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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. Future accreted terranes: a compilation of island arcs, oceanic plateaus, submarine ridges, seamounts, and continental fragments

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

    Allochthonous accreted terranes are exotic geologic units that originated from anomalous crustal regions on a subducting oceanic plate and were transferred to the overriding plate by accretionary processes during subduction. The geographical regions that eventually become accreted allochthonous terranes include island arcs, oceanic plateaus, submarine ridges, seamounts, continental fragments, and microcontinents. These future allochthonous terranes (FATs) contribute to continental crustal growth, subduction dynamics, and crustal recycling in the mantle. We present a review of modern FATs and their accreted counterparts based on available geological, seismic, and gravity studies and discuss their crustal structure, geological origin, and bulk crustal density. Island arcs have an average crustal thickness of 26 km, average bulk crustal density of 2.79 g cm-3, and three distinct crustal units overlying a crust-mantle transition zone. Oceanic plateaus and submarine ridges have an average crustal thickness of 21 km and average bulk crustal density of 2.84 g cm-3. Continental fragments presently on the ocean floor have an average crustal thickness of 25 km and bulk crustal density of 2.81 g cm-3. Accreted allochthonous terranes can be compared to these crustal compilations to better understand which units of crust are accreted or subducted. In general, most accreted terranes are thin crustal units sheared off of FATs and added onto the accretionary prism, with thicknesses on the order of hundreds of meters to a few kilometers. However, many island arcs, oceanic plateaus, and submarine ridges were sheared off in the subduction interface and underplated onto the overlying continent. Other times we find evidence of terrane-continent collision leaving behind accreted terranes 25-40 km thick. We posit that rheologically weak crustal layers or shear zones that were formed when the FATs were produced can be activated as detachments during subduction, allowing parts of the FAT

  3. Future accreted terranes: a compilation of island arcs, oceanic plateaus, submarine ridges, seamounts, and continental fragments

    NASA Astrophysics Data System (ADS)

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

    2014-07-01

    Allochthonous accreted terranes are exotic geologic units that originated from anomalous crustal regions on a subducting oceanic plate and were transferred to the overriding plate during subduction by accretionary processes. The geographical regions that eventually become accreted allochthonous terranes include island arcs, oceanic plateaus, submarine ridges, seamounts, continental fragments, and microcontinents. These future allochthonous terranes (FATs) contribute to continental crustal growth, subduction dynamics, and crustal recycling in the mantle. We present a review of modern FATs and their accreted counterparts based on available geological, seismic, and gravity studies and discuss their crustal structure, geological origin, and bulk crustal density. Island arcs have an average crustal thickness of 26 km, average bulk crustal density of 2.79 g cm-3, and have 3 distinct crustal units overlying a crust-mantle transition zone. Oceanic plateaus and submarine ridges have an average crustal thickness of 21 km and average bulk crustal density of 2.84 g cm-3. Continental fragments presently on the ocean floor have an average crustal thickness of 25 km and bulk crustal density of 2.81 g cm-3. Accreted allochthonous terranes can be compared to these crustal compilations to better understand which units of crust are accreted or subducted. In general, most accreted terranes are thin crustal units sheared off of FATs and added onto the accretionary prism, with thicknesses on the order of hundreds of meters to a few kilometers. In addition many island arcs, oceanic plateaus, and submarine ridges were sheared off in the subduction interface and underplated onto the overlying continent. And other times we find evidence of collision leaving behind accreted terranes 25 to 40 km thick. We posit that rheologically weak crustal layers or shear zones that were formed when the FATs were produced can be activated as detachments during subduction, allowing parts of the FAT crust to

  4. 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.

  5. 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

  6. 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.

  7. Pillow basalts of the Angayucham terrane: Oceanic plateau and island crust accreted to the Brooks Range

    NASA Astrophysics Data System (ADS)

    Pallister, John S.; Budahn, James R.; Murchey, Benita L.

    1989-11-01

    from obduction faulting, but the lack of fault slabs of gabbro or peridotite suggests that obduction faults did not penetrate below oceanic layer 2, a likely occurrence if layer 2 were anomalously thick, as in the vicinity of an oceanic island. The presence of basaltic tuff interbeds indicates proximity to an explosive basaltic eruptive center. The juxtaposition of submarine basalts of differing chemical affinity and age, adjacent to higher-grade Paleozoic metamorphic rocks of the Brooks Range to the north, may be explained by obduction of internally complex (thickened) oceanic crust formed in an ocean plateau setting. Emplacement and rotation of thrust plates to steep attitudes occurred during accretion of the Brooks Range passive margin, probably beginning in the Late to Middle Jurassic.

  8. Pillow basalts of the Angayucham terrane: oceanic plateau and island crust accreted to the Brooks Range

    USGS Publications Warehouse

    Pallister, J.S.; Budahn, J.R.; Murchey, B.L.

    1989-01-01

    The Angayucham Mountains (north margin of the Yukon-Koyukuk province) are made up of an imbricate stack of four to eight east-west trending, steeply dipping, fault slabs composed of Paleozoic, Middle to Late Triassic, and Early Jurassic oceanic upper crustal rocks. Field relations and geochemical characteristics of the basaltic rocks suggest that the fault slabs were derived from an oceanic plateau or island setting and were emplaced onto the Brooks Range continental margin. The basalts are variably metamorphosed to prehnite-pumpellyite and low-greenschist facies. Major element analyses suggest that many are hypersthene-normative olivine tholeiites. The Triassic and Jurassic basalts are geochemically most akin to modern oceanic plateau and island basalts. Field evidence also favors an oceanic plateau or island setting. The great composite thickness of pillow basalt probably resulted from obduction faulting, but the lack of fault slabs of gabbro or peridotite suggests that obduction faults did not penetrate below oceanic layer 2, a likely occurrence if layer 2 were anomalously thick, as in the vicinity of an oceanic island. -from Authors

  9. 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

  10. 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

  11. The Guerrero suspect terrane (western Mexico) and coeval arc terranes (the Greater Antilles and the Western Cordillera of Colombia): a late Mesozoic intra-oceanic arc accreted to cratonal America during the Cretaceous

    NASA Astrophysics Data System (ADS)

    Tardy, M.; Lapierre, H.; Freydier, C.; Coulon, C.; Gill, J.-B.; de Lepinay, B. Mercier; Beck, C.; Martinez R., J.; O. Talavera, M.; E. Ortiz, H.; Stein, G.; Bourdier, J.-L.; Yta, M.

    1994-02-01

    The Guerrero suspect terrane, composed of Late Jurassic-Early Cretaceous sequences, extends from Baja California to Acapulco and is considered to be coeval with the late Mesozoic igneous and sedimentary arc sequences of the Greater Antilles, the West Indies, Venezuela and the Western Cordillera of Colombia. These sequences represent the remnants of an arc which accreted to the North American and northern South American cratons at the end of the Cretaceous. In western Mexico, the arc sequences built on continental crust consist of high-K calc-alkaline basalts, andesites and rhyolites enriched in LREE with abundant siliceous pyroclastic rocks interbedded either with Aptian-Albian reefal limestones or red beds. They do not show magmatic changes during the arc development. In contrast, the arc sequences built on oceanic crust show an evolution with time. Arc activity began with the development of depleted low K-tholeiitic mafic suite (Guanajuato igneous sequence), followed first by mature tholeiitic basalts and then by calc-alkaline olivine basalts interbedded with micritic limestones and radiolarian oozes of Early Cretaceous age. At the end of the arc growth, during Aptian-Albian times, calc-alkaline pillow basalts and and esites poured out in the volcanic front while shoshonitic olivine basalts extruded in the back arc. The tholeiitic and shoshonitic mafic rocks as well as the calc-alkaline lavas are mildly enriched in LREE, Y and Nb and show high ɛNd ratios, typical of oceanic arcs. In contrast, the calc-alkaline mafic suite enriched in LREE, Y and Nb exhibits lower ɛNd ratios suggesting that it was derived by the partial melting of a mantle source contaminated either by Paleozoic subducted sediments or old source enrichments (OIB). The Cretaceous arc rocks of the Greater Antilles, interbedded with and/or capped by Aptian-Albian limestones, the Cretaceous andesites of northern Colombia, the Cretaceous tholeiitic and calc-alkaline volcanic rocks of Venezuela, and

  12. 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).

  13. Continental collision with a sandwiched accreted terrane: Insights into Himalayan-Tibetan lithospheric mantle tectonics?

    NASA Astrophysics Data System (ADS)

    Kelly, Sean; Butler, Jared P.; Beaumont, Christopher

    2016-12-01

    Many collisional orogens contain exotic terranes that were accreted to either the subducting or overriding plate prior to terminal continent-continent collision. The ways in which the physical properties of these terranes influence collision remain poorly understood. We use 2D thermomechanical finite element models to examine the effects of prior 'soft' terrane accretion to a continental upper plate (retro-lithosphere) on the ensuing continent-continent collision. The experiments explore how the style of collision changes in response to variations in the density and viscosity of the accreted terrane lithospheric mantle, as well as the density of the pro-lithospheric mantle, which determines its propensity to subduct or compress the accreted terrane and retro-lithosphere. The models evolve self-consistently through several emergent phases: breakoff of subducted oceanic lithosphere; pro-continent subduction; shortening of the retro-lithosphere accreted terrane, sometimes accompanied by lithospheric delamination; and, terminal underthrusting of pro-lithospheric mantle beneath the accreted terrane crust or mantle. The modeled variations in the properties of the accreted terrane lithospheric mantle can be interpreted to reflect metasomatism during earlier oceanic subduction beneath the terrane. Strongly metasomatized (i.e., dense and weak) mantle is easily removed by delamination or entrainment by the subducting pro-lithosphere, and facilitates later flat-slab underthrusting. The models are a prototype representation of the Himalayan-Tibetan orogeny in which there is only one accreted terrane, representing the Lhasa terrane, but they nonetheless exhibit processes like those inferred for the more complex Himalayan-Tibetan system. Present-day underthrusting of the Tibetan Plateau crust by Indian mantle lithosphere requires that the Lhasa terrane lithospheric mantle has been removed. Some of the model results support previous conceptual interpretations that Tibetan

  14. Caledonian and Proterozoic terrane accretion in the southwest Baltic Sea

    NASA Astrophysics Data System (ADS)

    Meissner, R.; Krawczyk, C. H.

    1999-12-01

    A marine seismic reflection survey was carried out in 1996 by DEKORP, Potsdam, and BGR, Hannover in the SW Baltic Sea. Several tectonic lineaments were crossed nearly perpendicularly, for example, the Caledonian Deformation Front which is assumed to mark the northern boundary of the terrane Avalonia which accreted to Baltica ca. 400 Ma. Here, a bivergent collision is clearly observed, confirming early ideas from the BABEL survey. The NE-dipping reflections in the uppermost mantle are interpreted as signs of the subducted Tornquist Ocean. A similar tectonic style of compression and indentation is observed in the Proterozoic crust northeast of Bornholm, where in addition a remarkable crustal thickening and a strong increase of reflection power is observed. The three DEKORP lines in this area provide a certain three-dimensional control and allows extension of similar observations from the BABEL line A southward. This Proterozoic terrane accretion seems to be connected to major tectonic lineaments in southern Sweden, either to the Gothian Thrust or the Sveconorwegian Front.

  15. Accreted terranes and mineral deposits of Myanmar

    NASA Astrophysics Data System (ADS)

    Fan, Pow-foong; Ko, Ko

    There are three terranes in Myanmar: (1) Shan-West Malaysia-Sumatra, (2) Central Burma Basin, and (3) Arakan Yoma. The Shan-West Malaysia-Sumatra terrane can be divided into three subterranes: (a) West Kachin, (b) East Kachin-Shan, and (c) Karen-Tenasserim. The Shan-West Malaysia-Sumatra terrane consists of jadeite of gem quality in the West Kachin subterrane ruby and other gems, Paleozoic strata-bound lead-zinc and iron deposits in the East Kachin-Shan subterrane; and a tin and tungsten mineralized belt in the Karen-Tenasserim subterrane. The volcanic arc divides the Central Myanmar Basin terrane into forearc and back-arc basins; the oil-bearing fields are located in the forearc basin. In the Arakan Yoma terrane, chromium and nickel of late Cretaceous-early Tertiary age occur within ultramafic belts.

  16. Oceanic mafic magmatism in the Siletz terrane, NW North America: Fragments of an Eocene oceanic plateau?

    NASA Astrophysics Data System (ADS)

    Phillips, Bethan A.; Kerr, Andrew C.; Mullen, Emily K.; Weis, Dominique

    2017-03-01

    The Siletz terrane, a predominantly mafic accreted oceanic terrane, is located in the Cascadia forearc region of Oregon, Washington and Vancouver Island. The terrane represents a late Palaeocene-Eocene large igneous province that consists of pillow lavas, massive flows and intrusive sheets. Previously it has been proposed that the Siletz terrane represents either an accreted oceanic plateau, hotspot island chain, backarc basin, island arc, or a sequence of slab window volcanics. A province-wide geochemical reassessment of the terrane, including new high precision Sr-Pb-Nd-Hf isotope data, has been used to assess the validity of the proposed tectonomagmatic models for the Siletz terrane. The trace element data show little evidence of crustal contamination, or an arc signature, and the samples have rare earth element (REE) patterns that are flat to light REE enriched. These features are similar to other oceanic plateaus such as the Ontong Java and the Caribbean. Initial isotope ratios range from 206Pb/204 Pb: 18.751 to 19.668, 207Pb/204Pb: 15.507 to 15.661, 208Pb/204Pb: 38.294 to 39.2128, 176Hf/177Hf: 0.28300 to 0.28316 (εHf: 9.0 to 14.5), 143Nd/144Nd: 0.51282 to 0.51299 (εNd: 5.0 to 8.1) and 87Sr/86Sr: 0.70302 to 0.70380. These data are consistent with a mantle source of the Siletz terrane that appears to have been heterogeneous and slightly enriched. The enriched signature has characteristics of both EM2 and HIMU components and this, combined with a calculated mantle potential temperature well above ambient mantle, indicates derivation of the Siletz magmatism from a mantle plume, possibly the Yellowstone Hotspot. We therefore conclude that the Siletz terrane represents an accreted oceanic plateau.

  17. Middle Jurassic terrane accretion along the western edge of the Intermontane superterrane, southwestern British Columbia

    NASA Astrophysics Data System (ADS)

    Rusmore, Margaret E.; Potter, Christopher J.; Umhoefer, Paul J.

