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Sample records for rift oblicuo aplicaciones

  1. Rift propagation

    NASA Technical Reports Server (NTRS)

    Parmentier, E. M.; Schubert, G.

    1989-01-01

    A model for rift propagation which treats the rift as a crack in an elastic plate which is filled from beneath by upwelling viscous asthenosphere as it lengthens and opens. Growth of the crack is driven by either remotely applied forces or the pressure of buoyant asthenosphere in the crack and is resisted by viscous stresses associated with filling the crack. The model predicts a time for a rift to form which depends primarily on the driving stress and asthenosphere viscosity. For a driving stress on the order of 10 MPa, as expected from the topography of rifted swells, the development of rifts over times of a few Myr requires an asthenosphere viscosity of 10 to the 16th Pa s (10 to the 17th poise). This viscosity, which is several orders of magnitude less than values determined by postglacial rebound and at least one order of magnitude less than that inferred for spreading center propagation, may reflect a high temperature or large amount of partial melting in the mantle beneath a rifted swell.

  2. A simple continental rift classification

    NASA Astrophysics Data System (ADS)

    Merle, Olivier

    2011-12-01

    A continental rift is conventionally described as a thinning process of the lithosphere ultimately leading to the rupture of the continent and the formation of a mid-oceanic ridge. Rifting is the initial and fundamental process by which the separation of two continents into two tectonic plates takes place. Previous classifications, particularly the one into "active" and "passive" rifting, are briefly presented, together with their limitations. The new classification presented here links continental rifts to the major plate tectonics structures which are at the origin of their formation. Thus, four types of rift can be defined: the subduction-related rift, the plume-related rift, the mountain-related rift and the transform-related rift. A number of examples representative of these four types of rift are then presented. This classification is shown to lie at the heart of our understanding of the major plate tectonic processes at work on Earth.

  3. Continental rifting: a planetary perspective

    SciTech Connect

    Muehlberger, W.R.

    1985-01-01

    The only inner planet that has abundant evidence of regional extension, and the consequent generation of rifts in the earth. The absence of plate motion on the other inner planets limits their rifts to localized bulges or subsidence areas. The rifting of oceanic lithosphere is seldom preserved in the geological record. Thus, such rifting must be inferred via plate tectonic interpretation: if there is rifting, then there must be subduction whose results are commonly well preserved. Modern continental rifts are found in many tectonic settings: continental breakup, extension transverse to collisional stresses, or wide regions of nearly uniform extension. Recognition of these settings in older rocks becomes more difficult the farther back in geologic time you travel. Rift basin fillings typically show rapid lateral and vertical facies and thickness changes, bimodal volcanism, and distinctive rift-drift sequences. Proterozoic rifts and aulacogens are well-documented in North America; ex. Keweenawan, western margin of Labrador fold belt, Belt-Uinta and the Wopmay-Athapuscow regions. Documented Archean rifts are rare. In Quebec, the truncated margin of the Minto craton bounded on the south by a 2.8 Ga greenstone belt implies an earlier rift event. The oldest proposed rift dated at 3.0 Ga contains the Pongola Supergroup in southeastern Africa. The presence of Archean dikes demonstrates a rigid crust and andesites as old as 3.5 Ga imply plate tectonics and thus, at least, oceanic rifting.

  4. Intracontinental Rifts As Glorious Failures

    NASA Astrophysics Data System (ADS)

    Burke, K.

    2012-12-01

    Rifts: "Elongate depressions overlying places where the lithosphere has ruptured in extension" develop in many environments because rocks are weak in extension (Sengor 2nd edn. Springer Encycl. Solid Earth Geophys.). I focus on intra-continental rifts in which the Wilson Cycle failed to develop but in which that failure has led to glory because rocks and structures in those rifts throw exceptional light on how Earth's complex continental evolution can operate: The best studied record of human evolution is in the East African Rift; The Ventersdorp rifts (2.7 Ga) have yielded superb crustal-scale rift seismic reflection records; "Upside-down drainage" (Sleep 1997) has guided supra-plume-head partial melt into older continental rifts leading Deccan basalt of ~66Ma to erupt into a Late Paleozoic (~ 300Ma) rift and the CAMP basalts of ~201 Ma into Ladinian, ~230 Ma, rifts. Nepheline syenites and carbonatites, which are abundant in rifts that overlie sutures in the underlying mantle lithosphere, form by decompression melting of deformed nepheline syenites and carbonatites ornamenting those sutures (Burke et al.2003). Folding, faulting and igneous episodes involving decompression melting in old rifts can relate to collision at a remote plate margin (Guiraud and Bosworth 1997, Dewey and Burke 1974) or to passage of the rift over a plume generation zone (PGZ Burke et al.2008) on the Core Mantle Boundary (e.g.Lake Ellen MI kimberlites at ~206 Ma).

  5. Continental rifting - Progress and outlook

    SciTech Connect

    Baker, B.H.; Morgan, P.

    1981-07-21

    It is noted that in spite of the flood of new data on continental rifts in the last 15 years, there is little consensus about the basic mechanisms and causes of rifting. The remarkable similarities in rift cross sections (shown in a figure), are considered to suggest that the anomalous lithospheric structure of rifts is more dependent on lithosphere properties than the mode of rifting. It is thought that there is a spectrum of rifting processes for which two fundamental mechanisms can be postulated: an active mechanism, whereby thermal energy is transmitted into the lithosphere from the underlying asthenosphere, and a passive mechanism by which mechanical energy is transmitted laterally through the lithosphere as a consequence of plate interactions at a distance. In order to permit the concept of the two fundamentally different mechanisms to be tested, a tentative classification is proposed that divides rifts into two basic categories: active rifting and passive rifting. Here, the magnitude of active rifting will depend on the rate at which lithosphere moves over the thermal source, with rifts being restricted to stationary or slow-moving plates.

  6. Rift Valley Fever Review

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Rift Valley fever (RVF) is a disease of animals and humans that occurs in Africa and the Arabian Peninsula. A Phlebovirus in the family Bunyaviridae causes the disease that is transmitted by mosquitoes. Epidemics occur during years of unusually heavy rainfall that assessment models are being develo...

  7. Rift-flank uplift and rift dynamics, a new perspective

    NASA Astrophysics Data System (ADS)

    Koehn, D.; Sachau, T.; Aanyu, K.

    2009-04-01

    In this contribution we present a new model of passive rifting and related rift-flank uplift. The numerical model is based on a lattice spring network coupled with a viscous particle model so that we can simulate visco-elasto-plastic behaviour with dynamic fault development. In our model we show that rift flank uplift can be achieved best when extension in the crust is localized and the lower crust is strong so that major rift faults transsect the whole crust. Uplift of rift flanks follows a smooth function whereas down-throw in the rift basin takes place in steps. The geometry of the developing faults has also an influence on the uplift, in this case displacement along major rift faults produces higher flanks than distributed displacement on many faults. Our model also shows that the relative elastic thickness of the crust has only a minor influence on the uplift since fault depth and elastic thickness are not independent. In addition we show with a second set of simulations and analytically that a strain misfit between the upper and lower boundaries of a stretched crust leads to an active uplift driven by elastic forces. We compare the numerical simulations, the analytical solution and real surface data from the Albertine rift in the East African Rift System and show that our new model can reproduce realistic features. Our two-layer beam model with strain misfit can also explain why a thick crust in the simulations can have an even higher rift flank than a thin crust even though the thin crust topography has a higher curvature. We discuss the implications of our simulations for real rift systems and for the current theory of rift flank uplift.

  8. East African Rift Valley, Kenya

    NASA Technical Reports Server (NTRS)

    1990-01-01

    This rare, cloud free view of the East African Rift Valley, Kenya (1.5N, 35.5E) shows a clear view of the Turkwell River Valley, an offshoot of the African REift System. The East African Rift is part of a vast plate fracture which extends from southern Turkey, through the Red Sea, East Africa and into Mozambique. Dark green patches of forests are seen along the rift margin and tea plantations occupy the cooler higher ground.

  9. Rifts in Spreading Wax Layers

    E-print Network

    Rolf Ragnarsson; J. Lewis Ford; Christian D. Santangelo; Eberhard Bodenschatz

    1995-10-19

    We report experimental results on the rift formation between two freezing wax plates. The plates were pulled apart with constant velocity, while floating on the melt, in a way akin to the tectonic plates of the earth's crust. At slow spreading rates, a rift, initially perpendicular to the spreading direction, was found to be stable, while above a critical spreading rate a "spiky" rift with fracture zones almost parallel to the spreading direction developed. At yet higher spreading rates a second transition from the spiky rift to a zig-zag pattern occurred. In this regime the rift can be characterized by a single angle which was found to be dependent on the spreading rate. We show that the oblique spreading angles agree with a simple geometrical model. The coarsening of the zig-zag pattern over time and the three-dimensional structure of the solidified crust are also discussed.

  10. Rio Grande rift: An overview

    NASA Astrophysics Data System (ADS)

    Olsen, Kenneth H.; Scott Baldridge, W.; Callender, Jonathan F.

    1987-11-01

    The Rio Grande rift of the southwestern United States is one of the world's principal continental rift systems. It extends as a series of asymmetrical grabens from central Colorado, through New Mexico, to Presidio, Texas, and Chihuahua, Mexico—a distance of more than 1000 km. Although the Rio Grande rift is closely related in timing and structural style to the contiguous Basin and Range extensional province, the two can be distinguished by a variety of geological and geophysical signatures. Rifts (both oceanic and continental) can be defined as elongate depressions overlying places where the entire lithosphere has ruptured in extension. The lithosphere of the Rio Grande rift conforms to this definition, in that: (1) the crust is moderately thinned—Moho depths range from about 45 km under the flanks to about 33 km beneath the rift axis. (2) anomalously low P n velocities (7.6-7.8 km s -1) beneath the rift and a long wavelength gravity low suggest that the asthenosphere is in contact with the base of the crust. The P-velocity is abnormally low (6.4-6.5 km s -1) in the lower half of the crust beneath the rift, suggesting high crustal temperatures. However, associated seismic and volcanologic data indicate the sub-rift lower crust is not dominated by a massive composite mafic intrusion such as is sometimes inferred for the East African rifts. Seismic and magnetotelluric data suggest the presence of a thin (< 1 km) sill-like contemporary midcrustal magma body which may perhaps extend intermittently along much of the length of the rift. Seismic and structural studies indicate a dominant horizontal fabric in the upper and middle crust. The brittle-ductile transition is at depths -15 km except for the major volcanic fields, where it rises to 2-3 km. Structural development of the rift occurred mainly during two time intervals: the early phase beginning at -30 Ma. and lasting 10-12 m.y., and the late phase extending from -10 to 3 Ma. The early phase involved extensive low-angle normal faulting throughout the rift region which was subsequently offset by high-angle normal faulting during the later deformational event. Volcanism of the Rio Grande rift is minor compared to some other continental rifts. Most of the volcanism is basaltic and occurred less than about 5 m.y. ago. Compositions range from alkalic to tholeiitic, with no unique spatial or temporal pattern. Magmas were probably derived from a variety of depths, indicating an unintegrated heat source with only local melting. Basaltic andesites and related calc-alkaline rocks erupted in the southern rift between about 30 and 18 m.y. ago were not uniquely related to the rifting process. Rather, the thermal pulse which generated these magmas was part of the previous, subduction-related event. Our interpretation of existing data concerning the evolution of the Rio Grande rift does not fit either simple active or passive "end-member" models. In particular, there is no compelling evidence for a major thermal event in the mantle uniquely associated with rifting. Yet heat—inherited from the immediately-preceding deformational regime—was certainly a critical factor in, and was probably a necessary condition for, rifting.

  11. Cenozoic rifting in the West Antarctic Rift System

    NASA Astrophysics Data System (ADS)

    Granot, R.; Cande, S. S.; Stock, J. M.; Clayton, R. W.; Davey, F. J.

    2004-12-01

    The West Antarctic Rift System (WARS) experienced two episodes of Cenozoic rifting. Seafloor spreading at the Adare spreading axis, north of the Ross Sea, from Middle Eocene to Late Oligocene time (43 - 26 Ma), was directly linked with motions within the WARS. For this time interval, marine magnetic anomalies within the Adare Basin and structural features within the Ross Sea constrain the motion between East and West Antarctica. During this episode, widespread intrusive activity took place in the continental part of the rift. Subsequent Late Oligocene until present-day (26 - 0 Ma) extension was characterized by a transition to volcanic activity. Yet, the details of extension during this episode have been poorly resolved. We present preliminary results of new seismic reflection and seafloor mapping data acquired on geophysical cruise 07-01 aboard the R/VIB Nathaniel Palmer in the northern part of the rift. Our results suggest that the style of deformation changed from spreading-related faulting into diffuse normal faulting (tilted blocks) that trend NE-SW with little resultant E-W extension. Recent volcanism is distributed throughout but tends to align with the NE-SW trend, into a localized zone. Formation of the Terror Rift, Ross Sea, within the same time frame suggests that the pole of rotation has changed its position, reflecting a change in the relative magnitudes of tensile stresses along the rift. Moreover, this change was accompanied with a sharp decrease of extension rates.

  12. Cenozoic rifting in the West Antarctic Rift System

    NASA Astrophysics Data System (ADS)

    Granot, R.; Cande, S. S.; Stock, J. M.; Clayton, R. W.; Davey, F. J.

    2007-12-01

    The West Antarctic Rift System (WARS) experienced two episodes of Cenozoic rifting. Seafloor spreading at the Adare spreading axis, north of the Ross Sea, from Middle Eocene to Late Oligocene time (43 - 26 Ma), was directly linked with motions within the WARS. For this time interval, marine magnetic anomalies within the Adare Basin and structural features within the Ross Sea constrain the motion between East and West Antarctica. During this episode, widespread intrusive activity took place in the continental part of the rift. Subsequent Late Oligocene until present-day (26 - 0 Ma) extension was characterized by a transition to volcanic activity. Yet, the details of extension during this episode have been poorly resolved. We present preliminary results of new seismic reflection and seafloor mapping data acquired on geophysical cruise 07-01 aboard the R/VIB Nathaniel Palmer in the northern part of the rift. Our results suggest that the style of deformation changed from spreading-related faulting into diffuse normal faulting (tilted blocks) that trend NE-SW with little resultant E-W extension. Recent volcanism is distributed throughout but tends to align with the NE-SW trend, into a localized zone. Formation of the Terror Rift, Ross Sea, within the same time frame suggests that the pole of rotation has changed its position, reflecting a change in the relative magnitudes of tensile stresses along the rift. Moreover, this change was accompanied with a sharp decrease of extension rates.

  13. Rift basins - Origin, history, and distribution

    NASA Technical Reports Server (NTRS)

    Burke, K. C.

    1985-01-01

    Rifts are elongate depressions overlying places where the lithosphere has ruptured in extension. Where filled with sediment they may contain exploitable quantities of oil and gas. Because rits form in a variety of tectonic settings, it is helpful to define the particular tectonic environment in which a specific rift or set of rifts has developed. A useful approach has been to relate that environment to the Wilson Cycle of the opening and the closing of oceans. This appreciation of tectonic setting can help in better understanding of the depositional, structural and thermal history of individual rift systems. The global distribution of rifts can also be related to tectonic environment. For example, rifts associated with continental rupture at a temporary still-stand of a continent over the mantle convective system (rifts like those active in East Africa today) can be distinguished from those associated with continental collision (rifts like the Cenozoic rifts of China).

  14. Rift Stability and Localization in Devana Chasma, Venus

    NASA Astrophysics Data System (ADS)

    Martone, A. A.; Montési, L. G. J.

    2015-05-01

    The rift zone of Devana Chasma greatly resembles Earth's rift zones, especially the East African Rift, despite the lack of global plate tectonics. The stability of rifts on Venus will be characterized, and will include lithosphere weakening processes.

  15. Continental rift evolution: From rift initiation to incipient break-up in the Main Ethiopian Rift, East Africa

    NASA Astrophysics Data System (ADS)

    Corti, Giacomo

    2009-09-01

    The Main Ethiopian Rift is a key sector of the East African Rift System that connects the Afar depression, at Red Sea-Gulf of Aden junction, with the Turkana depression and Kenya Rift to the South. It is a magmatic rift that records all the different stages of rift evolution from rift initiation to break-up and incipient oceanic spreading: it is thus an ideal place to analyse the evolution of continental extension, the rupture of lithospheric plates and the dynamics by which distributed continental deformation is progressively focused at oceanic spreading centres. The first tectono-magmatic event related to the Tertiary rifting was the eruption of voluminous flood basalts that apparently occurred in a rather short time interval at around 30 Ma; strong plateau uplift, which resulted in the development of the Ethiopian and Somalian plateaus now surrounding the rift valley, has been suggested to have initiated contemporaneously or shortly after the extensive flood-basalt volcanism, although its exact timing remains controversial. Voluminous volcanism and uplift started prior to the main rifting phases, suggesting a mantle plume influence on the Tertiary deformation in East Africa. Different plume hypothesis have been suggested, with recent models indicating the existence of deep superplume originating at the core-mantle boundary beneath southern Africa, rising in a north-northeastward direction toward eastern Africa, and feeding multiple plume stems in the upper mantle. However, the existence of this whole-mantle feature and its possible connection with Tertiary rifting are highly debated. The main rifting phases started diachronously along the MER in the Mio-Pliocene; rift propagation was not a smooth process but rather a process with punctuated episodes of extension and relative quiescence. Rift location was most probably controlled by the reactivation of a lithospheric-scale pre-Cambrian weakness; the orientation of this weakness (roughly NE-SW) and the Late Pliocene (post 3.2 Ma)-recent extensional stress field generated by relative motion between Nubia and Somalia plates (roughly ESE-WNW) suggest that oblique rifting conditions have controlled rift evolution. However, it is still unclear if these kinematical boundary conditions have remained steady since the initial stages of rifting or the kinematics has changed during the Late Pliocene or at the Pliocene-Pleistocene boundary. Analysis of geological-geophysical data suggests that continental rifting in the MER evolved in two different phases. An early (Mio-Pliocene) continental rifting stage was characterised by displacement along large boundary faults, subsidence of rift depression with local development of deep (up to 5 km) asymmetric basins and diffuse magmatic activity. In this initial phase, magmatism encompassed the whole rift, with volcanic activity affecting the rift depression, the major boundary faults and limited portions of the rift shoulders (off-axis volcanism). Progressive extension led to the second (Pleistocene) rifting stage, characterised by a riftward narrowing of the volcano-tectonic activity. In this phase, the main boundary faults were deactivated and extensional deformation was accommodated by dense swarms of faults (Wonji segments) in the thinned rift depression. The progressive thinning of the continental lithosphere under constant, prolonged oblique rifting conditions controlled this migration of deformation, possibly in tandem with the weakening related to magmatic processes and/or a change in rift kinematics. Owing to the oblique rifting conditions, the fault swarms obliquely cut the rift floor and were characterised by a typical right-stepping arrangement. Ascending magmas were focused by the Wonji segments, with eruption of magmas at surface preferentially occurring along the oblique faults. As soon as the volcano-tectonic activity was localised within Wonji segments, a strong feedback between deformation and magmatism developed: the thinned lithosphere was strongly modified by the extensive magma intrusion and extension was facilitated

  16. Cenozoic rift formation in the northern Caribbean

    NASA Technical Reports Server (NTRS)

    Mann, P.; Burke, K.

    1984-01-01

    Rifts form in many different tectonic environments where the lithosphere is put into extension. An outline is provided of the distribution, orientation, and relative ages of 16 Cenozoic rifts along the northern edge of the Caribbean plate and it is suggested that these structures formed successively by localized extension as the Caribbean plate moved eastward past a continental promontory of North America. Evidence leading to this conclusion includes (1) recognition that the rifts become progressively younger westward; (2) a two-phase subsidence history in a rift exposed by upthrusting in Jamaica; (3) the absence of rifts east of Jamaica; and (4) the observation that removal of 1400 km of strike-slip displacement on the Cayman Trough fault system places the Paleogene rifts of Jamaica in an active area of extension south of Yucatan where the rifts of Honduras and Guatemala are forming today.

  17. Parga Chasma: Coronae and Rifting on Venus

    NASA Technical Reports Server (NTRS)

    Smrekar, S. E.; Stofan, E. R.; Buck, W. R.; Martin, P.

    2005-01-01

    The majority of coronae (quasicircular volcano-tectonic features) are found along rifts or fracture belts, and the majority of rifts have coronae [e.g. 1,2]. However, the relationship between coronae and rifts remains unclear [3-6]. There is evidence that coronae can form before, after, or synchronously with rifts [3,4]. The extensional fractures in the rift zones have been proposed to be a result of broad scale upwelling and traction on the lower lithosphere [7]. However, not all rift systems have a significant positive geoid anomaly, as would be expected for an upwelling site [8]. This could be explained if the rifts lacking anomalies are no longer active. Coronae are generally accepted to be sites of local upwelling [e.g. 1], but the observed rifting is frequently not radial to the coronae and extends well beyond the coronae into the surrounding plains. Thus the question remains as to whether the rifts represent regional extension, perhaps driven by mantle tractions, or if the coronae themselves create local thinning and extension of the lithosphere. In the first case, a regional extension model should be consistent with the observed characteristics of the rifts. In the latter case, a model of lithospheric loading and fracturing would be more appropriate. A good analogy may be the propagation of oceanic intraplate volcanoes [9].

  18. Continental rift zones without Moho uplift

    NASA Astrophysics Data System (ADS)

    Thybo, H.; Lyngsie, S.; Nielsen, C.

    2008-05-01

    Rifting is a fundamental plate tectonic process that creates elongated depressions in the Earth's surface, which become filled with sedimentary and volcanic material, as it is presently observed at the Baikal, East African, Rhine Graben and Rio Grande Rift Zones. All rifting models predict Moho uplift due to crustal thinning, and reduced seismic velocity in the uppermost mantle due to decompression or heating from the Earth's interior. However, recently acquired data from the presently active Baikal Rift zone in Siberia and the failed Dniepr-Donets rift zone in Ukraine are examples where there is no Moho topography that can related to the rifting process. Further, data from the Kenya Rift Zone shows sign of less Moho uplift than expected from the actual extension. At all these rift zones, we observe a localized zone in the lower crust which has exceptionally high seismic velocity and is highly reflective. We suggest that rift related crustal thinning took place, but the expected Moho up-warp was compensated by magmatic intrusion in the lower crust at the high-velocity zone. This finding has significant implications for modelling of the evolution of sedimentary basins around rift structures.

  19. Magmatism in rifting and basin formation

    NASA Astrophysics Data System (ADS)

    Thybo, H.

    2008-12-01

    Whether heating and magmatism cause rifting or rifting processes cause magmatic activity is highly debated. The stretching factor in rift zones can be estimated as the relation between the initial and the final crustal thickness provided that the magmatic addition to the crust is insignificant. Recent research demonstrates substantial magmatic intrusion into the crust in the form of sill like structures in the lowest crust in the presently active Kenya and Baikal rift zones and the DonBas palaeo-rift zone in Ukraine. This result may be surprising as the Kenya Rift is associated with large amounts of volcanic products, whereas the Baikal Rift shows very little volcanism. Identification of large amounts of magmatic intrusion into the crust has strong implications for estimation of stretching factor, which in the case of Baikal Rift Zone is around 1.7 but direct estimation gives a value of 1.3-1.4 if the magmatic addition is not taken into account. This may indicate that much more stretching has taken place on rift systems than hitherto believed. Wide sedimentary basins may form around aborted rifts due to loading of the lithosphere by sedimentary and volcanic in-fill of the rift. This type of subsidence will create wide basins without faulting. The Norwegian- Danish basin in the North Sea area also has subsided gradually during the Triassic without faulting, but only few rift structures have been identified below the Triassic sequences. We have identified several mafic intrusions in the form of large batholiths, typically more than 100 km long, 20-40 km wide and 20 km thick. The associated heating would have lifted the surface by about 2 km, which may have been eroded before cooling. The subsequent contraction due to solidification and cooling would create subsidence in a geometry similar to basins that developed by loading. These new aspects of magmatism will be discussed with regard to rifting and basin formation.

  20. Continental Rifts: Evolution, Structure and Tectonics

    NASA Astrophysics Data System (ADS)

    Bally, A. W.

    Twenty one “friends of continental rifts” wrote Continental Rifts: Evolution, Structureand Tectonics. They define the object of their passion as elongate tectonic depressions along which the entire lithosphere has been modified by extension. Strictly speaking, passive margins and highly extended terranes such as the Basin and Range are not included in this definition, but the authors consider them to be related to continental rifts. The authors hail from academia and set as their main goal “an improved understanding of the fundamental lithospheric processes of rifting, with primary focus on deep structures and processes associated with rifting.” Consequently, many well-known extensional systems, for example, the North Sea grabens, the Suez Basin, onshore and offshore eastern China, and large areas of southeast Asia, are barely considered. Rift aficionados from the petroleum industry will find very little to interest them in this book.

  1. Control of rift obliquity on the evolution and segmentation of the main Ethiopian rift

    NASA Astrophysics Data System (ADS)

    Corti, Giacomo

    2008-04-01

    The main Ethiopian rift is an active rift in the break-up stage, and it marks the incipient boundary between the Nubia and Somalia plates. Rifting started with the activation of large boundary faults and diffuse volcanism, followed by focused magmatism and faulting in the rift floor, along step-like segments oblique to the rift axis that now act as a protoridge for future seafloor spreading. This concentration of volcano-tectonic activity has been thought to be either magma assisted or controlled by a change in rift kinematics, with a late oblique rifting phase that would have caused the development of the step-like fault segments that focused magma upwelling. Geodetic, seismic and stress-field data confirm current oblique rifting kinematics, but plate kinematics models do not predict a change in Nubia-Somalia motion in the past 11 million years. Here, I use lithospheric-scale analogue models of oblique rifting to analyse the development of the main Ethiopian rift. I find that neither magma weakening nor a change in plate kinematics are required to simulate a two-phase evolution with successive activation of differently oriented fault systems. I conclude that rift evolution and segmentation are controlled by rift obliquity, independent of magmatic processes.

  2. Rift reactivation and migration during multiphase extension

    NASA Astrophysics Data System (ADS)

    Naliboff, John; Buiter, Susanne J. H.

    2015-07-01

    Passive margins may undergo multiple phases of extension with distinct structural, petrological and sedimentary processes before achieving breakup. Observations of rift axis migration through time may reflect cooling, hardening and subsequent abandonment of the rift axis during either long-term periods of slow extension or periods of tectonic quiescence. Here, we use 2D thermo-mechanical numerical models to examine rift reactivation and migration during multiphase extension where a period of tectonic quiescence separates phases of extension. Our goals are to identify the rheological mechanism(s) controlling rift reactivation versus migration and determine if cooling phases may help explain recent interpretations of passive margin architecture and evolution. Our numerical experiments indicate that the relative integrated brittle strength between the initial rift and surrounding regions, rather than the total integrated strength, largely controls rift reactivation versus migration. The tectonic quiescence (cooling) duration required to induce rift migration ranges between 20 and 60 Myr (minimum bounds). This range reflects variations in extension velocity, magnitude of shear zone healing, crustal rheology and asthenospheric rheology. Reactivated rifts after extensive (>20 Myr) cooling periods in some cases develop asymmetric margins with deformation patterns stepping toward the future rift, such as characterizing most of the Atlantic conjugate margins.

  3. The origin of along-rift variations in faulting and magmatism in the Ethiopian Rift

    NASA Astrophysics Data System (ADS)

    Keir, Derek; Bastow, Ian D.; Corti, Giacomo; Mazzarini, Francesco; Rooney, Tyrone O.

    2015-03-01

    The geological record at rifts and margins worldwide often reveals considerable along-strike variations in volumes of extruded and intruded igneous rocks. These variations may be the result of asthenospheric heterogeneity, variations in rate, and timing of extension; alternatively, preexisting plate architecture and/or the evolving kinematics of extension during breakup may exert first-order control on magmatism. The Main Ethiopian Rift (MER) in East Africa provides an excellent opportunity to address this dichotomy: it exposes, along strike, several sectors of asynchronous rift development from continental rifting in the south to incipient oceanic spreading in the north. Here we perform studies of volcanic cone density and rift obliquity along strike in the MER. By synthesizing these new data in light of existing geophysical, geochemical, and petrological constraints on magma generation and emplacement, we are able to discriminate between tectonic and mantle geodynamic controls on the geological record of a newly forming magmatic rifted margin. The timing of rift sector development, the three-dimensional focusing of melt, and the ponding of plume material where the rift dramatically narrows each influence igneous intrusion and volcanism along the MER. However, rifting obliquity plays an important role in localizing intrusion into the crust beneath en echelon volcanic segments. Along-strike variations in volumes and types of igneous rocks found at rifted margins thus likely carry information about the development of strain during rifting, as well as the physical state of the convecting mantle at the time of breakup.

  4. Rio Grande rift: problems and perspectives

    SciTech Connect

    Baldridge, W.S.; Olsen, K.H.; Callender, J.F.

    1984-01-01

    Topics and ideas addressed include: (1) the regional extent of the Rio Grande rift; (2) the structure of the crust and upper mantle; (3) whether the evidence for an axile dike in the lower crust is compelling; (4) the nature of faulting and extension in the crust; and (5) the structural and magmatic development of the rift. 88 references, 5 figures.

  5. Elliptical caldera formation throughout the Kenyan Rift

    NASA Astrophysics Data System (ADS)

    Robertson, Elspeth; Biggs, Juliet; Cashman, Katharine; Floyd, Michael; Vye-Brown, Charlotte

    2014-05-01

    Many of the world's calderas are elliptical in shape, and their orientation is often used as a proxy for the local stress regime. However, in some rift settings, pre-existing structural trends have been shown to control caldera orientation. We test these competing hypotheses in the Kenyan Rift, which consists of two distinct rift segments, with different tectonic and magmatic characteristics. Of the fourteen Quaternary volcanoes lying along the central rift axis, seven have undergone caldera collapse and six are highly elliptical. We present a remote-sensing study that investigates the structural and tectonic control on caldera ellipticity and orientation within the Kenyan Rift. Satellite-based mapping using ArcGIS on imagery derived from ASTER and GDEM data to identify the orientations of the main East African Rift border faults, intra-rift faults and the geometry of Kenyan calderas to determine the extensional setting, horizontal compressive stress orientations and the pre-existing rift fabric direction. Other data sources included the GPS-derived plate-kinematic model of East Africa and information from the literature. We find that deformation in the Kenyan Rift is characterised by orthogonal extension in the north and oblique opening in the south, suggesting that both tectonic stresses and magmatic pressures drive intra-rift fault formation. The long axis elongation of calderas are orientated NW-SE in the north, aligned with pre-existing structures and perpendicular to recent rift-faults. In contrast, the long axes are aligned NE-SW in the southern group of volcanoes, at an angle which is highly oblique to the recent rift faults, but aligned with pre-existing structures. Thus we conclude that in oblique continental rifts, pre-existing structures play a dominant role in the rise of magma through the crust. Understanding the geometry of caldera systems gives us important information as to the structural controls on magmatic and tectonic behaviour in extensional settings and the mechanisms by which continental rifts evolve from fault-controlled basins into mid-ocean ridges.

  6. Pre-rift basement structure and syn-rift faulting at the eastern onshore Gulf of Corinth Rift

    NASA Astrophysics Data System (ADS)

    Kranis, Haralambos; Skourtsos, Emmanuel; Gawthorpe, Robert; Leeder, Mike; Stamatakis, Michael

    2015-04-01

    %B We present results of recent field-based research with a view to providing information about and constraints on the initiation and evolution of the Gulf of Corinth (GoC) Rift. The onshore geology and structure of the GoC rift has been studied intensively and extensively; however most research efforts have focused on the western and partly the central parts. The last few years, efforts are being made to extend the scope of research in less-studied areas, such as the eastern southern onshore part of the GoC rift, trying to address two major issues in rift initiation and evolution, namely syn-rift faulting and pre-rift basement structure. While fault spacing and length appears to be well-constrained for the western and central parts of the GoC Rift, further east -and especially in the uplifted onshore southern part- this is thought to increase dramatically, as there are practically no mapped faults. We argue, however, that this may be a false image, owing to (i) the difficulty in identifying fault structures within a thick, fairly monotonous syn-rift sequence; (ii) the lesser attention this part has drawn; and (ii) the fact that the published summary geological and tectonic maps of the GoC area are based on the dated geological maps that cover the eastern and northern onshore shoulders of the Rift. Moreover, new field data provide new information on pre-rift structure: while only the topmost thrust sheet of the Hellenide nappe stack (Pindos Unit) was thought to crop out at the eastern southern onshore part, we mapped the underlying, non-metamorphic carbonate Unit (Tripolis Unit), which crops out within the footwall of a key intra-basin block (Xylokastro block). A minor outcrop further east, may also belong to this Unit, providing basement control, in connection with recently published offshore fault data. The mapping of these outcrops, combined with a revised stratigraphical framework for the early syn-rift deposits, allows the identification and mapping of faults within this less studied area. Regarding the pre-rift basement structure, not only the known, or inferred, pre-rift heterogeneities along rift axis, but also ongoing lithospheric processes affect the evolution of the GoC Rift. The suggestion that the exposure of the deeper metamorphic Units at the southern border of the rift is related to an earlier (?Miocene) age, thus constituting an inherited structure, is at odds with results from geothermochronological data from the southern Peloponnesus and Kythira, which suggest that these Units lay at a depth of at least 4 km in the Middle ((?)Late) Miocene. This means that the metamorphic outcrops at the southern border of the GoC Rift are the product of a later (possibly Quaternary) process of uplift, probably resulting from localized N-S culmination, whose locus is in the central Peloponnesus. Moreover, current (and Upper Quaternary) uplift rates along the southern shoulder of the GoC may also be related to and/or reflect the uplift of the nappe pile including the lower, metamorphic basement Units.

  7. Cenozoic rift tectonics of the Japan Sea

    SciTech Connect

    Kimura, K.

    1988-08-01

    The Japan Sea is one of the back-arc basins in trench-arc systems bordering the western Pacific. Recent paleomagnetic works suggest the Japan Sea opened during early to middle Miocene. Radiometric and microfossil ages of the Cenozoic onland sequences in the Japanese Islands elucidate the rift tectonics of the Japan Sea. The rifting history is summarized as follows: nonmarine volcanic formations of prerift stage before 50 Ma, rift-onset unconformity at 40 Ma, nonmarine volcanic formations of synrift stage 20-33 Ma, breakup unconformity 19 Ma showing the opening of the Japan Sea, marine volcanic and sedimentary formations of synrift stage 14.5-18 Ma, beginning of regional subsidence 14.5 Ma corresponding to the end of the Japan Sea opening, marine sedimentary formations of postdrift stage after 14.5 Ma. Rifting is not limited to the synrift stage but is continued to the syndrift stage. Rifting led to a horst-and-graben structure. Thus, the Cenozoic onland sequences in the Japanese Islands are suited for a study of rift tectonics because the sequences were subaerially exposed by the late Miocene-Holocene island-arc tectonics. Rift tectonics cannot be studied as easily in most Atlantic-type passive margins.

  8. Investigation of rifting processes in the Rio Grande Rift using data from unusually large earthquake swarms

    SciTech Connect

    Sanford, A.; Balch, R.; House, L.; Hartse, H.

    1995-12-01

    San Acacia Swarm in the Rio Grande Rift. Because the Rio Grande rift is one of the best seismically instrumented rift zones in the world, studying its seismicity provides an exceptional opportunity to explore the active tectonic processes within continental rifts. We have been studying earthquake swarms recorded near Socorro in an effort to link seismicity directly to the rifting process. For FY94, our research has focused on the San Acacia swarm, which occurred 25 km north of Socorro, New Mexico, along the accommodation zone between the Albuquerque-Belen and Socorro basins of the central Rio Grande rift. The swarm commenced on 25 February 1983, had a magnitude 4.2 main shock on 2 March and ended on 17 March, 1983.

  9. Geodynamics of the Shanxi Rift system, China

    NASA Astrophysics Data System (ADS)

    Xu, Xiwei; Ma, Xingyuan

    1992-07-01

    The Pliocene-Quaternary Shanxi Rift system of northern China transsects the domal axis of the Shanxi Highlands and is defined by an S-shaped string of asymmetric basins, extending from Huailai-Yanqing in the north via Taiyuan to Yuncheng in the south. This rift system has a length of about 1200 km and a width of 20-80 km; it is characterized by strong earthquakes. Its central, NNE-trending, transtensional segment forms the main element of the Shanxi Rift system. Basin subsidence commenced during the early Pliocene along NNE oriented master-faults characterized by large strike-slip rates (5.68-7 mm/yr); crustal extension amounts to some 1.4 km; basins are separated by push-up blocks; destructive earthquakes ( M = 7-8) are unevenly distributed. The ENE trending northern and southern branches of the Shanxi Rift system are characterized by orthogonal crustal extension along ENE striking normal faults, amounting to about 8 km; these areas are characterized by a basin-and-range type structural style and weak seismic activity. The Shanxi Rift system developed on a some 40 km thick, thermally cool Precambrian crustal segment that was overprinted during Mesozoic diastrophic events. There is only minor rift-related volcanic activity. The Moho is pulled up by a few kilometers only under the axial rift zone; sub-Moho compressional velocities are in the 8.0-8.1 km/s range. The Shanxi Rift system evolved in response to the build-up of regional stress fields related to the collision of India and Eurasia; its localization involved reactivation of pre-existing fracture systems.

  10. The 1974 Ethiopian rift geodimeter survey

    NASA Technical Reports Server (NTRS)

    Mohr, P.

    1977-01-01

    The field techniques and methods of data reduction for five successive geodimeter surveys in the Ethiopian rift valley are enlarged upon, with the considered conclusion that there is progressive accumulation of upper crustal strain, consonant with on-going rift extension. The extension is restricted to the Quaternary volcanotectonic axis of the rift, namely the Wonji fault belt, and is occurring at rates of 3 to 6 mm/yr in the northern sector of the rift valley. Although this concurs with the predictions of platetectonic analysis of the Afar triple junction, it is considered premature to endorse such a concurrence on the basis of only 5 years of observations. This is underlined by the detection of local tectonic contractions and expansions associated with geothermal and gravity anomalies in the central sector of the rift valley. There is a hint of a component of dextral slip along some of the rift-floor fault zones, both from geological evidence and from the strain patterns detected in the present geodetic surveys.

  11. Rift initiation with volatiles and magma

    NASA Astrophysics Data System (ADS)

    Ebinger, Cynthia; Muirhead, James; Roecker, Steve; Tiberi, Christel; Muzuka, Alfred; Ferdinand, Rrichard; Mulibo, Gabrile; Kianji, Gladys

    2015-04-01

    Rift initiation in cratonic lithosphere remains an outstanding problem in continental tectonics, but strain and magmatism patterns in youthful sectors of the East African rift provide new insights. Few teleseisms occur in the Eastern rift arm of the East African rift system, except the southernmost sector in northern Tanzania where extension occurs in Archaean lithosphere. The change in seismic energy release occurs over a narrow along-axis zone, and between sectors with and without volcanoes in the central rift valley. Are these differences in strain behavior indicative of along-strike variations in a) rheology; b) strain transfer from border faults to magma intrusion zones; c) dike vs fault slip; and/or d) shallow vs deep magma chambers? We present time-space relations of seismicity recorded on a 38-station array spanning the Kenya-Tanzania border, focal mechanisms for the largest events during those time periods, and compare these to longer-term strain patterns. Lower crustal seismicity occurs along the rift length, including sectors on and off craton, and those with and without central rift valley volcanoes, and we see no clear along-strike variation in seismogenic layer thickness. One explanation for widespread lower crustal seismicity is high gas pressures and volatile migration from active metasomatism of upper mantle and magma degassing, consistent with very high volatile flux along fault zones, and widespread metasomatism of xenoliths. Volatile release and migration may be critical to strength reduction of initially cold, strong cratonic lithosphere. Seismicity patterns indicate strain (and fluid?) transfer from the Manyara border fault to Gelai shield volcano (faulting, diking) via Oldoinyo Lengai volcano. Our focal mechanisms and Global CMTs from an intense fault-dike episode (2007) show a local, temporally stable, rotation from ~E-W extension to NE-SE extension in this linkage zone, consistent with longer term patterns recorded in vent and eruptive chain alignments.

  12. Tectono-Sedimentary Analysis of Rift Basins: Insights from the Corinth Rift, Greece

    NASA Astrophysics Data System (ADS)

    Gawthorpe, Robert; Ford, Mary

    2015-04-01

    Existing models for the tectono-sedimentary evolution of rift basins are strongly linked the growth and linkage of normal fault segments and localization of fault activity. Early stages of faulting (rift initiation phase) are characterized by distributed, short, low displacement fault segments, subdued fault-related topography and small depocentres within which sedimentation keeps pace with subsidence. Following linkage and displacement localization (rift climax phase), deformation if focused onto major, crustal-scale fault zones with kilometre-scale displacement. These major faults generate pronounced tilted fault-block topography, with subsidence rates that outpace sedimentation causing a pronounced change to deep-water deposition. Such models have been successful in helping to understand the gross structural and sedimentary evolution of rift basins, but recent work has suggested that pre-existing structures, normal fault interaction with pre-rift salt and antecedent drainage systems significantly alter this initiation-to-climax perspective of rift basin development. The E-W-striking, Pliocene-Pleistocene Corinth rift, central Greece, is an excellent natural laboratory for studying the tectono-sedimentary evolution of rift basins due to its young age, excellent onshore exposure of syn-rift structure and stratigraphy and extensive offshore seismic data. The rift cuts across the NW-SE-striking Hellenide mountain belt and has migrated northward and westward during its evolution. The Hellenide mountain belt significantly influences topography and drainage in the west of the rift. High topography and large antecedent drainage systems, focused along palaeovalleys, provided high sediment flux to NE-flowing alluvial systems that overfilled early-rift depocentres. Further east, away from the main antecedent drainage networks, contemporaneous deposits comprise deep-lacustrine turbidite channel and lobe complexes and basinal marls. Thus the stratigraphic expression within the Pliocene rift fill is similar to rift initiation in high sediment flux locations in the west and rift climax in low sediment flux locations in the east. Major shifts in the locus of fault activity within the Corinth Rift further complicate tectono-stratigraphy analysis of its basin fill. Pliocene depocentres are largely located onshore, south of the present-day Gulf of Corinth and involved activity that was distributed among north- and south-dipping faults. A northward shift in the southern rift margin in the early Pleistocene, established the present-day Gulf of Corinth as the site of several main depocentres and caused abandonment, uplift and reworking of a large portion of the Pliocene rift. Changes in the locus of fault activity during the Pleistocene record a change from activity on north- and south-dipping faults to mainly north-dipping faults. Such shifts in fault activity have a profound effect on the basin fill, with new footwall areas subject to subaerial exposure and incision while contemporaneous hangingwall depocentres undergo rapid subsidence and drowning. Such local complexity is not surprising, but factors such as major antecedent sediment transport pathways and marked temporal and spatial shifts in fault activity make application of conventional tectono-sedimentary subdivsions of pre-, syn-, and post-rift difficult to apply at the basin-scale.

  13. Variation in styles of rifting in the Gulf of California.

    PubMed

    Lizarralde, Daniel; Axen, Gary J; Brown, Hillary E; Fletcher, John M; González-Fernández, Antonio; Harding, Alistair J; Holbrook, W Steven; Kent, Graham M; Paramo, Pedro; Sutherland, Fiona; Umhoefer, Paul J

    2007-07-26

    Constraints on the structure of rifted continental margins and the magmatism resulting from such rifting can help refine our understanding of the strength of the lithosphere, the state of the underlying mantle and the transition from rifting to seafloor spreading. An important structural classification of rifts is by width, with narrow rifts thought to form as necking instabilities (where extension rates outpace thermal diffusion) and wide rifts thought to require a mechanism to inhibit localization, such as lower-crustal flow in high heat-flow settings. Observations of the magmatism that results from rifting range from volcanic margins with two to three times the magmatism predicted from melting models to non-volcanic margins with almost no rift or post-rift magmatism. Such variations in magmatic activity are commonly attributed to variations in mantle temperature. Here we describe results from the PESCADOR seismic experiment in the southern Gulf of California and present crustal-scale images across three rift segments. Over short lateral distances, we observe large differences in rifting style and magmatism--from wide rifting with minor synchronous magmatism to narrow rifting in magmatically robust segments. But many of the factors believed to control structural evolution and magmatism during rifting (extension rate, mantle potential temperature and heat flow) tend to vary over larger length scales. We conclude instead that mantle depletion, rather than low mantle temperature, accounts for the observed wide, magma-poor margins, and that mantle fertility and possibly sedimentary insulation, rather than high mantle temperature, account for the observed robust rift and post-rift magmatism. PMID:17653189

  14. Rifting Attractor Structures in the Baikal Rift System: Location and Effects

    NASA Astrophysics Data System (ADS)

    Klyuchevskii, Anatoly V.

    2014-07-01

    The current geodynamics and tectonophysics of the Baikal rift system (BRS) as recorded in lithospheric stress and strain are discussed in the context of self organization of nonlinear dissipative dynamic systems and nonlinear media. The regional strain field inferred from instrumental seismic moment and fault radius data for almost 70,000 MLH ? 2.0 events of 1968 through 1994 shows a complex pattern with zones of high strain anisotropy in the central part and both flanks of the rift system (the South Baikal, Hovsgöl, and Muya rift basins, respectively). The three zones of local strain anisotropy highs coincide with domains of predominantly vertical stress where earthquakes of different magnitudes are mostly of normal slip geometry. Pulse-like reversals of principal stresses in the high-strain domains appear to be nonlinear responses of the system to subcrustal processes. In this respect, the BRS lithosphere is interpreted in terms of the self organization theory as a geological dissipative system. Correspondingly, the domains of high strain anisotropy and stress change, called rifting attractor structures (RAS), are the driving forces of its evolution. The location and nonlinear dynamics of the rifting attractors have controlled lithospheric stress and strain of the rift system over the period of observations, and the same scenario may have been valid also in the Mesozoic-Cenozoic rifting history. The suggested model of a positive-feedback (fire-like) evolution of nonlinear dynamical systems with rifting attractors opens a new perspective on the current geodynamics and tectonophysics of the Baikal rift system.

  15. Earthquake clusters in Corinth Rift

    NASA Astrophysics Data System (ADS)

    Mesimeri, Maria; Papadimitriou, Eleftheria; Karakostas, Vasilios; Tsaklidis, George

    2013-04-01

    Clusters commonly occur as main shock-aftershock (MS-AS) sequences but also as earthquake swarms, which are empirically defined as an increase in seismicity rate above the background rate without a clear triggering main shock earthquake. Earthquake swarms occur in a variety of different environments and might have a diversity of origins, characterized by a high b-value in their magnitude distribution. The Corinth Rift, which was selected as our target area, appears to be the most recent extensional structure, with a likely rate of fault slip of about 1cm/yr and opening of 7mm/yr. High seismic activity accommodates the active deformation with frequent strong (M?6.0) events and several seismic excitations without a main shock with clearly discriminative magnitude. Identification of earthquake clusters that occurred in this area in last years and investigation of their spatio-temporal distribution is attempted, with the application of known declustering algorithms, aiming to associate their occurrence with certain patterns in seismicity behavior. The earthquake catalog of the National Hellenic Seismological Network is used, and a certain number of clusters were extracted from the dataset, with the MS-AS sequences being distinguished from earthquake swarms. Spatio-temporal properties of each subset were analyzed in detail, after determining the respective completeness magnitude. This work was supported in part by the THALES Program of the Ministry of Education of Greece and the European Union in the framework of the project entitled "Integrated understanding of Seismicity, using innovative Methodologies of Fracture mechanics along with Earthquake and non-extensive statistical physics - Application to the geodynamic system of the Hellenic Arc, SEISMO FEAR HELLARC".

  16. Thermomechanical models of the Rio Grande rift

    SciTech Connect

    Bridwell, R.J.; Anderson, C.A.

    1980-01-01

    Fully two-dimensional, coupled thermochemical solutions of a continental rift and platform are used to model the crust and mantle structure of a hot, buoyant mantle diapir beneath the Rio Grande rift. The thermomechanical model includes both linear and nonlinear laws of the Weertman type relating shear stress and creep strain rate, viscosity which depends on temperature and pressure, and activation energy, temperature-dependent thermal conductivity, temperature-dependent coefficient of thermal expansion, the Boussinesq approximation for thermal bouyancy, material convection using a stress rate that is invariant to rigid rotations, an elastically deformable crust, and a free surface. The model determines the free surface velocities, solid state flow field in the mantle, and viscosity structure of lithosphere and asthenosphere. Regional topography and crustal heat flow are simulated. A suite of symmetric models, assumes continental geotherms on the right and the successively increasing rift geotherms on the left. These models predict an asthenospheric flow field which transfers cold material laterally toward the rift at > 300 km, hot, buoyant material approx. 200 km wide which ascends vertically at rates of 1 km/my between 175 to 325 km, and spreads laterally away from the rift at the base of the lithosphere. Crustal spreading rates are similar to uplift rates. The lithosphere acts as stiff, elastic cap, damping upward motion through decreased velocities of 1 km/10 my and spreading uplift laterally. A parameter study varying material coefficients for the Weertman flow law suggests asthenospheric viscosities of approx. 10/sup 22/ to 10/sup 23/ poise. Similar studies predict crustal viscosities of approx. 10/sup 25/ poise. The buoyant process of mantle flow narrows and concentrates heat transport beneath the rift, increases upward velocity, and broadly arches the lithosphere. 10 figures, 1 table.

  17. Formation and deformation of hyperextended rift systems: Insights from rift domain mapping in the Bay of Biscay-Pyrenees

    NASA Astrophysics Data System (ADS)

    Tugend, J.; Manatschal, G.; Kusznir, N. J.; Masini, E.; Mohn, G.; Thinon, I.

    2014-07-01

    The Bay of Biscay and the Pyrenees correspond to a Lower Cretaceous rift system including both oceanic and hyperextended rift domains. The transition from preserved oceanic and rift domains in the West to their complete inversion in the East enables us to study the progressive reactivation of a hyperextended rift system. We use seismic interpretation, gravity inversion, and field mapping to identify and map former rift domains and their subsequent reactivation. We propose a new map and sections across the system illustrating the progressive integration of the rift domains into the orogen. This study aims to provide insights on the formation of hyperextended rift systems and discuss their role during reactivation. Two spatially and temporally distinct rift systems can be distinguished: the Bay of Biscay-Parentis and the Pyrenean-Basque-Cantabrian rifts. While the offshore Bay of Biscay represent a former mature oceanic domain, the fossil remnants of hyperextended domains preserved onshore in the Pyrenean-Cantabrian orogen record distributed extensional deformation partitioned between strongly segmented rift basins. Reactivation initiated in the exhumed mantle domain before it affected the hyperthinned domain. Both domains accommodated most of the shortening. The final architecture of the orogen is acquired once the conjugate necking domains became involved in collisional processes. The complex 3-D architecture of the initial rift system may partly explain the heterogeneous reactivation of the overall system. These results have important implications for the formation and reactivation of hyperextended rift systems and for the restoration of the Bay of Biscay and Pyrenean domains.

  18. Syn-rift unconformities punctuating the lower-middle Cambrian transition in the Atlas Rift, Morocco

    NASA Astrophysics Data System (ADS)

    Álvaro, J. Javier; Ezzouhairi, Hassan; Clausen, Sébastien; Ribeiro, M. Luisa; Solá, Rita

    2015-04-01

    The Cambrian Tamdroust and Bab n'Ali Volcanic Complexes represent two magmatic episodes developed in the latest Ediacaran-Cambrian Atlas Rift of Morocco. Their rifting pulses were accompanied by accumulation of volcanosedimentary edifices (dominated by effusive lava flows in the former and explosive acidic aprons in the latter) associated with active tilting and uplift. Sealing of their peneplaned horst-and-graben palaeotopographies led to the onset of distinct onlapping geometries and angular discordances capping eroded basements ranging from the Ediacaran Ouarzazate Supergroup to the Cambrian Asrir Formation. Previous interpretations of these discordances as pull-apart or compressive events are revised here and reinterpreted in an extensional (rifting) context associated with active volcanism. The record of erosive unconformities, stratigraphic gaps, condensed beds and onlapping patterns across the traditional "lower-middle Cambrian" (or Cambrian Series 2-3) transition of the Atlas Rift must be taken into consideration for global chronostratigraphic correlation based on their trilobite content.

  19. Volcanic field elongation, vent distribution and tectonic evolution of continental rift: The Main Ethiopian Rift example

    NASA Astrophysics Data System (ADS)

    Mazzarini, Francesco; Le Corvec, Nicolas; Isola, Ilaria; Favalli, Massimiliano

    2015-04-01

    Magmatism and faulting operate in continental rifts and interact at a variety of scales, however their relationship is complex. The African rift, being the best example for both active continental rifting and magmatism, provides the ideal location to study the interplay between the two mechanisms. The Main Ethiopian Rift (MER), which connects the Afar depression in the north with the Turkana depression and Kenya Rift to the south, consists of two distinct systems of normal faults and its floor is scattered with volcanic fields formed by tens to several hundreds monogenetic, generally basaltic, small volcanoes and composite volcanoes and small calderas. The distribution of vents defines the overall shape of the volcanic field. Previous work has shown that the distribution of volcanic vents and the shape of a field are linked to its tectonic environment and its magmatic system. In order to distinguish the impact of each mechanism, we analyzed four volcanic fields located at the boundary between the central and northern MER, three of them (Debre Zeyit, Wonji and Kone) grew in the rift valley and one (Akaki) on the western rift shoulder. The elongation and shape of the fields were analyzed based on their vent distribution using the Principal Component Analysis (PCA), the Vent-to-Vent Distance (VVD), and the two dimensional symmetric Gaussian kernel density estimate methods. We extracted from these methods several parameters characterizing the spatial distribution of points (e.g., eccentricity (e), eigenvector index (evi), angular dispersion (Da)). These parameters allow to define at least three types of shape for volcanic fields: strong elongate (line and ellipse), bimodal/medium elongate (ellipse) and dispersed (circle) shapes. Applied to the natural example, these methods well differentiate each volcanic field. For example, the elongation of the field increases from shoulder to rift axis inversely to the angular dispersion. In addition, the results show that none of the analyzed fields has its shape parallel to the actual trend of youngest and active faulting and volcanism. The alignment analysis shows that the feeders located along the actual rift axis (Wonji and Kone) are parallel to the NNE trend of the youngest fault system. This parallelism decreases as we move to the rift border. Our results suggest that the shape of volcanic fields is controlled mainly by large crustal to lithosphere scale structures (main border faults of the rift) and/or by the Lithosphere-Asthenosphere-Border (LAB) geometry, whereas diking, occurring at shallower levels, is principally controlled by upper crustal stress and strain state.

  20. Structure and kinematics of the Taupo Rift, New Zealand

    NASA Astrophysics Data System (ADS)

    Seebeck, Hannu; Nicol, Andrew; Villamor, Pilar; Ristau, John; Pettinga, Jarg

    2014-06-01

    The structure and kinematics of the continental intra-arc Taupo Rift have been constrained by fault-trace mapping, a large catalogue of focal mechanisms (N = 202) and fault slip striations. The mean extension direction of ~137° is approximately orthogonal to the regional trend of the rift and arc front (? = 84° and 79°, respectively) and to the strike of the underlying subducting Pacific Plate. Bending and rollback of the subduction hinge strongly influence the location, orientation, and extension direction of intra-arc rifting in the North Island. In detail, orthogonal rifting (? = 85-90°) transitions northward to oblique rifting (? = 69-71°) across a paleovertical-axis rotation boundary where rift faults, extension directions, and basement fabric rotate by ~20-25°. Toward the south, extension is orthogonal to normal faults which are parallel to, and reactivate, steeply dipping basement fabric. Basement reactivation facilitates strain partitioning with a portion of margin-parallel motion in the overriding plate mainly accommodated east of the rift by strike-slip faults in the North Island Fault System (NIFS). Toward the north where the rift and NIFS intersect, ~4 mm/yr strike slip is transferred into the rift with net oblique extension accommodating a component of margin-parallel motion. The trend and kinematics of the Taupo Rift are comparable to late Miocene-Pliocene intra-arc rifting in the Taranaki Basin, indicating that the northeast strike of the subducting plate and the southeast extension direction have been uniform since at least 4 Ma.

  1. Seismic anisotropy beneath the incipient Okavango rift: Implications for rifting initiation

    NASA Astrophysics Data System (ADS)

    Yu, Youqiang; Gao, Stephen S.; Moidaki, Moikwathai; Reed, Cory A.; Liu, Kelly H.

    2015-11-01

    This study represents the first shear-wave splitting investigation of the Okavango rift zone (ORZ), an incipient continental rift belonging to the East African rift system in northern Botswana. Analysis of broadband seismic data recorded along a 750 km long profile of 22 stations traversing the ORZ and adjacent Congo and Kalahari cratons and several Precambrian orogenic zones reveals dominantly NE-SW fast orientations, which are parallel to both the absolute plate motion direction (based on the NNR-NUVEL-1A model) and the trend of most tectonic boundaries, including that of the ORZ. Spatial coherence analysis of the splitting parameters and correspondence between the observed fast orientations and the trend of tectonic features indicate that the main source of observed anisotropy is most likely in the upper asthenosphere, probably due to simple shear associated with the relative movement of the lithosphere against the asthenosphere. The presence of consistently rift-parallel fast orientations and normal splitting times in the ORZ and most parts of southern Africa implies that neither an upper mantle plume nor small-scale convection is the dominant source for rift initiation and development. The first shear-wave splitting measurements in the vicinity of the ORZ favor a model in which continental rifting develops in response to intra-plate relative movement of continental blocks along zones of weakness produced by ancient tectonic events.

  2. Rift Valley fever: A neglected zoonotic disease?

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Rift Valley fever (RVF) is a serious viral disease of animals and humans in Africa and the Middle East that is transmitted by mosquitoes. First isolated in Kenya during an outbreak in 1930, subsequent outbreaks have had a significant impact on animal and human health, as well as national economies. ...

  3. The 1973 Ethiopian-Rift geodimeter survey

    NASA Technical Reports Server (NTRS)

    Mohr, P. A.

    1974-01-01

    Remeasurement of the Adama, Lake Langana, and Arba Minch (Lake Margherita) geodimeter networks in 1973 has enabled Mohr's interpretation concerning possible surface ground deformation in the Ethiopian rift to be considerably developed. Extension appears to have occurred across the Mojjo-Adama horst at a rate of about 1 cm yr/1. The opposing rims of the Adama graben have not moved significantly relative to one another (between 1969 and 1973), but stations on the sliced graben floor show possible movement with a large rift-trend component. In the Wolenchiti quadrilateral, significant movement of station RABBIT is confirmed, but the radical change of vector (that of 1970-1971 to that of 1971-1973) casts doubt on a tectonic cause and seems to indicate that stations on steep hillslopes are liable to be unstable. South of the quadrilateral and east of the Adama graben, alternating rift-trend zones of extension and shortening appear to coexist. In the Lake Langana network, significant movements of the order of 0.5 cm yr/1 are directed perpendicular to the rift floor faulting.

  4. RIFT VALLEY FEVER POTENTIAL, ARABIAN PENINSULA

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Rift Valley fever (RVF) was first confirmed outside of Africa in September 2000. This outbreak, which occurred in southwestern coastal Saudi Arabia and neighboring coastal areas of Yemen, followed elevated rainfall levels in nearby highlands which flooded the coastal areas, providing ideal environm...

  5. Molecular Rift: Virtual Reality for Drug Designers.

    PubMed

    Norrby, Magnus; Grebner, Christoph; Eriksson, Joakim; Boström, Jonas

    2015-11-23

    Recent advances in interaction design have created new ways to use computers. One example is the ability to create enhanced 3D environments that simulate physical presence in the real world-a virtual reality. This is relevant to drug discovery since molecular models are frequently used to obtain deeper understandings of, say, ligand-protein complexes. We have developed a tool (Molecular Rift), which creates a virtual reality environment steered with hand movements. Oculus Rift, a head-mounted display, is used to create the virtual settings. The program is controlled by gesture-recognition, using the gaming sensor MS Kinect v2, eliminating the need for standard input devices. The Open Babel toolkit was integrated to provide access to powerful cheminformatics functions. Molecular Rift was developed with a focus on usability, including iterative test-group evaluations. We conclude with reflections on virtual reality's future capabilities in chemistry and education. Molecular Rift is open source and can be downloaded from GitHub. PMID:26558887

  6. Petrofabrics of olivine in a rift axis and rift shoulder and their implications for seismic anisotropy beneath the Rio Grande rift

    NASA Astrophysics Data System (ADS)

    Park, Munjae; Jung, Haemyeong; Kil, Youngwoo

    2015-04-01

    Mantle-derived xenoliths associated with continental rifting can provide important information about the mantle structure and the physicochemical properties of deformation processes in the upper mantle. Metasomatized spinel peridotites from Adam's Diggings (AD) at a rift shoulder and Elephant Butte (EB) at a rift axis in the Rio Grande rift (RGR) were investigated to understand the deformation processes and seismic anisotropy occurring in the upper mantle. As determined through analysis of the lattice preferred orientation (LPO) of olivine by using a scanning electron microscope equipped with electron backscatter diffraction (SEM/EBSD), AD peridotites exhibited C-type LPO of olivine indicating a dominant slip system of (100)[001] at the rift shoulder, whereas EB peridotites exhibited A-type LPO indicating a dominant slip system of (010)[100] at the rift axis. Both geochemical data and microstructural observations indicate that the localized mantle enrichment processes, including melts with hydrous fluids, controlled multiple mantle metasomatisms and deformation of rocks under wet conditions (with olivine C-type LPO) at the rift shoulder (AD), whereas mantle depletion by decompression partial melting caused deformation of rocks under dry conditions (with olivine A-type LPO) at the rift axis (EB). These observations provide evidence for localized hydration and physicochemical heterogeneity of the upper mantle in the Rio Grande rift (RGR) zone. Seismic anisotropy observed beneath this zone can be attributed to the transtensional rupture, such as inhomogeneous stretching, and the petrofabrics of olivine beneath the study area.

  7. Controls on (anomalous) topography in rifted margin settings

    NASA Astrophysics Data System (ADS)

    Huismans, Ritske S.

    2015-04-01

    Contrasting end members of volcanic and non-volcanic passive margin formation show a large variability in basin shape and structure, subsidence history, and associated topographic evolution of the onshore rifted margins. The large range of structural style and associated topography of these systems imply a strong variability in the underlying thermo-mechanical conditions at the time of rifting. Rift - passive margin styles ranging from narrow to ultra wide are explained using forward numerical models with varying rheological structure, with strong crust lithosphere leading to narrow rift formation associated with highly elevated rift shoulders and conversely weak crust lithosphere resulting in highly stretched wide rifted conjugate margins and little flank morphology. In some cases rifted margins appear to indicate the formation of anomalous post rift topography. A number of mechanisms including small-scale convective removal of the lower lithosphere, lithosphere counter-flow, and dynamic topography, have been invoked to explain the anomalous topography. Forward numerical models are used to predict the magnitude and characteristic topography associated with each of these mechanisms and to evaluate their potential for explaining these apparent anomalous characteristics of rifts and rifted margins.

  8. Lista de admitidos en el plazo de inscripcin de septiembre Ttulo Propio de Experto Universitario en Desarrollo de Aplicaciones Web

    E-print Network

    Escolano, Francisco

    en Desarrollo de Aplicaciones Web con Java EE y JavaScript (Curso 2014-2015) Admitidos Lista de Aplicaciones Web con Java EE y Java Script Nombre Apellidos Puntuación LUCAS MARTIN POGGI RANWEZ 7,92 JOSE

  9. How many rifts are there in West Africa?

    NASA Astrophysics Data System (ADS)

    Freeth, S. J.

    1984-02-01

    The West African Rift System has, for the last ten years, been thought to consist of five interconnected rifts extending from the Gulf of Guinea deep into the heart of Africa. Careful re-examination of the geophysical evidence makes it quite clear that there are only three interconnected rifts in West Africa; the Lower Benue Rift which extends to the northeast from the Gulf of Guinea to a triple junction near Chum, and the Gongola and Yola Rifts which extend to the north and east, respectively, from the Chum triple junction. These three rifts opened during the earlier part of the Mesozoic and were subsequently filled with Cretaceous sediments. The evidence for two further rifts, the Ati Rift and the Fort Archambault Rift which were thought to extend to the northeast and southeast, respectively, from a triple junction at the eastern end of the Yola Rift, does not stand up to re-examination. The "Ati Rift" was thought to follow a major linear positive gravity anomaly which had been mapped beneath the Quaternary sediments of the Chad Basin. The main gravity anomaly is separated from the Yola Rift by over 300 km and is probably due to a linear body of basic volcanic or volcano-clastic rocks associated with a suture of Pan-African age. Within the gap, between the main anomaly and the Yola Rift, there are three localised positive anomalies which relate to a gabbro of Precambrian age, a band of dense meta-sediments within the Basement Complex and an acid igneous complex of Palaeogene age. The anomaly as a whole is therefore a sequence of unrelated anomalies, none of which are due to features of Mesozoic age. The "Fort Archambault Rift" was thought to follow a major linear negative gravity anomaly which has been mapped beneath the Quaternary sediments of the Chad Basin. To a large extent the negative anomaly overlies the fosse de Baké-Birao (Baké-Birao Basin) which is itself part of a far larger structure that extends, parallel to the southern margin of the West African Rift System, from Douala on the Gulf of Guinea to Birao near the C.A.R. frontier with Sudan. The Douala-Birao Structure may possibly be loosely related to the West African Rift System in that it would appear to be a compressional structure formed at the same time as the Coniacian-Santonian phase of folding which is observed in the West African rifts. However, the two structures are clearly separated from each other and are quite different in character and to a lesser extent in age.

  10. Aplicaciones de Arc GISAplicaciones de Arc GIS en Recursos de Aguaen Recursos de Agua

    E-print Network

    Gilbes, Fernando

    Posteriormente fue expandido para el análisis espacial y diversas aplicaciones.diversas aplicaciones. ArcGIS en a Recursos Hídricos.Recursos Hídricos. Resultado ArcGIS Hydro data model conocido comoResultado ArcGIS HydroGIS.temporal que opera en ArcGIS. Prepara los datos para modelaciones hidrológicas ePrepara los datos para

  11. The North Polar Spur and Aquila Rift

    NASA Astrophysics Data System (ADS)

    Sofue, Yoshiaki

    2015-03-01

    Soft X-ray intensity at 0.89 keV along the North Polar Spur (NPS) is shown to follow the extinction law due to the interstellar gas in the Aquila Rift by analysing the ROSAT archival data, which proves that the NPS is located behind the rift. The Aquila-Serpens molecular clouds, where the X-ray optical depth exceeds unity, are shown to have a mean LSR velocity of v = 7.33 ± 1.94 km s-1, corresponding to a kinematic distance of r = 0.642 ± 0.174 kpc. Assuming a shell structure, a lower limit of the distance to NPS is derived to be 1.01 ± 0.25 kpc, with the shell centre being located farther than 1.1 kpc. Based on the distance estimation, we argue that the NPS is a Galactic halo object.

  12. The Midcontinent rift system in Kansas

    SciTech Connect

    Berendsen, P. . Kansas Geological Survey)

    1993-03-01

    A sequence of rift-related mafic volcanic rocks, volcanoclastic-, and clastic sedimentary rocks are recognized in cuttings and cores from about seventy wells in Kansas. The age (1,097.5 Ma) for gabbro in the Poersch [number sign]1 well in northern Kansas, as well as the general petrographic characteristics of the sedimentary rocks throughout the area favors a correlation with established Keweenawan stratigraphy in the Lake Superior region. Rift-related northeast-trending faults and older northwest-trending faults divide the area up into a number of orthogonal fault blocks or basins. Depending upon the tectonic history of the individual basin all or part of the Keweenawan section may be preserved. It is believed that large amounts of Keweenawan clastic sedimentary rock were eroded from the nemaha uplift east of the central graben of the rift and transported in an easterly direction. Prior to deposition of Paleozoic rocks the area was peneplaned. Correlation of various stratigraphic units over any distance is complicated by tectonic activity occurring at several times during the Precambrian and Paleozoic. Stratabound or stratiform deposits can occur both in the Precambrian as well as the overlying Paleozoic rocks. The possibility of massive sulfides to occur in the mafic intrusive rocks must not be excluded. In the core from the Poersch [number sign]1 well sulfides are recognized in gabbroic sills or dikes. Dark, fissile shale, similar to the Nonesuch Shale in the [number sign]1--4 Finn well averages 0.75% organic carbon. Thermal maturation within the rift probably ranges from within the oil window to over maturity.

  13. Seismotectonics of Reelfoot rift basement structures

    SciTech Connect

    Dart, R.L.; Swolfs, H.S. )

    1993-03-01

    Contour maps of the Precambrian basement surface show major northwest-trending structural features within the boundaries of the northeast-oriented Reelfoot rift. These northwest-trending features, southeast of New Madrid, Missouri, consist of a trough flanked on the northeast by a 2-km-high ridge. These features correlate with similar features on an updated depth-to-magnetic basement map. The boundary between the trough and the ridge slopes gently to the southwest. The upward projection of this boundary into the overlying Paleozoic strata may be expressed on a structure-contour map of the Cambrian rocks. The vertical relief of this boundary on the younger datum is inferred to be about 1 km. This Precambrian trough-ridge structure may correlate with a southwest dipping, west-northwest-striking normal fault inferred by Schwalb (1982) to offset rocks of the Cambrian-Ordovician Knox Megagroup that subcrop at the Paleozoic surface. Schwalb (1982) inferred 1.22 km of vertical relief on this fault near the bootheel of Missouri. The nature and significance of this tectonic-structural boundary is unclear, but at the top of the Precambrian basement rocks, it coincides with the southwestern terminus of the New Madrid seismic zone (NMSZ) near the end of the Blytheville arch in northeastern Arkansas. Since the mid-1970's, when instrumental recording began, some of the earthquakes in the NMSZ having the largest magnitudes occurred in this area. The authors working hypothesis is that this trough-ridge structural boundary may concentrate stress and/or may be a barrier that defines the southwestern limit of the seismically active axial fault zone in the rift. Future study will concentrate on improving the understanding of the influence of rift-bounding faults on the lateral extent of this structure, as well as constructing a tectonic stress model of seismically active rift faults and this trough-ridge structure.

  14. Early Paleozoic sedimentation in Reelfoot rift

    SciTech Connect

    Houseknecht, D.W.; Weaverling, P.H.

    1983-09-01

    Analysis of subsurface data from deep tests drilled in the northern Mississippi embayment and southern Mid-Continent suggests that earliest Paleozoic sedimentation was dominated by the tectonic evolution of the Reelfoot rift. Throughout most of the Mid-Continent, the Upper Cambrian Lamotte (Mt. Simon) sandstone rests nonconformably on Precambrian basement and is overlain by the Bonneterre (Eau Claire) Formation. However, in the area of the Reelfoot rift, both the Lamotte and Bonneterre grade into thick, basinal shales that locally display evidence of episodic deposition of coarse clastics, perhaps on submarine fans. Moreover, two major sedimentary units are present beneath the Lamotte-Bonneterre basinal facies within the Reelfoot rift. Immediately underlying the Lamotte-Bonneterre shale is a carbonate stratum (probably dolomite) that thickens to more than 1,000 ft (300 m) along the axis of the basin in eastern Arkansas. Underlying this carbonate is a detrital unit that grades from arkosic sandstone near the northern terminus of the basin to a basinal shale southward. This basinal shale is at least several hundred feet thick near the axis of the basin. These two strata occupy the stratigraphic position of the Conasauga (Middle Cambrian) and Rome (Lower Cambrian) Formations of the southern Appalachians. The axial and transverse distribution of these strata suggests that the Reelfoot evolved as paired grabens or half grabens during the Early and Middle Cambrian. Subsequently, the Reelfoot remained the axis for more widespread subsidence and sedimentation throughout much of the Paleozoic.

  15. Volcano Deformation in the Main Ethiopian Rift

    NASA Astrophysics Data System (ADS)

    Biggs, J.; Bastow, I.; Keir, D.; Hutchison, W.

    2012-04-01

    Magmatism strongly influences continental rift development yet the mechanism, distribution and timescales on which melt is emplaced and erupted through the shallow crust are not well characterized. The Main Ethiopian Rift (MER) has experienced significant volcanism and the mantle beneath is characterized by high temperatures and partial melt. Despite its magma-rich geological record, only one eruption has been historically recorded and no dedicated monitoring networks exist. Consequently, the present-day magmatic processes in the region remain poorly documented, and the associated hazard neglected. We use satellite-based InSAR observations to demonstrate that significant deformation has occurring at 4 volcanic edifices in the MER (Alutu, Corbetti, Bora and Haledebi) from 1993-2010. This raises the number of volcanoes known to be deforming in East Africa beyond 12, comparable to many subduction arcs despite the smaller number of recorded eruptions. The largest displacements are at Alutu volcano, the site of a geothermal plant, which showed two pulses of rapid inflation (10-15 cm) in 2004 and 2008 separated by gradual subsidence. Our observations indicate a shallow (<10 km), frequently replenished zone of magma storage associated with volcanic edifices and add to the growing body of observations that indicate shallow magmatic processes operating on a decadal timescale are ubiquitous throughout the East African Rift. In the absence of detailed historical records of volcanic activity, satellite-based observations of monitoring parameters, such as deformation, could play an important role in assessing volcanic hazard.

  16. Style of rifting and the stages of Pangea breakup

    NASA Astrophysics Data System (ADS)

    Frizon de Lamotte, Dominique; Fourdan, Brendan; Leleu, Sophie; Leparmentier, François; Clarens, Philippe

    2015-05-01

    Pangea results from the progressive amalgamation of continental blocks achieved at 320 Ma. Assuming that the ancient concept of "active" versus "passive" rifting remains pertinent as end-members of more complex processes, we show that the progressive Pangea breakup occurred through a succession of rifting episodes characterized by different tectonic evolutions. A first episode of passive continental rifting during the Upper Carboniferous and Permian led to the formation of the Neo-Tethys Ocean. Then at the beginning of Triassic times, two short episodes of active rifting associated to the Siberian and Emeishan large igneous provinces (LIPs) failed. The true disintegration of Pangea resulted from (1) a Triassic passive rifting leading to the emplacement of the central Atlantic magmatic province (200 Ma) LIP and the subsequent opening of the central Atlantic Ocean during the lowermost Jurassic and from (2) a Lower Jurassic active rifting triggered by the Karoo-Ferrar LIP (183 Ma), which led to the opening of the West Indian Ocean. The same sequence of passive then active rifting is observed during the Lower Cretaceous with, in between, the Parana-Etendeka LIP at 135 Ma. We show that the relationships between the style of rifts and their breakdown or with the type of resulting margins (as magma poor or magma dominated) are not straightforward. Finally, we discuss the respective role of mantle global warming promoted by continental agglomeration and mantle plumes in the weakening of the continental lithosphere and their roles as rifting triggers.

  17. The MOZART Project - MOZAmbique Rift Tomography

    NASA Astrophysics Data System (ADS)

    Fonseca, J. F.; Chamussa, J. R.; Domingues, A.; Helffrich, G. R.; Fishwick, S.; Ferreira, A. M.; Custodio, S.; Brisbourne, A. M.; Grobbelaar, M.

    2012-12-01

    Project MOZART (MOZAmbique Rift Tomography) is an ongoing joint effort of Portuguese, Mozambican and British research groups to investigate the geological structure and current tectonic activity of the southernmost tip of the East African Rift System (EARS) through the deployment of a network of 30 broad band seismic stations in Central and Southern Mozambique. In contrast with other stretches of the EARS to the North and with the Kapvaal craton to the West and South, the lithosphere of Mozambique was not previously studied with a dense seismographic deployment on account of past political instability, and many questions remain unanswered with respect to the location and characteristics of the EARS to the south of Tanzania. In recent years, space geodesy revealed the existence of three microplates in and off Mozambique - Victoria, Rovuma, Lwandle - whose borders provide a connection of the EARS to the South West Indian Ridge as required by plate tectonics. However, the picture is still coarse concerning the location of the rift structures. The 2006 M7 Machaze earthquake in Central Mozambique highlighted the current tectonic activity of the region and added a further clue to the location of the continental rift, prompting the MOZART deployment. Besides helping unravel the current tectonics, the project is expected to shed light on the poorly known Mesoproterozoic structure described by Arthur Holmes in 1951 as the Mozambique Belt, and on the mechanisms of transition from stable craton to rifted continental crust, through the development of a tomographic model for the lithosphere. The MOZART network is distributed South of the Zambezi river at average inter-station spaces of the order of 100 km and includes four stations across the border in South Africa. Data exchange was agreed with AfricaArray. The deployment proceeded in two phases in March 2011, and November and December 2011. Decommissioning is foreseen for August 2013. We report preliminary results for this previously unexplored region concerning the seismicity and ambient noise (see also Domingues et al, this conference), receiver function analysis, surface wave dispersion and SEM forward modelling. These preliminary results will pave the way for a tomographic model of the lithosphere, to be developed in the next stage of the project.

  18. Variable styles of rifting expressed in crustal structure across three rift segments of the Gulf of California

    NASA Astrophysics Data System (ADS)

    Lizarralde, D. D.; Axen, G. J.; Brown, H. E.; Fletcher, J. M.; Fernandez, A. G.; Harding, A. J.; Holbrook, W. S.; Kent, G. M.; Paramo, P.; Sutherland, F. H.; Umhoefer, P. J.

    2007-05-01

    We present a summary of results from a crustal-scale seismic experiment conducted in the southern Gulf of California. This experiment, the PESCADOR experiment, imaged crustal structure across three rift segments, the Alarcon, Guaymas, and San José del Cabo to Puerto Vallarta (Cabo-PV) segments, using seismic refraction/wide-angle reflection data acquired with airgun sources and recorded by closely spaced (10-15 km) ocean-bottom seismometers (OBSs). The imaged crustal structure reveals a surprisingly large variation in rifting style and magmatism between these segments: the Alarcon segment is a wide rift with apparently little syn-rift magmatism; the Guaymas segment is a narrow, magmatically robust rift; and the Cabo-PV segment is a narrow, magmatically "normal" rift. Our explanation for the observed variability is non-traditional in that we do not invoke mantle temperature, the factor commonly invoked to explain end-member volcanic and non-volcanic rifted margins, as the source of the considerable, though non-end-member variability we observe. Instead, we invoke mantle depletion related to pre-rift arc volcanism to account for observed wide, magma-poor rifting and mantle fertility and possibly the influence of sediments to account for robust rift and post-rift magmatism. These factors may commonly vary over small lateral spatial scales in regions that have transitioned from convergent to extensional tectonics, as is the case for the Gulf of California and many other rifts. Our hypothesis suggests that substantial lateral variability may exist within the uppermost mantle beneath the Gulf of California today, and it is hoped that ongoing efforts to image upper mantle structure here will provide tests for this hypothesis.

