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Sample records for accreted oceanic terranes

  1. Continental accretion: From oceanic plateaus to allochthonous terranes

    USGS Publications Warehouse

    Ben-Avraham, Z.; Nur, A.; Jones, D.; Cox, A.

    1981-01-01

    Some of the regions of the anomalously high sea-floor topography in today's oceans may be modern allochthonous terranes moving with their oceanic plates. Fated to collide with and be accreted to adjacent continents, they may create complex volcanism, cut off and trap oceanic crust, and cause orogenic deformation. The accretion of plateaus during subduction of oceanic plates may be responsible for mountain building comparable to that produced by the collision of continents. Copyright ?? 1981 AAAS.

  2. Geodynamic models of terrane accretion: Testing the fate of island arcs, oceanic plateaus, and continental fragments in subduction zones

    NASA Astrophysics Data System (ADS)

    Tetreault, J. L.; Buiter, S. J. H.

    2012-08-01

    Crustal growth at convergent margins can occur by the accretion of future allochthonous terranes (FATs), such as island arcs, oceanic plateaus, submarine ridges, and continental fragments. Using geodynamic numerical experiments, we demonstrate how crustal properties of FATs impact the amount of FAT crust that is accreted or subducted, the type of accretionary process, and the style of deformation on the overriding plate. Our results show that (1) accretion of crustal units occurs when there is a weak detachment layer within the FAT, (2) the depth of detachment controls the amount of crust accreted onto the overriding plate, and (3) lithospheric buoyancy does not prevent FAT subduction during constant convergence. Island arcs, oceanic plateaus, and continental fragments will completely subduct, despite having buoyant lithospheric densities, if they have rheologically strong crusts. Weak basal layers, representing pre-existing weaknesses or detachment layers, will either lead to underplating of faulted blocks of FAT crust to the overriding plate or collision and suturing of an unbroken FAT crust. Our experiments show that the weak, ultramafic layer found at the base of island arcs and oceanic plateaus plays a significant role in terrane accretion. The different types of accretionary processes also affect deformation and uplift patterns in the overriding plate, trench migration and jumping, and the dip of the plate interface. The resulting accreted terranes produced from our numerical experiments resemble observed accreted terranes, such as the Wrangellia Terrane and Klamath Mountain terranes in the North American Cordilleran Belt.

  3. Circum-Pacific accretion of oceanic terranes to continental blocks: accretion of the Early Permian Dun Mountain ophiolite to the E Gondwana continental margin, South Island, New Zealand

    NASA Astrophysics Data System (ADS)

    Robertson, Alastair

    2016-04-01

    Accretionary orogens, in part, grow as a result of the accretion of oceanic terranes to pre-existing continental blocks, as in the circum-Pacific and central Asian regions. However, the accretionary processes involved remain poorly understood. Here, we consider settings in which oceanic crust formed in a supra-subduction zone setting and later accreted to continental terranes (some, themselves of accretionary origin). Good examples include some Late Cretaceous ophiolites in SE Turkey, the Jurassic Coast Range ophiolite, W USA and the Early Permian Dun Mountain ophiolite of South Island, New Zealand. In the last two cases, the ophiolites are depositionally overlain by coarse clastic sedimentary rocks (e.g. Permian Upukerora Formation of South Island, NZ) that then pass upwards into very thick continental margin fore-arc basin sequences (Great Valley sequence, California; Matai sequence, South Island, NZ). Field observations, together with petrographical and geochemical studies in South Island, NZ, summarised here, provide evidence of terrane accretion processes. In a proposed tectonic model, the Early Permian Dun Mountain ophiolite was created by supra-subduction zone spreading above a W-dipping subduction zone (comparable to the present-day Izu-Bonin arc and fore arc, W Pacific). The SSZ oceanic crust in the New Zealand example is inferred to have included an intra-oceanic magmatic arc, which is no longer exposed (other than within a melange unit in Southland), but which is documented by petrographic and geochemical evidence. An additional subduction zone is likely to have dipped westwards beneath the E Gondwana margin during the Permian. As a result, relatively buoyant Early Permian supra-subduction zone oceanic crust was able to dock with the E Gondwana continental margin, terminating intra-oceanic subduction (although the exact timing is debatable). The amalgamation ('soft collision') was accompanied by crustal extension of the newly accreted oceanic slab, and

  4. Terrane accretion: Insights from numerical modelling

    NASA Astrophysics Data System (ADS)

    Vogt, Katharina; Gerya, Taras

    2016-04-01

    The oceanic crust is not homogenous, but contains significantly thicker crust than norm, i.e. extinct arcs, spreading ridges, detached continental fragments, volcanic piles or oceanic swells. These (crustal) fragments may collide with continental crust and form accretionary complexes, contributing to its growth. We analyse this process using a thermo-mechanical computer model (i2vis) of an ocean-continent subduction zone. In this model the oceanic plate can bend spontaneously under the control of visco-plastic rheologies. It moreover incorporates effects such as mineralogical phase changes, fluid release and consumption, partial melting and melt extraction. Based on our 2-D experiments we suggest that the lithospheric buoyancy of the downgoing slab and the rheological strength of crustal material may result in a variety of accretionary processes. In addition to terrane subduction, we are able to identify three distinct modes of terrane accretion: frontal accretion, basal accretion and underplating plateaus. We show that crustal fragments may dock onto continental crust and cease subduction, be scrapped off the downgoing plate, or subduct to greater depth prior to slab break off and subsequent exhumation. Direct consequences of these processes include slab break off, subduction zone transference, structural reworking, formation of high-pressure terranes, partial melting and crustal growth.

  5. Gondwanaland origin, dispersion, and accretion of East and Southeast Asian continental terranes

    NASA Astrophysics Data System (ADS)

    Metcalfe, I.

    1994-10-01

    East and Southeast Asia is a complex assembly of allochthonous continental terranes, island arcs, accretionary complexes and small ocean basins. The boundaries between continental terranes are marked by major fault zones or by sutures recognized by the presence of ophiolites, mélanges and accretionary complexes. Stratigraphical, sedimentological, paleobiogeographical and paleomagnetic data suggest that all of the East and Southeast Asian continental terranes were derived directly or indirectly from the Iran-Himalaya-Australia margin of Gondwanaland. The evolution of the terranes is one of rifting from Gondwanaland, northwards drift and amalgamation/accretion to form present day East Asia. Three continental silvers were rifted from the northeast margin of Gondwanaland in the Silurian-Early Devonian (North China, South China, Indochina/East Malaya, Qamdo-Simao and Tarim terranes), Early-Middle Permian (Sibumasu, Lhasa and Qiangtang terranes) and Late Jurassic (West Burma terrane, Woyla terranes). The northwards drift of these terranes was effected by the opening and closing of three successive Tethys oceans, the Paleo-Tethys, Meso-Tethys and Ceno-Tethys. Terrane assembly took place between the Late Paleozoic and Cenozoic, but the precise timings of amalgamation and accretion are still contentious. Amalgamation of South China and Indochina/East Malaya occurred during the Early Carboniferous along the Song Ma Suture to form "Cathaysialand". Cathaysialand, together with North China, formed a large continental region within the Paleotethys during the Late Carboniferous and Permian. Paleomagnetic data indicate that this continental region was in equatorial to low northern paleolatitudes which is consistent with the tropical Cathaysian flora developed on these terranes. The Tarim terrane (together with the Kunlun, Qaidam and Ala Shan terranes) accreted to Kazakhstan/Siberia in the Permian. This was followed by the suturing of Sibumasu and Qiangtang to Cathaysialand in the

  6. Magmatic and kinematic history of Siletzia, a Paleocene-Eocene accreted oceanic terrane in the Oregon Coast Range

    NASA Astrophysics Data System (ADS)

    Wells, R. E.; Bukry, D.; Wooden, J. L.; Friedman, R. M.; Haeussler, P. J.

    2010-12-01

    The basalt basement of the Oregon and Washington Coast Ranges, known as the Siletz terrane or Siletzia after the type Siletz River Volcanics of Oregon, consists of more than 2 million km3 of Paleocene and early Eocene tholeiitic and alkalic basalt sutured to North America in Eocene time. Siletzia is up to 30 km thick in Oregon and thins northward; it is thought to be an oceanic plateau or island chain, possibly created near the Yellowstone hotspot. Most Siletz lavas were erupted between 56 and 49 Ma based on 40Ar/39Ar and U-Pb ages of flows. The ages are consistent with measured magnetic polarities of lavas and coccolith zones (CP8b to 11) from interbedded sediments, and they become younger to the north. Sedimentary interbeds also contain continentally-derived cobbles near Roseburg, OR and in the Olympic Mountains, suggesting some of the basalt flows were erupted close to the margin. Some researchers consider Siletzia the product of marginal rifting, slab window or hot spot magmatism during ridge subduction. Siletzia may have been much larger; similar rocks are found as far north as the Yakutat terrane in Alaska and south into California. Geologic mapping near Roseburg, Tillamook, and the Willapa Hills, along with U/Pb, 40Ar/39Ar, paleomagnetism, and coccolith zones provide constraints on Siletzia's final docking in Oregon at 51 Ma. Low thermal maturity for Tyee basin fill deposited on Siletzia and its shallowing upward section are incompatible with rifting. Slip vectors from the basin-bounding faults indicate margin-normal thrusting ocurred from the start of basin filling. Margin-parallel folding and thrusting of Siletzia, which was disrupted by subsequent clockwise block rotation, can be restored to its original NW strike and indicates a NW-trending continental margin at the time of collision. After collision, Siletzia migrated modestly northward (< 300 km from paleomagnetic evidence), rotating clockwise into its present position. Collision was followed by a

  7. Oceanic terranes of S-Central America - 200 Million years of accretion history recorded on the W-edge of the Caribbean Plate.

    NASA Astrophysics Data System (ADS)

    Baumgartner, P. O.; Flores, K.; Bandini, A.; Buchs, D.; Andjic, G.; Baumgartner-Mora, C.

    2012-04-01

    (Chortis Block s. str.), and became exhumed again by the earliest Cretaceous (139 Ma phengite age). A pre-Albian basaltic plateau-like basement is suspected but yet undated in the Matambú Terrane (Central Nicoya Peninsula). It is overlain by the Albian Loma Chumico Formation. A pre-Turonian basement hosting the 90 Ma old Tortugal Picrites and alkaline baselts characterizes the Manzanillo Terrane ( around the Nicoya Gulf, Costa Rica). The overlying Coniacian-Santonian to early Campanian Berrugate Formation represents the first Cretaceous evolved arc activity. It must be located on the edge of the MCOT, since the CLIP is still forming at that time. To the SE of the S-Nicoya fault zone, Turonian-Santonian (~90-85 Ma) oceanic plateaus represent outcrops of the CLIP. These include parts of Herradura (Costa Rica) and the Azuero Plateau (Coiba, Sona and Azuero, Panama). Late Campanian to Paleocene arcs rest on the CLIP: The Golfito Complex (Costa Rica) and the Azuero Arc (Panama), possibly also the San Blas Complex (Panama) and the Serrania de Baudo (W-Colombia). Late Cretaceous to Eocene plateau/seamount basalts and oceanic sediments became accreted during the Early Tertiary: The Tulin Group (Herradura), Quepos, The Osa Igneous Complex, Burica, the Osa Mélange (Costa Rica/Panama), and the Azuero Accretionary Complex (Panama).

  8. An accreted continental terrane in northwestern Peru

    NASA Astrophysics Data System (ADS)

    Mourier, T.; Laj, C.; Mégard, F.; Roperch, P.; Mitouard, P.; Farfan Medrano, A.

    1988-04-01

    A paleomagnetic study of over 250 cores from 26 sites sampled in Early to Late Cretaceous and Paleogene volcanic, plutonic and sedimentary formations of the Lancones basin in the Piura province of northern Peru, indicates that most of these lithologies carry a stable primary remanent magnetization whose direction is significantly different from that of coeval formations of stable South America. A clockwise rotation ranging from 90° for the lowermost units to 35° for the uppermost ones has been documented, although the lack of precise chronology has not allowed a detailed temporal description. Four sites from Late Carboniferous (Pennsylvanian) formations in the Amotape-Tahuin Range also show a 110° clockwise rotation and yield evidence for a northward displacement. When considered together with previous geological studies, these data are consistent with the hypothesis of the accretion of an Amotape-Tahuin continental terrane to the Peruvian margin in Neocomian times. The accretion was followed by in situ rotation, suggesting a dextral shear regime. These results indicate that the geodynamical evolution of northern Peru is more closely related to the processes observed in Ecuador than to those classically assumed for the Central Andes of Peru.

  9. A tectonic reconstruction of accreted terranes along the paleo-Pacific margin of Gondwana

    NASA Astrophysics Data System (ADS)

    Bammel, Brandon

    The southern oceanic margin of Gondwana was nearly 40,000 km long or 24,854.8 miles. The southern margin was the result of the Terra Australis orogen. Spanning 18,000 km or 11,184.7 miles and is proposed as one of the largest and longest lived orogens in Earth history. The paleo-Pacific margin of Gondwana consisted of segments of the Australian-Antarctic craton, southern South America (modern Argentina and Chile), southern South Africa, Marie Byrdland, New Zealand and its adjacent continental shelf, the Ellsworth Mountains, and the Transantarctic Mountains. The process of terrane accretion has played a substantial part in the assembly of the continents as they look today. The paleo-Pacific margin of Gondwana was an active region of terrane accretion from the Neoproterozoic to the Late Mesozoic. This research study examines the accretion of terranes across the paleo-Pacific Gondwana margin to provide a comprehensive reconstruction. A paleogeographic basemap was created using PALEOMAP Project maps and the geology data was provided by the School of Geoscience from the University of Witwatersrand of South Africa. Location and data analyzed for terranes were collected building a PDF library of journal articles across numerous geological publications.

  10. The Late Triassic bivalve Monotis in accreted terranes of Alaska

    USGS Publications Warehouse

    Silberling, Norman J.; Grant-Mackie, J. A.; Nichols, K.M.

    1997-01-01

    Late Triassic bivalves of the genus Monotis occur in at least 16 of the lithotectonic terranes and subterranes that together comprise nearly all of Alaska, and they also occur in the Upper Yukon region of Alaska where Triassic strata are regarded as representing non-accretionary North America. On the basis of collections made thus far, 14 kinds of Monotis that differ at the species or subspecies level can be recognized from alaska. These are grouped into the subgenera Monotis (Monotis), M. (Pacimonotis), M. (Entomonotis), and M. (Eomonotis). In places, Monotis shells of one kind or another occur in rock-forming abundance. On the basis of superpositional data from Alaska, as well as from elsewhere in North America and Far Eastern Russia, at least four distince biostratigraphic levels can be discriminated utilizing Monotis species. Different species of M. (Eomonotis) characterize two middle Norian levels, both probably within the supper middle Norian Columbianus Ammonite Zone. Two additional levels are recognized in the lower upper Norian Cordilleranus Ammonite Zone utilizing species of M. (Monotis) or M. (Entomonotis), both of which subgenera are restricted to the late Norian. An attached-floating mode of life is commonly attributed to Monotis; thus, these bivalves would have been pseudoplanktonic surface dwellers that were sensitive to surface-water temperature and paleolatitude. Distinctly different kinds of Monotis occur at different paleolatitudes along the Pacific and Arctic margins of the North American craton inboard of the accreted terranes. Comparison between thse craton-bound Monotis faunas and those of the Alaskan terranes in southern Alaska south of the Denali fault were paleoequatorial in latitude during Late Triassic time. Among these terranes, the Alexander terrane was possibly in the southern hemisphere at that time. Terranes of northern Alaska, on the other hand, represent middle, possibly high-middle, northern paleolatitudes.

  11. Mesozoic terrane accretion and formation of the Idaho batholith

    NASA Astrophysics Data System (ADS)

    Gaschnig, R. M.; Vervoort, J. D.; Tikoff, B.

    2014-12-01

    The Mesozoic suturing of the Blue Mountains province to North America was accompanied by a complex history of magmatism that ultimately culminated in the formation of the Idaho batholith. The oceanic terranes of the Blue Mountains were most likely assembled offshore in the Late Jurassic and accreted to North America in the Early Cretaceous, probably south of their current latitude. Suturing was followed by a renewal of subduction outboard of the Blue Mountains, leading to tonalitic and trondhjemitic magmatism in the eastern portion of the province from about 130 to 110 Ma. Additional compositionally diverse magmatism within the suture occurred from about 110 to 100 Ma and included the reworking of crustal material from both sides of the suture. This resulting mid-Cretaceous intrusive suite was collapsed by dextral transpressional deformation from ~102 to 92 Ma, leading to the formation of the western Idaho shear zone and transporting the Blue Mountains to their present latitude. During the later stages of WISZ deformation, continental arc magmatism migrated east of the WISZ and produced the early metaluminous phases of the Idaho batholith. By 90 Ma, a large metaluminous continental arc connecting to the Sierra Nevada in the south and Coast Mountains in the north existed in Idaho, but subsequent crustal thickening on the continental side of the WISZ led to pervasive crustal melting in two pulses between ~83 and 54 Ma, which obliterated much of the earlier magmatic history of the batholith. Further magmatism occurred in the Eocene in response to changing plate boundary conditions and onset of extension.

  12. Terrane Stations: intra-oceanic subduction assembled western North America

    NASA Astrophysics Data System (ADS)

    Sigloch, K.; Mihalynuk, M. G.

    2012-12-01

    The western quarter of North America consists of accreted terranes, crustal blocks that were added to the margin in a series of collisions over the past 200 million years - but why? The most widely accepted explanation posits a scenario analogous to Andean subduction, with these terranes conveyed to the continental margin while the oceanic Farallon plate subducted under it. Yet purely Andean-style subduction under North America is questionable as a terrane delivery mechanism, since no comparable accretion sequence took place along the South American margin, and since North American terranes are of very varied provenance. We consider this geological question directly related to a geodynamical one: Why has it been so difficult to reconcile - even on the largest scale - the geometries and locations of slabs in the lower-mantle, as imaged by seismic tomography, with Cretaceous plate reconstructions of the North American west coast (unless anomalous mantle rheology or ad hoc shifts of absolute reference frame are invoked)? This problem was recognized soon after the discovery of the massive, lower-mantle "Farallon slabs" by Grand (1994), but has recently been aggravated by the discovery of additional, more westerly deep slabs (Sigloch et al. 2008), thanks to USArray. Not all of these slabs can be Farallon, unless very non-vertical and/or uneven slab sinking behavior is allowed for. As a joint solution, we offer a radical reinterpretation of paleogeography and test it quantitatively: The seas west of Cretaceous North America must have resembled today's western Pacific. The Farallon and two more plates subducted into the intra-oceanic trenches of a vast archipelago in the eastern Panthalassa (proto-Pacific) ocean, both from the east and the west. The trenches remained stationary throughout much of Jurassic and Cretaceous times, depositing the massive, near-vertical slab walls imaged in the lower mantle today. On their overriding plates, island arcs and subduction complexes

  13. Argon-40/argon-39 whole-rock slate ages from the Robertson Bay terrane, northern Victoria Land: Documenting diachronous orogeny as a result of terrane accretion

    SciTech Connect

    Dallmeyer, R.D.

    1987-09-01

    In models of tectonic convergence, oceanic material enters subduction zones, descends, and is deformed. Over time subduction cuts across areas of deformation and should become younger in a direction opposite the dip of the subduction zone. Typically, geologists have found it difficult to document this because of insufficient cross-strike exposure, uncertainties in interpretation of radiometric ages, and overprinting of later tectonothermal events. The Robertson Bay and Bowers terranes of northern Victoria Land, which were accreted to the east antarctic craton (Gondwana margin) during the Ordovician Ross Orogeny, offer a unique opportunity to document a potentially diachronous deformation associated with an Ordovician orogeny. Exposed over a large area, these terranes display nearly uniform lithology, low metamorphic grade, and structural style and are combined with the absence of a regionally significant tectonothermal over print. Reconnaissance age analysis by potassium/argon and argon-40/argon-39 methods suggests that ages decrease eastward across the Robertson Bay terrane. The objective is systematically to collect samples of and perform argon-40/argon-39 age analysis on a whole-rock suite from the Bowers and Robertson Bay terranes. The resultant ages would more clearly document the suggested diachronism and allow more rigorous evaluation of the various tectonothermal models proposed for the evolution of the Gondwana margin.

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

    USGS Publications Warehouse

    Fruehn, J.; Von Huene, R.; Fisher, M.A.

    1999-01-01

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

  15. Crustal structure of accreted terranes in southern Alaska, Chugach Mountains and Copper River Basin, from seismic refraction results

    USGS Publications Warehouse

    Fuis, G.S.; Ambos, E.L.; Mooney, W.D.; Christensen, N.I.; Geist, E.

    1991-01-01

    Seismic refraction data were collected along a 320-km-long "transect' line in southern Alaska, crossing the Prince William, Chugach, Peninsular, and Wrangellia terranes, and along several shorter lines within individual terranes. Velocity structure in the upper crust (less than 9-km depth) differs among the four terranes. In contrast, layers in the middle crust (9- to 25-km depth) in some case extend across projected terrane boundaries. The top of a gently north dipping sequence of low- and high-velocity layers (5.7-7.8 km/s), more than 10 km thick, extends from near the surface in the southern Chugach terrane to more than 20-km depth beneath the southern Peninsular terrane. This sequence, truncated by the suture between the Prince William and Chugach terranes, is interpreted to be an underplated "terrane' made up of fragments of the Kula plate and its sedimentary overburden that were accreted during subduction in the late Mesozoic and/or early Tertiary, during or between times of accretion of the Prince William and Chugach terranes. -from Authors

  16. Igneous history of the Koyukuk terrane, western Alaska: constraints on the origin, evolution, and ultimate collision of an accreted island arc terrane

    USGS Publications Warehouse

    Box, S.E.; Patton, W.W., Jr.

    1989-01-01

    The Koyukuk terrane consists of volcanic, volcaniclastic, and plutonic rocks which range from Late Paleozoic to Early Cretaceous in age. The terrane crops out in a U-shaped belt which is roughly paralleled by outer belts of ultramafic rocks, oceanic plate basalts and cherts, and retrograded blueschist facies rocks of continental protolith. These rocks have been interpreted as components of a volcanic arc terrane that collided with the North American continental margin in Early Cretaceous time. The Koyukuk terrane consists of four time-stratigraphic units: (1) pre-Middle Jurassic basalts, (2) Middle and Late Jurassic granitic rocks, (3) lower Lower Cretaceous volcanic rocks, and (4) upper Lower Cretaceous volcanic rocks. Limited chemical data from the basalts of unit 1 indicate that they were erupted in a nonarc tectonic environment, possibly in an oceanic island or back arc setting. Units, 2, 3, and 4 have the characteristics of subduction-related volcanic rocks. -from Authors

  17. Three-dimensional velocity structure of Siletzia and other accreted terranes in the Cascadia forearc of Washington

    USGS Publications Warehouse

    Parsons, T.; Wells, R.E.; Fisher, M.A.; Flueh, E.; ten Brink, U.S.

    1999-01-01

    Eocene mafic crust with high seismic velocities underlies much of the Oregon and Washington forearc and acts as a backstop for accretion of marine sedimentary rocks from the obliquely subducting Juan de Fuca slab. Arc-parallel migration of relatively strong blocks of this terrane, known as Siletzia, focuses upper crustal deformation along block boundaries, which are potential sources of earthquakes. In a three-dimensional velocity model of coastal Washington, we have combined surface geology, well data, and travel times from earthquakes and controlled source seismic experiments to resolve the major boundaries of the Siletz terrane with the adjacent accreted sedimentary prism and volcanic arc. In southern Washington and northern Oregon the Siletz terrane appears to be a thick block (???20 km) that extends west of the coastline and makes a high-angle contact with the offshore accreted sedimentary prism. On its east flank the high-velocity Siletz terrane boundary coincides with an en echelon zone of seismicity in the arc. In northern Washington the western edge of Siletzia makes a lower-angled, fault-bound contact with the accretionary prism. In addition, alternating, east-west trending uplifts and downwarps of the Siletz terrane centered on the antiformal Olympic Mountains may reflect focusing of north-south compression in the northern part of the Siletz terrane. This compressional strain may result from northward transport and clockwise rotation of the Siletz terrane into the relatively fixed Canadian Coast Mountains restraining bend along the coast.

  18. Tectonic accretion and underplating of mafic terranes in the Late Eocene intraoceanic fore-arc of New Caledonia (Southwest Pacific): geodynamic implications

    NASA Astrophysics Data System (ADS)

    Cluzel, Dominique; Aitchison, Jonathan C.; Picard, Christian

    2001-10-01

    This paper deals with the tectonic events that result in the accretion of mafic terranes in the fore-arc region and a close juxtaposition of ultramafic rocks, low grade and high-grade mafic terranes in many collisional orogens. The example is taken from New Caledonia where tectonic accretion, subduction, underplating and obduction of mafic terranes took place during the late Eocene in an intra-oceanic forearc setting. The late Eocene tectonic complex comprised three major terranes: an overlying ultramafic, mainly harzburgitic allochthon named the Ophiolitic Nappe, an intermediate mafic, mainly basaltic off-scraped melange, composed of kilometre-scale slices of oceanic upper crust, called the Poya Terrane, parts of which have been metamorphosed into an eclogite/blueschist facies complex, the Pouebo Terrane; and a lower, continental basement formed by the Norkolk Ridge terranes. Based upon exhaustive sampling of the mafic terranes and field surveys, our tectonic, micropaleontologic and geochemical data reveal that Poya and Pouebo terranes rocks originally formed within one single Campanian to late Paleocene oceanic basin, floored by tholeiitic basalt associated with some minor seamount-related intraplate alkali basalt. The tholeiitic basalt displays a continuous range of compositions spanning between "undepleted" and "depleted" end-members; the former being volumetrically predominant. The overall geochemical and isotopic features indicate an origin from a prominently heterogeneous mantle source during the opening of a marginal basin, the South Loyalty Basin, which almost completely disappeared during Eocene convergence. The opening of this basin originally located to the east of the Norfolk Ridge was synchronous with that of Tasman Sea basin as a consequence of oceanward migration of the west-dipping Pacific subduction zone. Establishing the origin of the ultramafic Ophiolitic Nappe is beyond the scope of this paper; however, it appears to be genetically unrelated to

  19. Paleozoic to Jurassic terrane accretion along the northeastern margin of Tibet plateau

    NASA Astrophysics Data System (ADS)

    Neubauer, Franz; Liu, Yongjiang; Genser, Johann; Ren, Shoumai

    2010-05-01

    The Tibet plateau is considered to have been constructed by a number of basement terranes accreted to the Eurasian margin during Paleozoic to Mesozoic times, and accretion is interpreted to have progressed southwards. The northern margin, exposed in Altyn and Qilian Mountains, is generally considered as an Lower Paleozoic orogen including previously subducted ultra-high successions (Yin and Harrison, 2000; Xiao et al., 2009). Previous tectonic models of the Qaidam block and adjacent mountain ranges at the northeastern margin of the Tibet plateau assumed a minor role of Indosinian tectonism in that region, and firm evidence was only reported from eastern Kunlun Mountains (e.g., Liu et al., 2005). Based on four sources of new data, we propose that the Indosinian tectonism was much more widespread in the northeastern Qaidam block and adjacent mountain ranges, Altyn and Qilian Mts., as believed before and we propose a new tectonic model. The new data sources comprise: (1) 40Ar/39Ar dating of detrital white mica of Jurassic to Pliocene sandstones from the north-eastern Qaidam basin fill; (2) interpretation of Ordovician, Devonian and Jurassic sedimentary successions, from which we interpret the Jurassic successions as intramontane molasse to the Indosinian orogen; (3) 40Ar/39Ar dating of detrital white mica in recent rivers from the southern Qilian Shan revealing possible basement sources in the Qilian Shan draining towards the Qaidam basin; and (4) structural study of basement rocks and subordinate 40Ar/39Ar mineral ages of metamorphic basement rocks. An Ordovician greywacke exposed in the eastern Qaidam basin (W of Delinghua) yields three 40Ar/39Ar age groups of detrital white mica: 900-922, 610-654 and 527-554 Ma. Furthermore, similar old age groups centering at ca. 670 and 1010 Ma are virtually widespread in recent rivers from Qilian Mountains and clearly demonstrate the presence of Panafrican and Grenvillian tectonic elements in the southern Qilian Mts. at the

  20. Native terranes: examples from the Cordillera

    SciTech Connect

    Gray, G.G.

    1985-01-01

    The suspect terrane concept has provided a new working hypothesis which has been invaluable for interpreting complex continental margin areas. This concept is restrictive, however, because it allows for only exotic or suspect terrane origins, while there is a growing body of evidence that many terranes have developed in situ. The authors propose that such in situ units be called native terranes. Stratigraphic, metamorphic, and disrupted terranes in the Klamath Mountains of northern California and southern Oregon are examples of native terranes. A variety of oceanic crust and Tethyan seamount-derived units were accreted in a Late Triassic subduction zone in this area. Each of these terranes is highly disrupted, and contains blocks derived from North American units to the east. The incorporation of North American blocks into these terranes, coupled with paleomagnetic data and intrusive relationships, indicates that these exotic block-bearing disrupted terranes developed in their present position adjacent to North America and are therefore native. A coeval blueschist terrane developed inboard of these melanges and is also considered native. A coherent Middle Jurassic arc (.) sequence built upon this melange basement constitutes a native stratigraphic terrane. The present melange basement constitutes a native stratigraphic terrane. The present boundaries of these terranes are typically thrust faults and normal faults which postdate accretion. Similar mixtures of North American and exotic blocks have been noted in coeval accretionary belts in the Sierra Nevada foothills, east-central Oregon and eastern British Columbia.

  1. Diapirs of the Mediterranean ridge: The tectonic regime of an incipient accreted terrane

    NASA Technical Reports Server (NTRS)

    Mart, Y.

    1988-01-01

    The occurrence of diapirs in the Mediterranean ridge stems mostly from the massive deposition of salt and gypsum in the Mediterranean basin during the late Miocean. The diapiric emplacement of the evaporitic sequence is not obvious, because the mobilization of the salt beds and the initiation of the diapiric upward flow are constrained by the relatively shallow thickness of the Plio-Pleistocene sedimentary overburden and by the low heat flow that prevails in the eastern Mediterranean. The diapirs consist also of early Cretaceous shales as well as other gravitationally metastable strata which are less mobile than salt. Studies of subduction trenches and their surroundings show that shallow ridges occur seaward of the trenches in many places. The collisional motion between the African and the Eurasian plates would further enhance accretion of sediments in the Mediterranean ridge, which would attain subaerial exposure, and eventually would become a mountain range accreted to southern Europe. The numerous diapirs of salt and shales that occur in the ridge would be common features in the future accreted terrane, indicating an intermediate extensional phase in the tectonic history of the development of crustal growth.

  2. Timing of terrane accretion in eastern and east-central Maine

    NASA Astrophysics Data System (ADS)

    Ludman, Allan

    1986-05-01

    The Norumbega fault zone is often cited as a post-Acadian suture between exotic blocks, even though stratigraphic, structural, and metamorphic data indicate that there is little offset of the Silurian-Devonian strata that the zone cuts in eastern Maine. Similarly, the Kingman fault zone has been shown by gravity and geochemical studies to separate distinct crustal blocks, whereas mapping shows that it lies entirely within a Silurian turbidite package. These conflicts are resolved if the two fault zones represent boundaries between Ordovician or older crustal blocks that had accreted to form a composite terrane prior to deposition of the cover sequences. The faults now mapped within these younger rocks formed by reactivation of the pre-Silurian boundaries during late Acadian time; movement continued until the late Carboniferous. Most of the accretionary history of Maine had thus ended before the Silurian. A complex composite terrane may have formed during Cambrian-Ordovician time that (1) interacted with cratonic North America during the Taconian orogeny and (2) became the “basement” upon which the Silurian and Lower Devonian strata of eastern Maine were deposited.

  3. Middle Jurassic strata link Wallowa, Olds Ferry, and Izee terranes in the accreted Blue Mountains island arc, northeastern Oregon

    SciTech Connect

    White, J.D.L. ); Vallier, T. ); Stanley, G.D. Jr. ); Ash, S.R. ); White, D.L.

    1992-08-01

    Middle Jurassic strata atop the Wallowa terrane in northeastern Oregon link the Wallowa, Izee, and Olds Ferry terranes as related elements of a single long-lived and complex oceanic feature, the Blue Mountains island arc. Middle Jurassic strata in the Wallowa terrane include a dacitic ash-flow deposit and contain fossil corals and bivalves of North American affinity. Plant fossils in fluvial sandstones support a Jurassic age and indicate a seasonal temperate climate. Corals in a transgressive sequence traditionally overlying the fluvial units are of Bajocian age and are closely related to endemic varieties of the Western Interior embayment. They are unlike Middle Jurassic corals in other Cordilleran terranes; their presence suggests that the Blue Mountains island arc first approached the North American craton at high paleolatitudes in Middle Jurassic time. The authors consider the Bajocian marine strata and underlying fluvial volcaniclastic units to be a basin-margin equivalent of the Izee terrane, a largely Middle Jurassic (Bajocian) succession of basinal volcaniclastic and volcanic rocks known to overlie the Olds Ferry and Baker terranes.

  4. Zircon dating of oceanic crustal accretion.

    PubMed

    Lissenberg, C Johan; Rioux, Matthew; Shimizu, Nobumichi; Bowring, Samuel A; Mével, Catherine

    2009-02-20

    Most of Earth's present-day crust formed at mid-ocean ridges. High-precision uranium-lead dating of zircons in gabbros from the Vema Fracture Zone on the Mid-Atlantic Ridge reveals that the crust there grew in a highly regular pattern characterized by shallow melt delivery. Combined with results from previous dating studies, this finding suggests that two distinct modes of crustal accretion occur along slow-spreading ridges. Individual samples record a zircon date range of 90,000 to 235,000 years, which is interpreted to reflect the time scale of zircon crystallization in oceanic plutonic rocks. PMID:19179492

  5. Biogeographic complexity in Triassic bivalves of the Wallowa terrane, northwestern US: oceanic islands, not continents, provide the best analogues

    SciTech Connect

    Newton, C.R.

    1987-12-01

    High levels of endemism and complex, overlapping biogeographic patterns characterize modern molluscan faunas of the Hawaiian Islands and the Triassic bivalve fauna of the Wallowa volcanic-arc terrane in Hells Canyon, Oregon. Such biogeographic complexities and high levels of endemism in many modern and Mesozoic island settings constrain the use of faunal data as a primary basis for paleogeographic reconstruction of accreted terranes. Large, taxonomically diver samples are required to identify genuine biogeographic patterns in these insular settings. Selective use of individual species, genera, or families to reconstruct terrane paleogeography may give misleading results.

  6. Hf Isotopes and Geochemical Evidence Constrain the Nature and Sources of Melting During and After Progressive Accretion of the Wrangellia Composite Terrane to the Southern Alaska Margin

    NASA Astrophysics Data System (ADS)

    Todd, E.; Jones, J. V., III; Kylander-Clark, A. R.

    2015-12-01

    Plutonic rocks in the western Alaska Range were emplaced prior to, during, and after accretion of the Wrangellia Composite Terrane (WCT) to the southern Alaska margin (locally, Farewell terrane, FT). Docking between (mostly) oceanic WCT and (mostly) Paleozoic continental FT was done largely by ca. 80 Ma on the basis of youngest detrital zircon ages from an overlapping flysch basin and the oldest post-deformational plutons. Plutons before and during progressive basin closure and terrane accretion (~100-76 Ma) were emplaced in WCT basement or proximal to the WCT-FT margin, are calcalkaline diorite to granite, and likely products of the migrating arc associated with closure of the intervening ocean basin. Plutons emplaced after 76 Ma are organized axially and cross into both sides of the inferred suture zone, suggesting an association with faults formed during crustal shortening and transcurrent deformation. These Late Cretaceous gabbro to granodiorite plutons have arc to collisional affinity, some with "adakitic" compositions, possibly due to crustal thickening associated with WCT collision. In contrast, younger Paleocene plutons are spatially scattered and widespread fractionated granites. Hf isotopes and U/Pb ages were measured in zircons from ~110 to ~30 Ma plutons by LA-ICPMS using the split-stream configuration. Maximum eHf decreases gradually over time (+15 to +12) suggesting either more enriched mantle or an increasing role of crustal components in the melt source and/or during magma ascent and emplacement. However, most Late Cretaceous and a subset of Paleocene plutons have anomalously low eHf (+6 to -2). Paleocene granite isotopes correlate with location and basement type; plutons emplaced in Paleozoic basement have lower eHf compared with those in Mesozoic basement. This pattern, most extreme in Paleocene plutons, is also seen in Cretaceous to Eocene plutons where similar-aged rocks were emplaced in both domains, suggesting strong basement control on Hf

  7. Bokan Mountain peralkaline granitic complex, Alexander terrane (southeastern Alaska): evidence for Early Jurassic rifting prior to accretion with North America

    USGS Publications Warehouse

    Dostal, Jaroslav; Karl, Susan M.; Keppie, J. Duncan; Kontak, Daniel J.; Shellnutt, J. Gregory

    2013-01-01

    The circular Bokan Mountain complex (BMC) on southern Prince of Wales Island, southernmost Alaska, is a Jurassic peralkaline granitic intrusion about 3 km in diameter that crosscuts igneous and metasedimentary rocks of the Alexander terrane. The BMC hosts significant rare metal (rare earth elements, Y, U, Th, Zr, and Nb) mineralization related to the last stage of BMC emplacement. U–Pb (zircon) and 40Ar/39Ar (amphibole and whole-rock) geochronology indicates the following sequence of intrusive activity: (i) a Paleozoic basement composed mainly of 469 ± 4 Ma granitic rocks; (ii) intrusion of the BMC at 177 ± 1 Ma followed by rapid cooling through ca. 550 °C at 176 ± 1 Ma that was synchronous with mineralization associated with vertical, WNW-trending pegmatites, felsic dikes, and aegirine–fluorite veins and late-stage, sinistral shear deformation; and (iii) intrusion of crosscutting lamprophyre dikes at >150 Ma and again at ca. 105 Ma. The peralkaline nature of the BMC and the WNW trend of associated dikes suggest intrusion during NE–SW rifting that was followed by NE–SW shortening during the waning stages of BMC emplacement. The 177 Ma BMC was synchronous with other magmatic centres in the Alexander terrane, such as (1) the Dora Bay peralkaline stock and (2) the bimodal Moffatt volcanic suite located ~30 km north and ~100 km SE of the BMC, respectively. This regional magmatism is interpreted to represent a regional extensional event that precedes deposition of the Late Jurassic – Cretaceous Gravina sequence that oversteps the Wrangellia and Alexander exotic accreted terranes and the Taku and Yukon–Tanana pericratonic terranes of the Canadian–Alaskan Cordillera.

  8. Aleutian terranes from Nd isotopes

    NASA Technical Reports Server (NTRS)

    Kay, R. W.; Kay, S. M.; Rubenstone, J. L.

    1986-01-01

    Nd isotope ratios substantiate the identification of oceanic crustal terranes within the continental crustal basement of the Aleutian island arc. The oceanic terranes are exposed in the westernmost Aleutians, but to the east, they are completely buried by isotopically distinct arc-volcanic rocks. Analogous oceanic terranes may be important components of the terrane collages that comprise the continents.

  9. Hurricane Mountain Formation melange: history of Cambro-Ordovician accretion of the Boundary Mountains terrane within the northern Appalachian orthotectonic zone

    SciTech Connect

    Boone, G.M.; Boudette, E.L.

    1985-01-01

    The Hurricane Mountain Formation (HMF) melange and associated ophiolitic and volcanogenic formations of Cambrian and lowermost Ordovician age bound the SE margin of the Precambrian Y (Helikian) Chain Lakes Massif in western Maine. HMF melange matrix, though weakly metamorphosed, contains a wide variety of exotic greenschist to amphibolite facies blocks as components of its polymictic assemblage, but blocks of high-grade cratonal rocks such as those of Chain Lakes or Grenville affinity are lacking. Formations of melange exposed in structural culminations of Cambrian and Ordovician rocks NE of the HMF in Maine and in the Fournier Group in New Brunswick are lithologically similar and probably tectonically correlative with the HMF; taken together, they may delineate a common pre-Middle Ordovician tectonic boundary. The authors infer that the Hurricane Mountain and St. Daniel melange belts define the SE and NW margins of the Boundary Mountains accreted terrane (BMT), which may consist of cratonal basement of Chain Lakes affinity extending from eastern Gaspe (deBroucker and St. Julien, 1985) to north-central New Hampshire. The Laurentian continental margin, underlain by Grenville basement, underplated the NW margin of this terrane, marked by the SDF suture zone, in late Cambrian to early Ordovician time, while terranes marked by Cambrian to Tremadocian (.) lithologies dissimilar to the Boundary Mountains terrane were accreted to its outboard margin penecontemporaneously. The docking of the Boundary Mountains terrane and the initiation of its peripheral melanges are equated to the Penobscottian disturbance.

  10. Late Cretaceous - Paleogene forearc sedimentation and accretion of oceanic plateaus and seamounts along the Middle American convergent margin (Costa Rica)

    NASA Astrophysics Data System (ADS)

    Baumgartner, Peter O.; Baumgartner-Mora, Claudia; Andjic, Goran

    2016-04-01

    -derived plateaus and seamounts. Scarce redeposited remnants of Campanian-Maastrichtian and Late Paleocene-Early Eocene shallow water limestones are associated either with shoals on oceanic seamounts such as the Tulín and Quepos Terranes, or on accreted and uplifted plateaus, such as the Inner Osa Igneous Complex. The latter was probably accreted during the Early Paleocene and partly uplifted and maintained in the photic zone during the Late Paleocene - Late Eocene, as indicated by shallow water material both in place (Burica Peninsula, western Panama) and resedimented in deep water hemipelagic series. The Paleocene-Middle Eocene period is punctuated by the accretion of large pieces of plateaus and oceanic islands that may have temporarily extinguished the arc in southern Costa Rica. Only distal (airborne and suspension) volcanic material is known from that time. By Late Eocene, arc-volcanic activity resumed. The accretion of small seamounts and mass wasting of earlier accreted material from the hanging wall created the Osa Mélange. It contains scarce remnants of the insular shallow water carbonates along with a big volume of arc-derived detritals, including upper Eocene shallow water resediments.

  11. COMPOSITIONALLY DRIVEN CONVECTION IN THE OCEANS OF ACCRETING NEUTRON STARS

    SciTech Connect

    Medin, Zach; Cumming, Andrew E-mail: cumming@physics.mcgill.ca

    2011-04-01

    We discuss the effect of chemical separation as matter freezes at the base of the ocean of an accreting neutron star, and argue that the retention of light elements in the liquid acts as a source of buoyancy that drives a slow but continual mixing of the ocean, enriching it substantially in light elements, and leading to a relatively uniform composition with depth. We first consider the timescales associated with different processes that can redistribute elements in the ocean, including convection, sedimentation, crystallization, and diffusion. We then calculate the steady-state structure of the ocean of a neutron star for an illustrative model in which the accreted hydrogen and helium burn to produce a mixture of O and Se. Even though the H/He burning produces only 2% oxygen by mass, the steady-state ocean has an oxygen abundance more than 10 times larger, almost 40% by mass. Furthermore, we show that the convective motions transport heat inward, with a flux of {approx}0.2 MeV nucleon{sup -1} for an O-Se ocean, heating the ocean and steepening the outward temperature gradient. The enrichment of light elements and heating of the ocean due to compositionally driven convection likely have important implications for carbon ignition models of superbursts.

  12. Seismic anisotropy of the subcrustal lithosphere in Europe: Another clue to recognition of accreted terranes?

    NASA Astrophysics Data System (ADS)

    Babuška, Vladislav; Plomerová, J.

    P-wave residuals computed relative to a reference Earth model and normalized for effects originating in focal regions and along ray paths in the deep mantle provide information on deep lithospheric structure. The variations of the directionally independent representative average residuals, which are computed for waves arriving from different azimuths and with steep incidence angles, reflect compositional and thermal inhomogeneities. In our model they are attributed to variations of the lithosphere thickness. On the other hand, the variations of relative residuals that depend on the angles of azimuth and incidence form spatial patterns suggesting the existence of large-scale dipping anisotropic structures in the subcrustal lithosphere. The P-velocity anisotropy of these structures (9-11% on the average) agrees with the anisotropy of olivine ultramafites as measured in the laboratory. Orientations of the deep anisotropic structures change in the vicinity of important tectonic suture zones, for example, at the suture between the Saxothuringicum and Moldanubicum in central Europe, at the Insubric line in the Alps, and at the deep contact between the Rhodopean Massif and the Moesian Platform in the central Balkans. The structures probably retain preferred orientations of olivine crystals originating from an ancient oceanic lithosphere and may thus represent relict paleosubduction zones by which the continental lithosphere grew in the past.

  13. Stable Isotope Evidence for a Complex Fluid Evolution of the Northwestern British Columbia Coast Ranges Related to Terrane Accretion

    NASA Astrophysics Data System (ADS)

    Moertle, J.; Holk, G. J.

    2015-12-01

    Stable isotope geochemistry reveals a complex fluid evolution for the Western Metamorphic Belt (WMB), Coast Ranges Batholith (CRB), Central Gneiss Complex (CGC) and Coast Ranges Megalineament (CRM). These fluids are a product of a complex tectonic history related to terrane accretion that includes oblique convergence, metamorphism, magmatism, and orogenic collapse. From W-to-E, these fluid systems are as follows. High-pressure greenschist-to-amphibolite facies metasedimentary rocks of the WMB record variable mineral δD (-61 to -104‰) and δ18O (e.g., quartz +9.6 to +13.4‰) values with multiple minerals in apparent isotopic equilibrium (T ~ 450-550°C) suggest a low W/R system dominated by metamorphic fluids. Variable and non-equilibrium δD (-53 to -143‰) and δ18O (e.g., biotite +2.3 to +5.3‰) values from diorites of the Quottoon pluton affected by the ductile CRM suggest a complex evolution that involved both metamorphic and meteoric-hydrothermal fluids in this dextral shear zone; these results differ from those 300 km along strike to the north that documented only metamorphic fluids in the CRM (Goldfarb et al., 1988). Our data and those of Magaritz and Taylor (1976) from granulite facies metasediments of the CGC and plutons of the western CRB reveal homogeneous δD values (-62 to -78‰) and a restricted range of δ18O values (e.g., quartz +8.5 to +11.5‰) with all minerals in equilibrium at T > 570°C indicate a system dominated by magmatic fluids. Calculated whole-rock δ18O values (~ +7‰) for the Quottoon pluton and CRB intrusive rocks suggest a mantle origin for these magmas. Reinterpretation of very low δD (< -150‰) and quartz-feldspar δ18O pairs that display extreme disequilibrium (feldspar δ18O values as low as -5‰) from the Ponder pluton, eastern CRB, and Hazelton Group point reveals that the major meteoric-hydrothermal system that affected these rocks was related to Eocene detachment faulting along the Shames Lake fault system, a

  14. Accretion of southern Alaska

    USGS Publications Warehouse

    Hillhouse, J.W.

    1987-01-01

    Paleomagnetic data from southern Alaska indicate that the Wrangellia and Peninsular terranes collided with central Alaska probably by 65 Ma ago and certainly no later than 55 Ma ago. The accretion of these terranes to the mainland was followed by the arrival of the Ghost Rocks volcanic assemblage at the southern margin of Kodiak Island. Poleward movement of these terranes can be explained by rapid motion of the Kula oceanic plate, mainly from 85 to 43 Ma ago, according to recent reconstructions derived from the hot-spot reference frame. After accretion, much of southwestern Alaska underwent a counterclockwise rotation of about 50 ?? as indicated by paleomagnetic poles from volcanic rocks of Late Cretaceous and Early Tertiary age. Compression between North America and Asia during opening of the North Atlantic (68-44 Ma ago) may account for the rotation. ?? 1987.

  15. Character, distribution, and tectonic significance of accretionary terranes in the Central Alaska Range

    NASA Astrophysics Data System (ADS)

    Jones, David L.; Silberling, N. J.; Gilbert, Wyatt; Coney, Peter

    1982-05-01

    Triassic red beds, basalt, and interbedded basalt and limestone; later Mesozoic rocks are sandstone, chert, and argillite, (8) Westfork terrane, Jurassic chert, sandstone, conglomerate, and Triassic (?) and Jurassic crystal tuff, (9) Broad Pass terrane, upper Paleozoic chert, tuff, and argillite, with blocks of Devonian and older limestone locally associated with serpentinite. These diverse terranes, of mixed oceanic and continental affinities, are now juxtaposed to form a complex sequence of folded and faulted rootless nappes. Major suture zones between them are occupied by intensely deformed upper Mesozoic flysch. All of the terranes differ markedly from Wrangellia, the southermost coherent accreted terrane in Alaska. Paleomagnetic measurements from Triassic rocks prove that Wrangellia originated near the equator, probably in the southern heimsphere. Paleolatitudes for terranes in the central Alaska Range are not yet determined, but other lines of evidence (biogeographic and lithologic) suggest large-scale northern transport for some terranes.

  16. Continental growth by successive accretion of oceanic lithosphere: Evidence from tilted seismic anisotropy

    NASA Astrophysics Data System (ADS)

    Babuska, V.; Plomerova, J.; Karato, S. I.

    2012-04-01

    Although many studies indicate that subduction-related accretion, subduction-driven magmatism and tectonic stacking are major crustal-growth mechanisms, how the mantle lithosphere forms remains enigmatic. Cook (AGU Geod. Series 1986) published a model of continental 'shingling' based on seismic reflection data indicating dipping structures in the deep crust of accreted terranes. Helmstaedt and Gurney (J. Geoch. Explor. 1995) and Hart et al. (Geology 1997) suggest that the Archean continental lithosphere consists of alternating layers of basalt and peridotite derived from subducted and obducted Archean oceanic lithosphere. Peridotite xenoliths from the Mojavian mantle lithosphere (Luffi et al., JGR 2009), as well as xenoliths of eclogites underlying the Sierra Nevada batholith in California (Horodynskij et al., EPSL 2007), are representative for oceanic slab fragments successively attached to the continent. Recent seismological findings also seem to support a model of continental lithosphere built from systems of paleosubductions of plates of ancient oceanic lithosphere (Babuska and Plomerova, AGU Geoph. Monograph 1989), or by stacking of the plates (Helmstaedt and Schulze, Geol. Soc. Aust. Spec. Publ. 1989). Seismic anisotropy in the oceanic mantle lithosphere, explained mainly by the olivine A- (or D-) type fabric (Karato et al., Annu. Rev. Earth Planet. Sci. 2008), was discovered almost a half century ago (Hess, Nature 1964). Though it is difficult to determine seismic anisotropy within an active subducting slab (e.g., Healy et al., EPSL 2009; Eberhart-Phillips and Reyners, JGR 2009), field observations and laboratory experiments indicate the oceanic olivine fabric might be preserved there to a depth of at least 200-300 km. Dipping anisotropic fabrics in domains of the European mantle lithosphere were interpreted as systems of 'frozen' paleosubductions (Babuska and Plomerova, PEPI 2006), and the lithosphere base as a boundary between a fossil anisotropy in the

  17. Structural analysis of terrane accretions in the eastern Brooks Range and adjacent areas in central Alaska and Canada. Technical progress report No. 6

    SciTech Connect

    Coney, P.J.; Harms, T.A.

    1985-03-14

    Work on radiolarian separation and identification of samples from the Sylvester Allochthon is nearing completion. A preliminary chronostratigraphic chart showing the age range and lithologic type of all dated units has been drawn. The comparative petrography of lithologies from the Sylvester Allochthon and the Angayucham and Mosquito Terranes, and from deformed clastic rocks of the Venetie Terrane is being studied. Several preliminary structure sections have been drawn across the Sylvester Terrane. (ACR)

  18. Zedong terrane revisited: An intra-oceanic arc within Neo-Tethys or a part of the Asian active continental margin?

    NASA Astrophysics Data System (ADS)

    Zhang, Liang-Liang; Liu, Chuan-Zhou; Wu, Fu-Yuan; Ji, Wei-Qiang; Wang, Jian-Gang

    2014-02-01

    Precise timing of the India-Asia collision is important to constrain the evolution history of both the Himalayan orogen and the Tibetan Plateau. It has been proposed that the Indian plate first collided with an intra-oceanic arc at ˜55 Ma, and then the composite terrane collided with the Asian continent at ˜35 Ma. The Zedong terrane has been suggested to represent the vestige of such an intra-oceanic arc developed within the Neo-Tethys Ocean, as some volcanic rocks with high K2O have been classified as shoshonites. In this study, we present detailed geochemical and geochronological data of various types of magmatic rocks (including volcanic, cumulate and granitic rocks) widely exposed in the Zedong terrane to constrain the formation age and tectonic setting of the Zedong terrane. We found that the Zedong volcanic rocks belong to calc-alkaline series rather than shoshonites and high K2O contents in some volcanic rocks resulted from alteration. The basalts are highly enriched in LREE and LILE, but strongly depleted in HFSE, indicating they were derived from a metasomatized mantle. Presence of hornblende phenocryst in both gabbros and hornblendites indicates that the cumulates were produced from hydrous basalts through crystallization. The granitic rocks have adakite-like compositional characteristics, i.e., high Sr/Y ratios but low Y contents, which were formed by melting of a thickened lower crust. Zircons from six samples, including a volcanic rock (an andesite), three cumulates (a hornblendites, a hornblende-bearing gabbro and a gabbro) and two granitic (a tonalite and a granodiorite) rocks, have been dated to yield identical ages of ˜155-160 Ma. This suggests that the volcanic eruption and plutonic emplacement were coevally developed in the Zedong terrane. Zircons from both the andesite and the cumulates have similar positive ɛHf(t) values (˜+11.6 to +16.7), indicating they were stemmed from similarly depleted mantle sources. Meanwhile, zircons from the

  19. Circum-North Pacific tectonostratigraphic terrane map

    USGS Publications Warehouse

    Nokleberg, Warren J.; Parfenov, Leonid M.; Monger, James W.H.; Baranov, Boris B.; Byalobzhesky, Stanislav G.; Bundtzen, Thomas K.; Feeney, Tracey D.; Fujita, Kazuya; Gordey, Steven P.; Grantz, Arthur; Khanchuk, Alexander I.; Natal'in, Boris A.; Natapov, Lev M.; Norton, Ian O.; Patton, William W., Jr.; Plafker, George; Scholl, David W.; Sokolov, Sergei D.; Sosunov, Gleb M.; Stone, David B.; Tabor, Rowland W.; Tsukanov, Nickolai V.; Vallier, Tracy L.; Wakita, Koji

    1994-01-01

    after accretion of most terranes in the region; (2) Cenozoic and Mesozoic basinal deposits that occur within a terrane or on the craton; (3) plutonic rocks. The postaccretion igneous units are identified by age-lithologic abbreviations and by name. These overlap assemblages and basinal deposits formed mainly during sedimentation and magmatism that occurred after accretion of terranes to each other or to a continental margin. Overlap assemblages provide minimum ages on the timing of accretion of terranes. Some Cenozoic and Mesozoic overlap assemblages and basinal deposits, as well as fragments of terranes, are extensively offset by movement along postaccretion faults. In addition, in onshore areas, the map depicts major preaccretion plutonic rocks that are limited to individual terranes. and in offshore areas. the map depicts major oceanic plates,-ocean floor magnetic lineations. oceanic spreading ridges, and seamounts. The map consists of five sheets. Sheets I and 2 depict, at a scale of I :5.000.000. the tectonostratigraphic terranes. preaccretion plutonic rocks, and postaccretion Cenozoic and Mesozoic overlap sedimentary, volcanic. and plutonic assemblages, and basinal deposits for the Circum- orth Pacific including the Russian Far East, northern Hokkaido Island of Japan, Alaska. the Canadian Cordillera, part of the U.S.A. Pacific Northwest. and adjacent offshore areas. Sheet 3 provides the list of map units for Sheets I and 2. Sheet 4 is a index map showing generalized onshore terranes and overlap assemblages for onshore parts of the Circum-North Pacific at a scale of I: I 0,000,000. Sheet 4 is a guide to the more complicated onshore features depicted on Sheets I and 2. Sheet 5 is an index map showing the major geographic regions for the Circum-North Pacific. Significant differences exist between the representation of onshore and offshore geology on Sheets I and 2. These are: (I) compared to the onshore part of the map, the offshore part is depicted in a more

  20. Reconstructing Ophiolites: Reassessing Assumptions From the Oceanic Crust and Related Terranes

    NASA Astrophysics Data System (ADS)

    Karson, J. A.

    2014-12-01

    The internal structure of ophiolite complexes has long been used as a window into the inaccessible parts of the oceanic lithosphere and by inference, processes beneath spreading centers. However, even the best preserved ophiolite complexes have been tilted, folded, faulting and dismembered during post-spreading tectonic events. Some degree of reconstruction is required to restore ophiolite structures to their appropriate relative orientations in order to relate them to processes beneath spreading centers. A number of assumptions about ophiolite structures have been used to guide reconstructions including: lava flows (horizontal, especially sheet-like lavas), dikes in lavas and sheeted dike complexes (vertical and parallel to spreading centers), the contacts between major rock units (horizontal, analogous to the seismic structure of oceanic crust) and the mafic/ultramafic contact representing the geologic expression of the Moho (horizontal). Based in part on these assumptions the internal structure of rock units, metamorphic relationships, and the kinematics of faults and deformation fabrics are also inferred. The spreading direction is seldom constrained in ophiolites making it difficult to assess the geometry of asymmetrical features, such as the dip of dikes, faults, or igneous layering, relative to spreading axes. Observations from exposures of upper crustal rock units (lavas, transition zones, dike and upper gabbroic rocks) along major tectonic escarpments in oceanic crust formed at fast to intermediate rates, as well as the uplifted and glaciated Tertiary basaltic crust of Iceland, raise questions about several of the assumptions used in ophiolite reconstructions. Alternative reconstructions may provide new ideas about spreading processes.

  1. Accretion tectonics and crustal structure in Alaska

    USGS Publications Warehouse

    Coney, P.J.; Jones, D.L.

    1985-01-01

    The entire width of the North American Cordillera in Alaska is made up of "suspect terranes". Pre-Late Cretaceous paleogeography is poorly constrained and the ultimate origins of the many fragments which make up the state are unclear. The Prince William and Chugach terranes accreted since Late Cretaceous time and represent the collapse of much of the northeast Pacific Ocean swept into what today is southern Alaska. Greater Wrangellia, a composite terrane now dispersed into fragments scattered from Idaho to southern Alaska, apparently accreted into Alaska in Late Cretaceous time crushing an enormous deep-marine flysch basin on its inboard side. Most of interior eastern Alaska is the Yukon Tanana terrane, a very large entirely fault-bounded metamorphic-plutonic assemblage covering thousands of square kilometers in Canada as well as Alaska. The original stratigraphy and relationship to North America of the Yukon-Tanana terrane are both obscure. A collapsed Mesozoic flysch basin, similar to the one inboard of Wrangellia, lies along the northern margin. Much of Arctic Alaska was apparently a vast expanse of upper Paleozoic to Early Mesozoic deep marine sediments and mafic volcanic and plutonic rocks now scattered widely as large telescoped sheets and Klippen thrust over the Ruby geanticline and the Brooks Range, and probably underlying the Yukon-Koyukuk basin and the Yukon flats. The Brooks Range itself is a stack of north vergent nappes, the telescoping of which began in Early Cretaceous time. Despite compelling evidence for thousands of kilometers of relative displacement between the accreted terranes, and large amounts of telescoping, translation, and rotation since accretion, the resulting new continental crust added to North America in Alaska carries few obvious signatures that allow application of currently popular simple plate tectonic models. Intraplate telescoping and strike-slip translations, delamination at mid-crustal levels, and large-scale lithospheric

  2. Bedrock geology and tectonic evolution of the Wrangellia, Peninsular, and Chugach terranes along the Trans-Alaska Crustal Transect in the Chugach Mountains and southern Copper River Basin, Alaska

    USGS Publications Warehouse

    Plafker, G.; Nokleberg, W.J.; Lull, J.S.

    1989-01-01

    The Trans-Alaskan Crustal Transect in the southern Copper River Basin and Chugach Mountains traverses the margins of the Peninsular and Wrangellia terranes, and the adjacent accretionary oceanic units of the Chugach terrane to the south. The southern Wrangellia terrane margin consists of a polymetamorphosed magmatic arc complex at least in part of Pennsylvanian age (Strelna Metamorphics and metagranodiorite) and tonalitic metaplutonic rocks of the Late Jurassic Chitina magmatic arc. The southern Peninsular terrane margin is underlain by rocks of the Late Triassic(?) and Early Jurassic Talkeetna magmatic arc (Talkeetna Formation and Border Ranges ultramafic-mafic assemblage) on Permian or older basement rocks. The Peninsular and Wrangellia terranes are parts of a dominantly oceanic superterrane (composite Terrane II) that was amalgamated by Late Triassic time and was accreted to terranes of continental affinity north of the Denali fault system in the mid- to Late Cretaceous. The Chugach terrane in the transect area consists of three successively accreted units. A regional thermal event that culminated in early middle Eocene time (48-52 Ma) resulted in widespread greenschist facies metamorphism and plutonism. -from Authors

  3. Time-dependent, Compositionally Driven Convection in the Oceans of Accreting Neutron Stars

    NASA Astrophysics Data System (ADS)

    Medin, Zach; Cumming, Andrew

    2015-03-01

    We discuss the effect of convection driven by chemical separation at the ocean-crust boundary of accreting neutron stars. We extend the steady-state results of Medin & Cumming to transient accretors, by considering the time-dependent cases of heating during accretion outbursts and cooling during quiescence. During accretion outbursts, inward heat transport has only a small effect on the temperature profile in the outer layers until the ocean is strongly enriched in light elements, a process that takes hundreds of years to complete. During quiescence, however, inward heat transport rapidly cools the outer layers of the ocean while keeping the inner layers hot. We find that this leads to a sharp drop in surface emission at around a week followed by a gradual recovery as cooling becomes dominated by the crust. Such a dip should be observable in the light curves of these neutron star transients, if enough data is taken at a few days to a month after the end of accretion. If such a dip is definitively observed, it will provide strong constraints on the chemical composition of the ocean and outer crust.

  4. Exotic terranes of western California

    USGS Publications Warehouse

    McWilliams, M.O.; Howell, D.G.

    1982-01-01

    Numerous distinct geological terranes compose the North American Cordillera1; there may be as many as 50 terranes in California alone2. Critical to deciphering the history of Cordilleran tectonic assembly is an understanding of the displacement history of individual terranes. It is therefore important to know: (1) whether a terrane has undergone significant motion with respect to the stable craton (that is, whether it is allochthonous or exotic); (2) if so, when relative motion started and stopped; (3) from where an individual terrane originated; and (4) the nature of interterrane movements. We consider here the problem of determining whether the now-juxtaposed Salinian and Stanley Mountain terranes of California became amalgamated at or near their present position with respect to cratonic North America, or if they collided at a considerable distance from their present positions and were later accreted to North America as a composite package. The palaeomagnetic data that we present indicate that the latter was the case. ?? 1982 Nature Publishing Group.

  5. Cooling of the magma ocean due to accretional disruption of the surface insulating layer

    NASA Technical Reports Server (NTRS)

    Sasaki, Sho

    1992-01-01

    Planetary accretion has been considered as a process to heat planets. Some fraction of the kinetic energy of incoming planetesimals is trapped to heat the planetary interior (Kaula, 1979; Davies, 1984). Moreover, blanketing effect of a primary atmosphere (Hayashi et al., 1979; Sasaki, 1990) or a degassed atmosphere (Abe and Matsui, 1986; Zahnle et al., 1988) would raise the surface temperature of the Earth-size planets to be higher than the melting temperature. The primordial magma ocean was likely to be formed during accretion of terrestrial planets. In the magma ocean, if crystallized fractions were heavier than melt, they would sink. But if solidified materials were lighter than the melt (like anorthosite of the lunar early crust) they would float to form a solid shell surrounding the planet. (In an icy satellite, solidified water ice should easily float on liquid water because of its small density.) The surface solid lid would prevent efficient convective heat transfer and slow the interior cooling. Consider that the accretion of planetesimals still continues in this cooling stage. Shock disruption at planetesimal impact events may destroy the solid insulating layer. Even if the layer survives impacts, the surface layer is finally overturned by Rayleigh-Taylor instability, since accreting materials containing metals are heavier than the surface solidified lid of silicates.

  6. The open scars of Latin America: The Bolivian Orocline as a basament-related hinge, and the influence of accreted terranes on the paleomagnetic rotational patterns of the Chilean forearc.

    NASA Astrophysics Data System (ADS)

    Peña Gomez, M. A.; Arriagada, C.; Gómez, I.; Roperch, P. J.

    2015-12-01

    We made a paleomagnetic study in two separate zones of the Chilean forearc, between 18-22ºS and between 28-32ºS, sampling igneous and sedimentary rocks with ages ranging from Triassic to Miocene. More than 500 samples showed a stable magnetization, with hematite and magnetite being the principal carriers of magnetism. The rotation pattern obtained, added to previously published paleomagnetic data, show a continuous database for the Chilean forearc, between 19 and 35ºS, allowing us to separate distinct patterns in 4 major rotational zones: (1) Between 18-19.5ºS there is a strong anticlockwise rotational pattern, in agreement with the data known in southern Peru. (2) Between 19.5-22.5ºS, there is little to no rotation, with the southern limit being related to a major structural feature: The Antofagasta-Calama Lineament. (3) Between 22.5-29ºS there is a strong clockwise rotation pattern of nearly 30º. (4) Between 29-32ºS there is again a little to non-rotational pattern, in the area of the Pampean flat-slab. Overlapping these zones and the recognized accreted terranes boundaries shows a clear spatial relation between these and the limits of the rotated zones. We propose that the limits of this rotational domains can be linked to basament hinge-like weakness zones that helped to create the margin curvatures observed today. Under this model, the bolivian orocline would be the result of the opening of a hinge, helped by other geodynamics features like sea mountains and ridges, at the limit between the old accreted paleozoic terranes of Antofalla and Arequipa.

  7. Tectonics and terranes of the Southeastern Caribbean

    SciTech Connect

    Speed, R.C. )

    1993-02-01

    The southeastern Caribbean plate (Ca) is comprised of the following terranes: Tobago, Grenada Basin, St. Vincent, Araya-Margarita, and Paria-Trinidad-Barbardos (PTB). All are alient relative to South America (SA) east of Caracas except for PTB, which is of continental provenance and parautochthonous and lies within the principal movement zone of the Ca-Sa plate boundary. The Tobago terrane extends between the eastern Venezuelan coastline and the Grenada Basin. On its south, the Tobago terrane overrode PTB and the South American passive margin during Neogene oblique collision. The Mesozoic tectonostratigraphy of the Tobago terrane is not unlike that of the Colombian Basin, suggesting the Tobago may belong to the Pacific-derived Caribbean plate. The Grenada Basin terrane consists of Eocene and older oceanic crust that now occupies the southern Lesser Antilles arc platform and the southern Grenada Basin. Such crust abducted southward below the Tobago terrane in mid-Cenozoic time, probably taking up boundary-normal shortening during oblique collision of the Ca and Sa plates. The oceanic crust of the GB terrane arose by backarc spreading of unknown orientation. The St. Vincent terrane extends north in the Antilles from St. Vincent; it is defined by thick crust, perhaps an old arc basement. The Araya-Margarita terrane is a probable subduction complex of Mesozoic age of metamorphism that has been transported far eastward from an unknown site with the Ca plate.

  8. Tectono-stratigraphic terrane map of Alaska

    SciTech Connect

    Nokleberg, W.J.; Brew, D.A.; Grantz, A.; Plafker, G.; Moore, T.E.; Patton, W.W. Jr. ); Mollstalcup, E.J. ); Miller, T.P. )

    1993-04-01

    A new terrane map compelled at a scale of 2.5 million is a comprehensive portrayal of the major tectono-stratigraphic terranes, pre-accretionary plutonic rocks, faults or sutures that bound terranes, and younger overlap sedimentary , volcanic, and plutonic assemblages of Alaska. Terranes are divided by tectonic affinity into cratonal, passive continental margin, metamorphosed continental margin, continental margin arc, island arc, oceanic crust, sea mount, ophiolite, accretionary wedge, subduction zone, turbidite basin, and metamorphic environments. Overlap assemblages consist of sequences of sedimentary, volcanic, and plutonic rocks that link or weld together adjacent terranes after emplacement, and provide important constraints on the timing of tectonic juxtaposition. Groups of terranes and overlap assemblages, with similar tectonic environments and geologic histories, can be correlated within Alaska and into the adjacent Canadian Cordillera. These groups include: (1) highly deformed and metamorphosed continental margin terranes (Seward, Coldfoot, Ruby, Yukon-Tanana, Kootenay) that are interpreted either as displaced fragments of the North American or other continental margins; (2) ophiolite terranes (Angayucham, Tozitna, Inoko, Seventymile, Slide Mountain) that are interpreted as remnants of one or more major, long-lived, Paleozoic and early Mesozoic oceanic basins; (3) Jurassic and Early Cretaceous island arc terranes (Koyukuk, Togiak, Nyac) that are interpreted as remnants of a discontinuous, short-lived, Mesoxoic island arc system; and (4) the Late Jurassic and Early Cretaceous Kahiltna and Gravina-Nutzotin overlap assemblages that are interpreted as parts of a major arc and flysch sequence.

  9. Continental accretion: contrasting Mesozoic and Early Proterozoic tectonic regimes in North America

    NASA Astrophysics Data System (ADS)

    Condie, Kent C.; Chomiak, Beverly

    1996-11-01

    Mesozoic terranes, tectonic setting may differ, whereas in most Early Proterozoic terranes tectonic setting appears to have remained the same. Unlike the Mesozoic terranes, which were fragmented during collision and displaced along transcurrent faults, Early Proterozoic terranes show no evidence of major transcurrent offset. Using accretion age windows of 120 My for the Mesozoic and 115 My for the Early Proterozoic, we obtain total crustal accretion rates of 1.33 km 3/y and 1.73 km 3/y, respectively, for 6000 km of strike length in each case. Early Proterozoic crustal accretion in southwestern North America was strikingly different from that in northwestern North America during the Mesozoic. Mesozoic accretion involves transformation of mafic oceanic terranes into continental crust. In contrast, most of the juvenile Early Proterozoic crust appears to have evolved directly into mature continental crust without passing through an 'oceanic' stage. This probably occurred in a continental margin arc system. Our results also indicate that oceanic terranes cannot evolve into continental crust as closed chemical systems. Although some Mesozoic oceanic terranes began to evolve into continental crust before accretion to North America, most of the transition occurred during and shortly after accretion. This may have been accomplished by incompatible element enrichment associated with subduction-related processes beneath collisionally thickened crust. The accreted Mesozoic crust has not yet evolved into mature continental crust and whether it will depends on the duration of subduction processes along the continental margin in the future.

  10. The Khida terrane - Geology of Paleoproterozoic rocks in the Muhayil area, eastern Arabian Shield, Saudi Arabia

    USGS Publications Warehouse

    Stoeser, D.B.; Whitehouse, M.J.; Stacey, J.S.

    2001-01-01

    The bulk of the Arabian Shield of Saudi Arabia is underlain by Neoproterozoic terranes of oceanic affinity that were accreted during Pan-African time (about 680- 640Ma). Geologicalmappingandisotopicinvestigations during the 1980’s,however, provided the first evidence for Paleoproterozoic continental crust within the east- central part of the shield in Saudi Arabia. These studies delineated an older basement domain, herein referred to as the Khida terrane (Fig. l), which is defined as that part of the southern Afif composite terrane underlain by Paleoproterozoicto Archean continental crust (Stoeser and Stacey, 1988). The isotopic and geochronologic work to support our current studies within the Khida terrane are discussed in a companion abstract (Whitehouse et al., this volume). The regional geology and geochronology of the region has been summarized in detail by Johnson (1996). The current study is based on the continued use of samples previously collected in the Khida area by the authors and others as well as new field work conducted by us in 1999. This work further defines the occurrence of late Paleoproterozoic rocks at Jabal Muhayil, which is located at the eastern margin of the exposed terrane (Fig. 1). Our isotopic work is at an early stage and this abstract partly relates geologic problems that remain to be resolved. 

  11. Tracing source terranes using U-Pb-Hf isotopic analysis of detrital zircons: provenance of the Orhanlar Unit of the Palaeotethyan Karakaya subduction-accretion complex, NW Turkey

    NASA Astrophysics Data System (ADS)

    Ustaömer, Timur; Ayda Ustaömer, Petek; Robertson, Alastair; Gerdes, Axel

    2016-04-01

    Sandstones of the Late Palaeozoic-Early Mesozoic Karakaya Complex are interpreted to have accumulated along an active continental margin related to northward subduction of Palaeotethys. The age of deposition and provenance of the sandstones are currently being determined using radiometric dating of detrital zircons, coupled with dating of potential source terranes. Our previous work shows that the U-Pb-Hf isotopic characteristics of the sandstones of all but one of the main tectonostratigraphic units of the Karakaya Complex are compatible with a provenance that was dominated by Triassic and Permo-Carboniferous magmatic arc-type rocks, together with a minor contribution from Lower to Mid-Devonian igneous rocks (Ustaömer et al. 2015). However, one of the tectono-stratigraphic units, the Orhanlar Unit, which occurs in a structurally high position, differs in sedimentary facies and composition from the other units of the Karakaya Complex. Here, we report new isotopic age data for the sandstones of the Orhanlar Unit and also from an extensive, associated tectonic slice of continental metamorphic rocks (part of the regional Sakarya Terrane). Our main aim is to assess the provenance of the Orhanlar Unit sandstones in relation to the tectonic development of the Karakaya Complex as a whole. The Orhanlar Unit is composed of shales, sandstone turbidites and debris-flow deposits, which include blocks of Devonian radiolarian chert and Carboniferous and Permian neritic limestones. The sandstones are dominated by rock fragments, principally volcanic and plutonic rocks of basic-to-intermediate composition, metamorphic rocks and chert, together with common quartz, feldspar and mica. This modal composition contrasts significantly with the dominantly arkosic composition of the other Karakaya Complex sandstones. The detrital zircons were dated by the U-Pb method, coupled with determination of Lu-Hf isotopic compositions using a laser ablation microprobe attached to a multicollector

  12. Tracing source terranes using U-Pb-Hf isotopic analysis of detrital zircons: provenance of the Orhanlar Unit of the Palaeotethyan Karakaya subduction-accretion complex, NW Turkey

    NASA Astrophysics Data System (ADS)

    Ustaömer, Timur; Ayda Ustaömer, Petek; Robertson, Alastair; Gerdes, Axel

    2016-04-01

    Sandstones of the Late Palaeozoic-Early Mesozoic Karakaya Complex are interpreted to have accumulated along an active continental margin related to northward subduction of Palaeotethys. The age of deposition and provenance of the sandstones are currently being determined using radiometric dating of detrital zircons, coupled with dating of potential source terranes. Our previous work shows that the U-Pb-Hf isotopic characteristics of the sandstones of all but one of the main tectonostratigraphic units of the Karakaya Complex are compatible with a provenance that was dominated by Triassic and Permo-Carboniferous magmatic arc-type rocks, together with a minor contribution from Lower to Mid-Devonian igneous rocks (Ustaömer et al. 2015). However, one of the tectono-stratigraphic units, the Orhanlar Unit, which occurs in a structurally high position, differs in sedimentary facies and composition from the other units of the Karakaya Complex. Here, we report new isotopic age data for the sandstones of the Orhanlar Unit and also from an extensive, associated tectonic slice of continental metamorphic rocks (part of the regional Sakarya Terrane). Our main aim is to assess the provenance of the Orhanlar Unit sandstones in relation to the tectonic development of the Karakaya Complex as a whole. The Orhanlar Unit is composed of shales, sandstone turbidites and debris-flow deposits, which include blocks of Devonian radiolarian chert and Carboniferous and Permian neritic limestones. The sandstones are dominated by rock fragments, principally volcanic and plutonic rocks of basic-to-intermediate composition, metamorphic rocks and chert, together with common quartz, feldspar and mica. This modal composition contrasts significantly with the dominantly arkosic composition of the other Karakaya Complex sandstones. The detrital zircons were dated by the U-Pb method, coupled with determination of Lu-Hf isotopic compositions using a laser ablation microprobe attached to a multicollector

  13. Assembly of the Lhasa and Qiangtang terranes in central Tibet by divergent double subduction

    NASA Astrophysics Data System (ADS)

    Zhu, Di-Cheng; Li, Shi-Min; Cawood, Peter A.; Wang, Qing; Zhao, Zhi-Dan; Liu, Sheng-Ao; Wang, Li-Quan

    2016-02-01

    Integration of lithostratigraphic, magmatic, and metamorphic data from the Lhasa-Qiangtang collision zone in central Tibet (including the Bangong suture zone and adjacent regions of the Lhasa and Qiangtang terranes) indicates assembly through divergent double sided subduction. This collision zone is characterized by the absence of Early Cretaceous high-grade metamorphic rocks and the presence of extensive magmatism with enhanced mantle contributions at ca. 120-110 Ma. Two Jurassic-Cretaceous magmatic arcs are identified from the Caima-Duobuza-Rongma-Kangqiong-Amdo magmatic belt in the western Qiangtang Terrane and from the Along Tso-Yanhu-Daguo-Baingoin-Daru Tso magmatic belt in the northern Lhasa Terrane. These two magmatic arcs reflect northward and southward subduction of the Bangong Ocean lithosphere, respectively. Available multidisciplinary data reconcile that the Bangong Ocean may have closed during the Late Jurassic-Early Cretaceous (most likely ca. 140-130 Ma) through arc-arc "soft" collision rather than continent-continent "hard" collision. Subduction zone retreat associated with convergence beneath the Lhasa Terrane may have driven its rifting and separation from the northern margin of Gondwana leading to its accretion within Asia.

  14. An 80 million year oceanic redox history from Permian to Jurassic pelagic sediments of the Mino-Tamba terrane, SW Japan, and the origin of four mass extinctions

    NASA Astrophysics Data System (ADS)

    Wignall, Paul B.; Bond, David P. G.; Kuwahara, Kiyoko; Kakuwa, Yoshitaka; Newton, Robert J.; Poulton, Simon W.

    2010-03-01

    Fabric and pyrite framboid size analysis of Permian to Jurassic samples from the Mino-Tamba terrane of Japan provide an 80 myr redox history from the Panthalassa Ocean. Fully oxygenated conditions dominated but were punctuated by three phases of anoxia (euxinia) during the Permo-Triassic boundary, the late Early Triassic (Spathian Stage) and the late Early Jurassic (Toarcian Stage). A Permo-Triassic superanoxic event of 10 myr duration is thus resolved into a complex redox history of dysoxic-anoxic background conditions punctuated by short-lived euxinic pulses. Both the end-Permian and Toarcian anoxic episodes began abruptly with the development of siliceous claystones, characterized by a framboid-dominated pyrite fraction and no organic C enrichment, interpreted to record euxinic conditions. These facies are overlain by organic-rich shales with slightly larger framboids, more crystalline pyrite and bioturbation indicating accumulation beneath a better-ventilated, higher productivity water column. Thus, an initial phase of ocean-wide stagnation was followed by improved circulation attributed to the onset of warm saline bottom water circulation. Five radiolarian crises during our study interval show a complex relationship with redox conditions. The latest Permian mass extinction coincided with the onset of ocean euxinia but two other crises (Middle Permian and end Triassic), occurred during fully oxygenated intervals. Radiolarian radiation events are also not consistently linked with redox conditions: diversification in the early Late Permian coincides with the onset of weakly dysoxic conditions, late Early Triassic and late Early Jurassic radiations coincide with euxinic events, and the basal Jurassic radiation occurred during a fully oxic phase.

  15. Nd isotopic anatomy of a pebble conglomerate from the Murihiku terrane of New Zealand: Record of a varied provenance along the Mesozoic Gondwanaland margin

    NASA Astrophysics Data System (ADS)

    Frost, Carol D.; Mortimer, Nick; Goles, Gordon G.

    2005-12-01

    The Murihiku terrane is a volcano-sedimentary terrane of Late Permian to Early Cretaceous age that forms part of a collage of accreted terranes in the Eastern Province of New Zealand. These terranes record a history of deposition along the Gondwanaland margin, and include terranes of oceanic character (e.g. Brook Street terrane) and terranes dominated by continental detritus (e.g. Torlesse terrane). The Murihiku terrane is of particular interest because it is relatively large, one of the least structurally deformed terranes, and preserves a long (˜120 m.y) record of sedimentation. Previous studies have suggested that the Murihiku terrane preserves a record of change in provenance from predominantly mafic, juvenile sources to dominantly felsic volcanic sources in Middle to Late Triassic time interpreted as containing contributions from Precambrian continental crust, and modest amounts of detritus supplied directly from this old crust [Frost, C.D., Coombs, D.S., 1989. Nd isotope character of New Zealand sediments: Implications for terrane concepts and crustal evolution. American Journal of Science 289, 744-770.; Roser, B.P., Coombs, D.S., Korsch, R.J., Campbell, J.D., 2002. Whole-rock geochemical variations and evolution of the arc-derived Murihiku Terrane, New Zealand. Geological Magazine 139, 665-685.]. We present Nd isotopic data from a Late Triassic (Rhetian; Otapirian local stage; 200-206 Ma) roundstone cobble conglomerate from near Gore, South Island, New Zealand. The sandstone matrix of this conglomerate has an initial ɛNd of + 3.0 and Sm/Nd of 0.219, values that are intermediate between the more radiogenic Nd isotopic and higher Sm / Nd ratios of older Murihiku siltstones and less radiogenic, lower Sm / Nd, younger siltstones. Clasts within this matrix are mainly volcanic: we analyzed basaltic-andesite, andesite and dacite pebbles along with a volcanogenic sandstone pebble and hypabyssal quartz diorite clast. These pebbles have initial ɛNd between + 3

  16. A simple tectonic model for crustal accretion in the Slave Province: A 2.7-2.5 Ga granite greenstone terrane

    NASA Technical Reports Server (NTRS)

    Hoffman, P. F.

