caspian seas with neutral to negative isostatic anomalies (Artemiev, 1973; Artemieva et al., 2006). We suggest that the positive anomaly considers with ascending of a mantle plume and .negative anomalies - with descending mantle flows on each its side (Sharkov, 2011). The main feature of this segment of the AHICZ is a presence of two late Cenozoic andesite-latite volcanic arcs: Anatolian-Caucasian and Caucasian-Elbursian, which jointed in area of TCTU. Such type of volcanism is also traced to the south practically through the whole zone of the syntaxis till to Van Lake area in Turkey. These volcanics are close to subduction-related magmas on their petrological and geochemical features, however, there are no clear evidences of the present-day subduction in the area of Neogene-Quaternary volcanism: crustal-level earthquakes predominated here (Gugunava, 1981; Sandvol et al., 2003). It suggests that such type of magmatism appeared as a result of interaction of a mantle plume head with continental crust under condition of continental plates collision (Lebedev et al., 2011). So, the Caucasus is an area of the present-day active interaction of deep-seated mantle processes with shallow lithosphere under conditions of the largest modern convergence zone.

Testing 8000 years of submarine paleoseismicity record offshore western Algeria : First evidence for irregular seismic cycles

NASA Astrophysics Data System (ADS)

Ratzov, G.; Cattaneo, A.; Babonneau, N.; Yelles, K.; Bracene, R.; Deverchere, J.

2012-12-01

It is commonly assumed that stress buildup along a given fault is proportional to the time elapsed since the previous earthquake. Although the resulting « seismic gap » hypothesis suits well for moderate magnitude earthquakes (Mw 4-5), large events (Mw>6) are hardly predictable and depict great variation in recurrence intervals. Models based on stress transfer and interactions between faults argue that an earthquake may promote or delay the occurrence of next earthquakes on adjacent faults by increasing or lowering the level of static stress. The Algerian margin is a Cenozoic passive margin presently inverted within the slow convergence between Africa and Eurasia plates (~3-6 mm/yr). The western margin experienced two large earthquakes in 1954 (Orléansville, M 6.7) and 1980 (El Asnam, M 7.3), supporting an interaction between the two faults. To get meaningful statistics of large earthquakes recurrence intervals over numerous seismic cycles, we conducted a submarine paleoseismicity investigation based on turbidite chronostratigraphy. As evidenced on the Cascadia subduction zone, synchronous turbidites accumulated over a large area and originated from independent sources are likely triggered by an earthquake. To test the method on a slowly convergent margin, we analyze turbidites from three sediment cores collected during the Maradja (2003) and Prisme (2007) cruises off the 1954-1980 source areas. We use X-ray radioscopy, XRF major elements counter, magnetic susceptibility, and grain-size distribution to accurately discriminate turbidites from hemipelagites. We date turbidites by calculating hemipelagic sedimentation rates obtained with radiocarbon ages, and interpolate the rates between turbidites. Finally, the age of events is compared with the only paleoseismic study available on land (El Asnam fault). Fourteen possible seismic events are identified by the counting and correlation of turbidites over the last 8 ka. Most events are correlated with the paleoseismic record of the El Asnam fault, but uncorrelated events suggest that other faults were active. Only the 1954 event (not the 1980) triggered a turbidity current, implying that the sediment buffer on the continental shelf could not be reloaded in 26 years, thus arguing for a minimum time resolution of our method. The new paleoseismic catalog shows a recurrence interval of 300-700 years for most events, but also a great interval of >1200 years without any major earthquake. This result suggests that the level of static stress may have drastically dropped as a result of three main events occurring within the 800 years prior the quiescence period.

A synthesis of Jurassic and Early Cretaceous crustal evolution along the southern margin of the Arctic Alaska–Chukotka microplate and implications for defining tectonic boundaries active during opening of Arctic Ocean basins

USGS Publications Warehouse

Till, Alison B.

2016-01-01

A synthesis of Late Jurassic and Early Cretaceous collision-related metamorphic events in the Arctic Alaska–Chukotka microplate clarifies its likely movement history during opening of the Amerasian and Canada basins. Comprehensive tectonic reconstructions of basin opening have been problematic, in part, because of the large size of the microplate, uncertainties in the location and kinematics of structures bounding the microplate, and lack of information on its internal deformation history. Many reconstructions have treated Arctic Alaska and Chukotka as a single crustal entity largely on the basis of similarities in their Mesozoic structural trends and similar late Proterozoic and early Paleozoic histories. Others have located Chukotka near Siberia during the Triassic and Jurassic, on the basis of detrital zircon age populations, and suggested that it was Arctic Alaska alone that rotated. The Mesozoic metamorphic histories of Arctic Alaska and Chukotka can be used to test the validity of these two approaches.A synthesis of the distribution, character, and timing of metamorphic events reveals substantial differences in the histories of the southern margin of the microplate in Chukotka in comparison to Arctic Alaska and places specific limitations on tectonic reconstructions. During the Late Jurassic and earliest Cretaceous, the Arctic Alaska margin was subducted to the south, while the Chukotka margin was the upper plate of a north-dipping subduction zone or a zone of transpression. An early Aptian blueschist- and greenschist-facies belt records the most profound crustal thickening event in the evolution of the orogen. It may have resulted in thicknesses of 50–60 km and was likely the cause of flexural subsidence in the foredeep of the Brooks Range. This event involved northern Alaska and northeasternmost Chukotka; it did not involve central and western Chukotka. Arctic Alaska and Chukotka evolved separately until the Aptian thickening event, which was likely a result of the rotation of Arctic Alaska into central and western Chukotka. In northeastern Chukotka, the thickened rocks are separated from the relatively little thickened continental crust of the remainder of Chukotka by the oceanic rocks of the Kolyuchin-Mechigmen zone. The zone is a candidate for an Early Cretaceous suture that separated most of Chukotka from northeast Chukotka and Alaska. Albian patterns of magmatism, metamorphism, and deformation in Chukotka and the Seward Peninsula may represent an example of escape tectonics that developed in response to final amalgamation of Chukotka with Eurasia.

Multichannel Seismic Images of Cascadia Forearc Structure at the Oregon Margin

NASA Astrophysics Data System (ADS)

Han, S.; Carbotte, S. M.; Carton, H. D.; Canales, J.; Nedimovic, M. R.

2013-12-01

We present new Multichannel Seismic (MCS) images of the Cascadia forearc and downgoing Juan de Fuca plate offshore Oregon. The data were collected during the Cascadia Ridge-to-Trench experiment conducted in June-July 2012 aboard the R/V Langseth. 2D processing including geometry definition, filtering and editing, deconvolution, amplitude correction, velocity analysis, CMP stacking, and post-stack time migration, has been conducted. The new images confirm some previous observations on the location of the plate boundary and structure of the forearc and also reveal new features of the Oregon margin. West of the deformation front, the Juan de Fuca Plate has a dip of ~1.5o and sediment thickness is > 3 km. A bright Moho reflection and reflections from faults cutting through the crust are imaged. The subducting oceanic crust can be traced continuously landward at least to 15 km from the deformation front. One major forearc basin and a smaller basin 10 km from its west end are imaged. Sediments in both basins are folded with wavelengths of 4-6 km and several faults are identified in the larger basin. Beneath the major basin, a low-frequency reflection is imaged at 3.7 s TWTT similar to that imaged by Trehu et al (1995) and interpreted as originating from the top of Siletz terrane. About 70-80 km from the deformation front, a shallowly dipping reflection is imaged at 7.3 s, which likely corresponds to the top of the downgoing plate. Based on existing velocity models for the margin, the location of this reflection is approximately coincident with the July 2004 earthquake cluster interpreted to have occurred at the plate boundary. This bright reflection is presumably similar in origin to the 'bright spot' imaged from two prior multichannel and wide-angle seismic reflection surveys lines located 40 km and 60 km north of our line. The brightness of the reflection may reflect high pore fluid pressure at the plate interface. Just 4 km west of this presumed top-of-subducting plate reflection, there is another deep reflection at around 7 s dipping landward. This reflection may correspond to the base of the Siletz terrane, which would imply a subduction channel beneath the Siletz terrane. Alternatively, this reflection may be related to a subducted seamount identified from magnetic anomalies by Trehu et al (2012). In addition, we image several small diffractors at 5-7 s TWTT to the west, which are likely related to heterogeneities within the accretionary complex. MCS images of the Cascadia forearc at the Oregon margin illustrating these features will be presented and will be compared with the forearc structure imaged along our Washington MCS line from the same survey.

Zircon U-Pb ages, Hf isotope data, and tectonic implications of Early-Middle Triassic granitoids in the Ailaoshan high-grade metamorphic belt of Southeast Tibet

NASA Astrophysics Data System (ADS)

Wu, Wenbin; Liu, Junlai; Chen, Xiaoyu; Zhang, Lisheng

2017-04-01

The Ailaoshan tectonic belt, where the effects of the Paleo-Tethyan ocean evolution and Indian-Eurasian plate collision are superimposed, is one of the most significant geological discontinuities in western Yunnan province of southeast Tibet. An Ailaoshan micro-block within the belt is bounded by the Ailaoshan suture zone to the west and the Red River Fault to the east, and consists of low- and high-grade metamorphic belts. Late Permian-Middle Triassic granitoids that are widely distributed to the west of the Ailaoshan suture zone and within the Ailaoshan micro-block may yield significant information on the Tethyan tectonic evolution of the Ailaoshan tectonic belt. This study reports new LA-ICP-MS zircon U-Pb geochronology and Hf isotope data of four granitoids from the Ailaoshan high-grade metamorphic belt. Zircon grains from the Yinjie granitoid do not have inherited cores and yield a weighted mean U-Pb age of 247.1 ± 2.0 Ma. The zircon ɛ Hf( t) values range from 7.8 to 12.1, and Hf model ages from 775 to 546 Ma, indicating that the granitoid was derived from juvenile crust. The rims of zircons from the Majie and Yuanjiang granitoids yield weighted mean U-Pb ages of 239.5 ± 1.8 and 237.9 ± 2.6 Ma, respectively, whereas the cores yield ages of 1608-352 Ma. The ɛ Hf( t) values of zircon rims range from -20.4 to -5.3, yielding Hf model ages from 2557 to 1606 Ma and suggesting that the source magma of the Majie and Yuanjiang granitoids was derived from ancient crust. An additional granitoid located near the Majie Village yields a zircon U-Pb age of 241.2 ± 1.0 Ma. Based on our geochronological and geochemical data, combined with geological observations, we propose that the Ailaoshan micro-block was derived from the western margin of the Yangtze block, and is comparable to the Zhongzan and Nam Co micro-blocks. The presence of late Permian mafic rocks with rift-related geochemical characteristics within the Ailaoshan micro-block, together with granitoids derived from partial melting of ancient/juvenile crust, indicates the presence of an Ailaoshan rift. This possible rift may correspond to the Ganzi-Litang Ocean to the northwest and the Jinping-Song Da rift to the southeast. It is suggested that westward subduction of the Jinshajiang-Ailaoshan-Song Ma oceanic lithosphere triggered the separation of the Zhongzan, Ailaoshan, and Nam Co micro-blocks from the western passive continental margin of the Yangtze block through the opening of the Ganzi-Litang-Ailaoshan-Jinping-Song Da ocean/rift. This ocean/rift may represent a subsidiary branch of the Paleo-Tethyan Ocean along the western margin of the Yangtze block.

Contrasting upper-mantle shear wave anisotropy across the transpressive Queen Charlotte margin

NASA Astrophysics Data System (ADS)

Cao, Lingmin; Kao, Honn; Wang, Kelin

2017-10-01

In order to investigate upper mantle and crustal anisotropy along the transpressive Queen Charlotte margin between the Pacific (PA) and North America (NA) plates, we conducted shear wave splitting analyses using 17 seismic stations in and around the island of Haida Gwaii, Canada. Despite the limited station coverage at present, our reconnaissance study does reveal a systematic pattern of mantle anisotropy in this region. Fast directions derived from teleseismic SKS-phase splitting are mostly margin-parallel (NNW-SSE) near the plate boundary but transition to predominantly E-W-trending farther away. We propose that the former is associated with the absolute motion of PA, and the latter reflects a transition from this direction to that of the absolute motion of NA. The broad width of the zone of transition from the PA to NA direction is probably caused by the very obliquely subducting PA slab that travels primarily in the margin-parallel direction. Anisotropy of Haida Gwaii based on local earthquakes features a fast direction that cannot be explained with regional stresses and is probably associated with local structural fabric within the overriding crust. Our preliminary shear wave splitting measurements and working hypotheses based on them will serve to guide more refined future studies to unravel details of the geometry and kinematics of the subducted PA slab, as well as the viscous coupling between the slab and upper mantle in other transpressive margins.

A new genus and species of marine catfishes (Siluriformes; Ariidae) from the upper Eocene Birket Qarun Formation, Wadi El-Hitan, Egypt.

PubMed

El-Sayed, Sanaa E; Kora, Mahmoud A; Sallam, Hesham M; Claeson, Kerin M; Seiffert, Erik R; Antar, Mohammed S

2017-01-01

Wadi El-Hitan, the UNESCO World Heritage Site, of the Fayum Depression in the northeast part of the Western Desert of Egypt, has produced a remarkable collection of Eocene vertebrates, in particular the fossil whales from which it derives its name. Here we describe a new genus and species of marine catfishes (Siluriformes; Ariidae), Qarmoutus hitanensis, from the base of the upper Eocene Birket Qarun Formation, based on a partial neurocranium including the complete left side, partial right dentary, left suspensorium, two opercles, left pectoral girdle and spine, nuchal plates, first and second dorsal spines, Weberian apparatus and a disassociated series of abdominal vertebrae. All of the elements belong to the same individual and some of them were found articulated. Qarmoutus gen. nov. is the oldest and the most complete of the Paleogene marine catfishes unearthed from the Birket Qarun Formation. The new genus exhibits distinctive features not seen in other African Paleogene taxa, such as different sculpturing on the opercle and pectoral girdle with respect to that on the neurocranium and nuchal plates, denticulate ornamentation on the skull bones arranged in longitudinal rows and forming a radiating pattern on the sphenotic, pterotic, extrascapular and the parieto-supraoccipital, indentations or pitted ornamentation on the nuchal plates as well as the parieto-supraoccipital process, strut-like radiating pattern of ornamentation on the opercle from the proximal articulation to margins, longitudinal, curved, reticulate ridges and tubercular ornamentations on the cleithrum, sinuous articulation between the parieto-supraoccipital process and the anterior nuchal plate, long, narrow, and arrowhead shaped nuchal shield, very small otic capsules restricted to the prootic. Multiple parsimony and Bayesian morphological phylogenetic analyses of Ariidae, run with and without "molecular scaffolds", yield contradictory results for the placement of Qarmoutus; the genus is either a phylogenetically basal ariid, or it is deeply nested within the ariid clade containing New World species of Sciades.