  19. Geophysical glimpses into the Ferrigno Rift at the northwestern tip of the West Antarctic Rift System

    NASA Astrophysics Data System (ADS)

    Bingham, Robert; Ferraccioli, Fausto

    2014-05-01

    The West Antarctic Rift System (WARS) forms one of the largest continental rift systems on Earth. The WARS is of major significance as it forms the lithospheric cradle for the marine-based and potentially unstable West Antarctic Ice Sheet (WAIS). Seismic refraction, reflection, aeromagnetic, gravity and drilling in the Ross Sea have revealed most of what we know about its structure, tectonic and magmatic patterns and sedimentary basins. Aerogeophysical research and passive seismic networks have considerably extended our knowledge of the WARS and its influence on the overlying WAIS in the Siple Coast and Amundsen Sea Embayment (ASE) regions. The Bellingshausen Sea Embayment region has however remained largely unexplored, and hence the possible extent of the WARS in this sector has remained poorly constrained. Here we use a combination of reconnaissance ground-based and airborne radar observations, airborne gravity, satellite gravity and aeromagnetic data to investigate the WARS in the Bellingshausen Sea Embayment, in the area of the Ferrigno Ice Stream (Bingham et al., 2012, Nature). This region is of high significance, as it one of the main sectors of the WAIS that is currently exhibiting rapid ice loss, thought to be driven primarily by oceanic warming. Assessing geological controls on subice topography and ice dynamics is therefore of prime importance in this part of the WAIS. Ground-based and airborne radar image a subglacial basin beneath the Ferrigno Ice Stream that is up to 1.5 kilometres deep and that connects the ice-sheet interior to the Bellingshausen Sea margin. We interpret this basin as a narrow, glacially overdeepened rift basin that formed at the northwestern tip of the WARS. Satellite gravity data cannot resolve such a narrow rift basin but indicate that the crust beneath the region is likely thinned, lending support to the hypothesis that this area is indeed part of the WARS. Widely-spaced aeromagnetic data image a linear low along the inferred Ferrigno rift, but provide no evidence for high-amplitude aeromagnetic anomalies, typically associated with Cenozoic magmatism within the WARS. However, the reconnaissance character of these data, do not enable us to rule out the presence of magmatism within this part of the rift and cannot disclose the potential greater variability in subglacial geology either. Bingham et al. (2012) proposed the glacially overdeepened Ferrigno rift basins provided major controls for a palaeo-ice stream on the adjacent continental shelf during glacial maxima. The palaeo-ice stream, in turn, eroded the 'Belgica' trough, which today routes warm open ocean water back to the ice front to potentially reinforce dynamic thinning. Dynamic thinning in the Bellingshausen Sea region appears to be steered back to the ice-sheet interior along the Ferrigno rift system. We conclude that detailed aerogeophysical studies of the inferred rift basins that cut across the WAIS margin in the Bellingshausen Sea sector are a high priority to: a) better comprehend the structure and the tectono-magmatic evolution of the WARS and; b) to test the hypothesis that these rifts play a key role in rapidly transmitting oceanic-driven change inland, potentially promoting accelerated ice-sheet instability.

  20. Volcanic rifting at Martian grabens Daniel Me`ge,1

    E-print Network

    Mege, Daniel

    with terrestrial rifts. The DEMs were obtained from Mars Observer Laser Altimeter, Viking Orbiter stereo images February 2003; published 22 May 2003. [1] A large fraction of surface extension on Mars occurred. A Martian rift evolution model is proposed on the basis of new geomorphological and structural

  1. Crustal structure of central Lake Baikal: Insights into intracontinental rifting

    E-print Network

    ten Brink, Uri S.

    Crustal structure of central Lake Baikal: Insights into intracontinental rifting Uri S. ten Brink Related to Geographic Region: Asia; KEYWORDS: Lake Baikal, continental rifts, Newark basin, crustal of Baikal, located in the interior of the largest continental mass on Earth, is thought to represent

  2. Midplate seismicity exterior to former rift-basins

    USGS Publications Warehouse

    Dewey, J.W.

    1988-01-01

    Midplate seismicity associated with some former rift-zones is distributed diffusely near, but exterior to, the rift basins. This "basin-exterior' seismicity cannot be attributed to reactivation of major basin-border faults on which uppercrustal extension was concentrated at the time of rifting, because the border faults dip beneath the basins. The seismicity may nonetheless represent reactivation of minor faults that were active at the time of rifting but that were located outside of the principal zones of upper-crustal extension; the occurrence of basin-exterior seismicity in some present-day rift-zones supports the existence of such minor basin-exterior faults. Other hypotheses for seismicity exterior to former rift-basins are that the seismicity reflects lobes of high stress due to lithospheric-bending that is centered on the axis of the rift, that the seismicity is localized on the exteriors of rift-basins by basin-interiors that are less deformable in the current epoch than the basin exteriors, and that seismicity is localized on the basin-exteriors by the concentration of tectonic stress in the highly elastic basin-exterior upper-crust. -from Author

  3. Fault Orientations at Obliquely Rifted Margins: Where? When? Why?

    NASA Astrophysics Data System (ADS)

    Brune, Sascha

    2015-04-01

    Present-day knowledge of rifted margin formation is largely based on 2D seismic lines, 2D conceptual models, and corroborated by 2D numerical experiments. However, the 2D assumption that the extension direction is perpendicular to the rift trend is often invalid. In fact, worldwide more than 75% of all rifted margin segments have been formed under significant obliquity exceeding 20° (angle measured between extension direction and rift trend normal): During formation of the Atlantic Ocean, oblique rifting dominated at the sheared margins of South Africa and Patagonia, the Equatorial Atlantic margins, separation of Greenland and North America, and it played a major role in the protracted rift history of the North East Atlantic. Outside the Atlantic Ocean, oblique rifting occurred during the split between East and West Gondwana, the separation of India and Australia, India and Madagascar, Australia and Antarctica, as well as Arabia and Africa. It is presently observed in the Gulf of California, the Aegean and in the East African Rift. Despite its significance, the degree to which oblique lithospheric extension affects first-order rift and passive margin properties like surface stress pattern, fault azimuths, and basin geometry, is still not entirely clear. This contribution provides insight in crustal stress patterns and fault orientations by applying a 3D numerical rift model to oblique extensional settings. The presented forward experiments cover the whole spectrum of oblique extension (i.e. rift-orthogonal extension, low obliquity, high obliquity, strike-slip deformation) from initial deformation to breakup. They are conducted using an elasto-visco-plastic finite element model and involve crustal and mantle layers accounting for self-consistent necking of the lithosphere. Results are thoroughly compared to previous analogue experiments, which yields many similarities but also distinct differences for late rift stages and for high obliquity. Even though the model setup is very simple (horizontally layered, no inherited faults, constant extension velocity and direction), its evolution exhibits a variety of fault orientations that are solely caused by the three-dimensionality of oblique rift systems. Allowing new insights on fault patterns of the proximal and distal margins, the model shows that individual fault populations are activated in a characteristic multi-phase evolution driven by lateral density variations of the evolving rift system. Moreover, the model depicts strain partitioning between rift-parallel and rift-perpendicular far-field velocity components that are accommodated by strike-slip faults in the rift centre and normal faults at the rift sides, respectively. Oblique extensional systems worldwide differ in many aspects and clearly one suit of models cannot explain all rifted margin structures at the same time. However, the distinct pattern of fault populations discussed in this study and their sequence of activity compares very well to previous studies of the Gulf of Aden and holds implications for many other rifted margins worldwide. Note that in nature, the resulting stress and fault pattern will also be affected by inherited heterogeneities, surface processes, as well as melting and dyke dynamics.

  4. Rifting and lower crustal reflectivity: A case study of the intracratonic Dniepr-Donets rift zone, Ukraine

    NASA Astrophysics Data System (ADS)

    Lyngsie, Stig B.; Thybo, Hans; Lang, Rasmus

    2007-12-01

    Intracratonic rifting, caused by late Devonian extensional stresses in the East European Craton, created the largest rift zone in Europe, the Pripyat-Dniepr-Donets rift (southeast Ukraine). The rift basin is approximately 2000 km long, up to 170 km wide, and 22 km deep. Wide-angle refraction and reflection seismic data from the Donbas Basin deep seismic Refraction and Reflection Experiments (DOBRE'99) project together with gravity and magnetic data are analyzed for the structure and evolution of the Donbas Fold Belt, which is the uplifted and deformed part of the Dniepr-Donets Basin. The seismic data are used for identification of large-scale crustal structures and modeling of the seismic velocities of the crust and uppermost mantle. A ray-trace-based velocity and density model is derived by joint inversion of gravity and traveltime data. The inversion result reveals a zone of high density and velocity beneath the basin at middle to lower crustal levels, slightly offset to the NE of the rift axis. Full waveform synthetic seismograms, matching the observed data, show high-amplitude and low-frequency arrivals from this high-density body as well as from the Moho. We interpret the high-amplitude, low-frequency signals as reflections from layered magmatic rocks, which intruded into the ductile lower crust during the main rift phase and subsequently were sorted by fractional crystallization. The intrusive material thickened the lower crust by approximately 50%. This may explain the enigmatic flat Moho topography across the rift zone which has been significantly stretched (? = 1.3). The rifting initiated in the late Devonian (Frasnien) as a consequence of back-arc extension in relation to subduction of the Paleo-Tethys Ocean. The subducting oceanic slab may have enriched the mantle with volatiles and created convection, leading to strong partial melting, upwelling, and continued rifting in the Famennien. We interpret the asymmetrical rift geometry as a combination of pure and simple shear tectonic processes.

  5. Papers presented to the Conference on the Processes of Planetary Rifting

    NASA Technical Reports Server (NTRS)

    1981-01-01

    The basic problems of processes of planetary rifting are addressed from the following viewpoints: (1) speculation as to the origin and development of rifts; (2) rifts on other planets; (3) tectonics; (4) geology; (5) chemistry of the lithosphere; (6) physics of the lithosphere; and (7) resources associated with rifting. The state of ignorance on the subject and its remedy is debated.

  6. A Blowing Snow Model for Ice Shelf Rifts

    NASA Astrophysics Data System (ADS)

    Leonard, K. C.; Tremblay, L.; Macayeal, D. R.

    2005-12-01

    Ice melange (a mixture of snow, marine ice, and ice talus) may play various roles in the rates of propagation of iceberg-calving rifts through Antarctic ice shelves. This modeling study examines the role of windblown snow in the formation and maintenance of ice melange in the "nascent rift" in the Ross Ice Shelf (78 08'S, 178 29'W). The rift axis is perpendicular to the regional wind direction, allowing us to employ a two-dimensional blowing snow model. The Piektuk-Tuvaq blowing snow model (Dery and Tremblay, 2004) adapted the Piektuk blowing snow model for use in sea ice environments by including parameterization for open-water leads within the sea ice. This version of the model was used to study the initial conditions of a freshly-opened rift, as the input of blowing snow into the seawater within the rift promotes marine ice formation by cooling and freshening the surface water. We adapted the Piektuk-Tuvaq model both for the local climatic conditions and to incorporate the geometry of the rift, which is 30m deep and 100m wide (far deeper than a lead). We present the evolution of the topography within the rift for two cases. The first is an ice melange composed exclusively of snow and marine ice, the second uses an initial topography including large chunks of ice talus.

  7. Composition of the crust beneath the Kenya rift

    USGS Publications Warehouse

    Mooney, W.D.; Christensen, N.I.

    1994-01-01

    We infer the composition of the crust beneath and on the flanks of the Kenya rift based on a comparison of the KRISP-90 crustal velocity structure with laboratory measurements of compressional-wave velocities of rock samples from Kenya. The rock samples studied, which are representative of the major lithologies exposed in Kenya, include volcanic tuffs and flows (primarily basalts and phonolites), and felsic to intermediate composition gneisses. This comparison indicates that the upper crust (5-12 km depth) consists primarily of quartzo-feldspathic gneisses and schists similar to rocks exposed on the flanks of the rift, whereas the middle crust (12-22 km depth) consists of more mafic, hornblende-rich metamorphic rocks, probably intruded by mafic rocks beneath the rift axis. The lower crust on the flanks of the rift may consist of mafic granulite facies rocks. Along the rift axis, the lower crust varies in thickness from 9 km in the southern rift to only 2-3 km in the north, and has a seismic velocity substantially higher than the samples investigated in this study. The lower crust of the rift probably consists of a crust/mantle mix of high-grade metamorphic rocks, mafic intrusives, and an igneous mafic residuum accreted to the base of the crust during differentiation of a melt derived from the upper mantle. ?? 1994.

  8. The formation of the northwestern Dniepr-Donets Basin: 2-D forward and reverse syn-rift and post-rift modelling

    NASA Astrophysics Data System (ADS)

    Kusznir, N. J.; Stovba, S. M.; Stephenson, R. A.; Poplavskii, K. N.

    1996-12-01

    Forward and reverse modelling of structure and stratigraphy have been used to investigate the syn-rift (Devonian) and early post-rift (Carboniferous) evolution of the northwestern Dniepr-Donets Basin. Modelling shows that basin formation is consistent with Frasnian-Famennian rifting followed by post-rift subsidence starting in the early Tournaisian. Forward syn-rift modelling, using the flexural cantilever model of rift basin formation, satisfactorily models the observed syn-rift stratigraphic thicknesses and structure within the basin using total syn-rift extension of approximately 15 km in the region studied with a maximum ? stretching factor of 1.3. Forward structural and stratigraphic modelling suggests that the Devonian rifting, which formed the northwestern Dniepr-Donets Basin, was accompanied by regional uplift of the order of 300 m. Both forward and reverse (flexural backstripping) modelling of post-rift stratigraphy through the Carboniferous suggest that post-rift thermal subsidence, which commenced in the Tournaisian following Devonian rifting, was augmented by additional regional subsidence of the order of 300 m in middle Carboniferous times. It is suggested that this transient regional uplift event, which accompanied rifting and decayed in middle Carboniferous times, was generated by a mantle plume.

  9. Magmatism and the geodynamics of rifting of the Pripyat-Dnieper-Donets rift, East European Platform

    NASA Astrophysics Data System (ADS)

    Wilson, Marjorie; Lyashkevich, Zoya M.

    1996-12-01

    The distribution of volcanism in the Late Devonian Pripyat-Dnieper-Donets rift within the East European Platform, based upon the results of deep drilling, indicates that pre-existing basement structures and the major rift-bounding faults played an important role in channelling the magmas towards the surface. Major- and trace-element and Sr sbnd Nd isotopic studies of the most primitive basic and ultrabasic magmas, combined with estimates of the amount of extension (? = 1.1 to 1.3), strongly suggest that magmatism was triggered by the upwelling of a thermally and geochemically anomalous mantle plume from the deep mantle during the Late Frasnian. The peak of the magmatism occurred in the Famennian, coincident with the maximum amount of lithospheric extension. Magmatism, rifting and domal basement uplife were contemporaneous at several localities within the EEP, suggesting that the thermal and geodynamic evolution of the platform could have been influenced by a cluster of mantle plumes during the Late Devonian.

  10. The role of inheritance in structuring hyperextended rift systems

    NASA Astrophysics Data System (ADS)

    Manatschal, Gianreto; Lavier, Luc; Chenin, Pauline

    2015-04-01

    A long-standing question in Earth Sciences is related to the importance of inheritance in controlling tectonic processes. In contrast to physical processes that are generally applicable, assessing the role of inheritance suffers from two major problems: firstly, it is difficult to appraise without having insights into the history of a geological system; and secondly all inherited features are not reactivated during subsequent deformation phases. Therefore, the aim of our presentation is to give some conceptual framework about how inheritance may control the architecture and evolution of hyperextended rift systems. We use the term inheritance to refer to the difference between an "ideal" layer-cake type lithosphere and a "real" lithosphere containing heterogeneities and we define 3 types of inheritance, namely structural, compositional and thermal inheritance. Moreover, we assume that the evolution of hyperextended rift systems reflects the interplay between their inheritance (innate/"genetic code") and the physical processes at play (acquired/external factors). Thus, by observing the architecture and evolution of hyperextended rift systems and integrating the physical processes, one my get hints on what may have been the original inheritance of a system. Using this approach, we focus on 3 well-studied rift systems that are the Alpine Tethys, Pyrenean-Bay of Biscay and Iberia-Newfoundland rift systems. For the studied examples we can show that: 1) strain localization on a local scale and during early stages of rifting is controlled by inherited structures and weaknesses 2) the architecture of the necking zone seems to be influenced by the distribution and importance of ductile layers during decoupled deformation and is consequently controlled by the thermal structure and/or the inherited composition of the curst 3) the location of breakup in the 3 examples is not significantly controlled by the inherited structures 4) inherited mantle composition and rift-related mantle processes may control the rheology of the mantle, the magmatic budget, the thermal structure and the localization of final rifting Conversely, the deformation in hyperextended domains is strongly controlled by weak hydrated minerals (e.g. clay, serpentinite) that result form the breakdown of feldspar and olivine due to fluid and reaction assisted deformation and is consequently not inherited but the result of rift induced processes. These key observations show that both inheritance and rift-induced processes play a significant role in the development of magma-poor rift systems and that the role of inheritance may change as the physical conditions vary during the evolving rifting and as rift-induced processes (serpentinization; magma) become more important. Thus, it is not only important to determine the "genetic code" of a rift system, but also to understand how it interacts and evolves during rifting. Understand how far these new ideas and concepts derived from the southern North Atlantic and Alpine Tethys can be translated to other less explored hyperextended rift systems will be one of the challenges of the future research in rifted margins.

  11. Regional magnetic anomaly constraints on continental rifting

    NASA Technical Reports Server (NTRS)

    Vonfrese, R. R. B.; Hinze, W. J.; Olivier, R.; Bentley, C. R.

    1985-01-01

    Radially polarized MAGSAT anomalies of North and South America, Europe, Africa, India, Australia and Antarctica demonstrate remarkably detailed correlation of regional magnetic lithospheric sources across rifted margins when plotted on a reconstruction of Pangea. These major magnetic features apparently preserve their integrity until a superimposed metamorphoric event alters the magnitude and pattern of the anomalies. The longevity of continental scale magnetic anomalies contrasts markedly with that of regional gravity anomalies which tend to reflect predominantly isostatic adjustments associated with neo-tectonism. First observed as a result of NASA's magnetic satellite programs, these anomalies provide new and fundamental constraints on the geologic evolution and dynamics of the continents and oceans. Accordingly, satellite magnetic observations provide a further tool for investigating continental drift to compliment other lines of evidence in paleoclimatology, paleontology, paleomagnetism, and studies of the radiometric ages and geometric fit of the continents.

  12. Diffuse Radiation from the Aquila Rift

    E-print Network

    Jyothy, S N; Karuppath, Narayanankutty; Sujatha, N V

    2015-01-01

    We present an analysis of the diffuse ultraviolet (UV) background in a low latitude region near the Aquila Rift based on observations made by the Galaxy Evolution Explorer (GALEX). The UV background is at a level of about 2000 ph cm^-2 s^-1 sr^-1 \\AA^-1 with no correlation with either the Galactic latitude or the 100 micron infrared (IR) emission. Rather, the UV emission falls off with distance from the bright B2 star HIP 88149, which is in the centre of the field. We have used a Monte Carlo model to derive an albedo of 0.6 - 0.7 in the UV with a phase function asymmetry factor (g) of 0.2 - 0.4. The value for the albedo is dependent on the dust distribution while g is determined by the extent of the halo.

  13. Diffuse Radiation from the Aquila Rift

    NASA Astrophysics Data System (ADS)

    Jyothy, S. N.; Murthy, Jayant; Karuppath, Narayanankutty; Sujatha, N. V.

    2015-12-01

    We present an analysis of the diffuse ultraviolet (UV) background in a low latitude region near the Aquila Rift based on observations made by the Galaxy Evolution Explorer (GALEX). The UV background is at a level of about 2000 ph cm-2 s-1 sr-1 Å-1 with no correlation with either the Galactic latitude or the 100 ?m infrared (IR) emission. Rather, the UV emission falls off with distance from the bright B2 star HIP 88149, which is in the centre of the field. We have used a Monte Carlo model to derive an albedo of 0.6-0.7 in the UV with a phase function asymmetry factor (g) of 0.2-0.4. The value for the albedo is dependent on the dust distribution while g is determined by the extent of the halo.

  14. Low lower crustal velocity across Ethiopia: Is the Main Ethiopian Rift a narrow rift in a hot craton?

    USGS Publications Warehouse

    Keranen, K.M.; Klemperer, S.L.; Julia, J.; Lawrence, J. F.; Nyblade, A.A.

    2009-01-01

    [1] The Main Ethiopian Rift (MER) is a classic narrow rift that developed in hot, weak lithosphere, not in the initially cold, thick, and strong lithosphere that would be predicted by common models of rift mode formation. Our new 1-D seismic velocity profiles from Rayleigh wave/receiver function joint inversion across the MER and the Ethiopian Plateau indicate that hot lower crust and upper mantle are present throughout the broad region affected by Oligocene flood basalt volcanism, including both the present rift and the adjacent Ethiopian Plateau hundreds of kilometers from the rift valley. The region of hot lithosphere closely corresponds to the region of flood basalt volcanism, and we interpret that the volcanism and thermal perturbation were jointly caused by impingement of the Afar plume head. Across the affected region, Vs is 3.6-3.8 km/s in the lowermost crust and ???4.3 km/s in the uppermost mantle, both ??0.3 km/s lower than in the eastern and western branches of the East African Rift System to the south. We interpret the low Vs in the lower crust and upper mantle as indicative of hot lithosphere with partial melt. Our results lead to a hybrid rift mode, in which the brittle upper crust has developed as a narrow rift along the Neoproterozoic suture between East and West Gondwana, while at depth lithospheric deformation is distributed over the broad region (??400 km wide) thermally perturbed by the broad thermal upwelling associated with the Afar plume head. Development of both the East African Rift System to the south (in cold, strong lithosphere) and the MER to the north (in hot, weak lithosphere) as narrow rifts, despite their vastly different initial thermal states and depth-integrated lithospheric strength, indicates that common models of rift mode formation that focus only on temperature, thickness, and vertical strength profiles do not apply to these classic continental rifts. Instead, inherited structure and associated lithospheric weaknesses are the primary control on the mode of extension. ?? 2009 by the American Geophysical Union.

  15. An updated global earthquake catalogue for stable continental regions: Reassessing the correlation with ancient rifts

    USGS Publications Warehouse

    Schulte, S.M.; Mooney, W.D.

    2005-01-01

    We present an updated global earthquake catalogue for stable continental regions (SCRs; i.e. intraplate earthquakes) that is available on the Internet. Our database contains information on location, magnitude, seismic moment and focal mechanisms for over 1300 M (moment magnitude) ??? 4.5 historic and instrumentally recorded crustal events. Using this updated earthquake database in combination with a recently published global catalogue of rifts, we assess the correlation of intraplate seismicity with ancient rifts on a global scale. Each tectonic event is put into one of five categories based on location: (i) interior rifts/taphrogens, (ii) rifted continental margins, (iii) non-rifted crust, (iv) possible interior rifts and (v) possible rifted margins. We find that approximately 27 per cent of all events are classified as interior rifts (i), 25 per cent are rifted continental margins (ii), 36 per cent are within non-rifted crust (iii) and 12 per cent (iv and v) remain uncertain. Thus, over half (52 per cent) of all events are associated with rifted crust, although within the continental interiors (i.e. away from continental margins), non-rifted crust has experienced more earthquakes than interior rifts. No major change in distribution is found if only large (M ??? 6.0) earthquakes are considered. The largest events (M ??? 7.0) however, have occurred predominantly within rifts (50 per cent) and continental margins (43 per cent). Intraplate seismicity is not distributed evenly. Instead several zones of concentrated seismicity seem to exist. This is especially true for interior rifts/taphrogens, where a total of only 12 regions are responsible for 74 per cent of all events and as much as 98 per cent of all seismic moment released in that category. Of the four rifts/taphrogens that have experienced the largest earthquakes, seismicity within the Kutch rift, India, and the East China rift system, may be controlled by diffuse plate boundary deformation more than by the presence of the ancient rifts themselves. The St. Lawrence depression, Canada, besides being an ancient rift, is also the site of a major collisional suture. Thus only at the Reelfoot rift (New Madrid seismic zone, NMSZ, USA), is the presence of features associated with rifting itself the sole candidate for causing seismicity. Our results suggest that on a global scale, the correlation of seismicity within SCRs and ancient rifts has been overestimated in the past. Because the majority of models used to explain intraplate seismicity have focused on seismicity within rifts, we conclude that a shift in attention more towards non-rifted as well as rifted crust is in order. ?? 2005 RAS.

  16. The geology and geophysics of the Oslo rift

    NASA Technical Reports Server (NTRS)

    Ruder, M. E.

    1981-01-01

    The regional geology and geophysical characteristics of the Oslo graben are reviewed. The graben is part of a Permian age failed continental rift. Alkali olivine, tholefitic, and monzonitic intrusives as well as basaltic lavas outline the extent of the graben. Geophysical evidence indicates that rifting activity covered a much greater area in Skagerrak Sea as well as the Paleozoic time, possibly including the northern Skagerrak Sea as well as the Oslo graben itself. Much of the surficial geologic characteristics in the southern part of the rift have since been eroded or covered by sedimentation. Geophysical data reveal a gravity maximum along the strike of the Oslo graben, local emplacements of magnetic material throughout the Skagerrak and the graben, and a slight mantle upward beneath the rift zone. Petrologic and geophysical maps which depict regional structure are included in the text. An extensive bibliography of pertinent literature published in English between 1960 and 1980 is also provided.

  17. Sociocultural and Economic Dimensions of Rift Valley Fever

    PubMed Central

    Muga, Geoffrey Otieno; Onyango-Ouma, Washington; Sang, Rosemary; Affognon, Hippolyte

    2015-01-01

    Health researchers have advocated for a cross-disciplinary approach to the study and prevention of infectious zoonotic diseases, such as Rift Valley Fever. It is believed that this approach can help bring out the social determinants and effects of the zoonotic diseases for the design of appropriate interventions and public health policy. A comprehensive literature review using a systematic search strategy was undertaken to explore the sociocultural and economic factors that influence the transmission and spread of Rift Valley Fever. Although the findings reveal a paucity of social research on Rift Valley Fever, they suggest that livestock sacrificial rituals, food preparation and consumption practices, gender roles, and inadequate resource base for public institutions are the key factors that influence the transmission. It is concluded that there is need for cross-disciplinary studies to increase the understanding of Rift Valley Fever and facilitate appropriate and timely response and mitigation measures. PMID:25688166

  18. Lithospheric thinning beneath rifted regions of Southern California.

    PubMed

    Lekic, Vedran; French, Scott W; Fischer, Karen M

    2011-11-11

    The stretching and break-up of tectonic plates by rifting control the evolution of continents and oceans, but the processes by which lithosphere deforms and accommodates strain during rifting remain enigmatic. Using scattering of teleseismic shear waves beneath rifted zones and adjacent areas in Southern California, we resolve the lithosphere-asthenosphere boundary and lithospheric thickness variations to directly constrain this deformation. Substantial and laterally abrupt lithospheric thinning beneath rifted regions suggests efficient strain localization. In the Salton Trough, either the mantle lithosphere has experienced more thinning than the crust, or large volumes of new lithosphere have been created. Lack of a systematic offset between surface and deep lithospheric deformation rules out simple shear along throughgoing unidirectional shallow-dipping shear zones, but is consistent with symmetric extension of the lithosphere. PMID:21979933

  19. Relationship of coronae, regional plains and rift zones on Venus

    NASA Astrophysics Data System (ADS)

    Krassilnikov, A. S.; Kostama, V.-P.; Aittola, M.; Guseva, E. N.; Cherkashina, O. S.

    2012-08-01

    Coronae and rifts are the most prominent volcano-tectonic features on the surface of Venus. Coronae are large radial-concentric structures with diameters of 100 to over 1000 km. They have varied topographical shapes, radial and concentric fracturing and compressional tectonic structures are common for their annuli. Massive volcanism is also connected with some of the structures. Coronae are interpreted to be the result of updoming and fracturing on the surface due to interaction of mantle diapirs with the lithosphere and its subsequent gravitational relaxation. According to Stofan et al. (2001), two types of coronae are observed: type 1 - coronae that have annuli of concentric ridges and/or fractures (407 structures), and type 2 that have similar characteristics to type 1 but lack a complete annulus of ridges and fractures (107 structures). We analyzed 20% of this coronae population (we chose each fifth structure from the Stofan et al. (2001) catalog; 82 coronae of type 1 and 22 coronae of type 2, in total 104 coronae) for the (1) spatial distribution of rift structures and time relationship of rift zones activity with time of regional volcanic plains emplacement, and (2) tectonics, volcanism, age relative to regional plains and relationship with rifts. Two different age groups of rifts on Venus were mapped at the scale 1:50 000 000: old rifts that predate and young rifts that postdate regional plains. Most of young rifts inherit strikes of old rifts and old rifts are reworked by them. This may be evidence of rift-produced uplift zones that were probably mostly stable during both types of rifts formation. Evolution of distribution of rift systems with time (decreasing of distribution and localization of rift zones) imply thickening of the lithosphere with time. Coronae-producing mantle diapirism and uplift of mantle material in rift zones are not well correlated at least in time in most cases, because majority of coronae (77%) of both types has no genetic association with rifts. Majority of coronae (72%) were mostly active before regional plains formation, and only 3% appear to have begun to form after the plains emplacement, which may be also due to thickening of the lithosphere. According to the relationship with regional plains type 2 coronae are in general older than type 1 coronae. Three types of corona-related volcanic activity were observed: shield volcanoes and their clusters, as well as extensive lobate lava flows and smooth volcanic plains. Shield volcanoes during coronae evolution were mostly active before regional plains emplacement. Most active phase of volcanism of corona may not coincide with the time of the major tectonic activity of corona, as majority of coronae (77%) were most active before regional plains formation, but almost half of all coronae have traces of post regional plains volcanism. Detailed mapping and stratigraphic analysis of seven regions with 34 examples of coronae showed a similarity in the sequence of regional geologic units.

  20. Crustal structure of central Lake Baikal: Insights into intracontinental rifting

    E-print Network

    ten Brink, Uri S.; Taylor, Michael Halford

    2002-07-16

    The Cenozoic rift system of Baikal, located in the interior of the largest continental mass on Earth, is thought to represent a potential analog of the early stage of breakup of supercontinents. We present a detailed P wave velocity structure...

  1. Sociocultural and economic dimensions of Rift Valley fever.

    PubMed

    Muga, Geoffrey Otieno; Onyango-Ouma, Washington; Sang, Rosemary; Affognon, Hippolyte

    2015-04-01

    Health researchers have advocated for a cross-disciplinary approach to the study and prevention of infectious zoonotic diseases, such as Rift Valley Fever. It is believed that this approach can help bring out the social determinants and effects of the zoonotic diseases for the design of appropriate interventions and public health policy. A comprehensive literature review using a systematic search strategy was undertaken to explore the sociocultural and economic factors that influence the transmission and spread of Rift Valley Fever. Although the findings reveal a paucity of social research on Rift Valley Fever, they suggest that livestock sacrificial rituals, food preparation and consumption practices, gender roles, and inadequate resource base for public institutions are the key factors that influence the transmission. It is concluded that there is need for cross-disciplinary studies to increase the understanding of Rift Valley Fever and facilitate appropriate and timely response and mitigation measures. PMID:25688166

  2. Thermal and mechanical development of the East African Rift System

    E-print Network

    Ebinger, Cynthia Joan

    1988-01-01

    The deep basins, uplifted flanks, and volcanoes of the Western and Kenya rift systems have developed along the western and eastern margins of the 1300 km-wide East African plateau. Structural patterns deduced from field, ...

  3. How much magma is required to rift a continent?

    NASA Astrophysics Data System (ADS)

    Bialas, Robert W.; Buck, W. Roger; Qin, Ran

    2010-03-01

    Many continental rifts are associated with and preceded by the formation of large igneous provinces. Partial melting of a large region of anomalously hot, upwelling mantle is required to explain these huge volumes of pre- and syn-rift basaltic magma. The uplift associated with a shallow region of hot mantle is often invoked as a cause of the "rift-push" force that may drive the observed extension. The rift-push force is likely to be an order of magnitude too small to drive tectonic stretching of normal continental lithosphere, but may be sufficient to drive opening of lithosphere-cutting dikes. Intrusion of a sequence of such dikes along a narrow zone may provide enough heat to significantly weaken the lithosphere. Here we use a newly developed 2D numerical approach, in which magma filled dikes open in continental lithosphere in response to an evolving model stress field, and the advection and diffusion of heat is tracked, to investigate the relation between the volume of magma and the amount of lithospheric weakening. The available rifting force, lithospheric structure, and parameters controlling magma flux are varied in the numerical simulations. For continental lithosphere ? 60 km thick ( HL = H1200 °C ) heat diffusion is unimportant and only a small amount of magma is required (< 4 km of maximum cumulative dike opening) to weaken the lithosphere such that it may rift without additional magma. The volume of magma per unit length of rift required to transition from magmatic to tectonic rifting using our method is less than half of that reported by an earlier study. For reasonable plate-tectonic time scales, the amount of magma required for this transition is nearly independent of the rate of magmatic injection, with the exception of highly extrusive cases. Thus, magma assisted rifts may transition to tectonic rifts with rapid magma injection over a short period of time (˜ 1 m.y. as seen for some "volcanic margins" like the North Atlantic) or with slow rates over longer time scales (> 10-20 m.y. similar to "magmatic rifts" like the Red Sea).

  4. Neotectonic activity along the Shanxi rift system, China

    NASA Astrophysics Data System (ADS)

    Xu, Xiwei; Ma, Xingyuan; Deng, Qidong

    1993-03-01

    The Shanxi rift system is one of the most outstanding Pliocene-Quaternary continental rift systems and strong earthquake belts in China. It extends as a series of en echelon left-stepping asymmetrical half-graben basins on the Shanxi Highlands over a distance of more than 1200 km. It describes a sinous S-shaped curve with a NNE-trending transtensional segment in the middle, and NE-ENE-trending extensional domains on both terminal segments. The latter are characterized by apparently synchronous, high-angle normal faulting, accommodating large vertical and relatively smaller lateral strains (3.5-8.5%), which produces the modern basin and range structure. The rift system has been intermittently active since the Pliocene. Geomorphological, neotectonic and seismic studies indicate that the rift system is at present still developing, as demonstrated by the occurrence of strong destructive historical earthquakes of magnitudes 7-8 and the large slip rates on the NNE-trending transtensional faults in the middle segment. The slip rates of these faults reached 4.9-6.4 mm per year during the Holocene. Geophysical studies show that the rifting occurred in a thickened crust, and no compelling evidence exists for the major thermal event in the mantle uniquely associated with the rifting. The development of the Shanxi rift system is consistent with the regional brittle strain pattern of a right-lateral shear belt and a regional stress field of ENE-WSW compression and NNW-SSE extension of the North China subplate. This structural setting corroborates the hypothesis that the deformation is in response to the escape tectonics caused by the Himalayan indenter from the southwest, and at the same time by the counter-clockwise rotation of the intervening crustal blocks. This provides the mode of formation of the Shanxi rift system.

  5. Does rift lateral propagation depend on climate and surface processes?

    NASA Astrophysics Data System (ADS)

    Steer, P.; Huismans, R. S.; Cowie, P. A.; Allken, V.; Thieulot, C.

    2012-12-01

    Initiation and linkage of rift segments is a fundamental aspect of tectonics in extensional settings. Despite that, the factors controlling the structural and topographic expression of individual rift segments are still not well resolved. Here, we investigate the impact of surface processes, including both erosion and deposition, on the lateral dynamic propagation of individual rift segments. We use a fully-coupled 3D model which is based on the landscape evolution model CASCADE and the 3D tectonics code FANTOM. We present results of numerical simulations designed to study the response of viscous-plastic crustal materials subjected to extension and to surface processes. In particular, we focus on the evolution in time and space of both the tectonic structures and of the surface morphology of the rift segments, under different climatic conditions. At first order, we show that the dynamic lateral propagation of a single rift segment, composed of two conjugate shear zones, is dictated not only by its rheological properties but also by the efficiency of erosion and sedimentation acting on its surface, which in turn depends on climate and precipitation. Especially, we demonstrate that increasing the efficiency of fluvial erosion or decreasing the viscosity of the lower crust, increase the velocity of rift lateral propagation and also tend to localize strain on only one of the shear zones, resulting in a half-graben like structure. This illustrates the strong coupling between tectonic, surface processes and climate, and demonstrates that surface processes, by enhancing localization of deformation, act as a positive forcing to rift lateral propagation. We suggest that our results may have important consequences for understanding the propagation of rift structures for varying climatic conditions in space and time.

  6. Crustal structure of central Lake Baikal: Insights into intracontinental rifting

    USGS Publications Warehouse

    ten Brink, U.S.; Taylor, M.H.

    2002-01-01

    The Cenozoic rift system of Baikal, located in the interior of the largest continental mass on Earth, is thought to represent a potential analog of the early stage of breakup of supercontinents. We present a detailed P wave velocity structure of the crust and sediments beneath the Central Basin, the deepest basin in the Baikal rift system. The structure is characterized by a Moho depth of 39-42.5 km; an 8-km-thick, laterally continuous high-velocity (7.05-7.4 km/s) lower crust, normal upper mantle velocity (8 km/s), a sedimentary section reaching maximum depths of 9 km, and a gradual increase of sediment velocity with depth. We interpret the high-velocity lower crust to be part of the Siberian Platform that was not thinned or altered significantly during rifting. In comparison to published results from the Siberian Platform, Moho under the basin is elevated by <3 km. On the basis of these results we propose that the basin was formed by upper crustal extension, possibly reactivating structures in an ancient fold-and-thrust belt. The extent and location of upper mantle extension are not revealed by our data, and it may be offset from the rift. We believe that the Baikal rift structure is similar in many respects to the Mesozoic Atlantic rift system, the precursor to the formation of the North Atlantic Ocean. We also propose that the Central Baikal rift evolved by episodic fault propagation and basin enlargement, rather than by two-stage rift evolution as is commonly assumed.

  7. ** Mdulo de Aplicaciones y Soporte (Lunes, Martes y Mircoles): -En el cuatrimestre B se imparten las asignaturas del Mdulo de Aplicaciones y Soporte (optativas), agrupadas

    E-print Network

    Villanueva, Alicia

    ** Módulo de Aplicaciones y Soporte (Lunes, Martes y Miércoles): - En el cuatrimestre B se imparten ISC - 18:00 - 19:00 18:00-18:30 18:30 - 20:00 20:00 - 21:30 Lunes Martes Miércoles PSE PAE ILN SGE LTD

  8. Feedback Between Surface Processes and Rift-Passive Margin Formation

    NASA Astrophysics Data System (ADS)

    Huismans, R. S.; Steer, P.; Cowie, P. A.; Kitterod, I.

    2013-12-01

    We use new state of the art computational modeling techniques to model crust and lithosphere deformation coupled to surface processes in 2D and 3D. To date few 2D models exist that are able to bridge scales ranging from the lithosphere to sedimentary basin fill. Here we model in 2D the interaction of lithosphere deformation, high-resolution shear zone formation, and sedimentary basin fill. The models indicate a strong interaction and feedback between the structural style of deformation at crust and lithosphere scales and efficiency of sedimentary basin fill and rift flank erosion both in 2D and 3D. In 2D high deposition rates enhance rift localization and increase rift boundary fault offset. Rift flank erosion similarly enhance fault offset. We furthermore use large deformation 3D forward coupled tectonic-surface process modeling techniques to investigate the effect of surface process efficiency on rift linkage. The models indicate that the style of rift linkage in 3D is strongly controlled by fluvial surface process efficiency and by the amount of offset between pre-existing weakness zones.

  9. Feedback between surface processes and rift-passive margin formation

    NASA Astrophysics Data System (ADS)

    Huismans, Ritske S.; Steer, Philippe; Cowie, Patience

    2014-05-01

    We use new state of the art computational modeling techniques to model crust and lithosphere deformation coupled to surface processes in 2D and 3D. To date few 2D models exist that are able to bridge scales ranging from the lithosphere to sedimentary basin fill. Here we model in 2D the interaction of lithosphere deformation, high-resolution shear zone formation, and sedimentary basin fill. The models indicate a strong interaction and feedback between the structural style of deformation at crust and lithosphere scales and efficiency of sedimentary basin fill and rift flank erosion both in 2D and 3D. In 2D high deposition rates enhance rift localization and increase rift boundary fault offset. Rift flank erosion similarly enhance fault offset. We furthermore use large deformation 3D forward coupled tectonic-surface process modeling techniques to investigate the effect of surface process efficiency on rift linkage. The models indicate that the style of rift linkage in 3D is strongly controlled by fluvial surface process efficiency and by the amount of offset between pre-existing weakness zones.

  10. Sedimentary deposits in response to rift geometry in Malawi, Africa

    SciTech Connect

    Bishop, M.G. )

    1991-03-01

    Sedimentary deposits of the Malawi continental rift basin are a direct result of topography and tectonics unique to rift structure. Recent models describe rifts as asymmetric half-graben connected in series by transfer of accommodation zones. Half-graben consist of roughly parallel, tilted fault blocks stepping up from the bounding fault zone where maximum subsidence occurs. The rift becomes a local baselevel and depocenter as regional drainage is shed away by the rift shoulders. Most of the sediments are derived locally due to internal drainage of connected basins, individual basins, and individual fault blocks. The patterns of sedimentation and facies associations depend on structural position at both fault block and half-graben scales. Drainage is directed and dammed by tilted fault blocks. Forward-tilted fault blocks form basinward-thickening sediment wedges filled with facies of axial fluvial systems, alluvial fault-scarp fans, and ponded swamp and lake deposits. These deposits are asymmetrically shifted toward the controlling fault and onlap the upthrown side of the block, ordinarily the site of erosion or nondeposition. Rivers entering the lake on back tilted fault blocks form large deltas resulting in basinward fining and thinning sediment wedges. Lacustrine, nearshore, shoreline, and lake shore plain deposits over multiple fault blocks record lake levels, water chemistry, and tectonic episodes. Tectonic movement periodically changes the basin depth, configuration, and baselevel. This movement results in widespread unconformities deposition and reworking of sediments within the rift.

  11. Buried Mesozoic rift basins of Moroccan Atlantic continental margin

    SciTech Connect

    Mohamed, N.; Jabour, H.; El Mostaine, M.

    1995-08-01

    The Atlantic continental margin is the largest frontier area for oil and gas exploration in Morocco. Most of the activity has been concentrated where Upper Jurassic carbonate rocks have been the drilling objectives, with only one significant but non commercial oil discovery. Recent exploration activities have focused on early Mesozoic Rift basins buried beneath the post-rift sediments of the Middle Atlantic coastal plain. Many of these basins are of interest because they contain fine-grained lacustrine rocks that have sufficient organic richness to be classified as efficient oil prone source rock. Location of inferred rift basins beneath the Atlantic coastal plain were determined by analysis of drilled-hole data in combination with gravity anomaly and aeromagnetic maps. These rift basins are characterized by several half graben filled by synrift sediments of Triassic age probably deposited in lacustrine environment. Coeval rift basins are known to be present in the U.S. Atlantic continental margin. Basin modeling suggested that many of the less deeply bored rift basins beneath the coastal plain are still within the oil window and present the most attractive exploration targets in the area.

  12. Turbidite systems of lacustrine rift basins: Examples from the Lake Kivu and Lake Albert rifts, East Africa

    NASA Astrophysics Data System (ADS)

    Zhang, Xuewei; Scholz, Christopher A.

    2015-07-01

    The Holocene turbidite systems of Lake Kivu and the Pliocene turbidite systems of Lake Albert in the East African Rift were examined using high-resolution 2-D and 3-D seismic reflection data and sediment core information. Based on investigations of seismic facies and lithofacies, several key turbidity-flow depositional elements were observed, including channels, overbank levees with sediment waves, and depositional lobes. Analyses of the sources of the recent and ancient turbidite systems in these two extensional basins suggest that flood-induced hyperpycnal flows are important triggers of turbidity currents in lacustrine rift basins. From source to sink, sediment dispersal, facies distribution, and depositional thickness of the turbidite systems are strongly influenced by rift topography. The Lake Kivu and Lake Albert rifts serve as excellent analogues for understanding the sedimentary patterns of lacustrine turbidites in extensional basins.

  13. Along-strike supply of volcanic rifted margins: Implications for plume-influenced rifting and sudden along-strike transitions between volcanic and non-volcanic rifted margins

    NASA Astrophysics Data System (ADS)

    Ranero, C. R.; Phipps Morgan, J.

    2006-12-01

    The existence of sudden along-strike transitions between volcanic and non-volcanic rifted margins is an important constraint for conceptual models of rifting and continental breakup. We think there is a promising indirect approach to infer the maximum width of the region of upwelling that exists beneath a rifted margin during the transition from rifting to seafloor-spreading. We infer this width of ~30km from the minimum length of the ridge-offsets that mark the limits of the `region of influence' of on-ridge plumes on the axial relief, axial morphology, and crustal thickness along the ridge and at the terminations of fossil volcanic rifted margins. We adopt Vogt's [1972] hypothesis for along-ridge asthenospheric flow in a narrow vertical slot beneath the axis of plume-influenced `macro-segments' and volcanic rifted margins. We find that: (1) There is a threshold distance to the lateral offsets that bound plume-influenced macrosegments; all such `barrier offsets' are greater than ~30km, while smaller offsets do not appear to be a barrier to along-axis flow. This pattern is seen in the often abrupt transitions between volcanic and non-volcanic rifted margins; these transitions coincide with >30km ridge offsets that mark the boundary between the smooth seafloor morphology and thick crust of a plume- influenced volcanic margin and a neighboring non-volcanic margin, as recorded in 180Ma rifting of the early N. Atlantic, the 42Ma rifting of the Kerguelen-Broken Ridge, and the 66Ma Seychelles-Indian rifting in the Indian Ocean. (2) A similar pattern is seen in the often abrupt transitions between `normal' and plume-influenced mid- ocean ridge segments, which is discussed in a companion presentation by Phipps Morgan and Ranero (this meeting). (3) The coexistance of adjacent volcanic and non-volcanic rifted margin segments is readily explained in this conceptual framework. If the volcanic margin macrosegment is plume-fed by hot asthenosphere along an axial ridge slot, while adjacent non-volcanic margin segments stretch and upwell ambient cooler subcontinental mantle, then there will be a sudden transition from volcanic to non-volcanic margins across a transform offset. (4) A 30km width for the region of ridge upwelling and melting offers a simple conceptual explanation for the apparent 30km threshold length for the existence of strike-slip transform faults and the occurrence of non-transform offsets at smaller ridge offset-distances. (5) The conceptual model leads to the interpretation of the observed characteristic ~1000km-2000km-width of plume-influenced macro- segments as a measure of the maximum potential plume supply into a subaxial slot of 5-10 cubic km per yr. (6) If asthenosphere consumption by plate-spreading is less than plume-supply into a macro-segment, then the shallow seafloor and excess gravitational spreading stresses associated with a plume-influenced ridge can lead to growth of the axial slot by ridge propagation. We think this is a promising conceptual framework with which to understand the differences between volcanic and non-volcanic rifted margins.

  14. Geodynamic models of the Wilson Cycle: From rifts to mountains to rifts

    NASA Astrophysics Data System (ADS)

    Buiter, Susanne; Tetreault, Joya; Torsvik, Trond

    2015-04-01

    The Wilson Cycle theory that oceans close and reopen along the former suture is a fundamental concept in plate tectonics. The theory suggests that subduction initiates at a passive margin, closing the ocean, and that future continental extension localises at the ensuing collision zone. Each stage of the Wilson Cycle will therefore be characterised by inherited structural and thermal heterogeneities. Here we investigate the role of Wilson Cycle inheritance by considering the influence of (1) passive margin structure on continental collision and (2) collision zones on passive margin formation. Passive margins may be preferred locations for subduction initiation because inherited faults and areas of exhumed serpentinized mantle may weaken a margin enough to localise shortening. If subduction initiates at a passive margin, the shape and structure of the passive margins will affect future continental collision. Our review of present-day passive margins along the Atlantic and Indian Oceans reveals that most passive margins are located on former collision zones. Continental break-up occurs on relatively young sutures, such as Morocco-Nova Scotia, and on very old sutures, such as the Greenland-Labrador and East Antarctica-Australia systems. This implies that it is not always post-collisional collapse that initiates the extensional phase of a Wilson Cycle. We highlight the impact of collision zone inheritance on continental extension and rifted margin architecture. We show numerical experiments of one Wilson Cycle of subduction, collision, and extension. Subduction initiates at a tapered passive margin. Closure of a 60 Ma ocean leads to continental collision and slab break-off, followed by some tens of kilometres of slab eduction. Mantle flow above the sinking detached slab enhances deformation in the rift area. The resulting rift exposes not only continental crust, but also subduction-related sediments and oceanic crust remnants. Renewed subduction in the post-collision phase is enabled by lithosphere delamination and slab rollback, leading to back-arc extension in a style similar to the Tyrrhenian Sea.

  15. Rifting, Volcanism, and the Geochemical Character of the Mantle Beneath the West Antarctic Rift System (Invited)

    NASA Astrophysics Data System (ADS)

    Mukasa, S. B.; Aviado, K. B.; Rilling-Hall, S.; Bryce, J. G.; Cabato, J.

    2013-12-01

    The West Antarctic Rift System (WARS) is one of the largest extensional alkali volcanic provinces on Earth, but the mechanisms responsible for generating the massive amounts of its associated magmatism remain controversial. The failure of both passive and active decompression melting models to adequately explain the observed lava volumes has prompted debate about the relative roles of thermal plume-related melting and ancient subduction-related flux melting. 40Ar/39Ar dating and geochemical analyses of the lavas, as well as volatile and trace-element determinations of olivine-hosted melt inclusions shed light on the relationship between rifting and volcanism, and also improve our understanding of the geochemical character of the mantle beneath the WARS. Results show that the magmatism post-dates the main phase of extension along the Terror Rift within the WARS, which supports a decompression-melting model without the benefit of a significant thermal anomaly. However, the observed large magma volumes seem to require a volatile-fluxed mantle, a notion supported by a long history of subduction (>500 Myr) along the paleo-Pacific margin of Gondwana. In fact, the legacy of that subduction may manifest itself in the high H2O concentrations of olivine-hosted melt inclusions (up to 3 wt% in preliminary results from ion probe measurements). The major oxide compositions of lavas in the WARS are best matched to experimental melts of garnet pyroxenite and carbonated peridotite sources. The Pb and Nd isotopic systems are decoupled from each other, suggesting removal of fluid-mobile elements from the mantle source possibly during the long history of subduction along this Gondwana margin. Extremely unradiogenic 187Os/188Os ranging to as low as 0.1081 × 0.0001 hints at the involvement of lithospheric components in generation of magmas in the WARS.

  16. Investigation of rifting processes in the Rio Grande Rift using data from an unusually large earthquake swarm. Final report, October 1, 1992--September 30, 1993

    SciTech Connect

    Sanford, A.; Balch, R.; Hartse, H.; House, L.

    1995-03-01

    Because the Rio Grande Rift is one of the best seismically instrumented rift zones in the world, studying its seismicity provides an exceptional opportunity to elucidate the active tectonic processes within continental rifts. Beginning on 29 November 1989, a 15 square km region near Bernardo, NM, produced the strongest and longest lasting sequence of earthquakes in the rift in 54 years. Our research focuses on the Bernardo swarm which occurred 40 km north of Socorro, New Mexico in the axial region of the central Rio Grande rift. Important characteristics concerning hypocenters, fault mechanisms, and seismogenic zones are discussed.

  17. Tectonostratigraphic development of the Interior Sudan rifts, Central Africa

    NASA Astrophysics Data System (ADS)

    McHargue, Tim R.; Heidrick, Tom L.; Livingston, Jack E.

    1992-10-01

    In the Muglad, Melut and Blue Nile rift basins of Interior Sudan three major episodes of rifting, concomitant subsidence and nonmarine/nonvolcanic sedimentation are recognized. These three rifting cycles, which spanned 140 to 95 Ma (Fl), 95 to 65 Ma (F2), and 65 to 30 Ma (F3), resulted in the accumulation of up to 5400, 4200 and 5400 m of sediments, respectively. In the Muglad Basin, the best known and largest of the Sudan rift basins, each rifting cycle consists of (1) a basal sandstone unit (at least near rift margins), that is followed by (2) an upward coarsening section of lacustrine shale grading through marginal lacustrine mudstone and sandstone into fluvial mudstone and sandstone, and (3) a capping blanket of fluvial and alluvial sandstone. The shale-dominated portions of these cycles were deposited in a closed-drainage basin during active faulting. The fluvial and alluvial blanket sands were deposited in an open-drainage basin during the thermal sag phase following each tectonic cycle. The Early Cretaceous F1 intracontinental rifts of Interior Sudan were linked to major rifts/spreading centres in the Proto-South Atlantic by the dextral WSW-trending Central African Shear Zone and to the Indian Ocean via the NW-trending Anza rift in Kenya. In the Muglad Basin, F1 deformation involved high strain rates, rapid syn-rift crustal stretching and subsidence, and the formation of deep, fault-bounded tensional and transtensional pull-apart basins. During the F2 and F3 deformations, the rates of subsidence and stretching were much lower and were focused within smaller geographic areas. Structural elements include asymmetric half-grabens and less common full-grabens with central highs. The three superimposed tectonic episodes resulted in the subsidence of NNW- to NW-trending rift sub-basins; this gave rise to a wide variety of normal fault geometries, displacements, and growth histories. Planar domino-style and listric normal F1 fault arrays are modeled. The rotated F1 basement blocks typically are asymmetric and low-standing, and favour NE-directed growth and material transport. The F2 and F3 normal faults, which have both NE- and SW-directed polarities, often are listric and decouple younger syn-rift strata from older F1 rotated and locked planar basement faults. Palinspastic restorations and forward modeling of three regional cross-sections suggest that F1 faults account for 65-80% of total crustal extension. Stretching factors across the Interior Sudan rifts vary between 1.25 and 1.40. This range is considered to be a minimum since restored sections do not cross F3 depocenters. Total crustal extension across the Muglad plus the Melut rift basins exceeds 75 km.

  18. Crust and Mantle Structure of a Closed Rift System from the Superior Province Rifting Earthscope Experiment (SPREE) (Invited)

    NASA Astrophysics Data System (ADS)

    van der Lee, S.; Wolin, E.; Bollmann, T. A.; Tekverk, K.

    2013-12-01

    The existence of the 1.1 Ga Mid-continent Rift System (MRS) in the Great Lakes region of North America is well known on account of its prominent gravity and magnetic anomalies. These elongated anomalies are associated with dense igneous rocks, which surface in sparse outcrops and are imaged in a handful of active source profiles. Part of the MRS cuts across the Archean Superior Craton while other parts cut through at least three different Proterozoic terranes, though there are indications that offsets between rift segments, such as the Belle Plaine Fault, may follow pre-existing terrane boundaries. The total volume of igneous rock imaged in active source data is consistently estimated as at least one million km3, which is enough for a sea floor of the size of the current Gulf of California, or five times the size of Lake Baikal. However, cessation of rifting and closure of the rift uplifted the igneous rocks along the axes, causing lateral gravity gradients of 150 mgal over 50 km between the gravity high above the uplifted igneous rift axis and the low above the sediment deposits in the original rift flanks. Our seismic experiment (SPREE) covers an area around a one thousand km long segment of the MRS. A long, interrupted line of stations follows the rift axis, another line cuts across this high gravity gradient, yet another line cuts across the Belle Plaine rift axis offset, and a TA-like station group north of Lake Superior complements surrounding Transportable Array coverage. The Superior Province Rifting Earthscope Flexible Array (FA) Experiment (SPREE) has been running for two years with a data return of over 96%. Preliminary SPREE and other analyses show puzzling low velocities along the rift axis and complex Moho structure beneath thickened crust. Other crustal features include a large diversity of sediments, from soggy Quaternary mud through meta-sedimentary Proterozoic rocks. At the time of writing we are quantifying the effects of this complex geological history on our seismic data and attempting to analyze and interpret the residual data in the context of this complex geologic and rich MRS studies history. We will present and discuss constraints from SPREE data on lithospheric structure beneath the MRS from receiver functions, noise analysis, surface waves, and teleseismic travel times.

  19. Stein et al.: MCR Rift/LIP 2/23/15 1 North America's Midcontinent Rift: when rift met LIP1

    E-print Network

    Stein, Seth

    ABSTRACT15 16 Rifts are segmented linear depressions, filled with sedimentary and igneous rocks,17 contains an enormous volume of igneous rocks, mostly flood basalt. We show that24 MCR volcanic rocks by younger sediments, but its two arms are easily traced because the igneous rocks are41 dense and highly

  20. East Antarctic rifting triggers uplift of the Gamburtsev Mountains.

    PubMed

    Ferraccioli, Fausto; Finn, Carol A; Jordan, Tom A; Bell, Robin E; Anderson, Lester M; Damaske, Detlef

    2011-11-17

    The Gamburtsev Subglacial Mountains are the least understood tectonic feature on Earth, because they are completely hidden beneath the East Antarctic Ice Sheet. Their high elevation and youthful Alpine topography, combined with their location on the East Antarctic craton, creates a paradox that has puzzled researchers since the mountains were discovered in 1958. The preservation of Alpine topography in the Gamburtsevs may reflect extremely low long-term erosion rates beneath the ice sheet, but the mountains' origin remains problematic. Here we present the first comprehensive view of the crustal architecture and uplift mechanisms for the Gamburtsevs, derived from radar, gravity and magnetic data. The geophysical data define a 2,500-km-long rift system in East Antarctica surrounding the Gamburtsevs, and a thick crustal root beneath the range. We propose that the root formed during the Proterozoic assembly of interior East Antarctica (possibly about 1 Gyr ago), was preserved as in some old orogens and was rejuvenated during much later Permian (roughly 250 Myr ago) and Cretaceous (roughly 100 Myr ago) rifting. Much like East Africa, the interior of East Antarctica is a mosaic of Precambrian provinces affected by rifting processes. Our models show that the combination of rift-flank uplift, root buoyancy and the isostatic response to fluvial and glacial erosion explains the high elevation and relief of the Gamburtsevs. The evolution of the Gamburtsevs demonstrates that rifting and preserved orogenic roots can produce broad regions of high topography in continental interiors without significantly modifying the underlying Precambrian lithosphere. PMID:22094700

  1. Field Studies of Geothermal Reservoirs Rio Grande Rift, New Mexico

    SciTech Connect

    James C Witcher

    2002-07-30

    The Rio Grande rift provides an excellent field laboratory to study the nature of geothermal systems in an extensional environment. Much of the geologic complexity that is found in the Basin and Range is absent because the rift is located on cratonic crust with a thin and well-characterized Phanerozoic stratigraphy and tectonic history. On the other hand, the Neogene thermo-tectonic history of the rift has many parallels with the Basin and Range to the west. The geology of the southern Rio Grande rift is among the best characterized of any rift system in the world. Also, most geologic maps for the region are rather unique in that detailed analyses of Quaternary stratigraphic and surficial unit are added in concert with the details of bedrock geology. Pleistocene to Holocene entrenchment of the Rio Grande and tributaries unroofs the alteration signatures and permeability attributes of paleo outflow plumes and upflow zones, associated with present-day, but hidden or ''blind,'' hydrothermal systems at Rincon and San Diego Mountain.

  2. Magmatic activity and plate motion during the latent stage of Midcontinent Rift development

    E-print Network

    Swanson-Hysell, N. L.

    The Keweenawan Midcontinent Rift of North America records significant continental rifting between ca. 1110 and 1085 Ma, and preserves the most detailed paleomagnetic record of plate motion of any continent in Precambrian ...

  3. From hyper-extended rifts to orogens: the example of the Mauléon rift basin in the Western Pyrenees (SW France)

    NASA Astrophysics Data System (ADS)

    Masini, E.; Manatschal, G.; Tugend, J.

    2011-12-01

    An integral part of plate tectonic theory is that the fate of rifted margins is to be accreted into mountain belts. Thus, rift-related inheritance is an essential parameter controlling the evolution and architecture of collisional orogens. Although this link is well accepted, rift inheritance is often ignored. The Pyrenees, located along the Iberian and European plate boundary, can be considered as one of the best places to study the reactivation of former rift structures. In this orogen the Late Cretaceous and Tertiary convergence overprints a Late Jurassic to Lower Cretaceous complex intracontinental rift system related to the opening of the North Atlantic. During the rifting, several strongly subsiding basins developed in the axis of the Pyrenees showing evidence of extreme crustal extension and even locale mantle exhumation to the seafloor. Although the exact age and kinematics of rifting is still debated, these structures have an important impact in the subsequent orogenic overprint. In our presentation we discuss the example of the Mauléon basin, which escaped from the most pervasive deformations because of its specific location at the interface between the western termination of the chain and the Bay of Biscay oceanic realm. Detailed mapping combined with seismic reflection, gravity data and industry wells enabled to determine the 3D rift architecture of the Mauléon basin. Two major diachronous detachment systems can be mapped and followed through space. The Southern Mauléon Detachment (SMD) develops first, starts to thin the crust and floors the Southern Mauléon sub-Basin (SMB). The second, the Northern Mauléon Detachment (SMD) is younger and controls the final crustal thinning and mantle exhumation to the north. Both constitute the whole Mauléon basin. Like at the scale of the overall Pyrenees, the reactivation of the Mauléon Basin increases progressively from west to east, which enables to document the progressive reactivation of an aborted hyper-extended rift system. In our presentation, we discuss the compressional reactivation of the rift structures by the study of dip sections across the basin, from weakly reactivated sections in the west to strongly reactivated sections in the east. Comparing the sections, it results that the compression reactivated the rift structures (mainly the detachment faults) and that this reactivation occurred in 2 steps. It corresponds to the reactivation through time of the NMB before the SMB. This evolution is in line with an early proto-subduction of the hyper-extended domain beneath the European plate whereas the NMB sediments are wedged, folded and thrust onto the Iberia and Europe margins ("thin-skin" tectonics). The second step occurs when the deformation started to migrate southward resulting in the formation of the axial Pyrenees nappe stack (thick-skin tectonics). These results suggest that the inherited rift structures strongly controlled the initial convergence. Future work will revisit the more reactivated Albian basins throughout the chain to investigate how far the results from western Pyrenees can be used to understand the Central and Eastern Pyrenees. Moreover, this field-oriented study can serve as an example of how rift structures may control style and timing of orogenic processes.

  4. A groundwater convection model for Rio Grande rift geothermal resources

    NASA Technical Reports Server (NTRS)

    Morgan, P.; Harder, V.; Daggett, P. H.; Swanberg, C. A.

    1981-01-01

    It has been proposed that forced convection, driven by normal groundwater flow through the interconnected basins of the Rio Grande rift is the primary source mechanism for the numerous geothermal anomalies along the rift. A test of this concept using an analytical model indicates that significant forced convection must occur in the basins even if permeabilities are as low as 50-200 millidarcies at a depth of 2 km. Where groundwater flow is constricted at the discharge areas of the basins forced convection can locally increase the gradient to a level where free convection also occurs, generating surface heat flow anomalies 5-15 times background. A compilation of groundwater data for the rift basins shows a strong correlation between constrictions in groundwater flow and hot springs and geothermal anomalies, giving strong circumstantial support to the convection model.

  5. Groundwater convection model for Rio Grande rift geothermal resources

    SciTech Connect

    Morgan, P.; Harder, V.; Swanberg, C.A.; Daggett, P.H.; Ruscetta, C.A.; Foley, D.

    1981-05-01

    It has been proposed that forced convection, driven by normal groundwater flow through the interconnected basins of the Rio Grande rift is the primary source mechanism for the numerous geothermal anomalies along the rift. A test of this concept using an analytical model indicates that significant forced convection must occur in the basins even if permeabilities are as low as 50-200 millidarcies at a depth of 2 km. Where groundwater flow is constricted at the discharge areas of the basins forced convection can locally increase the gradient to a level where free convection also occurs, generating surface heat flow anomalies 5-15 times background. A compilation of groundwater data for the rift basins shows a strong correlation between constrictions in groundwater flow and hot springs and geothermal anomalies, giving strong circumstantial support to the convection model.

  6. The First Prediction of a Rift Valley Fever Outbreak

    NASA Technical Reports Server (NTRS)

    Anyamba, Assaf; Chretien, Jean-Paul; Small, Jennifer; Tucker, Compton J.; Formenty, Pierre; Richardson, Jason H.; Britch, Seth C.; Schnabel, David C.; Erickson, Ralph L.; Linthicum, Kenneth J.

    2009-01-01

    El Nino/Southern Oscillation (ENSO) related anomalies were analyzed using a combination of satellite measurements of elevated sea surface temperatures, and subsequent elevated rainfall and satellite derived normalized difference vegetation index data. A Rift Valley fever risk mapping model using these climate data predicted areas where outbreaks of Rift Valley fever in humans and animals were expected and occurred in the Horn of Africa from December 2006 to May 2007. The predictions were subsequently confirmed by entomological and epidemiological field investigations of virus activity in the areas identified as at risk. Accurate spatial and temporal predictions of disease activity, as it occurred first in southern Somalia and then through much of Kenya before affecting northern Tanzania, provided a 2 to 6 week period of warning for the Horn of Africa that facilitated disease outbreak response and mitigation activities. This is the first prospective prediction of a Rift Valley fever outbreak.

  7. Constraints on rift thermal processes from heat flow and uplift

    NASA Technical Reports Server (NTRS)

    Morgan, P.

    1983-01-01

    The implications of heat flow data available from five major Cenozoic continental rift systems for the processes of continental rifting are discussed, and simple thermal models of lithospheric thinning which predict uplift are used to further constrain the thermal processes in the lithosphere during rifting. Compilations of the heat flow data are summarized and the salient results of these compilations are briefly discussed. The uplift predictions of the slow and rapid thinning models, in which thinning is assumed to occur at a respectively slower and faster rate than heat can be conducted into the lithosphere, are presented. Comparison of uplift rates with model results indicates that the lithosphere is in a state between the two models. While uplift is predicted to continue after thinning has ceased due to thermal relaxation of the lithosphere, the rapid thinning model is always predicted to apply to surface heat flow, and an anomaly in this flow is not predicted to develop until after thinning has stopped.

  8. Relative Timing of CAMP, Rifting, Continental Breakup, and Basin Inversion: Tectonic Significance

    E-print Network

    1 Relative Timing of CAMP, Rifting, Continental Breakup, and Basin Inversion: Tectonic Significance in eastern North America provides a temporal benchmark for assessing the relative timing of rifting, drift, and geochronological data favor a diachronous rift-drift transition (seafloor spreading began earlier in the south

  9. 6Seismic stratigraphy and subsidence history of the United Arab Emirates (UAE) rifted margin

    E-print Network

    Watts, A. B. "Tony"

    and overlying foreland basins M. Y. Ali, A. B. Watts, and M. P. Searle Introduction Rifted continental margins rifted continental margin. The rifted margin sequence (that includes Araej, Sila, Thamama and Wasia continental margin. The upper foreland basin (Pabdeh basin) is infilled by shales, marls and limestones

  10. Palaeohydrology of hydrocarbon maturation, migration and accumulation in the Dead Sea Rift

    E-print Network

    Gvirtzman, Haim

    Palaeohydrology of hydrocarbon maturation, migration and accumulation in the Dead Sea Rift H in the Dead Sea rift, which took place during the last 3±6 Myr, were signi®cantly affected by density, using the Dead Sea rift as an example. Numerical models have been used extensively in recent years

  11. Mantle transition zone discontinuities beneath the Baikal rift and adjacent areas

    E-print Network

    Gao, Stephen Shangxing

    a single long-running seismic station, TLY, located at the SW tip of Lake Baikal. A clear uplift of the 410Mantle transition zone discontinuities beneath the Baikal rift and adjacent areas Kelly H. Liu1 November 2006. [1] Like most other major continental rifts, the Baikal rift zone (BRZ) in Siberia

  12. Deep structure and origin of the Baikal rift zone Dapeng Zhao a,, Jianshe Lei a

    E-print Network

    Gao, Stephen Shangxing

    -east. Lake Baikal occupies only about a third of the rift zone. It is the deepest lake (1620m) in the worldDeep structure and origin of the Baikal rift zone Dapeng Zhao a,, Jianshe Lei a , Tomofumi Inoue: S. King Abstract P-wave velocity images are determined under the Baikal rift zone in Siberia

  13. Flexural analysis of uplifted rift flanks on Venus

    NASA Technical Reports Server (NTRS)

    Evans, Susan A.; Simons, Mark; Solomon, Sean C.

    1992-01-01

    Knowledge of the thermal structure of a planet is vital to a thorough understanding of its general scheme of tectonics. Since no direct measurements of heat flow or thermal gradient are available for Venus, most estimates have been derived from theoretical considerations or by analog with the Earth. The flexural response of the lithosphere to applied loads is sensitive to regional thermal structure. Under the assumption that the yield strength as a function of depth can be specified, the temperature gradient can be inferred from the effective elastic plate thickness. Previous estimates of the effective elastic plate thickness of Venus range from 11-18 km for the foredeep north of Uorsar Rupes to 30-60 km for the annular troughs around several coronae. Thermal gradients inferred for these regions are 14-23 K km(exp -1) and 4-9 K km(exp -1) respectively. In this study, we apply the same techniques to investigate the uplifted flanks of an extensional rift. Hypotheses for the origin of uplifted rift flanks on Earth include lateral transport of heat from the center of the rift, vertical transport of heat by small-scale convection, differential thinning of the lithosphere, dynamical uplift, and isostatic response to mechanical uploading of the lithosphere. The 1st hypothesis is considered the dominant contributor to terrestrial rift flanks lacking evidence for volcanic activity, particularly for rift structures that are no longer active. In this study, we model the uplifted flanks of a venusian rift as the flexural response to a vertical end load.

  14. Thermal perturbations beneath the incipient Okavango Rift Zone, northwest Botswana

    NASA Astrophysics Data System (ADS)

    Leseane, Khumo; Atekwana, Estella A.; Mickus, Kevin L.; Abdelsalam, Mohamed G.; Shemang, Elisha M.; Atekwana, Eliot A.

    2015-02-01

    We used aeromagnetic and gravity data to investigate the thermal structure beneath the incipient Okavango Rift Zone (ORZ) in northwestern Botswana in order to understand its role in strain localization during rift initiation. We used three-dimensional (3-D) inversion of aeromagnetic data to estimate the Curie Point Depth (CPD) and heat flow under the rift and surrounding basement. We also used two-dimensional (2-D) power-density spectrum analysis of gravity data to estimate the Moho depth. Our results reveal shallow CPD values (8-15 km) and high heat flow (60-90 mW m-2) beneath a ~60 km wide NE-trending zone coincident with major rift-related border faults and the boundary between Proterozoic orogenic belts. This is accompanied by thin crust (<30 km) in the northeastern and southwestern parts of the ORZ. Within the Precambrian basement areas, the CPD values are deeper (16-30 km) and the heat flow estimates are lower (30-50 mW m-2), corresponding to thicker crust (~40-50 km). We interpret the thermal structure under the ORZ as due to upward migration of hot mantle fluids through the lithospheric column that utilized the presence of Precambrian lithospheric shear zones as conduits. These fluids weaken the crust, enhancing rift nucleation. Our interpretation is supported by 2-D forward modeling of gravity data suggesting the presence of a wedge of altered lithospheric mantle centered beneath the ORZ. If our interpretation is correct, it may result in a potential paradigm shift in which strain localization at continental rift initiation could be achieved through fluid-assisted lithospheric weakening without asthenospheric involvement.

  15. Sensitivity of the East African rift lakes to climate variability

    NASA Astrophysics Data System (ADS)

    Olaka, L.; Trauth, M. H.

    2009-04-01

    Lakes in the East African Rift have provided excellent proxies to reconstruct past climate changes in the low latitudes. The lakes occupy volcano-tectonic depressions with highly variable climate and hydrological setting, that present a good opportunity to study the climatic and hydrogeological influences on the lake water budget. Previous studies have used lake floor sediments to establish the sensitivity of the East African rift lakes. This study focuses on geomorphology and climate to offer additional or alternative record of lake history that are key to quantifying sensitivity of these lakes as archives to external and internal climatic forcings. By using the published Holocene lake areas and levels, we analyze twelve lakes on the eastern arm of the East African rift; Ziway, Awassa, Turkana, Suguta, Baringo, Nakuru, Elmenteita, Naivasha, Natron, Manyara and compare with Lake Victoria, that occupies the plateau between the east and the western arms of the rift. Using the SRTM data, Hypsometric (area-altitude) analysis has been used to compare the lake basins between latitude 80 North and 30 South. The mean elevation for the lakes, is between 524 and 2262 meters above sea level, the lakes' hypsometric integrals (HI), a measure of landmass volume above the reference plane, vary from 0.31 to 0.76. The aridity index (Ai), defined as Precipitation/ Evapotranspiration, quantifies the water available to a lake, it encompasses land cover and climatic effects. It is lowest (arid) in the basin between the Ethiopian rift and the Kenyan rift and at the southern termination of the Kenyan Rift in the catchments of lake Turkana, Suguta, Baringo and Manyara with values of 0.55, 0.43, 0.43 and 0.5 respectively. And it is highest (wet) in the catchments of, Ziway, Awassa, Nakuru and Naivasha as 1.33,1.03 and 1.2 respectively, which occupy the highest points of the rift. Lake Victoria has an index of 1.42 the highest of these lakes and receives a high precipitation. We use a simple model written on a Matlab code to illustrate the lake volume and area response to climate of surficialy closed, graben shaped and panshaped lake basins. From preliminary results, lake basins that are sensitive to climate variability have a high HI and high aridity index, which will be presented in this conference

  16. Continental rifting and the origin of Beta Regio, Venus

    NASA Technical Reports Server (NTRS)

    Mcgill, G. E.; Steenstrup, S. J.; Barton, C.; Ford, P. G.

    1981-01-01

    Topographic maps based on Pioneer Venus altimetry suggest that Beta Regio, an elevated feature centered at 27 deg N, 282 deg E, is analogous to domes associated with continental rift systems on earth. This interpretation is consistent with the commonly quoted analogy between the East African rift system and the topography of the region from Beta Regio southward to Phoebe Regio. If Beta Regio is a dome, major structural uplift of the crust of Venus is implied, suggesting a more dynamic upper mantle than would be the case if Beta Regio were simply a large volcanic construct.

  17. Lithosphere Response to Intracratonic Rifting: Examples from Europe and Siberia

    NASA Astrophysics Data System (ADS)

    Artemieva, I. M.; Thybo, H.; Herceg, M.; Cherepanova, Y. V.; Chemia, Z.; Cammarano, F.

    2012-12-01

    Several cratons have experienced a significant modification of their crustal and mantle lithosphere structure during Phanerozoic large-scale lithosphere-mantle interactions. In Eurasia, the most prominent examples include the Dniepre-Donets rift in the East European craton, the Oslo graben in the Baltic shield, the Viluy rift and the Baikal rift in Siberia. Despite some similarities, mostly in the crustal structure, there are also significant differences in the lithospheric structure of these rifts. Besides, a large lithosphere-scale Riphean suture/rift runs across the East European craton. While this suture can be recognized in the crustal structure, it is not clearly seen in the structure of the lithospheric mantle. In contrast, Phanerozoic processes associated with emplacement of large magma volumes had a strong effect on modification of the lithosphere structure, primarily by infiltration of basaltic magmas and consequently in a change in mantle density and seismic velocities. Although kimberlite magmatism is commonly not considered as a rifting events, its deep causes may be similar to the mantle-driven rifting and, as a consequence, modification of mantle density and velocity structure may also be expected. We present a new model for the structure of the crust in an area that encompasses the East European craton, the West Siberian basin, and the Siberian cratons. The region includes a nearly continuous age record for lithosphere evolution over ca. 3.6-3.8 billion years. The crustal model is based on critically assessed results from various seismic studies, including reflection and refraction profiles and receiver function studies. We also use global shear-wave tomography models, gravity constraints based on GOCE data, and thermal models for the lithosphere to speculate on thermo-chemical heterogeneity of the mantle. An analysis of the lithosphere structural heterogeneity is presented in relation to geotectonic setting and mantle geodynamics based on interpretations of geophysical models. The results indicate that there is no simple rule for lithosphere modification by intracontinental rifting. The pattern of is controlled by the pre-existing tectonic setting and the intensity of lithosphere-mantle interaction. The results are summarized in a series of maps of lateral variations in lithosphere structure, including the depth to the LAB and compositional heterogeneity of the lithosphere as reflected in it seismic wave velocity and density structure.

  18. Intermittent upwelling of asthenosphere beneath the Gregory Rift, Kenya

    SciTech Connect

    Tatsumi, Yoshiyuki Kyoto Univ. ); Kimura, Nobukazu ); Itaya, Tetsumaru ); Koyaguchi, Takehiro ); Suwa, Kanenori )

    1991-06-01

    K-Ar dates and chemical compositions of basalts in the Gregory Rift, Kenya, demonstrate marked secular variation of lava chemistry. Two magmatic cycles characterized by incompatible element relative depletion are recognized; both occurring immediately after the peak of basaltic volcanism and coeval with both trachyte/phonolite volcanism and domal uplift of the region. These cycles may be attributed to increasing degree of partial melting of mantle source material in association with thinning of the lithosphere by thermal erosion through contact with hot upwelling asthenospheric mantle. Cyclic variation in asthenosphere upwelling may be considered an important controlling process in the evolution of the Gregory Rift.

  19. APLICACIONES DE BABA Y BTH EN BRINZALES DE PINUS PINASTER PARA LA INDUCIN DE

    E-print Network

    APLICACIONES DE BABA Y BTH EN BRINZALES DE PINUS PINASTER PARA LA INDUCIÓN DE RESISTENCIA ANTE actuar como elicitadores de los mecanismos defensivos de Pinus pinaster ante este patógeno. El vivero del País Vasco, sobre Pinus radiata y P. pinaster (LANDE- RAS et al., 2005). Recientemente, F

  20. 280 VISIN Y VISUALIZACIN APLICACIONES EN LA EDUCACIN DEL DISEO ARQUITECTONICO Y CAD

    E-print Network

    280 VISIÓN Y VISUALIZACIÓN APLICACIONES EN LA EDUCACIÓN DEL DISEÑO ARQUITECTONICO Y CAD Arizona of 3D modeling and rendering were offered only to graduate students ten years ago in academics, the CAD school provides several elective courses to teach CAD and 2D CG with AutoCAD (AutoDesk 2005), Photoshop

  1. Uso del conocimiento de la arquitectura Fermi para mejorar el rendimiento en aplicaciones

    E-print Network

    Llanos, Diego R.

    Uso del conocimiento de la arquitectura Fermi para mejorar el rendimiento en aplicaciones CUDA Yuri arquitectura Fermi de NVIDIA introduce nuevos criterios a la hora de selec- cionar los tama~nos y la geometr arquitectura para aplicar adecua- damente t´ecnicas de optimizaci´on de c´odigo. Fermi [2] [3] [4] es la

  2. What created the Proterozoic Ladoga rift (SE Baltic shield): Testing rifting versus supercontinent reconfiguration origin by geophysical data

    NASA Astrophysics Data System (ADS)

    Artemieva, I. M.; Shulgin, A.