    1986-01-01

    A prograding (direction unspecified) trench-arc system is favored as a simple yet comprehensive model for crustal generation in a 250,000 sq km granite-greenstone terrain. The model accounts for the evolutionary sequence of volcanism, sedimentation, deformation, metamorphism and plutonism, observed througout the Slave province. Both unconformable (trench inner slope) and subconformable (trench outer slope) relations between the volcanics and overlying turbidities; and the existence of relatively minor amounts of pre-greenstone basement (microcontinents) and syn-greenstone plutons (accreted arc roots) are explained. Predictions include: a varaiable gap between greenstone volcanism and trench turbidite sedimentation (accompanied by minor volcanism) and systematic regional variations in age span of volcanism and plutonism. Implications of the model will be illustrated with reference to a 1:1 million scale geological map of the Slave Province (and its bounding 1.0 Ga orogens).

  17. Geochemical and isotopic perspectives on the origin and evolution of the Siletzia Terrane.

    NASA Astrophysics Data System (ADS)

    Phillips, B. A.; Weis, D.; Mullen, E.; Kerr, A. C.

    2015-12-01

    The Siletzia terrane, located in the Cascadia forearc region of Oregon, Washington and Vancouver Island, consists of a series of accreted basaltic pillow lavas, massive flows and intrusive sheets. It represents a late Paleocene-Eocene oceanic large igneous province (LIP), previously proposed to represent an accreted oceanic plateau, hotspot island chain, backarc basin, island arc, or a sequence of slab window volcanics formed by ridge subduction. A province-wide geochemical reassessment of the terrane, including new high precision Sr-Pb-Nd-Hf isotope data on basaltic samples, has been used to assess the validity of the proposed tectonomagmatic models for Siletzia. The trace element data show REE patterns that are flat to LREE enriched with an absence of any arc signatures. These features are comparable to other oceanic plateaus such as the Ontong Java and the Caribbean and so therefore support a mantle plume origin. Initial isotope ratios range from 206Pb/204Pb = 18.869 - 19.673, 207Pb/204Pb = 15.527 - 15.609, 208Pb/204Pb = 38.551 - 39.220, ɛHf = +9.0 - 14.8, ɛNd = +5.0 - 8.0 and 87Sr/86Sr = 0.70304 - 0.70397. The isotope signatures become more varied southward across the terrane and reveal two trends: i) HIMU-DMM and ii) another extending from DMM towards the Imnaha component, thought to represent the mantle plume source of the Columbia River Basalts and Yellowstone 1,2. The data may support the previously proposed idea that the volcanism of the Siletzia terrane represents initial melting of the mantle plume head of the Yellowstone hotspot 3,4,5. Other evidence indicating a LIP origin includes the relatively rapid eruption/intrusion of an estimated magma volume of 2.6 x 106 km3 6 between ~56-49 Ma 5, which, in conjunction with our new elemental and isotopic data, indicates that the Siletzia terrane most likely represents an accreted oceanic plateau. 1. Wolff et al., (2008) Nature Geoscience 1, 177-180. 2. Jean et al., (2014) EPSL 389, 119-131 3. Duncan (1982

  18. Numerical modeling of the subduction initiation after accretion of oceanic island.

    NASA Astrophysics Data System (ADS)

    Simakin, A.

    2012-04-01

    Accretion of the large terrains leads to the temporal blockage of subduction and accumulation of the oceanic slab material. New subduction front started in the thickened contact zone. This process is modeled numerically in 2D. We apply constant velocity condition at the inlet vertical boundary. Another vertical boundary is treated as a free slide one, Winkler boundary condition is applied to the lower boundary. Sticky air used to represent stress free upper boundary. Scenario of the new front initiation depends on the assumed rheology of oceanic slab and docked island. At the application of the purely viscous rheology of all components of the system: island viscosity 1023 Pas, upper mantle viscosity 3·1019 Pas and slab viscosity in the range 6·1020-6·1022 Pas, we find a variety of dynamic styles. At low slab viscosity in the time scale of several millions years plate is thickened and experienced RTI instability. New subduction front is started after plate break up near the island edge. At the more realistic η=6·1022 Pas oceanic slab is folded before plunging into the mantle. In the intermediate range thickening of the oceanic slab takes place with low angle subduction followed by accelerated submergence of the widen slab tip. Too large time of the transient process and too large scale of oceanic slab accumulation contradict to the observations. Visco-plastic rheology of the crustal rocks brings model closer to the real world. At the early stage of deformation conjugate "viscous faults" form in the oceanic slab in respond to the shortening. Later on sliding along these faults doubles oceanic plate thickness at the contact with docked island. Permanent fault (with dip away from island) was created to accommodate bending of oceanic plate. Thickened plate tip starts to descend with low angle of ca 35o. The most important observation is breakage of island edge that is carried downward with subducted oceanic plate. We compare our results with data on the current

  19. LA ICP MS and Ion Probe U-Pb dating of igneous and metasedimentary units in the NE Pontides, NE Turkey: evidence of Peri-Gondwanan terrane accretion, Late Palaeozoic magmatism/metamorphism and Early Mesozoic extension along the S Eurasian margin

    NASA Astrophysics Data System (ADS)

    Ustaömer, Timur; Robertson, Alastair H. F.; Gerdes, Axel; Ayda Ustaömer, P.

    2010-05-01

    experienced lead loss, with a lower intercept at 326 Ma. One magmatic zircon from this intrusion gave an age of 358 Ma (early Carboniferous), interpreted as the crystallisation age. Metamorphic rims of these zircons cluster around 330 Ma, viewed as the time of peak Variscan metamorphism. We interpret the E Pontide region (e.g. Artvin area) as part of an active S-Eurasian continental margin during Late Palaeozoic. Accretion/collision of Peri-Gondwanan terrane(s) was likely responsible for Variscan deformation/metamorphism. Newly accreted Peri-Gondwanan crust was intruded by granitic rocks during early Carboniferous, possibly in response to delamination/slab-break off processes. Following exhumation, the Eurasian margin remained relatively inactive and erosional during Late Carboniferous-Triassic. Related to regional northward subduction of Palaeotethys, the S-Eurasian margin underwent tectonic extension and deep-marine basin formation during Early Jurassic. The dyke swarm and Toarcian felsic plutons were emplaced into extended crust behind a continental margin magmatic arc. Short-lived Mid-Jurassic compression may reflect collision of an oceanic edifice (seamount/continental fragment) with the subduction trench. Extension resumed during Late Jurassic associated with Oxfordian magmatism. A S-facing subsiding passive margin existed during Late Jurassic-Early Cretaceous, followed by northward subduction and arc magmatism (E Pontide Arc). SSZ-type ophiolites were regionally obducted during latest Cretaceous, followed by Eocene telescoping of the Eurasian margin during final closure of Izmir-Ankara-Erzincan ocean.

  20. The Pelagonian terrane in Greece: a piece of peri-Gondwanan mosaic of the Eastern Mediterranean and a new piece of information about the geological evolution of Avalonia

    NASA Astrophysics Data System (ADS)

    Zlatkin, Olga; Avigad, Dov; Gerdes, Axel

    2016-04-01

    The North-East Mediterranean region is a crustal mosaic comprised of proximal (Cadomian) and exotic (Avalonian) peri-Gondwanan terranes that were accreted to the European margin and repeatedly reshaped during several orogenic events, including Caledonian, Variscan and Alpine. The Pelagonian terrane in Greece is a peri-Gondwanan terrane of the Avalonian affinity: the properties of its >700 Ma-aged "Proto-Pelagonian" basement are attesting its peri-Amazonian origin. Our new survey reveals the Proto-Pelagonian rocks in most of the basement outcrops. Additionally, 600 Ma-aged orthogneisses were discovered, which is typical for the Avalonian magmatic arc. Here we also report the data obtained on the overstep Late Ediacaran to Early Mesozoic Pelagonian rock section, using U-Pb-Hf isotope geochemistry, and supported by structural and lithological observations. The Late Ediacaran Pelagonian metasedimentary sequence yields mainly magmatic ages between 750-560 Ma with Hf-TDM ages of 1.0-1.4 Ga, indicating the detrital transport exclusively from the Avalonian microcontinent that was insular at that time. These data are well correlated with the correspondent Avalonian sequences from Atlantic Canada and the British Midlands. It is generally suggested that Avalonian terranes were detached from Gondwana by the Rheic Ocean opening in the Early Ordovician and accreted to the European margin in the course of the Caledonian orogeny, while the Cadomian terranes have detached and accreted later, during the Variscan convergence. Despite this, no Caledonian magmatism is manifested within Pelagonian basement. Moreover, the post-Caledonian zircon data displays no Caledonian zircon ages, with a gap between 520 and 350 Ma. Voluminous intrusion of late-Variscan (ca. 300 Ma) Pelagonian granites indicates the upper-plate position for the Pelagonian terrane at that time. It suggests that the Pelagonian terrane wasn't involved in the Caledonian orogeny, but had remained adjacent to Gondwana or

  1. Tectonomagmatic setting and provenance of the Santa Marta Schists, northern Colombia: Insights on the growth and approach of Cretaceous Caribbean oceanic terranes to the South American continent

    NASA Astrophysics Data System (ADS)

    Cardona, A.; Valencia, V.; Bustamante, C.; García-Casco, A.; Ojeda, G.; Ruiz, J.; Saldarriaga, M.; Weber, M.

    2010-10-01

    Metamorphosed volcano-sedimentary rocks accreted to the northern South American continental margin are major vestiges of the Caribbean oceanic plate evolution and its interactions with the continent. Selected whole rock geochemistry, Nd-Sr isotopes and detrital zircon geochronology were obtained in metabasic and metasedimentary rocks from the Santa Marta and San Lorenzo Schists in northernmost Colombia. Trace element patterns are characterized by primitive island arc and MORB signatures. Similarly initial 87Sr/ 86Sr-ɛ Nd isotopic relations correlate with oceanic arcs and MORB reservoirs, suggesting that the protoliths were formed within a back-arc setting or at the transition between the inta-oceanic arc and the Caribbean oceanic crust. Trace element trends from associated metasedimentary rocks show that the provenance was controlled by a volcanic arc and a sialic continental domain, whereas detrital U/Pb zircons from the Santa Marta Schists and adjacent southeastern metamorphic units show Late Cretaceous and older Mesozoic, Late Paleozoic and Mesoproterozoic sources. Comparison with continental inland basins suggests that this arc-basin is allocthonous to its current position, and was still active by ca. 82 Ma. The geological features are comparable to other arc remnants found in northeastern Colombia and the Netherland Antilles. The geochemical and U/Pb detrital signatures from the metasedimentary rocks suggest that this tectonic domain was already in proximity to the continental margin, in a configuration similar to the modern Antilles or the Kermadec arc in the Pacific. The older continental detritus were derived from the ongoing Andean uplift feeding the intra-oceanic tectonic environment. Cross-cutting relations with granitoids and metamorphic ages suggest that metamorphism was completed by ca. 65 Ma.

  2. Continental growth through accreted oceanic arc: Zircon Hf-O isotope evidence for granitoids from the Qinling orogen

    NASA Astrophysics Data System (ADS)

    Wang, Hao; Wu, Yuan-Bao; Gao, Shan; Qin, Zheng-Wei; Hu, Zhao-Chu; Zheng, Jian-Ping; Yang, Sai-Hong

    2016-06-01

    The continental crust is commonly viewed as being formed in subduction zones, but there is no consensus on the relative roles of oceanic or continental arcs in the formation of the continental crust. The main difficulties of the oceanic arc model are how the oceanic arcs can be preserved from being subducted, how we can trace the former oceanic arcs through their high-Si products, and how the oceanic arcs can generate the high-Si, K-rich granitoid composition similar to the upper continental crust. The eastern Qinling orogen provides an optimal place to address these issues as it preserves the well-exposed Erlangping oceanic arc with large amounts of granitoids. In this study, we present an integrated investigation of zircon U-Pb ages and Hf-O isotopes for four representative granitoid plutons in the Erlangping unit. In situ zircon SIMS U-Pb dating indicated that the Zhangjiadazhuang, Xizhuanghe, and Taoyuan plutons formed at 472 ± 7, 458 ± 6 and 443 ± 5 Ma, respectively, all of which postdated the deep subduction of the Qinling microcontinent under the Erlangping oceanic arc. The Zhangjiadazhuang, Xizhuanghe, and Taoyuan plutons are sodic granitoid and have highly positive εHf(t) (+7.6 to +12.9) and relatively low δ18O (4.7-5.0‰) values, which were suggested to result from prompt remelting of hydrothermally altered lower oceanic crust of the accreted Erlangping oceanic arc. The zircon grains from the Manziying monzogranitic pluton show similar Hf-O isotopic compositions to those of the Xizhuanghe pluton, and thus the Manziying monzogranitic pluton was likely derived from the dehydration melting of previous tonalites as exemplified by the Xizhuanghe pluton. The deep subduction of Qinling microcontinent resulted in the accretion of the Erlangping oceanic arc, which implies that arc-continent collision provides an effective way for preventing oceanic arcs from being completely subducted. The highly positive εHf(t) and relatively low δ18O values of zircon

  3. Pebbles from Barkerville and Slide Mountain terranes in a Quesnel terrane conglomerate: Evidence for pre-Jurassic deformation of the Barkerville and Slide Mountain terranes

    NASA Astrophysics Data System (ADS)

    McMullin, David W. A.; Greenwood, Hugh J.; Ross, John V.

    1990-10-01

    Rocks of the Quesnel Lake area belong to three terranes. These are, from east to west, the Barkerville terrane (a continental prism sequence), the Slide Mountain terrane (an ocean-floor sequence), and the Quesnel terrane (an island-arc-marginal basin sequence). The major deformation of these rocks occurred during the Jurassic. There has been renewed discussion recently as to whether the Barkerville terrane was deformed prior to the Jurassic. Two conglomerate localities within the Quesnel terrane contain clasts we identify as being derived from deformed rocks of the Barkerville and Slide Mountain terranes. These include gneiss, orthoquartzite, graphitic phyllite, and grit from the Barkerville, and serpentine-talc and chromite fragments from the Slide Mountain terrane. Some clasts inferred to be from the Barkerville terrane show two predeposition foliations, implying two phases of deformation prior to the deposition of the conglomerate (Middle Jurassic at the latest). One of these events may be deformation associated with the intrusion of Devonian granitoid bodies. Deformation must also have accompanied the emplacement of the Slide Mountain terrane some time between its deposition (Mississippian-Permian) and its erosion (Triassic-Jurassic).

  4. Jurassic plume-origin ophiolites in Japan: accreted fragments of oceanic plateaus

    NASA Astrophysics Data System (ADS)

    Ichiyama, Yuji; Ishiwatari, Akira; Kimura, Jun-Ichi; Senda, Ryoko; Miyamoto, Tsuyoshi

    2014-07-01

    The Mikabu and Sorachi-Yezo belts comprise Jurassic ophiolitic complexes in Japan, where abundant basaltic to picritic rocks occur as lavas and hyaloclastite blocks. In the studied northern Hamamatsu and Dodaira areas of the Mikabu belt, these rocks are divided into two geochemical types, namely depleted (D-) and enriched (E-) types. In addition, highly enriched (HE-) type has been reported from other areas in literature. The D-type picrites contain highly magnesian relic olivine phenocrysts up to Fo93.5, and their Fo-NiO trend indicates fractional crystallization from a high-MgO primary magma. The MgO content is calculated as high as 25 wt%, indicating mantle melting at unusually high potential temperature ( T p) up to 1,650 °C. The E-type rocks represent the enrichment in Fe and LREE and the depletion in Mg, Al and HREE relative to the D-type rocks. These chemical characteristics are in good accordance with those of melts from garnet pyroxenite melting. Volcanics in the Sorachi-Yezo belts can be divided into the same types as the Mikabu belt, and the D-type picrites with magnesian olivines also show lines of evidence for production from high T p mantle. Evidence for the high T p mantle and geochemical similarities with high-Mg picrites and komatiites from oceanic and continental large igneous provinces (LIPs) indicate that the Mikabu and Sorachi-Yezo belts are accreted oceanic LIPs that were formed from hot large mantle plumes in the Late Jurassic Pacific Ocean. The E- and D-type rocks were formed as magmas generated by garnet pyroxenite melting at an early stage of LIP magmatism and by depleted peridotite melting at the later stage, respectively. The Mikabu belt characteristically bears abundant ultramafic cumulates, which could have been formed by crystal accumulation from a primary magma generated from Fe-rich peridotite mantle source, and the HE-type magma were produced by low degrees partial melting of garnet pyroxenite source. They should have been formed

  5. Evidence for Late Eocene emplacement of the Malaita Terrane, Solomon Islands: Implications for an even larger Ontong Java Nui oceanic plateau

    NASA Astrophysics Data System (ADS)

    Musgrave, Robert J.

    2013-06-01

    Most tectonic models for the Solomon Islands Arc invoke a Miocene collision with the Ontong Java Plateau (OJP) to halt cessation of Pacific Plate subduction, initiate Australian Plate subduction, and emplace the Malaita Terrane, which shares the characteristic basement age and geochemistry of OJP. Existing paleomagnetic evidence, however, required the Malaita Terrane to have been fixed to the arc from at least the Late Eocene. New sampling has yielded a paleomagnetic pole from Aptian-Albian limestones and mudstones that falls between the apparent polar wander paths for the Australian Plate and OJP, confirming the extended period of residence of the Malaita Terrane on the arc. Arc-derived turbidities within Late Eocene through Miocene limestones on Malaita and Santa Isabel, and related clasts in broadly contemporary sandstones and conglomerates on Santa Isabel, also attest to early emplacement. Modeling the emplacement at 35 Ma satisfies both the paleomagnetic data and the sediment provenance. Continuing the reconstruction to 125 Ma leaves the Malaita Terrane far from OJP at the time of plateau formation. OJP is now understood to have formed as part of a larger Ontong Java Nui, also comprising the Hikurangi and Manihiki plateaus, separated by spreading during the Cretaceous. Restoring the separation of the known elements, and invoking an additional triple junction, unites the (now largely subducted) Malaita Terrane with the rest of Ontong Java Nui. Subduction of substantial areas of the Ontong Java Nui plateau, with little geological signal other than a reduction in arc volcanism, is a corollary.

  6. Gondwanan basement terranes of the Variscan-Appalachian orogen: Baltican, Saharan and West African hafnium isotopic fingerprints in Avalonia, Iberia and the Armorican Terranes

    NASA Astrophysics Data System (ADS)

    Henderson, Bonnie J.; Collins, William Joseph; Murphy, James Brendan; Gutierrez-Alonso, Gabriel; Hand, Martin

    2016-06-01

    Iberia, Avalonia and the "Armorican" terranes form key constituents of the Variscan-Appalachian orogen, but their Neoproterozoic origins along the northern Gondwanan margin continue to be strongly debated. Here, we present a new detrital zircon U-Pb-Hf dataset from Neoproterozoic-Silurian sedimentary sequences in NW Iberia and Avalonia, in conjunction with the comprehensive existing datasets from potential source cratons, to demonstrate that the provenance of each terrane is relatively simple and can be traced back to three major cratons. The enigmatic Tonian-Stenian detrital zircons in autochthonous Iberian rocks were derived from the Saharan metacraton in the latest Neoproterozoic-early Cambrian. Avalonia is commonly considered to have been derived from the Amazonian margin of Gondwana, but the hafnium isotopic characteristics of the detrital zircon grains in early Neoproterozoic rocks bear much stronger similarities to Baltica. The hafnium isotopic array also suggests the early Avalonian oceanic arc was built on a sliver of "Grenvillian-type crust" (~ 2.0-1.0 Ga) possibly of Baltican affinity at ~ 800 Ma, prior to accretion with a continental margin at ~ 640 Ma. The Upper Allochthon of Iberia is frequently linked to the West African Craton in the late Neoproterozoic-early Cambrian, however the hafnium isotopic array presented here does not support this connection; rather it is more similar to the hafnium array from Avalonia. The Armorican terranes have strong detrital zircon isotopic links to the West African Craton during the late Neoproterozoic-Cambrian.

  7. Variscan terrane boundaries in the Odenwald-Spessart basement, Mid-German Crystalline Zone: New evidence from ocean ridge, intraplate and arc-derived metabasaltic rocks

    NASA Astrophysics Data System (ADS)

    Will, T. M.; Lee, S.-H.; Schmädicke, E.; Frimmel, H. E.; Okrusch, M.

    2015-04-01

    different age must have occurred during the formation of the continental arc precursor rocks. Retrogressed eclogites from the eastern Odenwald are tholeiitic metabasalts with chondrite-normalised flat rare earth element patterns, intermediate to high TiO2 concentrations and very low Th/Nb ratios. The least retrogressed eclogites lack negative Nb-Ta anomalies. In addition, the rocks have depleted Nd isotope compositions (εNd360 Ma = 7.1-8.5), Nd model ages of 470-425 Ma, superchondritic 147Sm/144Nd ratios and 87Sr/86Sr initial ratios similar to bulk Earth. These features are consistent with the notion that the eclogite protoliths originated in a shallow, depleted mid-ocean ridge mantle and were emplaced in a mid-ocean ridge setting. During crustal emplacement, the composition of these rocks was variably modified by subduction-related fluids and/or assimilation of crustal material. The hitherto unknown differences in the composition and genesis of the various amphibolites in the southern and northern Spessart basement require revision of existing tectonostratigraphic models for the area. Instead of representing one contiguous unit, the various amphibolites are interpreted to belong to two different terranes that became juxtaposed in the Spessart-Odenwald basement. These terranes are separated by a major fault (Otzberg-Michelbach Fault Zone), along which rocks of inferred peri-Gondwana origin, now exposed in the western Odenwald and the northernmost Spessart were thrust onto probable Baltica/Avalonia-derived rocks that are exposed as a tectonic window in the eastern Odenwald and the central and southern Spessart. It is further suggested that the Otzberg-Michelbach Fault Zone is part of the lithospheric-scale Rheic Suture.

  8. Late Triassic Batang Group arc volcanic rocks in the northeastern margin of Qiangtang terrane, northern Tibet: partial melting of juvenile crust and implications for Paleo-Tethys ocean subduction

    NASA Astrophysics Data System (ADS)

    Zhao, Shao-Qing; Tan, Jun; Wei, Jun-Hao; Tian, Ning; Zhang, Dao-Han; Liang, Sheng-Nan; Chen, Jia-Jie

    2015-03-01

    The Batang Group (BTG) volcanic rocks in the Zhiduo area, with NW-trending outcrops along the northeastern margin of the Qiangtang terrane (northern Tibet), are mainly composed of volcaniclastic rocks, dacite and rhyolite. Major and trace element, Sr and Nd isotope, zircon U-Pb and Hf isotope data are presented for the BTG dacites. Laser ablation inductively coupled plasma mass spectrometry zircon U-Pb dating constrains the timing of volcanic eruption as Late Triassic (221 ± 1 Ma). Major and trace element geochemistry shows that the BTG volcanic rocks are classified as calc-alkaline series. All samples are enriched in large-ion lithophile elements and light rare earth elements with negative-slightly positive Eu anomalies (Eu/Eu* = 0.47-1.15), and depleted in high field strength elements and heavy rare earth elements. In addition, these rocks possess less radiogenic Sr [(87Sr/86Sr) i = 0.7047-0.7078], much radiogenic Nd (ɛNd( t) = -4.2 to -1.3) and Hf (ɛHf( t) = 4.0-6.6) isotopes, suggesting that they probably originated from partial melting of a crustal source containing a mantle-derived juvenile component. The inferred magma was assimilated by crustal materials during ascending and experienced significant fractional crystallization. By combining previously published and the new data, we propose that the BTG volcanic rocks were genetically related to southwestward subduction of the Ganzi-Litang ocean (a branch of Paleo-Tethys) in the northeastern margin of the Qiangtang terrane. Given the coeval arc-affinity magmatic rocks in the region, we envisage that the Ganzi-Litang ocean may extend from the Zhongdian arc through the Yidun terrane to the Zhiduo area, probably even further northwest to the Tuotuohe area.

  9. Tectonic evolution of the East Junggar terrane, CAOB

    NASA Astrophysics Data System (ADS)

    Xu, Xing-Wang

    2016-04-01

    The East Junggar terrane is one of the important tectonic units of the Central Asian Orogenic Belt (CAOB; Zonenshain et al., 1990). Debate surrounds the tectonics of the East Junggar area, including tectonic setting, age, basement nature, subduction polarity and collisional time between the East Junggar terrane and Junggar block (e.g., Xiao et al., 2008, 2011; Long et al., 2012; Huang et al., 2012). Among the two popular models, one suggests that the Junggar is a continental block (e.g. Zhang et al., 1984, 1993; Watson et al., 1987; Xiao et al., 1992; He et al., 1994; Li et al., 2000; Charvet et al., 2001, 2007; Xu et al., 2003; Zhao et al., 2003; Buslov et al., 2004; Xu and Ma, 2004; Dong et al., 2009; Bazhenov et al., 2012; Choulet et al., 2012; Zhang et al., 2012). The other model proposes that the Junggar has a basement of Paleozoic oceanic crust (e.g., Carroll et al., 1990; Zheng et al., 2007) or oceanic island arc complexes (e.g., Coleman, 1989; Chen and Jahn, 2004; Windley et al., 2007) of the Altaid Paleozoic rocks (e.g., Sengör et al., 1993; Sengör and Natal'in, 1996; Allen and Vincent, 1997; Filippova et al., 2001; Xiao et al., 2004a, 2004b, 2008, 2009, 2010a, 2010b, 2012). The tectonics in the Eastern Junggar area are interpreted to be related to late Paleozoic intra-oceanic accretion induced by northward subduction of the Junggar oceanic lithosphere (e.g. Xiao et al., 2008, 2009; Biske and Seltmann, 2010; Wan et al., 2011; Yang et al., 2011) or by the southward subduction of the Paleo-Asian oceanic lithosphere (Zhang et al., 2004; Wong et al., 2010; Su et al., 2012). Recently, we did detailed field survey and petrological, geochemical and chronological analysis of the metamorphosed volcanic rocks and magmatic rocks, and new discovered gneiss and magnetite quartzite enclaves from the Taheir tectonic window in the East Junggar region which is situated between the Zaisan-Erqis-the Main Mongolian Lineament-suture and the Kelameili suture. The new results

  10. Igneous Cooling Rate constraints on the Accretion of the lower Oceanic Crust in Mid-ocean Ridges: Insights from a new Thermo-mechanical Model

    NASA Astrophysics Data System (ADS)

    Garrido, C. J.; Machetel, P.

    2005-12-01

    We report the results of a new thermo-mechanical model of crustal flow beneath fast spreading mid-ocean ridges to investigate both the effect of deep, near off-axis hydrothermal convection on the thermal structure of the magma chamber and the role of variable number of melt intrusions on the accretion of the oceanic crust. In our model the melt is injected at the center of the axial magma chamber with a 'needle' with adjustable porosity at different depths allowing the simulation of different arrangements of melt injection and supply within the magma chamber. Conversely to previous models, the shape of the magma chamber -defined as the isotherm where 95% solidification of the melt occurs- is not imposed but computed from the steady state reached by the thermal field considering the heat diffusion and advection and the latent heat of crystallization. The motion equation is solved for a temperature and phase dependent viscosity. The thermal diffusivity is also dependent on temperature and depth, with a higher diffusivity in the upper plutonic crust to account for more efficient hydrothermal cooling at these crustal levels. In agreement with previous non-dynamic thermal models, our results show that near, deep off-axis hydrothermal circulation strongly affects the shape of the axial magma by tightening isotherms in the upper half of the plutonic oceanic crust where hydrothermal cooling is more efficient. Different accretion modes have however little effect on the shape of the magma chamber, but result in variable arrangements of flow lines ranging from tent-shape in a single-lens accretion scenario to sub-horizontal in "sheeted-sill" intrusion models. For different intrusion models, we computed the average Igneous Cooling Rates (ICR) of gabbros by dividing the crystallization temperature interval of gabbros by the integrated time, from the initial intrusion to the point where it crossed the 950 °C isotherm where total solidification of gabbro occurs, along individual

  11. Petrogenesis of greenstones from the Mino Tamba belt, SW Japan: Evidence for an accreted Permian oceanic plateau

    NASA Astrophysics Data System (ADS)

    Ichiyama, Yuji; Ishiwatari, Akira; Koizumi, Kazuto

    2008-01-01

    Permian greenstones in the Jurassic Mino-Tamba accretionary complex, southwest Japan, are divided into three distinct series on the basis of their geological occurrence, mineralogy, and geochemistry. A low-Ti series (LTS) is associated with Lower Permian chert and limestone, and is the most voluminous of the three series. The LTS shows slightly more enriched geochemical and isotopic characteristics than MORB. A transition series (TS) is mainly associated with Lower Permian chert, and has more enriched geochemical signatures than MORB. Its isotopic characteristics are divided into enriched and depleted types. A high-Ti series (HTS) occurs as sills and hyaloclastites within Middle Permian chert and as dikes intruding the TS. Some HTS rocks have high MgO contents. The HTS is characterized by enrichment in incompatible trace elements and an isotopic composition comparable to HIMU-type basalt. The geochemistry of the voluminous LTS is similar to that of the oceanic basalt series of the Kerguelen plateau, suggesting production by partial melting of a shallow mantle plume head below thick oceanic lithosphere in Early Permian time. We infer that the TS formed simultaneously at the margins of the mantle plume head. In contrast, the HTS may have resulted from partial melting of a deep mantle plume tail in Middle Permian time. Permian greenstones in the Mino-Tamba belt may have thus formed by superplume activity in an intra-oceanic setting. Given the presence of two known contemporary continental flood basalt provinces (Siberia and Emeishan) and some accreted oceanic plateau basalts, the vast magmatism of the Mino-Tamba oceanic plateau suggests a large-scale superplume pulse in Permian time. Accretion of oceanic plateaux may have played an important role in the growth of continental margins and island arcs in Japan and elsewhere in the circum-Pacific region.

  12. Nd, Pb, Sr, and O isotopic characterization of Saudi Arabian Shield terranes

    USGS Publications Warehouse

    Stoeser, D.B.; Frost, C.D.

    2006-01-01

    New Nd, Sr and O isotopic data for granitoid rocks of the Saudi Arabian Shield are presented together with published Nd, Pb, Sr and O isotopic data and all available geologic and geochronologic information to re-evaluate the terranes defined for the Saudi Arabian part of the Arabian-Nubian Shield. Three groups of terranes are identified: 1) the western arc terranes, 2) the eastern arc terranes, and 3) the Khida terrane. The Khida terrane is the only terrane composed of pre-Neoproterozoic continental crust. The western arc terranes are of oceanic arc affinity, and have the least radiogenic Pb and Sr and most radiogenic Nd isotopic compositions and some of the lowest ??18O values of any rocks of the Saudi Arabian Shield. Although some previous studies have characterized the eastern arc terranes as of continental affinity, this study shows that they too are composed of Neoproterozoic oceanic arcs, although their sources have slightly elevated 208Pb/204Pb, Nd, Sri, and ??18O values compared to the western arc terranes. These data suggest that either the isotopic composition of the mantle source for the western arc terranes is more depleted than that of the eastern arc terranes or the eastern arc terranes have been mixed with a small amount of cratonic source material, or both. We further elaborate on the Hulayfah-Ad Dafinah fault zone as a major boundary within the Saudi Arabian portion of the East African Orogen. With further study, its northern extension may be shown to pass through what has been defined as the Hail terrane, and its southern extension appears to lie under cover east of the Tathlith-Malahah terrane and extend into Yemen. It may represent the collision zone between East and West Gondwana, and at the very least it is an important suture between groups of arc terranes of contrasting isotopic composition caught between two converging continents.

  13. Volcanism, jump and propagation on the Sheba ridge, eastern Gulf of Aden: segmentation evolution and implications for oceanic accretion processes

    NASA Astrophysics Data System (ADS)

    d'Acremont, Elia; Leroy, Sylvie; Maia, Marcia; Gente, Pascal; Autin, Julia

    2010-02-01

    The rifting between Arabia and Somalia, which started around 35 Ma, was followed by oceanic accretion from at least 17.6 Ma leading to the formation of the present-day Gulf of Aden. Bathymetric, gravity and magnetic data from the Encens-Sheba cruise are used to constrain the structure and segmentation of the oceanic basin separating the conjugate continental margins in the eastern part of the Gulf of Aden between 51°E and 55.5°E. Data analysis reveals that the oceanic domain along this ridge section is divided into two distinct areas. The Eastern area is characterized by a shorter wavelength variation of the axial segmentation and an extremely thin oceanic crust. In the western segment, a thicker oceanic crust suggests a high melt supply. This supply is probably due to an off-axis melting anomaly located below the southern flank of the Sheba ridge, 75 km east of the major Alula-Fartak transform fault. This suggests that the axial morphology is produced by a combination of factors, including spreading rate, melt supply and the edge effect of the Alula-Fartak transform fault, as well as the proximity of the continental margin. The oceanic domains have undergone two distinct phases of accretion since the onset of seafloor spreading, with a shift around 11 Ma. At that time, the ridge jumped southwards, in response to the melting anomaly. Propagating ridges were triggered by the melting activity, and propagated both eastward and westward. The influence of the melting anomaly on the ridges decreased, stopping their propagation since less than 9 Ma. From that time up to the present, the N025°E-trending Socotra transform fault developed in association with the formation of the N115°E-trending segment #2. In recent times, a counter-clockwise rotation of the stress field associated with kinematic changes could explain the structural morphology of the Alula-Fartak and Socotra-Hadbeen fracture zones.

  14. Constraints on the accretion of the gabbroic lower oceanic crust from plagioclase lattice preferred orientation in the Samail ophiolite

    NASA Astrophysics Data System (ADS)

    VanTongeren, J. A.; Hirth, G.; Kelemen, P. B.

    2015-10-01

    Oceanic crust represents more than 60% of the earth's surface and despite a large body of knowledge regarding the formation and chemistry of the extrusive upper oceanic crust, there still remains significant debate over how the intrusive gabbroic lower oceanic crust is accreted at the ridge axis. The two proposed end-member models, the Gabbro Glacier and the Sheeted Sills, predict radically different strain accumulation in the lower crust during accretion. In order to determine which of these two hypotheses is most applicable to a well-studied lower crustal section, we present data on plagioclase lattice preferred orientations (LPO) in the Wadi Khafifah section of the Samail ophiolite. We observe no systematic change in the strength of the plagioclase LPO with height above the crust-mantle transition, no dominant orientation of the plagioclase a-axis lineation, and no systematic change in the obliquity of the plagioclase LPO with respect to the modal layering and macroscopic foliation evident in outcrop. These observations are most consistent with the Sheeted Sills hypothesis, in which gabbros are crystallized in situ and fabrics are dominated by compaction and localized extension rather than by systematically increasing shear strain with increasing depth in a Gabbro Glacier. Our data support the hypothesis of MacLeod and Yaouancq (2000) that the rotation of the outcrop-scale layering from sub-horizontal in the layered gabbros to sub-vertical near the sheeted dikes is due to rapid vertical melt migration through upper gabbros close to the axial magma chamber. Additionally, our results support the hypothesis that the majority of extensional strain in fast spreading ridges is accommodated in partially molten regions at the ridge axis, whereas in slow and ultra-slow ridges large shear strains are accommodated by plastic deformation.

  15. Ordovician ocean plate stratigraphy and thrust duplexes of the Ballantrae Complex, SW Scotland: Implications for the pelagic deposition rate and forearc accretion in the closing Iapetus Ocean

    NASA Astrophysics Data System (ADS)

    Fujisaki, Wataru; Asanuma, Hisashi; Suzuki, Kazue; Sawaki, Yusuke; Sakata, Shuhei; Hirata, Takafumi; Maruyama, Shigenori; Windley, Brian F.

    2015-11-01

    The Ballantrae Complex (at Bennane Lea in SW Scotland) contains important ocean plate stratigraphy (basalt, chert, mudstone, sandstone) in an accretionary prism that is associated with a classic Ordovician ophiolite. We used the ocean plate stratigraphy to sub-divide the prism into 11 tectonic units. To determine the depositional age of bedded cherts, zircons were separated from 9 tuff beds from 6 different units. All the tuffs have early to middle Ordovician ages, even though their present positions are mutually distant. These ages are consistent with microfossil records of radiolaria and graptolites. The stratigraphic-structural relationships demonstrate that the ocean plate stratigraphy has been repeated by bedding-parallel thrusts; this is typical of a modern accretionary duplex. We calculated the sedimentation rate of Early to Middle Ordovician bedded cherts at Bennane Lea on the basis of U-Pb zircon ages obtained from several tuff beds; the data indicate that the depositional rate (0.6-3 m/myr) was as slow as that of Mesozoic-Cenozoic equivalents defined by radiolaria. The age spectra of detrital zircons from Ballantrae sandstones show prominent single peaks at ca. 467 and 478 Ma, and a lack of Precambrian zircons. Integration of our new zircon ages with published isotopic data and palaeo-geographic maps indicates that the sandstones were deposited near an intra-oceanic arc and far from any continent containing Precambrian rocks. The pelagic-to-clastic sediments at Bennane Lea were deposited in the closing Iapetus Ocean from ca. 477 Ma to ca. 464 Ma, when they were accreted with the intra-oceanic arc of Ballantrae.

  16. The African Plate: A history of oceanic crust accretion and subduction since the Jurassic

    NASA Astrophysics Data System (ADS)

    Gaina, C.; Torsvik, T. H.; Labails, C.; van Hinsbergen, D.; Werner, S.; Medvedev, S.

    2012-04-01

    Initially part of Gondwana and Pangea, and now surrounded almost entirely by spreading centres, the African plate moved relatively slowly for the last 200 million years. Yet both Africa's cratons and passive margins were affected by tectonic stresses developed at distant plate boundaries. Moreover, the African plate was partly underlain by hot mantle (at least for the last 300 Ma) - either a series of hotspots or a superswell, or both - that contributed to episodic volcanism, basin-swell topography, and consequent sediment deposition, erosion, and structural deformation. A systematic study of the African plate boundaries since the opening of surrounding oceanic basins is presently lacking. This is mainly because geophysical data are sparse and there are still controversies regarding the ages of oceanic crust. The publication of individual geophysical datasets and more recently, global Digital Map of Magnetic Anomalies (WDMAM, EMAG2) prompted us to systematically reconstruct the ages and extent of oceanic crust around Africa for the last 200 Ma. Location of Continent Ocean Boundary/Continent Ocean Transition and older oceanic crust (Jurassic and Cretaceous) are updates in the light of gravity, magnetic and seismic data and models of passive margin formation. Reconstructed NeoTethys oceanic crust is based on a new model of microcontinent and intr-oceanic subduction zone evolution in this area.The new set of oceanic palaeo-age grid models constitutes the basis for estimating the dynamics of oceanic crust through time and will be used as input for quantifying the paleo-ridge push and slab pull that contributed to the African plate palaeo-stresses and had the potential to influence the formation of sedimentary basins.

  17. Geochemical investigation of Gabbroic Xenoliths from Hualalai Volcano: Implications for lower oceanic crust accretion and Hualalai Volcano magma storage system

    NASA Astrophysics Data System (ADS)

    Gao, Ruohan; Lassiter, John C.; Barnes, Jaime D.; Clague, David A.; Bohrson, Wendy A.

    2016-05-01

    The patterns of axial hydrothermal circulation at mid-ocean ridges both affect and are influenced by the styles of magma plumbing. Therefore, the intensity and distribution of hydrothermal alteration in the lower oceanic crust (LOC) can provide constraints on LOC accretion models (e.g., "gabbro glacier" vs. "multiple sills"). Gabbroic xenoliths from Hualalai Volcano, Hawaii include rare fragments of in situ Pacific lower oceanic crust. Oxygen and strontium isotope compositions of 16 LOC-derived Hualalai gabbros are primarily within the range of fresh MORB, indicating minimal hydrothermal alteration of the in situ Pacific LOC, in contrast to pervasive alteration recorded in LOC xenoliths from the Canary Islands. This difference may reflect less hydrothermal alteration of LOC formed at fast ridges than at slow ridges. Mid-ocean ridge magmas from slow ridges also pond on average at greater and more variable depths and undergo less homogenization than those from fast ridges. These features are consistent with LOC accretion resembling the "multiple sills" model at slow ridges. In contrast, shallow magma ponding and limited hydrothermal alteration in LOC at fast ridges are consistent with the presence of a long-lived shallow magma lens, which limits the penetration of hydrothermal circulation into the LOC. Most Hualalai gabbros have geochemical and petrologic characteristics indicating derivation from Hualalai shield-stage and post-shield-stage cumulates. These xenoliths provide information on the evolution of Hawaiian magmas and magma storage systems. MELTS modeling and equilibration temperatures constrain the crystallization pressures of 7 Hualalai shield-stage-related gabbros to be ∼2.5-5 kbar, generally consistent with inferred local LOC depth. Therefore a deep magma reservoir existed within or at the base of the LOC during the shield stage of Hualalai Volcano. Melt-crust interaction between Hawaiian melts and in situ Pacific crust during magma storage partially

  18. Are oceanic plateaus sites of komatiite formation?

    NASA Astrophysics Data System (ADS)

    Storey, M.; Mahoney, J. J.; Kroenke, L. W.; Saunders, A. D.

    1991-04-01

    During Cretaceous and Tertiary time a series of oceanic terranes were accreted onto the Pacific continental margin of Colombia. The island of Gorgona is thought to represent part of the most recent, early Eocene, terrane-forming event. Gorgona is remarkable for the occurrence of komatiites of middle Cretaceous age, having MgO contents up to 24%. The geochemistry of spatially and temporally associated tholeiites suggests that Gorgona is an obducted fragment of the oceanic Caribbean Plateau, postulated by Duncan and Hargraves (1984) to have formed at 100 to 75 Ma over the Galapagos hotspot. Further examples of high-MgO oceanic lavas that may represent fragments of the Caribbean Plateau occur in allochthonous terranes on the island of Curaçao in the Netherlands Antilles and in the Romeral zone ophiolites in the southwestern Colombian Andes. These and other examples suggest that the formation of high-MgO liquids may be a feature of oceanic-plateau settings. The association of Phanerozoic komatiites with oceanic plateaus, coupled with thermal considerations, provides a plausible analogue for the origin of some komatiite-tholeiite sequences in Archean greenstone belts.

  19. Application of cladistics to terrane history—parsimony analysis of qualitative geological data

    NASA Astrophysics Data System (ADS)

    Young, Gavin C.

    Hypotheses of terrane dispersal or accretion can be represented graphically as branching diagrams (cladograms), but an assessment of competing hypotheses of terrane history requires a method of analysis of supporting evidence which resolves the most parsimonious explanation of all available data. Cladistics is a rigorous analytical method first developed for phylogeny reconstruction (i.e. biological history), but applicable to any hierarchical data set. Given appropriate definitions, the various types of geological, geophysical and biological data used to support hypotheses of fragmentation or fusion history for geological regions (terranes) assumed to have had independent geological histories can be organized hierarchically. Terrane fragmentation is equivalent to phylogenetic splitting of biological taxa, and standard algorithms for parsimony analysis may be directly applied. Terrane accretion may be represented as a coalescing area cladogram, and the supporting evidence also forms a hierarchical data set, but with two main differences. The less general attributes historically precede the more general (the reverse applies in phylogeny reconstruction), and the branching points (nodes on the cladogram), unlike hypothetical common ancestors in phylogeny reconstruction, represent defined geographic areas, with a geological structure which can be investigated. In cladistic reconstruction of evolutionary history the common ancestors are hypothetical, and their attributes can only be inferred from the distribution of attributes amongst the terminals (known biological taxa); in contrast, the end product of terrane accretion is a composite structure (geological province) within which juxtaposition of terranes may eliminate some of the possible historical sequences which led to its formation.

  20. Ophiolitic terranes of northern and central Alaska and their correlatives in Canada and northeastern Russia

    SciTech Connect

    Patton, W.W. Jr. )

    1993-04-01

    All of the major ophiolitic terranes (Angayucham, Tozitna, Innoko, Seventymile, and Goodnews terranes) in the northern and central Alaska belong to the Tethyan-type' of Moores (1982) and were obducted onto Paleozoic and Proterozoic continental and continental margin terranes in Mesozoic time. Tethyan-type' ophiolitic assemblages also occur in the Slide Mountain terrane in the Canadian Cordillera and extend from western Alaska into northeastern Russia. Although investigators have suggested widely different ages from their times of abduction onto the continent, these ophiolitic terranes display some remarkably similar features: (1) they consist of a stack of imbricated thrust slices dominated by ocean floor sediments, basalt, and high-level gabbro of late Paleozoic and Triassic age; (2) their mafic-ultramafic complexes generally are confined to the uppermost thrust sheets; (3) they lack the large tectonic melanges zones and younger accretionary flysch deposits associated with the ophiolitic terranes of southern Alaska and the Koryak region of northeastern Russia; (4) blueschist mineral assemblages occur in the lower part of these ophiolite terranes and (or) in the underlying continental terranes; and (5) they are bordered on their outboard' side by Mesozoic intraoceanic volcanic arc terranes. Recent geochemical and geologic studies of the mafic-ultramafic complexes in the Anagayucham and Tozitna terranes strongly suggest they were generated in a supra-subduction zone (SSZ) and that they are directly overlain by volcanic rocks of the Koyukuk terrane.

  1. Franciscan complex calera limestones: Accreted remnants of farallon plate oceanic plateaus

    USGS Publications Warehouse

    Tarduno, J.A.; McWilliams, M.; Debiche, M.G.; Sliter, W.V.; Blake, M.C.

    1985-01-01

    The Calera Limestone, part of the Franciscan Complex of northern California, may have formed in a palaeoenvironment similar to Hess and Shatsky Rises of the present north-west Pacific1. We report here new palaeomagnetic results, palaeontological data and recent plate-motion models that reinforce this assertion. The Calera Limestone may have formed on Farallon Plate plateaus, north of the Pacific-Farallon spreading centre as a counterpart to Hess or Shatsky Rises. In one model2, the plateaus were formed by hotspots close to the Farallon_Pacific ridge axis. On accretion to North America, plateau dissection in the late Cretaceous to Eocene (50-70 Myr) could explain the occurrence of large volumes of pillow basalt and exotic blocks of limestone in the Franciscan Complex. Partial subduction of the plateaus could have contributed to Laramide (70-40 Myr) compressional events3. ?? 1985 Nature Publishing Group.

  2. Evolution of eclogite facies metamorphism in the St. Cyr klippe, Yukon-Tanana terrane, Yukon, Canada

    NASA Astrophysics Data System (ADS)

    Petrie, Meredith Blair

    The St. Cyr klippe hosts well preserved to variably retrogressed eclogites found as sub-meter to hundreds of meter scale lenses within quartzofeldspathic schists in the Yukon-Tanana terrane, Canadian Cordillera. The St. Cyr area consists of structurally imbricated, polydeformed, and polymetamorphosed units of continental arc and oceanic crust. The eclogite-bearing quartzofeldspathic schists form a 30 by 6 kilometer thick, northwest-striking, coherent package. The schists consist of metasediments and felsic intrusives that are intercalated on the tens of meter scale. The presence of phengite and Permian age zircon crystallized under eclogite facies metamorphic conditions indicates that the eclogite was metamorphosed in situ with its quartzofeldspathic host. I investigated the metamorphic evolution of the eclogite-facies rocks in the St. Cyr klippe using isochemical phase equilibrium thermodynamic (pseudosection) modeling. I constructed P-T pseudosections in the system Na2O-K2O-CaO-FeO-O2-MnO-MgO-Al2O 3-SiO2-TiO2-H2O for the bulk-rock composition of an eclogite and a host metatonalite. In combination with petrology and mineral compositions, St. Cyr eclogites followed a five-stage clockwise P-T path. Peak pressure conditions for the eclogites and metatonalites reached up to 3.2 GPa, well within the coesite stability field, indicating the eclogites reached ultrahigh-pressure conditions. Decompression during exhumation occurred with a corresponding temperature increase. SHRIMP-RG zircon dating shows that the protolith of the eclogites formed within the Yukon-Tanana terrane during early, continental arc activity, between 364 and 380 Ma, while the metatonalite protolith formed at approximately 334 Ma, during the Little Salmon Cycle of the Klinkit phase of Yukon-Tanana arc activity. Both the eclogites and the metatonalites were then subducted to mantle depths and metamorphosed to ultrahigh-pressure conditions during the late Permian, between 266 and 271 Ma. The results of

  3. Wandering terranes in southern Alaska: The Aleutia Microplate and implications for the Bering Sea

    NASA Astrophysics Data System (ADS)

    Marlow, Michael S.; Cooper, Alan K.

    1983-04-01

    Paleomagnetic and geological data suggest that much of southern Alaska is a collage of tectonostratigraphic terranes which originated in Mesozoic time at paleolatitudes far south of their present position. The time of `docking' of the terranes against cratonic Alaska is critical to defining their amalgamated size and extent during their northward motion as well as their role in the evolution of the Bering Sea. One of the largest of the tectonostratigraphic terranes, the Peninsular terrane of south central and southwestern Alaska, extends offshore along the outer Bering Sea continental margin (Beringia). Paleomagnetic data suggest that this terrane has moved northward through all of Cenozoic time, but geologic data imply that the terrane had accreted to Alaska by the end of the Mesozoic. In early Cenozoic time the eastern part of the Aleutian arc appears to have been superimposed on the Peninsular terrane, and postulated northward Cenozoic motion of the terrane would therefore have required northward motion of the arc. Two accretion models, based on docking times for terranes in Alaska, are proposed, and they illustrate that large areas of the abyssal Bering Sea, the Alaska Peninsula, the Aleutian arc, and the Beringian continental margin may be part of a superterrane or microplate called Aleutia (microplate as defined by Beck et al. (1980), i.e., a microplate is a displaced segment of lithosphere that has crustal roots, whereas a superterrane is an amalgamation of terranes which may or may not be rootless). Model A implies that the Aleutian arc developed in situ on the southern edge of Aleutia after the microplate had docked. In model B, the final docking time of the Peninsular terrane is late Cenozoic, which implies that the Aleutia microplate encompasses a mammoth area that includes parts of southern Alaska, the Alaska Peninsula, the southern Beringian margin, the abyssal Bering Sea (Kula plate), and the Aleutian arc. If model A is correct, the docking time of

  4. Accretion, subduction, and underplating along the southern Alaska continental margin

    SciTech Connect

    Plafker, G.; Ambos, E.L.; Fuis, G.S.; Mooney, W.D.; Nokleberg, W.J.; Campbell, D.L.

    1985-01-01

    In 1984-1985 the Trans Alaska Crustal Transect (TACT) program completed geologic, seismic refraction, gravity, and magnetic studies along a 350-km-long corridor that extends northward from the Gulf of Alaska coast near Cordova to the Denali fault at the Richardson Highway. From south to north, this segment of the transect traverses: 1) part of the Prince William terrance (PWT), composed of an accreted Paleocene and Eocene deep-sea fan complex, oceanic volcanic rocks, and pelagic sediments; 2) the Chugach terrane (CGT) composed of a) accreted Late Cretaceous flysch and oceanic basaltic rocks, b) accreted and subducted (.) Late Jurassic to Early Cretaceous sheared melange, and c) subducted Early (.) Jurassic or older blueschist/greenschist; and 3) Wrangellia-Peninsular terranes (WRT/PET) consisting primarily of late Paleozoic intraoceanic andesitic arc rocks with associated mafic and ultramafic plutonic rocks, an overlying distinctive Triassic sedimentary and volcanic sequence, and superposed intrusive and extrusive magmatic rocks of the Jurassic Talkeetna arc. At the southern margin of both the CGT and WRT/PET, shallow high-velocity zones characterized by positive gravity and magnetic anomalies reflect uplift of mafic and ultramafic basement along these thrusts. The Contact and Border Ranges fault systems appear to merge into a subhorizontal low-velocity zone of uncertain origin that underlies the CGT and southern WRT/PET at 5-9 km depth. A few kilometers beneath the shallow low-velocity zone in a 30-km-thick stack of eight northward-dipping layers of alternating high and low velocity, interpreted as subducted and underplated mantle and oceanic crust rocks. Distribution of earthquake hypocenters suggests that active subduction involves at least the lowest two and possibly the lower four layers.

  5. Central Appalachian Exotic Terranes and Exposures of Former Orogenic Middle Crust

    NASA Astrophysics Data System (ADS)

    Martin, A. J.

    2015-12-01

    In the northern and southern Appalachians, rocks that formed Paleozoic orogenic middle crust mostly are exposed within or directly inboard of terranes that originated near Gondwana. Most outcrops of the Paleozoic orogenic middle crust of the eastern edge of Laurentia occur adjacent to these exotic terranes. However, a narrow belt of Paleozoic orogenic middle crust is exposed in the Piedmont of the central Appalachians despite the absence of recognized exotic terranes. The presence of these deformed, amphibolite facies rocks raises the questions: "Did central Appalachian orogeny occur in the absence of accreted exotic terranes?" and, more generally, "Is exotic terrane collision required for exhumation of Appalachian former middle crust?" Previous U/Pb isotopic dating of spots in detrital zircon revealed the presence of Gondwanan terranes in three locations in the central Appalachians: central Virginia, central Maryland, and southeastern Pennsylvania. Two new samples collected near the discovery locations in Virginia and Maryland yielded prominent peaks in zircon U/Pb age distributions at ca. 630-610 Ma, confirming the Gondwanan affinity of these rocks. Hf isotopic compositions of spots in these upper Neoproterozoic zircon grains range to both more and less depleted than spots in zircon from the few possible Laurentian granitic sources, consistent with derivation of the zircon from Gondwana. Abundant 1700-1000 Ma detrital zircon rules out the West Africa Craton as a potential source; Amazonia is the most likely ultimate source of the zircon. The extent of the exotic terrane(s) in the central Appalachian Piedmont remains enigmatic due to uncertain connections between isolated exposures of the terrane(s). Nevertheless, the discovery of one or more exotic terranes in the central Appalachian Piedmont underscores the relationship between exotic terranes and exposed former middle crust in the Appalachians. This relationship may be a feature of several other major

  6. Scaphopoda from the Alexander Terrane, Southeast Alaska-The first occurrence of Scaphopoda in the Silurian

    USGS Publications Warehouse

    Rohr, D.M.; Blodgett, R.B.; Baichtal, J.

    2006-01-01

    The scaphopods Dentalium hecetaensis n. sp. and Rhytiodentalium cf. kentuckyensis Pojeta et Runnegar, 1979, are described from Ludlow-age strata of the Heceta Limestone on Prince of Wales Island, Southeast Alaska. This is the first occurrence of Silurian scaphopods known to date. They are part of a diverse macrobenthic fauna of the Alexander terrane, an accreted southern Alaskan terrane of Siberian or Uralian affinities. ?? 2006 Nanjing Institute of Geology and Palaeontology, CAS.

  7. Velocity-porosity relationships for slope apron and accreted sediments in the Nankai Trough Seismogenic Zone Experiment, Integrated Ocean Drilling Program Expedition 315 Site C0001

    NASA Astrophysics Data System (ADS)

    Hashimoto, Y.; Tobin, H. J.; Knuth, M.

    2010-12-01

    In this study, we focused on the porosity and compressional wave velocity of marine sediments to examine the physical properties of the slope apron and the accreted sediments. This approach allows us to identify characteristic variations between sediments being deposited onto the active prism and those deposited on the oceanic plate and then carried into the prism during subduction. For this purpose we conducted ultrasonic compressional wave velocity measurements on the obtained core samples with pore pressure control. Site C0001 in the Nankai Trough Seismogenic Zone Experiment transect of the Integrated Ocean Drilling Program is located in the hanging wall of the midslope megasplay thrust fault in the Nankai subduction zone offshore of the Kii peninsula (SW Japan), penetrating an unconformity at ˜200 m depth between slope apron sediments and the underlying accreted sediments. We used samples from Site C0001. Compressional wave velocity from laboratory measurements ranges from ˜1.6 to ˜2.0 km/s at hydrostatic pore pressure conditions estimated from sample depth. The compressional wave velocity-porosity relationship for the slope apron sediments shows a slope almost parallel to the slope for global empirical relationships. In contrast, the velocity-porosity relationship for the accreted sediments shows a slightly steeper slope than that of the slope apron sediments at 0.55 of porosity. This higher slope in the velocity-porosity relationship is found to be characteristic of the accreted sediments. Textural analysis was also conducted to examine the relationship between microstructural texture and acoustic properties. Images from micro-X-ray CT indicated a homogeneous and well-sorted distribution of small pores both in shallow and in deeper sections. Other mechanisms such as lithology, clay fraction, and abnormal fluid pressure were found to be insufficient to explain the higher velocity for accreted sediments. The higher slope in velocity-porosity relationship for

  8. Peninsular terrane basement ages recorded by Paleozoic and Paleoproterozoic zircon in gabbro xenoliths and andesite from Redoubt volcano, Alaska

    USGS Publications Warehouse

    Bacon, Charles R.; Vazquez, Jorge A.; Wooden, Joseph L.

    2012-01-01

    Historically Sactive Redoubt volcano is an Aleutian arc basalt-to-dacite cone constructed upon the Jurassic–Early Tertiary Alaska–Aleutian Range batholith. The batholith intrudes the Peninsular tectonostratigraphic terrane, which is considered to have developed on oceanic basement and to have accreted to North America, possibly in Late Jurassic time. Xenoliths in Redoubt magmas have been thought to be modern cumulate gabbros and fragments of the batholith. However, new sensitive high-resolution ion microprobe (SHRIMP) U-Pb ages for zircon from gabbro xenoliths from a late Pleistocene pyroclastic deposit are dominated by much older, ca. 310 Ma Pennsylvanian and ca. 1865 Ma Paleoproterozoic grains. Zircon age distributions and trace-element concentrations indicate that the ca. 310 Ma zircons date gabbroic intrusive rocks, and the ca. 1865 Ma zircons also are likely from igneous rocks in or beneath Peninsular terrane basement. The trace-element data imply that four of five Cretaceous–Paleocene zircons, and Pennsylvanian low-U, low-Th zircons in one sample, grew from metamorphic or hydrothermal fluids. Textural evidence of xenocrysts and a dominant population of ca. 1865 Ma zircon in juvenile crystal-rich andesite from the same pyroclastic deposit show that this basement has been assimilated by Redoubt magma. Equilibration temperatures and oxygen fugacities indicated by Fe-Ti–oxide minerals in the gabbros and crystal-rich andesite suggest sources near the margins of the Redoubt magmatic system, most likely in the magma accumulation and storage region currently outlined by seismicity and magma petrology at ∼4–10 km below sea level. Additionally, a partially melted gabbro from the 1990 eruption contains zircon with U-Pb ages between ca. 620 Ma and ca. 1705 Ma, as well as one zircon with a U-Th disequilibrium model age of 0 ka. The zircon ages demonstrate that Pennsylvanian, and probably Paleoproterozoic, igneous rocks exist in, or possibly beneath, Peninsular

  9. Constraints on the accretion of the gabbroic lower oceanic crust from plagioclase lattice preferred orientation in the Samail ophiolite

    NASA Astrophysics Data System (ADS)

    VanTongeren, J. A.; Hirth, G.; Kelemen, P. B.

    2015-12-01

    The debate over the processes of igneous accretion of gabbroic lower crust at submarine spreading centers is centered on two end-member hypotheses: Gabbro Glaciers and Sheeted Sills. In order to determine which of these two hypotheses is most applicable to a well-studied lower crustal section, we present newly published data (VanTongeren et al., 2015 EPSL v. 427, p. 249-261) on plagioclase lattice preferred orientations (LPO) in the Wadi Khafifah section of the Samail ophiolite, Oman. Based on our results we provide five critical observations that any model for the accretion of the lower oceanic crust must satisfy: (1) There is a distinctive change in the orientation of the outcrop-scale layering from near-vertical to sub-horizontal that is also reflected in the plagioclase fabrics in the uppermost ~1000-1500 m of the gabbroic crust; (2) The distinction between the upper gabbros and lower gabbros is not a geochemical boundary. Rather, the change in outcrop-scale orientation from near-vertical to sub-horizontal occurs stratigraphically lower in the crust than a change in whole-rock geochemistry; (3) There is no systematic difference in plagioclase fabric strength in any crystallographic axis between the upper gabbros and the lower gabbros; (4) Beneath the abrupt transition from sub-vertical to sub-horizontal fabric, there is no systematic change in the geographic orientation of the plagioclase fabric, or in the development of a dominant lineation direction within the upper gabbros or the lower gabbros; (5) In the lower gabbros, the obliquity between the (010) and the modal layering remains approximately constant and indicates a consistent top to the right sense of shear throughout the stratigraphy. Our observations are most consistent with the Sheeted Sills hypothesis, in which the majority of lower crustal gabbros are crystallized in situ and fabrics are dominated by compaction and localized extension rather than by systematically increasing shear strain with

  10. Seismic structure and crustal accretion along an intermediate-rate mid-ocean ridge segment

    NASA Astrophysics Data System (ADS)

    Weekly, Robert Todd

    Epicenters and magnitudes for 36,523 earthquakes recorded along the Endeavour segment between August 2003 and October 2006 are automatically determined using a local ocean-bottom seismometer (OBS) network. The catalog is dominated by two swarm sequences in January and February 2005 in the vicinity of the Endeavour overlapping spreading center, which included earthquakes in West Valley, the northern portion of the Endeavour segment, southwest Endeavour Valley and the Endeavour vent fields. These swarms are attributed to volcanism including a dike intrusion on the northern Endeavour in February 2005 and smaller diking events on the propagating tip of the West Valley segment in both swarms. The dike on the northern Endeavour propagated to the south, which is inconsistent with magma sourced from the axial magma chamber beneath the elevated central portion of the segment. Following the swarms, seismic activity on the Endeavour segment decreased on average to ˜15% of pre-swarm values and almost ceased at the segment ends. I infer that a six-year non-eruptive event that started with a swarm in 1999 and finished with the 2005 swarms ruptured the entire segment and relieved plate-spreading stresses. The inferred coupling between the 1999 and 2005 events, the observation of extensive precursory activity prior to the 2005 swarms, and the interaction between seismically active regions during the swarms is consistent with static triggering with delays influenced by viscoelastic relaxation, hydraulic diffusion and magma withdrawal and replenishment. The isotropic and anisotropic P-wave velocity structure of the upper oceanic crust on the Endeavour Segment of the Juan de Fuca Ridge is studied using refracted travel time data collected by an active-source, three-dimensional tomography experiment. The isotropic velocity structure is characterized by low crustal velocities in the overlapping spreading centers (OSCs) at the ends of the segment. These low velocities are indicative of

  11. Gondwanan/peri-Gondwanan origin for the Uchee terrane, Alabama and georgia: Carolina zone or Suwannee terrane(?) and its suture with Grenvillian basement of the Pine Mountain window

    USGS Publications Warehouse

    Steltenpohl, M.G.; Mueller, P.M.; Heatherington, A.L.; Hanley, T.B.; Wooden, J.L.

    2008-01-01

    Taconian) has been reported. This younger history, together with the ages of metaigneous rocks and evidence for pre-Grenville basement, suggests the Uchee terrane is likely of Gondwanan origin and may he related to Carolina zone terranes that accreted during the Alleghanian orogeny. ?? 2008 Geological Society of America.

  12. Accretion of the Archean Slave province

    NASA Astrophysics Data System (ADS)

    Kusky, T. M.

    1989-01-01

    Continental rift models have long been applied to the Archean Slave province of northwestern Canada. A reassessment of these models shows them to be incompatible with observed geological relations and suggests that contractional tectonic models may be more appropriate than extensional ones. Regions composed of different rock suites (e.g., orthogneisses vs. mafic volcanics) are separated by high-strain zones recording large displacements. It is proposed that the high-strain zones separate four distinct terranes that have been juxtaposed during collisional orogenesis. From west to east, these include the Anton terrane, interpreted as an Archean microcontinent; the Sleepy Dragon terrane, possibly an exhumed more eastern part of the Anton terrane; the Contwoyto terrane, a westward-verging fold and thrust belt containing tectonic slivers of greenstone volcanics; and the Hackett River volcanic terrane, interpreted as an Archean island arc. The Contwoyto and Hackett River terranes represent a paired accretionary prism and island-arc system that formed above an east-dipping subduction zone. These collided with the Anton microcontinent, producing a basement nappe, expressed as the Sleepy Dragon terrane, during the main accretion event within the Slave province. The whole tectonic assemblage was intruded by late-kinematic to postkinematic granitoids.

  13. Origin and pre-Cenozoic evolution of the Qiangtang terrane basement, central Tibet

    NASA Astrophysics Data System (ADS)

    Zhao, Zhongbao; Bons, Paul D.; Wang, Genhou

    2013-04-01

    subduction of the Proto- -Tethys. Late Triassic syn-collisional granites and rapidly exhumed high-pressure rocks indicate the closure of these oceans and final amalgamation of the re-united Qiangtang Terranes with Eurasia. Liu, Y, Santosh, M., Zhao, Z.B., Niu, W.C., Wang, G.H. 2011. Evidence for palaeo-Tethyan-oceanic subduction within central Qiangtang, northern Tibet, Lithos 127, 39-53

  14. Gulf of California analogue for origin of Late Paleozoic ocean basins adjacent to western North America

    SciTech Connect

    Murchey, B.L. )

    1993-04-01

    Ocean crust accreted to the western margin of North America following the Late Devonian to earliest Missippian Antler orogeny is not older than Devonian. Therefore, ocean crust all along the margin of western North America may have been very young following the Antler event. This situation can be compared to the present-day margin of North America which lies adjacent to young ocean crust as a result of the subduction of the Farallon plate and arrival of the East Pacific spreading ridge. Syn- and post-Antler rifting that occurred along the North American margin may well be analogous to the formation of the Gulf of California by the propagation of the East Pacific spreading ridge. Black-arc rifting associated with the subduction of very old ocean crust seems a less likely mechanism for the early stages of ocean basin formation along the late Paleozoic margin of western North America because of the apparent absence of old ocean crust to the west of the arc terranes. The eastern Pacific basins were as long-lived as any truly oceanic basins and may have constituted, by the earliest Permian, a single wedge-shaped basin separated from the western Pacific by rifted fragments of North American arc-terranes. In the Permian, the rifted arcs were once again sites of active magmatism and the eastern Pacific basins began to close, from south (Golconda terrane) to north. Final closure of the northernmost eastern Pacific basin (Angayucham in Alaska) did not occur until the Jurassic.

  15. Paleomagnetic data from Alaska: reliability, interpretation and terrane trajectories

    NASA Astrophysics Data System (ADS)

    Harbert, William

    1990-11-01

    Virtually the entire body of paleomagnetic data collected from southern Alaska depicts a clear decrease in paleolatitude with increasing age, strongly suggesting that southern Alaska represents a displaced terrane. In this paper, paleomagnetic studies from southern Alaska have been classified with respect to a Quality Index that is based on four criteria. These criteria are the presence of both polarities of magnetic remanence, stepwise thermal or alternating field demagnetization of specimens, principal component analysis of demagnetization data, and a successful fold test. Of the 51 studies compiled, only four from southern Alaska and one from western Canada are demonstrated to satisfy all criteria and fall therefore in the category of Group 3, ("highly reliable"). Two studies from southern Alaska satisfy three of the four criteria, lacking both polarities of characteristic remanence, and are judged to be of Group 2 ("reliable"). Two of these paleomagnetic studies constrain the accretion time of the southern Alaska terrane to the relatively stationary region of central Alaska north of the Denali fault. Four paleomagnetic studies from the southern Alaska terrane show a distinct paleolatitude anomaly when compared with their expected paleolatitudes from the North American apparent polar wander path. Using the model of Engebretson et al. (1984), a series of models are presented to best fit these highly reliable and reliable paleomagnetic studies. The model preferred in this article assumes an accretion time with North America of 50 Ma, and documents pre-50 Ma displacement of the southern Alaska terrane on the Kula plate. If the Ghost Rocks paleomagnetic magnetizations (Plumley et al., 1983) are assumed to be of earliest Tertiary age, this model fits all of the low paleolatitudes observed in southern Alaska. Models incorporating coastwise translation of the southern Alaska terrane along the western boundary of the North America plate and a 50 Ma suturing age of this

  16. Crustal accretion along the global mid-ocean ridge system based on basaltic glass and olivine-hosted melt inclusion compositions

    NASA Astrophysics Data System (ADS)

    Wanless, V. D.; Behn, M. D.

    2015-12-01

    The depth and distribution of crystallization at mid-ocean ridges controls the overall architecture of the oceanic crust, influences hydrothermal circulation, and determines geothermal gradients in the crust and uppermost mantle. Despite this, there is no overall consensus on how crystallization is distributed within the crust/upper mantle or how this varies with spreading rate. Here, we examine crustal accretion at mid-ocean ridges by combining crystallization pressures calculated from major element barometers on mid-ocean ridge basalt (MORB) glasses with vapor-saturation pressures from melt inclusions to produce a detailed map of crystallization depths and distributions along the global ridge system. We calculate pressures of crystallization from >11,500 MORB glasses from the global ridge system using two established major element barometers (1,2). Additionally, we use vapor-saturation pressures from >400 olivine-hosted melt inclusions from five ridges with variable spreading rates to constrain pressures and distributions of crystallization along the global ridge system. We show that (i) crystallization depths from MORB glasses increase and become less focused with decreasing spreading rate, (ii) maximum glass pressures are greater than the maximum melt inclusion pressure, which indicates that the melt inclusions do not record the deepest crystallization at mid-ocean ridges, and (iii) crystallization occurs in the lower crust/upper mantle at all ridges, indicating accretion is distributed throughout the crust at all spreading rates, including those with a steady-state magma lens. Finally, we suggest that the remarkably similar maximum vapor-saturation pressures (~ 3000 bars) in melt inclusion from all spreading rates reflects the CO2 content of the depleted upper mantle feeding the global mid-ocean ridge system. (1) Michael, P. & W. Cornell (1998), Journal of Geophysical Research, 103(B8), 18325-18356; (2) Herzberg, C. (2004), Journal of Petrology, 45(12), 2389.

  17. The Wisconsin magmatic terrane: An Early Proterozoic greenstone-granite terrane formed by plate tectonic processes

    NASA Technical Reports Server (NTRS)

    Schulz, K. J.; Laberge, G. L.

    1986-01-01

    The Wisconsin magmatic terrane (WMT) is an east trending belt of dominantly volcanic-plutonic complexes of Early Proterozoic age (approx. 1850 m.y.) that lies to the south of the Archean rocks and Early Proterozoic epicratonic sequence (Marquette Range Supergroup) in Michigan. It is separated from the epicratonic Marquette Range Supergroup by the high-angle Niagara fault, is bounded on the south, in central Wisconsin, by Archean gneisses, is truncated on the west by rocks of the Midcontinent rift system, and is intruded on the east by the post-orogenic Wolf river batholith. The overall lithologic, geochemical, metallogenic, metamorphic, and deformational characteristics of the WMT are similar to those observed in recent volcanic arc terranes formed at sites of plate convergence. It is concluded that the WMT represents an evolved oceanic island-arc terrane accreated to the Superior craton in the Early Proterozoic. This conclusion is strengthened by the apparent absence of Archean basement from most of the WMT, and the recent recognition of the passive margin character of the epicratonic Marquette Range Supergroup.

  18. Kinematics of the mosquito terrane, Coldfoot Area, Alaska: Keys to Brooks Range tectonics: Final report, Project No. 2

    SciTech Connect

    Harms, T.A.; Coney, P.J.

    1988-04-01

    Within the large-scale geometry of the Brooks Range, the Angayucham terrane occurs as a vast overthrust sheet. From the north flank of the Ruby terrane it underlies the Koyukuk basin and stretches north as the roof thrust to the various nappe terranes of the Brooks Range. The tectonic relationship of the Ruby terrane to the south flank of the Brooks Range lies largely obscured beneath the Angayucham in the eastern apex of the Koyukuk basin. The Mosquito terrane occurs as a window through the Angayucham at this juncture. The composition and structures of the Mosquito terrane reveal that is the result of shear along a sub-horizontal step or flange within the prominent, through-going dextral strike-slip fault system which cuts across the eastern Koyukuk basin and southeastern Brooks Range. Units of the Mosquito were derived from both the Angayucham and Ruby terranes. A consistent tectonic fabric imposed upon them is kinematically linked to the strike-slip system and indicates a northeasterly direction of transport across the terrane. The presence of Ruby-correlative units within the Mosquito suggests the Ruby underlies the Angayucham and that it is in contact with terrances of the southern Brooks Range at that structural level along high-angle strike-slip faults. These relationships demonstrate that an episode of dextral transpression is the latest in the history of terrane accretion and tectonic evolution of the Brooks Range. 35 refs.

  19. The Sikhote-Alin orogenic belt, Russian South East: Terranes and the formation of continental lithosphere based on geological and isotopic data

    NASA Astrophysics Data System (ADS)

    Khanchuk, A. I.; Kemkin, I. V.; Kruk, N. N.

    2016-04-01

    The Sikhote-Alin orogenic belt, Russian South East, consists of folded terranes made up of Jurassic and Early Cretaceous accretionary prisms, turbidite basins, and island arc terranes that are overlapped unconformably by undeformed upper Cenomanian to Cenozoic volcanic deposits. The Jurassic and Early Cretaceous accretionary prisms, together with the Early Cretaceous island arc, are related to subduction of the Paleo-Pacific plate. The turbidite basin, which began to form at the beginning of the Early Cretaceous, is related to left-lateral movement of the Paleo-Pacific plate along the Paleo-Asian continental margin. The collage of terranes that make up the Sikhote-Alin orogenic belt was amalgamated in two stages. The first began after Jurassic subduction beneath the Asian continent was terminated, and the second took place in the late Albian, when the Early Cretaceous island arc collided with the continental margin. Intense deformation of the terranes took place along the continental margin in the form of large-scale translations from south to north, together with oroclinal folding. The deformation resulted in rapid thickening of sediments in the upper crust, resulting in turn in the formation of granitic-metamorphic material in the continental lithosphere. In the southwestern part of the Sikhote-Alin orogen, granites were intruded during the Hauterivian-Aptian, while the entire orogenic belt was affected by intrusions in the late Albian-early Cenomanian. Synorogenic intraplate volcanic rocks and alkaline ultramafic-mafic intrusions also testify to the fact that the orogenic processes in the Sikhote-Alin were related to a transform continental margin, and not to subduction. Geochemical and Nd isotopic data indicate, the primary continental crust of the Sikhote-Alin was of a "hybrid" nature, consisting of juvenile basic components accreted from an oceanic plate and recycled sedimentary material derived from the erosion of ancient blocks.

  20. Geochemistry and U-Pb zircon ages of plutonic rocks from the Algodões granite-greenstone terrane, Troia Massif, northern Borborema Province, Brazil: Implications for Paleoproterozoic subduction-accretion processes

    NASA Astrophysics Data System (ADS)

    Costa, Felipe Grandjean da; Palheta, Edney Smith de Moraes; Rodrigues, Joseneusa Brilhante; Gomes, Iaponira Paiva; Vasconcelos, Antonio Maurilio

    2015-04-01

    The Algodões metavolcano-sedimentary sequence is located at the northern margin of the Archean/Paleoproterozoic Troia Massif, northern Borborema Province (NE Brazil). It represents a well-preserved Paleoproterozoic greenstone-like sequence affected by two major plutonic events. The early plutonism, represented by the Cipó orthogneisses, mainly comprises biotite-bearing metatonalites, which share similar geochemical signatures with Archean tonalite-trondjhemite-granodiorite (TTG). For these rocks, we report U-Pb (LA-ICPMS) zircon ages of 2189 ± 14 Ma and 2180 ± 15 Ma. A subsequent plutonic magmatism occurred at ˜2150-2130 Ma and is mainly represented by hornblende-bearing dioritic to tonalitic orthogneisses of the Madalena Suite and São José da Macaoca Complex. Geochemical data indicate that these dioritic/tonalitic orthogneisses have adakitic characteristics and strongly suggest mantle-related magmas. A (sensu stricto) granite plutonism (Serra da Palha orthogneisses) also intruded the Algodões sequence and yielded U-Pb (LA-ICPMS) zircon age of 2150 ± 16 Ma. These granitic orthogneisses show high-K content, A-type characteristics and probably derived from partial melting of a crustal (tonalitic) source. We suggest that the early ˜2190-2160 Ma TTG plutons probably developed in intra-oceanic arc setting, whereas the following ˜2150-2130 Ma adakitic plutons and A-type granitic magmatism developed in response to arc-continent collision.

  1. Deformation and sedimentation along a developing terrane suture: Eastern Sunda forearc, Indonesia

    SciTech Connect

    Reed, D.L.; Silver, E.A.; Prasetyo, H.; Meyer, A.W.

    1986-12-01

    The collision of the eastern Sunda arc with northwest Australia has resulted in the development of a suture between the Sumba ridge and Sawu-Timor terranes along a zone of intraforearc convergence. The developing suture varies from the low-angle Sawu thrust, with attendant mud diapirs in the Sumba basin, to high-angle reverse faults near a basement high of the underthrust Sumba ridge terrane. Bottom currents, associated with the flow of Pacific Ocean deep water into the Indian Ocean, have eroded the terranes and subsequently deposited the detritus in an assemblage of contourites along the suture. This study reveals the high structural variability of a terrane suture and the oceanographic influence on the deposition of overlap assemblages.

  2. The Mars Hill Terrane: An enigmatic southern Appalachian terrane

    SciTech Connect

    Raymond, L.A.; Johnson, P.A. . Dept. of Geology)

    1994-03-01

    The Mars Hill Terrane (MHT) in the Appalachian Blue Ride Belt is bordered by complex, locally reactivated thrust and strike-slip faults. On the east, the MHT is bounded by the allochthonous, ensimatic Toe Terrane (TT) across the diachronous, ductile Holland Mountain-Soque River Fault System. The MHT is separated on the northwest from ensialic Laurentian basement (LB), by the Fries-Hayesville Fault System. On the south, the MHT is truncated by the Shope Fork Fault. The MHT is characterized by migmatitic biotite-pyroxene-hornblende gneiss, but contains 1--1.8 b.y. old quartz-feldspar gneisses, plus ultramafic rocks, calc-silicate rocks, mica schists and gneisses, and Neoproterozoic Bakersville gabbros. This rock assemblage contrasts with that of the adjoining terranes. The only correlative units between the MHT and adjoining terranes are Neoproterozoic gabbro, Ordovician-Devonian granitoid plutons, and ultramafic rocks. Gabbro links the MHT with LB rocks. Apparently similar calc-silicate rocks differ petrographically among terranes. During Taconic or Acadian events, both the TT and MHT reached amphibolite to granulite metamorphic grade, but the LB did not exceed greenschist grade. The data conflict. The O-D plutons, ultramafic rocks, and metamorphic histories suggest that the TT had docked with the MHT by Ordovician time. The premetamorphic character of the Holland Mtn.-Soque River Fault System supports that chronology. Neoproterozoic gabbros suggest a MHT-LB link by Cambrian time, but the LB experienced neither O-D plutonism nor Paleozoic amphibolite-granulite facies metamorphism.

  3. Cimmerian terranes: Preface

    NASA Astrophysics Data System (ADS)

    Zanchi, Andrea; Fürsich, Franz Theodor; Santosh, M.

    2015-04-01

    The Cimmerian orogeny affected the southern Eurasian margin from Anatolia to Tibet and was caused by the collision of several microplates detached from northern Gondwana at the end of the Palaeozoic, because of the opening of the Neotethys Ocean. The resulting geodynamic scenario led to mountain building all along the southern Eurasian margin in the early Mesozoic, producing an intricate tectonic framework. In fact, large parts of the present day mountain chains extending along southern Eurasia were formed during the Cimmerian events, whose vestiges are still poorly known.

  4. Tectonic evolution of the East Junggar terrane, CAOB

    NASA Astrophysics Data System (ADS)

    Xu, Xing-Wang

    2016-04-01

    The East Junggar terrane is one of the important tectonic units of the Central Asian Orogenic Belt (CAOB; Zonenshain et al., 1990). Debate surrounds the tectonics of the East Junggar area, including tectonic setting, age, basement nature, subduction polarity and collisional time between the East Junggar terrane and Junggar block (e.g., Xiao et al., 2008, 2011; Long et al., 2012; Huang et al., 2012). Among the two popular models, one suggests that the Junggar is a continental block (e.g. Zhang et al., 1984, 1993; Watson et al., 1987; Xiao et al., 1992; He et al., 1994; Li et al., 2000; Charvet et al., 2001, 2007; Xu et al., 2003; Zhao et al., 2003; Buslov et al., 2004; Xu and Ma, 2004; Dong et al., 2009; Bazhenov et al., 2012; Choulet et al., 2012; Zhang et al., 2012). The other model proposes that the Junggar has a basement of Paleozoic oceanic crust (e.g., Carroll et al., 1990; Zheng et al., 2007) or oceanic island arc complexes (e.g., Coleman, 1989; Chen and Jahn, 2004; Windley et al., 2007) of the Altaid Paleozoic rocks (e.g., Sengör et al., 1993; Sengör and Natal'in, 1996; Allen and Vincent, 1997; Filippova et al., 2001; Xiao et al., 2004a, 2004b, 2008, 2009, 2010a, 2010b, 2012). The tectonics in the Eastern Junggar area are interpreted to be related to late Paleozoic intra-oceanic accretion induced by northward subduction of the Junggar oceanic lithosphere (e.g. Xiao et al., 2008, 2009; Biske and Seltmann, 2010; Wan et al., 2011; Yang et al., 2011) or by the southward subduction of the Paleo-Asian oceanic lithosphere (Zhang et al., 2004; Wong et al., 2010; Su et al., 2012). Recently, we did detailed field survey and petrological, geochemical and chronological analysis of the metamorphosed volcanic rocks and magmatic rocks, and new discovered gneiss and magnetite quartzite enclaves from the Taheir tectonic window in the East Junggar region which is situated between the Zaisan-Erqis-the Main Mongolian Lineament-suture and the Kelameili suture. The new results

  5. Siberia, the wandering northern terrane, and its changing geography through the Palaeozoic

    NASA Astrophysics Data System (ADS)

    Cocks, L. Robin M.; Torsvik, Trond H.