    1988-10-01

    Two small lower Mesozoic terranes, the Bridge River and Cadwallader, lie along the southwestern margin of the Intermontane superterrane and represent fragments of a volcanic arc and marginal basin that bordered North America in the early Mesozoic. During Middle Jurassic time, these terranes were juxtaposed and deformed. This event was synchronous with deformation in northern and central British Columbia, and it probably records accretion of the Cadwallader and Stikine volcanic arcs against Quesnellia during closure of the Bridge River-Cache Creek ocean basin.

  18. Deformation during terrane accretion in the Saint Elias orogen, Alaska

    USGS Publications Warehouse

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

    2004-01-01

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

  19. 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.

  20. 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

  1. Late Jurassic adakitic granodiorite in the Dong Co area, northern Tibet: Implications for subduction of the Bangong-Nujiang oceanic lithosphere and related accretion of the Southern Qiangtang terrane

    NASA Astrophysics Data System (ADS)

    Fan, Jian-Jun; Li, Cai; Wu, Hao; Zhang, Tian-Yu; Wang, Ming; Chen, Jing-Wen; Xu, Jian-Xin

    2016-11-01

    In this paper we present new major and trace element chemical data, zircon U-Pb age data, and zircon Hf isotopic compositions for the Dong Co granodiorites (DGs) that were emplaced directly in the Dong Co ophiolites (DOs) in the middle segment of the Bangong-Nujiang suture zone (BNSZ), northern Tibet. We use these new data to discuss the genesis of the DGs and implications for the evolution of the region. The DGs have high contents of SiO2, Al2O3, Na2O, and Sr, high values of Mg#, low contents of Yb and Y, and markedly high Sr/Y ratios, indicating an adakitic affinity, and we show that they were derived from the partial melting of subducted oceanic crust. The DGs contain large numbers of inherited zircons that have similar shapes and peak ages to the detrital zircons in the accretionary wedge graywackes that surround the DGs, suggesting that many of these sediments were assimilated during the formation of the DGs. Five DG samples yield the youngest zircon U-Pb ages of 155-160 Ma, indicating that the DGs formed during the Late Jurassic. Based on the present results and regional geological data, we infer that the Bangong-Nujiang oceanic lithosphere was subducted northwards beneath the Southern Qiangtang terrane during the formation of a continental arc system during the Late Jurassic. The Bangong-Nujiang oceanic lithosphere retreated rapidly after its initial subduction, and the Southern Qiangtang terrane to the north of the Bangong-Nujiang Ocean acquired a massive amount of new oceanic material in the form of an accretionary wedge. This wedge, which included the DOs and the Mugagangri Group, became sufficiently large (both in size and width) for arc-type magmas (DGs) to develop within it during the Late Jurassic.

  2. Mesozoic oceanic terranes - key to our understanding of Meso-Pacific History

    NASA Astrophysics Data System (ADS)

    Krylov, K. A.

    2004-12-01

    Along the eastern Pacific margin of Asia Mesozoic oceanic complexes are widely distributed. The structure and geometry of these accreted oceanic complexes vary from: (1) dismembered ophiolite nappes; (2) blocks in serpentinite melange and olistostromes; (3) offscraped fragments of the oceanic crust in a sheared argillite matrix which is interpreted to represent an accretionary prism sequence. The sedimentary strata associated with the oceanic complexes are mostly various types of siliceous sediments: radiolaria chert and jasper, grey cherts and siliceouc argillite and in lesser quantities, limestones. In many ophiolitic sections, sediments form the cover sequences of the various MORB-type lavas. The following sedimentary sequences have been mapped and described in the Koryak Mountains: The Pekul'ney Ridge, Mainitz, Alkatvaam, Ekonay, Kuyul (Penzhina) Ukelayat and Olutor terranes. Similar sequences are known from Kamchatka, Sakhalin, Taigonos and Elistratov Peninsulas, in Udskaya Guba, Priamurie, and in Sikhote-Alin. The composition and character of microfauna in these mostly pelagic sequences indicate accumulation of siliceous strata at lower latitudes than their present localities (Bragin, 1991; Vishnevskaya, 1990). Paleomagnetic data available on the oceanic complexes tend to support their allochthoneity, and indicate significant northward motion. As such, these oceanic complexes are particularly important for paleotectonic reconstructions of the Paleo Pacific ocean floor. Analysis of all data and paleomagnetic results from Late Jurassic - to Cretaceous accreted oceanic units allow one to conclude that the kinematics of the Koryak and Kamchatka oceanic terranes and their accretion is defined by the presence and interactions of four oceanic plates, the Izanagi, Farallon, Kula and Pacific plates with the continental margins of North American and Siberian continents. The older, Triassic - to Middle Jurassic complexes are the main source of information on the

  3. Rise and Fall of the Eastern Great Indonesian Arc Recorded by the Assembly, Dispersion and Accretion of the Banda Terrane, Timor.

    NASA Astrophysics Data System (ADS)

    Harris, R.

    2006-12-01

    New age, petrochemical and structural data indicate that the Banda Terrane is a remnant of a Jurassic to Eocene arc trench system that formed the eastern part of the Great Indonesian arc. The arc system rifted apart during Eocene to Miocene supra-subduction zone sea floor spreading, which dispersed ridges of Banda Terrane embedded in young oceanic crust as far south as Sumba and Timor. In Timor the Banda Terrane is well exposed as high-level thrust sheets that were detached from the edge of the Banda Sea upper plate and uplifted by thrust understacking associated with Late Miocene to Present arc-continent collision with the passive margin of NW Australia. Banda Terrane thrust sheets contain a distinctive assemblage of medium grade metamorphic rocks overlain by Cretaceous to Miocene forearc basin deposits. New U/Pb age data indicate igneous zircons are less than 162 Ma with a cluster of ages at 83 Ma and 35 Ma. 40Ar/39Ar plateau ages of various mineral phases all cluster between 32 39 Ma. These data yield a cooling curve that shows exhumation from around 550 °C to the surface between 36 28 Ma. After this time there is no evidence of metamorphism of the Banda Terrane, including its accretion to the edge of the Australian continental margin during the Pliocene. These data link the Banda Terrane to similar rocks and events documented throughout the eastern edge of the Sunda Shelf and the Banda Sea floor. Closing of the Banda Sea basins puts the allochthonous part of the Banda Terrane on a collision course with autochthonous Banda Terrane fragments imbedded in the Banda Sea floor, and eventually with Sulawesi, where most of these fragments started their journey over 50 m.y. ago.

  4. 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.

  5. 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.

    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

  6. From Back-Arc Drifting to Arc Accretion: the Late Jurassic-Early Cretaceous Evolution of the Guerrero Terrane in Central Mexico (Sierra de Guanajuato)

    NASA Astrophysics Data System (ADS)

    Martini, M.; Solari, L.; Centeno-García, E.; Mori, L.; Camprubi, A.

    2011-12-01

    Three paleogeographic scenarios have been proposed for the Mesozoic volcano-sedimentary successions that compose the Guerrero terrane, western Mexico. In the "type 1" scenario the Guerrero terrane is an exotic Pacific arc accreted to nuclear Mexico by the consumption of a pre-Cretaceous oceanic basin, named Arperos Basin. The "type 2" scenario considers the Guerrero terrane as a fringing multi-arc system, accreted by the closure of relatively small pre-Cretaceous oceanic basins at multiple subduction zones with varying polarities. Alternatively, in the "type 3" scenario the Guerrero terrane is interpreted as a North American west-facing para-autochthonous arc, which drifted into the paleo-Pacific domain by the opening of the Cretaceous back-arc oceanic Arperos Basin, and subsequently accreted back to the Mexican mainland. In order to test these reconstructions and understand the dynamics of the arc accretion, we present here a combined study that includes sandstone provenance, U-Pb geochronology, and structural data from the Arperos Basin in the Sierra de Guanajuato, central Mexico. Our data document that the Arperos Basin developed in a back-arc setting, and evolved from continental to oceanic conditions from the Late Jurassic to the Early Cretaceous. Sandstone provenance analysis shows an asymmetric distribution of the infill sources for the Arperos Basin: continent-recycled sedimentary rocks were deposited along its north-eastern side, whereas magmatic arc-recycled clastic rocks developed at its south-western side. Such an asymmetric distribution closely fits with sedimentological models proposed for present-day continent-influenced back-arc basins. Based on these evidences, we favor a "type 3" scenario for the Guerrero terrane, which is then considered to represent a detached slice of the Mexican leading-edge that drifted in the paleo-Pacific domain during Late Jurassic-lower Early Cretaceous back-arc extension, and subsequently accreted back to the Mexican

  7. Hydrothermal alteration of a seamount complex on La Palma, Canary Islands: Implications for metamorphism in accreted terranes

    NASA Astrophysics Data System (ADS)

    Schiffman, P.; Staudigel, H.

    1994-02-01

    The hydrothermal metamorphism of a sequence of Pliocene-age seamount extrusive and volcaniclastic rocks on La Palma, Canary Islands, is characterized by a relatively complete low-pressure-high-temperature facies series encompassing the zeolite, prehnite-pumpellyite, and greenschist facies. The observed mineral zonations imply metamorphic gradients of 200-300 °C/km. The metamorphism of the seamount, at least in its core region, is distinct from ocean-floor metamorphism: the former is characterized by a serially continuous facies series encompassing zeolite, prehnite-pumpellyite, and greenschist assemblages, and the latter by a discontinuous metamorphic gradient in which prehnite-pumpellyite assemblages are absent. These metamorphic features, presumably reflecting fundamental thermal-tectonic differences between extending oceanic crust at mid- oceanic ridges vs. the more static crust underlying seamount volcanoes, should aid in the recognition of incoherent fragments of seamount metamorphic rocks within accreted terranes which typically have undergone subsequent higher pressure-temperature regional metamorphism, albeit to comparable grades.

  8. Cordilleran suspect terranes

    USGS Publications Warehouse

    Coney, P.J.; Jones, D.L.; Monger, J.W.H.

    1980-01-01

    Over 70% of the North American Cordillera is made up of 'suspect terranes'. Many of these geological provinces are certainly allochthonous to the North American continent and seem to have been swept from far reaches of the Pacific Ocean before collision and accretion into the Cordilleran margin mostly in Mesozoic to early Cenozoic time. ?? 1980 Nature Publishing Group.

  9. 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

  10. Paleoproterozoic Cordilleran-style accretion along the south eastern margin of the eastern Dharwar craton: Evidence from the Vinjamuru arc terrane of the Krishna orogen, India

    NASA Astrophysics Data System (ADS)

    Chatterjee, Chiranjeeb; Vadlamani, Ravikant; Kaptan, Om Prakash

    2016-10-01

    Cordilleran-style accretion involving development of an intra-oceanic arc due to ocean-ward migration following the earlier choking of the subduction zone at an active continental margin, caused probably by the North China crustal ribbon that had by ~ 1.78 Ga accreted to the EDC margin. The formation of the outboard intra-oceanic Ongole arc terrane occurred thereafter and was eventually accreted (and metamorphosed) to the Vinjamuru arc terrane between ~ 1.63 and 1.55 Ga to form the two arc terranes of the Krishna orogen; we discount any continental collision at this stage as tectonics along this margin, post 1.5 Ga, switched to an extensional regime.

  11. Distribution of mineral deposits in accreted terranes and cratonal rocks of western United States.

    USGS Publications Warehouse

    Albers, J.P.

    1983-01-01

    The western margin of the USA, covering approx 777 000 km2, is an agglomeration of tectonostratigraphic terrains accreted to the North American craton mainly during Mesozoic time. The terrains represent a number of fundamental crustal types: oceanic crust, island-arc crust, melange, various combinations of the preceding three, batholithic, miogeoclinal and platform. The distribution patterns of types of mineral deposits show that miogeoclinal terrains of the craton are characterized by replacement and vein-type Pb-Zn-Ag, skarn W deposits, Mo and Sn, whereas accreted terrains contain all the known volcanic massive sulphide deposits, all chromite and chert-associated Mn, and all the large Au quartz-vein deposits, except Goldfield, Nevada. Carlin-type disseminated fine-grained Au deposits occur mostly in windows of Palaeozoic miogeoclinal rocks in Nevada, but the only known fine-grained Au deposit in California is in very youthful volcanic rocks overlying oceanic-crust terrain. Large bedded-type baryte deposits, although in the same area and showing the same trend as disseminated Au in Nevada, are in allochthonous oceanic terrain. Hg and Sb are dominantly in accreted terrains, but Sb also forms important deposits in cratonal rocks. Most of the large Fe deposits are in the craton, but a few are in accreted island-arc terrains.-P.Br.

  12. 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.

  13. Provenance changes for sandstones from the Sierra de los Cuarzos (central Mexico): the possible record of a terrane accretion

    NASA Astrophysics Data System (ADS)

    Palacios García, N. B.; Martini, M.

    2012-04-01

    The Guerrero terrane is composed of Late Jurassic-Early Cretaceous arc successions exposed along the western Pacific margin of Mexico. Several lines of evidence indicate that the Guerrero terrane represents the Mexican leading-edge of the North American plate, which was drifted in the paleo-Pacific domain during Late Jurassic-Early Cretaceous back-arc spreading, and subsequently accreted back to the Mexican continental core before the Albian. In this paper, we present new stratigraphic data and a detailed provenance analysis of sandstones from the Sierra de Los Cuarzos area, which is located in central Mexico, ~50 km to the east of the Guerrero terrane suture belt. In the Sierra de Los Cuarzos is exposed a Mesozoic succession composed of: 1) quartz-rich turbidites grading upward to 2) calcareous slump deposits, which are overlain by 3) volcaniclastic sandstone and scarce conglomerate hosting 20 cm- to 100 m-wide blocks and slabs of basalt. Sandstone provenance and paleocurrent marks indicate that turbidites and slumps deposits were fed by sources from the Mexican continental core. Similar Late Jurassic-Early Cretaceous turbidites and calcareous slump deposits are exposed in the Sierra de Guanajuato, ~50 km to the west of the Sierra de Los Cuarzos area, and are preliminarily correlated with the lower units (1 and 2) of the study area. On the other hand, provenance analysis indicates that volcaniclastic sandstones from unit 3 were principally fed by the arc successions exposed in the Guerrero terrane. The drastic change in provenance of sandstones from the Sierra de los Cuarzos suggests a complex depositional history, characterized by the contribution of distinct supplying sources during the infilling of the basin. In this paper, it is explored the possibility of a syn-tectonic origin for the volcaniclastic rocks of the Sierra de Los Cuarzos, related to the accretion of the Guerrero terrane to the Mexican continental core.