    2014-12-01

    Mesoproterozoic mafic magmatism at the southern part of the Baltic Shield (the Lake Ladoga region) is conventionally ascribed to epicratonic rifting. The region hosts a series of mafic dykes and sills of Mesoproterozoic ages, including a ca. 1.53-1.46 Ga sheet-like gabbro-dolerite sills and the Salmi plateau-basalts from the Lake Ladoga region. Based on chiefly geochemical data, the region is conventionally interpreted as an intracratonic Ladoga rift (graben). We question the validity of this geodynamic interpretation by analyzing regional geophysical data (crustal structure, heat flow, Bouguer gravity anomalies, magnetic anomalies, and mantle Vs velocities). We provide a complete list of tectonic, magmatic, and geophysical characteristics typical of continental rifts in general and demonstrate that, except for magmatic and, perhaps, some gravity signature, the Lake Ladoga region lacks any other rift features. We also compare the geophysical data from the Lake Ladoga region with a similar in age Midcontinent rift (USA) and the Valday rift (NW Russia), and provide alternative explanations for the Mesoproterozoic geodynamic evolution of the southern Baltic Shield. We propose that Mesoproterozoic mafic intrusions in southern Fennoscandia may be associated with a complex deformation pattern during reconfiguration of (a part of) Nuna (Columbia) supercontinent, which led to magma intrusions as a series of mafic dykes along lithosphere weakness zones and ponding of small magma pockets within the cratonic lithosphere. Consequent magma cooling and its partial transition to eclogite facies could have led to the formation of a series of basement depressions, similar to the intracratonic basins of North America, while spatially heterogeneous thermo-chemical subsidence, with phase transitions locally speeded by the presence of (subduction-related) fluids, could have produced a series of faults bounding graben-like structures.

  3. Recent geodynamics and evolution of the Moma rift, Northeast Asia.

    NASA Astrophysics Data System (ADS)

    Imaev, V. S.; Imaeva, L. P.; Kozmin, B. M.; Fujita, K. S.; Mackey, K. G.

    2009-04-01

    The Cenozoic Moma rift system is a major tectonic feature in northeast Russia. It is composed of a series of basins (Selennyakh, Kyrin,Lower Moma,Upper Moma,etc.) filled with up to one km thick and bounded by the Chersky Range (up to 3100 m high) on the southwest and the Moma Range (up to 2400 m high) on the northeast. Northeast of the Moma Range is the Indigirka-Zyryanka foreland basin, composed of thick, up to 2.5 km, Eocene, Oligocene, and Miocene coal-bearing sequences, while on the southwestern side of the Chersky Range there are a number of piedmont basins (Tuostakh, Upper Adycha, Derbeke, etc.) containing up to several hundred meters of Miocene and Oligocene coal-bearing deposits. Despite considerable study over the past half-century, there is considerable debate over the origin, present-day tectonics, and evolution of the Moma rift system. The Cenozoic deposits of the basins generally become younger from northwest to southeast with the exception of the Seimchan-Buyunda basin. In the northeast, fan-shaped coal-bearing basins (e.g., Nenneli, Olzhoi, Selennyakh, Uyandina, Tommot, and others) are filled with Miocene to Pliocene deposits, while basins in the southeast (e.g., Taskan) are filled with Neogene sediments. The Seimchan-Buyunda basin, however, has sediments of Oligocene age. The Moma rift system is reflected a major step in the gravity field, presumably separating denser rocks of the Kolyma-Omolon superterrain from somewhat less dense rocks of the Verkhoyansk fold belt (margin of the North Asian Craton). Analysis of travel-times of Pn and Pg waves from local earthquakes indicates an area of thinned crust (30-35 km) southwest of the Moma rift system, extending as a "tongue" from the Lena River delta and the Laptev Sea to the upper part of the Kolyma River, as compared to 40-45 km in the surrounding areas. This region of thinned crust also coincides with a region of high heat flow values measured in boreholes of the Chersky Range (up to 88 mW/m2). Hot springs with temperatures up to +20°C are found within the Moma and Selnnyakh basins proper.The crustal inhomogeneity is also reflected in the upper mantle as indicated by a 40° rotation of the Rayleigh wave polarization angle from teleseisms recorded at Tiksi that cross the Moma rift system as opposed to those that do not. Cenozoic volcanism, chemically similar to basalts and rhyolites from rift zones elsewhere is found in the Moma rift proper. Balagan-Tas is a basaltic cinder cone which has been dated at 286,000 years based on Ar-Ar dating, while Uraga-Khaya is an undated, presumed Quaternary, rhyolitic dome. All these factors indicate that the Moma rift system originated as a continental rift, probably as an extension of the Arctic (Gakkel) Mid-Ocean Ridge. At the present, however, compressional conditions prevail within the Moma rift zone. Seismicity is generally absent from the rift basins proper or their margins; most seismicity is concentrated to the southwest of the Moma rift basins along major strike-slip fault systems. Focal mechanisms of the largest earthquakes in the Chersky Range also all show transpression. Field mapping indicates that the majority of the faults mapped in the field are strike-slip, thrust and reverse faults (86%) with only a small number of normal faults (14%) and that the Cenozoic deposits within the Moma rift are intensely folded. Re-leveling surveys conducted along the Indigirka River, which cuts across the Moma rift system, reveal a moderate rate of presnt-day vertical uplift (up to +4 mm/yr). Thus, the Moma rift system is no longer acting as a rift, but is undergoing transpression. This conclusion is also supported by recent plate motion calculations based on GPS and VLBI data, as well as slip-vectors of earthquakes, which indicates that the Euler pole between North America and Eurasia is located around 68-70°N, near the coast of the Laptev Sea. This places the Moma rift system in a zone of convergence between North America and Eurasia; this geometry also supports the extrusion of the Okhotsk Sea plate. Poles of ro

  4. Depositional and tectonic framework of the rift basins of Lake Baikal from multichannel seismic data

    USGS Publications Warehouse

    Hutchinson, D.R.; Golmshtok, A.J.; Zonenshain, L.P.; Moore, T.C.; Scholz, C.A.; Klitgord, Kim D.

    1992-01-01

    Recent multichannel seismic reflection data from Lake Baikal, located in a large, active, continental rift in central Asia, image three major stratigraphic units totalling 3.5 to 7.5 km thick in four subbasins. A major change in rift deposition and faulting between the oldest and middle-rift units probably corresponds to the change from slow to fast rifting. A brief comparison of the basins of Lake Baikal with those of the East African rift system highlights differences in structural style that can be explained by differences in age and evolution of the surrounding basement rocks. -from Authors

  5. Large-scale variation in lithospheric structure along and across the Kenya rift

    USGS Publications Warehouse

    Prodehl, C.; Mechie, J.; Kaminski, W.; Fuchs, K.; Grosse, C.; Hoffmann, H.; Stangl, R.; Stellrecht, R.; Khan, M.A.; Maguire, Peter K.H.; Kirk, W.; Keller, Gordon R.; Githui, A.; Baker, M.; Mooney, W.; Criley, E.; Luetgert, J.; Jacob, B.; Thybo, H.; Demartin, M.; Scarascia, S.; Hirn, A.; Bowman, J.R.; Nyambok, I.; Gaciri, S.; Patel, J.; Dindi, E.; Griffiths, D.H.; King, R.F.; Mussett, A.E.; Braile, L.W.; Thompson, G.; Olsen, K.; Harder, S.; Vees, R.; Gajewski, D.; Schulte, A.; Obel, J.; Mwango, F.; Mukinya, J.; Riaroh, D.

    1991-01-01

    The Kenya rift is one of the classic examples of a continental rift zone: models for its evolution range from extension of the lithosphere by pure shear1, through extension by simple shear2, to diapiric upwelling of an asthenolith3. Following a pilot study in 19854, the present work involved the shooting of three seismic refraction and wide-angle reflection profiles along the axis, across the margins, and on the northeastern flank of the rift (Fig. 1). These lines were intended to reconcile the different crustal thickness estimates for the northern and southern parts of the rift4-6 and to reveal the structure across the rift, including that beneath the flanks. The data, presented here, reveal significant lateral variations in structure both along and across the rift. The crust thins along the rift axis from 35 km in the south to 20 km in the north; there are abrupt changes in Mono depth and uppermost-mantle seismic velocity across the rift margins, and crustal thickening across the boundary between the Archaean craton and PanAfrican orogenic belt immediately west of the rift. These results suggest that thickened crust may have controlled the rift's location, that there is a decrease in extension from north to south, and that the upper mantle immediately beneath the rift may contain reservoirs of magma generated at greater depth.

  6. Closing of the Midcontinent-Rift - a far-field effect on Grenvillian compression

    USGS Publications Warehouse

    Cannon, W.F.

    1994-01-01

    The Midcontinent rift formed in the Laurentian supercontinent between 1109 and 1094 Ma. Soon after rifting, stresses changed from extensional to compressional, and the central graben of the rift was partly inverted by thrusting on original extensional faults. Thrusting culminated at about 1060 Ma but may have begun as early as 1080 Ma. On the southwest-trending arm of the rift, the crust was shortened about 30km; on the southeast-trending arm, strike-slip motion was dominant. The rift developed adjacent to the tectonically active Grenville province, and its rapid evolution from an extensional to a compressional feature at c1080 Ma was coincident with renewal of northwest-directed thrusting in the Grenville, probably caused by continent-continent collision. A zone of weak lithosphere created by rifting became the locus for deformation within the otherwise strong continental lithosphere. Stresses transmitted from the Grenville province utilized this weak zone to close and invert the rift. -Author

  7. Predicting the Next Outbreak of Rift Valley Fever (RVF)

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Rift Valley fever (RVF) is a mosquito-borne zoonotic disease of domestic ruminants in Africa. The disease is most severe in cattle, sheep, and goats, and it causes high mortality in young animals and abortion in adults. Exotic aanimal breeds from areas where RVF is not endemic tend to be more suscep...

  8. Rift Valley Fever Overview and Recent Developments at USDA

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Rift Valley fever (RVF) is a mosquito-borne viral disease with significant health and economic impacts to domestic animals and humans in much of sub-Saharan Africa. Human infections are believed to occur mainly from mosquito bites and from infectious aerosols. The available strategies for protection...

  9. Forecast and Outbreak of Rift valley fever in Sudan, 2007

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Background Rift Valley fever (RVF) outbreaks occur during heavy rainfall in various sub-Saharan countries including Kenya, Somalia, and Tanzania and more recently in Saudi Arabia and Yemen. Given the wide geographic and ecological range of RVF virus, it is necessary to monitor large areas for condit...

  10. Rift Valley fever Entomology, Ecology, and Outbreak Risk Factors

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Rift Valley fever (RVF) is a mosquito-borne zoonotic disease of domestic ruminants and humans in Africa. The disease is most severe in cattle, sheep, and goats, and it causes high mortality in young animals and abortion in adults. Exotic aanimal breeds from areas where RVF is not endemic tend to be ...

  11. Seismicity of the Earth 1900-2013 East African Rift

    USGS Publications Warehouse

    Hayes, Gavin P.; Jones, Eric S.; Stadler, Timothy J.; Barnhart, William D.; McNamara, Daniel E.; Benz, Harley M.; Furlong, Kevin P.; Villaseñor, Antonio

    2014-01-01

    Rifting in East Africa is not all coeval; volcanism and faulting have been an ongoing phenomenon on the continent since the Eocene (~45 Ma). The rifting began in northern East Africa, and led to the separation of the Nubia (Africa) and Arabia plates in the Red Sea and Gulf of Aden, and in the Lake Turkana area at the Kenya-Ethiopia border. A Paleogene mantle superplume beneath East Africa caused extension within the Nubia plate, as well as a first order topographic high known as the African superswell which now includes most of the eastern and southern sectors of the Nubia plate. Widespread volcanism erupted onto much of the rising plateau in Ethiopia during the Eocene-Oligocene (45–29 Ma), with chains of volcanoes forming along the rift separating Africa and Arabia. Since the initiation of rifting in northeastern Africa, the system has propagated over 3,000 km to the south and southwest, and it experiences seismicity as a direct result of the extension and active magmatism.

  12. Innovative tephra studies in the East African Rift System

    NASA Astrophysics Data System (ADS)

    WoldeGabriel, Giday; Hart, William K.; Heiken, Grant

    Geosciences investigations form the foundation for paleoanthropological research in the East African Rift System. However, innovative applications of tephra studies for constraining spatial and temporal relations of diverse geological processes, biostratigraphic records, and paleoenvironmental conditions within the East African Rift System were fueled by paleoanthropological investigations into the origin and evolution of hominids and material culture. Tephra is a collective, size-independent term used for any material ejected during an explosive volcanic eruption.The East African Rift System has become a magnet for paleoanthropological research ever since the discovery of the first hominids at Olduvai Gorge, in Tanzania, in the 1950s [Leakey et al., 1961]. Currently, numerous multidisciplinary scientific teams from academic institutions in the United States and Western Europe make annual pilgrimages for a couple of months to conduct paleoanthropological field research in the fossil-rich sedimentary deposits of the East African Rift System in Ethiopia, Kenya, and Tanzania. The field expedition consists of geological, paleontological, archaeological, and paleoenvironmental investigations.

  13. Assessing the extent of carbonate deposition in early rift settings

    E-print Network

    Purkis, Sam

    Company in San Ramon, California, performs carbonate research, techni- cal service projects, consulting- graphic, and diagenetic problems that pertain to carbonate reservoirs and exploration plays in mostAssessing the extent of carbonate deposition in early rift settings Paul M. (Mitch) Harris, James

  14. Potential Effects of Rift Valley Fever in the United States

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Rift Valley fever virus (RVFV) has been the cause of disease outbreaks throughout Africa and the Arabian Peninsula, and the infection often results in heavy economic costs through loss of livestock. If RVFV, which is common to select agent lists of the US Department of Health and Human Services and ...

  15. Mapping of the major structures of the African rift system

    NASA Technical Reports Server (NTRS)

    Mohr, P. A. (principal investigator)

    1973-01-01

    The author has identified the following significant results. ERTS-1 imagery of the African rift system has already proved of great value in structural geological studies. One of the interesting megastructures expressed on the imagery occurs some 40 km east of the eastern margin of the main Ethiopian rift, in Arussi province, and extending between latitude 71/2 and 81/4 deg N. The Badda-Encuolo ridge proves to have been a line of major Tertiary volcanism and probably supplied the thick Trap Series flood basalt sequence exposed farther east in the canyons of the Webi Shebeli drainage system. The ridge itself was built up by the waning activity of the Sagatu line of volcanism. Serendipitious has been the discovery on Mt. Badda of several deeply glaciated valleys, many of which show clearly on the ERTS-1 imagery. It seems that Mt. Badda was one of the most important glacial centers in eastern Africa during the Pleistocene. Three major late-Tertiary trachytic centers lie between the Badda-Encuolo ridge and the rift valley. The relationships of these three volcanoes to each other and to the rift faulting is revealed for the first time by the ERTS-1 imagery, as is the form of the cladera of Baltata and the crater of Chilalo.

  16. Potential for Rift Valley to be Introduced into North America

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Rift Valley fever (RVF) is a mosquito-borne zoonotic disease of domestic ruminants in Africa. The disease is most severe in cattle, sheep, and goats, causing mortality in young animals and abortion in adults. Human infection causes significant morbidity and mortality. RVF occurs in sub-Saharan Afri...

  17. Monitoring Volcanic Gases on Kilauea's East Rift Zone

    USGS Multimedia Gallery

    Hawaiian Volcano Observatory Geochemist Jeff Sutton and CSAV international volcanology students visit a continuous gas monitoring site on Kilauea's east rift zone during field studies portion of the summer training course. Instrumentation at this site measures ambient concentration of noxious sulfur...

  18. Monitoring Volcanic Gases on Kilauea's East Rift Zone II

    USGS Multimedia Gallery

    Hawaiian Volcano Observatory Geochemist Jeff Sutton and CSAV international volcanology students visit a continuous gas monitoring site on Kilauea's east rift zone during field studies portion of the summer training course. Instrumentation at this site measures ambient concentration of noxious su...

  19. The protracted development of focused magmatic intrusion during continental rifting

    NASA Astrophysics Data System (ADS)

    Rooney, Tyrone O.; Bastow, Ian D.; Keir, Derek; Mazzarini, Francesco; Movsesian, Emily; Grosfils, Eric B.; Zimbelman, James R.; Ramsey, Michael S.; Ayalew, Dereje; Yirgu, Gezahegn

    2014-06-01

    The transition from mechanical thinning toward focused magmatic intrusion during continental rifting is poorly constrained; the tectonically active Main Ethiopian Rift (MER) provides an ideal study locale to address this issue. The presence of linear magmatic-tectonic belts in the relatively immature central MER may indicate that the transition from mechanical to magmatic rifting is more spatially distributed and temporally protracted than has previously been assumed. Here we examine lava geochemistry and vent distribution of a Pliocene-Quaternary linear magmatic chain along the western margin of the central MER—the Akaki Magmatic Zone. Our results show limited variability in parental magma that evolve in a complex polybaric fractionation system that has not changed significantly over the past 3 Ma. Our results suggest the following: (1) channeling of plume material and the localization of shear- or topography-induced porosity modulates melt intrusion into the continental lithosphere. (2) Pre-existing lithospheric structures may act as catalysts for intrusion of magmas into the lithospheric mantle. (3) The midcrustal to upper crustal strain regime dictates the surface orientation of volcanic vents. Therefore, although linear magmatic belts like those in the central MER may young progressively toward the rift axis and superficially resemble oceanic style magmatism, they actually represent prebreakup magmatism on continental crust. The oldest linear magmatic belts observed seismically and magnetically at the edge of the ocean basins thus may not, as is often assumed, actually mark the onset of seafloor spreading.

  20. Masirah Graben, Oman: A hidden Cretaceous rift basin

    SciTech Connect

    Beauchamp, W.H.; Ries, A.C.; Coward, M.P.

    1995-06-01

    Reflection seismic data, well data, geochemical data, and surface geology suggest that a Cretaceous rift basin exists beneath the thrusted allochthonous sedimentary sequence of the Masirah graben, Oman. The Masirah graben is located east of the Huqf uplift, parallel to the southern coast of Oman. The eastern side of the northeast-trending Huqf anticlinorium is bounded by an extensional fault system that is downthrown to the southeast, forming the western edge of the Masirah graben. This graben is limited to the east by a large wedge of sea floor sediments and oceanic crust, that is stacked as imbricate thrusts. These sediments/ophiolites were obducted onto the southern margin of the Arabian plate during the collision of the Indian/Afghan plates at the end of the Cretaceous. Most of the Masirah graben is covered by an allochthonous sedimentary sequence, which is complexly folded and deformed above a detachment. This complexly deformed sequence contrasts sharply with what is believed to be a rift sequence below the ophiolites. The sedimentary sequence in the Masirah graben was stable until further rifting of the Arabian Sea/Gulf of Aden in the late Tertiary, resulting in reactivation of earlier rift-associated faults. Wells drilled in the Masirah graben in the south penetrated reservoir quality rocks in the Lower Cretaceous Natih and Shuaiba carbonates. Analyses of oil extracted from Infracambrian sedimentary rocks penetrated by these wells suggest an origin from a Mesozoic source rock.

  1. Seismic structure of the uppermost mantle beneath the Kenya rift

    USGS Publications Warehouse

    Keller, Gordon R.; Mechie, J.; Braile, L.W.; Mooney, W.D.; Prodehl, C.

    1994-01-01

    A major goal of the Kenya Rift International Seismic Project (KRISP) 1990 experiment was the determination of deep lithospheric structure. In the refraction/wide-angle reflection part of the KRISP effort, the experiment was designed to obtain arrivals to distances in excess of 400 km. Phases from interfaces within the mantle were recorded from many shotpoints, and by design, the best data were obtained along the axial profile. Reflected arrivals from two thin (< 10 km), high-velocity layers were observed along this profile and a refracted arrival was observed from the upper high-velocity layer. These mantle phases were observed on record sections from four axial profile shotpoints so overlapping and reversed coverage was obtained. Both high-velocity layers are deepest beneath Lake Turkana and become more shallow southward as the apex of the Kenya dome is approached. The first layer has a velocity of 8.05-8.15 km/s, is at a depth of about 45 km beneath Lake Turkana, and is observed at depths of about 40 km to the south before it disappears near the base of the crust. The deeper layer has velocities ranging from 7.7 to 7.8 km/s in the south to about 8.3 km/s in the north, has a similar dip as the upper one, and is found at depths of 60-65 km. Mantle arrivals outside the rift valley appear to correlate with this layer. The large amounts of extrusive volcanics associated with the rift suggest compositional anomalies as an explanation for the observed velocity structure. However, the effects of the large heat anomaly associated with the rift indicate that composition alone cannot explain the high-velocity layers observed. These layers require some anisotropy probably due to the preferred orientation of olivine crystals. The seismic model is consistent with hot mantle material rising beneath the Kenya dome in the southern Kenya rift and north-dipping shearing along the rift axis near the base of the lithosphere beneath the northern Kenya rift. This implies lithosphere thickening towards the north and is consistent with a thermal thinning of the lithosphere from below in the south changing to thinning of the lithosphere due to stretching in the north. ?? 1994.

  2. The Porcupine Basin: from rifting to continental breakup

    NASA Astrophysics Data System (ADS)

    Reston, Timothy; Gaw, Viola; Klaeschen, Dirk; McDermott, Ken

    2015-04-01

    Southwest of Ireland, the Porcupine Basin is characterized by axial stretching factors that increase southward to values greater than six and typical of rifted margins. As such, the basin can be regarded as a natural laboratory to investigate the evolution and symmetry of rifting leading towards continental separation and breakup, and in particular the processes of mantle serpentinisation, and the onset of detachment faulting. We have processed through to prestack depth migration a series of E-W profiles crossing the basin at different axial stretching factors and linked by a N-S profile running close to the rift axis. Our results constrain the structure of the basin and have implications for the evolution of rifted margins. In the north at a latitude of 52.25N, no clear detachment is imaged, although faults do appear to cut down into the mantle, so that serpentinisation may have started. Further south (51.75N), a bright reflection (here named P) cuts down to the west from the base of the sedimentary section, is overlain by small fault blocks and appears to represent a detachment fault. P may in part follow the top of partially serpentinized mantle: this interpretation is consistent with gravity modelling, with numerical models of crustal embrittlement and mantle serpentinization during extension and with wide-angle data (see posters of Prada and of Watremez). Furthermore, P closely resembles the S reflection west of Iberia, where such serpentinites are well documented. P develops where the crust was thinned to less than 3 km during rifting, again similar to S. Although overall the basin remains symmetrical, the consistent westward structural dip of the detachment implies that, at high stretching factors, extension became asymmetric. Analysis of the depth sections suggests that the detachment may have been active as a rolling hinge rooting at low-angle beneath the Porcupine Bank, consistent with the presence of a footwall of serpentinites. This requires very weak fault rocks, such as serpentinites. Reconstructions suggest that the detachment developed after the onset of serpentinisation and thus represents late stage of faulting within a complex polyphase rift history. Farther south still, a N-S running profile shows that P cuts up to form the top of the basement, and locally forms the top of what we interpret as exhumed mantle, since buried by postrift sediments. Thus detachment here appear to have been both responsible for the late-stage extension of the crust and the unroofing of the mantle. The same processes are likely to have occurred at magma poor rifted margins.

  3. The role of structural inheritance in continental rifting

    NASA Astrophysics Data System (ADS)

    Buiter, Susanne; Tetreault, Joya

    2015-04-01

    In nature we observe that passive margins tend to originate on continental collision zones. This is not surprising as continents are long-lived and therefore have undergone multiple deformation phases, producing many regions with inherited structures. Collision zones can act as intrinsic rift-localizers for several reasons: rifting at a suture may be initiated by extensional collapse of the orogen, the thicker crustal root of orogens and their associated increase in heat producing elements makes orogens thermally weak, and inherited thrust faults form large-scale heterogeneities. When investigating continental extension geodynamically, numerical experiments often simplify such inheritance and start from laterally homogeneous crustal layers with a prescribed inhomogeneity that initiates deformation. These inhomogeneities represent thermal or structural remnants from previous deformation phases and are imposed as a thermal anomaly, a variation in Moho geometry, or an inherited weak region. However, imposed initial heterogeneities do not fully capture the structural and thermal complexities of continental sutures. Here we present 2-D numerical experiments that investigate the role of inherited crustal structures in continental rifting and passive margin formation. We first examine a series of experiments in which we explicitly prescribe collisional structures in the initial setup, such as increased Moho depth and inherited thrust faults. Different prescribed collisional structures result in different rift to break-up durations, crustal shear zone patterns, and margin symmetry. Our second series of experiments actually creates an inherited collision zone through subduction and closure of an ocean. We use this set-up to investigate how extension localizes on a former continental collision zone. Passive margin architecture strongly depends on the duration of post-collision thermal equilibration time, with a long pause between collision and initiation of extension producing wide, faulted margins that take long to break-up. We find that the two approaches of prescribing or creating a suture zone to initiate a continental rift result in different dynamics of the crust and mantle, thereby impacting rift geometry, rift to break-up duration and exhumation of subduction-related sediments and oceanic crust.

  4. Crustal structure beneath the Kenya Rift from axial profile data

    USGS Publications Warehouse

    Mechie, J.; Keller, Gordon R.; Prodehl, C.; Gaciri, S.; Braile, L.W.; Mooney, W.D.; Gajewski, D.; Sandmeier, K.-J.

    1994-01-01

    Modelling of the KRISP 90 axial line data shows that major crustal thinning occurs along the axis of the Kenya Rift from Moho depths of 35 km in the south beneath the Kenya Dome in the vicinity of Lake Naivasha to 20 km in the north beneath Lake Turkana. Low Pn velocities of 7.5-7.7 km/s are found beneath the whole of the axial line. The results indicate that crustal extension increases to the north and that the low Pn velocities are probably caused by magma (partial melt) rising from below and being trapped in the uppermost kilometres of the mantle. Along the axial line, the rift infill consisting of volcanics and a minor amount of sediments varies in thickness from zero where Precambrian crystalline basement highs occur to 5-6 km beneath the lakes Turkana and Naivasha. Analysis of the Pg phase shows that the upper crystalline crust has velocities of 6.1-6.3 km/s. Bearing in mind the Cainozoic volcanism associated with the rift, these velocities most probably represent Precambrian basement intruded by small amounts of igneous material. The boundary between the upper and lower crusts occurs at about 10 km depth beneath the northern part of the rift and 15 km depth beneath the southern part of the rift. The upper part of the lower crust has velocities of 6.4-6.5 km/s. The basal crustal layer which varies in thickness from a maximum of 2 km in the north to around 9 km in the south has a velocity of about 6.8 km/s. ?? 1994.

  5. Fluid history in hyper-extended rifted margins: Examples from the fossil Alpine and western Pyrenean rift systems and the present-day Iberia rifted continental margin.

    NASA Astrophysics Data System (ADS)

    Pinto, Victor Hugo; Manatschal, Gianreto; Karpoff, Anne Marie; Masini, Emmanuel; Lemarchand, Damien; Hayman, Nicholas; Trow, Rudolph; Viana, Adriano

    2013-04-01

    The evolution of deep-water, magma-poor rifted margins is intimately linked with complex and polyphase fault structures. These structures, known as detachment faults, are responsible for extreme crustal thinning and mantle exhumation. During the evolution of detachment faults fluid-rock interaction plays an important role, changing the chemical and physical properties of rocks. These processes likely have major implications for the strain localization and structural evolution of the margin. The change in rock chemistry and rheology is best indicated by the breakdown of feldspars and olivine into clay and serpentine minerals and the pervasive cementation and precipitation of quartz along the fault zones. Although the chemical and mineral reactions are well known, it is still unclear to what extent these reactions lead to changes in the overall rheology of the extending lithosphere and how they can affect the thermal evolution of the hyper-extended rifted margins. In order to answer to these questions it is important to understand the origin, timing, pathways and composition of the fluids generated during rifting. Are fluids solely of marine origin or do they have a metamorphic- or mantle-derived component? Can we determine the range of temperature and consequently at what depth these fluids are formed? And can we constrain the age of their migration? These questions can be addressed in the well-known hyper-extended rift systems such as the Alpine Tethys margins exposed in the Alps, the Mauléon basin in the Western Pyrenees and the Deep Iberia margin drilled and seismically imaged offshore Portugal. All of these rift settings show evidence for detachment systems associated with hyper-extension and mantle exhumation. The aim of this ongoing study is to characterize the fluid signature in hyper-extended domain in magma-poor rifted margins. Including different sites with different degrees of compressional and metamorphic overprint enables us to compare results and to define the general importance of fluid systems in the development of hyper-extended rifts systems. The first results show that in all three geological settings fluid percolation can be recognized in fault rocks linked to the detachment systems. Evidence for the presence of fluids comes from the analyses of hydration reactions in fault zones. In the Alps the major and trace elements show a gain in elements typical from mantle rocks (Mg, Ni, Cu, Co, V). In the Pyrenees, microstructural studies show that detachment faulting crossed a range of crustal depths providing constraints on the depths of fluid migration. Future analyses will focus on additional major and trace elements and isotopic ratios (Sr and B) of hydrated rocks recovered from these hyper-extended domains, which will be linked with the temporal and spatial evolution of the major detachment structures.

  6. Geochronological and geochemical assessment of Cenozoic volcanism from the Terror Rift region of the West Antarctic Rift System

    NASA Astrophysics Data System (ADS)

    Rilling, Sarah E.

    The work presented in this dissertation explains results from three different methods to determine the relation between tectonism and rift-related volcanism in the Terror Rift region of the West Antarctic Rift System (WARS). Alkaline lavas from seven submarine features, Beaufort Island and Franklin Islands, and several locations near Mt Melbourne were dated by 40Ar/39Ar geochronology and analyzed for elemental and isotopic chemical signatures. Each chapter addresses a different aspect of the hypothesis that the presence of volatiles, primarily H2O or CO2, in the magma source has led to anomalously high volumes of magmatism after rift-related decompressional melting rather than requiring an active mantle plume source. Chapter 2 provides the temporal framework, illustrating that the sampled features range in age from 6.7 Ma to 89 ka, post-dating the main Miocene age phase of Terror Rift extension. Chapter 3 illustrates the traditional enriched elemental and isotopic chemical signatures to support the overall homogeneity of these lavas and previously analyzed areas of the WARS. This chapter also provides a new model for the generation of the Pb isotopic signatures consistent with a history of metasomatism in the magma source. Chapter 4 provides an entirely new chemical dataset for the WARS. The first platinum group element (PGE) abundances and extremely unradiogenic Os isotopic signatures of Cenozoic lavas from Antarctica provide the strongest evidence of melting contributions from a lithospheric mantle source. The combined results from these three studies consistently support the original hypothesis of this dissertation. New evidence suggests that WARS related lavas are not related to a mantle plume(s) as previously proposed. Instead, they are generated by passive, decompressional melting of a source, likely a combination of the asthenospheric and lithospheric mantle, which has undergone previous melting events and metasomatism.

  7. Colorado Basin Structure and Rifting, Argentine passive margin

    NASA Astrophysics Data System (ADS)

    Autin, Julia; Scheck-Wenderoth, Magdalena; Loegering, Markus; Anka, Zahie; Vallejo, Eduardo; Rodriguez, Jorge; Marchal, Denis; Reichert, Christian; di Primio, Rolando

    2010-05-01

    The Argentine margin presents a strong segmentation with considerable strike-slip movements along the fracture zones. We focus on the volcanic segment (between the Salado and Colorado transfer zones), which is characterized by seaward dipping reflectors (SDR) all along the ocean-continent transition [e.g. Franke et al., 2006; Gladczenko et al., 1997; Hinz et al., 1999]. The segment is structured by E-W trending basins, which differs from the South African margin basins and cannot be explained by classical models of rifting. Thus the study of the relationship between the basins and the Argentine margin itself will allow the understanding of their contemporary development. Moreover the comparison of the conjugate margins suggests a particular evolution of rifting and break-up. We firstly focus on the Colorado Basin, which is thought to be the conjugate of the well studied Orange Basin [Hirsch et al., 2009] at the South African margin [e.g. Franke et al., 2006]. This work presents results of a combined approach using seismic interpretation and structural, isostatic and thermal modelling highlighting the structure of the crust. The seismic interpretation shows two rift-related discordances: one intra syn-rift and the break-up unconformity. The overlying sediments of the sag phase are less deformed (no sedimentary wedges) and accumulated before the generation of oceanic crust. The axis of the Colorado Basin trends E-W in the western part, where the deepest pre-rift series are preserved. In contrast, the basin axis turns to a NW-SE direction in its eastern part, where mainly post-rift sediments accumulated. The most distal part reaches the margin slope and opens into the oceanic basin. The general basin direction is almost orthogonal to the present-day margin trend. The most frequent hypothesis explaining this geometry is that the Colorado Basin is an aborted rift resulting from a previous RRR triple junction [e.g. Franke et al., 2002]. The structural interpretation partly supports this hypothesis and shows two main directions of faulting: margin-parallel faults (~N30°) and rift-parallel faults (~N125°). A specific distribution of the two fault sets is observed: margin-parallel faults are restrained to the most distal part of the margin. Starting with a 3D structural model of the basin fill based on seismic and well data the deeper structure of the crust beneath the Colorado Basin can be evaluate using isostatic and thermal modelling. Franke, D., et al. (2002), Deep Crustal Structure Of The Argentine Continental Margin From Seismic Wide-Angle And Multichannel Reflection Seismic Data, paper presented at AAPG Hedberg Conference "Hydrocarbon Habitat of Volcanic Rifted Passive Margins", Stavanger, Norway Franke, D., et al. (2006), Crustal structure across the Colorado Basin, offshore Argentina Geophysical Journal International 165, 850-864. Gladczenko, T. P., et al. (1997), South Atlantic volcanic margins Journal of the Geological Society, London 154, 465-470. Hinz, K., et al. (1999), The Argentine continental margin north of 48°S: sedimentary successions, volcanic activity during breakup Marine and Petroleum Geology 16(1-25). Hirsch, K. K., et al. (2009), Tectonic subsidence history and thermal evolution of the Orange Basin, Marine and Petroleum Geology, in press, doi:10.1016/j.marpetgeo.2009.1006.1009

  8. Deformation and seismicity associated with continental rift zones propagating toward continental margins

    NASA Astrophysics Data System (ADS)

    Lyakhovsky, V.; Segev, A.; Schattner, U.; Weinberger, R.

    2012-01-01

    We study the propagation of a continental rift and its interaction with a continental margin utilizing a 3-D lithospheric model with a seismogenic crust governed by a damage rheology. A long-standing problem in rift-mechanics, known as thetectonic force paradox, is that the magnitude of the tectonic forces required for rifting are not large enough in the absence of basaltic magmatism. Our modeling results demonstrate that under moderate rift-driving tectonic forces the rift propagation is feasible even in the absence of magmatism. This is due to gradual weakening and "long-term memory" of fractured rocks that lead to a significantly lower yielding stress than that of the surrounding intact rocks. We show that the style, rate and the associated seismicity pattern of the rift zone formation in the continental lithosphere depend not only on the applied tectonic forces, but also on the rate of healing. Accounting for the memory effect provides a feasible solution for thetectonic force paradox. Our modeling results also demonstrate how the lithosphere structure affects the geometry of the propagating rift system toward a continental margin. Thinning of the crystalline crust leads to a decrease in the propagation rate and possibly to rift termination across the margin. In such a case, a new fault system is created perpendicular to the direction of the rift propagation. These results reveal that the local lithosphere structure is one of the key factors controlling the geometry of the evolving rift system and seismicity pattern.

  9. Fault evolution in the Potiguar rift termination, Equatorial margin of Brazil

    NASA Astrophysics Data System (ADS)

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

    2014-10-01

    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 fault architecture and to analyse 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 Potiguar rift is a Neocomian structure located in 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- to EW-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 post-rift 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.

  10. Aerosol and Cloud Microphysical Characteristics of Rifts and Gradients in Maritime Stratocumulus Clouds

    NASA Technical Reports Server (NTRS)

    Sharon, Tarah M.; Albrecht, Bruce A.; Jonsson, Haflidi H.; Minnis, Patrick; Khaiyer, Mandana M.; Van Reken, Timothy; Seinfeld, John; Flagan, Rick

    2008-01-01

    A cloud rift is characterized as a large-scale, persistent area of broken, low reflectivity stratocumulus clouds usually surrounded by a solid deck of stratocumulus. A rift observed off the coast of Monterey Bay, California on 16 July 1999 was studied to compare the aerosol and cloud microphysical properties in the rift with those of the surrounding solid stratus deck. Variables measured from an instrumented aircraft included temperature, water vapor, and cloud liquid water. These measurements characterized the thermodynamic properties of the solid deck and rift areas. Microphysical measurements made included aerosol, cloud drop and drizzle drop concentrations and cloud condensation nuclei (CCN) concentrations. The microphysical characteristics in a solid stratus deck differ substantially from those of a broken, cellular rift where cloud droplet concentrations are a factor of 2 lower than those in the solid cloud. Further, CCN concentrations were found to be about 3 times greater in the solid cloud area compared with those in the rift and aerosol concentrations showed a similar difference as well. Although drizzle was observed near cloud top in parts of the solid stratus cloud, the largest drizzle rates were associated with the broken clouds within the rift area. In addition to marked differences in particle concentrations, evidence of a mesoscale circulation near the solid cloud rift boundary is presented. This mesoscale circulation provides a mechanism for maintaining a rift, but further study is required to understand the initiation of a rift and the conditions that may cause it to fill.

  11. Contour mapping of relic structures in the Precambrian basement of the Reelfoot rift, North American midcontinent

    USGS Publications Warehouse

    Dart, R.L.; Swolfs, H.S.

    1998-01-01

    A new contour map of the basement of the Reelfoot rift constructed from drill hole and seismic reflection data shows the general surface configuration as well as several major and minor structural features. The major features are two asymmetric intrarift basins, bounded by three structural highs, and the rift margins. The basins are oriented normal to the northeast trend of the rift. Two of the highs appear to be ridges of undetermined width that extend across the rift. The third high is an isolated dome or platform located between the basins. The minor features are three linear structures of low relief oriented subparallel to the trend of the rift. Two of these, located within the rift basins, may divide the rift basins into paired subbasins. These mapped features may be the remnants of initial extensional rifting, half graben faulting, and basement subsidence. The rift basins are interpreted as having formed as opposing half graben, and the structural highs are interpreted as having formed as associated accommodation zones. Some of these features appear to be reactivated seismogenic structures within the modem midcontinent compressional stress regime. A detailed knowledge of the geometries of the Reelfoot rift's basement features, therefore, is essential when evaluating their seismic risk potential.

  12. Inversion tectonics of a back arc rift basin: A case study in Niigata, central Japan

    NASA Astrophysics Data System (ADS)

    Sato, H.; Abe, S.; Kawai, N.; Saito, H.; Kato, N.; Ishiyama, T.; Iwasaki, T.; Kurashimo, E.; Inaba, M.; Van Horne, A.

    2011-12-01

    A back arc rift basin, formed during the Miocene opening of the Japan Sea, now uplifted and exposed in Niigata, central Japan, provides a unique opportunity to study a back arc rift formed on a short time scale and in a still active setting. We used onshore-offshore deep seismic reflection profiling to examine the crustal architecture of the back arc basin, in particular the geometry of the source faults, along four seismic lines between 2008 and 2011. We further applied refraction tomography analysis to distinguish between previously undifferentiated syn-rift volcanics and pre-rift basement rock based on P-wave velocity. Our findings indicate that the Miocene rift structure created during the extensional phase regulates the style of deformation and the geometry of the source faults in the current compressional regime. Syn-rift volcanics with a maximum thickness of 6 km filled the fault controlled basins as rifting proceeded. The volcanism was bimodal, comprising a reflective unit of mafic rocks around the rift axis and a non-reflective unit of felsic rocks near the margins of the basins. Once rifting ended, thermal subsidence, and subsequently, mechanical subsidence related to the onset of the compressional regime, allowed deposition of up to 5 km of post-rift, deep marine to fluvial sedimentation, including the Teradomari Fm., an over-pressured mudstone in the middle of the section that later became an important shallow detachment layer. Continued compression has caused fault-related fold and wedge thrusting in the post-rift sedimentary strata which are highly deformed by thin-skin style deformation. Since the Pliocene, normal faults created during the rift phase have been reactivated as reverse faults, including a shallow detachment in the Teradomari Fm. which forms a complicated shortened deformation structure. Quaternary geomorphology suggests ongoing shortening. Transform faults inherited from the rift stage control the extent of present day reverse source faults and more importantly, earthquake magnitude.

  13. Radial Anisotropy beneath the Main Ethiopian Rift and Afar Depression

    NASA Astrophysics Data System (ADS)

    Accardo, N. J.; Gaherty, J. B.; Jin, G.; Shillington, D. J.

    2014-12-01

    The Main Ethiopian Rift (MER) and Afar uniquely capture the final stages of transition from continental rifting in the broader East African Rift System to incipient seafloor spreading above a mantle hotspot. Studies of the region increasingly point to magmatism as a controlling factor on continental extension. However, the character and depth extent of these melt products remain contentious. Radial anisotropy derived from surface waves provides a unique diagnostic constraint on the presence of oriented melt pockets versus broader oriented anisotropic fabrics. This study investigates the thermal and radially anisotropic structure beneath the broader MER and Afar to resolve the magmatic character of the region and ultimately to understand the role of magmatism in present day rift development. We utilize 104 stations from 4 collocated arrays in the MER/Afar region to constrain radial anisotropy within the upper mantle via the inversion of Love- and Rayleigh-wave observations between 25 and 100 s period. We employ a multi-channel cross-correlation algorithm to obtain inter-station phase and amplitude information. The multi-channel phase observations are inverted for dynamic phase velocity across the array, which are then corrected for focusing and multipathing using the amplitude observations via Helmholtz tomography. We jointly invert Love- and Rayleigh-wave structural phase velocity measurements employing crustal constraints from co-located active source experiments to obtain estimates of Vsv and Vsh between 50 - 170 km depth. Preliminary results readily reveal the distinct shear velocity structure beneath the MER and Afar. Within the MER, shear velocity structure suggests pronounced low velocities accompanied by strong anisotropy between 80 - 140 km depth beneath the western Ethiopian plateau and rift valley. Within Afar, shear velocity structure is more varied with the slowest velocities found at shallow depths (less than 70 km depth), accompanied by weak anisotropy. The pronounced changes in the depth extent of slow velocities and strength of anisotropy interpreted to be associated with asthenosphere may reflect variations in the distribution and magnitude of temperature anomalies/melt between continental rifting in the MER and incipient, hot-spot influenced seafloor spreading in Afar.

  14. Albertine Rift, Uganda: Deformation-Sedimentation-Erosion relationships

    NASA Astrophysics Data System (ADS)

    Simon, Brendan; Guillocheau, François; Robin, Cécile; Dauteuil, Olivier; Nalpas, Thierry; Bourges, Philippe; Bez, Martine; Lays, Philippe

    2014-05-01

    The Albertine Rift is the northern part of the western branch of the East African Rift that runs over a distance of around 2000 km from Lake Albert in the north to Lake Malawi in the south. Lake Albert Basin is assumed to be a classical half-graben initiated around 12 Ma and oriented NNW-SSW, with a major northwesterly bounding fault - the Bunia fault - located along the western Congolese shoreline (Ebinger, 1989; Pickford & al., 1993). The aim of this study is to understand the relationships between deformation, erosion, and sedimentation of the rift through time by restoring (1) the timing and amplitude of vertical movements (subsidence, uplift), (2) the geometry and paleo-environmental evolution (including climate) of the sedimentary infilling and (3) the geomorphological evolution of the surrounding area and associated erosion budget. Seismic data and outcrops studies suggest a much more complex history than previously described. (1) The age model, mainly based on mammal fossils (Pickford et al., 1993; Van Damme and Pickford, 2003), is debated, but the early stage of the rift is probably Middle Miocene. (2) No half-graben geometry has been characterized: the infilling consists of juxtaposed tabular compartments with sharp thicknesses variations along bounding faults, in response of either low rate extensional or combined strike-slip/extensional movements. The following onshore-offshore evolution is proposed: - Middle Miocene (~ 13 Ma) to Late Miocene (?): rifting 1 - differential subsidence along N60° faults - major deepening from fluvio-deltaic to deep lacustrine environments (maximum flooding at 8 Ma) - uplift, erosion and reworking of weathered profiles - first generation of pediments. - Late Miocene (?) to Late Pliocene (~ 3 Ma): quiescence phase - homogenous subsidence - lacustrine clays interbedded with sandy flood-lobes - uplift, erosion and reworking of ferruginous laterite (iron duricrusts) - second generation of pediments. - Late Pliocene (~ 3Ma) to Early Pleistocene (~ 2 Ma): rifting 2 - major uplift and growth of the Ruwenzori Mountains (5000 m of elevation) - differential subsidence - deltaic to wave-dominated coast (shoreface) sandy deposits - pediments degradation by fluvial erosion. - Middle-Late Pleistocene: late regional uplift and tilting - drainage inversion and present-day scarp formation.

  15. Continental rift development in Precambrian and Phanerozoic Europe: EUROPROBE and the Dnieper-Donets Rift and Polish Trough basins

    NASA Astrophysics Data System (ADS)

    Stephenson, R. A.

    1993-07-01

    Upper crustal structure and lower crustal and mantle character of the European continental lithosphere differ significantly from west to east. The processes of rifting of Phanerozoic western Europe's hotter, thinner crust compared with the more stable, thicker (45 vs 30 km) Proterozoic crust of the East European Craton is being investigated by EUROPROBE case studies of western and eastern European sedimentary basins. The Dnieper—Donets basin transects the southwestern part of the East European platform in a NW—SE direction, lying between the Ukrainian Shield and the Voronezh Massif. Rifting took place from early Frasnian until perhaps the late Visean and was accompanied by major volcanic activity. The distribution of both was affected by pre-existing basement fault systems. The syn- and post-rift sedimentary succession may be as thick as 20 km in the Donets Trough. Basin evolution and the present crustal geometry have been much affected by "inversion" during the Permian in association with Uralian orogenesis. Tectonic events of Bathonian—Oxfordian (˜ 168-154 Ma) and Turonian—Santonian (˜ 91-84 Ma) age are also recorded in the basin stratigraphy. The Polish Trough lies along the boundary between the Phanerozoic and Proterozoic European crustal domains, coincident with the Trans-European Suture Zone. The presence of this rheological boundary may be paramount in structurally controlling the position of the Trough. Tectonic subsidence analysis indicates an initial (late) Rotliegendes—Early Triassic syn-rift phase of development. Subsequently, episodes of increased tectonic subsidence rate occurred during the Oxfordian—Tithonian (? 157-146 Ma) and beginning in the Cenomanian (? 95-90 Ma). The Oxfordian—Tithonian episode is likely the signature of a second extensional event (correlated with intensification of rifting within the Arctic—North Atlantic rift system) while the Cenomanian and later is a precursor to mild compressional, intraplate orogenic movements. Present-day crustal structure reflects the "docking" of Phanerozoic crust against the Proterozoic during Palaeozoic orogenies, Permo—Carboniferous wrenching and transtensional modifications leading to formation of the Polish Trough, as well as modifications during the Late Cretaceous—Early Tertiary inversion process.

  16. Novel approaches to develop Rift Valley fever vaccines

    PubMed Central

    Indran, Sabarish V.; Ikegami, Tetsuro

    2012-01-01

    Rift Valley fever (RVF) is endemic to sub-Saharan Africa, and has spread into Madagascar, Egypt, Saudi Arabia, and Yemen. Rift Valley fever virus (RVFV) of the family Bunyaviridae, genus Phlebovirus causes hemorrhagic fever, neurological disorders or blindness in humans, and high rate abortion and fetal malformation in ruminants. RVFV is classified as a Category A Priority pathogen and overlap select agent by CDC/USDA due to its potential impact on public health and agriculture. There is a gap in the safety and immunogenicity in traditional RVF vaccines; the formalin-inactivated RVFV vaccine TSI-GSD-200 requires three doses for protection, and the live-attenuated Smithburn vaccine has a risk to cause abortion and fetal malformation in pregnant ruminants. In this review, problems of traditional vaccines and the safety and efficacy of recently reported novel RVF candidate vaccines including subunit vaccines, virus vector, and replicons are discussed. PMID:23112960

  17. Dike intrusions during rifting episodes obey scaling relationships similar to earthquakes

    NASA Astrophysics Data System (ADS)

    Passarelli, L.; Rivalta, E.; Shuler, A.

    2014-01-01

    As continental rifts evolve towards mid-ocean ridges, strain is accommodated by repeated episodes of faulting and magmatism. Discrete rifting episodes have been observed along two subaerial divergent plate boundaries, the Krafla segment of the Northern Volcanic Rift Zone in Iceland and the Manda-Hararo segment of the Red Sea Rift in Ethiopia. In both cases, the initial and largest dike intrusion was followed by a series of smaller intrusions. By performing a statistical analysis of these rifting episodes, we demonstrate that dike intrusions obey scaling relationships similar to earthquakes. We find that the dimensions of dike intrusions obey a power law analogous to the Gutenberg-Richter relation, and the long-term release of geodetic moment is governed by a relationship consistent with the Omori law. Due to the effects of magma supply, the timing of secondary dike intrusions differs from that of the aftershocks. This work provides evidence of self-similarity in the rifting process.

  18. Dike intrusions during rifting episodes obey scaling relationships similar to earthquakes

    PubMed Central

    L., Passarelli; E., Rivalta; A., Shuler

    2014-01-01

    As continental rifts evolve towards mid-ocean ridges, strain is accommodated by repeated episodes of faulting and magmatism. Discrete rifting episodes have been observed along two subaerial divergent plate boundaries, the Krafla segment of the Northern Volcanic Rift Zone in Iceland and the Manda-Hararo segment of the Red Sea Rift in Ethiopia. In both cases, the initial and largest dike intrusion was followed by a series of smaller intrusions. By performing a statistical analysis of these rifting episodes, we demonstrate that dike intrusions obey scaling relationships similar to earthquakes. We find that the dimensions of dike intrusions obey a power law analogous to the Gutenberg-Richter relation, and the long-term release of geodetic moment is governed by a relationship consistent with the Omori law. Due to the effects of magma supply, the timing of secondary dike intrusions differs from that of the aftershocks. This work provides evidence of self-similarity in the rifting process. PMID:24469260

  19. Dike intrusions during rifting episodes obey scaling relationships similar to earthquakes.

    PubMed

    Passarelli, L; Rivalta, E; Shuler, A

    2014-01-01

    As continental rifts evolve towards mid-ocean ridges, strain is accommodated by repeated episodes of faulting and magmatism. Discrete rifting episodes have been observed along two subaerial divergent plate boundaries, the Krafla segment of the Northern Volcanic Rift Zone in Iceland and the Manda-Hararo segment of the Red Sea Rift in Ethiopia. In both cases, the initial and largest dike intrusion was followed by a series of smaller intrusions. By performing a statistical analysis of these rifting episodes, we demonstrate that dike intrusions obey scaling relationships similar to earthquakes. We find that the dimensions of dike intrusions obey a power law analogous to the Gutenberg-Richter relation, and the long-term release of geodetic moment is governed by a relationship consistent with the Omori law. Due to the effects of magma supply, the timing of secondary dike intrusions differs from that of the aftershocks. This work provides evidence of self-similarity in the rifting process. PMID:24469260

  20. Rifted Continental Margins: The Case for Depth-Dependent Extension

    NASA Astrophysics Data System (ADS)

    Huismans, Ritske S.; Beaumont, Christopher

    2015-04-01

    Even though many basic properties of non-volcanic rifted margins are predicted by uniform extension of the lithosphere, uniform extension fails to explain other important characteristics. Particularly significant discrepancies are observed at: 1) the Iberia-Newfoundland conjugate margins (Type I), where large tracts of continental mantle lithosphere are exposed at the seafloor, and at; 2) ultra-wide central South Atlantic margins (Type II) where continental crust spans wide regions below which it appears that lower crust and mantle lithosphere were removed. Neither corresponds to uniform extension in which crust and mantle thin by the same factor. Instead, either the crust or mantle lithosphere has been preferentially removed during extension. We show that the Type I and II styles are respectively reproduced by dynamical numerical lithospheric stretching models (Models I-A/C and II-A/C) that undergo depth-dependent extension. In this notation A and C imply underplating of the rift zone during rifting by asthenosphere and lower cratonic lithosphere, respectively. We also present results for models with a weak upper crust and strong lower crust, Models III-A/C, to show that lower crust can also be removed from beneath the rift zone by horizontal advection with the mantle lithosphere. From the model results we infer that these Types I, II, and III margin styles are controlled by the strength of the mid/lower crust, which determines the amount of decoupling between upper and lower lithosphere during extension and the excision of crust or mantle. We also predict the styles of sedimentary basins that form on these margins as a test of the concepts presented.

  1. Erosion of Terrestrial Rift Flank Topography: A Quantitative Study

    NASA Technical Reports Server (NTRS)

    Weissel, Jeffrey K.

    1999-01-01

    Many rifted or passive continental margins feature a seaward-facing erosional escarpment which abruptly demarcates deeply weathered, low relief, interior uplands from a deeply incised, high relief coastal zone. It is generally accepted that these escarpments originate at the time of continental rifting and propagate inland through the elevated rift flank topography at rates on the order of 1 km/Myr over the course of a margin's history. Considering the length of passive margins worldwide and an average rift flank plateau height of several hundred meters, it is clear that sediment eroded from passive margins is an important component of the mass flux from continents to oceans through geologic time. The overall goal of the research reported here is to develop a quantitative understanding of the kinematics of escarpment propagation across passive margins and the underlying geological processes responsible for this behavior. Plateau-bounding escarpments in general exhibit two basic forms depending on the direction of surface water drainage on the plateau interior relative to the escarpment. Where surface water flows away from the escarpment, the escarpment takes the form of subdued embayments and promontories, such that its overall trend remains fairly straight as it evolves with time. Where upland streams flow across the escarpment, it takes the form of dramatic, narrow gorges whose heads appear to propagate up the plateau drainage systems as large-scale knickpoints. From work on the Colorado Plateau, Schmidt (1987) noted that the Colorado River is located much closer to the Grand Canyon's south rim, a drainage divide escarpment, than to the north rim, which is a gorge-like escarpment. The main implication is that the gorge-like form might be associated with higher long-term average erosion rates compared to the drainage divide escarpment type.

  2. The Sagatu Ridge dike swarm, Ethiopian rift margin. [tectonic evolution

    NASA Technical Reports Server (NTRS)

    Mohr, P. A.; Potter, E. C.

    1976-01-01

    A swarm of dikes forms the core of the Sagatu Ridge, a 70-km-long topographic feature elevated to more than 4000 m above sea level and 1500 m above the level of the Eastern (Somalian) plateau. The ridge trends NNE and lies about 50 km east of the northeasterly trending rift-valley margin. Intrusion of the dikes and buildup of the flood-lava pile, largely hawaiitic but with trachyte preponderant in the final stages, occurred during the late Pliocene-early Pleistocene and may have been contemporaneous with downwarping of the protorift trough to the west. The ensuing faulting that formed the present rift margin, however, bypassed the ridge. The peculiar situation and orientation of the Sagatu Ridge, and its temporary existence as a line of crustal extension and voluminous magmatism, are considered related to a powerful structural control by a major line of Precambrian crustal weakness, well exposed further south. Transverse rift structures of unknown type appear to have limited the development of the ridge to the north and south.

  3. Mapping of the major structures of the African rift system

    NASA Technical Reports Server (NTRS)

    Mohr, P. A. (principal investigator)

    1972-01-01

    The author has identified the following significant results. ERTS-1 imagery of the African rift system resolves the major Cenozoic faults, zones of warping, and the associated volcanism. It also clearly depicts the crustal grain of the Precambrian rocks where these are exposed. New structural features, or new properties of known features such as greater extent, continuity, linearity, etc., are revealed by the ERTS-1 imagery. This applies to the NE-SW fracture zones in Yemen, the Aswa mylonite zone at the northern end of the Western Rift, the Nandi fault of western Kenya, the arcuate faults of the Elgeyo escarpment in the Gregory rift, and hemi-basins of warped Tertiary lavas on the Red Sea margin of Yemen, matching those of the Ethiopian plateau-Afar margin. A tentative scheme is proposed, relating the effect on the pattern of Cenozoic faulting of the degree of obliquity to Precambrian structural trend. Some ground-mapped lithological boundaries are obscure on ERTS-1 imagery. The present approaches to mapping of Precambrian terrain in Africa may require radical revision with the input of satellite imagery.

  4. Faulting of the lithosphere during extension and related rift-flank uplift: a numerical study

    NASA Astrophysics Data System (ADS)

    Sachau, Till; Koehn, Daniel

    2010-10-01

    In this contribution, we present a new model of passive rifting and related rift-flank uplift. The numerical model is based on a lattice spring network coupled with a viscous particle model so that we can simulate visco-elasto-plastic behaviour with dynamic fault development. In our model, we show that rift-flank uplift can be achieved best when extension in the crust is localized and the lower crust is strong so that major rift faults transect the whole crust. Uplift of rift flanks follows a smooth function whereas down-throw in the rift basin takes place in steps. The geometry of the developing faults has also an influence on the uplift; in this case, displacement along major rift faults produces higher flanks than distributed displacement on many faults. Our model also shows that the relative elastic thickness of the crust has only a minor influence on the uplift since fault depth and elastic thickness are not independent. In addition, we show with a second set of simulations and analytically that a strain misfit between the upper and lower boundaries of a stretched crust, which is created by the horizontal extension, leads to an active uplift driven by elastic forces. We compare the numerical simulations, the analytical solution and real surface data from the Albertine rift in the East African Rift System and show that our new model can reproduce realistic features. Our two-layer beam model with strain misfit can also explain why a thick crust in the simulations can have an even higher rift flank than a thin crust even though the thin crust topography has a higher curvature. We discuss the implications of our simulations for real rift systems and for the current theory of rift-flank uplift.

  5. A model for Iapetan rifting of Laurentia based on Neoproterozoic dikes and related rocks

    USGS Publications Warehouse

    Burton, William C.; Southworth, Scott

    2010-01-01

    Geologic evidence of the Neoproterozoic rifting of Laurentia during breakup of Rodinia is recorded in basement massifs of the cratonic margin by dike swarms, volcanic and plutonic rocks, and rift-related clastic sedimentary sequences. The spatial and temporal distribution of these geologic features varies both within and between the massifs but preserves evidence concerning the timing and nature of rifting. The most salient features include: (1) a rift-related magmatic event recorded in the French Broad massif and the southern and central Shenandoah massif that is distinctly older than that recorded in the northern Shenandoah massif and northward; (2) felsic volcanic centers at the north ends of both French Broad and Shenandoah massifs accompanied by dike swarms; (3) differences in volume between massifs of cover-sequence volcanic rocks and rift-related clastic rocks; and (4) WNW orientation of the Grenville dike swarm in contrast to the predominately NE orientation of other Neoproterozoic dikes. Previously proposed rifting mechanisms to explain these features include rift-transform and plume–triple-junction systems. The rift-transform system best explains features 1, 2, and 3, listed here, and we propose that it represents the dominant rifting mechanism for most of the Laurentian margin. To explain feature 4, as well as magmatic ages and geochemical trends in the Northern Appalachians, we propose that a plume–triple-junction system evolved into the rift-transform system. A ca. 600 Ma mantle plume centered east of the Sutton Mountains generated the radial dike swarm of the Adirondack massif and the Grenville dike swarm, and a collocated triple junction generated the northern part of the rift-transform system. An eastern branch of this system produced the Long Range dike swarm in Newfoundland, and a subsequent western branch produced the ca. 554 Ma Tibbit Hill volcanics and the ca. 550 Ma rift-related magmatism of Newfoundland.

  6. Varying styles of magmatic strain accommodation across the East African Rift

    NASA Astrophysics Data System (ADS)

    Muirhead, James D.; Kattenhorn, Simon A.; Le Corvec, Nicolas

    2015-09-01

    Observations of active dike intrusions provide present day snapshots of the magmatic contribution to continental rifting. However, unravelling the contributions of upper crustal dikes over the timescale of continental rift evolution is a significant challenge. To address this issue, we analyzed the morphologies and alignments of >1500 volcanic cones to infer the distribution and trends of upper crustal dikes in various rift basins across the East African Rift (EAR). Cone lineament data reveal along-axis variations in the distribution and geometries of dike intrusions as a result of changing tectonomagmatic conditions. In younger (<10 Ma) basins of the North Tanzanian Divergence, dikes are largely restricted to zones of rift-oblique faulting between major rift segments, referred to here as transfer zones. Cone lineament trends are highly variable, resulting from the interplay between (1) the regional stress field, (2) local magma-induced stress fields, and (3) stress rotations related to mechanical interactions between rift segments. We find similar cone lineament trends in transfer zones in the western branch of the EAR, such as the Virunga Province, Democratic Republic of the Congo. The distributions and orientations of upper crustal dikes in the eastern branch of the EAR vary during continental rift evolution. In early-stage rifts (<10 Ma), upper crustal dikes play a limited role in accommodating extension, as they are confined to areas in and around transfer zones. In evolved rift basins (>10 Ma) in Ethiopia and the Kenya Rift, rift-parallel dikes accommodate upper crustal extension along the full length of the basin.

  7. Seismic structure of the Central US crust and shallow upper mantle: Uniqueness of the Reelfoot Rift

    NASA Astrophysics Data System (ADS)

    Pollitz, Fred F.; Mooney, Walter D.

    2014-09-01

    Using seismic surface waves recorded with Earthscope's Transportable Array, we apply surface wave imaging to determine 3D seismic velocity in the crust and uppermost mantle. Our images span several Proterozoic and early Cambrian rift zones (Mid-Continent Rift, Rough Creek Graben-Rome trough, Birmingham trough, Southern Oklahoma Aulacogen, and Reelfoot Rift). While ancient rifts are generally associated with low crustal velocity because of the presence of thick sedimentary sequences, the Reelfoot Rift is unique in its association with low mantle seismic velocity. Its mantle low-velocity zone (LVZ) is exceptionally pronounced and extends down to at least 200 km depth. This LVZ is of variable width, being relatively narrow (?50 km wide) within the northern Reelfoot Rift, which hosts the New Madrid Seismic Zone (NMSZ). We hypothesize that this mantle volume is weaker than its surroundings and that the Reelfoot Rift consequently has relatively low elastic plate thickness, which would tend to concentrate tectonic stress within this zone. No other intraplate ancient rift zone is known to be associated with such a deep mantle low-velocity anomaly, which suggests that the NMSZ is more susceptible to external stress perturbations than other ancient rift zones.

  8. Rift flank segmentation, basin initiation and propagation: a neotectonic example from Lake Baikal

    USGS Publications Warehouse

    Agar, S.M.; Klitgord, Kim D.

    1995-01-01

    New surficial data (field, Landsat TM and topography) define morpho-tectonic domains and rift flank segmentation in the Ol'khon region of the Central Baikal rift. Deformation, drainage and depositional patterns indicate a change in the locus of active extension that may relate to a recent (rift with concomitant shifts in depocentres. Within the hanging wall of the new western border fault, distinct segments control the location of drainage paths and syn-rift deposits. Morphology, sediment thicknesses and fault scarp amplitude indicate that a segmented rift flank graben has propagated southwards along the rift flank and is still actively fragmenting. These surficial data are used to constrain a model for the time-dependent topographic variations during progressive subsidence along a rift flank, involving the transfer of footwall units to hanging-wall domains. Rapid changes in border fault footwall relief in this model are associated with change in the active border fault location with widespread mass-wasting. The model shows that time-dependent histories need to be integrated with flexural uplift models for active normal faults. The active, syn-rift depositional systems of the Ol'khon region provide a valuable analogue for the early evolution of continental margins and the structural controls on syn-rift hydrocarbon sources and reservoirs.

  9. Tectonics and stratigraphy of the East Brazil Rift system: an overview

    NASA Astrophysics Data System (ADS)

    Hung Kiang Chang; Kowsmann, Renato Oscar; Figueiredo, Antonio Manuel Ferreira; Bender, AndréAdriano

    1992-10-01

    The East Brazilian Rift system (Ebris) constitutes the northern segment of the South Atlantic rift system which developed during the Mesozoic breakup of South America and Africa. Following crustal separation in the Late Aptian, it evolved into a passive continental margin. Along the continental margin six basins are recognized, while three onshore basins form part of an aborted rift. Three continental syn-rift stratigraphic sequences are recognized, spanning Jurassic to Barremian times. The Jurassic (Syn-rift I) and Neocomian (Syn-rift II) phases were most active in the interior rift basins. During the Barremian (Syn-rift III), rift subsidence rates were twice as large as during the Neocomian (Syn-rift II), both in the interior rift and in the marginal rift segments, indicating that rift axis did not migrate from the interior to the marginal setting. Rift magmatism was centered on the southern EBRIS and peaked between 130 and 120 Ma during syn-rift phase II. Rift phase III was followed by a transitional marine, evaporitic megasequence of Aptian age, which directly overlies the rift unconformity and a marine drift megasequence which spans Albian to Recent times. During the Late Cretaceous, sedimentation rates responded to first-order eustatic sea-level fluctuations. Tertiary accelerated sedimentation rates can be related to local clastic supply which filled in spaces inherited from previous starved conditions. Between 60 and 40 Ma, post-rift magmatism, centered on the Abrolhos and Royal Charlotte banks, is probably related to development of a hot spot associated with the Vitória-Trindade Seamount Chain. Although crossing three distinct Precambrian tectono-thermal provinces, ranging from Archean through Late Proterozoic, rift structures follow a general NE trend, subparallel to the principal basement fabric. A NW-SE oriented stress field appears to be compatible with both Neocomian and Barremian phases of crustal extension. Profiles transverse to the rift axis indicate crustal stretching factors ranging between ? = 2.16 and 2.88. In the shallow portions of the rift, surface extension and crustal thinning seem to be compatible; however, in the deep portions of the basins, this relationship could not be tested. Reinterpretation of refraction profiles, north and south of the Walvis-São Paulo Ridge transform, indicates that seafloor spreading, from M3 anomaly to Aptian off Pelotas Basin, was taken up by crustal extension in the São Paulo Plateau. Differences in stretching rates may have been accommodated by extension across the Ponta Grossa Arch. The Early Aptian syn-rift/post-rift transition in the EBRIS marginal basins does not coincide with the onset of the drift phase during the Early Albian. This apparent discrepancy may be explained by a change from distributed margin-wide extension to a focused mode of extension near the future continent/ocean boundary.

  10. Hydrothermal vents is Lake Tanganyika, East African Rift system

    SciTech Connect

    Tiercelin, J.J.; Pflumio, C.; Castrec, M.

    1993-06-01

    Sublacustrine hydrothermal vents with associated massive sulfides were discovered during April 1987 at Pemba and Cape Banza on the Zaire side of the northern basin of Lake Tanganyika, East African Rift system. New investigations by a team of ten scuba divers during the multinational (France, Zaire, Germany, and Burundi) TANGANYDRO expedition (August-October 1991) found hydrothermal vents down to a depth of 46 m along north-trending active faults bounding the Tanganyika rift on the western side. Temperatures from 53 to 103 {degrees}C were measured in hydrothermal fluids and sediments. Veins of massive sulfides 1-10 cm thick (pyrite and marcasite banding) were found associated with vents at the Pemba site. At Cape Banza, active vents are characterized by 1-70-cm-high aragonite chimneys, and there are microcrystalline pyrite coatings on the walls of hydrothermal pipes. Hydrothermal fluid end members show distinctive compositions at the two sites. The Pemba end member is a NaHCO{sub 3}-enriched fluid similar to the NaHCO{sub 3} thermal fluids form lakes Magadi and Bogoria in the eastern branch of the rift. The Cape Banza end member is a solution enriched in NaCl. Such brines may have a deep-seated basement origin, as do the Uvinza NaCl brines on the eastern flank of the Tanganyika basin. Geothermometric calculations have yielded temperatures of fluid-rock interaction of 219 and 179 {degrees}C in the Pemba and Cape Banza systems, respectively. Abundant white or reddish-brown microbial colonies resembling Beggiatoa mats were found surrounding the active vents. Thermal fluid circulation is permitted by opening of cracks related to 130{degrees}N normal-dextral faults that intersect the north-south major rift trend. The sources of heat for such hydrothermal systems may relate to the existence of magmatic bodies under the rift, which is suggested by the isotopic composition of carbon dioxide released at Pemba and Cape Banza. 21 refs., 2 figs.

  11. The evolving contribution of border faults and intra-rift faults in early-stage East African rifts: insights from the Natron (Tanzania) and Magadi (Kenya) basins

    NASA Astrophysics Data System (ADS)

    Muirhead, J.; Kattenhorn, S. A.; Dindi, E.; Gama, R.

    2013-12-01

    In the early stages of continental rifting, East African Rift (EAR) basins are conventionally depicted as asymmetric basins bounded on one side by a ~100 km-long border fault. As rifting progresses, strain concentrates into the rift center, producing intra-rift faults. The timing and nature of the transition from border fault to intra-rift-dominated strain accommodation is unclear. Our study focuses on this transitional phase of continental rifting by exploring the spatial and temporal evolution of faulting in the Natron (border fault initiation at ~3 Ma) and Magadi (~7 Ma) basins of northern Tanzania and southern Kenya, respectively. We compare the morphologies and activity histories of faults in each basin using field observations and remote sensing in order to address the relative contributions of border faults and intra-rift faults to crustal strain accommodation as rifting progresses. The ~500 m-high border fault along the western margin of the Natron basin is steep compared to many border faults in the eastern branch of the EAR, indicating limited scarp degradation by mass wasting. Locally, the escarpment shows open fissures and young scarps 10s of meters high and a few kilometers long, implying ongoing border fault activity in this young rift. However, intra-rift faults within ~1 Ma lavas are greatly eroded and fresh scarps are typically absent, implying long recurrence intervals between slip events. Rift-normal topographic profiles across the Natron basin show the lowest elevations in the lake-filled basin adjacent to the border fault, where a number of hydrothermal springs along the border fault system expel water into the lake. In contrast to Natron, a ~1600 m high, densely vegetated, border fault escarpment along the western edge of the Magadi basin is highly degraded; we were unable to identify evidence of recent rupturing. Rift-normal elevation profiles indicate the focus of strain has migrated away from the border fault into the rift center, where faults pervasively dissect 1.2-0.8 Ma trachyte lavas. Unlike Natron, intra-rift faults in the Magadi basin exhibit primarily steep, little-degraded fault scarps, implying greater activity than Natron intra-rift faults. Numerous fault-associated springs feed water into perennial Lake Magadi, which has no surface drainage input, yet survives despite a high evaporation rate that has created economically viable evaporite deposits. Calcite vein-filled joints are common along fault zones around Lake Magadi, as well as several cm veins around columnar joints that imply isotropic expansion of the fracture network under high pressures of CO2-rich fluids. Our work indicates that the locus of strain in this portion of the EAR transfers from the border fault to the center of the rift basin some time between 3 and 7 million years after rift initiation. This transition likely reflects the evolving respective roles of crustal flexure and magma budget in focusing strain, as well as the hydrothermal fluid budget along evolving fault zones.

  12. Petrological constraints on the crustal structure under rift zones

    NASA Astrophysics Data System (ADS)

    Maclennan, J.; Gaetani, G. A.; Hartley, M. E.; Neave, D.; Winpenny, B.

    2012-12-01

    When magmatism occurs in extensional settings, the transfer and release of heat by magma transport and storage may control the thermal structure of the crust and uppermost mantle underlying the rift zone. The composition and mineralogy of the material added to the solid crust during magmatism is also dictated by the conditions of crystallisation. The temperature, pressure and composition of this material controls the physical response of the crust on timescales relevant both for geophysical imaging of the deep structure of rift zones and for the dynamical development of the structure of zones of extension. Examination of the petrology of the products of rift zone magmatism can be used to provide constraints on the depth distribution of crystallisation and the composition of the solid material accreted to the crust and should be a key consideration in geophysical and dynamical investigations of rift zones. A number of complementary petrological techniques have been used to determine crystallisation depths under the rift zones of Iceland. These techniques include: 1) a parametrisation of clinopyroxene-liquid equilibrium; 2) a parametrisation of the composition of liquid in joint equilibrium with the phases olivine, plagioclase and clinopyroxene; 3) assessment of the order of appearance of phases on the liquidus; 4) the relationship between melt CO2 content and pressure. However, the application of each technique is dependent upon a number of assumptions about the achievement of equilibrium between phases and also the interpolation of parametrisations between experimental conditions. Most rift zone eruptions carry large crystals that have formed in the magma storage zone and these crystals are often referred to as phenocrysts, a term that implies equilibrium between the crystals and the melts that carry them to the surface for eruption. However, careful examination of the trace element composition of Icelandic clinopyroxene phenocrysts shows that they are rarely in equilibrium with their carrier liquids. In order to avoid significant systematic errors in estimates of crystallisation depths it is therefore necessary to use both the major and trace element composition of the clinopyroxenes to establish the range of potential equilibrium liquids at the depth of crystallisation and to identify if such liquids correspond to the composition of lavas from the same magmatic system. Existing parametrisations of experimental equilibrium between clinopyroxene and basaltic liquid are prone to large errors at crustal pressures. In order to address this problem, a series of experiments was conducted using a primitive Icelandic basalt composition as a starting material. These experiments indicate that the parametrisations can robustly be used to establish the importance of crystallisation close to the Moho under Iceland, at depths of over 20 km. It is likely that crystallisation also occurs in the middle and lower crust, generating cumulate gabbro material. However, systematic errors in available parameterisations of clinopyroxene-liquid equilibrium hinder barometry over this pressure range. In order to better establish crystallisation conditions for rift zone magmas, further experimental constraints for crustal pressures are required, along with development of more systematic studies of the volatile contents of melt inclusions.

  13. Fluoride and Geothermal Activities In Continental Rift Zones, Ethiopia

    NASA Astrophysics Data System (ADS)

    Weldesenbet, S. F.

    2012-12-01

    The Central Main Ethiopian Rift basin is a continental rift system characterized by volcano-tectonic depression endowed with huge geothermal resource and associated natural geochemical changes on groundwater quality. Chemical composition of groundwater in the study area showed a well defined trend along flow from the highland and escarpment to the rift floor aquifer. The low TDS (< 500mg/l) Ca-Mg-HCO3 dominated water at recharge area in the highlands and escarpments evolve progressively into Ca-Na-HCO3 and Na-Ca-HCO3 type waters along the rift ward groundwater flow paths. These waters finally appear as moderate TDS (mean 960mg/l) Na-HCO3 type and as high TDS (> 1000 mg/l) Na-HCO3-Cl type in volcano-lacustrine aquifers of the rift floor. High concentrations of fluoride (up to 97.2 mg/l) and arsenic (up to 98?g/l) are recognized feature of groundwaters which occur mostly in the vicinity of the geothermal fields and the rift lakes in the basin. Fluoride and arsenic content of dry volcaniclastic sediments close to these areas are in the range 666-2586mg/kg and 10-13mg/kg respectively. The relationship between fluoride and calcium concentrations in groundwaters showed negative correlation. Near-equilibrium state attained between the mineral fluorite (CaF2) and the majority of fluoride-rich (>30mg/l) thermal groundwater and shallow cold groundwater. This indicated that the equilibrium condition control the high concentration of fluoride in the groundwaters. Whereas undersaturation state of fluorite in some relatively low-fluoride (<30mg/l) thermal waters indicated a dilution by cold waters. Laboratory batch leaching experiments showed that fast dissolution of fluoride from the sediment samples suddenly leached into the interacting water at the first one hour and then remain stable throughout the experiment. The concentrations of leached fluoride from the hot spring deposits, the lacustrine sediments, and the pyroclastic rock are usually low (1% of the total or less than the content in the sediment or rock) but strongly correlated with the concentrations in groundwaters in the local vicinity. The readily leachable hot spring deposits and local lacustrine sediments, which were leached easily as high as three fold of other sediments leachability, are considered as the reservoir for the potential fluoride contamination of the rift groundwater. Leaching of fluoride in the sub-surface system is simulated with sediment-packed column leached by flowing water and applying temporary interruption of flow during the experiment. The result indicated that a sharp increase of fluoride concentration (up to 58mg/kg) observed in leachates before one pore-volume of water eluted from the column. The concentration of leached fluoride consequently declined with the increased flowing pore-volume of water and finally the lowest concentrations of leached fluoride occurred in the end of the experiment. Flow interruption during column leaching experiment causes a noticeable fluoride concentration perturbation due to the heterogeneity of the sediment.

  14. High Fluoride and Geothermal Activities In Continental Rift Zones, Ethiopia

    NASA Astrophysics Data System (ADS)

    Weldesenbet, S. F.; Wohnlich, S.

    2012-12-01

    The Central Main Ethiopian Rift basin is a continental rift system characterized by volcano-tectonic depression endowed with huge geothermal resource and associated natural geochemical changes on groundwater quality. Chemical composition of groundwater in the study area showed a well defined trend along flow from the highland and escarpment to the rift floor aquifer. The low TDS (< 500mg/l) Ca-Mg-HCO3 dominated water at recharge area in the highlands and escarpments evolve progressively into Ca-Na-HCO3 and Na-Ca-HCO3 type waters along the rift ward groundwater flow paths. These waters finally appear as moderate TDS (mean 960mg/l) Na-HCO3 type and as high TDS (> 1000 mg/l) Na-HCO3-Cl type in volcano-lacustrine aquifers of the rift floor. High concentrations of fluoride (up to 97.2 mg/l) and arsenic (up to 98?g/l) are recognized feature of groundwaters which occur mostly in the vicinity of the geothermal fields and the rift lakes in the basin. Fluoride and arsenic content of dry volcaniclastic sediments close to these areas are in the range 666-2586mg/kg and 10-13mg/kg respectively. The relationship between fluoride and calcium concentrations in groundwaters showed negative correlation. Near-equilibrium state attained between the mineral fluorite (CaF2) and the majority of fluoride-rich (>30mg/l) thermal groundwater and shallow cold groundwater. This indicated that the equilibrium condition control the high concentration of fluoride in the groundwaters. Whereas undersaturation state of fluorite in some relatively low-fluoride (<30mg/l) thermal waters indicated a dilution by cold waters. Laboratory batch leaching experiments showed that fast dissolution of fluoride from the sediment samples suddenly leached into the interacting water at the first one hour and then remain stable throughout the experiment. The concentrations of leached fluoride from the hot spring deposits, the lacustrine sediments, and the pyroclastic rock are usually low (1% of the total or less than the content in the sediment or rock) but strongly correlated with the concentrations in groundwaters in the local vicinity. The readily leachable hot spring deposits and local lacustrine sediments, which were leached easily as high as three fold of other sediments leachability, are considered as the reservoir for the potential fluoride contamination of the rift groundwater. Leaching of fluoride in the sub-surface system is simulated with sediment-packed column leached by flowing water and applying temporary interruption of flow during the experiment. The result indicated that a sharp increase of fluoride concentration (up to 58mg/kg) observed in leachates before one pore-volume of water eluted from the column. The concentration of leached fluoride consequently declined with the increased flowing pore-volume of water and finally the lowest concentrations of leached fluoride occurred in the end of the experiment. Flow interruption during column leaching experiment causes a noticeable fluoride concentration perturbation due to the heterogeneity of the sediment.

  15. Geometry and architecture of faults in a syn-rift normal fault array: The Nukhul half-graben, Suez rift, Egypt

    NASA Astrophysics Data System (ADS)

    Wilson, Paul; Gawthorpe, Rob L.; Hodgetts, David; Rarity, Franklin; Sharp, Ian R.