    2007-05-01

    The old terrane of Siberia occupied a very substantial area in the centre of today's political Siberia and also adjacent areas of Mongolia, eastern Kazakhstan, and northwestern China. Siberia's location within the Early Neoproterozoic Rodinia Superterrane is contentious (since few if any reliable palaeomagnetic data exist between about 1.0 Ga and 540 Ma), but Siberia probably became independent during the breakup of Rodinia soon after 800 Ma and continued to be so until very near the end of the Palaeozoic, when it became an integral part of the Pangea Supercontinent. The boundaries of the cratonic core of the Siberian Terrane (including the Patom area) are briefly described, together with summaries of some of the geologically complex surrounding areas, and it is concluded that all of the Palaeozoic underlying the West Siberian Basin (including the Ob-Saisan Surgut area), Tomsk Terrane, Altai-Sayan Terranes (including Salair, Kuznetsk Alatau, Batenov, Kobdin and West Sayan), Ertix Terrane, Barguzin Terrane, Tuva-Mongol Terrane, Central Mongolia Terrane Assemblage, Gobi Altai and Mandalovoo Terranes, Okhotsk Terrane and much of the Verkhoyansk-Kolyma region all formed parts of peri-Siberia, and thus rotated with the main Siberian Craton as those areas were progressively accreted to the main Siberian Terrane at various times during the latest Neoproterozoic and Palaeozoic. The Ertix Terrane is a new term combining what has been termed the "Altay Terrane" or "NE Xinjiang" area of China, and the Baytag, Baaran and Bidz terranes of Mongolia. The Silurian Tuvaella brachiopod fauna is restricted only to today's southern parts of peri-Siberia. Thus, allowing for subsequent rotation, the fauna occurs only in the N of the Siberian Terrane, and, as well as being a helpful indicator of what marginal terranes made up peri-Siberia, is distinctive as being the only Silurian fauna known from northern higher latitudes globally. In contrast, the other terranes adjacent to peri

  6. Provenance analysis of the Mesozoic Hoh-Xil-Songpan-Ganzi turbidites in northern Tibet: Implications for the tectonic evolution of the eastern Paleo-Tethys Ocean

    NASA Astrophysics Data System (ADS)

    Ding, L.; Yang, D.; Cai, F. L.; Pullen, A.; Kapp, P.; Gehrels, G. E.; Zhang, L. Y.; Zhang, Q. H.; Lai, Q. Z.; Yue, Y. H.; Shi, R. D.

    2013-01-01

    Mesozoic strata of the Hoh-Xil-Songpan-Ganzi complex in northern Tibet are exposed in a vast (> 370,000 km2) triangle-shaped orogenic belt bound by the Longmen Shan thrust belt in the east, the Kunlun terrane and North China block in the north, and the Qiangtang terrane and Yidun arc in the south. These strata consist of Middle-Upper Triassic submarine fan and deep marine facies rocks that were deposited in the Paleo-Tethys Ocean. Late Triassic-Early Jurassic contractional deformation in the eastern Hoh-Xil-Songpan-Ganzi complex marks the demise of the Paleo-Tethys Ocean basin and the accretion of the Gondwana-derived Qiangtang terrane to Eurasia. We conducted geological mapping, regional stratigraphic analyses, and U-Pb geochronology of detrital zircons (n = 4128) on the Mesozoic sequences exposed in the Hoh-Xil-Songpan-Ganzi complex, Kunlun terrane, and Qiangtang terrane. We identify for the first time marine silciclastic sandstone and shale of Jurassic age in the northwestern Hoh-Xil-Songpan-Ganzi complex that unconformably overlie Upper Triassic turbidites. Zircon age data indicate that the Middle-Upper Triassic marine gravity-flow deposits of the Hoh-Xil-Songpan-Ganzi complex were shed from the North and South China blocks, and Middle-Late Triassic ultrahigh-pressure Qinling-Dabie orogenic belt, as well as the Kunlun and Qiangtang terranes. In addition, the detrital zircon results suggest vast sediment source to sink distances (>1500 km) for the Middle-Upper Triassic Hoh-Xil-Songpan-Ganzi strata, which is consistent with tectonic models for the Paleo-Tethys Ocean basin that incorporate significant components of horizontal tectonic transport like opening of large back-arc basins in response to oceanic slab rollback.

  7. Intra-oceanic subduction shaped the assembly of Cordilleran North America.

    PubMed

    Sigloch, Karin; Mihalynuk, Mitchell G

    2013-04-01

    The western quarter of North America consists of accreted terranes--crustal blocks added over the past 200 million years--but the reason for this is unclear. The widely accepted explanation posits that the oceanic Farallon plate acted as a conveyor belt, sweeping terranes into the continental margin while subducting under it. Here we show that this hypothesis, which fails to explain many terrane complexities, is also inconsistent with new tomographic images of lower-mantle slabs, and with their locations relative to plate reconstructions. We offer a reinterpretation of North American palaeogeography and test it quantitatively: collision events are clearly recorded by slab geometry, and can be time calibrated and reconciled with plate reconstructions and surface geology. The seas west of Cretaceous North America must have resembled today's western Pacific, strung with island arcs. All proto-Pacific plates initially subducted into almost stationary, intra-oceanic trenches, and accumulated below as massive vertical slab walls. Above the slabs, long-lived volcanic archipelagos and subduction complexes grew. Crustal accretion occurred when North America overrode the archipelagos, causing major episodes of Cordilleran mountain building. PMID:23552944

  8. Pre-Mesozoic terranes and the tectonic framework of the Gulf Coastal Plain

    SciTech Connect

    Thomas, W.A. . Dept. of Geological Sciences)

    1993-03-01

    Pre-Mesozoic rocks beneath the Gulf Coastal Plain reflect the late Precambrian (Pc)-Cambrian (Cb) rifted continental margin and the late Paleozoic Appalachian-Ouachita orogen (AOO). The AL promontory of Pc continental crust is bounded by a NW-striking transform margin (AL-OK transform) and a NE-striking rifted margin (southern Blue Ridge rift). Terrane accretion during the AOO differed markedly on the orthogonal adjacent sides of the AL promontory (ALp). Late Paleozoic compressional fabrics and terrane-boundary sutures, as well as extensional fabrics of the older rifted margin, influenced the geometry of Mesozoic extension and opening of the Gulf of Mexico. Along the SW side of the ALp, arc-continent collision resulted in accretion of an arc and subduction complex onto the margin of N American crust. The Ouachita allochthon includes off-shelf passive-margin rocks in an accretionary prism and synorogenic turbidites that represent a forearc basin and trench. Carbonate-shelf strata of the N American passive margin remained in place beneath the Ouachita allochthon. Along the southeast side of the ALp, passive-margin carbonate-shelf rocks are imbricated in the Appalachian thrust belt and bordered by an internal metamorphic belt of accreted terranes; both are underlain by relatively shallow Pc basement. The SE-dipping Suwannee-Wiggins suture terminates the shallow continental crust, truncates previously accreted terranes, and forms the boundary between N America and the Suwannee terrane. Mesozoic extensional structures include NE-and NW-striking fault systems. A NE-striking Triassic graben overlies the Suwannee-Wiggins suture, suggesting that Mesozoic extension used the Late Paleozoic compressional fabric of the suture. A NW-striking system of Triassic fault-bounded basins coincides with the trace of the Cb AL-OK transform fault, suggesting that the older crustal boundary controlled the location of a Mesozoic transform/transfer fault system.

  9. Insights into Oceanic Crust Accretion from a Comparison of Rock Magnetic and Silicate Fabrics from Lower Crustal Gabbros from Hess Deep Rift

    NASA Astrophysics Data System (ADS)

    Horst, A. J.; Morris, A.; Friedman, S. A.; Cheadle, M. J.

    2014-12-01

    The mechanisms of lower crustal accretion remain a long-standing question for those who study fast-spreading mid-ocean ridges. One of the goals of Integrated Ocean Drilling Program (IODP) Expedition 345 is to test accretionary models by investigating the structure of the lower oceanic crust exposed within the Hess Deep Rift. Located near the tip of the westward-propagating Cocos-Nazca spreading center, Hess Deep Rift exposes crust formed at the East Pacific Rise. During IODP Expedition 345, primitive gabbroic rocks were recovered from a dismembered lower crustal section at ~4850 meters below sealevel. Constraints on physical processes during magmatic accretion are provided by the relative orientation and strength of rock fabrics. We present anisotropy of magnetic susceptibility (AMS) fabric data from gabbros recovered from the two deepest holes (U1415J and U1415P). AMS measurements provide petrofabric data that may be used to constrain magma emplacement and subsequent magmatic flow. Bulk susceptibility ranges from 1.15 x 10-4 to 5.73 x 10-2 SI, with a majority of the samples having susceptibility greater than 10-3 SI, suggesting magnetite is the dominant contributor to the AMS signal. Low-temperature demagnetization data show Verwey transitions near 125K indicating the presence of nearly stoichiometric magnetite in most samples. AMS reveals dominantly oblate fabrics with a moderate degree of anisotropy (P') ranging from 1.01 to 1.38 (average P' = 1.13). Fabric strength varies within each of the petrologically-defined units recovered from different crustal blocks. Additional remanence anisotropy fabric analyses of a few specimens reveal nearly identical directions of principal axes compared to AMS, but with larger degrees of anisotropy. Electron backscatter diffraction (EBSD) data from one sample shows a moderate plagioclase crystallographic preferred orientation best defined by a b-axis maxima that is coincident with the AMS minimum principal axis. This comparison

  10. Middle Proterozoic age for the Montpelier Anorthosite, Goochland terrane, eastern Piedmont, Virginia

    USGS Publications Warehouse

    Aleinikoff, J.N.; Horton, J.W., Jr.; Walter, M.

    1996-01-01

    Uranium-lead dating of zircons from the Montpelier Anorthosite confirms previous interpretations, based on equivocal evidence, that the Goochland terrane in the eastern Piedmont of Virginia contains Grenvillian basement rocks of Middle Proterozoic age. A very few prismatic, elongate, euhedral zircons, which contain 12-29 ppm uranium, are interpreted to be igneous in origin. The vast majority of zircons are more equant, subangular to anhedral, contain 38-52 ppm uranium, and are interpreted to be metamorphic in origin. One fraction of elongate zircon, and four fragments of a very large zircon (occurring in a nelsonite segregation) yield an upper intercept age of 1045 ?? 10 Ma, interpreted as the time of anorthosite crystallization. Irregularly shaped metamorphic zircons are dated at 1011 ?? 2 Ma (weighted average of the 207Pb/206Pb ages). The U-Pb isotopic systematics of metamorphic titanite were reset during the Alleghanian orogeny at 297 ?? 5 Ma. These data provide a minimum age for gneisses of the Goochland terrane that are intruded by the anorthosite. Middle Proterozoic basement rocks of the Goochland terrane may be correlative with those in the Shenandoah massif of the Blue Ridge tectonic province, as suggested by similarities between the Montpelier Anorthosite and the Roseland anorthosite. Although the areal extent of Middle Proterozoic basement and basement-cover relations in the eastern Piedmont remain unresolved, results of this investigation indicate that the Goochland terrane is an internal massif of Laurentian crust rather than an exotic accreted terrane.

  11. Volcanic accretion, tectonic extension and the second-order segmentation of slow and ultraslow-spreading mid-ocean ridges

    NASA Astrophysics Data System (ADS)

    Cannat, M.; Sauter, D.; Escartin, J.

    2011-12-01

    In this presentation we compare the segmentation and seafloor geology record of slow and ultraslow ridges with variable volcanic input. The easternmost Southwest Indian Ridge (SWIR), where long stretches of the axis lack volcanism is our volcanism-poor end-member, which we contrast with volcanically more active parts of the Mid-Atlantic Ridge (MAR). Keeping the differences of spreading rates in perspective, we use this comparison to analyze and discuss the respective roles of tectonic extension, which ultimately leads to the exhumation of deeply-derived rocks (ultramafics and gabbros), and of volcanic accretion, in shaping the geometry of the plate boundary. Second-order segments at slow and ultraslow ridges are typically 30 to 100 km-long, and separated by transform, or so-called "non-transform" discontinuities. Segment centers typically have a thicker crust, and in most cases have a thinner axial lithosphere, than segment ends. Although we do not resolve the controversy of whether these characteristics are produced by discrete melt and/or mantle diapirs in the subaxial asthenosphere (eg Lin et al. 1990), or by melt channeling toward regions of thinner axial lithosphere (eg Magde and Sparks, 1997), we show that melt supply and volcanism are needed to initiate second-order ridge segmentation. Axial valley bounding faults in our SWIR volcanism-poor end-member go un-segmented for up to 170 km along-axis.

  12. Vestiges of The Peri-rodinian Ocean: Sliced, Diced, Recycled But Preserved In Younger Orogens

    NASA Astrophysics Data System (ADS)

    Murphy, J. B.; Nance, R. D.; Keppie, J. D.; Dostal, J.

    Just as the amalgamation and dispersal of Pangea exerted a first-order influence on tectonothermal events in the Phanerozoic, Middle to late Proterozoic global scale tec- tonics were profoundly influenced by the amalgamation and subsequent breakup of the supercontinent Rodinia. Most tectonic studies of the evolution of Rodinia concentrate on the near-field effects of these events such as the collisional orogenies that result in its amalgamation and the sedimentary rift-drift record of its breakup. However, ves- tiges of the far field effects of these events are also preserved and their tectonothermal evolution can provide additional constraints on supercontinent configuration and the timing of breakup. During the time interval of Rodinian amalgamation, for exam- ple, ensimatic subduction and plume activity in the peri-Rodinian ocean resulted in the formation of oceanic crust with ca. 1.0 Ga Sm-Nd depleted mantle model (TDM) ages. Vestiges of this crust are now preserved in the terranes that subsequently accreted along the margins of the dispersing continents, including the peri-Gondwanan terranes (e.g. Avalonia, Carolina, Iberia) of eastern North America and Europe, the Tocantins orogenic belt of Brazil, and the terranes of the Arabian-Nubian Shield. The Neopro- terozoic tectonothermal evolution of these terranes was a far-field response to Rodinia breakup, in the same way that the Mesozoic and Cenozoic evolution of western North America was a far-field response to the breakup of Pangea. Hence, the Neoproterozoic histories of these terranes are geodynamically linked to that of Rodinia and so provide constraints on its configuration and the timing of its major tectonothermal events.

  13. Along-axis variability in crustal accretion at the Mid-Atlantic Ridge: Results from the OCEAN study

    USGS Publications Warehouse

    Henstock, T.J.; White, Robert S.; McBride, J.H.

    1996-01-01

    The OCEAN experiment is an integrated geophysical study of a region of the Cape Verde abyssal plain that formed at 140 Ma. Deep seismic reflection and ocean bottom hydrophone (OBH) refraction data were acquired along lines parallel and perpendicular to the paleoridge axis trend identified from a detailed magnetic anomaly survey. The igneous basement is overlain by about 1.3 km of sediment which enables improved imaging of intracrustal structure beyond that possible near the Mid-Atlantic Ridge axis. We describe the results of a 150-km long profile oriented parallel to magnetic anomalies M15 and M16, along which deep seismic reflection data collected by the British Institutions Reflection Profiling Syndicate are complemented by refraction data constrained by four OBHs. The line spans an entire spreading segment between two fracture zones; the northern of which has an offset of 40 km and the other (central) has an offset of only 10 km. Away from the fracture zones, the mean igneous crustal thickness is 7.2 km; near both fracture zones, thinning of up to 4 km is observed, giving a mean igneous crustal thickness over the whole segment of approximately 6.5 km. Differences are seen between the two fracture zones in their seismic velocity structure, in the associated basement topography, and in the presence of a strong reflection extending into the mantle beneath the northern fracture zone. The boundary between oceanic layers 2 and 3 correlates with variably coherent normal incidence reflections and a change in the character of the reflectivity. A number of planar reflections up to 10 km in length are present within the middle and lower crust, dipping outward from beneath low-amplitude basement highs at ??? 15??; these appear to be present only within layer 3. The Moho has several expressions in the reflection data, including isolated reflection events, a local increase in reflected amplitudes, and a downward decrease in coherent reflections. At the center of the segment

  14. Features and geotectonic evolution of the Alxa Terrane at North Qilian Mountains in China

    NASA Astrophysics Data System (ADS)

    Wu, Xiaozhi; Zhengmin, Min

    2015-04-01

    The Alxa Terrane in west China, covered with Badain Jaran and Tengger Deserts at the earth's surface, lies geographically on the north of the Qilian-Mountains Structural Belt and the Qinghai-Tibet Plateau with intense tectonic activities. The Mongolian Plateau with Cenozoic activities and the Ordos Plateau are on the north and east of the terrane separately. Tectonically the terrane lies among the Central Asian Orogenic Belt, the Qilian-Qinling Orogenic Belt in the Tarim Plate, and the North China Craton. In view of its special geotectonic location, the knowledge about the Alxa Terrane generation and evolution would be significant to the understanding of plates and terranes convergence and evolution in west China and to hydrocarbon exploration in those small and medium basins in the Hexi Corridor. The conclusions include (1) the Alxa Terrane is a component part in the west of the North China Plate instead of a part separated from the Tarim Plate. Neoarchean rocks occurring in the Beidashan area in west Alxa are mainly composed of granodiorite gneiss with typical TTG gneiss features. The age of the magmatic zircon nucleus is about 2522±30 Ma, which is basically consistent with that of TFG gneiss pervasively distributing in the North China Craton; (2) the Alxa Terrane was a relatively isolated small terrane in the Archaean and Proterozoic Eras. The lithologies of the crystalline basement are different from those in the Tarim and North China Plates. Tectothermal events took place 800-1000 Ma and 400-600 Ma ago separately in Alxa, which had few signatures in the North China Plate. The North China Plate and the Alxa Terrane converged in the south and diverged in the north due to the impact of the Caledonian Movement and then merged at the early stage of the Middle Ordovician. During the Middle Hercynian Movement, the Paleoasian Ocean in the north closed and new crust appeared in the Early Permian to form the trench-arc-basin system at the north margin. During the Late

  15. Compositional Variability of the Mantle beneath West Antarctica and its Relationship to Terrane Tectonics: Evidence from Mantle Xenoliths

    NASA Astrophysics Data System (ADS)

    Ross, A. J.; Gibson, S. A.; Leat, P. T.; Vaughan, A. P. M.

    2009-04-01

    This work examines the petrography and mineral chemistry of sixteen previously undescribed mantle xenoliths suites from the West Antarctic Peninsula. The xenoliths are from the Jones Mountains (a Palaeozoic volcanic arc terrane on the margin of Gondwana), Adelaide Island (a Mesozoic volcanic arc terrane) and Alexander Island (an accretionary complex). They were entrained by subduction and rift-related magmatism, including 50 Ma calc-alkaline lamprophyres (Adelaide Island) and 10-5 Ma alkali basalts (Alexander Island and Jones Mountains). The xenoliths range in composition from pyroxenites (Adelaide Island) to spinel peridotites (Alexander Island and Jones Mountains). At Alexander Island, the spinel peridotites occur as both ‘fertile' lherzolites and ‘depleted' harzburgites. The xenoliths show a wide variation in mineral chemistry, for example olivine varies from Fo91.5 in the harzburgites to Fo71 in the pyroxenites. Significant variations have also been observed in the major-element chemistry of the pyroxenes. Those in the spinel peridotites are Cr-diopsides whereas those in the pyroxenites are Al-augites. Rare-earth element patterns of clinopyroxenes and Cr/(Cr+Al) ratios of spinels provide information on the origin of the xenoliths. We conclude that the harzburgites experienced a complex evolution involving extraction of up to 20% melt, perhaps in the mantle wedge, followed by accretion on to the base of the lithosphere and enrichment in Cr by large degree hydrous melts (boninites). There is also evidence of enrichment in strongly incompatible trace elements by carbonate melts and fluids from the subducted Phoenix plate. However, some of the spinel lherzolites from Alexander Island, and also those from the Jones Mountains, have compositions that are similar to fertile mantle and have not been subjected to large scale melting. The pyroxenites from Adelaide Island are believed to represent samples of veined lithospheric mantle caused by percolation and reaction

  16. Provenance of Permian-Triassic Gondwana Sequence Units Accreted to the Banda Arc: Constraints from U/Pb and Hf Analysis of Zircons and Igneous Geochemistry

    NASA Astrophysics Data System (ADS)

    Flores, J. A.; Spencer, C. J.; Harris, R. A.; Hoiland, C.

    2011-12-01

    Analysis of zircons from Australian affinity Permo-Triassic units of the Timor region yield age distributions with large peaks at 230-400 Ma and 1750-1900 Ma (n=435). Similar zircon age peaks are also found in rocks from NE Australia and the eastern Cimmerian block. It is likely that these terranes, which are now widely separated, were once part of the northern edge of Gondwana near what is now the NW margin of Australia. The Cimmerian Block was removed from Gondwana during Early Permian rifting and initiation of the Neo-Tethys Ocean. Hf analysis of zircon from the Aileu Complex in Timor and Kisar shows bimodal (juvenial and evolved) magmatism in the Gondwana Sequence of NW Australia at ~300 Ma. The magmatic event produced basalt with rift valley and ocean floor geochemical affinities, and rhyolite. Similar rock types and isotopic signatures are also found in Permo-Triassic igneous units throughout the Cimmerian continental block. The part of the Cimmerian Block with zircon distributions most like the Gondwana Sequence of NW Australia is the terranes of northern Tibet and Malaysia. The large 1750-1900 Ma zircon peak is much more wide spread, and appears in terranes from Baoshan (SW China) to Borneo. The Permo-Triassic rocks of the Timor region fill syn-rift intracratonic basins that successfully rifted in the Jurassic to form the NW margin of Australia. This passive continental margin first entered the Sunda Trench in the Timor region at around 8 Ma causing the Permo-Triassic rocks to accrete to the edge of the Asian Plate and emerge as a series of mountainous islands in the young collision zone. Eventually, the Australian continental margin will collide with the southern edge of the Asian plate and these Gondwana terranes will rejoin. However, it may be difficult to reconstruct the various ventures of they made over the past 300 Ma.

  17. Upper crustal seismic structure of the Endeavour segment, Juan de Fuca Ridge from traveltime tomography: Implications for oceanic crustal accretion

    NASA Astrophysics Data System (ADS)

    Weekly, Robert T.; Wilcock, William S. D.; Toomey, Douglas R.; Hooft, Emilie E. E.; Kim, Eunyoung

    2014-04-01

    isotropic and anisotropic P wave velocity structure of the upper oceanic crust on the Endeavour segment of the Juan de Fuca Ridge is studied using refracted traveltime data collected by an active-source, three-dimensional tomography experiment. The isotropic velocity structure is characterized by low crustal velocities in the overlapping spreading centers (OSCs) at the segment ends. These low velocities are indicative of pervasive tectonic fracturing and persist off axis, recording the history of ridge propagation. Near the segment center, velocities within the upper 1 km show ridge-parallel bands with low velocities on the outer flanks of topographic highs. These features are consistent with localized thickening of the volcanic extrusive layer from eruptions extending outside of the axial valley that flow down the fault-tilted blocks that form the abyssal hill topography. On-axis velocities are generally relatively high beneath the hydrothermal vent fields likely due to the infilling of porosity by mineral precipitation. Lower velocities are observed beneath the most vigorous vent fields in a seismically active region above the axial magma chamber and may reflect increased fracturing and higher temperatures. Seismic anisotropy is high on-axis but decreases substantially off axis over 5-10 km (0.2-0.4 Ma). This decrease coincides with an increase in seismic velocities resolved at depths ≥1 km and is attributed to the infilling of cracks by mineral precipitation associated with near-axis hydrothermal circulation. The orientation of the fast-axis of anisotropy is ridge-parallel near the segment center but curves near the segment ends reflecting the tectonic fabric within the OSCs.

  18. Neoproterozoic oceanic arc remnants in the Moroccan Anti-Atlas: reconstructing deep to shallow arc crustal sequence and tracking Pan-African subduction-accretion processes

    NASA Astrophysics Data System (ADS)

    Triantafyllou, Antoine; Berger, Julien; Baele, Jean-Marc; Bruguier, Olivier; Diot, Hervé; Ennih, Nasser; Plissart, Gaëlle; Monnier, Christophe; Spagna, Paul; Watlet, Arnaud; Vandycke, Sara

    2015-04-01

    established that they were recrystallized under garnet-granulites P-T conditions (up to ~1000°C at 12 kbar). Preliminary geochemical data of hornblende-gabbros and garnet-bearing granulites portray similar trace geochemical signatures ((La/Sm)N: 0.8-1.6 ; (Nb/La) < 0.46) as studied paleo-arc complexes. These P-T results and new geochemical data argue that Asmlil mafic complex could represent a deep arc root comparable to the deep section of exposed oceanic arcs (i.e. Kohistan, Talkeetna, Amalaoulaou). We propose that Iriri and Asmlil units depict the deep-to-shallow sequence of a single Cryogenian oceanic arc (760-740 Ma), as discrete exposures along the southern edge of Anti-Atlas ophiolitic assemblages. Nevertheless, this primary arc has been likely broke up and intruded by subsequent hydrous arc-related magmas under medium- to high-grade P-T conditions (700 to 650 Ma). We interpret this period as an oceanic pre-collision stage when subduction geometry is intensively perturbed (c.g. composite subductions, polarity inversion), doping production of typical hydrous arc magma that intrudes original arc. This complex arc melange has been lastly accreted and sealed on the West African Craton margin.

  19. Kilbuck terrane: Oldest known rocks in Alaska

    SciTech Connect

    Box, S.E. ); Moll-Stalcup, E.J.; Wooden, J.L. ); Bradshaw, J.Y. )

    1990-12-01

    The Kilbuck terrane in southwestern Alaska is a narrow, thin crustal sliver or flake of amphibolite facies orthogneiss. The igneous protolith of this gneiss was a suite of subduction-related plutonic rocks. U-Pb data on zircons from trondhjemitic and granitic samples yield upper-intercept (igneous) ages of 2,070 {plus minus}16 and 2,040 {plus minus}74 Ma, respectively. Nd isotope data from these rocks suggest that a diorite-tonalite-trondhjemite suite ({epsilon}{sub Nd}(T) = +2.1 to +2.7; T is time of crystallization) evolved from partial melts of depleted mantle with no discernible contamination by older crust, whereas a coeval granitic pluton ({epsilon}{sub Nd}(T) = {minus}5.7) contains a significant component derived from Archean crust. Orthogneisses with similar age and Nd isotope characteristics are found in the Idono complex 250 km to the north. Early Proterozoic rocks are unknown elsewhere in Alaska. However, Phanerozoic plutons cutting several continental terranes in Alaska (southern Brooks Range and Ruby, Seward, and Yukon-Tanana terranes) have Nd isotope compositions indicative of Early Proterozoic (or older) crustal components that could be correlative with rocks of the Kilbuck terrane. Rocks with similar igneous ages in cratonal North America are rare, and those few that are known have Nd isotope compositions distinct from those of the Kilbuck terrane. Conversely, provinces with Nd model ages of 2.0-2.1 Ga are characterized by extensive 1.8 Ga or younger plutonism, which is unknown in the Kilbuck terrane. At present the case for a North American parentage of the Kilbuck terrane is not compelling. The possibility that the Kilbuck terrane was displaced from provinces of similar age in other cratons (e.g., Australian, Baltic, Guiana, and west African shields), or from the poorly dated Siberian craton, cannot be excluded.

  20. A kinematic model for the formation of the Siletz-Crescent forearc terrane by capture of coherent fragments of the Farallon and Resurrection plates

    USGS Publications Warehouse

    McCrory, Patricia A.; Wilson, Douglas S.

    2013-01-01

    The volcanic basement of the Oregon and Washington Coast ranges has been proposed to represent a pair of tracks of the Yellowstone hotspot formed at a mid-ocean ridge during the early Cenozoic. This interpretation has been questioned on many grounds, especially that the range of ages does not match the offshore spreading rates and that the presence of continental coarse clastic sediments is difficult to reconcile with fast convergence rates between the oceanic plates and North America. Updates to basement geochronology and plate motion history reveal that these objections are much less serious than when they were first raised. Forward plate kinematic modeling reveals that predicted basement ages can be consistent with the observed range of about 55–49 Ma, and that the entire basement terrane can form within about 300 km of continental sources for clastic sediments. This kinematic model indicates that there is no firm reason to reject the near-ridge hotspot hypothesis on the basis of plate motions. A novel element of the model is the Resurrection plate, previously proposed to exist between the Farallon and Kula plates. By including the defunct Resurrection plate in our reconstruction, we are able to model the Farallon hotspot track as docking against the Oregon subduction margin starting about 53 Ma, followed by docking of the Resurrection track to the north starting about 48 Ma. Accretion of the Farallon plate fragment and partial subduction of the Resurrection fragment complicates the three-dimensional structure of the modern Cascadia forearc. We interpret the so-called “E” layer beneath Vancouver Island to be part of the Resurrection fragment. Our new kinematic model of mobile terranes within the Paleogene North American plate boundary allows reinterpretation of the three-dimensional structure of the Cascadia forearc and its relationship to ongoing seismotectonic processes.

  1. Structural analysis of the Carolina-Inner Piedmont terrane boundary: Implications for the age and kinematics of the central Piedmont suture, a terrane boundary that records Paleozoic Laurentia-Gondwana interactions

    NASA Astrophysics Data System (ADS)

    West, Thomas E.

    1998-06-01

    New field mapping along the Carolina-Inner Piedmont terrane boundary in South Carolina and eastern Georgia reveals preaccretionary, synaccretionary, and postaccretionary faults. The dextral strike-slip Lowndesville shear zone is adjacent to a ˜50-km-long segment of the terrane boundary. However, the Lowndesville shear zone is correlated eastward with the dextral strike-slip Beaver Creek shear zone, which is within the Carolina terrane and predates ˜415 Ma. The Lowndesville shear zone is overprinted by a dextral strike-slip phyllonite zone, named the Deal Creek shear zone. The Deal Creek shear zone is correlated eastward with the Gold Hill-Silver Hill shear zone which is also within the Carolina terrane and records dextral strike-slip motion between ˜400 and ˜325 Ma. The Cross Anchor and Mulberry Creek faults both truncate the Lowndesville and the Deal Creek shear zones and form the terrane boundary. The Mulberry Creek fault, probably of Triassic-Jurassic age, juxtaposes the Lowndesville shear zone adjacent to the Carolina-Inner Piedmont terrane boundary west of Waterloo, South Carolina. The Cross Anchor fault is the terrane boundary east of Waterloo, South Carolina, and forms the southeastern boundary of the Whitmire reentrant. Crosscutting relationships indicate that the Cross Anchor fault is the oldest fault which juxtaposes the Carolina and Inner Piedmont terranes in the study area. These structural interpretations and available geochronological data indicate that the Cross Anchor fault is a ˜325 Ma thrust fault and may be the central Piedmont suture. An early Alleghanian suture resolves the problem of inserting the Carolina terrane into the western Iapetus Ocean.

  2. Simultaneous batholith emplacement, terrane/continent collision, and oroclinal bending in the Blue Mountains Province, North American Cordillera

    NASA Astrophysics Data System (ADS)

    Žák, Jiří; Verner, Kryštof; Tomek, Filip; Holub, František V.; Johnson, Kenneth; Schwartz, Joshua J.

    2015-06-01

    The North American Cordillera is a classic example of accretionary orogen, consisting of multiple oceanic terranes attached to the western margin of Laurentia during the Mesozoic times. Although the Cordillera is linear for most parts, terrane boundaries are at a high angle to the overall structural grain in several segments of the orogen, which has been a matter of longstanding controversy as to how and when these orogenic curvatures formed. This paper discusses mechanisms, kinematics, and timing of initiation of one of these major curvatures, the Blue Mountains Province in northeastern Oregon. Here magmatic fabric patterns and anisotropy of magnetic susceptibility in the Wallowa batholith record three phases of progressive deformation of the host Wallowa terrane during Early Cretaceous. First is terrane-oblique ~NE-SW shortening, interpreted as recording attachment of the amalgamated oceanic and fringing terranes to the continental margin during dextral convergence at ~140 Ma. Deformation subsequently switched to pure shear-dominated ~NNE-SSW shortening associated with crustal thickening, caused by continued impingement of the amalgamated Blue Mountains superterrane into a presumed westward concave reentrant in the continental margin at ~135-128 Ma. Upon impingement (at ~126 Ma), the northern portion of the superterrane became "locked," leading to reorientation of the principal shortening direction to ~NNW-SSE while its still deformable southern portion rotated clockwise about a vertical axis. We thus propose oblique bending as the main mechanism of the orocline formation whereby horizontal compressive forces resulting from plate convergence acted at an angle to the terrane boundaries.

  3. Southeastern Alaska tectonostratigraphic terranes revisited

    SciTech Connect

    Brew, D.A.; Ford, A.B.

    1985-04-01

    The presence of only three major tectonostratigraphic terranes (TSTs) in southeastern Alaska and northwestern British Columbia (Chugach, Wrangell, and Alexander) is indicated by critical analysis of available age, stratigraphic, and structural data. A possible fourth TST (Stikine) is probably an equivalent of part or all of the Alexander. The Yakutat block belongs to the Chugach TST, and both are closely linked to the Wrangell and Alexander(-Stikine) TSTs; the Gravina TST is an overlap assemblage. THe Alexander(-Stikine) TSTs is subdivided on the basis of age and facies. The subterranes within it share common substrates and represent large-scale facies changes in a long-lived island-arc environment. The Taku TSTs is the metamorphic equivalent of the upper part (Permian and Upper Triassic) of the Alexander(-Stikine) TSTs with some fossil evidence preserved that indicates the age of protoliths. Similarly, the Tracy Arm TST is the metamorphic equivalent of (1) the lower (Ordovician to Carboniferous) Alexander TST without any such fossil evidence and (2) the upper (Permian to Triassic) Alexander(-Stikine) with some newly discovered fossil evidence. Evidence for the ages of juxtaposition of the TSTs is limited. The Chugach TST deformed against the Wrangell and Alexander TSTs in late Cretaceous. Gravina rocks were deformed at the time and also earlier. The Wrangell TST was stitched to the Alexander(-Stikine) by middle Cretaceous plutons but may have arrived before its Late Jurassic plutons were emplaced. The Alexander(-Stikine) and Cache Creek TSTs were juxtaposed before Late Triassic.

  4. Paleomagnetism and question of original location of the Permian Brook Street Terrane, New Zealand

    NASA Astrophysics Data System (ADS)

    Haston, Roger B.; Luyendyk, Bruce P.; Landis, C. A.; Coombs, D. S.

    1989-08-01

    Over 400 rock samples from 30 sites were collected for paleomagnetic study from the volcanogenic section in the Brook Street terrane within the Takitimu Mountains in western Southland, New Zealand. The section includes igneous and sedimentary rocks of the Permian Takitimu Group and White Hill Intrusive Suite. Many of the samples show a partial or complete remagnetization in the present field because of a recent acquisition of viscous remanent magnetization. An Early Permian direction (inclination = 46.1°, declination = 257°), isolated from the Heartbreak and Chimney Peaks formations of the Takitimu Group, indicates a low paleolatitude to midpaleolatitude, position (27° ± 5°) for the Brook Street terrane. Directions from the Late Permian (?) White Hill Intrusives (inclination = 64.6°, declination = 173.3°) suggest a slightly higher paleolatitude than the Early Permian Takitimu Group directions and 70°-90° of intervening clockwise rotation. Plate reconstructions and paleomagnetic data predict a high paleolatitude for the New Zealand margin of Gondwana throughout the late Paleozoic and Mesozoic. The low paleolatitude to middle paleolatitude, implied by the Early Permian Brook Street result, together with the oceanic nature of the Brook Street arc, suggest that the Brook Street terrane is allochthonous to the margin of Gondwana. A published Late Triassic/Early Jurassic paleomagnetic pole from the adjacent Murihiku terrane indicates a high paleolatitude. This suggests that the Brook Street and Murihiku terranes are genetically distinct.

  5. Australian Palaeozoic palaeomagnetism and tectonics—I. Tectonostratigraphic terrane constraints from the Tasman Fold Belt

    NASA Astrophysics Data System (ADS)

    Powell, C. McA.; Li, Z. X.; Thrupp, G. A.; Schmidt, P. W.

    The Tasman Fold Belt (TFB) of Eastern Australia can be divided into three meridional orogenic realms: the Kanmantoo, Lachlan-Thomson and New England Orogens. The geological histories of the orogens overlap, but each is distinctive. The Kanmantoo Orogen was provenance-linked to the Australian craton in the Early Cambrian, and accreted to Australia by Late Cambrian. There are many possible tectonostratigraphic terranes in the Lachlan Fold Belt (LFB) but these can be simplified to two major amalgamated terranes by the Middle Silurian. All the LFB terranes appear provenance-linked in the Ordovician, and were progressively covered, from the west, during the Late Silurian to Late Devonian, by a quartzose overlap assemblage. The New England Orogen has a fragmentary Early Palaeozoic history, but from the Devonian onwards its geology is related to a series of volcanic island and continental margin magmatic arcs. There is some evidence of provenance-linking between the Lachlan and New England Orogens in the Devono-Carboniferous but docking is not demonstrated until the mid-Carboniferous. The few reliable pre-Late Carboniferous palaeomagnetic poles available from the TFB come from the eastern LFB. The poles post-date accretion of the LFB to the Australian craton. Thus, the possibility that parts of the Lachlan-Thomson and New England Orogens contain exotic elements is yet to be tested palaeomagnetically.

  6. Native Terranes of the Central Klamath Mountains, California

    NASA Astrophysics Data System (ADS)

    Gray, Gary G.

    1986-12-01

    The Klamath Mountains of northern California and southern Oregon contain several good examples of terranes which have developed in situ. The term "native" is proposed for five of these terranes and all terranes whose development can be tied to an adjacent cratonal area. The Klamath terranes discussed herein include disrupted, stratigraphic, and metamorphic types. Three disrupted terranes, the Rattlesnake Creek, eastern Hayfork, and North Fork, contain fossiliferous blocks derived from both North American and exotic sources. The unique mixed faunal assemblage, stratigraphic ties to North America in the source terranes of the blocks, and paleomagnetic evidence indicate that the tectonic and sedimentary processes responsible for mixing these blocks occurred in proximity to North America, not distant from the terranes' present positions. Coeval blueschist metamorphism in a fourth, inboard terrane, the Stuart Fork, suggests that all four terranes developed during a Late Triassic to Early Jurassic subduction event. A fifth, stratigraphic terrane, the western Hayfork, was constructed upon the assembled disrupted and metamorphic terranes in the Middle Jurassic. Disrupted terranes with similar mixtures of North American and exotic faunas occur throughout the Cordillera from central California possibly as far north as British Columbia. Late Triassic deformation has been documented in several of these terranes, suggesting that (1) subduction operated along at least this portion of North America during the Late Triassic-Early Jurassic, and (2) many additional Cordilleran terranes should also be considered native.

  7. Use and abuse of crustal accretion calculations

    NASA Astrophysics Data System (ADS)

    Pallister, John S.; Cole, James C.; Stoeser, Douglas B.; Quick, James E.

    1990-01-01

    Recent attempts to calculate the average growth rate of continental crust for the Late Proterozoic shield of Arabia and Nubia are subject to large geological uncertainties, and widely contrasting conclusions result from dissimilar boundary conditions. The four greatest sources of divergence are (1) the extent of 620-920 Ma arc-terrane crust beneath Phanerozoic cover; (2) the extent of pre-920 Ma continental crust within the arc terranes; (3) the amount of postaccretion magmatic addition and erosion; and (4) the aggregate length and average life span of Late Proterozoic magmatic-arc systems that formed the Arabian-Nubian Shield. Calculations restricted to the relatively well known Arabian segment of the Arabian-Nubian Shield result in average crustal growth rates and arc accretion rates comparable to rates for modern arc systems, but we recognize substantial uncertainty in such results. Critical review of available geochemical, isotopic, and geochronological evidence contradicts the often stated notion that intact, pre-920 Ma crust is widespread in the eastern Arabian Shield. Instead, the arc terranes of the region apparently were "contaminated" with sediments derived, in part, from pre-920 Ma crust. Available geologic and radiometric data indicate that the Arabian-Nubian Shield and its "Pan-African" extensions constitute the greatest known volume of arc-accreted crust on Earth that formed in the period 920-620 Ma. Thus, the region may truly represent a disproportionate share of Earth's crustal growth budget for this time period.

  8. The Grenvillian Namaqua-Natal fold belt adjacent to the Kaapvaal Craton: 1. Distribution of Mesoproterozoic collisional terranes deduced from results of regional surveys and selected profiles in the western and southern parts of the fold belt

    NASA Astrophysics Data System (ADS)

    Colliston, W. P.; Schoch, A. E.; Cole, J.

    2014-12-01

    Sixteen tectonic terranes of the Namaqua-Natal metamorphic complex are distinguished (the Aggeneys, Agulhas, Bladgrond, Gamka, Grootdrink, Grünau, Fraserburg, Upington, Margate, Mossel, Mzumbe, Okiep, Olifantshoek, Steinkopf, Pofadder, and Tugela terranes). Evidence obtained from field investigations in the outcrop regions of Namaqualand and Natal are correlated with the geophysical data, enabling recognition of terrane suboutcrops in the regions covered by Phanerozoic deposits in the south. This is illustrated by nine selected profiles over the western and southern parts of the metamorphic complex. Four terranes that have not been observed in outcrop are postulated (Agulhas, Fraserburg, Gamka and Mossel terranes) and may represent extensions of some of the Natal terranes (Mzumbe and Margate terranes). The depth to Moho is generally about 40 km, diminishing dramatically at the present continental edge to as little as 15 km. Listric thrust ramps may originate on rises of the gently undulating topography of the Moho zone. Zones of thrusting and later shearing often exploit older structures and fabrics. The terranes that participated in the Grenvillian Namaqua-Natal Orogeny exhibit a dichotomy of vergences regionally. Those that moved to the northeast and north accreted onto the Archaean Kaapvaal Craton before becoming part of the Kalahari Craton. Terranes with vergences to the south and southwest were amalgamated onto other Archaean cratons. All of the composite cratons took part in the assembly of Rodinia.

  9. Tectonostratigraphic reconstruction Cretaceous volcano-sedimentary in the northwestern Andes: from extensional tectonics to arc accretion.

    NASA Astrophysics Data System (ADS)

    Zapata, S.; Patino, A. M.; Cardona, A.; Mejia, D.; Leon, S.; Jaramillo, J. S.; Valencia, V.; Parra, M.; Hincapie, S.

    2014-12-01

    Active continental margins characterized by continuous convergence experienced overimposed tectonic configurations that allowed the formation of volcanic arcs, back arc basins, transtensional divergent tectonics or the accretion of exotic volcanic terranes. Such record, particularly the extensional phases, can be partially destroyed and obscure by multiple deformational events, the accretion of exotic terranes and strike slip fragmentation along the margin. The tectonic evolution of the northern Andes during the Mesozoic is the result of post Pangea extension followed by the installation of a long-lived Jurassic volcanic arc (209 - 136 ma) that apparently stops between 136 Ma and 110 Ma. The Quebradagrande Complex has been define as a single Lower Cretaceous volcano-sedimentary unit exposed in the western flank of the Central Cordillera of the Colombian Andes that growth after the Late Jurassic to Early Cretaceous magmatic hiatus. The origin of this unit have been related either to an oceanic volcanic arc or a marginal basin environment. The existence of such contrasting models reflect the regional perspective followed in published studies and the paucity of detail analysis of the volcano-sedimentary sequences.We integrate multiple approaches including structural mapping, stratigraphy, geochemistry, U-Pb provenance and geochronology to improve the understanding of this unit and track the earlier phases of accumulation that are mask on the overimposed tectonic history. Our preliminary results suggest the existence of different volcano-sedimentary units that accumulated between 100 Ma and 82 Ma.The older Lower Cretaceous sequences was deposited over Triassic metamorphic continental crust and include a upward basin deepening record characterized by thick fan delta conglomerates, followed by distal turbidites and a syn-sedimentary volcanic record at 100 ma. The other sequence include a 85 - 82 Ma fringing arc that was also formed close to the continental margin or

  10. Architecture of Off-Axis Magma Bodies at EPR 9o37-40'N and Implications for Oceanic Crustal Accretion (Invited)

    NASA Astrophysics Data System (ADS)

    Han, S.; Carbotte, S. M.; Carton, H. D.; Mutter, J. C.; Aghaei, O.; Nedimovic, M. R.; Canales, J.

    2013-12-01

    Oceanic crust is formed by decompression melting of upwelling mantle beneath mid-ocean ridges. At fast spreading ridges, although the mantle melting region is several hundred kilometers wide, crustal accretion is believed to be concentrated in a narrow zone a few kilometers wide centered beneath the ridge axis. However, mid-ocean ridge studies over the past two decades have provided increasing evidence that melt focusing may not occur entirely within this narrow zone. Here, we present 3D multichannel seismic (MCS) images from the East Pacific Rise 9o37-40'N extending to 11 km on the ridge flanks. In the axial region, we observe two axial magma bodies underlying the seafloor discontinuity at ~9°37'N at a depth of 1.5-1.6 km, with an overlapping geometry similar to that of the seafloor structures. On the ridge flanks, a series of off-axis melt lenses (OAML) are imaged, located from 2 -10 km from ridge axis, at 700 to 1520 ms twtt below seafloor (bsf) (~1.6 to 4.5 km), and with various sizes from 0.46 km2 to 5.15 km2. The largest body is centered 3.9 km east of the ridge axis and is composed of a series of small discontinuous upward dipping bodies at the western edge of a larger, continuous flat-topped lens. The flat-topped crest of the OAML lies at approximately the same depth beneath layer 2A as the axial magma lens, from which we infer that this OAML has formed by aggregation of smaller melt bodies ascending along the western edge of the main body that accumulate at the base of the sheeted dike section. A cluster of reflectors underlies the OAML at 1260-1510 ms bsf that may be deeper lenses feeding melts to the upper lens. Moho traveltime anomalies associated with this OAML suggest a lower crust that is partially molten with velocities reduced by 8-18% and/or thicker than normal by up to 1 km. The data indicate that melt delivery pathways to the OAML are independent of the axial system. Local volcanic edifices are found above two of the three OAMLs in our study

  11. Isotopic age constraints on provenance of exotic terranes, latest Permian collision and fast Late Triassic post-collisional cooling and tectonic exhumation of the Korean collision belt

    NASA Astrophysics Data System (ADS)

    de Jong, Koenraad; Han, Seokyoung; Ruffet, Gilles; Yi, Keewook

    2016-04-01

    The Korean peninsula is located in the eastern margin of the Eurasian continent where major late Palaeozoic to early Mesozoic continental collision zones, like the Central Asian Orogenic Belt and the Central China Orogen, merge with circum-Pacific subduction-accretion systems. We present an integrated view of the Korean collision belt using recent Ar/Ar laser-probe step-heating single grain ages from the uppermost Gyeonggi Massif, central Korea's Palaeoproterozoic high-grade granite-gneiss terrane affected by Permo-Triassic metamorphism, the bordering Hongseong zone and the overlying Imjingang belt and the correlative Taean Formation, as well as SHRIMP isotopic ages of detrital zircons from meta-sandstones from the latter metamorphic marine turbidite sequences. We show that early Paleozoic isolated exotic terranes form part of the collision belt and were reworked in Permo-Triassic time. Age spectra of zircons from mature meta-sandstones in the Misan Formation (Imjingang Belt) and Taean Formation do not match the age distribution of the Gyeonggi Massif, to which both are usually assigned, as they show only subordinate 1.9-1.8 Ga and ~2.5 Ga age modes but dominant 441-426 Ma and 978-919 Ma peaks. Much of the sediment appears to have been derived from distant, exotic middle Paleozoic and Early Neoproterozoic magmatic sources, not present in Gyeonggi or other Korean basement massifs. The youngest concordant zircon ages are: 394, 398 and 402 Ma, showing that both formations are at least of Early Devonian age. Terranes with a substratum with Early Neoproterozoic and Silurian-Devonian granitoids are present in the South Chinese Cathaysia Terrane and in the Qinling Terrane (Central China Orogen). Both formations may, hence, represent the submarine fan part of a routing system and a delta-shelf system originally situated in China. The Taean Formation and Imjingang Belt are thus exotic Paleozoic terranes tectonically emplaced in the Korean collision belt. Muscovite, biotite

  12. Fragments of the Vendian-Paleozoic oceanic crust of the Paleo-Asian Ocean in foldbelts (Altai-Sayan, Central Asia)

    NASA Astrophysics Data System (ADS)

    Safonova, I.; Buslov, M.

    2003-04-01

    Four main accretion-collision stages of the evolution of the Paleo-Asian ocean have been recognized in Altai-Sayan: 1) Early-Middle Cambrian, 2) Late Cambrian-Early Ordovician, 3) Devonian-Early Carboniferous and 4) Late Paleozoic. The 1st and 2nd stages characterize evolution of the Kuznetsk-Altai island-arc system, which was accreted to the Siberian continent. The third stage refers to two collisional events during the closing of the Paleo-Asian Ocean: the Gondwana-derived Altai-Mongolian terrane collided with the Siberian continent (D3), and the latter collided with the Kazakhstan continent (C2). Each stage was recorded in folded rock units. Paleooceanic crust fragments occur within accretionary wedges and suture zone. Their geological identification was supported by geochemical data. The better studied areas are Kurai and Katun accretionary wedges, Charysh-Terekta shear zone, and Chara ophiolitic suture zone. Identification of the Vendian-Early Carboniferous oceanic crust extended our knowledge about the Paleo-Asian Ocean evolution. The Kurai and Katun accretionary wedges recorded the Vendian-Cambrian stage of the Kuznetsk-Altai island arc evolution. The Charysh-Terekta zone resulted from the Late Devonian collision of the Altai-Mongolian terrane and the Siberian continent. The Chara ophiolitic suture was formed after the Late Carboniferous-Permian collision of the Siberian and Kazakhstan continents. The Kurai accretionary wedge is composed of the tectonic sheets of the Baratal paleoisland and Chagan-Uzun ophiolites. The Katun accretionary wedge involves paleo-oceanic island rock units: limestones, dolomites, siliceous shales, and basaltic flows. The Charysh-Terekta zone is composed of several tectonic lenses (e.g. Zasurin Formation) comprising sandstones, cherts, pillow-basalts, volcanoclastics, and gabbro-diabase sills and dikes. The Chara ophiolitic belt consists of several melange zones with high-pressure metamorphic rocks (metabasaltic rocks) metamorphosed

  13. Origin of Siletzia, an Accreted Large Igneous Province in the Cascadia Forearc, and the Early History of the Yellowstone Hotspot

    NASA Astrophysics Data System (ADS)

    Wells, R. E.; Bukry, D.; Friedman, R. M.; Pyle, D. G.; Duncan, R. A.; Haeussler, P. J.; Wooden, J.

    2014-12-01

    Siletzia as named by Irving (1979) is a Paleogene large igneous province forming the oceanic basalt basement of coastal OR, WA and S. BC that was accreted to North America in the early Eocene. U-Pb (magmatic, detrital zircon) and 40Ar/39Ar ages constrained by mapping, global coccolith (CP) zones, and magnetic polarities permit correlation of basalts with the geomagnetic polarity time scale of Gradstein et al. (2012). Siletzia was rapidly erupted 56-49 Ma (Chron 25-22), and accretion was completed between 51 and 49 Ma in Oregon. Magmatism continued until ca. 46 Ma with emplacement of a basalt sill complex during or shortly after accretion. Siletzia's great crustal thickness, rapid eruption, and timing of accretion are consistent with formation as an oceanic plateau. Eight m.y. after accretion, margin-parallel extension and regional dike swarms mark the Tillamook magmatic episode in the forearc (41.6 Ma; CP zone 14a; Chron 19r). We examined the origin of Siletzia and the possible role of a long-lived Yellowstone hotspot (YHS) in an open source plate modeling program. In most reference frames, the YHS is on or near an inferred northeast-striking Kula- Farallon and/or Resurrection-Farallon ridge 60 to 50 Ma. The YHS thus could have provided a 56-49 Ma source on the Farallon plate for Siletzia, which accreted to North America by 50 Ma. A sister plateau, the Eocene basalt basement of the Yakutat terrane, now in Alaska, formed on the adjacent Kula (or Resurrection) plate and accreted to coastal British Columbia at about the same time. Following accretion of Siletzia, the leading edge of North America overrode the YHS ca. 42 Ma. The encounter with an active YHS may explain the voluminous high-Ti tholeiitic to alkalic magmatism of the 42-34 Ma Tillamook episode and extension in the forearc. Clockwise rotation of western Oregon about a pole in the backarc has since moved the Tillamook center and underlying Siletzia northward ~250 km from the probable hotspot track on North

  14. To accrete or not accrete, that is the question

    USGS Publications Warehouse

    von, Huene R.

    1986-01-01

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

  15. Tectonically reset Rb-Sr system during Late Ordovician terrane assembly in lapetus, western Ireland

    SciTech Connect

    Kennan, P.S.; Murphy, F.C.

    1987-12-01

    The uncertainty of a ca. 460 Ma age of mylonitization of acid igneous rocks in the western Irish Caledonides required reevaluation of the published Rb-Sr whole-rock data. The authors found that the data support an alternative ca. 426 +/- 10 Ma age of mylonitic resetting. This time of deformation relates to the assembly of suspect terranes during Late Ordovician closure of the Iapetus ocean.

  16. Late Paleozoic orogeny in Alaska's Farewell terrane

    USGS Publications Warehouse

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

    2003-01-01

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

  17. The Record of Collision and Accretion in the History of a Convergent Margin

    NASA Astrophysics Data System (ADS)

    Moresi, L. N.; Betts, P. G.; Miller, M. S.; Cayley, R. A.

    2014-12-01

    Convergent margins become congested when they try to swallow buoyant, exotic crust or an oceanic swell associated with anomalously buoyant plume material. Mountain belts (orogens) that form at these convergent plate margins are the sites of significant lateral continental growth. Modern examples of accretionary margins are the North American Cordillera and southwest Pacific. The geologic record is riddled with accretionary orogens, such as the Tasmanides along the eastern margin of the supercontinent Gondwana and the Altaides that formed on the southern margin of Laurasia. In modern and ancient examples of long lived accretionary orogens, the overriding plate is subjected to episodes of crustal extension and back arc basin development, often related to subduction roll back and transient episodes of orogenesis and crustal shortening, coincident with accretion of exotic crust. In previous work, (Mason et al, 2010), we found that buoyant material ingested by a subduction zone produces a relative advance of the local region of the trench (either reduced rollback or absolute advance) naturally leading to the characteristic indentation of the plate boundary by the plateau. Depending on the strength and buoyancy of the incoming anomaly relative to the oceanic lithosphere, it may be subducted or it may be accreted with the associated formation of a slab window. Extending this model to ocean-continent convergent zones (Moresi et al, 2014), we show how the indentation of buoyant exotic material also dominates terrane accretion. When large blocks of material congest a subduction zone, the subduction zone needs to undergo signficiant re-arrangement for convergence to continue. We have modelled this process and observe characteristic patterns in the deformation of the over-riding plate, in the timing of the escape of material from behind the indenter, and in the oroclinal geometry that remains once the collision has completed. References Mason, W. G., Moresi, L., Betts, P. G

  18. Architecture of on- and off-axis magma bodies at EPR 9°37-40‧N and implications for oceanic crustal accretion

    NASA Astrophysics Data System (ADS)

    Han, Shuoshuo; Carbotte, Suzanne M.; Carton, Hélène; Mutter, John C.; Aghaei, Omid; Nedimović, Mladen R.; Canales, J. Pablo

    2014-03-01

    Crustal accretion at fast-spreading mid-ocean ridges is believed to be concentrated in a narrow zone up to a few kilometers wide centered beneath the ridge axis. However, there is increasing evidence for off-axis magmatism occurring beyond this narrow zone. Here, we present 3D multichannel seismic (MCS) images from the East Pacific Rise 9°37-40‧N extending to 11 km on the ridge flanks. In the axial region, two offset axial magma bodies underlie a small ridge-axis discontinuity at ∼9°37‧N, displaying an overlapping geometry similar to that of the seafloor structures above. On the ridge flanks, a series of off-axis magma lenses (OAML) are imaged: they are located 2-10 km from the ridge axis, at 700 to 1520 ms two-way travel time below seafloor (bsf) (∼1.6 to 4.5 km bsf), with variable areas ranging from 0.5 km2 to 5.2 km2. The largest body is centered 4 km east of the ridge axis and is composed of a large, continuous, flat-topped lens and a series of small, discontinuous, westward-dipping bodies along its western edge. The flat crest of the OAML lies at approximately the same depth beneath layer 2A as the axial magma lens and we infer that this OAML has formed by aggregation of ascending melts that accumulate at the base of the sheeted dike section. A cluster of reflections underlying the OAML at 1260-1510 ms bsf are observed that may be deeper lenses feeding melts to the upper lens. This largest OAML is associated with Moho travel time anomalies of 120-260 ms within a zone that extends up to 2 km from the edge of the OAML, suggesting a lower crust that is partially molten with lower crustal velocities reduced by 8-18% and/or thicker than normal by up to 1 km. Local volcanic edifices are found above two of the three OAMLs imaged in our study area and are inferred to be the eruptive products of the OAMLs. From the volume of these edifices and the Moho travel time anomalies we estimate the potential contribution of off-axis magmatism to the total volume of

  19. Lower precambrian of the Keivy Terrane, Northeastern Baltic Shield: A stratigraphic succession or a collage of tectonic sheets?

    NASA Astrophysics Data System (ADS)

    Balagansky, V. V.; Raevsky, A. B.; Mudruk, S. V.

    2011-03-01

    spatiotemporal succession that resulted in the formation of a Paleoproterozoic supercontinent and the Baltic Shield as its fragment. This succession began with the amalgamation and deformation of the Archean terranes in the northeast of the Baltic Shield during the Lapland-Kola Orogeny, the Keivy Terrane showing a record of the earliest reworking (1.97-1.93 Ga). The succession completed in the southern and southwestern parts of the shield (1.80 Ga) after the Svecofennian Orogeny, expressed in the accretion of island-arc terranes composed of Paleoproterozoic juvenile crust to the continent.

  20. The age and tectonic setting of the Puncoviscana Formation in northwestern Argentina: An accretionary complex related to Early Cambrian closure of the Puncoviscana Ocean and accretion of the Arequipa-Antofalla block

    NASA Astrophysics Data System (ADS)

    Escayola, Mónica P.; van Staal, Cees R.; Davis, William J.

    2011-12-01

    TIMS and SHRIMP U-Pb zircon geochronology of selected parts of the Puncoviscana Formation suggest its deposition took place mainly during the Early Cambrian, coeval with 540-535 Ma calc-alkaline Pampean arc volcanism mainly preserved as tuff beds in the oldest identified parts of this unit. Syn- to post-tectonic plutons constrains the Tilcarian-Pampean orogeny to have occurred between ca. 530 Ma and deposition of the unconformably overlying Middle-Upper Cambrian Meson Group. Deposition of the Puncoviscana Formation continued after the onset of the Tilcarian-Pampean orogeny. We propose that the Puncoviscana Formation rocks older than 530 Ma were deposited in the arc-trench gap of the west-facing Pampean arc and/or the associated trench, whereas the rocks younger than 530 Ma were deposited in a syn-collision foreland basin. The Puncoviscana Formation rocks were progressively assembled into a west-younging accretionary complex, consistent with the style of deformation and low-grade metamorphism. The age of the syn-collision plutons (≤530 Ma) suggest the foredeep deposits record the transition from trench to foreland basin, due to arrival of the Arequipa-Antofalla block at the west-facing trench at ca. 530 Ma. Our geochronological and Pb-isotope investigations suggest that the Arequipa-Antofalla terrane was a coherent, ribbon-shaped crustal block that also included the western part of the Pampia terrane. A compilation of existing U-Pb zircon studies suggests that the Pampean arc extended along the length of the proto-Andean margin of West Gondwana, represented by the previously amalgamated Amazonia and Rio de La Plata cratons, and probably was initiated during the late Ediacaran after 600 Ma. Following earlier workers, we reaffirm that the Arequipa-Antofalla block was originally separating Laurentia and Amazonia in Rodinia. It probably rifted from Laurentia during the Ediacaran between 600 and 570 Ma, following an earlier departure of Amazonia (˜650 Ma?). The

  1. A mid-Permian chert event: widespread deposition of biogenic siliceous sediments in coastal, island arc and oceanic basins

    USGS Publications Warehouse

    Murchey, B.L.; Jones, D.L.

    1992-01-01

    Radiolarian and conodont of Permian siliceous rocks from twenty-three areas in teh the circum-Pacific and Mediterranean regions reveal a widespread Permian Chert Event during the middle Leonardian to Wordian. Radiolarian- and (or) sponge spicule-rich siliceous sediments accumulated beneath high productivity zones in coastal, island arc and oceanic basins. Most of these deposits now crop out in fault-bounded accreted terranes. Biogenic siliceous sediments did not accumulate in terranes lying beneath infertile waters including the marine sequences in terranes of northern and central Alaska. The Permian Chert Event is coeval with major phosphorite deposition along the western margin of Pangea (Phosphoria Formation and related deposits). A well-known analogue for this event is middle Miocene deposition of biogenic siliceous sediments beneath high productivity zones in many parts of the Pacific and concurrent deposition of phosphatic as well as siliceous sediments in basins along the coast of California. Interrelated factors associated with both the Miocene and Permian depositional events include plate reorientations, small sea-level rises and cool polar waters. ?? 1992.

  2. Late Carboniferous collision between the Tarim and Kazakhstan-Yili terranes in the western segment of the South Tian Shan Orogen, Central Asia, and implications for the Northern Xinjiang, western China

    NASA Astrophysics Data System (ADS)

    Han, Bao-Fu; He, Guo-Qi; Wang, Xue-Chao; Guo, Zhao-Jie

    2011-12-01

    The Tian Shan of Central Asia is located in the southwestern part of the Central Asian Orogenic Belt (CAOB, also known as the Central Asian Orogenic System or CAOS). Formation of the South Tian Shan Orogen is a diachronous, scissors-like process during the Paleozoic and its western segment in China-Kyrgyzstan contiguous regions is accepted as the site of the final collision zone between the Tarim craton to the south and the Kazakhstan-Yili terrane to the north in the Late Paleozoic. However, when the final collision occurred is still in hot debate. Particularly, an end-Permian to Triassic collisional model is recently proposed for the western segment of the South Tian Shan Orogen. This even leads to the speculation that the complicated accretion-collision processes in the Northern Xinjiang of western China, which involved the terrane amalgamation in the East and West Junggar and the collision between the Altai and Kazakhstan terranes and between the Yili-Central Tian Shan and Junggar terranes, were finally terminated during the end-Permian to mid-Triassic, rather than the Late Paleozoic as usually accepted. Obviously, the western segment of the South Tian Shan Orogen also presents the key issue associated with the termination time of accretion-collision processes in the Northern Xinjiang. A collisional model that is derived from the knowledge of the Himalayan Orogen is helpful for establishing a sequence of major tectonothermal events in the western segment of the South Tian Shan Orogen and constraining the time of collision between the Tarim craton and the Kazakhstan-Yili terrane. For the western segment of the South Tian Shan Orogen, the end-Permian to Triassic collisional model is mainly based on Triassic zircon U-Pb ages of 234 to 226 Ma from the West Tian Shan eclogite and two suspected Late Permian radiolarian specimens Albaillella excelsa Ishiga, Kito and Imoto (?) from the Baleigong ophiolitic mélange. Actually, the poor preservation of the two radiolarian

  3. Geology and metallogeny of the Ar Rayn terrane, eastern Arabian shield: Evolution of a Neoproterozoic continental-margin arc during assembly of Gondwana within the East African orogen

    USGS Publications Warehouse

    Doebrich, J.L.; Al-Jehani, A. M.; Siddiqui, A.A.; Hayes, T.S.; Wooden, J.L.; Johnson, P.R.

    2007-01-01

    characteristics of the Ar Rayn terrane are analogous to the Andean continental margin of Chile, with opposite subduction polarity. The Ar Rayn terrane represents a continental margin arc that lay above a west-dipping subduction zone along a continental block represented by the Afif composite terrane. The concentration of epithermal, porphyry Cu and IOCG mineral systems, of central arc affiliation, along the AAF suggests that the AAF is not an ophiolitic suture zone, but originated as a major intra-arc fault that localized magmatism and mineralization. West-directed oblique subduction and ultimate collision with a land mass from the east (East Gondwana?) resulted in major transcurrent displacement along the AAF, bringing the eastern part of the arc terrane to its present exposed position, juxtaposed across the AAF against a back-arc basin assemblage represented by the Abt schist of the Ad Dawadimi terrane. Our findings indicate that arc formation and accretionary processes in the Arabian shield were still ongoing into the latest Neoproterozoic (Ediacaran), to about 620-600 Ma, and lead us to conclude that evolution of the Ar Rayn terrane (arc formation, accretion, syn- to postorogenic plutonism) defines a final stage of assembly of the Gondwana supercontinent along the northeastern margin of the East African orogen. ?? 2007 Elsevier B.V. All rights reserved.

  4. Resolving Variations in the Tectonostratigraphic Terrane Structure of New England using Receiver Functions

    NASA Astrophysics Data System (ADS)

    Schuh, John

    Passive teleseismic data were collected with a 17-station broadband seismic array deployed from Vermont to Massachusetts. The purpose of the array was to detect changes in crustal seismic velocity structure related to the regional tectonostratigraphic terranes using receiver functions. Ps conversions from the Moho and mid-crust were observed and a cross-section of the crustal structure beneath the seismic array was produced. The crustal cross-section reveals a synclinal structure related to the Taconic orogeny, a remnant Iapetan oceanic slab, a plausible surface-location of the Red Indian Line, and several terrane boundaries that can be projected from their proposed surface locations into the deeper crust based on crustal-horizon offsets observed in the receiver function data.

  5. Thermal structure of the accreting earth

    NASA Technical Reports Server (NTRS)

    Turcotte, D. L.; Pflugrath, J. C.

    1985-01-01

    The energy associated with the accretion of the earth and the segregation of the core is more than sufficient to melt the entire earth. In order to understand the thermal evolution of the early earth it is necessary to study the relevant heat transfer mechanisms. In this paper the existence of a global magma ocean is postulated and calculations are carried out of the heat flux through it in order to determine its depth. In the solid mantle heat is transferred by the upward migration of magma. This magma supplies the magma ocean. The increase in the mantle liquidus with depth (pressure) is the dominant effect influencing heat transfer through the magma ocean. It is found that a magma ocean with a depth of the order of 20 km would have existed as the earth accreted. It is concluded that the core segregated and an atmosphere was formed during accretion.

  6. Thermal structure of the accreting earth

    SciTech Connect

    Turcotte, D.L.; Pflugrath, J.C.

    1985-02-15

    The energy associated with the accretion of the earth and the segregation of the core is more than sufficient to melt the entire earth. In order to understand the thermal evolution of the early earth it is necessary to study the relevant heat transfer mechanisms. In this paper we postulate the existence of a global magma ocean and carry out calculations of the heat flux through it in order to determine its depth. In the solid mantle heat is transferred by the upward migration of magma. This magma supplies the magma ocean. The increase in the mantle liquidus with depth (pressure) is the dominant effect influencing heat transfer through the magma ocean. We find that a magma ocean with a depth of the order of 20 km would have existed as the earth accreted. We conclude that the core segregated and an atmosphere was formed during accretion.

  7. Paleomagnetic contributions to the Klamath Mountains terrane puzzle-a new piece from the Ironside Mountain batholith, northern California

    USGS Publications Warehouse

    Mankinen, Edward A.; Gromme, C. Sherman; Irwin, W. Porter

    2013-01-01

    We obtained paleomagnetic samples from six sites within the Middle Jurassic Ironside Mountain batholith (~170 Ma), which constitutes the structurally lowest part of the Western Hayfork terrane, in the Klamath Mountains province of northern California and southern Oregon. Structural attitudes measured in the coeval Hayfork Bally Meta-andesite were used to correct paleomagnetic data from the batholith. Comparing the corrected paleomagnetic pole with a 170-Ma reference pole for North America indicates 73.5° ± 10.6° of clockwise rotation relative to the craton. Nearly one-half of this rotation may have occurred before the terrane accreted to the composite Klamath province at ~168 Ma. No latitudinal displacement of the batholith was detected.

  8. Geochemistry of accreted metavolcanic rocks from the Neoproterozoic Gwna Group of Anglesey-Lleyn, NW Wales, U.K.: MORB and OIB in the Iapetus Ocean

    NASA Astrophysics Data System (ADS)

    Saito, Takuya; Uno, Masaoki; Sato, Tomohiko; Fujisaki, Wataru; Haraguchi, Satoru; Li, Yi-bing; Sawaki, Yusuke; Yamamoto, Shinji; Maruyama, Shigenori

    2015-11-01

    The Gwna Group in Anglesey-Lleyn, NW Wales, UK, is a Neoproterozoic accretionary complex that consists of basalt, bedded chert, red claystone, and trench turbidite that have been intercalated in coherent and incoherent mélanges that are considered typical Ocean plate stratigraphy (OPS). The sediments in the OPS can be useful for constraining the geological environment in the Iapetus Ocean. Most basalts in this area have undergone hydrothermal alteration, greenschist facies regional metamorphism, and surface oxidation. This indicates that immobile elements such as Al2O3 and TiO2, Rare Earth Elements (REE) and High Field Strength Elements (HSFE) are appropriate for discriminating the origin of the basalts in the Gwna Group. Most basalts showing light REE-enriched pattern in CI chondrite-normalized spider diagrams in within-plate basalt (WPB) fields, and some have flat patterns in spider diagrams in mid-oceanic ridge basalt (MORB) fields. In view of these relations, we conclude that the former erupted in an oceanic island. Oceanic island basalts (OIB) are common in Phanerozoic accretionary complexes, and this study presents the first evidence of OIB in a Neoproterozoic accretionary complex of the Gwna Group in Anglesey-Llyen and Llyen area. The OIB-like basalts are locally capped by red hematite-rich claystones. This indicates that a fully oxic pelagic condition was present around the oceanic island in the Iapetus Ocean in the Neoproterozoic, which is consistent with the redox condition estimated from contemporaneous shallow marine sediments. On the other hand, the presence of black mudstones on top of MORB-like meta-basalts suggests that deep-sea anoxia conditions were prevalent during the end-Proterozoic.

  9. The western transverse ranges microplate as a native terrane

    SciTech Connect

    Campbell, M.D.; Reed, W.E. )

    1994-04-01

    Palocurrent measurements from the entire Cretaceous section of the western Transverse Ranges microplate (WTRM) yield a northerly flow direction. Point count data indicate a mixed provenance for both conglomerates and associated sandstones. The dominant provenance was mixed magmatic arc/recycled orogen and disected/transitional arc terranes. Petrographic, quantitative SEM and microprobe analysis also indicate the presence of diagnostic Franciscan mineralogy in these sediments, including glaucophane, riebeckite, lawsonite, and serpentine, suggesting derivation from a subduction complex. Olistoclasts of chert, jadeitic graywacke, serpentine and blueschist are found intermixed within the arc-derived sediments. Olistoclasts range in size from sub-millimeter to centimeter scale and olistoliths range up to 150 m. Well preserved internal bedding in some of the olistoliths suggest emplacement by landsliding indicating very short transport distance. This Franciscan material represents the oldest melange-derived material reported from this part of California and documents uplift and erosion of the subduction complex earlier than previously suggested. These data are consistent with deposition in a Cretaceous fore-arc basin located west or south of the San Diego area. The allochthonous WTRM of southern California can be reconstructed to an originally north-south oriented fore-arc basin. After deposition of the Sespe Formation (22 Ma [+-]) the microplate was slivered by strike-slip faults and rotated clockwise approximately 90[degrees], after which, the block again accreted against the continental margin. Our reconstruction suggest that depositional and structural trends for Eocene and Cretaceous sediments is likely to be different from that in the Miocene Monterey pay zones in the Santa Barbara channel region. If our reconstruction is correct, exploration strategy for Eocene and Cretaceous petroleum in the southern California Bight should take this tectonic model into account.

  10. Kilbuck terrane: oldest known rocks in Alaska

    USGS Publications Warehouse

    Box, S.E.; Moll-Stalcup, E. J.; Wooden, J.L.; Bradshaw, J.Y.

    1990-01-01

    The Kilbuck terrane in southwestern Alaska is a narrow, thin crustal sliver or flake of amphibolite facies orthogneiss. The igneous protolith of this gneiss was a suite of subduction-related plutonic rocks. U-Pb data on zircons from trondhjemitic and granitic samples yield upper-intercept (igneous) ages of 2070 ?? 16 and 2040 ?? 74 Ma, respectively. Nd isotope data from these rocks suggest that a diorite-tonalite-trondhjemite suite (??Nd[T] = +2.1 to +2.7; T is time of crystallization) evolved from partial melts of depleted mantle with no discernible contamination by older crust, whereas a coeval granitic pluton (??Nd[T] = -5.7) contains a significant component derived from Archean crust. Orthogneisses with similar age and Nd isotope characteristics are found in the Idono complex 250 km to the north. Early Proterozoic rocks are unknown elsewhere in Alaska. The possibility that the Kilbuck terrane was displaced from provinces of similar age in other cratons (e.g., Australian, Baltic, Guiana, and west African shields), or from the poorly dated Siberian craton, cannot be excluded. -from Authors

  11. North America as an exotic terrane'' and the origin of the Appalachian--Andean Mountain system

    SciTech Connect

    Dalziel, I.W.D; Gahagan, L.M. . Inst. for Geophysics); Dalla Salda, L.H. . Centro de Investigaciones Geologicas)

    1992-01-01

    North America was sutured to Gondwana in the terminal Alleghanian event of Appalachian orogenesis, thus completing the late Paleozoic assembly of Pangea. The suggestion that the Pacific margins of East Antarctica-Australia and Laurentia may have been juxtaposed during the Neoproterozoic prompts reevaluation of the widely held assumptions that the ancestral Appalachian margin rifted from northwestern Africa during the earliest Paleozoic opening of Iapetus, and remained juxtaposed to that margin, even though widely separated from it at times, until the assembly of Pangea. The lower Paleozoic carbonate platform of northwestern Argentina has been known for a long time to contain Olenellid trilobites of the Pacific or Columbian realm. Although normally regarded as some kind of far-travelled terrane that originated along the Appalachian margin of Laurentia, it has recently been interpreted as a fragment detached from the Ouachita embayment of Laurentia following Taconic-Famatinian collision with Gondwana during the Ordovician. The Oaxaca terrane of Mexico, on the other hand, contains a Tremadocian trilobite fauna of Argentine-Bolivian affinities, and appears to have been detached from Gondwana following the same collision. The Wilson cycle'' of Iapetus ocean basin opening and closing along the Appalachian and Andean orogens may have involved more than one such continental collision during clockwise drift of Laurentia around South America following late Neoproterozoic to earliest Cambrian separation. Together with the collisions of baltic and smaller terranes with Laurentia, this could explain the protracted Paleozoic orogenic history of both the Appalachian and proto-Andean orogens.

  12. Nature and melting processes of the lithosphere beneath the North-East Qiangqtang terrane, Central Tibet, during Eocene times.

    NASA Astrophysics Data System (ADS)

    Goussin, Fanny; Guillot, Stéphane; Schulmann, Karel; Cordier, Carole; Oliot, Emilien; Replumaz, Anne; Roperch, Pierrick; Dupont-Nivet, Guillaume

    2016-04-01

    At the time of the collision with India (~55Ma), the southern margin of Asia was a composite continental domain resulting from an already long history of successive accretions of different terranes having different rheologies. Knowledge about the structure, composition and thermal state of the Tibetan lithosphere through time is thus fundamental to understand the respective contributions of pre-Cenozoïc and Cenozoïc tectonics in the building of the Plateau to its present-day elevations. We focused on the boundary between the Qiangtang terrane to the south, and the Songpan-Ganze terrane to the north. We jointly studied deep crustal xenoliths and associated (ultra-)potassic magmatism from the Eocene basins of Nangqian and Xialaxiu (Qinghai Province, China), north of the Qiangtang terrane. The aims were to retrieve the composition and the thermal state of the lower crust during Eocene times, to study the behavior of the lower crust and lithospheric mantle of the Eastern Qiangtang terrane and the adjacent Songpan-Ganze terrane at the time of the collision, and the link with the magmatic activity. Crustal xenoliths are of two types: biotite-rich, amphibole bearing metasediments; and garnet-bearing quartzo-feldspathic gneisses. Such assemblages are typical of very high-grade amphibolite and granulite facies metamorphism; further study should allow us to quantify the pressures and temperatures those rocks experienced until the time they were sampled by their host lavas. Major element geochemistry places the c.a. 51-49 Ma (Spurlin et al., 2005) Xialaxiu volcanic field in a fairly differentiated (SiO2~65-70 wt%) high-K field of the calc-alcaline series. Trace element analysis suggests a strong crustal contamination of the primary mantellic melts. C.a. 38-37 Ma (Spurlin et al., 2005) Nangqian magmatic bodies span across the alkaline series, with high to extreme (K2O~6wt%) values. Complex major and trace element patterns, coupled with high-resolution microprobe data on

  13. A geologic guide to Wrangell-Saint Elias National Park and Preserve, Alaska; a tectonic collage of northbound terranes

    USGS Publications Warehouse

    Winkler, Gary R.; with contributions by MacKevett, E. M., Jr.; Plafker, George; Richter, D.H.; Rosenkrans, D.S.; Schmoll, H.R.

    2000-01-01

    Wrangell-Saint Elias National Park and Preserve, the largest unit in the U.S. National Park System, encompasses near 13.2 million acres of geological wonderments. This geologic guide presents history of exploration and Earth-science investigation; describes the complex geologic makeup; characterizes the vast college of accretion geologic terranes in this area of Alaska's continental margin; recapitulates the effects of earthquakes, volcanoes, and glaciers; characterizes the copper and gold resources of the parklands; and describes outstanding locales within the park and preserve area. A glossary of geologic terms and a categorized list of additional sources of information complete this report.

  14. High-resolution geology, petrology and age of a tectonically accreted section of Paleoarchean oceanic crust, Barberton greenstone belt, South Africa

    NASA Astrophysics Data System (ADS)

    Grosch, Eugene; Vidal, Olivier; McLoughlin, Nicola; Whitehouse, Martin

    2015-04-01

    The ca. 3.53 to 3.29 Ga Onverwacht Group of the Barberton greenstone belt (BGB), South Africa records a rare sequence of exceptionally well-preserved volcanic, intrusive and volcani-clastic Paleaoarchean rocks. Numerous conflicting models exist for the geologic evolution and stratigraphy of this early Archean greenstone belt, ranging from plume-type dynamics to modern-style plate tectonics. Although much work has focussed on the komatiites of the ca. 3.48 Ga Komati Formation since their discovery in 1969, far less petrological attention has been given to the younger oceanic rock sequences of the Kromberg type-section in the mid-Onverwacht Group. In this study, we present new field observations from a detailed re-mapping of the Kromberg type-section, and combine this with high-resolution lithological observations from continuous drill core of the Barberton Scientific Drilling Project [1]. The new mapping and field observations are compared to a recent preliminary study of the Kromberg type-section [2]. A U-Pb detrital provenance study was conducted on a reworked, volcani-clastic unit in the upper Kromberg type-section for the first time. This included U-Pb age determination of 110 detrital zircons by secondary ion microprobe analyses (SIMS), providing constraints on maximum depositional age, provenance of the ocean-floor detritus, and timing for the onset of Kromberg ocean basin formation. These new zircon age data are compared to a previous U-Pb detrital zircon study conducted on the structurally underlying sediments of the ca. 3.43 Ga Noisy formation [3]. A multi-pronged petrological approach has been applied to various rock units across the Kromberg, including thermodynamic modelling techniques applied to metabasalts and metapyroxenites for PT-estimates, bulk- and in-situ isotope geochemistry providing constraints on protolith geochemistry and metamorphic history. Consequently, it is shown that this previously poorly studied Kromberg oceanic rock sequence of the

  15. The Costa Rican Jurassic to Miocene oceanic complexes: Origin, tectonics and relations

    NASA Astrophysics Data System (ADS)

    Denyer, Percy; Gazel, Esteban

    2009-12-01

    The occurrences of oceanic assemblages on the Pacific shore of Costa Rica are part of an intricate group of complexes with different tectonic origins. Although they are dismembered and disrupted, they are the only available inland source of information to decipher the evolution of this active margin. Six main regions are described in this paper: (1) Santa Elena Peninsula, constituted by a supra-subduction zone (Santa Elena Nappe), that is overthrusting an igneous-sedimentary Aptian-Cenomanian sequence (Santa Rosa Accretionary Complex), which includes OIB (Ocean Island Basalts) portions, (2) the Nicoya Complex, which is a Jurassic-Cretaceous chert sediment pile disrupted and detached from its original basement by multiple magmatic events that occurred during the formation of the CLIP (Caribbean Large Igneous Province), (3) the Tortugal area formed by the Tortugal Suite with OIB signature and surrounded by Nicoya Complex outcrops, (4) the Herradura Block composed of the Tulín Formation to Maastrichtian to Lower Eocene OIB accreted oceanic island and the Nicoya Complex as basement, (5) Quepos Block correlated with the Tulín Formation, (6) the Osa-Burica Block composed of the Golfito and Burica Terranes (geochemically and chronologically correlated to the Nicoya Complex), Rincón Block (Early Paleocene to Early Eocene accreted seamounts), and the Miocene Osa-Caño Accretionary Complex. The Santa Rosa Accretionary Complex together with the Tortugal Suite have OIB signatures and possibly without Galapagos hotspot geochemical affinity. These coincidences would be explained by the hypothetical existence of an "autochthonous" Cretaceous basement formed by these two regions together with the rest of the Caribbean. Costa Rican basement is constituted by several CLIP portions and seamounts accreted from the end of Cretaceous in the northwest to the Miocene in the southeast, forming the diverse oceanic occurrences of the Pacific, which are mainly connected to the Galapagos

  16. Is the Rattlesnake Creek terrane out of place with respect to other terranes in the Klamath Mountains, CA

    SciTech Connect

    Wyld, S.J.; Wright, J.E. . Dept. of Geology and Geophysics)

    1993-04-01

    The Rattlesnake Creek terrane (RCT) of the western Klamath Mtns. (KM) represents an Upper Triassic to Lower Jurassic arc assemblage constructed on a serpentinite matrix melange basement. Following Saleeby (1992), melange basement is believed to be the disrupted remnants of a Paleozoic( ) oceanic fracture zone. Overlying arc volcanics include a lower group of pillowed to massive basalts with tholeiitic IAB chemistry and an upper group of cpx-phyric lavas and volcaniclastics with calc-alkaline to shoshinitic IAB chemistry. Volcanogenic rocks are interbedded with chert, argillite and epiclastic rocks derived from a terrigenous source. Gabbroic to dioritic intrusions, dated at 212--198 Ma (U/Pb zircon), intrude both melange basement and overlying arc strata and are interpreted as the intrusive roots of the arc. Collectively, these relations suggest that the RCT did not originate in or develop adjacent to the rest of the KM province, although cross-cutting relations require that the RCT was situated adjacent to the KM by [approximately]170 Ma. The authors propose a model in which the RCT was translated, probably by fore-arc strike slip faulting in response to oblique subduction, from a point of origin elsewhere along the early Mesozoic Cordillera margin, most likely the western Sierra Nevada.