A new genus and species of marine catfishes (Siluriformes; Ariidae) from the upper Eocene Birket Qarun Formation, Wadi El-Hitan, Egypt

PubMed Central

2017-01-01

Wadi El-Hitan, the UNESCO World Heritage Site, of the Fayum Depression in the northeast part of the Western Desert of Egypt, has produced a remarkable collection of Eocene vertebrates, in particular the fossil whales from which it derives its name. Here we describe a new genus and species of marine catfishes (Siluriformes; Ariidae), Qarmoutus hitanensis, from the base of the upper Eocene Birket Qarun Formation, based on a partial neurocranium including the complete left side, partial right dentary, left suspensorium, two opercles, left pectoral girdle and spine, nuchal plates, first and second dorsal spines, Weberian apparatus and a disassociated series of abdominal vertebrae. All of the elements belong to the same individual and some of them were found articulated. Qarmoutus gen. nov. is the oldest and the most complete of the Paleogene marine catfishes unearthed from the Birket Qarun Formation. The new genus exhibits distinctive features not seen in other African Paleogene taxa, such as different sculpturing on the opercle and pectoral girdle with respect to that on the neurocranium and nuchal plates, denticulate ornamentation on the skull bones arranged in longitudinal rows and forming a radiating pattern on the sphenotic, pterotic, extrascapular and the parieto-supraoccipital, indentations or pitted ornamentation on the nuchal plates as well as the parieto-supraoccipital process, strut-like radiating pattern of ornamentation on the opercle from the proximal articulation to margins, longitudinal, curved, reticulate ridges and tubercular ornamentations on the cleithrum, sinuous articulation between the parieto-supraoccipital process and the anterior nuchal plate, long, narrow, and arrowhead shaped nuchal shield, very small otic capsules restricted to the prootic. Multiple parsimony and Bayesian morphological phylogenetic analyses of Ariidae, run with and without “molecular scaffolds”, yield contradictory results for the placement of Qarmoutus; the genus is either a phylogenetically basal ariid, or it is deeply nested within the ariid clade containing New World species of Sciades. PMID:28248973

Crustal Seismicity and Geomorphic Observations of the Chiripa-Haciendas Fault System: The Guanacaste Volcanic Arc Sliver of Western Costa Rica

NASA Astrophysics Data System (ADS)

Lewis, J. C.; Montero Pohly, W. K.; Araya, M. C.

2017-12-01

It has recently been shown that contemporary northwest motion of the Nicoya Peninsula of Costa Rica reflects a tectonic sliver that includes much of the upper-plate arc, referred to as the Guanacaste Volcanic Arc Sliver (GVAS). Here we characterize historical seismicity and geomorphic expressions of faults that define the northeastern margin of the GVAS. Several crustal earthquakes and their aftershocks provide constraints on the geometry and/or kinematics of the fault system. These include the Armenia earthquake of July 12, 2011, the Bijagua earthquake of January 27, 2002, the Tilarán earthquake of April 13, 1973 and two much older events. We summarize these earthquakes in the context of recent fault mapping and focal mechanism solutions, and suggest that most of the deformation can be explained by slip on steeply dipping NW-striking fault planes accommodating dextral slip. Streams that cross the major fault traces we have mapped also show deflections consistent with dextral slip. These include map-view apparent offsets of 6.5 km for the Haciendas River, 1.0 km for the Orosi River and 0.6 km for the Pizote River. Although preservation is poor, we document stream terrace risers that reveal truncations and/or offsets consistent with dextral slip. Additional constraints on the fault system are apparent as it is traced into Lake Nicaragua. Previous workers have shown that earthquake clusters accommodate a combination of dextral slip on NW-strike faults and sinistral slip NE-strike faults, the latter described as part of a system of bookshelf fault blocks. Whether the northeastern margin of the GVAS under Lake Nicaragua is a single fault strand or an array of bookshelf blocks remains an open question. An equally important gap in our understanding is the kinematic link of the fault system to the east where the GVAS originates. Our results set the stage for expanded studies that will be essential to understanding the relative contributions of Cocos Ridge collision and coupling between the North American plate and the Caribbean Plate in driving the sliver motion. They are also important in the context of natural hazards, both seismic ground accelerations and mass-wasting. This was made clear by the 1973 Tilarán earthquake, which included fatalities, and several damaging moderate-magnitude crustal earthquakes on 3 July 2016.

Reconciling the geological history of western Turkey with plate circuits and mantle tomography

NASA Astrophysics Data System (ADS)

van Hinsbergen, Douwe J. J.; Kaymakci, Nuretdin; Spakman, Wim; Torsvik, Trond H.

2010-09-01

We place the geological history since Cretaceous times in western Turkey in a context of convergence, subduction, collision and slab break-off. To this end, we compare the west Anatolian geological history with amounts of Africa-Europe convergence calculated from the Atlantic plate circuit, and the seismic tomography images of the west Anatolian mantle structure. Western Turkish geology reflects the convergence between the Sakarya continent (here treated as Eurasia) in the north and Africa in the south, with the Anatolide-Tauride Block (ATB) between two strands of the Neotethyan ocean. Convergence between the Sakarya and the ATB started at least ~ 95-90 Myr ago, marked by ages of metamorphic soles of ophiolites that form the highest structural unit below Sakarya. These are underlain by high-pressure, low-temperature metamorphic rocks of the Tavşanlı and Afyon zones, and the Ören Unit, which in turn are underlain by the Menderes Massif derived from the ATB. Underthrusting of the ATB below Sakarya was since ~ 50 Ma, associated with high-temperature metamorphism and widespread granitic magmatism. Thrusting in the Menderes Massif continued until 35 Ma, after which there is no record of accretion in western Turkey. Plate circuits show that since 90 Ma, ~ 1400 km of Africa-Europe convergence occurred, of which ~ 700 km since 50 Ma and ~ 450 km since 35 Ma. Seismic tomography shows that the African slab under western Turkey is decoupled from the African Plate. This detached slab is a single, coherent body, representing the lithosphere consumed since 90 Ma. There was no subduction re-initiation after slab break-off. ATB collision with Europe therefore did not immediately lead to slab break-off but instead to delamination of subducting lithospheric mantle from accreting ATB crust, while staying attached to the African Plate. This led to asthenospheric inflow below the ATB crust, high-temperature metamorphism and felsic magmatism. Slab break-off in western Turkey probably occurred ~ 15 Myr ago, after which overriding plate compression and rotation accommodated ongoing Africa-Europe convergence. Slab break-off was accommodated along a vertical NE trending subduction transform edge propagator (STEP) fault zone, accelerating southwestward slab retreat of the Aegean slab. The SE Aegean slab edge may have existed already since early Miocene times or before, but started to rapidly roll back along the southeastern Aegean STEP in middle Miocene times, penetrating the Aegean region in the Pliocene.

Where does subduction initiate and die? Insights from global convection models with continental drift

NASA Astrophysics Data System (ADS)

Ulvrova, Martina; Williams, Simon; Coltice, Nicolas; Tackley, Paul

2017-04-01

Plate tectonics is a prominent feature on Earth. Together with the underlying convecting mantle, plates form a self-organized system. In order to understand the dynamics of the coupled system, subduction of the lithospheric plates plays the key role since it links the exterior with the interior of the planet. In this work we study subduction initiation and death with respect to the position of the continental rafts. Using thermo-mechanical numerical calculations we investigate global convection models featuring self-consistent plate tectonics and continental drifting employing a pseudo-plastic rheology and testing the effect of a free surface. We consider uncompressible mantle convection in Boussinesq approximation that is basaly and internaly heated. Our calculations indicate that the presence of the continents alterns stress distribution within a certain distance from the margins. Intra-oceanic subudction initiation is favorable during super-continent cycles while the initiation at passive continental margin prevails when continents are dispersed. The location of subduction initiation is additionally controlled by the lithospheric strength. Very weak lithosphere results in domination of intra-oceanic subduction initiation. The subduction zones die more easily in the vicinity of the continent due to the strong rheological contrast between the oceanic and continental lithosphere. In order to compare our findings with subduction positions through time recorded on Earth, we analyse subduction birth in global plate reconstruction back to 410 My.

Revisit of Criteria and Evidence for the Tectonic Erosion vs Accretion in East Asian Margin

NASA Astrophysics Data System (ADS)

Kimura, G.; Hamahashi, M.

2015-12-01

Accretionary and erosive margins provide tectonic end-members in subduction zone and how these tectonic processes might be recorded and recognizable in ancient subduction complexes remains a challenging issue. Tectonic erosion includes sediment subduction and basal erosion along the plate boundary megathrust and drags down the crust of the upper plate into the mantle. Geologic evidence for the erosion is commonly based on lost geological tectono-stratigraphic data, i.e. gaps in the record and indirect phenomena such as subsidence of the forearc slopes. A topographically rough surface such as seamount has been suggested to work like an erosive saw carving the upper plate. Another mechanism of basal erosion has been suggested to be hydrofracturing of upper plate materials due to dehydration-induced fluid pressures, resulting in entrainment of upper plate materials into the basal décollement. Considering the interaction between the ~30 km thick crust of the upper plate and subducting oceanic plate, a subduction dip angle of ~15°, and convergent rate of ~10 cm/year, at least ~1 Ma of continuous basal erosion is necessary to induce clear subsidence of the forearc because the width of plate interface between the upper crustal and subducting plates is about 115 km (30/cos15°). In several examples of subduction zones, for example the Japan Trench and the Middle America Trench off Costa Rica, the subsidence of a few thousand metres of the forearc, combined with a lack of accretionary prism over a period of several million years, suggest that the erosive condition needs to be maintained for several to tens of million years.Such age gaps in the accretionary complex, however, do not automatically imply that tectonic erosion has taken place, as other interpretations such as no accretion, cessation of subduction, and/or later tectonic modification, are also possible. Recent drilling in the forearc of the Nankai Trough suggests that the accretion was ceased between ~12 Ma to ~8 Ma due to the transference of subduction from the Pacific Plate to the Philippine Sea Plate, as opposed to the continuous subduction of the Phillipine Sea Plate with subduction erosion.

Textures of water-rich mud sediments from the continental margin offshore Costa Rica (IODP expeditions 334 and 344)

NASA Astrophysics Data System (ADS)

Kuehn, Rebecca; Stipp, Michael; Leiss, Bernd

2017-04-01

During sedimentation and burial at continental margins, clay-rich sediments develop crystallographic preferred orientations (textures) depending on the ongoing compaction as well as size distribution and shape fabrics of the grains. Such textures can control the deformational properties of these sediments and hence the strain distribution in active continental margins and also the frictional behavior along and around the plate boundary. Strain-hardening and discontinuous deformation may lead to earthquake nucleation at or below the updip limit of the seismogenic zone. We want to investigate the active continental margin offshore Costa Rica where the oceanic Cocos plate is subducted below the Caribbean plate at a rate of approximately 9 cm per year. The Costa Rica trench is well-known for shallow seismogenesis and tsunami generation. As it is an erosive continental margin, both the incoming sediments from the Nazca plate as well as the slope sediments of the continental margin can be important for earthquake nucleation and faulting causing sea-floor breakage. To investigate texture and composition of the sediments and hence their deformational properties we collected samples from varying depth of 7 different drilling locations across the trench retrieved during IODP expeditions 334 and 344 as part of the Costa Rica Seismogenesis Project (CRISP). Texture analysis was carried out by means of synchrotron diffraction, as only this method is suitable for water-bearing samples. As knowledge on the sediment composition is required as input parameter for the texture data analysis, additional X-ray powder diffraction analysis on the sample material has been carried out. Samples for texture measurements were prepared from the original drill cores using an internally developed cutter which allows to produce cylindrical samples with a diameter of about 1.5 cm. The samples are oriented with respect to the drill core axis. Synchrotron texture measurements were conducted at the ESRF (European Synchrotron Radiation Facility) in Grenoble and the DESY (German Electron Synchrotron) in Hamburg. Samples were measured in transmission mode perpendicular to their cylinder axis with a beam diameter of 500 µm. Measurements were taken from 0 to 175° in 5° steps resulting in 36 images from a 2D image plate detector. Measurement time was in a range from 1 to 3 seconds. Due to the different, low symmetric mineral phases a large number of mostly overlapping reflections results. Such data can only be analyzed by the Rietveld method, in our case implemented in the software package MAUD (Materials Analysis Using Diffraction). Preliminary results show distinct textures depending on the composition and the origin of the samples, i.e. on drilling location and depth, which may be critical for strain localization and faulting of these samples. The results are also important for the analysis of experimentally deformed samples from the same drill cores which showed structurally weak and structurally strong deformation behavior during triaxial compression.

New Insight Into the Crustal Structure of the Continental Margin offshore NW Sabah/Borneo

NASA Astrophysics Data System (ADS)

Barckhausen, U.; Franke, D.; Behain, D.; Meyer, H.

2002-12-01

The continental margin offshore NW Sabah/Borneo (Malaysia) has been investigated with reflection and refraction seismics, magnetics, and gravity during the recent cruise BGR01-POPSCOMS. A total of 4000 km of geophysical profiles has been acquired, thereof 2900 km with reflection seismics. The focus of investigations was on the deep water areas. The margin looks like a typical accretionary margin and was presumably formed during the subduction of a proto South China Sea. Presently, no horizontal movements between the two plates are being observed. Like in major parts of the South China Sea, the area seaward of the Sabah Trough consists of extended continental lithosphere which is characterised by a pattern of rotated fault blocks and half grabens and a carbonate platform of Early Oligocene to Early Miocene age. We found evidence that the continental crust also underlies the Sabah Trough and the adjacent continental slope, a fact that raises many questions about the tectonic history and development of this margin. The tectonic pattern of the Dangerous Grounds' extended continental crust can be traced a long way landward of the Sabah Trough beneath the sedimentary succession of the upper plate. The magnetic anomalies which are dominated by the magnetic signatures of relatively young volcanic features also continue under the continental slope. The sedimentary rocks of the upper plate, in contrast, seem to generate hardly any magnetic anomalies. Based on the new data we propose the following scenario for the development of the NW Sabah continental margin: Seafloor spreading in the present South China Sea started at about 30 Ma in the Late Oligocene. The spreading process separated the Dangerous Grounds area from the SE Asian continent and ceased in late Early Miocene when the oceanic crust of the proto South China Sea was fully subducted in eastward direction along the Borneo-Palawan Trough. During Lower and/or Middle Miocene, Borneo rotated counterclockwise and was thrusted onto the edge of the rifted continental block of the Dangerous Grounds. The subducted oceanic crust of the proto South China Sea must today be located below the Eastern part of Sabah and not along the present NW Sabah Trough.