    2009-08-01

    The geometry and architecture of a well exposed syn-rift normal fault array in the Suez rift is examined. At pre-rift level, the Nukhul fault consists of a single zone of intense deformation up to 10 m wide, with a significant monocline in the hanging wall and much more limited folding in the footwall. At syn-rift level, the fault zone is characterised by a single discrete fault zone less than 2 m wide, with damage zone faults up to approximately 200 m into the hanging wall, and with no significant monocline developed. The evolution of the fault from a buried structure with associated fault-propagation folding, to a surface-breaking structure with associated surface faulting, has led to enhanced bedding-parallel slip at lower levels that is absent at higher levels. Strain is enhanced at breached relay ramps and bends inherited from pre-existing structures that were reactivated during rifting. Damage zone faults observed within the pre-rift show ramp-flat geometries associated with contrast in competency of the layers cut and commonly contain zones of scaly shale or clay smear. Damage zone faults within the syn-rift are commonly very straight, and may be discrete fault planes with no visible fault rock at the scale of observation, or contain relatively thin and simple zones of scaly shale or gouge. The geometric and architectural evolution of the fault array is interpreted to be the result of (i) the evolution from distributed trishear deformation during upward propagation of buried fault tips to surface faulting after faults breach the surface; (ii) differences in deformation response between lithified pre-rift units that display high competence contrasts during deformation, and unlithified syn-rift units that display low competence contrasts during deformation, and; (iii) the history of segmentation, growth and linkage of the faults that make up the fault array. This has important implications for fluid flow in fault zones.

  16. Stress perturbation associated with the Amazonas and other ancient continental rifts

    USGS Publications Warehouse

    Zoback, M.L.; Richardson, R.M.

    1996-01-01

    The state of stress in the vicinity of old continental rifts is examined to investigate the possibility that crustal structure associated with ancient rifts (specifically a dense rift pillow in the lower crust) may modify substantially the regional stress field. Both shallow (2.0-2.6 km depth) breakout data and deep (20-45 km depth) crustal earthquake focal mechanisms indicate a N to NNE maximum horizontal compression in the vicinity of the Paleozoic Amazonas rift in central Brazil. This compressive stress direction is nearly perpendicular to the rift structure and represents a ???75?? rotation relative to a regional E-W compressive stress direction in the South American plate. Elastic two-dimensional finite element models of the density structure associated with the Amazonas rift (as inferred from independent gravity modeling) indicate that elastic support of this dense feature would generate horizontal rift-normal compressional stresses between 60 and 120 MPa, with values of 80-100 MPa probably most representative of the overall structure. The observed ???75?? stress rotation constrains the ratio of the regional horizontal stress difference to the rift-normal compressive stress to be between 0.25 and 1.0, suggesting that this rift-normal stress may be from 1 to 4 times larger than the regional horizontal stress difference. A general expression for the modification of the normalized local horizontal shear stress (relative to the regional horizontal shear stress) shows that the same ratio of the rift-normal compression relative to the regional horizontal stress difference, which controls the amount of stress rotation, also determines whether the superposed stress increases or decreases the local maximum horizontal shear stress. The potential for fault reactivation of ancient continental rifts in general is analyzed considering both the local stress rotation and modification of horizontal shear stress for both thrust and strike-slip stress regimes. In the Amazonas rift case, because the observed stress rotation only weakly constrains the ratio of the regional horizontal stress difference to the rift-normal compression to be between 0.25 and 1.0, our analysis is inconclusive because the resultant normalized horizontal shear stress may be reduced (for ratios >0.5) or enhanced (for ratios <0.5). Additional information is needed on all three stress magnitudes to predict how a change in horizontal shear stress directly influences the likelihood of faulting in the thrust-faulting stress regime in the vicinity of the Amazonas rift. A rift-normal stress associated with the seismically active New Madrid ancient rift may be sufficient to rotate the horizontal stress field consistent with strike-slip faults parallel to the axis of the rift, although this results in a 20-40% reduction in the local horizontal shear stress within the seismic zone. Sparse stress data in the vicinity of the seismically quiescent Midcontinent rift of the central United States suggest a stress state similar to that of New Madrid, with the local horizontal shear stress potentially reduced by as much as 60%. Thus the markedly different levels of seismic activity associated with these two subparallel ancient rifts is probably due to other factors than stress perturbations due to dense rift pillows. The modeling and analysis here demonstrate that rift-normal compressive stresses are a significant source of stress acting on the lithosphere and that in some cases may be a contributing factor to the association of intraplate seismicity with old zones of continental extension.

  17. Rifted continental margins: geometric control on crustal architecture and melting

    NASA Astrophysics Data System (ADS)

    Lundin, Erik; Redfield, Tim; Peron-Pinvidic, Gwenn

    2014-05-01

    A new model is provided for the distribution of magma-poor and magma-rich rifted margins. The South Atlantic, Central Atlantic, North Atlantic - Arctic (Eurasia Basin), and Red Sea all are magma-rich at their distal ends and magma-poor at their proximal ends (with respect to their poles of rotation). The well-known architectural zonation across fully developed magma-poor margins (limited crustal stretching, hyperextension, exhumed mantle, oceanic crust) is also observed along the lengths of many margins at the super-regional scale. Zones of exhumed mantle, marking magma-poor margin, can be mapped for thousands of kilometers. Likewise can zones of seaward dipping reflectors (SDR) marking magma-rich margins. At this scale, the age of the oceanic crust becomes younger in the direction of the rotation pole, implying that the continents ruptured by rift tip propagation (and rotation pole propagation). Propagation is also manifested by the age of pre-break-up magmatism, break-up unconformity, and margin uplift. Hence, the classic cross-sectional depiction of margin evolution has a third dimension. The degree of melting follows the same pattern. At the distal end of e.g. the South Atlantic, SDR zones are wide and gradually thin toward the rotation pole. Eventually exhumed mantle takes over, marking the transition to the magma-poor margins, which remain to the proximal end of rifting. SDR zones also thin laterally from ca 10-15 km thickness at the continent-ocean boundary (COB) to ca 7 km thick oceanic crust beyond the SDRs. Outcrop data demonstrate that also exhumed mantle contains up to ca 12% melt, infiltrated in the peridotites. Thus, melting is largest at the distal ends near the COB, and decreases both laterally toward the evolving ocean and along strike toward the rift tip. Accepting that continents are rigid to a first order, the linear rate of extension at any given location along an evolving rift and ocean, is governed by the angular rate of opening, the distance to the rotation pole, and the rate of propagation of the pole. For a fixed angular rate, the linear extension rate increases away from the pole. Numerical models reveal that both mantle temperature and rate of extension can govern the degree of melting. However, the above empirical observations suggest that to a first order the rifted margin architecture, including the degree of melting, is governed by the linear rate of extension, which is a direct outcome of geometric rules of plate tectonics. Rapid pole propagation, or a pole jump, will induce a rapid increase in the linear rate. Magma-rich margins seem to form when continents break at a high extension/strain rate caused by rapid propagation; this occurs at the distal end of a rupturing plate. Our testable model questions the common ad hoc introduction of mantle plumes to explain "excess" melting along magma-rich margins. This does not rule out that mantle heterogeneities may exist, but such heterogeneities appear second order when it comes to generating magma-rich margins.

  18. A network-based meta-population approach to model Rift Valley fever epidemics

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Rift Valley fever virus (RVFV) has been expanding its geographical distribution with important implications for both human and animal health. The emergence of Rift Valley fever (RVF) in the Middle East, and its continuing presence in many areas of Africa, has negatively impacted both medical and vet...

  19. The origin and geologic evolution of the East Continent Rift Basin

    SciTech Connect

    Drahovzal, J.A. . Kentucky Geological Survey)

    1992-01-01

    The East Continent Rift Basin (ECRB) is a newly recognized, dominantly sedimentary-volcanic Proterozoic rift basin that apparently represents the southern extension of the Keweenawan Midcontinent Rift. The ECRB extends from central Michigan at least as far south as south-central Kentucky. The inferred age of the rift fill is approximately 1,000 Ma. Evidence supporting a rift origin for the ECRB includes: interbedding of continental flood basalts and felsic volcanics with siliciclastics; sedimentary fill consisting of distal, arid-climate alluvial fan sediments that lack metamorphic lithologies; close proximity and similar lithologic succession to the Keweenawan rift-fill rocks of the Michigan Basin; and inferred marginal block faulting of Granite-Rhyolite Province rocks near the western edge of the ECRB. ECRB evolution is interpreted as follows: (1) formation of Granite-Rhyolite Province rocks (1,500--1,340 Ma); (2) Keweenawan crustal extension and rifting with development of central mafic complexes, emplacement of volcanic rocks, and deposition of siliciclastic fill from eroded marginal Granite-Rhyolite Province tilted fault blocks (ca 1,000 Ma); (3) overthrusting of the Grenville allochthon and associated foreland thrusting and folding of the rift sequence rocks together with deposition of foreland basin sediments (975---890 Ma); (4) Late Proterozoic erosional removal of the foreland basin sediments and interpreted wrench faulting along the Grenville Front (post-975 to pre-570 Ma); and (5) tectonic inversion, with the ECRB area remaining relatively high during major cambrian subsidence in central Kentucky (590--510 Ma).

  20. Origin of the West Taiwan basin by orogenic loading and flexure of a rifted continental margin

    E-print Network

    Watts, A. B. "Tony"

    Origin of the West Taiwan basin by orogenic loading and flexure of a rifted continental margin A. T-type continental margin. The most likely source of the loading is Taiwan, where oblique convergence between of the West Taiwan basin by orogenic loading and flexure of a rifted continental margin, J. Geophys. Res., 107

  1. Trace element characteristics of lithospheric and asthenospheric mantle in the Rio Grande rift region

    SciTech Connect

    Perry, F.V.

    1994-06-01

    Trace element analyses of 10 mafic volcanic rocks from the Colorado Plateau transition zone, Colorado Plateau, Rio Grande rift, and Great Plains were obtained to characterize the trace element characteristics of asthenospheric and lithospheric mantle beneath these regions. Characterization of these mantle reservoirs using the trace element contents of basalts allows one to track the response of the lithosphere to continental rifting and extension.

  2. Melting during late-stage rifting in Afar is hot and deep.

    PubMed

    Ferguson, D J; Maclennan, J; Bastow, I D; Pyle, D M; Jones, S M; Keir, D; Blundy, J D; Plank, T; Yirgu, G

    2013-07-01

    Investigations of a variety of continental rifts and margins worldwide have revealed that a considerable volume of melt can intrude into the crust during continental breakup, modifying its composition and thermal structure. However, it is unclear whether the cause of voluminous melt production at volcanic rifts is primarily increased mantle temperature or plate thinning. Also disputed is the extent to which plate stretching or thinning is uniform or varies with depth with the entire continental lithospheric mantle potentially being removed before plate rupture. Here we show that the extensive magmatism during rifting along the southern Red Sea rift in Afar, a unique region of sub-aerial transition from continental to oceanic rifting, is driven by deep melting of hotter-than-normal asthenosphere. Petrogenetic modelling shows that melts are predominantly generated at depths greater than 80?kilometres, implying the existence of a thick upper thermo-mechanical boundary layer in a rift system approaching the point of plate rupture. Numerical modelling of rift development shows that when breakup occurs at the slow extension rates observed in Afar, the survival of a thick plate is an inevitable consequence of conductive cooling of the lithosphere, even when the underlying asthenosphere is hot. Sustained magmatic activity during rifting in Afar thus requires persistently high mantle temperatures, which would allow melting at high pressure beneath the thick plate. If extensive plate thinning does occur during breakup it must do so abruptly at a late stage, immediately before the formation of the new ocean basin. PMID:23823795

  3. Geoscience Methods Lead to Paleo-anthropological Discoveries in Afar Rift, Ethiopia

    NASA Astrophysics Data System (ADS)

    WoldeGabriel, Giday; Renne, Paul R.; Hart, William K.; Ambrose, Stanley; Asfaw, Berhane; White, Tim D.

    2004-07-01

    With few exceptions, most of the hominid evolutionary record in Africa is closely associated with the East African Rift System. The exceptions are the South African and Chadian hominids collected from the southern and west-central parts of the continent, respectively. The Middle Awash region stands alone as the most prolific paleoanthropological area ever discovered (Figure 1). Its paleontological record has yielded over 13,000 vertebrate fossils, including several hominid taxa, ranging in age from 5.8 Ma to the present. The uniqueness of the Middle Awash hominid sites lies in their occurrence within long, > 6 Ma volcanic and sedimentary stratigraphic records. The Middle Awash region has yielded the longest hominid record yet available. The region is characterized by distinct geologic features related to a volcanic and tectonic transition zone between the continental Main Ethiopian and the proto-oceanic Afar Rifts. The rift floor is wider-200 km-than other parts of the East African Rift (Figure 1). Moreover, its Quaternary axial rift zone is wide and asymetrically located close to the western margin. The fossil assemblages and the lithostratigraphic records suggest that volcanic and tectonic activities within the broad rift floor and the adjacent rift margins were intense and episodic during the late Neogene rift evolution.

  4. Variations in Mid-Continent Rift magma volumes consistent with microplate evolution

    E-print Network

    Stein, Seth

    active East African and Baikal rifts, has two major arms meeting in the Lake Superior region. One extends arms. The volume of magma increases towards the Lake Superior region, consistent with magma flowing the rift is hidden beneath Phanerozoic sedimentary rocks except where it is exposed in the Lake Superior

  5. Stratigraphy, structure, and extent of the East Continent Rift Basin

    SciTech Connect

    Wickstrom, L.H. )

    1992-01-01

    The proven existence of pre-Mt. Simon sedimentary rocks named the Middle Run Formation in southwestern Ohio led to the establishment of the Cincinnati Arch Consortium, a joint industry-government partnership to investigate the areal extent, nature, and origin of this new unit. Utilizing available well, seismic, and potential-field data, the consortium has shown that the Middle Run was deposited in a Precambrian rift basin, named the East Continent Rift Basin (ECRB). These data indicate the ECRB assemblage consists of a large folded and faulted wedge of interlayered volcanic and sedimentary rocks, unconformably overlain by Cambrian strata. This wedge is estimated to be thickest (up to about 22,000 feet) on the western edge, where it is in fault contact with Grenville Province rocks. To the west, the ECRB may extend as far as central Illinois and postdates the Precambrian Granite-Rhyolite Province rocks. The contact between the ECRB and this older province appears to be in part an angular unconformity and in part block faulted. The northern limit of this basin was not encountered in the study area; this may indicate a connection with the Midcontinent Rift in Michigan. In central Kentucky, the boundary conditions are more complex. It appears that the ECRB is constricted between a large block of the Granite-Rhyolite Province to the west and the Grenville Front on the east. Large Cambrian extensional structures (Rough Creek Graben and Rome Trough) were overprinted on the Granite-Rhyolite and Grenville Provinces. The ECRB may have acted as a stable block between these Cambrian features. The relationships of the ECRB to overlying Paleozoic features may be profound. Indeed, the ECRB may prove to be the reason for the very existence of the Cincinnati and Kankakee Arches.

  6. Early Continental Rifting of the South China Sea

    NASA Astrophysics Data System (ADS)

    Lee, C.; Chiu, M.; Chan, C.

    2010-12-01

    Combined two years (2007 and 2008) of OBS and MCS studies in the northern slope of the South China Sea, we suggest that the early rifting, probably during 60 - 30 mabp, is an asymmetrical Atlantic-type continental rifting. The crust thin out from 35-40 km of possible continental crust to about 10-15 km of typical oceanic crust. Along the continent-ocean boundary, we observe an intrusion of the high P-wave velocity (about 7.5-8.0 km/sec). This is possible of mantle exhumation as comparable to other Atlantic-type continental margins. The OBS result is revealed by the gravity data. Along the upper layers of the continental crust as well as the oceanic crust, the MCS and multi-beam bathymetry data show that they are covered by numerous submarine seamounts. This probably relate to a volcanic origin of the Cenozoic sea-floor spreading during 30-15 mabp as mapped by previous magnetic anomalies in this region. The sea-floor spreading spread apart in the central, NW and SW sub-basins with several different episodes. Lack of the deep crustal data in the southern slope of the South China Sea, particularly around the Sprately area, the interpretation is speculative. However, several very large-size atolls (150 - 200 km in diameter), such as the Chen-Ho, Shun-Zu, Chung-Yeh and Chiu-Cheng fringing reefs, are sub-parallel located along the south margins. We interpret that these are the upper portions of the continental rifting. Combined the two tectonic stories in the northern and southern slope of the South China Sea, we believe that it is in consistent with the complicate nature of the South China Sea crust.

  7. Modern and ancient mineralization in the Salton Trough Rift

    SciTech Connect

    McKibben, M.A. . Dept. Earth Sciences)

    1992-01-01

    The Salton Trough of SW North America is an active continental rift, the landward extension of the divergent tectonics of the Gulf of California. Shallow magmatic heat sources, thick porous sediments, tectonic activity and saline lakes interact to yield a variety of Pleistocene to modern hydrothermal systems. The oldest mineralization, the fish Creek evaporite, is a CASO[sub 4] deposit formed by a pre-rift Tertiary marine incursion. 4--5 million years ago the prograding Colorado River delta bisected the Trough, influencing the character of Pliocene and younger hydrothermal activity. The northern part of the Trough became a closed basin filled intermittently by large freshwater lakes. Along the W margin of the rift lies the Modoc hot spring gold deposit. This deposit occurs at the intersection of a range-front growth fault with fossil lake levels, suggesting paleohydrologic control by ancient lakes. Active geothermal systems within the Trough include low-T systems such as Heber and East Mesa, localized along high-angle faults where shallow groundwaters are conductively heated above basement highs. These blind systems have no surface expression and only moderate geophysical anomalies. High-T (> 250 C) active systems occur in sediment filled pull-apart basins developed over spreading center fragments (e.g., Salton Sea, Brawley, Cerro Prieto). These systems exhibit high heat flow, strong gravity and magnetic anomalies, and often have surface manifestations such as Quaternary volcanoes and thermal features. Many contain hot metalliferous brines that have evolved in the saline lake environment of the northern Trough.

  8. ICESat's new perspective on ice shelf rifts: The vertical dimension H. A. Fricker, J. N. Bassis, and B. Minster

    E-print Network

    Eustice, Ryan

    detail over features such as crevasses, through-cutting rifts, ice shelf margins, ice fronts, icebergs and grounding zones. In this paper we focus on rifts; another paper in this Special Section consid- ers icebergs for the calving of large tabular icebergs. Rift propa- gation rates on the Amery Ice Shelf have been shown

  9. Crustal architecture of an inverted back arc rift basin, Niigata, central Japan

    NASA Astrophysics Data System (ADS)

    Sato, H.; Abe, S.; Kawai, N.; Saito, H.; Kato, N.; Ishiyama, T.; Iwasaki, T.; Kurashimo, E.; Inaba, M.; Van Horne, A.

    2012-04-01

    A back arc rift basin, formed during the Miocene opening of the Japan Sea, now uplifted and exposed in Niigata, central Japan, provides an exceptional opportunity to study a back arc rift formed on a short time scale and in a still active setting for the present day shortening deformation. Due to stress build up before the 2011 Tohoku earthquake (M9), two damaging earthquakes (M6.8) occurred in 2004 and 2007 in this inverted rift basin. Deep seismic profiling was performed along four seismic lines between 2008 and 2011. We used onshore-offshore deep seismic reflection profiling to examine the crustal architecture of the back arc basin, in particular the geometry of the source faults. We further applied refraction tomography analysis to distinguish between previously undifferentiated syn-rift volcanics and pre-rift Mesozoic rock based on P-wave velocity. Our findings indicate that the Miocene rift structure created during the extensional phase regulates the style of deformation and the geometry of the source faults in the current compressional regime. Syn-rift volcanics with a maximum thickness of 6 km filled the fault controlled basins as rifting proceeded. The volcanism was bimodal, comprising a reflective unit of mafic rocks around the rift axis and a non-reflective unit of felsic rocks near the margins of the basins. Once rifting ended, thermal subsidence, and subsequently, mechanical subsidence related to the onset of the compressional regime, allowed deposition of up to 5 km of post-rift, deep marine to fluvial sedimentation, including the Teradomari Formation, an over-pressured mudstone in the middle of the section that later became an important shallow detachment layer. Continued compression has caused fault-related fold and wedge thrusting in the post-rift sedimentary strata which are highly deformed by thin-skin style deformation. Since the Pliocene, normal faults created during the rift phase have been reactivated as reverse faults, including a shallow detachment in the Teradomari Formation which forms a complicated shortened deformation structure. Quaternary geomorphology suggests ongoing shortening. Transform faults inherited from the rift stage control the extent of present day reverse source faults and more importantly, earthquake magnitude.

  10. Syn-rift evolution of the Pripyat Trough: constraints from structural and stratigraphic modelling

    NASA Astrophysics Data System (ADS)

    Kusznir, N. J.; Kovkhuto, A.; Stephenson, R. A.

    1996-12-01

    The Pripyat Trough is a Late Devonian rift basin. It forms part of the larger Pripyat-Dniepr-Donets rift system which has a length in excess of 800 km and separates the Ukrainian Shield from the Voronezh Massif. The Pripyat Trough contains a well resolved stratigraphic sub-division within the syn-rift basin fill sequence which allows the duration and rate of rifting to be determined using 2-D forward and reverse structural and stratigraphic modelling. The analysis shows that rifting was extremely rapid. Sequential decompaction and flattening of 2-D cross-sections has been applied to six syn-rift time horizons between top Upper Devonian (364 Ma) and top Middle Devonian (377 Ma) and used to quantify the syn-rift development of basin cross-sectional area in time. The evolution of basin cross-sectional area shows that some initial rifting had occurred prior to the Middle Frasnian (369 Ma); however, most rifting occurred in the Famennian (367-364 Ma). Forward syn-rift modelling using the flexural cantilever model of rift basin formation has also been applied to quantify the magnitude of extension within the Pripyat Trough in time. Forward syn-rift models are constrained by the intra syn-rift flattened and decompacted cross-sections and the observed evolution of the cross-sectional area. Most rapid rift basin formation is shown to have occurred in the Famennian with over 66% of basin cross-sectional area forming in less than 5 Myr, and with approximately 80% or more of total extension occurring in less than 3 Myr. This period of most rapid extension during the Famennian coincides with the most active period of volcanicity. Total Devonian extension across the Pripyat Trough is estimated to be of the order of 11-14 km with a maximum ? stretching factor of approximately 1.12. Extensional strain rates are estimated to be of the order of 0.8 × 10 -15 s -1.

  11. Rifting and Post-Rift Reactivation of The Eastern Sardinian Margin (Western Tyrrhenian Back-Arc Basin) Evidenced by the Messinian Salinity Crisis Markers and Salt Tectonics

    NASA Astrophysics Data System (ADS)

    Gaullier, V.; Chanier, F.; Vendeville, B.; Lymer, G.; Lofi, J.; Sage, F.; Maillard, A.; Thinon, I.

    2014-12-01

    The Eastern Sardinian margin formed during the opening of the Tyrrhenian Sea, a back-arc basin created by continental rifting and oceanic spreading related to the eastward migrating Apennine subduction system from middle Miocene to Pliocene times. We carried out the "METYSS" project aiming at better understanding the Miocene-Pliocene relationships between crustal tectonics and salt tectonics in this key-area, where rifting is pro parte coeval with the Messinian Salinity Crisis (MSC, 5.96-5.33 Ma) and Messinian salt décollement creates thin-skinned tectonics. Thereby, we use the MSC seismic markers and the deformation of viscous salt and its brittle overburden as proxies to better delineate the timing of rifting and post-rift reactivation, and especially to quantifying vertical and horizontal movements. Our mapping of the Messinian Erosion Surface and of Messinian Upper and Mobile Units shows that a rifted basin already existed by the Messinian times, revealing a major pre-MSC rifting episode across the entire domain. Because salt tectonics can create fan-shaped geometries in sediments, syn-rift deposits have to be carefully re-examined in order to decipher the effects of crustal tectonics (rifting) and salt tectonics. Our data surprisingly showed that there are no clues for Messinian syn-rift sediments along the East-Sardinia Basin and Cornaglia Terrace, hence no evidence for rifting after Late Tortonian times. Nevertheless, widespread deformation occurred during the Pliocene and is attributed to post-rift reactivation. Some Pliocene vertical movements have been evidenced by discovering localized gravity gliding of the salt and its Late Messinian (UU) and Early Pliocene overburden. To the South, crustal-scale southward tilting triggered along-strike gravity gliding of salt and cover recorded by upslope extension and downslope shortening. To the North, East of the Baronie Ridge, there was some post-salt crustal activity along a narrow N-S basement trough, bounded by crustal faults. The salt geometry would suggest that nothing happened after Messinian times, but some structural features (confirmed by analogue modelling) show that basement fault slip was accommodated by lateral salt flow, which thinned upslope and inflated downslope, while the overlying sediments remained sub-horizontal.

  12. Rift flank uplift and thermal evolution of an intracratonic rift basin (eastern Canada) determined by combined apatite and zircon (U-Th)/He thermochronology

    NASA Astrophysics Data System (ADS)

    Hardie, Rebecca; Schneider, David; Metcalf, James; Flowers, Rebecca

    2015-04-01

    As a significant portion of the world's oil reserves are retrieved from rift systems, a better understanding of the timing of thermal evolution and burial history of these systems will increase the potential for the discovery of hydrocarbon-bearing rifts. The Ottawa Embayment of the St. Lawrence Platform of eastern Canada is a reactivated intracratonic rift basin related to the opening of the Iapetus Ocean at ca. 620-570 Ma, followed by the formation of the well-developed continental passive margin. Siliciclastic sediments derived from the adjacent uplifted Neoproterozoic Grenville basement provide the basin fill material. Apatite and zircon (U-Th)/He thermochronology allows for low-temperature analysis across the exposed crystalline rift flank into the synrift sedimentary sequence to resolve the unroofing, burial and subsidence history of the region. Samples were collected along a ~250 km NE-SW transect, oblique to the axis of the rift, from Mont-Tremblant, Québec (~900 m) to the central axis of the Paleozoic rift in the Southern Ontario Lowlands (~300 m). Targets included Neoproterozoic metamorphic rocks of the Grenville Province along the rift flank and basinal Cambro-Ordovician Potsdam Group. Samples from the rift flank yield zircon ages from ca. 650 Ma to ca. 560 Ma and apatite ages from ca. 290 Ma to ca. 190 Ma, with a weak positive correlation between age and grain size. Zircon ages demonstrate a strong negative correlation with radiation damage: as eU increases, age decreases. By incorporating (U-Th)/He ages with regional constraints in the thermal modelling program HeFTy, viable temperature time paths for the region can be determined. Through inverse and forward modeling, preliminary rift flank (U-Th)/He ages correspond to post-Grenville cooling with <4 km of post-Carboniferous burial. The data define slow and long episodes of syn- to post-rift cooling with rates between 0.4 and 0.1 °C/Ma. (U-Th)/He dating of samples along the full-length of the transect will resolve thermal changes in the basin-orogen system and improve our understanding of the rift related history of the region.

  13. An epidemic of Rift Valley fever in Egypt

    PubMed Central

    Imam, Imam Z. E.; Karamany, R. El; Darwish, Medhat A.

    1979-01-01

    During the epidemic of Rift Valley fever (RVF) that occurred in Egypt and other areas of North Africa in 1977, the virus was isolated from various species of domestic animal and rats (Rattus rattus frugivorus) as well as man. The highest number of RVF virus isolates were obtained from sheep; only one isolate was recovered from each of the other species tested, viz. cow, camel, goat, horse, and rat. RVF virus was reisolated from both camel and horse sera, apparently for the first time. PMID:314355

  14. Graben formation during the Bárðarbunga rifting event in central Iceland

    NASA Astrophysics Data System (ADS)

    Ruch, Joël; Jónsson, Sigurjón; Wang, Teng; Xu, Wenbin; Trippanera, Daniele

    2015-04-01

    On the 16th of August 2014, an intense seismic swarm was detected at the Bárðarbunga caldera (central Iceland), which migrated to the east and then to the northeast during the following days. The swarm, highlighting magma propagation pathway from the caldera, migrated laterally during the following two weeks over 40 km. By the end of August, a volcanic eruption had started along a north-south oriented fissure located ~45 km from the caldera. Here we focus on the near-field deformation related to the dike emplacement in the shallow crust, which generated in few days an 8 km long by 0.8 km wide graben (depression) structure. The new graben extends from the northern edge of the Vatnajökull glacier and to the north to the eruptive fissure. We analyze the temporal evolution of the graben by integrating structural mapping using multiple acquisitions of TerraSAR-X amplitude radar images, InSAR and ground-truth data with GPS and structural measurements. Pixel-offset tracking of radar amplitude images shows clearly the graben subsidence, directly above the intrusion pathway, of up to 6 meters in the satellite line-of-sight direction. We installed a GPS profile of 15 points across the graben in October 2014 and measured its depth up to 8 meters, relative to the flanks of the graben. Field structural observations show graben collapse structures that typically accompany dike intrusions, with two tilted blocks dipping toward the graben axis, bordered by two normal faults. Extensive fractures at the center of the graben and at the graben edges show a cumulative extension of ~8 meters. The formation of the graben was also accompanied by strong seismic activity locally, constraining the time frame period of the main graben formation subsidence. Our results show a rare case of a graben formation captured from space and from ground observations. Such structures are the dominant features along rift zones, however, their formation remain poorly understood. The results also provide information about transient deformation occurring along rift zones, suggesting that rapid dike intrusions modify the topography permanently in only a few days. This supports that rift morphologies are primarily generated during rifting events rather than by long-term regional tectonic processes.

  15. Present-day Kinematics of the East African Rift

    NASA Astrophysics Data System (ADS)

    Stamps, D. S.; Calais, E.; Saria, E.; Mbede, E.; Ebinger, C.; Delvaux, D.; Kervyn, F.; Combrinck, L.; Hartnady, C.; Nocquet, J.; Fernandes, R.

    2007-12-01

    The East African Rift (EAR), a ~5000 km-long series of seismically active structures that mark the divergent boundary between the Somalia and Nubia plates, is often cited as a modern archetype for rifting and continental breakup. Paradoxically, its current kinematics is the least well-known of all major plate boundaries, owing to its tremendous extent, difficult access, and lack of geodetic data. The existence of two "new" tectonic plates within the EAR (Victoria and Rovuma) between the main Nubian and Somalian plates has recently been proposed through combined analysis of earthquake slip vector and (sparse) space-geodetic data (Calais et al., 2006). A third, dominantly oceanic Lwandle plate was recently postulated by a re-analysis of the post 3.2 Ma spreading rates and transform-fault azimuths along the Southwest Indian Ridge (SWIR; Horner-Johnson et al., 2007). Here we use an updated geodetic solution -- a combination of all continuous GPS stations on the Nubia, Somalia, and Antarctic plates, campaign GPS measurements in Africa, and a global DORIS solution -- together with earthquake slip vectors in the EAR and transform azimuth and spreading rates along the SWIR. We show that the space geodetic data and the 3.2 Ma average oceanic data along the SWIR are consistent with each other at the 95% confidence level. The data support a kinematic model with (1) a present-day Nubia-Somalia Euler pole located to the SE of the southern tip of Africa, (2) the existence of three distinct microplates embedded in the EAR: Victoria, Rovuma, and Lwandle. Additional geodetic data in the EAR are now needed to confirm and further refine this model. References Calais, E., C. Hartnady, C. Ebinger, and J.M. Nocquet, Kinematics of the East African Rift from GPS and earthquake slip vector data, In: Yirgu, G., Ebinger, C.J. & Maguire, P.K.H. (eds) Structure and Evolution of the Rift Systems within the Afar volcanic province, Northeast Africa, Geological Society Special Publications, 259, p.9-22, 2006. Horner-Johnson, B., R.G. Gordon, D.F. Argus, Plate kinematic evidence for the existence of a distinct plate between the Nubian and Somalian plates along the Southwest Indian Ridge, Journal of Geophysical Research, 112, 2007

  16. Strain partitioning in hyper-extended, strongly segmented rift systems: insights from the Cretaceous Bay of Biscay- Pyrenean rift system and comparison with present-day mature rift-transform margins

    NASA Astrophysics Data System (ADS)

    Manatschal, G.; Tugend, J.; Masini, E.; Kusznir, N. J.

    2013-12-01

    Continental margins are often subdivided into transform and volcanic and non-volcanic rifted margins, although, in reality, such end-member type margins do not exist and the distribution of strain and magma leading to lithospheric breakup is more complex. Key questions related to the development of oblique and/or segmented rift-transform margins include the importance of inheritance, the partitioning of deformation in time and space, the interplay between deformation and magmatism, the timing and location of breakup, and the isostatic evolution of these systems during and after final rifting. At present-day continental margins the initial stages associated with the development of highly segmented rift-transform margins are often masked by thick sedimentary sequences and the relation between the rift structures, syn-tectonic sediments and magmatic additions remain poorly understood. Moreover, it looks as if the oceanic transform faults do not develop from transfer or transform faults within continental rifts, suggesting that the continental and oceanic systems are decoupled within the ocean continent transition. In this study we use the Bay of Biscay - Pyrenean system to understand how deformation was distributed in time and space during the evolution of a highly segmented rift-transform system along the Iberian/European plate boundary during Late Jurassic to Mid Cretaceous time. We will show that the rift basins (Parentis, Arzacq-Mauléon, Cantabrian basins) that developed along this embryonic plate boundary record a complex poly-phase deformation history, showing locale evidence for extreme crustal thinning and locally also mantle exhumation. Because these basins are preserved to the west (Bay of Biscay-Parentis), but reactivated and exposed in the east (Pyrenees), the basins and related structures can be studied using geological and geophysical methods. In our presentation we will show new observations and preliminary results that enable discussion about how a segmented rift-transform plate boundary formed in time and space. We will also show that the poly-phase evolution recorded along the European-Iberian plate boundary has important kinematic implications for the pre-breakup evolution that cannot be taken into account by kinematic models based on magnetic anomaly restorations only. These results, combined with those of present day margins, may give some insights on the pre-breakup evolution and processes that are at the origin of highly segmented rift-transform margins as seen in the Equatorial Atlantic.

  17. Late Mesozoic and Cenozoic rifting and its dynamic setting in Eastern China and adjacent areas

    NASA Astrophysics Data System (ADS)

    Ren, Jianye; Tamaki, Kensaku; Li, Sitian; Junxia, Zhang

    2002-02-01

    During the Late Mesozoic and Cenozoic, extension was widespread in Eastern China and adjacent areas. The first rifting stage spanned in the Late Jurassic-Early Cretaceous times and covered an area of more than 2 million km 2 of NE Asia from the Lake Baikal to the Sikhot-Alin in EW direction and from the Mongol-Okhotsk fold belt to North China in NS direction. This rifting was characterized by intracontinental rifts, volcanic eruptions and transform extension along large-scale strike-slip faults. Based on the magmatic activity, filling sequence of basins, tectonic framework and subsidence analysis of basins, the evolution of this area can be divided into three main developmental phases. The first phase, calc-alkaline volcanics erupted intensely along NNE-trending faults, forming Daxing'anling volcanic belt, NE China. The second phase, Basin and Range type fault basin system bearing coal and oil developed in NE Asia. During the third phase, which was marked by the change from synrifting to thermal subsidence, very thick postrift deposits developed in the Songliao basin (the largest oil basin in NE China). Following uplift and denudation, caused by compressional tectonism in the near end of Cretaceous, a Paleogene rifting stage produced widespread continental rift systems and continental margin basins in Eastern China. These rifted basins were usually filled with several kilometers of alluvial and lacustrine deposits and contain a large amount of fossil fuel resources. Integrated research in most of these rifting basins has shown that the basins are characterized by rapid subsidence, relative high paleo-geothermal history and thinned crust. It is now accepted that the formation of most of these basins was related to a lithospheric extensional regime or dextral transtensional regime. During Neogene time, early Tertiary basins in Eastern China entered a postrifting phase, forming regional downwarping. Basin fills formed in a thermal subsidence period onlapped the fault basin margins and were deposited in a broad downwarped lacustrine depression. At the same time, within plate rifting of the Lake Baikal and Shanxi graben climaxed and spreading of the Japan Sea and South China Sea occurred. Quaternary rifting was marked by basalt eruption and accelerated subsidence in the area of Tertiary rifting. The Okinawa Trough is an active rift involving back-arc extension. Continental rifting and marginal sea opening were clearly developed in various kind of tectonic settings. Three rifting styles, intracontinental rifting within fold belt, intracontinental rifting within craton and continental marginal rifting and spreading, are distinguished on the basis of nature of the basin basement, tectonic location of rifting and relations to large strike-slip faults. Changes of convergence rates of India-Eurasia and Pacific-Eurasia may have caused NW-SE-trending extensional stress field dominating the rifting. Asthenospheric upwelling may have well assisted the rifting process. In this paper, a combination model of interactions between plates and deep process of lithosphere has been proposed to explain the rifting process in East China and adjacent areas. The research on the Late Mesozoic and Cenozoic extensional tectonics of East China and adjacent areas is important because of its utility as an indicator of the dynamic setting and deformational mechanisms involved in stretching Lithosphere. The research also benefits the exploration and development of mineral and energy resources in this area.

  18. Next-generation Geotectonic Data Analysis: Using pyGPlates to quantify Rift Obliquity during Supercontinent Dispersal

    NASA Astrophysics Data System (ADS)

    Butterworth, Nathaniel; Brune, Sascha; Williams, Simon; Müller, Dietmar

    2015-04-01

    Fragmentation of a supercontinent by rifting is an integral part of plate tectonics, yet the dynamics that govern the success or failure of individual rift systems are still unclear. Recently, analytical and thermo-mechanical modelling has suggested that obliquely activated rifts are mechanically favoured over orthogonal rift systems. Hence, where two rift zones compete, the more oblique rift proceeds to break-up while the less oblique one stalls and becomes an aulacogen. This implies that the orientation and shape of individual rift systems affects the relative motion of Earth's continents during supercontinent break-up. We test this hypothesis using the latest global plate tectonic reconstructions for the past 200 million years. The analysis is performed using pyGPlates, a recently developed Python library that allows script-based access to the plate reconstruction software GPlates. We quantify rift obliquity, extension velocity and their temporal evolution for all small-scale rift segments that constituted a major rift system during the last 200 million years. Boundaries between continental and oceanic crust (COBs) mark the end of rifting and the beginning of sea floor spreading, which is why we use a global set of updated COBs in order to pinpoint continental break-up and as a proxy for the local trend of former rift systems. Analysing the entire length of all rift systems during the last 200 My, we find a mean obliquity of ~40° (measured as the angle between extension direction and local rift trend normal), with a standard deviation of 25°. More than 75% of all rift segments exceeded an obliquity of 20° highlighting the fact that oblique rifting is the rule, not the exception. More specifically, East and West Gondwana split along the East African coast with a mean obliquity of 45°. While rifting of the central and southern South Atlantic segment involved a low obliquity of 10°, the Equatorial Atlantic opened under a high angle of 60°. The separation of Australia and Antarctica involved a protracted extension history involving two stages with ~25° prior to 100 Ma followed by more than 50° obliquity. In many cases both obliquity and extension velocity increase during rift evolution (e.g. South Atlantic, India-Antarctica, Australia-Antarctica, Gulf of California), suggesting an underlying geodynamic correlation. Considering that most conceptual models of rift evolution assume 2D deformation, we here quantify the degree to which 2D rift models are globally applicable, and highlight the importance of 3D models where oblique rifting is the dominant mode of deformation.

  19. Gas Geochemistry of Volcanic and Geothermal Areas in the Kenya Rift: Implications for the Role of Fluids in Continental Rifting

    NASA Astrophysics Data System (ADS)

    Lee, H.; Fischer, T. P.; Ranka, L. S.; Onguso, B.; Kanda, I.; Opiyo-Akech, N.; Sharp, Z. D.; Hilton, D. R.; Kattenhorn, S. A.; Muirhead, J.

    2013-12-01

    The East African Rift (EAR) is an active continental rift and ideal to investigate the processes of rift initiation and the breaking apart of continental lithosphere. Mantle and crust-derived fluids may play a pivotal role in both magmatism and faulting in the EAR. For instance, large quantities of mantle-derived volatiles are emitted at Oldoinyo Lengai volcano [1, 2]. Throughout the EAR, CO2-dominated volatile fluxes are prevalent [3, 4] and often associated with faults (i.e. Rungwe area, Tanzania, [5, 6]). The purpose of this study is to examine the relationship between volcanism, faulting and the volatile compositions, focusing on the central and southern Kenyan and northern Tanzanian section of the EAR. We report our analysis results for samples obtained during a 2013 field season in Kenya. Gases were sampled at fumaroles and geothermal plants in caldera volcanoes (T=83.1-120.2°C) and springs (T=40-79.6°C and pH 8.5-10) located near volcanoes, intra-rift faults, and a transverse fault (the Kordjya fault, a key fluid source in the Magadi rift) by 4N-NaOH solution-filled and empty Giggenbach bottles. Headspace gases were analyzed by a Gas Chromatograph and a Quadrupole Mass Spectrometer at the University of New Mexico. Both N2/Ar and N2/He ratios of all gases (35.38-205.31 and 142.92-564,272, respectively) range between air saturated water (ASW, 40 and ?150,000) and MORB (100-200 and 40-50). In addition, an N2-Ar-He ternary diagram supports that the gases are produced by two component (mantle and air) mixing. Gases in the empty bottles from volcanoes and springs have N2 (90.88-895.99 mmom/mol), CO2 (2.47-681.21 mmom/mol), CH4 (0-214.78 mmom/mol), O2 (4.47-131.12 mmom/mol), H2 (0-35.78 mmom/mol), Ar (0.15-10.65 mmom/mol), He (0-2.21 mmom/mol), and CO (0-0.08 mmom/mol). Although some of the samples show an atmospheric component, CO2 is a major component in most samples, indicating both volcanoes and springs are emitting CO2. Gases from volcanoes are enriched in CH4 and H2, denoting their sources are reduced (oxygen-poor) magma chambers or hydrothermal systems. 40Ar/36Ar ratios (average of all samples=299.15) are similar to our air standard value (299.65×4.05), however, some volcanoes (~308.75) and springs (~321.96) have slightly higher ratios. The springs with elevated 40Ar imply that both the intra-rift (Lake Bogoria) and transverse (Lake Magadi) faults are possibly pathways to carry volatiles from deep sources to the surface. In future work, we will carry out wet chemistry and ion chromatography analyses of the NaOH solutions, measure 3He/4He ratios and complete C, N, and S isotope analyses to further constrain fluid sources and migration processes. [1] Fischer et al., 2009, Nature 459. [2] de Moor et al., 2013, EPSL 361. [3] Sawyer et al., 2008, G-cubed 9. [4] Tassi et al., 2009, G-cubed 10. [5] Barry et al., 2013, Chem Geol 339. [6] de Moor et al., 2013, Chem Geol 339.

  20. ALVIN investigation of an active propagating rift system, Galapagos 95.5° W

    USGS Publications Warehouse

    Hey, R.N.; Sinton, J.M.; Kleinrock, M.C.; Yonover, R.N.; MacDonald, K.C.; Miller, S.P.; Searle, R.C.; Christie, D.M.; Atwater, T.M.; Sleep, N.H.; Johnson, H. Paul; Neal, C.A.

    1992-01-01

    ALVIN investigations have defined the fine-scale structural and volcanic patterns produced by active rift and spreading center propagation and failure near 95.5° W on the Galapagos spreading center. Behind the initial lithospheric rifting, which is propagating nearly due west at about 50 km m.y.?1, a triangular block of preexisting lithosphere is being stretched and fractured, with some recent volcanism along curving fissures. A well-organized seafloor spreading center, an extensively faulted and fissured volcanic ridge, develops ~ 10 km (~ 200,000 years) behind the tectonic rift tip. Regional variations in the chemical compositions of the youngest lavas collected during this program contrast with those encompassing the entire 3 m.y. of propagation history for this region. A maximum in degree of magmatic differentiation occurs about 9 km behind the propagating rift tip, in a region of diffuse rifting. The propagating spreading center shows a gentle gradient in magmatic differentiation culminating at the SW-curving spreading center tip. Except for the doomed rift, which is in a constructional phase, tectonic activity also dominates over volcanic activity along the failing spreading system. In contrast to the propagating rift, failing rift lavas show a highly restricted range of compositions consistent with derivation from a declining upwelling zone accompanying rift failure. The lithosphere transferred from the Cocos to the Nazca plate by this propagator is extensively faulted and characterized by ubiquitous talus in one of the most tectonically disrupted areas of seafloor known. The pseudofault scarps, where the preexisting lithosphere was rifted apart, appear to include both normal and propagator lavas and are thus more lithologically complex than previously thought. Biological communities, probably vestimentiferan tubeworms, occur near the top of the outer pseudofault scarp, although no hydrothermal venting was observed.

  1. 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.

  2. Tracing mantle-reacted fluids in magma-poor rifted margins: The example of Alpine Tethyan rifted margins

    NASA Astrophysics Data System (ADS)

    Pinto, Victor Hugo G.; Manatschal, Gianreto; Karpoff, Anne Marie; Viana, Adriano

    2015-09-01

    The thinning of the crust and the exhumation of subcontinental mantle in magma-poor rifted margins is accompanied by a series of extensional detachment faults. We show that exhumation along these detachments is intimately related to migration of fluids leading to changes in mineralogy and chemistry of the mantle, crustal, and sedimentary rocks. Using field observation and analytical methods, we investigate the role of fluids in the fossil distal margins of the Alpine Tethys. Using Cr-Ni-V, Fe, and Mn as tracers, we show that fluids used detachment faults as pathways and interacted with the overlying crust and sediments. These observations allow us to discuss when, where, and how this interaction happened during the formation of the rifted margin. The results show that: (i) serpentinization of mantle rocks during their exhumation results in the depletion of elements and migration of mantle-reacted fluids that are channeled along active detachment system; (ii) in earlier-stages, these fluids affected the overlying syntectonic sediments by direct migration from the underlying detachments;(iii) in later-stages, these fluids arrived at the seafloor, were introduced into, or "polluted" the seawater and were absorbed by post tectonic sediments. We conclude that a significant amount of serpentinization occurred underneath the hyperextended continental crust, and that the mantle-reacted fluids might have modified the chemical composition of the sediments and seawater. We propose that the chemical signature of serpentinization related to mantle exhumation is recorded in the sediments and may serve as a proxy to date serpentinization and mantle exhumation at present-day magma-poor rifted margins.

  3. Construction and destruction of a volcanic island developed inside an oceanic rift: Graciosa Island, Terceira Rift, Azores

    NASA Astrophysics Data System (ADS)

    Sibrant, A. L. R.; Marques, F. O.; Hildenbrand, A.

    2014-09-01

    There is a great lack of knowledge regarding the evolution of islands inside active oceanic rifts, in particular the meaning of the different evolutionary steps. Therefore, we conducted an investigation in Graciosa Island, which lies at the northwestern end of the Terceira Rift in the Azores Triple Junction, with the objective of constraining the evolution of the island in terms of volcanic growth and mass wasting, in particular the meaning and age of the destruction events. From digital elevation model (DEM) analysis, stratigraphic and tectonic observations, K/Ar dating on key samples, and available bathymetry and gravity data, we propose that Graciosa comprises five main volcanic complexes separated by major unconformities related to large scale mass wasting: (1) The older volcanic edifice (Serra das Fontes Complex) grew until ca. 700 ka, and was affected by a major flank collapse towards the southwest, which removed the whole SW flank, the summit and a part of the NE flank. (2) The Baía do Filipe Complex developed between at least 472 ka and 433 ka in two different ways: in the SW (presently offshore) as a main volcano, and in the NE unconformably over the sub-aerial remnants of the Serra das Fontes Complex, as secondary volcanic edifices. (3) The Baía do Filipe Complex was affected by a major flank collapse towards the SW, again removing most of the edifice. (4) The remnants of the Baía do Filipe Complex were covered in unconformity by the Serra Dormida Complex between ca. 330 and 300 ka, which in turn was unconformably covered by the younger Basaltic Cover Complex between ca. 300 ka and 214 ka. These two units were affected by a third major sector collapse that removed the whole western flank, the summit and part of the eastern flank of the Serra Dormida and Basaltic Cover complexes. (5) Despite the relatively young age of Graciosa, the collapse scars are not well preserved, and not active anymore. (6) A central-type volcano has been growing since at least 60 ka at the southeastern end of the island, inside the scar left by a fourth sector collapse towards the SE, which affected most previous complexes. Contemporaneously, parasitic strombolian cones formed all over the island. Despite the location of Graciosa inside the active Terceira Rift, the new data indicate that the evolution of the island has been driven by a competition between volcano growth and repeated destruction by catastrophic sector collapses, rather than by slow incremental faulting associated with the tectonics of the rift.

  4. Using Lake Superior Parks to Present the Midcontinent Rift

    NASA Astrophysics Data System (ADS)

    Stein, C. A.; Stein, S. A.; Blavascunas, E.

    2014-12-01

    Some of the Midwest's most spectacular scenery occurs near Lake Superior, in places like Pictured Rocks and Apostle Islands National Lakeshores, Isle Royale National Park, Interstate Park, and Porcupine Mountains State Park. These landscapes provide an enormous, but underutilized opportunity for park interpreters and educators to explain some of the most exciting concepts of modern geology. A crucial aspect of doing this is recognizing that many of the rocks and landforms in individual parks are pieces of a huge regional structure. This structure, called the Midcontinent Rift System (MCRS), is a 1.1 billion year old 3000 km (2000 mile) long scar along which the North American continent started to tear apart, just as Africa is splitting today along the East African Rift, but for some reason failed to form a new ocean. Drawing on our experience as researchers and teachers studying the MCRS (Steins) and as an interpreter at Isle Royale National Park (Blavascunas), we seek to give interpreters a brief introduction to MCRS to help them present information about what geologists know already and what they are learning from continuing research. Our goal is to help interpreters visualize how what they see at a specific site fits into an exciting regional picture spanning much of the Midwest.

  5. Rift Valley Fever: Recent Insights into Pathogenesis and Prevention?

    PubMed Central

    Boshra, Hani; Lorenzo, Gema; Busquets, Núria; Brun, Alejandro

    2011-01-01

    Rift Valley fever virus (RVFV) is a zoonotic pathogen that primarily affects ruminants but can also be lethal in humans. A negative-stranded RNA virus of the family Bunyaviridae, this pathogen is transmitted mainly via mosquito vectors. RVFV has shown the ability to inflict significant damage to livestock and is also a threat to public health. While outbreaks have traditionally occurred in sub-Saharan Africa, recent outbreaks in the Middle East have raised awareness of the potential of this virus to spread to Europe, Asia, and the Americas. Although the virus was initially characterized almost 80 years ago, the only vaccine approved for widespread veterinary use is an attenuated strain that has been associated with significant pathogenic side effects. However, increased understanding of the molecular biology of the virus over the last few years has led to recent advances in vaccine design and has enabled the development of more-potent prophylactic measures to combat infection. In this review, we discuss several aspects of RVFV, with particular emphasis on the molecular components of the virus and their respective roles in pathogenesis and an overview of current vaccine candidates. Progress in understanding the epidemiology of Rift Valley fever has also enabled prediction of potential outbreaks well in advance, thus providing another tool to combat the physical and economic impact of this disease. PMID:21450816

  6. Upwarp of anomalous asthenosphere beneath the Rio Grande rift

    USGS Publications Warehouse

    Parker, E.C.; Davis, P.M.; Evans, J.R.; Iyer, H.M.; Olsen, K.H.

    1984-01-01

    Continental rifts are possible analogues of mid-ocean ridges, although major plate tectonic features are less clearly observed1. Current thermal models of mid-ocean ridges2-4 consist of solid lithospheric plates overlying the hotter, less viscous asthenosphere, with plate thickness increasing away from the ridge axis. The lithospheric lower boundary lies at or near the melting point isotherm, so that at greater depths higher temperatures account for lower viscosity, lower seismic velocities and possibly partial melting. Upwarp of this boundary at the ridge axis concentrates heat there, thus lowering densities by expansion and raising the sea floor to the level of thermal isostatic equilibrium. At slow spreading ridges, a major central graben forms owing to the mechanics of magma injection into the crust5. Topography, heat flow, gravity and seismic studies support these models. On the continents, a low-velocity channel has been observed, although it is poorly developed beneath ancient cratons6-9. Plate tectonic models have been applied to continental basins and margins10-12, but further similarities to the oceanic models remain elusive. Topographic uplift is often ascribed to Airy type isostatic compensation caused by crustal thickening, rather than thermal compensation in the asthenosphere. Here we discuss the Rio Grande rift, in southwestern United States. Teleseismic P-wave residuals show that regional uplift is explained by asthenosphere uplift rather than crustal thickening. ?? 1984 Nature Publishing Group.

  7. Correlation of geophysical datasets in rifted margin studies

    NASA Astrophysics Data System (ADS)

    Peron-Pinvidic, Gwenn; Terje Osmundsen, Per; Ebbing, Jörg

    2015-04-01

    A robust interpretation protocol for defining offshore rifted margin architecture includes interpretation of seismic reflection data supplemented by refraction and/or potential field modeling. In combination, this workflow is believed to provide better constraints on sedimentary, basement and Moho geometries at depth and/or the presence of magmatic material. Interpretation of the new generation of long-offset seismic reflection data shows that conflicts may arise between structural observations made from high-resolution seismic reflection profiles and a simple translation of density and velocity values into specific rock-types. We illustrate variations over this topic using three type-examples from the Mid-Norwegian rifted system. We show, for instance, that dense sediments wrongly interpreted as crystalline basement, can lead to incorrect mapping of the top of basement and thus to a wrong distribution of crustal and sedimentary material in the margin. This would directly impact margin restoration exercises, modeled plate kinematics and basin analyses. Our examples show that, in the absence of a seismic reflection dataset with good local coverage and high resolution, interpretation of potential field and/or velocity models in terms of structures or lithologies should be handled with care in order to avoid misunderstanding of the margin's tectonic and stratigraphic evolution.

  8. Generation of Continental Rifts, Basins and Swells by Lithosphere Instabilities

    NASA Astrophysics Data System (ADS)

    Milelli, L.; Fourel, L.; Jaupart, C. P.

    2012-12-01

    Domal uplifts, volcanism, basin formation and rifting have often struck the same continent in different areas at the same time. Their characteristics and orientations are difficult to reconcile with mantle convection or tectonic forces and suggest a driving mechanism that is intrinsic to the continent. The rifts seem to develop preferentially at high angles to the edge of the continent whereas swells and basins seem confined to the interior. Another intriguing geometrical feature is that the rifts often branch out in complicated patterns at their landward end. In Western Africa, for example, magmatic activity currently occurs in a number of uplifted areas including the peculiar Cameroon Volcanic Line that stretches away from the continental margin over about 1000 km. Magmatic and volcanic activity has been sustained along this line for 70 My with no age progression. The mantle upwelling that feeds the volcanoes is not affected by absolute plate motions and hence is attached to the continent. The Cameroon Volcanic Line extends to the Biu swell to the North and the Jos plateau to the West defining a striking Y-shaped pattern. This structure segues into several volcanic domes including the Air, the Hoggar, the Darfur, the Tibesti and the Haruj domes towards the Mediterranean coast. Another example is provided by North America, where the late Proterozoic-early Ordovician saw the formation of four major basins, the Michigan, Illinois, Williston and Hudson Bay, as well as of major rifts in southern Oklahoma and the Mississipi Valley within a short time interval. At the same time, a series of uplifts developed, such as the Ozark and Nashville domes. Motivated by these observations, we have sought an explanation in the continental lithosphere itself. We describe a new type of convective instability at the base of the lithosphere that leads to a remarkable spatial pattern at the scale of an entire continent. We carried out fluid mechanics laboratory experiments on buoyant blocks of finite size that became unstable due to cooling from above and describe the peculiar horizontal planform that developed. Dynamical behaviour depends on three dimensionless numbers, a Rayleigh number for the unstable block, a buoyancy number that scales the intrinsic density contrast to the thermal one and the aspect ratio of the block. Within the block, instability develops in two different ways in an outer annulus and in an inner region. In the outer annulus, upwellings and downwellings take the form of radial rolls spaced regularly. In the interior region, the planform adopts the more familiar form of polygonal cells. Translated to geological conditions, such instabilities should manifest themselves as linear rifts striking at a right angle to the continent-ocean boundary and an array of domal uplifts, volcanic swells and basins in the continental interior. The laboratory data lead to simple scaling laws for the dimensions and spacings of the convective structures. For the sub-continental lithospheric mantle, these dimensions and distances take values in the 500-1000 km range, close to geological examples. The large intrinsic buoyancy of Archean lithospheric roots prevents this type of instability, which explains why the widespread volcanic activity that currently affects Western Africa is confined to post-Archean domains.

  9. Geology and Petrology of the Southeast Mariana Forearc Rift

    NASA Astrophysics Data System (ADS)

    Ribeiro, J. M.; Anthony, E. Y.; Bloomer, S. H.; Girard, G.; Ishizuka, O.; Kelley, K. A.; Manton, W. I.; Martinez, F.; Merle, S. G.; Ohara, Y.; Reagan, M. K.; Ren, M.; Stern, R. J.

    2011-12-01

    The southernmost Mariana convergent margin is tectonically and magmatically very active, with submarine arc volcanoes that are sub-parallel to the Malaguana-Gadao Ridge backarc spreading center at ~110km from the trench axis. This activity reflects widening of the S. Mariana Trough. Stretching formed 3 southeast-facing, broad rifts extending from the trench to an extinct arc volcano chain (~80km from the trench axis) that is mostly composed of outcrops and fragments of pillow lavas partially covered by sediments. The 3 rifts comprise the S.E. Mariana Forearc Rift (SEMFR) and are 50-56km long and 3600 to 8200m deep, with axial valleys that narrow near the extinct arc. We studied the SEMFR using one Shinkai 6500 dive in 2008 and two Shinkai 6500 dives and 7 deep-tows in 2010. Near the trench, the SEMFR flanks are very steep and dominated by talus slopes of lava, fine-grained gabbro, diabase and peridotite, sometimes covered by thin volcaniclastic sediments. Few outcrops of pillow lavas, lava flows and volcaniclastics are observed, strongly suggesting that SEMFR morphology is dominated by faulting and landsliding. Lava outcrops are smoother and better preserved towards the extinct arc, suggesting that magmatic activity dominates that part of the rift. 40Ar-39Ar ages of 3 SEMFR lavas are 3.0-3.7Ma, so post-magmatic rifting is younger than ~3Ma. SEMFR pillow lavas are vesicular and microporphyritic with crystallite-rich glassy rinds, indicating they erupted underwater at near-liquidus conditions. In contrast, the lava flows are more crystallized and less vesicular. SEMFR lavas exhibit similar ranges in mineral composition with 2 kinds of plagioclase (An>80% and An<80%), clinopyroxene (Mg#?80% and Mg#<80%), olivine (Fo>90 and Fo<90), suggesting magma mixing. Gabbroic rocks are slightly altered and have olivine and clinopyroxene compositions similar to those of the lavas, but contain less anorthitic plagioclase with a wider range in composition (An20-70) than the lavas. One sample of the extinct arc lava is vesicular and aphyric, consistent with near-liquidus eruption conditions. SEMFR lavas are low-K calk-alkaline basalts to andesites, whereas lava from the extinct arc is tholeiitic basaltic andesite similar to those erupted from the Malaguana-Gadao Ridge. These lavas are enriched in LILE and depleted in HFSE, consistent with subduction zone settings. Their similar Yb-normalized trace element patterns (Pearce et al., 2005), with different enrichment in LILE, suggest that SEMFR lavas were produced from a relatively homogenous mantle source that interacted with different proportions of subduction-derived components. Only SEMFR lavas sampled near the extinct arc exhibit different petrographic and geochemical features (higher abundances in olivine and clinopyroxene, lower HREE patterns, lower Nb/Yb and higher Ba/Th), indicating that a more depleted mantle source interacted with a greater subduction input.

  10. Stable isotope variation in tooth enamel from Neogene hippopotamids: monitor of meso and global climate and rift dynamics on the Albertine Rift, Uganda

    NASA Astrophysics Data System (ADS)

    Brachert, Thomas Christian; Brügmann, Gerhard B.; Mertz, Dieter F.; Kullmer, Ottmar; Schrenk, Friedemann; Jacob, Dorrit E.; Ssemmanda, Immaculate; Taubald, Heinrich

    2010-10-01

    The Neogene was a period of long-term global cooling and increasing climatic variability. Variations in African-Asian monsoon intensity over the last 7 Ma have been deduced from patterns of eolian dust export into the Indian Ocean and Mediterranean Sea as well as from lake level records in the East African Rift System (EARS). However, lake systems not only depend on rainfall patterns, but also on the size and physiography of river catchment areas. This study is based on stable isotope proxy data (18O/16O, 13C/12C) from tooth enamel of hippopotamids (Mammalia) and aims in unravelling long-term climate and watershed dynamics that control the evolution of palaeolake systems in the western branch of the EARS (Lake Albert, Uganda) during the Late Neogene (7.5 Ma to recent). Having no dietary preferences with respect to wooded (C3) versus grassland (C4) vegetation, these territorial, water-dependant mammals are particularly useful for palaeoclimate analyses. As inhabitants of lakes and rivers, hippopotamid tooth enamel isotope data document mesoclimates of topographic depressions, such as the rift valleys and, therefore, changes in relative valley depth instead of exclusively global climate changes. Consequently, we ascribe a synchronous maximum in 18O/16O and 13C/12C composition of hippopotamid enamel centred around 1.5-2.5 Ma to maximum aridity and/or maximum hydrological isolation of the rift floor from rift-external river catchment areas in response to the combined effects of rift shoulder uplift and subsidence of the rift valley floor. Structural rearrangements by ~2.5 Ma within the northern segment of the Albertine Rift are well constrained by reversals in river flow, cannibalisation of catchments, biogeographic turnover and uplift of the Rwenzori horst. However, a growing rain shadow is not obvious in 18O/16O signatures of the hippopotamid teeth of the Albertine Rift. According to our interpretation, this is the result of the overriding effect of evaporation on 18O/16O responding to aridification of the basin floor by a valley air circulation system through relative deepening of the valley. On the other hand, a synchronous arid pulse is not so clearly recorded in palaeosol data and mammalian fauna of the eastern branch of the EARS. This discrepancy indicates that rift mesoclimates may represent an underestimated aspect in previous palaeoclimate reconstructions from rift valley data and represent a clear limitation to attempts at global climate reconstructions. The results of this study also suggest that using 18O/16O data as a proxy to rain shadow evolution must take into account relative basin subsidence to properly document mountain range uplift.

  11. Structural geology of the African rift system: Summary of new data from ERTS-1 imagery. [Precambrian influence

    NASA Technical Reports Server (NTRS)

    Mohr, P. A.

    1974-01-01

    ERTS imagery reveals for the first time the structural pattern of the African rift system as a whole. The strong influence of Precambrian structures on this pattern is clearly evident, especially along zones of cataclastic deformation, but the rift pattern is seen to be ultimately independent in origin and nature from Precambrian tectonism. Continuity of rift structures from one swell to another is noted. The widening of the Gregory rift as its northern end reflects an underlying Precambrian structural divergence, and is not a consequence of reaching the swell margin. Although the Western Rift is now proven to terminate at the Aswa Mylonite Zone, in southern Sudan, lineaments extend northeastwards from Lake Albert to the Eastern Rift at Lake Stefanie. The importance of en-echelon structures in the African rifts is seen to have been exaggerated.

  12. New perspectives on the evolution of narrow, modest extension continental rifts: Embryonic core complexes and localized, rapid Quaternary extension in the Rio Grande rift, central New Mexico

    NASA Astrophysics Data System (ADS)

    Ricketts, J.; Karlstrom, K. E.; Kelley, S.

    2013-12-01

    Updated models for continental rift zones need to address the role and development of low-angle normal fault networks, episodicity of extension, and interaction of 'active and passive' driving mechanisms. In the Rio Grande rift, USA, low-angle normal faults are found throughout the entire length of the rift, but make up a small percentage of the total fault population. The low-angle Jeter and Knife Edge faults, for example, crop out along the SW and NE margins of the Albuquerque basin, respectively. Apatite fission track (AFT) age-elevation data and apatite (U-Th)/He (AHe) ages from these rift flank uplifts record cooling between ~21 - 16 Ma in the NE rift flank and ~20 - 10 Ma in the SW, which coincides with times of rapid extension and voluminous syntectonic sedimentation. The timing of exhumation is also similar to rift flanks farther north in active margins based on AFT data alone. In addition, synthetic faults in the hanging wall of each low-angle fault become progressively steeper and younger basinward, and footwall blocks are the highest elevation along the rift flanks. These observations are consistent with a model where initially high-angle faults are shallowed in regions of maximum extension. As they rotate, new intrabasinal faults emerge which also can be rotated if extension continues. These relationships are similarly described in mature core complexes, and if these processes continued in the Rio Grande rift, it could eventually result in mid-crustal ductily deformed rocks in the footwall placed against surficial deposits in the hanging wall across faults that have been isostatically rotated to shallow dips. Although existing data are consistent with highest strain rates during a pulse of extension along the entire length of the rift 20-10 Ma., GPS-constrained measurements suggest that the rift is still actively-extending at 1.23-1.39 nstr/yr (Berglund et al., 2012). Additional evidence for Quaternary extension comes from travertine deposits that are cut by multiple tensile vein sets along the western margin of the Albuquerque basin in the Lucero uplift. At this location, U-series ages on travertine deposits are used to calculate strain rates at this location. These strain rates (15-105 nstr/yr) are higher than both the modern strain rates as well as the average long-term strain rates (3-14 nstr/yr) obtained from restored cross-sections across different basins in the rift. To explain these observations, we propose a model involving high fluid pressures, which promote the formation of tensile veins that are oriented with respect to the modern day stress field in the rift. These regions of anomalously-high strain need not be widespread, and are only active on timescales of the hydraulic system, but they are nevertheless an underappreciated mechanism of progressive extension in the rift. Berglund, H.T., Sheehan, A.F., Murray, M.H., Roy, M., Lowry, A.R., Nerem, R.S., and Blume, F., 2012, Distributed deformation across the Rio Grande Rift, Great Plains, and Colorado Plateau: Geology, v. 40, p. 23-26.

  13. Transient cracks and triple junctions induced by Cocos-Nazca propagating rift

    NASA Astrophysics Data System (ADS)

    Schouten, H.; Smith, D. K.; Zhu, W.; Montesi, L. G.; Mitchell, G. A.; Cann, J. R.

    2009-12-01

    The Galapagos triple junction is a ridge-ridge-ridge triple junction where the Cocos, Nazca, and Pacific plates meet around the Galapagos microplate (GMP). On the Cocos plate, north of the large gore that marks the propagating Cocos-Nazca (C-N) Rift, a 250-km-long and 50-km-wide band of NW-SE-trending cracks crosscuts the N-S-trending abyssal hills of the East Pacific Rise (EPR). These appear as a succession of minor rifts, accommodating some NE-SW extension of EPR-generated seafloor. The rifts successively intersected the EPR in triple junctions at distances of 50-100 km north of the tip of the C-N Rift. We proposed a simple crack interaction model to explain the location of the transient rifts and their junction with the EPR. The model predicts that crack locations are controlled by the stress perturbation along the EPR, induced by the dominant C-N Rift, and scaled by the distance of its tip to the EPR (Schouten et al., 2008). The model also predicts that tensile stresses are symmetric about the C-N Rift and thus, similar cracks should have occurred south of the C-N Rift prior to formation of the GMP about 1 Ma. There were no data at the time to test this prediction. In early 2009 (AT 15-41), we mapped an area on the Nazca plate south of the C-N rift out to 4 Ma. The new bathymetric data confirm the existence of a distinctive pattern of cracks south of the southern C-N gore that mirrors the pattern on the Cocos plate until about 1 Ma, and lends support to the crack interaction model. The envelope of the symmetric cracking pattern indicates that the distance between the C-N Rift tip and the EPR varied between 40 and 65 km during this time (1-4 Ma). The breakdown of the symmetry at 1 Ma accurately dates the onset of a southern plate boundary of the GMP, now Dietz Deep Rift. At present, the southern rift boundary of the GMP joins the EPR with a steep-sided, 80 km long ridge. This ridge releases the stress perturbation otherwise induced along the EPR by elastic interaction with the C-N Rift and prevents the formation of minor rifts of the type in the North of the C-N Rift. However, the seafloor displays traces of rifts formed as the Dietz Deep Rift was approaching the EPR. In fact, the present day ridge appears to have originated as one of these minor rifts, probably stabilized by enhanced magma supply from a nearby volcano at the southwestern end of Dietz Deep.

  14. Long-term cooling history of the Albertine Rift: new evidence from the western rift shoulder, D.R. Congo

    NASA Astrophysics Data System (ADS)

    Bauer, F. U.; Glasmacher, U. A.; Ring, U.; Grobe, R. W.; Mambo, V. S.; Starz, M.

    2015-02-01

    To determine the long-term landscape evolution of the Albertine Rift in East Africa, low-temperature thermochronology was applied and the cooling history constrained using thermal history modelling. Acquired results reveal (1) "old" cooling ages, with predominantly Devonian to Carboniferous apatite fission-track ages, Ordovician to Silurian zircon (U-Th)/He ages and Jurassic to Cretaceous apatite (U-Th-Sm)/He ages; (2) protracted cooling histories of the western rift shoulder with major phases of exhumation in mid-Palaeozoic and Palaeogene to Neogene times; (3) low Palaeozoic and Neogene erosion rates. This indicates a long residence time of the analysed samples in the uppermost crust, with the current landscape surface at a near-surface position for hundreds of million years. Apatite He cooling ages and thermal history models indicate moderate reheating in Jurassic to Cretaceous times. Together with the cooling age distribution, a possible Albertine high with a distinct relief can be inferred that might have been a source area for the Congo Basin.

  15. The development of extension and magmatism in the Red Sea rift of Afar

    NASA Astrophysics Data System (ADS)

    Keir, Derek; Bastow, Ian D.; Pagli, Carolina; Chambers, Emma L.

    2013-11-01

    Despite the importance of continental breakup in plate tectonics, precisely how extensional processes such as brittle faulting, ductile plate stretching, and magma intrusion evolve in space and time during the development of new ocean basins remains poorly understood. The rifting of Arabia from Africa in the Afar depression is an ideal natural laboratory to address this problem since the region exposes subaerially the tectonically active transition from continental rifting to incipient seafloor spreading. We review recent constraints on along-axis variations in rift morphology, crustal and mantle structure, the distribution and style of ongoing faulting, subsurface magmatism and surface volcanism in the Red Sea rift of Afar to understand processes ultimately responsible for the formation of magmatic rifted continental margins. Our synthesis shows that there is a fundamental change in rift morphology from central Afar northward into the Danakil depression, spatially coincident with marked thinning of the crust, an increase in the volume of young basalt flows, and subsidence of the land towards and below sea-level. The variations can be attributed to a northward increase in proportion of extension by ductile plate stretching at the expense of magma intrusion. This is likely in response to a longer history of localised heating and weakening in a narrower rift. Thus, although magma intrusion accommodates strain for a protracted period during rift development, the final stages of breakup are dominated by a phase of plate stretching with a shift from intrusive to extrusive magmatism. This late-stage pulse of decompression melting due to plate thinning may be responsible for the formation of seaward dipping reflector sequences of basalts and sediments, which are ubiquitous at magmatic rifted margins worldwide.

  16. Crustal structure of the northern mississippi embayment and a comparison with other continental rift zones

    USGS Publications Warehouse

    Mooney, W.D.; Andrews, M.C.; Ginzburg, A.; Peters, D.A.; Hamilton, R.M.

    1983-01-01

    Previous geological and geophysical investigations have suggested that the Mississippi Embayment is the site of a Late Precambrian continental rift that was reactivated in the Mesozoic. New information on the deep structure of the northern Mississippi Embayment, gained through an extensive seismic refraction survey, supports a rifting hypothesis. The data indicate that the crust of the Mississippi Embayment may be characterized by six primary layers that correspond geologically to unconsolidated Mesozoic and Tertiary sediments (1.8 km/s), Paleozoic carbonate and clastic sedimentary rocks (5.9 km/s), a low-velocity layer of Early Paleozoic sediments (4.9 km/s), crystalline upper crust (6.2 km/s), lower crust (6.6 km/s), modified lower crust (7.3 km/s), and mantle. Average crustal thickness is approximately 41 km. The presence and configuration of the low-velocity layer provide new evidence for rifting in the Mississippi Embayment. The layer lies within the northeast-trending upper-crustal graben reported by Kane et al. (1981), and probably represents marine shales deposited in the graben after rifting. The confirmation and delineation of a 7.3 km/s layer, identified in previous studies, implies that the lower crust has been altered by injection of mantle material. Our results indicate that this layer reaches a maximum thickness in the north-central Embayment and thins gradually to the southeast and northwest, and more rapidly to the southwest along the axis of the graben. The apparent doming of the 7.3 km/s layer in the north-central Embayment suggests that rifting may be the result of a triple junction located in the Reelfoot Basin area. The crustal structure of the Mississippi Embayment is compared to other continental rifts: the Rhinegraben, Limagnegraben, Rio Grande Rift, Gregory Rift, and the Salton Trough. This comparison suggests that alteration of the lower crust is a ubiquitous feature of continental rifts. ?? 1983.

  17. The tectonic evolution of the southeastern Terceira Rift/São Miguel region (Azores)

    NASA Astrophysics Data System (ADS)

    Weiß, B. J.; Hübscher, C.; Lüdmann, T.

    2015-07-01

    The eastern Azores Archipelago with São Miguel being the dominant subaerial structure is located at the intersection of an oceanic rift (Terceira Rift) with a major transform fault (Gloria Fault) representing the westernmost part of the Nubian-Eurasian plate boundary. The evolution of islands, bathymetric highs and basin margins involves strong volcanism, but the controlling geodynamic and tectonic processes are currently under debate. In order to study this evolution, multibeam bathymetry and marine seismic reflection data were collected to image faults and stratigraphy. The basins of the southeastern Terceira Rift are rift valleys whose southwestern and northeastern margins are defined by few major normal faults and several minor normal faults, respectively. Since São Miguel in between the rift valleys shows an unusual W-E orientation, it is supposed to be located on a leaky transform. South of the island and separated by a N120° trending graben system, the Monacco Bank represents a N160° oriented flat topped volcanic ridge dominated by tilted fault blocks. Up to six seismic units are interpreted for each basin. Although volcanic ridges hamper a direct linking of depositional strata between the rift and adjacent basins, the individual seismic stratigraphic units have distinct characteristics. Using these units to provide a consistent relative chrono-stratigraphic scheme for the entire study area, we suggest that the evolution of the southeastern Terceira Rift occurred in two stages. Considering age constrains from previous studies, we conclude that N140° structures developed orthogonal to the SW-NE direction of plate-tectonic extension before ~ 10 Ma. The N160° trending volcanic ridges and faults developed later as the plate tectonic spreading direction changed to WSW-ENE. Hence, the evolution of the southeastern Terceira Rift domain is predominantly controlled by plate kinematics and lithospheric stress forming a kind of a re-organized rift system.

  18. The mode of rifting of the Tyrrhenian Sea

    NASA Astrophysics Data System (ADS)

    Milia, Alfonsa; Torrente, Maurizio M.

    2014-05-01

    The back-arc evolution of the Tyrrhenian Sea has mainly been attributed to the roll-back towards the south-east of the subducting Ionian plate that could have provided the bulk of the space required for this extension. The Tyrrhenian Sea is a triangular basin characterized by two large bathyal basins (Vavilov and Marsili basins) that are covered by some hundred meters of sediments, and a number of peri- Tyrrhenian basins filled by thousands of meters of clastic and/or volcaniclastic sediments. The stratigraphic record of these basins offers an opportunity to study the timing and kinematics of the basin-forming faults that are relevant for the creation of a model on the opening of the Tyrrhenian Sea. Basin analysis was performed using interpretation of seismic reflection profiles and well logs. The interpretation of these data was made using seismic and sequence stratigraphy and structural geology in a GIS-dedicated environment. The sequence stratigraphy interpretation of the deepest wells were performed using discontinuities and trends in wireline log pattern. Systems tracts and transgressive-regressive cycles were identified in well log succession and seismic profiles. The sequence stratigraphy approach allows the identification of 4th-order depositional sequences (100 ka). The geologic evolution, in terms of age of basin formation, style of deformation, timing of activity of the fault bounding basins, tectonic subsidence, post-rift infill and volcanic activity, was analyzed for several peri-Tyrrhenian basins. The study reconstructed the three-dimensional architecture of the peri-Tyrrhenian basins and illustrated the link between the bathyal basin and the Tyrrhenian margin. We document that during the evolution of the Tyrrhenian region several basins opened contemporaneously with different direction of extension and a progressive change in rifting direction occurred along the Campania Margin. The mode of rifting of the Tyrrhenian Sea was characterized by different styles of the extensional basins and detachment faulting. Taking into account the published geological data, we propose a kinematic evolution of the Tyrrhenian basin over the last 12 Ma.

  19. Geodynamic Evolution of the Southern Flank of the Corinth Rift

    NASA Astrophysics Data System (ADS)

    Jousselin, D.; Bourlange, S.; Ford, M.; Le Carlier, C.; Rohais, S.

    2005-12-01

    The Gulf of Corinth is the most seismically active area of Europe extending in a N-S direction at a rate of 12mm/yr. Many studies have focused on currently active normal faults, which bound the southern flank of the Corinth rift. However, many prominent faults, that are considered no longer active, outcrop over a broader area along with their syn-rift sedimentary sequence. The normal fault network and related synrift succession have been mapped and studied at a scale of 1:25,000. The geometry of the fault network in 3D has been reconstructed in the gOcad 3D geomodelling package. Based on these detailed field data we propose a geodynamic model for the evolution of the region in three steps over the last two million years. From 2 to 1 Ma, extension affected a broad area at least twenty kilometers wide. Alluvial conglomerates and sandstones, deposited in grabens and half grabens, were derived from erosion of the pre-existing (Hellenic) high relief to the south in the Peloponnese massif. Around 1Ma, a regional uplift led to the migration of the depocenter to the north. New faults are activated closer to the coast, in places generating forced folds that tilted synrift fluvial formations northward over a 2-3 kilometers wide area. Considerable accommodation space was generated by these faults and, with an increased sediment supply from the south, a series of giant marine Gilbert deltas developed in their hangingwalls. Finally, from 0.5 Ma to present, the activation of a series of major new faults further to the north (near the current coastline) coincided with regional uplift of the south coastal belt, and initiated the formation of a second generation of Gilbert delta, currently building out into the gulf. The faults can be divided into three families. The main one (74 data) is oriented N110, dipping 45 to 60° to the north. It includes 5 major faults spaced 3 to 5 km from south to north, with a throw larger than 1000m. A secondary set (41 data) strikes N60, dipping to the NW, it includes only one fault with a throw larger than 500m; the other secondary set (43 data) is antithetic to the two first ones as it strikes N60 to N110, dipping to the south, the largest fault it includes has a throw of about 400m. Most faults are planar and the importance of listric faults seems minor in the rift history.

  20. A NEW (or old) RIFT IN EASTERN CALIFORNIA

    NASA Astrophysics Data System (ADS)

    Calzia, J.