  17. The Western Sierras Pampeanas: Protracted Grenville-age history (1330-1030 Ma) of intra-oceanic arcs, subduction-accretion at continental-edge and AMCG intraplate magmatism

    NASA Astrophysics Data System (ADS)

    Rapela, C. W.; Pankhurst, R. J.; Casquet, C.; Baldo, E.; Galindo, C.; Fanning, C. M.; Dahlquist, J. M.

    2010-01-01

    basic amphibolites with geochemical fingerprints of emplacement in a more mature crust, and (ii) a 1027 ± 17 Ma TTG juvenile suite, which is the youngest Grenville-age magmatic event registered in the Western Sierras Pampeanas. The geodynamic history in both study areas reveals a complex orogenic evolution, dominated by convergent tectonics and accretion of juvenile oceanic arcs to the continent.

  18. The basement of the Eastern Cordillera, Colombia: An allochthonous terrane in northwestern South America

    NASA Astrophysics Data System (ADS)

    Forero Suarez, A.

    The fault system of the Borde Llanero of Colombia represents the limit between two early Paleozoic geologic provinces: the Guiana Shield (Gondwana) to the east, and an allochthonous terrane — formerly a piece of the North American continent — to the west. The Baudó Range, the Western Cordillera, and the western flank of the Central Cordillera are the result of post-Jurassic accretion. In contrast the pre-Emsian metamorphic rocks of the eastern flank of the Central Cordillera, of the Eastern Cordillera of Colombia, and of the Mérida Andes correspond to an allochthonous terrane that was accreted to the north-western continental border of South America during the collision between North America and Gondwana in Silurian-Early Devonian times. Geochronologic and petrographic data indicate the presence of the Grenvillian granulite belt, represented by the Garzón-Sierra Nevada de Santa Marta belt. This belt is separated from the Guiana Shield by a magmatic tract which is parallel to the Borde Llanero of Venezuela and Colombia. The late Paleozoic regional metamorphism in the Northern Andes of Colombia occurred during Late Silurian-Early Devonian times. Since the late Emsian, a sedimentary cycle was initiated on this allochthonous basement. The faunal records of northwestern South America and the North American continent are indistinguishable for that time. This similarity clearly shows that both northwestern South America and the North American regions of the Appalachians and New Mexico belong to the same paleobiogeographic province. The faunal communication in this case supports the idea of the immediate neighborhood of the two continents.

  19. Linking Tengchong Terrane in SW Yunnan with Lhasa Terrane in southern Tibet through magmatic correlation

    NASA Astrophysics Data System (ADS)

    Xie, Jincheng; Zhu, Dicheng; Dong, Guochen; Zhao, Zhidan; Wang, Qing

    2016-04-01

    New zircon U-Pb data, along with the data reported in the literature, reveal five phases of magmatic activity in the Tengchong Terrane since the Early Paleozoic with spatial and temporal variations summarized as: Cambrian-Ordovician (500-460 Ma) to the eastern, minor Triassic (245-206 Ma) in the eastern and western, abundant Early Cretaceous (131-114 Ma) in the eastern, extensive Late Cretaceous (77-65 Ma) in the central, and Paleocene-Eocene (65-49 Ma) in the central and western Tengchong Terrane, in which the Cretaceous-Eocene magmatism was migrated from east to west (Xu et al., 2012). The increased zircon eHf(t) of the Early Cretaceous granitoids from -12.3 to -1.4 at ca. 131-122 Ma to -4.6 to +7.1 at ca. 122-114 Ma identified for the first time in this study and the magmatic flare-up at ca. 53 Ma in the central and western Tengchong Terrane (Wang et al., 2014, Ma et al., 2015) indicate the increased contributions from mantle- or juvenile crust-derived components. The spatial and temporal variations and changing magmatic compositions with time in the Tengchong Terrane closely resemble the Lhasa Terrane in southern Tibet. Such similarities, together with the data of stratigraphy and paleobiogeography (Zhang et al., 2013), enable us to propose that the Tengchong Terrane in SW Yunnan is most likely linked with the Lhasa Terrane in southern Tibet, both of which experience similar tectonomagmatic histories since the Early Paleozoic. References Ma, L.Y., Wang, Y.J., Fan, W.M., Geng, H.Y., Cai, Y.F., Zhong, H., Liu, H.C., Xing, X.W., 2014. Petrogenesis of the early Eocene I-type granites in west Yingjiang (SW Yunnan) and its implication for the eastern extension of the Gangdese batholiths. Gondwana Research 25, 401-419. Wang, Y.J., Zhang, L.M., Cawood, P.A., Ma, L.Y., Fan, W.M., Zhang, A.M., Zhang, Y.Z., Bi, X.W., 2014. Eocene supra-subduction zone mafic magmatism in the Sibumasu Block of SW Yunnan: Implications for Neotethyan subduction and India-Asia collision

  20. Channelling of hydrothermal fluids during the accretion and evolution of the upper oceanic crust: Sr isotope evidence from ODP Hole 1256D

    NASA Astrophysics Data System (ADS)

    Harris, Michelle; Coggon, Rosalind M.; Smith-Duque, Christopher E.; Cooper, Matthew J.; Milton, James A.; Teagle, Damon A. H.

    2015-04-01

    ODP Hole 1256D in the eastern equatorial Pacific is the first penetration of a complete section of fast spread ocean crust down to the dike-gabbro transition, and only the second borehole to sample in situ sheeted dikes after DSDP Hole 504B. Here a high spatial resolution record of whole rock and mineral strontium isotopic compositions from Site 1256 is combined with core observations and downhole wireline geophysical measurements to determine the extent of basalt-hydrothermal fluid reaction and to identify fluid pathways at different levels in the upper ocean crust. The volcanic sequence at Site 1256 is dominated by sheet and massive lava flows but the Sr isotope profile shows only limited exchange with seawater. However, the upper margins of two anomalously thick (>25 m) massive flow sequences are strongly hydrothermally altered with elevated Sr isotope ratios and appear to be conduits of lateral low-temperature off-axis fluid flow. Elsewhere in the lavas, high 87Sr/86Sr are restricted to breccia horizons. Mineralised hyaloclastic breccias in the Lava-Dike Transition are strongly altered to Mg-saponite, silica and pyrite, indicating alteration by mixed seawater and cooled hydrothermal fluids. In the Sheeted Dike Complex 87Sr/86Sr ratios are pervasively shifted towards hydrothermal fluid values (∼0.705). Dike chilled margins display secondary mineral assemblages formed during both axial recharge and discharge and have higher 87Sr/86Sr than dike cores, indicating preferential fluid flow along dike margins. Localised increases in 87Sr/86Sr in the Dike-Gabbro Transition indicates the channelling of fluids along the sub-horizontal intrusive boundaries of the 25 to 50 m-thick gabbroic intrusions, with only minor increases in 87Sr/86Sr within the cores of the gabbro bodies. When compared to the pillow lava-dominated section from Hole 504B, the Sr isotope measurements from Site 1256 suggest that the extent of hydrothermal circulation in the upper ocean crust may be

  1. Petrology and geochemistry of primitive lower oceanic crust from Pito Deep: Implications for the accretion of the lower crust at the Southern East Pacific Rise

    USGS Publications Warehouse

    Perk, N.W.; Coogan, L.A.; Karson, J.A.; Klein, E.M.; Hanna, H.D.

    2007-01-01

    A suite of samples collected from the uppermost part of the plutonic section of the oceanic crust formed at the southern East Pacific Rise and exposed at the Pito Deep has been examined. These rocks were sampled in situ by ROV and lie beneath a complete upper crustal section providing geological context. This is only the second area (after the Hess Deep) in which a substantial depth into the plutonic complex formed at the East Pacific Rise has been sampled in situ and reveals significant spatial heterogeneity in the plutonic complex. In contrast to the uppermost plutonic rocks at Hess Deep, the rocks studied here are generally primitive with olivine forsterite contents mainly between 85 and 88 and including many troctolites. The melt that the majority of the samples crystallized from was aggregated normal mid-ocean ridge basalt (MORB). Despite this high Mg# clinopyroxene is common despite model predictions that clinopyroxene should not reach the liquidus early during low-pressure crystallization of MORB. Stochastic modeling of melt crystallisation at various levels in the crust suggests that it is unlikely that a significant melt mass crystallized in the deeper crust (for example in sills) because this would lead to more evolved shallow level plutonic rocks. Similar to the upper plutonic section at Hess Deep, and in the Oman ophiolite, many samples show a steeply dipping, axis-parallel, magmatic fabric. This suggests that vertical magmatic flow is an important process in the upper part of the seismic low velocity zone beneath fast-spreading ridges. We suggest that both temporal and spatial (along-axis) variability in the magmatic and hydrothermal systems can explain the differences observed between the Hess Deep and Pito Deep plutonics. ?? Springer-Verlag 2007.

  2. Reconstruction of ocean plate stratigraphy in the Gwna Group, NW Wales: Implications for the subduction-accretion process of a latest Proterozoic trench-forearc

    NASA Astrophysics Data System (ADS)

    Asanuma, Hisashi; Okada, Yoshihiro; Fujisaki, Wataru; Suzuki, Kazue; Sato, Tomohiko; Sawaki, Yusuke; Sakata, Shuhei; Yamamoto, Shinji; Hirata, Takafumi; Maruyama, Shigenori; Windley, Brian F.

    2015-11-01

    The Gwna Group in Anglesey island and Lleyn peninsula, Wales consists of a latest Proterozoic volcano-sedimentary trench mélange, which has a complicated accretionary structure, and is poorly constrained by isotopic ages. The mélange contains oceanic-trench rocks including pillow basalts, cherts, mudstones and sandstones, which have not previously been interpreted as ocean plate stratigraphy (OPS). We reconstructed imbricated OPS at 5 localities in the coastal Lleyn peninsula. In order to constrain the depositional U-Pb age of the upper clastic sediments, detrital zircons, separated from 9 clastic sediments, were analyzed with a Nu AttoM single-collector inductively-coupled plasma-mass spectrometer. The ages indicate that there are two Gwna Groups (maximum depositional ages of: 1 at 608-601 Ma, and 2 at 564-539 Ma) that were deposited between the late Neoproterozoic and the Middle Cambrian contemporaneously with dated calc-alkaline arc magmatism and regional metamorphism in the Anglesey-Lleyn complex. The age spectra of the detrital zircons show a prominent peak at ca. 650-600 Ma, and several Proterozoic and Archean ages. To account for the older ages, we integrated our new isotopic data with published radiometric and fossil ages, and conclude that the clastic sediments at the top of the OPS were deposited in a trench on the western active margin of Avalonia when it was close to the Amazonian craton, and that the Gwna Group OPS began to be incorporated into an accretionary wedge in an active subduction zone in the latest Proterozoic.

  3. The radiolarian evidence for the accretion of the Fu-saki Formation with the inferred oceanic plate stratigraphy: A case of weakly-metamorphosed accretionary complex in Ishigaki Jima, southern Ryukyu Arc, Japan

    NASA Astrophysics Data System (ADS)

    Nakae, Satoshi

    2013-09-01

    The island of Ishigaki Jima, located in the western part of the southern Ryukyu Arc, Japan, is underlain by a basement comprising the Tumuru and Fu-saki formations. The former is a pelitic glaucophane schist with a metamorphic age of 220-190 Ma, and the latter is a weakly metamorphosed accretionary complex, composed mainly of chert, mudstone and sandstone with minor amounts of limestone and mafic rocks. The Fu-saki Formation was weakly metamorphosed at ∼140 Ma. Latest Carboniferous-Early Jurassic microfossils have been obtained from the limestones, cherts and siliceous mudstones of this formation, but no fossils have been collected from the phyllitic mudstones. The radiolarian fauna of the phyllitic mudstones described herein indicates a late Pliensbachian-early Toarcian (Early Jurassic) age. This result, when combined with existing data, enables the reconstruction of an oceanic plate stratigraphy, showing a succession of (in ascending order) Upper Carboniferous-Triassic cherts, Sinemurian-lower Pliensbachian siliceous mudstones and upper Pliensbachian-lower Toarcian phyllitic mudstones and sandstones. The radiolarians from the phyllitic mudstones are important in constraining the timing of the accretion of the Fu-saki Formation to the base of the Tumuru Formation.

  4. Geochronology and geochemistry of Paleozoic plutons in the Alxa Terrane: petrogenesis and tectonic implications

    NASA Astrophysics Data System (ADS)

    Liu, Qian; Zhao, Guochun

    2016-04-01

    Situated between the Tarim Craton and the North China Craton (NCC), the Paleozoic magmatic record in the Alxa Terrane places important constraints on the accretionary orogenesis of the southern Paleo-Asian Ocean (PAO) forming the southern section of the Central Asian Orogenic Belt. New results of LA-ICPMS zircon U-Pb ages and whole-rock major- and trace-element compositions reveal two groups of diorites and granitoids in the Alxa Terrane. One group consists of diorites and granitoids that were emplaced at 458-440 Ma, characterized by lower Al2O3/TiO2 ratios and higher TiO2 contents, implying high temperature - low pressure crystallization conditions and a shallow source region. In contrast, the second group consists of granitoids that were formed at 417-407 Ma, displaying low high rare earth elements, very high Sr/Y ratios and mostly positive Eu anomalies, suggesting low temperature - high pressure crystallization conditions and source regions at deep crustal levels where garnet is stable in the residual phase. Both of two groups are mostly calc-alkaline to high-K calc-alkaline, and depleted in Nb, Ta and Ti and enriched in Ba, K and Sr, suggesting an arc affinity related to a PAO oceanic subduction regime since the Late Ordovician. Both zircon ɛHf(t) and whole-rock ɛNd(t) values decrease from 458 Ma to 440 Ma but increase from 417 Ma to 407 Ma, whereas whole-rock initial 87Sr/86Sr ratios display an opposite trend. Such an isotopic change suggests a tectonic switch from an advancing to a retreating subduction setting in the Early Devonian. Compiled with previous studies, we suggest that the early Paleozoic magmatic arc existing in the Alxa Terrane represented the western extension of the super-large early Paleozoic active continental margin on the northern margin of the NCC.

  5. Tectonostratigraphy and provenance of an accretionary complex within the Yarlung-Zangpo suture zone, southern Tibet: Insights into subduction-accretion processes in the Neo-Tethys

    NASA Astrophysics Data System (ADS)

    Cai, Fulong; Ding, Lin; Leary, Ryan J.; Wang, Houqi; Xu, Qiang; Zhang, Liyun; Yue, Yahui

    2012-10-01

    Accretionary complexes record critical information about the history of subduction and accretion along the southern margin of Asia prior to the India-Asia collision. This paper presents detailed field mapping, petrographic and detrital zircon U-Pb data from an accretionary complex within the Yalung-Zangpo suture zone, southern Tibet. From structurally higher to lower levels, the accretionary complex consists of a serpentinite mélange, the chert-matrix Tangga mélange, the mud-matrix Pomunong mélange, and the coherent Rongmawa Formation. The Tangga mélange consists of Late Triassic-Early Cretaceous abyssal chert and siliceous shale with blocks of chert and mafic to ultra-mafic rocks. The Tangga mélange was accreted beneath ophiolitic rocks during the Aptian. The Pomunong mélange consists of a Late Jurassic-Early Cretaceous hemipelagic siliceous shale and chert matrix with blocks from Early Permian seamounts and from Late Cretaceous trench-fill sandstones; it was accreted beneath the Tangga mélange after the Aptian but prior to 71 Ma. Structurally beneath these mélanges to the south, the uppermost Cretaceous Rongmawa Formation consists of turbiditic sandstone, pelagic chert, and siliceous shale and records a transition in depositional setting from lower abyssal plain to upper trench. Detrital zircons from sandstone blocks of the Pomunong mélange and the coherent Rongmawa Formation display similar U-Pb age spectra and are dominated by peaks at 71-231 Ma, 481-693 Ma, and 701-1372 Ma. These age peaks overlap with igneous crystallization ages and detrital zircon ages from sedimentary strata in the Lhasa terrane. Our data indicate that the serpentinite mélange, Tangga mélange, Pomunong mélange and Rongmawa Formation comprise a southward-younging accretionary complex that developed during the northward subduction of Neo-Tethyan oceanic lithosphere beneath the south margin of the Lhasa terrane and that all exposed ophiolitic and accretionary complex assemblages

  6. Two radically different exhumation models for the Qiangtang Terrane, Central Tibet

    NASA Astrophysics Data System (ADS)

    Zhao, Zhongbao; Bons, Paul D.

    2014-05-01

    A century after the discovery of blueschist rocks in the central Qiangtang Terrane, the debate on their origin is still ongoing. Although all authors agree that the blueschists derive from subduction of oceanic crust, two radically different models are proposed. The "underthrust model" envisages early Mesozoic subduction mélange to have been thrust southwards (from the Jinsa Suture Zone, 200 km to the North) under Paleozoic crust of the Qiangtang Terrane. The subduction mélange, with high-pressure rocks, are thought to have exhumed by normal faulting in a large metamorphic core complex. The alternative "in-situ subduction model" puts the suture with northward subduction closer by at the Longmu Co-Shuanhu Suture Zone that separates the North and South Qiangtang terranes. In this model, allochtonous mélange is thrust over autochtonous Paleozoic basement in a south-vergent imbricate thrusts system. The models have far-reaching consequences for the crustal structure of central Tibet, as the first predicts the middle and lower crust to consist of subduction mélange overlain by Qiangtang basement and onshore Carboniferous-Triassic sediments, whereas the second predicts these rocks to overlie the Qiangtang basement of North-Gondwana provenance. To resolve this issue we combined detailed mapping in the central Qiangtang Terrane with 3D structural modelling using ASTER satellite images and the software package Move(TM). The high topography makes it possible to constrain the location and orientation of large-scale (>kms) structures, such as major faults. We found that Ordovician to Carboniferous sediments unconformably overly low-metamorphic grade pre-Ordovician basement rocks, together forming the autochtonous Paleozoic basement. This is overlain by a stack of shallowly dipping thrust sheets of subduction mélange, high-pressure rocks, as well as Permo-Triassic sediments. With the subduction mélange overlying the autochtonous basement, we prefer the in-situ subduction

  7. Crustal and upper mantle structure of central Qiangtang terrane (Tibet Plateau) imaged with magnetotelluric data

    NASA Astrophysics Data System (ADS)

    Zeng, S.; Hu, X.; Li, J.

    2013-12-01

    Since the Tethys Ocean closed, the ongoing collision between India and Aisa continents has created the Tibet Plateau, which is the most spectacular topographic feature on the surface of the earth. In the last decades, a large number of geological and geophysical studies have been undertaken in the Tibet Plateau, but most of these studies were focused on southern Tibet, where the collision of the Indian tectonic plate with Eurasia was occurred, and southeast Tibet, where lateral extrusion of crustal material may be occurred, absent in the central Tibet. As research continues, it has become clear that a complete understanding of the formation and deformation of the Tibet Plateau requires a study of the entire plateau. The Qiangtang terrane is located in the central Tibet Plateau. In 1993-1994, three profiles of broadband MT data (320 Hz to 2000 s) along N-S trending ranges from 86°E to 91°E were collected by China University of Geoscience in central Qiangtang terrane for the purpose of oil and gas exploration, the previous interpretation was focused on the shallow structures. In this study, we reanalyze the three MT profiles to produce more detailed images of the deep electrical structure of the Qiangtang terrane. Dimensionality analysis and geoelectric strike analysis of these data show that they appear to be two dimensional. 2-D inversion model show that there is a pervasive conductivity layer in the mid- to lower crustal and upper mantle, especially in the north Qiangtang terrane, which was considered to be the result of partial melt. The partial melt fraction is sufficient for crustal flow to occur. The similarity of the inversion models of the three profiles show that there is west-east crustal flow along the Jinsha River suture in central Qiangtang terrane, which seems to be western extension of the crustal flow observed in southeast Tibet by Bai et al. (2010). The inversion results also show difference of the electrical structure between the west and east

  8. Precise U-Pb Zircon Constraints on the Earliest Magmatic History of the Carolina Terrane.

    PubMed

    Wortman; Samson; Hibbard

    2000-05-01

    -type basement. This observation, together with the juvenile Nd isotopic character of the Virgilina volcanic arc sequence, suggests that the oldest part of the Carolina terrane was built on oceanic crust away from a continental crustal influence. PMID:10769159

  9. Two flysch belts having distinctly different provenance suggest no stratigraphic link between the Wrangellia composite terrane and the paleo-Alaskan margin

    USGS Publications Warehouse

    Hults, Chad P.; Wilson, Frederic H.; Donelick, Raymond A.; O'Sullivan, Paul B.

    2013-01-01

    The provenance of Jurassic to Cretaceous flysch along the northern boundary of the allochthonous Wrangellia composite terrane, exposed from the Lake Clark region of southwest Alaska to the Nutzotin Mountains in eastern Alaska, suggests that the flysch can be divided into two belts having different sources. On the north, the Kahiltna flysch and Kuskokwim Group overlie and were derived from the Farwell and Yukon-Tanana terranes, as well as smaller related terranes that were part of the paleo-Alaskan margin. Paleocurrent indicators for these two units suggest that they derived sediment from the north and west. Sandstones are predominantly lithic wacke that contain abundant quartz grains, lithic rock fragments, and detrital mica, which suggest that these rocks were derived from recycled orogen and arc sources. Conglomerates contain limestone clasts that have fossils matching terranes that made up the paleo-Alaskan margin. In contrast, flysch units on the south overlie and were derived from the Wrangellia composite terrane. Paleocurrent indicators for these units suggest that they derived sediment from the south. Sandstones are predominantly feldspathic wackes that contain abundant plagioclase grains and volcanic rock fragments, which suggest these rocks were derived from an arc. Clast compositions in conglomerate south of the boundary match rock types of the Wrangellia composite terrane. The distributions of detrital zircon ages also differentiate the flysch units. Flysch units on the north average 54% Mesozoic, 14% Paleozoic, and 32% Precambrian detrital zircons, reflecting derivation from the older Yukon-Tanana, Farewell, and other terranes that made up the paleo-Alaskan margin. In comparison, flysch units on the south average 94% Mesozoic, 1% Paleozoic, and 5% Precambrian zircons, which are consistent with derivation from the Mesozoic oceanic magmatic arc rocks in the Wrangellia composite terrane. In particular, the flysch units on the south contain a large

  10. Uniform accretion of oceanic crust south of the Garrett transform at 14°15‧S on the East Pacific Rise

    NASA Astrophysics Data System (ADS)

    Kent, Graham M.; Harding, Alistair J.; Orcutt, John A.; Detrick, Robert S.; Mutter, John C.; Buhl, Peter

    1994-05-01

    Using migrated common depth point reflection profiles, we find the structural differences along the ultrafast spreading (>150 mm/yr) East Pacific Rise south of the Garrett fracture zone are second-order, suggesting a remarkably uniform process of crustal accretion. The rise axis south of the Garrett transform is underlain by a narrow (<1.0 km) melt lens which shows great along-strike continuity. The depth of the axial melt sill is approximately 1200 m beneath the seafloor which is about 400 m shallower than along the slower spreading East Pacific Rise at 9°30'N. This observation strengthens the argument that the depth to the top of the crustal velocity inversion is spreading rate dependent. Melt sill width, however, shows little variation along the East Pacific Rise, suggesting no dependence of magma chamber size on spreading rate. The melt reservoir decreases in width toward/across the 14°27'S ridge axis discontinuity by a modest 250-300 m and appears to be continuous across this feature. Given the small aspect ratio (˜1.0 km by ˜50 m by tens of kilometers) of the axial melt lens, the previously recorded jump in MgO content across the 14°27'S offset is likely the result of a mixing boundary which is sustained through an along-strike impedance in convection. Wide-angle reflections originating at the base of seismic layer 2A, assumed to coincide with the extrusive layer, reveal a twofold to threefold increase (200-250 to 500-600 m) in thickness within 1-2 km of the rise axis. The pattern of extrusive thickening imaged south of the Garrett transform is similar to that observed along the slower spreading (110-120 mm/yr) East Pacific Rise at 9°N. Outside of the neovolcanic zone mean extrusive thickness is relatively invariant along a profile and from profile to profile. This implies a degree of temporal stability of the along-strike magma supply when integrated over the 10 kyr that corresponds to the width of the neovolcanic zone. The inferred uniformity of off

  11. Archaean Greenstone Belt Architecture and Stratigraphy: are Comparisons With Ophiolites and Oceanic Plateaux Valid?

    NASA Astrophysics Data System (ADS)

    Bedard, J. H.; Bleeker, W.; Leclerc, F.

    2005-12-01

    Archaean greenstone belts and coeval plutonic belts (dominated by TTGs, tonalite-tronhjemite-granodiorite), are commonly interpreted to represent assembled fragments of oceanic crust, oceanic plateaux or juvenile arc terranes, variably reworked by Archaean orogenic processes related to the operation of plate tectonics. However, many of the lava successions that have been interpreted to represent accreted oceanic plateaux are demonstrably ensialic, can be correlated over long distances along-strike, have depositional contacts onto older continental crustal rocks, show tholeiitic to calc-alkaline cyclicity, and have isotopic signatures indicating assimilation of older felsic crust. Inferred Archaean ophiolites do not have sheeted dyke complexes or associated mantle rocks, and cannot be proven to be oceanic terranes formed by seafloor-spreading. Archaean supracrustal sequences are typically dominated by tholeiitic to komatiitic lavas, typically interpreted to represent the products of decompression melting of mantle plumes. Subordinate proportions of andesites, dacites and rhyolites also occur, and these, together with the coeval TTGs, are generally interpreted to represent arc magmas. In the context of uniformitarian interpretations, the coeval emplacement of putative arc- and plume-related magmas requires extremely complex geodynamic scenarios. However, the relative rarity of the archetypal convergent margin magma type (andesite) in Archaean sequences, and the absence of Archaean blueschists, ultra-high-pressure terranes, thrust and fold belts, core complexes and ophiolites, along with theoretical arguments against Archaean subduction, together imply that Archaean cratonic crust was not formed through uniformitarian plate-tectonic processes. A simpler interpretation involves soft intraoceanic collisions of thick (30-50km), plume-related, basaltic-komatiitic oceanic plateaux, with ongoing mafic magmatism leading to anatexis of the hydrated plateau base to generate

  12. Geochemistry of siliciclastic rocks in the Peninsular, Chugach, and Prince William terranes: Implications for the tectonic evolution of south central Alaska

    SciTech Connect

    Gilbert, S.A.; Casey, J.F. . Dept. of Geosciences); Bradley, D. . Branch of Alaskan Geology); Kusky, T. . Dept. of Geology)

    1992-01-01

    According to some interpretations, south-central Alaska consists of a series of unrelated terranes juxtaposed by dominantly strike-slip motions some time after formation. Alternatively, these so-called terranes may be related components of a seaward-facing arc, forearc, and accretionary prism. To shed new light on the tectonic history of this area, 150 samples of siliciclastic rocks were analyzed for major, trace, and rare earth elements (REE). Shales were sampled from the Upper Cretaceous Matanuska and Paleogene Chickaloon Fms. of the Peninsular Terrane (forearc basin); argillaceous melange matrix from the Mesozoic McHugh Complex and slate from turbidites of the Upper Cretaceous Valdez Group of the Chugach Terrane (landward part of accretionary prism); and slate from turbidites of the Paleogene Orea Group of the Prince William Terrane (seaward part of accretionary prism). One tectonic model that may fit these geochemical data requires an early linkage between the Peninsular and Chugach-Prince William composite terranes. The geochemical signatures suggest that the McHugh Complex was derived from a mafic volcanic source and may represent an early accretionary stage of sediments derived from an oceanic arc. The progressive continental enrichment of the Valdez and Orca Groups may reflect later accretionary processes during and/or after the collision of the Talkectna arc with the North American continent. The similar increasingly continental source documented in the geochemistry of the forearc basin shales of the Matanuska and Chickaloon Fms. may suggest: that the presently defined Peninsular, Chugach, and Prince William terranes collectively represent one continuously evolving, seaward facing arc, forearc, and accretionary prism complex.

  13. Revisiting the Baranof-Leech River hypothesis for early Tertiary coastwise transport of the Chugach-Prince William terrane

    NASA Astrophysics Data System (ADS)

    Cowan, Darrel S.

    2003-08-01

    According to the Baranof-Leech River hypothesis originally proposed in 1982, (1) schists on southern Baranof Island in southeastern Alaska were contiguous with the Leech River schist on southern Vancouver Island until 40 Ma, and (2) both rock units were part of the 2200 km long Chugach-Prince William terrane, which was displaced northward about 1100 km after 40 Ma. Isotopic data obtained since 1982 show that the syn-magmatic metamorphism that produced the Baranof and Leech River schists occurred at 50 Ma, not at 40 Ma. Large-magnitude coastwise slip of the terrane is therefore post-50 Ma. Igneous rocks in the Baranof and Leech River units are part of the Sanak-Baranof magmatic belt of forearc magmatism, which has been ascribed to the early Tertiary subduction of an oceanic ridge. The slab window also gave rise to early Eocene, near-trench plutonic and volcanic rocks on North American basement in the North Cascades of Washington State, and probably to coeval igneous rocks on the western coast of Vancouver Island. These igneous suites in the forearc fix the location of the intersection of the ridge with the continental margin 50 Myr ago at latitude ca. 48-49°N (present-day coordinates). Paleomagnetic data obtained since 1982 imply that before 50 Ma, the parts of the Chugach-Prince William terrane that were to become the Baranof and Leech River schists were south of 48-49°N. From 61 to 50 Ma, the northward movement of the terrane relative to North America can be reconciled with the southward migration of forearc magmatism in the Chugach-Prince William terrane if the ridge-trench intersection was fixed at 48°N (present-day coordinates). The Border Ranges fault system is the on-land structure that most likely accommodated hundreds of kilometers of post-early Eocene displacement.

  14. Laurentian and Baltican components of Terranes in NW Washington: Implications for Displacement of Paleozoic Terranes

    NASA Astrophysics Data System (ADS)

    Schermer, E. R.; Brown, N.; Gehrels, G. E.

    2015-12-01

    New field, U-Pb, and Lu-Hf data constrain the geologic history, age, and origin of the Yellow Aster Complex (YAC) in NW Washington, and suggest that this Paleozoic arc terrane originated along the paleo-Arctic margin of NE Laurentia. Field work shows the oldest YAC consists of quartzo-feldspathic paragneiss (meta-arkosic sandstone + conglomerate) and quartzose calc-silicate gneiss (meta-calcareous siltstone) in gradational contact. Paragneisses are cut by syn- and post-tectonic intrusions, and faulted against granitic orthogneiss. U-Pb results show that 1) maximum depositional ages of paragneisses are Silurian to early Devonian (399 to 434 Ma); 2) quartzose calc-silicate gneisses show a broad age peak from 1000-1900 Ma, while quartzofeldspathic gneisses contain several distinct Precambrian age peaks, including at 1.8-2.0 Ga and 2.4-2.5 Ga; 3) Both gneisses contain early Paleozoic grains with peaks at ~400-420 and ~450-460 Ma; 4) pre-tectonic orthogneiss and syn- and post-tectonic dikes range from 410 to 398 Ma; 4) All intrusive rocks contain apparently xenocrystic ~450 Ma grains. Lu-Hf data show that nearly all Paleozoic grains have negative epsilon Hf values, and zircons in the meta-arkose samples are more highly evolved than those in the calc-silicate. Several meta-arkose samples yield epsilon Hf values of -40 to -50, which is rare in the North American Cordillera, and requires the involvement of Early Archean crustal components. The most likely source region is Greenland, which implies derivation from the paleo-Arctic margin of northeastern Laurentia or Baltica. The chemistry and petrology of the igneous rocks suggest the terrane was in a continental arc setting during or very shortly after deposition of the sedimentary rocks. The data suggest that sedimentation, deformation, metamorphism, and magmatism all occurred within a brief (~15 m.y.) period in the early Devonian. These relationships suggest a Caledonian origin for YAC prior to translation to the

  15. Seismic images of a Grenvillian terrane boundary

    USGS Publications Warehouse

    Milkereit, B.; Forsyth, D. A.; Green, A.G.; Davidson, A.; Hanmer, S.; Hutchinson, Deborah R.; Hinze, W. J.; Mereu, R. F.

    1992-01-01

    A series of gently dipping reflection zones extending to mid-crustal depths is recorded by seismic data from Lakes Ontario and Erie. These prominent reflection zones define a broad complex of southeast-dipping ductile thrust faults in the interior of the Grenville orogen. One major reflection zone provides the first image of a proposed Grenvillian suture—the listric boundary zone between allochthonous terranes of the Central Gneiss and Central Metasedimentary belts. Curvilinear bands of reflections that may represent "ramp folds" and "ramp anticlines" that originally formed in a deep crustal-scale duplex abut several faults. Vertical stacking of some curvilinear features suggests coeval or later out-of-sequence faulting of imbricated and folded thrust sheets. Grenvillian structure reflections are overlain by a thin, wedge-shaped package of shallow-dipping reflections that probably originates from sediments deposited in a local half graben developed during a period of post-Grenville extension. This is the first seismic evidence for such extension in this region, which could have occurred during terminal collapse of the Grenville orogen, or could have marked the beginning of pre-Appalachian continental rifting.

  16. Tectonic slicing of subducting oceanic crust along plate interfaces: Numerical modeling

    NASA Astrophysics Data System (ADS)

    Ruh, J. B.; Le Pourhiet, L.; Agard, Ph.; Burov, E.; Gerya, T.

    2015-10-01

    Multikilometer-sized slivers of high-pressure low-temperature metamorphic oceanic crust and mantle are observed in many mountain belts. These blueschist and eclogite units were detached from the descending plate during subduction. Large-scale thermo-mechanical numerical models based on finite difference marker-in-cell staggered grid technique are implemented to investigate slicing processes that lead to the detachment of oceanic slivers and their exhumation before the onset of the continental collision phase. In particular, we investigate the role of the serpentinized subcrustal slab mantle in the mechanisms of shallow and deep crustal slicing. Results show that spatially homogeneous serpentinization of the sub-Moho slab mantle leads to complete accretion of oceanic crust within the accretionary wedge. Spatially discontinuous serpentinization of the slab mantle in form of unconnected patches can lead to shallow slicing of the oceanic crust below the accretionary wedge and to its deep slicing at mantle depths depending on the patch length, slab angle, convergence velocity and continental geothermal gradient. P-T paths obtained in this study are compared to natural examples of shallow slicing of the Crescent Terrane below Vancouver Island and deeply sliced crust of the Lago Superiore and Saas-Zermatt units in the Western Alps.

  17. Origin and Evolution of the Lhasa Terrane, Tibetan Plateau: Constraints from the Petrological and Geochronological Studies of the High-grade Metamorphic Rocks

    NASA Astrophysics Data System (ADS)

    Dong, X.; Zhang, Z.; Liu, F.; Lin, Y.

    2011-12-01

    The Lhasa terrane, the southern Tibet, between the Bangong-Nujiang suture and the Indus-Yarlung Tsangpo suture zones, experienced the multi-stages of orogenies related to the Tethys subduction and continent collision. Therefore, the Lhasa terrane is the key that reveal the formation and evolution of Tibetan Plateau. A series of high-grade metamorphic rocks outcrop in the central and southern Lhasa terrane. Previous studies show that some metamorphic rocks are the Precambrian metamorphic basement of the Lhasa terrane, named as the Nyainqentanglha rock group, Bomi rock group, Nyingchi rock group or Gangdise rock group. However, our work shows that the protoliths of the high-grade metamorphic rocks consist of Paleozoic, Mesozoic to Cenozoic sedimentary rocks and intrusion rocks, and experienced the multi stages of tectonic-thermal events during Paleozoic to Cenozoic. Most meta-sedimentary rocks of the southern Lhasa terrane were formed during Cambrian to Carboniferous, consisting of gneiss, amphibolite, schist, quartzite and marble. The distribution of inherited detrital zircons ages of meta-sedimentary rocks, together with the regional comparisons, show that the meta-sedimentary rocks have the similar material provenance with the Tethyan Himalayan sequence, recording the tectonic-thermal events related to the Grenville and Pan-African orogenies. The oldest metamorphic intrusion rock is the Cambrian granite (496 Ma) caused by the subduction of oceanic crust beneath the Gondwana supercontinent during Andean-type orogeny. The Late Devonian (367 Ma) meta-granitoids are common in the southern margin of the Lhasa terrane. The Gangdise batholith formed during Mesozoic and Cenozoic is also the main component of high-grade metamorphic rocks of the southern Lhasa terrane. The southern Lhasa terrane understood the multi-stages of metamorphism, including a Triassic metamorphic belt and a Mesozoic to Cenozoic compound metamorphic belt, i.e. the Late Paleozoic to Early Mesozoic

  18. Petrology and tectonics of Phanerozoic continent formation: From island arcs to accretion and continental arc magmatism

    USGS Publications Warehouse

    Lee, C.-T.A.; Morton, D.M.; Kistler, R.W.; Baird, A.K.

    2007-01-01

    Mesozoic continental arcs in the North American Cordillera were examined here to establish a baseline model for Phanerozoic continent formation. We combine new trace-element data on lower crustal xenoliths from the Mesozoic Sierra Nevada Batholith with an extensive grid-based geochemical map of the Peninsular Ranges Batholith, the southern equivalent of the Sierras. Collectively, these observations give a three-dimensional view of the crust, which permits the petrogenesis and tectonics of Phanerozoic crust formation to be linked in space and time. Subduction of the Farallon plate beneath North America during the Triassic to early Cretaceous was characterized by trench retreat and slab rollback because old and cold oceanic lithosphere was being subducted. This generated an extensional subduction zone, which created fringing island arcs just off the Paleozoic continental margin. However, as the age of the Farallon plate at the time of subduction decreased, the extensional environment waned, allowing the fringing island arc to accrete onto the continental margin. With continued subduction, a continental arc was born and a progressively more compressional environment developed as the age of subducting slab continued to young. Refinement into a felsic crust occurred after accretion, that is, during the continental arc stage, wherein a thickened crustal and lithospheric column permitted a longer differentiation column. New basaltic arc magmas underplate and intrude the accreted terrane, suture, and former continental margin. Interaction of these basaltic magmas with pre-existing crust and lithospheric mantle created garnet pyroxenitic mafic cumulates by fractional crystallization at depth as well as gabbroic and garnet pyroxenitic restites at shallower levels by melting of pre-existing lower crust. The complementary felsic plutons formed by these deep-seated differentiation processes rose into the upper crust, stitching together the accreted terrane, suture and former

  19. Paleogeographic reconstruction of northwestern Oregon based on Eocene freshwater deposition in accreted terrane

    SciTech Connect

    Ries, J.E.

    1989-03-01

    Freshwater deposits exposed in the Coast Range of Oregon have been identified by the absence of marine organisms, significant floral remains, and the identification of a freshwater fish assemblage. These facies have been correlated with foraminiferal and lithologic horizons from test wells from the Mist Gas field of northwestern Oregon. Consistent records of inner neritic and marginal marine deposition in the Narizian stage, upper Cowlitz Formation, suggest the existence of an Eocene volcanic archipelago. Foraminiferal correlation through this stage is complicated by the absence of stratigraphically significant species in several of the wells. Floral remains from exposed sections have provided diverse elements, allowing paleogeographic reconstruction. A sea level coastal swamp was dominated by a subtropical flora consisting of Sabalites, Platanophyllum, and Equisetum. The swamp was apparently backed by higher altitude volcanic uplands dominated by a more temperate flora including Cornus, Chamaecyparis, Ailanthus, Pinus, and Picea.

  20. Geophysical modeling of the northern Appalachian Brompton-Cameron, Central Maine, and Avalon terranes under the New Jersey Coastal Plain

    USGS Publications Warehouse

    Maguire, T.J.; Sheridan, R.E.; Volkert, R.A.

    2004-01-01

    A regional terrane map of the New Jersey Coastal Plain basement was constructed using seismic, drilling, gravity and magnetic data. The Brompton-Cameron and Central Maine terranes were coalesced as one volcanic island arc terrane before obducting onto Laurentian, Grenville age, continental crust in the Taconian orogeny [Rankin, D.W., 1994. Continental margin of the eastern United States: past and present. In: Speed, R.C., (Ed.), Phanerozoic Evolution of North American Continent-Ocean Transitions. DNAG Continent-Ocean Transect Volume. Geological Society of America, Boulder, Colorado, pp. 129-218]. Volcanic island-arc rocks of the Avalon terrane are in contact with Central Maine terrane rocks in southern Connecticut where the latter are overthrust onto the Brompton-Cameron terrane, which is thrust over Laurentian basement. Similarities of these allochthonous island arc terranes (Brompton-Cameron, Central Maine, Avalon) in lithology, fauna and age suggest that they are faulted segments of the margin of one major late Precambrian to early Paleozoic, high latitude peri-Gondwana island arc designated as "Avalonia", which collided with Laurentia in the early to middle Paleozoic. The Brompton Cameron, Central Maine, and Avalon terranes are projected as the basement under the eastern New Jersey Coastal Plain based on drill core samples of metamorphic rocks of active margin/magmatic arc origin. A seismic reflection profile across the New York Bight traces the gentle dipping (approximately 20 degrees) Cameron's Line Taconian suture southeast beneath allochthonous Avalon and other terranes to a 4 sec TWTT depth (approximately 9 km) where the Avalonian rocks are over Laurentian crust. Gentle up-plunge (approximately 5 degrees) projections to the southwest bring the Laurentian Grenville age basement and the drift-stage early Paleozoic cover rocks to windows in Burlington Co. at approximately 1 km depth and Cape May Co. at approximately 2 km depths. The antiformal Shellburne

  1. Forming the lunar farside highlands by accretion of a companion moon

    NASA Astrophysics Data System (ADS)

    Jutzi, M.; Asphaug, E.

    2011-08-01

    The most striking geological feature of the Moon is the terrain and elevation dichotomy between the hemispheres: the nearside is low and flat, dominated by volcanic maria, whereas the farside is mountainous and deeply cratered. Associated with this geological dichotomy is a compositional and thermal variation with the nearside Procellarum KREEP (potassium/rare-earth element/phosphorus) Terrane and environs interpreted as having thin, compositionally evolved crust in comparison with the massive feldspathic highlands. The lunar dichotomy may have been caused by internal effects (for example spatial variations in tidal heating, asymmetric convective processes or asymmetric crystallization of the magma ocean) or external effects (such as the event that formed the South Pole/Aitken basin1 or asymmetric cratering). Here we consider its origin as a late carapace added by the accretion of a companion moon. Companion moons are a common outcome of simulations of Moon formation from a protolunar disk resulting from a giant impact, and although most coplanar configurations are unstable, a ~1,200-km-diameter moon located at one of the Trojan points could be dynamically stable for tens of millions of years after the giant impact. Most of the Moon's magma ocean would solidify on this timescale, whereas the companion moon would evolve more quickly into a crust and a solid mantle derived from similar disk material, and would presumably have little or no core. Its likely fate would be to collide with the Moon at ~2-3 km s-1, well below the speed of sound in silicates. According to our simulations, a large moon/Moon size ratio (~0.3) and a subsonic impact velocity lead to an accretionary pile rather than a crater, contributing a hemispheric layer of extent and thickness consistent with the dimensions of the farside highlands and in agreement with the degree-two crustal thickness profile. The collision furthermore displaces the KREEP-rich layer to the opposite hemisphere, explaining

  2. From intra-oceanic subduction to arc accretion and arc-continent collision: Insights from the structural evolution of the Río San Juan metamorphic complex, northern Hispaniola

    NASA Astrophysics Data System (ADS)

    Escuder-Viruete, Javier; Valverde-Vaquero, Pablo; Rojas-Agramonte, Yamirka; Jabites, Janet; Pérez-Estaún, Andrés

    2013-01-01

    The Río San Juan metamorphic complex exposes a segment of a high-pressure subduction-accretionary complex built during Caribbean island arc-North America continental margin convergence. It is composed of accreted arc- and oceanic-derived metaigneous rocks, serpentinized peridotites and minor metasediments forming a structural pile. Combined detailed mapping, structural and metamorphic analysis, and geochronology show that the deformation can be divided into five main events (D1-D5). An early subduction-related D1 deformation and M1 metamorphism produced greenschist (mafic rocks of the Gaspar Hernández peridotite-tectonite), blueschist and eclogite (metamafic blocks in the Jagua Clara mélange), high-P epidote-amphibolite and eclogite (Cuaba unit), and lower blueschist and greenschist-facies conditions (Morrito unit). This was followed by M2 decompression and cooling in the blueschist, greenschist and low-P amphibolite-facies conditions. The shape of the retrograde P-T path, the age of the exhumation-related D2 structures, and the tectonic significance of D2 deformation are different in each structural unit. Published U-Pb and 40Ar/39Ar plateau ages and T-t/P-t estimations reveal diachronic Turonian-Coniacian to Maastrichtian retrograde M2 metamorphism in the different structural units of the complex, during a consistent D2 top-to-the-NE/ENE tectonic transport. Regionally, a similar top-to-the-ENE tectonic transport also took place in the metasedimentary nappes of the Samaná complex during the Eocene to earliest Miocene. This kinematic compatibility indicates a general northeastward progradation of deformation in the northern Caribbean convergent margin, as the successive tectonic incorporation of arc, oceanic and continental-derived terrains to the developing Caribbean subduction-accretionary complex took place. D3-D5 deformations are discontinuous and much less penetrative, recording the evolution from ductile to brittle conditions of deformation in the complex

  3. Observations of accreting pulsars

    NASA Technical Reports Server (NTRS)

    Prince, Thomas A.; Bildsten, Lars; Chakrabarty, Deepto; Wilson, Robert B.; Finger, Mark H.

    1994-01-01

    We discuss recent observations of accreting binary pulsars with the all-sky BATSE instrument on the Compton Gamma Ray Observatory. BATSE has detected and studied nearly half of the known accreting pulsar systems. Continuous timing studies over a two-year period have yielded accurate orbital parameters for 9 of these systems, as well as new insights into long-term accretion torque histories.

  4. Late Devonian-Early Carboniferous magmatism in the Lhasa terrane and its tectonic implications: Evidences from detrital zircons in the Nyingchi Complex

    NASA Astrophysics Data System (ADS)

    Guo, Liang; Zhang, Hong-Fei; Harris, Nigel; Xu, Wang-Chun; Pan, Fa-Bin

    2016-02-01

    The Late Paleozoic tectonic evolution of the Lhasa terrane remains poorly understood due to the paucity of the Late Paleozoic magmatic rocks exposed at the surface. Detrital zircons in the sedimentary rocks can provide a record of magmatic rocks that have been eroded. Here we report detrital zircon U-Pb ages, trace-element and Hf isotopic data of metasedimentary rocks from the Nyingchi Complex in the eastern Himalayan syntaxis. Detrital zircons from the metasedimentary rocks yield major age populations of 330-364 Ma, 490-800 Ma, 1000-1200 Ma, and 1500-1800 Ma. The weighted mean ages of the youngest three detrital zircons indicate Carboniferous (~ 330 Ma) depositional age for their sedimentary protoliths. Provenance analysis indicates that the sedimentary detritus was sourced from the Lhasa terrane itself. The presence of abundant 330-364 Ma detrital zircons indicates that the Lhasa terrane was characterized by Late Devonian-Early Carboniferous magmatism. The trace-element compositions of the 330-364 Ma detrital zircons indicate that their magmatic host rocks mainly include mafic rocks and granitoids, and minor carbonatite. Some mafic host rocks probably formed in rift-related tectonic setting, and the others formed in arc-related tectonic settings. The granitic host rocks were S-type granites. The 330-391 Ma zircons have negative εHf(t) values (- 19.3 to - 2.5), suggesting that their magmatic host rocks resulted from partial melting of the enriched mantle or ancient crustal materials. Combined with previous studies, we propose that the Late Devonian-Early Carboniferous magmatic rocks in the Lhasa terrane probably formed in an arc-back-arc system which resulted from the southward subduction of the Paleo-Tethys oceanic crust. The back-arc basin developed as the Sumdo Paleo-Tethys ocean, which began to shrink as oceanic crust subducted northwards underneath the North Lhasa terrane during the Late Carboniferous-Permian and finally closed during the Triassic.

  5. Geomorphic terranes of the central Klamath Mountains: Applications to ecosystem management

    SciTech Connect

    De La Fuente, J.; Biery, E.; Creasy, M.; Elder, D.; Haessig, P.; Laurent, T.; Snavely, W. )

    1993-04-01

    Five geomorphic terranes have been identified in the Dillon Mountain area, about 20 miles southwest of Happy Camp, California. These terranes are defined as lands with similar geologic histories, where modern geomorphic processes are similar, and where soils and biotic communities are similar. They include: (1) slump/earthflow terrane; (2) glacial deposit terrane; (3) mountain slope terrane; (4) headwall terrane (steep, fan-shaped headwaters of first order drainages); and (5) inner gorge terrane (the steep landform which develops adjacent to rapidly downcutting streams). These primary geomorphic terranes are further subdivided on a basis of lithology, slope gradient, and geomorphic setting. Geomorphic terrane maps are derived from primary data layers in a geographic information system (GIS). The primary data layers include field-generated lithology, structure, and geomorphology. Slope gradient information is also used, and is derived from digital terrain data, modified by field observations. The distribution of geomorphic terranes is strongly influenced by local stratigraphy, which includes portions of the Western Jurassic Belt (Galice Formation), and the Western Paleozoic and Triassic Belt (Rattlesnake Creek, and Hayfork terranes). Tectonic and climatic events of the Pleistocene Epoch also played a major role in the formation and distribution of geomorphic terranes. These included rapid uplift, seismic activity, and alternating glacial and interglacial conditions. Work is underway to refine the geomorphic terranes by applying other variables such as bedrock structure, precipitation zones, and elevation zones.

  6. Thondhjemite of the Talkeetna Mountains: An unusually large low-K pluton in Alaska's Peninsular terrane

    SciTech Connect

    Ford, A.B.; Arth, J.G.; Csejtey, B. )

    1993-04-01

    An unusually large, elongate Jurassic pluton of trondhjemite, about 120- by 10--15 km in dimensions, intruded Jurassic plutonic and metamorphic rocks of the Peninsular terrane in the central Talkeetna Mountains of south-central Alaska. Muscovite and biotite yield minimum ages of 150--145 Ma. The N40[degree]E-trending body is concordant with regional structures. It is the youngest member of a subduction-related Jurassic plutonic suite in the Peninsular terrane that, along with Wrangellia, was accreted to the North American continent in the middle Cretaceous. Rocks, commonly sheared, are medium to coarse grained and leucocratic (CI = 3--9). Biotite is the chief mafic mineral. Minor muscovite and garnet are common and green hornblende rare. Samples (n = 27) from the body's entire length have an average Mg[number sign] of 45 and an SiO[sub 2] continuum of 67--74% (avg. 70.7%). High Al[sub 2]O[sub 3] (14.4--17.9%, avg. 16.5%) is typical of continental trondhjemite. Averages for Zr (109 ppm) and Nb (3.5 ppm) and the ratios K/Rb (491) and Zr/Nb (34) are typical of orogenic igneous rocks of subduction origin. Four samples analyzed have low ([sup 87]Sr/[sup 86]Sr)[sub i] (avg. 0.7036). Very low Rb/Sr (avg. 0.027) is similar to Idaho batholith trondhjemites. REE patterns with low to moderate LREE and HREE with flat patterns and low contents suggest residual garnet or hornblende during partial melting or fractionation. The pluton appears homogeneous in outcrop. However, some geographic variations in chemistry, as in SiO[sub 2] contents and especially in Eu/Eu[sup *], suggest existence of perhaps three regionally separate plumbing systems, or chambers in which different processes such as plagioclase accumulation or hornblende fractionation were active.

  7. The evolution of the neoproterozoic São Gabriel juvenile terrane, southern Brazil based on SHRIMP and LA-ICP-MS U-Pb ages and ?18O data on detrital zircon

    NASA Astrophysics Data System (ADS)

    Lena, L. O.; Pimentel, M. M.; Philipp, R. P.; Armstrong, R. A.; Sato, K.

    2013-12-01

    . This is explained by the installation of a continental magmatic arc along the western margin of the Rio de La Plata Craton. Period III took place between ca. 690-650 represented by the accretion of the arc system to the Rio de La Plata Craton. In this timeframe only continental crust-derived magmas are recognized and no mantle δ18O values are observed. In summary, our data provides an insight into the progressive evolution of the São Gabriel terrane from an intra-oceanic subduction zone at ca. 879-750 Ma to a continental arc setting (ages < 690 Ma) into three distinct periods, which preceded ocean closure and continental collision at the end of the Neoproterozoic.

  8. Implications of magmatic records for Neotethyan subduction beneath the Eurasian margin (Lhasa terrane, southern Tibet)

    NASA Astrophysics Data System (ADS)

    Tan, Jieqing; Aitchison, Jonathan

    2014-05-01

    Evidence for magmatism is widely developed in the Lhasa terrane of southern Tibet. Much of this is related to northward subduction of the Neotethyan Ocean prior the India-Eurasia collision. To better understand the tectono-magmatism, we systematically studied the published data for Middle Jurassic-Eocene igneous rocks in southern Tibet. Many of these rocks formed during two important intervals from ca. 110-80 Ma and ca. 65-40 Ma. On the basis of the reported rocks in this area, we considered the possibility that a Neotethyan mid-ocean ridge was subducted during the early peak episode (ca. 110-80 Ma). With this ridge subduction system, hot asthenosphere rose up through a slab window causing both oceanic slab and mantle wedge melting that resulted in peak volcanism during the Late Cretaceous. As young and hot crust at a mid-ocean ridge has a relatively low density, and thus potentially positive buoyancy, the subduction of a buoyant mid-ocean ridge may have led to a reduction in the angle of subduction. Evidence for termination of arc magmatism by the flat subducted oceanic slab is recorded by a magmatic gap ca. 80-65 Ma. Around ca. 65 Ma, the magmatic record appears again accompanied by a southward migration that represents resumption of an oceanic slab subduction at a normal subduction angle. Subsequently, magmatism lasts to ca. 36 Ma before the India-Eurasia collision and reached a peak of activity associated with a magmatic flare-up at 50 Ma. In this subduction system, some magmatic processes triggered formation of porphyry ore deposits and affected the temporal and spatial distribution of ores.

  9. Paleomagnetism of the Late Cretaceous ignimbrite from the Okhotsk-Chukotka Volcanic Belt, Kolyma-Omolon Composite Terrane: Tectonic implications

    NASA Astrophysics Data System (ADS)

    Otofuji, Yo-ichiro; Zaman, Haider; Shogaki, Gen; Seki, Hanae; Polin, Vladimir F.; Miura, Daisuke; Ahn, Hyeon-Seon; Ivanov, Yurii; Minyuk, Porel; Zimin, Peter

    2015-11-01

    New Late Cretaceous paleomagnetic results from the Okhotsk-Chukotka Volcanic Belt in the Kolyma-Omolon Composite Terrane yield stable and consistent remanent directions. The Late Cretaceous (86-81 Ma) ignimbrites from the Kholchan and Ola suites were sampled at 19 sites in the Magadan area (60.4° N, 151.0° E). We isolated the characteristic paleomagnetic directions from 16 sampled sites using an alternating field demagnetization procedure. The primary nature of these directions is ascertained by dual polarities and positive fold tests. A tilt-corrected mean direction (D = 42.8°, I = 84.7°, k = 46.0, α95 = 10.0°) yields a paleomagnetic pole of 66.7° N, 168.5° E (A95 = 18.8°) which appears almost identical to the 90-67 Ma pole reported from the Lake El'gygytgyn area of the Okhotsk-Chukotka Volcanic Belt (Chukotka Terrane). This consistency suggests that the Kolyma-Omolon Composite Terrane and Chukotka Terrane has acted as a single tectonic unit since 80 Ma without any significant internal deformation. Accordingly, we calculate a combined 80 Ma characteristic paleomagnetic pole (Long. = 164.7° E, Lat. = 68.0°, A95 = 10.9°, N = 12) for the Kolyma-Omolon-Chukotka Block which falls 16.5-17.5° south of the same age poles from Europe and East Asia. We ascribe this discrepancy in pole positions to tectonic activity in the area and infer a southward displacement of 1640 ± 1380 km for the Kolyma-Omolon-Chukotka Block with respect to the North American and Eurasian blocks since 80 Ma; more than 260 km of it is attributed to tectonic displacement in the Arctic Ocean due to the opening of the Canadian Basin.

  10. Accretion and Subduction of Oceanic Lithosphere: 2D and 3D Seismic Studies of Off-Axis Magma Lenses at East Pacific Rise 9°37-40'N Area and Downgoing Juan de Fuca Plate at Cascadia Subduction Zone

    NASA Astrophysics Data System (ADS)

    Han, Shuoshuo

    Two thirds of the Earth's lithosphere is covered by the ocean. The oceanic lithosphere is formed at mid-ocean ridges, evolves and interacts with the overlying ocean for millions of years, and is eventually consumed at subduction zones. In this thesis, I use 2D and 3D multichannel seismic (MCS) data to investigate the accretionary and hydrothermal process on the ridge flank of the fast-spreading East Pacific Rise (EPR) at 9°37-40'N and the structure of the downgoing Juan de Fuca plate at the Cascadia subduction zone offshore Oregon and Washington. Using 3D multichannel seismic (MCS) data, I image a series of off-axis magma lenses (OAML) in the middle or lower crust, 2-10 km from the ridge axis at EPR 9°37-40'N. The large OAMLs are associated with Moho travel time anomalies and local volcanic edifices above them, indicating off-axis magmatism contributes to crustal accretion though both intrusion and eruption (Chapter 1). To assess the effect of OAMLs on the upper crustal structure, I conduct 2-D travel time tomography on downward continued MCS data along two across-axis lines above a prominent OAML in our study area. I find higher upper crustal velocity in a region ~ 2 km wide above this OAML compared with the surrounding crust. I attribute these local anomalies to enhanced precipitation of alteration minerals in the pore space of upper crust associated with high-temperature off-axis hydrothermal circulation driven by the OAML (Chapter 2). At Cascadia, a young and hot end-member of the global subduction system, the state of hydration of the downgoing Juan de Fuca (JdF) plate is important to a number of subduction processes, yet is poorly known. As local zones of higher porosity and permeability, faults constitute primary conduits for seawater to enter the crust and potentially uppermost mantle. From pre-stack time migrated MCS images, I observe pervasive faulting in the sediment section up to 200 km from the deformation front. Yet faults with large throw and

  11. Tubiphytes-archaeolithoporella-girvanella reefal facies in Permian buildup, Mino terrane, central Japan

    NASA Astrophysics Data System (ADS)

    Sano, Hiroyoshi; Horibo, Kenji; Kumamoto, Yasuko

    1990-10-01

    The Lower to Middle Permian Okumino buildup of the Mino terrane, central Japan, formed a carbonate cap on a seamount which was sitting in an open-ocean realm. Microscopic examination reveals considerable amounts of Tubiphytes, Archaeolithoporella, and Girvanella in these rocks. These low laminar encrusting organisms together with cystopore bryozoa and syndepositional radial-fibrous cements formed bindstones. The bindstones are interpreted as having formed wave-resistant algal reefal mounds on the marginal terrace of the Okumino buildup which also has the lagoonal flat, sand bar or shoal, and foreslope facies. The Okumino buildup is closer in its biotic association of major encrusting organisms to the Trogkofel buildup in southern Alps than to the Capitan Reef Complex in New Mexico and Texas. The similarity implies that Tubiphytes and Archaeolithoporella were the most predominant and significant rock-forming encrusting organisms in Early to early Middle Permian times.

  12. Accretion/underplating, detachment and exhumation: short/long-term rheology of the subduction plate interface

    NASA Astrophysics Data System (ADS)

    Agard, Philippe; Angiboust, Samuel; Plunder, Alexis; Guillot, Stéphane; Yamato, Philippe; Oncken, Onno; Ruh, Jonas; Burov, Evgueni; Bonnet, Guillaume

    2016-04-01

    The presence of km-scale accreted terranes/units in both ancient and present-day subduction zones attests to changes in strain localization along the plate interface, whereby these terranes/units get detached from the downgoing slab (or, in places, are eroded away from the tip of the upper plate) and either directly exhumed or accreted/underplated below the upper plate before final exhumation. The rock record (P-T-t data) indicates that, for a given subduction zone, exhumation is episodic: no more than a few My compared to the ~100 My lifetime of typical subduction zones. Not much is known, however, regarding this process and important open questions remain: what exactly is episodic (i.e., detachment from the slab and/or exhumation?), for how long and where? How is mechanical coupling impacted by the initial structure of the incoming plates (structural/lithological heterogeneities, thermo-fluid regime, geodynamic boundary conditions, etc...)? We herein present both new and literature structural and P-T-t data ranging from shallow (i.e., 15-20 km) to intermediate depths (~100 km) along the subduction interface, that span a range from long-term to short-lived events of underplating and/or exhumation, and confront them with the recent wealth of geophysical data gathered on subduction zones. Structural and petrological data indicate that the slicing of km-scale units mostly occurs at specific depths where major mechanical changes occur along the plate interface: at 30-40 km (downdip of the seismogenic zone) and 70-80 km (where mechanical coupling between the two plates resumes and where eclogites get critically dense). This suggests that switches in mechanical coupling (i.e., in the rheology of the material) are key in controlling the ability to detach pieces from the slab (and that later exhumation is rather controlled by large-scale, lithospheric-scale boundary conditions). The study of rock remnants detached from the slab and underplated during subduction infancy (i

  13. Petrology and tectonic significance of gabbros, tonalites, shoshonites, and anorthosites in a late Paleozoic arc-root complex in the Wrangellia Terrane, southern Alaska

    SciTech Connect

    Beard, J.S. ); Barker, F. )

    1989-11-01

    Plutonic rocks intrusive into the late Paleozoic Tetelna Formation of southern Alaska are the underpinnings of the late Paleozoic Skolai arc of the Wrangellia Terrane. There are four groups of intrusive rocks within the Skolai arc: (1) Gabbro-diorite plutons that contain gabbroic to anorthositic cumulates along with a differentiated series of gabbros and diorites of basaltic to andesitic composition; (2) Silicic intrusions including tonalite, granodiorite, and granite; (3) Monzonitic to syenitic plutonic rocks of the Ahtell complex and related dikes and sills; (4) Fault-bounded bytownite anorthosite of uncertain age and association. These anorthosites may be related to post-Skolai, Nikolai Greenstone magmatism. The silicic rocks yield discordant U-Pb zircon ages of 290-320 Ma (early to late Pennsylvanian). The monzonitic rocks of the Ahtell complex have shoshonitic chemistry. Similar shoshonitic rocks are widespread in both the Wrangellia terrane and the neighboring Alexander terrane and intrude the contact between the two. In modern oceanic arcs, shoshonitic rocks are typically associated with tectonic instability occurring during the initial stages of subduction or just prior to or during termination or flip of an established subduction zone. The nature of any tectonic instability which may have led to the cessation of subduction in the Skolai arc is unclear. Possibilities include collision of the arc with a ridge, an oceanic plateau, another arc, or a continental fragment. One possibility is that the shoshonitic magmatism marks the late Paleozoic amalgamation of Wrangellia and the Alexander terrane. The scarcity of arc rocks predating the shoshonites in the Alexander terrane supports this possibility, but structural corroboration is lacking.

  14. Theory of wind accretion

    NASA Astrophysics Data System (ADS)

    Shakura, N. I.; Postnov, K. A.; Kochetkova, A. Yu.; Hjalmarsdotter, L.

    2014-01-01

    A review of wind accretion in high-mass X-ray binaries is presented. We focus attention to different regimes of quasi-spherical accretion onto the neutron star: the supersonic (Bondi) accretion, which takes place when the captured matter cools down rapidly and falls supersonically toward NS magnetospghere, and subsonic (settling) accretion which occurs when plasma remains hot until it meets the magnetospheric boundary. Two regimes of accretion are separated by an X-ray luminosity of about 4 × 1036 erg/s. In the subsonic case, which sets in at low luminosities, a hot quasi-spherical shell must be formed around the magnetosphere, and the actual accretion rate onto NS is determined by ability of the plasma to enter the magnetosphere due to Rayleigh-Taylor instability. We calculate the rate of plasma entry the magnetopshere and the angular momentum transfer in the shell due to turbulent viscosity appearing in the convective differentially rotating shell. We also discuss and calculate the structure of the magnetospheric boundary layer where the angular momentum between the rotating magnetosphere and the base of the differentially rotating quasi-spherical shell takes place. We show how observations of equilibrium X-ray pulsars Vela X-1 and GX 301-2 can be used to estimate dimensionless parameters of the subsonic settling accretion theory, and obtain the width of the magnetospheric boundary layer for these pulsars.

  15. TRANSPORT OF CHEMICAL CONTAMINANTS IN KARST TERRANES: OUTLINE AND SUMMARY

    EPA Science Inventory

    Chemical spills that reach an aquifer in karst terranes do not behave like those in granular or highly fractured aquifers. pills reaching diffuse-flow aquifers display relatively slow transport, are radially dispersive, and can be tracked through the use of monitoring wells. pill...

  16. Geophysical constraints for terrane boundaries in southern Mongolia

    NASA Astrophysics Data System (ADS)

    Guy, Alexandra; Schulmann, Karel; Munschy, Marc; Miehe, Jean-Marc; Edel, Jean-Bernard; Lexa, Ondrej; Fairhead, Derek

    2014-10-01

    The Central Asian Orogenic Belt (CAOB) is a typical accretionary orogen divided into numerous lithostratigraphic terranes. In theory, these terranes should be characterized by contrasting magnetic and gravity signatures owing to their dissimilar petrophysical properties. To test this hypothesis, the extent of tectonostratigraphic terranes in southern Mongolia was compared with the potential field data. The analysis reveals that the terrane boundaries are not systematically defined by strong gravity and magnetic gradients. The correlation of the magnetic signal with the geology reveals that the magnetic highs coincide with Late Carboniferous to Early Permian volcanic-plutonic belts. The matched filtering shows a good continuity of signal along the boundaries of these high magnetic anomalies toward the deeper crustal levels which may indicate the presence of deeply rooted tectonomagmatic zones. The axes of high-density bodies in the western and central parts of the study area are characterized by periodic alternations of NW-SE trending gravity anomalies corresponding to up to 20 km wide cleavage fronts of Permo-Triassic age. The matched filtering analysis shows good continuity of signal to the depth of these gravity highs which may indicate presence of deeply rooted high-strain zones. The magnetic signal is interpreted to be as the result of a giant Permo-Triassic magmatic event associated with lithosphere-scale deformation, whereas the gravity pattern is related to the postaccretionary shortening of the CAOB between the North China and Siberia cratons.

  17. The Nature of Mare Basalts in the Procellarum KREEP Terrane

    NASA Technical Reports Server (NTRS)

    Haskin, Larry A.; Gillis, Jeffrey J.; Korotev, Randy L.; Jolliff, Bradley L.

    2000-01-01

    Unlike Apollo 12 and 15 basalts, many mare lavas of the Procellarum KREEP Terrane (PKT) have Th concentrations of 2.5-6 ppm and perhaps greater, as well as high TiO2. Lunar "picritic" volcanic glasses from the PKT have a similar range.

  18. Episodic dike intrusions in the northwestern Sierra Nevada, California: Implications for multistage evolution of a Jurassic arc terrane

    SciTech Connect

    Dilek, Y.; Moores, E.M. ); Thy, P. )

    1991-02-01

    In the northwestern Sierra Nevada, California, volcanic and plutonic rocks of the Smartville and Slate Creek complexes, both fragments of a Jurassic arc terrane, are tectonically juxtaposed against ophiolitic and marine rocks that represent late Paleozoic-early Mesozoic oceanic basement. This oceanic basement is intruded by Early Jurassic dikes that are coeval with hypabyssal and plutonic rocks within the Smartville and Slate Creek complexes. These dikes have geochemical characteristics reflecting a depleted and metasomatized source, as commonly observed in modern fore-arc settings and incipient volcanic arcs, and are interpreted to be the conduits for the Early Jurassic arc volcanism, which was built on and across the disrupted oceanic basement. Late Jurassic sheeted dikes intruding the Smartville complex have basaltic compositions compatible with an intra-arc or back-arc origin and indicate that a spreading event occurred within the arc in early Late Jurassic time. These interpretations support models for a complex multistage evolution via episodic magmatism and deformation within a singly ensimatic Jurassic arc terrane west of the North American continent.

  19. Detailed structure and stratigraphy of the eastern Marble Mountain terrane, Klamath Mountains, CA

    SciTech Connect

    Miller, D.E.; Hacker, B.R. . Dept. of Geology)

    1993-04-01

    Amphibolite-grade rocks in the eastern Marble Mountains (MM), N. California, consist of several fault-bounded, SSE-dipping lithotectonic units. Each unit is ca. 2 km thick and is characterized by differences in rock type, metamorphism, and structural style. The lowermost unit composed of well-foliated and lineated epidote amphibolite grading upward into clinopyroxene-bearing amphibolite with a consistent NE-SW lineation. Structurally overlying these rocks are andalusite- and staurolite-bearing, epidote-amphibolite facies rocks (Wright Lake assemblage (WLa)) that have variable foliation and lineation orientations. The WLa consists of meta-supracrustal rocks with well-preserved relict textures, and massive, meta-ultramafic rock. Supracrustal rocks include polymict conglomerate and breccia, fine- to medium-grained clastic rocks, alkalic pillow basalt, chert, and carbonate. Conglomerate clasts include partially recrystallized granitoids and quartzite. Previous studies have interpreted the WLa to represent a fragment of oceanic crust, but coeval coarse-grained sedimentation and alkalic volcanism, small volume of mafic volcanics, conglomerate composition, and lack of oceanic plutonic and hypabyssal rocks suggest deposition in an arc-related rift or transtensional basin. Previous studies have also described the terrane as melange, but recognition of local pseudostratigraphy allows mapping of multiply folded, isoclinal, nappe-like structures. Small-scale nappes are generally 100+ m thick and are imbricated with massive meta-ultramafic rocks along gently to steeply east-dipping shear zones. Shear zones are characterized by metamorphosed ultramafic fault rocks that suggest a range of brittle to ductile behavior. Regionally distributed, Ar/Ar hornblende ages of 149.9[+-]0.4, 150.3[+-]0.6, 152.1[+-]4.7, 152.5[+-]2.5 Ma and Ar/Ar biotite ages of 148.8[+-]2.6 and 149.9[+-]0.4 Ma indicate the MM terrane cooled rapidly through ca. 500--300 C in the Late Jurassic.

  20. Provenance of sandstones in the Golconda terrane, north central Nevada

    SciTech Connect

    Jones, E.A. )

    1991-02-01

    The upper Paleozoic Golconda terrane of north-central Nevada is a composite of several structurally bounded subterranes made of clastic, volcanic, and carbonate rocks. The clastic rocks provide important clues for the interpretation of the provenance and paleogeographic settings of the different lithologic assemblages found in these subterranes. Two petrographically distinct sandstones are identified in the Golconda terrane in the Osgood Mountains and the Hot springs Range of north-central Nevada. The sandstone of the Mississippian Farrel Canyon Formation, part of the Dry Hills subterrane, is characterized by quartzose and sedimentary and lithic-rich clasts with a small feldspar component. in contrast, the sandstone of the Permian Poverty Peak (II) subterrane is a silty quartzarenite with no lithic component, and a very limited feldspar component. The sandstone of the Farrel Canyon Formation is similar to nonvolcanic sandstones reported from elsewhere in the Golconda terrane. Modal data reflect a provenance of a recycled orogen and permit the interpretation that it could have been derived from the antler orogen as has been proposed for other sandstones of the golconda terrane. The sandstone of the Poverty Peak (II) subterrane is more mature than any of the other sandstones in either the Golconda terrane, the Antler overlap sequence, or the Antler foreland basin sequence. Modal data put the Poverty Peak (II) sandstone in the continental block provenance category. The distinct extrabasinal provenances represented in these different sandstones support the idea that the Golconda basin was made up of complex paleogeographic settings, which included multiple sources of extrabasinal sediment.

  1. Tectonostratigraphic Terranes of the Circum-Pacific Region

    NASA Astrophysics Data System (ADS)

    Van der Voo, Rob

    Have you always wondered where the Tujunga, Baldy, and Cortez terranes might be located today, let alone during the Cretaceous or early Tertiary? This book may provide the answer, because in a little less than 600 pages for $32, which includes a marvelously produced color map of the entire Circum-Pacific region, one can read almost everything one wants to know about Earth's “ring of fire” and its displaced or suspect terranes. The printing, proofreading, illustrations, and references are all of the highest caliber, and the book is handsomely produced indeed. In page-by-page reading, I found maybe five typographical errors, but I will spare you the details.The contents of the book are divided into five parts, comprising principles or applications of terrane analysis and four unequally long parts on the four quadrants of the Pacific coasts. The northeast quadrant includes Alaska, the Canadian Cordillera, the U.S. coastal and Rocky Mountain belts, and Mexico; the northwest includes Kamchatka, northeast Asia, China, Japan, Taiwan, and the Philippines; the southwest section has articles on Australia, Malaya, Indonesia, New Zealand, and Antarctica; and the southeast comprises the Andes from Colombia to southern Chile. The book offers introductory text for beginning students of terrane analysis, as well as plenty of useful details and data for the expert who needs a handy reference volume. Subject matter or emphasis ranges from hydrocarbon generation in marginal basins, biogeography, paleomagnetism, geochronology, and structural and metamorphic aspects to stratigraphy and shows how the entire discipline of geological sciences is contributing to terrane analysis. There is literally something here for everyone in solid Earth science.

  2. New COCORP profiling in the southeastern United States. Part II: Brunswick and east coast magnetic anomalies, opening of the north-central Atlantic Ocean

    NASA Astrophysics Data System (ADS)

    Nelson, K. D.; McBride, J. H.; Arnow, J. A.; Oliver, J. E.; Brown, L. D.; Kaufman, S.

    1985-10-01

    New COCORP profiles on the coastal plain of Georgia and northern Florida support the hypothesis that the Brunswick anomaly marks a late Paleozoic suture. They do not support the alternate view that this anomaly is caused by a Mesozoic rift basin. The trend of the Brunswick anomaly relative to the Appalachian gravity gradient indicates that in westernmost Georgia and adjacent Alabama, African basement (Suwannee terrane) is in proximity to autochthonous North American basement (Grenville). Farther east one or more Paleozoic accreted terranes intervene between the North American and African sides of the orogen. Offshore, the Brunswick anomaly closely parallels the east coast magnetic anomaly. This relationship implies that the east coast magnetic anomaly marks not only the present continental/oceanic crustal transition but also the northward continuation of the late Paleozoic suture between North America and Africa. Transitional crust beneath the Carolina Trough, Baltimore Canyon trough, and correlative parts of the African Atlantic margin is thus likely to have formed within a preexisting suture zone. The dramatic change in character of the U.S. Atlantic margin southward from the Carolina Trough to the Blake Plateau probably reflects the fact that two different types of prerift crust are juxtaposed in this region. *And Department of Geological Sciences, Cornell University, Ithaca, New York 14853

  3. Rotating Bondi Accretion Flow

    NASA Astrophysics Data System (ADS)

    Park, Myeong-Gu; Han, Du-Hwan

    2016-06-01

    The characteristics of accretion flow onto a black hole are determined by the physical condition of gas at large radius. When the gas has no angular momentum and is polytropic, the accretion flow becomes the classic Bondi flow. The mass accretion rate in such case is an eigenvalue and uniquely determined by the density and the temperature of the surrounding gas for a given black hole mass. When the gas has angular momentum above some critical value, the angular momentum of the gas should be removed by viscosity to reach the black hole horizon. We study, within the slim disk approximation, rotating polytropic accretion flow with alpha viscosity as an an extension of the Bondi flow. The characteristics of the accretion flow are now determined by the temperature, density, and angular momentum of the gas at the outer boundary. We explore the effects of the viscosity parameter and the outer boundary radius on the physical characteristic of the flow, especially on the mass accretion rate, and compare the result with previous works of Park (2009) and Narayan & Fabian (2011).

  4. A major 2.1 Ga event of mafic magmatism in west Africa: An Early stage of crustal accretion

    NASA Astrophysics Data System (ADS)

    Abouchami, Wafa; Boher, Muriel; Michard, Annie; Albarede, Francis

    1990-10-01

    environments. Back-arc or low-Ti continental flood basalts provide a marginally good agreement but still face some difficulties. Oceanic flood basalts similar to those which form oceanic plateaus (e.g. in the Nauru basin) and later accreted to continents as allochtonous terranes represent the most acceptable modern analogue of many Proterozoic basalts. It is suggested that deep plumes piercing young lithosphere can generate huge amounts of tholeiites in a short time. Birimian basalts, like many Early Proterozoic basalts, may also be viewed as recent equivalents of the Archean greenstone belts. The modern komatiite of Gorgona Island is suggested to fit this model of intraplate volcanism. Although the 2.1 Ga magmatic event in West Africa has gone virtually unnoticed in the literature, it extends over several thousand kilometers and compares with the distribution of mantle-derived magmatic activity in other major orogenic provinces (e.g. Superior). It shows that the growth rate of continents cannot be extrapolated from the data obtained solely from the best studied continents (North America, Europe, Australia). If such large crustal segments were overlooked, a spurious pattern of episodic activity of the mantle could arise.

  5. From intra-oceanic subduction to arc accretion and oblique arc-continent collision: Insights from the P-T-D-t path of the southern Río San Juan metamorphic complex, northern Dominican Republic

    NASA Astrophysics Data System (ADS)

    Escuder-Viruete, J.; Valverde-Vaquero, P.; Rojas-Agramonte, Y.; Gabites, J.; Pérez-Estaún, A.

    2012-04-01

    +zoisite-bearing felsic melts exhibit a positive Ce anomaly relative to chondrite, an overall flat HREE pattern, and no significant negative Eu anomaly, which provide evidence for zircon growth under eclogite facies conditions. Structural and sedimentary data confirm the existence of a regional, Late Oligocene to Miocene contractional event in the complex (D3), as well as two additional late brittle deformations (D4 and D5). The compilation of this pressure-temperature-deformation-time path supports that an early subduction-related D1 deformation and high-P M1 metamorphism, was followed by a D2 extensional shearing deformation and M2 retrograde decompression, at exhumation rates of 3-4 km/Ma. The dominant tectonic mechanism responsible for exhumation of high-P rocks in the Cuaba Unit was therefore the D2 extensional tectonics, which caused tectonic juxtaposition and local metamorphic pressure gaps within the structural pile. This P-T-D-t evolution indicates that the Cuaba Unit experienced initial subduction, subsequent underplating below the forearc wedge of an island-arc, and final exhumation in the accretionary prism. At the regional scale, the Cuaba unit could be a fragment of the proto-Pacific arc that collided and was accreted bellow the E-facing, intra-oceanic, Caribbean island-arc, prior to final oblique collision with the paleo-continental margin of North America.

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

    NASA Technical Reports Server (NTRS)

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

    2012-01-01

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

  7. Ediacaran-Devonian opening and closing of the complex Iapetus Ocean and the formation of an accretionary orogen in the northern Appalachians and British Caledonides

    NASA Astrophysics Data System (ADS)

    van Staal, Cees; Zagorevski, Alexandre

    2013-04-01

    Prior to the formation of Pangea, the Northern Appalachians-British Caledonides accretionary orogen formed over ca. 150 my by piecemeal accretion of outboard terranes to a progressively growing composite Laurentian margin. The accreted material originated either in seaways and marginal basins in the peri-Gondwanan or peri-Laurentian realms and comprises micro-continental ribbons with arc supra-structure and, to a lesser extent, supra-subduction zone oceanic slivers. The preponderance of micro-continental ribbons in Iapetus necessitates detailed knowledge of the opening history in order to understand the closure. The final opening of Iapetus took place between 550 and 540 Ma following a ca. 70 my period of rifting and formation of extensive hyper-extended, non volcanic segments with adjacent seaways partially underlain by exhumed lithospheric mantle along the Laurentian margin. The conjugate margin to Laurentia likely was represented by Arequipa-Antofalla, which was left behind when Amazonia departed earlier during the Ediacaran. Subduction in Iapetus initiated at ca. 515 Ma at opposite margins, probably as a result of a major plate reorganization following the terminal amalgamation of Gondwana. Shortly thereafter Ganderia and Avalonia diachronously rifted-off Gondwana and drifted towards Laurentia, opening the Rheic Ocean in their wake. Meguma could have travelled with Avalonia or as a separate microcontinent. Closure of the main tract of the Iapetus Ocean took place during the Late Ordovician following arc-arc collision. Iapetus' main closure did not lead to widespread orogenesis; most collisional damage took place as a result of arrival of continental ribbons at the composite Laurentian margin following closure of narrow oceanic, Iapetus-related seaways or marginal basins The resultant deformation is grouped into Cambro-Ordovician Taconic-Grampian, Silurian Salinic-Scandian and Devonian Acadian orogenies; however, these orogenies are all composite and involve

  8. Zircon U-Pb ages and geochemistry of granitoids in the Truong Son terrane, Vietnam: Tectonic and metallogenic implications

    NASA Astrophysics Data System (ADS)

    Shi, Mei-Feng; Lin, Fang-Cheng; Fan, Wen-Yu; Deng, Qi; Cong, Feng; Tran, My-Dung; Zhu, Hua-Ping; Wang, Hong

    2015-04-01

    Truong Son terrane, one of the most important tectonic and metallogenic terranes in Indochina block, is composed of many volcano-plutonic complexes. Reported here is geochronological and geochemical data obtained from six different volcano-plutonic complexes. The new data reveals that the granite from the Hai Van complex is 438 Ma in age, and shows collision-related geochemical characteristics; whereas another five samples from five volcano-plutonic complexes present consistent emplacement and crystallization ages ranging from 261 to 242 Ma. Dien Bien granodiorite, Phia Bioc monzogranite and Dong Trau rhyolite display typical subduction-related calc-alkaline affinity (e.g., depletion in Nb-Ta and Ti and enrichment in Rb and La), while monzogranite from Song Ma complex displays collision-related shoshoniteseries and granites from Bengiang-Queson complex are related to post-collision calc-alkaline series. Based on these observations, in combination with the previous published geochronological data, we propose that at least four major stages of magmatic activities occurred during the Paleozoic and the Early Mesozoic through the Truong Son terrane: Ordovician-Silurian (420-470 Ma), Late Carboniferous-Early Permian (280-300 Ma), Late Permian to Mid-Triassic (245-270 Ma) and Middle-Late Triassic (200-245 Ma). These magmatic activities are not only attributed to the Tethyan Song Ma ocean southwestward subduction but also related to Paleo Tamky-Phuoc Son oceanic bidirectional subduction. Synthesized with regional metallogenic data, we identified three metallogenic epochs: (1) Late Carboniferous-Early Permian (280-300 Ma) arc-magmatic hydrothermal Cu-Au-Fe polymetallic metallogenic system related to the Tamky-Phuoc Son ocean north-dipping subduction; (2) Late Permian-Middle Triassic (245-280 Ma) arc-magmatic hydrothermal Cu-Au-Fe and orogenic W-Sn-Au polymetallic metallogenic system, which linked to both Paleo-Tethyan Song Ma ocean south-dipping subduction and Tamky

  9. Origin of Silurian reefs in the Alexander Terrane of southeastern Alaska

    SciTech Connect

    Soja, C.M. )

    1991-04-01

    Lower to Upper Silurian (upper Llandovery-Ludlow) limestones belonging to the Heceta Formation record several episodes of reef growth in the Alexander terrane of southeastern Alaska. As the oldest carbonates of wide-spread distribution in the region, the Heceta limestones represent the earliest development of a shallow-marine platform within the Alexander arc and the oldest foundation for reef evolution. These deposits provide important insights into the dynamic processes, styles, and bathymetry associated with reef growth in tectonically active oceanic islands. Massive stromatoporoids, corals, and red algae are preserved in fragmental rudstones and represent a fringing reef that formed at the seaward edge of the incipient marine shelf. Accessory constituents in this reef include crinoids and the cyanobacterium Girvanella. Small biostromes were constructed by ramose corals and stromatoporoids on oncolitic substrates in backreef or lagoonal environments. These buildups were associated with low-diversity assemblages of brachiopods and with gastropods, amphiporids, calcareous algae and cyanobacteria. Microbial boundstones reflect the widespread encrustation of cyanobacteria and calcified microproblematica on shelly debris as stromatolitic mats that resulted in the development of a stromatactoid-bearing mud mound and a barrier reef complex. Epiphytaceans, other microbes, and aphrosalpingid sponges were the primary frame-builders of the barrier reefs. These buildups attained significant relief at the shelf margin and shed detritus as slumped blocks and debris flows into deep-water sites along the slope. The similarity of these stromatolitic-aphrosalpingid reefs to those from Siluro-Devonian strata of autochthonous southwestern Alaska suggests paleobiogeographic ties of the Alexander terrane to cratonal North America during the Silurian.