Crustal structure of the rifted volcanic margins and uplifted plateau of Western Yemen from receiver function analysis

NASA Astrophysics Data System (ADS)

Ahmed, Abdulhakim; Tiberi, Christel; Leroy, Sylvie; Stuart, Graham W.; Keir, Derek; Sholan, Jamal; Khanbari, Khaled; Al-Ganad, Ismael; Basuyau, Clémence

2013-06-01

We analyse P-wave receiver functions across the western Gulf of Aden and southern Red Sea continental margins in Western Yemen to constrain crustal thickness, internal crustal structure and the bulk seismic velocity characteristics in order to address the role of magmatism, faulting and mechanical crustal thinning during continental breakup. We analyse teleseismic data from 21 stations forming the temporary Young Conjugate Margins Laboratory (YOCMAL) network together with GFZ and Yemeni permanent stations. Analysis of computed receiver functions shows that (1) the thickness of unextended crust on the Yemen plateau is ˜35 km; (2) this thins to ˜22 km in coastal areas and reaches less than 14 km on the Red Sea coast, where presence of a high-velocity lower crust is evident. The average Vp/Vs ratio for the western Yemen Plateau is 1.79, increasing to ˜1.92 near the Red Sea coast and decreasing to 1.68 for those stations located on or near the granitic rocks. Thinning of the crust, and by inference extension, occurs over a ˜130-km-wide transition zone from the Red Sea and Gulf of Aden coasts to the edges of the Yemen plateau. Thinning of continental crust is particularly localized in a <30-km-wide zone near the coastline, spatially co-incident with addition of magmatic underplate to the lower crust, above which on the surface we observe the presence of seaward dipping reflectors (SDRs) and thickened Oligo-Miocene syn-rift basaltic flows. Our results strongly suggest the presence of high-velocity mafic intrusions in the lower crust, which are likely either synrift magmatic intrusion into continental lower crust or alternatively depleted upper mantle underplated to the base of the crust during the eruption of the SDRs. Our results also point towards a regional breakup history in which the onset of rifting was synchronous along the western Gulf of Aden and southern Red Sea volcanic margins followed by a second phase of extension along the Red Sea margin.

Geophysical reassessment of the role of ancient crustal structures on the development of western Laurentia and Selwyn Basin, Yukon and Northwest Territories, Canada.

NASA Astrophysics Data System (ADS)

Hayward, N.

2017-12-01

The structure of the western margin of the North American craton (Laurentia) in the northern Canadian Cordillera and its role in the development of the Neoproterozoic-Early Paleozoic Selwyn Basin are reassessed through 3D inversion of a new compilation of aeromagnetic data and archival Bouguer gravity data. The region's tectonic history is obscured by partial burial beneath Selwyn Basin, and a tectonic overprint that includes terrane accretion, regional plutonism, and strike-slip faults with displacements of 100s and perhaps 1000s of kilometers. Despite the implied complexity, preliminary geological and geophysical based interpretations of the structure of the western margin of Laurentia, have been adopted with few refinements in over two decades. Regionally continuous, NE-trending, crustal lineaments, including the Fort Norman line and Leith Ridge fault, were interpreted as having had long-standing influence on the craton development, its western margin, and overlapping sedimentary basin. New results reveal limited evidence for the regional continuity of the NE-trending lineaments. Instead, models suggest that the structure of the Laurentian margin is characterised by segmentation on numerous shorter structures of varied strike. The western margin of the craton and its structures are bound by a NW-trending structure that connects with the Richardson Trough to the north and may have been active during rifting of the Misty Creek embayment. This boundary also marks the easternmost limit of both granitic intrusions in Selwyn Basin, which gravity models suggest are of greater extent than reflected on geological maps, and SEDEX occurrences. An ENE-trending structure beneath northern Selwyn Basin is interpreted as marking the southern edge of a previously unidentified cratonic promontory, akin to the Liard line that marks a transfer fault that bounds the promontory of the Macdonald Platform, south of Selwyn Basin. The ENE-trending structure is traced from the Tintina fault in the west to near to the Great Bear magmatic zone. The structure's regional continuity also limits the interpretation of a post-Cretaceous structure, inboard of the Tintina fault that could be responsible for 1000's km of dextral strike-slip ascribed to the Baja-BC terrane translation model.

40Ar/39Ar geochronology of subaerial lava flows of Barren Island volcano and the deep crust beneath the Andaman Island Arc, Burma Microplate

NASA Astrophysics Data System (ADS)

Ray, Jyotiranjan S.; Pande, Kanchan; Bhutani, Rajneesh

2015-06-01

Little was known about the nature and origin of the deep crust beneath the Andaman Island Arc in spite of the fact that it formed part of the highly active Indonesian volcanic arc system, one of the important continental crust forming regions in Southeast Asia. This arc, formed as a result of subduction of the Indian Plate beneath the Burma Microplate (a sliver of the Eurasian Plate), contains only one active subaerial magmatic center, Barren Island volcano, whose evolutional timeline had remained uncertain. In this work, we present results of the first successful attempt to date crustal xenoliths and their host lava flows from the island, by incremental heating 40Ar/39Ar method, in an attempt to understand the evolutionary histories of the volcano and its basement. Based on concordant plateau and isochron ages, we establish that the oldest subaerial lava flows of the volcano are 1.58 ± 0.04 (2σ) Ma, and some of the plagioclase xenocrysts have been derived from crustal rocks of 106 ± 3 (2σ) Ma. Mineralogy (anorthite + Cr-rich diopside + minor olivine) and isotopic compositions (87Sr/86Sr < 0.7040; ɛNd > 7.0) of xenoliths not only indicate their derivation from a lower (oceanic) crustal olivine gabbro but also suggest a genetic relationship between the arc crust and the ophiolitic basement of the Andaman accretionary prism. We speculate that the basements of the forearc and volcanic arc of the Andaman subduction zone belong to a single continuous unit that was once attached to the western margin of the Eurasian Plate.

Reconstructing Plate Boundaries in the Jurassic Neo-Tethys From the East and West Vardar Ophiolites (Greece and Serbia)

NASA Astrophysics Data System (ADS)

Maffione, Marco; van Hinsbergen, Douwe J. J.

2018-03-01

Jurassic subduction initiation in the Neo-Tethys Ocean eventually led to the collision of the Adria-Africa and Eurasia continents and the formation of an 6,000 km long Alpine orogen spanning from Iberia to Iran. Reconstructing the location and geometry of the plate boundaries of the now disappeared Neo-Tethys during the initial moments of its closure is instrumental to perform more realistic plate reconstructions of this region, of ancient ocean basins in general, and on the process of subduction initiation. Neo-Tethyan relics are preserved in an ophiolite belt distributed above the Dinaric-Hellenic fold-thrust belt. Here we provide the first quantitative constraints on the geometry of the spreading ridges and trenches active in the Jurassic Neo-Tethys using a paleomagnetically based net tectonic rotation analysis of sheeted dykes and dykes from the West and East Vardar Ophiolites of Serbia (Maljen and Ibar) and Greece (Othris, Pindos, Vourinos, and Guevgueli). Based on our results and existing geological evidence, we show that initial Middle Jurassic ( 175 Ma) closure of the western Neo-Tethys was accommodated at a N-S trending, west dipping subduction zone initiated near and parallel to the spreading ridge. The West Vardar Ophiolites formed in the forearc parallel to this new trench. Simultaneously, the East Vardar Ophiolites formed above a second N-S to NW-SE trending subduction zone located close to the European passive margin. We tentatively propose that this second subduction zone had been active since at least the Middle Triassic, simultaneously accommodating the closure of the Paleo-Tethys and the back-arc opening of Neo-Tethys.

Mantle heterogeneities beneath the Northeast Indian Ocean as sampled by intra-plate volcanism at Christmas Island

NASA Astrophysics Data System (ADS)

Taneja, Rajat; Rushmer, Tracy; Blichert-Toft, Janne; Turner, Simon; O'Neill, Craig

2016-10-01

The intra-plate region of the Northeast Indian Ocean, located between the Ninetyeast Ridge and the North West Shelf of Australia, contains numerous submerged seamounts and two sub-aerially exposed volcanic island groups. While the Cocos (Keeling) Archipelago is a coral atoll, Christmas Island is the only sub-aerially exposed volcanic island and contains Late Cretaceous, Eocene and Pliocene lavas. The lavas are predominantly basaltic in composition, except for one sampled flow that is trachytic. Although the evolution of the western margin of Australia, and the seismicity in the intra-plate region, has received considerable attention, the origin of the seamount province in the Northeast Indian Ocean is still a matter of debate. In order to constrain the origin of volcanism on Christmas Island and the associated Seamount Province we analysed 14 Christmas Island samples for major and trace element abundances and 12 of these for Nd, Hf and Pb isotope compositions. The trace element patterns of the lavas are similar to many ocean island basalts, while high 208Pb/204Pb and 207Pb/204Pb at a given 206Pb/204Pb suggest affiliation with the DUPAL anomaly. The reconstructed position of Christmas Island during the Eocene (44-37 Ma) places the island in close proximity to the (present-day) upper mantle low-seismic velocity anomalies. Moreover, an enriched mantle (EM-2) type component in addition to the DUPAL anomaly is observed in the Eocene volcanic phase. The younger Pliocene ( 4 Ma) sequences at Christmas Island are inferred to be the product of partial melting of existing material induced by lithospheric flexure.

Sedimentary structure and tectonic setting of the abyssal basins adjoining the southeast part of the Ontong Java Plateau, western Pacific Ocean

NASA Astrophysics Data System (ADS)

Shimizu, S.; Masato, N.; Miura, S.; Suetsugu, D.

2017-12-01

Ontong Java Plateau(OJP) in the western Pacific Ocean is one of the largest oceanic plateau in the world. Radioactive ages of drilling samples indicate that the most part of the OJP was emplaced about 122 Ma (Mahoney et al., 1993). Taylor (2006) proposed that the OJP formed as a single large volcanic province together with the Manihiki and Hikurangi plateaus. OJP is surrounding by East Mariana, Pigafetta, Nauru, Ellice, Stewart, and Lyra basins. The East Mariana and Pigafetta basins were formed at the Pacific-Izanagi ridge and the Nauru basin was formed at Pacific-Phoenix ridges (Nakanishi et al., 1992). The tectonic history of the Ellice, Stewart, and Lyra basins is still unknown because of lack of magnetic anomaly lineations. Tectonic setting during the OJP formation is thus a matter of controversy. To expose the tectonic setting of the Ellice, Stewart, and Lyra basins, we conducted the Multi-Channel Seismic (MCS) survey in the basins during the research cruise by R/V Mirai of JAMSTEC in 2014. We present our preliminary results of the MCS survey in the Stewart basin(SB) and Ellice Basin(EB). After the regular data processing, we compared the seismic facies of MCS profile with DSDP Site 288 and ODP Site 1184 to assign ages to seismic reflectors. Our processing exposed several remarkable structures in the basins. The graben structures deformed only the igneous basement in the northwestern and northeastern and southwestern margins of the SB. This suggests the graben structures were formed before sedimentary layer deposited. Taylor (2006) proposed that the basin was formed by the NW-SE rifting during the separation of OJP and Manihiki Plateau around 120 Ma. Neal (1997) proposed that the NE-SW rifting formed the basin around 80 Ma. Our study supports the rifting model proposed by Neal et al. (1997) because the displacement of graben in northeastern and southwestern margins of the SB is larger than that in northwestern of the SB. We found several igneous diapirs in the SB and EB. Several diapirs intrude into Oligocene sediments, implying that the volcanism occurred after the formation of the basins. On the southern edge of SB is the outer rise called Stewart Arch (Phinney et al., 1999). We identified normal faults near the Stewart Arch. Those faults caused by the plate bending owing to the subduction of the Pacific plate.

The Yilgarn Craton western Australia: A tectonic synthesis

NASA Technical Reports Server (NTRS)

Fripp, R. E. P.

1986-01-01

The Yilgarn Craton in Western Australia is one of the larger contiguous preserved Archaean crustal fragments, with an area of about 650,000 square kilometres. Of this, by area, about 70% is granitoid and 30% greenstone. The Craton is defined by the Darling Fault on its western margin, by Proterozoic deformation belts on its southern and northwestern margins, and by unconformable younger sediments on its eastern and northeastern margins. A regional geotectonic synthesis at a scale of 1:500,000 is being prepared. This is based largely upon the 1:250,000 scale mapping of the Geological Survey of Western Australia together with interpretation using geophysical data, mainly airborne magnetic surveys. On a regional basis the granitoids are classied as pre-, syn- and post-tectonic with respect to greenstone belt deformation. The post-tectonic granitoids yield Rb-Sr isochrons of about 2.6 b.y., close to Rb-Sr ages for the greenstones themselves which are up to about 2.8 b.y. old, although data for the latter is sparse. Contacts between earlier granitoids and greenstones which are not obscured by the post-tectonic granitoids are most commonly tectonic contacts, intensely deformed and with mylonitic fabrics. The general concensus however is that there is a pre-tectonic, pre-greenhouse sialic gneiss preserved in places. A discussion follows.

Changes in Student Knowledge and Views of Geohazards, Societal Risks, and Monitoring at Active Plate Boundaries Using a Data-Rich Curriculum

NASA Astrophysics Data System (ADS)

Selkin, P. A.; Goodell, L. P.; Teasdale, R.

2015-12-01

The "Living on the Edge: Building Resilient Societies on Active Plate Margins" curriculum consists of six data-rich activities, each intended for a 50-minute class, in which students assess risk at active plate boundaries due to earthquakes and volcanoes. Developed as part of the InTeGrate NSF STEP Center the peer-reviewed, publically available materials (http://serc.carleton.edu/104296) have been used at several institutions in diverse classroom settings including small laboratory sections, large lecture courses, medium-sized upper division courses and professional development programs for middle and high school teachers. Pre- and post-instruction surveys measured content knowledge and geoscience literacy, self-efficacy in using geologic data to assess hazards and risk, and attitudes towards the value of monitoring plate margins. The activities have overall positive effects on knowledge of geohazard concepts. Views about the value of scientific practice also became more positive: 74% of students indicated they "agree" or "strongly agree" that monitoring geologic activity has value to them personally (even if they don't live on an active plate margin) and 94% indicated that such monitoring is valuable to society. Most became more confident in evaluating geologic hazard and risk (>60% of students self-described increased confidence by one or more Likert levels). Student knowledge of both the types and limits of data in forecasting geological hazards and their effects also improved. However, attitudes toward sustainability and geoscience careers did not change. Learning and attitudinal improvements are true for all classroom types, but the degree of change varies with class size and the amount of time spent on activities. Learning data and instructor feedback suggest that interactive classroom activities that use real-world data to address societally relevant issues increase student learning and enhance students' ability to synthesize scientific information.