  14. 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.

  15. The Precambrian terranes of Yemen and their correlation with those of Saudi Arabia and Somalia: Implications for the accretion of Gondwana

    USGS Publications Warehouse

    Windley, B.F.; Whitehouse, M.J.; Stoeser, D.B.; Al-Khirbash, S.; Ba-Bttat, M. A. O.; Al-Ghotbah, A.

    2001-01-01

    Most of the basement of Yemen consists of early Precambrian continental high-grade terranes and Neoproterozoic low-grade island arcs that were accreted together to form an arc-continent collage during the Pan-African orogeny (Windley et al., 1996; Whitehouse et al., 1998; Whitehouse et al., in press). The suture zones between the arc and gneiss terranes are major crustal- scale tectonic boundaries. The terranes are situated east of the Nabitah suture and of the collage of low-grade, mainly island arc terranes of the Arabian Shield, but they have been reworked by a Neoproterozoic event associated with island arc accretion. Further east in Yemen are mostly unconformable, very weakly deformed and very low-grade or unmetamorphosed sediments. Thus Yemen provides key information on the broad zone of Neoproterozoic reworking associated with the collisional boundary between western and eastern Gondwana. 

  16. 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

  17. 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.

  18. 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.

  19. Ocean plateau-seamount origin of basaltic rocks, Angayucham terrane, central Alaska

    USGS Publications Warehouse

    Barker, F.; Jones, D.L.; Budahn, J.R.; Coney, P.J.

    1988-01-01

    The Angayucham terrane of north-central Alaska (immediately S of the Brooks Range) is a large (ca. 500 km E-W), allochthonous complex of Devonian to Lower Jurassic pillow basalt, diabase sills, gabbro plutons, and chert. The mafic rocks are transitional normal-to-enriched, mid-ocean-ridge (MORB) type tholeiites (TiO2 1.2-3.4%, Nb 7-23 ppm, Ta 0.24-1.08 ppm, Zr 69-214 ppm, and light REE's slightly depleted to moderately enriched). Geologic and geochemical constraints indicate that Angayucham terrane is the upper "skin' (ca. 3-4 km thick) of a long-lived (ca. 170-200 ma) oceanic plateau whose basaltic-gabbroic rocks are like those of seamounts of the East Pacific Rise. -Authors

  20. 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.

  1. 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.

  2. Thermochronology in southeast Alaska and southwest Yukon: Implications for North American Plate response to terrane accretion

    NASA Astrophysics Data System (ADS)

    Enkelmann, Eva; Piestrzeniewicz, Adam; Falkowski, Sarah; Stübner, Konstanze; Ehlers, Todd A.

    2017-01-01

    This study presents the first comprehensive dataset of low-temperature thermochronology from 43 bedrock samples collected north of the active Yakutat-North American plate boundary. Our apatite and zircon (U-Th)/He and fission-track data reveal the cooling history of the inboard Wrangellia Composite Terrane that is dominated by rapid cooling after Late Jurassic to Early Cretaceous arc magmatism followed by very little cooling and exhumation until today. Deformation resulting in rock exhumation due to the collision of the Yakutat microplate is spatially very limited (20-30 km) and is concentrated mainly in the Chugach-Prince William Terrane and rocks near the Border Ranges Fault. Focused exhumation from greater depths of ca. 10 km with very high rates (>5 km/Myr) is localized at the syntaxis region, starting ca. 10 Ma and shifted south through time. The rapid exhumation rates are explained by the development of strong feedbacks between tectonically driven surface uplift and erosion, which started already before glaciation of the area. The shift in the location towards the south is a consequence of continuous readjusting between tectonics and climate, which is changing on local and global scales since the Late Miocene.

  3. Early Precambrian gneiss terranes and Pan-African island arcs in Yemen: Crustal accretion of the eastern Arabian Shield

    NASA Astrophysics Data System (ADS)

    Windley, Brian F.; Whitehouse, Martin J.; Ba-Bttat, Mahfood A. O.

    1996-02-01

    Within the Precambrian of Yemen, we have identified four gneiss terranes and two island-arc terranes on the basis of existing literature, mapping, and our own field observations, together with new Sm-Nd isotopic data. The two western gneiss terranes can be correlated with well-documented terranes (Asir and Afif) in Saudi Arabia. To the east of these, the Abas and Al-Mahfid gneiss terranes yield Sm-Nd model ages (tDM) of 1.7 2.3 Ga and 1.3 2.7 Ga, respectively, and cannot be correlated with any documented terranes in Saudi Arabia. These two terranes are separated by a Pan-African island-arc terrane that has been obducted onto one or both of the gneiss terranes, and a second arc bounds the Al-Mahfid gneiss terrane to the east. Our discovery of extensive Proterozoic to late Archean gneisses in Yemen provides important constraints upon the much-discussed tectonic framework of northeast Gondwana and the rate of Pan-African crustal growth. The terranes in Yemen may be correlated with comparable terranes on the eastern margin of the Arabian Shield and in northern Somalia. Thus Yemen provides a link between the arc collage of the Arabian Shield and the gneissic Mozambique belt of East Africa.

  4. Geophysical Modeling of Tectonostratigraphic Terrane Boundaries and Crustal Structure Across a Pacific Ocean-Gulf of Mexico Transect, Southern Mexico

    NASA Astrophysics Data System (ADS)

    Urrutia-Fucugauchi, J.; Flores-Ruiz, J. H.; Spranger, M.

    2006-12-01

    Geophysical models of terrane boundaries and lithospheric structure beneath southern Mexico derived from gravity and aeromagnetic surveys are presented. The transect crosses southern Mexico from the active Pacific margin to the passive Gulf of Mexico margin, across four distinct terranes (Xolapa, Oaxaca, Juarez and Maya) with Precambrian, Paleozoic and Mesozoic basements and contrasting tectonostratigraphic records. The crust/mantle boundary displays a smooth large amplitude variation along the transect from Puerto Escondido at the Pacific margin to Los Tuxtlas-Alvarado at the Gulf of Mexico, roughly between 28 km and 44 km deep. Crustal thickness variations correspond well with inferred terrane distribution and major surface discontinuities. Suture zones are complex as a result of the kinematics of terrane accretion, contrasting crustal rheological properties, shallow level detachments, post-accretion deformation, thermal conditions and characteristics of relative terrane/plate motions. Pre-suturing characteristics of terranes including crustal structure are difficult to document because of deformation resulting from suturing and any subsequent post-accretion processes. In a simplified way, gravity anomalies from the Pacific margin to the Gulf of Mexico show: large positive 50 mgal anomaly above the continental slope units and the intrusive and metamorphic rocks of the Xolapa complex, then anomalies increasingly negative (with minimum values of -180 mgal over the Acatlan and Oaxaca metamorphics. The Juchatengo mylonitic zone is characterized by a gradient change, while minimum gravity values approximately coincide with the wide mylonitic zone north of Oaxaca city. The Juarez terrane and the region over the Sierra de Juarez is characterized by positive gradient. Finally, the Gulf coastal plain is marked by a positive anomaly in the order of -40 mgal. Geophysical models are combined with the seismic models of the Geolimex profile and used to evaluate the crustal

  5. 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.

  6. Paleostress Inversions of Post-Triassic Brittle Faults in Paleozoic Accreted Terranes and the Early Jurassic Higganum Dike, Eastern Connecticut

    NASA Astrophysics Data System (ADS)

    Smith, M. R.; Crespi, J. M.

    2012-12-01

    The geological development of a passive margin begins with the extension and attenuation of the lithosphere and ultimately leads to seafloor spreading. This is commonly referred to as the rift-drift tectonic model. Geological observations and tectonic modeling recognize the process to be significantly more complex than described. Reverse faults and folds in early Mesozoic rift basins of eastern North America are evidence for a compressional event occurring sometime around the rift-drift transition. While most research studies have concentrated on structures in synrift strata, we concentrate on brittle structures found in crystalline bedrock. Our study focuses on eastern Connecticut and includes fault-slip data from various Paleozoic accreted terranes and the early Jurassic Higganum dike. This allows us to restrict our analysis to Mesozoic rifting and subsequent development of the passive margin. Paleostress inversion of the fault-slip data combined with age relations observed in the field produces four tectonic phases. At the regional-scale, phase 1 consists primarily of normal faults striking in a variety of directions but mainly ~NE-SW and displays a NW-SE σ3. At one site, however, phase 1 consists of a conjugate set of ~NW-SE striking normal faults and displays a NE-SW σ3. Faults in phase 2 have a variety of orientations but are dominated by a conjugate set of ~N-S and ~E-W-striking strike-slip faults and displays a NW-SE σ1. Phase 3 consists of a conjugate set of ~NW-SE and ~NE-SW-striking strike-slip faults and displays a N-S σ1. Phase 4 consists of a conjugate set of ~NE-SW and ~WNW-ESE-striking strike-slip faults and displays an ENE-WSW σ1. Phase 1 stress orientations are characteristic of main-stage rifting. Phase 2 faults may have developed synchronously with structural inversion of the rift basins. The local evidence in phase 1 for NE-SW extension is suggested to be the transition from rifting to postrift inversion or may indicate flattening strain

  7. 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

  8. 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.

  9. Origin of narrow terranes and adjacent major terranes occurring along the denali fault in the eastern and central alaska range, alaska

    USGS Publications Warehouse

    Nokleberg, W.J.; Richter, D.H.

    2007-01-01

    Several narrow terranes occur along the Denali fault in the Eastern and Central Alaska Range in Southern Alaska. These terranes are the Aurora Peak, Cottonwood Creek, Maclaren, Pingston, and Windy terranes, and a terrane of ultramafic and associated rocks. Exterior to the narrow terranes to the south is the majorWrangellia island arc composite terrane, and to the north is the major Yukon Tanana metamorphosed continental margin terrane. Overlying mainly the northern margin of the Wrangellia composite terrane are the Kahiltna overlap assemblage to the west, and the Gravina- Nutzotin-Gambier volcanic-plutonic- sedimentary belt to the east and southeast. The various narrow terranes are interpreted as the result of translation of fragments of larger terranes during two major tectonic events: (1) Late Jurassic to mid-Cretaceous accretion of the Wrangellia island arc composite terrane (or superterrane composed of the Wrangellia, Peninsular, and Alexander terranes) and associated subduction zone complexes; and (2) starting in about the Late Cretaceous, dextral transport of the Wrangellia composite terrane along the Denali fault. These two major tectonic events caused: (1) entrapment of a lens of oceanic lithosphere along the suture belt between the Wrangellia composite terrane and the North American Craton Margin and outboard accreted terranes to form the ultramafic and mafic part of the terrane of ultramafic and associated rocks, (2) subsequent dextral translation along the Denali fault of the terrane of ultramafic and associated rocks, (3) dextral translation along the Denali fault of the Aurora Peak, Cottonwood Creek, and Maclaren and continental margin arc terranes from part of the Coast plutonic-metamorphic complex (Coast-North Cascade plutonic belt) in the southwest Yukon Territory or Southeastern Alaska, (4) dextral translation along the Denali fault of the Pingston passive continental margin from a locus along the North American Continental Margin, and (5

  10. Formation of Oceanic Lithosphere by Basal Magma Accretion

    NASA Astrophysics Data System (ADS)

    Hamza, V. M.; Cardoso, R. R.; Alexandrino, C. H.

    2009-12-01

    The thermal models of the lithosphere proposed to date have failed to provide satisfactory accounts of some of the important features in large-scale variations of ocean floor bathymetry and heat flow. The systematic difference between model calculations and observational data have given rise to the so-called “oceanic heat flow paradox”, for which no satisfactory solution has been found for over the last forty years. In the present work, we point out that this paradox is a consequence of the assumption that lateral temperature variations are absent in the sub-lithospheric mantle. In the present work we propose a simple magma accretion model and examine its implications for understanding the thermal field of oceanic lithosphere. The new model (designated VBA) assumes existence of lateral variations in magma accretion rates and temperatures at the boundary zone between the lithosphere and the asthenosphere, similar in character to those observed in magma solidification processes in the upper crust. However, unlike the previous thermal models of the lithosphere, the ratio of advection to conduction heat transfer (the Peclet number) is considered a space dependent variable. The solution to the problem of variable basal heat input has been obtained by the method of integral transform. The results of VBA model simulations reveal that the thickness of the young lithosphere increases with distance from the ridge axis, at rates faster than those predicted by Half-Space Cooling and Plate models. Another noteworthy feature of the new model is its ability to account for the main observational features in the thermal behavior of both young and old oceanic lithosphere. Thus, heat flow and bathymetry variations calculated on the basis of the VBA model provide vastly improved fits to respective observational datasets. More importantly, the improved fits to bathymetry and heat flow have been achieved for the entire age range of oceanic lithosphere and without the need to invoke

  11. Setting and occurrence of Late Paleozoic radiolarians in the Sylvester allochthon, part of a proto-Pacific ocean floor terrane in the Canadian Cordillera

    USGS Publications Warehouse

    Harms, T.A.; Murchey, B.L.

    1992-01-01

    Late Paleozoic radiolarians have been used to establish th allochthon of the Slide Mountain terrane in British Columbia, and have thereby greatly clarified the geology and tectonic history of the terrane. As the Sylvester radiolarian fauna is limited, age assignments were based on a few distinctive and diagnostic robust forms. Radiolarians occur in cherts from a wide variety of different oceanic sequences that are structurally juxtaposed within the Sylvester allochthon. Like others in a suite of correlative terranes that lie along the length of the Cordillera, the Sylvester allochthon and the radiolarian bearing cherts in it derive from the telescoping together of slices from what was, in the late Paleozoic, a large area of the proto-Pacific ocean. ?? 1992.

  12. 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

  13. From Back-arc Drifting to Arc Accretion: the Late Jurassic-Early Cretaceous Evolution of the Guerrero Terrane Recorded by a Major Provenance Change in Sandstones from the Sierra de los Cuarzos, Central Mexico

    NASA Astrophysics Data System (ADS)

    Palacios Garcia, N. B.; Martini, M.