  10. Ringed Accretion Disks: Instabilities

    NASA Astrophysics Data System (ADS)

    Pugliese, D.; Stuchlík, Z.

    2016-04-01

    We analyze the possibility that several instability points may be formed, due to the Paczyński mechanism of violation of mechanical equilibrium, in the orbiting matter around a supermassive Kerr black hole. We consider a recently proposed model of a ringed accretion disk, made up by several tori (rings) that can be corotating or counter-rotating relative to the Kerr attractor due to the history of the accretion process. Each torus is governed by the general relativistic hydrodynamic Boyer condition of equilibrium configurations of rotating perfect fluids. We prove that the number of the instability points is generally limited and depends on the dimensionless spin of the rotating attractor.

  11. Terrane Definition From Textural Measures of Aeromagnetic Data

    NASA Astrophysics Data System (ADS)

    Gettings, M.

    2007-12-01

    The vertical and horizontal magnetization in the Earth's crust is an anisotropic multifractal distribution, and this results in a horizontal multifractal distribution of magnetic anomalies at and above the Earth's surface. Discreet lithologic terranes are frequently observed to exhibit a characteristic pattern or "texture" of anomalies in aeromagnetic maps. Multifractal measures provide some tools useful in quantifying different textures and the scaling properties of aeromagnetic anomalies in map view can be used to define boundaries between terranes of different magnetic textures. If the source depths are not too large, the magnetic textural measures of lithologic units can be used to map their extent beneath cover. Although magnetic anomaly textures are generally visible to the eye on aeromagnetic anomaly images, the actual boundary between two textures is frequently difficult to determine with certainty. The use of quantitative textural measures provides a more objective framework for the boundary definition problem. A high resolution aeromagnetic survey over an area of highly variable geology has been used as a test area for these studies. Two robust measures that have proven useful for textural analysis are: the number of extrema per unit area; and the surface area per unit area in a window moving over the gridded aeromagnetic data. The former measures the "noisiness" of the data, and the latter depends on anomaly amplitudes and discriminates between large and small magnetizations. Window sizes for texture analysis are typically a few km square because geologic terranes of interest are generally of the order of tens of km in characteristic dimension. Other measures investigated are based on the scaling properties of the field within the window computed from the structure function for various exponents. The minima of the structure function define the characteristic sizes of anomalies, analogous to the power spectrum for a periodic function, and the maxima

  12. Hydrocarbon gas potential of accretionary melange terranes: an example from the olympic peninsula, Washington

    SciTech Connect

    Kvenvolden, K.A.; Snavely, P.D. Jr.

    1985-01-01

    Convergence between the oceanic and North American plates during middle late Eocene and late middle Miocene times produced two principal accretionary terranes of melange and broken formation on the continental margin of Washington. Hydrocarbon analyses of these melange units were undertaken to evaluate their source rock potential for oil and gas and to assess the generative processes operating in these thick melange wedges. The results of pyrolysis, vitrinite reflectance, and visual kerogen analyses of samples of these melanges are consistent and in good agreement, showing mainly Type III organic matter that is marginally mature to mature with respect to gas generation. Coastal exposure of Ozette melange commonly have a petroliferous odor which contains methane through at least the pentanes as prominent constitutents. Hydrocarbon gases from seeps and from an abandoned well in the study area have been molecular compositions and methane carbon isotopic values indicating related sources. The authors evidence suggest that the Ozette assemblage melange is the principal source for thermogenic hydrocarbon gases. Potential exploration targets may exist in western Washington where melange and broken formation are thrust beneath the Eocene oceanic crust (Crescent Formation). Gas generated from the underplated rocks could have migrated through the upper plate into structures in the Tertiary strata that overlie these Eocene basalts.

  13. Magnetospheric accretion in EX Lupi

    NASA Astrophysics Data System (ADS)

    Abraham, Peter; Kospal, Agnes; Bouvier, Jerome

    2016-08-01

    We propose to observe EX Lup, the prototype of the EXor class of young eruptive stars, in order to understand how the accretion process works in the quiescent system. Here, we request 2.6 hours of telescope time on Spitzer, to carry out a mid-infrared photometric monitoring, which we will supplement with simultaneous ground-based optical and near-infrared data. The multi-wavelength light curves will allow us to reliably separate the effects of fluctuating accretion rate from the rotation of the star. By analyzing the variations of the accretion rate we will determine whether EX Lup accretes through a few stable accretion columns or several short-lived random accretion streams. With this campaign, EX Lup will become one of the T Tauri systems where the accretion process is best understood.

  14. Forming the lunar farside highlands by accretion of a companion moon.

    PubMed

    Jutzi, M; Asphaug, E

    2011-08-01

    The most striking geological feature of the Moon is the terrain and elevation dichotomy between the hemispheres: the nearside is low and flat, dominated by volcanic maria, whereas the farside is mountainous and deeply cratered. Associated with this geological dichotomy is a compositional and thermal variation, with the nearside Procellarum KREEP (potassium/rare-earth element/phosphorus) Terrane and environs interpreted as having thin, compositionally evolved crust in comparison with the massive feldspathic highlands. The lunar dichotomy may have been caused by internal effects (for example spatial variations in tidal heating, asymmetric convective processes or asymmetric crystallization of the magma ocean) or external effects (such as the event that formed the South Pole/Aitken basin or asymmetric cratering). Here we consider its origin as a late carapace added by the accretion of a companion moon. Companion moons are a common outcome of simulations of Moon formation from a protolunar disk resulting from a giant impact, and although most coplanar configurations are unstable, a ∼1,200-km-diameter moon located at one of the Trojan points could be dynamically stable for tens of millions of years after the giant impact. Most of the Moon's magma ocean would solidify on this timescale, whereas the companion moon would evolve more quickly into a crust and a solid mantle derived from similar disk material, and would presumably have little or no core. Its likely fate would be to collide with the Moon at ∼2-3 km s(-1), well below the speed of sound in silicates. According to our simulations, a large moon/Moon size ratio (∼0.3) and a subsonic impact velocity lead to an accretionary pile rather than a crater, contributing a hemispheric layer of extent and thickness consistent with the dimensions of the farside highlands and in agreement with the degree-two crustal thickness profile. The collision furthermore displaces the KREEP-rich layer to the opposite hemisphere

  15. Massive star formation by accretion. I. Disc accretion

    NASA Astrophysics Data System (ADS)

    Haemmerlé, L.; Eggenberger, P.; Meynet, G.; Maeder, A.; Charbonnel, C.

    2016-01-01

    Context. Massive stars likely form by accretion and the evolutionary track of an accreting forming star corresponds to what is called the birthline in the Hertzsprung-Russell (HR) diagram. The shape of this birthline is quite sensitive to the evolution of the entropy in the accreting star. Aims: We first study the reasons why some birthlines published in past years present different behaviours for a given accretion rate. We then revisit the question of the accretion rate, which allows us to understand the distribution of the observed pre-main-sequence (pre-MS) stars in the HR diagram. Finally, we identify the conditions needed to obtain a large inflation of the star along its pre-MS evolution that may push the birthline towards the Hayashi line in the upper part of the HR diagram. Methods: We present new pre-MS models including accretion at various rates and for different initial structures of the accreting core. We compare them with previously published equivalent models. From the observed upper envelope of pre-MS stars in the HR diagram, we deduce the accretion law that best matches the accretion history of most of the intermediate-mass stars. Results: In the numerical computation of the time derivative of the entropy, some treatment leads to an artificial loss of entropy and thus reduces the inflation that the accreting star undergoes along the birthline. In the case of cold disc accretion, the existence of a significant swelling during the accretion phase, which leads to radii ≳ 100 R⊙ and brings the star back to the red part of the HR diagram, depends sensitively on the initial conditions. For an accretion rate of 10-3M⊙ yr-1, only models starting from a core with a significant radiative region evolve back to the red part of the HR diagram. We also obtain that, in order to reproduce the observed upper envelope of pre-MS stars in the HR diagram with an accretion law deduced from the observed mass outflows in ultra-compact HII regions, the fraction of the

  16. Ultramafic Terranes and Associated Springs as Analogs for Mars and Early Earth

    NASA Technical Reports Server (NTRS)

    Blake, David; Schulte, Mitch; Cullings, Ken; DeVincezi, D. (Technical Monitor)

    2002-01-01

    Putative extinct or extant Martian organisms, like their terrestrial counterparts, must adopt metabolic strategies based on the environments in which they live. In order for organisms to derive metabolic energy from the natural environment (Martian or terrestrial), a state of thermodynamic disequilibrium must exist. The most widespread environment of chemical disequilibrium on present-day Earth results from the interaction of mafic rocks of the ocean crust with liquid water. Such environments were even more pervasive and important on the Archean Earth due to increased geothermal heat flow and the absence of widespread continental crust formation. The composition of the lower crust and upper mantle of the Earth is essentially the-same as that of Mars, and the early histories of these two planets are similar. It follows that a knowledge of the mineralogy, water-rock chemistry and microbial ecology of Earth's oceanic crust could be of great value in devising a search strategy for evidence of past or present life on Mars. In some tectonic regimes, cross-sections of lower oceanic crust and upper mantle are exposed on land as so-called "ophiolite suites." Such is the case in the state of California (USA) as a result of its location adjacent to active plate margins. These mafic and ultramafic rocks contain numerous springs that offer an easily accessible field laboratory for studying water/rock interactions and the microbial communities that are supported by the resulting geochemical energy. A preliminary screen of Archaean biodiversity was conducted in a cold spring located in a presently serpentinizing ultramafic terrane. PCR and phylogenetic analysis of partial 16s rRNA, sequences were performed on water and sediment samples. Archaea of recent phylogenetic origin were detected with sequences nearly identical to those of organisms living in ultra-high pH lakes of Africa.

  17. Accretion disk electrodynamics

    NASA Technical Reports Server (NTRS)

    Coroniti, F. V.

    1985-01-01

    Accretion disk electrodynamic phenomena are separable into two classes: (1) disks and coronas with turbulent magnetic fields; (2) disks and black holes which are connected to a large-scale external magnetic field. Turbulent fields may originate in an alpha-omega dynamo, provide anomalous viscous transport, and sustain an active corona by magnetic buoyancy. The large-scale field can extract energy and angular momentum from the disk and black hole, and be dynamically configured into a collimated relativistic jet.

  18. Matter accreting neutron stars

    NASA Technical Reports Server (NTRS)

    Meszaros, P.

    1981-01-01

    Some of the fundamental neutron star parameters, such as the mass and the magnetic field strength, were experimentally determined in accreting neutron star systems. Some of the relevant data and the models used to derive useful information from them, are reviewed concentrating mainly on X-ray pulsars. The latest advances in our understanding of the radiation mechanisms and the transfer in the strongly magnetized polar cap regions are discussed.

  19. Accretion disk coronae

    NASA Technical Reports Server (NTRS)

    White, N. E.; Holt, S. S.

    1981-01-01

    Recent observations of partial X-ray eclipses from 4U1822-37 have shown that the central X-ray source in this system is diffused by a large Compton-thick accretion disk corona (ADC). Another binary, 4U2129-47, also displays a partial eclipse and contains an ADC. The possible origin of an ADC is discussed and a simple hydrostatic evaporated ADC model is developed which, when applied to 4U1822-37, 4U2129+47 and Cyg X-3, can explain their temporal and spectral properties. The quasi-sinusoidal modulation of all three sources can be reconciled with the partial occultation of the ADC by a bulge at the edge of the accretion disk which is caused by the inflowing material. The height of this bulge is an order of magnitude larger than the hydrostatic disk height and is the result of turbulence in the outer region of the disk. The spectral properties of all three sources can be understood in terms of Compton scattering of the original source spectrum by the ADC. Spectral variations with epoch in Cyg X-3 are probably caused by changes in the optical depth of the corona. A consequence of our model is that any accreting neutron star X-ray source in a semi-detached binary system which is close to its Eddington limit most likely contains an optically thick ADC.

  20. MASS ACCRETION RATE OF ROTATING VISCOUS ACCRETION FLOW

    SciTech Connect

    Park, Myeong-Gu

    2009-11-20

    The mass accretion rate of transonic spherical accretion flow onto compact objects such as black holes is known as the Bondi accretion rate, which is determined only by the density and the temperature of gas at the outer boundary. A rotating accretion flow has angular momentum, which modifies the flow profile from the spherical Bondi flow, and hence its mass accretion rate, but most work on disc accretion has taken the mass flux to be given with the relation between that parameter and external conditions left uncertain. Within the framework of a slim alpha disk, we have constructed global solutions of the rotating, viscous, hot accretion flow in the Paczynski-Wiita potential and determined its mass accretion rate as a function of density, temperature, and angular momentum of gas at the outer boundary. We find that the low angular momentum flow resembles the spherical Bondi flow and its mass accretion rate approaches the Bondi accretion rate for the same density and temperature at the outer boundary. The high angular momentum flow on the other hand is the conventional hot accretion disk with advection, but its mass accretion rate can be significantly smaller than the Bondi accretion rate with the same boundary conditions. We also find that solutions exist only within a limited range of dimensionless mass accretion rate m-dotident toM-dot/M-dot{sub B}, where M-dot is the mass accretion rate and M-dot{sub B} is the Bondi accretion rate: when the temperature at the outer boundary is equal to the virial temperature, solutions exist only for 0.05approxaccretion rate is roughly independent of the radius of the outer boundary but inversely proportional to the angular momentum at the outer boundary and proportional to the viscosity parameter, m-dotapprox =9.0 alphalambda{sup -1} when 0.1 approx

  1. Transcurrent displacement of tectonic terranes in the central Appalachian Piedmont

    SciTech Connect

    Hill, M.L. . Dept. of Geology)

    1994-03-01

    The metamorphic Piedmont of southeastern Pennsylvania is crosscut by a significant system of steeply dipping ductile shear zones with late- to post-orogenic transcurrent displacement. Geologic evidence suggests that at least some of these shear zones are responsible for hundreds of kilometers of dextral displacement, juxtaposing lithologic units with very different origins and tectonic histories across relatively narrow deformation zones. As a result, the present distribution of lithologies does not reflect Taconic (or earlier) collision at this place on the Laurentian margin. Type-section Wissahickon schist, exposed in the Philadelphia terrane, was metamorphosed to amphibolite facies before or during the Taconic orogeny. This terrane, bounded to the west by the Rosemont shear zone and to the north by the Cream Valley-Huntingdon Valley shear zone system, has been translated southwestward with respect to autochthonous Laurentian lithologies by dextral displacement on these shear zones since peak metamorphism of the schist. Published Late Ordovician ages from the Rosemont shear zone provide a minimum age for peak metamorphism of Wissahickon schist and a maximum age for displacement on the younger Huntingdon Valley-Cream Valley shear zone system. Therefore, peak metamorphic mineral assemblages in Wissahickon schist cannot be used to constrain Taconic collision models for this part of the Appalachians; rather, these mineral assemblages record evidence of (possibly earlier) orogeny elsewhere.

  2. Lineaments in basement terrane of the Peninsular Ranges, Southern California

    NASA Technical Reports Server (NTRS)

    Merifield, P. M. (Principal Investigator); Lamar, D. L.

    1974-01-01

    The author has identified the following significant results. ERTS and Skylab images reveal a number of prominent lineaments in the basement terrane of the Peninsular Ranges, Southern California. The major, well-known, active, northwest trending, right-slip faults are well displayed; northeast and west to west-northwest trending lineaments are also present. Study of large-scale airphotos followed by field investigations have shown that several of these lineaments represent previously unmapped faults. Pitches of striations on shear surfaces of the northeast and west trending faults indicate oblique slip movement; data are insufficient to determine the net-slip. These faults are restricted to the pre-tertiary basement terrane and are truncated by the major northwest trending faults. They may have been formed in response to an earlier stress system. All lineaments observed in the space photography are not due to faulting, and additional detailed geologic investigations are required to determine the nature of the unstudied lineaments, and the history and net-slip of fault-controlled lineaments.

  3. Reconnaissance paleomagnetic study of the Eocene Admiralty Island volcanics, southeast Alaska: evidence for pre-late Eocene accretion

    SciTech Connect

    Panuska, B.C.; Decker, J.

    1985-01-01

    Paleomagnetic data have shown that many of the terranes in southern and southeastern Alaska originated in equatorial paleolatitudes. The ages(s) of accretion of these terranes is much debated and paleomagnetic studies constraining this age are limited. As part of a larger study, reconnaissance samples of the Admiralty Island Volcanics (Eocene) were collected at Deepwater Point and Little Pybus Bay on the southern coast of Admiralty Island. Thermal or AF cleaning effectively isolated stable magnetic components in most specimens. Homoclinal dip of the flows precludes a fold test and reversals were not observed. However, 3 penecontemporaneous feeder dikes have magnetic directions which are statistically different from the magnetic directions of the flows they intrude (baked contact test). In addition, the flows have not been affected by a regional overprinting observed in most pre-Tertiary rocks. Thus, these magnetic directions are provisionally interpreted as primary. Assuming a reversed geomagnetic polarity during the eruption of the flows, the mean direction is not significantly different than the expected North American direction. Although more data are necessary to prove a primary remanence and to insure that secular variation has been averaged out, the preliminary evidence suggests that the Southern Alaska superterrane had accreted to North American by Eocene time. These results are similar to findings in south central Alaska, which also suggest that the major terrane translation and had been completed by the early Tertiary.

  4. Core Forensics: Earth's Accretion and Differentiation

    NASA Astrophysics Data System (ADS)

    Badro, J.; Brodholt, J. P.; Siebert, J.; Piet, H.; Ryerson, F. J.

    2013-12-01

    estimate outer-core density and bulk sound velocity, and combine it with seismology to define a range of possible compositions of the core that satisfies the observations, (3) a refined core formation model bringing together the continuousness of the overall process with the discreetness of the final impacts, and equilibrium thermodynamics with the non-equilibrium nature of certain processes (giant impacts, deep magma ocean). We propose a few strong constraints that come out from our models: (1) the Earth accreted in a rather oxidizing environment, (2) yielding an oxygen-rich core, in a (3) deep magma ocean (~1500 km) that could have (4) never been fully molten or fully equilibrated, at least during core extraction, despite the giant impacts.

  5. Launching jets from accretion belts

    NASA Astrophysics Data System (ADS)

    Schreier, Ron; Soker, Noam

    2016-05-01

    We propose that sub-Keplerian accretion belts around stars might launch jets. The sub-Keplerian inflow does not form a rotationally supported accretion disk, but it rather reaches the accreting object from a wide solid angle. The basic ingredients of the flow are a turbulent region where the accretion belt interacts with the accreting object via a shear layer, and two avoidance regions on the poles where the accretion rate is very low. A dynamo that is developed in the shear layer amplifies magnetic fields to high values. It is likely that the amplified magnetic fields form polar outflows from the avoidance regions. Our speculative belt-launched jets model has implications on a rich variety of astrophysical objects, from the removal of common envelopes to the explosion of core collapse supernovae by jittering jets.

  6. Early Jurassic Volcanism in the South Lhasa Terrane, Southern Tibet: Record of Back-arc Extension in the Active Continental Margin

    NASA Astrophysics Data System (ADS)

    Wei, Y.; Zhao, Z.; Zhu, D. C.; Wang, Z.; Liu, D.; Mo, X.

    2015-12-01

    Indus-Yarlung Zangbo Suture Zone (IYZSZ) represents the Mesozoic remnants of the Neo-Tethyan Ocean lithosphere after its northward subduction beneath the Lhasa Terrane. The evolution of the Neo-Tethyan Ocean prior to India-Asia collision remains unclear. To explore this period of history, we investigate zircon U-Pb geochronology, geochemistry and Nd-Hf isotopes of the Early Jurassic bimodal-like volcanic sequence around Dagze area, south Tibet. The volcanic sequence comprises calc-alkaline basalts to rhyolites whereas intermediate components are volumetrically restricted. Zircons from a basaltic andesite yielded crystallization age of 178Ma whereas those from 5 silicic rocks were dated at 183-174Ma, which suggest that both the basaltic and the silicic rocks are coeval. The basaltic rocks are enriched in LREE and LILE, and depleted in HFSE, with Epsilon Nd(t) of 1.6-4.0 and zircon Epsilon Hf(t) of 0.7-11.8, which implies that they were derived from a heterogenetic mantle source metasomatized by subduction components. Trace element geochemistry shows that the basaltic rocks are compositionally transitional from normal mid-ocean ridge basalts (N-MORB) to island arc basalts (IAB, e.g. Zedong arc basalts of ~160-155Ma in the south margin of Lhasa Terrane), with the signature of immature back-arc basin basalts. The silicic rocks display similar Nd-Hf isotopic features of the Gangdese batholith with Epsilon Nd(t) of 0.9-3.4 and zircon Epsilon Hf(t) of 2.4-17.7, indicating that they were possibly generated by anatexis of basaltic juvenile lower crust, instead of derived from the basaltic magma. These results support an Early to Middle Jurassic (183-155Ma) model that the back-arc extension tectonic setting were existing in the active continental margin in the south Lhasa Terrane.

  7. Accreting X-ray Pulsars

    NASA Technical Reports Server (NTRS)

    Wilson-Hodge, Colleen A.

    2009-01-01

    This presentation describes the behavior of matter in environments with extreme magnetic and gravitational fields, explains the instability/stability of accretion disks in certain systems, and discusses how emergent radiation affects accretion flow. Magnetic field measurements are obtained by measuring the lowest cyclotron absorption line energy, observing the cutoff of accretion due to centrifugal inhibition and measuring the spin-up rate at high luminosity.

  8. Titan's Cold Accretion and its Internal Structure

    NASA Astrophysics Data System (ADS)

    Estrada, Paul R.; Mosqueira, I.

    2010-10-01

    Recent Cassini radio tracking data has provided a normalized moment of inertia for Titan of 0.34 (Iess et al. 2010). Given that the quadrupole field is consistent with hydrostatic equilibrium, a two-layer interior model implies incomplete differentiation with a 700 km water-ice shell and an undifferentiated ice and rock-metal interior. We investigate the accretional history of Titan in connection with its internal structure. Our formation model allows for a size distribution of impactors with upper size cut-off constrained by Hyperion's size and a variable power-law exponent (Mosqueira et al. 2010). The burial of impact energy takes place in a lengthscale of order of the impactor radius, as indicated by numerical simulations (e.g., Pierazzo et al. 1997) applied to our energy regime of interest. Our thermal model includes radiogenic heating due to short and long-lived radionuclides, latent heat of melting, gravitational energy release due to sinking rock, heat of accretion and radiative cooling. We find that melting in the interior takes place well before the satellite reaches its final size. As a result, we expect the formation of an ocean overlying a silicate carapace, which may spend a considerable amount of time in contact with the liquid layer. Such a framework not only facilitates the transport of heat from the interior, but also can help both in leaching Ar40 into the ocean and then releasing into the atmosphere. We consider a range of parameters such as the degree of hydration of the rock component, the fraction of the impact energy that is deposited at the surface of the satellite, and accretion times. But we do not yet consider the effects of small admixtures of contaminants. We argue that models that form Titan in a cold environment may have allowed for the interior to remain cold enough as to preclude complete differentiation.

  9. Turbulent Distortion of Condensate Accretion

    NASA Technical Reports Server (NTRS)

    Hazoume, R.; Orou Chabi, J.; Johnson, J. A., III

    1997-01-01

    When a simple model for the relationship between the density-temperature fluctuation correlation and mean values is used, we determine that the rate of change of turbulent intensity can influence directly the accretion rate of droplets. Considerable interest exists in the accretion rate for condensates in nonequilibrium flow with icing and the potential role which reactant accretion can play in nonequilibrium exothermic reactant processes. Turbulence is thought to play an important role in such flows. It has already been experimentally determined that turbulence influences the sizes of droplets in the heterogeneous nucleation of supersaturated vapors. This paper addresses the issue of the possible influence of turbulence on the accretion rate of droplets.

  10. Chronologic constraints on the tectonic evolution of the Wilson Lake terrane of the Grenville Province, Canada

    NASA Astrophysics Data System (ADS)

    Reno, B. L.; Korhonen, F. J.; Stout, J. H.; Waight, T.

    2010-12-01

    The Wilson Lake terrane in central Labrador, Canada is one of a number of terranes that make up the Grenville Province, representing the northern extent of the Grenville Orogen in North America. Many of these terranes record evidence of two orogenies: the Labradorian Orogeny at ca. 1710-1600 Ma, and the Grenville Orogeny at ca. 1080-980 Ma. The rocks in the Wilson Lake terrane are interpreted to have been subjected to peak pressures of ~0.95 GPa and ~930°C during the Labradorian Orogeny (Korhonen et al., in prep., Stability of sapphirine + quartz in the Wilson Lake terrane: calculated equilibria in NCKFMASHTO). The final amalgamation of the Wilson Lake terrane over the underlying Parautochthonous Belt is interpreted to have occurred during the Grenville Orogeny, when the terrane was subjected to a lower-T (500-350°C) overprinting. However, petrologic and chronologic evidence for the Grenville orogeny is limited in the Wilson Lake terrane. Here we present results from a monazite chemical (U-Th)-Pb chronologic study in order to provide constraints on the metamorphic history of the Wilson Lake terrane. Monazite was analyzed in samples of orthopyroxene + sillimanite + quartz bearing and sapphirine + quartz bearing gneisses from throughout the Wilson Lake terrane. These samples contain two distinct populations of monazite: 1) a population of large (up to ~500 μm) monazite exhibits distinct core and rim zoning in yttrium X-ray compositional maps, and occurs predominately in the melanosome of the rocks, and 2) a population of smaller (up to ~50 μm) unzoned monazite rarely occurs in quartz-rich layers of the rocks. In a majority of the melanosome-hosted monazite, (U-Th)-Pb chemical ages yield cores and rims with statistically similar Labradorian ages of ca. 1705-1675 Ma. However, one sample from the middle of the terrane yields monazite grains with Labradorian age cores (ca. 1710 Ma) and post-Labradorian rims (ca. 1590 Ma). Monazite from the second, quartz

  11. Exploring a contagion model for karst terrane evolution

    SciTech Connect

    Kemmerly, P.R.

    1985-01-01

    The theoretical and geomorphic implications of a contagion model of karst depression and initiation are explored with particular emphasis on (1) identifying the parent versus daughter depression subpopulations; (2) analyzing the spatial characteristics of each subpopulation; and (3) defining the contagious karst mechanism and hot it is transmitted along solution-enlarged joints. The contagious karst mechanism suggests that the presence of one or more parent depressions does increase the the probability of daughter depressions developing along solution-enlarged joints that radiate outward from beneath parent depressions. In karst terranes where the contagious model applies, a well defined infrastructure exists with several important elements. The interaction of these elements in the infrastructure result in depressions occurring in clusters. The clusters tend to be randomly distributed and consist typically of a centrally located parent depression surrounded by numerous daughter depressions.

  12. Terrane-controlled crustal shear wave splitting in Taiwan

    NASA Astrophysics Data System (ADS)

    Okaya, David; Christensen, Nikolas I.; Ross, Zachary E.; Wu, Francis T.

    2016-01-01

    Taiwan is the result of arc-continent collision associated with the convergence of the Philippine Sea plate with the eastern Eurasian plate continental margin. The locus of deformation is found in eastern Taiwan in the form of mountain building (Central Range) with underlying thickened lithosphere. Rapid tectonic exhumation in the Central Range has uncovered low-to-high-grade metamorphic rocks marked by steep cleavage. We carried out a crustal seismic anisotropy study across Taiwan, producing a database of over 27,000 local earthquake shear wave splitting measurements. Additionally, we carried out rock physics measurements of metamorphic outcrop samples to quantify shear wave rock anisotropy. We produced a map of station-averaged splitting measurements across Taiwan. Patterns of fast shear wave directions correlate with tectonic terranes produced by plate convergence. Deformation-related mineral-preferred orientation in the metamorphic rocks produces a significant amount of the crustal anisotropy in the Taiwan collision zone.

  13. RADIATIVELY EFFICIENT MAGNETIZED BONDI ACCRETION

    SciTech Connect

    Cunningham, Andrew J.; Klein, Richard I.; McKee, Christopher F.; Krumholz, Mark R.; Teyssier, Romain

    2012-01-10

    We have carried out a numerical study of the effect of large-scale magnetic fields on the rate of accretion from a uniform, isothermal gas onto a resistive, stationary point mass. Only mass, not magnetic flux, accretes onto the point mass. The simulations for this study avoid complications arising from boundary conditions by keeping the boundaries far from the accreting object. Our simulations leverage adaptive refinement methodology to attain high spatial fidelity close to the accreting object. Our results are particularly relevant to the problem of star formation from a magnetized molecular cloud in which thermal energy is radiated away on timescales much shorter than the dynamical timescale. Contrary to the adiabatic case, our simulations show convergence toward a finite accretion rate in the limit in which the radius of the accreting object vanishes, regardless of magnetic field strength. For very weak magnetic fields, the accretion rate first approaches the Bondi value and then drops by a factor of {approx}2 as magnetic flux builds up near the point mass. For strong magnetic fields, the steady-state accretion rate is reduced by a factor of {approx}0.2 {beta}{sup 1/2} compared to the Bondi value, where {beta} is the ratio of the gas pressure to the magnetic pressure. We give a simple expression for the accretion rate as a function of the magnetic field strength. Approximate analytic results are given in the Appendices for both time-dependent accretion in the limit of weak magnetic fields and steady-state accretion for the case of strong magnetic fields.

  14. Age and origin of the Lomonosov Ridge: a key continental fragment in Arctic Ocean reconstructions

    NASA Astrophysics Data System (ADS)

    Marcussen, Christian; Knudsen, Christian; Hopper, John R.; Funck, Thomas; Ineson, Jon R.; Bjerager, Morten

    2015-04-01

    -Ordovician M'Clintock orogeny that affected the Pearya terrane, which forms the northernmost part of Ellesmere Island. The Pearya Terrane has long been considered an exotic terrane in the Canadian Arctic because the Proterozoic and early Paleozoic geological history has much in common with rocks on Svalbard, along East Greenland, and other parts of the Caledonides, but not with the rocks of the Franklinian Basin and other parts of the northern Laurentian margin. The M'Clintock orogeny is interpreted to be related to the earliest phase of the Caledonian orogenic activity and the Pearya terrane is a commonly considered as small piece of Baltica that accreted to northern Ellesmere Island, probably sometime in the late Paleozoic during the Ellesmerian orogeny. These results from the Lomonosov Ridge provide an important new constraint on post-Caledonian plate tectonic configurations in the Arctic region, prior to the opening of the Amerasia and Eurasia basins.

  15. Late Triassic paleomagnetic result from the Baoshan Terrane, West Yunnan of China: Implication for orientation of the East Paleotethys suture zone and timing of the Sibumasu-Indochina collision

    NASA Astrophysics Data System (ADS)

    Zhao, Jie; Huang, Baochun; Yan, Yonggang; Zhang, Donghai

    2015-11-01

    In order to better understand the paleogeographic position of the Baoshan Terrane in the northernmost part of the Sibumasu Block during formation of the Pangea supercontinent, a paleomagnetic study has been conducted on Late Triassic basaltic lavas from the southern part of the Baoshan Terrane in the West Yunnan region of Southwest China. Following detailed rock magnetic investigations and progressive thermal demagnetization, stable characteristic remanent magnetizations (ChRMs) were successfully isolated from Late Triassic Niuhetang lava flows. The ChRMs are of dual polarity and pass fold and reversal tests with magnetic carriers dominated by magnetite and subordinate oxidation-induced hematite; we thus interpret them as a primary remanence. This new paleomagnetic result indicates that the Baoshan Terrane was located at low paleolatitudes of ∼15°N in the Northern Hemisphere during Late Triassic times. Together with available paleomagnetic data from the Baoshan Terrane and surrounding areas, a wider paleomagnetic comparison supports the view that the East Paleotethys Ocean separated the Sibumasu and Indochina blocks and closed no later than Late Triassic times. We argue that the currently approximately north-to-south directed Changning-Menglian suture zone is very likely to have been oriented nearly east-to-west at the time of the Sibumasu-Indochina collision.

  16. AGN flickering and chaotic accretion

    NASA Astrophysics Data System (ADS)

    King, Andrew; Nixon, Chris

    2015-10-01

    Observational arguments suggest that the growth phases of the supermassive black holes in active galactic nuclei have a characteristic time-scale ˜105 yr. We show that this is the time-scale expected in the chaotic accretion picture of black hole feeding, because of the effect of self-gravity in limiting the mass of any accretion-disc feeding event.

  17. Morphodynamics of Accreting Beaches

    NASA Astrophysics Data System (ADS)

    Ruggiero, P.; Gelfenbaum, G.; Sherwood, C. R.; Kaminsky, G. M.

    2002-12-01

    Beaches along the Pacific Northwest coast of the US have been shown to have large seasonal variability in shoreline position with several 10's of meters of recession occurring during the winter (high-energy waves) and typically similar scales of beach recovery during the summer (low-energy waves). However, many beaches along the Columbia River littoral cell (northwest Oregon and southwest Washington) have exhibited net residual progradation of several meters per year over decades, resulting in significant shoreline realignment. This historical shoreline advance has been primarily due to the dispersal of sand from the flanks of the ebb-tidal deltas following jetty construction at the entrances to the Columbia River and Grays Harbor. The installation of jetties removed the shallow shoals from the influence of tidal currents, resulting in a shoreface profile that was too shallow for the inherent wave energy. Onshore transport of large quantities of sand occurred over the next several decades, decreasing through time. While much of the original source material is now exhausted, many beaches today are still rapidly accreting on inter-annual time scales. Gradients in alongshore sediment transport, net onshore directed cross-shore sediment transport within the surf zone, and cross-shore feeding from a shoreface out of equilibrium with forcing conditions may each be partially responsible for this continued accretion. The primary morphodynamic mechanism for sub-aerial beach growth, and shoreline progradation on a seasonal scale, is hypothesized to be the development, onshore migration, and welding of inter-tidal (swash) bars to the upper beach face. To investigate the processes and morphodynamics associated with accreting beaches we have completed two field experiments and are applying computational models that link measured sediment transport to wave and current forcing. Experiments completed in Spring 2001 and Summer 2002 combined process measurements with observations of

  18. Dispersed Remnants of a Northeast Pacific Fringing Arc: Upper Paleozoic Terranes of Permian McCLOUD Faunal Affinity, Western U.S.

    NASA Astrophysics Data System (ADS)

    Miller, M. Meghan

    1987-12-01

    Two fragmentary, subparallel belts of terranes within the western North American Cordillera contain upper Paleozoic rocks and are characterized by contrasting lithotectonic assemblages and contrasting Permian faunal affinity. These two belts are (A) volcanic-arc related successions of Permian McCloud faunal affinity (McCloud belt) and (B) subduction-related accretionary complexes of Permian Tethy an faunal affinity (Cache Creek belt). This paper supports the hypothesis that the fragmentary terranes of the McCloud belt once constituted parts of a northeast Pacific fringing-arc system and, in constrast to some earlier interpretations, concludes that the volcanic arc evolved above an eastward dipping subduction zone. The absolute distance between this arc and western North America during the late Paleozoic cannot be constrained, however, there is little evidence to suggest closure of a major (>10³ km) ocean basin or protracted periods of westward dipping subduction. Parts of Devonian to Permian volcanic island arc sequences of the western U.S. Cordillera are represented in the northern Sierra, eastern Klamath, Bilk Creek, Grindstone, and Chilliwack terranes. These scattered volcanic arc remnants share several fundamental characteristics: (1) The sequences were constructed across continental-affinity basement assemblages. (2) They underwent similar tectonic evolution during late Paleozoic time, such as coeval pulses in volcanism and related depositional histories. (3) They contain Early Permian McCloud-type fauna, of distinctive biogeographic affinity. (4) McCloud belt terranes are spatially and possibly genetically related to westward lying accretionary complexes of the Cache Creek belt which contain fragments of Upper Triassic blueschist and Permian limestone blocks bearing Tethyan Permian fusulinids and corals. Based on the presence of distinctive Early Permian McCloud fauna, the island arc remnants discussed in this paper are referred to as the McCloud belt. The

  19. Oceanic island arc stratigraphy in the Caribbean region: don't take it for granite

    NASA Astrophysics Data System (ADS)

    Larue, D. K.; Smith, A. L.; Schellekens, J. H.

    1991-11-01

    reflect arc development on thick oceanic crust (for example, a plateau). Submarine volcanic arc facies associations (OADS II) occur only in the northern Virgin Islands (Water Island Formation) and Bonaire (Washikemba Formation). Typically, great structural thicknesses (5-10 km) of arc-related strata are preserved, although condensed arc sequences built upon accreted terranes are present locally (southwest Puerto Rico, Grenada). On Grenada, the active magmatic platform is underlain by overthrust forearc(?) basin strata, indicating that magmatic platforms may grow by means other than volcanic aggradation.

  20. Microwave ice accretion meter

    NASA Technical Reports Server (NTRS)

    Magenheim, Bertram (Inventor); Rocks, James K. (Inventor)

    1984-01-01

    A system for indicating ice thickness and rate of ice thickness growth on surfaces is disclosed. The region to be monitored for ice accretion is provided with a resonant surface waveguide which is mounted flush, below the surface being monitored. A controlled oscillator provides microwave energy via a feed point at a controllable frequency. A detector is coupled to the surface waveguide and is responsive to electrical energy. A measuring device indicates the frequency deviation of the controlled oscillator from a quiescent frequency. A control means is provided to control the frequency of oscillation of the controlled oscillator. In a first, open-loop embodiment, the control means is a shaft operated by an operator. In a second, closed-loop embodiment, the control means is a processor which effects automatic control.

  1. Tracing trends in erosion and exhumation during the Middle-Late Paleozoic tectonic evolution of the Farewell terrane, SW Alaska

    NASA Astrophysics Data System (ADS)

    Hampton, B. A.; Malkowski, M. A.; Bradley, D. C.; Fujita, K.; O'Sullivan, P. B.

    2010-12-01

    The Farewell terrane of southwest and west-central Alaska is located at the northernmost endpoint of the North American Cordillera and is just beyond the present-day margin of the Amerasia Basin. The initial geologic framework of this region has been constrained, yet the origin, Paleozoic tectonic development, and paleogeography of the Farewell terrane prior to opening of the Arctic Ocean remain unknown. In southwestern Alaska, the Farewell is defined by a three-part succession that consist of (1) Neoproterozoic-Devonian carbonate rocks and subordinate clastic strata of the Nixon Fork subterrane, (2) Cambrian-Devonian clastic and carbonate units of the Dillinger subterrane, and (3) Devonian-Permian(?) siliciclastic strata of the Mystic subterrane. Although previous studies have suggested a Siberian origin for the oldest parts of the Farewell based on faunal data, it has yet to be determined if the Dillinger and Mystic subterranes share links with regions to the north (e.g. Siberia, Baltica, Greenland) or with the northern and western regions of Laurentia. Here we present U-Pb detrital zircon data as well as modal composition trends from Paleozoic strata of the Dillinger and Mystic subterranes that reflect an upsection transition in detrital contribution from middle to top of the Farewell terrane. U-Pb detrital zircon age spectra from Silurian-Devonian strata of the Dillinger subterrane reveal a range of Precambrian and Paleozoic ages with primary occurrences between 400-440 and 1000-2000 Ma. Isolated age peaks occur at 430, 500, 890, 1100, and1400 Ma. The oldest strata from the overlying Mystic subterrane contain primary peaks at 380, 420, 925 Ma with an elevated occurrence of Proterozoic ages between 500-2000 Ma. Younger Devonian-Permian age strata of the Mystic subterrane yield primary age peaks between 300-350 and 420-450 Ma with smaller peaks between 1800-2000 Ma. Modal composition trends from the Dillinger and Mystic subterranes reveal pervasive occurrences of

  2. Extension, disruption and translation of an orogenic wedge by exhumation of large ultrahigh pressure terranes: Two examples from the Norwegian Caledonides

    NASA Astrophysics Data System (ADS)

    Brueckner, H. K.; Cuthbert, S. J.

    2012-12-01

    We propose the thrust-dominated accretion of an orogenic wedge during continental subduction can be succeeded by stretching, disruption and passive transport of the frontal part of the wedge on top of an exhuming high pressure/ultrahigh pressure (HP/UHP) metamorphic terrane. Initial thrusting occurs when cratons collide and one subducts beneath the other into the mantle. The subducted craton undergoes HP/UHP metamorphism while an accretionary orogenic wedge develops at its junction with the overlying craton. The subsequent exhumation of the HP/UHP terrane either by true extension and/or buoyancy-driven extrusion reverses the shear traction along its upper boundary from its earlier foreland-directed thrust motion to hinterland-directed normal displacement. This normal-sense shear stretches the orogenic wedge and can potentially detach a fragment of its frontal part away from the rearward part, allowing it to be carried passively towards the foreland on the exhuming plate with the length of displacement a function of the amount of exhumation of the HP/UHP terrane. The Jotun and Trondheim Basin Nappe Complexes of the Caledonide orogenic wedge in southern and central Scandinavia were thrust (sensu stricto) E/SE onto the Baltic Shield during the Scandian Orogeny when the western edge of Baltica subducted into the mantle beneath Laurentia to form the HP/UHP Western Gneiss Complex (WGC). Kinematic indicators along the basal décollements of orogenic wedge allochthons indicate a change in shear sense from top-E/SE to top-W/NW at the same time (≈415 Ma) radiometric ages indicate the WGC began exhumation from the mantle. The shear traction along the top of the exhuming WGC stretched the Jotun and Trondheim Basin allochthons, then broke them into segments, and finally separated the frontal part of some of the allochthons away from the main body, causing them to be carried passively E/SE as the WGC continued to exhume out of the mantle. The lack of fragmentation and absence

  3. Pennsylvanian pluton stitching of Wrangellia and the Alexander terrane, Wrangell Mountains, Alaska

    SciTech Connect

    Gardner, M.C.; Bergman, S.C.; Cushing, G.W. ); Plafker, G. ); Campbell, R.B.; Dodds, C.J. ); McClelland, W.C. ); Mueller, P.A. ); MacKevett, E.M. Jr.

    1988-11-01

    A quartz monzonite-syenite-alkali granite plutonic complex in eastern Alaska crosscuts the contact of the Alexander terrane and Wrangellia and intrudes the basement rocks of both terranes. Zircon U-Pb data indicate an intrusion age of 309 {plus minus} 5 Ma (Middle Pennsylvanian) for the pluton, and {sup 40}K-{sup 40}Ar age for hornblende separates indicate cooling to about 450 C during Middle Pennsylvanian-Early Permian time. The new field relations and age data demonstrate the Wrangellia and the Alexander terrane were contiguous during the Middle Pennsylvanian. This conclusion provides an important new constraint on paleogeographic reconstructions of the northwest Cordillera, and necessitates reassessment of stratigraphic and paleomagnetic data that were cited as evidence that the terranes evolved separately until the late Mesozoic.

  4. Paleozoic paleomagnetism and northward drift of the Alexander Terrane, southeastern Alaska

    NASA Astrophysics Data System (ADS)

    van Der Voo, Rob; Jones, Meridee; Gromme, C. Sherman; Eberlein, G. Donald; Churkin, Michael, Jr.

    1980-10-01

    Paleozoic limestone, graywacke, sandstone, milestone, red beds and volcanic rocks of the Alexander terrane, southeastern Alaska, have yielded six paleomagnetic pole positions after thermal and alternating-field demagnetization. These poles are from sample groups of late Middle Ordovician, Late Ordovician, Devonian, Late Devonian, and early and late Carboniferous age. To test various tectonic models for the structural development of this part of western North America, the paleomagnetic results are compared to those for the North American craton. It is found that the observed inclination and declination values deviate significantly from the values predicted for the present-day position of the Alexander terrane (55.5N, 133.5W). Better matching can be obtained for a paleoposition of the terrane at about 40N, 120W, in the present position of western Nevada and northeastern California. In addition, an in situ 25° clockwise rotation of the terrane is required to restore it to its original position.

  5. ACCRETION OUTBURSTS IN CIRCUMPLANETARY DISKS

    SciTech Connect

    Lubow, S. H.; Martin, R. G.

    2012-04-20

    We describe a model for the long-term evolution of a circumplanetary disk that is fed mass from a circumstellar disk and contains regions of low turbulence (dead zones). We show that such disks can be subject to accretion-driven outbursts, analogous to outbursts previously modeled in the context of circumstellar disks to explain FU Ori phenomena. Circumplanetary disks around a proto-Jupiter can undergo outbursts for infall accretion rates onto the disks in the range M-dot{sub infall} approx. 10{sup -9} to 10{sup -7} M{sub Sun} yr{sup -1}, typical of accretion rates in the T Tauri phase. During outbursts, the accretion rate and disk luminosity increases by several orders of magnitude. Most of the planet mass growth during planetary gas accretion may occur via disk outbursts involving gas that is considerably hotter than predicted by steady state models. For low infall accretion rates M-dot{sub infall} {approx}< 10{sup -10} M{sub sun} yr{sup -1} that occur in late stages of disk accretion, disk outbursts are unlikely to occur, even if dead zones are present. Such conditions are favorable for the formation of icy satellites.

  6. Modes of crustal accretion and their implications for hydrothermal circulation

    NASA Astrophysics Data System (ADS)

    Theissen-Krah, Sonja; Rüpke, Lars H.; Hasenclever, Jörg

    2016-02-01

    Hydrothermal convection at mid-ocean ridges links the ocean's long-term chemical evolution to solid earth processes, forms hydrothermal ore deposits, and sustains the unique chemosynthetic vent fauna. Yet the depth extent of hydrothermal cooling and the inseparably connected question of how the lower crust accretes remain poorly constrained. Here based on coupled models of crustal accretion and hydrothermal circulation, we provide new insights into which modes of lower crust formation and hydrothermal cooling are thermally viable and most consistent with observations at fast-spreading ridges. We integrate numerical models with observations of melt lens depth, thermal structure, and melt fraction. Models matching all these observations always require a deep crustal-scale hydrothermal flow component and less than 50% of the lower crust crystallizing in situ.

  7. Properties of accretion disk coronae

    NASA Technical Reports Server (NTRS)

    Wilms, J.; Dove, J.; Staubert, R.; Begelman, M. C.

    1997-01-01

    The properties of accretion disk corona in a parameter regime suitable for Galactic black hole candidates are considered and the results of an analysis of these properties using a self-consistent Monte Carlo code are presented. Examples of the coronal temperature structure, the shape and angular dependency of the spectrum and the maximum temperature allowed for each optical depth of the corona are presented. It is shown that the observed spectrum of the Galactic black hole candidate Cygnus X-1 cannot be explained by accreting disk corona models with a slab geometry, where the accretion disk is sandwiched by the comptonizing medium.

  8. Wind accretion: Theory and observations

    NASA Astrophysics Data System (ADS)

    Shakura, N. I.; Postnov, K. A.; Kochetkova, A. Yu.; Hjalmarsdotter, L.; Sidoli, L.; Paizis, A.

    2015-07-01

    A review of wind accretion in high-mass X-ray binaries is presented. We focus on different regimes of quasi-spherical accretion onto the neutron star (NS): the supersonic (Bondi) accretion, which takes place when the captured matter cools down rapidly and falls supersonically towards the NS magnetosphere, and subsonic (settling) accretion which occurs when plasma remains hot until it meets the magnetospheric boundary. These two regimes of accretion are separated by an X-ray luminosity of about 4 × 1036 erg s-1. In the subsonic case, which sets in at lower luminosities, a hot quasi-spherical shell must form around the magnetosphere, and the actual accretion rate onto NS is determined by the ability of the plasma to enter the magnetosphere due to Rayleigh-Taylor instability. In turn, two regimes of subsonic accretion are possible, depending on plasma cooling mechanism (Compton or radiative) near the magnetopshere. The transition from the high-luminosity with Compton cooling to the lowluminosity (Lx ≲ 3 × 1035 erg s-1) with radiative cooling can be responsible for the onset of the off states repeatedly observed in several low-luminosity slowly accreting pulsars, such as Vela X-1, GX 301-2, and 4U 1907+09. The triggering of the transitionmay be due to a switch in the X-ray beam pattern in response to a change in the optical depth in the accretion column with changing luminosity. We also show that in the settling accretion theory, bright X-ray flares (~1038-1040 erg) observed in supergiant fast X-ray transients (SFXT) can be produced by sporadic capture of magnetized stellar wind plasma. At sufficiently low accretion rates, magnetic reconnection can enhance the magnetospheric plasma entry rate, resulting in copious production of X-ray photons, strong Compton cooling and ultimately in unstable accretion of the entire shell. A bright flare develops on the free-fall time scale in the shell, and the typical energy released in an SFXT bright flare corresponds to the mass

  9. Displaced terranes and crustal evolution of the Levant and the eastern Mediterranean

    NASA Astrophysics Data System (ADS)

    Ben-Avraham, Zvi; Ginzburg, Avihu

    1990-08-01

    Geophysical data from the Levant and the eastern Mediterranean suggest that this area can be divided into distinct crustal units of different origins. For example, a marked difference in crustal structure and structural style exists in the Levant between Samaria and Judea, south of the Carmel structure, and the Galilee and Lebanon, north of it. On the other hand, several microcontinental fragments such as Cyprus, and the Eratosthenes and Anaximander Seamounts are embedded within the oceanic crust of the eastern Mediterranean. The present location of these units has resulted from complex tectonic processes which were active during the mid Paleozoic-Mesozoic evolution of this area. During this period several crustal units were accreted to the northern margin of Africa while others were detached away from it. The crustal evolution of the eastern Mediterranean and the Levant is intimately linked with the closure of the Paleo-Tethys, the formation of the Neo-Tethys, and its subsequent closure.

  10. Phase separation in the crust of accreting neutron stars.

    PubMed

    Horowitz, C J; Berry, D K; Brown, E F

    2007-06-01

    Nucleosynthesis, on the surface of accreting neutron stars, produces a range of chemical elements. We perform molecular dynamics simulations of crystallization to see how this complex composition forms new neutron star crust. We find chemical separation, with the liquid ocean phase greatly enriched in low atomic number elements compared to the solid crust. This phase separation should change many crust properties such as the thermal conductivity and shear modulus. PMID:17677319

  11. Far-travelled permian chert of the North Fork terrane, Klamath mountains, California

    USGS Publications Warehouse

    Mankinen, E.A.; Irwin, W.P.; Blome, C.D.

    1996-01-01

    Permian chert in the North Fork terrane and correlative rocks of the Klamath Mountains province has a remanent magnetization that is prefolding and presumably primary. Paleomagnetic results indicate that the chert formed at a paleolatitude of 8.6?? ?? 2.5?? but in which hemisphere remains uncertain. This finding requires that these rocks have undergone at least 8.6?? ?? 4.4?? of northward transport relative to Permian North America since their deposition. Paleontological evidence suggests that the Permian limestone of the Eastern Klamath terrane originated thousands of kilometers distant from North America. The limestone of the North Fork terrane may have formed at a similar or even greater distance as suggested by its faunal affinity to the Eastern Klamath terrane and more westerly position. Available evidence indicates that convergence of the North Fork and composite Central Metamorphic-Eastern Klamath terranes occurred during Triassic or Early Jurassic time and that their joining together was a Middle Jurassic event. Primary and secondary magnetizations indicate that the new composite terrane containing these and other rocks of the Western Paleozoic and Triassic belt behaved as a single rigid block that has been latitudinally concordant with the North American craton since Middle Jurassic time.

  12. He-accreting white dwarfs: accretion regimes and final outcomes

    NASA Astrophysics Data System (ADS)

    Piersanti, L.; Tornambé, A.; Yungelson, L. R.

    2014-12-01

    The behaviour of carbon-oxygen (CO) white dwarfs (WDs) subject to direct helium accretion is extensively studied. We aim to analyse the thermal response of an accreting WD to mass deposition at different timescales. The analysis has been performed for initial WD masses and accretion rates in the range 0.60-1.02 M⊙ and 10-9-10-5 M⊙ yr-1, respectively. Thermal regimes in the parameter space MWD-dot{M}_He leading to formation of red-giant-like structures, steady burning of He, and mild, strong and dynamical flashes have been identified and the transition between these regimes has been studied in detail. In particular, the physical properties of WDs experiencing the He-flash accretion regime have been investigated to determine the mass retention efficiency as a function of the accretor total mass and accretion rate. We also discuss to what extent the building up of a He-rich layer via H burning could be described according to the behaviour of models accreting He-rich matter directly. Polynomial fits to the obtained results are provided for use in binary population synthesis computations. Several applications for close binary systems with He-rich donors and CO WD accretors are considered and the relevance of the results for interpreting He novae is discussed.

  13. Effects of the Yakutat terrane collision with North America on the neighboring Pacific plate

    NASA Astrophysics Data System (ADS)

    Reece, R.; Gulick, S. P.; Christeson, G. L.; Barth, G. A.; van Avendonk, H.

    2011-12-01

    High-resolution bathymetry data show a 30 km N-S trending ridge within the deep-sea Surveyor Fan between the mouths of the Yakutat Sea Valley and Bering Trough in the Gulf of Alaska. The ridge originates in the north, perpendicular to and at the base of the continental slope, coincident with the Transition Fault, the strike-slip boundary between the Yakutat terrane (YAK) and the Pacific plate (PAC). The ridge exhibits greatest relief adjacent to the Transition Fault, and becomes less distinct farther from the shelf edge. Seismic reflection data reveal a sharp basement high beneath the ridge (1.1 sec of relief above "normal" basement in two-way travel time) as well as multiple similarly oriented strike-slip fault segments. The ridge, basement high, and faults are aligned and co-located with an intraplate earthquake swarm on the PAC, which includes four events > 6.5 Mw that occurred from 1987-1992. The swarm is defined by right-lateral strike-slip events, and is collectively called the Gulf of Alaska Shear Zone (GASZ). Based on the extent of historic seismicity, the GASZ extends at least 230 km into the PAC, seemingly ending at the Kodiak-Bowie Seamount Chain. Farther southwest, between the Kodiak-Bowie and Patton-Murray Seamount Chains, there is a large regional bathymetric low with an axis centered along the Aja Fracture Zone, perpendicular to the GASZ and Aleutian Trench. Basement and overlying sediment in the low are irregularly, but pervasively faulted. The GASZ and faulted bathymetric low could represent PAC deformation due to PAC-YAK coupling whereby YAK resistance to subduction is expressed as deformation in the thinner (weaker) PAC crust. The YAK is an allochthonous, basaltic terrane coupled to the PAC that began subducting at a low angle beneath North America (NA) ~25-40 Ma. Due to its 15-25 km thickness, the YAK is resistant to subduction compared to the normal oceanic crust of the PAC. As a result the plates developed differential motion along the

  14. Nd-Hf isotopic mapping of Late Mesozoic granitoids in the East Qinling orogen, central China: Constraint on the basements of terranes and distribution of Mo mineralization

    NASA Astrophysics Data System (ADS)

    Wang, Xiaoxia; Wang, Tao; Ke, Changhui; Yang, Yang; Li, Jinbao; Li, Yinghong; Qi, Qiuju; Lv, Xingqiu

    2015-05-01

    during Archaean to Neoproterozic, different from a typical accretion orogen. The old sources of the granitoids and basements of the terranes constrain the distribution, scale and number of the Mo mineralization and deposits. Mo mineralization is closely related to the small granitic bodies with old continental component sources and Mo deposits are mainly hosted by the terranes with oldest basement. The scale and number of the Mo mineralization and deposits decreased from the southern margin of the NCB to SQB.

  15. Dynamics of continental accretion.

    PubMed

    Moresi, L; Betts, P G; Miller, M S; Cayley, R A

    2014-04-10

    Subduction zones become congested when they try to consume buoyant, exotic crust. The accretionary mountain belts (orogens) that form at these convergent plate margins have been the principal sites of lateral continental growth through Earth's history. Modern examples of accretionary margins are the North American Cordilleras and southwest Pacific subduction zones. The geologic record contains abundant accretionary orogens, such as the Tasmanides, along the eastern margin of the supercontinent Gondwana, and the Altaïdes, which formed on the southern margin of Laurasia. In modern and ancient examples of long-lived accretionary orogens, the overriding plate is subjected to episodes of crustal extension and back-arc basin development, often related to subduction rollback and transient episodes of orogenesis and crustal shortening, coincident with accretion of exotic crust. Here we present three-dimensional dynamic models that show how accretionary margins evolve from the initial collision, through a period of plate margin instability, to re-establishment of a stable convergent margin. The models illustrate how significant curvature of the orogenic system develops, as well as the mechanism for tectonic escape of the back-arc region. The complexity of the morphology and the evolution of the system are caused by lateral rollback of a tightly arcuate trench migrating parallel to the plate boundary and orthogonally to the convergence direction. We find geological and geophysical evidence for this process in the Tasmanides of eastern Australia, and infer that this is a recurrent and global phenomenon. PMID:24670638

  16. Metamorphism of San Antonio Terrane metapelites, San Gabriel Mountains, California

    SciTech Connect

    Archuleta, L.; Ishimatsu, J.; Schneiderman, J.S. . Geology Dept.)

    1993-04-01

    Pelitic schists and gneisses from the San Antonio terrane in the eastern San Gabriel Mountains consist of garnet, biotite, plagioclase, quartz, sillimanite, cordierite, hercynite [+-] alkali feldspar. Large garnet porphyroblasts contain quartz, plagioclase and sillimanite inclusions. Cordierite occurs as haloes around garnet porphyroblasts and as small subgrains always associated with hercynite and together replacing sillimanite blades. Hercynite additionally appears to have nucleated on the edges of sillimanite blades. Contrary to previous investigations, hercynite appears to be a late mineral phase. Reaction textures described above have been used to calculate a set of net-transfer reactions that can be used (1) to characterize all possible exchanges of matter between minerals in the system and (2) to construct a reaction space for the system. Fourteen thin sections with large garnet porphyroblasts and abundant biotite were used for microprobe analysis. Detailed probe analyses show well-developed zoning in the plagioclase and alkali feldspar whose character varies depending on location in the thin section relative to neighboring minerals. Generally, large plagioclase porphyroblasts display normal zoning and are not as calcium-rich as plagioclase inclusions in the garnet. Garnet porphyroblasts have flat zoning profiles due to high temperatures of metamorphism. Pressures and temperatures of metamorphism have been calculated from these assemblages using garnet-biotite geothermometry and quartz-garnet-aluminosilicate-plagioclase geobarometry.

  17. Geophysical constraints on the lunar Procellarum KREEP Terrane

    NASA Astrophysics Data System (ADS)

    Grimm, Robert E.

    2013-04-01

    The Moon's Procellarum KREEP Terrane (PKT) is distinguished by unique geochemistry and extended volcanic history. Previous thermal-conduction models using enhanced radionuclide abundances in subcrustal potassium, rare earth elements, and phosphorus (KREEP) predicted the existence of a contemporary upper-mantle melt zone as well as heat flow consistent with Apollo measurements. Here I show that such models also predict large gravity or topography anomalies that are not observed. If the topography is suppressed by a rigid lithosphere, it is possible to eliminate the gravity anomaly and still match heat flow by completely fractionating the excess radionuclides into a thin crust. This implies that upper-mantle heat sources for mare volcanism were spatially discontinuous or transient and that radionuclides defining the PKT are not necessarily directly related to mare volcanic sources. However, the mantle temperature of a crustally fractionated PKT is insufficient to match the observed electrical conductivity: globally enhanced mantle heating or a thick megaregolith may be required. Alternatively, upper-mantle enrichment in iron, hydrogen, or aluminum can provide the requisite conductivity. Iron is the most plausible: the derived lower limit to the upper-mantle magnesium number 75-80% is consistent with seismic modeling. Regardless of the specific mechanism for electrical-conductivity enhancement, the overall excellent match to simple thermal-conduction models indicates that the lunar upper mantle is not convecting at present.

  18. Galactic Fountains and Gas Accretion

    NASA Astrophysics Data System (ADS)

    Marinacci, F.; Binney, J.; Fraternali, F.; Nipoti, C.; Ciotti, L.; Londrillo, P.

    2010-06-01

    Star-forming disc galaxies such as the Milky Way need to accrete >~1 Msolar of gas each year to sustain their star formation. This gas accretion is likely to come from the cooling of the hot corona, however it is still not clear how this process can take place. We present simulations supporting the idea that this cooling and the subsequent accretion are caused by the passage of cold galactic-fountain clouds through the hot corona. The Kelvin-Helmholtz instability strips gas from these clouds and the stripped gas causes coronal gas to condense in the cloud's wake. For likely parameters of the Galactic corona and of typical fountain clouds we obtain a global accretion rate of the order of that required to feed the star formation.

  19. Structure, age, and geodynamic settings of early Neoproterozoic magmatic complexes of the Central Asian fold belt exemplified by the Holbo Nur zone of Songin terrane

    NASA Astrophysics Data System (ADS)

    Yarmolyuk, V. V.; Kozlovsky, A. M.; Salnikova, E. B.; Kozakov, I. K.; Kovach, V. P.; Kotov, A. B.; Tomurtogoo, O.

    2015-11-01

    The Holbo Nur zone of Songin terrane in the Central Asian fold belt is a rare example of slightly metamorphosed paleoceanic complexes that were formed at the initial stages of the Paleoasian ocean evolution. The zone encompasses three tectonic slices. Rhyolites of the island arc series have been dated back to 888 ± 2 Ma; the formation of the island arc lasted from 888 to 859 Ma. The differences in the geological structure, geochemical and isotopic characteristics of igneous rocks constituting the tectonic slices of the Holbo Nur zone imply spatial isolation of these complexes during their formation in the Paleoasian ocean. They were tectonically juxtaposed between 859 and 790 Ma ago, which is evident from postkinematic granitoids.

  20. The Ust-Belaya ophiolite terrane, West Koryak Orogen: Isotopic dating and paleotectonic interpretation

    NASA Astrophysics Data System (ADS)

    Palandzhyan, S. A.

    2015-03-01

    The Ust-Belaya ophiolite terrane in the West Koryak Orogen, which is the largest in northeastern Asia, consists of three nappe complexes. The upper Ust-Belaya Nappe is composed of a thick (>5 km) sheet of fertile peridotites and mafic rocks (remnants of the proto-Pacific lithosphere); its upper age boundary is marked by Late Neoproterozoic plagiogranites. In the middle Tolovka-Otrozhny Nappe, the Late Precambrian lherzolite-type ophiolites are supplemented by fragments of tectonically delaminated harzburgite-type ophiolites, which make up the Tolovka rock association. The isotopic age of metadacite (K-Ar method, whole-rock sample) and zircons from plagiogranite porphyry (U-Pb method, SHRIMP) determines the upper chronological limit of the Tolovka ophiolites as 262-265 Ma ago. It is suggested that igneous rocks of these ophiolites were generated in a backarc basin during the Early Carboniferous and then incorporated into the fold-nappe structure in the Mid-Permian. This was the future basement of the Koni-Taigonos arc, where the Early Carboniferous ophiolites together with Late Neoproterozoic precursors were subject to low-temperature metamorphism and intruded by plagiogranite porphyry dikes in Permian-Triassic. The polymicte serpentinite mélange, which was formed in the accretionary complex of the Koni-Taigonos arc comprises rock blocks of the upper units of Late Precambrian ophiolites (in particular, plagiogranite), the overlying Middle to Upper Devonian and Early Carboniferous deposits, as well as Early Carboniferous (?) Tolovka ophiolites and meta-ophiolites. Mélange of this type with inclusions of Late Precambrian "oceanic" granitoids also developed in the lower Utyosiki Nappe composed of Middle Jurassic-Lower Cretaceous sedimentary and volcanic sequences, the formation of which was related to the next Uda-Murgal island-arc systems.

  1. ACCRETING CIRCUMPLANETARY DISKS: OBSERVATIONAL SIGNATURES

    SciTech Connect

    Zhu, Zhaohuan

    2015-01-20

    I calculate the spectral energy distributions of accreting circumplanetary disks using atmospheric radiative transfer models. Circumplanetary disks only accreting at 10{sup –10} M {sub ☉} yr{sup –1} around a 1 M{sub J} planet can be brighter than the planet itself. A moderately accreting circumplanetary disk ( M-dot ∼10{sup −8} M{sub ⊙} yr{sup −1}; enough to form a 10 M{sub J} planet within 1 Myr) around a 1 M{sub J} planet has a maximum temperature of ∼2000 K, and at near-infrared wavelengths (J, H, K bands), this disk is as bright as a late-M-type brown dwarf or a 10 M{sub J} planet with a ''hot start''. To use direct imaging to find the accretion disks around low-mass planets (e.g., 1 M{sub J} ) and distinguish them from brown dwarfs or hot high-mass planets, it is crucial to obtain photometry at mid-infrared bands (L', M, N bands) because the emission from circumplanetary disks falls off more slowly toward longer wavelengths than those of brown dwarfs or planets. If young planets have strong magnetic fields (≳100 G), fields may truncate slowly accreting circumplanetary disks ( M-dot ≲10{sup −9} M{sub ⊙} yr{sup −1}) and lead to magnetospheric accretion, which can provide additional accretion signatures, such as UV/optical excess from the accretion shock and line emission.

  2. Baseline Assessment of Net Calcium Carbonate Accretion Rates on U.S. Pacific Reefs.

    PubMed

    Vargas-Ángel, Bernardo; Richards, Cristi L; Vroom, Peter S; Price, Nichole N; Schils, Tom; Young, Charles W; Smith, Jennifer; Johnson, Maggie D; Brainard, Russell E

    2015-01-01

    This paper presents a comprehensive quantitative baseline assessment of in situ net calcium carbonate accretion rates (g CaCO3 cm(-2) yr(-1)) of early successional recruitment communities on Calcification Accretion Unit (CAU) plates deployed on coral reefs at 78 discrete sites, across 11 islands in the central and south Pacific Oceans. Accretion rates varied substantially within and between islands, reef zones, levels of wave exposure, and island geomorphology. For forereef sites, mean accretion rates were the highest at Rose Atoll, Jarvis, and Swains Islands, and the lowest at Johnston Atoll and Tutuila. A comparison between reef zones showed higher accretion rates on forereefs compared to lagoon sites; mean accretion rates were also higher on windward than leeward sites but only for a subset of islands. High levels of spatial variability in net carbonate accretion rates reported herein draw attention to the heterogeneity of the community assemblages. Percent cover of key early successional taxa on CAU plates did not reflect that of the mature communities present on surrounding benthos, possibly due to the short deployment period (2 years) of the experimental units. Yet, net CaCO3 accretion rates were positively correlated with crustose coralline algae (CCA) percent cover on the surrounding benthos and on the CAU plates, which on average represented >70% of the accreted material. For foreeefs and lagoon sites combined CaCO3 accretion rates were statistically correlated with total alkalinity and Chlorophyll-a; a GAM analysis indicated that SiOH and Halimeda were the best predictor variables of accretion rates on lagoon sites, and total alkalinity and Chlorophyll-a for forereef sites, demonstrating the utility of CAUs as a tool to monitor changes in reef accretion rates as they relate to ocean acidification. This study underscores the pivotal role CCA play as a key benthic component and supporting actively calcifying reefs; high Mg-calcite exoskeletons makes CCA

  3. Baseline Assessment of Net Calcium Carbonate Accretion Rates on U.S. Pacific Reefs

    PubMed Central

    Vargas-Ángel, Bernardo; Richards, Cristi L.; Vroom, Peter S.; Price, Nichole N.; Schils, Tom; Young, Charles W.; Smith, Jennifer; Johnson, Maggie D.; Brainard, Russell E.

    2015-01-01

    This paper presents a comprehensive quantitative baseline assessment of in situ net calcium carbonate accretion rates (g CaCO3 cm-2 yr-1) of early successional recruitment communities on Calcification Accretion Unit (CAU) plates deployed on coral reefs at 78 discrete sites, across 11 islands in the central and south Pacific Oceans. Accretion rates varied substantially within and between islands, reef zones, levels of wave exposure, and island geomorphology. For forereef sites, mean accretion rates were the highest at Rose Atoll, Jarvis, and Swains Islands, and the lowest at Johnston Atoll and Tutuila. A comparison between reef zones showed higher accretion rates on forereefs compared to lagoon sites; mean accretion rates were also higher on windward than leeward sites but only for a subset of islands. High levels of spatial variability in net carbonate accretion rates reported herein draw attention to the heterogeneity of the community assemblages. Percent cover of key early successional taxa on CAU plates did not reflect that of the mature communities present on surrounding benthos, possibly due to the short deployment period (2 years) of the experimental units. Yet, net CaCO3 accretion rates were positively correlated with crustose coralline algae (CCA) percent cover on the surrounding benthos and on the CAU plates, which on average represented >70% of the accreted material. For foreeefs and lagoon sites combined CaCO3 accretion rates were statistically correlated with total alkalinity and Chlorophyll-a; a GAM analysis indicated that SiOH and Halimeda were the best predictor variables of accretion rates on lagoon sites, and total alkalinity and Chlorophyll-a for forereef sites, demonstrating the utility of CAUs as a tool to monitor changes in reef accretion rates as they relate to ocean acidification. This study underscores the pivotal role CCA play as a key benthic component and supporting actively calcifying reefs; high Mg-calcite exoskeletons makes CCA

  4. Final amalgamation of the Central Asian Orogenic Belt in NE China: Paleo-Asian Ocean closure versus Paleo-Pacific plate subduction - A review of the evidence

    NASA Astrophysics Data System (ADS)

    Wilde, Simon A.

    2015-11-01

    The Central Asian Orogenic Belt (CAOB) evolved through complex closure of the Paleo-Asian Ocean from the Neoproterozoic to the late Phanerozoic. This caused the Chinese cratons to collide with Eurasia and led to the formation of the world's largest Phanerozoic orogenic belt. Ocean closure commenced in the west and was completed in the east near Changchun. Closure of the Paleo-Asian Ocean in NE China was along the Solonker-Xar Moron-Changchun-Yanji suture and this was likely completed in the Late Permian, although associated activity continued into the Triassic. There was an overlap in the latest Permian-Early Triassic between terminal activity associated with Paleo-Asian Ocean closure and the onset of tectonism associated with subduction of the Paleo-Pacific plate. This switch in geodynamic setting occurred at ~ 260-250 Ma, and is reflected by a relaxing of north-south directed compression and the onset of east-west directed processes related to Paleo-Pacific subduction. By the Early Jurassic, events associated with the westward advance of the Paleo-Pacific plate dominated, leading to extensive development of I-type granites as far inland as the Great Xing'an Range. From ~ 140 Ma, the Paleo-Pacific plate retreated eastward, resulting in an extensional setting in the Early Cretaceous, the effects of which were enhanced by regional thinning of the lithosphere, commonly attributed to delamination. Throughout this period, the eastern Asian margin was tectonically complex. The north-south oriented Jiamusi-Khanka(-Bureya) block was rifted away from the eastern margin of the CAOB in the Late Triassic, but was then re-united in the Jurassic by westward-advancing subduction that affected both the western and eastern margins of the block. Accretionary complexes continued to evolve in the Cretaceous along the whole eastern margin of Asia, with final accretion of the Nadanhada Terrane (part of the Sikhote-Alin accretionary terrane) with the CAOB at ~ 130 Ma, followed by the

  5. Mediterranean-style closure of the Paleo-Tethys ocean

    NASA Astrophysics Data System (ADS)

    Pullen, A.; Kapp, P.; Gehrels, G. E.

    2008-12-01

    The Alpine-Himalayan belt expresses lateral variations in the architecture of continent-continent collisions and includes two end-member examples. One end-member, the Mediterranean, formed from a complex interaction between the Eurasian and Africa-Apulian plates which initiated rollback of Alpine-Tethys lithosphere and opened new ocean basins coeval with plate convergence. As the other end-member, the India-Asia collision is generally considered to have initiated following the closure of a single large ocean basin through the subduction of Neo-Tethys oceanic lithosphere beneath Eurasia. We propose that Mediterranean-style tectonics may have dominated during the closure of the Paleo-Tethys in central Asia based on new and previous observations. These include: (1) coeval Mid-Triassic HP-UHP metamorphism within the Qinling-Dabie orogen, Qiangtang metamorphic belt, and Indochina block; (2) coeval Late Triassic exhumation of the Qinling-Dabie orogen and Qiangtang metamorphic belt and accumulation of vast volumes of Upper Triassic flysch within the Paleo-Tethys ocean; (3) the north-south orientation of the Longman Shan thrust belt, the triangular map pattern of the Hoh-Xil-Songpan-Ganzi flysch complex, the oroclinal map patterns of Paleo-Tethys arc terranes, and the abrupt eastern termination of the Kunlun; (4) wide distribution of 224-210 Ma adakitic rocks; and (5) evidence for Late Triassic extension within terranes adjacent to the Paleo-Tethys. We propose near synchronous Mid-Triassic continent-continent collisions between the Qiangtang terrane of western Cimmerian-Gondwanan affinity and a Paleo-Tethys arc terrane or the Kunlun terrane, eastern Cimmeria-Indochina and South China block, and the South China and North China blocks. Resistance to continental subduction and oceanic slab breakoff led to a decrease in the northward drift of Gondwana-derived terranes and promoted rollback of subducting remnant Paleo-Tethys oceanic lithosphere. In particular, we suggest that

  6. Crustal melting and magma mixing in a continental arc setting: Evidence from the Yaloman intrusive complex in the Gorny Altai terrane, Central Asian Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Chen, Ming; Sun, Min; Buslov, Mikhail M.; Cai, Keda; Zhao, Guochun; Kulikova, Anna V.; Rubanova, Elena S.

    2016-05-01

    Devonian granitoids from the Gorny Altai terrane show remarkable temporal-spatial-petrogenetic affinities to the counterparts from the Altai-Mongolian terrane, indicating that these two terranes were possibly under subduction of the same oceanic plate (i.e., the Ob-Zaisan Ocean). The voluminous granitoids signify significant crustal recycling and growth as a response to the underplating of extensive mantle-derived basaltic melts.

  7. A kinematic model for the formation of the Siletz terrane by capture of coherent fragments of the Farallon and Resurrection plates (Invited)

    NASA Astrophysics Data System (ADS)

    Wilson, D. S.; McCrory, P. A.

    2010-12-01

    The details of the formation of the mostly Eocene basaltic Siletz terrane in the coast ranges of Oregon and Washington have remained enigmatic. Early models [e.g. Duncan, 1982 JGR], noting younger ages for basalts and overlying foramifera in the center of the terrane near the Columbia River and older ages to the north and south, called for an interpretation of a captured fossil spreading ridge between the Kula and Farallon plates. However, later work showed two problems with this simple model [Wells et al., 1984 Tectonics]. First, the difference between old and young ages was larger than predicted by the relatively fast spreading rate. Second, the presence of proximal continental sediments interbedded with some of the older basalts conflicted with the large amounts of convergence predicted by the generally used models for motion of the Kula and Farallon plates relative to North America. The recent suggestion that an additional oceanic plate, the Resurrection plate, was present between the Kula and Farallon plates and subducting beneath eastern Alaska and western Canada during the early Cenozoic [Haeussler et al., 2003 GSAB] permits important revisions to the interpretation of regional tectonic history. We propose a model in which the older (~55 Ma) parts of the Siletz terrane formed at the Resurrection-Farallon ridge, not far offshore from the Klamath Mountains. Proximity to the Yellowstone hotspot could have generated thick oceanic crust in a setting analogous to Iceland. When the Juan de Fuca (Vancouver) plate separated from the Farallon plate at ~52 Ma, we propose that a small Siletz plate also separated from the northern Juan de Fuca plate, eventually to be captured in the Oregon coast range. A fragment of the Resurrection plate may have separated later (~48 Ma), eventually captured in the Washington coast range and constituting the southward-younging part of the terrane. Cessation of motion of these fragments relative to adjacent parts of North America may

  8. Is the Liverpool Land Eclogite Terrane, Eastern Greenland Caledonides, Baltican, Laurentian, or Both?

    NASA Astrophysics Data System (ADS)

    Brueckner, H. K.; Johnston, S. M.; Belousova, E. A.; Griffin, W. L.; Hartz, E. H.

    2014-12-01

    The Liverpool land Eclogite Terrane (LLET) is a small (≈200 km2) high pressure metamorphic terrane at the southern tip of the Eastern Greenland Caledonides. Recent publications propose the LLET is a subducted Baltic terrane that underplated the Eastern Greenland Caledonides during the ca. 400 Ma Scandian Orogeny and was stranded in Greenland (Laurentia) during the opening of the Atlantic. However Johnston et al. (2011) proposed the LLET is actually a composite terrane comprised of a mid-crustal, high-pressure (granulite facies) terrane, the Jaettedal Complex, tectonically juxtaposed during the Scandian orogeny against a higher pressure (eclogite-facies), peridotite-bearing, metamorphic terrane, the Tvaerdal complex. The composite Tvaerdal/Jaettedal Terrane (now the LLET) was subsequently exhumed from beneath a Laurentian plutonic continental arc terrane, the Hurry Inlet Complex. U-Pb Precambrian ages from the Tvaerdal Complex obtained by LA-ICPMS from detrital zircons (this study) as well as published zircon dates do not distinctly match the published Proterozoic zircon age patterns of basins and crystalline complexes from either Laurentia or Baltica, therefore not resolving the provenance of the Tvaerdal Complex. However, Paleozoic zircon U-Pb dates (this study and the literature) demonstrate the Tvaerdal Complex underwent relatively short-lived high pressure/ultrahigh pressure metamorphism and subsequent anatectic melting between 415-385 Ma while recent work by Johnston et al. (2014) indicates the Jaettedal Complex underwent an earlier, much longer evolution between 460-412 Ma. The older ages from the Jaettedal coincide with ages obtained from the overlying Hurry Inlet Complex as well as other Greenland igneous terranes suggesting the Jaettedal Complex is a Laurentian terrane involved in the lengthy evolution of a continental arc complex along the eastern Greenland margin during the closure of Iapetus. The paucity of concordant 460-412 ages from the Tvaerdal

  9. How do accretion discs break?

    NASA Astrophysics Data System (ADS)

    Dogan, Suzan

    2016-07-01

    Accretion discs are common in binary systems, and they are often found to be misaligned with respect to the binary orbit. The gravitational torque from a companion induces nodal precession in misaligned disc orbits. In this study, we first calculate whether this precession is strong enough to overcome the internal disc torques communicating angular momentum. We compare the disc precession torque with the disc viscous torque to determine whether the disc should warp or break. For typical parameters precession wins: the disc breaks into distinct planes that precess effectively independently. To check our analytical findings, we perform 3D hydrodynamical numerical simulations using the PHANTOM smoothed particle hydrodynamics code, and confirm that disc breaking is widespread and enhances accretion on to the central object. For some inclinations, the disc goes through strong Kozai cycles. Disc breaking promotes markedly enhanced and variable accretion and potentially produces high-energy particles or radiation through shocks. This would have significant implications for all binary systems: e.g. accretion outbursts in X-ray binaries and fuelling supermassive black hole (SMBH) binaries. The behaviour we have discussed in this work is relevant to a variety of astrophysical systems, for example X-ray binaries, where the disc plane may be tilted by radiation warping, SMBH binaries, where accretion of misaligned gas can create effectively random inclinations and protostellar binaries, where a disc may be misaligned by a variety of effects such as binary capture/exchange, accretion after binary formation.

  10. Oceanic Plateaus

    NASA Astrophysics Data System (ADS)

    Kerr, A. C.

    2003-12-01

    al., 1992; Stein and Hofmann, 1994), as well as environmental effects on the global biosphere (e.g., Caldeira and Rampino, 1990; Courtillot et al., 1996; Kerr, 1998). Oceanic plateaus can also become accreted to continental margins, and it has been proposed that these plateaus have been significant contributors to the growth of continental crust (e.g., Abbott, 1996; Albarede, 1998).The most recent major phase of oceanic plateau formation was in the Cretaceous when the Ontong Java, Manihiki, Hess Rise, and the Caribbean-Colombian plateaus formed in the Pacific, while in the Indian Ocean the Kerguelen plateau was developing. The areas, volume maximum thicknesses and ages of the larger of these plateaus are given in Table 1. The Ontong Java is the largest of the Cretaceous plateaus. It covers an area of 1.9×106 km2, and has an estimated total volume of 4.4×107 km3 (Eldholm and Coffin, 2000). Although early seismic refraction data suggested that the OJP was as thick as 43 km ( Furomoto et al., 1976), a more recent synthesis based on existing seismic and new gravity data ( Gladczenko et al., 1997) has indicated the average thickness to be ˜32 km. Table 1. Ages and dimensions of Jurassic-Cretaceous oceanic plateaus Oceanic plateauMean age (Ma)Area (106 km2)Thickness range (km)Volume (106 km3) Hikurangiearly-mid Cretaceous0.710-152.7 Shatsky Rise1470.210-282.5 Magellan Rise1450.5101.8 Manihiki1230.8>208.8 Ontong Java121(90)1.915-3244.4 Hess Rise990.8>159.1 Caribbean881.18-204.4 South Kerguelen1101.0˜226.0 Central Kerguelen/Broken Ridge861.019-219.1 Sierra Leone Rise˜730.9>102.5 Maud Rise˜>730.2>101.2 After Eldholm and Coffin (2000).