    2009-12-01

    The Eastern California Shear Zone (ECSZ) includes a broad network of right-lateral faults in the Mojave Desert and southern Basin-Ranges of California. Seven large earthquakes, including the 1992 M7.4 Landers and 1999 M7.1 Hector Mine earthquakes, have occurred within this zone in the last 60 years. This severe seismic activity is complicated by two puzzling facts: 1) the hypocenter of the Landers and Hector Mine earthquakes occur at shallow crustal depths (10±4 km); and 2) the aftershock patterns associated with these earthquakes cross cut the structural grain of the ECSZ at an acute angle. GPS data indicate that crustal rocks within the Mojave Desert segment of the ECSZ are moving as much as 14 mm/year to the northwest relative to Station FUNE in Death Valley; crustal rocks within the Basin-Ranges segment are moving 12.5 mm/year northwestward. Dokka and Travis (1990) concluded that the ECZS accommodates 20-25 percent of relative plate motion along the North American-Pacific plate margin inboard of the San Andreas Fault Zone. Nur and others (1993) reported that the large earthquakes may occur along a new fault system that cuts across the older faults; they named this young fault the Landers-Mojave earthquake line. Neither model, however, considers Quaternary volcanic centers in eastern CA and NV just east of and locally within the ECSZ. Rocks within these volcanic centers, including (from north to south) Lathrop Wells, Cima, Pisgah, Amboy, and Obsidian Butte, are younger than but chemically and isotopically similar to Neogene volcanic rocks that bound the rift province of the northern Gulf of California. The seismic and GPS data, combined with petrologic data from nearby volcanic centers, suggests that the ECZS is the active margin of an incipient transtensional rift as the Gulf of California propagates northward. Given the volume of volcanic rocks and prehistoric seismic history in California, it is not certain if this rift is growing or dying; only time will tell.

  1. Dyke intrusion dynamics during the ongoing rifting episode in Afar

    NASA Astrophysics Data System (ADS)

    Jacques, E.; Grandin, R.; Nercessian, A.; Ayele, A.; Keir, D.; Doubre, C.; Socquet, A.; Lemarchand, A.

    2010-12-01

    Dyke intrusion is one of the main processes of crustal accretion occurring along magma-assisted divergent plate boundaries, in particular along Mid-Ocean Ridges. Seismology, through the analysis of earthquake migration coeval to dyke intrusion, is one of the few tools, with geodesy, which allows studying the dynamics of this fundamental process. In Afar (Ethiopia), the major Dabbahu-Manda Hararo rifting episode started in September 2005 with the intrusion a 60 km-long, 5 m-wide mega-dyke. Between June 2006 and May 2010, 13 smaller dykes (~10 km long) were emplaced, giving an outstanding opportunity to perform such studies. A few weeks after the rifting episode commenced, a regional seismological network of broadband stations was installed in Afar (part of a multinational project including UK, US and Ethiopia). In November 2007, in collaboration with the Geophysical Observatory of Addis Ababa University, IPGP (Institut de Physique du Globe de Paris) installed a local telemetered seismological network including 5 short-period stations around the southern half of the Dabbahu-Manda Hararo rift. We focus here on the microseismicity related to five dyke intrusions, which occurred between June 2006 and February 2009. The dykes propagated away from the Walis magma reservoir (WMR, 12.3°N, 40.6°E) at velocities ranging from 0.5km/h to 2km/h. Earthquake migrations during dyke intrusions involved a narrow (less than 5 km wide), seismically-active strip around the dykes, and seem to show the following pattern: first, near the tip of the propagating dyke, probably in the damage zone, small to moderate earthquakes are detected. Subsequent, more energetic earthquakes may have been triggered only after sufficient dyke inflation had occurred. Earthquake sequences, which occurred north of WMR, migrated faster (~2km/h) than those that took place south of WMR (0.5-1km/h), and liberated a greater amount of seismic energy. This is in keeping with results of normal stress modeling derived from InSAR, which indicate that before the mega-dyke of September 2005, differential stress in the region 10 km north of WMR may have been greater than south of the reservoir. This suggests a heterogeneous state of stress, with a tectonic stretching greater north of WMR than toward the south. We plan to extend the local seismological network to better constrain seismic rupture mechanisms and the precise depth at which earthquakes occur during dyke propagation.

  2. Lithospheric Decoupling and Rotations: Hints from Ethiopian Rift

    NASA Astrophysics Data System (ADS)

    Muluneh, A. A.; Cuffaro, M.; Doglioni, C.; Kidane, T.

    2014-12-01

    Plates move relative to the mantle because some torques are acting on them. The shear in the low-velocity zone (LVZ) at the base of the lithosphere is the expression of these torques. The decoupling is allowed by the low viscosity in the LVZ, which is likely few orders of magnitudes lower than previously estimated. The viscosity value in the LVZ controls the degree of coupling/decoupling between the lithosphere and the underlying mantle. Lateral variations in viscosity within the LVZ may explain the velocity gradient among tectonic plates as the one determining the Ethiopian Rift (ER) separating Africa from Somalia. While it remains not fully understood the mechanisms of the torques acting on the lithosphere (thermally driven mantle convection or the combination of mantle convection with astronomical forces such as the Earth's rotation and tidal drag), the stresses are transmitted across the different mechanical layers (e.g., the brittle upper crust, down to the viscous-plastic ductile lower crust and upper mantle). Differential basal shear traction at the base of the lithosphere beneath the two sides of the East African Rift System (EARS) is assumed to drive and sustain rifting. In our analysis, the differential torques acting on the lithospheric/crustal blocks drive kinematics and block rotations. Since, ER involves the whole lithosphere, we do not expect large amount of rotation. Rotation can be the result of the whole plate motion on the sphere moving along the tectonic equator, or the second order sub-rotation of a single plate. Further rotation may occur along oblique plate boundaries (e.g., left lateral transtensional setting at the ER). Small amount of vertical axis rotation of blocks in northern ER could be related to the presence of local, shallower decollement layers. Shallow brittle-ductile transition (BDT) zone and differential tilting of crustal blocks in the northern ER could hint a possibility of detachment surface between the flow in the lower crust relative to the brittle crust above. Our study suggests that kinematics of crustal blocks in the ER is controlled by Africa and Somalia plates interaction at different scale and layers.

  3. No thermal anomalies in the mantle transition zone beneath an incipient continental rift: evidence from the first receiver function study across the Okavango Rift Zone, Botswana

    NASA Astrophysics Data System (ADS)

    Yu, Y.; Liu, K. H.; Moidaki, M.; Reed, C. A.; Gao, S. S.

    2015-08-01

    Mechanisms leading to the initiation and early-stage development of continental rifts remain enigmatic, in spite of numerous studies. Among the various rifting models, which were developed mostly based on studies of mature rifts, far-field stresses originating from plate interactions (passive rifting) and nearby active mantle upwelling (active rifting) are commonly used to explain rift dynamics. Situated atop of the hypothesized African Superplume, the incipient Okavango Rift Zone (ORZ) of northern Botswana is ideal to investigate the role of mantle plumes in rift initiation and development, as well as the interaction between the upper and lower mantle. The ORZ developed within the Neoproterozoic Damara belt between the Congo Craton to the northwest and the Kalahari Craton to the southeast. Mantle structure and thermal status beneath the ORZ are poorly known, mostly due to a complete paucity of broad-band seismic stations in the area. As a component of an interdisciplinary project funded by the United States National Science Foundation, a broad-band seismic array was deployed over a 2-yr period between mid-2012 and mid-2014 along a profile 756 km in length. Using P-to-S receiver functions (RFs) recorded by the stations, the 410 and 660 km discontinuities bordering the mantle transition zone (MTZ) are imaged for the first time. When a standard Earth model is used for the stacking of RFs, the apparent depths of both discontinuities beneath the Kalahari Craton are about 15 km shallower than those beneath the Congo Craton. Using teleseismic P- and S-wave traveltime residuals obtained by this study and lithospheric thickness estimated by previous studies, we conclude that the apparent shallowing is the result of a 100-150 km difference in the thickness of the lithosphere between the two cratons. Relative to the adjacent tectonically stable areas, no significant anomalies in the depth of the MTZ discontinuities or in teleseismic P- and S-wave traveltime residuals are found beneath the ORZ. These observations imply an absence of significant thermal anomalies in the MTZ and in the upper mantle beneath the incipient rift, ruling out the role of mantle plumes in the initiation of the ORZ. We propose that the initiation and development of the ORZ were the consequences of relative movements between the South African block and the rest of the African plate along a zone of lithospheric weakness between the Congo and Kalahari cratons. An area of thinner-than-normal MTZ is found at the SW corner of the study area. This anomaly, if confirmed by future studies, could suggest significant transferring of heat from the lower to the upper mantle.

  4. Active volcanism on Venus in the Ganiki Chasma rift zone

    NASA Astrophysics Data System (ADS)

    Shalygin, E. V.; Markiewicz, W. J.; Basilevsky, A. T.; Titov, D. V.; Ignatiev, N. I.; Head, J. W.

    2015-06-01

    Venus is known to have been volcanically resurfaced in the last third of solar system history and to have undergone a significant decrease in volcanic activity a few hundred million years ago. However, fundamental questions remain: Is Venus still volcanically active today, and if so, where and in what geological and geodynamic environment? Here we show evidence from the Venus Express Venus Monitoring Camera for transient bright spots that are consistent with the extrusion of lava flows that locally cause significantly elevated surface temperatures. The very strong spatial correlation of the transient bright spots with the extremely young Ganiki Chasma, their similarity to locations of rift-associated volcanism on Earth, provide strong evidence for their volcanic origin and suggests that Venus is currently geodynamically active.

  5. The Risk of Nosocomial Transmission of Rift Valley Fever.

    PubMed

    Al-Hamdan, Nasser A; Panackal, Anil A; Al Bassam, Tami H; Alrabea, Abdullah; Al Hazmi, Mohammed; Al Mazroa, Yagoub; Al Jefri, Mohammed; Khan, Ali S; Ksiazek, Thomas G

    2015-12-01

    In 2000, we investigated the Rift Valley fever (RVF) outbreak on the Arabian Peninsula-the first outside Africa-and the risk of nosocomial transmission. In a cross-sectional design, during the peak of the epidemic at its epicenter, we found four (0.6%) of 703 healthcare workers (HCWs) IgM seropositive but all with only community-associated exposures. Standard precautions are sufficient for HCWs exposed to known RVF patients, in contrast to other viral hemorrhagic fevers (VHF) such as Ebola virus disease (EVD) in which the route of transmission differs. Suspected VHF in which the etiology is uncertain should be initially managed with the most cautious infection control measures. PMID:26694834

  6. The Risk of Nosocomial Transmission of Rift Valley Fever

    PubMed Central

    Al-Hamdan, Nasser A.; Panackal, Anil A.; Al Bassam, Tami H.; Alrabea, Abdullah; Al Hazmi, Mohammed; Al Mazroa, Yagoub; Al Jefri, Mohammed; Khan, Ali S.; Ksiazek, Thomas G.

    2015-01-01

    In 2000, we investigated the Rift Valley fever (RVF) outbreak on the Arabian Peninsula—the first outside Africa—and the risk of nosocomial transmission. In a cross-sectional design, during the peak of the epidemic at its epicenter, we found four (0.6%) of 703 healthcare workers (HCWs) IgM seropositive but all with only community-associated exposures. Standard precautions are sufficient for HCWs exposed to known RVF patients, in contrast to other viral hemorrhagic fevers (VHF) such as Ebola virus disease (EVD) in which the route of transmission differs. Suspected VHF in which the etiology is uncertain should be initially managed with the most cautious infection control measures. PMID:26694834

  7. Rift Valley Fever Outbreaks in Mauritania and Related Environmental Conditions

    PubMed Central

    Caminade, Cyril; Ndione, Jacques A.; Diallo, Mawlouth; MacLeod, Dave A.; Faye, Ousmane; Ba, Yamar; Dia, Ibrahima; Morse, Andrew P.

    2014-01-01

    Four large outbreaks of Rift Valley Fever (RVF) occurred in Mauritania in 1998, 2003, 2010 and 2012 which caused lots of animal and several human deaths. We investigated rainfall and vegetation conditions that might have impacted on RVF transmission over the affected regions. Our results corroborate that RVF transmission generally occurs during the months of September and October in Mauritania, similarly to Senegal. The four outbreaks were preceded by a rainless period lasting at least a week followed by heavy precipitation that took place during the second half of the rainy season. First human infections were generally reported three to five weeks later. By bridging the gap between meteorological forecasting centers and veterinary services, an early warning system might be developed in Senegal and Mauritania to warn decision makers and health services about the upcoming RVF risk. PMID:24413703

  8. The Influence of Mantle Petrology on Basin Subsidence During Rifting

    NASA Astrophysics Data System (ADS)

    Simon, N. S.; Podladchikov, Y. Y.

    2005-12-01

    The density distribution of the lithosphere is non-linear and discontinuous due to complex mineralogy and, most importantly, phase transitions. It was shown that the garnet-spinel and spinel plagioclase transitions in the lithospheric mantle have the most profound effect on uplift/subsidence during lithospheric extension. Here we evaluate the influence of changes in mantle composition on lithospheric density and behaviour during stretching. Lithospheric mantle enters the plagioclase stability field if the crust is thin and the geotherm is relatively hot (stretching factor 2 to 3, depending on composition). This causes a large decrease in density (80-100 kg/m3). The extent of syn-rift uplift and post-rift subsidence recorded in the basin is directly proportional to the total amount of plagioclase in the column. This is controlled by the depth of the plagioclase-in reaction and the bulk Al2O3 in the mantle. The depth to which plagioclase is stable is mainly governed by bulk Na2O. Since Al2O3 and Na2O decrease with decreasing fertility of the mantle the effect of phase transitions on basin subsidence is most pronounced for relatively fertile mantle (and strong extension). The incompatible nature and strong mobility of sodium, however, may lead to large changes in bulk Na2O during minor melting (Na extraction) or metasomatism (Na addition). The sensitivity of mantle mineralogy to variations in sodium can therefore cause sudden changes in density without changes in P-T regime. The combination of 2-D finite element deformation modeling and phase diagram calculations allows us to evaluate and quantify the feedback between geochemical variations and lithospheric stretching.

  9. Littoral sedimentation of rift lakes: an illustrated overview from the modern to Pliocene Lake Turkana (East African Rift System, Kenya)

    NASA Astrophysics Data System (ADS)

    Schuster, Mathieu; Nutz, Alexis

    2015-04-01

    Existing depositional models for rift lakes can be summarized as clastics transported by axial and lateral rivers, then distributed by fan-deltas and/or deltas into a standing water body which is dominated by settling of fine particles, and experiencing occasional coarser underflows. Even if known from paleolakes and modern lakes, reworking of clastics by alongshore drift, waves and storms are rarely considered in depositional models. However, if we consider the lake Turkana Basin (East African Rift System, Kenya) it is obvious that this vision is incomplete. Three representative time slices are considered here: the modern Lake Turkana, the Megalake Turkana which developed thanks to the African Humid Period (Holocene), and the Plio-Pleistocene highstand episodes of paleolake Turkana (Nachukui, Shungura and Koobi Fora Formations, Omo Group). First, remarkable clastic morphosedimentary structures such as beach ridges, spits, washover fans, lagoons, or wave-dominated deltas are very well developed along the shoreline of modern lake Turkana, suggesting strong hydrodynamics responsible for a major reworking of the fluvial-derived clastics all along the littoral zone (longshore and cross-shore transport) of the lake. Similarly, past hydrodynamics are recorded from prominent raised beach ridges and spits, well-preserved all around the lake, above its present water-level (~360 m asl) and up to ~455 m. These large-scale clastic morphosedimentary structures also record the maximum extent of Megalake Turkana during the African Humid Period, as well as its subsequent regression forced by the end of the Holocene climatic optimum. Several hundreds of meters of fluvial-deltaic-lacustrine deposits spanning the Pliocene-Pleistocene are exposed in the Turkana basin thanks to tectonic faulting. These deposits are world famous for their paleontological and archeological content that documents the very early story of Mankind. They also preserve several paleolake highstand episodes with typical sedimentary facies and structures/bodies reflecting important littoral hydrodynamics distributed from the backshore up to the lower shoreface zones. As a consequence, this preliminary overview from the Lake Turkana Basin, suggests that littoral hydrodynamics are important processes of erosion, transport an redeposition of clastics in rift lakes, and should thus be considered in the next generation of depositional models.

  10. Climatic influence on rift structure: Axial rift evolution in Guaymas Basin during the past 250,000 years

    NASA Astrophysics Data System (ADS)

    Kluesner, J. W.; Lonsdale, P.; Kirtland, S. E.

    2009-12-01

    Newly collected, high-resolution seismic profiles in the Southern Trough of Guaymas Basin display temporal changes in axial graben structure; ages of these structures correlate to glacial-interglacial cycles over the past quarter millennium. During glacial periods the southern spreading trough in Guaymas Basin is overwhelmed with turbidites, whereas during interglacials, extension out-paces sedimentation and the trough deepens. Glacial low-stand shedding--caused by lower sea level, shelf exposure, and enhanced precipitation--results in the erosion of shelf sediments and transportation of these to the deeper basins in the Gulf. Major offshore canyon systems directly southwest of the Southern Trough capture eroded sediments from the exposed continental shelf and funnel these sediments into the axial graben as turbidites. In contrast, the higher sea level and reduced precipitation characteristic of interglacials limits the introduction of terrestrial sediments into the axial trough; sedimentation during these periods predominately consists of redistributed pelagic sediments swept into the trough by tidal and bottom currents. These sediments, dominated by diatomaceous ooze, appear as acoustically transparent layers in seismic profiles, whereas turbidites composed of fluvial and reworked shelf material appear as high-amplitude reflector packages. Systematic changes between the two sediment types are apparent on seismic profiles within the Southern Trough of Guaymas Basin as well as Carmen Basin. In addition, the regional extent of the youngest acoustically transparent layer imaged with multichannel seismic profiling approximately matches modern distribution and deposition patterns of sediments dominated by diatomaceous ooze, further supporting our interpretation that the acoustically transparent layers represent sediment packages composed mainly of pelagic material. Plotting paleo-temperature and sea-level curves along the MCS profile crossing the Southern Trough shows a striking correlation between climatic cycles, sedimentation rate, acoustic character of sediment packages, and changes in rift structure. Off-axis intrusion of young sills masks past trough floor structures resulting in only the paleo-rift flanks being imaged on seismic profiles. Assuming a constant spreading rate of 45 km/My and the relationship between different sediment packages and changes in climate, sediments imaged on off-axis flanks in the Southern Trough of Guaymas Basin record the evolution of the axial graben and flanks during the past 250,000 years. The relationship between climatic change and rift structure observed in the Southern Trough, Guaymas Basin may provide insight into how sedimentation and axial structure of a young intercontinental ocean basin have been modulated by climate change.

  11. Heat flow in the Nipigon arm of the Keweenawan rift, northwestern Ontario, Canada

    E-print Network

    Rolandone, Frederique

    by Keweenawan diabase sills. It has been interpreted as a failed arm of the ca. 1100 Ma Keweenawan rift. Six new diabase sills ca. 1109 Ma [Davis and Sutcliffe, 1985; Sutcliffe, 1991]. Prior to the Keweenawan event

  12. Magmatism on rift flanks: Insights from ambient noise phase velocity in Afar region

    NASA Astrophysics Data System (ADS)

    Korostelev, Félicie; Weemstra, Cornelis; Leroy, Sylvie; Boschi, Lapo; Keir, Derek; Ren, Yong; Molinari, Irene; Ahmed, Abdulhakim; Stuart, Graham W.; Rolandone, Frédérique; Khanbari, Khaled; Hammond, James O. S.; Kendall, J. M.; Doubre, Cécile; Ganad, Ismail Al; Goitom, Berhe; Ayele, Atalay

    2015-04-01

    During the breakup of continents in magmatic settings, the extension of the rift valley is commonly assumed to initially occur by border faulting and progressively migrate in space and time toward the spreading axis. Magmatic processes near the rift flanks are commonly ignored. We present phase velocity maps of the crust and uppermost mantle of the conjugate margins of the southern Red Sea (Afar and Yemen) using ambient noise tomography to constrain crustal modification during breakup. Our images show that the low seismic velocities characterize not only the upper crust beneath the axial volcanic systems but also both upper and lower crust beneath the rift flanks where ongoing volcanism and hydrothermal activity occur at the surface. Magmatic modification of the crust beneath rift flanks likely occurs for a protracted period of time during the breakup process and may persist through to early seafloor spreading.

  13. A joint receiver function and gravity study of crustal structure beneath the incipient Okavango Rift, Botswana

    NASA Astrophysics Data System (ADS)

    Yu, Youqiang; Liu, Kelly H.; Reed, Cory A.; Moidaki, Moikwathai; Mickus, Kevin; Atekwana, Estella A.; Gao, Stephen S.

    2015-10-01

    Rifting incorporates the fundamental processes concerning the breakup of continental lithosphere and plays a significant role in the formation and evolution of sedimentary basins. In order to decipher the characteristics of rifting at its earliest stage, we conduct the first teleseismic crustal study of one of the world's youngest continental rifts, the Okavango Rift Zone (ORZ), where the magma has not yet breached the surface. Results from receiver function stacking and gravity modeling indicate that the crust/mantle boundary beneath the ORZ is uplifted by 4-5 km, and the initiation of the ORZ is closely related to lithospheric stretching. Possible decompression melting of the subcrustal lithosphere occurs beneath the ORZ, as evidenced by a relatively low upper mantle density based on the gravity modeling.

  14. CASERTZ aeromagnetic data reveal late Cenozoic flood basalts (?) in the West Antarctic rift system

    USGS Publications Warehouse

    Behrendt, John C.

    1994-01-01

    The late Cenozoic volcanic and tectonic activity of the enigmatic West Antarctic rift system, the least understood of the great active continental rifts, has been suggested to be plume driven. In 1991-1992, as part of the CASERTZ (Corridor Aerogeophysics of the Southeast Ross Transect Zone) program, an ~25 000 km aeromagnetic survey over the ice-covered Byrd subglacial basin shows magnetic "texture' critical to interpretations of the underlying extended volcanic terrane. The aeromagnetic data reveal numerous semicircular anomalies ~100-1100 nT in amplitude, interpreted as having volcanic sources at the base of the ice sheet; they are concentrated along north-trending magnetic lineations interpreted as rift fabric. The CASERTZ aeromagnetic results, combined with >100 000 km of widely spaced aeromagnetic profiles, indicate at least 106 km3 of probable late Cenozoic volcanic rock (flood basalt?) in the West Antarctic rift beneath the ice sheet and Ross Ice Shelf. -from Authors

  15. Workshop on The Rio Grande Rift: Crustal Modeling and Applications of Remote Sensing

    NASA Technical Reports Server (NTRS)

    Blanchard, D. P. (editor)

    1980-01-01

    The elements of a program that could address significant earth science problems by combining remote sensing and traditional geological, geophysical, and geochemical approaches were addressed. Specific areas and tasks related to the Rio Grande Rift are discussed.

  16. 77 FR 68783 - Prospective Grant of Exclusive License: Veterinary Vaccines for Rift Valley Fever Virus

    Federal Register 2010, 2011, 2012, 2013, 2014

    2012-11-16

    ...: Veterinary Vaccines for Rift Valley Fever Virus AGENCY: Centers for Disease Control and Prevention (CDC... use of veterinary vaccines, to practice the inventions listed in the patent applications referred...

  17. Inflation rates, rifts, and bands in a p?hoehoe sheet flow

    USGS Publications Warehouse

    Hoblitt, Richard P.; Orr, Tim R.; Heliker, Christina; Denlinger, Roger P.; Hon, Ken; Cervelli, Peter F.

    2012-01-01

    The margins of sheet flows—p?hoehoe lavas emplaced on surfaces sloping Inflation and rift-band formation is probably cyclic, because the pattern we observed suggests episodic or crude cyclic behavior. Furthermore, some inflation rifts contain numerous bands whose spacing and general appearances are remarkably similar. We propose a conceptual model wherein the inferred cyclicity is due to the competition between the fluid pressure in the flow's liquid core and the tensile strength of the viscoelastic layer where it is weakest—in inflation rifts. The viscoelastic layer consists of lava that has cooled to temperatures between 800 and 1070 °C. This layer is the key parameter in our model because, in its absence, rift banding and stepwise changes in the flow height would not occur.

  18. Origin of three-armed rifts in volcanic islands: the case of El Hierro (Canary Islands)

    NASA Astrophysics Data System (ADS)

    Galindo Jiménez, Inés; Becerril Carretero, Laura; Martí Molist, Joan; Gudmundsson, Agust

    2015-04-01

    Rifts zones in volcanic oceanic islands are common structures that have been explained through several theories/models. However, despite all these models it is as yet unclear whether it is the intense intrusive activity or the sector collapses that actually control the structural evolution and geometry of oceanic-island rift zones. Here we provide a new hypothesis to explain the origin and characteristics of the feeding system of oceanic-island rift zones based on the analysis of more than 1700 surface, subsurface (water galleries), and submarine structural data from El Hierro (Canary Islands). El Hierro's geological structure is primarily controlled by a three-armed rift-zone, the arms striking NE, WSW and S. Between the rift axes there are three valleys formed during huge landslides: El Golfo, El Julan, and Las Playas. Our results show: (1) a predominant NE-SW strike of structural elements, which coincides with the main regional trend of the Canary Archipelago as a whole; (2) a clear radial strike distribution of structural elements for the whole volcanic edifice (including submarine flanks) with respect to the centre of the island; (3) that the rift zones are mainly subaerial structures and do not propagate through the submarine edifice; (4) that it is only in the NE rift that structures have a general strike similar to that of the rift as a whole, and; (5) that in the W and S rifts there is not clear main direction, showing the structural elements in the W rift a fan distribution coinciding with the general radial pattern in the island as a whole. Based on these data, we suggest that the radial-striking structures reflect comparatively uniform stress fields that operated during the constructive episodes, mainly conditioned by the combination of overburden pressure, gravitational spreading, and magma-induced stresses. By contrast, in the shallower parts of the edifice, that is, the NE-SW, N-S and WNW-ESE-striking structures, reflect local stress fields related to the formation of mega-landslides and mask the general radial pattern. Thus, the rift zones on El Hierro are shallow structures that commonly capture and divert ascending magma towards different parts of the island but do not condition magma ascent at depth.

  19. Structural characteristics and tectonic evolution of the east-margin blocks of the Suez rift

    NASA Astrophysics Data System (ADS)

    Moustafa, Adel R.

    1993-08-01

    Detailed field study of the east-margin blocks of the Suez rift indicates that rift initiation was marked by 22-24 Ma old volcanicity followed by second-order (relatively small) extensional faulting without block rotation. Slight rotation of first-order (large) rift blocks took place shortly before or during the deposition of the earliest rift sediments (Nukhul Formation). The shallow-water environment of deposition of the Nukhul Formation became deeper during the deposition of the overlying Rudeis Formation in response to rapid tectonic subsidence. A major event (mid-clysmic, 17 Ma ago) led to the rejuvenation of many faults bounding the large rift blocks, the development of several intra-block faults, and an abrupt change in the environment of deposition. At that time, tectonic deformation increased towards the rift axis where the width of the active part of the rift decreased. As a result of this mid-clysmic event, coarse elastics were deposited close to the major faults; later these deposits were faulted in a younger event. NW-NNW (clysmic) and N-NNE (transfer) faults are the two predominant normal fault sets in the east-margin blocks and link up in a characteristic zig-zag pattern. Block rotation is related to slip on NW-NNW listric faults. Fault plane slickenside lineations indicate predominant NE-SW extension as well as a subordinate E-W to ESE-WNW extension. The latter is perhaps related to post-Miocene mild deformation. Slip on the major faults of the rift is associated with drape folding and footwall deformation. Both lead to the development of rift-facing monoclines. The deformed parts of the footwall are spindle-shaped and link to the main faults via N-NNE transfer faults. Footwall deformation also results where a major fault bends out leaving a protrusion of the footwall. Rift extension is transferred beyond the rift boundary via some transfer faults and affects some parts of the shoulder.

  20. Imaging continental breakup using teleseismic body waves: The Woodlark Rift, Papua New Guinea

    NASA Astrophysics Data System (ADS)

    Eilon, Zachary; Abers, Geoffrey A.; Gaherty, James B.; Jin, Ge

    2015-09-01

    This study images the upper mantle beneath the D'Entrecasteax Islands, Papua New Guinea, providing insight into mantle deformation beneath a highly rifted continent adjacent to propagating spreading centers. Differential travel times from P and S-wave teleseisms recorded during the 2010-2011 CDPapua passive seismic experiment are used to invert for separate VP and VS velocity models of the continental rift. A low-velocity structure marks the E-W axis of the rift, correlating with the thinnest crust, high heat flow, and a linear trend of volcanoes. This slow region extends 250 km along strike from the oceanic spreading centers, demonstrating significant mantle extension ahead of seafloor breakup. The rift remains narrow to depth indicating localization of extension, perhaps as a result of mantle hydration. A high-VP structure at depths of 90-120 km beneath the north of the array is more than 6.5% faster than the rift axis and contains well-located intermediate depth earthquakes. These independent observations place firm constraints on the lateral thermal contrast at depth between the rift axis and cold lithosphere to the north that may be related to recent subduction, although the polarity of subduction cannot be resolved. This geometry is gravitationally unstable; downwelling or small-scale convection could have facilitated rifting and rapid lithospheric removal, although this may require a wet mantle to be realistic on the required time scales. The high-V structure agrees with the maximum P,T conditions recorded by young ultra-high pressure rocks exposed on the rift axis and may be implicated in their genesis.

  1. Rifting to drifting transition of the Southwest Subbasin of the South China Sea

    NASA Astrophysics Data System (ADS)

    Song, Taoran; Li, Chun-Feng

    2015-09-01

    Continental rupturing process and related dynamics on the onset of seafloor spreading remain poorly understood in the opening of the South China Sea. To constrain the timing and cause of major tectonic events, we focus on the rifting-to-drifting transition of the Southwest Subbasin, which has very wide extended continental margins. By carefully interpreting rifting structures and carbonate platforms and reefs, we distinguished two major unconformities, i.e., the breakup unconformity (BRU) and the mid-Miocene unconformity, in the two conjugate margins of the Southwest Subbasin. The age of the BRU in our study area is near the Oligocene/Miocene boundary (~23 Ma). Pre-stack depth migration of a recently acquired multichannel reflection seismic profile reveals complex structures and strong lateral velocity variations associated with a 3.5 km thick syn-rifting sequence developed right at the continent-ocean boundary (COB) of the Southwest Subbasin. This syn-rifting sequence is bounded landwards by a large seaward dipping fault, and tapers out seawards. An erosional truncation, which represents the mid-Miocene unconformity landwards but the older breakup unconformity on the seaward side, occurred at the top of this sequence. The overall transitional deformation style from the rifting to drifting suggests a successive episode of rifting, faulting, compression, tilting, and erosion at the COB during the crustal thinning and mantle upwelling. Localized thick syn-rifting deposition and early deposition beneath the BRU in the oceanic domain exist only at the seaward concave part of the COB, indicating discrete rifting and seafloor spreading prior to the buildup of a unified spreading center for the entire subbasin.

  2. Petroleum geology of Cretaceous-Tertiary rift basins in Niger, Chad, and Central African Republic

    SciTech Connect

    Genik, G.J. )

    1993-08-01

    This overview of the petroleum geology of rift basins in Niger, Chad, and Central African Republic (CAR) is based on exploration work by Exxon and partners in the years 1969-1991. The work included 50,000 km of modern reflection seismic, 53 exploration wells, 1,000,000 km[sup 2] of aeromagnetic coverage, and about 10,500 km of gravity profiles. The results outline ten Cretaceous and Tertiary rift basins, which constitute a major part of the West and Central African rift system (WCARS). The rift basins derive from a multiphased geologic history dating from the Pan-African (approximately 750-550 Ma) to the Holocene. WCARS in the study area is divided into the West African rift subsystem (WAS) and the Central African rift subsystem (WAS) and the Central African rift subsystem (CAS). WAS basins in Niger and Chad are chiefly extensional, and are filled by up to 13,000 m of Lower Cretaceous to Holocene continental and marine clastics. The basins contain five oil (19-43[degrees]API) and two oil and gas accumulations in Upper Cretaceous and Eocene sandstone reservoirs. The hydrocarbons are sourced and sealed by Upper Cretaceous and Eocene marine and lacustrine shales. The most common structural styles and hydrocarbon traps usually are associated with normal fault blocks. CAS rift basins in Chad and CAR are extensional and transtensional, and are filled by up to 7500 m of chiefly Lower Cretaceous continental clastics. The basins contain eight oil (15-39[degrees]API) and one oil and gas discovery in Lower and Upper Cretaceous sandstone reservoirs. The hydrocarbons are sourced by Lower Cretaceous shales and sealed by interbedded lacustrine and flood-plain shales. Structural styles range from simple fault blocks through complex flower structures. The main hydrocarbon traps are in contractional anticlines. Geological conditions favor the discovery of potentially commercial volumes of oil in WCARS basins, of Niger, Chad and CAR. 108 refs., 24 figs., 4 tabs.

  3. Imaging the midcontinent rift beneath Lake Superior using large aperture seismic data

    USGS Publications Warehouse

    Trehu, Anne M.; Morel-a-l'Huissier, Patrick; Meyer, R.; Hajnal, Z.; Karl, J.; Mereu, R. F.; Sexton, J.; Shay, J.; Chan, W. K.; Epili, D.; Jefferson, T.; Shih, X. R.; Wendling, S.; Milkereit, B.; Green, A.; Hutchinson, Deborah R.

    1991-01-01

    We present a detailed velocity model across the 1.1 billion year old Midcontinent Rift System (MRS) in central Lake Superior. The model was derived primarily from onshore-offshore large-aperture seismic and gravity data. High velocities obtained within a highly reflective half-graben that was imaged on coincident seismic reflection data demonstrate the dominantly mafic composition of the graben fill and constrain its total thickness to be at least 30km. Strong wide-angle reflections are observed from the lower crust and Moho, indicating that the crust is thickest (55–60km) beneath the axis of the graben. The total crustal thickness decreases rapidly to about 40 km beneath the south shore of the lake and decreases more gradually to the north. Above the Moho is a high-velocity lower crust interpreted to result from syn-rift basaltic intrusion into and/or underplating beneath the Archean lower crust. The lower crust is thickest beneath the axis of the main rift half-graben. A second region of thick lower crust is found approximately 100km north of the axis of the rift beneath a smaller half graben that is interpreted to reflect an earlier stage of rifting. The crustal model presented here resembles recent models of some passive continental margins and is in marked contrast to many models of both active and extinct Phanerozoic continental rift zones. It demonstrates that the Moho is a dynamic feature, since the pre-rift Moho is probably within or above the high-velocity lower crust, whereas the post-rift Moho is defined as the base of this layer. In the absence of major tectonic activity, however, the Moho is very stable, since the large, abrupt variations in crustal thickness beneath the MRS have been preserved for at least a billion years.

  4. Remote sensing studies and morphotectonic investigations in an arid rift setting, Baja California, Mexico 

    E-print Network

    El-Sobky, Hesham Farouk

    2009-05-15

    -1 REMOTE SENSING STUDIES AND MORPHOTECTONIC INVESTIGATIONS IN AN ARID RIFT SETTING, BAJA CALIFORNIA, MEXICO A Dissertation by HESHAM FAROUK EL-SOBKY Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment... of the requirements for the degree of DOCTOR OF PHILOSOPHY August 2007 Major Subject: Geology REMOTE SENSING STUDIES AND MORPHOTECTONIC INVESTIGATIONS IN AN ARID RIFT SETTING, BAJA CALIFORNIA, MEXICO A Dissertation by HESHAM FAROUK EL...

  5. Deformation during the 1975-1984 Krafla rifting crisis, NE Iceland, measured from historical optical imagery

    NASA Astrophysics Data System (ADS)

    Hollingsworth, James; Leprince, SéBastien; Ayoub, FrançOis; Avouac, Jean-Philippe

    2012-11-01

    We measure the displacement field resulting from the 1975-1984 Krafla rifting crisis, NE Iceland, using optical image correlation. Images are processed using the COSI-Corr software package. Surface extension is accommodated on normal faults and fissures which bound the rift zone, in response to dike injection at depth. Correlation of declassified KH-9 spy and SPOT5 satellite images reveals extension between 1977-2002 (2.5 m average opening over 80 km), while correlation of aerial photos between 1957-1990 provide measurements of the total extension (average 4.3 m opening over 80 km). Our results show ˜8 m of opening immediately north of Krafla caldera, decreasing to 3-4 m at the northern end of the rift. Correlation of aerial photos from 1957-1976 reveal a bi-modal pattern of opening along the rift during the early crisis, which may indicate either two different magma sources located at either end of the rift zone (a similar pattern of opening was observed in the 2005 Afar rift crisis in East Africa), or variations in rock strength along the rift. Our results provide new information on how past dike injection events accommodate long-term plate spreading, as well as providing more details on the Krafla rift crisis. This study also highlights the potential of optical image correlation using inexpensive declassified spy satellite and aerial photos to measure deformation of the Earth's surface going back many decades, thus providing a new tool for measuring Earth surface dynamics, e.g. glaciers, landsliding, coastal erosion, volcano monitoring and earthquake studies, when InSAR and GPS data are not available.

  6. Formation of the Shanxi Rift in North China: The control of preexisting lithospheric weakness

    NASA Astrophysics Data System (ADS)

    Lin, F.; Liu, M.; Ye, J.

    2012-12-01

    The Shanxi Rift is an active seismic zone in North China, developed mainly since Pliocene (~5 Ma). Its formation has been associated with the Indo-Asian collision; other hypothesized causes include a regional extensional stress field associated with subduction of the western pacific plate and mantle upwelling under the North China Plain. However, these mechanisms do not explain why the rift system did not form along the western boundary of the North China Plain, where lithospheric thickness changes sharply from more than 150 km under the Ordos block and the Taihangshan Mountains to the west, to less than 70 km under the North China Plain. We have used a viscoplastic finite element model to explore the conditions for localized rifting in North China. Our results show that, for all the hypothesized causes, the preferred site of rifting would be along the boundary zone of changing lithospheric thickness. The only way to initiate the Shanxi rift in its current location, which is between the Ordos block and the Taihangshan Mountains with thick lithosphere, is to have preexisting lithospheric weakening there. This lithospheric weakness was likely formed during the collision between the Easter North China block and the Western North China block during the Paleoproterozoic (~1.8 Ga). Hence the ancient tectonic event still controls the young continental rifting.

  7. Structural evolution of the southern transfer zone of the Gulf of Suez rift, Egypt

    NASA Astrophysics Data System (ADS)

    Abd-Allah, Ali M. A.; Abdel Aal, Mohamed H.; El-Said, Mohamed M.; Abd El-Naby, Ahmed

    2014-08-01

    We present a detailed study about the initiation and reactivations of Zeit-El Tor transfer zone, south Gulf of Suez rift, and its structural setting and tectonic evolution with respect to the Cretaceous-Cenozoic tectonic movements in North Egyptian margin. NE trending zone of opposed-dipping faults (22 km wide) has transferred the NE and SW rotations of the sub-basins in central and south Gulf of Suez rift, respectively. The evolution of this zone started by reactivation of the NE oriented late Neoproterozoic fractures that controlled the occurrence of Dokhan Volcanics in the rift shoulders. Later, the Syrian Arc contraction reactivated these fractures by a sinistral transpression during the Late Cretaceous-Eocene time. N64°E extension of the Oligo-Miocene rift reactivated the NE fractures by a sinistral transtension. During this rifting, the NE trending faults forming the transfer zone were more active than the rift-bounding faults; the Upper Cretaceous reverse faults in the blocks lying between these NE trending faults were rotated; and drape-related reverse faults and the positive flower structures were formed. Tectonic inversion from contraction to extension controlled the distribution and thickness of the Upper Cretaceous-Miocene rocks.

  8. Basaltic volcanism in Ethiopia: Constraints on continental rifting and mantle interactions

    SciTech Connect

    Hart, W. K.; WoldeGabriel, G.; Walter, R. C.; Mertzman, S. A.

    1989-06-10

    Middle to late Cenozoic mafic lavas from the Ethiopian volcanic province exhibit considerable chemical and isotopic diversity that is linked to eruption age and eruption location. These variations provide a geochemical framework in which continental rifting can be examined. Trace element and Sr, Nd, and Pb isotopic data are interpreted to indicate involvement of up to two depleted and two enriched mantle reservoirs throughout Cenozoic rift development in Ethiopia. Superimposed on the characteristics imparted by varying degrees of melting of these distinct reservoirs are the effects of crystal fractionation and, in some instances, crustal contamnation. Initial stages of Oligocene rifting and volcanism, as manifested by the rift-bounding plateau flood basalts, are attributed to asthenospheric upwelling and melting of a heterogeneous, enriched subcontinental lithospheric mantle. Mildly alkaline lavas were produced from an enriched source with characteristics similar to those of the inferred source of other mantle/minus/derived lavas and xenoliths from east Africa (LoNd array, EMI to HIMU). Contemporaneous tholeiitic lavas were derived from a source similar to that producing oceanic basalts from Samoa and the Society Islands (EMII). As lithospheric thinning and rifting continued into the Miocene, upwelling depleted asthenosphere (depleted OIB reservoir, PREMA) interacted with the lithospheric sources producing lavas with hybrid elemental and isotopic characteristics (11-6 Ma plateau and rift margin basalts).

  9. Structure and kinematics of the Livingstone Mountains border fault zone, Nyasa (Malawi) Rift, southwestern Tanzania

    NASA Astrophysics Data System (ADS)

    Wheeler, Walter H.; Karson, Jeffrey A.

    Reconnaissance mapping of the Livingstone Mountains border fault zone (LMBFZ) at the northern end of the Nyasa (Malawi) Rift in SW Tanzania constrains the geometry and movement history of this typical rift border fault. The fault is a narrow zone of complex brittle deformation, striking 320°, that overprints and reactivates an older ductile shear zone. Long, straight, NW-trending border fault segments are offset by minor NE-trending faults. These two orthogonal fault sets integrate along strike to produce an overall curved fault trace that is concave towards a major depositional basin in the rift. A typical section through the fault zone shows an E to W progression from gneissic country rock through ductilely deformed country rock, into a zone overprinted by closely spaced fractures and grading into an intensely fractured, massive, flinty, aphanitic mylonite band at the lakeshore. Pseudotachylite veins, probably generated during seismic movement on the border fault, are common within and near the aphanitic mylonite. Slickensides indicate dextral oblique-slip, whereas shear belts and rolled porphyroclasts with complex tails in the older ductile shear zone indicate sub-horizontal sinistral motion. The adjacent rift basin is typical of other East African Rift Basins, and contains at least 4 km of Recent to perhaps Mesozoic sediment. Whereas the minimum net slip on the LMBFZ, in the dominant slickenside direction, is on the order of 10 km, regional geologic considerations suggest that dominantly strike-slip motion preceded the oblique-slip phase that produced the LMBFZ and the adjacent rift basin.

  10. A new geodynamical thermal model of rift evolution, with application to the Dnieper Donets Basin, Ukraine

    NASA Astrophysics Data System (ADS)

    Starostenko, V. I.; Danilenko, V. A.; Vengrovitch, D. B.; Kutas, R. I.; Stovba, S. M.; Stephenson, R. A.; Kharitonov, O. M.

    1999-11-01

    A model of the lithosphere, incorporating both dynamic and thermal processes, has been developed by solving a coupled system of differential equations governing stress and temperature in a 2-D block-structured geophysical medium. Designed to study the roles of tectonic and geothermal factors in continental rift formation and evolution, the model incorporates syn-sedimentary and/or erosional faulting of an upper crustal layer and allows the thermal regime of the lithosphere to be calculated through time. The method has been applied to the formation and evolution of the northwest Dnieper-Donets Basin (DDB) along one regional profile controlled by seismic and other geophysical and subsurface data. The results are compared with those published earlier for the same profile using different methods of modelling the rift and early post-rift development of the region. The final basement geometry at the end of the rifting stage predicted by the new model satisfactorily corresponds with geological data and is qualitatively similar to that predicted by the previously published models. However, the new results imply an important role for an active mechanism during rifting that generates greater mantle thinning than crustal thinning and elevated temperatures in the upper mantle beneath the rift.

  11. Accumulation of fossil fuels and metallic minerals in active and ancient rift lakes

    USGS Publications Warehouse

    Robbins, E.I.

    1983-01-01

    A study of active and ancient rift systems around the world suggests that accumulations of fossil fuels and metallic minerals are related to the interactions of processes that form rift valleys with those that take place in and around rift lakes. The deposition of the precursors of petroleum, gas, oil shale, coal, phosphate, barite, Cu-Pb-Zn sulfides, and uranium begins with erosion of uplifted areas, and the consequent input of abundant nutrients and solute loads into swamps and tectonic lakes. Hot springs and volcanism add other nutrients and solutes. The resulting high biological productivity creates oxidized/reduced interfaces, and anoxic and H2S-rich bottom waters which preserves metal-bearing organic tissues and horizons. In the depositional phases, the fine-grained lake deposits are in contact with coarse-grained beach, delta, river, talus, and alluvial fan deposits. Earthquake-induced turbidites also are common coarse-grained deposits of rift lakes. Postdepositional processes in rifts include high heat flow and a resulting concentration of the organic and metallic components that were dispersed throughout the lakebeds. Postdepositional faulting brings organic- and metal-rich sourcebeds in contact with coarse-grained host and reservoir rocks. A suite of potentially economic deposits is therefore a characteristic of rift valleys. ?? 1983.

  12. Three-Dimensional Modelling of Rift Basin Evolution

    NASA Astrophysics Data System (ADS)

    Longshaw, S. M.; Finch, E.; Turner, M.; Gawthorpe, R. L.

    2009-12-01

    Numerical simulation of faulting is gaining interest, typically utilising the Discrete Element Modelling (DEM) technique. As computational power increases, so does the ability to run larger and more complex experiments. This research aims to explore rift basin evolution, with emphasis placed on the interaction of multiple faults as they evolve. Importance is placed on the technical aspects of implementing a complex and large DEM, specifically parallelism and whether currently popular software libraries known as physics engines can be a useful tool in their development. The technique employed is a variation of Mora and Place’s Lattice-Solid Model (1994). We use a 3D block of spheres, initially packed into a defined volume, interconnected using spring-damper structures. Elemental position is controlled throughout the simulation using Newtonian physics, while underlying plate movement is introduced by applying a specific force to each element. The spring-damper components provide a compressive force in the system, restricting each element to its neighbours, until the point the spring-damper has reached a pre-defined maximum extension, at which point it breaks resulting in a fault. Physical calculations are performed using using a publically available engine. The technical implementation of a physics engine is markedly more advanced than that found in many DEM codes used to date, specifically that of the collision detection system, which is implemented as a broad and then narrow phase process, which is solved as a Linear Complementarity Problem. In order to make use of large, distributed computing resources, a method by which to split the DEM into smaller sub-DEMs and then calculate each on an individual CPU has been defined. Typically results are accurate to within four decimal places compared to idealised values; this is comparable to other integration techniques such as Verlet or RK4. Through splitting the model and using multiple instances of a physics engine, it has been possible to limit the scale of simulation to available computing hardware, rather than software libraries. Extension of the crust results in an initial phase of a large number of small faults over which extensional strain is accommodated. As extension progresses, these faults are noted to either link to become larger structures or become inactive and passively rotated in the hanging-wall and footwalls of neighbouring structures. This does not mean, however, that they cannot be reactivated later during rift development. As deformation within the system reaches the rift climax phase, extension is focused on a small number of large faults within the basin. The timing, location, linkage and strain accommodated by these structures is determined during extension and examined as it progresses in three-dimensions. It can also be concluded that using a physics engine in the design of a geological DEM can be beneficial for development time, speed of execution and ensuring physical correctness. Model splitting provides a neat and scalable method by which to distribute computation of a large DEM over multiple CPUs and memory resources.

  13. The development of the Midcontinent Rift in the context of rapid paleogeographic change

    NASA Astrophysics Data System (ADS)

    Swanson-Hysell, N.; Vaughn, A. A.; Mustain, M. R.; Feinberg, J. M.

    2012-12-01

    Despite being active for >20 million years and resulting in the thinning of pre-rift crust by a factor of 3 or more, the 1.1 Ga Midcontinent Rift failed to dismember the Laurentian craton. This failure resulted in the preservation of a thick record of rift-related volcanic and sedimentary rocks that give geoscientists a powerful window into the development of this ancient rift. Most models for the development of the Midcontinent Rift attribute its origin to the upwelling and decompression melting of a mantle plume. On the basis of the great volume of generated magma and interpretation of geochemical data, it is argued that the early stage plateau flood basalts of the rift (~1110-1105 Ma) and the main stage volcanics that erupted into the central basin (~1100-1095 Ma) were both dominated by plume-sourced melts. However, this model needs to be reconciled with paleomagnetic data from rift volcanics that reveal a significant decrease in inclination between the early and main stage volcanics. New data we have developed from 90+ flows of the early stage Osler Volcanic Group bolster evidence from the succession at Mamainse Point that this change in inclination is the result of fast equatorward plate motion during the early stage and into the main stage of rift volcanism. Even with >20° of latitudinal motion from the time of initial volcanism to eruption of the thick main stage volcanics, magmatism was largely confined to the same geographic region in a relatively narrow central basin. If a long-lived plume was in a fixed position relative to Earth's spin axis, the large relative motion of Laurentia would make it unable to continue to be a source of melt to the rift. Two possible explanations to reconcile a plume-contribution in the main stage with this latitudinal change are: (1) That the active contribution from an underlying plume was limited to the early stage of volcanism, but substantial volume of material accreted to the lithosphere that was subsequently sampled during the main stage. While such a scenario could explain the geochemical signature, it removes an active plume as a heat source contributing to voluminous melt generation. (2) That the North American plate and a deep-seated mantle plume traveled in unison to lower latitudes as a result of large-scale true polar wander. An interpretation of an active contribution from a plume throughout rift development implicitly favors this true polar wander hypothesis.

  14. Tomography of the East African Rift System in Mozambique

    NASA Astrophysics Data System (ADS)

    Domingues, A.; Silveira, G. M.; Custodio, S.; Chamussa, J.; Lebedev, S.; Chang, S. J.; Ferreira, A. M. G.; Fonseca, J. F. B. D.

    2014-12-01

    Unlike the majority of the East African Rift, the Mozambique region has not been deeply studied, not only due to political instabilities but also because of the difficult access to its most interior regions. An earthquake with M7 occurred in Machaze in 2006, which triggered the investigation of this particular region. The MOZART project (funded by FCT, Lisbon) installed a temporary seismic network, with a total of 30 broadband stations from the SEIS-UK pool, from April 2011 to July 2013. Preliminary locations of the seismicity were estimated with the data recorded from April 2011 to July 2012. A total of 307 earthquakes were located, with ML magnitudes ranging from 0.9 to 3.9. We observe a linear northeast-southwest distribution of the seismicity that seems associated to the Inhaminga fault. The seismicity has an extension of ~300km reaching the Machaze earthquake area. The northeast sector of the seismicity shows a good correlation with the topography, tracing the Urema rift valley. In order to obtain an initial velocity model of the region, the ambient noise method is used. This method is applied to the entire data set available and two additional stations of the AfricaARRAY project. Ambient noise surface wave tomography is possible by computing cross-correlations between all pairs of stations and measuring the group velocities for all interstation paths. With this approach we obtain Rayleigh wave group velocity dispersion curves in the period range from 3 to 50 seconds. Group velocity maps are calculated for several periods and allowing a geological and tectonic interpretation. In order to extend the investigation to longer wave periods and thus probe both the crust and upper mantle, we apply a recent implementation of the surface-wave two-station method (teleseismic interferometry - Meier el al 2004) to augment our dataset with Rayleigh wave phase velocities curves in a broad period range. Using this method we expect to be able to explore the lithosphere-asthenosphere depth range beneath Mozambique.

  15. The accuracy of the Oculus Rift virtual reality head-mounted display during cervical spine mobility measurement.

    PubMed

    Xu, Xu; Chen, Karen B; Lin, Jia-Hua; Radwin, Robert G

    2015-02-26

    An inertial sensor-embedded virtual reality (VR) head-mounted display, the Oculus Rift (the Rift), monitors head movement so the content displayed can be updated accordingly. While the Rift may have potential use in cervical spine biomechanics studies, its accuracy in terms of cervical spine mobility measurement has not yet been validated. In the current study, a VR environment was designed to guide participants to perform prescribed neck movements. The cervical spine kinematics was measured by both the Rift and a reference motion tracking system. Comparison of the kinematics data between the Rift and the tracking system indicated that the Rift can provide good estimates on full range of motion (from one side to the other side) during the performed task. Because of inertial sensor drifting, the unilateral range of motion (from one side to neutral posture) derived from the Rift is more erroneous. The root-mean-square errors over a 1-min task were within 10° for each rotation axis. The error analysis further indicated that the inertial sensor drifted approximately 6° at the beginning of a trial during the initialization. This needs to be addressed when using the Rift in order to more accurately measure cervical spine kinematics. It is suggested that the front cover of the Rift should be aligned against a vertical plane during its initialization. PMID:25636855

  16. Genetic features of petroleum systems in rift basins of eastern China

    USGS Publications Warehouse

    Qiang, J.; McCabe, P.J.

    1998-01-01

    Most oil-bearing basins in eastern China are Mesozoic-Cenozoic continental rifts which have played a habitat for oil and gas in China. Investigation of the petroleum systems may give a better understanding of the oil and gas habitats in these basins. Of the essential elements of the petroleum system, the source rock is the most important in rift basins. However, rift tectonic evolution controls all the essential elements and processes nevessary for a petroleum system. A four stage evolution model is suggested for the controls in the rift basin. A rift basin may consist of sub-basins, depressions, sub-depressions, and major, moderate, and minor uplifts. A depression or sub-depression has its own depocentre (mainly occupied by source rock) and all kinds of lacustrine sediments, and thus has all the essential elements of a petroleum system. However, only those depressions or sub-depressions which are rich in organic matter and deeply buried to generate oil and gas form petroleum systems. Immature oil, another characteristic, complicates the petroleum system in the rift basins. Three types of oil and gas habitats are described as a result of this analysis of the petroleum systems of the 26 largest oil and gas fields discovered in eastern China rift basins: uplifts between oil source centres are the most prospective areas for oil and gas accumulations, slopes connecting oil source centres and uplifts are the second, and the third type is subtle traps in the soil source centre.Most oil-bearing basins in eastern China are Mesozoic-Cenozoic continental rifts which have played a habitat for oil and gas in China. Investigation of the petroleum systems may give a better understanding of the oil and gas habitats in these basins. Of the essential elements of the petroleum system, the source rock is the most important in rift basins. However, rift tectonic evolution controls all the essential elements and processes necessary for a petroleum system. A four stage evolution model is suggested for the controls in the rift basin. A rift basin may consist of sub-basins, depressions, sub-depressions, and major, moderate, and minor uplifts. A depression or sub-depression has its own depocentre (mainly occupied by source rock) and all kinds of lacustrine sediments, and thus has all the essential elements of a petroleum system. However, only those depressions or sub-depressions which are rich in organic matter and deeply buried to generate oil and gas form petroleum systems. Immature oil, another characteristic, complicates the petroleum system in the rift basins. Three types of oil and gas habitats are described as a result of this analysis of the petroleum systems of the 26 largest oil and gas fields discovered in eastern China rift basins: uplifts between oil source centres are the most prospective areas for oil and gas accumulations, slopes connecting oil source centres and uplifts are the second, and the third type is subtle traps in the oil source centre.

  17. 563Studinger et al.--Airborne geophysical mapping of the West Antarctic Rift System Mesozoic and Cenozoic extensional tectonics of the West Antarctic Rift

    E-print Network

    Studinger, Michael

    and Cenozoic extensional tectonics of the West Antarctic Rift System from high-resolution airborne geophysical shoulder close to the Whitmore Mountains block might have been reactivated during regional Cenozoic right Byrd Land (Fig. 1). In contrast, models of the Mesozoic and Cenozoic evolution of the region between

  18. Permo-Triassic anatexis, continental rifting and the disassembly of western Pangaea

    NASA Astrophysics Data System (ADS)

    Cochrane, Ryan; Spikings, Richard; Gerdes, Axel; Ulianov, Alexey; Mora, Andres; Villagómez, Diego; Putlitz, Benita; Chiaradia, Massimo

    2014-03-01

    Crustal anatectites are frequently observed along ocean-continent active margins, although their origins are disputed with interpretations varying between rift-related and collisional. We report geochemical, isotopic and geochronological data that define an ~ 1500 km long belt of S-type meta-granites along the Andes of Colombia and Ecuador, which formed during 275-223 Ma. These are accompanied by amphibolitized tholeiitic basaltic dykes that yield concordant zircon U-Pb dates ranging between 240 and 223 Ma. A model is presented which places these rocks within a compressive Permian arc setting that existed during the amalgamation of westernmost Pangaea. Anatexis and mafic intrusion during 240-223 Ma are interpreted to have occurred during continental rifting, which culminated in the formation of oceanic crust and initiated the break-up of western Pangaea. Compression during 275-240 Ma generated small volumes of crustal melting. Rifting during 240-225 Ma was characterized by basaltic underplating, the intrusion of tholeiitic basalts and a peak in crustal melting. Tholeiitic intrusions during 225-216 Ma isotopically resemble depleted mantle and yield no evidence for contamination by continental crust, and we assign this period to the onset of continental drift. Dissected ophiolitic sequences in northern Colombia yield zircon U-Pb dates of 216 Ma. The Permo-Triassic margin of Ecuador and Colombia exhibits close temporal, faunal and geochemical similarities with various crustal blocks that form the basement to parts of Mexico, and thus these may represent the relict conjugate margin to NW Gondwana. The magmatic record of the early disassembly of Pangaea spans ~ 20 Ma (240-216 Ma), and the duration of rifting and rift-drift transition is similar to that documented in Cretaceous-Tertiary rift settings such as the West Iberia-Newfoundland conjugate margins, and the Taupo-Lau-Havre System, where rifting and continental disassembly also occurred over periods lasting ~ 20 Ma.

  19. Tectonic and eustatic controls on carbonate platforms of the Jurassic High Atlas rift of Morocco

    SciTech Connect

    Crevello, P. )

    1990-05-01

    Stratigraphic studies of Lower and Middle Jurassic carbonates along the southern margin of the High Atlas rift document five major stages of platform development a response to rift tectonics and eustasy. The five stages are as follows. (1) Early Sinemurian marine transgression over Triassic-Liassic continental red beds and basalts lead to the development of regionally extensive, cyclic carbonate platforms. (2) Middle to late Sinemurian marked the initiation of Liassic rifting, with synrift platforms restricted to the rift margin and to localized horsts within the rift axis. Synrift platforms developed diverse depositional systems, with a marked change to rimmed margins flanked by steep slopes and deep (400-500 m) marine basins. Two orders of cyclicity shallowing-upward cycles and bundles of cycles dominated the platform tops. (3) Late Pliensbachian subaerial exposure, resulting in termination of Liassic platform development, was recorded by regressive seaward shifts in facies belts, microkarstification, and deposition of continental red beds across the platforms. (4) Early to middle Toarcian transgression, yielding landward shifts in facies belts on platform tops, was signified by deposition of noncyclic, skeletal carbonate sequences. (5) Middle Toarcian platform drowning was followed by deposition of Toarcian-Aalenian amonite-bearing marine shales (100 m thick) blanketing (downlapping, width source from the south) the southern platform, whereas only a condensed sequence (2-3 m thick) of glauconitic, ammonite-skeletal carbonate strata covered the submerged, isolated, axial rift platform of Jebel Bou Dahar. Prolonged sediment starvation on Bou Dahar combined with depositional onlap of its slope by basin-filling shales (Toarcian to Bajocian) and distal carbonate turbidites (Aalenian), also sourced from the southern margin of the rift, produccd a drowning onlap unconformity.

  20. Climate-disease connections: Rift Valley Fever in Kenya

    NASA Technical Reports Server (NTRS)

    Anyamba, A.; Linthicum, K. J.; Tucker, C. J.

    2001-01-01

    All known Rift Valley fever(RVF) outbreaks in Kenya from 1950 to 1998 followed periods of abnormally high rainfall. On an interannual scale, periods of above normal rainfall in East Africa are associated with the warm phase of the El Nino/Southern Oscillation (ENSO) phenomenon. Anomalous rainfall floods mosquito-breeding habitats called dambos, which contain transovarially infected mosquito eggs. The eggs hatch Aedes mosquitoes that transmit the RVF virus preferentially to livestock and to humans as well. Analysis of historical data on RVF outbreaks and indicators of ENSO (including Pacific and Indian Ocean sea surface temperatures and the Southern Oscillation Index) indicates that more than three quarters of the RVF outbreaks have occurred during warm ENSO event periods. Mapping of ecological conditions using satellite normalized difference vegetation index (NDVI) data show that areas where outbreaks have occurred during the satellite recording period (1981-1998) show anomalous positive departures in vegetation greenness, an indicator of above-normal precipitation. This is particularly observed in arid areas of East Africa, which are predominantly impacted by this disease. These results indicate a close association between interannual climate variability and RVF outbreaks in Kenya.

  1. A statistical model of Rift Valley fever activity in Egypt

    PubMed Central

    Hassan, Ali N.; Beier, John C.

    2014-01-01

    Rift Valley fever (RVF) is a viral disease of animals and humans and a global public health concern due to its ecological plasticity, adaptivity, and potential for spread to countries with a temperate climate. In many places, outbreaks are episodic and linked to climatic, hydrologic, and socioeconomic factors. Although outbreaks of RVF have occurred in Egypt since 1977, attempts to identify risk factors have been limited. Using a statistical learning approach (lasso-regularized generalized linear model), we tested the hypotheses that outbreaks in Egypt are linked to (1) River Nile conditions that create a mosquito vector habitat, (2) entomologic conditions favorable to transmission, (3) socio-economic factors (Islamic festival of Greater Bairam), and (4) recent history of transmission activity. Evidence was found for effects of rainfall and river discharge and recent history of transmission activity. There was no evidence for an effect of Greater Bairam. The model predicted RVF activity correctly in 351 of 358 months (98.0%). This is the first study to statistically identify risk factors for RVF outbreaks in a region of unstable transmission. PMID:24581353

  2. Middle Stone Age starch acquisition in the Niassa Rift, Mozambique

    NASA Astrophysics Data System (ADS)

    Mercader, Julio; Bennett, Tim; Raja, Mussa

    2008-09-01

    The quest for direct lines of evidence for Paleolithic plant consumption during the African Middle Stone Age has led scientists to study residues and use-wear on flaked stone tools. Past work has established lithic function through multiple lines of evidence and the spatial breakdown of use-wear and microscopic traces on tool surfaces. This paper focuses on the quantitative analysis of starch assemblages and the botanical identification of grains from flake and core tools to learn about human ecology of carbohydrate use around the Niassa woodlands, in the Mozambican Rift. The processing of starchy plant parts is deduced from the occurrence of starch assemblages that presumably got attached to stone tool surfaces by actions associated with extractive or culinary activities. Specifically, we investigate starch grains from stone tools recently excavated in northern Mozambique at the site of Mikuyu; which presumably spans the middle to late Pleistocene and represents similar sites found along the Malawi/Niassa corridor that links East, Southern, and Central Africa. Starch was extracted and processed with a diverse tool kit consisting of scrapers, cores, points, flakes, and other kinds of tools. The microbotanical data suggests consumption of seeds, legumes, caryopses, piths, underground storage organs, nuts, and mesocarps from more than a dozen families. Our data suggest a great antiquity for starch use in Africa as well as an expanded diet and intensification.

  3. Probable rift origin of Canada Basin, Arctic Ocean

    USGS Publications Warehouse

    Tailleur, Irvin L.

    1973-01-01

    Formation of the Canada basin by post-Triassic rifting seems the most workable and logical hypothesis on the basis of available information. Speculated counterclockwise rotation of the Alaska-Chukchi continental edge best rationalizes the complex geology of northern Alaska, whereas the assumption that a single continental block was present before the Jurassic makes the best palinspastic fit for Arctic America. The Arctic Ocean is the focus of present-day spreading and probably was the focus of earlier stages of spreading in which spread of the Canada basin would have been an initial stage. Spread of the Canada basin is probable if the Atlantic formed by sea-floor spreading, because analogies between the Arctic and Atlantic edges indicate a common origin for the ocean basins. Late Cretaceous and younger deflections of the Cordillera in the Arctic and diabasic emplacements in the northern Arctic Islands may reflect later stages of spreading. Pre-Mesozoic plate tectonism may be represented by the widespread Proterozoic diabasic emplacements in the Canadian Arctic and by the Franklinian-lnnuitian tract, where the volcanogenic rocks and deformation resulted not from a classical eugeosyncline-miogeosyncline couple, but from the junction of a mid-Paleozoic continental edge and another plate on closure of a pre-Arctic Ocean.

  4. Probable rift origin of the Canada basin, Arctic Ocean

    USGS Publications Warehouse

    Tailleur, Irvin L.

    1973-01-01

    Formation of the Canada basin by post-Triassic rifting seems the most workable and logical hypothesis with information available. Speculated counterclockwise rotation of the Alaska-Chukchi continental edge best rationalizes the complex geology of northern Alaska, whereas a single continental block before the Jurassic makes the best palinspastic fit for Arctic America. The Arctic Ocean is the focus of present-day spreading and probably was the focus of earlier stages of spreading in which spread of the Canada basin would be an initial stage. If the Atlantic formed by seafloor spreading, spread of the Canada basin is probable because analogies between the Arctic and Atlantic edges indicate a common origin for the ocean basins. Late Cretaceous and younger deflections of the Cordillera in the Arctic and diabasic emplacements in the northern Arctic Islands may reflect later stages of spreading. Pre-Mesozoic plate tectonism may be represented by the widespread Proterozoic diabasic emplacements in the Canadian Arctic and by the Franklinian-Innuitian tract where the volcanogenic rocks and deformation resulted not from a classical eugeosyncline-miogeosyncline couple but from the junction of a mid-Paleozoic continental edge and another plate on closure of a pre-Arctic Ocean.

  5. Rift Valley fever: an uninvited zoonosis in the Arabian peninsula.

    PubMed

    Balkhy, Hanan H; Memish, Ziad A

    2003-02-01

    Rift Valley fever (RVF) is an acute viral disease, affecting mainly livestock but also humans. The virus is transmitted to humans through mosquito bites or by exposure to blood and bodily fluids. Drinking raw, unpasteurized milk from infected animals can also transmit RVF. Routine vaccination of livestock in Africa has been prohibitively expensive, leading to endemicity of RVF in most African countries. Reports in September 2000 first documented RVF occurring outside of Africa in the Kingdom of Saudi Arabia and Yemen. Prior to this outbreak, the potential for RVF spread into the Arabian Peninsula had already been exemplified by a 1977 Egyptian epidemic. This appearance of RVF outside the African Continent might be related to importation of infected animals from Africa. In the most recent outbreak patients presented with a febrile haemorrhagic syndrome accompanied by liver and renal dysfunction. By the end of the outbreak, April 2001 statistics from the Saudi Ministry of Health documented a total of 882 confirmed cases with 124 deaths. Both the severity of disease and the relatively high 14% death rate might be a consequence of underreporting of less severe disease. Travellers to endemic areas may be at risk of acquiring the disease if exposed to animals or their body fluids directly or through mosquito bites. Special education regarding both modes of transmission and the geographical distribution of this disease needs to be given to travellers at risk. PMID:12615379

  6. ENAM: A community seismic experiment targeting rifting processes and post-rift evolution of the Mid Atlantic US margin

    NASA Astrophysics Data System (ADS)

    Van Avendonk, H. J.; Magnani, M. B.; Shillington, D. J.; Gaherty, J. B.; Hornbach, M. J.; Dugan, B.; Long, M. D.; Lizarralde, D.; Becel, A.; Benoit, M. H.; Harder, S. H.; Wagner, L. S.; Christeson, G. L.

    2014-12-01

    The continental margins of the eastern United States formed in the Early Jurassic after the breakup of supercontinent Pangea. The relationship between the timing of this rift episode and the occurrence of offshore magmatism, which is expressed in the East Coast Magnetic Anomaly, is still unknown. The possible influence of magmatism and existing lithospheric structure on the rifting processes along margin of the eastern U.S. was one of the motivations to conduct a large-scale community seismic experiment in the Eastern North America (ENAM) GeoPRISMS focus site. In addition, there is also a clear need for better high-resolution seismic data with shallow penetration on this margin to better understand the geological setting of submarine landslides. The ENAM community seismic experiment is a project in which a team of scientists will gather both active-source and earthquake seismic data in the vicinity of Cape Hatteras on a 500 km wide section of the margin offshore North Carolina and Virginia. The timing of data acquisition in 2014 and 2015 facilitates leveraging of other geophysical data acquisition programs such as Earthscope's Transportable Array and the USGS marine seismic investigation of the continental shelf. In April of 2014, 30 broadband ocean-bottom seismometers were deployed on the shelf, slope and abyssal plain of the study site. These instruments will record earthquakes for one year, which will help future seismic imaging of the deeper lithosphere beneath the margin. In September and October of 2014, regional marine seismic reflection and refraction data will be gathered with the seismic vessel R/V Marcus Langseth, and airgun shots will also be recorded on land to provide data coverage across the shoreline. Last, in the summer of 2015, a land explosion seismic refraction study will provide constraints on the crustal structure in the adjacent coastal plain of North Carolina and Virginia. All seismic data will be distributed to the community through IRIS/DMC and the LDEO/UTIG Seismic data center. Two workshops are planned for 2015, where new users get an opportunity to engage in basic processing and analysis of the new data set.

  7. Morpho-structural evolution of a volcanic island developed inside an active oceanic rift: S. Miguel Island (Terceira Rift, Azores)

    NASA Astrophysics Data System (ADS)

    Sibrant, A. L. R.; Hildenbrand, A.; Marques, F. O.; Weiss, B.; Boulesteix, T.; Hübscher, C.; Lüdmann, T.; Costa, A. C. G.; Catalão, J. C.

    2015-08-01

    The evolution of volcanic islands is generally marked by fast construction phases alternating with destruction by a variety of mass-wasting processes. More specifically, volcanic islands located in areas of intense regional deformation can be particularly prone to gravitational destabilisation. The island of S. Miguel (Azores) has developed during the last 1 Myr inside the active Terceira Rift, a major tectonic structure materializing the present boundary between the Eurasian and Nubian lithospheric plates. In this work, we depict the evolution of the island, based on high-resolution DEM data, stratigraphic and structural analyses, high-precision K-Ar dating on separated mineral phases, and offshore data (bathymetry and seismic profiles). The new results indicate that: (1) the oldest volcanic complex (Nordeste), composing the easternmost part of the island, was dominantly active between ca. 850 and 750 ka, and was subsequently affected by a major south-directed flank collapse. (2) Between at least 500 ka and 250 ka, the landslide depression was massively filled by a thick lava succession erupted from volcanic cones and domes distributed along the main E-W collapse scar. (3) Since 250 kyr, the western part of this succession (Furnas area) was affected by multiple vertical collapses; associated plinian eruptions produced large pyroclastic deposits, here dated at ca. 60 ka and less than 25 ka. (4) During the same period, the eastern part of the landslide scar was enlarged by retrogressive erosion, producing the large Povoação valley, which was gradually filled by sediments and young volcanic products. (5) The Fogo volcano, in the middle of S. Miguel, is here dated between ca. 270 and 17 ka, and was affected by, at least, one southwards flank collapse. (6) The Sete Cidades volcano, in the western end of the island, is here dated between ca. 91 and 13 ka, and experienced mutliple caldera collapses; a landslide to the North is also suspected from the presence of a subtle morphologic scar covered by recent lava flows erupted from alignments of basaltic strombolian cones. The predominance of the N150° and N75° trends in the island suggest that the tectonics of the Terceira Rift controlled the location and the distribution of the volcanism, and to some extent the various destruction events.

  8. Surface displacements on faults triggered by slow magma transfers between dyke injections in the 2005-2010 rifting episode at Dabbahu-Manda-Hararo rift (Afar, Ethiopia)

    NASA Astrophysics Data System (ADS)

    Dumont, S.; Socquet, A.; Grandin, R.; Doubre, C.; Klinger, Y.

    2016-01-01

    The rifting episode that occurred in Dabbahu-Manda-Hararo (Ethiopia) between 2005 and 2010 during which 14 dyke intrusions were emitted, was a unique opportunity to study interactions between tectonic deformation and magmatic processes. While magmatism has been shown to control primarily the spatial and temporal distribution of dyke intrusions during this accretion sequence, the role of faults in accommodating plate spreading in rift segments is poorly understood. During interdyking periods, transient ground deformation due to magma movement is generally observed. Investigating such a small-scale deformation and in particular the movement along faults during these periods will help understanding the factors that trigger fault movement in magmatic rifts. We analyse fault activity during three interdyking periods: 2006 December-June (d0-d1), 2007 January-July (d5-d6) and 2009 November-January (d10-d11). The time-space evolution of surface displacements along ˜700 faults is derived from pairs of ascending and descending SAR interferograms. Surface slip distributions are then compared with codyking ground deformation fields. The results show that faults are mainly activated above the areas affected by magma emplacement during interdyking periods. A detailed analysis of brittle deformation during the six months following the 2005 September intrusion shows asymmetric deformation on the rift shoulders, with significant opening on faults located to the west of the dyke. We explain this feature by the activation of westward dipping pre-existing faults, with block rotations in between. In addition, we observe that the strip encompassing the activated faults narrows by 30 per cent from co- to interdyking period. This suggests that magma keeps migrating to shallower depths after the dyke intrusion. During a rifting episode, activation of faults in a pre-existing fracture network therefore seems to be mainly controlled by deep magma processes.

  9. Strain partitioning evolution and segmentation in hyperextended rift systems: insights from the Bay of Biscay and Pyrenees

    NASA Astrophysics Data System (ADS)

    Tugend, Julie; Manatschal, Gianreto; Kusznir, Nick J.

    2014-05-01

    The understanding of the formation of hyper-extended domains has greatly benefited from combined studies at present-day and fossil rift systems preserved in collisional orogens. However, even though domains of extreme crustal and lithosphere thinning have been increasingly recognized, the spatial and temporal evolution of their tectonic processes remains poorly constrained. The Bay of Biscay and Pyrenees correspond to a Late Jurassic to Mid Cretaceous rift system including both oceanic and hyper-extended rift domains. The transition from preserved oceanic and rift domains to the West to their complete inversion in the East provide simultaneous access to seismically imaged and exposed parts of a hyper-extended rift system. We combine seismic interpretations and gravity inversion results with field mapping to identify and map former rift domains from the Bay of Biscay margins to their fossil analogues preserved in the Pyrenean orogen. This onshore/offshore map of the rift systems enables us to investigate the spatial and temporal evolution and the strain distribution related to the formation of a strongly segmented rift system preserved at the transition between the European and Iberian plate boundary. The restoration of the hyper-extended domains reveals the occurrence of spatially disconnected rift systems separated by weakly thinned continental ribbons (e.g. Landes High, Ebro block). While the offshore Bay of Biscay represent a former mature oceanic domain, the fossil remnants of hyper-extended domains preserved onshore in the Pyrenean-Cantabrian orogen record distributed extensional deformation partitioned between strongly segmented rift basins (e.g. Basque-Cantabrian, Arzacq-Mauléon basins). Rift system segmentation controls lateral variations of architecture and may be partly inherited from the pre-rift structuration. The relative timing of hyper-extensional processes is diachronous between the different rift systems recording the polyphased evolution of the European - Iberian plate boundary. Based on the subsidence and deformation history, we propose a scenario illustrating the strain partitioning evolution between the different rift systems. The results of this work may provide insights on the spatial and temporal evolution of the embryonic stages of other segmented rifted continental margins.

  10. Complexities in rift initiation and development within the Iceland Plateau, North-Atlantic.

    NASA Astrophysics Data System (ADS)

    Brandsdóttir, Bryndís; Hooft, Emilie; Mjelde, Rolf; Murai, Yoshio

    2015-04-01

    Spreading north of Iceland has been complex since the break-up of the North Atlantic in late Paleocene-early Eocene. Magnetic anomalies within the Iceland Plateau, west of the Ægir Ridge and along the Greenland-Iceland-Faeroe Ridge are irregular, formed by plate boundary complexities at breakup, branched accretion zones, westward rifting relocations, and large overlapping rifts. The 700 km long KRISE7 seismic refraction/reflection and gravity profile, straddles 66.5°N, between the Kolbeinsey and Ægir Ridges, crossing the three physiographic provinces that characterize this region: the Iceland Shelf, Iceland Plateau and Norway Basin. On the basis of crustal thickness and velocity structure, combined with older seismic reflection profiles and drill cores, these provinces correspond to three individual spreading rifts that were active at different time periods. The deep, fan shaped Norway basin was formed during the initial opening of the Atlantic by spreading at the now extinct Ægir Ridge. The oldest crust in the western Norway Basin has thickness 8-10 km and this thins to 4-5 km at the Ægir Ridge reflecting the progressive abandonment of spreading at this rift axis. The eastern Iceland Plateau was the locus of an extinct spreading center, which was segmented and overlapped the Ægir Ridge by 300 km. Spreading on the Iceland Plateau rift occurred simultaneously with that on the Ægir Ridge prior to 26 Ma, when the Kolbeinsey Ridge was initiated by a westward rift jump. The Iceland Plateau rift formed by rifting along the continent-ocean transition at the former central E-Greenland margin, associated with the formation of the Jan Mayen Ridge. Lower crustal domes and corresponding gravity highs across the Iceland Plateau mark the location of the extinct rift axis. The crust at the Iceland Plateau rift is thicker (12-15 km) that that at the conjugate Ægir Ridge and formed under active upwelling conditions (normal lower crustal velocities), which we attribute to the influence of the Iceland plume. The overlapping geometry of the Ægir and Iceland Plateau rift, led to progressive south to north abandonment of spreading on the Ægir Ridge. Rifting at the Kolbeinsey Ridge in the last 26 Ma formed the shallowest physiographic province, the Iceland Shelf. Initiation of the Kolbeinsey Ridge is recorded in very thick crust (24-28 km) at the eastern Iceland Shelf, due to significant increase in melt flux (active, plume driven upwelling) which caused the spreading axis to jump from the Iceland Plateau to the new Kolbeinsey ridge. Magmatism at the new Kolbeinsey Ridge was also associated with unusual large amounts of extrusive volcanism as recorded by an extremely thick (6 km) layer of low velocities in this region. As the Iceland plume approached the Mid-Atlantic ridge from the west, three separate spreading centers formed - each progressively further west. Magma from the plume generated new zones of weakness at the continent-ocean boundary and thus facilitated new spreading rifts. The plume influence is documented in thicker crust as each successive spreading center was initiated.