Dry Juan de Fuca slab revealed by quantification of water entering Cascadia subduction zone

NASA Astrophysics Data System (ADS)

Canales, J. P.; Carbotte, S. M.; Nedimovic, M. R.; Carton, H. D.

2017-12-01

Water is carried by subducting slabs as a pore fluid and in structurally bound minerals, yet no comprehensive quantification of water content and how it is stored and distributed at depth within incoming plates exists for any segment of the global subduction system. Here we use controlled-source seismic data collected in 2012 as part of the Ridge-to-Trench seismic experiment to quantify the amount of pore and structurally bound water in the Juan de Fuca plate entering the Cascadia subduction zone. We use wide-angle OBS seismic data along a 400-km-long margin-parallel profile 10-15 km seaward from the Cascadia deformation front to obtain P-wave tomography models of the sediments, crust, and uppermost mantle, and effective medium theory combined with a stochastic description of crustal properties (e.g., temperature, alteration assemblages, porosity, pore aspect ratio), to analyze the pore fluid and structurally bound water reservoirs in the sediments, crust and lithospheric mantle, and their variations along the Cascadia margin. Our results demonstrate that the Juan de Fuca lower crust and mantle are much drier than at any other subducting plate, with most of the water stored in the sediments and upper crust. Previously documented, variable but limited bend faulting along the margin, which correlates with degree of plate locking, limits slab access to water, and a warm thermal structure resulting from a thick sediment cover and young plate age prevents significant serpentinization of the mantle. Our results have important implications for a number of subduction processes at Cascadia, such as: (1) the dryness of the lower crust and mantle indicates that fluids that facilitate episodic tremor and slip must be sourced from the subducted upper crust; (2) decompression rather than hydrous melting must dominate arc magmatism in northern-central Cascadia; and (3) dry subducted lower crust and mantle can explain the low levels of intermediate-depth seismicity in the Juan de Fuca slab.

Cenozoic evolution of the Yakutat-North American collision zone and structural accommodation of St. Elias syntaxis exhumation, Alaska/Yukon

NASA Astrophysics Data System (ADS)

Falkowski, Sarah; Enkelmann, Eva; Ehlers, Todd

2016-04-01

Active convergent margins potentially pose multiple natural hazards to human life and infrastructure. Tectonic strain may be further focused where convergent margins are warped into broad syntaxes. However, the processes responsible for upper plate deformation in these settings are not well understood. The St. Elias syntaxis in southeast Alaska and southwest Yukon is located at the eastern corner of the Yakutat microplate, which indents into the North American Plate and subducts at a flat angle beneath Alaska. High rates of long-term glacial erosion and exhumation (>2 mm/yr) are found on the southern, coastal flanks of the St. Elias orogen, but the deepest and most rapid exhumation is focused at the St. Elias syntaxis. In this location, transform motion transitions into subduction of the wedge-shaped, oceanic plateau of the Yakutat microplate. In order to map the spatio-temporal pattern of exhumation in the Yakutat-North American collision zone, we conducted zircon and apatite fission-track analyses of predominantly detrital, sand-sized material and five bedrock samples from 47 different glacio-fluvial catchments covering an area of ~45,000 km2 around the St. Elias syntaxis. Integration of the new thermochronologic data with prior work and other geologic and geophysical observations yielded information on past terrane accretion events at the North American margin since the late Mesozoic and the evolution of exhumation at the St. Elias syntaxis in the context of the ongoing Yakutat-North American plate collision. Our results indicate a migrating focus of the most rapid exhumation from north to south and from the upper (North American Plate) to the lower (Yakutat microplate) plate in the syntaxis area over the past ~10 Myr. This migration occurred in response to a change in plate motions, increasingly thicker crust of the subducting Yakutat microplate, and changes in surface processes after glaciation began that resulted in modification of the rheology. We propose a positive, two-sided flower structure to have accommodated the rapid, and temporarily deep (~10 km), exhumation.

Inversion for the driving forces of plate tectonics

NASA Technical Reports Server (NTRS)

Richardson, R. M.

1983-01-01

Inverse modeling techniques have been applied to the problem of determining the roles of various forces that may drive and resist plate tectonic motions. Separate linear inverse problems have been solved to find the best fitting pole of rotation for finite element grid point velocities and to find the best combination of force models to fit the observed relative plate velocities for the earth's twelve major plates using the generalized inverse operator. Variance-covariance data on plate motion have also been included. Results emphasize the relative importance of ridge push forces in the driving mechanism. Convergent margin forces are smaller by at least a factor of two, and perhaps by as much as a factor of twenty. Slab pull, apparently, is poorly transmitted to the surface plate as a driving force. Drag forces at the base of the plate are smaller than ridge push forces, although the sign of the force remains in question.

Lithosphere Structure of the Rivera Plate - Jalisco Block Contact Zone: Septentrional Region of the Islas Marías (Mexico)

NASA Astrophysics Data System (ADS)

Madrigal-Ávalos, L. A.; Nunez, D.; Escalona-Alcazar, F. D. J.; Nuñez-Cornu, F. J.; Barba, D. C., Sr.; Danobeitia, J.

2017-12-01

The western margin of Mexico is a tectonic complex region where large earthquakes occurred with very destructive consequences, including the generation of big tsunamis. This fact is mainly the result of the Rivera plate subduction beneath the North American plate and the Jalisco Block implying a high potential seismic risk. In the north, between the Tamayo Fracture Zone and the Mesoamerican Trench, the Islas Marías region is a complex tectonic limit within the interaction of the Rivera plate oceanic crust and the Jalisco Block continental crust. In order to know the shallow and deep structure of the Rivera plate - Jalisco Block contact zone and to be able to determine these potential seismic sources, the TSUJAL geophysical experiment was carried out from 2012 to 2016. As part of this project, we present the results of the processed and analyzed MCS and WAS data along the TS09 and RTSIM01 seismic transects, respectively, across the septentrional region of Islas Marías. These marine seismic lines are coincident with 110 km length for MCS and 240 km for WAS, and perpendicular to the coastline with SW-NE orientation. The seismic sources used in this work aboard RRS James Cook consisted of 12 guns with a total capacity for WAS data of 5800 in3 every 120 s and 3540 in3 every 50 m for MCS data. The MCS data were acquired with a 5.85 km length streamer with a 468 active channels, while the WAS data were recorded by a network of 4 OBS and 27 land seismic stations. After data processing and joint interpretation, it was possible to determine that shallow structure is mainly constituted by normal faults associated to graben structures forming sedimentary basins with non-deformed sediments in the basement. While the deep structure is characterized by depths from 9 to 12 km in the oceanic crust and 18 to 21 km in the continental crust. The deepest layers of the upper mantle were determined up to 35 km depth. In this study, it was possible to calculate a dip angle between 6° and 8°.

The Continental Margins of the Western North Atlantic.

ERIC Educational Resources Information Center

Schlee, John S.; And Others

1979-01-01

Presents an interpretation of geological and geophysical data, which provides a summary of the structural and sedimentary history of the United States Atlantic Margin. The importance of an understanding of the development of the outer continental shelf to future hydrocarbon exploration is detailed. (BT)

The Colorado front range: anatomy of a Laramide uplift

USGS Publications Warehouse

Kellogg, Karl; Bryant, Bruce; Reed, John C.

2004-01-01

Along a transect across the Front Range from Denver to the Blue River valley near Dillon, the trip explores the geologic framework and Laramide (Late Cretaceous to early Eocene) uplift history of this basement-cored mountain range. Specific items for discussion at various stops are (1) the sedimentary and structural record along the upturned eastern margin of the range, which contains several discontinuous, east-directed reverse faults; (2) the western structural margin of the range, which contains a minimum of 9 km of thrust overhang and is significantly different in structural style from the eastern margin; (3) mid- to late-Tertiary modifications to the western margin of the range from extensional faulting along the northern Rio Grande rift trend; (4) the thermal and uplift history of the range as revealed by apatite fission track analysis; (5) the Proterozoic basement of the range, including the significance of northeast-trending shear zones; and (6) the geologic setting of the Colorado mineral belt, formed during Laramide and mid-Tertiary igneous activity.

Intra-continental subduction and contemporaneous lateral extrusion of the upper plate: insights into Alps-Adria interactions

NASA Astrophysics Data System (ADS)

van Gelder, Inge; Willingshofer, Ernst; Sokoutis, Dimitrios; Cloetingh, Sierd

2017-04-01

A series of physical analogue experiments were performed to simulate intra-continental subduction contemporaneous with lateral extrusion of the upper plate to study the interferences between these two processes at crustal levels and in the lithospheric mantle. The lithospheric-scale models are specifically designed to represent the collision of the Adriatic microplate with the Eastern Alps, simulated by an intra-continental weak zone to initiate subduction and a weak confined margin perpendicular to the direction of convergence in order to allow for extrusion of the lithosphere. The weak confined margin is the analog for the opening of the Pannonian back-arc basin adjacent to the Eastern Alps with the direction of extension perpendicular to the strike of the orogen. The models show that intra-continental subduction and coeval lateral extrusion of the upper plate are compatible processes. The obtained deformation structures within the extruding region are similar compared to the classical setup where lateral extrusion is provoked by lithosphere-scale indentation. In the models a strong coupling across the subduction boundary allows for the transfer of abundant stresses to the upper plate, leading to laterally varying strain regimes that are characterized by crustal thickening near a confined margin and dominated by lateral displacement of material near a weak lateral confinement. During ongoing convergence the strain regimes propagate laterally, thereby creating an area of overlap characterized by transpression. In models with oblique subduction, with respect to the convergence direction, less deformation of the upper plate is observed and as a consequence the amount of lateral extrusion decreases. Additionally, strain is partitioned along the oblique plate boundary leading to less subduction in expense of right lateral displacement close to the weak lateral confinement. Both oblique and orthogonal subduction models have a strong resemblance to lateral extrusion tectonics of the Eastern Alps, where subduction of the adjacent Adriatic plate beneath the Eastern Alps is debated. Our results highlight that both indentation and subduction of Adria are valid collisional mechanisms to provoke lateral extrusion-type deformation within the Eastern Alps lithosphere, i.e. the upper plate. Moreover, the insights suggest that the Oligocene to Late Miocene structural evolution of the Eastern Alps is best described by phases of oblique and subsequent orthogonal subduction which is in line with Miocene rotations of the Adriatic plate. Furthermore, oblique subduction of the Adriatic plate provides a viable mechanism to explain the rapid decrease in slab length beneath the Eastern Alps towards the Pannonian Basin, also implying that the Adriatic slab can behave and form independently with regards to the adjacent subduction of Adria beneath the Dinarides.

FIXED END OF MIDDLE SPAN. WESTERN SIDE SHOWING WELDING OF ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

FIXED END OF MIDDLE SPAN. WESTERN SIDE SHOWING WELDING OF TOP PLATE ADDED TO STRENGTHEN THE BRIDGE. - Spile Bridge Road Bridge, Spanning Black Lake Outlie at Spile Bridge Road, Oswegatchie, St. Lawrence County, NY

Role of the offshore Pedro Banks left-lateral strike-slip fault zone in the plate tectonic evolution of the northern Caribbean

NASA Astrophysics Data System (ADS)

Ott, B.; Mann, P.; Saunders, M.

2013-12-01

Previous workers, mainly mapping onland active faults on Caribbean islands, defined the northern Caribbean plate boundary zone as a 200-km-wide bounded by two active and parallel strike-slip faults: the Oriente fault along the northern edge of the Cayman trough with a GPS rate of 14 mm/yr, and and the Enriquillo-Plaintain Garden fault zone (EPGFZ) with a rate of 5-7 mm/yr. In this study we use 5,000 km of industry and academic data from the Nicaraguan Rise south and southwest of the EPGFZ in the maritime areas of Jamaica, Honduras, and Colombia to define an offshore, 700-km-long, active, left-lateral strike-slip fault in what has previously been considered the stable interior of the Caribbean plate as determined from plate-wide GPS studies. The fault was named by previous workers as the Pedro Banks fault zone because a 100-km-long segment of the fault forms an escarpment along the Pedro carbonate bank of the Nicaraguan Rise. Two fault segments of the PBFZ are defined: the 400-km-long eastern segment that exhibits large negative flower structures 10-50 km in width, with faults segments rupturing the sea floor as defined by high resolution 2D seismic data, and a 300-km-long western segment that is defined by a narrow zone of anomalous seismicity first observed by previous workers. The western end of the PBFZ terminates on a Quaternary rift structure, the San Andres rift, associated with Plio-Pleistocene volcanism and thickening trends indicating initial rifting in the Late Miocene. The southern end of the San Andreas rift terminates on the western Hess fault which also exhibits active strands consistent with left-lateral, strike-slip faults. The total length of the PBFZ-San Andres rift-Southern Hess escarpment fault is 1,200 km and traverses the entire western end of the Caribbean plate. Our interpretation is similar to previous models that have proposed the "stable" western Caribbean plate is broken by this fault whose rate of displacement is less than the threshold recognizable from the current GPS network (~3 mm/yr). The Late Miocene age of the fault indicates it may have activated during the Late Miocene to recent Hispaniola-Bahamas oblique collision event.

Transform push, oblique subduction resistance, and intraplate stress of the Juan de Fuca plate

USGS Publications Warehouse

Wang, K.; He, J.; Davis, E.E.

1997-01-01

The Juan de Fuca plate is a small oceanic plate between the Pacific and North America plates. In the southernmost region, referred to as the Gorda deformation zone, the maximum compressive stress a, constrained by earthquake focal mechanisms is N-S. Off Oregon, and possibly off Washington, NW trending left-lateral faults cutting the Juan de Fuca plate indicate a a, in a NE-SW to E-W direction. The magnitude of differential stress increases from north to south; this is inferred from the plastic yielding and distribution of earthquakes throughout the Gorda deformation zone. To understand how tectonic forces determine the stress field of the Juan de Fuca plate, we have modeled the intraplate stress using both elastic and elastic-perfectly plastic plane-stress finite element models. We conclude that the right-lateral shear motion of the Pacific and North America plates is primarily responsible for the stress pattern of the Juan de Fuca plate. The most important roles are played by a compressional force normal to the Mendocino transform fault, a result of the northward push by the Pacific plate and a horizontal resistance operating against the northward, or margin-parallel, component of oblique subduction. Margin-parallel subduction resistance results in large N-S compression in the Gorda deformation zone because the force is integrated over the full length of the Cascadia subduction zone. The Mendocino transform fault serves as a strong buttress that is very weak in shear but capable of transmitting large strike-normal compressive stresses. Internal failure of the Gorda deformation zone potentially places limits on the magnitude of the fault-normal stresses being transmitted and correspondingly on the magnitude of strike-parallel subduction resistance. Transform faults and oblique subduction zones in other parts of the world can be expected to transmit and create stresses in the same manner. Copyright 1997 by the American Geophysical Union.