    2014-12-01

    The Guerrero terrane composed of Middle Jurassic-Early Cretaceous arc assemblages, were drifted from the North American continental mainland during lower Early Cretaceous spreading in the Arperos back arc basin, and subsequently accreted back to the continental margin in the late Aptian. Although the accretion of the Guerrero terrane represents one of the major tectonic processes that shaped the southern North American Pacific margin, the stratigraphic record related to such a regional event was not yet recognized in central Mexico. Due to the Sierra de los Cuarzos is located just 50 km east of the Guerrero terrane suture belt, its stratigraphic record should be highly sensitive to first order tectonic changes and would record a syn-tectonic deposits related to this major event. In that study area, were identified two main Upper Jurassic-Lower Cretaceous clastic units. The Sierra de los Cuarzos formation represents the lowermost exposed stratigraphic record. Sedimentary structures, sandstones composition, and U-Pb detrital zircon ages document that the Sierra de los Cuarzos formation reflects a vigorous mass wasting along the margin of the North American continental mainland, representing the eastern side of the Arperos back arc basin. Sandstones of the Sierra de los Cuarzos formation are free from detrital contributions related to the Guerrero terrane juvenile sources, indicating that the Arperos Basin acted like an efficient sedimentological barrier that inhibited the influence of the arc massifs on the continental mainland deposits. The Sierra de los Cuarzos formation is overlain by submarine slope deposits of the Pelones formation, which mark a sudden change in the depositional conditions. Provenance analysis documents that sandstones from the Pelones formation were fed by the mafic to intermediate arc assemblages of the Guerrero terrane, as well as by quartz-rich sources of the continental mainland, suggesting that, by the time of deposition of the Pelones

  14. 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)

  15. The quantum event of oceanic crustal accretion: impacts of diking at mid-ocean ridges.

    PubMed

    Delaney, J R; Kelley, D S; Lilley, M D; Butterfield, D A; Baross, J A; Wilcock, W S; Embley, R W; Summit, M

    1998-07-10

    Seafloor diking-eruptive events represent the irreducible, quantum events of upper oceanic crustal accretion. They record events by which a large portion of the oceanic crust has formed through geological history. Since 1993, the U.S. Navy's real-time Sound Surveillance System has allowed location of ongoing acoustic signatures of dike emplacement and basalt eruptions at ridge crests in the northeast Pacific. These diking-eruptive events trigger a sequence of related, rapidly evolving physical, chemical, and biological processes. Magmatic volatiles released during these events may provide nutrients for communities of subsea-floor microorganisms, some of which thrive in high-temperature anaerobic environments. Many of the organisms identified from these systems are Archaea. If microorganisms can thrive in the water-saturated pores and cracks within deep, volcanically active portions of our planet, other hydrothermally active planets may harbor similar life forms.

  16. The Quantum Event of Oceanic Crustal Accretion: Impacts of Diking at Mid-Ocean Ridges

    PubMed

    Delaney; Kelley; Lilley; Butterfield; Baross; Wilcock; Embley; Summit

    1998-07-10

    Seafloor diking-eruptive events represent the irreducible, quantum events of upper oceanic crustal accretion. They record events by which a large portion of the oceanic crust has formed through geological history. Since 1993, the U.S. Navy's real-time Sound Surveillance System has allowed location of ongoing acoustic signatures of dike emplacement and basalt eruptions at ridge crests in the northeast Pacific. These diking-eruptive events trigger a sequence of related, rapidly evolving physical, chemical, and biological processes. Magmatic volatiles released during these events may provide nutrients for communities of subseafloor microorganisms, some of which thrive in high-temperature anaerobic environments. Many of the organisms identified from these systems are Archaea. If microorganisms can thrive in the water-saturated pores and cracks within deep, volcanically active portions of our planet, other hydrothermally active planets may harbor similar life forms.

  17. 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.; 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

  18. Timing of the final closure of the Paleo-Asian Ocean in the Alxa Terrane: Constraints from geochronology and geochemistry of Late Carboniferous to Permian gabbros and diorites

    NASA Astrophysics Data System (ADS)

    Liu, Qian; Zhao, Guochun; Han, Yigui; Eizenhöfer, Paul R.; Zhu, Yanlin; Hou, Wenzhu; Zhang, Xiaoran

    2017-03-01

    Connecting the North China Craton to the east and the Tarim Craton to the west, the Alxa Terrane is a key place in investigating the timing of the final closure of the Paleo-Asian Ocean (PAO). New LA-ICPMS zircon U-Pb dating results reveal ca. 300-268 Ma gabbros and diorites in the Bayan Nuru area in the eastern part of the Alxa Terrane. The 300 Ma gabbros show plagioclase accumulations with anorthite compositions, arc-like geochemical affinities with relative enrichment in large ionic lithophile elements and depletion in high field strength elements (e.g., Ti, Nb and Ta), as well as negative εHf(t) and εNd(t) values and high initial 87Sr/86Sr ratios. These features indicate a magma source of an enriched lithospheric mantle metasomatized by high fluid activities. In comparison, the 280-268 Ma gabbros and diorites also have arc-like geochemical affinities but show increasingly evolved isotope compositions, implying more sediment inputs. Compiled zircon εHf(t) and whole-rock εNd(t) values of the magmatic rocks in the Alxa Terrane decrease from the Late Carboniferous to the Early Permian, and increase from the Middle Permian to the Triassic. The significantly large variation in zircon εHf(t) and whole-rock εNd(t) values at ca. 280-265 Ma likely reflects a tectonic switch from a subduction setting to a post-collisional setting, corresponding to the timing of the final closure of the PAO in the Alxa Terrane. Thus, the PAO progressively closed from west to east along the northern margin of the Tarim Craton, the Alxa Terrane, and then the northern margin of the North China Craton during Late Carboniferous to Middle Triassic time.

  19. 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

  20. Early Paleozoic subduction processes of the Paleo-Asian Ocean: Insights from geochronology and geochemistry of Paleozoic plutons in the Alxa Terrane

    NASA Astrophysics Data System (ADS)

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

    2016-10-01

    The Alxa Terrane is situated in a key area between the North China and Tarim cratons. Paleozoic magmatic records in this terrane place important constraints on the subduction processes of the southern Paleo-Asian Ocean. New data of zircon U-Pb ages and whole-rock elemental and isotopic data reveal two groups of intermediate to felsic plutons in the Alxa Terrane. One group consists of diorites and granitoids that were emplaced at ca. 460-440 Ma and characterized by lower Al2O3/TiO2 ratios and higher TiO2 contents, implying high temperature-low pressure crystallization conditions and a shallow source region. The second group is dominated by granitoids aged at ca. 420-407 Ma and displays high Sr and Ba, low Y and high rare earth elements, with very high Sr/Y ratios and mostly positive Eu anomalies. These characteristics imply low temperature-high pressure crystallization conditions and source regions at deep crustal levels where garnet is stable in the residual phase. Both of the two groups are mostly calc-alkaline to high-K calc-alkaline, depleted in Nb, Ta and Ti and enriched in Ba, K and Sr, indicative of an arc affinity most likely related to the southward subduction of the Paleo-Asian Ocean. Zircon εHf(t) and whole-rock εNd(t) values of these magmatic rocks decrease from 458 Ma to 440 Ma and 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 regime at 407 Ma. Synthesized data from this and previous studies suggest that the 460-400 Ma magmatic arc in the Alxa Terrane represented the western extension of the Paleozoic arc belt on the northern margin of the North China Craton.

  1. 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

    interpolated and we discuss two different APW path alternatives that result in two very different convergence scenarios between Siberia and Baltica/Kazakh terranes. There are a newly-constructed succession of palaeogeographic maps of Siberia and its nearby areas at various times from the Cambrian to the Permian as, firstly, the peri-Siberian terranes and, secondly, the remainder of the Central Asian terranes accreted to it. Prior to the Early Ordovician, Siberia was in the southern hemisphere, but after that it drifted northwards and for most of the Phanerozoic it has been one of the few larger terranes in the northern hemisphere. The Cambrian and Ordovician maps are provisional for the Altai-Sayan and Mongolian areas, whose geology is highly complex and whose detailed palaeogeography is unresolved. The terms "Altaids" and "Paleo-Asian Ocean" have been used in so many different ways by so many different authors over so many geological periods that we reject their use. Wider issues considered include the possible links between the Cambrian Radiation (often wrongly termed "Explosion"), when metazoan animals first gained hard parts, and True Polar Wander (TPW). New Early Cambrian palaeomagnetic data from Siberia do not show rapid APW (< 10 cm/yr.) or dramatic velocity changes (< 4 cm/yr). It is concluded that the Cambrian Radiation occurred over a period approaching 20 Myr, and that rapid and large-scale TPW did not take place in the Cambrian. In addition, there are no traces of glaciogenic deposits in the very large area of Siberia during the Neoproterozoic, casting some doubt on the "Snowball Earth" hypothesis.

  2. Preferential rifting of continents - A source of displaced terranes

    NASA Technical Reports Server (NTRS)

    Vink, G. E.; Morgan, W. J.; Zhao, W.-L.

    1984-01-01

    Lithospheric rifting, while prevalent in the continents, rarely occurs in oceanic regions. To explain this preferential rifting of continents, the total strength of different lithospheres is compared by integrating the limits of lithospheric stress with depth. Comparisons of total strength indicate that continental lithosphere is weaker than oceanic lithosphere by about a factor of three. Also, a thickened crust can halve the total strength of normal continental lithosphere. Because the weakest area acts as a stress guide, any rifting close to an ocean-continent boundary would prefer a continental pathway. This results in the formation of small continental fragments or microplates that, once accreted back to a continent during subduction, are seen as displaced terranes. In addition, the large crustal thicknesses associated with suture zones would make such areas likely locations for future rifting episodes. This results in the tendency of new oceans to open along the suture where a former ocean had closed.

  3. TIME-DEPENDENT, COMPOSITIONALLY DRIVEN CONVECTION IN THE OCEANS OF ACCRETING NEUTRON STARS

    SciTech Connect

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

    2015-03-20

    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 and 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. The Basement of the Central Andes: The Arequipa and Related Terranes

    NASA Astrophysics Data System (ADS)

    Ramos, Victor A.

    2008-05-01

    The basement of the Central Andes provides insights for the dispersal of Rodinia, the reconstruction of Gondwana, and the dynamics of terrane accretion along the Pacific. The Paleoproterozoic Arequipa terrane was trapped during collision between Laurentia and Amazonia in the Mesoproterozoic. Ultrahigh-temperature metamorphism correlates with the collapse of the Sunsás-Grenville orogen after 1000 Ma and is related to slab break-off and dispersal of Rodinia. The Antofalla terrane separated in the Neoproterozoic, forming the Puncoviscana basin. Its closure was coeval with the collision of the eastern Sierras Pampeanas. The rift-drift transitions of the early Paleozoic clastic platform showed a gradual younging to the north, in agreement with counterclockwise rotation based on paleomagnetic data of Antofalla. North of Arequipa arc magmatism and high-grade metamorphism are linked to collision of the Paracas terrane in the Ordovician, during the Famatinian orogeny in the Sierras Pampeanas. The early Paleozoic history of the Arequipa massif is explained by a backarc, which further south changed to open oceanic conditions and subsequent collision. The Antofalla terrane reaccreted to the continental margin by the late Ordovician. These accretions and subsequent separations during the Mesoproterozoic, Neoproterozoic early Cambrian, and late Cambrian middle Ordovician are explained by changes in absolute motion of the Gondwana supercontinent during plate global reorganization.

  5. Petrogenesis of Middle-Late Triassic volcanic rocks from the Gangdese belt, southern Lhasa terrane: Implications for early subduction of Neo-Tethyan oceanic lithosphere

    NASA Astrophysics Data System (ADS)

    Wang, Chao; Ding, Lin; Zhang, Li-Yun; Kapp, Paul; Pullen, Alex; Yue, Ya-Hui

    2016-10-01

    The Gangdese belt is dominantly composed of igneous rocks that formed during the northward subduction of Neo-Tethyan oceanic lithosphere beneath the Lhasa terrane and has played a crucial role in understanding the pre-collisional evolution of southern Tibet. This paper presents new geochronological and geochemical (whole-rock major and trace element and Sr-Nd and zircon Hf isotope) data for recently identified volcanic rocks exposed in Changguo area, southernmost part of the Lhasa terrane. Zircon U-Pb dating from six samples yields consistent ages of 237.1 ± 1.1 Ma to 211.7 ± 1.5 Ma for magma emplacement through volcanic eruption, showing the Middle-Late Triassic magmatic activity in the southernmost Gangdese Belt. The Changguo volcanic rocks are mainly composed of basaltic and andesitic rocks and exhibit LILE enrichment and HFSE depletion. They also exhibit relatively uniform Nd-Hf isotopic compositions (εNd(t) = + 5.20 to + 7.74 and εHf(t)zircon = + 10.2 to + 15.9). The basaltic magmas were likely sourced from partial melting of sub-arc mantle wedge that was metasomatized by not only the aqueous fluid derived from subducting altered oceanic crust but also hydrous melt derived from subducting seafloor sediments, and subsequently experienced fractional crystallization and juvenile crustal contamination during ascent. The andesitic magmas were generated by partial melting of mafic-ultramafic metasomes through melt/fluid-peridotite reaction at slab-mantle interface. Taking into account the temporal and spatial distribution of the Early Mesozoic magmatic rocks and regional detrital zircon data, we further propose that the northward subduction of Neo-Tethyan oceanic lithosphere beneath the Lhasa terrane commenced by Middle Triassic.

  6. Numerical model of crustal accretion and cooling rates of fast-spreading mid-ocean ridges

    NASA Astrophysics Data System (ADS)

    Machetel, P.; Garrido, C. J.