  11. Internal structure of oceanic lithosphere: A perspective from tectonic windows

    NASA Astrophysics Data System (ADS)

    Karson, Jeffrey A.

    layered structure occur near transform faults, but also occur along typical rift valley walls tens of kilometers away from obvious ridge-axis discontinuities. Under some circumstances, large-scale stretching and thinning of the lithosphere results in the formation of "oceanic core complexes" analogous to those of highly extended continental terranes. Tectonic windows into regions of variable magma budget commonly show that: 1) some of the ophiolite rock units are missing, 2) contacts between major rock units are neither horizontal nor laterally continuous, and 3) internal structures of major rock units are highly variable. In several intensively studied locations, volcanic units appear to lie directly (unconformably) over variably deformed volumes of metadiabase, metagabbro, or serpentinite. These relations reflect a sputtering magma supply and/or heterogeneous magmatic accretion across axial valleys over periods of tens to hundreds of thousands of years. Structurally complex oceanic crust that lacks a simple layered structure is likely to be a typical product of many slow-spreading ridges. At intermediate- to fast-spreading ridges the magma budget appears to be consistently higher and accordingly magmatic construction dominates. Spreading center morphology suggests only limited faulting; however, available geologic data suggest that the oceanic crust created in these environments may be more variable and geologically complex than generally thought. Although tectonic windows in fast-spread crust are rare, the crustal structures seen in them show variations on the ophiolite model. Surficial lavas are likely to blanket complicated structures that result from subsurface crustal collapse related to growth faulting, dike intrusion, and inflation/deflation of axial summit regions and underlying magma chambers. These mechanisms accommodate significant vertical mass transport at various crustal levels that are required to generate the observed thicknesses of major rock units

  12. Evolution of a steam atmosphere during Earth's accretion.

    PubMed

    Zahnle, K J; Kasting, J F; Pollack, J B

    1988-01-01

    We have modeled the evolution of an impact-generated steam atmosphere surrounding an accreting Earth. The model assumes Safronov accretion; it includes degassing of planetesimals upon impact, thermal blanketing by a steam atmosphere, interchange of water between the surface and the interior, shock heating and convective cooling of Earth's interior, and hydrogen escape, both by a solar extreme ultraviolet (EUV) powered planetary wind and by impact erosion (atmospheric cratering). The model does not include atmophiles other than water, chemical reaction of water with metallic iron, core formation, compression, and spatial and temporal inhomogeneity of accretion. If the incoming planetesimals were too dry or the EUV flux too high, very little water would accumulate at the surface. Essentially all water retained by such a planet would be through rehydration of silicates. If rehydration were inefficient, very little water would be retained in any form. Degassing of wetter planetesimals produces a steam atmosphere over a magma ocean, the energy of accretion being sufficient to maintain a runaway greenhouse atmosphere. The mass of the atmosphere is limited by water's solubility in the (partial) melt. This type of solution is produced for a wide range of model parameters. During accretion, approximately 30 bars of water could have kept the surface at 1500 degrees K. As the accretional energy input declined below the runaway greenhouse threshold, the steam atmosphere rained out. Outgassing of dissolved water at the close of accretion is quantitatively important. These models can leave from approximately 100 to more than 300 bars of water at the surface at the close of accretion. In general, most of the water accreted remains dissolved in the mantle. H2 could have escaped as rapidly as it formed only if the planetesimals were relatively dry. Consequently H2 should have accumulated until it reached chemical equilibrium with water vapor. Impact erosion (escape caused by impact

  13. Anatexis of garnet amphibolites from a subduction zone metamorphic terrane

    SciTech Connect

    Sorensen, S.S.; Barton, M.D.; Ernst, W.G.

    1985-01-01

    Concomitant rehydration, metasomatism and amphibolitization of eclogite blocks from a mafic/ultramafic complex of the Catalina Schist terrane, southern California, at estimated metamorphic P approx. 8-12 kb, T approx. 600/sup 0/-700/sup 0/C was apparently accompanied by partial melting of some blocks. Mobilizates of An approx./sub 10-20/ plagioclase (PL) +/- zoisite (ZO) + quartz (QZ) + celadonitic (Si approx. 3.3 p.f.u.) white mica (WM) +/- tourmaline range from stringers and dikelets (approx. 1 cm-0.5 m) in migmatitic amphibolite blocks to dikes approx. 30 m x 3 m which intrude the surrounding, locally enstatite + chlorite +/- talc +/- aluminous actinolite +/- anthophyllite-bearing ultramafic matrix. The uniform phase proportions and the coarse-grained (PL to approx. 20 cm) pegmatitic, graphic, and myrmekitic textures displayed by the dikes and dikelets suggest that they crystallized from silicate melts. WM and ZO appear to be magmatic phases. Fe-rich GT is migmatitic portions of blocks exhibits higher Mg/(Mg + Ca) p.f.u. than GT in restitic portions of blocks; rims are richer in Mg than cores. Field relations, microprobe mineral chemistry, and bulk compositions suggest the pegmatites are low fractions of amphibolite-derived partial melt. Abundant fluid inclusions occur in GT, QZ, PL and clinopyroxene. T/sub h/ for primary H/sub 2/O-rich, low salinity L + V inclusions in GT and QZ from a migmatite range from 136-169/sup 0/C; estimates of T limits for entrapment are 530-640/sup 0/C at 8 kb, 650-780/sup 0/C at 10 kb. H/sub 2/O-rich fluids evidently enabled metasomatism, amphibolitization, and anatexis of (originally) eclogitic rocks at the P-T conditions reflected by the metamorphic mineral assemblages.

  14. Cyclicity in Silurian island-arc carbonates, Alexander terrane, Alaska

    SciTech Connect

    Kittredge, L.E.; Soja, C.M. . Dept. of Geology)

    1993-03-01

    Silurian carbonates from Alaska (Alexander terrane) record the evolution of a submarine platform during waning volcanism in an island arc. A detailed stratigraphic analysis of a 47 meter-thick sequence revealed the existence of cyclically repeated limestones: coral-stromatoporoid wackestones alternate with oncoid packstones and bioturbated, silty lime mudstones. The coral-stromatoporoid deposits are characterized by a low-diversity assemblage of dendroid corals, massive stromatoporoids, Atrypoidea brachiopods, and rare occurrences of biostromes associated with Solenopora, high-spired gastropods, and crinoids. Oncoids typically are 2-6 mm in diameter and form massive, meter-thick units. Coated grains are symmetrically developed, have a shell or algal nucleus, and are also a minor component of coral-stromatoporoid beds. These lithologic units form seven, shallowing-upwards cycles (parasequences) that range in thickness from 3-9 meters. Coral-stomatoporoid wackestones form the base of each cycle and grade upwards into oncoid packstones with silty, lime mudstones at the top. This succession of lithofacies within each cycle reflects an increase in energy levels from relatively deeper water environments to relatively shallower ones. The lack of abrasion in the corals and stromatoporoids suggests predominantly quiet-water conditions in shallow subtidal areas affected by periodic turbulence. Comparison with correlative sections in Alaska and lack of correspondence with global sea level curves suggest that the primary cause of cyclicity was tectonic perturbations with secondary eustatic effects. Cyclic deposition in peri/subtidal sites was terminated by rapid drowning of the carbonate platform during late Silurian orogenesis.

  15. Classical Accreting Pulsars with NICER

    NASA Technical Reports Server (NTRS)

    Wilson-Hodge, Colleen A.

    2014-01-01

    Soft excesses are very common center dot Lx > 1038 erg/s - reprocessing by optically thick material at the inner edge of the accretion disk center dot Lx < 1036 erg/s - photoionized or collisionally heated diffuse gas or thermal emission from the NS surface center dot Lx 1037 erg/s - either or both types of emission center dot NICER observations of soft excesses in bright X-ray pulsars combined with reflection modeling will constrain the ionization state, metalicity and dynamics of the inner edge of the magnetically truncated accretion disk Reflection models of an accretion disk for a hard power law - Strong soft excess below 3 keV from hot X-ray heated disk - For weakly ionized case: strong recombination lines - Are we seeing changes in the disk ionization in 4U1626-26? 13 years of weekly monitoring with RXTE PCA center dot Revealed an unexpectedly large population of Be/X-ray binaries compared to the Milky Way center dot Plotted luminosities are typical of "normal" outbursts (once per orbit) center dot The SMC provides an excellent opportunity to study a homogenous population of HMXBs with low interstellar absorption for accretion disk studies. Monitoring with NICER will enable studies of accretion disk physics in X-ray pulsars center dot The SMC provides a potential homogeneous low-absorption population for this study center dot NICER monitoring and TOO observations will also provide measurements of spinfrequencies, QPOs, pulsed fluxes, and energy spectra.

  16. Jets from magnetized accretion disks

    NASA Astrophysics Data System (ADS)

    Matsumoto, Ryoji

    When an accretion disk is threaded by large scale poloidal magnetic fields, the injection of magnetic helicity from the accretion disk drives bipolar outflows. We present the results of global magnetohydrodynamic (MHD) simulations of jet formation from a torus initially threaded by vertical magnetic fields. After the torsional Alfvén waves generated by the injected magnetic twists propagate along the large-scale magnetic field lines, magnetically driven jets emanate from the surface of the torus. Due to the magnetic pinch effect, the jets are collimated along the rotation axis. Since the jet formation process extracts angular momentum from the disk, it enhances the accretion rate of the disk material. Through three-dimensional (3D) global MHD simulations, we confirmed previous 2D results that the magnetically braked surface of the disk accretes like an avalanche. Owing to the growth of non-axisymmetric perturbations, the avalanche flow breaks up into spiral channels. Helical structure also appears inside the jet. When magnetic helicity is injected into closed magnetic loops connecting the central object and the accretion disk, it drives recurrent magnetic reconnection and outflows.

  17. Characterizing Accreting White Dwarf Pulsators

    NASA Astrophysics Data System (ADS)

    Szkody, Paula; Mukadam, Anjum

    2014-02-01

    Understanding the population, mass distribution, and evolution of accreting white dwarfs impacts the entire realm of binary interaction, including the creation of Type Ia supernovae. We are concentrating on accreting white dwarf pulsators, as the pulsation properties allow us a view of how the accretion affects the interior of the star. Our ground- based photometry on 11 accreting pulsators with corresponding temperatures from HST UV spectra suggest a broad instability strip in the range of 10500 to 16000K. Additionally, tracking a post-outburst heated white dwarf as it cools and crosses the blue edge and resumes pulsation provides an independent method to locate the empirical instability strip. Determining a post-outburst cooling curve yields an estimate of the amount of heating and the accreted mass during the outburst. We request additional photometry of 2 objects that present unique properties: GW Lib which has not yet returned to its pre-outburst pulsation spectrum after 6 yrs, and EQ Lyn which returned to its pre- outburst pulsation after 3 yrs but is now turning on and off without ongoing outbursts. Following the pulsation spectrum changes over stretches of several nights in a row will provide specific knowledge of the stability of the observed modes.

  18. Metamorphism of tectonic terranes in the eastern marginal zone of the Appalachian orogen, New England

    SciTech Connect

    Hepburn, J.C.; Olszewski, W.J.; Guidotti, C.V.

    1985-01-01

    Southeastern New England is subdivided into three major fault bounded tectonic terranes, each with a distinctive metamorphic history. The easternmost, the Avalon Terrane, has generally been metamorphosed no higher than the lower greenschist facies. Evidence for pre-Alleghanian metamorphism includes a Proterozoic Z(.) event, contact metamorphism adjacent to Ord.-Dev. alkaline plutons, and granulite facies crustal xenoliths in Mesozoic dikes. To the west the Nashoba Terrane has been deformed and polymetamorphosed to the sill. and 2nd sill. zones between approximately 415 and 450 m.y., based on ages of associated granitic and migmatitic rocks. 730 m.y. assumed basement gneisses (fish Brook) have likely experienced Late PC metamorphism. In the Merrimack Trough, here including the Massabesic Gneiss, the metamorphic grade ranges from the greenschist facies on the east to the 2nd sillimanite zones on the west toward the Massabesic. The two metamorphic events present here must predate the intrusion of the Exeter Diorite (473 m.y .), indicating one or both may be PC. To the east, the fault bounded Rye Formation has also experienced two pre- 470 m.y. metamorphisms (and -sill.) However, the terrane east of the Turtle Head Fault Zone (THFZ) has many similarities to the Boston Platform including general (Late PC.) lower greenschist metamorphism. Also, the area between the Norumbega FZ and the THFZ has experienced high grade metamorphism of probable Silurian age and thus may be similar to the Nashoba Terrane.

  19. A northern Cordilleran ocean-continent transect: Sitka Sound, Alaska, to Atlin Lake, British Columbia

    USGS Publications Warehouse

    Brew, D.A.; Karl, S.M.; Barnes, D.F.; Jachens, R.C.; Ford, A.B.; Horner, R.

    1991-01-01

    The 155 km wide, 310 km long Sitka Sound - Atlin Lake continent-ocean transect includes almost all the geologic, geophysical, and geotectonic elements of the Canadian Cordillera. It crosses the Chugach, Wrangellia, Alexander, Stikine, and Cache Creek terranes, the Gravina and Laberge overlap assemblages, intrusive and metamorphic belts, and neotectonic faults that bound major blocks. -from Authors

  20. Evolution of Massive Protostars Via Disk Accretion

    NASA Astrophysics Data System (ADS)

    Hosokawa, Takashi; Yorke, Harold W.; Omukai, Kazuyuki

    2010-09-01

    Mass accretion onto (proto-)stars at high accretion rates \\dot{M}_* > 10^{-4} M_{⊙} yr^{-1} is expected in massive star formation. We study the evolution of massive protostars at such high rates by numerically solving the stellar structure equations. In this paper, we examine the evolution via disk accretion. We consider a limiting case of "cold" disk accretion, whereby most of the stellar photosphere can radiate freely with negligible backwarming from the accretion flow, and the accreting material settles onto the star with the same specific entropy as the photosphere. We compare our results to the calculated evolution via spherically symmetric accretion, the opposite limit, whereby the material accreting onto the star contains the entropy produced in the accretion shock front. We examine how different accretion geometries affect the evolution of massive protostars. For cold disk accretion at 10-3 M sun yr-1, the radius of a protostar is initially small, R *sime a few R sun. After several solar masses have accreted, the protostar begins to bloat up and for M * ~= 10 M sun the stellar radius attains its maximum of 30-400 R sun. The large radius ~100 R sun is also a feature of spherically symmetric accretion at the same accreted mass and accretion rate. Hence, expansion to a large radius is a robust feature of accreting massive protostars. At later times, the protostar eventually begins to contract and reaches the zero-age main sequence (ZAMS) for M * ~= 30 M sun, independent of the accretion geometry. For accretion rates exceeding several 10-3 M sun yr-1, the protostar never contracts to the ZAMS. The very large radius of several hundreds R sun results in the low effective temperature and low UV luminosity of the protostar. Such bloated protostars could well explain the existence of bright high-mass protostellar objects, which lack detectable H II regions.

  1. Mafic mantle sources indicated by the olivine-spinifex basalt-ferropicrite lavas in the accreted Permian oceanic LIP fragments and Miocene low-Ni basalt and adakite lavas in central Japan

    NASA Astrophysics Data System (ADS)

    Ishiwatari, A.; Ichiyama, Y.; Yamazaki, R.; Katsuragi, T.; Tsuchihashi, H.

    2008-12-01

    Melting of mafic (eclogitic) rocks in the peridotite mantle diapir may be important to generate a large quantity of magma in a short period of time as required for the LIP basaltic magmatism (e.g. Takahashi et al. 1998; EPSL, 162, 63-). Ferropicritic rocks also occur in some LIPs, and Ichiyama et al. (2006; Lithos, 89, 47-) propose a non-peridotitic, Ti- and Fe-rich eclogitic source (recycled oceanic ferrogabbro?) entrained in the peridotitic LIP mantle plume for the origin of ferropicritic rocks, that occur with olivine-spinifex basalt (Ichiyama et al., 2007; Island Arc, 16, 493-) in a Permian LIP fragment that was captured in the Jurassic Tamba accretionary complex in central Japan. Although Ti-poor ferrokomatiitic magma might form through high- degree melting of a primitive chondritic mantle (25wt% MgO and 25wt% Fe+FeO), Ti- and HFSE-rich ferropicritic and meimechitic magmas can not form in this way. On the other hand, Miocene volcanic rocks distributed along the Japan Sea coast of central Japan also represent a product of large-scale arc magmatism that happened coeval to the spreading of the Japan Sea floor. The chemical and isotopic signatures of the magmas are consistent with the secular change of tectonic setting from continental arc (22- 20 Ma) to island arc (15-11 Ma) (Shuto et al. 2006; Lithos, 86, 1-). Some adakites have already been found from these Miocene volcanic rocks by Shuto"fs group, and mafic rock melting in either subducting slab or lower arc crust has been proposed. We have recently found a wide distribution of low-Ni basalt from Fukui City. The low-Ni basalt contains olivine phenocrysts which are one order of magnitude poorer in Ni (less than 0.02 wt% NiO at Fo87) than those in normal basalt (more than 0.2 wt% NiO at Fo87). The rock is also poor in bulk-rock Ni, rich in K and Ti, and may have formed from an olivine-free pyroxenitic source. Close association of adakite and low-Ni basalt with normal tholeiitic basalt, calc-alkaline andesite

  2. Crustal structure of Wrangellia and adjacent terranes inferred from geophysical studies along a transect through the northern Talkeetna Mountains

    USGS Publications Warehouse

    Glen, J.M.G.; Schmidt, J.; Pellerin, L.; McPhee, D.K.; O'Neill, J. M.

    2007-01-01

    Recent investigations of the Talkeetna Mountains in south-central Alaska were undertaken to study the region's framework geophysics and to reinterpret structures and crustal composition. Potential field (gravity and magnetic) and magnetotelluric (MT) data were collected along northwest-trending profiles as part of the U.S. Geological Survey's Talkeetna Mountains transect project. The Talkeetna Mountains transect area comprises eight 1:63,360 quadrangles (???9500 km2) in the Healy and Talkeetna Mountains 1?? ?? 3?? sheets that span four major lithostratigraphic terranes (Glen et al., this volume) including the Wrangellia and Peninsular terranes and two Mesozoic overlap assemblages inboard (northwest) of Wrangellia. These data were used here to develop 21/2-dimensional models for the three profiles. Modeling results reveal prominent gravity, magnetic, and MT gradients (???3.25 mGal/ km, ???100nT/km, ???300 ohm-m/km) corresponding to the Talkeetna Suture Zone-a first-order crustal discontinuity in the deep crust that juxtaposes rocks with strongly contrasting rock properties. This discontinuity corresponds with the suture between relatively dense magnetic crust of Wrangellia (likely of oceanic composition) and relatively less dense transitional crust underlying Jurassic to Cretaceous flysch basins developed between Wrangellia and North America. Some area of the oceanic crust beneath Wrangellia may also have been underplated by mafic material during early to mid-Tertiary volcanism. The prominent crustal break underlies the Fog Lakes basin approximately where theTalkeetna thrust faultwaspreviouslymappedas a surface feature. Potential fieldand MT models, however, indicate that the Talkeetna Suture Zone crustal break along the transect is a deep (2-8 km), steeply west-dipping structure-not a shallow east-dipping Alpine nappe-like thrust. Indeed, most of the crustal breaks in the area appear to be steep in the geophysical data, which is consistent with regional geologic

  3. Black hole accretion disc impacts

    NASA Astrophysics Data System (ADS)

    Pihajoki, P.

    2016-04-01

    We present an analytic model for computing the luminosity and spectral evolution of flares caused by a supermassive black hole impacting the accretion disc of another supermassive black hole. Our model includes photon diffusion, emission from optically thin regions and relativistic corrections to the observed spectrum and time-scales. We test the observability of the impact scenario with a simulated population of quasars hosting supermassive black hole binaries. The results indicate that for a moderate binary mass ratio of 0.3, and impact distances of 100 primary Schwarzschild radii, the accretion disc impacts can be expected to equal or exceed the host quasar in brightness at observed wavelength λ = 510 nm up to z = 0.6. We conclude that accretion disc impacts may function as an independent probe for supermassive black hole binaries. We release the code used for computing the model light curves to the community.

  4. Episodic Accretion in Young Stars

    NASA Astrophysics Data System (ADS)

    Audard, M.; Ábrahám, P.; Dunham, M. M.; Green, J. D.; Grosso, N.; Hamaguchi, K.; Kastner, J. H.; Kóspál, Á.; Lodato, G.; Romanova, M. M.; Skinner, S. L.; Vorobyov, E. I.; Zhu, Z.

    In the last 20 years, the topic of episodic accretion has gained significant interest in the star-formation community. It is now viewed as a common, although still poorly understood, phenomenon in low-mass star formation. The FU Orionis objects (FUors) are long-studied examples of this phenomenon. FU Orionis objects are believed to undergo accretion outbursts during which the accretion rate rapidly increases from typically 10-7 to a few 10-4 M⊙ yr-1, and remains elevated over several decades or more. EXors, a loosely defined class of pre-main-sequence stars, exhibit shorter and repetitive outbursts, associated with lower accretion rates. The relationship between the two classes, and their connection to the standard pre-main-sequence evolutionary sequence, is an open question: Do they represent two distinct classes, are they triggered by the same physical mechanism, and do they occur in the same evolutionary phases? Over the past couple of decades, many theoretical and numerical models have been developed to explain the origin of FUor and EXor outbursts. In parallel, such accretion bursts have been detected at an increasing rate, and as observing techniques improve, each individual outburst is studied in increasing detail. We summarize key observations of pre-main-sequence star outbursts, and review the latest thinking on outburst triggering mechanisms, the propagation of outbursts from star/disk to disk/jet systems, the relation between classical EXors and FUors, and newly discovered outbursting sources — all of which shed new light on episodic accretion. We finally highlight some of the most promising directions for this field in the near- and long-term.

  5. Origin of granulite terranes and the formation of the lowermost continental crust.

    PubMed

    Bohlen, S R; Mezger, K

    1989-04-21

    Differences in composition and pressures of equilibration between exposed, regional granulite terranes and suites of granulite xenoliths of crustal origin indicate that granulite terranes do not represent exhumed lowermost crust, as had been thought, but rather middle and lower-middle crustal levels. Application of well-calibrated barometers indicate that exposed granulites record equilibration pressures of 0.6 to 0.8 gigapascal (20 to 30 kilometers depth of burial), whereas granulite xenoliths, which also tend to be more mafic, record pressures of at least 1.0 to 1.5 gigapascals (35 to 50 kilometers depth of burial). Thickening of the crust by the crystallization of mafic magmas at the crust-mantle boundary may account for both the formation of regional granulite terranes at shallower depths and the formation of deep-seated mafic crust represented by many xenolith suites. PMID:17738304

  6. Paleozoic paleomagnetism and northward drift of the Alexander terrane, southeastern Alaska.

    USGS Publications Warehouse

    Van Der Voo, R.; Jones, M.; Gromme, C.S.; Eberlein, G.D.; Churkin, M., Jr.

    1980-01-01

    Paleozoic limestone, greywacke, sandstone, mudstone, red beds and volcanic rocks of the Alexander terrane, SE Alaska, have yielded six paleomagnetic pole positions after thermal and alternating-field demagnetization. These poles are from sample groups of late Middle Ordovician, Late Ordovician, Devonian, Late Devonian, and early and late Carboniferous age. To test various tectonic models for the structural development of this part of western N America, the paleomagnetic results are compared to those for the N American craton. It is found that the observed inclination and declination values deviate significantly from the values predicted for the present-day position of the Alexander terrance (55.5 N, 133.5 W). Better matching can be obtained for a paleoposition of the terrane at about 40 N, 120 W, in the present position of western Nevada and NE California. In addition, an in situ 25o clockwise rotation of the terrane is required to restore it to its original position.-Authors

  7. Exhumation of (ultra-)high-pressure terranes: concepts and mechanisms

    NASA Astrophysics Data System (ADS)

    Warren, C. J.

    2013-02-01

    The formation and exhumation of high and ultra-high-pressure, (U)HP, rocks of crustal origin appears to be ubiquitous during Phanerozoic plate subduction and continental collision events. Exhumation of (U)HP material has been shown in some orogens to have occurred only once, during a single short-lived event; in other cases exhumation appears to have occurred multiple discrete times or during a single, long-lived, protracted event. It is becoming increasingly clear that no single exhumation mechanism dominates in any particular tectonic environment, and the mechanism may change in time and space within the same subduction zone. Subduction zone style and internal force balance change in both time and space, responding to changes in width, steepness, composition of subducting material and velocity of subduction. In order for continental crust, which is relatively buoyant compared to the mantle even when metamorphosed to (U)HP assemblages, to be subducted to (U)HP conditions, it must remain attached to a stronger and denser substrate. Buoyancy and external tectonic forces drive exhumation, although the changing spatial and temporal dominance of different driving forces still remains unclear. Exhumation may involve whole-scale detachment of the terrane from the subducting slab followed by exhumation within a subduction channel (perhaps during continued subduction) or a reversal in motion of the entire plate (eduction) following the removal of a lower part of the subducting slab. Weakening mechanisms that may be responsible for the detachment of deeply subducted crust from its stronger, denser substrate include strain weakening, hydration, melting, grain size reduction and the development of foliation. These may act locally to form narrow high-strain shear zones separating stronger, less-strained crust or may act on the bulk of the subducted material, allowing whole-scale flow. Metamorphic reactions, metastability and the composition of the subducted crust all affect

  8. Dislocations of the cretaceous and cenozoic complexes of the northern part of the West Sakhalin Terrane

    NASA Astrophysics Data System (ADS)

    Golozubov, V. V.; Kasatkin, S. A.; Malinovskii, A. I.; Nechayuk, A. E.; Grannik, V. M.

    2016-07-01

    The contemporary structure of the West Sakhalin Terrane started to form in the Pleistocene and the process of its formation continues up to now in a setting of ENE (60°-90°) shortening. Evidence of the preceding NE (30°-45°) compression was revealed during the study. This compression prevailed in the Eocene-Pliocene. Under the settings of NE (30°-45°) compression, dextral displacements occurred along the West Sakhalin and Tym'-Poronai fault systems, bounding the West Sakhalin Terrane.

  9. Integrated seismic model of the crust and upper mantle of the Trans-European Suture zone between the Precambrian craton and Phanerozoic terranes in Central Europe

    NASA Astrophysics Data System (ADS)

    Wilde-Piórko, Monika; Świeczak, Marzena; Grad, Marek; Majdański, Mariusz

    2010-01-01

    The structure and evolution of the Trans-European Suture zone (TESZ), contact between Precambrian Europe to the northeast and Phanerozoic terranes to the southwest is one of the main tectonic questions in Europe. The knowledge of the crustal structure, lithosphere-asthenosphere boundary and mantle transition zone between two seismic discontinuities at depths "410" and "660" km, is one of the most important issues to understand the Earth's dynamics. To create a mantle model of the TESZ and surroundings we used different seismic data collected along the 950 km long POLONAISE'97 profile P4. Previous results of 2-D ray-tracing and P-wave travel time modelling and new results of P-wave travel time residuals methods and receiver function sections provide facts about the seismic structure from the surface down to 900 km depth. In the TESZ a large basin, about 125 km wide, is filled with sedimentary strata (Vp < 6.0 km s - 1 ) to about 20 km depth. This basin is asymmetric with its northeast margin being most abrupt. The crystalline crust under this basin is only about 20 km thick today indicating that the lithosphere of Baltica was either thinned drastically or terminated along the northeast margin of the basin. The East European craton (EEC) has a ~ 45 km thick three-layered crust. The crust of the accreted terranes to the southwest is relatively thin (~ 30 km) and similar to that found in other non-cratonal areas of Western Europe. The lower crust is relatively fast (Vp > 7.0 km s - 1 ) along most of the P4 profile. However, lower values to the southwest may indicate the termination of Baltica. High velocity (~ 8.35 km s - 1 ) uppermost mantle lies beneath the Avalonia/Variscan terranes, and may be due to rifting and/or subduction. The seismic lithosphere thickness for the EEC is about 200 km, while it is only 90 km in the Palaeozoic platform (PP). The mantle transition zone is shallower and about 30 km thicker under the EEC, which could be due to thermal conditions

  10. Stellar X-ray accretion signatures

    NASA Astrophysics Data System (ADS)

    Schneider, C.; Guenther, M.

    2016-06-01

    Accretion is observed in a wide range objects with partially overlapping properties. In this contribution, we study accretion in young stars, where we can directly observe the accretion shock on the stellar surface in the X-ray regime. High-resolution grating spectroscopy allows us to infer the properties of the accretion streams. I will present results from our recent 250 ks XMM-Newton/Chandra program targeting the prototypical T Tau system such as strong X-ray variability despite constant mass accretion, abundances typical for accreting stars, but line ratios typically not found in accreting stars. Finally, I will compare these results with other systems focusing on potentially different accretion modes.

  11. Viscosity in spherically symmetric accretion

    NASA Astrophysics Data System (ADS)

    Ray, Arnab K.

    2003-10-01

    The influence of viscosity on the flow behaviour in spherically symmetric accretion has been studied here. The governing equation chosen has been the Navier-Stokes equation. It has been found that at least for the transonic solution, viscosity acts as a mechanism that detracts from the effectiveness of gravity. This has been conjectured to set up a limiting scale of length for gravity to bring about accretion, and the physical interpretation of such a length scale has been compared with the conventional understanding of the so-called `accretion radius' for spherically symmetric accretion. For a perturbative presence of viscosity, it has also been pointed out that the critical points for inflows and outflows are not identical, which is a consequence of the fact that under the Navier-Stokes prescription, there is a breakdown of the invariance of the stationary inflow and outflow solutions - an invariance that holds good under inviscid conditions. For inflows, the critical point gets shifted deeper within the gravitational potential well. Finally, a linear stability analysis of the stationary inflow solutions, under the influence of a perturbation that is in the nature of a standing wave, has indicated that the presence of viscosity induces greater stability in the system than has been seen for the case of inviscid spherically symmetric inflows.

  12. Counter-rotating accretion discs

    NASA Astrophysics Data System (ADS)

    Dyda, S.; Lovelace, R. V. E.; Ustyugova, G. V.; Romanova, M. M.; Koldoba, A. V.

    2015-01-01

    Counter-rotating discs can arise from the accretion of a counter-rotating gas cloud on to the surface of an existing corotating disc or from the counter-rotating gas moving radially inwards to the outer edge of an existing disc. At the interface, the two components mix to produce gas or plasma with zero net angular momentum which tends to free-fall towards the disc centre. We discuss high-resolution axisymmetric hydrodynamic simulations of viscous counter-rotating discs for the cases where the two components are vertically separated and radially separated. The viscosity is described by an isotropic α-viscosity including all terms in the viscous stress tensor. For the vertically separated components, a shear layer forms between them and the middle part of this layer free-falls to the disc centre. The accretion rates are increased by factors of ˜102-104 over that for a conventional disc rotating in one direction with the same viscosity. The vertical width of the shear layer and the accretion rate are strongly dependent on the viscosity and the mass fraction of the counter-rotating gas. In the case of radially separated components where the inner disc corotates and the outer disc rotates in the opposite direction, a gap between the two components opens and closes quasi-periodically. The accretion rates are ≳25 times larger than those for a disc rotating in one direction with the same viscosity.

  13. Perturbation growth in accreting filaments

    NASA Astrophysics Data System (ADS)

    Clarke, S. D.; Whitworth, A. P.; Hubber, D. A.

    2016-05-01

    We use smoothed particle hydrodynamic simulations to investigate the growth of perturbations in infinitely long filaments as they form and grow by accretion. The growth of these perturbations leads to filament fragmentation and the formation of cores. Most previous work on this subject has been confined to the growth and fragmentation of equilibrium filaments and has found that there exists a preferential fragmentation length-scale which is roughly four times the filament's diameter. Our results show a more complicated dispersion relation with a series of peaks linking perturbation wavelength and growth rate. These are due to gravo-acoustic oscillations along the longitudinal axis during the sub-critical phase of growth. The positions of the peaks in growth rate have a strong dependence on both the mass accretion rate onto the filament and the temperature of the gas. When seeded with a multiwavelength density power spectrum, there exists a clear preferred core separation equal to the largest peak in the dispersion relation. Our results allow one to estimate a minimum age for a filament which is breaking up into regularly spaced fragments, as well as an average accretion rate. We apply the model to observations of filaments in Taurus by Tafalla & Hacar and find accretion rates consistent with those estimated by Palmeirim et al.

  14. Dynamics of subduction, accretion, exhumation and slab roll-back: Mediterranean scenarios

    NASA Astrophysics Data System (ADS)

    Tirel, C.; Brun, J.; Burov, E. B.; Wortel, M. J.; Lebedev, S.

    2010-12-01

    A dynamic orogen reveals various tectonic processes brought about by subduction: accretion of oceanic and continental crust, exhumation of UHP-HP rocks, and often, back-arc extension. In the Mediterranean, orogeny is strongly affected by slab retreat, as in the Aegean and Tyrrhenian Seas. In order to examine the different dynamic processes in a self-consistent manner, we perform a parametric study using the fully coupled thermo-mechanical numerical code PARAFLAM. The experiments reproduce a subduction zone in a slab pull mode, with accretion of one (the Tyrrhenian case) and two continental blocks (the Aegean case) that undergo, in sequence, thrusting, burial and exhumation. The modeling shows that despite differences in structure between the two cases, the deformation mechanisms are fundamentally similar and can be described as follows. The accretion of a continental block at the trench beneath the suture zone begins with its burial to UHP-HP conditions and thrusting. Then the continental block is delaminated from its subducting lithosphere. During the subduction-accretion process, the angle of the subducting slab increases due to the buoyancy of the continental block. When the oceanic subduction resumes, the angle of the slab decreases to reach a steady-state position. The Aegean and Tyrrhenian scenarios diverge at this stage, due naturally to the differences of their accretion history. When continental accretion is followed by oceanic subduction only, the continental block that has been accreted and detached stays at close to the trench and does not undergo further deformation, despite the continuing rollback. The extensional deformation is located further within the overriding plate, resulting in continental breakup and the development of an oceanic basin, as in the Tyrrhenian domain. When the continental accretion is followed first by oceanic subduction and then by accretion of another continental block, however, the evolution of the subduction zone is

  15. Archean terrane docking: upper crust collision tectonics, Abitibi greenstone belt, Quebec, Canada

    NASA Astrophysics Data System (ADS)

    Mueller, W. U.; Daigneault, R.; Mortensen, J. K.; Chown, E. H.

    1996-11-01

    The northern (NVZ) and southern volcanic zones (SVZ) of the Abitibi greenstone belt are separated by the major E-trending Destor-Porcupine-Manneville fault zone (DPMFZ). The DPMFZ is interpreted to be the locus of Archean terrane docking between the older diffuse volcanic arc of the NVZ (2730-2710 Ma) and the younger arc segments of the SVZ (2705-2698 Ma). Two distinct evolutionary phases can be documented along the DPMFZ of the Abitibi greenstone belt and include (1) arc-arc collision occurring between 2697 and 2690 Ma, and (2) arc fragmentation between 2689 and 2680 Ma. Identification of these two events along the DPMFZ is based on detailed structural studies, sedimentary basin analysis, and precise UPb age determinations. The thrusting event, representative of the arc-arc collision phase, is characterized by shallow north-dipping foliations (20-40°) and dip-parallel stretching lineations in the eastern Manneville segment of the DPMFZ. Local overturned mafic pillowed units suggest recumbent folding. Late strike-slip or transcurrent movement displayed in the late-orogenic sedimentary Duparquet Basin records the arc fragmentation phase. Basin geometry, E-trending en-echelon folds, shallow E-plunging stretching lineations and a late NE-striking cleavage cross-cutting the folds support a dextral shear sense along the western Destor-Porcupine segment of the DPMFZ. The sedimentary facies observed in the basin are consistent with those of modern strike-slip basins located along the East Anatolian fault, Turkey (Hazar Lake) and the Hope fault, New Zealand (Hanmer Basin). Precise UPb zircon age determinations from porphyry stocks located at the northern and southern limits of the Duparquet Basin, yielded 2681 ± 1 Ma and 2689 +3.2-2.9 Ma, respectively. These ages constrain the rapid change from thrusting to transcurrent movement. It is apparent that once thrusting ceased the response to oblique subduction continued in the form of strike-slip displacement. Modern

  16. Towards combined modeling of planetary accretion and differentiation

    NASA Astrophysics Data System (ADS)

    Golabek, G. J.; Gerya, T.; Morishima, R.; Tackley, P. J.; Labrosse, S.

    2011-12-01

    Results of current 1D models on planetesimal accretion yield an onion-like thermal structure with very high internal temperatures due to powerful short-lived radiogenic heating in the planetesimals. These lead to extensive silicate melting in the parent bodies. Yet, magma ocean and impact processes are not considered in these models and core formation is, if taken into account, assumed to be instantaneous with no feedback on the mantle evolution. It was pointed out that impacts can not only deposit heat deep into the target body, which is later buried by ejecta of further impacts [1], but also that impacts expose in the crater region originally deep-seated layers, thus cooling the interior [2]. This combination of impact effects becomes even more important when we consider that planetesimals of all masses contribute to planetary accretion. This leads occasionally to collisions between bodies with large ratios between impactor and target mass. Thus, all these processes can be expected to have a profound effect on the thermal evolution during the epoch of planetary accretion and may have implications for the onset of mantle convection and cannot be described properly in 1D geometry. Here we present a new methodology, which can be used to simulate the internal evolution of a planetary body during accretion and differentiation: Using the N-body code PKDGRAV [3] we simulate the accretion of planetary embryos from an initial annulus of several thousand planetesimals. The growth history of the largest resulting planetary embryo is used as an input for the thermomechanical 2D code I2ELVIS [4]. The thermomechanical model takes recent parametrizations of impact processes like impact heating and crater excavation [5] into account. The model also includes both long- and short-lived radiogenic isotopes and a more realistic treatment of largely molten silicates [6]. Results show that late-formed planetesimals do not experience silicate melting and avoid thermal alteration

  17. Towards combined modeling of planetary accretion and differentiation

    NASA Astrophysics Data System (ADS)

    Golabek, G. J.; Gerya, T. V.; Morishima, R.; Tackley, P. J.; Labrosse, S.

    2012-09-01

    accretion yield an onion-like thermal structure with very high internal temperatures due to powerful short-lived radiogenic heating in the planetesimals. These lead to extensive silicate melting in the parent bodies. Yet, magma ocean and impact processes are not considered in these models and core formation is, if taken into account, assumed to be instantaneous with no feedback on the mantle evolution. It was pointed out that impacts can not only deposit heat deep into the target body, which is later buried by ejecta of further impacts [1], but also that impacts expose in the crater region originally deep-seated layers, thus cooling the interior [2]. This combination of impact effects becomes even more important when we consider that planetesimals of all masses contribute to planetary accretion. This leads occasionally to collisions between bodies with large ratios between impactor and target mass. Thus, all these processes can be expected to have a profound effect on the thermal evolution during the epoch of planetary accretion and may have implications for the onset of mantle convection and cannot be described properly in 1D geometry. Here we present a new methodology, which can be used to simulate the internal evolution of a planetary body during accretion and differentiation: Using the N-body code PKDGRAV[3] we simulate the accretion of planetary embryos from an initial annulus of several thousand planetesimals. The growth history of the largest resulting planetary embryo is used as an input for the thermomechanical 2D code I2ELVIS [4]. The thermomechanical model takes recent parametrizations of impact processes like impact heating and crater excavation [5] into account. The model also includes both long- and short-lived radiogenic isotopes and a more realistic treatment of largely molten silicates [6]. Results show that late-formed planetesimals do not experience silicate melting and avoid thermal alteration, whereas in early-formed bodies accretion and iron

  18. Archaean Crustal Growth, Proterozoic Terrane Amalgamation and the Pan-African Orogeny, as Recorded in the NE African Sedimentary Record.

    NASA Astrophysics Data System (ADS)

    Najman, Y.; Fielding, L.; Millar, I.; Butterworth, P.; Andò, S.; Padoan, M.; Barfod, D. N.; Kneller, B. C.

    2015-12-01

    The cratons of Central Africa are formed of various blocks of Archaean and Palaeoproterozoic crust, flanked or truncated by Palaeoproterozoic to Mesoproterozoic orogenic belts. The geology of east Africa has largely been shaped by the events of the Pan-African Orogeny when east and west Gondwana collided to form 'Greater Gondwana' at the end of the Neoproterozoic. The Pan-African orogeny in NE Africa involved the collision of Archaean cratons and the Saharan Metacraton with the Arabian Nubian Shield, a terrane comprising Neoproterozoic juvenile oceanic island arcs. Phanerozoic cover sedimentary rocks, eroded from the Pan-African orogenies, blanket much of NE Africa. Detrital data from these Phanerozoic cover sedimentary rocks, and modern rivers draining both the cover the basement, provide a wealth of information on basement evolution, of particular relevance for regions where the basement itself is poorly exposed due to ancient or modern sedimentary cover. From samples collected in Uganda, Ethiopia, Sudan and Egypt, we provide combined U-Pb and Hf-isotope zircon, U-Pb rutile and Ar-Ar mica datasets, heavy mineral analyses, and bulk trace element data, from Archaean basement, Phanerozoic cover and modern river sediment from the Nile and its tributaries to document the evolution of the North African crust. The data document early crust-forming events in the Congo Craton and Sahara Metacraton, phased development of the Arabian Nubian Shield culminating in the Neoproterozoic assembly of Gondwana during the Pan African Orogeny, and the orogen's subsequent erosion, with deposition of voluminous Phanerozoic cover.

  19. Stratigraphy, geochronology, and accretionary terrane settings of two Bronson Hill arc sequences, northern New England

    USGS Publications Warehouse

    Moench, R.H.; Aleinikoff, J.N.

    2002-01-01

    off the southern Laurentian margin, but northwest of the principal Iapetan suture, or Red Indian line (RIL). The Boil Mountain-Jim Pond-Hurricane Mountain sequence was ramped northwestward over the Chain Lakes massif at ???475 Ma, on the basal Boil Mountain surface. This obduction probably occurred slightly before obduction on the Baie Verte-Brompton surface (BBL), farther NW, over the Laurentian margin, and was followed by Dead River flysch sedimentation, which ended with the abrupt onset of Ammonoosuc-sequence arc magmatism at ???470 Ma. Ammonoosuc eruptions probably ended at ???460 Ma, when Iapetus closed along the Red Indian line. During a following magmatic hiatus of ???3-5 m.y., now represented by portions of the Partridge Formation that overlie the Ammonoosuc Volcanics, subduction polarity reversed, and subduction resumed below the northwest-dipping Brunswick subduction complex (BSC) of New Brunswick, Canada. Quimby-sequence magmatism (???456-435 Ma) on the the newly accreted Laurentian margin occurred above the BSC, whose footwall is now buried to the southeast by mainly Silurian clastic sediments of the Merrimack-Fredericton trough, deposited in the "Fredericton Sea". In Silurian to Early Devonian time, the NW-dipping BSC footwall was paired with a SE-dipping subduction zone that produced arc magmas of the Coastal Volcanic belt, built on the composite Avalon and adjacent peri-Avalonian terranes. Orogen-normal extension produced by rapid rollback of both subduction zones narrowed the Fredericton Sea, produced the Central Maine and Connecticut Valley-Gaspe?? basins, and culminated in the Acadian orogeny when the sea completely closed in Early Devonian time. Published by Elsevier Science Ltd.

  20. Stratigraphy, geochronology, and accretionary terrane settings of two Bronson Hill arc sequences, northern New England

    USGS Publications Warehouse

    Moench, R.H.; Aleinikoff, J.N.

    2003-01-01

    the southern Laurentian margin, but northwest of the principal Iapetan suture, or Red Indian line. The Boil Mountain-Jim Pond-Hurricane Mountain sequence was ramped northwestward over the Chain Lakes massif at ???475 Ma, on the basal Boil Mountain surface. This obduction probably occurred slightly before obduction on the Baie Vert-Brompton surface (BBL), farther NW, over the Laurentian margin, and was followed by Dead River flysch sedimentation, which ended with the abrupt onset of Ammonoosuc-sequence arc magmatism at ???470 Ma. Ammonoosuc eruptions probably ended at ???460 Ma, when Iapetus closed along the Red Indian line. During a following magmatic hiatus of ???3-5 m.y., now represented by portions of the Partridge Formation that overlie the Ammonoosuc Volcanics, subduction polarity reversed, and subduction resumed below the northwest-dipping Brunswick subduction complex (BSC) of New Brunswick, Canada. Quimby-sequence magmatism (???456-435 Ma) on the newly accreted Laurentian margin occurred above the BSC, whose footwall is now buried to the southeast by mainly Silurian clastic sediments of the Merrimack-Fredericton trough, deposited in the "Fredericton Sea". In Silurian to Early Devonian time, the NW-dipping BSC footwall was paired with a SE-dipping subduction zone that produced arc magmas of the Coastal Volcanic belt, built on the composite Avalon and adjacent peri-Avalonian terranes. Orogen-normal extension produced by rapid rollback of both subduction zones narrowed the Fredericton Sea, produced the Central Maine and Connecticut Valley-Gaspe?? basins, and culminated in the Acadian orogeny when the sea completely closed in Early Devonian time. Published by Elsevier Science Ltd.

  1. Stratigraphy, geochronology, and accretionary terrane settings of two Bronson Hill arc sequences, northern New England

    NASA Astrophysics Data System (ADS)

    Moench, Robert H.; Aleinikoff, John N.

    2002-01-01

    southern Laurentian margin, but northwest of the principal Iapetan suture, or Red Indian line (RIL). The Boil Mountain-Jim Pond-Hurricane Mountain sequence was ramped northwestward over the Chain Lakes massif at ∼475 Ma, on the basal Boil Mountain surface. This obduction probably occurred slightly before obduction on the Baie Verte-Brompton surface (BBL), farther NW, over the Laurentian margin, and was followed by Dead River flysch sedimentation, which ended with the abrupt onset of Ammonoosuc-sequence arc magmatism at ∼470 Ma. Ammonoosuc eruptions probably ended at ∼460 Ma, when Iapetus closed along the Red Indian line. During a following magmatic hiatus of ∼3-5 m.y., now represented by portions of the Partridge Formation that overlie the Ammonoosuc Volcanics, subduction polarity reversed, and subduction resumed below the northwest-dipping Brunswick subduction complex (BSC) of New Brunswick, Canada. Quimby-sequence magmatism (∼456-435 Ma) on the the newly accreted Laurentian margin occurred above the BSC, whose footwall is now buried to the southeast by mainly Silurian clastic sediments of the Merrimack-Fredericton trough, deposited in the “Fredericton Sea”. In Silurian to Early Devonian time, the NW-dipping BSC footwall was paired with a SE-dipping subduction zone that produced arc magmas of the Coastal Volcanic belt, built on the composite Avalon and adjacent peri-Avalonian terranes. Orogen-normal extension produced by rapid rollback of both subduction zones narrowed the Fredericton Sea, produced the Central Maine and Connecticut Valley-Gaspé basins, and culminated in the Acadian orogeny when the sea completely closed in Early Devonian time.

  2. Stratigraphy, geochronology, and accretionary terrane settings of two Bronson Hill arc sequences, northern New England

    NASA Astrophysics Data System (ADS)

    Moench, Robert H.; Aleinikoff, John N.

    the southern Laurentian margin, but northwest of the principal Iapetan suture, or Red Indian line. The Boil Mountain-Jim Pond-Hurricane Mountain sequence was ramped northwestward over the Chain Lakes massif at ∼475 Ma, on the basal Boil Mountain surface. This obduction probably occurred slightly before obduction on the Baie Vert-Brompton surface (BBL), farther NW, over the Laurentian margin, and was followed by Dead River flysch sedimentation, which ended with the abrupt onset of Ammonoosuc-sequence arc magmatism at ∼470 Ma. Ammonoosuc eruptions probably ended at ∼460 Ma, when Iapetus closed along the Red Indian line. During a following magmatic hiatus of ∼3-5 m.y., now represented by portions of the Partridge Formation that overlie the Ammonoosuc Volcanics, subduction polarity reversed, and subduction resumed below the northwest-dipping Brunswick subduction complex (BSC) of New Brunswick, Canada. Quimby-sequence magmatism (∼456-435 Ma) on the newly accreted Laurentian margin occurred above the BSC, whose footwall is now buried to the southeast by mainly Silurian clastic sediments of the Merrimack-Fredericton trough, deposited in the “Fredericton Sea”. In Silurian to Early Devonian time, the NW-dipping BSC footwall was paired with a SE-dipping subduction zone that produced arc magmas of the Coastal Volcanic belt, built on the composite Avalon and adjacent peri-Avalonian terranes. Orogen-normal extension produced by rapid rollback of both subduction zones narrowed the Fredericton Sea, produced the Central Maine and Connecticut Valley-Gaspé basins, and culminated in the Acadian orogeny when the sea completely closed in Early Devonian time.

  3. The Hikurangi Plateau: Tectonic Ricochet and Accretion

    NASA Astrophysics Data System (ADS)

    Willis, David; Moresi, Louis; Betts, Peter; Whittaker, Joanne

    2015-04-01

    80 million years between interactions with different subduction systems provided time for the Hikurangi Plateau and Pacific Ocean lithosphere to cool, densify and strengthen. Neogene subduction of the Hikurangi Plateau occurring orthogonal to its Cretaceous predecessor, provides a unique opportunity to explore how changes to the physical properties of oceanic lithosphere affect subduction dynamics. We used Underworld to build mechanically consistent collision models to understand the dynamics of the two Hikurangi collisions. The Hikurangi Plateau is a ~112 Ma, 15km thick oceanic plateau that has been entrained by subduction zones immediately preceding the final break-up of Eastern Gondwana and currently within the active Hikurangi Margin. We explore why attempted subduction of the plateau has resulted in vastly different dynamics on two separate occasions. Slab break-off occured during the collision with Gondwana, currently there is apparent subduction of the plateau underneath New Zealand. At ~100Ma the young, hot Hikurangi Plateau, positively buoyant with respect to the underlying mantle, impacted a Gondwana Margin under rapid extension after the subduction of an mid-ocean ridge 10-15Ma earlier. Modelling of plateaus within young oceanic crust indicates that subduction of the thickened crust was unlikely to occur. Frontal accretion of the plateau and accompanying slab break-off is expected to have occured rapidly after its arrival. The weak, young slab was susceptible to lateral propagation of the ~1500 km window opened by the collision, and break-off would have progressed along the subduction zone inhibiting the "step-back" of the trench seen in older plates. Slab break-off coincided with a world-wide reorganisation of plate velocites, and orogenic collapse along the Gondwana margin characterised by rapid extension and thinning of the over-riding continental plate from ~60 to 30km. Following extension, Zealandia migrated to the NW until the Miocene allowing the

  4. Uvá complex, the oldest orthogneisses of the Archean-Paleoproterozoic terrane of central Brazil

    NASA Astrophysics Data System (ADS)

    Jost, Hardy; Junior, Farid Chemale; Fuck, Reinhardt Adolfo; Dussin, Ivo Antônio

    2013-11-01

    The Archean-Paleoproterozoic terrane of central Brazil is an exotic and allochthonous part of the Tocantins Province, a large Brasiliano/Pan-African orogen of the South American Platform formed during the Brasiliano orogeny. The terrane amalgamated to the province during the late stages of the orogeny as a crustal segment consisting of six Archean orthogneiss complexes and five low-grade metamorphic, in part Paleoproterozoic (Rhyacian) greenstone belts. The Uvá complex is the southernmost orthogneiss association of the Archean-Paleoproterozoic terrane of central Brazil. New U-Pb LA-ICP-MS data from zircon crystals show that the complex formed at least during two magmatic stages. The older consists of polydeformed tonalite and granodiorite batholitic and diorite stock protoliths with igneous age of 3040 Ma to 2930 Ma. The youngest comprises tonalite, monzogranite and granodiorite tabular bodies formed between 2876 and 2846 Ma. As compared to the orthogneisses of the northern portion of the terrane, both the oldest and youngest granitogenesis stages of the Uvá complex are, in average, about 150 Ma older. This suggests that the northern and southern orthogneisses formed during different times as independent crustal segments, but when and why they amalgamated is still under investigation.

  5. Chemistry of the Calcalong Creek lunar meteorite and its relationship to lunar terranes

    NASA Astrophysics Data System (ADS)

    Hill, D. H.; Boynton, W. V.

    2003-04-01

    The Calcalong Creek lunar meteorite is a polymict breccia that contains clasts of both highlands and mare affinity. Reported here is a compilation of major, minor, and trace element data for bulk, clast, and matrix samples determined by instrumental neutron activation analysis (INAA). Petrographic information and results of electron microprobe analyses are included. The relationship of Calcalong Creek to lunar terranes, especially the Procellarum KREEP Terrane and Feldspathic Highlands Terrane, is established by the abundance of thorium, incompatible elements and their KREEP-like CI chondrite normalized pattern, FeO, and TiO2. The highlands component is associated with Apollo 15 KREEP basalt but represents a variant of the KREEP-derived material widely found on the moon. Sources of Calcalong Creek's mare basalt components may be related to low-titanium (LT) and very low-titanium (VLT) basalts seen in other lunar meteorites but do not sample the same source. The content of some components of Calcalong Creek are found to display similarities to the composition of the South Pole-Aitken Terrane. What appear to be VLT relationships could represent new high aluminum, low titanium basalt types.

  6. Latest Carboniferous closure of the Junggar Ocean constrained by geochemical and zircon U-Pb-Hf isotopic data of granitic gneisses from the Central Tianshan block, NW China

    NASA Astrophysics Data System (ADS)

    Zhang, Xiaoran; Zhao, Guochun; Eizenhöfer, Paul R.; Sun, Min; Han, Yigui; Hou, Wenzhu; Liu, Dongxing; Wang, Bo; Liu, Qian; Xu, Bing

    2015-12-01

    Once situated between the Central Tianshan and Junggar terranes during Paleozoic time, the Junggar Ocean was a major southern branch of the Paleo-Asian Ocean. Thus, when and how it was closed are essential in understanding the final assembly of the Central Asian Orogenic Belt. However, the exact closure time of the Junggar Ocean remains unresolved due to the lack of reliable timing of collision-related regional metamorphism. This paper reports whole-rock geochemical and zircon U-Pb and Lu-Hf isotopic data for granitic gneisses from the northern margin of the Central Tianshan block, which can provide crucial constraints on the final closure of the Junggar Ocean. Mineral assemblages and geochemical suggest that the protoliths of the Central Tianshan gneisses are weakly peraluminous high-K calc-alkaline I-type granites, possessing typical subduction-related features such as strong enrichment in LREE and LILE and depletion in HFSE. Negative Eu anomalies (δEu = 0.46-0.81) and highly variable zircon Hf isotope compositions indicate various amounts of residual plagioclase in the source and crustal contamination during magma formation. LA-ICPMS U-Pb dating on magmatic-type zircons (72%), characterized by euhedral to subhedral shapes, concentric oscillatory zoning, high Th/U ratios (0.30-2.05) and large ranges of εHf(t) values (- 3.4 to + 8.7; up to 6 epsilon units in each sample), yields consistent weighted mean 206Pb/238U ages of ca. 325-320 Ma, interpreted as the crystallization ages of the granitic protoliths. Geochemical signatures and dominantly positive zircon εHf(t) values reveal that the protoliths were emplaced in a continental arc setting, pinpointing the development of a late Early to early Late Carboniferous continental arc system on the northern margin of the Central Tianshan block, probably related to the southward subduction of the Junggar oceanic plate. Meanwhile, younger ages at ca. 303-301 Ma were obtained on recrystallized zircon-rims and unzoned

  7. Basaltic Martian analogues from the Baikal Rift Zone and Mongolian terranes

    NASA Astrophysics Data System (ADS)

    Gurgurewicz, J.; Kostylew, J.

    2007-08-01

    In order to compare the results of studies of the western part of the Valles Marineris canyon on Mars there have been done field works on terrestrial surface areas similar with regard to geological setting and environmental conditions. One of the possible terrestrial analogues of the Valles Marineris canyon is the Baikal Rift Zone [1]. Field investigations have been done on the south end of the Baikal Lake, in the Khamar-Daban massif, where the outcrops of volcanic rocks occur. The second part of the field works has been done in the Mongolian terranes: Mandalovoo, Gobi Altay and Bayanhongor, because of environmental conditions being similar to those on Mars. The Mandalovoo terrane comprises a nearly continuous Paleozoic islandarc sequence [2]. In the Gobi Altay terrane an older sequence is capped by younger Devonian-Triassic volcanic-sedimentary deposits [2]. The Bayanhongor terrane forms a northwest-trending, discontinuous, narrow belt that consists of a large ophiolite allochton [3]. The collected samples of basalts derive from various geologic environments. The CORONA satellite-images have been used for the imaging of the Khamar-Daban massif and the Mandalovoo terrane. These images have the same spatial resolution and range as the Mars Orbiter Camera images of the Mars Global Surveyor mission. In the Mandalovoo terrane these images allowed to find an area with large amounts of tectonic structures, mainly faults (part of the Ongi massif), similar to the studied area on Mars. Microscopic observations in thin sections show diversification of composition and structures of basalts. These rocks have mostly a porphyric structure, rarely aphyric. The main components are plagioclases, pyroxenes and olivines phenocrysts, in different proportions. The groundmass usually consist of plagioclases, pyroxenes and opaques. The most diversified are basalts from the Mandalovoo terrane. Infrared spectroscopy has been used to analyse the composition of the rock material and compare

  8. New Insights from Seismic Imaging over the Youanmi Terrane, Western Australia

    NASA Astrophysics Data System (ADS)

    Ahmadi, Omid; Juhlin, Christopher

    2014-05-01

    The Youanmi terrane is located in the central parts of the Yilgarn craton, Western Australia, an Archean granite-greenstone unit containing numerous mineral deposits such as gold, base metals, nickel, uranium and gemstones. The terrane is surrounded by the Kalgoorlie and Narryer terranes to the east and west, respectively. To the southwest it is bounded by the South West terrane. In order to study the transitions between the Youanmi terrane and the surrounding terranes, as well as identifying potential mineral rich areas, the Geological Survey of Western Australia acquired three deep crustal 2D seismic profiles with a total length of about 700 km in 2010. Correlated record lengths of 20 seconds allow the deep structure of the crust to be investigated with the data, down to Moho depths and greater. Initial processing using a conventional 2D flow show a highly reflective crust with several interesting features. We have now reprocessed the data following mainly the previous processing flow, but with a focus on the shallower crust, less than 10 seconds (about 27 km). Due to the complex geology in the region, 3D aspects of the structures need to be considered in the data processing. Therefore, we investigated the effect of cross-dip corrections to the data. The cross-dip correction has two advantages; (i) reflections are more coherent and enhanced after the correction and (ii) the orientation and dip angle of the geological structures of the corresponding reflections can be identified in the cross-line direction. Where the profiles intersect each other sparse 3D processing can be performed. First arrival travel-time tomography was also tested on parts of the dataset. Travel-time inversion may provide better velocity models at shallow depths than standard reflection seismic processing provides. Preliminary results show that the travel-time tomography has a depth of investigation of about 1 km, a depth that is of interest for mining purposes. Therefore, the tomography

  9. The evolution of a Gondwanan collisional orogen: A structural and geochronological appraisal from the Southern Granulite Terrane, South India

    NASA Astrophysics Data System (ADS)

    Plavsa, Diana; Collins, Alan S.; Foden, John D.; Clark, Chris

    2015-05-01

    Gondwana amalgamated along a suite of Himalayan-scale collisional orogens, the roots of which lace the continents of Africa, South America, and Antarctica. The Southern Granulite Terrane of India is a generally well-exposed, exhumed, Gondwana-forming orogen that preserves a record of the tectonic evolution of the eastern margin of the East African Orogen during the Ediacaran-Cambrian (circa 600-500 Ma) as central Gondwana formed. The deformation associated with the closure of the Mozambique Ocean and collision of the Indian and East African/Madagascan cratonic domains is believed to have taken place along the southern margin of the Salem Block (the Palghat-Cauvery Shear System, PCSS) in the Southern Granulite Terrane. Investigation of the structural fabrics and the geochronology of the high-grade shear zones within the PCSS system shows that the Moyar-Salem-Attur shear zone to the north of the PCSS system is early Paleoproterozoic in age and associated with dextral strike-slip motion, while the Cauvery shear zone (CSZ) to the south of the PCSS system can be loosely constrained to circa 740-550 Ma and is associated with dip-slip dextral transpression and north side-up motion. To the south of the proposed suture zone (the Cauvery shear zone), the structural fabrics of the Northern Madurai Block suggest four deformational events (D1-D4), some of which are likely to be contemporaneous. The timing of high pressure-ultrahigh temperature metamorphism and deformation (D1-D3) in the Madurai Block (here interpreted as the southern extension of Azania) is constrained to circa 550-500 Ma and interpreted as representing collisional orogeny and subsequent orogenic collapse of the eastern margin of the East African Orogen. The disparity in the nature of the structural fabrics and the timing of the deformation in the Salem and the Madurai Blocks suggest that the two experienced distinct tectonothermal events prior to their amalgamation along the Cauvery shear zone during the

  10. Obscured accretion from AGN surveys

    NASA Astrophysics Data System (ADS)

    Vignali, Cristian

    2014-07-01

    Recent models of super-massive black hole (SMBH) and host galaxy joint evolution predict the presence of a key phase where accretion, traced by obscured Active Galactic Nuclei (AGN) emission, is coupled with powerful star formation. Then feedback processes likely self-regulate the SMBH growth and quench the star-formation activity. AGN in this important evolutionary phase have been revealed in the last decade via surveys at different wavelengths. On the one hand, moderate-to-deep X-ray surveys have allowed a systematic search for heavily obscured AGN, up to very high redshifts (z~5). On the other hand, infrared/optical surveys have been invaluable in offering complementary methods to select obscured AGN also in cases where the nuclear X-ray emission below 10 keV is largely hidden to our view. In this review I will present my personal perspective of the field of obscured accretion from AGN surveys.

  11. Cyclotron Resonance in Accreting Pulsars

    NASA Astrophysics Data System (ADS)

    Bhattacharya, Dipankar

    2016-07-01

    Cyclotron Resonance Absorption/Scattering features provide direct measurement of magnetic field strength in the line forming region. This has enabled the estimation of magnetic field strengths of nearly two dozen neutron stars in accreting high mass binary systems. With improved spectroscopic sensitivity, new X-ray observatories such as NuSTAR, Astrosat and Hitomi are opening the doors to studying detailed features such as the line shape and phase dependence with high significance. Such studies will help understand the nature of matter accumulation in, and outflow from, the magnetically confined accretion column on the neutron star. This talk will describe the results of MHD simulations of the matter flow in such systems, the diagnostics of such flows using cyclotron lines, and comparison with recent observations from NuSTAR and Astrosat.

  12. Brook street and Murihiku terranes of New Zealand in the context of a mobile South Pacific Gondwana margin

    NASA Astrophysics Data System (ADS)

    Bradshaw, J. D.

    1994-10-01

    Data from geochemistry, isotope studies and paleomagnetism suggest that both the Permian arc-dominated Brook Street terrane and Triassic-Jurassic Murihiku forearc basin terrane are unrelated to each other and that both are allochthonous with respect to the Gondwana margin. The Murihiku terrane lies farther from the Gondwana margin but its fauna and flora have strong Gondwana affinities and closely resemble those of autochthonous epi-Gondwana basins in Australia and Argentina. All terranes within New Zealand are elongate parallel to the Gondwana margin but very few can be clearly identified in the Australian and Antarctic continental crust that abuts New Zealand in Cretaceous Gondwana reconstructions. Similarly, some Australia terranes do not appear to continue into New Zealand or Antarctica. This suggests that breakup was influenced significantly by old terrane boundaries and also that there was considerable terrane mobility in the active Gondwana margin before the Cretaceous. A systematic comparison of arc chemistry, geochronology, biota, and paleomagnetism is desirable to constrain the development of the South Pacific margin between Australia and South America in the Mesozoic.

  13. Stratigraphic and isotopic link between the northern Stikine terrane and an ancient continental margin assemblage, Canadian Cordillera

    SciTech Connect

    Jackson, J.L.; Gehrels, G.E.; Patchett, P.J. ); Mihalynuk, M.G. )

    1991-12-01

    Geologic and isotopic data strongly imply a Late Triassic depositional link between a juvenile volcanic arc (northern Stikine terrane) and an outboard ancient continental margin assemblage (Nisling terrane) in the Canadian Cordillera. Two sandstone samples and a schist clast from a conglomerate layer at the base of the Upper Triassic Stuhini Group (northern Stikine terrane) have Nd-depleted mantle model ages of 1400-1430 and 1600 Ma, respectively; other Stuhini Group rocks have model ages of 390,660 and 690 Ma. Three samples of Nisling terrane schist and gneiss yield Nd model ages of 910, 1770, and 2450 Ma and highly radiogenic {sup 87}Sr/{sup 86}Sr ratios. These isotopic data corroborate stratigraphic evidence that detritus at the base of northern Stikine was shed from the Nisling terrane and strengthen interpretations that these terranes became linked by Late Triassic time. Thus, Upper Triassic strata of the northern Stikine terrane may have accumulated on top of or adjacent to an exotic continental fragment, a rifted fragment of the North American margin, or the in situ North American margin.

  14. Persistent Patterns in Accretion Disks

    SciTech Connect

    Amin, Mustafa A.; Frolov, Andrei V.; /KIPAC, Menlo Park

    2006-04-03

    We present a set of new characteristic frequencies associated with accretion disks around compact objects. These frequencies arise from persistent rotating patterns in the disk that are finite in radial extent and driven purely by the gravity of the central body. Their existence depends on general relativistic corrections to orbital motion and, if observed, could be used to probe the strong gravity region around a black hole. We also discuss a possible connection to the puzzle of quasi-periodic oscillations.

  15. Spiral Waves in Accretion Disks

    NASA Astrophysics Data System (ADS)

    Harlaftis, Emilios

    A review with the most characteristic spiral waves in accretion disks of cataclysmic variables will be presented. Recent work on experiments targeting the detection of spiral waves from time lapse movies of real disks and the study of permanent spiral waves will be discussed. The relevance of spiral waves with other systems such as star-planet X-ray binaries and Algols will be reviewed.

  16. Accretion disks around black holes

    NASA Technical Reports Server (NTRS)

    Abramowicz, M. A.

    1994-01-01

    The physics of accretion flow very close to a black hole is dominated by several general relativistic effects. It cannot be described by the standard Shakura Sunyaev model or by its relativistic version developed by Novikov and Thome. The most important of these effects is a dynamical mass loss from the inner edge of the disk (Roche lobe overflow). The relativistic Roche lobe overflow induces a strong advective cooling, which is sufficient to stabilize local, axially symmetric thermal and viscous modes. It also stabilizes the non-axially-symmetric global modes discovered by Papaloizou and Pringle. The Roche lobe overflow, however, destabilizes sufficiently self-gravitating accretion disks with respect to a catastrophic runaway of mass due to minute changes of the gravitational field induced by the changes in the mass and angular momentum of the central black hole. One of the two acoustic modes may become trapped near the inner edge of the disk. All these effects, absent in the standard model, have dramatic implications for time-dependent behavior of the accretion disks around black holes.

  17. Magnetically Torqued Thin Accretion Disks

    NASA Astrophysics Data System (ADS)

    Kluźniak, W.; Rappaport, S.

    2007-12-01

    We compute the properties of a geometrically thin, steady accretion disk surrounding a central rotating, magnetized star. The magnetosphere is assumed to entrain the disk over a wide range of radii. The model is simplified in that we adopt two (alternate) ad hoc, but plausible, expressions for the azimuthal component of the magnetic field as a function of radial distance. We find a solution for the angular velocity profile tending to corotation close to the central star and smoothly matching a Keplerian curve at a radius where the viscous stress vanishes. The value of this ``transition'' radius is nearly the same for both of our adopted B-field models. We then solve analytically for the torques on the central star and for the disk luminosity due to gravity and magnetic torques. When expressed in a dimensionless form, the resulting quantities depend on one parameter alone, the ratio of the transition radius to the corotation radius. For rapid rotators, the accretion disk may be powered mostly by spin-down of the central star. These results are independent of the viscosity prescription in the disk. We also solve for the disk structure for the special case of an optically thick alpha disk. Our results are applicable to a range of astrophysical systems including accreting neutron stars, intermediate polar cataclysmic variables, and T Tauri systems.

  18. A Novel μCT Analysis Reveals Different Responses of Bioerosion and Secondary Accretion to Environmental Variability

    PubMed Central

    Silbiger, Nyssa J.; Guadayol, Òscar; Thomas, Florence I. M.; Donahue, Megan J.

    2016-01-01

    Corals build reefs through accretion of calcium carbonate (CaCO3) skeletons, but net reef growth also depends on bioerosion by grazers and borers and on secondary calcification by crustose coralline algae and other calcifying invertebrates. However, traditional field methods for quantifying secondary accretion and bioerosion confound both processes, do not measure them on the same time-scale, or are restricted to 2D methods. In a prior study, we compared multiple environmental drivers of net erosion using pre- and post-deployment micro-computed tomography scans (μCT; calculated as the % change in volume of experimental CaCO3 blocks) and found a shift from net accretion to net erosion with increasing ocean acidity. Here, we present a novel μCT method and detail a procedure that aligns and digitally subtracts pre- and post-deployment μCT scans and measures the simultaneous response of secondary accretion and bioerosion on blocks exposed to the same environmental variation over the same time-scale. We tested our method on a dataset from a prior study and show that it can be used to uncover information previously unattainable using traditional methods. We demonstrated that secondary accretion and bioerosion are driven by different environmental parameters, bioerosion is more sensitive to ocean acidity than secondary accretion, and net erosion is driven more by changes in bioerosion than secondary accretion. PMID:27074001

  19. A Novel μCT Analysis Reveals Different Responses of Bioerosion and Secondary Accretion to Environmental Variability.

    PubMed

    Silbiger, Nyssa J; Guadayol, Òscar; Thomas, Florence I M; Donahue, Megan J

    2016-01-01

    Corals build reefs through accretion of calcium carbonate (CaCO3) skeletons, but net reef growth also depends on bioerosion by grazers and borers and on secondary calcification by crustose coralline algae and other calcifying invertebrates. However, traditional field methods for quantifying secondary accretion and bioerosion confound both processes, do not measure them on the same time-scale, or are restricted to 2D methods. In a prior study, we compared multiple environmental drivers of net erosion using pre- and post-deployment micro-computed tomography scans (μCT; calculated as the % change in volume of experimental CaCO3 blocks) and found a shift from net accretion to net erosion with increasing ocean acidity. Here, we present a novel μCT method and detail a procedure that aligns and digitally subtracts pre- and post-deployment μCT scans and measures the simultaneous response of secondary accretion and bioerosion on blocks exposed to the same environmental variation over the same time-scale. We tested our method on a dataset from a prior study and show that it can be used to uncover information previously unattainable using traditional methods. We demonstrated that secondary accretion and bioerosion are driven by different environmental parameters, bioerosion is more sensitive to ocean acidity than secondary accretion, and net erosion is driven more by changes in bioerosion than secondary accretion. PMID:27074001

  20. Crustal structure of the Archaean granite-greenstone terrane in the northern portion of the Kaapvaal Craton

    NASA Technical Reports Server (NTRS)

    Debeer, J. H.; Stettler, E. H.; Barton, J. M., Jr.; Vanreenen, D. D.; Bearncombe, J. R.

    1986-01-01

    Recent investigations of the electrical resistivity, gravity and aeromagnetic signatures of the various granite-greenstone units in the northern portion of the Kaapvaal craton have revealed three features of significance: (1) the Archean greenstone belts are shallow features, rarely exceeding 5 km in depth; (2) the high resistivity upper crustal layer typical of the lower grade granite-greenstone terranes is absent in the granulite facies terrane; and (3) the aeromagnetic lineation patterns allow the granite-greenstone terrane to be subdivided into geologically recognizable tectono-metamorphic domains on the basis of lineation frequency and direction. A discussion follows.

  1. Stratigraphy, petrology, and structure of the Pingston terrane, Mount Hayes C-5 and C-6 quadrangles, eastern Alaska Range, Alaska

    NASA Astrophysics Data System (ADS)

    Nokleberg, W. J.; Schwab, C. E.; Miyaoka, R. T.; Buhrmaster, C. L.

    Recent field, petrologic, and structural studies of the Pingston terrane in the Mount Hayes C-5 and C-6 quandrangles reveal that in this area the terrane: (1) has a highly distinctive stratigraphy, age, petrology (relict textures, relict minerals, and metamorphic facies), and structure; and (2) differs markedly from that described in previous studies. These more recent studies indicate that the major rock types, in order of decreasing abundance, are meta-andesite, metadacite and metarhyodacite flows and (or) tuff, metabasalt, metagabbro, metavolcanic graywacke, metagray-wacke, metasiltstone, metaquartzite or metachert, and very sparse marble. The general petrography of the major rock units in the Pingston terrane is given.

  2. The Khida terrane - Geochronological and isotopic evidence for Paleoproterozoic and Archean crust in the eastern Arabian Shield of Saudi Arabia

    USGS Publications Warehouse

    Whitehouse, M.J.; Stoeser, D.B.; Stacey, J.S.

    2001-01-01

    The Khida terrane of the eastern Arabian Shield of Saudi Arabia has been proposed as being underlain by Paleoproterozoic to Archean continental crust (Stoeser and Stacey, 1988). Detailed geological aspects of the Khida terrane, particularly resulting from new fieldwork during 1999, are discussed in a companion abstract (Stoeser et al., this volume). We present conventional and ion- microprobe U-Pb zircon geoenronology, Nd whole-rock, and feldspar Pb isotopic data that further elucidate the pre-Pan-African evolution of the Khida terrane. Locations for the Muhayil samples described below are shown in figure 2 of Stoeser et al. (this volume). 

  3. Controls on the Geometry of Accretion Reflectors

    NASA Astrophysics Data System (ADS)

    Wolovick, M.; Bell, R. E.; Buck, W. R.; Creyts, T. T.

    2012-12-01

    Basal accretion occurs when meltwater refreezes onto the base of an ice sheet. Thick packages (900-1100m) of accretion ice are identified in radio-echo sounding data as plume-shaped reflectors above the basal reflector and below isochronous layers of meteoric ice. Accretion reflectors have been imaged in both Antarctica and Greenland rising to a height of 1/3-1/2 of the ice sheet thickness and extending in the flow direction as far as 100 km. Here we use a two-dimensional thermomechanical higher order flowline model coupled to a basal hydrology model to investigate the freezing rates and energy budgets of basal accretion processes. Simple order-of-magnitude estimates for the freezing rate based on the observed height of the reflectors and the assumption that all ice under the observed reflector consists of accretion ice indicate very large freezing rates, on the order of 10-100 cm/yr. We test two end-member possibilities for the formation of the basal accretion bodies: high accretion rates and complex basal deformation. The first possibility is that the freezing rates are very large. The second possibility is that the ice under the observed reflector is a mixture of accreted and meteoric ice. If the ice below the accretion reflector is a mixture, the freezing rates can be much smaller than the simple estimates. If the freezing rates are small, then complex basal deformation must be invoked to cause accretion ice to override meteoric ice to a height of 1/3-1/2 the ice thickness. In the basal deformation case, low freezing rates predict a maximum thickness of 100-200m of accretion ice. The remaining ice beneath the reflector will be deformed meteoric ice. Both cases make testable predictions. If the accretion rates are very high and supercooling is the dominant process, accretion cannot use up all of the subglacial water. In this high rate scenario there will be water at the melting point exiting the accretion site. Alternatively if the accretion is part of a complex

  4. Planetary migration, accretion, and atmospheres

    NASA Astrophysics Data System (ADS)

    Dobbs-Dixon, Ian M.

    This dissertation explores three distinct projects in the field of planetary formation and evolution: type I migration, cessation of mass accretion, and the atmospheric dynamics of hot Jupiters. All three of these projects touch on outstanding or unresolved issues in the field. Each attempts to unify analytic and numerical approaches in order to physically motivate solutions while simultaneously probing areas currently inaccessible to purely analytic approaches. The first section, type I migration, explores the outstanding problem of the rapid inward migration of low mass planets embedded in protoplanetary disks. Analytic estimates of migration predict characteristic timescales that are much shorter then either observed disk lifetimes or theoretical core-accretion formation timescales. If migration is actually as efficient as these analytic estimates predict, planet formation across the observed range of masses and semimajor axis' is difficult. Here I introduce several new formalisms to both allow the disk to adiabatically adjust to the presence of a planet and include the effect of axisymmetric disk self-gravity. I find that these modifications increase migration timescales by approximately 4 times. In addition to these numerical improvements, I present simulations of migration in lower sound-speed regions of the disk on the grounds that self shadowing within the disk could yield substantially cooler gas temperatures then those derived by most irradiated disk models. In such regions the planetary perturbation excites a secondary instability, leading to the formation of vortices. These vortices cause a substantial reduction in the net torque, increasing migration timescales by up to approximately 200 times the analytically predicted rate. The second section addresses the mechanism for shutting off accretion onto giant planets. According to the conventional sequential accretion scenario, giant planets acquire a majority of their gas in a runaway phase. Conventional

  5. Paleomagnetic study on the Triassic rocks from the Lhasa Terrane, Tibet, and its paleogeographic implications

    NASA Astrophysics Data System (ADS)

    Zhou, Yanan; Cheng, Xin; Yu, Lei; Yang, Xingfeng; Su, Hailun; Peng, Ximing; Xue, Yongkang; Li, Yangyang; Ye, Yakun; Zhang, Jin; Li, Yuyu; Wu, Hanning

    2016-05-01

    We present paleomagnetic results from the newly discovered Early-Middle and Late Triassic marine sediments of the Lhasa Terrane. Orientated samples were collected from 32 sites (330 samples) on the north side of the Dibu Co Lake (84.7°E, 30.9°N), Coqên County, in the western region of the Lhasa terrane. Rock magnetic data revealed that most of the samples were dominated by magnetite and/or pyrrhotite. The stepwise demagnetization curves illustrated three-components: a low temperature component (Component A) near the present-day field (PDF), a secondary remanent magnetization (Component B) that may be from the Cretaceous Period, and a high-temperature component (Component C). The Component C were isolated from the Early-Middle Triassic rocks in 8 sites (47 specimens) and from the Late Triassic rocks in 6 sites (37 specimens). The Component C of the Early-Middle Triassic rocks passed a reversal test (B class, 95% confidence level) and a fold test (99% confidence level), that of the Late Triassic rocks passed a fold test (95% confidence level). The corresponding paleopoles for the Early-Middle and Late Triassic periods of the Lhasa Terrane were at 18.9°N, 208.4°E with A95 = 3.9° and 19.6°N, 211.8°E with A95 = 10.7°, respectively. We suggest that the Lhasa Terrane maintained a relative stable latitude (16.5 ± 3.9°S and 18.4 ± 10.7°S) in the southern hemisphere during the Triassic Period before moving northwards and amalgamating with the main body of Eurasia. The Qiangtang and Lhasa terranes, which were located at the mid-low latitudes of the southern hemisphere, might have been isolated between Eurasia and Gondwanaland since the Early Triassic Period. The Meso-Tethys, potentially represented by the Bangong-Nujiang suture zone (BNS) between the Lhasa and Qiangtang terranes, opened up in the Early-Middle Triassic Period and expanded during the entire course of the Triassic Period.

  6. Star formation sustained by gas accretion

    NASA Astrophysics Data System (ADS)

    Sánchez Almeida, Jorge; Elmegreen, Bruce G.; Muñoz-Tuñón, Casiana; Elmegreen, Debra Meloy

    2014-07-01

    Numerical simulations predict that metal-poor gas accretion from the cosmic web fuels the formation of disk galaxies. This paper discusses how cosmic gas accretion controls star formation, and summarizes the physical properties expected for the cosmic gas accreted by galaxies. The paper also collects observational evidence for gas accretion sustaining star formation. It reviews evidence inferred from neutral and ionized hydrogen, as well as from stars. A number of properties characterizing large samples of star-forming galaxies can be explained by metal-poor gas accretion, in particular, the relationship among stellar mass, metallicity, and star-formation rate (the so-called fundamental metallicity relationship). They are put forward and analyzed. Theory predicts gas accretion to be particularly important at high redshift, so indications based on distant objects are reviewed, including the global star-formation history of the universe, and the gas around galaxies as inferred from absorption features in the spectra of background sources.

  7. Erosion and accretion along the arctic coast of Alaska. The influence of ice and climate

    USGS Publications Warehouse

    Barnes, Peter W.; Rollyson, Bonnie P.

    1991-01-01

    Coastline comparison on 1951 and 1981 charts to determine erosion and accretion showed that ocean-facing coastal bluffs were retreating while deltas were rapidly expanding. Where the coast is fronted by a lagoon, and coast-parallel sand and gravel islands, bluff retreat was reduced. The extensive bluff erosion was volumetrically balanced by accretion at the mouths of deltas. Coastal erosion is driven by ice-related processes, aided by the presence of an ice-eroded shelf. Rapid delta expansion is interpreted to have begun in the last 200 years, perhaps related to observed permafrost warming.

  8. Circum-arctic plate accretion - Isolating part of a pacific plate to form the nucleus of the Arctic Basin

    USGS Publications Warehouse

    Churkin, M., Jr.; Trexler, J.H., Jr.

    1980-01-01

    A mosaic of large lithospheric plates rims the Arctic Ocean Basin, and foldbelts between these plates contain numerous allochthonous microplates. A new model for continental drift and microplate accretion proposes that prior to the late Mesozoic the Kula plate extended from the Pacific into the Arctic. By a process of circumpolar drift and microplate accretion, fragments of the Pacific basin, including parts of the Kula plate, were cut off and isolated in the Arctic Ocean, the Yukon-Koyukuk basin in Alaska, and the Bering Sea. ?? 1980.