Virtual Research Expeditions along Plate Margins: Examples from an Online Oceanography Course

NASA Astrophysics Data System (ADS)

Reed, D. L.; Moore, G. F.; Bangs, N. L.; Tobin, H. J.

2010-12-01

An undergraduate online course in oceanography is based on the participation of each student in a series of virtual, at-sea, research expeditions, two of which are used to examine the tectonic processes at plate boundaries. The objective is to leverage the results of major federal research initiatives in the ocean sciences into effective learning tools with a long lifespan for use in undergraduate geoscience courses. These web-based expeditions examine: (1) hydrothermal vents along the divergent plate boundary at the Explorer Ridge and (2) the convergent plate boundary fault along the Nankai Trough, which is the objective of the multi-year NanTroSEIZE drilling program. Here we focus on the convergent plate boundary in NanTroSEIZE 3-D, which is based on a seismic survey supported through NSF-MARGINS, IODP and CDEX in Japan to study the properties of the plate boundary fault system in the upper limit of the seismogenic zone off Japan. The virtual voyage can be used in undergraduate classes at anytime, since it is not directly tied to the finite duration of a specific seagoing project, and comes in two versions, one that is being used in geoscience major courses and the other in non-major courses, such as the oceanography course mentioned above and a lower-division global studies course with a science emphasis. NanTroSEIZE in 3-D places undergraduate learning in an experiential framework as students participate on the expedition and carry out research on the structure of the plate boundary fault. Students learn the scientific background of the program, especially the critical role of international collaboration, and meet the chief scientists before joining the 3-D seismic imaging expedition to identify the active faults that were the likely sources of devastating earthquakes and tsunamis in Japan in 1944 and 1948. The initial results of phase I ODP drilling that began in 2007 are also reviewed. Students document their research on a worksheet that accompanies the expedition, interpret a slice through the 3-D seismic volume, and compose an “AGU-style” abstract summarizing their work, which is submitted to the instructor for review. NanTroSEIZE in 3-D is openly available and can be accessed through the MARGINS Mini-lesson section of the Science Education Resource Center (SERC).

Abrupt plate acceleration during rifted margin formation: Cause and effect

NASA Astrophysics Data System (ADS)

Brune, Sascha; Williams, Simon; Butterworth, Nathaniel; Müller, Dietmar

2017-04-01

Extension rate is known to control key processes during rifted margin formation such as crust-mantle coupling, decompression melting, magmatism, and serpentinisation. Here we build on recent advances in plate tectonic reconstructions by quantifying the extension velocity history of Earth's major rifted margins during the last 240 million years. We find that many successful rifts start with a slow phase of extension followed by rapid acceleration that introduces a fast phase. The transition from slow to fast rifting takes place long before crustal break-up: approximately half of the present day rifted margin area was created during the slow, and the other half during the fast rift phase. We reproduce the rapid transition from slow to fast extension using analytical and numerical modelling with constant force boundary conditions. In these models, rift velocities are not imposed but instead evolve naturally in response to the changing strength of the rift. Our results demonstrate that abrupt plate acceleration during continental rifting is controlled by a rift-intrinsic strength-velocity feedback. The abruptness of rift acceleration is thereby governed by the nonlinearity of lithospheric localization. Realistic brittle and power-law rheologies lead to a speed-up duration between two and ten million years. For successful rifts that generate a new ocean basin, the duration of rift speed-up is notably almost independent of the applied extensional force. Instead, the force controls the duration of the slow phase: higher forces shorten the slow phase while lower forces prolong it. If the force is too low, however, delocalisation processes prevent the rift from reaching the point of speed-up and produce a failed rift, even if the extensional system was active for many million years.

MARGINS mini-lessons: A tour of the Mariana Subduction System (Invited)

NASA Astrophysics Data System (ADS)

Goodliffe, A. M.; Oakley, A.

2009-12-01

MARGINS mini-lessons provide an efficient way to quickly move cutting edge MARGINS research into the university classroom. Instructors who are not necessarily familiar with the MARGINS program can easily use mini-lessons in a variety of educational settings. The mini-lesson described herein is centered on bathymetric and multi-channel seismic data collected during a 2003 NSF-MARGINS funded marine geophysical survey in the Mariana Basin. Designed as an approximately sixty minute lecture segment, the lesson covers both the techniques used to collect marine geophysical data and a description of the geology of the system. All geological provinces are included, from the subducting Pacific Plate in the east to the remnant arc in the west. Representative seismic lines and bathymetric images are presented for each province, along with a description of key processes including deformation of the subducting plate, serpentinite mud volcanism, forearc faulting, potentially tsunamigenic landslides, arc volcanism, and backarc spreading. The Mariana subduction system mini-lesson requires a computer with an internet connection, powerpoint, Google Earth, and a web-browser. Questions are embedded in the powerpoint presentation that can be adapted to a specific interactive response system as needed. Optimally the lesson should be used in parallel with a GeoWall. A 3-dimensional ArcScene visualization of the Mariana system is available for download through the MARGINS mini-lessons web site. Such visualizations are particularly effective in helping students understand complex three-dimensional systems. If presented in a computer lab students will benefit from being able to explore the Mariana system using tools such as GeoMapApp.

Architecture of ductile-type passive margins: Geological constraints from the inverted Cretaceous basin of the North-Pyrenean Zone (`Chaînons Béarnais', Western Pyrenees)

NASA Astrophysics Data System (ADS)

Corre, B.; Lagabrielle, Y.; Labaume, P.; Lahfid, A.; Boulvais, P.; Bergamini, G.; Fourcade, S.; Clerc, C. N.; Asti, R.

2017-12-01

Subcontinental lithospheric mantle rocks are exhumed at the foot of magma-poor distal passive margins as a response to extreme stretching of the continental crust. The North-Pyrenean Zone (NPZ) exposes remnants of such extremely stretched paleo-passive margin that represent field analogues to study the processes of continental crust thinning and mantle exhumation. The NPZ results from the inversion of basins opened between the Iberia and Europa plates during Albo-Cenomanian times. The Chaînons Béarnais belt displays a fold-and-thrust structure involving the Mesozoic sedimentary cover associated with peridotite bodies in tectonic contact with Paleozoic basement lenses of small size. Continental extension developed under hot thermal conditions, as demonstrated by the syn-metamorphic Cretaceous ductile deformation affecting both the crustal basement and the Mesozoic cover. In this study, we present structural and geochemical data providing constraints to reconstruct the evolution of this paleo-margin. Field work confirms that the Mesozoic cover is intimately associated with mantle rocks and thin tectonic lenses of middle crust. Micro-structural studies show that the greenschist facies ductile deformation in the crust produced a mylonitic foliation which is always parallel to the crust/mantle contact. The crust/mantle detachment fault is a major shear zone characterized by anastomosed shear bands. It also shows that the pre-rift cover was detached from its bedrock at the Keuper evaporites level and was welded to mantle rocks during their exhumation at the foot of the margin. We show that: (i) the boudinaged pre-rift sediments have undergone drastic syn-metamorphic thinning with the genesis of a S0/S1 foliation and, (ii) the Paleozoic basement has been ductilely deformed, into thin tectonic lenses that remained welded to the exhumed mantle rocks. The ductile behavior is related to the presence of a thick pre- and syn-rift cover acting as an efficient thermal blanket. This new geological data set highlights important characteristics of ductile-type hyper-extended passive margin. Finally, we stress that studying field analogues represents a major tool to better understand the mechanisms of crustal thinning associated with mantle exhumation and their structural inheritance during tectonic inversion.

Insights on the seismotectonics of the western part of northern Calabria (southern Italy) by integrated geological and geophysical data: Coexistence of shallow extensional and deep strike-slip kinematics

NASA Astrophysics Data System (ADS)

Ferranti, L.; Milano, G.; Pierro, M.

2017-11-01

We assess the seismotectonics of the western part of the border area between the Southern Apennines and Calabrian Arc, centered on the Mercure extensional basin, by integrating recent seismicity with a reconstruction of the structural frame from surface to deep crust. The analysis of low-magnitude (ML ≤ 3.5) events occurred in the area during 2013-2017, when evaluated in the context of the structural model, has revealed an unexpected complexity of seismotectonics processes. Hypocentral distribution and kinematics allow separating these events into three groups. Focal mechanisms of the shallower (< 9 km) set of events show extensional kinematics. These results are consistent with the last kinematic event recorded on outcropping faults, and with the typical depth and kinematics of normal faulting earthquakes in the axial part of southern Italy. By contrast, intermediate ( 9-17 km) and deep ( 17-23 km) events have fault plane solutions characterized by strike- to reverse-oblique slip, but they differ from each other in the orientation of the principal axes. The intermediate events have P axes with a NE-SW trend, which is at odds with the NW-SE trend recorded by strike-slip earthquakes affecting the Apulia foreland plate in the eastern part of southern Italy. The intermediate events are interpreted to reflect reactivation of faults in the Apulia unit involved in thrust uplift, and appears aligned along an WNW-ESE trending deep crustal, possibly lithospheric boundary. Instead, deep events beneath the basin, which have P-axis with a NW-SE trend, hint to the activity of a deep overthrust of the Tyrrhenian back-arc basin crust over the continental crust of the Apulia margin, or alternatively, to a tear fault in the underthrust Apulia plate. Results of this work suggest that extensional faulting, as believed so far, does not solely characterizes the seismotectonics of the axial part of the Southern Apennines.

Pliocene-Quaternary basalts from the Harrat Tufail, western Saudi Arabia: Recycling of ancient oceanic slabs and generation of alkaline intra-plate magma

NASA Astrophysics Data System (ADS)

Bakhsh, Rami A.

2015-12-01

Harrat Tufail represents a Caenozoic basalt suite at the western margin of the Arabian plate. This rift-related suite includes voluminous Quaternary non-vesicular basalt (with fragments of earlier Pliocene vesicular flow) that forms a cap sheet over Miocene rhyolite and minor vesicular basalt. The contact between rhyolite and the basaltic cap is erosional with remarkable denudations indicating long time gap between the felsic and mafic eruptions. The geochemical data prove alkaline, sodic and low-Ti nature of the olivine basalt cap sheet. The combined whole-rock and mineral spot analyses by the electron microprobe (EMPA) suggest magma generation from low degree of partial melting (∼5%) from spinel- and garnet-lherzolite mantle source. Derivation from a mantle source is supported by low Na content in clinopyroxene (ferroan diopside) whereas high Mg content in ilmenite is an evidence of fractional crystallization trajectory. Accordingly, the Pliocene basaltic cap of Harrat Tufail is a product of mantle melt that originates by recycling in the asthenosphere during subduction of ancient oceanic slab(s). The whole-rock chemistry suggests an ancient ocean island basaltic slab (OIB) whereas the EMPA of Al-rich spinel inclusions in olivine phenocrysts are in favour of a mid-ocean ridge basaltic source (MORB). Calculations of oxygen fugacity based on the composition of co-existing Fe-Ti oxide suggest fluctuation from highly to moderately oxidizing conditions with propagation of crystallization (log10 fO2 from -22.09 to -12.50). Clinopyroxene composition and pressure calculation indicates low-pressure (0.4-2 kbar). Cores of olivine phenocrysts formed at highest temperature (1086-1151 °C) whereas the rims and olivine micro-phenocrysts formed at 712-9-796 °C which is contemporaneous to formation of clinopyroxene at 611-782 °C. Fe-Ti oxides crystallized over a long range (652-992 °C) where it started to form at outer peripheries of olivine phenocrysts and as interstitial phase with clinopyroxene.

Regional offshore geology of central and western Solomon Islands and Bougainville, Papua New Guinea

DOE Office of Scientific and Technical Information (OSTI.GOV)

Vedder, J.G.; Colwell, J.B.; Bruns, T.R.

The central and western Solomon Islands and the Bougainville regions are parts of a complex island-arc system that includes an intra-arc basin and remnants of both forearc and back-arc depositional wedges. These features formed in response to episodic Cenozoic tectonism along the convergent boundary between the Pacific and Australia-India plates. Presumed early Tertiary southwest-directed subduction of the Pacific plate and associated arc magmatism were curtailed by impingement of the leading edge of the Ontong Java Plateau. Aprons of back-arc and forearc sediment were derived from highstanding parts of the arc during the late Oligocene and early Miocene. Late Tertiary arc-polaritymore » reversal and northeastward-directed subduction of the Woodlark spreading system caused a renewal of island-arc magmatism that completed the construction of the Central Solomons Trough as an enclosed intra-arc basin. Interpretations of multichannel profiles from 1982 and 1984 CCOP/SOPAC Tripartite Cruises of the research vessel R/V S.P. Lee indicate that the Central Solomons Trough is a composite intra-arc basin containing as much as 5.5 km of late Oligocene(.) and younger sedimentary rocks. As many as five lenticular seismic-stratigraphic units can be identified on the basis of unconformities and abrupt velocity changes. Late Miocene and younger folds and faults deform the northeast and southwest flanks of the basin. Profiles across the Kilinailau Trench show Ontong Java Plateau rocks covered by 2-4 km of trench sediment. The inner trench wall consists of folded, upfaulted, and rotated blocks of trench and forearc strata. The deep-water basin northwest of Bougainville is a southeastward extension of the New Ireland forearc basin, the southern margin of which is formed by a subsided part of the early Cenozoic arc. There, Oligocene(.) and younger basin strata, as much as 7 km thick, are deformed by pre-Pliocene faults and folds.« less

3D geodynamic models for the development of opposing continental subduction zones: The Hindu Kush-Pamir example

NASA Astrophysics Data System (ADS)

Liao, Jie; Gerya, Taras; Thielmann, Marcel; Webb, A. Alexander G.; Kufner, Sofia-Katerina; Yin, An

2017-12-01

The development of opposing continental subduction zones remains scantly explored in three dimensions. The Hindu Kush-Pamir orogenic system at the western end of the Himalayan orogen provides a rare example of continental collision linked to two opposing intra-continental subduction zones. The subducted plates feature a peculiar 3D geometry consisting of two distinct lithospheric fragments with different polarities, subduction angles and slab-curvatures beneath the Hindu Kush and Pamir, respectively. Using 3D geodynamic modeling, we simulate possible development of two opposing continental subduction zones to understand the dynamic evolution of the Hindu Kush-Pamir orogenic system. Our geodynamic model reproduces the major tectonic elements observed: (1) the deeper subduction depth, the steeper dip angle and the southward offset of the Hindu Kush subduction zone relative to the Pamir naturally occur if convergence direction of the subducting Indian plate and dip-direction of the Hindu Kush subduction zone match. (2) The formation of the highly asymmetrically curved Pamir region and the south-dipping subduction is promoted by the initial geometry of the indenting Indian lithosphere together with the existence of a major strike-slip fault on the eastern margin of the Pamir region. (3) Subduction of only the lower continental crust during continental collision can occur if the coupling between upper and lower crusts is weak enough to allow a separation of these two components, and that (4) the subduction of mainly lower crust then facilitates that conditions for intermediate-depth seismicity can be reached. (5) The secondary tectonic features modeled here such as strike-slip-fault growth, north-northwest striking extension zone, and lateral flow of the thickened ductile upper crust are comparable to the current tectonics of the region. (6) Model results are further compared to the potentially similar orogenic system, i.e., the Alpine orogen, in terms of the curved Western Alpine arc and the two opposing subducted slabs beneath the Alps and the Dinarides.