    2013-10-01

    We designed a thermo-mechanical numerical model for fast-spreading mid-ocean ridge with variable viscosity, hydrothermal cooling, latent heat release, sheeted dyke layer, and variable melt intrusion possibilities. The model allows for modulating several accretion possibilities such as the "gabbro glacier" (G), the "sheeted sills" (S) or the "mixed shallow and MTZ lenses" (M). These three crustal accretion modes have been explored assuming viscosity contrasts of 2 to 3 orders of magnitude between strong and weak phases and various hydrothermal cooling conditions depending on the cracking temperatures value. Mass conservation (stream-function), momentum (vorticity) and temperature equations are solved in 2-D cartesian geometry using 2-D, alternate direction, implicit and semi-implicit finite-difference scheme. In a first step, an Eulerian approach is used solving iteratively the motion and temperature equations until reaching steady states. With this procedure, the temperature patterns and motions that are obtained for the various crustal intrusion modes and hydrothermal cooling hypotheses display significant differences near the mid-ocean ridge axis. In a second step, a Lagrangian approach is used, recording the thermal histories and cooling rates of tracers travelling from the ridge axis to their final emplacements in the crust far from the mid-ocean ridge axis. The results show that the tracer's thermal histories are depending on the temperature patterns and the crustal accretion modes near the mid-ocean ridge axis. The instantaneous cooling rates obtained from these thermal histories betray these discrepancies and might therefore be used to characterize the crustal accretion mode at the ridge axis. These deciphering effects are even more pronounced if we consider the average cooling rates occurring over a prescribed temperature range. Two situations were tested at 1275-1125 °C and 1050-850 °C. The first temperature range covers mainly the crystallization range

  7. 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.

  8. Tectonic accretion and the origin of the two major metamorphic and plutonic welts in the Canadian Cordillera

    NASA Astrophysics Data System (ADS)

    Monger, J. W. H.; Price, R. A.; Tempelman-Kluit, D. J.

    1982-02-01

    The Omineca Crystalline Belt and Coast Plutonic Complex are the two major regional tectonic welts in the Canadian Cordillera in which were concentrated intense deformation, regional metamorphism, granitic magmatism, uplift, and erosion. The welts, which formerly were thought to result from subduction of Pacific Ocean lithosphere beneath the western edge of North America, can now be viewed partly as the result of tectonic overlap and/or compressional thickening of crustal rocks during collisions between North America and two large, composite, allochthonous terranes that were accreted to its ancient western margin. The inner composite terrane, Terrane I, includes four smaller terranes that apparently were together by the end of Triassic time. The outer composite terrane, Terrane II, comprises two terranes, amalgamated by Late Jurassic time. The Omineca Crystalline Belt formed mainly from mid-Jurassic time onward, during and following the collision of Terrane I with North America. This belt straddles the zone of overlap of autochthonous and allochthonous terranes, and its characteristic metamorphism and structure are superimposed on both. The Coast Plutonic Complex formed mainly in Cretaceous to early Tertiary time during and following the attachment of Terrane II to the new, Jurassic, continental margin. It lies along the boundary of Terrane I and Terrane II and involves elements of both terranes. The collisions took place within the overall setting of the North American plate moving relatively westward into various Pacific plates from Jurassic time onward and in conjunction with subduction of Pacific Ocean lithosphere. *Present address: Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A OE8, Canada

  9. Central role of detachment faults in accretion of slow-spreading oceanic lithosphere.

    PubMed

    Escartín, J; Smith, D K; Cann, J; Schouten, H; Langmuir, C H; Escrig, S

    2008-10-09

    The formation of oceanic detachment faults is well established from inactive, corrugated fault planes exposed on sea floor formed along ridges spreading at less than 80 km Myr(-1) (refs 1-4). These faults can accommodate extension for up to 1-3 Myr (ref. 5), and are associated with one of the two contrasting modes of accretion operating along the northern Mid-Atlantic Ridge. The first mode is asymmetrical accretion involving an active detachment fault along one ridge flank. The second mode is the well-known symmetrical accretion, dominated by magmatic processes with subsidiary high-angle faulting and the formation of abyssal hills on both flanks. Here we present an examination of approximately 2,500 km of the Mid-Atlantic Ridge between 12.5 and 35 degrees N, which reveals asymmetrical accretion along almost half of the ridge. Hydrothermal activity identified so far in the study region is closely associated with asymmetrical accretion, which also shows high levels of near-continuous hydroacoustically and teleseismically recorded seismicity. Increased seismicity is probably generated along detachment faults that accommodate a sizeable proportion of the total plate separation. In contrast, symmetrical segments have lower levels of seismicity, which occurs primarily at segment ends. Basalts erupted along asymmetrical segments have compositions that are consistent with crystallization at higher pressures than basalts from symmetrical segments, and with lower extents of partial melting of the mantle. Both seismic evidence and geochemical evidence indicate that the axial lithosphere is thicker and colder at asymmetrical sections of the ridge, either because associated hydrothermal circulation efficiently penetrates to greater depths or because the rising mantle is cooler. We suggest that much of the variability in sea-floor morphology, seismicity and basalt chemistry found along slow-spreading ridges can be thus attributed to the frequent involvement of detachment faults

  10. 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.

  11. 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.

  12. 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.

  13. 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.

  14. Nd and Sr isotopic characterization of the Wrangellia terrane and implications for crustal growth of the Canadian cordillera

    SciTech Connect

    Samson, S.D.; Patchett, P.J.; Gehrels, G.E. ); Anderson, R.G. )

    1990-09-01

    Nd and Sr isotopic data are reported from samples of most of the major lithologic units of the Wrangellia terrane exposed on Vancouver Island and the Queen Charlotte Islands. Initial {epsilon}{sub Nd} values range from +1.0 to +7.3 and initial {sup 87}Sr/{sup 86}Sr ratios range from 0.70323 to 0.70481. These ratios are similar to those of modern island arcs and demonstrate the unevolved, juvenile nature of this terrane. These data indicate that Wrangellia probably resided in an intra-oceanic environment until its accretion to North America. Nd crustal residence ages for clastic sedimentary rocks of Wrangellia (mean T{sub CR} = 0.77 {plus minus} 0.33 Ga, T{sub DM} = 0.57 {plus minus} 0.35 Ga) are much closer to their stratigraphic ages than is the case for the ages of many other Phanerozoic sedimentary rocks, thus providing strong evidence for Phanerozoic crustal addition to North America. Early Proterozoic crustal components cannot comprise more than about 6% of Wrangellia; therefore the terrane consists of {approximately}94% new, mantle-derived crust. The amount of new crust in Wrangellia is very similar to the amount determined for two other large Cordilleran terranes, Alexander and Stikine. The accretion of juvenile terranes to a continent appears to be an important mechanism of crustal growth.

  15. 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. 

  16. 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

  17. The Geometry of Accreted "Packets" in Subduction Zones: Examples from the Eastern Belt of the Franciscan in California and the Torlesse Terrane in New Zealand

    NASA Astrophysics Data System (ADS)

    MacKinnon, T. C.

    2015-12-01

    Accretion is a complex process, the details of which cannot be resolved seismically. Well-exposed ancient examples offer a better opportunity. This study focuses on two areas with nearly 100% exposure: glaciated exposures of the Torlesse near Arthur's Pass, New Zealand and stream-cut exposures of the Franciscan Eastern Belt in Grindstone and Thomes Creeks, California. Rocks are mainly turbidites of prehnite-pumpellyite to lower blueschist grade, described as "broken formation" with no melange or exotic blocks present. The dominant structural features are steeply dipping beds cut by thrust faults oriented at a modest angle to bedding. Fault spacing ranges from ~100 to 600 meters with the angle between bedding and faults usually ranging from 0° to 35°. Between faults, bedding continuity, though locally disrupted, is generally well-preserved. Deformation associated with the faults is variable. In some cases, fault contacts are sharp and show little deformation of adjacent beds. In other cases, small folds, boudinage, and intense fracturing and veining define fault zones. Intensity of deformation varies along the fault plane and is commonly more intense on one side than the other. Of note are two additional faults present in the Thomes Creek area; they are out-of-sequence thrusts (OOST's) that cross-cut the other faults at high angles and are associated with a greater degree of deformation. Aside from these OOST's, deformation associated with the faults is roughly an order of magnitude less than that described in the literature for "megathrusts." The faults described above (excluding the OOST's) represent primary surfaces along which "packets" of relatively intact rocks were accreted. This study and others show that these fault-bounded packets can be traced along strike for a few km to at least 10 km or more where not cut by OOST's or younger faults. The faults appear to form after a period of diffuse, largely extensional shear in semi-consolidated sediments; as

  18. 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).

  19. Global variations in gravity-derived oceanic crustal thickness: Implications on oceanic crustal accretion and hotspot-lithosphere interactions

    NASA Astrophysics Data System (ADS)

    Lin, J.; Zhu, J.

    2012-12-01

    emplacements on the plate. (2) In contrast, crustal thickness near fast and intermediately fast spreading ridges typically does not exceed 7-8 km. The relatively weak lithosphere at fast and intermediately fast ridges might make it harder for excess magmatism to accrete. We further speculate that the relatively wide partial melting zones in the upper mantle beneath the fast and intermediately fast ridges might act as "buffer" zones, thus diluting the melt anomalies from the underlying hotspots or regions of mantle heterogeneities. (3) As the crustal age increases and the lithospheric plate thickens, regions of thickened crust start to develop on ocean basins that were originally created at fast and intermediately fast ridges. The integrated crustal volume for fast and intermediately fast ocean crust appears to reach peak values for certain geological periods, such as 40-50 Ma and 70-80 Ma. The newly constructed global models of gravity-derived crustal thickness, combining with geochemical and other constraints, can be used to investigate the processes of oceanic crustal accretion and hotspot-lithosphere interactions.

  20. 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

  1. Californian blueschists, subduction, and the significance of tectonostratigraphic terranes

    NASA Astrophysics Data System (ADS)

    Ernst, W. G.

    1984-07-01

    Glaucophane and related schists are present as tectonic fragments in ophiolitic suture zones and as discrete lithotectonic belts along the accreted Mesozoic/Tertiary Californian margin. Occurrences include parts of the Klamath Mountains, the western Sierran Foothills, the Coast Ranges, faulted marginal segments of the Mojave Desert, the Transverse Ranges, and the southern California borderland. These high-pressure, low-temperature blueschist assemblages reflect the thermal regime of subduction-zone environments. Considerable underflow accompanied drifting and the assembly of far-traveled tectonostratigraphic terranes, as documented by sea-floor magnetic anomaly patterns and age relationships of the oceanic crust-capped lithosphere: the eastern limbs of paleo-Pacific plates (especially the Farallon-Cocos), have been extensively or completely overridden by the westward-encroaching North American plate—7000 km since Early Cretaceous time and nearly 10 000 km since Jurassic time. Subduction is attested to by remnant high-pressure mineral assemblages scattered throughout California; by construction of related, roughly contemporaneous calc-alkaline volcanic-plutonic belts and forearc basin deposits; and by the stranding of ophiolitic complexes. Although substantial northward drift transported exotic oceanic and continental materials to the growing Californian crust and caused extensive dislocation of the post-Paleozoic continental margin, much of the plate motion evidently involved a large component of convergence and eastward underflow. Terrane shuffling has complicated the picture, but the dominant mechanism of continental growth at the Californian margin during Mesozoic and Paleogene time was subduction.

  2. Collisional orogenesis in the northern Canadian Cordillera: Implications for Cordilleran crustal structure, ophiolite emplacement, continental growth, and the terrane hypothesis

    NASA Astrophysics Data System (ADS)

    English, Joseph M.; Johnston, Stephen T.

    2005-04-01

    During Upper Triassic to Lower Jurassic time, arc magmatic rocks of the Stikine terrane, arc-marginal sediments of the Whitehorse Trough, igneous and mantle rocks of the Cache Creek 'ophiolite' and Kutcho assemblage, and oceanic sedimentary and volcanic rocks of the Cache Creek terrane represented a magmatic arc, forearc basin, forearc basement, and subduction complex, respectively. The Cache Creek subduction complex was thrust to the southwest over the Whitehorse Trough forearc basin and the Stikine terrane in the Middle Jurassic during collision with an inboard continental domain. Interpretation of the various lithotectonic assemblages of the northern Intermontane belt in terms of a new plate tectonic model has a number of important implications for the Canadian Cordillera: (a) the model allows comparisons to be drawn with available seismic reflection interpretations of Cordilleran crustal structure; (b) 'ophiolite' emplacement was achieved by ramping of forearc oceanic lithosphere onto thick crustal parts of a subducting plate during collisional orogenesis; (c) island-arc collision and accretion were the principal mechanisms for continental growth with relatively minor contributions from 'sliced-off' oceanic seamounts and/or plateaux; and (d) some terrane-bounding faults such as the Nahlin Fault do not represent major lithospheric-scale boundaries and their importance in tectonic reconstructions has been overemphasised.

  3. 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.

  4. Origin of plagiogranites in oceanic complexes: A case study of the Nicoya and Santa Elena terranes, Costa Rica

    NASA Astrophysics Data System (ADS)

    Whattam, Scott A.; Gazel, Esteban; Yi, Keewook; Denyer, Percy

    2016-10-01

    The origin of minor leucocratic intrusions known as "plagiogranites" in oceanic complexes dominated by basaltic compositions have been debated in the literature. Here we use well preserved plagiogranites within the contrasting Santa Elena Ophiolite and the Nicoya Complex, NW Costa Rica, to investigate the origin and age of these leucocratic intrusions. Magmatic zircons of plagiogranites of the Santa Elena ophiolite and the Nicoya Complex, yield weighted mean SHRIMP 206Pb/238U ages of 125.3 ± 2.0 Ma and 90.9 ± 2.0 Ma to 88.5 ± 2.0 Ma (n = 5), respectively. These ages record the main magmatic phases of formation of these intrusions preserved in each complex. Relatively flat, unfractionated chondrite-normalized REE patterns of plagiogranites from both complexes mimic signatures of related mafic rocks, which is consistent with formation via fractional crystallization from a mafic source as opposed to partial melting of a mafic (basalt, amphibolite) protolith. In the case of the Nicoya Complex, modelling suggests that the Nicoya plagiogranites are consistent as 10-15% residual liquids after fractional crystallization from its mafic parental source. Trace element systematics of the plagiogranites are consistent with an oceanic arc origin for Santa Elena and a mostly mid-ocean ridge to oceanic-plateau origin for Nicoya.

  5. 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

  6. Origin, transport, and emplacement of an exotic island-arc terrane exposed in eastern Kamchatka, Russia

    USGS Publications Warehouse

    Geist, Eric L.; Vallier, Tracy L.; Scholl, David W.