  11. Topography, river network and recent fault activity at the margins of the Central Main Ethiopian Rift (East Africa)

    NASA Astrophysics Data System (ADS)

    Molin, Paola; Corti, Giacomo

    2015-11-01

    Along its length, the Main Ethiopian Rift (MER) in East Africa records a transition from early fault-dominated morphology in the South to axial magma assisted-rifting typical of continental break-up in the North. It is one of the few locations on Earth offering a complete picture of the evolution of continental rifting and thus provides a unique opportunity to directly analyze how the drainage network reorganizes under extensional tectonic forcing. In this paper we present a new analysis of the river network and relative landforms-complemented with a summary of recent geological data-at both rift margins of the Central MER, a key sector of the rift capturing the phase of drainage reorganization between incipient and mature rifting. This analysis shows that hydrography is strongly influenced by recent tectonics. Rectangular drainage patterns, windgaps, and lacustrine/swampy areas formed by structural dams document that the rivers are in continuous competition with fault activity. The irregular longitudinal profiles (with knickpoints/knickzones in correspondence with faults) also suggest that rivers are in a transient state of disequilibrium related to recent tectonic activity at rift margins, in agreement with previous geological and seismological data. A more regional analysis extended to the adjoining Northern and Southern MER indicates that rifting evolves from initial stages characterized by margins poorly incised by rivers with gentle channel gradients (except in correspondence with faults), to mature phases in which rift margins are highly incised by a well organized fluvial network composed by concave and steep rivers. Our regional analysis also indicates a stronger and/or more recent tectonic activity at the rift margins proceeding to the south, in line with previous models of rift development.

  12. The Role of Rheological Weakening in the Formation of Narrow Rifts on Venus

    NASA Astrophysics Data System (ADS)

    Martone, Alexis; Montesi, Laurent

    2015-11-01

    The rift zones on Venus are remarkably similar to those seen on Earth, despite Venus’ current lack of plate tectonics. The Devana Chasma rift on Beta Regio accommodates extension in a narrow zone and is associated with volcanism. As a result, it has often been compared to the East African Rift (Burov and Gerya, 2014; Foster and Nimmo, 1996). It has been suggested that plate boundaries develop on Earth because an interconnected network of localized shear zones (areas of concentrated weakening) can form through the lithosphere (Regenauer-Lieb and Yuen, 2001). If Venusian rifts, such as Devana Chasma, are similar to terrestrial plate boundaries, then it is possible that shear zones should form in those locations.Montesi (2013) showed that water-bearing minerals, such as micas, which are probably not present on Venus, largely dominate weakening in the Earth’s crust. On Venus, melts are likely to play the role of the weak phase that allows for localization, due to its low viscosity relative to host rocks. Weakening due to grain size reduction is also possible if a dislocation-accommodated grain boundary sliding mechanism is active on Venus (Montesi, 2013).Rift stability for Venus-like conditions has been analyzed using the model of Buck (1991). This model links the evolution of lithospheric strength with the style of rifting (wide, narrow, or metamorphic core complex). The crust and mantle are assumed to be dry diabase and dry olivine, respectively (diabase rheological parameters are from Mackwell et. al. (1998), olivine rheological parameters are from Hirth and Kohlstedt (2003)). The crustal thickness and surface heat flux are varied based on estimated values from the literature (Nimmo and McKenzie, 1998; Buck, 2002). Without the inclusion of a weakening mechanism the large majority of model runs predict wide rifts developing. Adding a simplistic exponential decay to the lithospheric yield strength allows for more narrow rift formation to occur. Including explicit weakening processes, which may be associated with grain size reduction or development of a foliation, appears necessary to explain the presence of narrow rifts on Venus.

  13. Elevated Passive Continental Margins may form much Later than the time of Rifting

    NASA Astrophysics Data System (ADS)

    Chalmers, J. A.; Japsen, P.; Green, P. F.; Bonow, J.; Lidmar-Bergstrom, K.

    2007-12-01

    Many current models of the development of elevated passive continental margins assume that they are either the remains of foot-wall uplift at the time of rifting or due to underplating by magma from a plume or other mantle source. We have studied the rift and post-rift history of such a passive margin in West and South Greenland and have concluded that the present-day elevations developed 25-60 million years after cessation of rifting and local volcanism, suggesting that additional factors need to be considered when modelling such margins. The morphology of West Greenland is similar to that of other elevated passive margins ion many parts of the world. There are high-level, large-scale, quasi-planar landscapes (planation surfaces) at altitudes of 1-2 km cut by deeply incised valleys. The gradient from the highest ground to the coast is much steeper than that away from the coast. We combined analysis of the morphology of the landscape with studies of fission tracks and the preserved stratigraphic record both on- and off-shore. Rifting and the commencement of sea-floor spreading in the Early Paleogene was accompanied by voluminous high-temperature volcanism. Kilometer-scale uplift at the time of rifting was followed shortly afterwards by kilometer-scale subsidence and possibly by transgression of marine sediments across the rift margin. The present elevated margin formed during three episodes of uplift during the Neogene, 25-60 million years after the cessation of rifting and local volcanism. The quasi-planar planation surfaces presently at 1-2 km altitude are the end-products of denudation to near sea-level in the mid- and late Cenozoic and these surfaces were uplifted to their present altitudes during the Neogene events. Rivers then incised the summit surface to form valleys that were further enlarged and deepened by glaciers. Similar elevated margins exist all around the northern North Atlantic and in many other parts of the world; eastern North America, on both sides of the South Atlantic, western India, eastern Australia, and possibly in Antarctica. Our results show that we cannot simply assume that these elevations were produced either at the time of rifting or as underplating at the time of plume impact. There is, however, no general agreement as to what caused them and we suggest that the history of these margins need to be re-assessed in the light of our results.

  14. Elevated Passive Continental Margins may form much Later than the time of Rifting

    NASA Astrophysics Data System (ADS)

    Chalmers, J. A.; Japsen, P.; Green, P. F.; Bonow, J.; Lidmar-Bergstrom, K.

    2004-12-01

    Many current models of the development of elevated passive continental margins assume that they are either the remains of foot-wall uplift at the time of rifting or due to underplating by magma from a plume or other mantle source. We have studied the rift and post-rift history of such a passive margin in West and South Greenland and have concluded that the present-day elevations developed 25-60 million years after cessation of rifting and local volcanism, suggesting that additional factors need to be considered when modelling such margins. The morphology of West Greenland is similar to that of other elevated passive margins ion many parts of the world. There are high-level, large-scale, quasi-planar landscapes (planation surfaces) at altitudes of 1-2 km cut by deeply incised valleys. The gradient from the highest ground to the coast is much steeper than that away from the coast. We combined analysis of the morphology of the landscape with studies of fission tracks and the preserved stratigraphic record both on- and off-shore. Rifting and the commencement of sea-floor spreading in the Early Paleogene was accompanied by voluminous high-temperature volcanism. Kilometer-scale uplift at the time of rifting was followed shortly afterwards by kilometer-scale subsidence and possibly by transgression of marine sediments across the rift margin. The present elevated margin formed during three episodes of uplift during the Neogene, 25-60 million years after the cessation of rifting and local volcanism. The quasi-planar planation surfaces presently at 1-2 km altitude are the end-products of denudation to near sea-level in the mid- and late Cenozoic and these surfaces were uplifted to their present altitudes during the Neogene events. Rivers then incised the summit surface to form valleys that were further enlarged and deepened by glaciers. Similar elevated margins exist all around the northern North Atlantic and in many other parts of the world; eastern North America, on both sides of the South Atlantic, western India, eastern Australia, and possibly in Antarctica. Our results show that we cannot simply assume that these elevations were produced either at the time of rifting or as underplating at the time of plume impact. There is, however, no general agreement as to what caused them and we suggest that the history of these margins need to be re-assessed in the light of our results.

  15. Force Required to Breakup a Continent: Implications on Rifting Localization and Migration

    NASA Astrophysics Data System (ADS)

    Svartman Dias, A. E.; Lavier, L. L.; Hayman, N. W.

    2014-12-01

    The maximum force from ridge push available is about 5 TN/m, lower than that required by 2D and 3D numerical experiments to rift the lithosphere in the absence of magmatic input. We carry out 2D numerical experiments without any magmatic input to study the extensional force necessary to start a rift basin and to breakup a continent. We assume a range of initial temperature structure, crust and mantle initial thicknesses and composition. In a first step, we use velocity boundary conditions (1cm/yr) and we monitor the force necessary to breakup the continent. Results can be classified in two groups according to the amount of force needed to rift through time: (1) The initial force builds up rapidly to 12-20 TN/m within 0.4-1.0 Myr. This is followed by an exponential decrease due to early strain localization and lithospheric weakening. The force is < 5TN/m after 4.4-7.0 Myr of extension. Continental breakup occurs approximately 10 Myr after the onset of extension forming narrow conjugate margins. This group encompasses experiments with initial Tmoho < 650oC and crustal thicknesses ? 35 km, where crust and mantle deformation are coupled from the early stages of rifting. (2) The initial build-up is more discrete, from < 3 TN/m to 4-6.5 TN/m in the first 0.1 Myr, followed by a decrease to a nearly constant value of 3-5 TN/m from 0.4 Myr to 10 Myr, when strain starts localizing. The constant force through time reflects lithosphere strengthening and migration of the deformation. This rift migration forms a wide basin (> 250 km wide) that may evolve to form very asymmetric conjugate margins. Breakup occurs 18 Myr after the onset of rifting or later. This second group corresponds to experiments with initial Tmoho > 650 km and crustal thicknesses ? 35 km. High bending stresses result in upper crust brittle failure and on enhancement of lower crust lateral flow. Interaction between ductile failure in the lower crust and brittle failure in the upper crust controls the migration of the deformation. In a second step, we repeat the same experiments applying a constant extensional force ? 5 TN/m to test whether and how the limited ridge push force naturally available can rift and breakup a continent. We then compare our results to the evolution of aborted rifts and to along strike variations in rifted magins such as the South Atlantic.

  16. Crustal Deformation Field Around Rift Zone In Southeastern Afar Derived From Jers-1/in-sar

    NASA Astrophysics Data System (ADS)

    Ozawa, T.; Nogi, Y.; Shibuya, K.

    Afar is one of the major active rift zones recognized on the ground and located around the triple junction of Arabia, Somalia and Nubian plates. Afar is one of the major rift zones recognized on the ground. The crustal deformation of Afar has been deduced from paleomagnetism, geology and seismology by many scientists. The current crustal deformation must be detected by geodetic measurements. Ruegg et al. (J. Geophys. Res., 1984) showed the crustal deformation across the Asal-Ghoubbet rift with rate of about 60 mm/yr extension derived from triangulation and trilateration. Walpersdorf et al. (J. Geodyn., 1999) show the opening between South Djibouti and Yemen with rate of 16 mm/yr by GPS surveys. Denser observations are required for detailed crustal deformation, however it is difficult to construct such observation network because of harsh environment. The geodetic application of remote sensing is useful in this region, and we apply JERS-1 SAR interferometry in southeastern Afar, which is one of the most active deformation area. In this study, we use six SAR scenes observed from 1996/5/20 to 1997/5/7, and generate five interferograms; these repeat cycles are 88 (2 pairs), 176, 264, 352 days. First, we generate the digital elevation model (DEM) from two 88 repeat cycle pairs applying the multiple pass SAR interferometry method by Kwok and Fahnestock (IEEE Trans. Geosci. Remote Sensing, 1996). Next, the topographic fringes of all pairs are removed using the DEM. The crustal deformation derived from SAR interferometry increases with expanding repeat cycle. Finally, the velocity field is estimated by fitting to linear trend for each pixel. The spreading rate of Asal-Ghoubbet rift derived from SAR interferometry is good agreement with that by Ruegg et al. (J. Geophys. Res., 1984). We can see the crustal deformation with the subsidence sense in the west of Asal-Ghoubbet rift. This suggests that the extension is distinguished in this area. The subsidence sense deformation is also seen along the Manda-Inakir rift, and we can see the V shape of subsidence area in the east end of the Manda-Inakir rift, which is transition zone from the Manda-Inakir rift to the Mak'Arrassou strike slip zone. It may be caused that the stress along the tectonic line from the Manda-Inakir rift to the Mak'Arrasou strike slip zone is gradually change from the extension to the strike slip.

  17. Sediment infill within rift basins: Facies distribution and effects of deformation: Examples from the Kenya and Tanganyika Rifts, East Africa

    SciTech Connect

    Tiercelin, J.J.; Lezzar, K.E. ); Richert, J.P. )

    1994-07-01

    Oil is known from lacustrine basins of the east African rift. The geology of such basins is complex and different depending on location in the eastern and western branches. The western branch has little volcanism, leading to long-lived basins, such as Lake Tanganyika, whereas a large quantity of volcanics results in the eastern branch characterized by ephemeral basins, as the Baringo-Bogoria basin in Kenya. The Baringo-Bogoria basin is a north-south half graben formed in the middle Pleistocene and presently occupied by the hypersaline Lake Bogoria and the freshwater Lake Baringo. Lake Bogoria is fed by hot springs and ephemeral streams controlled by grid faults bounding the basin to the west. The sedimentary fill is formed by cycles of organic oozes having a good petroleum potential and evaporites. On the other hand, and as a consequence of the grid faults, Lake Baringo is fed by permanent streams bringing into the basin large quantities of terrigenous sediments. Lake Tanganyika is a meromictic lake 1470 m deep and 700 km long, of middle Miocene age. It is subdivided into seven asymmetric half grabens separated by transverse ridges. The sedimentary fill is thick and formed by organic oozes having a very good petroleum potential. In contrast to Bogoria, the lateral distribution of organic matter is characterized by considerable heterogeneity due to the existence of structural blocks or to redepositional processes.

  18. Rift migration explains continental margin asymmetry and crustal hyper-extension

    PubMed Central

    Brune, Sascha; Heine, Christian; Pérez-Gussinyé, Marta; Sobolev, Stephan V.

    2014-01-01

    When continents break apart, continental crust and lithosphere are thinned until break-up is achieved and an oceanic basin is formed. The most remarkable and least understood structures associated with this process are up to 200?km wide areas of hyper-extended continental crust, which are partitioned between conjugate margins with pronounced asymmetry. Here we show, using high-resolution thermo-mechanical modelling, that hyper-extended crust and margin asymmetry are produced by steady state rift migration. We demonstrate that rift migration is accomplished by sequential, oceanward-younging, upper crustal faults, and is balanced through lower crustal flow. Constraining our model with a new South Atlantic plate reconstruction, we demonstrate that larger extension velocities may account for southward increasing width and asymmetry of these conjugate magma-poor margins. Our model challenges conventional ideas of rifted margin evolution, as it implies that during rift migration large amounts of material are transferred from one side of the rift zone to the other. PMID:24905463

  19. Constraining the thermal and erosional evolution of the Rwenzori Mtns, Albertine Rift, by detrital thermochronology

    NASA Astrophysics Data System (ADS)

    Bauer, F. U.; Roller, S.; Grobe, R. W.; Glasmacher, U. A.; Hinderer, M.; Ring, U.; Mambo, V. S.

    2012-04-01

    In East Africa, the feedback between tectonic uplift, erosional denudation and associated possible climate changes is being studied by a multidisciplinary research group, 'RiftLink'. The group's focus is the Albertine Rift of the East African Rift System, and therein rising Rwenzori Mountains that stretch along the border of Uganda and Eastern D.R. Congo. Data from low-temperature thermochronology analysis of hardrocks comprising apatite fission-track (AFT), zircon and apatite (U-Th-Sm)/He dating (ZHe, AHe) and thermal modelling point to a prolonged cooling history with differentiated exhumation in Neogene times. The final rock uplift in Plio- to Pleistocene times, thereby, was very fast that the erosion could not keep pace [1]. In order to narrow the final exhumation stage detrital thermochronology has proven to be very useful. Therefore, sedimentary successions of the Albertine Rift valley in western Uganda and Eastern D.R. Congo were sampled to perform AFT, ZHe and AHe dating of detrital sediments. In the frame of the presentation we will present first results from the detrital thermochronology study of the Albertine Rift and will discuss its implications for the landscape evolution of this area.

  20. Crustal structure of the Southern Rio Grande rift determined from seismic refraction profiling

    NASA Technical Reports Server (NTRS)

    Sinno, Y. A.; Keller, G. R.; Harder, S. H.; Daggett, P. H.; Morgan, P.

    1986-01-01

    As part of a major cooperative seismic experiment, a series of seismic refraction profiles have been recorded in south-central New Mexico with the goal of determining the crustal structure in the southern Rio Grande rift. The data gathered greatly expand the seismic data base in the area, and consist of three interlocking regional profiles: a reversed E-W line across the rift, an unreversed N-S axial line, and an unreversed SW-SE line. The reversed E-W line shows no significant dip along the Moho (32 km thick crust) and a 7.7 km/s Pn velocity. Results from the N-S axial line and the NW-SE line indicate an apparent Pn velocity of 7.95 km/s and significant dip along the Moho with crustal thinning toward the south and southeast. When interpreted together, these data indicate a crustal thinning in the southern rift of 4-6 km with respect to the northern rift and the adjacent Basin and Range province, and establish the regional Pn velocity to be approximately 7.7 km/s. These results suggest that the Rio Grande rift can be identified as a crustal feature separate and distinct from the Basin and Range province.

  1. Inland extent of the Weddell Sea Rift imaged by new aerogeophysical data

    NASA Astrophysics Data System (ADS)

    Jordan, Tom A.; Ferraccioli, Fausto; Ross, Neil; Corr, Hugh F. J.; Leat, Philip T.; Bingham, Rob G.; Rippin, David M.; le Brocq, Anne; Siegert, Martin J.

    2013-02-01

    The Weddell Sea Rift was a major focus for Jurassic extension and magmatism during the early stages of Gondwana break-up and underlies the Weddell Sea Embayment, which separates East Antarctica from a collage of crustal blocks in West Antarctica. Newly-collected aerogeophysical data over the catchments of Institute and Möller ice streams reveal the inland extent of the Weddell Sea Rift against the Ellsworth-Whitmore block and a hitherto unknown major left-lateral strike slip boundary between East and West Antarctica. Aeromagnetic and gravity anomalies define the regional subglacial extent of Proterozoic basement, Middle Cambrian rift-related volcanic rocks, Jurassic intrusions and sedimentary rocks of inferred post-Jurassic age. 2D and 3D magnetic depth-to-source estimates were used to help constrain joint magnetic and gravity models for the region. The models reveal that Proterozoic crust similar to that exposed at Haag Nunataks, extends southeast of the Ellsworth Mountains to the margin of the Coastal Basins. Thick granitic Jurassic intrusions are modelled at the transition between the Ellsworth-Whitmore block and the thinner crust of the Weddell Sea Rift and within the Pagano Shear Zone. The crust beneath the inland extension of the Weddell Sea Rift is modelled as being either ~ 4 km thinner compared to the adjacent Ellsworth-Whitmore block or as underlain by an up to 8 km thick mafic underplate.

  2. Lower crustal bodies in the Møre volcanic rifted margin: Geophysical determination and geological implications

    NASA Astrophysics Data System (ADS)

    Nirrengarten, M.; Gernigon, L.; Manatschal, G.

    2014-12-01

    Understanding nature, structure and age of Lower Crustal Bodies (LCBs) and their relation to the crustal structure of the Møre margin (Mid-Norwegian margin), and in a more general way, of magma-rich rifted margins, is a key issue to decipher the tectono-magmatic processes found in volcanic rifted margins. In light of 2D potential field modelling combined with reflection and refraction seismic data, we reinvestigated the crustal nature of the Møre margin and adjacent Jan Mayen corridor. In the proximal domain, our study shows that the LCBs most likely represent inherited crustal bodies and not necessarily rift-related serpentinised mantle as previously proposed. To fit all geophysical observations, both lower and middle crustal layers need to be preserved over a large part of the Møre Basin. For the distal margin, the interpretation of the LCBs is more difficult. Our preferred interpretation is that they are mainly made of boudins of hyper-extended, pre-rift lower continental crustal rocks more or less intruded by Early Tertiary magmatic material. Our seismic, magnetic and gravity data does not easily support large scale exhumation of serpentinised mantle in the inner and is unlikely in the outer parts of the Møre Basin. The deep structures of the Mid-Norwegian magma rich rifted margin result from the poly-phase stretching and thinning of complex inherited crustal structures, locally intruded by Early Tertiary magmatic material.

  3. Rift border system: The interplay between tectonics and sedimentation in the Reconcavo basin, northeastern Brazil

    SciTech Connect

    Magnavita, L.P.; Silva, T.F. da

    1995-11-01

    A geometric and depositional model is proposed to explain the tectonic and sedimentary evolution of the main border of the Reconcavo basin. The architecture of the rift margin is characterized by a rift border system constituted by (1) a master fault, (2) a step, and (3) a clastic wedge. This footwall-derived clastic wedge is interpreted as alluvial fans and fan deltas composed of conglomerates that interfinger with hanging-wall strata. The analysis of the vertical distribution of coarse-grained components of this wedge suggests that its composition is geographically controlled, and no regular inverted stratigraphy is commonly described for this type of succession. During an initial lacustrine phase, turbidites accumulated farther from and parallel to the rift margin. The mapping of marker beds that bound these lacustrine turbidite deposits may be used to infer major periods of clastic influx and, therefore, to correlate with periods of fault-related subsidence or climatic fluctuations in the depositional basin and erosion of the sediment source area. Periods of limited back-faulting and basin expansion toward the main border are distinguished through patterns of progradation and aggradation indicating progressive retreat of the rift border and younging; in the footwall direction. The overall evolution of the rift border seems to be related to extension, block rotation, hanging-wall subsidence, and footwall uplift associated with the initial master fault, with limited propagation of faults away from the basin into the footwall.

  4. Oligo-Miocene rift of Sardinia and the early history of the Western Mediterranean Basin

    NASA Astrophysics Data System (ADS)

    Cherchi, A.

    1982-08-01

    The geodynamic evolution of the Western Mediterranean Basin, in spite of many studies, is still uncertain. There is some consensus for interpreting this basin as a kind of small oceanic marginal basin. Its opening has generally been related to a subduction process which was active during the Oligocene-Miocene somewhere east of Sardinia-Corsica1-7. As the margins of the basin are deeply buried below Miocene-to-present sediments, direct lithological and stratigraphical data which could explain the events responsible for its formation are rare8-10 or missing altogether. To obtain such data, detailed field studies have been undertaken in Sardinia (Fig. 1), and the first results are presented here. This approach is justified by the fact that in that island, Oligocene and Miocene sediments were deposited in a rift (fossa tettonica sarda of Verdabasso11), which is the easternmost arm of the complex rift system that affected the European plate during Oligocene and Miocene times. One of these arms evolved towards a small oceanic basin-the Western Mediterranean or Algero-Provençal Basin-while others such as the Gulf of Valencia and the Sardinia rift aborted and remained at the rift stage. Its exceptional exposures make it possible to examine the Sardinia rift to clarify the sequence of events which created it, and to establish a sedimentological model which we believe is directly applicable to the Western Mediterranean Basin.

  5. Rift migration explains continental margin asymmetry and crustal hyper-extension.

    PubMed

    Brune, Sascha; Heine, Christian; Pérez-Gussinyé, Marta; Sobolev, Stephan V

    2014-01-01

    When continents break apart, continental crust and lithosphere are thinned until break-up is achieved and an oceanic basin is formed. The most remarkable and least understood structures associated with this process are up to 200 km wide areas of hyper-extended continental crust, which are partitioned between conjugate margins with pronounced asymmetry. Here we show, using high-resolution thermo-mechanical modelling, that hyper-extended crust and margin asymmetry are produced by steady state rift migration. We demonstrate that rift migration is accomplished by sequential, oceanward-younging, upper crustal faults, and is balanced through lower crustal flow. Constraining our model with a new South Atlantic plate reconstruction, we demonstrate that larger extension velocities may account for southward increasing width and asymmetry of these conjugate magma-poor margins. Our model challenges conventional ideas of rifted margin evolution, as it implies that during rift migration large amounts of material are transferred from one side of the rift zone to the other. PMID:24905463

  6. Eruptions arising from tidally controlled periodic openings of rifts on Enceladus.

    PubMed

    Hurford, T A; Helfenstein, P; Hoppa, G V; Greenberg, R; Bills, B G

    2007-05-17

    In 2005, plumes were detected near the south polar region of Enceladus, a small icy satellite of Saturn. Observations of the south pole revealed large rifts in the crust, informally called 'tiger stripes', which exhibit higher temperatures than the surrounding terrain and are probably sources of the observed eruptions. Models of the ultimate interior source for the eruptions are under consideration. Other models of an expanding plume require eruptions from discrete sources, as well as less voluminous eruptions from a more extended source, to match the observations. No physical mechanism that matches the observations has been identified to control these eruptions. Here we report a mechanism in which temporal variations in tidal stress open and close the tiger-stripe rifts, governing the timing of eruptions. During each orbit, every portion of each tiger stripe rift spends about half the time in tension, which allows the rift to open, exposing volatiles, and allowing eruptions. In a complementary process, periodic shear stress along the rifts also generates heat along their lengths, which has the capacity to enhance eruptions. Plume activity is expected to vary periodically, affecting the injection of material into Saturn's E ring and its formation, evolution and structure. Moreover, the stresses controlling eruptions imply that Enceladus' icy shell behaves as a thin elastic layer, perhaps only a few tens of kilometres thick. PMID:17507977

  7. Synrift sedimentation in the Gulf of Suez rift controlled by eustatic sea level variations

    SciTech Connect

    Perry, S.K.; Schamel, S.

    1985-01-01

    Laterally persistent stratigraphic variations in the southern Gulf of Suez rift indicate that eustatic variations in sea level predominate over local tectonic effects in controlling Neogene basin-fill sedimentation. Low sea level allows extensive erosion of tilt-block crests and rift shoulders, sending clastic aprons into the intervening subbasins and allowing evaporite deposition. High sea level minimizes clastic input, allowing marls and shales to build up off structure and reefs to form on and around tilt blocks. Thus variations in sediment character indicate relative sea level. Early rifting events in the upper Oligocene are marked by deposition of continental red beds. Overlying lowest Miocene clastics and evaporites are cut by a lower Burdigalian unconformity, indicating a minor transgression in the Aquitanian. Above a disconformity, laterally varying clastics and evaporites suggest regression followed by intermittent shallow-water conditions. A higher unconformity is overlain by thick cyclic evaporites representing periodic flooding and drying of the rift, a result of sea level remaining close to the height of the Suez sill to the north. A major Messinian unconformity cuts the section, indicating major regression, and is overlain by largely clastic sediments of both continental and marine affinities, showing rapid sea level fluctuations. Regional synrift sedimentation has been controlled more by eustatic sea level change, modified by the Suez sill to the north, than by tectonic movements within the rift.

  8. Cenozoic thermal, mechanical and tectonic evolution of the Rio Grande rift

    NASA Technical Reports Server (NTRS)

    Morgan, P.; Seager, W. R.; Golombek, M. P.

    1986-01-01

    Two areas of New Mexico which exhibit complex but similar Cenozoic histories of extensional tectonism are analyzed. The first study area is the Basin and Range province and southern Rio Gande rift in southern New Mexico; the second study area is the central Rio Grande rift in central and northern New Mexico, the southern San Luis basin, the Espanola basin, and the Albuquerque basin. Two phases of extension were identified: the first phase which began in mid-Oligocene was characterized by local high-strain extension events, low-angle faulting, and the development of broad, shallow basins, all indicating an approximately NE-SW + or - 25 deg extension direction; the later phase which occurred primarily in the late Miocene, was characterized by synchronous, high-angle faulting, resulting in large vertical strains which produced the modern Rio Grande rift morphology. Extension direction was approximately E-W. Geotherms were estimated and lithospheric strength curves were calculated for these two phases of extension. A high geotherm was deduced for the early phase resulting in a shallow crustal brittle transition, and insignificant mantle strength. The lithosphere subsequently cooled, resulting in a significant zone of mantle strength beneath the Moho. It is concluded that the interrelationship among regional and local prerifting, synrifting, and postrifting events in the Rio Grande rift attests to the fact that the rifting (in the region studied) should be considered in the context of other geologic events.

  9. Stresses and seismicity at the present stage of evolution of the Baikal rift zone lithosphere

    NASA Astrophysics Data System (ADS)

    Klyuchevskii, A. V.

    2007-12-01

    The paper presents results of studying stresses and seismicity of the lithosphere in the Baikal rift zone at the present (instrumental) stage of its evolution. These results are well consistent with the theory of self-organization of complex dynamic systems and can be used for the classification of certain features and properties of the Baikal rift seismogenic system studied. Application of the theory of nonlinear dynamic systems to the analysis of stresses and seismicity allowed us to develop a concept of a complex spatiotemporal structure of the stress state of the lithosphere and seismicity in the region. In terms of this concept, the distribution of strong earthquakes in time is explained in terms of bifurcations (catastrophes) of stresses in the rift zone. Extrapolation of the results indicates that a stress catastrophe in lithosphere of the rift zone can take place in the next few years years, which increases the probability of occurrence of strong ( M ? 7) earthquakes in the Baikal region. A model with bifurcation of triple equilibrium most consistent with the phase image of regional stresses is proposed as a scenario of the stress state evolution in the lithosphere of the Baikal rift zone.

  10. Exhumation, rift-flank uplift, and the thermal evolution of the Rwenzori Mountains determined by combined (U-Th)/He and U-Pb thermochronometry

    E-print Network

    MacPhee, Daniel

    2006-01-01

    Rising over 5 km along the border of Uganda and the Democratic Republic of the Congo, the Rwenzori Mountains represent an extreme example of basement rift-flank uplift in the western branch of the East African Rift, a ...

  11. Evaporation Estimation of Rift Valley Lakes: Comparison of Models

    PubMed Central

    Melesse, Assefa M.; Abtew, Wossenu; Dessalegne, Tibebe

    2009-01-01

    Evapotranspiration (ET) accounts for a substantial amount of the water flux in the arid and semi-arid regions of the World. Accurate estimation of ET has been a challenge for hydrologists, mainly because of the spatiotemporal variability of the environmental and physical parameters governing the latent heat flux. In addition, most available ET models depend on intensive meteorological information for ET estimation. Such data are not available at the desired spatial and temporal scales in less developed and remote parts of the world. This limitation has necessitated the development of simple models that are less data intensive and provide ET estimates with acceptable level of accuracy. Remote sensing approach can also be applied to large areas where meteorological data are not available and field scale data collection is costly, time consuming and difficult. In areas like the Rift Valley regions of Ethiopia, the applicability of the Simple Method (Abtew Method) of lake evaporation estimation and surface energy balance approach using remote sensing was studied. The Simple Method and a remote sensing-based lake evaporation estimates were compared to the Penman, Energy balance, Pan, Radiation and Complementary Relationship Lake Evaporation (CRLE) methods applied in the region. Results indicate a good correspondence of the models outputs to that of the above methods. Comparison of the 1986 and 2000 monthly lake ET from the Landsat images to the Simple and Penman Methods show that the remote sensing and surface energy balance approach is promising for large scale applications to understand the spatial variation of the latent heat flux. PMID:22303142

  12. The Influence of Mantle Petrology on Basin Subsidence During Rifting Nina S.C. Simon & Yuri Y. Podladchikov

    E-print Network

    Simon, Nina

    The Influence of Mantle Petrology on Basin Subsidence During Rifting Nina S.C. Simon & Yuri Y-spinel and spinel plagioclase transitions in the lithospheric mantle have the most profound effect on uplift/subsidence- rift subsidence recorded in the basin is directly proportional to the total amount of plagioclase

  13. Characterization and Mapping of the Gene Conferring Resistance to Rift Valley Fever Virus Hepatic Disease in WF.LEW Rats 

    E-print Network

    Callicott, Ralph J.

    2010-01-14

    human response to Rift Valley fever virus infection. J Med Virol 1982;10(1):45-54.ib Epidemiol Biostat. Volume 3. New York: Karger; 1981. p 21-41 Ritter M, Bouloy M, Vialat P, Janzen C, Haller O, Frese M (2000) Resistance to Rift Valley fever virus...

  14. Mapping the Risk of Rift Valley fever re-emergence in Southern Africa using remote sensing data

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Rift Valley fever is a viral disease of animals and humans that occurs throughout sub-Saharan Africa, Egypt and the Arabian Peninsula. Outbreaks of the disease are episodic and closely linked to climate variability, especially widespread elevated rainfall that facilitates Rift Valley fever virus tra...

  15. TECTONICS, VOL. 2, NO. 2, PAGES 167-185, APRIL 1983 TECTONIC ANALYSIS OF THE DEAD SEA RIFT

    E-print Network

    Ze'ev, Reches

    TECTONICS, VOL. 2, NO. 2, PAGES 167-185, APRIL 1983 TECTONIC ANALYSIS OF THE DEAD SEA RIFT REGION Cretaceous to Eocene rock· in the folds and plateaus west of the Dead Sea rift. The second field, with domina. The first stress field is called the Syrian Arc stress, and the second is called the Dead Sea stress

  16. Cenozoic rifting and volcanism in eastern China: a mantle dynamic link to the IndoAsian collision?

    E-print Network

    Liu, Mian

    Cenozoic rifting and volcanism in eastern China: a mantle dynamic link to the Indo­Asian collision upwelling, rifting, and widespread Cenozoic volcanism in eastern China. D 2004 Elsevier B.V. All rights has been characterized by crustal shortening and mountain building throughout the Cenozoic (Allegre et

  17. Topographic and volcanic asymmetry around the Red Sea - Constraints on rift models

    NASA Technical Reports Server (NTRS)

    Dixon, Timothy H.; Ivins, Erik R.; Franklin, Brenda J.

    1989-01-01

    This paper describes a model which explains the topographic and volcanic asymmetry around the Red Sea. The model involves asthenospheric upwelling beneath a lithosphere of laterally variable strength in which a weak zone (e.g., a suture or a region with quartz-bearing lower crust) may have controlled the location of rifting. In this model, Tertiary volcanism in Saudi Arabia marks the location of initial upwelling, and uplift is due to crustal thickening associated with magmatic underplating and crustal intrusion. The model predicts that the incipient crustal rift and the locus of mantle upwelling will tend to align as rifting continues and stable seafloor spreading develops, implying relative migration of the lithosphere and asthenosphere.

  18. The Mesozoic rift basins of eastern North America: Potential reservoir or Explorationist's folly

    SciTech Connect

    Pyron, A.

    1991-08-01

    Mesozoic rift basins are found on the East Coast of North America from Georgia to Nova Scotia. The basins formed as a result of extensional activity associated with the breakup of Pangaea. The internal geometry of the basins includes a depositional sequence ranging from coarse fanglomerates to fine-grained siltstones and argillites. Since these Mesozoic rift basins were first studied, they have not been considered to be likely spots for hydrocarbon accumulations. Recently, geologists have reconsidered these Mesozoic basins and have developed a more synergistic approach that suggests that many of these rift basins might be suitable targets for exploration. By analogy, these Mesozoic basins are correlative to similar basins in northwestern Africa, where significant reserved of oil and natural gas have been developed. The similarity between the productive basins in northwestern Africa and the Mesozoic basins of North America and their proximity to major markets provides sufficient rationale to further investigate these basins.

  19. Deep magma body beneath the summit and rift zones of Kilauea Volcano, Hawaii

    USGS Publications Warehouse

    Delaney, P.T.; Fiske, R.S.; Miklius, Asta; Okamura, A.T.; Sako, M.K.

    1990-01-01

    A magnitude 7.2 earthquake in 1975 caused the south flank of Kilauea Volcano, Hawaii, to move seaward in response to slippage along a deep fault. Since then, a large part of the volcano's edifice has been adjusting to this perturbation. The summit of Kilauea extended at a rate of 0.26 meter per year until 1983, the south flank uplifted more than 0.5 meter, and the axes of both the volcano's rift zones extended and subsided; the summit continues to subside. These ground-surface motions have been remarkably steady and much more widespread than those caused by either recurrent inflation and deflation of the summit magma chamber or the episodic propagation of dikes into the rift zones. Kilauea's magmatic system is, therefore, probably deeper and more extensive than previously thought; the summit and both rift zones may be underlain by a thick, near vertical dike-like magma system at a depth of 3 to 9 kilometers.

  20. Evidence for a seismogenic upper mantle and lower crust in the Baikal rift

    SciTech Connect

    Deverchere, J.; Houdry, F. ); Diament, M. ); Solonenko, N.V.; Solonenko, A.V. )

    1991-06-01

    The high level seismicity of the Baikal rift zone and its spatial distribution in dense swarms and belts provide an opportunity to study the seismogenic behavior of a continental lithosphere submitted to extension in an early stage. Using data from a regional seismological network, the authors analyze a significantly large set of events from an earthquake swarm located east of the nearly aseismic northern Baikal lake. They find that at least 10% of the well-constrained events are located in the lower crust or the uppermost mantle. The fault plane solutions of earthquakes within the crust define a NW-SE extensional stress regime perpendicular to the rift axis. Results confirm the idea that zones of continental extension may exhibit significant rigidity. The authors propose to infer a migration of deformation from the northern Baikal lake to an initially stronger part of the lithosphere, i.e. the Barguzin rift and its extension to the east.

  1. Magmatic history of Red Sea rifting: perspective from the central Saudi Arabian coastal plain.

    USGS Publications Warehouse

    Pallister, J.S.

    1987-01-01

    An early stage of magmatism related to Red Sea rifting is recorded by a Tertiary dyke complex and comagmatic volcanic rocks exposed on the central Saudi Arabian coastal plain. Field relations and new K/Ar dates indicate episodic magmatism from approx 30 m.y. to the present day and rift-related magmatism as early as 50 m.y. Localized volcanism and sheeted dyke injection ceased at approx 20 m.y. and were replaced by the intrusion of thick gabbro dykes, marking the onset of sea-floor spreading in the central Red Sea. Differences in the depths and dynamics of mantle-melt extraction and transport may account for the transition from mixed alkaline-subalkaline bimodal magmatism of the pre-20 m.y. rift basin to exclusively subalkaline (tholeiitic) magmatism of the Red Sea spreading axis and the alkali basalt volcanism inland.-L.C.H.

  2. Oceanic rift propagation - a cause of crustal underplating and seamount volcanism

    SciTech Connect

    Calvert, A.J.; Hasselgren, E.A.; Clowes, R.M. )

    1990-09-01

    We present the first seismic reflection data across a pseudofault zone, the trace of a propagating rift away from a spreading axis. Strong reflections from the crust-mantle transition are discontinuous across the pseudofault. Deeper reflections, which originate near the base of the crust formed at the propagator tip, dip beneath the older oceanic crust and become subhorizontal. They are interpreted to represent the lower limit of an underplated subcrustal plutonic complex. An anomalously smooth basement surface indicative of massive lava flows and a 1200-m-high seamount are above the underplated zone. The sill complex extends tens of kilometres ahead of, and off-axis from, the former location of the propagating rift, indicating that the magmatic supply to the propagator tip was unusually vigorous. Other seamounts in the northeast Pacific lie close to pseudofaults and may have formed as a result of rift propagation.

  3. Soils, slopes and source rocks: Application of a soil chemistry model to nutrient delivery to rift lakes

    NASA Astrophysics Data System (ADS)

    Harris, Nicholas B.; Tucker, Gregory E.

    2015-06-01

    The topographic evolution of rift basins may be critical to the deposition of lacustrine source rocks such as the organic-rich Lower Cretaceous shales of the South Atlantic margin. Soils have been proposed as a key link between topography and source rock deposition by providing nutrients for the algae growth in rift lakes. Decreasing topographic relief from active rift to late rift has several effects on soils: soils become thicker and finer, erosion of dead surface and soil organic matter decreases, and the fractionation of precipitation between runoff and infiltration may favor increased infiltration. This hypothesis is tested by application of CENTURY, a complex box model that simulates transfer of nutrients within soil pools. The model is first applied to a rainforest soil, with several parameters individually varied. Infiltration experiments show that the concentrations of C, N and P in groundwater decrease rapidly as infiltration decreases, whether due to increased slope or to decreased precipitation. Increased erosion of surface plant litter and topsoil results in substantially decreased nutrient concentrations in groundwater. Increased sand content in soil causes an increase in nutrient concentration. We integrate these variables in analyzing topographic swathes from the Rio Grande Rift, comparing the southern part of the rift, where topography is relatively old and reduced, to the northern rift. C and P concentrations in groundwater increase as slope gradient decreases, resulting in substantially larger C and P concentrations in groundwater in the southern rift than the northern rift. Nitrogen concentrations in groundwater depends on whether infiltration varies as a function of slope gradient; in experiments where the fraction of infiltrated precipitation decreased with increasing slope, N concentrations was also substantially higher in the southern rift; but in experiments where that fraction was held constant, N concentrations was lower in the southern rift. These simulations demonstrate that evolving topography during rift development can significantly influence nutrient concentrations in groundwater and, if these nutrients flow into rift lakes and stimulate organic productivity, account for the deposition of rich oil-prone source rocks in late rift stages.

  4. Statistical mechanics and scaling of fault populations with increasing strain in the Corinth Rift

    NASA Astrophysics Data System (ADS)

    Michas, Georgios; Vallianatos, Filippos; Sammonds, Peter

    2015-12-01

    Scaling properties of fracture/fault systems are studied in order to characterize the mechanical properties of rocks and to provide insight into the mechanisms that govern fault growth. A comprehensive image of the fault network in the Corinth Rift, Greece, obtained through numerous field studies and marine geophysical surveys, allows for the first time such a study over the entire area of the Rift. We compile a detailed fault map of the area and analyze the scaling properties of fault trace-lengths by using a statistical mechanics model, derived in the framework of generalized statistical mechanics and associated maximum entropy principle. By using this framework, a range of asymptotic power-law to exponential-like distributions are derived that can well describe the observed scaling patterns of fault trace-lengths in the Rift. Systematic variations and in particular a transition from asymptotic power-law to exponential-like scaling are observed to be a function of increasing strain in distinct strain regimes in the Rift, providing quantitative evidence for such crustal processes in a single tectonic setting. These results indicate the organization of the fault system as a function of brittle strain in the Earth's crust and suggest there are different mechanisms for fault growth in the distinct parts of the Rift. In addition, other factors such as fault interactions and the thickness of the brittle layer affect how the fault system evolves in time. The results suggest that regional strain, fault interactions and the boundary condition of the brittle layer may control fault growth and the fault network evolution in the Corinth Rift.

  5. The Midcontinent rift system and the Precambrian basement in southern Michigan

    SciTech Connect

    Smith, W.A. . Dept. of Geology)

    1994-04-01

    The Precambrian basement within Michigan consists of at least three provinces, each characterized by distinctive potential field anomalies: (1) the Eastern Granite-Rhyolite Province (EGRP) in the south, (2) the Grenville Province in the southeast and (3) the Penokean Province to the north. Also located within the basement is the Mid-Michigan rift (MMR), which is the eastern arm of the Midcontinent rift system (MRS). Southwest and parallel to the MMR is a series of linear positive gravity anomalies which has been referred to as the Ft. Wayne rift (FWR) and the Southwest Michigan Anomaly (SWMA). The EGRP, which is characterized by undeformed and unmetamorphosed rhyolite to dacite and epizonal granites, was emplaced ca. 1510--1450 Ma. However, the EGRP may be comprised of several terranes of varying extent and origin based on analysis of potential field data and rock and mineral ages. The MMR and the FWR/SWMA are characterized by linear arrays of positive magnetic and gravity anomalies, which are probably due to thick accumulations of mafic igneous rocks within the rifts. The extent and trends of the FWR/SWMA have been largely inferred from geophysical data with a presumption of the age of about 1,100 Ma. The continuation of the MMR southward into Ohio and Kentucky as a sequence of gravity highs is questionable and needs further resolution. The FWR/SWMA may be part of the East Continent Rift Basin (ECRB). The ECRB, which is a large complex of related rift basins of Keweenawan age (1300 --1100 Ma), may be an extension of the MRS but it is not physically continuous with it. The ECRB lies to the west of the Grenville Front and extends at least from northwest Ohio to central Kentucky. Extensions of the ECRB north and south are speculative.

  6. A Joint Inversion for Velocity and Anisotropy Structure Beneath a Highly Extended Continental Rift

    NASA Astrophysics Data System (ADS)

    Eilon, Z.; Abers, G. A.; Gaherty, J. B.; Jin, G.

    2014-12-01

    We jointly invert body wave travel times for anisotropy and velocity variations within the Woodlark Rift, Papua New Guinea, one of the youngest (?6Ma) and most highly-extended (?190km) continental rifts known. We use data from the CDPapua passive seismic array deployed around the D'Entrecasteaux Islands; these islands lie within the maximally-thinned continental crust at the centre of the rift and host the world's youngest (5 - 7 Ma) UHP rocks. We have previously used SK(K)S splitting to identify strong anisotropy within this rift, with fast axis orientated parallel to extension, roughly N-S. Consistency of splitting direction across station, back azimuth and method of analysis indicates a simple anisotropic fabric beneath much of this region. Consequently, we simplify the anisotropic inversion by solving for perturbations to N-S and E-W shear velocities at each node. This work builds on our previous isotropic tomographic study by resolving tradeoffs between anisotropy and velocity heterogeneities. We cross-correlate shear wave arrivals separately on N-S and E-W components, using the Christoffel equations to show that the travel times of these orthogonal quasi-shear pulses distinctly record the fast and slow velocities within our model. We invert these data for velocities on an irregular mesh, using a finite frequency approach with a first fresnel zone approximation. Preliminary results identify the locus of the rift beneath the D'Entrecasteaux Islands, and demonstrate that substantial anisotropy is present beneath the region of major extension. This anisotropy is co-located with low seismic velocities that indicate almost total removal of lithosphere, consistent with gravity fitting. Therefore, we ascribe the anisotropy within the rift to LPO of highly sheared asthenospheric mantle as a result of the large magnitude of extension.

  7. The mechanism of post-rift fault activities in Baiyun sag, Pearl River Mouth basin

    NASA Astrophysics Data System (ADS)

    Sun, Zhen; Xu, Ziying; Sun, Longtao; Pang, Xiong; Yan, Chengzhi; Li, Yuanping; Zhao, Zhongxian; Wang, Zhangwen; Zhang, Cuimei

    2014-08-01

    Post-rift fault activities were often observed in deepwater basins, which have great contributions to oil and gas migration and accumulation. The main causes for post-rift fault activities include tectonic events, mud or salt diapirs, and gravitational collapse. In the South China Sea continental margin, post-rift fault activities are widely distributed, especially in Baiyun sag, one of the largest deepwater sag with its main body located beneath present continental slope. During the post-rift stage, large population of faults kept active for a long time from 32 Ma (T70) till 5.5 Ma (T10). Seismic interpretation, fault analysis and analogue modeling experiments indicate that the post-rift fault activities in Baiyun sag between 32 Ma (T70) and 13.8 Ma (T30) was mainly controlled by gravity pointing to the Main Baiyun sag, which caused the faults extensive on the side facing Main Baiyun sag and the back side compressive. Around 32 Ma (T70), the breakup of the continental margin and the spreading of the South China Sea shed a combined effect of weak compression toward Baiyun sag. The gravity during post-rift stage might be caused by discrepant subsidence and sedimentation between strongly thinned sag center and wing areas. This is supported by positive relationship between sedimentation rate and fault growth index. After 13.8 Ma (T30), fault activity shows negative relationship with sedimentation rate. Compressive uplift and erosion in seismic profiles as well as negative tectonic subsiding rates suggest that the fault activity from 13.8 Ma (T30) to 5.5 Ma (T10) might be controlled by the subductive compression from the Philippine plate in the east.

  8. Current kinematics and dynamics of Africa and the East African Rift System

    NASA Astrophysics Data System (ADS)

    Stamps, D. S.; Flesch, L. M.; Calais, E.; Ghosh, A.

    2014-06-01

    Although the East African Rift System (EARS) is an archetype continental rift, the forces driving its evolution remain debated. Some contend buoyancy forces arising from gravitational potential energy (GPE) gradients within the lithosphere drive rifting. Others argue for a major role of the diverging mantle flow associated with the African Superplume. Here we quantify the forces driving present-day continental rifting in East Africa by (1) solving the depth averaged 3-D force balance equations for 3-D deviatoric stress associated with GPE, (2) inverting for a stress field boundary condition that we interpret as originating from large-scale mantle tractions, (3) calculating dynamic velocities due to lithospheric buoyancy forces, lateral viscosity variations, and velocity boundary conditions, and (4) calculating dynamic velocities that result from the stress response of horizontal mantle tractions acting on a viscous lithosphere in Africa and surroundings. We find deviatoric stress associated with lithospheric GPE gradients are ˜8-20 MPa in EARS, and the minimum deviatoric stress resulting from basal shear is ˜1.6 MPa along the EARS. Our dynamic velocity calculations confirm that a force contribution from GPE gradients alone is sufficient to drive Nubia-Somalia divergence and that additional forcing from horizontal mantle tractions overestimates surface kinematics. Stresses from GPE gradients appear sufficient to sustain present-day rifting in East Africa; however, they are lower than the vertically integrated strength of the lithosphere along most of the EARS. This indicates additional processes are required to initiate rupture of continental lithosphere, but once it is initiated, lithospheric buoyancy forces are enough to maintain rifting.

  9. Dolomitization and fluid interaction in the Reelfoot rift, southeastern Missouri: Geochemical and petrologic studies

    SciTech Connect

    Shelton, K.L.; Haeussler, G.T.; Burstein, I.B. ); Gregg, J.M. . Dept. of Geology and Geophysics); Palmer, J.R. . Div. of Geology and Land Survey)

    1992-01-01

    Platform carbonates of the Bonneterre Dolomite (Cambrian) from 2--3 km deep within the Reelfoot Rift of SE Missouri record a diagenetic environment dominated by primitive basinal fluids emanating from a continental rift. The rifted area has been suggested as a possible source region for metals and sulfur for major Mississippi Valley-type (MVT) mineral deposits of the Viburnum Trend, hosted in shallower (< 1 km), stratigraphically equivalent rocks. Rift-related carbonates are replaced extensively by coarse crystalline, nonplanar dolomite; vug and fracture porosity is filled by dolomite and calcite cement, and less commonly by quartz, anhydrite and sulfide minerals. CL and microprobe analysis reveal a simple dolomite cement stratigraphy with an inner Fe-rich and an outer Fe-free zone. These cement zones and their chemistry are in marked contrast to those of more complexly zoned, MVT-related dolomite cements in shallower carbonates throughout SE MO and N Arkansas. Sr-87/Sr-86 values for rift-related, replacement (0.714--0.715) and cement (0.720--0.727) dolomites are highly radiogenic. [delta]C-13 and [delta]O-18 values become progressively more negative with paragenetic time. Fluid inclusion homogenization temperatures (250--180 C) indicate a diagenetic environment nearly 100 C hotter than that preserved in shallower MVT-related carbonates. If the rift environment is a source region for fluids and ore constituents for shallower MVT mineralization, it is only one of several sources or its fluids must have undergone extensive chemical interaction with rocks along their flow paths to have evolved into the more complex chemistry indicated for the shallower site of ore deposition.

  10. Motion in the north Iceland volcanic rift zone accommodated by bookshelf faulting

    E-print Network

    Green, Robert G.; White, Robert S.; Greenfield, Tim

    2013-12-01

    beneath Askja volcano on the north Ice- land rift. Bull. Volcanol. 72, 55–62 (2009). [23] Key, J., White, R. S., Soosalu, H. & Jakobsdóttir, S. S. Mul- tiple melt injection along a spreading segment at Askja, Ice- land. Geophysical Research Letters 38, 1... , permanent ice caps in blue-white, rivers and lakes in blue, tectonic fractures[5],[6],[15] in black lines and active fissure swarms of the volcanic rift systems overlain in beige12. WNW oriented fractures5,6,15 are red lines. Black triangles are seismic...

  11. Fractal nature and scaling of normal faults, Rio Grande rift, NM: Implications for growth and strain

    SciTech Connect

    Carter, K.E.

    1994-09-01

    In this paper I introduce a suite of Quaternary normal faults from within an active continental rift and characterize the nature of the relationship between fault dimensions. I address the statistical and geological significance of the fractal analysis used in that characterization and discuss the tectonic implications. Specifically, I suggest (1) scaling laws for a previously unanalyzed population of young normal faults in rift environment; (2) implications for fault growth models in this area, in particular, addressing self-similar growth implied from the population; and (3) estimates for the total strain in this part of the basin, considering the contribution of small to unobserved faults.

  12. Depositional setting and hydrocarbon source potential of the Miocene Gulf of Suez syn-rift evaporites

    SciTech Connect

    Richardson, M.; Arthur, M.A.; Quinn, J.S.; Whelan, J.K.; Katz, B.J. )

    1988-08-01

    The Red Sea rift basin and its northern continuation, the Gulf of Suez, has experienced continuous deposition of marine evaporites throughout much of its development from the early Miocene to the Pliocene resulting in the accumulation of up to 5 km of evaporite strata in the rift. In this paper, the geologic history of these evaporites are discussed, along with their petroleum source rock potential. The authors hypothesize that rapid deposition of organic matter occurred during episodic storms and freshening events in which a less saline surface layer developed.

  13. Lithological Influences on Occurrence of High-Fluoride Waters in The Central Kenya Rift

    NASA Astrophysics Data System (ADS)

    Olaka, L. A.; Musolff, A.; Mulch, A.; Olago, D.; Odada, E. O.

    2013-12-01

    Within the East African rift, groundwater recharge results from the complex interplay of geology, land cover, geomorphology, climate and on going volcano-tectonic processes across a broad range of spatial and temporal scales. The interrelationships between these factors create complex patterns of water availability, reliability and quality. The hydrochemical evolution of the waters is further complex due to the different climatic regimes and geothermal processes going on in this area. High fluoridic waters within the rift have been reported by few studies, while dental fluorosis is high among the inhabitants of the rift. The natural sources of fluoride in waters can be from weathering of fluorine bearing minerals in rocks, volcanic or fumarolic activities. Fluoride concentration in water depends on a number of factors including pH, temperature, time of water-rock formation contact and geochemical processes. Knowledge of the sources and dispersion of fluoride in both surface and groundwaters within the central Kenya rift and seasonal variations between wet and dry seasons is still poor. The Central Kenya rift is marked by active tectonics, volcanic activity and fumarolic activity, the rocks are majorly volcanics: rhyolites, tuffs, basalts, phonolites, ashes and agglomerates some are highly fractured. Major NW-SE faults bound the rift escarpment while the rift floor is marked by N-S striking faults We combine petrographic, hydrochemistry and structural information to determine the sources and enrichment pathways of high fluoridic waters within the Naivasha catchment. A total of 120 water samples for both the dry season (January-February2012) and after wet season (June-July 2013) from springs, rivers, lakes, hand dug wells, fumaroles and boreholes within the Naivasha catchment are collected and analysed for fluoride, physicochemical parameters and stable isotopes (?2 H, ?18 O) in order to determine the origin and evolution of the waters. Additionally, 30 soil and rock samples were also collected and analysed for fluoride, and rock samples were subjected to petrographic investigations and X-ray diffraction. The fluoride levels in surface and groundwater for the dry season range from 0.019 - 50.14 mg/L, on average above the WHO permissible limit. The high fluoride occurs both in the lake and groundwater. Preliminary petrographic studies show considerable fluoride in micas. The study is on-going and plans to present the relative abundances of fluoride in the lithology as the sources and the fluoride enrichment pathways of the groundwater within the Central Kenya rift.

  14. Brief Communication: Newly developing rift in Larsen C Ice Shelf presents significant risk to stability

    NASA Astrophysics Data System (ADS)

    Jansen, D.; Luckman, A. J.; Cook, A.; Bevan, S.; Kulessa, B.; Hubbard, B.; Holland, P. R.

    2015-06-01

    An established rift in the Larsen C Ice Shelf, formerly constrained by a suture zone containing marine ice, grew rapidly during 2014 and is likely in the near future to generate the largest calving event since the 1980s and result in a new minimum area for the ice shelf. Here we investigate the recent development of the rift, quantify the projected calving event and, using a numerical model, assess its likely impact on ice shelf stability. We find that the ice front is at risk of becoming unstable when the anticipated calving event occurs.

  15. Volcanic rocks and processes of the Mid-Atlantic Ridge rift valley near 36 ° 49? N

    USGS Publications Warehouse

    Hekinian, R.; Moore, J.G.; Bryan, W.B.

    1976-01-01

    The above relations indicate that the diverse lava types were erupted from a shallow, zoned magma chamber from fissures distributed over the width of the inner rift valley and elongate parallel to it. Differentiation was accomplished by cooling and crystallization of plagioclase, olivine, and clinopyroxene toward the margins of the chamber. The centrally located hills were built by the piling up of frequent eruption of mainly primitive lavas which also are the youngest flows. In contrast smaller and less frequent eruptions of more differentiated lavas were exposed on both sides of the rift valley axis.

  16. Deformation during the 1975-84 Krafla rifting crisis, NE Iceland, measured by optical image correlation

    NASA Astrophysics Data System (ADS)

    Hollingsworth, J.; Leprince, S.; Avouac, J.; Ayoub, F.

    2011-12-01

    In this study we combine results from optical image correlation of SPOT, KH-9 spy satellite and aerial photos, EDM data and high resolution topographic data to better constrain the 3D deformation associated with the 1975-84 Krafla rifting crisis, NE Iceland. Inversion of the various geodetic datasets yields new volumes for the amount of material injected into the crust during this rifting crisis. Correlation of aerial photos from 1957 and 1990 for the middle section of the 2 km-wide Krafla fissure swarm, along with DEM differencing of their respective 1957 and 1990 DEM's (extracted using photogrammetric techniques), provides constraints on the full 3D displacement field spanning the entire rifting period. Elastic dislocation modeling of this displacement data is then used to determine the geometry of faulting and diking in the crust. In contrast to leveling data from the northern end of the fissure swarm (Rubin, et al., 1988), we find that dikes do not extend into the upper 1-2 km, where extension is accommodated primarily by faulting in the fissure swarm. Dislocation modeling of a 4 m-wide dike injected between 2 km and 6 km in the crust produces a maximum surface strain which reaches the elastic yield limit for rock (derived from laboratory experiments of deformed granite) at two points spanning a 2 km-wide zone above the dike, and which corresponds with the location of the major rift-bounding faults of the Krafla fissure swarm. If dikes extend nearer to the surface, the predicted fissure zone width would be correspondingly smaller (consistent with the southern-end of the fissure swarm), while deeper diking produces a wider fissure swarm (consistent with the northern-end of the fissure swarm). The apparent northward increase in depth of diking is consistent with the flexural effects of rift-margin topography (Behn, et al., 2006); increased flexure in the south, where the Krafla caldera is located, results in the promotion of shallow diking, where as subdued topography in the north promotes deeper diking. Correlation of aerial photos between 1957 and 1976 (during the early stages of the rifting crisis) indicate 2 m extension, which is localized on faults along the northern end of the fissure swarm. No fault slip occurs in the central section of the fissure swarm during the same period, suggesting extension in the north during the early stages of rifting may result from dike injections sourced from the north (possibly offshore), rather than the Krafla caldera to the south. A similar variation in magmatic source region was also observed during the 2005-2009 Afar rifting crisis in East Africa.

  17. Late-Pliocene timing of Corinth (Greece) rift-margin fault migration

    NASA Astrophysics Data System (ADS)

    Leeder, M. R.; Mack, G. H.; Brasier, A. T.; Parrish, R. R.; McIntosh, W. C.; Andrews, J. E.; Duermeijer, C. E.

    2008-09-01

    Geochronological data for timing of Gulf of Corinth rift-margin fault migration is presented from the Megara basin, precursor to the active eastern Corinth rift. Fifteen sanidine separates from a thin tuff near the top of the sedimentary basin fill analysed by laser-fusion give apparent 40Ar- 39Ar ages from 2.78 to 4.53 Ma. A weighted mean age of 2.82 ± 0.06 Ma (at 2 ? error) for the three youngest aliquots (range of mean ages 2.78-2.86 Ma) is regarded as a maximum probable age for ash eruption. Together with magnetostratigraphy results, this age constrains timing of Megara basin abandonment and likely propagation of the active South Alkyonides coastal faults that presently bound the southern rift-margin to the late-Pliocene, ˜ 2.2 Ma. Initiation of the coastal faults caused uplift, fluvial incision and calcrete formation on geomorphic surfaces over the abandoned Megara basin fill. A petrocalcic laminar horizon from a supermature calcrete unconformably capping the fill gives a U-Pb age of 0.77 ± 0.08 Ma, dating a late stage in the history of calcrete development. The new age for initiation of active faulting in the eastern rift yields a low estimate of long-term mean extension along the active South Alkyonides coastal faults of ˜ 0.9-1.4 mm a - 1 , consistent with previous geological data. This rate is less than the 100-year GPS-determined geodetic extension rate of 6 ± 2.7 mm a - 1 measured along a ˜ 23°E meridional array just west of the Alkyonides gulf. It implies either that the geodetic rate declines rapidly over the ˜ 15 km distance into the eastern gulf or that the geodetic rate is unchanged but extra strain is taken up aseismically and/or along antithetic and unrecognised major intrabasinal faults. In the full graben of the central rift displacements are ˜ 3.5 km along the Xylocastro and Antikyra faults; the mean long term extension rate here since 2.2 Ma is ˜ 3.5 mm a - 1 , much less than current geodetic rates of ˜ 10 mm a - 1 . In the western rift, despite a lack of precise chronological data, geodetic and extension rates seem comparable. Overall, a late-Pliocene to early Pleistocene age is likely for initiation of the deep-marine Corinth rift, with no evidence for rift propagation, either eastwards or westwards. More generally, our results constrain timing of strain localisation and vertical axis rigid block rotation over the Aegean-Anatolian plate and demonstrate that intraplate deformation can be accomplished rapidly in response to regional-scale tectonic drivers.

  18. Ground motion and tectonics in the Terceira Island: Tectonomagmatic interactions in an oceanic rift (Terceira Rift, Azores Triple Junction)

    NASA Astrophysics Data System (ADS)

    Marques, F. O.; Catalão, J.; Hildenbrand, A.; Madureira, P.

    2015-05-01

    The interpretation of high-resolution topography/bathymetry, GPS and InSAR data, and detailed structural geology indicate that: (1) Terceira developed at the intersection of two major volcano-tectonic lineaments: WNW-ESE (local TR's direction) and NNW-SSE (submarine chain of volcanoes, here firstly recognised and coined Terceira Seamount Chain). (2) Terceira is affected by four main fault systems: the ca. N165° (normal faults dipping to east and west, mostly across the middle of the island), the N140° (normal faults mostly making up the Lajes Graben), the N110° (faults with oblique striations - normal dextral, making up the main volcanic lineament), and the more subtle N70° (the transform direction related to the Nubia/Eurasia plate boundary). Seismicity, GPS data and faults displacing the topography indicate that all systems are active. (3) The whole island is subsiding at a rate of ca. 5 mm/yr, as attested by both GPS and InSAR data, which is exceptionally high for the Azores islands. Common explanations like thermal contraction, or bending of the lithosphere, or magmatic processes, or collapse of the island under its own weight likely cannot justify the observed subsidence rate. The estimated average of TR's subsidence rate is also not enough, therefore we conclude that the measured 5 mm/yr can be a peak. (4) The fault geometry and kinematics are consistent with the current direction of maximum extension in the Azores (ca. N65°), and the rotation of Nubia relative to Eurasia. (5) Given that the NE shoulder of the Lajes Graben is moving upwards at 5 mm/yr and sits directly on the TR's NE shoulder, we conclude that the TR's shoulder is moving up, most likely as a result of the elastic rebound associated with rifting. The elastic rebound in both NE and SW TR's shoulders is most likely responsible for the observed ridge morphology all along the TR.

  19. Characterising East Antarctic Lithosphere and its Rift Systems using Gravity Inversion

    NASA Astrophysics Data System (ADS)

    Vaughan, Alan P. M.; Kusznir, Nick J.; Ferraccioli, Fausto; Leat, Phil T.; Jordan, Tom A. R. M.; Purucker, Michael E.; Golynsky, A. V. Sasha; Rogozhina, Irina

    2013-04-01

    Since the International Geophysical Year (1957), a view has prevailed that East Antarctica has a relatively homogeneous lithospheric structure, consisting of a craton-like mosaic of Precambrian terranes, stable since the Pan-African orogeny ~500 million years ago (e.g. Ferracioli et al. 2011). Recent recognition of a continental-scale rift system cutting the East Antarctic interior has crystallised an alternative view of much more recent geological activity with important implications. The newly defined East Antarctic Rift System (EARS) (Ferraccioli et al. 2011) appears to extend from at least the South Pole to the continental margin at the Lambert Rift, a distance of 2500 km. This is comparable in scale to the well-studied East African rift system. New analysis of RadarSat data by Golynsky & Golynsky (2009) indicates that further rift zones may form widely distributed extension zones within the continent. A pilot study (Vaughan et al. 2012), using a newly developed gravity inversion technique (Chappell & Kusznir 2008) with existing public domain satellite data, shows distinct crustal thickness provinces with overall high average thickness separated by thinner, possibly rifted, crust. Understanding the nature of crustal thickness in East Antarctica is critical because: 1) this is poorly known along the ocean-continent transition, but is necessary to improve the plate reconstruction fit between Antarctica, Australia and India in Gondwana, which will also better define how and when these continents separated; 2) lateral variation in crustal thickness can be used to test supercontinent reconstructions and assess the effects of crystalline basement architecture and mechanical properties on rifting; 3) rift zone trajectories through East Antarctica will define the geometry of zones of crustal and lithospheric thinning at plate-scale; 4) it is not clear why or when the crust of East Antarctica became so thick and elevated, but knowing this can be used to test models of Cenozoic ice sheet formation and stability. References Chappell, A.R. & Kusznir, N.J. 2008. Three-dimensional gravity inversion for Moho depth at rifted continental margins incorporating a lithosphere thermal gravity anomaly correction. Geophysical Journal International, 174 (1), 1-13. Ferraccioli, F., Finn, C.A., Jordan, T.A., Bell, R.E., Anderson, L.M. & Damaske, D. 2011. East Antarctic rifting triggers uplift of the Gamburtsev Mountains Nature, 479, 388-392. Golynsky, A.V. & Golynsky, D.A. 2009. Rifts in the tectonic structure of East Antarctica (in Russian). Russian Earth Science Research in Antarctica, 2, 132-162. Vaughan, A.P.M., Kusznir, N.J., Ferraccioli, F. & Jordan, T.A.R.M. 2012. Regional heat-flow prediction for Antarctica using gravity inversion mapping of crustal thickness and lithosphere thinning. Geophysical Research Abstracts, 14, EGU2012-8095.

  20. The development of the East African Rift system in north-central Kenya

    NASA Astrophysics Data System (ADS)

    Hackman, B. D.; Charsley, T. J.; Key, R. M.; Wilkinson, A. F.

    1990-11-01

    Between 1980 and 1986 geological surveying to produce maps on a scale of 1:250,000 was completed over an area of over 100,000 km 2 in north-central Kenya, bounded by the Equator, the Ethiopian border and longitudes 36° and 38 °E. The Gregory Rift, much of which has the structure of an asymmetric half-graben, is the most prominent component of the Cenozoic multiple rift system which extends up to 200 km to the east and for about 100 km to the west, forming the Kenya dome. On the eastern shoulder and fringes two en echelon arrays of late Tertiary to Quaternary multicentre shields can be recognized: to the south is the Aberdares-Mount Kenya-Nyambeni Range chain and, to the north the clusters of Mount Kulal, Asie, Huri Hills and Marsabit, with plateau lavas and fissure vents south of Marsabit in the Laisamis area. The Gregory Rift terminates at the southern end of Lake Turkana. Further north the rift system splays: the arcuate Kinu Sogo fault zone forms an offset link with the central Ethiopian Rift system. In the rifts of north-central Kenya volcanism, sedimentation and extensional tectonics commenced and have been continuous since the late Oligocene. Throughout this period the Elgeyo Fault acted as a major bounding fault. A comparative study of the northern and eastern fringes of the Kenya dome with the axial graben reinforces the impression of regional E-W asymmetry. Deviations from the essential N-trend of the Gregory Rift reflect structural weaknesses in the underlying Proterozoic basement, the Mozambique Orogenic Belt: thus south of Lake Baringo the swing to the southeast parallels the axes of the ca. 620 Ma phase folds. Secondary faults associated with this flexure have created a "shark tooth" array, an expression of en echelon offsets of the eastern margin of the Gregory Rift in a transtensional stress regime: hinge zones where major faults intersect on the eastern shoulder feature intense box faulting and ramp structures which have counterparts in the rift system in southern Ethiopia. The NE- and ENE-trending fissures of the eastern fringes of the Kenya dome, notably in the Meru-Nyambeni areaand in the Huri and Marsabit shields, parallel late orogenic structures dated at around 580-480 Ma. Alkaline trends characterize the petrochemistry of the Cenozoic volcanics: In the Gregory Rift, voluminous Miocene alkali basalts, associated with hawaiite/mugearite lavas, define a trend culminating in the Miocene flood phonolites of the eastern shoulderand in the trachyphonolites, trachytes and peralkaline rhyolites, with associated pyroclastics, in central volcanoes such as Korosi, Paka and Silali. Such trends may manifest in the products of a single volcanic centre, also regionally on a broadly cyclic basis. On the eastern flanks of the Kenya dome the flood phonolites are less evident, but the same alkaline trends dominate the lava sequences, supplemented by nephelinitic extrusives in parts of the Nyambeni Range and in the Laisamis area. Results from recent seismicity surveys in the Laisamis area indicate that crustal extension may be currently active on the eastern fringes of the Kenya dome, but manifest at greater depths than in the axial Gregory Rift-Lake Turkana zone: a correlation is suggested with the ultra-alkaline petrochemistry of some of the eastern multicentre shields.

  1. Middle to late cenozoic magmatism of the southeastern Colorado plateau and central Rio Grande rift (New Mexico and Arizona, U.S.A.) : a model for continental rifting

    USGS Publications Warehouse

    Baldridge, W.S.; Perry, F.V.; Vaniman, D.T.; Nealey, L.D.; Leavy, B.D.; Laughlin, A.W.; Kyle, P.; Bartov, Y.; Steinitz, G.; Gladney, E.S.

    1991-01-01

    The region of the present Rio Grande rift and southeastern Colorado Plateau underwent lithospheric extension during middle to late Cenozoic deformation affecting the entire southwestern U.S. Lithospheric mantle was disrupted, and in many regions displaced or replaced by asthenospheric mantle at depths from which basaltic magmas were derived and erupted to the surface. Study of the igneous rocks erupted or intruded during this deformation yields insights into processes of magmatism associated with extension of continental lithosphere. Magmatic rocks associated with an early (late Oligocene-early Miocene) ductile phase of extension are dominantly basaltic andesites and related, calc-alkaline intermediate to silicic derivative rocks. Mafic magmas were probably derived from isotopically "enriched" lithospheric mantle. Igneous rocks associated with a later (middle Miocene-Holocene), more brittle phase of extension include widespread basaltic rocks and localized central volcanoes of intermediate to silicic composition. Isotopic compositions of mafic rocks, which include both tholeiitic and alkalic basalts, correlate strongly with tectonic setting and lithospheric structure. Basalts erupted in areas of greatest crustal extension, such as the central and southern rift and Basin and Range province, were derived from isotopically "depleted" (correlated with "asthenospheric") mantle. Also, isotopic compositions of Pliocene to Holocene basalts are slightly more depleted than those of Miocene basalts, suggesting that subcrustal lithospheric mantle was thinned during late Miocene extension. Intermediate rocks of the central volcanoes formed by a complex combination of processes, probably dominated by fractional crystallization and by assimilation of upper and lower crust in isolated, small magma chambers. The petrologic, geochemical, and isotopic data are compatible with a model, derived first from geophysical data, whereby lithosphere is thinned beneath the central rift and southeastern Colorado Plateau, with greatest thinning centered beneath the axis of the rift. A lithospheric model involving uniform-sense simple shear does not appear compatible with the data as presently understood. ?? 1991.

  2. Rheological variations across an active rift system -- results from lithosphere-scale 3D gravity and thermal models of the Kenya Rift

    NASA Astrophysics Data System (ADS)

    Meeßen, Christian; Sippel, Judith; Cacace, Mauro; Scheck-Wenderoth, Magdalena; Fishwick, Stewart; Heine, Christian; Strecker, Manfred R.

    2015-04-01

    Due to its tectono-volcanic activity and economic (geothermal and petroleum) potential, the eastern branch of the East African Rift System (EARS) is one of the best studied extensional systems worldwide and an important natural laboratory for the development of geodynamic concepts on rifting and nascent continental break-up. The Kenya Rift, an integral part of the eastern branch of the EARS, has formed in the area of weak Proterozoic crust of the Mozambique mobile belt adjacent to the rheologically stronger Archean Tanzania craton. To assess the variations in lithospheric strength between different tectonic domains and their influence on the tectonic evolution of the region, we developed a set of structural, density, thermal and rheological 3D models. For these models we integrated multi-disciplinary information, such as published geological field data, sediment thicknesses, well information, existing structural models, seismic refraction and reflection data, seismic tomography, gravity and heat-flow data. Our main approach focused on combined 3D isostatic and gravity modelling. The resulting lithosphere-scale 3D density model provides new insights into the depth distribution of the crust-mantle boundary and thickness variations of different crustal density domains. The latter further facilitate interpretations of variations of lithologies and related physical rock properties. By considering lithology-dependent heat production and thermal conductivity, we calculate the conductive thermal field across the region of the greater Kenya Rift. Finally, the assessed variations in lithology and temperature allow deriving differences in the integrated strength of the lithosphere across the different tectonic domains.

  3. Deformation modelling of the 2014 Bárðarbunga rifting event in Iceland

    NASA Astrophysics Data System (ADS)

    Hooper, Andrew; Sigmundsson, Freysteinn; Hreinsdóttir, Sigrún; Heimisson, Elías; Ófeigsson, Benedikt; Dumont, Stéphanie; Parks, Michelle; Spaans, Karsten; Drouin, Vincent; Árnadóttir, Thóra; Vogfjörd, Kristín; Jónsdóttir, Kristín; Fridriksdóttir, Hildur; Hensch, Martin

    2015-04-01

    Between 16 August and 31 August 2014 a dike propagated from Bárðarbunga caldera, which culminated in an eruption at Holuhraun that is still ongoing at the time of writing. Previous models of other rifting events indicate either lateral dyke growth away from a feeding source, with propagation rates decreasing as the dyke lengthens, or magma flowing vertically into dykes from an underlying source, with the role of topography on the evolution of lateral dykes not clear. Our modelling shows how the segmented dyke intrusion in the Bárðarbunga volcanic system grew laterally for more than 45 kilometres at a variable rate, with topography influencing the direction of propagation. Barriers at the ends of each segment were overcome by the build-up of pressure in the dyke end; then a new segment formed and dyke lengthening temporarily peaked. The dyke propagation path is not simple, comprising many segments with differing orientations. We modelled the dyke propagation using deformation data from InSAR and GPS. Initial modelling of the dyke, with no a priori constraints on position, strike or dip, show the deformation data require the dyke to be approximately vertical and line up with the seismicity. We therefore fixed the dip to be vertical and the lateral position of the dyke to coincide with the earthquake locations. We modelled the dyke as a series of rectangular patches and estimated the opening and slip on each patch for each day between 16 August and 6 September. The results suggest that most of the magma injected into the dyke is shallower than the seismicity, which mostly spans the depth range from 5 to 8 km below sea level. Where constraints from InSAR and GPS are good, significant opening is all shallower than 5 km, and opening is up to 6 m. The total volume intruded into the dyke by 28 August was 0.48-0.51 km3. We also modelled the expected propagation direction of the dyke considering the regional stress field and the spatially-variable overburden. We find that our model agrees well with the actual propagation path as indicated by the seismicity.

  4. Tectonomagmatic evolution of the final stages of rifting along the deep conjugate Australian-Antarctic magma-poor rifted margins: Constraints from seismic observations

    NASA Astrophysics Data System (ADS)

    Gillard, Morgane; Autin, Julia; Manatschal, Gianreto; Sauter, Daniel; Munschy, Marc; Schaming, Marc

    2015-04-01

    The processes related to hyperextension, exhumed mantle domains, lithospheric breakup, and formation of first unequivocal oceanic crust at magma-poor rifted margins are yet poorly understood. In this paper, we try to bring new constraints and new ideas about these latest deformation stages by studying the most distal Australian-Antarctic rifted margins. We propose a new interpretation, linking the sedimentary architectures to the nature and type of basement units, including hyperextended crust, exhumed mantle, embryonic, and steady state oceanic crusts. One major implication of our study is that terms like prerift, synrift, and postrift cannot be used in such polyphase settings, which also invalidates the concept of breakup unconformity. Integration and correlation of all available data, particular seismic and potential field data, allows us to propose a new model to explain the evolution of magma-poor distal rifted margins involving multiple and complex detachment systems. We propose that lithospheric breakup occurs after a phase of proto-oceanic crust formation, associated with a substantial magma supply. First steady state oceanic crust may therefore not have been emplaced before ~53.3 Ma corresponding to magnetic anomaly C24. Observations of magma amount and its distribution along the margins highlight a close magma-fault relationship during the development of these margins.

  5. The transition from craton to rift in south eastern Africa: Preliminary surface wave tomography from the MOZambique Rift Tomography (MOZART) project

    NASA Astrophysics Data System (ADS)

    Fishwick, S.; Domingues, A.; Fonseca, J. F.; Chamussa, J. R.; Helffrich, G. R.; Grobbelaar, M.

    2012-12-01

    We present preliminary results from the MOZART (Mozambique Rift Tomography) experiment in the context of the transition from the old and cold lithosphere associated with the Kalahari Craton, to the continental lithosphere associated with the southern tip of the East African Rift System (EARS) in Mozambique. The initial stage of surface wave data recorded on the network of 30 broadband seismic stations deployed in 2011 are incorporated into an earlier continental scale tomographic model of Africa (Fishwick, 2010). These data provide a significant increase in potential resolution of upper mantle velocities, in a region where there has previously been little knowledge of the detailed lithospheric structure. Furthermore they allow a better comparison with the velocities observed beneath the Kaapvaal Craton where data from the dense network of seismometers deployed in the earlier Southern Africa Seismic Experiment (SASE) have already been included in the regional tomography. From the new tomographic models we investigate the lateral gradients in seismic velocity at upper mantle depths of 75-250km in order to place better constraints on the thermal and/or chemical transitions from the craton to the rift. In order to test the reliability of these estimates of the velocity gradient we compare different parameterisations and regularisations, but also investigate to what degree the addition of new data to the tomographic model alters the estimated velocities. The regional tomographic model produced will provide a useful new reference model for more detailed inter-station and array tomography when project MOZART is completed in mid-2013.

  6. The formation of the south-eastern part of the Dniepr Donets Basin: 2-D forward and reverse modelling taking into account post-rift redeposition of syn-rift salt

    NASA Astrophysics Data System (ADS)

    Stovba, S. M.; Maystrenko, Yu. P.; Stephenson, R. A.; Kusznir, N. J.

    2003-02-01

    Forward and reverse modelling of structure and stratigraphy has been used to investigate the syn-rift (Late Devonian) and early post-rift (Carboniferous) evolution of the south-eastern part of the Dniepr-Donets Basin (DDB). Modelling was carried out with and without taking into consideration the withdrawal and surface extrusion of Devonian salt during the formation of salt diapirs. The great thickness of Carboniferous deposits can be explained by the superimposed actions of three processes: post-rift thermal subsidence, withdrawal of Devonian salt from the mother layer during phases of salt diapir activity, and regional subsidence of the East European Platform. The effects of other tectonic and/or non-tectonic processes are not required. Forward syn-rift modelling using the flexural cantilever model of sedimentary basin formation predicts the total syn-rift extension across the southeastern DDB to be approximately 65 km with a maximum ? stretching factor of 2.4. Shallowing of the Moho during the syn-rift phase is estimated to be 15 km. The present-day Moho, after thermal subsidence and basin fill, is predicted to be 4-6 km shallower than surrounding regions. In the axial zone of the south-eastern DDB the thickness of the Devonian syn-rift sequence may have reached 7.5 km by the end of the rift stage. This is 3-3.5 km more than at present. The thicknesses reduction is due to the outflow of Devonian salt during post-rift periods of halokinetic activity in the early Visean, the middle Serpukhovian, and in the Early Permian. The withdrawal of salt from the mother layer produced additional accommodation space and up to 1.5-1.7 km of the total eventual thickness of the Carboniferous sedimentary succession can be explained as a result of this.