Reworked crustal of early Paleozoic WuYi Orogen revealed by receiver function data

NASA Astrophysics Data System (ADS)

Wei, Y.; Duan, Y.; Tian, X.; Zhao, Y.

2017-12-01

Intraplate orogenic belt, which occurs at the rigid and undeformable plate interiors, is a distinct new type of orogen rather than an interplate or plate marginal orogenic belt, whose deformation occurs exclusively at plate margins. Therefore, intraplate orogenic belts are the most obvious exception to the plate-tectonic paradigm, they are uncommon in Earth's history. The early Paleozoic Wuyi orogen in South China is one of the few examples of intraplate orogen, and is a key to understanding the process of intraplate orogenesis and global early Paleozoic geodynamics. In this study, we select teleseismic records from 45 mobile linear seismic stations deployed in Wuyi Mountain and 58 permanent stations setting in Jiangxi and Fujian provinces, from January 2011 to December 2012, and calculate the crustal thickness and average crustal Vp/Vs ratio using the H-κ stacking method. The main results include the following: 1) the crustal average Poission's ratio shows an increase tendency from land to sea, the interior of Wuyi orogen belt with an low ration less than 0.23, and the coastline with high ration which is up to 0.28, which indicate a very heterogeneous crustal structure and composition in Wuyi orogen and coast belt. 2) the crustal thickness ranges 28-34 km and shows a tendency of thinning from inland to coast in the region of SE China margin, which maight mean the eastern Eurasia lithospheric is extension and thinning induced by the subducted paleo-Pacific slab. To conclusion, we assume that Wuyi orogen experienced upper crustal thickening, lower crust and lithosphere delamination during the early Paleozoic orogeny, and lithosphere extension in Mesozoic. This research is founded by the Natural Science Foundation of China (41174052 and 41604048).

Tectonic implications of Mesozoic magmatism to initiation of Cenozoic basin development within the passive South China Sea margin

NASA Astrophysics Data System (ADS)

Mai, Hue Anh; Chan, Yu Lu; Yeh, Meng Wan; Lee, Tung Yi

2018-04-01

The South China Sea (SCS) is one of the classical example of a non-volcanic passive margin situated within three tectonic plates of the Eurasian, Indo-Australian and Philippine Sea plate. The development of SCS resulted from interaction of various types of plate boundaries, and complex tectonic assemblage of micro blocks and accretionary prisms. Numerous models were proposed for the formation of SCS, yet none can fully satisfy different aspects of tectonic forces. Temporal and geographical reconstruction of Cretaceous and Cenozoic magmatism with the isochrones of major basins was conducted. Our reconstruction indicated the SE margin of Asia had gone through two crustal thinning events. The sites for rifting development are controlled by localized thermal weakening of magmatism. NW-SE extension setting during Late Cretaceous revealed by magmatism distribution and sedimentary basins allow us to allocate the retreated subduction of Pacific plate to the cause of first crustal thinning event. A magmatic gap between 75 and 65 Ma prior to the initiation of first basin rifting suggested a significant modification of geodynamic setting occurred. The Tainan basin, Pearl River Mouth basin, and Liyue basins started to develop since 65 Ma where the youngest Late Cretaceous magmatism concentrated. Sporadic bimodal volcanism between 65 and 40 Ma indicates further continental extension prior to the opening of SCS. The E-W extension of Malay basin and West Natuna began since late Eocene followed by N-S rifting of SCS as Neotethys subducted. The SCS ridge developed between Pearl River Mouth basin and Liyue basin where 40 Ma volcanic activities concentrated. The interaction of two continental stretching events by Pacific followed by Neotethys subduction with localized magmatic thermal weakening is the cause for the non-volcanic nature of SCS.

Influence of increasing convergence obliquity and shallow slab geometry onto tectonic deformation and seismogenic behavior along the Northern Lesser Antilles zone

NASA Astrophysics Data System (ADS)

Laurencin, M.; Graindorge, D.; Klingelhoefer, F.; Marcaillou, B.; Evain, M.

2018-06-01

In subduction zones, the 3D geometry of the plate interface is one of the key parameters that controls margin tectonic deformation, interplate coupling and seismogenic behavior. The North American plate subducts beneath the convex Northern Lesser Antilles margin. This convergent plate boundary, with a northward increasing convergence obliquity, turns into a sinistral strike-slip limit at the northwestern end of the system. This geodynamic context suggests a complex slab geometry, which has never been imaged before. Moreover, the seismic activity and particularly the number of events with thrust focal mechanism compatible with subduction earthquakes, increases northward from the Barbuda-Anguilla segment to the Anguilla-Virgin Islands segment. One of the major questions in this area is thus to analyze the influence of the increasing convergence obliquity and the slab geometry onto tectonic deformation and seismogenic behavior of the subduction zone. Based on wide-angle and multichannel reflection seismic data acquired during the Antithesis cruises (2013-2016), we decipher the deep structure of this subduction zone. Velocity models derived from wide-angle data acquired across the Anegada Passage are consistent with the presence of a crust of oceanic affinity thickened by hotspot magmatism and probably affected by the Upper Cretaceous-Eocene arc magmatism forming the 'Great Arc of the Caribbean'. The slab is shallower beneath the Anguilla-Virgin Islands margin segment than beneath the Anguilla-Barbuda segment which is likely to be directly related to the convex geometry of the upper plate. This shallower slab is located under the forearc where earthquakes and partitioning deformations increase locally. Thus, the shallowing slab might result in local greater interplate coupling and basal friction favoring seismic activity and tectonic partitioning beneath the Virgin Islands platform.

Velocity structure of the mantle transition zone beneath the southeastern margin of the Tibetan Plateau

NASA Astrophysics Data System (ADS)

Li, Guohui; Bai, Ling; Zhou, Yuanze; Wang, Xiaoran; Cui, Qinghui

2017-11-01

P-wave triplications related to the 410 km discontinuity (the 410) were clearly observed from the vertical component seismograms of three intermediate-depth earthquakes that occurred in the Indo-Burma Subduction Zone (IBSZ) and were recorded by the Chinese Digital Seismic Network (CDSN). By matching the observed P-wave triplications with synthetics through a grid search, we obtained the best-fit models for four azimuthal profiles (I-IV from north to south) to constrain the P-wave velocity structure near the 410 beneath the southeastern margin of the Tibetan Plateau (TP). A ubiquitous low-velocity layer (LVL) resides atop the mantle transition zone (MTZ). The LVL is 25 to 40 km thick, with a P-wave velocity decrement ranging from approximately - 5.3% to - 3.6% related to the standard Earth model IASP91. An abrupt transition in the velocity decrement of the LVL was observed between profiles II and III. We postulate that the mantle structure beneath the southeastern margin of the TP is primarily controlled by the southeastern extrusion of the TP to the north combined with the eastward subduction of the Indian plate to the south, but not affected by the Emeishan mantle plume. We attribute the LVL to the partial melting induced by water and/or other volatiles released from the subducted Indian plate and the stagnant Pacific plate, but not from the upwelling or the remnants of the Emeishan mantle plume. A high-velocity anomaly ranging from approximately 1.0% to 1.5% was also detected at a depth of 542 to 600 km, providing additional evidence for the remnants of the subducted Pacific plate within the MTZ.

Peridinialean dinoflagellate plate patterns, labels and homologies

USGS Publications Warehouse

Edwards, L.E.

1990-01-01

Tabulation patterns for peridinialean dinoflagellate thecae and cysts have been traditionally expressed using a plate labelling system described by C.A. Kofoid in the early 1900's. This system can obscure dinoflagellate plate homologies and has not always been strictly applied. The plate-labelling system presented here introduces new series labels but incorporates key features and ideas from the more recently proposed systems of G.L. Eaton and F.J.R. Taylor, as modified by W.R. Evitt. Plate-series recognition begins with the cingulum (C-series) and proceeds from the cingulum toward the apex for the three series of the epitheca/epicyst and proceeds from the cingulum toward the antapex for the two series of the hypotheca/hypocyst. The epithecal/epicystal model consists of eight plates that touch the anterior margin of the cingulum (E-series: plates E1-E7, ES), seven plates toward the apex that touch the E-series plates (M-series: R, M1-M6), and up to seven plates near the apex that do not touch E-series plates (D-series: Dp-Dv). The hypothecal/hypocystal model consists of eight plates that touch the posterior margin of the cingulum (H-series: H1-H6,HR,HS) and three plates toward the antapex (T1-T3). Epithecal/epicystal tabulation patterns come in both 8- and 7- models, corresponding to eight and seven plates, respectively, in the E-series. Hypothecal/hypocystal tabulation patterns also come in both 8- and 7-models, corresponding to eight and seven plates, respectively, in the H-series. By convention, the 7-model epitheca/epicyst has no plates E1 and M1; the 7-model hypotheca/hypocyst has no plate H6. Within an 8-model or 7-model, the system emphasizes plates that are presumed to be homologous by giving them identical labels. I introduce the adjectives "monothigmate", "dithigmate," and "trithigmate" to designate plates touching one, two, and three plates, respectively, of the adjacent series. The term "thigmation" applies to the analysis of plate contacts between plate series as a guide to interpretation. Application of the proposed plate labelling system involves: (1) locating the cingulum and identifying the plate series, (2) identifying the landmark plates within each series, (3) assigning appropriate plate numbers to plates in the E- and H-series, (4) assigning appropriate plate numbers to the remaining plates using thigmation and interactions of diagonally opposite pairs of plates (quartets) as guides to interpretation. A "typical" gonyaulacoid tabulation pattern combines a 7-model epitheca/epicyst and an 8-model hypotheca/hypocyst. A "typical" peridinioid tabulation pattern combines an 8-model epitheca/epicyst and a 7-model hypotheca/hypocyst. The group that is presently termed partiform gonyaulacoid (which includes the modern genus Cladopyxis Stein and the fossil Microdinium Cookson and Eisenack) has an 8-model epitheca/epicyst and an 8-model hypotheca/hypocyst. ?? 1990.

The Cadiz margin study off Spain: An introduction

USGS Publications Warehouse

Nelson, C.H.; Maldonado, A.

1999-01-01

The Cadiz continental margin of the northeastern Gulf of Cadiz off Spain was selected for a multidisciplinary project because of the interplay of complex tectonic history between the Iberian and African plates, sediment supply from multiple sources, and unique Mediterranean Gateway inflow and outflow currents. The nature of this complex margin, particularly during the last 5 million years, was investigated with emphasis on tectonic history, stratigraphic sequences, marine circulation, contourite depositional facies, geotechnical properties, geologic hazards, and human influences such as dispersal of river contaminants. This study provides an integrated view of the tectonic, sediment supply and oceanographic factors that control depositional processes and growth patterns of the Cadiz and similar modem and ancient continental margins.

Rates and mechanics of rapid frontal accretion along the very obliquely convergent southern Hikurangi margin, New Zealand

NASA Astrophysics Data System (ADS)

Barnes, Philip M.; de Lépinay, Bernard Mercier

1997-11-01

Analysis of seismic reflection profiles, swath bathymetry, side-scan sonar imagery, and sediment samples reveal the three-dimensional structure, morphology, and stratigraphic evolution of the central to southern Hikurangi margin accretionary wedge, which is developing in response to thick trench fill sediment and oblique convergence between the Australian and Pacific plates. A seismic stratigraphy of the trench fill turbidites and frontal part of the wedge is constrained by seismic correlations to an already established stratigraphic succession nearby, by coccolith and foraminifera biostratigraphy of three core and dredge samples, and by estimates of stratigraphic thicknesses and rates of accumulation of compacted sediment. Structural and stratigraphic analyses of the frontal part of the wedge yield quantitative data on the timing of inception of thrust faults and folds, on the growth and mechanics of frontal accretion under variable convergence obliquity, and on the amounts and rates of horizontal shortening. The data place constraints on the partitioning of geological strain across the entire southern Hikurangi margin. The principal deformation front at the toe of the wedge is discontinuous and represented by right-stepping thrust faulted and folded ridges up to 1 km high, which develop initially from discontinuous protothrusts. In the central part of the margin near 41°S, where the convergence obliquity is 50°, orthogonal convergence rate is slow (27 mm/yr), and about 75% of the total 4 km of sediment on the Pacific Plate is accreted frontally, the seismically resolvable structures within 30 km of the deformation front accommodate about 6 km of horizontal shortening. At least 80% of this shortening has occurred within the last 0.4±0.1 m.y. at an average rate of 12±3 mm/yr. This rate indicates that the frontal 30 km of the wedge accounts for about 33-55% of the predicted orthogonal contraction across the entire plate boundary zone. Despite plate convergence obliquity of 50°, rapid frontal accretion has occurred during the late Quaternary with the principal deformation front migrating seaward up to 50 km within the last 0.5 m.y. (i.e., at a rate of 100 km/m.y.). The structural response to this accretion rate has been a reduction in wedge taper and, consequently, internal deformation behind the present deformation front. Near the southwestern termination of the wedge, where there is an along-the-margin transition to continental transpressional tectonics, the convergence obliquity increases to >56°, and the orthogonal convergence rate decreases to 22 mm/yr, the wedge narrows to 13 km and is characterized simply by two frontal backthrusts and landward-verging folds. These structures have accommodated not more than 0.5 km of horizontal shortening at a rate of < 1 mm/yr, which represents < 5% of the predicted orthogonal shortening across the entire plate boundary in southern North Island. The landward-vergent structural domain may represent a transition zone from rapid frontal accretion associated with low basal friction and high pore pressure ratio in the central part of the margin, to the northern South Island region where the upper and lower plates are locked or at least very strongly coupled.

Age and tectonic implications of some low-grade metamorphic rocks from the Yucatan Channel

USGS Publications Warehouse

Vedder, J.G.; MacLeod, N.S.; Lanphere, M.A.; Dillon, William P.