    1994-01-01

    The regional stratigraphy of eastern Kamchatka includes an exotic, Early-Late Cretaceous ophiolite and Late Cretaceous island-arc volcanic sequence. Integrating the existing geologic and geophysical data, we examine the origin, transport, emplacement, and postemplacement deformation of the island-arc terrane, which is named the Olyutorsky island arc. Results from several paleomagnetic studies consistently indicate that the island-arc terrane originated >1000 km to the south of where it is presently exposed. Although the formative paleolatitudes of the island-arc rocks approximately correspond to the location of the Izanagi-Farallon subduction zone, the age of the volcanic rocks postdates the cessation of Izanagi-Farallon convergence, thus indicating that an unnamed plate or back-arc basin existed in the northwest Pacific during Late Cretaceous time. We examine two possible models for northward transport of the island-arc terrane to Kamchatka: (1) infra-oceanic transport with the Pacific or Kula plates and (2) coastwise translation of the island-arc terrane after accretion to the Eurasian margin far to the south of Kamchatka. For both models, the dominant Eocene and Miocene deformation ages observed in eastern Kamchatka are used as two possible age limits for the cessation of northward transport. Although the observed paleolatitudes from paleomagnetic data correspond best with the infra-oceanic transport model, the provenance of the Paleogene "transport" stratigraphy indicates a near-shore sediment supply. Our preferred interpretation is that the island-arc terrane (1) accreted onto the Eurasian margin concurrent with cessation of island-arc volcanism (Maastrichtian-Danian) and (2) underwent northward coastwise translation along a major strike-slip fault zone ending by middle-late Eocene time (43-50 Ma). It is unclear whether the ophiolite was exposed during arc-continent collision or whether the ophiolite was obducted onto the island arc prior to collision. A

  7. 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

  8. Terrane amalgamation in the Philippine Sea margin

    NASA Astrophysics Data System (ADS)

    Hall, Robert; Nichols, Gary J.

    1990-09-01

    The Philippine Sea plate includes plateaus of thickened crust interpreted as imbricated ophiolite and arc-related terranes of late Mesozoic-early Tertiary age separated by thinner oceanic crust. The arrival of plateaus at the subducting southwest margin of the Philippine Sea plate has caused the Philippine Trench to propagate southward in increments and caused transfer of terranes to the Philippine margin. New data from the Halmahera region indicate that the position, nature and evolution of plate boundaries have been strongly influenced by the heterogeneous character of the Philippine Sea plate. At present the Philippine Trench terminates at an oceanic plateau which is structurally continuous with an old forearc and ophiolite terrane on Halmahera. The position of this terrane has caused Philippine Sea plate-Eurasia convergence to be transferred from subduction at the Philippine Trench to the Molucca Sea Collision Zone through a broad NE-SW dextral transpressional zone across Halmahera. This plate boundary configuration is unstable and requires the future development of a new subduction zone to the east of Halmahera which will result in amalgamation of the Halmahera ophiolite terrane to the Philippine margin. In the Halmahera region amalgamation of terranes to the evolving Philippine microcontinent is currently in process.

  9. Ocean Zircon - constraints on cooling histories, spreading rate, and modes of crustal accretion

    NASA Astrophysics Data System (ADS)

    John, B. E.; Cheadle, M. J.; Baines, G.; Grimes, C. B.; Wooden, J.

    2009-12-01

    Atlantis Bank (SWIR), Atlantis Massif and the Kane Megamullion (MAR) suggest that oceanic detachment faults form during periods of asymmetric spreading and consequent ridge migration (Atlantis Bank detachment fault accommodated roughly 80% of plate spreading). The average 206Pb/238U age is ~0.2 my older than the estimated magnetic age of ODP Hole 735B at Atlantis Bank, implying acquisition of magnetic remanence ~3 km off-axis, and crustal cooling rates of >1000°C/myr from 850-550°C. Dating of zircon from deep ODP/IODP vertical boreholes reveals the detailed chronology of emplacement of up to 1500 meters of gabbroic crust, illuminating not only crustal architecture, but a timescale of vertical crustal accretion of 100-200kyr for these holes. These data are consistent with the many sill model for the growth of slow spread ocean crust.

  10. 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.

  11. Geochemistry of Triassic flood basalts from the Yukon (Canada) segment of the accreted Wrangellia oceanic plateau

    NASA Astrophysics Data System (ADS)

    Greene, Andrew R.; Scoates, James S.; Weis, Dominique; Israel, Steve

    2009-06-01

    A large part of the accreted Middle to Late Triassic Wrangellia oceanic plateau is exposed as a linear belt (< 30 km × 300 km) in southwest Yukon. The first major- and trace-element, and isotopic compositions of the Nikolai Formation in Yukon are presented here, along with compositions for underlying Paleozoic arc rocks. The Nikolai Formation in Yukon is predominantly massive tholeiitic subaerial flows (~ 1000 m) with no intervening sediments and a thin zone of pillow breccia along the base (< 100 m). The Nikolai basalts unconformably overlie Late Paleozoic volcanic arc and marine sedimentary sequences and are overlain by Late Triassic limestone, which grades upwards into pelagic sediments. The Nikolai Formation is comprised of two distinct lava types: low-titanium basalts form most of the lower stratigraphy and high-titanium basalts form the upper parts of the volcanic stratigraphy. All of the low-titanium basalts (0.5-1.0 wt.% TiO 2; 5.6-11.3 wt.% MgO) have prominent negative HFSE anomalies, whereas the high-titanium basalts (1.4-2.3 wt.% TiO 2; 5.8-8.7 wt.% MgO) do not have HFSE anomalies and are more LREE-enriched. The low-titanium basalts are characterized by mostly higher initial ɛHf (+ 11.1 to + 15.8) and lower initial ɛNd (+ 2.3 to + 6.8) than the high-titanium basalts (initial ɛHf = + 10.4 to + 12.0; initial ɛNd = + 6.6 to + 9.0), and their Pb isotopic compositions overlap. Incongruent dynamic melting modeling of trace element compositions indicate the low-titanium basalts could have been derived from small degrees of melting (< 5%) of Paleozoic sub-arc lithospheric mantle that was HFSE-depleted and evolved with high 176Hf/ 177Hf. The high-titanium basalts formed from melting of Pacific plume-type mantle, similar to the source of the Caribbean Plateau. Plume-derived melts dominated the upper stratigraphy of the oceanic plateau as a result of increased decompression melting of the underlying mantle plume in response to thinning of the lithosphere.

  12. 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

  13. Construction of an Oceanic Plateau: Stratigraphic and Geochemical Perspectives from the Accreted Triassic Wrangellia Flood Basalts

    NASA Astrophysics Data System (ADS)

    Scoates, J. S.; Greene, A. R.; Weis, D. A.

    2009-12-01

    The accreted Wrangellia flood basalts and associated pre- and post-volcanic sedimentary rocks provide an unparalleled view of the architecture and compositional evolution of an oceanic plateau. Wrangellia may ultimately hold keys to answering some of the major questions about the development of oceanic plateaus, including how these large oceanic features initiate, build-up, and terminate, the timescale of eruptions, and the impact of very large-scale melting events and associated volcanism on the environment. The Wrangellia plateau extends ~2300 km in the Pacific Northwest as a discontinuous belt from central Alaska and western Yukon (Nikolai Formation: <3.5 km) to Vancouver Island (Karmutsen Formation: ~6 km). Wrangellia flood basalts are bounded by Middle to Late Triassic marine sediments and unconformably overlie Paleozoic arc volcanic sequences of different age. Based on combined radiometric (U-Pb, 40Ar/39Ar, K-Ar), paleontological, and magnetostratigraphic age constraints, the Wrangellia basalts were emplaced during a single phase of tholeiitic volcanism at ca. 230-225 Ma, and possibly within as little as 2 Myr, onto pre-existing submerged arc crust. The vast majority of the flood basalts are LREE-enriched high-Ti basalt (1.6-2.4 wt% TiO2; 6-8 wt% MgO) with relatively uniform isotopic compositions at 230 Ma (ɛHf = +9.1 to +12.6; ɛNd = +6.0 to +8.7). The lowest part of the stratigraphy in areas of Alaska and Yukon is LREE-depleted low-Ti basalt (0.4-1.2 wt% TiO2) with pronounced negative-HFSE anomalies and Hf isotopic compositions (ɛHf = +13.7 to +18.4) that are decoupled from Nd (ɛNd = +4.6 to +5.4) and displaced well above the OIB mantle array (ΔɛHf = +4 to +8). Submarine stratigraphy on northern Vancouver Island contains picritic basalts (9-20 wt%) depleted in LREE (La/YbCN = 0.5 ± 0.2) with overlapping initial ɛHf (+10.3 ± 2.1) and ɛNd (+7.7 ± 1.3) to the high-Ti basalts. A mantle plume initiation model for the Wrangellia flood basalts is

  14. 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

  15. Silurian Gastropoda from the Alexander terrane, southeast Alaska

    USGS Publications Warehouse

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

    2008-01-01

    Gastropods are described from Ludlow-age strata of the Heceta Limestone on Prince of Wales Island, southeast Alaska. They are part of a diverse megabenthic fauna of the Alexander terrane, an accreted terrane of Siberian or Uralian affinities. Heceta Limestone gastropods with Uralian affinities include Kirkospira glacialis, which closely resembles "Pleurotomaria" lindstromi Oehlert of Chernyshev, 1893, Retispira cf. R. volgulica (Chernyshev, 1893), and Medfracaulus turriformis (Chernyshev, 1893). Medfracaulus and similar morphotypes such as Coelocaulus karlae are unknown from rocks that are unquestionably part of the North American continent (Laurentia) during Late Silurian time. Beraunia is previously known only from the Silurian of Bohemia. Pachystrophia has previously been reported only from western North American terranes (Eastern Klamath, York, and Farewell terranes) and Europe. Bathmopterus Kirk, 1928, is resurrected and is only known from the Silurian of southeast Alaska. Newly described taxa include Hecetastoma gehrelsi n. gen. and n. sp. and Baichtalia tongassensis n. gen. and n. sp. ??2008 The Geological Society of America.

  16. Tectonic History and Metallogeny of the Chukchi Terrane

    NASA Astrophysics Data System (ADS)

    Byalobzhesky, S.; Goryachev, N. A.

    2004-12-01

    The Chukchi Terrane consists of the Anyui, Wrangel, Chaun and Bering Sub-Terranes; at present, it is the northern part of the Pacific Folded Belt. Its basement is composed of folded rocks of Proterozoic and Paleozoic, which crop out on Wrangel Island, the Kuul and Alyarmaut Uplifts, Chukotka and Seward Peninsulas. Early Precambrian blocks may be also present in it. The above-mentioned sub-terranes have a similar development history and are featured by the same type of Mesozoic metamorphism, magmatism and metallogeny. Its tectonic history has been as follows: 1. Late Proterozoic and Early Paleozoic (pre-Visean) magmatism, thrusting, isoclinal folding. Scarce occurrences of gold and stibnite mineralization. 2. An unconformity at the base of Visean - Mid Carboniferous section with basal conglomerates contain granite pebbles and boulders. 3. Permian uplifting (continental), reduced sedimentation. 4. Maximum sedimentation in Triassic time accompanied by Early Triassic rifting and dominating turbidite rocks. The problem is the provenance areas for large amounts of terrigenous rocks. 5. Lack of sediments characterisitc of the greatest part of early Jurassic (post-Sinemurian), middle Jurassic and late Jurassic (pre-Volgian) time periods. Uplifting is correlative with the extension stage in the South-Anyui Ocean. 6. Late Jurassic - Neocomian. Intense uplifting processes occured at the end of late Jurassic and in early Cretaceous and associated with intrusion of subduction- and collision-related granitoids (147-140 Ma). The Nutesyn marginal continental arc was forming over the southern periphery of terrane and flysch processes occurred there through Neocomian. Fault-related depressions were developing in the north of the area under consideration. Since late Jurassic, as a result of an approach (with a right-side fault shifting) of the Novosibirsk-Chukchi Super-Terrane and the northeastern edge of the Asiatic Craton, the Yuzhno-Anyui Ocean began to close from west to east

  17. 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.

  18. Evidence for Cambrian deformation in the Ellsworth-Whitmore Mountains terrane, Antarctica: Stratigraphic and tectonic implications

    NASA Astrophysics Data System (ADS)

    Duebendorfer, Ernest M.; Rees, Margaret N.

    1998-01-01

    The Ellsworth-Whitmore Mountains terrane is a large geologically and geophysically defined crustal block that lies between the Transantarctic Mountains and West Antarctica. The Cambrian position of the terrane is controversial, with many workers placing it between East Antarctica and southern Africa and distant from Cambrian orogenic belts. We present structural and stratigraphic evidence for Cambrian deformation in the Heritage Range, Ellsworth Mountains. From our revised stratigraphy and structural history of the Heritage Range, we propose that the Ellsworth-Whitmore Mountains block was located within the belt of Pan-African deformation, within the Late Cambrian continental arc, and was part of a collage of allochthonous terranes that included the Queen Maud terrane and probably the Bowers terrane of Antarctica. These terranes were situated outboard of Coats Land in the Cambrian and were subsequently translated and accreted to East Antarctica, probably during early Paleozoic time.

  19. 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.

  20. 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.

  1. Oceanic plateaus, the fragmentation of continents, and mountain building

    NASA Astrophysics Data System (ADS)

    Nur, Amos; Ben-Avraham, Zvi

    1982-05-01

    Many anomalous rises in today's oceans may be submerged continental fragments detached from previous continents, ancient island arcs, or basaltic piles formed by hot spots and spreading centers. These rises are embedded in their respective moving oceanic plates and are fated to be consumed at active margins. Where such rises are being consumed at present, e.g., the Nazca Ridge, they cause cessation of volcanism, disruption of the downgoing slab, and possible shifts in plate boundary configuration. Many past rises, including numerous continental fragments, have been recognized within mountain belts as allochthonous terranes. They constitute a large portion of the orogenic belts in the North Pacific from Mexico through western North America, Alaska, east Siberia, Japan and in New Zealand. The orogenic deformation in these belts is possibly the result of the accretion of the allochthonous terranes. Many terranes have been accreted with substantial deformation also in the Alpine chain, well before major continent-continent collisions. It is suggested, therefore, that the accretion of fragments may be the common process of the deformation phase of mountain building. Subduction of normal oceanic crust may be insufficient for deformation, whereas full continent-continent collision may not be necessary. The general validity of this conclusion depends critically on whether allochthonous terranes caused orogenic deformation in the Andes or not. Most of the accreted fragments with continental affinities in the Mesozoic-Cenozoic orogenic belts of the world can be traced back to the breakup of Gondwana, beginning with a Pacifica domain in the Permian through a larger India domain in the early Mesozoic and continuing through the separation of the Somalia plate in the near future. The reasons for this 250 million year breakup process are not known, but some kind of thermal process, possible of mantle-wide scale, is implied.