  7. Diachronous Growth of Normal Fault Systems in Multiphase Rift Basins: Structural Evolution of the East Shetland Basin, Northern North Sea

    NASA Astrophysics Data System (ADS)

    Claringbould, Johan S.; Bell, Rebecca E.; A-L. Jackson, Christopher; Gawthorpe, Robert L.; Odinsen, Tore

    2015-04-01

    Our ability to determine the structural evolution and interaction of fault systems (kinematically linked group of faults that are in the km to 10s of km scale) within a rift basin is typically limited by the spatial extent and temporal resolution of the available data and methods used. Physical and numerical models provide predictions on how fault systems nucleate, grow and interact, but these models need to be tested with natural examples. Although field studies and individual 3D seismic surveys can provide a detailed structural evolution of individual fault systems, they are often spatially limited and cannot be used examine the interaction of fault systems throughout the entire basin. In contrast, regional subsurface studies, commonly conducted on widely spaced 2D seismic surveys, are able to capture the general structural evolution of a rift basin, but lack the spatial and temporal detail. Moreover, these studies typically describe the structural evolution of rifts as comprising multiple discrete tectonic stages (i.e. pre-, syn- and post-rift). This simplified approach does not, however, consider that the timing of activity can be strongly diachronous along and between faults that form part of a kinematically linked system within a rift basin. This study focuses on the East Shetland Basin (ESB), a multiphase rift basin located on the western margin of the North Viking Graben, northern North Sea. Most previous studies suggest the basin evolved in response to two discrete phases of extension in the Permian-Triassic and Middle-Late Jurassic, with the overall geometry of the latter rift to be the result of selective reactivation of faults associated with the former rift. Gradually eastwards thickening intra-rift strata (deposited between two rift phases) that form wedges between and within fault blocks have led to two strongly contrasting tectonic interpretations: (i) Early-Middle Jurassic differential thermal subsidence after Permian-Triassic rifting; or (ii) Triassic syn-rift activity on west-dipping faults. Our analysis of regional 2D and basin-wide 3D 'mega-merge' seismic reflection data calibrated by wells allow us to re-evaluate the pre-Triassic-to-Cretaceous structural evolution of the ESB. Our results suggest that pre-Triassic extension was accommodated by diachronous growth of NW-SE-to-NE-SW-striking faults that dipped either to the east or the west. In the NW of the ESB, Triassic syn-rift deposits are observed along large (>20 km long), NE-SW-striking faults. Elsewhere in the basin, post-rift deposits gradually thicken eastward, suggesting differential Triassic post-rift thermal subsidence with its axis to the east of the ESB. Subsequent Early-to-Middle Jurassic deposits thicken eastward across large N-S striking faults, suggesting syn-depositional fault growth. Our observations suggest that, rather than forming in response to discrete periods of extension separated by periods of tectonic quiescence, the ESB witnessed diachronous fault system evolution with faults showing polyphase activity, cross-cutting relationships, and protracted growth from the pre-Triassic to Middle-Late Jurassic. The results of this work reveal the complex structural evolution of rifts, highlight the power of 3D mega-merge seismic reflection data, and demonstrate that the conventional rift package nomenclature of pre-, syn-, and post-rift is difficult to apply at the basin-scale.

  8. NEAR REAL-TIME SURVEILLANCE FOR RIFT VALLEY FEVER IN AFRICA AND THE ARABIAN PENINSULA

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Rift Valley fever (RVF) virus causes severe disease, abortion, and death in domestic animals in Africa and Arabia. Humans are infected by both Aedes and Culex species mosquitoes, which maintain enzootic and epizootic transmission, respectively, or through exposure to infected animal tissue. Outbrea...

  9. Evaluation of lamb and calf responses to Rift Valley fever MP-12 vaccination

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Rift Valley fever (RVF) is an important viral disease of animals and humans in Africa and the Middle East that is transmitted by mosquitoes. The disease is of concern to international agricultural and public health communities. The RVF MP-12 strain has been the most safety tested attenuated vaccine ...

  10. Magmatism on rift flanks: insights from Ambient-Noise Phase-velocity in Afar region

    NASA Astrophysics Data System (ADS)

    Korostelev, Félicie; Weemstra, Cornelis; Leroy, Sylvie; Boschi, Lapo; Ren, Yong; Ahmed, Abdulhakim; Keir, Derek; Stuart, Graham W.; Rolandone, Frédérique; Khanbari, Khaled; Hammond, James O. S.; Kendall, J. Michael; Doubre, Cécile; Ganad, Ismail Al

    2015-04-01

    During the breakup of continent in the presence of magma, strain is commonly assumed to initially occur by border faulting, and progressively migrate in space and time towards axial magma intrusion. Magmatic processes near the rift flanks are commonly ignored. We present phase-velocity maps of the crust and uppermost mantle of the conjugate margins of the southern Red Sea (Afar and Yemen) using ambient noise tomography to constrain crustal modification during breakup. Our images show that the low seismic velocities characterize not only upper crust beneath the axial volcanic systems, but also both upper and lower crust beneath rift flanks where ongoing volcanism and hydrothermal activity occurs at the surface. The results show that magmatic modification of the crust beneath rift flanks likely occurs for a protracted period of time during the breakup process, and may persist through to early seafloor spreading. Since ongoing flank magmatism during breakup impacts the thermal evolution of the lithosphere, it has implications for the subsidence history of the rift.

  11. Spreading process of the northern Mariana Trough: Rifting-spreading transition at 22N

    E-print Network

    Utrecht, Universiteit

    southward from incipient rifting to seafloor spreading within this region. This study aims to clarify of seafloor spreading after the transition. The new data set includes swath bathymetry with side- scan images is proved by seafloor-spreading fabric in the bathymetry, clear magnetic lineations, and the bull

  12. Evolution of magma-poor continental margins from rifting to seafloor spreading.

    PubMed

    Whitmarsh, R B; Manatschal, G; Minshull, T A

    2001-09-13

    The rifting of continents involves faulting (tectonism) and magmatism, which reflect the strain-rate and temperature dependent processes of solid-state deformation and decompression melting within the Earth. Most models of this rifting have treated tectonism and magmatism separately, and few numerical simulations have attempted to include continental break-up and melting, let alone describe how continental rifting evolves into seafloor spreading. Models of this evolution conventionally juxtapose continental and oceanic crust. Here we present observations that support the existence of a zone of exhumed continental mantle, several tens of kilometres wide, between oceanic and continental crust on continental margins where magma-poor rifting has taken place. We present geophysical and geological observations from the west Iberia margin, and geological mapping of margins of the former Tethys ocean now exposed in the Alps. We use these complementary findings to propose a conceptual model that focuses on the final stage of continental extension and break-up, and the creation of a zone of exhumed continental mantle that evolves oceanward into seafloor spreading. We conclude that the evolving stress and thermal fields are constrained by a rising and narrowing ridge of asthenospheric mantle, and that magmatism and rates of extension systematically increase oceanward. PMID:11557977

  13. The northwest-trending Euphrates graben sys-tem is an aborted intracontinental rift of Late

    E-print Network

    rift that also holds signifi- cant petroleum reserves, but a comprehensive anal- ysis of its history, from a possible Proterozoic suture 1173 ©Copyright 1998. The American Association of Petroleum Petroleum Company, Ministry of Petroleum and Mineral Resources, Damascus, Syria. Seismic and well data

  14. CURRENT ISSUES AND CONCERNS REGARDING RIFT VALLEY FEVER, AN EMERGING VIRUS THREAT

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Rift Valley fever (RVF) virus is a mosquito-borne zoonotic hemorrhagic disease that causes 100% abortions in cattle, sheep, and goats and is often fatal to young animals. Though currently confined mainly to Africa this disease could be introduced into the U.S. and spread via mosquitoes at least as ...

  15. Potential for North American mosquitoes to transmit Rift Valley fever virus

    Technology Transfer Automated Retrieval System (TEKTRAN)

    The recent outbreaks of disease caused by Rift Valley fever virus (RVFV) in Kenya, Mauritania, Yemen, Tanzania, Somalia, and Madagascar indicate the potential for RVFV to cause severe disease in both humans and domestic animals and its potential to be introduced into new areas, possibly even North A...

  16. Wilson cycles, tectonic inheritance, and rifting of the North American Gulf of Mexico continental margin

    E-print Network

    Huerta, Audrey D.

    Wilson cycles, tectonic inheritance, and rifting of the North American Gulf of Mexico continental during opening of the Gulf of Mexico. Unlike the Atlantic margins, where Wilson cycles were first recognized, breakup in the Gulf of Mexico did not initially focus within the orogen, but was instead

  17. Potential for mosquitoes (Diptera: Culicidae) from Florida to transmit rift valley fever virus

    Technology Transfer Automated Retrieval System (TEKTRAN)

    We evaluated 8 species of mosquitoes collected in Florida to determine which of these should be targeted for control should Rift Valley fever virus (RVFV) be detected in North America. Female mosquitoes that had fed on adult hamsters inoculated with RVFV were incubated for 7-21 d at 26°C, allowed to...

  18. Rift Valley fever virus-infected mosquito ova and associated pathology: possible implications for endemic maintenance

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Background: Endemic/enzootic maintenance mechanisms like vertical transmission, pathogen passage from infected adults to their offspring, are central in the epidemiology of zoonotic pathogens. In Kenya, Rift Valley fever virus (RVFV) may be maintained by vertical transmission in ground-pool mosquit...

  19. Current Status on the Development of Operator Safe Diagnostic Tools for Rift Valley Fever

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Rift Valley fever (RVF) is a disease of animals and humans that occurs in Africa and the Arabian Peninsula. It is caused by a Phlebovirus in the family Bunyaviridae. Mosquito-borne epidemics occur during years of unusually heavy rainfall. Domestic cattle, sheep and goats are highly susceptible to...

  20. Vector Competence of Selected African Mosquito (Diptera: Culicidae) Species for Rift Valley Fever Virus

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Outbreaks of Rift Valley fever (RVF) in Egypt, Yemen, and Saudi Arabia have indicated the potential for this disease to spread from its enzootic areas in sub-Saharan Africa. Because little is known about the potential for most African mosquito species to transmit RVF virus (RVFV), we conducted stud...

  1. Potential for North American Mosquitoes to Transmit Rift Valley Fever Virus

    Technology Transfer Automated Retrieval System (TEKTRAN)

    The recent outbreaks of disease caused by Rift Valley fever virus (RVFV) in Kenya, Mauritania, Yemen, Tanzania, Somalia, and Madagascar indicate the potential for RVFV to cause severe disease in both humans and domestic animals and its potential to be introduced into new areas, including North Ameri...

  2. Factors Affecting the Ability of American Mosquitoes to Transmit Rift Valley Fever Virus

    Technology Transfer Automated Retrieval System (TEKTRAN)

    The recent outbreaks of disease caused by Rift Valley fever virus (RVFV) in Kenya, Mauritania, Yemen, Tanzania, Somalia, and Madagascar indicate the potential for RVFV to cause severe disease in both humans and domestic animals and its potential to be introduced into new areas, including North Ameri...

  3. USDA, ARS, ABDRL Research on Countermeasures for Rift Valley Fever Virus

    Technology Transfer Automated Retrieval System (TEKTRAN)

    The United State Department of Agriculture, Agriculture Research Service has recently established research program to address countermeasures for of Rift Valley fever (RVF) virus (RVFV). The recent outbreak in Kenya, Tanzania and Somalia demonstrates the impact this virus can have on human and live...

  4. On rates and styles of late volcanism and rifting on Venus Alexander T. Basilevsky

    E-print Network

    Head III, James William

    On rates and styles of late volcanism and rifting on Venus Alexander T. Basilevsky Vernadsky the formation of regional plains on Venus. We analyzed the age relations of all known impact craters !30 km of such a transition occurring at different times in different areas of Venus. INDEX TERMS: 6295 Planetology: Solar

  5. Stratigraphic evidence for the Siberia-Laurentia connection and Early Cambrian rifting

    NASA Astrophysics Data System (ADS)

    Pelechaty, Shane M.

    1996-08-01

    Vendian to Cambrian age sedimentary rocks of the northern Siberian craton record Early Cambrian rifting from ˜ 543 to 530 Ma and the onset of regional thermal subsidence in early Tommotian time. A similar tectonic history in the Franklinian basin of northern Canada and Greenland supports the possibility that both basins formed conjugate margins. This correlation constrains both the configuration of the Siberia-Laurentia connection, also supported by paleomagnetic and paleoclimatic data from Siberia, and timing of continental breakup, which is further supported by regional trilobite biostratigraphy. Prior to breakup, Siberia and Laurentia formed a coherent continent that rifted from a western landmass (Australia Antarctica India South China) at ˜ 720 Ma, forming a continuous passive margin along western Siberia-Laurentia. Nearly orthogonal to this margin, the 723 Ma Natkusiak volcanic rocks and Franklin dike swarm in northern Canada are suggested to represent a failed rift that extended into Siberia-Laurentia. Subsequent Early Cambrian separation of Siberia-Laurentia was possibly influenced by crustal heterogeneity created by the failed rift.

  6. Chapter 34: Geology and petroleum potential of the rifted margins of the Canada Basin

    USGS Publications Warehouse

    Houseknecht, D.W.; Bird, K.J.

    2011-01-01

    Three sides of the Canada Basin are bordered by high-standing, conjugate rift shoulders of the Chukchi Borderland, Alaska and Canada. The Alaska and Canada margins are mantled with thick, growth-faulted sediment prisms, and the Chukchi Borderland contains only a thin veneer of sediment. The rift-margin strata of Alaska and Canada reflect the tectonics and sediment dispersal systems of adjacent continental regions whereas the Chukchi Borderland was tectonically isolated from these sediment dispersal systems. Along the eastern Alaska-southern Canada margin, termed herein the 'Canning-Mackenzie deformed margin', the rifted margin is deformed by ongoing Brooks Range tectonism. Additional contractional structures occur in a gravity fold belt that may be present along the entire Alaska and Canada margins of the Canada Basin. Source-rock data inboard of the rift shoulders and regional palaeogeographic reconstructions suggest three potential source-rock intervals: Lower Cretaceous (Hauterivian-Albian), Upper Cretaceous (mostly Turonian) and Lower Palaeogene. Burial history modelling indicates favourable timing for generation from all three intervals beneath the Alaska and Canada passive margins, and an active petroleum system has been documented in the Canning-Mackenzie deformed margin. Assessment of undiscovered petroleum resources indicates the greatest potential in the Canning-Mackenzie deformed margin and significant potential in the Canada and Alaska passive margins. ?? 2011 The Geological Society of London.

  7. Development of Enzyme-Linked Immunosorbent Assays Using Expressed Proteins of Rift Valley Fever Virus

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Rift Valley fever virus (RVFV) is a member of the genus Phlebovirus, family Bunyaviridae that can cause severe disease in both humans and animals. The disease is enzootic in sub-Saharan Africa and RVFV epidemics/epizootics occur periodically, primarily in eastern and southern Africa. Since the virus...

  8. Development of a Rift Valley fever virus viremia challenge model in sheep and goats

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Rift valley fever virus (RVFV), a member of the family Bunyaviridae, causes severe to fatal disease in newborn ruminants, as well as abortions in pregnant animals; both preventable by vaccination. Availability of a challenge model is a pre-requisite for vaccine efficacy trials. Several modes of ino...

  9. Mesozoic rift basins in western desert of Egypt, their southern extension and impact on future exploration

    SciTech Connect

    Taha, M.A. )

    1988-08-01

    Rift basins are a primary target of exploration in east, central, and west Africa. These intracratonic rift basins range in age from the Triassic to the Neogene and are filled with lagoonal-lacustrine sand-shale sequences. Several rift basins may be present in the Western Desert of Egypt. In the northeastern African platform, the Mesozoic Tethyan strand lines were previously interpreted to have limited southern extension onto the continent. This concept, based upon a relatively limited amount of subsurface data, has directed and focused the exploration for oil and gas to the northernmost 120 km of the Western Desert of Egypt. Recent well and geophysical data indicate a southerly extension of mesozoic rift basins several hundred kilometers inland from the Mediterranean Sea. Shushan/Faghur and Abu Gharadig/Bahrein basins may represent subparallel Mesozoic basins, trending northeast-southwest. Marine Oxfordian-Kimmeridgian sediments were recently reported from wells drilled approximately 500 km south of the present-day Mediterranean shoreline. The link of these basins with the Sirte basin to the southwest in Libya is not well understood. Exploration is needed to evaluate the hydrocarbon potential of such basins.

  10. SATELLITE-BASED RIFT VALLEY FEVER FORECASTS PREDICT A LARGE YELLOW FEVER EPIDEMIC IN SUDAN, 2005

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Sustained, heavy East African rainfall is associated with Aedes spp. breeding in Kenyan grasslands and Rift Valley Fever (RVF) epidemics. Validated RVF forecasts use satellite measurements of vegetation greenness (which increases after heavy rains) and other eco-climate indicators. These models may ...

  11. One Health –One World Approaches to Detection and Control of Rift Valley Fever

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Rift Valley fever (RVF) is a disease of animals and humans that occurs in Africa and the Arabian Peninsula. A Phlebovirus in the family Bunyaviridae causes the disease that is transmitted by mosquitoes. Epidemics occur during years of unusually heavy rainfall that assessment models are being develop...

  12. Insights into the mode of the South Georgia rift extension in eastern Georgia, USA

    NASA Astrophysics Data System (ADS)

    Clendenin, C. W.

    2013-11-01

    The South Georgia rift (SGR) lies oblique to the east coast margin of North America and across the Alleghenian suture between Laurentia and Africa in southern Georgia. Regionally, the SGR can be divided into a southwest compartment and a northeast compartment across the Jacksonville structure that is located in the vicinity of that suture. Analytical and numerical models are used to characterize the mode of rifting in the northeast compartment. Borehole, COCORP seismic, and regional geophysical information from the compartment, that were used previously to infer the geometry of the basin, are reassessed with the use of those models to analyze the lithospheric conditions influencing Triassic extension. This approach led to the interpretation of core complex mode extension and to the proposal of a model of progressive rifting. The model shows how the Riddleville and Main SGR basins are associated and how changes in structural style of those two basins resulted from changing lithospheric conditions during extension. The core complex model also indicated that extension was influenced by distributed deformation of a younger, warmer, and less stable lithosphere adjacent to the Permian suture; whereas extension in other east coast rifts that lie subparallel to structural fabric was probably localized by preexisting zones of weakness.

  13. Fission track analysis, rift shoulder uplift, and tectonic modeling of the Norwegian Continental Margin

    SciTech Connect

    Andriessen, P.; Van Der Beek, P.; Cloetingh, S.; Rohrman, M. )

    1993-09-01

    Apatite fission track analysis from southern Norway and Sweden, across the Permian Carboniferous Oslo rift, are presented and discussed in relation to different rifting scenarios. Vertical and horizontal apatite fission tack profiles in middle and southern Norway unravel the post-Carboniferous history of the Fennoscandian shield. Fission track apatite ages range from 240 Ma in the south to 160 Ma in the north, and according to spontaneous fission track length measurements, they must be interpreted as mixed ages, indicating minor amounts of Paleozoic-Mesozoic sedimentary cover. Apatite fission track length and age modeling suggest rapid cooling and uplift in the Tertiary for the southernmost part of Norway, suggesting a differential uplift of the basement. the obtained data are important for the reconstruction of burial and thermal histories of Cenozoic sedimentary basins of the Norwegian continental margin in the northern North Sea, where diverse rifting events, intraplate stress regimes, and inversion tectonics are involved. Fission track analysis puts constraints on tectonic modeling of uplift of rift flanks and the Norwegian continental margin and yields information for these assessment of hydrocarbon potentials of the sedimentary basins.

  14. 3D Graph Visualization with the Oculus Rift Virtual Graph Reality

    E-print Network

    Wismath, Stephen

    3D Graph Visualization with the Oculus Rift Virtual Graph Reality Farshad Barahimi, Stephen Wismath regarding three- dimensional (3D) representations of graphs. However, the actual usefulness of such 3D reality environment such as a CAVE, or · printed as a physical model with a 3D printer. Early studies

  15. INTRODUCTION The Dead Sea rift (Fig. 1) is a left-lateral

    E-print Network

    Gvirtzman, Haim

    INTRODUCTION The Dead Sea rift (Fig. 1) is a left-lateral strike-slip transform, separating several en echelon rhomb-shaped grabens, one of which contains the Dead Sea and the lowest land- surface elevations on Earth (approximately ­410 m). The Dead Sea basin evolved since 15 Ma. The basin fill has

  16. Brine freshwater interplay and effects on the evolution of saline lakes: The Dead Sea Rift

    E-print Network

    Torfstein, Adi

    Brine ­ freshwater interplay and effects on the evolution of saline lakes: The Dead Sea Rift-Plank-Institute f¨ur Chemie, Mainz, Germany. As an undergraduate student I was introduced to the wonders of the Dead-lacustrine exposed records of the Quaternary can be found in the Dead Sea Basin (DSB), Israel. The size

  17. Multi-year monitoring of rift propagation on the Amery Ice Shelf, East Antarctica

    E-print Network

    Eustice, Ryan

    Multi-year monitoring of rift propagation on the Amery Ice Shelf, East Antarctica H. A. Fricker,1 N on the Amery Ice Shelf, East Antarctica, Geophys. Res. Lett., 32, L02502, doi:10.1029/ 2004GL021036. 1 and Sandhager, 2005]. [4] AIS is the largest ice shelf in East Antarctica. Its last major calving event occurred

  18. Lava Flow From Fissure Eruption on Kilauea Volcano's East Rift Zone

    USGS Multimedia Gallery

    A new fissure eruption in September 2011 on the east flank of Pu‘u ‘?‘? on K?lauea Volcano's east rift zone reminds us of the need to be aware of our ever-changing volcanic environment on Hawai‘i Island. The fissure—the source of the lava flow—is sh...

  19. New Diagnostic Tools for Detecting Rift Valley Fever (RVF) and other Arboviruses

    Technology Transfer Automated Retrieval System (TEKTRAN)

    The outbreak of West Nile virus in the United Sates and the recent outbreak of Rift Valley fever (RVF) virus in East Africa have highlighted the need for validated early detection tools for arthropod-borne animal diseases. The Arthropod-Borne Animal Diseases Research Laboratory (ABADRL) has been inv...

  20. Broad accommodation of rift-related extension recorded by dyke intrusion in Saudi Arabia

    USGS Publications Warehouse

    Pallister, J.S.; McCausland, W.A.; Jonsson, Sigurjon; Lu, Zhiming; Zahran, H.M.; El, Hadidy S.; Aburukbah, A.; Stewart, I.C.F.; Lundgren, P.R.; White, R.A.; Moufti, M.R.H.

    2010-01-01

    The extensive harrat lava province of Arabia formed during the past 30 million years in response to Red Sea rifting and mantle upwelling. The area was regarded as seismically quiet, but between April and June 2009 a swarm of more than 30,000 earthquakes struck one of the lava fields in the province, Harrat Lunayyir, northwest Saudi Arabia. Concerned that larger damaging earthquakes might occur, the Saudi Arabian government evacuated 40,000 people from the region. Here we use geologic, geodetic and seismic data to show that the earthquake swarm resulted from magmatic dyke intrusion. We document a surface fault rupture that is 8?km long with 91?cm of offset. Surface deformation is best modelled by the shallow intrusion of a north-west trending dyke that is about 10?km long. Seismic waves generated during the earthquakes exhibit overlapping very low- and high-frequency components. We interpret the low frequencies to represent intrusion of magma and the high frequencies to represent fracturing of the crystalline basement rocks. Rather than extension being accommodated entirely by the central Red Sea rift axis, we suggest that the broad deformation observed in Harrat Lunayyir indicates that rift margins can remain as active sites of extension throughout rifting. Our analyses allowed us to forecast the likelihood of a future eruption or large earthquake in the region and informed the decisions made by the Saudi Arabian government to return the evacuees.

  1. Rift Valley Fever Virus Control: Integration of Virus, Host and Vector Studies

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Rift Valley fever (RVF) is a disease of animals and humans that occurs in Africa and the Arabian Peninsula. It is caused by a Phlebovirus in the family Bunyaviridae. Mosquito-borne epizootics occur during years of unusually heavy rainfall. Domestic cattle, sheep and goats are highly susceptible to i...

  2. Evolution of the NewfoundlandIberia conjugate rifted margins Alistair Crosby a,

    E-print Network

    Shillington, Donna J.

    Evolution of the Newfoundland­Iberia conjugate rifted margins Alistair Crosby a, , Nicky White on the well-studied Newfoundland­Iberia conjugate margins. We have tackled the problem of depth dependency particularly focussed on the Newfoundland­Iberia conjugate margins which have been studied in detail using deep

  3. Potential for stable flies and house flies (Diptera: Muscidae) to transmit Rift Valley fever virus

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Rift Valley fever (RVF), a disease of ruminants and humans, has been responsible for large outbreaks in Africa that have resulted in hundreds of thousands of human infections and major economic disruption due to loss of livestock and to trade restrictions. As indicated by the rapid spread of West N...

  4. A Cenozoic diffuse alkaline magmatic province (DAMP) in the southwest Pacific without rift or plume origin

    E-print Network

    Müller, Dietmar

    A Cenozoic diffuse alkaline magmatic province (DAMP) in the southwest Pacific without rift or plume of the Pacific Plate. A key to generating the Cenozoic magmatism is the combination of metasomatized lithosphere. The model may also provide a mechanism for warming south Pacific mantle and resulting Cenozoic alkaline

  5. Comparison of Rift Valley fever virus replication in North American livestock and wildlife cell lines

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Rift Valley fever virus (RVFV) causes outbreaks of endemic disease across Africa and the Arabian Peninsula, resulting in high morbidity and mortality among young domestic livestock, frequent abortions in pregnant animals, and potentially severe or fatal disease in humans. The possibility of RVFV spr...

  6. Rift Valley Fever Virus among Wild Ruminants, Etosha National Park, Namibia, 2011.

    PubMed

    Capobianco Dondona, Andrea; Aschenborn, Ortwin; Pinoni, Chiara; Di Gialleonardo, Luigina; Maseke, Adrianatus; Bortone, Grazia; Polci, Andrea; Scacchia, Massimo; Molini, Umberto; Monaco, Federica

    2016-01-01

    After a May 2011 outbreak of Rift Valley fever among livestock northeast of Etosha National Park, Namibia, wild ruminants in the park were tested for the virus. Antibodies were detected in springbok, wildebeest, and black-faced impala, and viral RNA was detected in springbok. Seroprevalence was high, and immune response was long lasting. PMID:26692385

  7. Non-cylindricity of mountain belts: a case for rheological and rift inheritance

    NASA Astrophysics Data System (ADS)

    Jammes, Suzon; Huismans, Ritske S.; Muñoz, Josep Anton

    2013-04-01

    Many mountain belts exhibit significant along strike variation in structural style with changes in the width of the orogen, the thickness of thick-skinned thrust sheets, the geometry and kinematics of the crustal-scale thrust system, and the degree of partitioning between pro- and retro-wedge deformation. While the main factors controlling structural style are understood to first order the cause of these lateral variations remains to be resolved. Here we focus on the Pyrenean example that is characterized by significant lateral variation in structural style with an antiformal stack in the eastern section and thick-skinned deformation in its western part. The Mesozoic rifting event preceding Pyrenean mountain building was similarly characterized by significant lateral variation in structure, with wide distributed extension in the eastern segment and very narrow rifting in its western part leading to mantle exhumation. We integrate the available geological and geophysical data with forward numerical models of both the rifting event and lithosphere scale inversion leading to mountain building. We show that a lateral variation in crustal strength related to inherited Variscan crustal composition explains both the variation in structural style during Mesozoic rifting as well as those observed during Pyrenean mountain building.

  8. Systematic destruction of K-Feldspar in deeply buried rift and passive margin sandstones 

    E-print Network

    Wilkinson, Mark; Milliken, Kitty; Haszeldine, Stuart

    2001-01-01

    Systematic patterns of detrital K-feldspar decline with increasing burial depth are revealed during deep burial (2.5-4.5km) of sandstones in diverse sedimentary basins from three rift and two passive margin setings in the North Sea and USA gulf...

  9. Riparian Rifts in the Space-Time Continuum Noel P. Gurwick1

    E-print Network

    Gurwick, Noel P.

    1 2 3 4 5 6 7 8 9 Riparian Rifts in the Space-Time Continuum Noel P. Gurwick1 , Peter M. Groffman residence time, water table depth). At small scales, denitrification potential depends primarily) lead to long contact times between nitrate-bearing groundwater and zones of high C supply. First

  10. Immunohistochemical Detection of Rift Valley Fever Virus with Non-Infectious, Recombinant Viral Protein Antibodies

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Rift Valley fever virus (RVFV) causes re-emerging disease outbreaks and abortion storms in mature cattle, sheep, and goats, and can cause 100% mortality in young animals. The spread of this exotic, insect transmitted virus is of particular concern because of its widely recognized potential for being...

  11. Seismic velocity structure of the rifted margin of the eastern Grand Banks of Newfoundland, Canada

    E-print Network

    Shillington, Donna J.

    seaward, the continental crust tapers rapidly beneath the continental slope to $6 km thickness to the southeast from 6 to 2 km, which we interpret as highly extended continental crust. Contrary to other seismic´collement in the final stages of continental rifting. The outer edge of highly extended continental crust borders a 25-km

  12. Rift Valley Fever Virus among Wild Ruminants, Etosha National Park, Namibia, 2011

    PubMed Central

    Aschenborn, Ortwin; Pinoni, Chiara; Di Gialleonardo, Luigina; Maseke, Adrianatus; Bortone, Grazia; Polci, Andrea; Scacchia, Massimo; Molini, Umberto; Monaco, Federica

    2016-01-01

    After a May 2011 outbreak of Rift Valley fever among livestock northeast of Etosha National Park, Namibia, wild ruminants in the park were tested for the virus. Antibodies were detected in springbok, wildebeest, and black-faced impala, and viral RNA was detected in springbok. Seroprevalence was high, and immune response was long lasting. PMID:26692385

  13. Rift Valley fever risk map model and seroprevalence in selected wild ungulates and camels from Kenya

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Since the first isolation of Rift Valley fever virus (RVFV) in the 1930s, there have been multiple epizootics and epidemics in animals and humans in sub-Saharan Africa. Prospective climate-based models have recently been developed that flag areas at risk of RVFV transmission in endemic regions based...

  14. Blood Meal Analysis of Mosquitoes Involved in a Rift Valley fever Outbreak

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Background: Rift Valley fever (RVF) is a zoonosis of domestic ruminants in Africa. Bloodfed mosquitoes collected during the 2006-2007 RVF outbreak in Kenya were analyzed to determine the virus infection status and animal source of the bloodmeals. Bloodmeals from individual mosquito abdomens were sc...

  15. The 12 September 1999 Upper East Rift Zone dike intrusion at Kilauea Volcano, Hawaii

    E-print Network

    Amelung, Falk

    ] Deformation associated with an earthquake swarm on 12 September 1999 in the Upper East Rift Zone of Kilauea, leveling data, and interferometric synthetic aperture radar (InSAR) data from the ERS-2 satellite also of 12 September 1999 at 0131 LT (Hawaiian Standard Time, UT À 10) a swarm of small earthquakes

  16. Propagated rifting in the Southwest Sub-basin, South China Sea: Insights from 3D analogue modeling

    NASA Astrophysics Data System (ADS)

    Ding, Weiwei; Li, Jiabiao

    2015-04-01

    Most of earth scientists agree that the South China Sea is a rifted marginal sea in the western Pacific. How and when the South China Sea rifted has long been a puzzling question and still debated, especially in the Southwest Sub-basin (SWSB). Analog modeling remains one of the useful tools for testing rift model and process. Here we present and discuss a series of analog modeling experiments designed to investigate the rifting process of the SWSB. Convincing geophysical data were compiled to provide truthful constraints to test the experimental results and interpretations. The results show that rigid tectonic blocks existed in the continental margin, such as the Zhongsha Islands and the Reed Bank, and played an important role in shaping up the continent-ocean boundary (COB) and the coupling between the crust and mantle. Our data suggest that the initial thermal condition and rheologial stratification of the lithosphere under the South China Sea controlled the rifting process of the SWSB. The first-stage seafloor spreading has weakened the lithosphere surrounding the East Sub-basin, and the extension was centered on the deep troughs between the rigid blocks. Rifting bagan in these deep troughs in the east part of the SWSB, and the break-up occurred in localized areas between the rigid blocks. The V-shaped configuration of the SWSB also argues for a propagated rifting model.

  17. Boundary separating the seismically active reelfoot rift from the sparsely seismic Rough Creek graben, Kentucky and Illinois

    USGS Publications Warehouse

    Wheeler, R.L.

    1997-01-01

    The Reelfoot rift is the most active of six Iapetan rifts and grabens in central and eastern North America. In contrast, the Rough Creek graben is one of the least active, being seismically indistinguishable from the central craton of North America. Yet the rift and graben adjoin. Hazard assessment in the rift and graben would be aided by identification of a boundary between them. Changes in the strikes of single large faults, the location of a Cambrian transfer zone, and the geographic extent of alkaline igneous rocks provide three independent estimates of the location of a structural boundary between the rift and the graben. The boundary trends north-northwest through the northeastern part of the Fluorspar Area Fault Complex of Kentucky and Illinois, and has no obvious surface expression. The boundary involves the largest faults, which are the most likely to penetrate to hypocentral depths, and the boundary coincides with the geographic change from abundant seismicity in the rift to sparse seismicity in the graben. Because the structural boundary was defined by geologic variables that are expected to be causally associated with seismicity, it may continue to bound the Reelfoot rift seismicity in the future.

  18. Sedimentary budgets of the Tanzania coastal basin and implications for uplift history of the East African rift system

    NASA Astrophysics Data System (ADS)

    Said, Aymen; Moder, Christoph; Clark, Stuart; Abdelmalak, Mohamed Mansour

    2015-11-01

    Data from 23 wells were used to quantify the sedimentary budgets in the Tanzania coastal basin in order to unravel the uplift chronology of the sourcing area located in the East African Rift System. We quantified the siliciclastic sedimentary volumes preserved in the Tanzania coastal basin corrected for compaction and in situ (e.g., carbonates) production. We found that the drainage areas, which supplied sediments to this basin, were eroded in four episodes: (1) during the middle Jurassic, (2) during the Campanian-Palaeocene, (3) during the middle Eocene and (4) during the Miocene. Three of these high erosion and sedimentation periods are more likely related to uplift events in the East African Rift System and earlier rift shoulders and plume uplifts. Indeed, rapid cooling in the rift system and high denudation rates in the sediment source area are coeval with these recorded pulses. However, the middle Eocene pulse was synchronous with a fall in the sea level, a climatic change and slow cooling of the rift flanks and thus seems more likely due to climatic and eustatic variations. We show that the rift shoulders of the East African rift system have inherited their present relief from at least three epeirogenic uplift pulses of middle Jurassic, Campanian-Palaeocene, and Miocene ages.

  19. Keweenaw hot spot: Geophysical evidence for a 1. 1 Ga mantle plume beneath the Midcontinent Rift System

    SciTech Connect

    Hutchinson, D.R. ); White, R.S. ); Cannon, W.F.; Schulz, K.J. )

    1990-07-10

    The Proterozoic Midcontinent Rift System of North America is remarkably similar to Phanerozoic rifted continental margins and flood basalt provinces. Like the younger analogues, the volcanism within this older rift can be explained by decompression melting and rapid extrusion of igneous material during lithospheric extension above a broad, asthenospheric, thermal anomaly which the authors call the Keweenaw hot spot. Great Lakes International Multidisciplinary Program on Crustal evolution seismic reflection profiles constrain end-member models of melt thickness and stretching factors, which yield an inferred mantle potential temperature of 1,500-1,570C during rifting. Combined gravity modeling and subsidence calculations are consistent with stretching factors that reached 3 or 4 before rifting ceased, and much of the lower crust beneath the rift consists of relatively high density intruded or underplated synrift igneous material. The isotopic signature of Keweenawan volcanic rocks, presented in a companion paper by Nicholson and Shirey (this issue), is consistent with the model of passive rifting above an asthenospheric mantle plume.

  20. Mechanical response of the south flank of kilauea volcano, hawaii, to intrusive events along the rift systems

    USGS Publications Warehouse

    Dvorak, J.J.; Okamura, A.T.; English, T.T.; Koyanagi, R.Y.; Nakata, J.S.; Sako, M.K.; Tanigawa, W.T.; Yamashita, K.M.

    1986-01-01

    Increased earthquake activity and compression of the south flank of Kilauea volcano, Hawaii, have been recognized by previous investigators to accompany rift intrusions. We further detail the temporal and spatial changes in earthquake rates and ground strain along the south flank induced by six major rift intrusions which occurred between December 1971 and January 1981. The seismic response of the south flank to individual rift intrusions is immediate; the increased rate of earthquake activity lasts from 1 to 4 weeks. Horizontal strain measurements indicate that compression of the south flank usually accompanies rift intrusions and eruptions. Emplacement of an intrusion at a depth greater than about 4 km, such as the June 1982 southwest rift intrusion, however, results in a slight extension of the subaerial portion of the south flank. Horizontal strain measurements along the south flank are used to locate the January 1983 east-rift intrusion, which resulted in eruptive activity. The intrusion is modeled as a vertical rectangular sheet with constant displacement perpendicular to the plane of the sheet. This model suggests that the intrusive body that compressed the south flank in January 1983 extended from the surface to about 2.4 km depth, and was aligned along a strike of N66??E. The intrusion is approximately 11 km in length, extended beyond the January 1983 eruptive fissures, which are 8 km in length and is contained within the 14-km-long region of shallow rift earthquakes. ?? 1986.

  1. Stability of rift axis magma reservoirs: Spatial and temporal evolution of magma supply in the Dabbahu rift segment (Afar, Ethiopia) over the past 30 kyr

    NASA Astrophysics Data System (ADS)

    Medynski, S.; Pik, R.; Burnard, P.; Vye-Brown, C.; France, L.; Schimmelpfennig, I.; Whaler, K.; Johnson, N.; Benedetti, L.; Ayelew, D.; Yirgu, G.

    2015-01-01

    Unravelling the volcanic history of the Dabbahu/Manda Hararo rift segment in the Afar depression (Ethiopia) using a combination of cosmogenic (36Cl and 3He) surface exposure dating of basaltic lava-flows, field observations, geological mapping and geochemistry, we show in this paper that magmatic activity in this rift segment alternates between two distinct magma chambers. Recent activity in the Dabbahu rift (notably the 2005-2010 dyking crises) has been fed by a seismically well-identified magma reservoir within the rift axis, and we show here that this magma body has been active over the last 30 kyr. However, in addition to this axial magma reservoir, we highlight in this paper the importance of a second, distinct magma reservoir, located 15 km west of the current axis, which has been the principal focus of magma accumulation from 15 ka to the subrecent. Magma supply to the axial reservoir substantially decreased between 20 ka and the present day, while the flank reservoir appears to have been regularly supplied with magma since 15 ka ago, resulting in less variably differentiated lavas. The trace element characteristics of magmas from both reservoirs were generated by variable degrees of partial melting of a single homogeneous mantle source, but their respective magmas evolved separately in distinct crustal plumbing systems. Magmatism in the Dabbahu/Manda Hararo rift segment is not focussed within the current axial depression but instead is spread out over at least 15 km on the western flank. This is consistent with magneto-telluric observations which show that two magma bodies are present below the segment, with the main accumulation of magma currently located below the western flank, precisely where the most voluminous recent (<15 ka) flank volcanism is observed at the surface. Applying these observations to slow spreading mid-ocean ridges indicates that magma bodies likely have a lifetime of a least 20 ka, and that the continuity of magmatic activity is maintained by a system of separate relaying reservoirs, which could in return control the location of spreading. This long term (>105 yr) alternation between distinct crustal reservoirs located broadly at the same location relative to the segment appears to be a key feature for organizing and maintaining active spreading centres over stable soft points in the mantle.

  2. Anomalous subsidence at South China Sea rifted margin: Sediments digging their own hole

    NASA Astrophysics Data System (ADS)

    Brune, Sascha; Clift, Peter; Quinteros, Javier

    2015-04-01

    Rifted continental margins subside as a consequence of combined crustal thinning and mantle lithosphere cooling. While standard models predict a slowing of subsidence after the end of rifting, the deep basins on the northern margin of the South China Sea, notably the Baiyun Sag exhibit subsidence that accelerated several million years after the end of active extension. Additionally, backstripping analysis at the South China margin has shown that the amount of subsidence is much greater than that predicted from the degree of brittle upper crustal extension seen in seismic profiles. Here we explain these observations by linking climate change onshore and deformation of the crystalline crust offshore: Early Miocene monsoon intensification increased erosion and thus the sediment flux to offshore basins after the cessation of active extension. When the sediment load encountered the weak crust of the South China Sea margin, it induced lower crustal flow away from the basin axis so that the deep Baiyun basin was formed nearly without brittle extension. We corroborate this concept using seismic observations and backstripping techniques, as well as thermo-mechanical forward modeling. The numerical forward model is a 2D version of the finite element code SLIM3D. The code includes nonlinear temperature- and stress-dependent elasto-visco-plastic rheology and is able to reproduces a wide range of rift-related deformation processes such as flexure, lower crustal flow, and faulting. We find that two factors allow to satisfy the observational constraints: (1) Post-rift increase of sediment load: The East Asian Summer Monsoon strengthened around the start of the Miocene (~23 Ma), several million years after continental rupture. Changes in the flora of continental China date from around this time and sedimentation rates across continental margins and deltas in South and Southeast Asia increased, as might be expected under the influence of heavier precipitation driving faster erosion in the source regions. (2) Weak continental crust: Extension in the South China Sea occurred in wide rift mode, which indicates the existence of a thick, weak crustal layer that decoupled deformation of brittle crust and mantle, ultimately forming symmetric margins with an extraordinary width of more than 300 km. Lateral crustal flow below the Baiyun basin is a consequence of the weakness of the crust, caused by high heat flow, a weak quartz-rheology and is triggered by sediment loading. We suggest that super-deep rift basins, like Baiyun Sag, are likely often linked to the flow of ductile crust in this fashion and would not be expected in cooler rift settings, such as found along the Atlantic margins.

  3. Geodynamic modeling of the Mid-Continental Rift System: Is a mantle plume required?

    NASA Astrophysics Data System (ADS)

    Moucha, R.; Rooney, T. O.; Stein, S. A.; Brown, E.

    2013-12-01

    The Mid-Continent Rift System (MCRS) is a 2000-km long trace of a massive igneous event that nearly split North America 1.1 billion years ago. The MCRS offers a snapshot of continental rifting and rift failure. The rift started, evolved, and terminated via a complex and not-yet-understood interplay of mantle dynamics, magmatism, and extension. In particular, details of the processes surrounding melt formation and the associated depletion of the lithospheric mantle, and re-thickening of the extended crust remain poorly constrained. To help unravel the complex history of the MCRS, we present a new geodynamic model for late Proterozoic rifting and compare a number of different extension scenarios in an effort to answer the following question: can the volume of magmas preserved in the MCRS be explained by extensional processes in the absence of a thermo-chemical mantle plume anomaly? In the late Proterozoic, inherently higher mantle potential temperatures may have permitted rifting at lower stress levels and favored more melt formation. Therefore, the inferred volume of magma in the MCRS may not have required a plume. To explore this idea, we compare models for the evolution of a rift under different ambient mantle temperatures and radiogenic heating conditions, i.e. Phanerozoic vs. Proterozoic, and in the absence or presence of a thermal-chemical plume. Our geodynamic model of the MCRS includes a temperature dependent visco-elasto-plastic rheology in the presence of partial melt that is dynamically determined according to given rock-type, pressure and temperature. The models include a 'sticky air' layer to mimic a free surface that is coupled to a surface processes model to account for erosion and sedimentation. The numerical method we use is based on the primitive variable particle-in-cell finite-difference method developed by Taras Gerya and others (e.g. Gerya, 2010). The advantage of this approach is that specific material properties are inherently traced through time negating the need for numerous high-resolution grids. Moreover, melt-extraction and the formation of a crust along with depletion and fertilization are also easily tracked.

  4. Melt generation and supply at a magmatic rift zone in Afar

    NASA Astrophysics Data System (ADS)

    Ferguson, D. J.; Pyle, D. M.; Maclennan, J.; Blundy, J. D.; Yirgu, G.; Bastow, I. D.; Plank, T.

    2012-12-01

    The thermochemical state of the mantle and the extension rate of the lithopshere play a fundamental role in governing melt volumes during continental rifting. Afar in northern Ethiopia has a prolonged history of extension and magmatism and is an ideal location to investigate mechanisms of melt production during the latter stages of continental break-up. However, current melting conditions here remain poorly constrained. In this study we use geochemical analysis of mafic lavas to develop a petrogenetic model for melt production beneath an active rift zone/segment in west-central Afar. Trace element and isotopic data show that melts are derived from a fertile mantle source, and all the lavas are enriched in incompatible trace elements compared to typical MORB compositions. We investigate primary melting using forward and inverse models of rare earth element (REE) concentrations in partial melts produced from a polybaric melting column. Best-fitting models of melt production with depth constrain melting to occur between depths of ~100-70 km, consistent with melting of upwelling mantle with a potential temperature of 1450 °C. The relatively deep termination of melting given by our results implies that asthenospheric ascent and melting beneath Afar are restricted by an upper thermo-mechanical boundary layer. This suggests that despite its long history of extension a significant thickness of the pre-rift African lithospheric plate still exists in this region. Major element thermobarometry shows that melts erupted at the rift axis preserve compositions consistent with melt-mantle equilibrium at the top of the melting column at ~70km depth, while off-axis lavas show evidence for major element re-equilibration at shallower depths (~60-40 km) within the upper mantle, in agreement with recent geophysical evidence for melt distribution beneath the rift zone. Overall, our results support rifting models where large extents of melt intrusion conserve plate thickness and may compensate for tectonic thinning. Melting beneath Afar still largely resembles a continental rather than oceanic rift system.

  5. Erosion controls on the metamorphic core complex dynamics and its relationship with syn- rift basin evolution

    NASA Astrophysics Data System (ADS)

    Wang, Kun; Burov, Evgueni; Gumiaux, Charles; Chen, Yan; Zhao, Liang

    2015-04-01

    The wide rifting evolves towards asymmetric extensional thinning of the entire crust and development of different characteristic features such as basins, half-grabens and metamorphic core complexes (MCC). In this context, formation of supra-detachment basins is also a common feature, along with the exhumation of metamorphic rocks and considerable displacements along the hanging wall. Initiation, geometry and mechanisms of metamorphic core complexes have been already largely debated on the basis field observations, analog and numerical models. For example, it has been well demonstrated that strain softening favors asymmetric deformation and accounts for different styles of brittle and ductile strain localization. However, the temporal and spatial relations between the dome formation and basin evolution are still poorly understood. In particular, most of the existing numerical models predict a topographical depression above the metamorphic dome, whereas in nature dome formation often corresponds to a topographical uplift. To explain these phenomena, we have integrated surface erosion, sedimentary processes and strain softening into a state-of-the-art 2-D numerical thermo-mechanical model of MCC development. In the numerical experiments, we first reproduce formation of a univergent MCC by implementing strain softening and testing a large spectrum of lithospheric structures. In the next series of experiments we apply erosion/sedimentation and test model sensitivity to different erosion parameters. The results show two distinctive stages of MCC dynamics and syn-rift basin development. One single broad basin forms above the dome and is divided onto an inactive basin located at the distal detachment and an active supradetachment basin that deepens with further extension, characterized by crustal necking and dome amplificationduring the MCC formation. It is noteworthy that without strain softening, erosion at of the rift flanks mayresult in complete burial of the dome below the sedimentary cover. The experiments also demonstrate strong dependence of the system evolution on the initial thermo-rheological structure. The geometry and topography of the rift system is largely controlled by syn-extensional erosion that also strongly affects vertical and lateral movements during the rifting phase. The predicted rift dynamics can be compared to the case of the wide rift system of the eastern part of North China Craton.

  6. Quantifying the temporal and spatial response of channel steepness to changes in rift basin architecture

    NASA Astrophysics Data System (ADS)

    Robinson, S. M.; Whipple, K. X.; Arrowsmith, R.; Heimsath, A. M.; Foster, M.; Cohen, A. S.

    2013-12-01

    Quantifying the temporal evolution of active continental rifts contributes to our understanding of fault system evolution and seismic hazards. Rift basins also preserve robust paleoenvironmental records that may help untangle feedbacks between climate and tectonics. Here we quantify the spatial and temporal history of rift flank uplift by exploiting landscape memory along footwall escarpments in the Malawi segment of the East Africa Rift. Our work helps answer a suite of questions that are widely applicable to continental rift settings; (1) Is rift-flank uplift well described by theoretical elliptical displacement patterns? (2) Do orographic climate patterns induced by rift topography affect rift-flank uplift? (3) Is footwall exhumation influenced by the evolution of depocenters and rift lakes? In Malawi, 100 km long border faults of alternating polarity bound half-graben sedimentary basins containing up to 4km of basin fill and water depths up to 700m. Orographically driven precipitation produces climatic gradients along footwall escarpments resulting in mean annual precipitation that varies spatially from 800 to 2500mm. Temporal oscillations in climate have also resulted in lake lowstands 500m below the modern Lake Malawi. We examine bedrock river profiles crossing the Livingstone and Usisya Border Faults in northern Malawi using the channel steepness index (Ksn). River profiles reveal consistent patterns of Ksn throughout the study area. Typical profiles contain a lower reach with Ksn values between 150-300 and an upper reach with values between 50-150. Some channels display an uppermost reach with values less than 100 that are interpreted to represent the pre to early-rift relief of the region. This progressive downstream steepening is consistent with acceleration of slip along the main border faults or reduction in climatically modulated erosional efficiency. Spatial patterns of Ksn and knickpoint elevations are not consistent with theoretical along fault displacement models or expected patterns from the observed climatic gradients. Instead, knickpoint elevations decrease from ~1000m to ~400m above watershed outlets from north to south. This pattern suggests that an acceleration of slip initiated in the northern part of the basin. Alternatively, this pattern is also consistent with increased hangingwall subsidence (instead of footwall uplift) relative to baselevel due to loading from depocenters. Unlike the Livingstone Fault, there is little along strike variation in hangingwall loading adjacent to the bordering Usisya Fault to the south. Ksn values of rivers crossing the Usisya border fault range from 100-250 in the middle section of the fault and decrease to <100 at the tips showing some consistency with elliptical displacement models. Our data indicating a discrete acceleration of footwall uplift are highly suggestive of a connection between basin architecture and the temporal and spatial distribution of this uplift. Erosion rate measurements beneath the knickpoint are needed to constrain the timing of this acceleration and establish a link to the basins well-documented depositional history.

  7. Development of the Barents Sea rift and its influence on sedimentation and hydrocarbon formation

    NASA Astrophysics Data System (ADS)

    Balanyuk, Inna; Dmitrievsky, Anatoly; Shapovalov, Sergey; Chaikina, Olga

    2010-05-01

    A special attention is given to the geodynamic active zone of the Barents Sea rift. Its development was accompanied by vigorous tectonic activity, propagation of deep faults, deep fractured zones that played an important role in fluid dynamic and thermobaric regime of the whole region. Geodynamic development of the Barents Sea rift not only played a substantial role in formation of as unique oil and gas fields as Shtokman, Prirazlomnoe and others, but created prerequisites for possible gas outbursts into near-surface sediments that could result, in some cases, in hydrocarbon formation. All the Barents Sea deposits are situated in the epicenter of the rift and, most important, over the zone of listric faults intersection, which set up a knot system over the mantle diapir. It is confirmed by prospecting seismology. Intrusion of hot mantle matter with further cooling down of abnormal lense might be a possible cause of appearance and evolution of ultradeep depressions. A high "seismic stratification" of the lower crust (nearly reaching the basement surface) at time scale about 8 sec. is typical for the deepest part of the depression. Supposing the "seismic stratified" lower crust correspond to "basalt" layer, this area is nearly upper crust ("granitic-gneiss") free. This fact confurmes conception on development of "granite free gaps" in the depression basement. Thick blocks of "seismically transparent" upper crust corresponding to the "granitic-gneiss" layer are marked out within Kolsk-Kanin monocline. An abrupt thickness decrease and appearance of "stratified" areas takes place at the southern edge of the depression. A filling of the over-rift sag with sediments, revival of the faults and their effect on the filtration processes and gas hydrates formation took place in the South Barents Sea depression. Repeating activation of the fault blocks in the basement, especially during late Jurassic - early Cretaceous period contributed to formation of the structures related to the greatest deposits of this depression. An extended field acoustic data collected in the Barents Sea led to understanding of general fundamental problems for all Arctic Seas and, first of all, the problem of Quaternary glaciations. An analysis of Eurasian-Arctic continental margin shows correspondence between the rift systems of the shelf with those of the ocean. This relation can be observed by an example of the central Arctic region. All the rift systems underlying the sediment basin are expressed in the sea bed relief as spacious and extensive graben valleys burnished by lobes. The rift structures and the sediment cover are connected by the systems of transversal (or oblique) faults. A study of the regional geologic structure of basins affords believing in the very complicated inner riftogenic structure of the base and the lower part of the sediment cover, which is represented by a combination of grabens and horsts bound by the same tectonic border in the form of high amplitude fault zones, and have a very high density of fault-fractured tectonics. In the same time, a branching of the rift structures related to rounding of large solid blocks of the crust, represented by Archaean or Baikal base prominences, takes place. Their frequent structural connection with the continental paleo-rift structures is ascertained, but the shelf rift system are not their straight prolongation and separated by base thresholds concealed under the sediment cover or by bulkheads expressed even in the modern relief. The information on geodynamic development of the region, thermal convection and modern sedimentation enable assessment of the real potentiality for underwater works in the region, potential gas hydrates resources, and will help to assign a strategy of prospecting work, to range the known fields, to carry out a regional survey for engineering work.

  8. Impact of geodynamic development of the Barents Sea deep rift on evolving petroleum systems

    NASA Astrophysics Data System (ADS)

    Balanyuk, I.; Dmitrievsky, A.

    2009-04-01

    All the Barents Sea deposits are situated in the epicenter on active geodynamic development of the Barents Sea rift and, most important, over the zone of listric faults intersection, which consist a knot system over the mantle diapir. This is confirmed by prospecting seismology. Intrusion of hot mantle matter with further cooling down of abnormal lense might be a possible cause of appearance and evolution of ultradeep depressions. A high "seismic stratification" of the lower crust (nearly reaching the basement surface) at time scale about 8 sec. is typical for the inner, the deepest part of the depression. Supposing the "seismic stratified" lower crust correspond to "basalt" layer, this area is nearly upper crust ("granitic-gneiss") free. This fact confurmes conception on development of "granite free gaps" in the depression basement. Thick blocks of "seismically transparent" upper crust corresponding to the "granitic-gneiss" layer are marked out within Kolsk-Kanin monocline. An abrupt thickness decrease and appearance of "stratified" areas takes place at the southern edge of the depression. A filling of the over-rift sag with sediments, revival of the faults and their effect on the filtration processes and gas hydrates formation took place in the South Barents Sea depression. Repeating activation of the fault blocks in the basement, especially during late Jurassic - early Cretaceous period contributed to formation of the structures related to the greatest deposits of the South Barents Sea depression. An extended field acoustic data collected in the Barents Sea led to understanding of general fundamental problems for all Arctic Seas and, first of all, the problem of Quaternary glaciations. An analysis of Eurasian-Arctic continental margin shows correspondence between the rift systems of the shelf with those of the ocean. This relation can be observed by an example of the central Arctic region. All the rift systems underlying the sediment basin are expressed in the sea bed relief as spacious and extensive graben valleys burnished by lobes. The rift structures and the sediment cover are connected by the systems of transversal (or oblique) faults. A study of the regional geologic structure of basins affords believing in the very complicated inner riftogenic structure of the base and the lower part of the sediment cover, which is represented by a combination of grabens and horsts bound by the same tectonic border in the form of high amplitude fault zones, and have a very high density of fault-fractured tectonics. In the same time, a branching of the rift structures related to rounding of large solid blocks of the crust, represented by Archaean or Baikal base prominences, takes place. Their frequent structural connection with the continental paleo-rift structures is ascertained, but the shelf rift system are not their straight prolongation and separated by base thresholds concealed under the sediment cover or by bulkheads expressed even in the modern relief. Filling of over rift flexure by sediments, revival of faults and their impact on flow processes and gas hydrate formation in under sea bed sediments have occurred in the South Barents depression. Geodynamic development of the Barents sea rift not only played a substantial role in formation of as unique fields as Shtokman, Prirazlomnoe and others, but created prerequisites for possible gas outbursts into near-surface sediments of the Barents Sea that could result, in some cases, in gas hydrate formation. Periodic activation of basic fault blocks, especially in late Jurassic - early Cretaceous time significantly contributed to formation of the structures related to the greatest fields of South Barents depression. The information on geodynamic development of the region, thermal convection and modern sedimentation enable assessment of the real potentiality for underwater works in the region, potential gas hydrates resources, and will help to assign a strategy of prospecting work, to range the known

  9. Rift processes and crustal structure of the Amundsen Sea Embayment, West Antarctica, from 3D potential field modelling

    NASA Astrophysics Data System (ADS)

    Kalberg, Thomas; Gohl, Karsten; Eagles, Graeme; Spiegel, Cornelia

    2015-12-01

    The Amundsen Sea Embayment of West Antarctica is of particular interest as it provides critical geological boundary conditions in better understanding the dynamic behavior of the West Antarctic Ice Sheet, which is undergoing rapid ice loss in the Amundsen Sea sector. One of the highly debated hypothesis is whether this region has been affected by the West Antarctic Rift System, which is one of the largest in the world and the dominating tectonic feature in West Antarctica. Previous geophysical studies suggested an eastward continuation of this rift system into the Amundsen Sea Embayment. This geophysical study of the Amundsen Sea Embayment presents a compilation of data collected during two RV Polarstern expeditions in the Amundsen Sea Embayment of West Antarctica in 2006 and 2010. Bathymetry and satellite-derived gravity data of the Amundsen Sea Embayment complete the dataset. Our 3-D gravity and magnetic models of the lithospheric architecture and development of this Pacific margin improve previous interpretations from 2-D models of the region. The crust-mantle boundary beneath the continental rise and shelf is between 14 and 29 km deep. The imaged basement structure can be related to rift basins within the Amundsen Sea Embayment, some of which can be interpreted as products of the Cretaceous rift and break-up phase and some as products of later propagation of the West Antarctic Rift System into the region. An estimate of the flexural rigidity of the lithosphere reveals a thin elastic thickness in the eastern embayment which increases towards the west. The results are comparable to estimates in other rift systems such as the Basin and Range province or the East African Rift. Based on these results, we infer an arm of the West Antarctic Rift System is superposed on a distributed Cretaceous rift province in the Amundsen Sea Embayment. Finally, the embayment was affected by magmatism from discrete sources along the Pacific margin of West Antarctica in the Cenozoic.

  10. Rift processes and crustal structure of the Amundsen Sea Embayment, West Antarctica, from 3D potential field modelling

    NASA Astrophysics Data System (ADS)

    Kalberg, Thomas; Gohl, Karsten; Eagles, Graeme; Spiegel, Cornelia

    2015-09-01

    The Amundsen Sea Embayment of West Antarctica is of particular interest as it provides critical geological boundary conditions in better understanding the dynamic behavior of the West Antarctic Ice Sheet, which is undergoing rapid ice loss in the Amundsen Sea sector. One of the highly debated hypothesis is whether this region has been affected by the West Antarctic Rift System, which is one of the largest in the world and the dominating tectonic feature in West Antarctica. Previous geophysical studies suggested an eastward continuation of this rift system into the Amundsen Sea Embayment. This geophysical study of the Amundsen Sea Embayment presents a compilation of data collected during two RV Polarstern expeditions in the Amundsen Sea Embayment of West Antarctica in 2006 and 2010. Bathymetry and satellite-derived gravity data of the Amundsen Sea Embayment complete the dataset. Our 3-D gravity and magnetic models of the lithospheric architecture and development of this Pacific margin improve previous interpretations from 2-D models of the region. The crust-mantle boundary beneath the continental rise and shelf is between 14 and 29 km deep. The imaged basement structure can be related to rift basins within the Amundsen Sea Embayment, some of which can be interpreted as products of the Cretaceous rift and break-up phase and some as products of later propagation of the West Antarctic Rift System into the region. An estimate of the flexural rigidity of the lithosphere reveals a thin elastic thickness in the eastern embayment which increases towards the west. The results are comparable to estimates in other rift systems such as the Basin and Range province or the East African Rift. Based on these results, we infer an arm of the West Antarctic Rift System is superposed on a distributed Cretaceous rift province in the Amundsen Sea Embayment. Finally, the embayment was affected by magmatism from discrete sources along the Pacific margin of West Antarctica in the Cenozoic.

  11. Diabase dikes of the Midcontinent Rift in Minnesota: a record of Keweenawan magmatism and tectonic development

    SciTech Connect

    Green, J.C.; Chandler, V.C.

    1985-01-01

    Swarms of both reversed-polarity (R, older) and normal-polarity (N, younger) basaltic dikes help to define the evolution of the Minnesota portion of the Midcontinent Rift of North America. Each swarm, representing fissure-feeders for a package of overlying plateau lavas now eroded away, shows the direction of least principal stress at that time and place in the complex evolution of this abortive but nearly-successful rift. Paleomagnetic pole determinations for Carlton county (CC) and Grand Portage (GP) R dikes are coincident along the Logan Loop of the Proterozoic track, showing essential contemporaneity, though their trends are different (N.30/sup 0/E vs. N. 70-90/sup 0/E.) and they are 250 km apart. These poles match that of the R lavas of the North Shore Volcanic Group (NSVG) and imply a younger age than the R Logan sills and dikes. The geochemistry of the dikes (71 analyses) also correlates well with NSVG flows, ranging from olivine tholeiites to transitional basalts and basaltic andesites and is similar to tholeiites of Columbia River, Parana, and Tertiary No. Atlantic provinces. Though each swarm shows a range of compositions, some are dominantly more evolved, whereas others contain more primitive dikes with higher Al, Mg/Fe, Cr and Ni and lower Fe, Ti, P and LIL's. An early major episode of rifting during the R polarity interval was followed by at least one major N episode in Minnesota before the final one along the present Lake Superior syncline axis. Rifting directions and mantle sources were different for each episode as shown by cross-trending dike sets, indicating complex rift development.

  12. 3D Geodynamic Modelling Reveals Stress and Strain Partitioning within Continental Rifting

    NASA Astrophysics Data System (ADS)

    Rey, P. F.; Mondy, L. S.; Duclaux, G.; Moresi, L. N.

    2014-12-01

    The relative movement between two divergent rigid plates on a sphere can be described using a Euler pole and an angular velocity. On Earth, this typically results in extensional velocities increasing linearly as a function of the distance from the pole (for example in the South Atlantic, North Atlantic, Woodlark Basin, Red Sea Basin, etc.). This property has strong implications for continental rifting and the formation of passive margins, given the role that extensional velocity plays on both rift style (wide or narrow), fault pattern, subsidence histories, and magmatism. Until now, this scissor-style opening has been approached via suites of 2D numerical models of contrasting extensional velocities, complimenting field geology and geophysics. New advances in numerical modelling tools and computational hardware have enabled us to investigate the geodynamics of this problem in a 3D self-consistent high-resolution context. Using Underworld at a grid resolution of 2 km over a domain of 500 km x 500 km x 180 km, we have explored the role of the velocity gradient on the strain pattern, style of rifting, and decompression melting, along the margin. We find that the three dimensionality of this problem is important. The rise of the asthenosphere is enhanced in 2D models compared to 3D numerical solutions, due to the limited volume of material available in 2D. This leads to oceanisation occurring significantly sooner in 2D models. The 3D model shows that there is a significant time and space dependent flows parallel to the rift-axis. A similar picture emerges from the stress field, showing time and space partitioning, including regions of compression separating areas dominated by extension. The strain pattern shows strong zonation along the rift axis, with increasingly localised deformation with extension velocity and though time.

  13. Inland thinning of West Antarctic Ice Sheet steered along subglacial rifts.

    PubMed

    Bingham, Robert G; Ferraccioli, Fausto; King, Edward C; Larter, Robert D; Pritchard, Hamish D; Smith, Andrew M; Vaughan, David G

    2012-07-26

    Current ice loss from the West Antarctic Ice Sheet (WAIS) accounts for about ten per cent of observed global sea-level rise. Losses are dominated by dynamic thinning, in which forcings by oceanic or atmospheric perturbations to the ice margin lead to an accelerated thinning of ice along the coastline. Although central to improving projections of future ice-sheet contributions to global sea-level rise, the incorporation of dynamic thinning into models has been restricted by lack of knowledge of basal topography and subglacial geology so that the rate and ultimate extent of potential WAIS retreat remains difficult to quantify. Here we report the discovery of a subglacial basin under Ferrigno Ice Stream up to 1.5?kilometres deep that connects the ice-sheet interior to the Bellingshausen Sea margin, and whose existence profoundly affects ice loss. We use a suite of ice-penetrating radar, magnetic and gravity measurements to propose a rift origin for the basin in association with the wider development of the West Antarctic rift system. The Ferrigno rift, overdeepened by glacial erosion, is a conduit which fed a major palaeo-ice stream on the adjacent continental shelf during glacial maxima. The palaeo-ice stream, in turn, eroded the 'Belgica' trough, which today routes warm open-ocean water back to the ice front to reinforce dynamic thinning. We show that dynamic thinning from both the Bellingshausen and Amundsen Sea region is being steered back to the ice-sheet interior along rift basins. We conclude that rift basins that cut across the WAIS margin can rapidly transmit coastally perturbed change inland, thereby promoting ice-sheet instability. PMID:22837002

  14. Petroleum geology of rift basins in Niger, Chad, and the Central African Republic

    SciTech Connect

    Genik, G.J. )

    1991-03-01

    Ten Cretaceous-Tertiary rift basins in Niger, Chad, and the Central African Republic (C.A.R.) are defined and the petroleum geology is overviewed. This paper is based on proprietary exploration results derived from more than 1 million km{sup 2} of aeromagnetics, 10,520 line km of gravity profiles, 49,721 km of reflection seismic, and 50 exploration wells. The data were acquired by Exxon with partners Shell, Chevron, Elf, Conoco, Texaco, and Amax Oil Gas, Inc., during the years 1969-1989. In Niger and Chad, the West African rift subsystem includes the extensional basins of Termit, Tefidet, Tenere, Grein/Kafra, N'Djel Edji, and Bongor. These rift basins contain up to 15,000 m of Cretaceous to Cenozoic continental and marine clastics. Key exploration elements are Tertiary and Cretaceous fluvial to tidal sandstone reservoirs, Tertiary and Cretaceous marine to lacustrine shale source rocks, and seals, with traps in normal fault blocks and anticlinal closures. There are six oil discoveries in the Termit basin. In Chad and the C.A.R., the Central African rift subsystem incorporates the extensional Doba and transtensional Doseo and Salamat basins flanking the Borogop dextral wrench fault. These basins contain up to 7,500 m of chiefly Cretaceous continental clastics. Key exploration elements are Lower and Upper Cretaceous fluvial to lacustrine sandstone reservoirs, Lower Cretaceous lacustrine shale source rocks, lacustrine to flood plain shale and mudstone seals, with traps in mainly faulted anticlinal closures. There are six oil discoveries in the Doba basin and three in the Doseo basin. The studied petroleum geology in the rifts of Niger, Chad, and the C.A.R. indicates that potentially commercial volumes of oil remain to be discovered.

  15. New Low-Temperature Thermochronology Reveals Contrasting Modes of Continental Extension Across the Sonoran Rifted Margin

    NASA Astrophysics Data System (ADS)

    Kohn, B. P.; Fletcher, J. M.; Gleadow, A. J.; Calmus, T.; Nourse, J. A.

    2003-12-01

    The Sonoran rifted margin extends 250 km from the western flanks of the Sierra Madre Occidental to the Gulf of California and contains a classic Basin and Range morphology that indicates "broad-rift" mode of continental extension. However, new low-temperature thermochronology reveals that the Sonoran rifted margin is also internally composed of at least two temporally and spatially distinct belts that display other distinct styles of extension. Mountain ranges that lie within a narrow belt (20 km wide) along the coast of the Gulf of California between Puerto Libertad and Bahia Kino yield highly discordant apatite fission track (AFT) ages that range from 5 to 54 Ma and likely reflect the strong tilting of these tectonic blocks. The widespread occurrence of AFT ages between 5 and 7 Ma, which are typically found in the deepest crustal levels of the tilt blocks, and the presence of Quaternary scarps indicate that extension in the coastal region largely occurred from late Miocene to recent times. We infer that this belt is dominated by a "narrow-rift" mode of extension where deformation has been focused to produce the Gulf depression. Well inland from the coast (175 km east) is a belt of metamorphic core complexes that extends more than 200 km from Magdalena to Mazatan and typically yields older and more concordant AFT ages from 14 to 23 Ma. However, the presence of ages as young as 8 to 11 Ma indicate that the "metamorphic-core-complex" mode of extension in this belt likely overlapped in time with the "narrow-rift" mode of extension in the Gulf of California. We conclude that the juxtaposition of major deformation belts each with different modes of continental extension reflects the diverse processes that have affected the Sonoran margin through time.

  16. Thermal and compaction processes in a young rifted basin containing evaporites: Gulf of Lions, France

    SciTech Connect

    Burrus, J.; Audebert, F. )

    1990-09-01

    The Gulf of Lions (northwestern Mediterranean) is a young Oligocene-Miocene rifted basin characterized by the presence of a thick evaporitic sequence. Observed data (surface heat flow, well temperatures, geopressures below the evaporites, crustal thinning) are used to model (1) the basin-forming mechanisms, and (2) the intrasedimentary processes that influence the temperature history, and thus, the maturation of source rocks. A two-dimensional finite-difference basin model, Temispack, simulates the sedimentation, compaction, and overpressures related to the low permeability of the post-rift sequence, the heat transfer from the rifted lithosphere into the sediments, and the level or maturation by using a kinetic approach. The classic passive margin model, which relates the history of heat flow and the rate of subsidence to a uniform thinning factor, yields subsidence and heat flow predictions that agree with observations. The recently accelerated subsidence observed in the deep margin is probably an effect of recent regional compression rather than the consequence of alternative rifting mechanisms. The history of maturation of organic matter was influenced by the variation of the regional heat flow due to the rifting, the effect of sedimentation (or blanketing), and the variability of the lithologies and porosities. The undercompaction of sediments underlying the evaporites is accounted for by the model, which suggests that hydraulic fracturing is taking place at present in the overpressured sediments. Fluid flow associated with the compaction disequilibrium has negligible thermal effect. Undercompacted shales below the evaporites have lower thermal diffusivity and thus higher temperatures, which has a significant effect on maturation. 15 figs., 4 tabs.

  17. Dniepr-Donets Rift: deep structure and evolution from DSS profiling

    NASA Astrophysics Data System (ADS)

    Ilchenko, T.

    1996-12-01

    Forward modelling has been applied to refraction and wide-angle reflection traveltime data along two profiles crossing the intracratonic Dniepr-Donets Rift (DDR), a Devonian sedimentary basin located in the southwestern part of the Eastern European Platform between the Ukrainian Shield and the Voronezh Massif. The 2-D P-wave velocity models show that the crystalline crust beneath the rift basin is thinner and characterised by higher velocity values (of almost 1.0 km/s) than beneath the Shield. Differences along the strike of the DDR are also indicated. The two profiles cross the Dniepr and Donets segments of the DDR and reveal, respectively, sedimentary infill thickness of 7 and 20 km and crystalline crustal thicknesses of 31 and 17 km. In both cases, the crystalline crust comprises two distinct velocity layers. Beneath the Dniepr basin, the Moho lies at 38 km and the upper mantle velocity is 8.4 km/s. In the Donets region, the crustal layers are underlain by velocity boundaries at depths of 37 and 42 km with velocities of 7.6 and 8.0 km/s, respectively. The velocity models are interpreted to indicate that a key process in the formation and evolution of the DDR was the exchange of material between the crust and uppermost mantle. It is hypothesised that the extended and faulted pre-rift shield crust was intruded by mantle melts during rifting and that crust/mantle material exchange may also have occurred during the subsequent cooling and subsidence stage of development. Rifting was less intense in the Dniepr segment of the DDR compared to the Donets. To explain other differences in uppermost mantle structure between the two segments of the DDR, it is postulated that density reduction of some portion of the Donets subcrustal mantle occurred in response to compressional deformation and inversion during the Permian Uralian orogeny.

  18. Numerical modeling of seismicity and geodynamics of the Kachchh rift zone, Gujarat, India

    NASA Astrophysics Data System (ADS)

    Vorobieva, Inessa; Mandal, Prantik; Gorshkov, Alexander

    2014-11-01

    The numerical block-and-fault model of lithosphere dynamics and seismicity (BAFD) is used to understand crustal motion and features of the observed seismicity in the Kachchh rift zone, Gujarat, Western India. The block-model allows simulating seismicity and geodynamics simultaneously unlike other modeling approaches for studying seismicity or geodynamics. The model structure of Kachchh rift zone is composed of seven major crustal blocks separated by fault planes. Based on the orientation of boundary crustal block movements, we develop a set of numerical experiments to analyze the spatial distribution of earthquakes, frequency-to-magnitude relationships, earthquake focal mechanisms, velocity field, and fault slip rates in the model. The main results of our modeling suggest that an NNW-SSE trending compression is a principal driving force in the Kachchh rift zone that explains basic features of the regional seismicity, direction of block motions, and the presence of an extensional stress regime associated with the Cambay rift zone. Large synthetic events occur on the fault segments associated with the Allah-Bund fault, Katrol hill fault and north Wagad fault which have been causative faults for the 1819 Mw7.7 Allah-Bund, 1956 Mw6.0 Anjar and 2001 Mw7.7 Bhuj earthquakes. The frequency-magnitude distribution for both synthetic seismicity and observed seismicity shows a similar slope. The focal mechanisms of the synthetic events are found to be consistent with those of earthquakes in the region. A special attention has been paid to study long-term and post-seismic deformations. Our results are in a qualitative agreement with the GPS post-seismic observations in the Kachchh rift zone. We infer that the observed seismicity and crustal block motions are a consequence of the dynamics of the entire regional fault and block system rather than that of a single causative fault only.

  19. Mindoro: a rifted microcontinent in collision with the Philippines volcanic arc; basin evolution and hydrocarbon potential

    NASA Astrophysics Data System (ADS)

    Bird, P. R.; Quinton, N. A.; Beeson, M. N.; Bristow, C.

    The Mindoro area is the easternmost part of the Palawan-Mindoro microcontinent which rifted away from the South China margin during the Early Oligocene. Sea floor spreading carried it southwards until the Late Miocene, when collision with the Philippines Arc in the east, and other terranes to the south began. This paper considers the collision tectonics which operated at the eastern end of the microcontinent, which resulted in the inversion and thrusting of its rifted margin. These processes are documented by seismic data and field outcrop. The syn- and post-rift sedimentary sequences contain prospective source and reservoir intervals. Subsequent burial and deformation have resulted in the generation of hydrocarbons, as proved by the existence of several oil seeps on Mindoro Island, and by hydrocarbon shows recorded in offshore exploration wells. The structural history of the area since the Eocene is divided into four phases: Syn-Rift:Early Eocene-Mid-OligoceneDrift:Late Oligocene-Mid-MioceneCollision:Mid-Late Miocenetranspression:Latest Miocene-Present The unconformities which separate these stratigraphic sequences can be identified on seismic data. The sequences show characteristic differences in stratigraphy and structure, which control their prospectivity. The Syn-Rift Sequence consists of non-marine arkosic sandstones at the base, becoming marine towards the top and including platform and reefal limestones. The Drift Sequence records an overall regression, beginning with the deposition of marine shales, sandstones and detrital limestones and passes upwards into deltaic sands, shales and coals. This sequence contains reservoir quality sandstones and several potential source horizons. On Mindoro Island, the Collision/Transpression Sequence interval is dominated by coarse clastics derived from the uplifted collision zone. In offshore areas to the south and west, away from the collision zone with the arc, a more uniform blanket of parallel bedded shale and calcaeous siltstones was deposited. These form a regional seal to structural traps which may contain significant volumes of hydrocarbons.

  20. Earthquakes and Geological Structures of the St. Lawrence Rift System

    NASA Astrophysics Data System (ADS)

    Lamontagne, M.; Ranalli, G.

    2013-12-01

    The St. Lawrence Rift System (SLRS), which includes the Ottawa-Bonnechère and Saguenay grabens, is located well inside the North American plate. Most historic and the some 350 earthquakes recorded yearly occur in three main seismically active zones, namely Charlevoix (CSZ), Western Quebec (WQSZ), and Lower St. Lawrence (LSLSZ)). Outside these areas, most of the Canadian Shield and bordering regions have had a very low level of earthquake activity. In the SLRS, moderate to large earthquakes (Moment magnitude (M) 5.5 to M 7) are known to have occurred since 1663 causing landslides and damage mostly to unreinforced masonry elements of buildings located on ground capable of amplifying ground motions. Most earthquakes in these seismic zones share common characteristics such as mid- to upper crustal focal depths, no known surface ruptures and proximity to SLRS faults. Variations also exist such as vast seismically-active region (WQSZ and LSLSZ), presence of a large water body (CSZ and LSLSZ), and absence of SLRS faults near concentration of earthquakes (WQSZ). The CSZ is the best studied seismic zone and there, earthquakes occur in the Canadian Shield, mostly in a 30 X 85 km rectangle elongated along the trend of the St. Lawrence River with local variations in focal depth distribution. Faults related to the SLRS and to a meteor impact structure exist and earthquakes occur along the SLRS faults as well as in between these faults. Overall, the SLRS faults are probably reactivated by the larger earthquakes (M ? 4.5) of the 20th century (CSZ in 1925; WQCSZ in 1935 and 1944; Saguenay in 1988) for which we have focal mechanisms. We propose that caution be exercised when linking historical events that have uncertain epicentres with SLRS faults. Similarly, SLRS faults should not be necessarily considered to be the reactivated structures for most small to moderate earthquakes (M < 4.5). A good example of this is the earthquakes of the WQSZ that tend to concentrate in a well-defined NW-SE alignment with no obvious geological control, except perhaps, a hypothetical hotspot track. Two local factors can lead to the occurrence of SLRS earthquakes: weak faults or enhanced stress levels. We propose that local conditions, concentrated in a few seismic zones, can alter these factors and lead to the occurrence of earthquakes, especially those with M < 4.5. At a continent-wide scale, the correlation between the SLRS and earthquakes is appealing. We suggest, however, that pre-existing faults related to the SLRS do not explain all features of the seismicity. Seismicity is concentrated in more active areas, some with conspicuous normal faults and some with suspected weakening mechanisms such as intense pre-fracturing (e.g. due to a meteorite impact), the passage over a hot spot, or the presence of intrusions and lateral crustal density variations.