1973-01-01

Phyllite and marble dredged from the lower part of the continental slope between Cuba and the Yucatan Peninsula seem to support the contention that a pre-early Tertiary metamorphic belt extends from the western Greater Antilles into northern Central America. The minimum K-Ar ages derived from the samples suggest that the metamorphic event was pre-Late Cretaceous, and evaluation of the K-Ar data implies that this metamorphic event is not older than Late Jurassic. Greater antiquity, however, is inferred from structural and stratigraphic relations in British Honduras, where the latest regional metamorphic event was post-Early Permian and pre-Middle Jurassic.  Rifting and extension related to plate motions along the British Honduras Quintana Roo margin through Mesozoic and earliest Cenozoic time presumably would preclude extensive regional metamorphism, permitting only limited development of schistose rocks there during that interval. The timing of metamorphic events in western Cuba is uncertain, but a pre-Middle Jurassic episode possibly is reflected in the phyllite and marble terranes of Isla de Pinos and Sierra de Trinidad. Local incipient metamorphism of Early and Middle Jurassic strata in the Sierra de los Organos may have resulted from severe tectonism that began in Late Cretaceous time and diminished in the Eocene.

Connecting the surface to the deep: Flat-slab subduction dynamics and the evolution of western Amazonia

NASA Astrophysics Data System (ADS)

Eakin, C. M.

2017-12-01

Plate tectonics is primarily driven by the subduction of cold dense oceanic slabs. It has yet to be fully understood however how variations in slab morphology and buoyancy influence the surrounding mantle dynamics, and what difference if any is seen at the surface. An excellent natural laboratory to answer such questions is found along the Andean margin where the world's largest flat slab is presently subducting beneath much of Peru. Following the deployment of broadband seismic arrays across the region, mantle flow both beneath and above the flat-slab is investigated using targeted shear-wave splitting techniques that detect seismic anisotropy and the pattern of mantle deformation. The along strike change in slab dip angle and buoyancy content is found to exert a strong control over the surrounding mantle flow field. Modeling of the induced mantle flow, and the dynamic topography at the surface that results, predicts a wave of dynamic subsidence that propagates away from the trench as the flat slab develops. This is found to correlate well with the record of widespread sediment deposition across western Amazonia during the Miocene. A combination of uplift, flexure and dynamic topography during slab flattening is proposed to explain the overall landscape evolution of the region and the subsequent configuration of the transcontinental Amazon drainage system we see today.

Slab geometry of the South American margin from joint inversion of body waves and surface waves

NASA Astrophysics Data System (ADS)

Porritt, R. W.; Ward, K. M.; Porter, R. C.; Portner, D. E.; Lynner, C.; Beck, S. L.; Zandt, G.

2016-12-01

The western margin of South America is a long subduction zone with a complex, highly three -dimensional geometry. The first order structure of the slab has previously been inferred from seismicity patterns and locations of volcanoes, but confirmation of the slab geometry by seismic imaging for the entire margin has been limited by either shallow, lithospheric scale models or broader, upper mantle images, often defined on a limited spatial footprint. Here, we present new teleseismic tomographic SV seismic models of the upper mantle from 10°S to 40°S along the South American subduction zone with resolution to a depth of 1000 km as inferred from checkerboard tests. In regions near the Peru Bolivia border (12°S to 18°S) and near central Chile and western Argentina (29.5°S to 33°S) we jointly invert the multi-band direct S and SKS relative delay times with Rayleigh wave phase velocities from ambient noise and teleseismic surface wave tomography. This self-consistent model provides information from the upper crust to below the mantle transition zone along the western margin in these two regions. This consistency allows tracing the slab from the South American coastline to the sub-transition zone upper mantle. From this model we image several features, but most notable is a significant eastward step near the southern edge of the margin (24°-30° S). West of this step, a large high shear velocity body is imaged in the base of and below the transition zone. We suggest this may be a stagnant slab, which is descending into the lower mantle now that it is no longer attached to the surface. This suggests a new component to the subduction history of western South America when an older slab lead the convergence before anchoring in the transition zone, breaking off from the surface, and being overtaken by the modern, actively subducting slab now located further east.

Heat flow anomalies on the Western Mediterranean margins: first results from the WestMedFlux-2016 cruise

NASA Astrophysics Data System (ADS)

Poort, Jeffrey; Lucazeau, Francis; Le Gal, Virginie; Rabineau, Marina; Dal Cin, Michela; Bouzid, Abderrezak; Palomino, Desirée; Leroux, Estelle; Akhmanov, Grigory; Battani, Anne; Bachir, Roza Si; Khlystov, Oleg; Koptev, Aleksandre

2017-04-01

While there is now a large consensus that Western Mediterranean basins developed in a Miocene back-arc setting due to slab roll-back and that some of its domains are floored by oceanic crust, there is still a lot of speculation on the configuration, nature and evolution of its margins and the ocean-continent transitions (OCT). A thick Messinian layer of evaporites in the deep basin obscures deep seismic reflectors, and only recently seismic refraction and wide-angle studies revealed a confident picture of basement configuration. In order to further constrain models of crustal structure and margin evolution, heat flow is one of the key parameters needed. Recent heat flow studies on other margins have shown the existence of a persistent thermal anomaly under rifted margins that urges to reconsider the classical models of its evolution. The young age of OCT and ceased oceanic formation in the Western Mediterranean make it an interesting test case for a thermo-mechanical study of its margins. The presence of halokinetic structuring and salt diapirs urges the need of close spaced heat flow measurement to evaluate heat refraction and advective heat transfer by fluid migration. During the WestMedFlux cruise on the research vessel L'Atalante, we collected a total of 150 new heat flow measurement (123 in pogo mode, 27 with a sediment corer) in the deep basin of the Western Mediterranean where heat flow data were sparse. Preliminary analysis of the heat flow data confirms two regional trends: in the southern Provencal basin an overall increase from west to east (from about 60 mW/m2 at the Golf of Lion towards 75 mW/m2 at the West-Sardinia margin), while in the northern part of the Algero-Balearic basin heat flow increases from east to west (from about 80 to 100 mW/m2). On this regional trends, several local anomalies are clearly differentiated. In the deep oceanic basin, strong anomalies seem to be merely associated to salt diapiric structures. On the OCT and on the rifted continent, both strongly reduced and elevated heat flow are observed and suggest other heat sink and sources. We will discuss on the different processes that might have affected the surface heat flow (e.g., bottom water currents, slope instabilities and focused fluid migrations) and try to link the large scale heat flow patterns with crustal nature, structuring of the margins and mantle dynamics.

Absolute Plate Motion Control Since the Triassic from the Cocos Slab and its Associated Subduction Record in Mexico

NASA Astrophysics Data System (ADS)

Boschman, L.; Van Hinsbergen, D. J. J.; Langereis, C. G.; Molina-Garza, R. S.; Kimbrough, D. L.; Spakman, W.

2017-12-01

A positive wave speed anomaly interpreted as the Cocos slab stretches from the uppermost mantle at the Middle America trench in the west, to the lowermost mantle below the Atlantic in the east. The length and continuity of this slab indicates long-lived, uninterrupted eastward subduction of the attached Cocos Plate and its predecessor, the Farallon Plate. The geological record of Mexico contains Triassic to present day evidence of subduction, of which the post-Late Cretaceous phase is of continental margin-style. Interpretations of the pre-Upper Cretaceous subduction-related rock assemblages are under debate, and vary from far-travelled exotic intra-oceanic island arc character to in-situ extended continental margin origin. We present new paleomagnetic data that show that Triassic, Jurassic and Cretaceous subduction-related rocks from the Vizcaíno Peninsula and the Guerrero terrane have a paleolatitudinal plate motion history that is equal to that of the North American continent. This suggests that these rock assemblages were part of the overriding plate and were perhaps only separated from the North American continent by temporal fore- or back-arc spreading. The entire Triassic-present day subduction record, and hence, reconstructed trench location, can therefore be linked to the Cocos slab, which provides control on longitudinal plate motion of North America since the time of Pangea. Compared to the latest state of the art mantle frames, in which longitudes are essentially unconstrained for pre-Cretaceous times, our reconstructed absolute position of North America requires a significant westward longitudinal shift for Mesozoic times.

Fluvial-deltaic sedimentation and stratigraphy of the ferron sandstone

USGS Publications Warehouse

Anderson, P.B.; Chidsey, T.C.; Ryer, T.A.

1997-01-01

East-central Utah has world-class outcrops of dominantly fluvial-deltaic Turonian to Coniacian aged strata deposited in the Cretaceous foreland basin. The Ferron Sandstone Member of the Mancos Shale records the influences of both tidal and wave energy on fluvial-dominated deltas on the western margin of the Cretaceous western interior seaway. Revisions of the stratigraphy are proposed for the Ferron Sandstone. Facies representing a variety of environments of deposition are well exposed, including delta-front, strandline, marginal marine, and coastal-plain. Some of these facies are described in detail for use in petroleum reservoir characterization and include permeability structure.

Oblique strike-slip motion off the Southeastern Continental Margin of India: Implication for the separation of Sri Lanka from India

NASA Astrophysics Data System (ADS)

Desa, Maria Ana; Ismaiel, Mohammad; Suresh, Yenne; Krishna, Kolluru Sree

2018-05-01

The ocean floor in the Bay of Bengal has evolved after the breakup of India from Antarctica since the Early Cretaceous. Recent geophysical investigations including updated satellite derived gravity map postulated two phases for the tectonic evolution of the Bay of Bengal, the first phase of spreading occurred in the NW-SE direction forming its Western Basin, while the second phase occurred in the N-S direction resulting in its Eastern Basin. Lack of magnetic data along the spreading direction in the Western Basin prompted us to acquire new magnetic data along four tracks (totaling ∼3000 km) to validate the previously identified magnetic anomaly picks. Comparison of the synthetic seafloor spreading model with the observed magnetic anomalies confirmed the presence of Mesozoic anomalies M12n to M0 in the Western Basin. Further, the model suggests that this spreading between India and Antarctica took place with half-spreading rates of 2.7-4.5 cm/yr. The trend of the fracture zones in the Western Basin with respect to that of the Southeastern Continental Margin of India (SCMI) suggests that SCMI is an oblique transform margin with 37° obliquity. Further, the SCMI consists of two oblique transform segments separated by a small rift segment. The strike-slip motion along the SCMI is bounded by the rift segments of the Northeastern Continental Margin of India and the southern margin of Sri Lanka. The margin configuration and fracture zones inferred in its conjugate Western Enderby Basin, East Antarctica helped in inferring three spreading corridors off the SCMI in the Western Basin of the Bay of Bengal. Detailed grid reconstruction models traced the oblique strike-slip motion off the SCMI since M12n time. The strike-slip motion along the short northern transform segment ended by M11n time. The longer transform segment, found east of Sri Lanka lost its obliquity and became a pure oceanic transform fault by M0 time. The eastward propagation of the Africa-Antarctica spreading center initiated the anticlockwise separation of Sri Lanka from India by M12n time. Seafloor spreading south of Sri Lanka due to the India-Antarctica spreading episode and the simultaneously occurring strike-slip motion east of Sri Lanka restricted this separation resulting in a failed rift. Thus Sri Lanka with strike-slip motion to its east, failed rift towards west, continental extension to its north and rifting to its south behaved as a short lived microplate during the Early Cretaceous period and remained attached to India thereafter.

Submarine slope failures along the convergent continental margin of the Middle America Trench

NASA Astrophysics Data System (ADS)

Harders, Rieka; Ranero, CéSar R.; Weinrebe, Wilhelm; Behrmann, Jan H.

2011-06-01

We present the first comprehensive study of mass wasting processes in the continental slope of a convergent margin of a subduction zone where tectonic processes are dominated by subduction erosion. We have used multibeam bathymetry along ˜1300 km of the Middle America Trench of the Central America Subduction Zone and deep-towed side-scan sonar data. We found abundant evidence of large-scale slope failures that were mostly previously unmapped. The features are classified into a variety of slope failure types, creating an inventory of 147 slope failure structures. Their type distribution and abundance define a segmentation of the continental slope in six sectors. The segmentation in slope stability processes does not appear to be related to slope preconditioning due to changes in physical properties of sediment, presence/absence of gas hydrates, or apparent changes in the hydrogeological system. The segmentation appears to be better explained by changes in slope preconditioning due to variations in tectonic processes. The region is an optimal setting to study how tectonic processes related to variations in intensity of subduction erosion and changes in relief of the underthrusting plate affect mass wasting processes of the continental slope. The largest slope failures occur offshore Costa Rica. There, subducting ridges and seamounts produce failures with up to hundreds of meters high headwalls, with detachment planes that penetrate deep into the continental margin, in some cases reaching the plate boundary. Offshore northern Costa Rica a smooth oceanic seafloor underthrusts the least disturbed continental slope. Offshore Nicaragua, the ocean plate is ornamented with smaller seamounts and horst and graben topography of variable intensity. Here mass wasting structures are numerous and comparatively smaller, but when combined, they affect a large part of the margin segment. Farther north, offshore El Salvador and Guatemala the downgoing plate has no large seamounts but well-defined horst and graben topography. Off El Salvador slope failure is least developed and mainly occurs in the uppermost continental slope at canyon walls. Off Guatemala mass wasting is abundant and possibly related to normal faulting across the slope. Collapse in the wake of subducting ocean plate topography is a likely failure trigger of slumps. Rapid oversteepening above subducting relief may trigger translational slides in the middle Nicaraguan upper Costa Rican slope. Earthquake shaking may be a trigger, but we interpret that slope failure rate is lower than recurrence time of large earthquakes in the region. Generally, our analysis indicates that the importance of mass wasting processes in the evolution of margins dominated by subduction erosion and its role in sediment dynamics may have been previously underestimated.

A Simplified View of the Geochemical Diversity Surrounding Home Plate

NASA Technical Reports Server (NTRS)

Yen, A. S.; Morris, R. V.; Clark, B. C.; Gellert, R.

2008-01-01

The Home Plate feature (Fig. 1) within the Inner Basin of the Columbia Hills consists of layered rocks and has been interpreted as an accumulation of pyroclastic deposits [1]. Samples analyzed by the Alpha Particle X-ray Spectrometer within 25 meters of the eastern margin of Home Plate exhibit a strikingly diverse range of geochemical compositions, including the highest levels of Mg, Si, K, Zn, and Ni measured at Gusev Crater. This wide range of chemical variability across the 40+ samples analyzed on and near Home Plate can be represented by contributions from only six primary components. This reconstruction is not reflected in the M ssbauer mineralogy suggesting that significant alteration of the contributing components has occurred.