  2. Oceanic plateaus, the fragmentation of continents, and mountain building

    SciTech Connect

    Nur, A.; Ben-Avraham, Z.

    1982-05-10

    Many anomalous rises in today's oceans may be submerged continental fragments detached from previous continents, ancient island arcs, or basaltic piles formed by hot spots and spreading centers. These rises are embedded in their respective moving oceanic plates and are fated to be consumed at active margins. Where such rises are being consumed at present, e.g., the Nazca Ridge, they cause cessation of volcanism, disruption of the downgoing slab, and possible shifts in plate boundary configuration. Many past rises, including numerous continental fragments have been recognized within mountain belts as allochthonous terranes. They constitute a large portion of the orogenic belts in the North Pacific from Mexico through western North America, Alaska, east Siberia, Japan and in New Zealand. The orogenic deformation in these belts is possibly the result of the accretion of the allochtronous terranes. Many terranes have been accreted with substantial deformation also in the Alpine chain, well before major continent-continent collisions. It is suggested, therefore, that the accretion of fragments may be the common process of the deformation phase of mountain building. Subduction of normal oceanic crust may be insufficient for deformation, whereas full continent-continent collision may be necessary. The general validity of this conclusion depends critically on whether allochthonous terranes caused orogenic deformation in the Andes or not. Most of the accreted fragments with continental affinites in the Mesozoic-Cenozoic orogenic belts of the world can be traced back to the breakup of Gondwana, beginning with a Pacifica domain in the Permian through a larger India domain in the early Mesozoic and continuing through the separation of the Somalia plate in the near future. The reasons for this 250 million year breakup process are not known, but some kind of thermal process, possible of mantle-wide scale, is implied.

  3. 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.

  4. 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.

  5. Cache Creek terrane entrapment: Oroclinal paradox within the Canadian Cordillera

    NASA Astrophysics Data System (ADS)

    Mihalynuk, Mitch G.; Nelson, Joanne; Diakow, Larry J.

    1994-06-01

    Exotic and far-traveled oceanic crustal rocks of the Cache Creek terrane (CC) are bordered by less exotic Quesnel (QN) and Stikine (ST) arc terranes to the east, north, and west. All of these terranes are enveloped by an arcuate belt of displaced continental margin rocks; the Kootenay (KO), Nisling (NS), and parts of the Yukon-Tanana (YTT) terranes, that have indirect ties to ancestral North America (NA). Initial 87Sr/86Sr isopleths conform to this arcuate pattern. Such a pattern of concentric belts presents a geological conundrum: How did the QN, ST, and CC come to be virtually enveloped by terranes with ties to NA? Past and current models that explain assembly of the Canadian Cordillera are deficient in their treatment of this problem. We propose that Early Mesozoic QN and ST were joined through their northern ends as two adjacent arc festoons that faced south toward the Cache Creek ocean (Panthalassa?). Oceanic plateau remnants within the CC today were transported from the Tethyan realm and collided with these arcs during subduction of the Cache Creek ocean. Counterclockwise oroclinal rotation of ST and NS terranes in the Late Triassic to Early Jurassic caused enclosure of the CC. Rotation continued until these terranes collided with QN in the Middle Jurassic. Paleomagnetic declination data provide support for this model in the form of large average anticlockwise rotations for Permian to Early Jurassic sites in ST but moderate clockwise rotations for sites in QN. Specific modern analogues for the Cordilleran orocline include the Yap trench, where the Caroline rise is colliding end-on with the Mariana Arc and the Banda Arc, located on the southeastern "tail" of the Asian plate, which is being deformed into a tight loop by interactions with the Australian and Pacific plates.

  6. Wrangellia flood basalts in Alaska: A record of plume-lithosphere interaction in a Late Triassic accreted oceanic plateau

    NASA Astrophysics Data System (ADS)

    Greene, Andrew R.; Scoates, James S.; Weis, Dominique

    2008-12-01

    The Wrangellia flood basalts are part of one of the best exposed accreted oceanic plateaus on Earth. They provide important constraints on the construction of these vast submarine edifices and the source and temporal evolution of magmas for a plume head impinging beneath oceanic lithosphere. Wrangellia flood basalts (˜231-225 Ma) extend ˜450 km across southern Alaska (Wrangell Mountains and Alaska Range) where ˜3.5 km of mostly subaerial flows are bounded by late Paleozoic arc volcanics and Late Triassic limestone. The vast majority of the flood basalts are light rare earth element (LREE) -enriched high-Ti basalt (1.6-2.4 wt % TiO2) with uniform ocean island basalt (OIB) -type Pacific mantle isotopic compositions (ɛHf(t) = +9.7 to +10.7; ɛNd(t) = +6.0 to +8.1; t = 230 Ma). However, the lowest ˜400 m of stratigraphy in the Alaska Range is LREE-depleted low-Ti basalt (0.4-1.2 wt % TiO2) with pronounced negative high field strength element (HFSE) anomalies and Hf isotopic compositions (ɛHf(t) = +13.7 to +18.4) that are decoupled from Nd (ɛNd(t) = +4.6 to +5.4) and displaced well above the OIB mantle array (ΔɛHf = +4 to +8). The radiogenic Hf of the low-Ti basalts indicates involvement of a component that evolved with high Lu/Hf over time but not with a correspondingly high Sm/Nd. The radiogenic Hf and HFSE-depleted signature of the low-Ti basalts suggest pre-existing arc lithosphere was involved in the formation of flood basalts that erupted early in construction of part of the Wrangellia plateau in Alaska. Thermal and mechanical erosion of the base of the lithosphere by the impinging plume head may have led to melting of arc lithosphere or interaction of plume-derived melts and subduction-modified mantle. The high-Ti lavas dominate the main phase of construction of the plateau and were derived from a depleted mantle source distinct from the source of MORB and with compositional similarities to that of ocean islands (e.g., Hawaii) and plateaus (e.g., Ontong

  7. Timing of tectonic emplacement of the ophiolites and terrane paleogeography in the Hellenides

    NASA Astrophysics Data System (ADS)

    Papanikolaou, Dimitrios

    2009-03-01

    The timing of tectonic emplacement of the ophiolites is analyzed in the four oceanic terranes of the Hellenides (H 2, H 4, H 6, H 8). The criteria for this analysis are based on: a) the post-emplacement sedimentary cover or intrusive rocks, b) the syn-emplacement tectonostratigraphic formations and c) the youngest rocks involved in the structure of the autochthon and the allochthon unit in each case. The timing becomes younger towards the more external tectonic units of the Hellenides with: (i) Late Eocene-Oligocene age in the external ophiolite belt of the Pindos-Cyclades oceanic terrane H 2, (ii) Late Jurassic-Early Cretaceous age in the internal ophiolite belt of the Vardar/Axios oceanic terrane H 4 , (iii) Post-Liassic-pre-Late Jurassic age in the ophiolites of Lesvos-Circum Rhodope oceanic terrane H 6 and (iv) Pre-Late Jurassic age in the ophiolites of Volvi-Eastern Rhodope terrane H 8. An ophiolite obduction model can be applied, with the ophiolitic nappes always emplaced on top of pre-Alpine continental terranes with Mesozoic shallow-water carbonate platforms. The geometry of the continental terranes drifting during the Mesozoic within the Tethys Ocean controls the number and dimensions of the Tethyan oceanic basins. Where a continental terrane dies out, the two adjacent oceanic basins merge into one larger basin. This seems to be the case of the Pelagonian terrane (H 3), which is terminated north of Skopje, where the Pindos oceanic basin (H 2) merges with the Vardar/Axios oceanic basin (H 4).

  8. 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.

  9. 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

  10. 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

  11. 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

  12. 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.

  13. Bowser basin, northern British Columbia: Constraints on the timing of initial subsidence and Stikinia-North America terrane interactions

    NASA Astrophysics Data System (ADS)

    Ricketts, Brian D.; Evenchick, Carol A.; Anderson, Robert G.; Murphy, Donald C.

    1992-12-01

    Clastic strata composing the northern Bowser basin record the accretion of Stikinia to the composite western edge of the North American plate (Cache Creek-Quesnellia-Slide Mountain-Kootenay North America) in early Middle Jurassic time and the concomitant demise of the intervening Cache Creek ocean. Initial flexural subsidence of the northern Bowser basin, resulting from thrust loading of Cache Creek terrane on Stikinia, is represented by an organic-rich shale of Aalenian age (the Abou Formation of the Spatsizi Group). Coarse-grained sediment first appeared in early Bajocian time following uplift and subaerial exposure of Cache Creek rocks in the upper plate. Thus, the inception of the Bowser basin was Aalenian, rather than Bajocian, as believed by earlier workers. Aalenian southwest-vergent thrusting at the composite western edge of North America is also known from southern British Columbia, a coincidence that implicates collision with Stikinia in the south as a cause of that deformation.

  14. Geology, petrology and tectonic significance of the Mesozoic Paleoceanic terranes of the Vizcaino Peninsula, Baja California Sur, Mexico

    NASA Astrophysics Data System (ADS)

    Moore, T. E.

    The Viscaino Terrane, which is the most outboard terrane of mainland Baja California, can be divided into three principal terranes on the Vizcaino Peninsula. The structurally lowest is the disrupted composite Puerto Nuevo terrane which is represented by metamorphosed blocks in a serpentinite-matrix melange. The Viscaino Norte terrane structurally overlies the Puerto Nuevo terrane along a low-angle contact. To the south, the Vizcaino Sur terrane exposes a sequence consisting of ophiolite, tuffaceous chart and limestone of the San Hipolito Formation of Late Triassic age. Based on the petrologic, geochemical, and stratigraphic characteristics, the ophiolites of the Vizcaino Norte and Vizcaino Sur terranes are interpreted to have been formed in one or more marginal basin island arc systems during the Late Triassic whereas the ophiolitic rocks of the Puerto Nuevo terrane most likely formed at a midocean ridge. The subsequent pre-Upper Jurassic island arc deposits of the terranes are entirely volcanogenic and biogenic and contain no evidence of close proximity to a cratonal source. It is suggested that for as much as 80 my, the terranes were probably not associated with a continental margin as a fringing island arc, but instead represent allochthonous fragments of a paleogeographically complex paleo-Pacific Ocean.

  15. 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.

  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. Ordovician and Triassic mafic dykes in the Wudang terrane: Evidence for opening and closure of the South Qinling ocean basin, central China

    NASA Astrophysics Data System (ADS)

    Nie, Hu; Wan, Xin; Zhang, He; He, Jian-Feng; Hou, Zhen-Hui; Siebel, Wolfgang; Chen, Fukun

    2016-12-01

    We report zircon ages and geochemical composition for mafic dykes that intruded Neoproterozoic volcanic-sedimentary sequences in the southern part of Wudang area, South Qinling. The results indicate that the dykes were emplaced during the Early Paleozoic (c. 460 Ma) and Early Mesozoic (c. 220 Ma). The dykes share similar major element composition, but have distinctive trace element pattern and Sr-Nd-Pb isotope distribution. Early Paleozoic mafic dykes are characterized by enrichment in LREEs, LILEs and HFSEs and EM II-type isotopic features. These geochemical features suggest derivation from an OIB-type mantle source that had undergone metasomatism during earlier subduction events. The Early Mesozoic mafic dykes can be subdivided into two distinct geochemical groups. Dykes of Group 1 are depleted in LREEs, LILEs and HFSEs and show depleted isotope compositions, indicating an origin by partial melting of asthenospheric mantle material. Dykes of Group 2 have high Rb-, Ba-, and K-contents and EM I-type isotopic features, suggesting input of lower crustal material to the magma source during Mesozoic subduction. We propose that the Early Paleozoic dykes are related to the opening of an oceanic basin separating South Qinling from the Yangtze Block, while the Early Mesozoic dykes were derived from partial melting of up-welling asthenosphere during the final amalgamation of these two blocks in the Early Mesozoic. A slab break-off model could explain not only the petrogenesis of the Mesozoic mafic dykes, but also the distinct geological features between the Dabie-Sulu and South Qinling orogens. We propose that slab break-off occurred at great depth in the Dabie-Sulu orogen and hence rare magmatism occurred. Whereas in South Qinling the break-off occurred at a shallow depth, the asthenospheric mantle material could rise further up into the overlying mantle where it experienced decompression and melting. As a consequence, crustal sections were heated up to produce extensive

  18. The odyssey of the Cache Creek terrane, Canadian Cordillera: Implications for accretionary orogens, tectonic setting of Panthalassa, the Pacific superwell, and break-up of Pangea

    NASA Astrophysics Data System (ADS)

    Johnston, S. T.; Borel, G. D.

    2007-01-01

    The Cache Creek terrane (CCT) of the Canadian Cordillera consists of accreted seamounts that originated adjacent to the Tethys Ocean in the Permian. We utilize Potential Translation Path plots to place quantitative constraints on the location of the CCT seamounts through time, including limiting the regions within which accretion events occurred. We assume a starting point for the CCT seamounts in the easternmost Tethys at 280 Ma. Using reasonable translation rates (11 cm/a), accretion to the Stikinia-Quesnellia oceanic arc, which occurred at about 230 Ma, took place in western Panthalassa, consistent with the mixed Tethyan fauna of the arc. Subsequent collision with a continental terrane, which occurred at about 180 Ma, took place in central Panthalassa, > 4000 km west of North America yielding a composite ribbon continent. Westward subduction of oceanic lithosphere continuous with the North American continent from 180 to 150 Ma facilitated docking of the ribbon continent with the North American plate. The paleogeographic constraints provided by the CCT indicate that much of the Canadian Cordilleran accretionary orogen is exotic. The accreting crustal block, a composite ribbon continent, grew through repeated collisional events within Panthalassa prior to docking with the North American plate. CCT's odyssey requires the presence of subduction zones within Panthalassa and indicates that the tectonic setting of the Panthalassa superocean differed substantially from the current Pacific basin, with its central spreading ridge and marginal outward dipping subduction zones. A substantial volume of oceanic lithosphere was subducted during CCT's transit of Panthalassa. Blanketing of the core by these cold oceanic slabs enhanced heat transfer out of the core into the lowermost mantle, and may have been responsible for the Cretaceous Normal Superchron, the coeval Pacific-centred mid-Cretaceous superplume event, and its lingering progeny, the Pacific Superswell. Far field

  19. 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.