  9. Lithium synthesis in microquasar accretion.

    PubMed

    Iocco, Fabio; Pato, Miguel

    2012-07-13

    We study the synthesis of lithium isotopes in the hot tori formed around stellar mass black holes by accretion of the companion star. We find that sizable amounts of both stable isotopes 6Li and 7Li can be produced, the exact figures varying with the characteristics of the torus and reaching as much as 10(-2) M⊙ for each isotope. This mass output is enough to contaminate the entire Galaxy at a level comparable with the original, pregalactic amount of lithium and to overcome other sources such as cosmic-ray spallation or stellar nucleosynthesis. PMID:23030150

  10. Pulsed Accretion onto Eccentric and Circular Binaries

    NASA Astrophysics Data System (ADS)

    Muñoz, Diego J.; Lai, Dong

    2016-08-01

    We present numerical simulations of circumbinary accretion onto eccentric and circular binaries using the moving-mesh code AREPO. This is the first set of simulations to tackle the problem of binary accretion using a finite-volume scheme on a freely moving mesh, which allows for accurate measurements of accretion onto individual stars for arbitrary binary eccentricity. While accretion onto a circular binary shows bursts with period of ∼ 5 times the binary period P b, accretion onto an eccentric binary is predominantly modulated at the period ∼ 1{P}{{b}}. For an equal-mass circular binary, the accretion rates onto individual stars are quite similar to each other, following the same variable pattern in time. By contrast, for eccentric binaries, one of the stars can accrete at a rate 10–20 times larger than its companion. This “symmetry breaking” between the stars, however, alternates over timescales of order 200P b and can be attributed to a slowly precessing, eccentric circumbinary disk. Over longer timescales, the net accretion rates onto individual stars are the same, reaching a quasi-steady state with the circumbinary disk. These results have important implications for the accretion behavior of binary T Tauri stars and supermassive binary black holes.

  11. Bondi accretion onto cosmological black holes

    NASA Astrophysics Data System (ADS)

    Karkowski, Janusz; Malec, Edward

    2013-02-01

    In this paper we investigate a steady accretion within the Einstein-Straus vacuole, in the presence of the cosmological constant. The dark energy damps the mass accretion rate and—above a certain limit—completely stops the steady accretion onto black holes, which, in particular, is prohibited in the inflation era and after (roughly) 1012 years from the big bang (assuming the presently known value of the cosmological constant). Steady accretion would not exist in the late phases of the Penrose’s scenario—known as the Weyl curvature hypothesis—of the evolution of the Universe.

  12. Accretion flows govern black hole jet properties

    NASA Astrophysics Data System (ADS)

    Koljonen, K.; Russell, D.; Fernández Ontiveros, J.; Miller-Jones, J.; Russell, T.; Curran, P.; Soria, R.; Markoff, S.; van der Horst, A.; Casella, P.

    2015-07-01

    The process of jet formation in accreting black holes, and the conditions under which it occurs is currently hotly debated, with competing models predicting the jet power to be governed by black hole spin, the magnetic field strength, the location of the jet base, the mass accretion rate and/or the properties of the inner accretion flow. We present new results that show empirical correlations between the accretion flow properties and the spectral energy distribution of the jets launched from accreting black holes. The X-ray power law is directly related to the particle energy distribution in the hot accretion flow. We find that the photon index of this power law correlates with the characteristic break frequency in the jet spectrum emitted near the jet base, and the jet luminosity up to the break frequency. The observed correlations can be explained by the energy distribution of electrons in the hot accretion flow being subsequently channeled into the jet. These correlations represent a new inflow--outflow connection in accreting black holes, and demonstrate that the spectral properties of the jet rely most critically on the conditions in the inner accretion flow, rather than other parameters such as the black hole mass or spin.

  13. Pulsed Accretion onto Eccentric and Circular Binaries

    NASA Astrophysics Data System (ADS)

    Muñoz, Diego J.; Lai, Dong

    2016-08-01

    We present numerical simulations of circumbinary accretion onto eccentric and circular binaries using the moving-mesh code AREPO. This is the first set of simulations to tackle the problem of binary accretion using a finite-volume scheme on a freely moving mesh, which allows for accurate measurements of accretion onto individual stars for arbitrary binary eccentricity. While accretion onto a circular binary shows bursts with period of ˜ 5 times the binary period P b, accretion onto an eccentric binary is predominantly modulated at the period ˜ 1{P}{{b}}. For an equal-mass circular binary, the accretion rates onto individual stars are quite similar to each other, following the same variable pattern in time. By contrast, for eccentric binaries, one of the stars can accrete at a rate 10–20 times larger than its companion. This “symmetry breaking” between the stars, however, alternates over timescales of order 200P b and can be attributed to a slowly precessing, eccentric circumbinary disk. Over longer timescales, the net accretion rates onto individual stars are the same, reaching a quasi-steady state with the circumbinary disk. These results have important implications for the accretion behavior of binary T Tauri stars and supermassive binary black holes.

  14. Geological investigations of pre-late Jurassic terranes in the southernmost Andes

    NASA Astrophysics Data System (ADS)

    Forsythe, R. D.

    Pre-Late Jurassic terranes of the Patagonian Archipelago were investigated. Their regional stratigraphic and structural characteristics were surveyed. Their significance in the late Paleozoic to early Mesozoic evolution of South America were determined. Pre-Late Jurassic rocks within the archipelago are distributed in two belts. Within the outer belt the Madre de Dios Archipielago was studied in detail. Pre-Late Jurassic rocks of this area are divisible into three mappable units. These three units are interpreted to be part of a late Paleozoic to early Mesozoic accretionary prism that was located along the ancestral Pacific margin of the South American sector of Gondwana. Within the inner belt, the region of Peninsula Staines was studied in detail. In this region greenschist facies metamorphism and pervasive deformation fabrics prevent stratigraphic subdivision of the terrane. However the lithologies present are correlative with the outer belt suggesting that they also were part of the late Paleozoic to early Mesozoic accretionary prism.

  15. Potassium metasomatism of volcanic and sedimentary rocks in rift basins, calderas and detachment terranes

    NASA Technical Reports Server (NTRS)

    Chapin, C. E.; drographic basins.

    1985-01-01

    The chemical, mineralogical, and oxygen-isotopic changes accompanying K-metasomatism are described. The similarities with diagenetic reactions in both deep marine and alkaline, saline-lake environments are noted. The common occurrence of K-metasomatism in upper-plate rocks of detachment terranes indicates that the early stage of severe regional extension causes crustal downwarping and, in arid to semi-arid regions, development of closed hydrographic basins.

  16. Paleomagnetic evidence that the central block of Salinia (California) is not a far-traveled terrane

    USGS Publications Warehouse

    Whidden, K.J.; Lund, S.P.; Bottjer, D.J.; Champion, D.; Howell, D.G.

    1998-01-01

    New paleomagnetic results from Late Cretaceous (75-85 m.y.) red beds on the central block of Salinia indicate that Salinia was located within 6?? (in latitude) of its current cratonal North American position during the Late Cretaceous (after correction for Neogene San Andreas Fault transport). The red beds formed as alluvial-fan overbank deposits with hematite cement deposited directly on Salinian granites in the La Panza Range. Paleomagnetic analysis shows two components of magnetization in the red beds, a low-blocking-temperature present-day overprint residing in goethite and a high-blocking-temperature (>600??) component residing in hematite. The hematite magnetization is a chemical remanent magnetization which formed soon after deposition during pedogenesis. The bedding-corrected hematite remanence contains a magnetic polarity stratigraphy with antipodal normal and reversed directions. Twenty-three Class I sites (??95 < 20??) have an average hematite direction with inclination = 54.4?? and declination = 18.2?? (??95 = 6.1??) after structural correction. These paleomagnetic data suggest that Salinia resided at about 35??N latitude during the Late Cretaceous, within 6?? of its current location adjacent to cratonal North America. By contrast, a summary of paleomagnetic data from the Peninsular Ranges terrane and the Sur-Obispo terrane, which are currently outboard of Salinia, shows northward transport of these terranes of 12,.o\\ to 22?? relative to their current locations in North America since the Cretaceous. The offsets increase systematically away from the craton with the most outboard Sur-Obispo terrane (which is composed of accretionary prism and distal forearc material) showing the largest degree of northward translation.

  17. Controls on accretion of flysch and melange belts at convergent margins: evidence from the Chugach Bay thrust and Iceworm melange, Chugach accretionary wedge, Alaska

    USGS Publications Warehouse

    Kusky, T.M.; Bradley, D.C.; Haeussler, P.J.; Karl, S.

    1997-01-01

    Controls on accretion of flysch and melange terranes at convergent margins are poorly understood. Southern Alaska's Chugach terrane forms the outboard accretionary margin of the Wrangellia composite terrane, and consists of two major lithotectonic units, including Triassic-Cretaceous melange of the McHugh Complex and Late Cretaceous flysch of the Valdez Group. The contact between the McHugh Complex and the Valdez Group on the Kenai Peninsula is a tectonic boundary between chaotically deformed melange of argillite, chert, greenstone, and graywacke of the McHugh Complex and a less chaotically deformed melange of argillite and graywacke of the Valdez Group. We assign the latter to a new, informal unit of formational rank, the Iceworm melange, and interpret it as a contractional fault zone (Chugach Bay thrust) along which the Valdez Group was emplaced beneath the McHugh Complex. The McHugh Complex had already been deformed and metamorphosed to prehnite-pumpellyite facies prior to formation of the Iceworm melange. The Chugach Bay thrust formed between 75 and 55 Ma, as shown by Campanian-Maastrichtian depositional ages of the Valdez Group, and fault-related fabrics in the Iceworm melange that are cut by Paleocene dikes. Motion along the Chugach Bay thrust thus followed Middle to Late Cretaceous collision (circa 90-100 Ma) of the Wrangellia composite terrane with North America. Collision related uplift and erosion of mountains in British Columbia formed a submarine fan on the Farallon plate, and we suggest that attempted subduction of this fan dramatically changed the subduction/accretion style within the Chugach accretionary wedge. We propose a model in which subduction of thinly sedimented plates concentrates shear strains in a narrow zone, generating melanges like the McHugh in accretionary complexes. Subduction of thickly sedimented plates allows wider distribution of shear strains to accommodate plate convergence, generating a more coherent accretionary style

  18. Paleoceanography of the tropical eastern pacific ocean.

    PubMed

    Grigg, R W; Hey, R

    1992-01-10

    The East Pacific Barrier (EPB) is the most effective marine barrier to dispersal of tropical shallow-water fauna in the world today. The fossil record of corals in the eastern Pacific suggests this has been true throughout the Cenozoic. In the Cretaceous, the EPB was apparently less effective in limiting dispersal. Equatorial circulation in the Pacific then appears to have been primarily east to west and the existence of oceanic atolls (now drowned guyots) in the eastern Pacific probably aided dispersal. Similarly, in the middle and early Mesozoic and late Paleozoic, terranes in the central tropical Pacific likely served as stepping stones to dispersal of tropical shelf faunas, reducing the isolating effect of an otherwise wider Pacific Ocean (Panthalassa). PMID:17756067

  19. Swept wing ice accretion modeling

    NASA Technical Reports Server (NTRS)

    Potapczuk, Mark G.; Bidwell, Colin S.

    1990-01-01

    An effort to develop a three-dimensional modeling method was initiated. This first step towards creation of a complete aircraft icing simulation code builds on previously developed methods for calculating three-dimensional flow fields and particle trajectories combined with a two-dimensional ice accretion calculation along coordinate locations corresponding to streamlines. This work is a demonstration of the types of calculations necessary to predict a three-dimensional ice accretion. Results of calculations using the 3-D method for a MS-317 swept wing geometry are projected onto a 2-D plane normal to the wing leading edge and compared to 2-D results for the same geometry. It is anticipated that many modifications will be made to this approach, however, this effort will lay the groundwork for future modeling efforts. Results indicate that the flow field over the surface and the particle trajectories differed for the two calculations. This led to lower collection efficiencies, convective heat transfer coefficients, freezing fractions, and ultimately ice accumulation for the 3-D calculation.

  20. Ringed Accretion Disks: Equilibrium Configurations

    NASA Astrophysics Data System (ADS)

    Pugliese, D.; Stuchlík, Z.

    2015-12-01

    We investigate a model of a ringed accretion disk, made up by several rings rotating around a supermassive Kerr black hole attractor. Each toroid of the ringed disk is governed by the general relativity hydrodynamic Boyer condition of equilibrium configurations of rotating perfect fluids. Properties of the tori can then be determined by an appropriately defined effective potential reflecting the background Kerr geometry and the centrifugal effects. The ringed disks could be created in various regimes during the evolution of matter configurations around supermassive black holes. Therefore, both corotating and counterrotating rings have to be considered as being a constituent of the ringed disk. We provide constraints on the model parameters for the existence and stability of various ringed configurations and discuss occurrence of accretion onto the Kerr black hole and possible launching of jets from the ringed disk. We demonstrate that various ringed disks can be characterized by a maximum number of rings. We present also a perturbation analysis based on evolution of the oscillating components of the ringed disk. The dynamics of the unstable phases of the ringed disk evolution seems to be promising in relation to high-energy phenomena demonstrated in active galactic nuclei.

  1. Correspondence of ores of silver and gold with basement terranes in the American southwest

    NASA Astrophysics Data System (ADS)

    Titley, S. R.

    1991-04-01

    The ratios of silver to gold produced from epigenetic ore districts of the American southwest reveal a consistency of value ranges, differing by an order of magnitude, that may be identified with either one or the other of two geologic terranes in which the ores occur. A discriminating value of the ratio is about 17.5∶1, the ratio of crustal abundance given by Ahrens (1965). (No further significance is attributed to this value, at this time, beyond the fact that it appears to establish a reasonable separation of values on the basis of geographic occurrence.) Ores relatively enriched in Ag occur in terranes floored by thick Proterozoic clastic and Paleozoic marine successions, and ores relatively enriched in Au lie above or within a Proterozoic basement dominated by maficfelsic volcanic (arc) successions. Proterozoic granites occur in each region. The values of the ratio are broadly consistent within each terrane, irrespective of the age of ore formation, the ore deposit style, associated igneous rocks, structural control, differing interpreted styles of subduction, and weathering histories. These characteristics and associations support a hypothesis that metallogenic signatures of ore districts in this region are fundamentally related to the crust in which the ores occur.

  2. Using multiple chemical systems in zircon to unravel the evolution of high-grade terranes

    NASA Astrophysics Data System (ADS)

    Clark, Chris; Taylor, Richard

    2016-04-01

    Since the turn of the century the rare earth element (REE) partitioning between zircon and garnet has facilitated the coupling of U-Pb ages to metamorphism, particularly in the granulite facies. The combination of in situ analysis and rapid data acquisition, particularly through combined techniques such as Laser Ablation Split Stream (LASS), means that complex terranes can be interrogated with increasing detail. However this detail provided by large datasets must also be combined with an understanding of the processes involved, for example the relative mobility of the REE and U-Pb systems with zircon grains that have withstood intense P-T conditions to varying degrees. For example, some high-temperature metapelites that seem to have all the right ingredients for the "equilibrium" to be achieved (e.g. they contain garnet, zircon, monazite and rutile, they've melted and experienced temperatures in excess of 900 °C) display variations in the REE partitioning between zircon and garnet that varies over the length-scale of a single thin section. This presentation seeks to highlight some complexities in the application of these undoublty useful techniques to high-temperature metamorphic rocks from a number of terranes and hopefully provide some useful comments on developing more efficient strategies to characterise the P-T-t evolution of high-grade terranes.

  3. Low Angle Contact Between the Oaxaca and Juárez Terranes Deduced From Magnetotelluric Data

    NASA Astrophysics Data System (ADS)

    Arzate-Flores, Jorge A.; Molina-Garza, Roberto; Corbo-Camargo, Fernando; Márquez-Ramírez, Víctor

    2016-04-01

    We present the electrical resistivity model along a profile perpendicular to the Middle America trench in southern Mexico that reveals previously unrecognized tectonic features at upper to mid-crustal depths. Our results support the hypotheses that the upper crust of the Oaxaca terrane is a residual ~20 km thick crust composed by an ~10 km thick faulted crustal upper layer and an ~10 km thick hydrated and/or mineralized layer. Oaxaca basement overthrust the younger Juárez (or Cuicateco) terrane. The electrical resistivity model supports the interpretation of a slab subducting at a low angle below Oaxaca. Uplift in the Oaxaca region appears to be related to fault reactivation induced by low angle subduction. In the Juárez terrane, isostatic forces may contribute to uplift because it is largely uncompensated. In the Sierra Madre del Sur, closer to the coast, uplift is facilitated by slab-dehydration driven buoyancy. Both gravity and resistivity models are consistent with a thinned upper crust in the northeast end of the profile.

  4. Deformation and chemical reaction in an ultramafic terrane boundary: the Livingstone Fault, New Zealand

    NASA Astrophysics Data System (ADS)

    Smith, S. A. F.; Crase, J. A.

    2015-12-01

    The Livingstone Fault is a >1000 km long terrane boundary that defines the eastern margin of the Dun Mountain Ophiolite Belt in New Zealand. The fault is spectacularly exposed where it juxtaposes ultramafic parts of the ophiolite belt (e.g. peridotite, serpentinite) against quartzofeldspathic rocks of the continental Caples Terrane. In such areas, the fault consists of a 50-400 m-wide foliated serpentinite shear zone entraining competent pods of massive serpentinite, Caples Terrane rocks and various volcanic rocks. The Livingstone Fault provides an excellent example of deformation styles (e.g. distributed vs. localized) and chemical reactions where peridotite, serpentinite and quartzofeldspathic rocks are juxtaposed, a common situation in many plate tectonic settings (e.g. portions of the San Andreas Fault, central and southern sectors of the Alpine Fault). We will present some initial results of fieldwork carried out in the Olivine Wilderness Area (NE of Milford Sound) focusing on: 1) the transition from intact peridotite to partly serpentinized peridotite to a fully serpentinized shear zone, 2) the distribution of strain within the serpentinite shear zone, 3) the significance of highly localized slip within entrained pods of peridotite and serpentinite, and 4) the nature and possible mechanical effects of talc-forming metasomatic reactions between serpentinite and quartzofeldspathic rocks.

  5. Lithospheric structures and Precambrian terrane boundaries in northeastern Botswana revealed through magnetotelluric profiling

    NASA Astrophysics Data System (ADS)

    Miensopust, M. P.; Jones, A. G.; Muller, M. R.; Garcia, X. A.; Evans, R. L.; Khoza, D. T.

    2010-12-01

    Within the framework of the Southern African MagnetoTelluric EXperiment (SAMTEX) a focused study was undertaken to gain better knowledge of the lithospheric geometries and structures of the westerly extension of the Zimbabwe Craton into Botswana, with the overarching aim to increase our understanding of southern African tectonics. The area of interest is located in northeastern Botswana, where Kalahari sands cover most of the geological terranes, and little is known about lithospheric structures and thickness. Some of the regional scale terrane boundary locations, defined based on potential field data, may be not sufficiently accurate for local scale studies. Investigation of the NNW-SSE orientated, 600 km long ZIM line profile crossing the Zimbabwe craton, Magondi mobile belt and Ghanzi-Chobe belt showed that the Zimbabwe craton is characterized by thick ( ˜ 220 km) resistive lithosphere, consistent with geochemical and geothermal estimates from kimberlite samples of the Orapa and Letlhakane pipes ( ˜ 175 km west of the profile). The lithospheric mantle of the Ghanzi-Chobe belt is resistive but the lithosphere is only about 180 km thick. At crustal depths a northwards-dipping boundary between the Ghanzi-Chobe and the Magondi belts is identified, and two mid- to lower-crustal conductors are discovered in the Magondi belt. The crustal terrane boundary between the Magondi and Ghanzi-Chobe belts is found to be located further to the north, and the southwestern boundary of the Zimbabwe craton might be further to the west, than previously inferred from potential field data.

  6. A magma ocean and the Earth's internal water budget

    NASA Technical Reports Server (NTRS)

    Ahrens, Thomas J.

    1992-01-01

    There are lines of evidence which relate bounds on the primordial water content of the Earth's mantle to a magma ocean and the accompanying Earth accretion process. We assume initially (before a magma ocean could form) that as the Earth accreted, it grew from volatile- (H2O, CO2, NH3, CH4, SO2, plus noble) gas-rich planetesimals, which accreted to form an initial 'primitive accretion core' (PAC). The PAC retained the initial complement of planetesimal gaseous components. Shock wave experiments in which both solid, and more recently, the gaseous components of materials such as serpentine and the Murchison meteorite have demonstrated that planetesimal infall velocities of less than 0.5 km/sec, induce shock pressures of less than 0.5 GPa and result in virtually complete retention of planetary gases.

  7. Tomographic images of subducted oceans matched to the accretionary records of orogens - Case study of North America and relevance to Central Asia

    NASA Astrophysics Data System (ADS)

    Sigloch, Karin; Mihalynuk, Mitchell G.; Hosseini, Kasra

    2016-04-01

    Accretionary orogens are the surface record of subduction on the 100-million-year timescale; they aggregate buoyant crustal welts that resisted subduction. The other record of subduction is found in the deep subsurface: oceanic lithosphere preserved in the mantle that records ocean basin closure between successive generations of arcs. Seismic tomography maps out these crumpled paleo-oceans down to the core-mantle boundary, where slab accumulates. One such accumulation of enormous scale is under Eastern Asia, recording the assembly of the Central Asian Orogenic Belt (CAOB). Deep CAOB slab has hardly been explored because tomographic image resolution in the lowermost mantle is limited, but this is rapidly improving. We present new images of the CAOB slabs from our P-wave tomography that includes core-diffracted waves as a technical novelty. The previous slab blur sharpens into the type of elongated geometries expected to trace paleo-trench lines. Since the North American Cordillera is younger than the CAOB (mostly <200 m.y. versus ~650-250 m.y.), its slabs have descended only to mid-mantle depths (<2000 km), where tomographic resolution is much better. Hence we can make a detailed, spatiotemporal match between 3-D slab geometries and the accretion history of the Cordillera - a blueprint for continental-scale investigations in other accretionary orogens, including what may become possible for the CAOB. Lower-mantle slabs beneath North America reveal evolving configurations of arc-trench positions back to the breakup of Pangea. These can be combined with quantitative plate reconstructions to show where and when the westward-drifting continent overrode pre-existing, intra-oceanic subduction zones, and accreted their associated arcs and basement terranes in Jurassic and Cretaceous times. Tectonic predictions from this "tomographic time machine" can be checked against the geological record. To demonstrate, we propose a resolution to the longstanding debate of how and when

  8. Accretion and core formation: constraints from metal-silicate partitioning.

    PubMed

    Wood, Bernard J

    2008-11-28

    Experimental metal-silicate partitioning data for Ni, Co, V, Cr, Nb, Mn, Si and W were used to investigate the geochemical consequences of a range of models for accretion and core formation on Earth. The starting assumptions were chondritic ratios of refractory elements in the Earth and the segregation of metal at the bottom of a magma ocean, which deepened as the planet grew and which had, at its base, a temperature close to the liquidus of the silicate. The models examined were as follows. (i) Continuous segregation from a mantle which is chemically homogeneous and which has a fixed oxidation state, corresponding to 6.26 per cent oxidized Fe. Although Ni, Co and W partitioning is consistent with chondritic ratios, the current V content of the silicate Earth cannot be reconciled with core segregation under these conditions of fixed oxidation state. (ii) Continuous segregation from a mantle which is chemically homogeneous but in which the Earth became more oxidized as it grew. In this case, the Ni, Co, W, V, Cr and Nb contents of core and mantle are easily matched to those calculated from the chondritic ratios of refractory elements. The magma ocean is calculated to maintain a thickness approximately 35 per cent of the depth to the core-mantle boundary in the accreting Earth, yielding a maximum pressure of 44GPa. This model yields a Si content of the core of 5.7 per cent, in good agreement with cosmochemical estimates and with recent isotopic data. (iii) Continuous segregation from a mantle which is not homogeneous and in which the core equilibrates with a restricted volume of mantle at the base of the magma ocean. This is found to increase depth of the magma ocean by approximately 50 per cent. All of the other elements (except Mn) have partitioning consistent with chondritic abundances in the Earth, provided the Earth became, as before, progressively oxidized during accretion. (iv) Continuous segregation of metal from a crystal-melt mush. In this case, pressures

  9. Microstructural and seismic properties of the upper mantle underneath a rifted continental terrane (Baja California): An example of sub-crustal mechanical asthenosphere?

    NASA Astrophysics Data System (ADS)

    Palasse, L. N.; Vissers, R. L. M.; Paulssen, H.; Basu, A. R.; Drury, M. R.

    2012-09-01

    The Gulf of California rift is a young and active plate boundary that links the San Andreas strike-slip fault system in California to the oceanic spreading system of the East Pacific Rise. The xenolith bearing lavas of the San Quintin volcanic area provide lower crust and upper mantle samples from beneath Baja California peninsula. The microstructures, crystallographic preferred orientations (CPO) and petrology of the San Quintin xenoliths suggest that the continental lithosphere in this region has undergone several stages of deformation, recrystallisation and melt-rock interaction. Melt-rock interactions have led to enrichment in olivine while fine-grained microstructures suggest intense deformation in an active shear zone in the shallow upper mantle. In this study we highlight the effect of the fine-grained mylonitic shear zone development in the upper mantle as an important process of weakening of continental lithosphere. The results of the microstructural study show a reduction in CPO strength with increasing grain size reduction. Most CPOs are consistent with dominant slip on the {0kl}[100] system. As a consequence, corresponding seismic anisotropies decrease for both P- and S-waves with increasing grain size reduction. The shallow crystallographic fabric can be related to active shear zones, which accommodate the relative motion between the Northern Baja terrane and the Pacific plate. Estimates of the strain rate, stress and viscosity indicate that the shallow mantle beneath Northern Baja is thermally and chemically lithospheric but mechanically has similar viscosity as the asthenosphere. The Northern Baja terrane is an interesting case of continental crust lying directly on low viscosity upper mantle.

  10. Early Neoproterozoic multiple arc-back-arc system formation during subduction-accretion processes between the Yangtze and Cathaysia blocks: New constraints from the supra-subduction zone NE Jiangxi ophiolite (South China)

    NASA Astrophysics Data System (ADS)

    Wang, Xin-Shui; Gao, Jun; Klemd, Reiner; Jiang, Tuo; Zhai, Qing-Guo; Xiao, Xu-Chang; Liang, Xin-Quan

    2015-11-01

    ophiolite originated from an isotopically homogeneous depleted mantle source. The diversity of MORB- to IAB-like basalts and the presence of Fe-Ti basalts favor a formation of the NE Jiangxi ophiolite during the initial rifting phase of an intra-oceanic back-arc basin between an oceanic arc (Huaiyu Terrane) and the continental margin of the Yangtze Block (Jiuling Terrane) at ca. 990 Ma. Both the present and previous studies imply that multiple arc-back-arc systems formed during long-lasting subduction-accretion processes between the Yangtze and Cathaysia blocks during the early Neoproterozoic.

  11. Interpretation of tectonic setting in the Phetchabun Volcanic Terrane, Northern Thailand: Evidence from enhanced airborne geophysical data

    NASA Astrophysics Data System (ADS)

    Sangsomphong, Arak; Thitimakorn, Thanop; Charusiri, Punya

    2015-08-01

    Re-processed aeromagnetic data with enhancement approaches of reduction to the pole, high pass filtering and shaded relief have been used to interpret complex subsurface structures of the Carboniferous to Triassic Phetchabun Volcanic Terrane (PVT) which is largely covered by thick Cenozoic sediment deposits. Interpretation of the enhanced aeromagnetic data reveals four distinct structural domains in the PVT, viz. Northern, Eastern, Central, and Western domains. Within these domains, high magnetic units are recognized, namely elongate, ring, circular, and dipolar spot units. The elongate unit in the Central domain is characterized by a deformation zone with northwest-southeast trending, sinistral shearing. East-west trending and the northeast-southwest trending faults cross-cut several magnetic units in the Central domain, with sinistral and dextral movements, respectively. Three major fault directions have been identified, including the northeast-southwest trending sinistral faults, north-south trending dextral faults, and northwest-southeast trending dextral faults. The younger spot units are small intrusive bodies largely situated along these latest fault segments. The aeromagnetic interpretation results, together with relevant current field verification, as well as previous geochronological and petrochemical investigations, have lead to the clarification of structural development in the PVT. The elongate units are interpreted to represent Late Carboniferous intrusive bodies. They occurred as a result of an eastward subduction of the Nakhonthai oceanic plate beneath the Indochina continental plate, along the Loei suture. The elongate units are also reflected in a north-south trending deformation zone formed by the east-west compressional tectonics. The ring units are considered to have formed in a Permo-Triassic volcanic arc, whereas the circular units represent equigranular intrusive bodies which formed in a response to the second phase of eastward subduction

  12. A SIGNATURE OF CHEMICAL SEPARATION IN THE COOLING LIGHT CURVES OF TRANSIENTLY ACCRETING NEUTRON STARS

    SciTech Connect

    Medin, Zach; Cumming, Andrew E-mail: cumming@physics.mcgill.ca

    2014-03-01

    We show that convection driven by chemical separation can significantly affect the cooling light curves of accreting neutron stars after they go into quiescence. We calculate the thermal relaxation of the neutron star ocean and crust including the thermal and compositional fluxes due to convection. After the inward propagating cooling wave reaches the base of the neutron star ocean, the ocean begins to freeze, driving chemical separation. The resulting convection transports heat inward, giving much faster cooling of the surface layers than found assuming the ocean cools passively. The light curves including convection show a rapid drop in temperature weeks after outburst. Identifying this signature in observed cooling curves would constrain the temperature and composition of the ocean as well as offer a real time probe of the freezing of a classical multicomponent plasma.

  13. Plasma physics of accreting neutron stars

    NASA Technical Reports Server (NTRS)

    Ghosh, Pranab; Lamb, Frederick K.

    1991-01-01

    Plasma concepts and phenomena that are needed to understand X- and gamma-ray sources are discussed. The capture of material from the wind or from the atmosphere or envelope of a binary companion star is described and the resulting types of accretion flows discussed. The reasons for the formation of a magnetosphere around the neutron star are explained. The qualitative features of the magnetospheres of accreting neutron stars are then described and compared with the qualitative features of the geomagnetosphere. The conditions for stable flow and for angular and linear momentum conservation are explained in the context of accretion by magnetic neutron stars and applied to obtain rough estimates of the scale of the magnetosphere. Accretion from Keplerian disks is then considered in some detail. The radial structure of geometrically thin disk flows, the interaction of disk flows with the neutron star magnetosphere, and models of steady accretion from Keplerian disks are described. Accretion torques and the resulting changes in the spin frequencies of rotating neutron stars are considered. The predicted behavior is then compared with observations of accretion-powered pulsars. Magnetospheric processes that may accelerate particles to very high energies, producing GeV and, perhaps, TeV gamma-rays are discussed. Finally, the mechanisms that decelerate and eventually stop accreting plasma at the surfaces of strongly magnetic neutron stars are described.

  14. Pulsed accretion in a variable protostar.

    PubMed

    Muzerolle, James; Furlan, Elise; Flaherty, Kevin; Balog, Zoltan; Gutermuth, Robert

    2013-01-17

    Periodic increases in luminosity arising from variable accretion rates have been predicted for some pre-main-sequence close binary stars as they grow from circumbinary disks. The phenomenon is known as pulsed accretion and can affect the orbital evolution and mass distribution of young binaries, as well as the potential for planet formation. Accretion variability is a common feature of young stars, with a large range of amplitudes and timescales as measured from multi-epoch observations at optical and infrared wavelengths. Periodic variations consistent with pulsed accretion have been seen in only a few young binaries via optical accretion tracers, albeit intermittently with accretion luminosity variations ranging from zero to 50 per cent from orbit to orbit. Here we report that the infrared luminosity of a young protostar (of age about 10(5) years) increases by a factor of ten in roughly one week every 25.34 days. We attribute this to pulsed accretion associated with an unseen binary companion. The strength and regularity of this accretion signal is surprising; it may be related to the very young age of the system, which is a factor of ten younger than the other pulsed accretors previously studied. PMID:23283175

  15. Accretion in the galactic halo

    NASA Astrophysics Data System (ADS)

    Stephens, Alex Courtney

    2000-10-01

    The Milky Way disk is enveloped in a diffuse, dynamically-hot collection of stars and star clusters collectively known as the ``stellar halo''. Photometric and chemical analyses suggest that these stars are ancient fossils of the galaxy formation epoch. Yet, little is known about the origin of this trace population. Is this system merely a vestige of the initial burst of star formation within the decoupled proto-Galaxy, or is it the detritus of cannibalized satellite galaxies? In an attempt to unravel the history of the Milky Way's stellar halo, I performed a detailed spectroscopic analysis of 55 metal-poor stars possessing ``extreme'' kinematic properties. It is thought that stars on orbits that either penetrate the remote halo or exhibit large retrograde velocities could have been associated with assimilated (or ``accreted'') dwarf galaxies. The hallmark of an accreted halo star is presumed to be a deficiency (compared with normal stars) of the α-elements (O, Mg, Si, Ca, Ti) with respect to iron, a consequence of sporadic bursts of star formation within the diminutive galaxies. Abundances for a select group of light metals (Li, Na, Mg, Si, Ca, Ti), iron-peak nuclides (Cr, Fe, Ni), and neutron-capture elements (Y, Ba) were calculated using line-strengths measured from high-resolution, high signal-to-noise spectral observations collected with the Keck I 10-m and KPNO 4-m telescopes. The abundances extracted from the spectra reveal: (1)The vast majority of outer halo stars possess supersolar [α/Fe] > 0.0) ratios. (2)The [α/Fe] ratio appears to decrease with increasing metallicity. (3)The outer halo stars have lower ratios of [α/Fe] than inner halo stars at a given metallicity. (4)At the largest metallicities, there is a large spread in the observed [α/Fe] ratios. (5)[α/Fe] anti-correlates with RAPO. (6)Only one star (BD+80° 245) exhibits the peculiar abundances expected of an assimilated star. The general conclusion extracted from these data is that the

  16. Structural Analyses of the Kahiltna Terrane: A Kinematic Record of the Collision of the Talkeetna Superterrane

    NASA Astrophysics Data System (ADS)

    Bier, S. E.; Fisher, D.

    2002-12-01

    Macro-, meso-, and microscale structural analyses from several localities across the ~1000 km Kahiltna Terrane provide valuable kinematic insights into the late Cretaceous collision between the Talkeetna superterrane and North America. The Kahiltna Terrane, a Jurassic-Cretaceous flysch basin inboard of the Talkeetna superterrane (Wrangellia, Peninsular, and Alexander terranes), contains incremental strain indicators that record a history of oblique collision and subsequent deformation in a strike-slip regime. A comparison of structural data from localities across the Kahiltna terrane suggests a unique history not yet described in previous work on south-central Alaskan tectonics. Data was collected from the Reindeer Hills area, the northwestern Talkeetna Mountains, Denali National Park, the Peters Hills, and the Tordrillo Mountains. In the Reindeer Hills, a melange zone occurs as a series of exposures dismembered by ongoing strike slip faulting between the flysch of the Kahiltna terrane and the precollisional edge of the North American continent. This melange is characterized by fault-bounded blocks of Paleozoic limestone and sandstone within an argillite matrix with a conspicuous scaly fabric. The blocks range in size from 10 cm to tens of meters; and melange fish indicate a south-directed shear sense. The melange is overlain by a red and green (Triassic-Jurassic?) conglomerate along an unconformity that likely marks the base of a perched slope basin near the toe of an accretionary wedge. The strike of bedding and cleavage in this area trends EW. The fold axes trend NW-SE and folds verge to the south. In the northwest corner of the Talkeetna Mountains, the structure is dominated by north vergent folds and faults. The strike of bedding trends ~025°; whereas the strike of the cleavage is ~060°. Both cleavage and bedding dip to the southeast. The fold axes trend roughly NE-SW. North of the Denali Fault System, in Denali National Park, strike of bedding is ~122° and

  17. The intra-oceanic Cretaceous (~ 108 Ma) Kata-Rash arc fragment in the Kurdistan segment of Iraqi Zagros suture zone: Implications for Neotethys evolution and closure

    NASA Astrophysics Data System (ADS)

    Ali, Sarmad A.; Ismail, Sabah A.; Nutman, Allen P.; Bennett, Vickie C.; Jones, Brian G.; Buckman, Solomon

    2016-09-01

    The Kata-Rash arc fragment is an allochthonous thrust-bound body situated near Penjween, 100 km northeast of Sulymannia city, Kurdistan Region, within the Iraqi portion of the Zagros suture zone. It forms part of the suprasubduction zone 'Upper Allochthon' terranes (designated as the Gimo-Qandil Group), which is dominated by calc-alkaline andesite and basaltic-andesite, rhyodacite to rhyolite, crosscut by granitic, granodioritic, and dioritic dykes. Previously, rocks of the Kata-Rash arc fragment were interpreted as a part of the Eocene Walash volcanic group. However, SHRIMP zircon U-Pb dates on them of 108.1 ± 2.9 Ma (Harbar volcanic rocks) and 107.7 ± 1.9 Ma (Aulan intrusion) indicate an Albian-Cenomanian age, which is interpreted as the time of igneous crystallisation. The Aulan intrusion zircons have initial εHf values of + 8.6 ± 0.2. On a Nb/Yb-Th/Yb diagram, all Kata-Rash samples fall within the compositional field of arc-related rocks, i.e. above the mid-ocean-ridge basalt (MORB)-ocean island basalt (OIB) mantle array. Primitive-mantle-normalised trace-element patterns for the Kata-Rash samples show enrichment in the large ion lithophile elements and depletion in the high-field-strength elements supporting their subduction-related character. Low Ba/La coupled with low La/Yb and Hf/Hf* < 1 for the Aulan sample with initial εHf of + 8.6 ± 0.2 is interpreted as the magma dominated by contributions from fluid fluxing of the mantle wedge and lesser contributions of low temperature melt from subducted slab sediment, in an oceanic setting. This mechanism can explain the sub-DM initial εHf value, without the need to invoke melting of significantly older (continental) crust in an Andean setting. We interpret the Kata-Rash igneous rocks as a fragment of the Late Cretaceous suprasubduction zone system (named here the Kata-Rash arc) that most likely developed within the Neotethys Ocean rather than at a continental margin. Subsequently during the latest Cretaceous

  18. Origin and models of oceanic transform faults

    NASA Astrophysics Data System (ADS)

    Gerya, Taras

    2012-02-01

    Mid-ocean ridges sectioned by transform faults represent prominent surface expressions of plate tectonics. A fundamental problem of plate tectonics is how this pattern has formed and why it is maintained. Gross-scale geometry of mid-ocean ridges is often inherited from respective rifted margins. Indeed, transform faults seem to nucleate after the beginning of the oceanic spreading and can spontaneously form at a single straight ridge. Both analog and numerical models of transform faults were investigated since the 1970s. Two main groups of analog models were developed: thermomechanical (freezing wax) models with accreting and cooling plates and mechanical models with non-accreting lithosphere. Freezing wax models reproduced ridge-ridge transform faults, inactive fracture zones, rotating microplates, overlapping spreading centers and other features of oceanic ridges. However, these models often produced open spreading centers that are dissimilar to nature. Mechanical models, on the other hand, do not accrete the lithosphere and their results are thus only applicable for relatively small amount of spreading. Three main types of numerical models were investigated: models of stress and displacement distribution around transforms, models of their thermal structure and crustal growth, and models of nucleation and evolution of ridge-transform fault patterns. It was shown that a limited number of spreading modes can form: transform faults, microplates, overlapping spreading centers, zigzag ridges and oblique connecting spreading centers. However, the controversy exists whether these patterns always result from pre-existing ridge offsets or can also form spontaneously at a single straight ridge during millions of year of accretion. Therefore, two types of transform fault interpretation exist: plate fragmentation structures vs. plate accretion structures. Models of transform faults are yet relatively scarce and partly controversial. Consequently, a number of first order

  19. The memory of the accreting plate boundary and the continuity of fracture zones

    USGS Publications Warehouse

    Schouten, Hans; Klitgord, Kim D.

    1982-01-01

    A detailed aeromagnetic anomaly map of the Mesozoic seafloor-spreading lineations southwest of Bermuda reveals the dominant magnetic grain of the oceanic crust and the character of the accreting boundary at the time of crustal formation. The magnetic anomaly pattern is that of a series of elongate lobes perpendicular to the fracture zone (flowline) trends. The linear sets of magnetic anomaly peaks and troughs have narrow regions of reduced amplitude anomalies associated with the fracture zones. During the period of Mesozoic geomagnetic polarity reversals (when 1200 km of central North Atlantic seafloor formed), the Atlantic accreting boundary consisted of stationary, elongate, spreading center cells that maintained their independence even though sometimes only minor spatial offsets existed between cells. Normal oceanic crustal structure was formed in the spreading center cells, but structural anomalies and discontinuities characteristic of fracture zones were formed at their boundaries, which parallel flowlines of Mesozoic relative plate motion in the central North Atlantic. We suggest that the memory for a stationary pattern of independent spreading center cells resides in the young brittle lithosphere at the accreting boundary where the lithosphere is weakest; here, each spreading center cell independently goes through its cylce of stress buildup, stress release, and crustal accretion, after which its memory is refreshed. The temporal offset between the peaks of the accretionary activity that takes place within each cell may provide the mechanism for maintaining the independence of adjacent spreading center cells through times when no spatial offset between the cells exists.

  20. A recent phase of accretion along the southern Costa Rican subduction zone

    NASA Astrophysics Data System (ADS)

    Bangs, Nathan L.; McIntosh, Kirk D.; Silver, Eli A.; Kluesner, Jared W.; Ranero, César R.

    2016-06-01

    In 2011 we acquired a 3D seismic reflection volume across the Costa Rica margin NW of the Osa Peninsula to investigate the complex structure and the development of the seismogenic zone within the Costa Rican subduction zone in the vicinity of recent International Ocean Drilling Program (IODP) drilling. In contrast to previous interpretations, these newly acquired seismic images show that the margin wedge is composed of a layered fabric that is consistent with clastic sediments, similar to materials recovered from IODP drilling, that have been thrust and thickened into thrust-bounded folded sequences. These structures are consistent with a balanced sequence that has been frontally accreted in the context of an accretionary model. We interpret these sequences as sediment originally deposited on the subducting crust in a trench basin created by the southward migration of the Cocos-Nazca-Caribbean triple junction, and accreted during recent margin subduction that also accelerated with passage of the triple junction. The margin is composed of relatively rapidly accreted sediment that was added to the margin during a phase of accretion within the last ∼5 Ma that was probably preceded throughout the Neogene by periods of non-accretion or erosion.

  1. Timing the structural events in the Palaeoproterozoic Bolé-Nangodi belt terrane and adjacent Maluwe basin, West African craton, in central-west Ghana

    NASA Astrophysics Data System (ADS)

    de Kock, G. S.; Théveniaut, H.; Botha, P. M. W.; Gyapong, W.

    2012-04-01

    The Maluwe basin, north-adjacent to the Sunyani basin, is the northernmost of the northeast-trending Eburnean volcaniclastic depositories in Ghana. These basins are separated from one another by remnants of Eoeburnean crust, all formed during the evolution of an arc-backarc basins complex in a Palaeoproterozoic intraoceanic environment. The Bolé-Nangodi belt terrane to the northwest, of mostly Eoeburnean crust is fault bounded with the Maluwe basin along the northeast-trending Bolé-Navrongo fault zone. The stratigraphic sequence, which was the key to unravelling the structural evolution of the study area, was established by means of field observations aided by precision SHRIMP geochronology. The quartzitic, pelitic, quartzofeldspathic and granitic gneisses of the Eoeburnean crust (>2150 Ma) experienced complex metamorphic mineral growth and migmatitization, mostly under static crustal conditions and were subjected to several deformation episodes. The foliated mafic and metasedimentary enclaves within the Ifanteyire granite establish deformation to have taken place prior to ˜2195 Ma, while the tectonically emplaced Kuri amphibolites within the 2187-Ma gneissic Gondo granite indicate a stage of rifting followed by collision. Deformation of granite dykes in the Gondo granites at ˜2150 Ma concluded the development of the Eoeburnean orogenic cycle (DEE). The Sawla Suite, contemporaneous with the deposition of the Maluwe Group, intruded the tectonic exhumed Bolé-Nangodi terrane during extension between ˜2137 and 2125 Ma. The rifting separated the Abulembire fragment from the Bolé-Nangodi terrane. During subsequent northwestward subduction of young back-arc basin oceanic crust the volcaniclastic strata of the Maluwe Group and Sawla granitoids were deformed (DE1) under chlorite/sericite greenschist-grade conditions. The NE-trending folds had subhorizontal axes and subvertical axial planes. Simultaneous to the DE1 orogenesis the molasses of the Banda Group was

  2. Effects of ice accretions on aircraft aerodynamics

    NASA Astrophysics Data System (ADS)

    Lynch, Frank T.; Khodadoust, Abdollah

    2001-11-01

    This article is a systematic and comprehensive review, correlation, and assessment of test results available in the public domain which address the aerodynamic performance and control degradations caused by various types of ice accretions on the lifting surfaces of fixed wing aircraft. To help put the various test results in perspective, overviews are provided first of the important factors and limitations involved in computational and experimental icing simulation techniques, as well as key aerodynamic testing simulation variables and governing flow physics issues. Following these are the actual reviews, assessments, and correlations of a large number of experimental measurements of various forms of mostly simulated in-flight and ground ice accretions, augmented where appropriate by similar measurements for other analogous forms of surface contamination and/or disruptions. In-flight icing categories reviewed include the initial and inter-cycle ice accretions inherent in the use of de-icing systems which are of particular concern because of widespread misconceptions about the thickness of such accretions which can be allowed before any serious consequences occur, and the runback/ridge ice accretions typically associated with larger-than-normal water droplet encounters which are of major concern because of the possible potential for catastrophic reductions in aerodynamic effectiveness. The other in-flight ice accretion category considered includes the more familiar large rime and glaze ice accretions, including ice shapes with rather grotesque features, where the concern is that, in spite of all the research conducted to date, the upper limit of penalties possible has probably not been defined. Lastly, the effects of various possible ground frost/ice accretions are considered. The concern with some of these is that for some types of configurations, all of the normally available operating margins to stall at takeoff may be erased if these accretions are not

  3. Accreting Neutron Stars as Astrophysical Laboratories

    NASA Technical Reports Server (NTRS)

    Chakrabarty, Deepto

    2004-01-01

    In the last year, we have made an extremely important breakthrough in establishing the relationship between thermonuclear burst oscillations in accreting neutron stars and the stellar spin. More broadly, we have continued t o make significant scientific progress in all four of the key focus areas identified in our original proposal: (1) the disk-magnetosphere interaction in neutron stars, (2) rapid variability in accreting neutron stars, (3) physics of accretion flows, and (4) fundamental properties of neutron stars. A list of all publications that have arising from this work since the start of our program is given.

  4. Lyman edges - Signatures of accretion disks

    NASA Astrophysics Data System (ADS)

    Kinney, A. L.

    1992-05-01

    Accretion disks are thought to provide the ultraviolet emission seen in the big blue bump of quasars. However, observations of the UV spectra of quasars do not show the additional signatures predicted by the accretion disk models. This paper will concentrate on just one of those signatures - the Lyman edge. Two studies are briefly discussed which explore the Lyman edge region of both high and low redshift quasars (Antonucci, Kinney, and Ford 1989 and Koratkar, Kinney, and Bohlin 1992). Both studies find that Lyman edges are not present in quasar spectra as frequently as predicted by the models or at the strength predicted by accretion disk models.

  5. Low palaeoelevation of the northern Lhasa terrane during late Eocene: Fossil foraminifera and stable isotope evidence from the Gerze Basin.

    PubMed

    Wei, Yi; Zhang, Kexin; Garzione, Carmala N; Xu, Yadong; Song, Bowen; Ji, Junliang

    2016-01-01

    The Lhasa terrane is a key region for understanding the paleoelevation of the southern Tibetan Plateau after India-Asia collision. The Gerze Basin, located in the northern part of the Lhasa terrane, is a shortening-related basin. We discovered Lagena laevis (Bandy) fossils in upper Eocene strata of the Gerze Basin. This type of foraminifera is associated with lagoon and estuarine environments, indicating that the northern part of the Lhasa terrane was near sea level during the late Eocene. We speculate that these foraminifera were transported inland by storm surges to low elevation freshwater lakes during times of marine transgressions. This inference is consistent with the relatively positive δ(18)O values in carbonate from the same deposits that indicate low palaeoelevations close to sea level. Considering the palaeoelevation results from the nearby Oligocene basins at a similar latitude and the volcanic history of the Lhasa terrane, we infer that large-magnitude surface uplift of the northern Lhasa terrane occurred between late Eocene and late Oligocene time. PMID:27272610

  6. Low palaeoelevation of the northern Lhasa terrane during late Eocene: Fossil foraminifera and stable isotope evidence from the Gerze Basin

    PubMed Central

    Wei, Yi; Zhang, Kexin; Garzione, Carmala N.; Xu, Yadong; Song, Bowen; Ji, Junliang

    2016-01-01

    The Lhasa terrane is a key region for understanding the paleoelevation of the southern Tibetan Plateau after India-Asia collision. The Gerze Basin, located in the northern part of the Lhasa terrane, is a shortening-related basin. We discovered Lagena laevis (Bandy) fossils in upper Eocene strata of the Gerze Basin. This type of foraminifera is associated with lagoon and estuarine environments, indicating that the northern part of the Lhasa terrane was near sea level during the late Eocene. We speculate that these foraminifera were transported inland by storm surges to low elevation freshwater lakes during times of marine transgressions. This inference is consistent with the relatively positive δ18O values in carbonate from the same deposits that indicate low palaeoelevations close to sea level. Considering the palaeoelevation results from the nearby Oligocene basins at a similar latitude and the volcanic history of the Lhasa terrane, we infer that large-magnitude surface uplift of the northern Lhasa terrane occurred between late Eocene and late Oligocene time. PMID:27272610

  7. Low palaeoelevation of the northern Lhasa terrane during late Eocene: Fossil foraminifera and stable isotope evidence from the Gerze Basin

    NASA Astrophysics Data System (ADS)

    Wei, Yi; Zhang, Kexin; Garzione, Carmala N.; Xu, Yadong; Song, Bowen; Ji, Junliang

    2016-06-01

    The Lhasa terrane is a key region for understanding the paleoelevation of the southern Tibetan Plateau after India-Asia collision. The Gerze Basin, located in the northern part of the Lhasa terrane, is a shortening-related basin. We discovered Lagena laevis (Bandy) fossils in upper Eocene strata of the Gerze Basin. This type of foraminifera is associated with lagoon and estuarine environments, indicating that the northern part of the Lhasa terrane was near sea level during the late Eocene. We speculate that these foraminifera were transported inland by storm surges to low elevation freshwater lakes during times of marine transgressions. This inference is consistent with the relatively positive δ18O values in carbonate from the same deposits that indicate low palaeoelevations close to sea level. Considering the palaeoelevation results from the nearby Oligocene basins at a similar latitude and the volcanic history of the Lhasa terrane, we infer that large-magnitude surface uplift of the northern Lhasa terrane occurred between late Eocene and late Oligocene time.

  8. Evidence From Detrital Zircon U-Pb Analysis for Suturing of Pre-Mississippian Terranes in Arctic Alaska

    NASA Astrophysics Data System (ADS)

    Moore, T. E.; Potter, C. J.; O'Sullivan, P. B.; Aleinikoff, J. N.

    2007-12-01

    900 Ma and 1200-1450 Ma that are not seen in the Neroukpuk sample, as well as a cluster of ages at 320-390 Ma. This sample indicates that the Neroukpuk is not the only source of zircons for Mississippian strata in the northeast Brooks Range. The samples from the northeastern Brooks Range contain zircon distributions similar to those reported from autochthonous North American strata in east-central Alaska and are strikingly different from those in western and southern parts of the Arctic Alaska terrane. Peaks at ~1.8 Ga are subdued or missing in the latter samples and 1.5-1.6 Ga grains, a magmatic gap in Laurentia, are present in the Lisburne Peninsula and North Slope samples. In view of these data, field and seismic evidence for significant Devonian deformation in northern Alaska, and likely plate configurations for that time, northern Alaska probably records early to mid-Paleozoic closing of the Iapatus Ocean and collisional suturing of non-Laurentian continental blocks with northwestern North America. This suggests that Devonian deformation in Arctic Alaska represents an important, northern element of the Caledonian deformational system that probably once linked up with Caledonian structures in the Canadian Arctic Islands and adjacent continental margin region.

  9. Extraterrestrial accretion and glacial cycles

    NASA Technical Reports Server (NTRS)

    Muller, R. A.

    1994-01-01

    We propose that the approx. 100-k.y. cycle seen in terrestrial glaciation is due to changes in meteor flux that come from changes in the Earth's orbit. This model can explain a 70-k.y. 'anomalous' period in climate data and the apparent discrepancy between present extraterrestrial fluxes and those in oceanic sediments. It can be tested by measuring Ir densities in sediments and ice during glacials and interglacials.

  10. Further paleomagnetic results for lower Permian basalts of the Baoshan Terrane, southwestern China, and paleogeographic implications

    NASA Astrophysics Data System (ADS)

    Xu, Yingchao; Yang, Zhenyu; Tong, Ya-Bo; Wang, Heng; Gao, Liang; An, Chunzhi

    2015-05-01

    The Baoshan Terrane of southwestern China is considered to have been part of the Cimmerian block during the late Paleozoic; consequently, knowledge of its paleoposition and geological evolution can provide constraints on the Permian breakup of northern East Gondwana. Therefore, we conducted paleomagnetic and rockmagnetic studies on lower Permian basalts from four localities in the Baoshan Terrane. The basalts hold a stable characteristic remanent magnetization (ChRM) at high temperatures (300-680 °C) that is carried by magnetite, maghemite, and hematite with both pseudo-single and multiple domains. To test the reliability of data from these volcanic rocks, we analyzed the geomagnetic secular variation (GSV) and reliability of both the present data and previous paleomagnetic data. The results from 23 sites yield a single reversed polarity directed downwards to the southwest, giving a site-mean direction of Dg/Ig = 156.7°/56.6° (kg = 8.0, α95 = 11.4°) before tilt correction, and Ds/Is = 218.3°/60.1° (ks = 14.1, α95 = 8.4°) after tilt correction. The result passed the fold test, but the GSV was able to be averaged out in only two sections. All available data were examined section-by-section using the angular dispersion (SB) of virtual geomagnetic poles (VGPs) to ensure that the GSV was completely averaged out. Because the dispersion in declinations is likely to have been affectedby subsequent tectonic deformation, the paleosecular variation (PSV) could not be evaluated from all the data amassed from different sections, and the PSV was able to be removed from only four (combined) sections. A small-circle fit of these VGPs gives an averaged paleocolatitude of 51.9° ± 3.7° (N = 31 sites) centered on 24°N, 99°E. The result indicates that the sampled area of the Baoshan Terrane was located at a latitude of 38°S ± 3.7° during the late early Permian. A comparison of this result with early Permian data from Gondwanan blocks suggests that the Baoshan Terrane

  11. Paleomagnetic constraints on the Mesozoic drift of the Lhasa terrane (Tibet) from Gondwana to Eurasia

    NASA Astrophysics Data System (ADS)

    Li, Zhenyu; Lippert, Peter; Ding, Lin; Song, Peiping; Yue, Yahui; van Hinsbergen, Douwe

    2016-04-01

    The Mesozoic plate tectonic history of Gondwana-derived crustal blocks of the Tibetan Plateau is hotly debated, but so far, paleomagnetic constraints quantifying their paleolatitudinal drift history remain sparse. Here, we compile existing data published mainly in Chinese literature and provide a new, high-quality, well-dated paleomagnetic pole from the ˜180 Ma Sangri Group volcanics of the Lhasa terrane. Our Sangri Group pole is calculated from pre-folding characteristic remanent magnetizations carried by thermoremanent magnetizations in low-Ti titanomagnetite and titanohematite in basalts and basaltic andesites that we have dated using zircon U-Pb geochronology. Forty-two lava sites (68%) meet our quality criteria and provide an average direction of D±ΔD = 341.9±3.4° , I±ΔI = -13.3±6.5° , A95 = 3.4, K = 42.9, n=42, corresponding to a paleolatitude of ˜6° S. The A95 value falls within the n-dependent confidence envelope of Deenen et al. (2011) (A95min=2.7; A95max=7.8), indicating that the data scatter can be straightforwardly explained by paleosecular variation of the paleomagnetic field alone. In addition, positive fold tests are consistent with a pre-folding remanence acquisition. Our new pole confirms a trend in existing data of variable quality that suggests the Lhasa terrane rifted from Gondwana in Late Triassic rather than Permian time, as widely perceived. A total northward drift of ˜ 4500 km between ˜220 and ˜130 Ma yields a reasonable average paleolatitudinal plate motion rate of 5 cm/yr. Our results are consistent with both an Indian or an Australian original position of the Lhasa terrane and cannot directly discriminate between these two interpretations. Nonetheless, we show that paleomagnetic data can provide a strong constraint on Mesozoic plate kinematics of the Tethyan realm. Our study also underscores the need for new, high-quality and well-dated paleomagnetic poles from the Paleozoic and Mesozoic of the Tibetan terranes.

  12. Geophysical Investigations of a Proterozoic Carbonatite Terrane, southeast Mojave Desert, California

    NASA Astrophysics Data System (ADS)

    Denton, K. M.; Ponce, D. A.; Miller, D. M.; Peacock, J.; Miller, J. S.

    2015-12-01

    One of the world's largest rare-earth element-rich carbonatite deposits is located in the eastern Mojave Desert at Mountain Pass, California. The eastern Mojave Desert carbonatite terrane consists of a ~1.7 Ga gneiss and schist rocks that are host to a ~1.417 Ga (Premo, 2013) ultrapotassic intrusive suite (shonkinite, syenite, and granite) and a ~1.375 Ga (DeWitt, 1983) carbonatite deposit . Regional geophysical data indicate that this carbonatite terrane occurs within a north-northwest trending ~1-km wide bench in a gravity high and along the eastern edge of a prominent magnetic high in the eastern Clark Mountain Range. To improve our understanding of the geophysical and structural framework of the eastern Mojave carbonatite terrane, we collected over 2,300 gravity stations and over 640 physical rock property samples. Carbonatite rocks typically have distinct gravity, magnetic, and radioactive signatures because they are relatively dense, often contain magnetite, and are commonly enriched in thorium and/or uranium. Contrary to this trend, our results show that the carbonatite deposit is essentially nonmagnetic with an average susceptibility of 0.18 x 10-3 SI (n=31), and the ultrapotassic intrusive suite is very weakly magnetic with an average susceptibility of 2.0 x 10-3 SI (n=36). However, these rocks are found along a steep gradient of a prominent aeromagnetic anomaly. The lack of magnetic signature from the rocks of the eastern Mojave carbonatite terrane suggests alteration of magnetic minerals. This is corroborated by its location within a broader alteration zone and observed magnetic low. If so, such an alteration event occurred after emplacement of the carbonatite deposit, which likely remobilized rare earth elements in the surrounding rocks. Further, an alteration event is consistent with geology, high rare-earth element concentration, and unusual geochemistry of the carbonatite deposit. Temporal constraints (DeWitt, 1987; Premo, 2013) also suggest

  13. Turbidite facies in an ancient subduction complex: Torlesse terrane, New Zealand

    USGS Publications Warehouse

    MacKinnon, T.C.; Howell, D.G.

    1984-01-01

    The Torlesse terrane of New Zealand is an ancient subduction complex consisting of deformed turbidite-facies rocks. These are mainly thick-bedded sandstone (facies B and C) with subordinate mudstone (facies D and E), comparable to inner- and middle-fan deposits of a submarine fan. Strata were deposited in trench-floor and trench-slope settings that received sandy sediment from slope-cutting submarine canyons. The dominance of sandstone suggests that some mudstone may have been selectively subducted. Construction of a detailed sediment dispersal model is not possible because tectonic deformation has largely destroyed original facies relationships and paleocurrent patterns. ?? 1984 Springer-Verlag New York Inc.

  14. The ``Procellarum KREEP terrane'': Implications for mare volcanism and lunar evolution

    NASA Astrophysics Data System (ADS)

    Wieczorek, Mark Allen

    Geophysical, remote sensing, and sample data all suggest that the Procellarum and Imbrium region of the Moon (here named the ``Procellarum KREEP Terrane'') is a unique geochemical province. Evidence in support of this hypothesis include the following observations: (1)Gamma-ray data obtained from orbiting spacecraft show that this region of the Moon is highly enriched in incompatible and heat producing elements (i.e., ``KREEP''). (2)Geochemical studies of Imbrium's ejecta suggest that a large portion of the lunar crust in this locale is composed of a material similar in composition to Apollo 15 KREEP basalt. And (3)geophysically derived crustal thickness maps show that only impact basins that formed within this region have been modified, most likely by viscous relaxation and/or voluminous KREEP basalt volcanism. KREEP basalt has about 300 times more uranium and thorium than ordinary chondrites so this infers that a large portion of Moon's heat-producing elements are located within this single crustal province. The spatial distribution of mare volcanism closely parallels the confines of the Procellarum KREEP terrane. We model the Moon's thermal evolution using a simple thermal conduction model and show that partial melting of the mantle beneath the Procellarum KREEP terrane is a likely outcome due to the high abundance of heat producing elements that are found there. Specifically, by placing a 10-km KREEP basalt layer at the base of the crust in this terrane, our model predicts that mare volcanism should span most of the Moon's history, and that the depth of melting in the mantle should increase with time to a maximum depth of about 600 km. We suggest that the 500-km seismic discontinuity that is observed in the Apollo seismic data may represent this maximum depth of melting. Furthermore, the KREEP-basalt layer in our model is found to remain partially molten for a few billion years. Thus, when the Imbrium basin formed this impact most likely excavated into a

  15. Triassic High-P Metamorphism of the central Qiangtang terrane, Tibet; constraints using mineral equilibria modelling and 40Ar/39Ar geochronology

    NASA Astrophysics Data System (ADS)

    Rajkumar, A.; Hui, L.; Clarke, G. L.; Aitchison, J. C.; Forster, M. A.

    2014-12-01

    The SE-trending Qiangtang metamorphic belt (QMB) stretches more than 500 km through the Qiangtang terrane in central Tibet and comprises tectonically disrupted blueschist and eclogite in lower-grade garnet-phengite-bearing schist and quartzite. These rocks record the closure of a paleo-Tethyan Triassic ocean that formerly separated Cathaysian and Gondwana components of Asia, now forming the northern and southern Qiangtang blocks. Eclogite is extensively recrystallized to high-P amphibolite and greenschist facies assemblages, formed during water ingression that accompanied terrane uplift. P-T pseudosections constructed in Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O (NCKMASHTO) in the context of petrography and mineral chemistry provides the ability to recover a dynamic PT history for the eclogite facies assemblages. Prograde (S1) assemblages for the Gemu Co eclogite are predicted to have formed at P≈21.5 kbars and T≈505°C and involved garnet, glaucophane, omphacite, rutile, lawsonite and chlorite, based on garnet composition and inferred pseudomorphs after lawsonite. Peak (S2) assemblages of garnet, barroisite, omphacite, rutile, epidote and quartz reflect P≈15 kbars and T≈570°C. Based on textural relations, post-peak stages can be divided into epidote-amphibolite and greenschist facies. The geothermal gradient for the prograde S1 assemblage and the peak S2 assemblage is 7.1 and 11.5°C/km respectively.40Ar/39Ar geochronology of phengitic mica using step heating in recrystallized eclogite components and surrounding garnet-mica schist components both yield maximum ages ranging 230-220 Ma. The congruency in ages of the deeply subducted high-pressure eclogites to the surrounding garnet phengite schists indicate they were the most probable source of fluids to extensively recrystallize most of the high-pressure eclogite components in the high-pressure belt. The P-T history of the high-P rocks of the QMB records the deep subduction of paleo-Tethyan oceanic crust to

  16. Accretion Timescales from Kepler AGN

    NASA Astrophysics Data System (ADS)

    Kasliwal, Vishal P.; Vogeley, Michael S.; Richards, Gordon T.

    2015-01-01

    We constrain AGN accretion disk variability mechanisms using the optical light curves of AGN observed by Kepler. AGN optical fluxes are known to exhibit stochastic variations on timescales of hours, days, months and years. The excellent sampling properties of the original Kepler mission - high S/N ratio (105), short sampling interval (30 minutes), and long sampling duration (~ 3.5 years) - allow for a detailed examination of the differences between the variability processes present in various sub-types of AGN such as Type I and II Seyferts, QSOs, and Blazars. We model the flux data using the Auto-Regressive Moving Average (ARMA) representation from the field of time series analysis. We use the Kalman filter to determine optimal mode parameters and use the Akaike Information Criteria (AIC) to select the optimal model. We find that optical light curves from Kepler AGN cannot be fit by low order statistical models such as the popular AR(1) process or damped random walk. Kepler light curves exhibit complicated power spectra and are better modeled by higher order ARMA processes. We find that Kepler AGN typically exhibit power spectra that change from a bending power law (PSD ~ 1/fa) to a flat power spectrum on timescales in the range of ~ 5 - 100 days consistent with the orbital and thermal timescales of a typical 107 solar mass black hole.

  17. Accretion of Ghost Condensate by Black Holes

    SciTech Connect

    Frolov, A

    2004-06-02

    The intent of this letter is to point out that the accretion of a ghost condensate by black holes could be extremely efficient. We analyze steady-state spherically symmetric flows of the ghost fluid in the gravitational field of a Schwarzschild black hole and calculate the accretion rate. Unlike minimally coupled scalar field or quintessence, the accretion rate is set not by the cosmological energy density of the field, but by the energy scale of the ghost condensate theory. If hydrodynamical flow is established, it could be as high as tenth of a solar mass per second for 10MeV-scale ghost condensate accreting onto a stellar-sized black hole, which puts serious constraints on the parameters of the ghost condensate model.

  18. Heat transfer on accreting ice surfaces

    NASA Technical Reports Server (NTRS)

    Yamaguchi, Keiko; Hansman, R. John, Jr.

    1993-01-01

    Based on previous observations of glaze ice accretion on aircraft surfaces, a multizone model with distinct zones of different surface roughness is demonstrated. The use of surface roughness in the LEWICE ice accretion prediction code is examined. It was found that roughness is used in two ways: (1) to determine the laminar to turbulent boundary-layer transition location; and (2) to calculate the convective turbulent heat-transfer coefficient. A two-zone version of the multizone model is implemented in the LEWICE code, and compared with experimental convective heat-transfer coefficient and ice accretion results. The analysis of the boundary-layer transition, surface roughness, and viscous flowfield effects significantly increased the accuracy in predicting heat-transfer coefficients. The multizone model was found to significantly improve the ice accretion prediction for the cases compared.

  19. Gravitomagnetic acceleration from black hole accretion disks

    NASA Astrophysics Data System (ADS)

    Poirier, J.; Mathews, G. J.

    2016-05-01

    We demonstrate how the motion of the neutral masses in an accretion disk orbiting a black hole creates a general-relativistic magnetic-like (gravitomagnetic) field that vertically accelerates neutral particles near an accretion disk upward and then inward toward the axis of the accretion disk. Even though this gravitomagnetic field is not the only mechanism contributing to the production of jets, it presents a novel means to identify one general relativistic effect from a much more complicated problem. In addition, as the accelerated material above or below the accretion disk nears the axis with a nearly vertical direction, a frame-dragging effect twists the trajectories around the axis thus contributing to the collimation of the jet.

  20. Planetary science: Iron fog of accretion

    DOE PAGESBeta

    Anderson, William W.

    2015-03-02

    Here, pinpointing when Earth's core formed depends on the extent of metal–silicate equilibration in the mantle. Vaporization and recondensation of impacting planetesimal cores during accretion may reconcile disparate lines of evidence.

  1. Accretion, winds and outflows in young stars

    NASA Astrophysics Data System (ADS)

    Günther, H. M.

    2013-02-01

    Young stars and planetary systems form in molecular clouds. After the initial radial infall an accretion disk develops. For classical T Tauri stars (CTTS, F-K type precursors) the accretion disk does not reach down to the central star, but it is truncated near the co-rotation radius by the stellar magnetic field. The inner edge of the disk is ionized by the stellar radiation, so that the accretion stream is funneled along the magnetic field lines. On the stellar surface an accretion shock develops, which is observed over a wide wavelength range as X-ray emission, UV excess, optical veiling and optical and IR emission lines. Some of the accretion tracers, e.g. Hα, can be calibrated to measure the accretion rate. This accretion process is variable on time scales of hours to years due to changing accretion rates, stellar rotation and reconfiguration of the magnetic field. Furthermore, many (if not all) accreting systems also drive strong outflows which are ultimately powered by accretion. However, the exact driving mechanism is still unclear. Several components could contribute to the outflows: slow, wide-angle disk winds, X-winds launched close to the inner disk rim, and thermally driven stellar winds. In any case, the outflows contain material of very different temperatures and speeds. The disk wind is cool and can have a molecular component with just a few tens of km s-1, while the central component of the outflow can reach a few 100 km s-1. In some cases the inner part of the outflow is collimated to a small-angle jet. These jets have an onion-like structure, where the inner components are consecutively hotter and faster. The jets can contain working surfaces, which show up as Herbig-Haro knots. Accretion and outflows in the CTTS phase do not only determine stellar parameters like the rotation rate on the main-sequence, they also can have a profound impact on the environment of young stars. This review concentrates on CTTS in near-by star forming regions where

  2. Thermonuclear flashes on accreting neutron stars

    NASA Technical Reports Server (NTRS)

    Joss, P. C.

    1979-01-01

    Observations of X-ray bursts from binary pulsars and globular clusters are reviewed. The previously proposed hypothesis is considered that such X-ray bursts result from thermonuclear flashes on accreting neutron stars. A general scenario for this mechanism is outlined, and numerical computations of the evolution of the surface layers of an accreting neutron star are discussed. The relation of these calculations to X-ray bursts and other phenomena is examined. Possible improvements in the numerical calculations are suggested.

  3. Asymmetric Accretion Flows within a Common Envelope

    NASA Astrophysics Data System (ADS)

    MacLeod, Morgan; Ramirez-Ruiz, Enrico

    2015-04-01

    This paper examines flows in the immediate vicinity of stars and compact objects dynamically inspiralling within a common envelope (CE). Flow in the vicinity of the embedded object is gravitationally focused, leading to drag and potentially to gas accretion. This process has been studied numerically and analytically in the context of Hoyle-Lyttleton accretion (HLA). Yet, within a CE, accretion structures may span a large fraction of the envelope radius, and in so doing sweep across a substantial radial gradient of density. We quantify these gradients using detailed stellar evolution models for a range of CE encounters. We provide estimates of typical scales in CE encounters that involve main sequence stars, white dwarfs, neutron stars, and black holes with giant-branch companions of a wide range of masses. We apply these typical scales to hydrodynamic simulations of three-dimensional HLA with an upstream density gradient. This density gradient breaks the symmetry that defines HLA flow, and imposes an angular momentum barrier to accretion. Material that is focused into the vicinity of the embedded object thus may not be able to accrete. As a result, accretion rates drop dramatically, by one to two orders of magnitude, while drag rates are only mildly affected. We provide fitting formulae to the numerically derived rates of drag and accretion as a function of the density gradient. The reduced ratio of accretion to drag suggests that objects that can efficiently gain mass during CE evolution, such as black holes and neutron stars, may grow less than implied by the HLA formalism.

  4. Disk accretion by magnetic neutron stars

    NASA Technical Reports Server (NTRS)

    Ghosh, P.; Lamb, F. K.

    1978-01-01

    A model for disk accretion by a rotating magnetic neutron star is proposed which includes a detailed description of matter flow in the transition region between the disk and the magnetosphere. It is shown that the disk plasma cannot be completely screened from the stellar magnetic field and that the resulting magnetic coupling between the star and the disk exerts a significant torque on the star. On the assumption that the distortion of the residual stellar field lines threading the disk is limited by reconnection, the total accretion torque on the star is calculated. The calculated torque gives period changes in agreement with those observed in the pulsating X-ray sources and provides a natural explanation of why a fast rotator like Her X-1 has a spin-up rate much below the conventional estimate for slow rotators. It is shown that for such fast rotators, fluctuations in the mass-accretion rate can produce fluctuations in the accretion torque about 100 times larger. For sufficiently fast rotators or, equivalently, for sufficiently low accretion rates, the star experiences a braking torque even while accretion continues and without any mass ejection from its vicinity.

  5. Bondi accretion in early-type galaxies

    NASA Astrophysics Data System (ADS)

    Korol, Valeriya; Ciotti, Luca; Pellegrini, Silvia

    2016-05-01

    Accretion on to central massive black holes in galaxies is often modelled with the Bondi solution. In this paper we study a generalization of the classical Bondi accretion theory, considering the additional effects of the gravitational potential of the host galaxy, and of electron scattering in the optically thin limit. We provide a general analysis of the bias in the estimates of the Bondi radius and mass accretion rate, when adopting as fiducial values for the density and temperature at infinity the values of these quantities measured at finite distance from the central black hole. We also give general formulae to compute the correction terms of the critical accretion parameter in relevant asymptotic regimes. A full analytical discussion is presented in the case of an Hernquist galaxy, when the problem reduces to the discussion of a cubic equation, therefore allowing for more than one critical point in the accretion structure. The results are useful for observational works (especially in the case of systems with a low Eddington ratio), as well as for numerical simulations, where accretion rates are usually defined in terms of the gas properties near the black hole.

  6. Bondi accretion in early-type galaxies

    NASA Astrophysics Data System (ADS)

    Korol, Valeriya; Ciotti, Luca; Pellegrini, Silvia

    2016-08-01

    Accretion on to central massive black holes in galaxies is often modelled with the Bondi solution. In this paper, we study a generalization of the classical Bondi accretion theory, considering the additional effects of the gravitational potential of the host galaxy, and of electron scattering in the optically thin limit. We provide a general analysis of the bias in the estimates of the Bondi radius and mass accretion rate, when adopting as fiducial values for the density and temperature at infinity the values of these quantities measured at finite distance from the central black hole. We also give general formulae to compute the correction terms of the critical accretion parameter in relevant asymptotic regimes. A full analytical discussion is presented in the case of a Hernquist galaxy, when the problem reduces to the discussion of a cubic equation, therefore, allowing for more than one critical point in the accretion structure. The results are useful for observational works (especially in the case of systems with a low Eddington ratio), as well as for numerical simulations, where accretion rates are usually defined in terms of the gas properties near the black hole.

  7. Heating and Cooling in Accreting Neutron Stars

    NASA Astrophysics Data System (ADS)

    Cumming, Andrew

    2015-10-01

    Neutron stars in low mass X-ray binaries accrete enough mass over their lifetimes to replace their entire crust. The accreted matter undergoes a series of nuclear reactions in the crust as it is compressed by continued accretion to higher density. These reactions, which include electron captures, neutron emissions, and pycnonuclear reactions, heat the crust and core of the neutron star. In this talk I will discuss what we can learn from observations of transiently accreting neutron stars in quiescence, when accretion has turned off and we can see emission from the neutron star directly. The quiescent luminosity of these neutron stars constrains the neutrino emissivity in the neutron star core. In systems with long accretion outbursts, observations of thermal relaxation of the crust in quiescence enable, for the first time, constraints on the thermal conductivity and heat capacity of the crust. In this way, low mass X-ray binary neutron stars offer a remarkable chance to constrain the properties of dense neutron-rich matter, such as neutron superfluidity and pasta phases in the inner crust of neutron stars.

  8. Oceanic Observations

    NASA Technical Reports Server (NTRS)

    Busalacchi, Antonio J.

    1997-01-01

    For many years, merchant ships and the naval fleets of various countries have been the major source of data over and in the open ocean. Oceanographic research experiments and process studies in the field have also contributed to the climatological data bases for the global ocean, but, for the most part, these have been limited in duration and extent. However, over the last 10 years under the auspices of the World Climate Research Program and the International Geosphere Biosphere Program the role of the oceans in global and climate change has taken on increased significance. This has created a need for a considerably improved understanding of the seasonal, interannual, decadal and longer time-scale variability of the physical and biogeochemical attributes of the global ocean. As a result, over the past 10 years several major international field programs have been implemented and have had a tremendous impact on the number of in situ observations obtained for the global ocean. The Tropical Ocean Global Atmosphere (TOGA) program, the World Ocean Circulation Experiment (WOCE), and the Joint Global Ocean Flux Study (JGOFS) were designed with observational, modelling, and process study components aimed at analyzing different aspects of the ocean's role in the coupled climate system. In parallel with the field programs, continuous space-based observations of sea surface temperature, sea surface topography, and sea surface winds spanning nearly a decade or longer have become a reality. During this same time period, numerical ocean models and computational power have advanced to the point where the oceanographic observations, both in situ and remotely sensed, can be assimilated into numerical ocean models in order to provide a four-dimensional (x-y-z-t) depiction of the evolving state of the global ocean.

  9. Accretion of the Archean Slave Province

    NASA Technical Reports Server (NTRS)

    Kusky, Timothy

    1988-01-01

    Detailed field studies of selected areas in the greenstone belts of the Slave Province of Canada were presented. This area was long cited as a type area by supporters of the (now generally abandoned) rift model of greenstone belts. It was shown that a plate tectonic interpretation accounted more successfully for the regional geology and identified four terranes that had experienced complex divergent and convergent histories between 2.7 and 3.4 Ga. A dismembered ophiolite was identified and a late episode of widespread granitic intrusion was recognized.

  10. Pine Mountain terrane, a complex window in the Georgia and Alabama Piedmont: evidence from the eastern termination

    SciTech Connect

    Hooper, R.J.; Hatcher, R.D. Jr.

    1988-04-01

    The Pine Mountain terrane is exposed in a complex window within the Piedmont of Georgia and Alabama. The eastern end of the terrane is framed by three ductile faults of demonstrably different ages. The polydeformed pre-thermal peak Box Ankle fault is truncated to the south by the younger pre-thermal peak Goat Rock fault, and to the north by the even younger post-thermal peak Towaliga fault. The three faults framing the eastern termination of the window are clearly neither (1) part of the same detachment nor (2) part of the Appalachian detachment.

  11. New late Precambrian-Cambrian U-PB zircon ages for zoned intrusives in the western Carolina terrane, Spartanburg and Union Counties, South Carolina

    SciTech Connect

    Dennis, A.J. . Physical Sciences); Wright, J.E. . Geology and Geophysics)

    1993-03-01

    The geology of the western Carolina terrane comprises zoned mafic-ultramafic intrusive complexes intruding a volcanic pile of basalts and basaltic andesites; this package is interpreted to represent an episode of intra-arc rifting prior to regional metamorphism and foliation formation. New U-Pb zircon ages from the Mean Crossroads complex in northwestern South Carolina along the central Piedmont suture confirm relative ages obtained by detailed mapping. Two foliated meta-diorites yield U-Pb dates of 580 Ma, interpreted to be crystallization ages. A foliated meta-quartz diorite yields a U-Pb date of 535 Ma interpreted to be a crystallization age. These ages are broadly contemporary with those inferred by other workers for the Battleground Formation in the type locality of the Kings Mountain belt. An undeformed, unmetamorphosed diorite intruding these metamorphosed zoned complex intrusives also yields an age of 535 Ma. Hence the authors believe that intra-arc rifting and regional metamorphism both occurred c. 535 Ma. While petrographic and Ar-Ar studies support subsequent regional metamorphic overprint(s), or at least static recrystallization and/or uplift through hornblende-biotite-muscovite blocking temperatures for Ar in mid- to late-Paleozoic time, the 535 Ma, undeformed, unmetamorphosed intrusive suggest late Precambrian regional metamorphism and deformation was the event responsible for regional greenschist-lower amphibolite facies metamorphism and foliation formation in this area of the Piedmont. This seems to contradict correlations with middle Ordovician fabric elements in the eastern Piedmont as well as the idea that this metamorphism and fabric development are related to presumed early Paleozoic accretion of the Carolina arc to Laurentia. They have also dated a foliated megacrystic granite that cuts the central Piedmont suture (325 Ma, U-Pb zircon), and the Bald Rock granite (326 Ma, U-Pb zircon).

  12. Paleomagnetic study of Jurassic and Cretaceous rocks from the Mixteca terrane (Mexico)

    NASA Astrophysics Data System (ADS)

    Böhnel, Harald

    1999-11-01

    Three sites from Cretaceous limestone and Jurassic sandstone in northern Oaxaca, Mexico, were studied paleomagnetically. Thermal demagnetization isolated site-mean remanence directions which differ significantly from the recent geomagnetic field. The paleopole for the Albian-Cenomanian Morelos formation is indistinguishable from the corresponding reference pole for stable North America, indicating tectonic stability of the Mixteca terrane since the Cretaceous. Rock magnetic properties and a positive reversal test for the Bajocian Tecomazuchil sandstone suggest that the remanence could be of primary origin, although no fold test could be applied. The Tecomazuchil paleopole is rotated 10°±5° clockwise and displaced 24°±5° towards the study area, with respect to the reference pole for stable North America. Similar values were found for the Toarcien-Aalenian Rosario Formation, with 35°±6° clockwise rotation and 33°±6° latitudinal translation. These data support a post-Bajocian southward translation of the Mixteca terrane by around 25°, which was completed in mid-Cretaceous time.

  13. Comparative analysis of core drilling and rotary drilling in volcanic terrane

    SciTech Connect

    Flynn, T.; Trexler, D.T.; Wallace, R.H. Jr.

    1987-04-01

    Initially, the goal of this report is to compare and contrast penetration rates of rotary-mud drilling and core drilling in young volcanic terranes. It is widely recognized that areas containing an abundance of recent volcanic rocks are excellent targets for geothermal resources. Exploration programs depend heavily upon reliable subsurface information, because surface geophysical methods may be ineffective, inconclusive, or both. Past exploration drilling programs have mainly relied upon rotary-mud rigs for virtually all drilling activity. Core-drilling became popular several years ago, because it could deal effectively with two major problems encountered in young volcanic terranes: very hard, abrasive rock and extreme difficulty in controlling loss of circulation. In addition to overcoming these difficulties, core-drilling produced subsurface samples (core) that defined lithostratigraphy, structure and fractures far better than drill-chips. It seemed that the only negative aspect of core drilling was cost. The cost-per-foot may be two to three times higher than an ''initial quote'' for rotary drilling. In addition, penetration rates for comparable rock-types are often much lower for coring operations. This report also seeks to identify the extent of wireline core drilling (core-drilling using wireline retrieval) as a geothermal exploration tool. 25 refs., 21 figs., 13 tabs.