  1. Crustal Rheology and Rifted Margin Architecture: Comparing Iberia-Newfoundland, Central South Atlantic, and South China Sea

    NASA Astrophysics Data System (ADS)

    Brune, Sascha

    2015-04-01

    Crustal rheology controls the style of rifting and ultimately the architecture of rifted margins: Hot, weak, or thick continental crust is dominated by ductile deformation and extends symmetrically into a wide rift system. Extension in cold, strong, or thin crust is accommodated by brittle faults and ductile shear zones that facilitate narrow rifts with asymmetric fault geometries. This recipe provides the standard framework to understand 2D rift geometry, however, a variety of processes exert significant control on subsequent rift evolution and ultimately on the architecture of rifted margins: inherited structures, melting and volcanism, 3D effects, extension rate, and weakening mechanisms. Numerical forward modelling studies have the opportunity to evaluate the influence of these processes on rift evolution in order to understand the complex interaction between rheology and tectonic history of specific margins. Here I compare the formation of three different magma-poor margin pairs, Iberia-Newfoundland, the Central South Atlantic Rift Segment, and the South China Sea margins within a numerical forward modelling framework. I apply a 2D version of the finite element code SLIM3D, which includes nonlinear temperature- and stress-dependent elasto-visco-plastic rheology and is able to reproduces a wide range of rift-related deformation processes such as flexure, lower crustal flow, and faulting. The Iberia-Newfoundland rifted margins are marked by moderate crustal asymmetry, with ~70 km of hyper-extended crust (less than 10 km thick) on the Iberian side and a very narrow margin on the Newfoundland counterpart. Similar to the Iberia-Newfoundland conjugates, the Central South Atlantic margins are predominantly asymmetric, however involve a much stronger degree of asymmetry with more than 200 km of hyper-extended crust offshore Angola, but only few tens of km at the Brazilian side. Kinematic and numerical modelling suggests that the asymmetry is caused by lateral migration of the rift centre, which generates sequential fault activity within the brittle crust. Rift migration results from two processes: (i) Strain hardening takes place in the rift centre due to cooling of upwelling mantle material. (ii) The formation of a low viscosity crustal pocket adjacent to the rift centre is caused by heat transfer from the mantle and viscous strain softening of the lower crust. These mechanisms generate a lateral strength contrast that promotes rift migration in a steady-state manner forming a wide sliver of hyper-extended crust on one margins side, while the conjugate margin becomes narrow. In contrast to these Atlantic examples where wide margins are formed diachronously, the South China Sea evolved in wide rift mode. Here, several hundred kilometres of highly attenuated continental crust are deformed simultaneously during ~40 My of extension. Numerical modelling suggests that the presence of weak, ductile crust enabled the formation of two wide and symmetric margins. Independent indicators for a weak crust come from super-deep basins on the northern margin. These basins appear to be created after the end of active extension and with a significant deficit in brittle faulting, which suggests that subsidence was controlled by sediment loading and accommodated by lower crustal flow, a style of basin formation that is only possible in the presence of low crustal viscosity.

  2. Faults kinematics, paleo- and present-day stress-strain state in the Central Baikal rift

    NASA Astrophysics Data System (ADS)

    Sankov, Vladimir; Lukhnev, Andrey; Miroshnitchenko, Andrey; Ashurkov, Sergey; Byzov, Leonid; Sankov, Alexey; Dembelov, Mihkail

    2013-04-01

    The central part of the Baikal rift system is commonly explained as a zone of continental extension separating of Siberian block attributed to North Eurasian plate and Transbaikal block attributed to Amur plate. The major and secondary NNE and NE active faults were recognized as normal faults with insignificant strike-slip component. We propose the interpretation of new data of investigation the junction zone of South Baikal, North Baikal and Barguzin basins using structural, geomorphological, seismological and GPS geodesy methods and paleostress reconstructions. Regionally the morphology of fault patterns shows an-echelon structures of right lateral type for NNE trending zones and left lateral type for NE and sublatitudinal zones. In agreement with these morphological features our stress reconstructions show strike-slip type of stress tensors for the initial (Oligocene and Miocene) stage of faulting along the basins borders. The second deformation stage (Pliocene-Pleistocene) is characterized by extension stress regime with the same (NW-SE) direction of minimum compression axis (Delvaux et al., 1997; Sankov et al., 1997). It corresponds with present-day stress field obtaining by earthquake focal mechanisms inversion (Petit et al., 1996). The only NE trending Argoda-Garga deformation zone on the eastern border of Barguzin basin is an example of initial stage deformation development during Late Pleistocene-Holocene. The left lateral strike-slips are dominated in this zone as evidenced by geomorphology and structural data. The results of our GPS measurements show SE block movements in the Central Baikal rift relative to Siberian block. The computation of relative horizontal deformations exhibits complicated strain distribution with prevailing of NW-SE elongation and NE-SW shortening. In general the results of GPS measurements show possibility of right lateral movements along NNE rift structures and extensional and left lateral movements along NE rift structures. We suppose the evolution mechanism of Central Baikal rift structures corresponding to paleo- and present-day stress-strain state and faults kinematics data. The realization of NW-SE blocks divergence in the intracontinental setting includes the stage of initial crustal deformations which characterizes mostly by horizontal movements and strike-slip to transtension stress regimes. The latest are a consequence of horizontal compression of the crust prevailing within the continental massive. Also the edition lithosphere compression of far field plate interaction origin can't be excluded. The pure extension deformation regime and normal faulting are attributed to second stage of crustal destruction accompanied by intensive rift shoulders uplift and sedimentary loading in the rift basins. These conditions are dominated recently in the Center of Baikal rift. In contrast the active strike-slip deformation regime can be discovered within interbasin blocks and on gentle slopes of rift half-grabens as a relic of initial stage of crustal extension.

  3. Precambrian alluvial fan and braidplain sedimentation patterns: Example from the Mesoproterozoic Rjukan Rift Basin, southern Norway

    NASA Astrophysics Data System (ADS)

    Köykkä, Juha

    2011-03-01

    An integrated approach of facies analysis, geochemistry and paleohydrology provides new insight into the sedimentology and paleogeography of alluvial sediments in the Precambrian. Here, alluvial fan and braidplain sedimentation patterns are documented in a Mesoproterozoic rift basin, called (known as) the Rjukan Rift Basin in southern Norway. The studied formation (Heddersvatnet Formation) consists of volcanoclastic breccias and conglomerates, cross-bedded sandstones with associated mudstones, and subaerial basaltic lava beds, deposited in a rift basin during the syn-rifting phase. Based on genetically related major lithofacies associations and individual minor lithofacies, the sedimentation is characterized by colluvial and screen apron deposits, subaerial debris flows, hyperconcentrated or sheet flood deposits in the proximal part, representing waning-flood cycles. The distal part contains shallow-water traction current deposits and associated subaerial continental flood basalts that flowed down the rift valleys. The synsedimentary intra-rift faulting formed seasonal or climatically controlled, intrafan ponded-lake with playa lake type cycles and shrinkage cracks. The geochemical composition of the of the studied sandstones and mudstones suggests a passive rifted continental margin with minor to moderate chemical weathering, possible in a semi-arid/arid paleoclimate. Clastic petrofacies of the sandstones indicate a locally uplifted and syn-rift paleotectonic setting during the main sedimentation stage, passing toward a post-rifting stage of the depositional basin. Small-scale structures within the sedimentary basin affected the depositional patterns of the Heddersvatnet Formation by creating local sites of uplift and erosion, controlling the sediment transport, and defining the accommodation space for deposition. The sedimentation of the Heddersvatnet Formation reflects a tectonic base level fall, which resulted in an accumulation of thick alluvial fan or braidplain sediments near uplifted flank(s) of the sedimentary basin. This was combined with intensive erosion of the basement in the pre-vegetation landscape, which resulted in changes in the hydraulic geometry of flow and sediment discharge of stream channels. The following criteria, observed from the Heddersvatnet Formation, are suggestive of an ancient alluvial fan and associated braidplain sedimentation: (1) deposition close to the source area; (2) slightly divergent paleoflows; (3) high-energy flows with waning flood-cycles; (4) abundance of poorly sorted and laterally extensive subaerial debris flows, and related streamflows, possible sheetfloods, hyperconcentrated flows, and their associations; (5) great distribution of grain size and lithofacies changes in proximal vs. distal parts; (6) limited suite of sedimentary structures (cross-stratification, ripple-marks, and desiccanation cracks); (7) fault bounded basin (graben) with hanging wall close to uplifted flank(s); (8) paleoslope estimation indicating at least moderate paleotopographic highs; (9) colluvial and scree apron breccias; (10) lack of any typical braided stream channel fill conglomerates and floodplains; and (11) channelized sediment bodies lacking extensive lateral continuity.

  4. Geologische Rundschau 80/1 I 155-170 [ Stuttgart 1991 A late Precambrian 710 Ma) high volcanicity rift

    E-print Network

    Stern, Robert J.

    volcanicity rift in the southern Eastern Desert of Egypt By ROBERTJ. STERN,Dallas, ALFREDKRONER-Metavulkanite in der sfid6stlichen Eastern Desert von )~gypten sind ei- ne schwach metamorphe bimodale Basalt

  5. Geophys. J. Int. (1998) 135, 10451059 The Oslo Rift: new palaeomagnetic and 40Ar/39Ar age constraints

    E-print Network

    Torsvik, Trond Helge

    1998-01-01

    with the Kiaman 1996). The latest Permian­Early Triassic period in the Oslo Reverse Superchron (KRS) (ca. 311 Carboniferous­Permian (294­274 Ma) and Kiaman-aged poles from the Oslo Rift. Key words: dykes, geochronology

  6. Evolution of oceanic margins : rifting in the Gulf of California and sediment diapirism and mantle hydration during subduction

    E-print Network

    Miller, Nathaniel Clark

    2013-01-01

    This thesis investigates three processes that control the evolution of oceanic margins. Chapter 2 presents seismic images of a ~2-km-thick evaporite body in Guaymas Basin, central Gulf of California. In rifts, evaporites ...

  7. Regional 3D Numerical Modeling of the Lithosphere-Mantle System: Implications for Continental Rift-Parallel Surface Velocities

    NASA Astrophysics Data System (ADS)

    Stamps, S.; Bangerth, W.; Hager, B. H.

    2014-12-01

    The East African Rift System (EARS) is an active divergent plate boundary with slow, approximately E-W extension rates ranging from <1-6 mm/yr. Previous work using thin-sheet modeling indicates lithospheric buoyancy dominates the force balance driving large-scale Nubia-Somalia divergence, however GPS observations within the Western Branch of the EARS show along-rift motions that contradict this simple model. Here, we test the role of mantle flow at the rift-scale using our new, regional 3D numerical model based on the open-source code ASPECT. We define a thermal lithosphere with thicknesses that are systematically changed for generic models or based on geophysical constraints in the Western branch (e.g. melting depths, xenoliths, seismic tomography). Preliminary results suggest existing variations in lithospheric thicknesses along-rift in the Western Branch can drive upper mantle flow that is consistent with geodetic observations.

  8. 77 FR 68783 - Prospective Grant of Co-Exclusive License: Veterinary Vaccines for Rift Valley Fever Virus

    Federal Register 2010, 2011, 2012, 2013, 2014

    2012-11-16

    ...: Veterinary Vaccines for Rift Valley Fever Virus AGENCY: Centers for Disease Control and Prevention (CDC...-exclusive license in Africa, in the field of use of veterinary vaccines, to practice the inventions...

  9. Ages of pre-rift basement and synrift rocks along the conjugate rift and transform margins of the Argintine Precordillera and Laurentia

    USGS Publications Warehouse

    Thomas, William A.; Tucker, Robert D.; Astini, Ricardo A.; Denison, Rodger E.

    2012-01-01

    New geochronologic data from basement rocks support the interpretation that the Argentine Precordillera (Cuyania) terrane was rifted from the Ouachita embayment of the Iapetan margin of Laurentia. New data from the Ozark dome show a range of ages in two groups at 1466 ± 3 to 1462 ± 1 Ma and 1323 ± 2 to 1317 ± 2 Ma, consistent with existing data for the Eastern Granite-Rhyolite province and Southern Granite-Rhyolite province, respectively. Similarly, a newly determined age of 1364 ± 2 Ma for the Tishomingo Granite in the Arbuckle Mountains confirms previously published analyses for this part of the Southern Granite-Rhyolite province. Along with previously reported ages from basement olistoliths in Ordovician slope deposits in the Ouachita embayment, the data for basement ages support the interpretation that rocks of the Southern Granite-Rhyolite province form the margin of Laurentian crust around the corner of the Ouachita embayment, which is bounded by the Ouachita rift and Alabama-Oklahoma transform fault. In contrast, both west and east of the corner of the Ouachita embayment, Grenville-Llano basement (approximately 1325–1000 Ma) forms the rifted margin of Laurentia. New U/Pb zircon data from basement rocks in the southern part of the Argentine Precordillera indicate crystallization ages of 1205 ± 1 Ma and 1204 ± 2 Ma, consistent with previously reported ages (approximately 1250–1000 Ma) of basement rocks from other parts of the Precordillera. These data document multiple events within the same time span as multiple events in the Grenville orogeny in eastern Laurentia, and are consistent with Grenville-age rocks along the conjugate margins of the Precordillera and Laurentia. Ages from one newly analyzed collection, however, are older than those from other basement rocks in the Precordillera. These ages, from granodioritic-granitic basement clasts in a conglomerate olistolith in Ordovician slope deposits, are 1370 ± 2 Ma and 1367 ± 5 Ma. These older ages from the Precordillera are consistent with indications that the Iapetan margin in the Ouachita embayment of Laurentia truncated the Grenville front and left older rocks of the Southern Granite-Rhyolite province (1390–1320 Ma) at the rifted margin. Chronostratigraphic correlations of synrift and post-rift sedimentary deposits on the Precordillera and on the Texas promontory of Laurentia document initial rifting in the Early Cambrian. Previously published data from synrift plutonic and volcanic rocks in the Wichita and Arbuckle Mountains along the transform-parallel intracratonic Southern Oklahoma fault system inboard from the Ouachita embayment document crystallization ages of 539–530 Ma. New data from synrift volcanic rocks in the Arbuckle Mountains in the eastern part of the Southern Oklahoma fault system yield ages of 539 ± 5 Ma and 536 ± 5 Ma, confirming the age of synrift volcanism.

  10. Transect across the West Antarctic rift system in the Ross Sea, Antarctica

    USGS Publications Warehouse

    Trey, H.; Cooper, A. K.; Pellis, G.; Della, Vedova B.; Cochrane, G.; Brancolini, Giuliano; Makris, J.

    1999-01-01

    In 1994, the ACRUP (Antarctic Crustal Profile) project recorded a 670-km-long geophysical transect across the southern Ross Sea to study the velocity and density structure of the crust and uppermost mantle of the West Antarctic rift system. Ray-trace modeling of P- and S-waves recorded on 47 ocean bottom seismograph (OBS) records, with strong seismic arrivals from airgun shots to distances of up to 120 km, show that crustal velocities and geometries vary significantly along the transect. The three major sedimentary basins (early-rift grabens), the Victoria Land Basin, the Central Trough and the Eastern Basin are underlain by highly extended crust and shallow mantle (minimum depth of about 16 km). Beneath the adjacent basement highs, Coulman High and Central High, Moho deepens, and lies at a depth of 21 and 24 km, respectively. Crustal layers have P-wave velocities that range from 5.8 to 7.0 km/s and S-wave velocities from 3.6 to 4.2 km/s. A distinct reflection (PiP) is observed on numerous OBS from an intra-crustal boundary between the upper and lower crust at a depth of about 10 to 12 km. Local zones of high velocities and inferred high densities are observed and modeled in the crust under the axes of the three major sedimentary basins. These zones, which are also marked by positive gravity anomalies, may be places where mafic dikes and sills pervade the crust. We postulate that there has been differential crustal extension across the West Antarctic rift system, with greatest extension beneath the early-rift grabens. The large amount of crustal stretching below the major rift basins may reflect the existence of deep crustal suture zones which initiated in an early stage of the rifting, defined areas of crustal weakness and thereby enhanced stress focussing followed by intense crustal thinning in these areas. The ACRUP data are consistent with the prior concept that most extension and basin down-faulting occurred in the Ross Sea during late Mesozoic time, with relatively small extension, concentrated in the western half of the Ross Sea, during Cenozoic time.

  11. A Spatial Analysis of Rift Valley Fever Virus Seropositivity in Domestic Ruminants in Tanzania

    PubMed Central

    Sindato, Calvin; Pfeiffer, Dirk U.; Karimuribo, Esron D.; Mboera, Leonard E. G.; Rweyemamu, Mark M.; Paweska, Janusz T.

    2015-01-01

    Rift Valley fever (RVF) is an acute arthropod-borne viral zoonotic disease primarily occurring in Africa. Since RVF-like disease was reported in Tanzania in 1930, outbreaks of the disease have been reported mainly from the eastern ecosystem of the Great Rift Valley. This cross-sectional study was carried out to describe the variation in RVF virus (RVFV) seropositivity in domestic ruminants between selected villages in the eastern and western Rift Valley ecosystems in Tanzania, and identify potential risk factors. Three study villages were purposively selected from each of the two Rift Valley ecosystems. Serum samples from randomly selected domestic ruminants (n = 1,435) were tested for the presence of specific immunoglobulin G (IgG) and M (IgM), using RVF enzyme-linked immunosorbent assay methods. Mixed effects logistic regression modelling was used to investigate the association between potential risk factors and RVFV seropositivity. The overall RVFV seroprevalence (n = 1,435) in domestic ruminants was 25.8% and speciesspecific seroprevalence was 29.7%, 27.7% and 22.0% in sheep (n = 148), cattle (n = 756) and goats (n = 531), respectively. The odds of seropositivity were significantly higher in animals sampled from the villages in the eastern than those in the western Rift Valley ecosystem (OR = 1.88, CI: 1.41, 2.51; p<0.001), in animals sampled from villages with soils of good than those with soils of poor water holding capacity (OR = 1.97; 95% CI: 1.58, 3.02; p< 0.001), and in animals which had been introduced than in animals born within the herd (OR = 5.08, CI: 2.74, 9.44; p< 0.001). Compared with animals aged 1–2 years, those aged 3 and 4–5 years had 3.40 (CI: 2.49, 4.64; p< 0.001) and 3.31 (CI: 2.27, 4.82, p< 0.001) times the odds of seropositivity. The findings confirm exposure to RVFV in all the study villages, but with a higher prevalence in the study villages from the eastern Rift Valley ecosystem. PMID:26162089

  12. Conjugate volcanic rifted margins, spreading and micro-continent: Lessons from the Norwegian-Greenland Sea

    NASA Astrophysics Data System (ADS)

    Gernigon, L.; Blischke, A.; Nasuti, A.; Sand, M.

    2014-12-01

    We have acquired and processed new aeromagnetic data that covers the entire Norway Basin oceanic spreading system located between the Møre volcanic rifted margin and its (intermediate) conjugate system, the Jan Mayen microcontinent (JMMC). The new compilation allows us to revisit its entire structure and spreading evolution from the Early Eocene breakup to the Late Oligocene abortion of the Aegir Ridge. We here discuss the dynamics of conjugate volcanic (rifted) margin formation and reconstruct the subsequent spreading evolution of the Norway Basin until its abortion. We have estimated a new set of Euler poles of rotation for the Norway Basin derived from more than 88,000 km of new magnetic profiles. The new compilation confirms that a fan-shaped spreading evolution of the Norway Basin was particularly active before the cessation of seafloor spreading and abortion of the Aegir Ridge. The Norway Basin shows a more complex system of micro-plates and asymmetric segments locally affected by episodic ridge jumps. The new observations have implications for the syn- and post-breakup stratigraphic and rifted-margin tectonic development of the JMMC and its conjugate margins. In particular, an important Mid-Eocene geodynamic event at around magnetic chron C21r is recognized in the Norway Basin. This event coincides with the onset of diking and rifting between the proto-JMMC and the East Greenland margin, leading to a second phase of breakup in the Norwegian-Greenland Sea ~26 My later in the Oligocene. Restored in its pre-breakup configuration, the new surveys also allow us to discuss further the tectonic and crustal evolution of the conjugate volcanic rifted margins and associated transform and oblique segments. The applicability of magma-poor concepts, off Norway, for example, remains questionable for us. The significant amount of breakup magmatism, the huge amount of pre-breakup sag sedimentation and the presence of thinned and preserved continental crust without the systematic occurrence of underlying and/or exhumed serpentinised terranes. The mid-Norwegian volcanic rifted margins appear quite different from the Iberian type, magma-poor margin, even if the processes leading to the early (and aborted) thinning events seems to be similar to some extent.

  13. The subsurface structure and stratigraphic architecture of rift-related units in the Lishu Depression of the Songliao Basin, China

    NASA Astrophysics Data System (ADS)

    Wang, Hongyu; Fan, Tailiang; Wu, Yue

    2015-03-01

    This contribution reports the basin configuration feature, stratigraphy and sedimentary architecture of the Lishu Depression in the Songliao Basin, China. The activity rate, distribution and style of local faulting demonstrate the timing and extent of regional rifting. Distinct episodes of compressional tectonic activity caused uplift and exposure of strata evident as the traditional syn- and post-rift stages of basin evolution. These episodes led to the sequential denudation of the Upper Jurassic Huoshiling Formation, Lower Cretaceous Yingcheng and Denglouku Formations, and corresponding regional unconformities. Acting in tandem with regional compression, activity along the major boundary faults influenced the evolving basin configuration, as well as seismic sequences and sedimentary patterns. Seismic, well log and drill core data described here show subdivision sections of the Lishu Depression strata according to discrete phases of the traditional syn-rift stage of deposition. We refer to these sub-stages as the initial rifting, the intensive rifting and the recession phases. The basin configuration shifted from a graben/half-graben configuration during the initial rifting phase, to a dustpan-shaped half-graben pattern during the subsequent phase of intensive rifting, and finally into a gentle sedimentary basin during the final recession phase. The early seismic sequence divides into a lowstand systems tract (LST), transgressive systems tract (TST) and highstand systems tract (HST). Evidence of the LST within the seismic sequence becomes less apparent with the intensive rifting phase, while the HST occupied an increasing proportion of the section. The shallow water depositional fill formed during the final recession phase consists only of TST and HST components. Depositional environment then shifts from alluvial fan and shallow lacustrine systems to fan delta, braided delta - lake, and finally to a braided fluvial setting. The vertical stacking pattern shifts from retrogradational, to progradational, to aggradational. Identification of sub-structural units and interpretation of their genetic relationships helps clarify basin evolution, and thus serves larger-scale continental basin analysis.

  14. Receiver function imaging of the onset of melting, implications for volcanism beneath the Afar Rift in contrast to hotspot environments

    NASA Astrophysics Data System (ADS)

    Rychert, C. A.; Harmon, N.; Hammond, J. O.; Laske, G.; Kendall, J.; Ebinger, C. J.; Shearer, P. M.; Bastow, I. D.; Keir, D.; Ayele, A.; Belachew, M.; Stuart, G. W.

    2012-12-01

    Heating, melting, and stretching destroy continents at volcanic rifts. Mantle plumes are often invoked to thermally weaken the continental lithosphere and accommodate rifting through the influx of magma. However the relative effects of mechanical stretching vs. melt infiltration and weakening are not well quantified during the evolution of rifting. S-to-p (Sp) imaging beneath the Afar Rift and hotspot regions such as Hawaii provides additional constraints. We use data from the Ethiopia/Kenya Broadband Seismic Experiment (EKBSE), the Ethiopia Afar Geophysical Lithospheric Experiment (EAGLE), a new UK/US led deployment of 46 stations in the Afar depression and surrounding area, and the PLUME experiment. We use two methodologies to investigate structure and locate robust features: 1) binning by conversion point and then simultaneous deconvolution in the frequency domain, and 2) extended multitaper followed by migration and stacking. We image a lithosphere-asthenosphere boundary at ~75 km beneath the flank of the Afar Rift vs. its complete absence beneath the rift, where the mantle lithosphere has been totally destroyed. Instead a strong velocity increase with depth at ~75 km depth matches geodynamic model predictions for a drop in melt percentage at the onset of decompression melting. The shallow depth of the onset of melting is consistent with a mantle potential temperature = 1350 - 1400°C, i.e., typical for adiabatic decompression melting. Therefore although a plume initially destroyed the mantle lithosphere, its influence directly beneath Afar today is minimal. Volcanism continues via adiabatic decompression melting assisted by strong melt buoyancy effects. This contrasts with a similar feature at much deeper depth, ~150 km, just west of Hawaii, where a deep thermal plume is hypothesized to impinge on the lithosphere. Improved high resolution imaging of rifting, ridges, and hotspots in a variety of stages and tectonic settings will increase constraints on the forces sustaining volcanism and the factors that dictate the style of breakup beneath rifts.

  15. Seismic anisotropy of the lithosphere/asthenosphere system beneath the Rwenzori region of the East-African Rift

    NASA Astrophysics Data System (ADS)

    Homuth, Benjamin; Löbl, Ulrike; Batte, Arthur; Link, Klemens; Kasereka, Celestine; Rümpker, Georg

    2014-05-01

    We present results from a temporary seismic network of 32 broad-band stations located around the Rwenzori region of the Albertine rift at the border between Uganda and DR Congo. The study aims to constrain seismic anisotropy and mantle deformation processes in relation to the formation of the rift zone. Shear-wave splitting measurements from local and teleseismic earthquakes are used to investigate the seismic anisotropy in the crust and upper mantle beneath the Rwenzori region. At most stations, shear-wave splitting parameters obtained from individual earthquakes exhibit only minor variations with backazimuth. We therefore employ a joint inversion of SKS waveforms to derive hypothetical one-layer parameters. The corresponding fast polarizations are generally rift-parallel and the average delay time is about 1 s. On the other hand, shear phases from local events within the crust are characterized by a bimodal pattern of fast polarizations and an average delay time of 0.04 s. This observation suggests that the dominant source region for seismic anisotropy beneath the rift is located within the mantle. We use finite-frequency waveform modeling to test different models of anisotropy within the lithosphere/asthenosphere system of the rift. The results show that the rift-parallel fast polarizations are consistent with HTI anisotropy caused by rift-parallel magmatic intrusions or lenses located within the lithospheric mantle - as it would be expected during the early stages of continental rifting. Furthermore, the short-scale spatial variations in the fast polarizations observed in the southern part of the study area can be explained by effects due to sedimentary basins of low isotropic velocity in combination with a shift in the orientation of anisotropic fabrics in the upper mantle. A uniform anisotropic layer in relation to large-scale asthenospheric mantle flow is less consistent with the observed splitting parameters.

  16. History of the development of the East African Rift System: A series of interpreted maps through time

    NASA Astrophysics Data System (ADS)

    Macgregor, Duncan

    2015-01-01

    This review paper presents a series of time reconstruction maps of the 'East African Rift System' ('EARS'), illustrating the progressive development of fault trends, subsidence, volcanism and topography. These maps build on previous basin specific interpretations and integrate released data from recent petroleum drilling. N-S trending EARS rifting commenced in the petroliferous South Lokichar Basin of northern Kenya in the Late Eocene to Oligocene, though there seem to be few further deep rifts of this age other than those immediately adjoining it. At various times during the Mid-Late Miocene, a series of small rifts and depressions formed between Ethiopia and Malawi, heralding the main regional rift subsidence phase and further rift propagation in the Plio-Pleistocene. A wide variation is thus seen in the ages of initiation of EARS basins, though the majority of fault activity, structural growth, subsidence, and associated uplift of East Africa seem to have occurred in the last 5-9 Ma, and particularly in the last 1-2 Ma. These perceptions are key to our understanding of the influence of the diverse tectonic histories on the petroleum prospectivity of undrilled basins.

  17. A structural analysis of the Midcontinent Rift in Michigan, based on a fault array analysis utilizing slickenside

    SciTech Connect

    Witthuhn, K.M.; Teyssier, C. . Dept. of Geology and Geophysics)

    1992-01-01

    The Midcontinent Rift is a 1.1 billion year old crustal structure which has been defined primarily on the basis of geophysical studies. It displays evidence of both rifting and subsequent closure. Many studies have examined the rift from a petrologic view but few have examined it in any detail from a structural geological viewpoint. This paper examines the structural aspects of the rift, elucidates the direction of closing, and constrains the timing of the system by utilizing paleostress stratigraphy. Establishment of the relative age and sense of movement on faults was accomplished by analyzing slickensides and crystallization on the fault plane. Detailed work on the Keweenaw Peninsula of Michigan suggests a southerly direction for the closing of the rift system, while similar work on Isle Royale suggests an east-southeasterly closing direction. Faults in both areas with opposite sense of movement (dextral vs. sinistral, normal vs. reverse) have similar attitudes indicating a reversal of maximum and minimum stress directions. Stress tensors from both limbs of the Lake Superior syncline were isolated using identical methods, suggesting the geometry or lithology of the system interfered with the manifestation of the far-field stresses. The results suggest then the direction of opening and closing of the rift was constrained by the geometry of the major and minor fault systems and anisotropies in the lava flows.

  18. Early Cretaceous rifting and exposure of periodotite on the Galicia continental margin: preliminary results of ocean drilling program Leg 103

    SciTech Connect

    Winterer, E.; Boillot, G.; Meyer, A.; Applegate, J.; Baltuck, M.; Bergen, J.; Comas, M.; Davies, T.; Dunham, K.; Evans, C.; Girardeau, J.

    1985-01-01

    Results of drilling near the ocean-continent boundary on the Galicia margin of Iberia shed new light on the timing of rifting and demonstrate the presence at the foot of the margin of a ridge of foliated, lineated, sheared and serpentinized harzburgite, probably representing oceanic mantle. Fifty km east of the periodotite ridge, on a continental fault block, the stratigraphic section sampled during Leg 103 above Hercynian basement comprises: (1) at least 250m of Upper Jurassic and possibly lowest Cretaceous limestone, dolomite and minor sandstone and claystone deposited in relatively shallow water before rifting began; (2) about 20m of Valanginian calpionellid marlstone, probably deposited in moderate depths at the onset of rifting; (3) from about 500 to 1500m of Valanginian and Hauterivian turbidite sandstone rich in terrestrial plant debris, and Barremian and Aptian( ) claystone and marlstone deposited in deeper water during rifting; and (4) about 700m of sediments deposited after Aptian time, when rifting ceased and oceanic spreading between Iberia and Newfoundland began. The lithology and seismic stratigraphy of the wedges of clastic sediments laid down during rifting show the progressive filling of basins that formed by episodic listric faulting that began very early in the Cretaceous and continued for about 25 my. The Lower Cretaceous turbidite sandstone cored on the Galicia margin correlates with thick Lower Cretaceous turbidites cored off Morocco during DSDP Leg 50, and with Wealden deltaic and fluviatile deposits on both sides of the Atlantic.

  19. What the volcanism of the East African Rift tells us on its evolution and dynamics: a reappraisal

    NASA Astrophysics Data System (ADS)

    Michon, Laurent

    2015-04-01

    The East African Rift (EAR) is one of the most studied tectonic structures on Earth. Classically, it is described as extending from Afar in the North to the Malawi rift in the South, along the eastern and western branches, respectively. A widely accepted consensus also exists on two main points: 1- the rift initiated first with plume emplacement below the northern part of the eastern branch and 2- extension and volcanism subsequently migrated southward along the western branch (e.g., Ebinger, 1989). However, an increasing amount of new geochronological data on the volcanic activity in the southern part of the East African Rift tends to weaken these interpretations and imposes a reassessment of the rift dynamics. The volcanic activity being one of the main characteristics of this rift, I use it here to determine the lateral extension of the rift system and to assess the rift activity through times. First, the volcanism unambiguously indicates that the rift is not limited to the African continent but can be traced in the Mozambique Channel and in Madagascar where it is closely related to active tectonics (graben and transfer faults) initiated since at least the Miocene. Moreover, the synthesis of more than 800 published geochronological data of volcanic products distributed over the overal East African Rift allows the distinction of two parts. The first part, the Northern EAR, corresponds to the sole eastern branch, which is characterized by volcanic plateaus resulting from huge magma flux during three main periods (32-28, 18-12, 6-0 Ma). Provinces of the second part, the Southern EAR (the western branch, the Mozambique Channel and Madagascar), share rift-related scattered volcanic centres characterized by coeval periods of activity since the Oligocene (28-24, 20-16 and 12-0 Ma). This synthesis highlights the lack of southward migration of the volcanism during the evolution of the East African Rift and instead reveals the almost synchronous development of the volcanism all along the rift system. Moreover, it suggests the existence of two parts that experienced different evolutions since around 30 Ma. The volcanism of the Northern EAR shows characteristics, which agrees with a mantle plume origin. On the contrary, the development of stationary volcanic centres in the Southern EAR despite the northward motion of the African plate disagrees with such an interpretation and instead suggests a tectonically-assisted origin of the volcanism along main Precambrian lithospheric structures. The occurrence of several pulses of volcanism in each part of the East African Rift, coeval with periods of crustal uplift, could stem from successive main tectonic phases favoured by mantle bursts fed by the African superswell. Reference: Ebinger, C.J., 1989, Tectonic development of the western branch of the East African rift system: Geological Society of America Bulletin, v. 101, p. 885-903.

  20. Stratigraphic and structural evolution of the Selenga Delta Accommodation Zone, Lake Baikal Rift, Siberia

    USGS Publications Warehouse

    Scholz, C.A.; Hutchinson, D.R.

    2000-01-01

    Seismic reflection profiles from the Lake Baikal Rift reveal extensive details about the sediment thickness, structural geometry and history of extensional deformation and syn-rift sedimentation in this classic continental rift. The Selenga River is the largest single source of terrigenous input into Lake Baikal, and its large delta sits astride the major accommodation zone between the Central and South basins of the lake. Incorporating one of the world's largest lacustrine deltas, this depositional system is a classic example of the influence of rift basin structural segmentation on a major continental drainage. More than 3700?km of deep basin-scale multi-channel seismic reflection (MCS) data were acquired during the 1989 Russian and the 1992 Russian–American field programs. The seismic data image most of the sedimentary section, including pre-rift basement in several localities. The MCS data reveal that the broad bathymetric saddle between these two major half-graben basins is underlain by a complex of severely deformed basement blocks, and is not simply a consequence of long-term deltaic deposition. Maximum sediment thickness is estimated to be more than 9?km in some areas around the Selenga Delta. Detailed stratigraphic analyses of the Selenga area MCS data suggest that modes of deposition have shifted markedly during the history of the delta. The present mode of gravity- and mass-flow sedimentation that dominates the northern and southern parts of the modern delta, as well as the pronounced bathymetric relief in the area, are relatively recent developments in the history of the Lake Baikal Rift. Several episodes of major delta progradation, each extending far across the modern rift, can be documented in the MCS data. The stratigraphic framework defined by these prograding deltaic sequences can be used to constrain the structural as well as depositional evolution of this part of the Baikal Rift. An age model has been established for this stratigraphy, by tying the delta sequences to the site of the Baikal Drilling Project 1993 Drill Hole. Although the drill hole is only 100?m deep, and the base of the cores is only ?670?ka in age, ages were extrapolated to deeper stratigraphic intervals using the Reflection-Seismic-Radiocarbon method of Cohen et al. (1993). The deep prograding delta sequences now observed in the MCS data probably formed in response to major fluctuations in sediment supply, rather than in response to shifts in lake level. This stratigraphic framework and age model suggest that the deep delta packages developed at intervals of approximately 400?ka and may have formed as a consequence of climate changes affiliated with the northern hemisphere glaciations. The stratigraphic analysis also suggests that the Selenga Basin and Syncline developed as a distinct depocentre only during the past ?2–3?Ma.

  1. Serological surveillance studies confirm the Rift Valley fever virus free status in South Korea.

    PubMed

    Kim, Hyun Joo; Park, Jee-Yong; Jeoung, Hye-Young; Yeh, Jung-Yong; Cho, Yun-Sang; Choi, Jeong-Soo; Lee, Ji-Youn; Cho, In-Soo; Yoo, Han-Sang

    2015-10-01

    Rift Valley fever is a mosquito-borne zoonotic disease of domestic ruminants. This disease causes abortions in pregnant animals, and it has a high mortality rate in newborn animals. Recently, a Rift Valley fever virus (RVFV) outbreak in the Arabian Peninsula increased its potential spread to new regions worldwide. In non-endemic or disease-free countries, early detection and surveillance are important for preventing the introduction of RVFV. In this study, a serological surveillance was conducted to detect antibodies against RVFV. A total of 2382 serum samples from goats and cattle were randomly collected from nine areas in South Korea from 2011 to 2013. These samples were tested for antibodies against RVFV, using commercial ELISA kits. None of the goats and cattle were positive for antibodies against RVFV. This finding suggests that this disease is not present in South Korea, and furthermore presents the evidence of the RVFV-free status of this country. PMID:26024956

  2. A new brachypterous scarab species, Orphnus longicornis (Coleoptera: Scarabaeidae: Orphninae), from the East African Rift.

    PubMed

    Frolov, Andrey; Akhmetova, Lilia

    2015-01-01

    The Afrotropical Region is the center of the diversity of the scarab beetle genus Orphnus MacLeay, 1819 (Coleoptera: Scarabaeidae: Orphninae), with 94 species occurring from Sahel in the north to Little Karoo in the south (Paulian, 1948; Petrovitz, 1971; Frolov, 2008). The East African Rift is one of the richest regions of the Afrotropics housing more than 20 species of Orphnus (Paulian, 1948; Frolov, 2013), most of which are endemic to this region. Yet the scarab beetle fauna of the East African Rift, and especially the Eastern Arc Mountains, is still inadequately studied. Examination of the material housed in the Museum of Natural History of Humboldt-Universität, Berlin, Germany (ZMHUB), revealed a series of brachypterous Orphnus beetles belonging to an undescribed species. The new species is described and illustrated below. PMID:26624632

  3. Magmatic constraints on rift development and the uplift of the Rwenzori Mountains in western Uganda

    NASA Astrophysics Data System (ADS)

    Foley, Stephen; Link, Klemens; Tiberindwa, John; Barifaijo, Erasmus

    2010-05-01

    The northern end of the western branch of the East African rift valley in western Uganda is located at the junction between a Proterozoic belt to the south and continuous Archaean crust of the Congo-Tanzania craton to the north (1). The difficulty of rift propagation through the thick Archaean lithosphere may explain the lack of a connection with the eastern rift branch around Lake Turkana. The >5,000m-high Rwenzori Mountains form a block lodged between two rift arms at this junction (2). The location and composition of Pleistocene to Recent volcanic rocks can be used to constrain models for the uplift of the Rwenzori Mountains that explain uplift by delamination of a lithosphere block beneath the rift (3). Six small volcanic fields are located in the Toro Ankole area east and southeast of Rwenzori; they contain carbonatite either as lava flows or as components in lapilli tuffs, as well as unusual K2O-rich, SiO2-poor melt compositions akin to leucitites and potassic melilitites that increase in importance towards the south. Their major and trace element and Sr-, Nd-, Hf-, and Os-isotope compositions are consistent with a source composed not just of peridotite, but also containing mica- and clinopyroxene-rich ultramafic assemblages (4). The requirement for assemblages of this type is greater than for potassic melilitites and nephelinites further south in the rift, and this agrees with their greater abundance as xenoliths in the lavas of the Toro Ankole area. The xenoliths contain no evidence for metasomatic replacement of pre-existing peridotite minerals, but rather appear to be formed by crystallization as liquidus minerals on the walls of magma channelways within the upper mantle. Some are zoned and contain vein margins, indicating more than one episode of melt infiltration. Re-melting of such assemblages partially meets the geochemical requirements of the volcanics (4) and has been shown to produce melts with similar major element compositions in experiments (5). The xenolith-richest occurrence is in the Katwe-Kikorongo field, furthest west within the rift at the location purported to be the site of lithospheric delamination (3). This may indicate that mica pyroxenite dyke zones formed by alkaline melts rising along the western edge of the Tanzania craton root provide zones of weakness for later delamination events. The elevated CO2-contents of the western rift volcanics can be explained by low-degree partial melting of carbonate-bearing peridotite at depths of 120-150km (5), thus constraining the source of the volcanic melts in this area to be deeper than those of the Virunga field to the south, in keeping with the postulation of a cratonic lithosphere root just east of the Rwenzori Mountains. (1) Link et al (2010) Internat. J. Earth Sci. submitted (2) Koehn et al (2010) Internat. J. Earth Sci. in press (3) Wallner and Schmeling (2010) Internat. J. Earth Sci. in press (4) Rosenthal et al (2009) Earth Planet. Sci. Lett. 284, 236-248 (5) Lloyd, Edgar and Arima (1985) Contrib. Mineral. Petrol. 91, 321-329 (6) Foley et al (2009) Lithos 112S, 274-283

  4. The transition from rifting to spreading in the Red Sea: No sign of discrete spreading nodes?

    NASA Astrophysics Data System (ADS)

    Augustin, N.; Devey, C. W.; Feldens, P.; van der Zwan, F. M.; Bantan, R.; Kwasnitschka, T.

    2012-12-01

    The rifting of a continent and its eventual splitting by the formation of an ocean basin is one of the decisive processes in plate tectonics - it is responsible for the formation of continental margins (home to most of the world's submarine hydrocarbon reserves) and has repeatedly divided and re-distributed the continental plates. The Red Sea is one of the few places on Earth where this process is presently occurring, with continental rifting occurring in the northern Red Sea (north of 23°N) and ocean floor creation in the south (south of 19.5°N). In the central region it has been proposed that the extension is accommodated in a series of discrete seafloor spreading cells in the so called "transition-" or "multi-deeps region". This suggestion is mainly based on interpretations of bathymetric, magnetic, gravimetric and seismic observations made in the late 1970's and mid 1980's at resolutions far below those possible today. During two major expeditions with the German and Dutch research vessels Poseidon (cruise #408) and Pelagia (cruise #350/51) in 2011 and 2012 we collected a continuous multibeam bathymetric dataset over and between the Red Sea deeps from the Thetis Deep at 23°N to the Red Sea Rift at 16.5°N, with a spatial acoustic resolution of 15-30 m over a total N-S distance of about 700 km. This data enables us to view in detail the bathymetric structures formed during extension and to make a detailed interpretation of the structural, tectonic, magmatic and sedimentary evolution of the Red Sea rift. Based on an analysis of the bathymetric datasets, combined with acoustic backscatter, shallow seismics, ground truthing sampling, and magmatic geochemical information, we arrive at a much simplier and less exotic model of the transition from rifting to spreading than previously proposed. Geomorphological features strongly indicate a continuous oceanic rift valley at least from Nereus Deep at 23°N southwards to the Danakil triple junction at 17°N. We can demonstrate that, although the previously described discrete deeps undoubtedly exist, they are not separated from one another by tectonic boundaries but rather represent a continuous deep spreading axis which is inundated and masked locally by massive sediment slumping or (in the case of the Red Sea evaporites) flow. This can also explain the relative paucity of large strike-slip earthquakes in the "transition region" - such earthquakes would be expected if the deeps were, as previously proposed, separated by transform- or "accommodation"-zones but are not required on a continuous spreading axis. The axis appears to become totally inundated with sediment and evaporites north of 23°N (Nereus Deep) - at what latitude seafloor spreading actually ceases is at present unknown. Geochemical indices of degree of melting vary in a systematic but not linear way along axis, with higher degrees of melting indicated for the axis around 18°N, falling off to the North and South of this. That the degree of melting falls off northwards (the direction in which rifting rate also decreases) is perhaps unsurprising, the decrease to the south seems to be related to the existence of a triple junction at 17°N and the partitioning of spreading between the Red Sea and the Danakil Rift.

  5. GALREX 2011: Extensive hydrothermal venting discovered along the eastern Galápagos Rift

    NASA Astrophysics Data System (ADS)

    Baker, E. T.; Walker, S. L.; White, S. M.; Embley, R. W.; Resing, J. A.; Lobecker, M.

    2011-12-01

    Leg 1 of the Galápagos Rift Expedition (GALREX) on the NOAA Ship Okeanos Explorer, July 2011, conducted a 400-km-long continuous CTD transect to map active hydrothermal areas between 89.33° and 87.75°W. Light backscattering (?NTU) and oxidation-reduction potential (ORP) sensors measured the relative concentration of suspended particles and reduced hydrothermal chemicals, respectively. GALREX was designed to complement a similar survey in 2005/2006 that surveyed the central Galápagos Rift from 94.9° to 89.6°W. That survey found only two active high-temperature vent fields, plus robust plume evidence for at least six smaller fields. The spatial density of hydrothermal plumes (ph) along the rift was only 0.11 (based on ?NTU anomalies), significantly less than expected for intermediate-rate (50-60 mm/yr) spreading but similar to other hotspot-affected ridges (e.g., near the Iceland, Ascension, and St. Paul-Amsterdam hotspots). This low ph value was hypothesized to be an expression of (1) reduced hydrothermal discharge on ridge sections with hotspot-thickened crust, (2), widespread low-temperature discharge undetectable by large-scale surveys, or (3) episodic venting. GALREX found a distinctly different hydrothermal environment on the eastern third of the rift. Overall, ph = 0.19, about twice that of the central Galápagos Rift. Strong venting was concentrated in two areas. Most remarkable was a 50-km-long section (88.56°-88.09°W) where continuous plumes with high (>0.2) ?NTU values rose as high as 250 m above the seafloor. This area is low amplitude valley and ridge topography, centered on a relatively recent lava flow at 88.33°W. The second area included the historical vent fields Rose Garden and Rosebud, and ranged from 86.25° to 85.87°W. In this area ?NTU was lower (~0.1) and plumes were patchy. ORP anomalies occasionally occurred in the absence of ?NTU anomalies, suggesting low-temperature, particle-poor vent sources. No anomalies were detected over the Rose Garden/Rosebud sites. The entire GALREX survey detected ~20 discrete ORP anomalies, with along-axis separations ranging from 2 to 110 km (median=14 km). Because ORP anomalies are very short lived, and thus do not advect far from their seafloor source, at least ~20 distinct vent "fields" must be active, a higher spatial frequency than found along the central Galápagos Rift. The eastern rift section is also notable for supporting two areas of extensive venting, each >40 km along axis, and each paved by apparently young (~10 yr) lavas. Each of these areas is ~3x longer than the longest hydrothermal area on the central rift section.

  6. Protection of Sheep against Rift Valley Fever Virus and Sheep Poxvirus with a Recombinant Capripoxvirus Vaccine?

    PubMed Central

    Soi, Reuben K.; Rurangirwa, Fred R.; McGuire, Travis C.; Rwambo, Paul M.; DeMartini, James C.; Crawford, Timothy B.

    2010-01-01

    Rift Valley fever (RVF) is an epizootic viral disease of sheep that can be transmitted from sheep to humans, particularly by contact with aborted fetuses. A capripoxvirus (CPV) recombinant virus (rKS1/RVFV) was developed, which expressed the Rift Valley fever virus (RVFV) Gn and Gc glycoproteins. These expressed glycoproteins had the correct size and reacted with monoclonal antibodies (MAb) to native glycoproteins. Mice vaccinated with rKS1/RVFV were protected against RVFV challenge. Sheep vaccinated with rKS1/RVFV twice developed neutralizing antibodies and were significantly protected against RVFV and sheep poxvirus challenge. These findings further document the value of CPV recombinants as ruminant vaccine vectors and support the inclusion of RVFV genes encoding glycoproteins in multivalent recombinant vaccines to be used where RVF occurs. PMID:20876822

  7. The continent-ocean transition of the rifted South China continental margin

    NASA Astrophysics Data System (ADS)

    Cameselle, Alejandra L.; Ranero, César R.; Franke, Dieter; Barckhausen, Udo

    2014-05-01

    The continent to ocean transition (COT) architecture of rifted margins represents a key aspect in the study of the variability of different rifting systems and thus, to understand lithospheric extension and final break-up processes. We used 2250 km of reprocessed multichannel seismic data along 4 regional lines and magnetic data acquired across the NW South China continental margin to investigate a previously poorly defined COT. The along-strike structure of the NW subbasin of the South China Sea presents different amounts of extension allowing the study of conjugate pairs of continental margins and their COT in a relative small region. The time-migrated seismic sections allow us to interpreted clear continental and oceanic domains from differences in internal reflectivity, faulting style, fault-block geometry, the seismic character of the top of the basement, the geometry of sediment deposits, and Moho reflections. The continental domain is characterized by arrays of normal faults and associated tilted blocks overlaid by syn-rift sedimentary units. The Moho is imaged as sub-horizontal reflections that define a fairly continuous boundary typically at 8-10 s TWT. Estimation of the thickness of the continental crust using 6 km/s average velocity indicates a ~22 km-thick continental crust under the uppermost slope thinning abruptly to ~9-6 km under the lower slope. The oceanic crust has a comparatively highly reflective top of basement, little-faulting, not discernible syn-tectonic strata, and fairly constant thickness (4-8 km) over tens of km distance defined by usually clear Moho reflections. The COT can be very well defined based on MSC images and occurs across a ~5-10 km narrow zone. Rifting in the NW subbasin resulted in asymmetric conjugate margins. Arrays of tilted fault blocks covered by abundant syn-rift sediment are displayed across the northwestern South China continental margin, whereas the conjugate Macclesfield Bank margin shows abrupt thinning and little faulting. Seismic profiles also show a clear change in the tectonic structure of the margin from NE to SW. On the two NE-most lines, the abrupt crustal thinning occurs over a 20-40 km wide area resulting in final breakup. To the SW, the area of stretched continental crust extends over a comparatively broader ~100-110 km segment of tilted fault-blocks. We interpret that the 3D structural variability and the narrow COT is related to the lateral NE to SW propagation of a spreading center. The early spreading center propagation in the NE suddenly stopped continental stretching during ongoing rifting, causing an abrupt break-up and a narrow COT. Later arrival of spreading center to the SW resulted in a comparatively broader segment of highly stretched continental crust. We suggest that the final structure of the northwest South China continental margin have been governed by the 3D interaction between rifting and oceanic spreading center propagation to a degree larger than by the local lithospheric structure during rifting.

  8. Rift Valley fever virus: A review of diagnosis and vaccination, and implications for emergence in Europe.

    PubMed

    Mansfield, Karen L; Banyard, Ashley C; McElhinney, Lorraine; Johnson, Nicholas; Horton, Daniel L; Hernández-Triana, Luis M; Fooks, Anthony R

    2015-10-13

    Rift Valley fever virus (RVFV) is a mosquito-borne virus, and is the causative agent of Rift Valley fever (RVF), a zoonotic disease characterised by an increased incidence of abortion or foetal malformation in ruminants. Infection in humans can also lead to clinical manifestations that in severe cases cause encephalitis or haemorrhagic fever. The virus is endemic throughout much of the African continent. However, the emergence of RVFV in the Middle East, northern Egypt and the Comoros Archipelago has highlighted that the geographical range of RVFV may be increasing, and has led to the concern that an incursion into Europe may occur. At present, there is a limited range of veterinary vaccines available for use in endemic areas, and there is no licensed human vaccine. In this review, the methods available for diagnosis of RVFV infection, the current status of vaccine development and possible implications for RVFV emergence in Europe, are discussed. PMID:26296499

  9. Animal movements in the Kenya Rift and evidence for the earliest ambush hunting by hominins

    PubMed Central

    Kübler, Simon; Owenga, Peter; Reynolds, Sally C.; Rucina, Stephen M.; King, Geoffrey C. P.

    2015-01-01

    Animal movements in the Kenya Rift Valley today are influenced by a combination of topography and trace nutrient distribution. These patterns would have been the same in the past when hominins inhabited the area. We use this approach to create a landscape reconstruction of Olorgesailie, a key site in the East African Rift with abundant evidence of large-mammal butchery between ~1.2 and ~0.5?Ma BP. The site location in relation to limited animal routes through the area show that hominins were aware of animal movements and used the location for ambush hunting during the Lower to Middle Pleistocene. These features explain the importance of Olorgesailie as a preferred location of repeated hominin activity through multiple changes in climate and local environmental conditions, and provide insights into the cognitive and hunting abilities of Homo erectus while indicating that their activities at the site were aimed at hunting, rather than scavenging. PMID:26369499

  10. Animal movements in the Kenya Rift and evidence for the earliest ambush hunting by hominins.

    PubMed

    Kübler, Simon; Owenga, Peter; Reynolds, Sally C; Rucina, Stephen M; King, Geoffrey C P

    2015-01-01

    Animal movements in the Kenya Rift Valley today are influenced by a combination of topography and trace nutrient distribution. These patterns would have been the same in the past when hominins inhabited the area. We use this approach to create a landscape reconstruction of Olorgesailie, a key site in the East African Rift with abundant evidence of large-mammal butchery between ~1.2 and ~0.5?Ma BP. The site location in relation to limited animal routes through the area show that hominins were aware of animal movements and used the location for ambush hunting during the Lower to Middle Pleistocene. These features explain the importance of Olorgesailie as a preferred location of repeated hominin activity through multiple changes in climate and local environmental conditions, and provide insights into the cognitive and hunting abilities of Homo erectus while indicating that their activities at the site were aimed at hunting, rather than scavenging. PMID:26369499

  11. Tectonic and sediment supply control of deep rift lake turbidite systems: Lake Baikal, Russia

    USGS Publications Warehouse

    Nelson, C.H.; Karabanov, E.B.; Colman, Steven M.; Escutia, C.

    1999-01-01

    Tectonically influenced half-graben morphology controls the amount and type of sediment supply and consequent type of late Quaternary turbidite systems developed in the active rift basins of Lake Baikal, Russia. Steep border fault slopes (footwall) on the northwest sides of half-graben basins provide a limited supply of coarser grained clastic material to multiple small fan deltas. These multiple sediment sources in turn laterally feed small (65 km) axially fed elongate mud-rich fans sourced by regional exterior drainage of the Selenga River that supplies large quantities of silt. Basin plain turbidites in the center of the linear basins and axial channels that are controlled by rift-parallel faults are fed from, and interfinger with, aprons and fans. The predictability of the turbidite systems in Lake Baikal provides the best example yet studied of how tectonics and sediment supply interact to control the development of a wide variety of coeval turbidite systems on a single basin floor.

  12. Thermochronologic constraints on Jurassic rift flank denudation in the Thiel Mountains, Antarctica

    USGS Publications Warehouse

    Fitzgerald, P.G.; Baldwin, S.L.

    2007-01-01

    The Thiel Mountains are part of the Transantarctic Mountains (TAM) and occupy a strategic position close to the East-West Antarctic boundary. They occur in a region of relatively subdued topography distal from high topography and high relief of most of the TAM adjacent to the West Antarctic rift system. Low-temperature thermochronology on samples collected from the Reed Ridge granite on the north flank of the Thiel Mountains constrain the thermal and hence tectonic history. Apatite fission track data plus thermal models indicate cooling from ca. 165-150 Ma. In conjunction with 40Ar/39Ar K-feldspar data, the results indicate cooling was due to relatively slow erosional denudation, and not thermal relaxation following Jurassic tholeiitic magmatism. Denudation was most likely associated with the formation of the Jurassic rift system across Antarctica that marked the initial breakup of Gondwana. This is the oldest episode of denudation associated with formation of the present day TAM

  13. Animal movements in the Kenya Rift and evidence for the earliest ambush hunting by hominins

    NASA Astrophysics Data System (ADS)

    Kübler, Simon; Owenga, Peter; Reynolds, Sally C.; Rucina, Stephen M.; King, Geoffrey C. P.

    2015-09-01

    Animal movements in the Kenya Rift Valley today are influenced by a combination of topography and trace nutrient distribution. These patterns would have been the same in the past when hominins inhabited the area. We use this approach to create a landscape reconstruction of Olorgesailie, a key site in the East African Rift with abundant evidence of large-mammal butchery between ~1.2 and ~0.5?Ma BP. The site location in relation to limited animal routes through the area show that hominins were aware of animal movements and used the location for ambush hunting during the Lower to Middle Pleistocene. These features explain the importance of Olorgesailie as a preferred location of repeated hominin activity through multiple changes in climate and local environmental conditions, and provide insights into the cognitive and hunting abilities of Homo erectus while indicating that their activities at the site were aimed at hunting, rather than scavenging.

  14. Dike intrusions during rifting episodes obey scaling relationships similar to earthquakes

    NASA Astrophysics Data System (ADS)

    Passarelli, Luigi; Rivalta, Eleonora; Shuler, Ashley

    2014-05-01

    Rifting episodes accommodate the relative motion of mature divergent plate boundaries with sequences of magma-filled dikes that compensate for the missing volume due to crustal splitting. Two major rifting episodes have been recorded since modern monitoring techniques are available: the 1975-1984 Krafla (Iceland) and the 2005-2010 Manda-Hararo (Ethiopia) dike sequences. The statistical properties of the frequency of dike intrusions during rifting have never been investigated in detail, but it has been suggested that they may have similarities with earthquake mainshock-aftershock sequences, for example they start with a large intrusion followed by several events of smaller magnitude. The scaling relationships of earthquakes have on the contrary been widely investigated: earthquakes have been found to follow a power law, the Gutenberg-Richter relation, from local to global scale, while the decay of aftershocks with time has been found to follow the Omori law. These statistical laws for earthquakes are the basis for hazard evaluation and the physical mechanisms behind them are the object of wide interest and debate. Here we investigate in detail the statistics of dikes from the Krafla and Manda-Hararo rifting episodes, including their frequency-magnitude distribution, the release of geodetic moment in time, the correlation between interevent times and intruded volumes. We find that the dimensions of dike intrusions obey a power law analogous to the Gutenberg-Richter relation, the long-term release of geodetic moment is governed by a relationship consistent with the Omori law, and the intrusions are roughly time-predictable. The need of magma availability affects however the timing of secondary dike intrusions: such timing is longer after large volume intrusions, contrarily to aftershock sequences where interevent times shorten after large events.

  15. Assessing Microseismicity of the Northern Mid-Continent Rift Zone and Surrounding Regions

    NASA Astrophysics Data System (ADS)

    Bartz, D.; Wysession, M. E.; Wiens, D. A.; Aleqabi, G. I.; Shore, P.; van der Lee, S.; Jurdy, D. M.; Stein, S. A.; Revenaugh, J.; Wolin, E.; Bollmann, T. A.; Frederiksen, A. W.; Darbyshire, F. A.

    2014-12-01

    SPREE is a flexible array (FA) deployment associated with the EarthScope project with the aim of better understanding the Mid-Continent Rift Zone (MCRZ). We have used data from SPREE FA and TA stations to detect and locate small earthquakes in the vicinity of the northwestern arm of the Mid-Continent Rift Zone, covering Minnesota, Wisconsin, and parts of Iowa, Michigan, and Ontario. This region, now stable craton, is a failed Precambrian rift marked by low levels of recorded seismicity, perhaps resulting in part from a historic dearth of stations deployed in the region. We assessed this possibility by taking advantage of the densest array that has been deployed in the region. Processing has allowed for regional stress constraints and a means of distinguishing earthquakes from mine blasts, and a quantitative method to differentiate natural earthquakes from mine blasting events, frequent here. After automated event identification, a manual review confirmed 14 earthquakes and several hundred blasting events with magnitudes ranging from M1 - M3. Observed seismicity is in reasonable agreement with available historical data. We use these earthquakes in conjunction with historical catalogs to estimate regional recurrence intervals for events of greater magnitudes. While initial earthquake/blast discrimination was done manually, the ratio of Rayleigh to P-wave amplitude appears to be a reliable discriminant for distinguishing between earthquakes and mine blasting in this region in a systematic way, with earthquakes displaying a systematically depressed ratio. Additionally, P-wave first-motion data and S/P amplitude ratios from natural events constrain focal mechanisms and regional stresses. These methods indicate roughly WSW-ENE compression, consistent with other determinations and absolute plate motion. The majority of events detected lie some distance from the actual rift; we thus conclude current seismicity bears no preferred association with the MCR.

  16. Rifting Process and Geomorphic Development of the Okinawa Tough, Southwest Japan

    NASA Astrophysics Data System (ADS)

    Sato, T.; Arai, K.; Inoue, T.; Matsumoto, D.

    2012-12-01

    The Ryukyu Island Arc extends from Kyushu to Taiwan, a distance of 1,200 km, along the Ryukyu Trench where the Philippine Sea Plate is subducting beneath the Eurasian Plate. The Okinawa Trough, a back arc basin has formed behind the Ryukyu Island Arc in late Pliocene to early Pleistocene. The research cruises of GH11 (from 14 July to 15 August, 2011) and GH12 (from 20 to 30 July, 2012) were carried out around the Okinawa Trough. More than 3,600 miles multi channel high-resolution seismic profiles were acquired during these cruises by the GI-gun (Generator 250 cu inch and Injector 105 cu. inch) systems with 16ch digital streamer cable. As a result, two unconformities and three depositional sequence divided by the unconformities are recognized in the trough. The lower and the midlle sequence are tilted and blocked by many normal faults, on the other hand the upper one is not tilted and shows the pattern of onlap fill. From this result, the upper sequence started to deposit after start of the rifting. Additionally, internal reflection of the upper sequence shows the cyclic activities of the rifting. The position of the rifting axis was revealed based on dip of the normal faults. As a result, rifting axis shows echelon arrangement and the displacement of the faults are varied with the segment of the arrangement. The location of the segment boundaries is correlated with geometrical boundary of the adjacent slope. Steep slope with incised valley is distributed in southwestern part where the displacement of the normal fault is large, on the other hand, gentle slope without incised valley is distributed in northeastern part where the displacement is small. This difference of the displacement strongly controls the geometry of the adjacent slope.

  17. Van Horn Sandstone, Trans-Pecos Texas: Evidence for Late Cambrian rifting along southern North America

    SciTech Connect

    Hongshuan, Ye; Soegaard, K. . Programs in Geosciences)

    1993-02-01

    The Van Horn Sandstone in the Trans-Pecos region of west Texas is interpreted as a rift sequence which developed in response to Cambrian breakup along the southern margin of the North American continent. The Van Horn Sandstone consists exclusively of braided alluvial sediments and occupies relatively small isolated basins in the vicinity of the town of Van Horn. The sandstone is in structural unconformable contact above intensely deformed Precambrian sediments which are < 1,123 Ma old. The Van Horn Sandstone is overlain by more than 650 meters of earliest Ordovician to Mississippian shallow-marine shelf sediments. Geohistory analysis of the overlying Paleozoic shelf sediments indicates that subsidence was driven by thermal contraction of the crust and that the shallow-marine sediments represent a drift sequence. Subsidence history curves correspond with theoretical thermal decay curves where [beta] = 1.2 and suggest that thermal subsidence commenced in Late Cambrian time about 510 Ma ago. Increased crustal attenuation, resulting in development of an ocean basin, occurred between Van Horn and the original location of deep water sediments presently exposed in the Marathon uplift to the south. Proposed Late Cambrian breakup south of Van Horn is coeval with rifting in the southern Oklahoma aulachogen and Rome trough in the Appalachian Mountains, but post-dates the main Late Proterozoic rifting event between 625 and 555 Ma along the eastern and western freeboard of North American. The significance of diachronous rifting in Eocambrian-Cambrian time is unclear at present but has consequences for fragmentation of the late Precambrian supercontinent Rodinia'.

  18. Master1RservoirsGologiquesDynamiquedesBassins-MichelSranne Cratonic Basins: Michigan Basin -> old rift ???

    E-print Network

    Cattin, Rodolphe

    emplaced in a vast spatial distribution over a period of about 1 million years, and are not limited&al,inRoberts&Bally,2012 Early Permian rifting Moho depth => crustal thinning Tertiary Tur-Maast Apt-Cen Neoc U.Jur Tr-Jur Permian Tertiary Tur-Maast Apt-Cen Neoc U.Jur Tr-M.Jur Permian 3- Intra-cratonic Basins long-term (Trias

  19. Nature of the Mantle Sources and Bearing on Tectonic Evolution in the West Antarctic Rift System

    NASA Astrophysics Data System (ADS)

    Mukasa, S. B.; Rilling-Hall, S.; Marcano, M. C.; Wilson, T. J.; Lawver, L. A.; LeMasurier, W. E.

    2012-12-01

    We collected samples from subaerial lava flows and dredged some Neogene basanitic lavas from seven volcanic edifices in the Ross Sea, Antarctica - a part of the West Antarctic Rift System (WARS) and one of the world's largest alkaline magmatic provinces - for a study aimed at two principal objectives: (1) Geochemical interrogation of the most primitive magmatic rocks to try and understand the nature of the seismically abnormal mantle domain recently identified beneath the shoulder of the Transantarctic Mountains (TAM), the Ross Sea Embayment and Marie Byrd Land; and (2) Using 40Ar/39Ar geochronology to establish a temporal link between magmatism and tectonism, particularly in the Terror Rift. We have attempted to answer the questions of whether magmatism is due to a hot mantle or wet mantle, and whether rifting in the area triggered magmatic activity or vice versa. Results show that the area does not have an age-progressive hotspot track, and the magmatism post-dates the main phase of extension along the Terror Rift within the WARS, which supports a decompression-melting model without the benefit of a significant thermal anomaly. In fact, preliminary volatile measurements on olivine-hosted melt inclusions have yielded water concentrations in excess of 2 wt%, indicating that flux melting was an important complementary process to decompression melting. The major oxide compositions of lavas in the WARS are best matched to experimental melts of carbonated peridotite, though garnet pyroxenite can also be a minor source. The Pb and Nd isotopic systems are decoupled from each other, suggesting removal of fluid-mobile elements from the mantle source possibly during the long history of subduction along the Paleo-Pacific margin of Gondwana. Extremely unradiogenic 187Os/188Os ranging to as low as 0.1081 ± 0.0001 hints at the involvement of lithospheric components in generation of magmas in the WARS.

  20. Early Pleistocene lake formation and hominin origins in the Turkana-Omo rift

    NASA Astrophysics Data System (ADS)

    Lepre, Christopher J.

    2014-10-01

    Prior research has correlated the formation of Plio-Pleistocene lakes in East Africa to global/regional climate changes and interpreted the lacustrine basins as significant settings of hominin evolution. Paleo-Lake Lorenyang from the Turkana-Omo rift is important to these issues, as its marginal deposits contain some of, if not the earliest currently known specimens of Acheulian stone tools and African Homo erectus. Magnetostratigraphic and sedimentological evidence indicates that the oldest preserved paleo-Lake Lorenyang deposits are dated at 2.148-2.128 Ma and derive from the NW Turkana basin, predating those from the Omo basin by ˜100 kyr and the NE Turkana basin by ˜190 kyr. Apparently, the lake expanded asynchronously in the rift, potentially due to a volcano-tectonic influence on the location of drainage networks, depositional slopes, or topographic elevation differences within and between the basins at the time of flooding. The onset of the lake temporally coincides with the eruption of basalt lava flows dated to 2.2-2.0 Ma that blocked the southeast outlet of the Turkana basin. This provides a plausible mechanism for hydrologic closure and lacustrine basin formation through volcano-tectonic impounding. It also points to a non-climatic cause for the initial formation of paleo-Lake Lorenyang at ˜2.14 Ma. First appearances for African H. erectus (˜1.87 Ma) and Acheulian tools (˜1.76 Ma) in the Turkana-Omo rift postdate the lake's initial formation by about 270 kyr and 380 kyr, respectively. Such timing differences contrast with studies that correlate all three to the 400-kyr-eccentricity maximum at 1.8 Ma. Although the Turkana-Omo rift is just one example, it does provide alternative insights to views that link climate, hominin evolution, and lake formation in East Africa.

  1. Volcanic rifts bracketing volcanoes: an analogue answer to an old unsolved problem

    NASA Astrophysics Data System (ADS)

    Mussetti, Giulio; van Wyk de Vries, Benjamin; Corti, Giacomo; Hagos, Miruts

    2015-04-01

    It has been observed in Central America that many volcanoes have volcanic alignments and faults at their east and west feet. A quick look at many rifts indicates that this also occurs elsewhere. While this feature has been noted for at least 30 years, no explanation has ever really been convincingly put forward. During analogue experiments on rifting volcanoes we have mixed the presence of a volcanic edifice with an underlying intrusive complex. The models use a rubber sheet that is extended and provides a broad area of extension (in contrast to many moving plate models that have one localised velocity discontinuity). This well suits the situation in many rifts and diffuse strike-slip zones (i.e. Central America and the East African Rift). We have noted the formation of localised extension bracketing the volcano, the location of which depends on the position of the analogue intrusion. Thus, we think we have found the answer to this long standing puzzle. We propose that diffuse extension of a volcano and intrusive complex generates two zones of faulting at the edge of the intrusion along the axis of greatest extensional strain. These serve to create surface faulting and preferential pathways for dykes. This positioning may also create craters aligned along the axis of extension, which is another notable feature of volcanoes in Central America. Paired volcanoes and volcanic uplifts in the Danakil region of Ethiopia may also be a consequence of such a process and lead us to draw some new preliminary cross sections of the Erta Ale volcanic range.

  2. Formation of the volcanic rifted margin off Argentina/Uruguay, South Atlantic

    NASA Astrophysics Data System (ADS)

    Franke, D.; Reichert, C.; Ladage, S.; Schnabel, M.; Schreckenberger, B.; Neben, S.; Hinz, K.

    2009-04-01

    The Federal Institute for Geosciences and Natural Resources (BGR), Germany has investigated the passive continental margins offshore Argentina and Uruguay since the early 90ies. Numerous marine geophysical surveys have meanwhile established a databasis of more than 25.000 km of regional multi-channel reflection seismic lines, accompanied with magnetic and gravity profiles. These data document that the Early Cretaceous South Atlantic continental break-up and initial sea-floor spreading were accompanied by large-scale, transient volcanism emplacing voluminous extrusives, manifested in the seismic data by huge wedges of seaward dipping reflectors (SDRs). These deeply buried and 60-120 km wide SDRs were emplaced episodically as suggested by at least three superimposed SDRS units. Distinct along-margin variations in the architecture, volume, and width of the SDRs wedges correlate with large scale margin segmentation. We identify at least four domains bounded by the Falkland Fracture Zone/Falkland Transfer, the Colorado Transfer, the Ventana Transfer and the Salado Transfer. The individual transfer zones may have acted as barriers for propagating rifts during the SDR emplacement phase, selectively directing rift segments in left stepping patterns along the western South Atlantic margin. The rift segments are offset systematically in a left stepping pattern along the western South Atlantic margin. Albeit we found extensive variations in the architecture, style and extent of the seaward dipping reflector sequences a general trend is that the largest volumes are emplaced close to the proposed transfer zones and the width of the SDRs wedges decreases northward within the individual margin segments. The different volcano-tectonic architectures of the margin segments and the distribution of the extruded magmas indicates that the emplacement of the volcanic material was controlled by the tectonic setting and the pre-rift lithosphere configuration within individual margin segments. We favour mainly melt generation from shallow sources as mechanism for the emplacement of large volumes of SDRs during breakup of the South Atlantic.

  3. Modelizacion, control e implementacion de un procesador energetico paralelo para aplicacion en sistemas multisalida

    NASA Astrophysics Data System (ADS)

    Ferreres Sabater, Agustin

    Cualquier sistema electronico que incluya un procesado o tratamiento de la senal, y ademas, algun tipo de actuador mecanico generalmente necesita, como minimo, dos tensiones diferentes de alimentacion. Excluyendo los sistemas de alimentacion distribuida, la solucion tecnica mas utilizada para proporcionar dos o mas tensiones consiste en las fuentes de alimentacion multisalida. En una fuente de alimentacion multisalida los diferentes circuitos que conforman cada salida comparten un mismo transformador de potencia optimizando coste, masa, y volumen. Las ventajas obtenidas con este procedimiento tienen en su contra el efecto que sobre cada salida individual provocan las demas en su conjunto debido, principalmente, a los efectos de los elementos parasitos de los componentes. Un cambio de carga en una de las salidas produce un transitorio que es visto por todas las demas como un efecto de impedancia cruzada, y al final del transitorio, la tension de cada salida es diferente respecto a la que tenian antes del transitorio. Este ultimo resultado se conoce como regulacion cruzada. La disminucion de los efectos de la regulacion cruzada ha sido objeto de estudio durante los ultimos anos. El objetivo ha sido el desarrollo de distintas estrategias que permiten, desde disminuir los efectos de la regulacion cruzada hasta los niveles deseables, a eliminarla completamente. El resultado final suele suponer una penalizacion sobre el diseno del sistema directamente proporcional al grado de regulacion a conseguir en las distintas salidas. Entre las soluciones propuestas para eliminar la regulacion cruzada las tecnicas de post-regulacion se han consolidado como la opcion mas aceptada ya que, pueden aplicarse a cualquier convertidor y no suponen ninguna complejidad adicional a la hora de plantear el diseno. En esta Tesis Doctoral se abordara el estudio de la tecnica conocida como postregulacion mediante transformador controlado, que si bien se ha empleado en convertidores resonantes, su modelizacion, y aplicacion en convertidores PWM, esta aun por estudiar y valorar. El primer Capitulo consiste en una breve introduccion al problema de la regulacion cruzada y la impedancia cruzada para posteriormente describir las tecnicas de post-regulacion actualmente mas empleadas, con especial atencion al post-regulador con transformador controlado. El Capitulo segundo trata del estudio de las caracteristicas estaticas del postregulador con transformador controlado. Partiendo de los estudios disponibles sobre el postregulador se plantean mejoras en su modo de actuacion y se discuten tres alternativas diferentes para controlar el transformador. Las dos primeras consisten en emplear un convertidor auxiliar Boost en sus dos modos de funcionamiento, continuo y discontinuo. La tercera consiste en controlar el transformador con una tension PWM directamente, sin filtrado. Finalmente se comprueba experimentalmente, para el estado estacionario, el funcionamiento del post-regulador para cada uno de los tres metodos de control. El Capitulo tercero trata de la dinamica de la salida controlada con el post-regulador cuando este emplea un convertidor auxiliar tipo Boost. Mediante la tecnica de promediado de variables de estado se propone el modelo de pequena senal, tanto para el modo continuo como para el modo discontinuo de funcionamiento del convertidor auxiliar. Los resultados mas significativos de esta seccion son las expresiones analiticas de las impedancias cruzadas y de la impedancia de la salida post-regulada. Como complemento al modelo de pequena senal se plantea un modelo de gran senal implementado sobre el simulador Pspice. Con este nuevo modelo se reproducen los resultados obtenidos con el modelo de pequena senal y ademas es posible simular los transitorios en las tensiones de salida ante cambios de carga. La modelizacion del convertidor cuando el transformador se controla con una tension PWM sin filtrar es el objetivo del Capitulo 4. En las secciones siguientes del Capitulo se plantea el correspondiente modelo de gran senal aplicado a un nuevo prototipo exper

  4. Preliminary estimate of crustal extension during Cambrian rifting in the southern midcontinent

    SciTech Connect

    McConnell, D.A.; Gilbert, M.

    1985-01-01

    The Southern Oklahoma Aulacogen is a prominent rift extending 500+ km NW into the southern Midcontinent from the probably Cambrian plate margin. The biomodal igneous floor of the rift has been strongly structurally inverted and is now partially exposed in the core of the Wichita Mountains. Utilizing structural and stratigraphic patterns, and character of the igneous sequence, the style of tectonism and magnitude of displacement can be inferred for the initiation of the rift. Five possible extensional pulses can be related to specific igneous features although these could be part of one continuous episode lasting from about 565 to 525 mybp. Pennsylvanian transpressive faults are assumed to be reactivated Cambrian normal faults (and they may even have an earlier parentage). Using known thickness of continuous rhyolite (1.4 km), an initial width of 60 km, and a half-graben configuration, an estimate of extension is possible independent of bounding fault dip. For a brittle-ductile transition between 10 to 15 km, the brittle extension in the upper crust varies between 8-5%. Using the minimum mafic volume needed within the ductile lower crust (40,000 km/sup 3/), and a total crustal thickness of 35 to 45 km, the lower crust was extended 8-6% to 10-7%. The upper and lower crustal estimates of extension are in good agreement confirming a relatively shallow brittle-ductile transition. This is consistent with concomitant igneous activity and an enhanced geothermal gradient.

  5. Tectonic rotations within the Rio Grande rift - Evidence from paleomagnetic studies

    NASA Technical Reports Server (NTRS)

    Brown, L. L.; Golombek, M. P.

    1985-01-01

    Paleomagnetic studies on Miocene Pliocene volcanic rocks from the Espanola basin of the Rio Grande rift, New Mexico, reveal directions discordant form the expected mean direction for North America. The Paliza Canyon Formation, Tschicoma Formation, and Lobato Basalt, all sampled in the Jemez Mountains west of the Pajarito fault zone, have mean declinations east of the expected mean. The Cerros del Rio volcanics, lying east of the Pajarito fault zone, have a westerly declination. Combined with published data on the Santa Fe Group sediments east of the fault zone, and the Valles Rhyolite, west of the fault zone, distinct rotations of the two areas are evident. The western block has rotated clockwise 12 deg, while the eastern block shows 16 deg of conter-clockwise motion. Differential rotations of 25-30 deg are calculated between the two blocks; 4 deg/m.y. is the minimum differential rotation for the past 5 m.y. Geologic explanations for these rotations include the opening of the Rio Grande rift in response to clockwise rotation of the Colorado Plateau and significant left slip along the Rio Grande rift.

  6. Hydrocarbon accumulation on rifted Continental Margin - examples of oil migration pathways, west African salt basins

    SciTech Connect

    Blackwelder, B.W.

    1989-03-01

    Examination of the oil fields in the Gabon, Lower Congo, and Cuanza basins allows modeling of oil migration and a more accurate ranking of prospects using geologic risk factors. Oil accumulations in these basins are in strata deposited during Cretaceous rift and drift phases, thus providing a diversity of geologic settings to examine. Oil accumulations in rift deposits are located on large faulted anticlines or in truncated units atop horst features. Many of these oil fields were sourced from adjacent organic shales along short direct migration paths. In Areas where source rock is more remote to fields or to prospective structures, faulting and continuity of reservoir rock are important to the migration of hydrocarbons. Because Aptian salts separate rift-related deposits from those of the drift stage, salt evacuation and faulting of the salt residuum are necessary for oil migration from the pre-salt sequences into the post-salt section. Oil migration within post-salt strata is complicated by the presence of salt walls and faulted carbonate platforms. Hydrocarbon shows in wells drilled throughout this area provide critical data for evaluating hydrocarbon migration pathways. Such evaluation in combination with modeling and mapping of the organic-rich units, maturation, reservoir facies, structural configurations, and seals in existing fields allows assessment of different plays. Based on this information, new play types and prospective structures can be ranked with respect to geologic risk.

  7. A satellite magnetic perspective of subduction zones, large igneous provinces, rifts, and diffuse plate boundary zones

    NASA Astrophysics Data System (ADS)

    Purucker, M. E.; Whaler, K. A.

    2008-12-01

    Large and intermediate-scale tectonic features such as subduction zones, large igneous provinces, rifts, and diffuse plate boundary zones are often seen to have a magnetic signature visible from the perspective of near-Earth magnetic field satellites such as CHAMP and Orsted. Why do these tectonic features have a magnetic signature, while others do not? A new model of the lithospheric field (MF-6, Maus et al., 2008) extending to spherical harmonic degree 120 (333 km wavelength) has been used to evaluate the magnetic state of the lithosphere under the assumption that the magnetization is either induced (with a seismi