Structural styles of the western onshore and offshore termination of the High Atlas, Morocco

NASA Astrophysics Data System (ADS)

Hafid, Mohamad; Zizi, Mahmoud; Bally, Albert W.; Ait Salem, Abdellah

2006-01-01

The present work aims (1) at documenting, by regional seismic transects, how the structural style varies in the western High Atlas system and its prolongation under the present-day Atlantic margin, (2) at understanding how this variation is related to the local geological framework, especially the presence of salt within the sedimentary cover, and (3) at discussing the exact geographic location of the northern front of the western High Atlas and how it links with the most western Atlas front in the offshore Cap Tafelney High Atlas. Previous work showed that the structural style of the Atlas belt changes eastward from a dominantly thick-skinned one in central and eastern High Atlas and Middle Atlas of Morocco to a dominantly thin-skinned one in Algeria and Tunisia. We propose here to show that a similar structural style change can be observed in the other direction of the Atlas Belt within its western termination, where the western High Atlas intersects at right angle the Atlantic passive margin and develops into a distinct segment, namely the High Atlas of Cap Tafelney, where salt/evaporite-based décollement tectonics prevail. To cite this article: M. Hafid et al., C. R. Geoscience 338 (2006).

Geological development and Phanerozoic crustal accretion in the western segment of the southern Tien Shan (Kyrgyzstan, Uzbekistan and Tajikistan)

NASA Astrophysics Data System (ADS)

Brookfield, M. E.

2000-12-01

The Tien Shan form a high intracontinental mountain belt, lying north of the main India-Asia collision mountains, and consist of re-activated Paleozoic orogens. The western segment of the southern Tien Shan lies northwest of the Pamir and west of the Talas-Fergana fault. The stratigraphy, lithology, igneous and metamorphic petrology and geochemistry of this segment indicate that it was formed by the assembly of Lower Paleozoic arcs which developed into microcontinents with Upper Paleozoic mature shelf and slope clastic and carbonate sediments. Precambrian continental crust is confined to two small blocks along its southern margin. The bulk of the southern Tien Shan consists of ?Vendian to Silurian oceanic and slope clastic rocks, resting on oceanic lithosphere, and overlain by thick passive margin Devonian to mid-Carboniferous mature shelf clastics and carbonates. These are unconformably overlain by syn- and post-orogenic immature clastic sediments derived from mountains on the north formed by closure of a Carboniferus southern Tajik and a northern Vendian to Carboniferous Turkestan ocean with the southern Tien Shan microcontinent sandwiched between. Associated with these collisions are late Carboniferous to Permian intrusives, which form three south to north (though overlapping) suites; a southern calc-alkaline granodiorite-granite suite, an intermediate gabbro-monzodiorite-granite suite, and a northern alkaline monzodiorite-granite-alaskite suite. The gabbro-monzodiorite-granite suite forms the earliest subduction-related magmatism of the southern Tien Shan: rare earth element patterns are consistent with derivation from a primitive or slightly enriched mantle. The other suites show more crustal contamination. Rb and Sr vary with depth and degree of partial melting and are consistent with progressive involvement of crustal material in partial melts during collision. The gradual change in composition within each complex, lasting in some cases from 295 to 250 Ma (the entire Permian), may be explained by a consecutive shift in the melting sedimentary cover of the subducting plate from oceanic crust through transitional crust to marginal continental crust. Like the Central Asian orogenic belt (the main focus of IGCP 420), the Tien Shan represent a net addition of continental crust during the Phanerozoic. Very little of the belt has any Precambrian precursor.

Sedimentary record of a fluctuating ice margin from the Pennsylvanian of western Gondwana: Paraná Basin, southern Brazil

NASA Astrophysics Data System (ADS)

Vesely, Fernando F.; Trzaskos, Barbara; Kipper, Felipe; Assine, Mario Luis; Souza, Paulo A.

2015-08-01

The Paraná Basin is a key locality in the context of the Late Paleozoic Ice Age (LPIA) because of its location east of the Andean proto-margin of Gondwana and west of contiguous interior basins today found in western Africa. In this paper we document the sedimentary record associated with an ice margin that reached the eastern border of the Paraná Basin during the Pennsylvanian, with the aim of interpreting the depositional environments and discussing paleogeographic implications. The examined stratigraphic succession is divided in four stacked facies associations that record an upward transition from subglacial to glaciomarine environments. Deposition took place during deglaciation but was punctuated by minor readvances of the ice margin that deformed the sediment pile. Tillites, well-preserved landforms of subglacial erosion and glaciotectonic deformational structures indicate that the ice flowed to the north and northwest and that the ice margin did not advance far throughout the basin during the glacial maximum. Consequently, time-equivalent glacial deposits that crop out in other localities of eastern Paraná Basin are better explained by assuming multiple smaller ice lobes instead of one single large glacier. These ice lobes flowed from an ice cap covering uplifted lands now located in western Namibia, where glacial deposits are younger and occur confined within paleovalleys cut onto the Precambrian basement. This conclusion corroborates the idea of a topographically-controlled ice-spreading center in southwestern Africa and does not support the view of a large polar ice sheet controlling deposition in the Paraná Basin during the LPIA.

Late Wisconsinan glaciation and postglacial relative sea-level change on western Banks Island, Canadian Arctic Archipelago

NASA Astrophysics Data System (ADS)

Lakeman, Thomas R.; England, John H.

2013-07-01

The study revises the maximum extent of the northwest Laurentide Ice Sheet (LIS) in the western Canadian Arctic Archipelago (CAA) during the last glaciation and documents subsequent ice sheet retreat and glacioisostatic adjustments across western Banks Island. New geomorphological mapping and maximum-limiting radiocarbon ages indicate that the northwest LIS inundated western Banks Island after ~ 31 14C ka BP and reached a terminal ice margin west of the present coastline. The onset of deglaciation and the age of the marine limit (22-40 m asl) are unresolved. Ice sheet retreat across western Banks Island was characterized by the withdrawal of a thin, cold-based ice margin that reached the central interior of the island by ~ 14 cal ka BP. The elevation of the marine limit is greater than previously recognized and consistent with greater glacioisostatic crustal unloading by a more expansive LIS. These results complement emerging bathymetric observations from the Arctic Ocean, which indicate glacial erosion during the Last Glacial Maximum (LGM) to depths of up to 450 m.

Recognition of Intermediate Crust (IC), its construction and its distinctive epeirogenic behaviour: an exciting new tool for plate kinematic analysis (PKA) of the Arctic margins and western Siberia

NASA Astrophysics Data System (ADS)

Osmaston, M. F.

2012-04-01

Identification of a microcontinental block within or near a continental margin raises two questions, addressed in this talk - How did it get there? What is the nature of the intervening crust? I will then illustrate briefly how, in the Arctic, the answers, although by no means restricted to that region, do seem to help us a lot to begin unravelling the ancient plate kinematics of its wide margins. The plate tectonics paradigm currently posits that the Earth has only two kinds of crust - continental and oceanic - and that the former may be stretched to form sedimentary basins or the latter may be modified by arc or collision until it looks continental. But global analysis of the dynamics of actual plate motions for the past 150 Ma indicates [1, 2, 3] that continental tectospheres must be immensely thicker than previously thought and almost certainly too thick to be stretched with the forces available. In the extreme case of cratons, these tectospheric keels may commonly extend to 600 km or more [3]. This thick-plate behaviour is attributable, not to cooling but to a petrological 'stiffening' effect, associated with a loss of water-weakening, which also applies to the LVZ below MORs [4, 5, 6]. The corresponding thick-plate version of the MOR process [1, 6] has a deep, narrow wall-accreting axial crack which inherently brings two outstanding additional benefits:- (i) why, at medium to fast spreading rates, MOR axes become straight and orthogonally segmented [7], (ii) not being driven by body forces, it can achieve the sudden jumps of axis, spreading-rate and direction widely recorded in mid-ocean and are necessary after generating the limited separations of microplates near margins. So in seeking the 'continent-ocean boundary' (COB) along passive margins, a site where stretching has often been invoked, we need instead to consider how this MOR process would be affected by the heavy concurrent sedimentation to be expected when splitting a continent. I reason that, by blocking the hydrothermal cooling, this must inhibit magnetic anomaly formation and prolong magmagenesis to give a thicker-than-oceanic mafic crust, which I have called Intermediate Crust (IC) [8, 9], to distinguish it from Mature Continental Crust (MCC). Seismologically, IC basement must look deceptively like that assigned to stretched MCC. For thermodynamic reasons [8, 9] the hydrous content of deep MCC and of deeply subducted UHP crustal slices gives them a big thermal epeirogenic sensitivity which IC lacks. The NE Atlantic offers an example of this distinction. Structurally, the MCC of Greenland and Norway must have been intimately juxtaposed by the Scandian collision, so it was concluded [9] that the crust of the Greenland-Norway continental shelves must mostly be IC of post-Scandian (early Devonian?) age, a character confirmed by their lack of epeirogenic response to laterally conducted heat from the opening N Atlantic, although drainage systems in Norway proper clearly show it. Geometrically, this separation appears to have changed direction sharply, the second and bigger stage also involving separation of Svalbard from near Tromsø, where it had provided northward continuation of a complete Caledonian transect, so it has an IC implication for much of the Barents Sea area (bar the Bjørnøya block). Moving quickly round to the NE side of Baltica, we can begin to trace the separative motions of the Novaya Zemlya - Pay Khoy (NZPK) strip of less-mature MCC, transverse to the Timanian belt, and the associated evolution of the Pechora basin system. In places, faulted IC/MCC epeirogenic contrasts seem to define the size and direction of the IC-generating separation with remarkable precision. A crucial opening-up of this analysis is provided by realizing that the Polar Ural stretch is not MCC, but is merely the huge 585 Ma Voykar-Synya ophiolite, with its metamorphics, resting on a now-crumpled boundary between IC of very different ages. For further understanding we need briefly to extend the analysis, first to the formation of the West Siberian Basin, the IC nature of whose crust (but not its low thermal epeirogenic sensitivity) has been obscured by Permo-Triassic addition of the Siberian flood basalts, and thence to the complicated MCC distribution seen in Khazakhstan. The pattern of the older elements of the latter appears to be a further, but more ancient, example of the plate dynamics evidence [1, 2, 3] for 'deep-keeled' cratons, in that, when they separate, horizontal inflows of mantle are induced which impinge on the lesser keels of MCC blocks in the region, potentially generating IC-floored basins in between them. In this way the geology which now lies between the Baltica and East Siberia cratons can in principle provide powerful constraints on the freedom with which palaeomagnetic data for them has been interpreted. A key element in this is that otherwise-missing components of the Timanian orogen are apparently to be found on the western edge of the Siberian platform. This means that the NZPK strip and Pechora Basin area were formed as part of the Timanian interplay between the then-roughly-parallel sides of the Baltica and Siberian cratons. A final step in this limited analysis suggests that the clockwise bending of northern Novaya Zemlya was the dextral consequence of the final (~2000 km) northwestward cratonic separation of Baltica plus NZPK etc. to form the IC of the West Siberian Basin during the 580-535 Ma interval. This may be older than any of the NZ rocks suitable for palaeomagnetic determination. Resolution of the conflict between this geologically and geometrically secured reconstruction and that prescribed palaeomagnetically for 'Rodinia' appears to lie in the possibility that at some point on the APWP there has been an inadvertent switch between following the N and the S pole, due to the habitual choosing of the lesser of two plate motion speeds during the interval between points on the curve. In that case some of the supposed assemblage will be geographically upside down in relation to the rest. Finally, a quick visit to the Chukchi-Bering-Alaska sector of the Arctic margin. Here, too, recognition of IC may provide a key. I suggest that northward emplacement of the Brooks Range Ophiolite at close to 170 Ma was swiftly followed by the (now) westward extraction of the Seward-Chukchi MCC assemblage to form, as IC, both the extensive Koyukuk basin system which lies south of Brooks Range and, ultimately, the Bering gap in the Pacific rim of cratonic keels, through which mantle to put at depth under the widening Eurasian Basin ocean floor has been drawn, initiating the Aleutian arc in the Palaeocene, trapping OC/IC behind it. I discuss a global reason for the initiation of the Bering gap, and its timing, in another session (GD6.1) of this meeting. [1] Osmaston M. F. (2006) Global tectonic actions emanating from Arctic opening in the circumstances of a two-layer mantle and a thick-plate paradigm involving deep cratonic tectospheres: the Eurekan (Eocene) compressive motion of Greenland and other examples. In, Proc. ICAM IV 2003 (ed. R. Scott & D. Thurston). OCS Study MMS 2006-003, pp.105-124: Also published on: http://www.mms.gov/alaska/icam [2] Osmaston M. F. (2007) Cratonic keels and a two-layer mantle tested: mantle expulsion during Arabia-Russia closure linked to westward enlargement of the Black Sea, formation of the Western Alps and subduction of the Tyrrhenian (not the Ionian) Sea. XXIV IUGG, Session JSS 011 Abstr #2105. http://www.iugg2007perugia.it/webbook/. [3] Osmaston M. F. (2009) Deep cratonic keels and a 2-layer mantle? Tectonic basis for some far-reaching new insights on the dynamical properties of the Earth's mantle: example motions from Mediterranean, Atlantic-Arctic and India. Geophys. Res. Abst. 11, EGU2009-6359 (Solicited). [4] Karato S. (1986) Does partial melting reduce the creep strength of the upper mantle? Nature 319, 309-310. [5] Hirth G. & Kohlstedt D. L. (1996) Water in the oceanic upper mantle: implication for rheology, melt extraction, and the evolution of the lithosphere. EPSL 144, 93-108. [6] Osmaston M. F. (2010) On the actual variety of plate dynamical mechanisms and how mantle evolution affected them through time, from core formation to the Indian collision. Geophys. Res. Abstr. 12, EGU2010-6101. [7] Osmaston M. F. (1995) A straightness mechanism for MORs: a new view of ocean plate genesis and evolution. In IUGG XXI Gen. Assy. Abstracts p. A472. [8] Osmaston M. F. (2011) An introduction to Intermediate Crust (IC): its formation, epeirogenic character, and plate tectonics significance. TSG Ann.Mtg. 2011, Durham University, Technical Programme p.45. [9] Osmaston M. F. (2008) Basal subduction tectonic erosion (STE), butter mélanges and the construction and exhumation of HP-UHP belts: the Alps example and some comparisons. Internat. Geol. Rev. 50(8), 685-754 DOI: 10.2747/00206814.50.8.685.

How does the Pacific Plate die, and what dies with it?

NASA Astrophysics Data System (ADS)

Gill, J.

2002-12-01

Investigation continues into the demise of the Pacific Plate by the subduction, which has been its principal driving force for about 50 m.y.. Key unanswered questions in the inquiry include the following. What was the motive for subduction to start? Where is the geochemical boundary between the Pacific and Indian Plates, and what is the reason for that difference in the first place? Why do marginal basins initiate above the subducting Pacific Plate, spread for 5-10 m.y., and then stop? How much and which parts of the Pacific Plate survive to an afterlife in the continents versus descent to Hades? Most of these questions reduce to: Why are the two largest islands in the Pacific Ocean (Hawaii and Viti Levu, Fiji) so different?