  14. Chain Lakes massif, west central Maine: northern Appalachian basement or suspect terrane

    SciTech Connect

    Cheatham, M.M.; Olszewski, W.J. Jr.; Gaudette, H.E.

    1985-01-01

    The Chain Lakes massif of west-central Main is a 3 km thick sequence of diamictite and aquagene metavolcanics and metasediments, which contrasts strikingly with its surrounding Paleozoic rocks in lithology, structural style and metamorphic grade. The rocks of the massif are characterized by mineral assemblages developed during two separate metamorphic events. The first, of second sillimanite grade, is reflected by qtz-oligoclase-Kspar-sillimanite-biotite and muscovite. The second metamorphism is a retrograde event of greenschist facies, and chlorite grade. Isotopic Rb-Sr and Sm-Nd whole rock, and Rb-Sr mineral analyses of samples of the diamictite members, now gneiss and granofels, indicate that the first prograde metamorphism occurred at 770 Ma. with the retrograde event at approximately 405 Ma. Due to the restricted range of /sup 147/Sm//sup 144/Nd, no Sm-Nd isochron age could be determined. However, model ages for both Sr and Nd are approximately 1500 Ma for derivation of the Chain Lakes protolith material from depleted mantle. Lithology, bounding formations, complexes and plutons, and the isotopic data support previous contentions that the Chain Lakes massif is a suspect terrane. However, similarities with Proterozoic rocks along the Eastern Margin, as well as recent suggestions of similar rocks underlying the Kearsarge-Central Main synclinorium may suggest the possible widespread occurrence of dismembered masses of a perhaps once coherent, Precambrian terrane underlying the Northern Appalachians.

  15. Petrology and geochemistry of the high-pressure Nilgiri Granulite Terrane, Southern India

    NASA Technical Reports Server (NTRS)

    Srikantappa, C.; Ashamanjari, K. G.; Raith, M.

    1988-01-01

    The Nilgiri granulite terrane in Southern India is predominantly composed of late Archaean medium- to coarse-grained enderbitic to charnockitic rocks. The dominant regional foliation strikes N60 to 70E with generally steep dips. Tight minor isoclinal folds have been observed in places. Granoblastic polygonal micro-structures are common and indicate thorough post-kinematic textural and chemical equilibration at conditions of the granulite facies (2.5 Ga ago). Late compressional deformation in connection with the formation of the Moyar and Bhavani shear zones to the north and south of the Nilgiri block, resulted in wide-spread development of weakly to strongly strained fabrics and was accompanied by minor rehydration. Enderbites and charnockites range from tonalitic to granodioritic in composition. A magmatogenic origin of the protoliths is inferred from their chemical characteristics which resemble those of the andesitic to dacitic members of Cordillera-type calc-alkaline igneous suites. A significant lithological feature of the Nilgiri granulite terrane are numerous extended bodies, lenses and pods of gabbroic and pyroxenitic rocks which are aligned conformable to the foliation of the enderbite-charnockite complex and which have also been deformed and metamorphosed at granulite facies conditions.

  16. Paleomagnetism of early Tertiary Alaska Peninsula rocks and implications for docking of peninsular terrane

    SciTech Connect

    Whitney, J.W.; Levinson, R.A.; Van Alstine, D.R.

    1985-04-01

    In order to refine the tectonic history of the peninsular terrane, Alaska, 22 sites (averaging 10 samples/site) in Paleogene Tertiary volcanic and sedimentary formations were sampled in the vicinity of Chignik, on the Pacific side of the Alaska Peninsula. Ten of the sites were drilled in the early Oligocene Meshik volcanics, ranging from andesite to basalt, and the other twelve sites were drilled in the late Eocene Tolstoi Formation sediments. Nine of the volcanic sites yielded stable R and/or N characteristic magnetization. Virtually no fine-grained, interbedded sediments occur with the Meshik volcanics at the sample sites, thus making reliable paleohorizontal determinations difficult. Although flow attitudes were tentatively used, it became rapidly apparent that problems of initial dip were insurmountable. As a result, all volcanic sites were considered unreliable for determining a meaningful paleomagnetic inclination. Upon thermal demagnetization, five of the sedimentary sites were judged stable. The mode of the paleomagnetic direction was calculated, D/I = 349.8/75.3(..beta..95 = 8.5), indicating no significant rotation. Uncertainties in structural corrections, however, may render only the inclination meaningful, which, from McFadden statistics yields, I = 75.9, ..cap alpha..95 = 7.9, corresponding to a paleolatitude of 63.3/sup 0/. This paleolatitude agrees with the expected value for the North American craton at 40 m.y. B.P., implying that the peninsular terrane had docked at at least that time.

  17. Chemical and dynamical perspectives on accretion and differentiation

    NASA Astrophysics Data System (ADS)

    Nimmo, F.

    2012-12-01

    The initial thermal and chemical state of a planet is largely determined by how it accreted. Although large bodies rapidly lose their memory of those initial conditions, smaller bodies do not: the Martian mantle has different isotopic reservoirs that were established early in its history and not subsequently homogenized [1], while the Martian dynamo may have been driven by an initially superheated core [2]. Accretion is also inefficient; impacts can modify planetary bulk compositions in subtle [3] or dramatic [4] ways. There are two main pathways for melting and differentiation of silicate bodies. Small rapidly-accreted bodies melt from the inside out due to 26Al decay, potentially leaving an unmelted carapace [5]. Large bodies melt due to release of gravitational energy via giant impacts. Both situations likely result in magma oceans, which may crystallize to yield unstable density structures [6]. The lifetime of magma oceans is highly uncertain and depends on whether a flotation crust develops, and whether a thick primordial atmosphere is present [7]. The Hf-W [8] and Pd-Ag [9] isotopic systems provide constraints on the timing and style of core formation. For instance, the rapid growth of planets in the ``Grand Tack'' model [10] may not be consistent with these constraints. The key uncertainty is the extent to which impactor cores equilibrate with the surrounding mantle during impacts. For example, the inferred rapid accretion of Mars [11] depends on an assumption of perfect re-equilibration. The physics of re-equilibration is imperfectly understood [12], and hard to model numerically [13]; laboratory experiments may provide a better approach [14]. Dynamical models suggest that the Earth's feeding zone moved outwards with time [15]. Isotopic [9] and element partitioning [16] models are consistent with this picture, suggesting that accreted material changed from volatile-poor and reduced to volatile-rich and oxidized as time progressed. [1] Halliday et al., SSR

  18. Lithospheric structure of southern Indian shield and adjoining oceans: integrated modelling of topography, gravity, geoid and heat flow data

    NASA Astrophysics Data System (ADS)

    Kumar, Niraj; Zeyen, H.; Singh, A. P.; Singh, B.

    2013-07-01

    For the present 2-D lithospheric density modelling, we selected three geotransects of more than 1000 km in length each crossing the southern Indian shield, south of 16°N, in N-S and E-W directions. The model is based on the assumption of local isostatic equilibrium and is constrained by the topography, gravity and geoid anomalies, by geothermal data, and where available by seismic data. Our integrated modelling approach reveals a crustal configuration with the Moho depth varying from ˜40 km beneath the Dharwar Craton, and ˜39 km beneath the Southern Granulite Terrane to about 15-20 km beneath the adjoining oceans. The lithospheric thickness varies significantly along the three profiles from ˜70-100 km under the adjoining oceans to ˜130-135 km under the southern block of Southern Granulite Terrane including Sri Lanka and increasing gradually to ˜165-180 km beneath the northern block of Southern Granulite Terrane and the Dharwar Craton. This step-like lithosphere-asthenosphere boundary (LAB) structure indicates a normal lithospheric thickness beneath the adjoining oceans, the northern block of Southern Granulite Terrane and the Dharwar Craton. The thin lithosphere below the southern block of Southern Granulite Terrane including Sri Lanka is, however, atypical considering its age. Our results suggest that the southern Indian shield as a whole cannot be supported isostatically only by thickened crust; a thin and hot lithosphere beneath the southern block of Southern Granulite Terrane including Sri Lanka is required to explain the high topography, gravity, geoid and crustal temperatures. The widespread thermal perturbation during Pan-African (550 Ma) metamorphism and the breakup of Gondwana during late Cretaceous are proposed as twin cause mechanism for the stretching and/or convective removal of the lower part of lithospheric mantle and its replacement by hotter and lighter asthenosphere in the southern block of Southern Granulite Terrane including Sri Lanka

  19. Supernova Light Curves Powered by Fallback Accretion

    NASA Astrophysics Data System (ADS)

    Dexter, Jason; Kasen, Daniel

    2013-07-01

    Some fraction of the material ejected in a core collapse supernova explosion may remain bound to the compact remnant, and eventually turn around and fall back. We show that the late time (gsimdays) power potentially associated with the accretion of this "fallback" material could significantly affect the optical light curve, in some cases producing super-luminous or otherwise peculiar supernovae. We use spherically symmetric hydrodynamical models to estimate the accretion rate at late times for a range of progenitor masses and radii and explosion energies. The accretion rate onto the proto-neutron star or black hole decreases as \\dot{M} \\propto t^{-5/3} at late times, but its normalization can be significantly enhanced at low explosion energies, in very massive stars, or if a strong reverse shock wave forms at the helium/hydrogen interface in the progenitor. If the resulting super-Eddington accretion drives an outflow which thermalizes in the outgoing ejecta, the supernova debris will be re-energized at a time when photons can diffuse out efficiently. The resulting light curves are different and more diverse than previous fallback supernova models which ignored the input of accretion power and produced short-lived, dim transients. The possible outcomes when fallback accretion power is significant include super-luminous (gsim 1044 erg s-1) Type II events of both short and long durations, as well as luminous Type I events from compact stars that may have experienced significant mass loss. Accretion power may unbind the remaining infalling material, causing a sudden decrease in the brightness of some long duration Type II events. This scenario may be relevant for explaining some of the recently discovered classes of peculiar and rare supernovae.

  20. SUPERNOVA LIGHT CURVES POWERED BY FALLBACK ACCRETION

    SciTech Connect

    Dexter, Jason; Kasen, Daniel

    2013-07-20

    Some fraction of the material ejected in a core collapse supernova explosion may remain bound to the compact remnant, and eventually turn around and fall back. We show that the late time ({approx}>days) power potentially associated with the accretion of this 'fallback' material could significantly affect the optical light curve, in some cases producing super-luminous or otherwise peculiar supernovae. We use spherically symmetric hydrodynamical models to estimate the accretion rate at late times for a range of progenitor masses and radii and explosion energies. The accretion rate onto the proto-neutron star or black hole decreases as M-dot {proportional_to}t{sup -5/3} at late times, but its normalization can be significantly enhanced at low explosion energies, in very massive stars, or if a strong reverse shock wave forms at the helium/hydrogen interface in the progenitor. If the resulting super-Eddington accretion drives an outflow which thermalizes in the outgoing ejecta, the supernova debris will be re-energized at a time when photons can diffuse out efficiently. The resulting light curves are different and more diverse than previous fallback supernova models which ignored the input of accretion power and produced short-lived, dim transients. The possible outcomes when fallback accretion power is significant include super-luminous ({approx}> 10{sup 44} erg s{sup -1}) Type II events of both short and long durations, as well as luminous Type I events from compact stars that may have experienced significant mass loss. Accretion power may unbind the remaining infalling material, causing a sudden decrease in the brightness of some long duration Type II events. This scenario may be relevant for explaining some of the recently discovered classes of peculiar and rare supernovae.

  1. Structural geology and kinematics associated with the collision of the Wrangellia composite terrane and North America, south-central Alaska

    NASA Astrophysics Data System (ADS)

    Bier, Sara Elizabeth

    The collision of the Insular superterrane, and thus, the Wrangellia composite terrane (WCT), with the Mesozoic margin of North America is one of the most important, yet enigmatic events in the tectonic history of North American cordillera. The location and therefore the nature of the collision of the Insular superterrane with North America remains controversial. In southern Alaska, the suture zone between the WCT and North America consists of the Kahiltna assemblage, Jurassic-Cretaceous submarine fan deposits. Structural investigation of the Kahiltna assemblage provides additional data on the kinematics of the collision and suggests an oblique collision with a significant component of right-lateral shearing. The first study of the dissertation presents the results across a transect at the northern end of Broad Pass where the depositional and deformational history of three tectonostratigraphic units enables determination of the tectonic evolution of the suture zone. The Reindeer Hills exposes melange units that include oceanic lithologies and represent a remnant of an accretionary complex that formed during subduction prior to the collision of the WCT. Structures within the Kahiltna assemblage in the Talkeetna Mountains indicate oblique northwest-directed thrusting and right-lateral shear during the collision of the WCT. The Jack River conglomerate, a fluvial clast-supported conglomerate unconformably overlies the Reindeers Hills melange and represents uplift, erosion, and deposition late in the collision. The second study is on the other side of Broad Pass, in the southern Alaska Range, and consists of a composite transect across the Peters and Dutch Hills and Chelatna Lake. Horizontal stretching lineations and steeply-dipping foliation indicate that deformation occurred during transpression as a result of an oblique collision. Strain analysis of pressure shadows indicate a counterclockwise rotation of the extension direction and thus, right-lateral shearing during

  2. Accreting neutron stars by QFT

    NASA Astrophysics Data System (ADS)

    Chen, Shao-Guang

    layer with thickness of 1 km then q = 1 (N1S1), the gravity from N1S1 inside and exterior will be completely shielded. Because of net nuν _{0} flux is the medium to produce and transmit gravity, q obstructed by the shielding layer lie on the density of layer matter and the section of single nucleon to electronic neutrino obtained by nuclear physics experiments is about 1.1*10 ({-) 43} cm (2) . The mass inside N1S1 for exterior has not gravity interaction, it equivalent to has not inertia as the mass vanish. The neutron star is as a empty shell thereby may rapidly rotating and has not upper limit of mass and radii by the gravity accretion of N1S1, which will influence the mechanisms of pulsars, quasars and X-rays generated. At N1S1 interior the mass for exterior has not gravity which is just we searching dark matter. The mass each part will each other shielding and gravity decrease to less than the pressure of the degenerate neutron gas. The neutron star cannot collapse into a singular point with infinite density, i.e., the black hole with infinite gravity cannot be formed or the neutron star is jest the black hole in observational meaning. By the gravity accrete of N1S1 the neutron star may enlarge its shell radii but thickness keep. Only a shell gravity may be not less than any a observed value which to be deemed as black hole. The neutron star has powerful gravity certainly accompany with great surface negative charge and it may rapidly to rotate, so that there is a powerful magnetic field surround it. The accreting neutron star is as a slowly expand empty shell with fixed thickness of 1 km, its spin period depend on its radii or total accretion mass.

  3. Disk Accretion and the Stellar Birthline

    NASA Astrophysics Data System (ADS)

    Hartmann, Lee; Cassen, Patrick; Kenyon, Scott J.

    1997-02-01

    We present a simplified analysis of some effects of disk accretion on the early evolution of fully convective, low-mass pre-main-sequence stars. Our analysis builds on the previous seminal work of Stahler, but it differs in that the accretion of material occurs over a small area of the stellar surface, such as through a disk or magnetospheric accretion column, so that most of the stellar photosphere is free to radiate to space. This boundary condition is similar to the limiting case considered by Palla & Stahler for intermediate-mass stars. We argue that for a wide variety of disk mass accretion rates, material will be added to the star with relatively small amounts of thermal energy. Protostellar evolution calculated assuming this ``low-temperature'' limit of accretion generally follows the results of Stahler because of the thermostatic nature of deuterium fusion, which prevents protostars from contracting below a ``birthline'' in the H-R diagram. Our calculated protostellar radii tend to fall below Stahler's at higher masses; the additional energy loss from the stellar photosphere in the case of disk accretion tends to make the protostar contract. The low-temperature disk accretion evolutionary tracks never fall below the deuterium-fusion birthline until the internal deuterium is depleted, but protostellar tracks can lie above the birthline in the H-R diagram if the initial radius of the protostellar core is large enough or if rapid disk accretion (such as might occur during FU Ori outbursts) adds significant amounts of thermal energy to the star. These possibilities cannot be ruled out by either theoretical arguments or observational constraints at present, so that individual protostars might evolve along a multiplicity of birthlines with a modest range of luminosity at a given mass. Our results indicate that there are large uncertainties in assigning ages for the youngest stars from H-R diagram positions, given the uncertainty in birthline positions. Our

  4. Chaotic cold accretion on to black holes

    NASA Astrophysics Data System (ADS)

    Gaspari, M.; Ruszkowski, M.; Oh, S. Peng

    2013-07-01

    Bondi theory is often assumed to adequately describe the mode of accretion in astrophysical environments. However, the Bondi flow must be adiabatic, spherically symmetric, steady, unperturbed, with constant boundary conditions. Using 3D adaptive mesh refinement simulations, linking the 50 kpc to the sub-parsec (sub-pc) scales over the course of 40 Myr, we systematically relax the classic assumptions in a typical galaxy hosting a supermassive black hole. In the more realistic scenario, where the hot gas is cooling, while heated and stirred on large scales, the accretion rate is boosted up to two orders of magnitude compared with the Bondi prediction. The cause is the non-linear growth of thermal instabilities, leading to the condensation of cold clouds and filaments when tcool/tff ≲ 10. The clouds decouple from the hot gas, `raining' on to the centre. Subsonic turbulence of just over 100 km s-1 (M > 0.2) induces the formation of thermal instabilities, even in the absence of heating, while in the transonic regime turbulent dissipation inhibits their growth (tturb/tcool ≲ 1). When heating restores global thermodynamic balance, the formation of the multiphase medium is violent, and the mode of accretion is fully cold and chaotic. The recurrent collisions and tidal forces between clouds, filaments and the central clumpy torus promote angular momentum cancellation, hence boosting accretion. On sub-pc scales the clouds are channelled to the very centre via a funnel. In this study, we do not inject a fixed initial angular momentum, though vorticity is later seeded by turbulence. A good approximation to the accretion rate is the cooling rate, which can be used as subgrid model, physically reproducing the boost factor of 100 required by cosmological simulations, while accounting for the frequent fluctuations. Since our modelling is fairly general (turbulence/heating due to AGN feedback, galaxy motions, mergers, stellar evolution), chaotic cold accretion may be common in

  5. The Cimmerian accretion of SE Pamir and its relationships with the surrounding Cimmerian blocks

    NASA Astrophysics Data System (ADS)

    Zanchi, Andrea; Angiolini, Lucia; Zanchetta, Stefano; Nicora, Alda; Vezzoli, Giovanni

    2013-04-01

    Here we show that the sedimentary succession of SE Pamir comprises a syn-rift succession (Bazar Dara Group) in the Carboniferous-Early Permian, followed by a marked deepening from the late Early Permian, with increasing volcanic activity and synsedimentary tectonics in the Gan and Takhtabulak formations, related to the opening of the Rushan ocean between South and Central Pamir; carbonate deposition characterizes most of the Triassic, but then Upper Triassic flysches record the progressive closure of the Rushan ocean. We also show that this Permian-Triassic stratigraphic and biotic evolution broadly matches that of Karakoram. We consider these blocks plus Central Pamir equivalent to the Qiantang Terrane of Tibet, all of Palaeozoic Gondwanan ancestry, which detached from Gondwana in the Early Permian to form the Cimmerian belt. This was dissected into distinct terranes separated by deep extensional basins (i.e. the Rushan basin between SE Pamir and Central Pamir, the Wakhan basin between SE Pamir and Karakoram; the Shuanghu basin in Qiangtang). The northward drift of this composite belt from the Gondwanan margin since the late Early Permian onward is well supported by statistical palaeobiogeographical se analyses which show that the biotic affinity of its blocks shifted from Gondwanan in the Asselian-Sakmarian (Early Permian) to Palaeoequatorial in the Roadian-Wordian (Middle Permian). We also show the occurrence of lowermost Jurassic deposits suturing intensively faulted and folded Permian and Triassic units, which record a marked Cimmerian unconformity, suggesting that South Pamir collided against Central Pamir along the Rushan-Pshart suture at the T-J boundary. The closure of the Rushan ocean was at least in part coeval to the closure of the Palaeotethys, which caused the collision of Central Pamir against North Pamir, at that time located at the southern Eurasian margin. Collision of Karakoram to South Pamir happened slightly later along the TBZ zone

  6. Effects of Ice Accretion on Aircraft Aerodynamics

    NASA Technical Reports Server (NTRS)

    Bragg, Michael B.

    1998-01-01

    The primary objective of this research was to support the development of a new ice accretion model by improving our physical understanding of the ice accretion process through experimental measurements. The focus was on the effect of the initial ice roughness (smooth/rough boundary) on the accretion process. This includes understanding the boundary-layer development over the roughness and especially its effect on the heat transfer which is fundamental to the ice accretion process. The research focused on acquiring the experimental data needed to formulate a new ice accretion physical model. Research was conducted to analyze boundary-layer data taken on a NACA 0012 airfoil with roughness to simulate the smooth/rough boundary. The effect of isolated roughness on boundary-layer transition was studied experimentally to determine if the classical critical roughness Reynolds number criteria could be applied to transition in the airfoil leading-edge area. The effect of simulated smooth/rough boundary roughness on convective heat transfer was studied to complete the study. During the course of this research the effect of free-stream wind tunnel turbulence on the boundary layer was measured. Since this quantity was not well known, research to accurately measure the wind tunnel turbulence in an icing cloud was undertaken. Preliminary results were attained and the final data were acquired, reduced and presented under a subsequent grant.

  7. Episodic Accretion among the Orion Protostars

    NASA Astrophysics Data System (ADS)

    Fischer, William J.; Safron, Emily; Megeath, S. Thomas

    2016-06-01

    Episodic accretion, where a young stellar object undergoes stochastic spikes in its disk-to-star accretion rate one or more times over its formation period, may be a crucial process in the formation of low-mass stars. These spikes result in a factor of 10 to 100 increase in the source luminosity over the course of several months that may persist for years. Six years after the Spitzer survey of the Orion molecular clouds, the WISE telescope mapped Orion with similar wavelength coverage. Thus, the two surveys can be used to explore the mid-infrared variability of young stars on this timescale, which is suitable for discovering episodic accretion events. Out of 319 Orion protostars that were targets of the Herschel Orion Protostar Survey, we identified two examples of episodic accretion with this method. One of them, HOPS 223, was previously known. The other, HOPS 383, is the first known example of episodic accretion in a Class 0 protostar (age < 0.2 Myr). With these and one other outburst that began early in the Spitzer mission, we estimate that the most likely interval between protostellar outbursts is 740 years, with a 90% confidence interval of 470 to 6200 years. These outbursts are weaker than the optically revealed FU Ori events. We will update the mid-infrared light curves of HOPS 223 and HOPS 383 with recent data from FORCAST aboard SOFIA; HOPS 223 shows signs of fading.

  8. MHD of accretion-disk flows

    NASA Astrophysics Data System (ADS)

    Yankova, Krasimira

    2015-01-01

    Accretion is one of the most important problems of astrophysics concerning the transfer of matter and the transformation of energy into space. Process represents a falling of the substance on a cosmic object from the surrounding area and is a powerful gravitational mechanism for the production of radiation. Accretion disc effectively converts the mass of the substance by viscous friction and released potential energy transformed into radiation by particle collisions. Accretion onto compact object shows high energy efficiency and temporal variability in a broad class of observational data in all ranges. In the disks of these objects are developed a series instabilities and structures that govern the distribution of the energy. They are expressed in many variety non-stationary phenomena that we observe. That is why we propose generalized model of magnetized accretion disk with advection, which preserves the nonlinearity of the problem. We study interaction of the plasmas flow with the magnetic field, and how this affects the self-organizing disk. The aim of the work is to describe the accretion flow in detail, in his quality of the open astrophysical system, to investigate the evolution and to reveal the mechanisms of the structuring the disk-corona system for to interpret correctly the high energy behavior of such sources.

  9. Aerodynamic Simulation of Ice Accretion on Airfoils

    NASA Technical Reports Server (NTRS)

    Broeren, Andy P.; Addy, Harold E., Jr.; Bragg, Michael B.; Busch, Greg T.; Montreuil, Emmanuel

    2011-01-01

    This report describes recent improvements in aerodynamic scaling and simulation of ice accretion on airfoils. Ice accretions were classified into four types on the basis of aerodynamic effects: roughness, horn, streamwise, and spanwise ridge. The NASA Icing Research Tunnel (IRT) was used to generate ice accretions within these four types using both subscale and full-scale models. Large-scale, pressurized windtunnel testing was performed using a 72-in.- (1.83-m-) chord, NACA 23012 airfoil model with high-fidelity, three-dimensional castings of the IRT ice accretions. Performance data were recorded over Reynolds numbers from 4.5 x 10(exp 6) to 15.9 x 10(exp 6) and Mach numbers from 0.10 to 0.28. Lower fidelity ice-accretion simulation methods were developed and tested on an 18-in.- (0.46-m-) chord NACA 23012 airfoil model in a small-scale wind tunnel at a lower Reynolds number. The aerodynamic accuracy of the lower fidelity, subscale ice simulations was validated against the full-scale results for a factor of 4 reduction in model scale and a factor of 8 reduction in Reynolds number. This research has defined the level of geometric fidelity required for artificial ice shapes to yield aerodynamic performance results to within a known level of uncertainty and has culminated in a proposed methodology for subscale iced-airfoil aerodynamic simulation.

  10. Torque Reversals in Disk Accreting Pulsars

    NASA Astrophysics Data System (ADS)

    Li, Jianke; Wickramasinghe, Dayal T.

    1998-07-01

    X-ray binaries in which the accreting component is a neutron star commonly exhibit significant changes in their spin. In the system Cen X-3, a disk accreting binary system, the pulsar was observed to spin up at a rate ḟ = 8 × 10-13 Hz s-1 when averaged over the past twenty years, but significant fluctuations were observed above this mean. Recent BASTE observations have disclosed that these fluctuations are much larger than previously noted, and appeared to be a system characteristic. The change in the spin state from spin-up to spin-down or vice-versa occurs on a time scale that is much shorter than the instrument can resolve (≤1 d), but appears always to be a similar amplitude, and to occur stochastically. These observations have posed a problem for the conventional torque-mass accretion relation for accreting pulsars, because in this model the spin rate is closely related to the accretion rate, and the latter needs to be finely tuned and to change abruptly to explain the observations. Here we review recent work in this direction and present a coherent picture that explains these observations. We also draw attention to some outstanding problems for future studies.

  11. Gamma-ray spectrometry of granitic suites of the Paranaguá Terrane, Southern Brazil

    NASA Astrophysics Data System (ADS)

    Weihermann, Jessica Derkacz; Ferreira, Francisco José Fonseca; Cury, Leonardo Fadel; da Silveira, Claudinei Taborda

    2016-09-01

    The Paranaguá Terrane, located in the coastal portion of the states of Santa Catarina, Paraná and São Paulo in Southern Brazil is a crustal segment constituted mainly by an igneous complex, with a variety of granitic rocks inserted into the Serra do Mar ridge. The average altitude is approximately 1200 m above sea level, with peaks of up to 1800 m. Due to the difficulty of accessing the area, a shortage of outcrops and the thick weathering mantle, this terrane is understudied. This research aims to evaluate the gamma-ray spectrometry data of the granitic suites of the Paranaguá Terrane, in correspondence with the geological, petrographical, lithogeochemical, relief and mass movement information available in the literature. Aerogeophysical data were acquired along north-south lines spaced at 500 m, with a mean terrain clearance of 100 m. These data cover potassium (K, %), equivalent in thorium (eTh, ppm) and equivalent in uranium (eU, ppm). After performing a critical analysis of the data, basic (K, eU, eTh) and ternary (R-K/G-eTh/B-eU) maps were generated and then superimposed on the digital elevation model (DEM). The investigation of the radionuclide mobility across the relief and weathering mantle consisted of an analysis of the schematic profiles of elevation related with each radionuclide; a comparison of the K, eU and eTh maps with their 3D correspondents; and the study of mass movements registered in the region. A statistical comparison of lithogeochemical (K, U, Th) and geophysical (K, eU, eTh) data showed consistency in all the granitic suites studied (Morro Inglês, Rio do Poço and Canavieiras-Estrela). Through gamma-ray spectrometry, it was possible to establish relationships between scars (from mass movements) and the gamma-ray responses as well as the radionuclide mobility and the relief and to map the granitic bodies.

  12. Abundance and distribution of radioelements in lunar terranes: Results of Chang'E-1 gamma ray spectrometer data

    NASA Astrophysics Data System (ADS)

    Chen, Jian; Ling, Zongcheng; Li, Bo; Zhang, Jiang; Sun, Lingzhi; Liu, Jianzhong

    2016-02-01

    The gamma ray spectrometer (GRS) onboard Chang'E-1 has acquired valuable datasets recording the gamma ray intensities from radioelements (Potassium (K), Thorium (Th) and Uranium (U), etc.) on lunar surface. We extracted the elemental concentrations from the GRS data with spectral fitting techniques and mapped the global absolute abundance of radioelements in terms of the ground truths from lunar samples and meteorites. The obtained global concentration maps of these radioelements indicate heterogeneous distribution among three major lunar crustal terranes (i.e., Procellarum KREEP Terrane (PKT), Feldspathic Highlands Terrane (FHT), and South Pole Aitken Terrane (SPAT)) in relation with their origin and distinct geologic history. The majority of radioelements are restricted in PKT, approving the scenario of KREEP (Potassium (K), rare earth elements (REE), Phosphorus (P)) residua concentrating under the Procellarum region. Moreover, we found the consistency of distribution for radioelements and basalts, concluding that the subsequent volcanism might be associated with local concentrations of radioelements in western Oceanus Procellarum and northwestern South Pole Aitken Basin. The prominent and asymmetric radioactive signatures were confirmed in SPAT comparing to FHT dominated by low level radioactivity, while the magnitudes are much lower than that of PKT, indicating a primary geochemical heterogeneity for the Moon.

  13. New paleomagnetic results from Ordovician sedimentary rocks from NW Anatolia: Tectonic implications for the paleolatitudinal position of the Istanbul Terrane

    NASA Astrophysics Data System (ADS)

    Oksum, Erdinc; Hisarlı, Z. Mümtaz; Çinku, Mualla Cengiz; Ustaömer, Timur; Orbay, Naci

    2015-11-01

    The Istanbul terrane, classically known as the "Paleozoic of Istanbul", is geologically one of the important continental components of NW Turkey. The terrane comprises an Early Ordovician to Early Carboniferous transgressive sedimentary sequence and appears as an exotic unit with respect to its present surroundings. The paleogeographical position of the Ordovician rocks is unknown. We have therefore conducted a paleomagnetic study from a total of 56 sites in red fluvial clastics of the Kurtkoy formation and shallow marine quartzites of the Aydos formation to determine the paleolatitude of the Istanbul terrane during the Ordovician. The Lower Ordovician group mean direction calculated from 17 reliable sites provides a mean inclination of I = 19.4°, (α95 = 2.3°, k = 146.0) accepting only inclination data. A paleolatitude of 16.4° is obtained after considering an inclination correction factor of f = 0.6 due to the E/I results. The Early Ordovician paleolatitude of the Istanbul terrane shows a lower paleolatitudinal position closer to the equatorial zone than previously inferred.

  14. U.S. EPA'S STRATEGY FOR GROUND WATER QUALITY MONITORING AT HAZARDOUS WASTE LAND DISPOSAL FACILITIES LOCATED IN KARST TERRANES

    EPA Science Inventory

    Ground water monitoring of hazardous waste land disposal units by a network of wells is ineffective when located in karstic terranes. The U.S. Environmental Protection Agency (EPA) is currently proposing to modify its current ground water quality monitoring requirement of one upg...

  15. Controls on accretion of flysch and mélange belts at convergent margins: Evidence from the Chugach Bay thrust and Iceworm mélange, Chugach accretionary wedge, Alaska

    NASA Astrophysics Data System (ADS)

    Kusky, Timothy M.; Bradley, Dwight C.; Haeussler, Peter J.; Karl, Sue

    1997-12-01

    Controls on accretion of flysch and mélange terranes at convergent margins are poorly understood. Southern Alaska's Chugach terrane forms the outboard accretionary margin of the Wrangellia composite terrane, and consists of two major lithotectonic units, including Triassic-Cretaceous mélange of the McHugh Complex and Late Cretaceous flysch of the Valdez Group. The contact between the McHugh Complex and the Valdez Group on the Kenai Peninsula is a tectonic boundary between chaotically deformed melange of argillite, chert, greenstone, and graywacke of the McHugh Complex and a less chaotically deformed mélange of argillite and graywacke of the Valdez Group. We assign the latter to a new, informal unit of formational rank, the Iceworm mélange, and interpret it as a contractional fault zone (Chugach Bay thrust) along which the Valdez Group was emplaced beneath the McHugh Complex. The McHugh Complex had already been deformed and metamorphosed to prehnite-pumpellyite facies prior to formation of the Iceworm mélange. The Chugach Bay thrust formed between 75 and 55 Ma, as shown by Campanian-Maastrichtian depositional ages of the Valdez Group, and fault-related fabrics in the Iceworm mélange that are cut by Paleocene dikes. Motion along the Chugach Bay thrust thus followed Middle to Late Cretaceous collision (circa 90-100 Ma) of the Wrangellia composite terrane with North America. Collision related uplift and erosion of mountains in British Columbia formed a submarine fan on the Farallon plate, and we suggest that attempted subduction of this fan dramatically changed the subduction/accretion style within the Chugach accretionary wedge. We propose a model in which subduction of thinly sedimented plates concentrates shear strains in a narrow zone, generating mélanges like the McHugh in accretionary complexes. Subduction of thickly sedimented plates allows wider distribution of shear strains to accommodate plate convergence, generating a more coherent accretionary style

  16. Geochemistry, petrography, and zircon U-Pb geochronology of Paleozoic metaigneous rocks in the Mount Veta area of east-central Alaska: implications for the evolution of the westernmost part of the Yukon-Tanana terrane

    USGS Publications Warehouse

    Dusel-Bacon, Cynthia; Day, Warren C.; Aleinikoff, John N.

    2013-01-01

    We report the results of new mapping, whole-rock major, minor, and trace-element geochemistry, and petrography for metaigneous rocks from the Mount Veta area in the westernmost part of the allochthonous Yukon–Tanana terrane (YTT) in east-central Alaska. These rocks include tonalitic mylonite gneiss and mafic metaigneous rocks from the Chicken metamorphic complex and the Nasina and Fortymile River assemblages. Whole-rock trace-element data from the tonalitic gneiss, whose igneous protolith was dated by SHRIMP U–Pb zircon geochronology at 332.6 ± 5.6 Ma, indicate derivation from tholeiitic arc basalt. Whole-rock analyses of the mafic rocks suggest that greenschist-facies rocks from the Chicken metamorphic complex, a mafic metavolcanic rock from the Nasina assemblage, and an amphibolite from the Fortymile River assemblage formed as island-arc tholeiite in a back-arc setting; another Nasina assemblage greenschist has MORB geochemical characteristics, and another mafic metaigneous rock from the Fortymile River assemblage has geochemical characteristics of calc-alkaline basalt. Our geochemical results imply derivation in an arc and back-arc spreading region within the allochthonous YTT crustal fragment, as previously proposed for correlative units in other parts of the terrane. We also describe the petrography and geochemistry of a newly discovered tectonic lens of Alpine-type metaharzburgite. The metaharzburgite is interpreted to be a sliver of lithospheric mantle from beneath the Seventymile ocean basin or from sub-continental mantle lithosphere of the allochthonous YTT or the western margin of Laurentia that was tectonically emplaced within crustal rocks during closure of the Seventymile ocean basin and subsequently displaced and fragmented by faults.

  17. Two Cenozoic tectonic events of N-S and E-W extension in the Lhasa Terrane: Evidence from geology and geochronology

    NASA Astrophysics Data System (ADS)

    Huang, Feng; Xu, Ji-Feng; Chen, Jian-Lin; Wu, Jian-Bin; Zeng, Yun-Chuan; Xiong, Qiu-Wei; Chen, Xue-Feng; Yu, Hong-Xia

    2016-02-01

    Cenozoic active structures in the Tibetan Plateau are mainly regional N-S trending extensional faults and grabens, and E-W trending extensional tracks that are related to the transition from syn- to post-collision between India and Asia. E-W trending tracks are parallel to the direction of Neo-Tethyan oceanic convergence and consist of extensional volcanic-sedimentary basins and magmatic dykes in the southern Lhasa Terrane, Tibet. N-S trending tracks comprise faults and grabens, which are widely developed in Tibet. It remains unknown how and when the geodynamic transition from E-W to N-S trending tectonic tracks occurred. This study describes both E-W and N-S trending tectonic tracks identified at Dazi area of southern Lhasa Terrane, where E-W trending mafic dykes intruded a granitoid and late-stage N-S trending felsic dykes cut across E-W trending mafic dykes. Zircons from four granitoid samples yield consistent crystallization ages of ca. 60 Ma and positive εHf(t) values (~+ 9). An altered dioritic vein, which cuts the mafic dykes, yields an age of ca. 53 Ma. These new dating results indicate that E-W trending dykes, which formed due to regional N-S extension, were emplaced between 60 and 53 Ma. In addition, two N-S trending monzonitic porphyritic dykes, which cut the mafic dykes, yield U-Pb zircon ages of ca. 17 Ma with moderate positive εHf(t) values (+ 3 to + 9.6), as well as a NNE-SSW trending quartz monzonitic dyke, which cuts all other types of dykes, yields U-Pb ages of ca. 13 Ma. This suggests that E-W extension took place between 17 and 13 Ma. These results, in combination with existing age data for Gangdese granitoids and mafic magmatism, indicate the occurrence of two major extensional events at 60-53 Ma and 17-13 Ma. In turn, this implies that the transition from E-W to N-S trending tectonic and the onset of E-W extension occurred at ca. 17 Ma or slightly earlier. Paleocene granitoids have geochemical characteristics that are indicative of both

  18. Doppler tomography of accretion in binaries

    NASA Astrophysics Data System (ADS)

    Steeghs, D.

    2004-03-01

    Since its conception, Doppler tomography has matured into a versatile and widely used tool. It exploits the information contained in the highly-structured spectral line-profiles typically observed in mass-transferring binaries. Using inversion techniques akin to medical imaging, it permits the reconstruction of Doppler maps that image the accretion flow on micro-arcsecond scales. I summarise the basic concepts behind the technique and highlight two recent results; the use of donor star emission as a means to system parameter determination, and the real-time movies of the evolving accretion flow in the cataclysmic variable WZ Sge during its 2001 outburst. I conclude with future opportunities in Doppler tomography by exploiting the combination of superior data sets, second generation reconstruction codes and simulated theoretical tomograms to delve deeper into the physics of accretion flows.

  19. Accretion disks in luminous young stellar objects

    NASA Astrophysics Data System (ADS)

    Beltrán, M. T.; de Wit, W. J.

    2016-01-01

    An observational review is provided of the properties of accretion disks around young stars. It concerns the primordial disks of intermediate- and high-mass young stellar objects in embedded and optically revealed phases. The properties were derived from spatially resolved observations and, therefore, predominantly obtained with interferometric means, either in the radio/(sub)millimeter or in the optical/infrared wavelength regions. We make summaries and comparisons of the physical properties, kinematics, and dynamics of these circumstellar structures and delineate trends where possible. Amongst others, we report on a quadratic trend of mass accretion rates with mass from T Tauri stars to the highest mass young stellar objects and on the systematic difference in mass infall and accretion rates.

  20. The accretion column of AE Aqr

    NASA Astrophysics Data System (ADS)

    Rodrigues, Claudia; Costa, D. Joaquim; Luna, Gerardo; Lima, Isabel J.; Silva, Karleyne M. G.; De Araujo, Jose Carlos N.; Coelho, Jaziel

    2016-07-01

    AE Aqr is a magnetic cataclysmic variable, whose white dwarf rotates at the very fast rate of 33 s modulating the flux from high-energies to optical wavelengths. There are many studies of the origin of its emission, which consider emission from a rotating magnetic field or from an accretion column. Recently, MAGIC observations have discarded AE Aqr emission in very high energy gamma-rays discarding non-thermal emission. Furthermore, soft and hard X-ray data from Swift and NuSTAR were fitted using thermal models. Here we present the modelling of AE Aqr X-ray spectra and light curve considering the emission of a magnetic accretion column using the Cyclops code. The model takes into consideration the 3D geometry of the system, allowing to properly represent the white-dwarf auto eclipse, the pre-shock column absorption, and the varying density and temperature of a tall accretion column.

  1. Strongly magnetized accretion discs require poloidal flux

    NASA Astrophysics Data System (ADS)

    Salvesen, Greg; Armitage, Philip J.; Simon, Jacob B.; Begelman, Mitchell C.

    2016-08-01

    Motivated by indirect observational evidence for strongly magnetized accretion discs around black holes, and the novel theoretical properties of such solutions, we investigate how a strong magnetization state can develop and persist. To this end, we perform local simulations of accretion discs with an initially purely toroidal magnetic field of equipartition strength. We demonstrate that discs with zero net vertical magnetic flux and realistic boundary conditions cannot sustain a strong toroidal field. However, a magnetic pressure-dominated disc can form from an initial configuration with a sufficient amount of net vertical flux and realistic boundary conditions. Our results suggest that poloidal flux is a necessary prerequisite for the sustainability of strongly magnetized accretion discs.

  2. Heat transfer on accreting ice surfaces

    NASA Technical Reports Server (NTRS)

    Yamaguchi, Keiko; Hansman, R. John, Jr.

    1990-01-01

    Based on previous observations of glaze ice accretion, a 'Multi-Zone' model with distinct zones of different surface roughness is demonstrated. The use of surface roughness in the LEWICE ice accretion prediction code is examined. It was found that roughness is used in two ways: to determine the laminar to turbulent transition location and to calculate the turbulent heat transfer coefficient. A two zone version of the Multi-Zone model is implemented in the LEWICE code, and compared with experimental heat transfer coefficient and ice accretin results. The analysis of the boundary layer transition, surface roughness, and viscous flow field effects significantly increased the accuracy in predicting heat transfer coefficients. The Multi-Zone model was found to greatly improve the ice accretion prediction for the cases compared.

  3. The accretion halo in AM Herculis systems

    NASA Technical Reports Server (NTRS)

    Achilleos, N.; Wickramasinghe, D. T.; Wu, Kinwah

    1992-01-01

    Previous phase-resolved spectropolarimetric observations of the AM Herculis systems V834 Centauri (E1405-451) and EF Eridani have shown broad, Zeeman-shifted absorption features during the bright phases. These features are thought to be nonphotospheric in origin, and to arise from a cool 'halo' of unshocked gas surrounding the accretion shock on the surface of the white dwarf primary. Preliminary models for the accretion halo region are presented and these models are used to perform a more detailed analysis of the relevant data for these two systems than has previously been done. To explain the observed halo Zeeman features, geometries which are consistent with the presence of linearly extended cyclotron emission regions are required. Such regions have previously been deduced from different considerations by other investigators. The estimated masses for the accretion halo are comparable to the mass of the cyclotron emission region.

  4. Accretion disks in Algols: Progenitors and evolution

    NASA Astrophysics Data System (ADS)

    Van Rensbergen, W.; De Greve, J. P.

    2016-08-01

    Context. There are only a few Algols with measured accretion disk parameters. These measurements provide additional constraints for tracing the origin of individual systems, narrowing down the initial parameter space. Aims: We investigate the origin and evolution of six Algol systems with accretion disks to find the initial parameters and evolutionary constraints for them. Methods: With a modified binary evolution code, series of close binary evolution are calculated to obtain the best match for observed individual systems. Results: Initial parameters for six Algol systems with accretion disks were determined matching both the present system parameters and the observed disk characteristics. Conclusions: When Roche lobe overflow (RLOF) starts during core hydrogen burning of the donor, the disk lifetime was found to be short. The disk luminosity is comparable to the luminosity of the gainer during a large fraction of the disk lifetime.

  5. Strongly magnetized accretion discs require poloidal flux

    NASA Astrophysics Data System (ADS)

    Salvesen, Greg; Armitage, Philip J.; Simon, Jacob B.; Begelman, Mitchell C.

    2016-05-01

    Motivated by indirect observational evidence for strongly magnetized accretion discs around black holes, and the novel theoretical properties of such solutions, we investigate how a strong magnetization state can develop and persist. To this end, we perform local simulations of accretion discs with an initially purely toroidal magnetic field of equipartition strength. We demonstrate that discs with zero net vertical magnetic flux and realistic boundary conditions cannot sustain a strong toroidal field. However, a magnetic pressure-dominated disc can form from an initial configuration with a sufficient amount of net vertical flux and realistic boundary conditions. Our results suggest that poloidal flux is a necessary prerequisite for the sustainability of strongly magnetized accretion discs.

  6. Exploring the disk accretion in DI Cep

    NASA Astrophysics Data System (ADS)

    Parihar, Padmakar Singh; Shantikumar, N. S.

    The low mass young stellar objects of class-II, popularly known as classical T Tauri stars (CTTS) supposed to be surrounded by thick flared disk and accretes disk material through strong stellar dipolar magnetic field. The disk accretion rate and its variation with time is poorly know. DI Cep is an interesting object, found to have unexpected hump around 5300 Å in the continuum excess emission spectrum, which cannot be explained by current models of YSOs. Over the last six years this object is being spectroscopically as well as photometrically monitored using HCT. The data have been analyzed and modeled using a simple modeling technique developed by us. In this paper, we report for the first time our results related to the disk accretion phenomena in DI Cep.

  7. P-T-t paths and differential Alleghanian loading and uplift of the Bronson Hill terrane south central New England

    USGS Publications Warehouse

    Wintsch, R.P.; Kunk, M.J.; Boyd, J.L.; Aleinikoff, J.N.

    2003-01-01

    Late Paleozoic U-Pb ages of sphene and 40Ar/39Ar cooling ages of amphibole and muscovite from rocks of the Bronson Hill terrane in Connecticut and central Massachusetts reflect a late Paleozoic (Alleghanian) overprint on Acadian metamorphic rocks. Prograde Alleghanian sphenes crystallized during the Late Pennsylvanian, and eliminate the possibility that amphibole ages reflect delayed Permian cooling from Devonian Acadian metamorphism. Fourteen new amphibole ages from Connecticut form a north-to-south trend of decreasing age from 294 to 245 Ma, while in Massachusetts four new amphibole ages together with three others from the literature produce a random Carboniferous pattern. Seven new muscovite ages support existing data indicating uniform cooling throughout the Bronson Hill terrane through ???350??C in the Early Triassic. The rate of Permian cooling defined by amphibole-muscovite pairs increases from ???4??C/my in northern Connecticut to ???50??C/my near Long Island Sound. Hinged loading and hinged but delayed exhumation in the southern part of the Bronson Hill terrane (with the hinge in central Connecticut) explain these ages and cooling rates as well as a southerly increasing metamorphic field gradient. One-dimensional thermal modeling indicates that loading of Bronson Hill rocks must have begun by the Late Mississippian. The time of peak Alleghanian metamorphic temperature decreases southward from Early Permian in northern Connecticut to Late Permian to the south. These results demonstrate that the metamorphic effects of the Alleghanian orogeny are not restricted to the Avalon terrane of southeastern New England. On the contrary, the Alleghanian orogeny reset 40Ar/39Ar mineral ages, recrystallized minerals, partially melted felsic rocks, and transposed fabrics at least as far west as the Bronson Hill terrane in south-central New England.

  8. Accretion-powered Compact Binaries

    NASA Astrophysics Data System (ADS)

    Mauche, Christopher W.

    2003-12-01

    Preface; The workshop logo; A short history of the CV workshop F. A. Córdova; Part I. Observations: 1. Low mass x-ray binaries A. P. Cowley, P. C. Schmidtke, D. Crampton, J. B. Hutchings, C. A. Haswell, E. L. Robinson, K. D. Horne, H. M. Johnston, S. R. Kulkarni, S. Kitamoto, X. Han, R. M. Hjellming, R. M. Wagner, S. L. Morris, P. Hertz, A. N. Parmar, L. Stella, P. Giommi, P. J. Callanan, T. Naylor, P. A. Charles, C. D. Bailyn, J. N. Imamura, T. Steiman-Cameron, J. Kristian, J. Middleditch, L. Angelini and J. P. Noris; 2. Nonmagnetic cataclysmic variables R. S. Polidan, C. W. Mauche, R. A. Wade, R. H. Kaitchuck, E. M. Schlegel, P. A. Hantzios, R. C. Smith, J. H. Wood, F. Hessman, A. Fiedler, D. H. P. Jones, J. Casares, P. A. Charles, J. van Paradijs, E. Harlaftis, T. Naylor, G. Sonneborn, B. J. M. Hassall, K. Horne, C. A. la Dous, A. W. Shafter, N. A. Hawkins, D. A. H. Buckley, D. J. Sullivan, F. V. Hessman, V. S. Dhillon, T. R. Marsh, J. Singh, S. Seetha, F. Giovannelli, A. Bianchini, E. M. Sion, D. J. Mullan, H. L. Shipman, G. Machin, P. J. Callanan, S. B. Howell, P. Szkody, E. M. Schlegel and R. F. Webbink; 3. Magnetic cataclysmic variables C. Hellier, K. O. Mason, C. W. Mauche, G. S. Miller, J. C. Raymond, F. K. Lamb, J. Patterson, A. J. Norton, M. G. Watson, A. R. King, I. M. McHardy, H. Lehto, J. P. Osborne, E. L. Robinson, A. W. Shafter, S. Balachandran, S. R. Rosen, J. Krautter, W. Buchholz, D. A. H. Buckley, I. R. Tuoly, D. Crampton, B. Warner, R. M. Prestage, B. N. Ashoka, M. Mouchet, J. M. Bonnet-Bidaud, J. M. Hameury, P. Szkody, P. Garnavich, S. Howell, T. Kii, M. Cropper, K. Mason, J. Bailey, D. T. Wickramasinghe, L. Ferrario, K. Beuermann, A. D. Schwope, H.-C. Thomas, S. Jordan, J. Schachter, A. V. Filippenko, S. M. Kahn, F. B. S. Paerels, K. Mukai, M. L. Edgar, S. Larsson, R. F. Jameson, A. R. King, A. Silber, R. Remillard, H. Bradt, M. Ishida, T. Ohashi and G. D. Schmidt; Part II. Accretion Theory: 4. Nonmagnetic W. Kley, F. Geyer, H. Herold, H

  9. Origin of deep crystal reflections: seismic profiling across high-grade metamorphic terranes in Canada

    USGS Publications Warehouse

    Green, A.; Milkereit, B.; Percival, J.; Davidson, A.; Parrish, R.; Cook, F.; Geis, W.; Cannon, W.; Hutchinson, D.; West, G.; Clowes, R.

    1990-01-01

    In an attempt to better understand the origin of deep crustal reflections LITHOPROBE has sponsored or co-sponsored Seismic reflection surveys across tracts of high-grade metamorphic rock in the Archean Superior craton, the Proterozoic Grenville orogen and the Phanerozoic Cordilleran orogen. Common to these three diverse terranes are near-surface zones of prominent Seismic reflectivity that are typically associated with velocity discontinuities at highly strained contacts between gneissic rocks of varying lithology. At some locations the reflective layering resulted from transposition and rearrangement of previously layered rocks (stratified assemblages, sills, etc.), whereas in other regions it was generated by extreme attenuation, stretching and ductile flow of weakly layered or irregularly organized rocks. It seems likely that compositionally layered gneissic rock is a common source of reflections in the deep crust, with reflections originating at lithological boundaries and zones of mylonite. ?? 1990.

  10. Comparison of radon in soil over faulted crystalline terranes: Glaciated versus unglaciated

    SciTech Connect

    Gates, A.E.; Malizzi, L.D. ); Gundersen, L.C.S. )

    1990-05-01

    Radon in soil correlates directly with the bedrock geology in unglaciated terranes. In the Hylas shear zone, Virginia, the rock units exhibit a marked contrast in uranium concentration which is reflected in soil radon. The soils are clay-rich and are derived directly from the underlying bedrock. Even small geologic features such as pegmatite veins and thin shear zones are discernable by differences in soil radon concentrations. This bedrock/soil radon correlation is obscured in areas with thick glacial covers. The Reservoir fault area, New Jersey, is proximal to a terminal moraine. Although the bedrock units exhibit a strong contrast in radioactivity and uranium concentration, soil radon concentrations in the overlying tills reflect the local chemical and physical properties of the till rather than the bedrock.

  11. Regional Geophysical Expression of a Carbonatite Terrane in the Eastern Mojave Desert, California

    NASA Astrophysics Data System (ADS)

    Denton, K. M.; Ponce, D. A.; Miller, D. M.

    2012-12-01

    A world-class, rare earth element carbonatite deposit is located near Mountain Pass, in the eastern Mojave Desert of California and is hosted by Proterozoic rocks that extend along the eastern margins of the Clark Mountain Range, Mescal Range, and Ivanpah Mountains in a north-northwest trending fault-bounded block. This Proterozoic block is generally composed of a complex of 1.7 - 1.6 Ga gneisses and schists that are intruded by ~1.4 Ga carbonatite and ultrapotassic mafic dikes. In the latter suite, common intrusive rock types include shonkinite, syenite, and alkali granites that are associated with carbonatite dikes. Regional geophysical data reveal that the carbonatite deposit itself occurs along the northeast edge of a prominent magnetic high with an amplitude of 200 nanoteslas, which appears to be related to the surrounding Proterozoic block. More than 340 gravity stations and 155 physical property samples were collected to augment existing geophysical data to determine the geophysical and geologic setting of the eastern Mojave Desert carbonatite terrane. Physical properties of representative rock types in the area show that 23 samples of carbonatite ore have an average saturated bulk density of 2,866 with a range of 2,440 to 3,192 kg/m3 and a magnetic susceptibility of 0.22 with a range of 0.03 to 0.61x 10-3 SI units, 17 samples of syenite have an average saturated bulk density of 2,670 with a range of 2,555 to 2,788 kg/m3 and a magnetic susceptibility of 3.50 with a range of 0.19 to 11.46 x 10-3 SI units, 19 samples of shonkinite dike have an average saturated bulk density of 2,800 with a range of 2,603 to 3,000 kg/m3 and a magnetic susceptibility of 0.71 with a range of 0.00 to 4.44 x 10-3 SI units, and 28 samples of Proterozoic gneiss have an average saturated bulk density of 2,734 with a range of 2,574 to 3,086 kg/m3 and a magnetic susceptibility of 1.23 with a range of 0.01 to 7.48 x 10-3 SI units. In general, carbonatites have distinctive gravity

  12. Regional geophysical expression of a carbonatite terrane in the eastern Mojave Desert, California

    USGS Publications Warehouse

    Ponce, David A.; Denton, Kevin M.; Miller, David M.

    2013-01-01

    A world-class, rare earth element carbonatite deposit is located near Mountain Pass, in the eastern Mojave Desert of California and is hosted by Proterozoic rocks that extend along the eastern margins of the Clark Mountain Range, Mescal Range, and Ivanpah Mountains in a north-northwest trending fault-bounded block. This Proterozoic block is generally composed of a complex of 1.7 - 1.6 Ga gneisses and schists that are intruded by ~1.4 Ga carbonatite and ultrapotassic mafic dikes. In the latter suite, common intrusive rock types include shonkinite, syenite, and alkali granites that are associated with carbonatite dikes. Regional geophysical data reveal that the carbonatite deposit itself occurs along the northeast edge of a prominent magnetic high with an amplitude of 200 nanoteslas, which appears to be related to the surrounding Proterozoic block. More than 340 gravity stations and 155 physical property samples were collected to augment existing geophysical data to determine the geophysical and geologic setting of the eastern Mojave Desert carbonatite terrane. Physical properties of representative rock types in the area show that 23 samples of carbonatite ore have an average saturated bulk density of 2,866 with a range of 2,440 to 3,192 kg/m3 and a magnetic susceptibility of 0.22 with a range of 0.03 to 0.61x 10-3 SI units, 17 samples of syenite have an average saturated bulk density of 2,670 with a range of 2,555 to 2,788 kg/m3 and a magnetic susceptibility of 3.50 with a range of 0.19 to 11.46 x 10-3 SI units, 19 samples of shonkinite dike have an average saturated bulk density of 2,800 with a range of 2,603 to 3,000 kg/m3 and a magnetic susceptibility of 0.71 with a range of 0.00 to 4.44 x 10-3 SI units, and 28 samples of Proterozoic gneiss have an average saturated bulk density of 2,734 with a range of 2,574 to 3,086 kg/m3 and a magnetic susceptibility of 1.23 with a range of 0.01 to 7.48 x 10-3 SI units. In general, carbonatites have distinctive gravity

  13. Accretion Flows in Magnetic White Dwarf Systems

    NASA Technical Reports Server (NTRS)

    Imamura, James N.

    2005-01-01

    We received Type A and B funding under the NASA Astrophysics Data Program for the analysis and interpretation of hard x-ray data obtained by the Rossi X-ray Timing Explorer and other NASA sponsored missions for Intermediate Polars (IPS) and Polars. For some targets, optical data was available. We reduced and analyzed the X-ray spectra and the X-ray and optical (obtained at the Cerro Tololo Inter-American Observatory) timing data using detailed shock models (which we constructed) to place constraints on the properties of the accreting white dwarfs, the high energy emission mechanisms of white dwarfs, and the large-scale accretion flows of Polars and IPS. IPS and Polars are white dwarf mass-transfer binaries, members of the larger class of cata,clysmic variables. They differ from the bulk of the cataclysmic variables in that they contain strongly magnetic white dwarfs; the white dwarfs in Polars have B, = 7 to 230 MG and those in IPS have B, less than 10 MG. The IPS and Polars are both examples of funneled accretion flows in strong magnetic field systems. The IPS are similar to x-ray pulsars in that accretion disks form in the systems which are disrupted by the strong stellar magnetic fields of the white dwarfs near the stellar surface from where the plasma is funneled to the surface of the white dwarf. The localized hot spots formed at the footpoints of the funnels coupled with the rotation of the white dwarf leads to coherent pulsed x-ray emission. The Polars offer an example of a different accretion topology; the magnetic field of the white dwarf controls the accretion flow from near the inner Lagrangian point of the system directly to the stellar surface. Accretion disks do not form. The strong magnetic coupling generally leads to synchronous orbital/rotational motion in the Polars. The physical system in this sense resembles the Io/Jupiter system. In both IPS and Polars, pulsed emission from the infrared to x-rays is produced as the funneled flows merge onto the

  14. Thermonuclear processes on accreting neutron stars

    NASA Technical Reports Server (NTRS)

    Joss, P. C.

    1981-01-01

    Theoretical models for X-ray burst sources that invoke thermonuclear flashes on the surface layers of an accreting neutron star are discussed. The historical development of X-ray burst observation is summarized, and a physical picture of a neutron star undergoing accretion is drawn. Detailed numerical computations of the evolution of the surface layers of such a star are reviewed. The need for general relativistic corrections to the model is pointed out. Finally, comparisons are made with observations of X-ray bursts, the rapid burster, fast X-ray transients, X-ray pulsars, and gamma-ray burst sources.

  15. Accretion disks in interacting binary stars

    NASA Technical Reports Server (NTRS)

    Lin, D. N. C.

    1991-01-01

    Accretion disks have most often been analyzed in cataclysmic variables (CVs); the structure and evolution of accretion disks is defined by angular momentum transfer processes. Detailed atmospheric models indicate that angular momentum transport is efficient, that CV outbursts are regulated by mass transfer variations in the disk, and that they may be initiated either from the inner and outer regions of the disk. Tidal effects on the companion are noted to be capable of inducing a significant departure from Keplerian flow near the outer region of the disk.

  16. The accretion of planets from planetesimals

    NASA Technical Reports Server (NTRS)

    Greenberg, R.; Hartmann, W. K.; Chapman, C. R.; Wacker, J. F.

    1978-01-01

    Collisional accretion appears to be a viable, and seemingly unavoidable, mechanism for intermediate-stage from a swarm of planetesimals into a system containing a few discrete seed planets. Some other mechanism must be invoked to explain growth of condensate grains up to at least tens of meters. Gravitational instability in the particulate disk seems a plausible means of achieving that early-stage growth up to kilometer-scale bodies. The last stage of growth in which the seed planets accrete the remaining material now presents difficulties due to the isolated, circular orbits generated from the intermediate collisional stage.

  17. Disk-overflow accretion in GK Persei?

    NASA Technical Reports Server (NTRS)

    Hellier, Coel; Livio, Mario

    1994-01-01

    We reanalyze the 1983 European X-ray Observatory Satellite (EXOSAT) observations of GK Per during an outburst to investigate the approximately 5000 s quasiperiodic modulation. We find that the spectral behavior is reminiscent of dipping low-mass X-ray binaries and note that the time scale is characteristic of the radius where an accretion stream overflowing the disk would collide back onto the disk. We suggest that structure caused by such disk-overflow accretion was periodically obscuring the white dwarf, producing the modulation.

  18. Effects of silicate weathering on water chemistry in forested, upland, felsic terrane of the USA

    SciTech Connect

    Stauffer, R.E.; Wittchen, B.D. )

    1991-11-01

    The authors use data from the US EPA National Surface Water Survey (NSWS), the USGS Bench-Mark Station monitoring program, and the National Acid Deposition Program (NADP) to evaluate the role of weathering in supplying base cations to surface waters in forested, upland, felsic terrane of the northeastern, northcentral, and northwestern (Idaho batholith) US. Multivariate regression reveals differential effects of discharge on individual base cations and silica, but no secular trend in the Ca/Na denudation rate over 24 yr (1965-1988) for the Wild River catchment in the White Mountains. Because the turn-over time for Na in the soil-exchange complex is only ca. 1.5 yr, the long-term behavior of the ratios Ca/Na and Si/Na in waters leaving this catchment indicates that weathering is compensating for base cation export. In every subregion, Ca and Mg concentrations in lakes are statistically linked to nonmarine Na, but the median Ca/Na ratio is greater than the ratio in local plagioclase. The authors attribute this inequality to nonstoichiometric weathering of calcium in juvenile (formerly glaciated) terrane, not to leaching of exchangeable cations by So{sub 4} because intraregional and cross-regional statistical analysis reveals no effect of atmospherically derived sulfate ion. The median base cation denudation rates (meq m{sup {minus}2}yr{sup {minus}1}) for these American lake regions are: Maine granites (108); western Adirondack felsic gneiss (85); Vermilion batholith (42); Idaho batholith (52). The regional rates are high enough to compensate for present wet deposition of acidifying anions except in some vulnerable lake watersheds in the western Adirondacks.

  19. Effects of silicate weathering on water chemistry in forested, upland, felsic terrane of the USA

    NASA Astrophysics Data System (ADS)

    Stauffer, Robert E.; Wittchen, Bruce D.

    1991-11-01

    We use data from the US EPA National Surface Water Survey (NSWS), the USGS Bench-Mark Station monitoring program, and the National Acid Deposition Program (NADP) to evaluate the role of weathering in supplying base cations to surface waters in forested, upland, felsic terrane of the northeastern, northcentral, and northwestern (Idaho batholith) United States. Multivariate regression reveals differential effects of discharge on individual base cations and silica, but no secular trend in the Ca/Na denudation rate over 24 yr (1965-1988) for the Wild River catchment in the White Mountains. Because the turn-over time for Na in the soil-exchange complex is only ca. 1.5 yr, the long-term behavior of the ratios Ca/Na and Si/Na in waters leaving this catchment indicates that weathering is compensating for base cation export. In every subregion, Ca and Mg concentrations in lakes are statistically linked to nonmarine Na, but the median Ca/Na ratio is greater than the ratio in local plagioclase. We attribute this inequality to nonstoichiometric weathering of calcium in juvenile (formerly glaciated) terrane, not to leaching of exchangeable cations by SO 4, because intraregional and cross-regional statistical analysis reveals no effect of atmospherically derived sulfate ion. The median base cation denudation rates (meq m -2 yr -1) for these American lake regions are: Maine granites (108); western Adirondack felsic gneiss (85); Vermilion batholith (42); Idaho batholith (52). The regional rates are high enough to compensate for present wet deposition of acidifying anions except in some vulnerable lake watersheds in the western Adirondacks.

  20. Paleomagnetic evidence for Post-Jurassic stability of southeastern Mexico: Maya Terrane

    NASA Astrophysics Data System (ADS)

    Guerrero, Jose C.; Herrero-Bervera, Emilio; Helsley, Charles E.

    1990-05-01

    The tectonic evolution of southeastern Mexico has been a subject of major controversy, not only in regard to past geometry but also in the timing of proposed geological events as well. For the past 10 years, most, if not all, investigators agree that the Gulf of Mexico Basin was formed by Late Jurassic time and that the Maya Terrane was in its current location prior to the Cretaceous. In order to gain further insight into the drift history of the Maya Terrane we have undertaken a paleomagnetic study of the uppermost Jurassic-Lower Cretaceous (Tithonian-lower Neocomian?) San Ricardo Formation in southeastern Mexico, at 93.7°W, 16.8°N. The sampling site is located east of the Isthmus of Tehuantepec, on the southwest side of the Maya block, at the base of the Yucatan Peninsula. A suite of 133 samples was collected in stratigraphic succession from a 114-m-thick sequence of red shales and sandstones near Cintalapa, Chiapas, Mexico. After progressive thermal demagnetization of all samples at six steps from 350°C to 630°C, 89 samples were selected for final paleopole analysis on the basis of their magnetic stability. Four different polarity intervals were observed, the sequence being from bottom to top: N, R, N, R which assists in the assessment of the reliability of the observations. The mean pole position obtained, 160.0°E, 69.8°N, agrees with the mean pole position of the upper part of the Morrison Formation of Colorado, a unit of virtually identical age. These results indicate that no discernible rotation or displacement of the Maya block has occurred since at least early Neocomian times.

  1. Sedimentology and tectonics of Devonian Nation River Formation, Alaska, part of yet another allochthonous terrane

    SciTech Connect

    Howell, D.G.; Murray, R.W.; Wiley, T.J.; Boundy-Sanders, S.; Kauffman-Linam, L.; Jones, D.L.

    1987-05-01

    Sandwiched between terra incognito of the Yukon Flats, Alaska, and the disrupted cratonal sequences of Yukon Territory, Canada, is a complex array of Proterozoic and Phanerozoic rock units composing a poorly defined group of tectonostratigraphic terranes. The Nation River formation (NRF) is a conspicuous siliciclastic submarine fan complex interbedded in a Paleozoic sequence characterized by deep-water cherts, siliceous shales, and platform to basin-plain carbonates. The NRF ranges from 500 to 2000 m thick. Where the basal part is exposed, NRF overlies the Devonian McCann Hill Chert, a deep-water radiolarian chert sequence. Above the NRF is either another radiolarian chert sequence, the Mississippian Ford Lake Shale, or Permian shallow-water Tahkandit Limestone or Step Conglomerate. NRF lithologies include fine-grained to pebbly turbidites assembled in both thinning- and fining-upward and thickening- and coarsening-upward cycles typical of middle to outer fan settings. Compositionally the grains are principally chert (green, gray, white, black, and rarely red) with minor amounts of vein quartz and quartz sandstone. Most of the chert seems to be replacement chert from a carbonate terrane, though some pebbles yield an Ordovician radiolarian assemblage. Paleocurrent flow directions based on thousands of bottom features (flutes, prods, and grooves) indicate, in present-day coordinates, flow toward the east. Individual azimuth directions are throughout the two easterly quadrants, by 60% of these data indicate flow between 045 and 150/sup 0/. This spread of data is consistent from outcrop to outcrop, indicating that there are no localized block rotations. Easterly flow has also been determined for the overlying Cretaceous units of the Kandik basin (Biederman Argillite and Kathul Graywacke).

  2. Carbonatite: A Geophysical investigation of a rare earth element terrane, eastern Mojave Desert, California

    NASA Astrophysics Data System (ADS)

    Denton, K. M.; Ponce, D. A.; Miller, D. M.; MacPherson-Krutsky, C. C.

    2013-12-01

    Geophysical investigations reveal gravity and magnetic anomalies related to a Proterozoic carbonatite terrane in the eastern Mojave Desert, host to one of the largest rare earth element carbonatite deposits in the world. The deposit is located near Mountain Pass, California and occurs in a north-northwest trending fault-bounded block that extends along the eastern parts of the Clarke Mountain Range, Mescal Range, and Ivanpah Mountains. This Early to Middle Proterozoic block is composed of a 1.7 Ga metamorphic complex of gneiss and schist intruded by a 1.4 Ga suite of ultrapotassic alkaline intrusive rocks that includes carbonatite. The intrusive suite (oldest to youngest) includes shonkinite, mesosyenite, syenite, quartz syenite, potassic granite, carbonatite, and late shonkinite dikes which are spatially and temporally associated with carbonatite intrusions and dikes. Regional geophysical data reveal that the carbonatite deposit occurs along a gravity high and the northeast edge of a prominent magnetic high with an amplitude of about 200 nanoteslas. More than 1400 gravity stations and over 200 physical property samples were collected to augment existing geophysical data and will be used to determine the geophysical and geologic setting that provide an improved structural interpretation of the eastern Mojave Desert carbonatite terrane. Physical properties of representative rock types in the area include carbonatite ore, syenite, shonkinite, gneiss, granite, and dolomite. Carbonatite intrusions typically have distinctive gravity, magnetic, and radiometric signatures because these deposits are relatively dense, contain magnetite, and are enriched in thorium or uranium. However, our results show that the main carbonatite body is essentially nonmagnetic. Thus, it is unlikely that carbonatite rocks are the source of the magnetic high associated with the Clark Mountain and Mescal Ranges. Instead, we suggest that weakly to moderately magnetic intrusive rocks or

  3. Age and duration of eclogite-facies metamorphism, North Qaidam HP/UHP terrane, Western China

    USGS Publications Warehouse

    Mattinson, C.G.; Wooden, J.L.; Liou, J.G.; Bird, D.K.; Wu, C.L.

    2006-01-01

    Amphibolite-facies para-and orthogneisses near Dulan, at the southeast end of the North Qaidam terrane, enclose minor eclogite and peridotite which record ultra-high pressure (UHP) metamorphism associated with the Early Paleozoic continental collision of the Qilian and Qaidam microplates. Field relations and coesite inclusions in zircons from paragneiss suggest that felsic, mafic, and ultramafic rocks all experienced UHP metamorphism and a common amphibolite-facies retrogression. SHRIMP-RG U-Pb and REE analyses of zircons from four eclogites yield weighted mean ages of 449 to 422 Ma, and REE patterns (flat HREE, no Eu anomaly) and inclusions of garnet, omphacite, and rutile indicate these ages record eclogite-facies metamorphism. The coherent field relations of these samples, and the similar range of individual ages in each sample suggests that the ???25 m.y. age range reflects the duration of eclogite-facies conditions in the studied samples. Analyses from zircon cores in one sample yield scattered 433 to 474 Ma ages, reflecting partial overlap on rims, and constrain the minimum age of eclogite protolith crystallization. Inclusions of Th + REE-rich epidote, and zircon REE patterns are consistent with prograde metamorphic growth. In the Lu??liang Shan, approximately 350 km northwest in the North Qaidam terrane, ages interpreted to record eclogite-facies metamorphism of eclogite and garnet peridotite are as old as 495 Ma and as young as 414 Ma, which suggests that processes responsible for extended high-pressure residence are not restricted to the Dulan region. Evidence of prolonged eclogite-facies metamorphism in HP/UHP localities in the Northeast Greenland eclogite province, the Western Gneiss Region of Norway, and the western Alps suggests that long eclogite-facies residence may be globally significant in continental subduction/collision zones.

  4. Stratigraphic and structural implications of conodont and detrital zircon U-Pb ages from metamorphic rocks of the Coldfoot terrane, Brooks Range, Alaska

    USGS Publications Warehouse

    Moore, T.E.; Aleinikoff, J.N.; Harris, A.G.

    1997-01-01

    New paleontologic and isotopic data from the Emma Creek and Marion Creek schists of the Coldfoot terrane, Arctic Alaska superterrane, central Brooks Range, suggest Devonian and possibly younger ages of deposition for their sedimentary protoliths. Conodonts from marble of the Emma Creek schist, intruded by a roughly 392 Ma orthogneiss, are late Lochkovian (early Early Devonian, between about 408 and 396 Ma) and Silurian to Devonian at two other locations. Spherical to oblong detrital zircons from quartz-mica schist of the overlying Marion Creek schist yield mostly discordant U-Pb data suggestive of provenance ages of 3.0, 2.0-1.8, and 1.5-1.4 Ga; however, several euhedral grains of zircon from Marion Creek quartz-mica schist have concordant U-Pb ages from 370 to 360 Ma. The Marion Creek schist in our study area therefore is at least 26 m.y. younger than the Emma Creek schist. The age data imply that the protolith of the Emma Creek schist is age correlative with Devonian carbonate rocks in the Hammond and North Slope terranes, whereas the Marion Creek schist is age correlative with Upper Devonian and Lower Mississippian clastic sedimentary rocks of the Endicott Group in the Endicott Mountains terrane and shale and carbonate units in the De Long Mountains and Sheenjek River terranes. Consequently, tectonic models restoring the entire Coldfoot terrane beneath partly or wholly coeval rocks of the Hammond, Endicott Mountains, De Long Mountains, and Sheenjek River terranes of the Arctic Alaska superterrane require revision. Alternative reconstructions, including restoration of the Coldfoot terrane inboard of the Endicott Mountains terrane or outboard of the De Long Mountains and Sheenjek River terranes are plausible but require either larger amounts of shortening than previously suggested or indicate problematic facies relations. copyright. Published in 1997 by the American Geophysical Union.

  5. Accretion dynamics of EX Lupi in quiescence. The star, the spot, and the accretion column

    NASA Astrophysics Data System (ADS)

    Sicilia-Aguilar, Aurora; Fang, Min; Roccatagliata, Veronica; Collier Cameron, Andrew; Kóspál, Ágnes; Henning, Thomas; Ábrahám, Peter; Sipos, Nikoletta

    2015-08-01

    Context. EX Lupi is a young, accreting M0 star and the prototype of EXor variable stars. Its spectrum is very rich in emission lines, including many metallic lines with narrow and broad components. The presence of a close companion has also been proposed, based on radial velocity signatures. Aims: We use the metallic emission lines to study the accretion structures and to test the companion hypothesis. Methods: We analyse 54 spectra obtained during five years of quiescence time. We study the line profile variability and the radial velocity of the narrow and broad metallic emission lines. We use the velocity signatures of different species with various excitation conditions and their time dependency to track the dynamics associated with accretion. Results: We observe periodic velocity variations in the broad and the narrow line components, consistent with rotational modulation. The modulation is stronger for lines with higher excitation potentials (e.g. He II), which are likely produced in a confined area very close to the accretion shock. Conclusions: We propose that the narrow line components are produced in the post-shock region, while the broad components originate in the more extended, pre-shock material in the accretion column. All the emission lines suffer velocity modulation due to the rotation of the star. The broad components are responsible for the line-dependent veiling observed in EX Lupi. We demonstrate that a rotationally modulated line-dependent veiling can explain the radial velocity signature of the photospheric absorption lines, making the close-in companion hypothesis unnecessary. The accretion structure is locked to the star and very stable during the five years of observations. Not all stars with similar spectral types and accretion rates show the same metallic emission lines, which could be related to differences in temperature and density in their accretion structure(s). The contamination of photospheric signatures by accretion

  6. Where a Neutron Star's Accretion Disk Ends

    NASA Astrophysics Data System (ADS)

    Kohler, Susanna

    2016-03-01

    In X-ray binaries that consist of a neutron star and a companion star, gas funnels from the companion into an accretion disk surrounding the neutron star, spiraling around until it is eventually accreted. How do the powerful magnetic fields threading through the neutron star affect this accretion disk? Recent observations provide evidence that they may push the accretion disk away from the neutron stars surface.Truncated DisksTheoretical models have indicated that neutron star accretion disks may not extend all the way in to the surface of a neutron star, but may instead be truncated at a distance. This prediction has been difficult to test observationally, however, due to the challenge of measuring the location of the inner disk edge in neutron-star X-ray binaries.In a new study, however, a team of scientists led by Ashley King (Einstein Fellow at Stanford University) has managed to measure the location of the inner edge of the disk in Aquila X-1, a neutron-star X-ray binary located 17,000 light-years away.Iron line feature detected by Swift (red) and NuSTAR (black). The symmetry of the line is one of the indicators that the disk is located far from the neutron star; if the inner regions of the disk were close to the neutron star, severe relativistic effects would skew the line to be asymmetric. [King et al. 2016]Measurements from ReflectionsKing and collaborators used observations made by NuSTAR and Swift/XRT both X-ray space observatories of Aquila X-1 during the peak of an X-ray outburst. By observing the reflection of Aquila X-1s emission off of the inner regions of the accretion disk, the authors were able to estimate the location of the inner edge of the disk.The authors find that this inner edge sits at ~15 gravitational radii. Since the neutron stars surface is at ~5 gravitational radii, this means that the accretion disk is truncated far from the stars surface. In spite of this truncation, material still manages to cross the gap and accrete onto the

  7. Pressure regimes and core formation in the accreting earth

    NASA Technical Reports Server (NTRS)

    Newsom, H. E.

    1992-01-01

    Recent work suggests that a large degree of melting is required to segregate metal from silicates, suggesting a connection with the formation of magma oceans. At low pressures metallic liquids do not wet silicate minerals, preventing the metal from aggregating into large masses that can sink. At high pressures, above 25 GPa, the dihedral angles of grains in contact with oxygen-rich metallic liquids may be reduced enough to allow percolation of metal, but this has not been confirmed. Physical models of core formation and accretion may therefore involve the formation of magma oceans and the segregation of metal at both high and low pressures. Models of core formation involving different pressure regimes are discussed as well as chemical evidence bearing on the models. Available geophysical data is ambiguous. The nature of the 670 km boundary (chemical difference or strictly phase change) between the upper and lower mantle is in doubt. There is some evidence that plumes are derived from the lower mantle, and seismic tomography strongly indicates that penetration of subducting oceanic crust into the lower mantle, but the tomography data also indicates that the 670 km discontinuity is a significant barrier to general mantle convection. The presence of the D' layer at the base of the lower mantle could be a reaction zone between the mantle and core indicating core-mantle disequilibrium, or D' layer could be subducted material. The abundance of the siderophile elements in the mantle could provide clues to the importance of high pressure processes in Earth, but partition coefficients at high pressures are only beginning to be measured.

  8. Formation of the Late Palaeozoic Konya Complex and comparable units in southern Turkey by subduction-accretion processes: Implications for the tectonic development of Tethys in the Eastern Mediterranean region

    NASA Astrophysics Data System (ADS)

    Robertson, Alastair H. F.; Ustaömer, Timur

    2009-07-01

    . Both the Permian and the Triassic sediments pass upwards without a break into a Middle Triassic-Upper Cretaceous platform carbonate succession, which was overthrust by accretionary melange and ophiolites during latest Cretaceous time. The Mesozoic carbonate platform and the underlying Konya Complex experienced polyphase deformation and partial high-temperature/low-pressure metamorphism related to Alpine (Late Cretaceous-Early Cenozoic) closure of Tethys. Comparison of Tauride and Pontide units suggests the deformed Konya Complex Upper Palaeozoic carbonate platform has Gondwanan affinities. We also compare the Complex with other Upper Palaeozoic units in southern Turkey, including the Tekedere unit (Lycian Nappes) and the Karaburun and Chios melanges. All of these units are interpreted to have formed by subduction/accretion processes. Alternative possible settings involve northward subduction, southward subduction, or terrane displacement (strike-slip). Northward subduction beneath Eurasia requires collisional assembly with Gondwana, possibly during the latest Triassic "Cimmerian orogeny" for which there is little evidence. A southward-dipping subduction zone would need to be located some distance outboard of the Gondwana margin as there is little evidence of Upper Palaeozoic arc magmatism in the Tauride platform. Emplacement as an exotic terrane, probably derived from further west in the Balkan region is also possible.

  9. Impact of erosion and accretion on the distribution of enterococci in beach sands

    NASA Astrophysics Data System (ADS)

    Gast, Rebecca J.; Gorrell, Levi; Raubenheimer, Britt; Elgar, Steve

    2011-09-01

    Bacterial pathogens in coastal sediments may pose a health risk to users of beaches. Although recent work shows that beach sands harbor both indicator bacteria and potential pathogens, it is neither known how deep within beach sands the organisms may persist nor if they may be exposed during natural physical processes. In this study, sand cores of approximately 100 cm depth were collected at three sites across the beach face in Kitty Hawk, North Carolina, before, during, and after large waves from an offshore hurricane. The presence of DNA from the fecal indicator bacterium Enterococci was detected in subsamples at different depths within the cores by PCR amplification. Erosion and accretion of beach sand at the three sites were also determined for each sampling day. The results indicate that ocean beach sands with persisting enterococci signals could be exposed and redistributed when wind, waves, and currents cause beach erosion or accretion.

  10. Planetary growth by the accretion of pebbles

    NASA Astrophysics Data System (ADS)

    Lambrechts, Michiel; Johansen, Anders; Bitsch, Bertram; Morbidelli, Alessandro

    2015-11-01

    Pebbles, approximately cm-sized solids that drift through a protoplanetary disc, provide a reservoir of material that can be efficiently accreted by planetary embryos due to the dissipating effect of gas drag (Lambrechts & Johansen, 2012).Here, we will highlight the robust implications of pebble accretion on the formation of planets throughout the protoplanetary disc.In the outer disc, icy pebbles form by coagulation and consequently start drifting inwards. Nevertheless, we find that the pebble surface densities are sufficiently high to form giant planets on wide orbits, before the gas disc disperses after a few Myr (Lambrechts & Johansen, 2014). Growth is only halted when cores reach sizes of around 10 Earth masses, when their gravity creates pressure bumps trapping the inwards drifting pebbles.This accretion cutoff triggers the attraction of a massive gaseous envelope. Additionally, the fast growth of giant planets prevents the loss of the cores by type-I migration (Lambrechts et al 2014, Bitsch et al 2015).Closer to the star, interior to the ice line, pebble accretion takes on a different form. There, chondrule-sized particles lead to the formation of much smaller, Mars-sized embryos, before the pebble flux is terminated by the growth of the gas giants (Morbidelli et al, 2015). We will also discuss ongoing work on the conditions under which much larger Super-Earths can form.