  20. 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.

  1. Tectonics and Terranes in the Undergraduate Curriculum.

    ERIC Educational Resources Information Center

    Lambert, Richard St J.

    1989-01-01

    Gives the background to the philosophy for the introductory teaching of global tectonics and the terrane concept. Provides a discussion of local and general considerations, the curriculum, and terrane tectonics. (RT)

  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. Gondwana dispersion and Asian accretion: Tectonic and palaeogeographic evolution of eastern Tethys

    NASA Astrophysics Data System (ADS)

    Metcalfe, I.

    2013-04-01

    Present-day Asia comprises a heterogeneous collage of continental blocks, derived from the Indian-west Australian margin of eastern Gondwana, and subduction related volcanic arcs assembled by the closure of multiple Tethyan and back-arc ocean basins now represented by suture zones containing ophiolites, accretionary complexes and remnants of ocean island arcs. The Phanerozoic evolution of the region is the result of more than 400 million years of continental dispersion from Gondwana and plate tectonic convergence, collision and accretion. This involved successive dispersion of continental blocks, the northwards translation of these, and their amalgamation and accretion to form present-day Asia. Separation and northwards migration of the various continental terranes/blocks from Gondwana occurred in three phases linked with the successive opening and closure of three intervening Tethyan oceans, the Palaeo-Tethys (Devonian-Triassic), Meso-Tethys (late Early Permian-Late Cretaceous) and Ceno-Tethys (Late Triassic-Late Cretaceous). The first group of continental blocks dispersed from Gondwana in the Devonian, opening the Palaeo-Tethys behind them, and included the North China, Tarim, South China and Indochina blocks (including West Sumatra and West Burma). Remnants of the main Palaeo-Tethys ocean are now preserved within the Longmu Co-Shuanghu, Changning-Menglian, Chiang Mai/Inthanon and Bentong-Raub Suture Zones. During northwards subduction of the Palaeo-Tethys, the Sukhothai Arc was constructed on the margin of South China-Indochina and separated from those terranes by a short-lived back-arc basin now represented by the Jinghong, Nan-Uttaradit and Sra Kaeo Sutures. Concurrently, a second continental sliver or collage of blocks (Cimmerian continent) rifted and separated from northern Gondwana and the Meso-Tethys opened in the late Early Permian between these separating blocks and Gondwana. The eastern Cimmerian continent, including the South Qiangtang block and

  5. 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.

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

    PubMed

    Sigloch, Karin; Mihalynuk, Mitchell G

    2013-04-04

    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.

  7. Structural analysis of the southern Peninsular, southern Wrangellia, and northern Chugach terranes along the Trans-Alaska Crustal Transect, northern Chugach Mountains, Alaska

    USGS Publications Warehouse

    Nokleberg, W.J.; Plafker, G.; Lull, J.S.; Wallace, W.K.; Winkler, G.R.

    1989-01-01

    Structural and tectonic analysis of the southern Peninsular, southern Wrangellia, and northern Chugach terranes, along the Trans-Alaska Crustal Transect in the northern Chugach Mountains documents a long succession of Early Jurassic through Cenozoic deformational events. The deformational events are generally characterized by distinctive structural fabrics and metamorphisms. Most of the events are interpreted to be related to subduction-related accretion or terrane accretion. Each period of subduction-related accretion consisted of underplating of the outboard unit beneath the adjacent inboard unit. The fabric associated with each subduction-related accretion consisted of folding, intense shearing, and local rolling of planar structures. Age and structural relationships suggest migration of the zone of subduction-related accretion from the BRFS to the north, through each accreting unit, to younger bounding thrust faults to the south. -from Author

  8. 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

  9. Cache Creek ocean: Closure or enclosure?

    NASA Astrophysics Data System (ADS)

    Nelson, Joanne; Mihalynuk, Mitch

    1993-02-01

    Exotic Tethyan faunas within the Cache Creek terrane contrast markedly with faunas and lithologic associations in the adjacent Quesnel and Stikine terranes. In northern British Columbia and southeast Yukon, all three terranes are enveloped in the north by pericontinental rocks of the Yukon-Tanana terrane, a geometry that imposes severe constraints on terrane assembly models for the northern Canadian Cordillera. Our solution to the problem invokes a northern join between the Stikinia and Quesnellia arcs through the Yukon-Tanana terrane, forming an orocline that encloses the Cache Creek terrane. This model involves (1) collision of a linear oceanic plateau at the cusp between Quesnellia and Stikinia, (2) anticlockwise rotation of Stikinia about an axis in the Yukon-Tanana terrane, (3) simultaneous enclosure of the Cache Creek ocean, and (4) emplacement of Quesnellia onto the margin of ancestral North America and the Cache Creek terrane onto Stikinia during final closure of the orocline. Early Mesozoic Paleomagnetic declinations in Stikinia are permissive of the large anticlockwise rotations predicted by the model. Similar large-scale rotations and ocean-basin enclosure are common features in the southwest Pacific. This model accounts for Paleozoic and younger linkages between Yukon-Tanana and both northern Stikinia and Quesnellia, the striking similarity between Triassic-Jurassic arcs east and west of the Cache Creek terrane, and the profound early Mesozoic deformational event in the Yukon-Tanana terrane.

  10. Magnetized accretion

    NASA Astrophysics Data System (ADS)

    Heyvaerts, J.

    This lecture reviews in simple terms the general subject of large scale magnetic field coupling to plasma flows in the vicinity of accreting compact stars. The relevant astrophysical phenomenology is summarized. Disk interaction with the magnetosphere of accreting stars is first discussed, in particular the structure of the magnetopause, its stability and plasma ejection in so-called propeller systems. The physics of accretion/ejection is then considered. Acceleration and focusing mechanisms of jets from accretion disks around compact stars or black holes and the question of the self-consistency of accretion and ejection are described. By contrast, small scale MHD turbulence in disks is not discussed, neither are accretion columns near the polar caps of neutron stars or white dwarfs. The reader is only assumed to have some basic knowledge of astrophysics and of fluid mechanics and electromagnetism.

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

    USGS Publications Warehouse

    Aleinikoff, J.N.; Horton, J.W.; 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.

  12. 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.

  13. The subduction-accretion history of the Bangong-Nujiang Ocean: Constraints from provenance and geochronology of the Mesozoic strata near Gaize, central Tibet

    NASA Astrophysics Data System (ADS)

    Li, Shun; Ding, Lin; Guilmette, Carl; Fu, Jiajun; Xu, Qiang; Yue, Yahui; Henrique-Pinto, Renato

    2017-04-01

    The Mesozoic strata, within the Bangong-Nujiang suture zone in central Tibet, recorded critical information about the subduction-accretion processes of the Bangong-Nujiang Ocean prior to the Lhasa-Qiangtang collision. This paper reports detailed field observations, petrographic descriptions, sandstone detrital zircon U-Pb ages and Hf isotopic analyses from an accretionary complex (preserved as Mugagangri Group) and the unconformably overlying Shamuluo Formation near Gaize. The youngest detrital zircon ages, together with other age constraints from literature, suggest that the Mugagangri Group was deposited during late Triassic-early Jurassic, while the Shamuluo Formation was deposited during late Jurassic-early Cretaceous. Based on the differences in lithology, age and provenance, the Mugagangri Group is subdivided into the upper, middle and lower subunits. These units are younging structurally downward/southward, consistent with models of progressive off-scrapping and accretion in a southward-facing subduction complex. The upper subunit, comprising mainly quartz-sandstone and siliceous mud/shale, was deposited in abyssal plain environment close to the Qiangtang passive margin during late Triassic, with sediments derived from the southern Qiangtang block. The middle and lower subunits comprise mainly lithic-quartz-sandstone and mud/shale, containing abundant ultramafic/ophiolitic fragments. The middle subunit, of late Triassic-early Jurassic age, records a transition in tectono-depositional setting from abyssal plain to trench-wedge basin, with sudden influx of sediments sourced from the central Qiangtang metamorphic belt and northern Qiangtang magmatic belt. The appearance of ultramafic/ophiolitic fragments in the middle subunit reflects the subduction initiation. The lower subunit was deposited in a trench-wedge basin during early Jurassic, with influx of Jurassic-aged zircons originating from the newly active southern Qiangtang magmatic arc. The lower subunit

  14. 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.

  15. 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

  16. 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

  17. Proterozoic subduction and terrane amalgamation in the southwestern Grenville province, Canada: Evidence from ultrapotassic to shoshonitic plutonism

    NASA Astrophysics Data System (ADS)

    Corriveau, Louise

    1990-07-01

    A late Grenvillian (1089-1076 Ma) subduction regime followed by terrane amalgamation is postulated as the paleoenvironment of a 400-km-long belt of potassium-rich alkaline and shoshonitic plutons in the Central metasedimentary belt of the southwestern Grenville province, Canada. Emplacement of the plutons postdates the regional metamorphism in the country rock, but predates major shear zones that form a structural boundary for the plutonic belt. The extent, timing, and magmatic affinities of the suite delineate the Gatineau domain within the current Mont-Laurier terrane of Quebec and are compelling evidence for the allochthonous nature of the Elzevir terrane and its extension eastward to Rideau Lake in Ontario and northward into the Gatineau domain. The belt trends northeast, the emplacement ages are younger to the southeast, and the magmatic affinities are those of island-arc ultrapotassic to shoshonitic rocks. This is interpreted to reflect the existence of a southeast-dipping, northeast-trending subduction zone beneath the combined Elzevir-Gatineau terrane between 1089 and 1076 Ma. Subsequent to subduction, amalgamation of the Elzevir terrane to the other terranes formed the Central metasedimentary belt, which then accreted and collided with the allochthonous polycyclic belt. These events provide evidence that the Ottawan orogeny commenced at ca. 1090 Ma in the Central metasedimentary belt. A modern analogue of this plutonism and its tectonic setting may have been the magmatism and arc-continent collision and subduction setting of the Sunda are, Indonesia.

  18. Foundering and Exhumation of UHP Terranes: Race Car or School Bus?

    NASA Astrophysics Data System (ADS)

    Kylander-Clark, A. R.; Hacker, B. R.

    2008-12-01

    Recent geochronologic data from the giant ultrahigh-pressure (UHP) terrane, in the Western Gneiss Region of Norway, indicate that subduction and exhumation were relatively slow (a few mm/yr), and that the terrane was exhumed to the surface as a relatively thick, coherent body. These conclusions are in stark contrast to those reached in previous studies of some of the best-studied, smaller UHP terranes and suggest that the processes that form and/or exhume small UHP terranes are fundamentally different from the processes that affect large UHP terranes. These differences may be the result of variations in the buoyancy forces of different proportions of subducted felsic crust, mafic crust, and mantle lithosphere. Initial collision occurs via the subduction of smaller portions of continental material, such as microcontinents or ribbon continents. Because the proportion of continental crust is small, the processes involved in early UHP terrane formation are dominated by the oceanic slab; subduction rates are fast because average plate densities are high, and, as a result, subduction angles are steep. Because these smaller, thinner portions of crust are weak, they deform easily and mix readily with the mantle. As the collision matures, thicker and larger portions of continental material-such as a continental margin-are subducted, and the subduction regime changes from one that was ocean dominated to one that is continent dominated. The increased buoyancy of the larger volume of continental crust resists the pull of the leading oceanic lithosphere; subduction shallows and plate rates slow. Because the downgoing continent is thick, it is strong, remains cohesive and has limited interaction with the mantle. Although the subduction regime during early orogenesis is distinct from that during late orogenesis, the degree of mountain building and crustal thickening may be similar in both stages as small volumes and fast flow rates of buoyant material give way to large volumes

  19. Nd isotopic characterization of metamorphic rocks in the Coast Mountains, Alaskan and Canadian Cordillera: Ancient crust bounded by juvenile terranes

    NASA Astrophysics Data System (ADS)

    Samson, Scott D.; Patchett, P. Jonathan; McClelland, William C.; Gehrels, George E.

    1991-08-01

    rocks that they are considered part of that terrane. Possible tectonic scenarios that can explain the present geometry of the YTT with respect to the Alexander-Wrangellia and Stikine terranes include: (1) The YTT is the upturned stratigraphic basement of the Stikine terrane, (2) part of the YTT was structurally emplaced beside the Stikine terrane in a transpressive tectonic regime, (3) the Stikine terrane and other inboard terranes are huge sheets that were thrust over the margin of the YTT before the final accretion of the Alexander-Wrangellia terrane.

  20. 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.

  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. The Ellsworth terrane, coastal Maine: Geochronology, geochemistry, and Nd-Pb isotopic composition - Implications for the rifting of Ganderia

    USGS Publications Warehouse

    Schulz, K.J.; Stewart, D.B.; Tucker, R.D.; Pollock, J.C.; Ayuso, R.A.

    2008-01-01

    The Ellsworth terrane is one of a number of fault-bounded blocks that occur along the eastern margin of Ganderia, the western-most of the peri-Gondwanan domains in the northern Appalachians that were accreted to Laurentia in the Paleozoic. Geologic relations, detrital zircon ages, and basalt geochemistry suggest that the Ellsworth terrane is part of Ganderia and not an exotic terrane. In the Penobscot Bay area of coastal Maine, the Ellsworth terrane is dominantly composed of bimodal basalt-rhyolite volcanic sequences of the Ellsworth Schist and unconformably overlying Castine Volcanics. We use new U-Pb zircon geochronology, geochemistry, and Nd and Pb isotopes for these volcanic sequences to constrain the petrogenetic history and paleotectonic setting of the Ellsworth terrane and its relationship with Ganderia. U-Pb zircon geochronology for rhyolites indicates that both the Ellsworth Schist (508.6 ?? 0.8 Ma) and overlying Castine Volcanics (503.5 ?? 2.5 Ma) are Middle Cambrian in age. Two tholefitic basalt types are recognized. Type Tb-1 basalt, present as pillowed and massive lava flows and as sills in both units, has depleted La and Ce ([La/Nd]N = 0.53-0.87) values, flat heavy rare earth element (REE) values, and no positive Th or negative Ta anomalies on primitive mantle-normalized diagrams. In contrast, type Th-2 basalt, present only in the Castine Volcanics, has stightly enriched LREE ([La/Yb]N = 1.42-2.92) values and no Th or Th anomalies. Both basalt types have strongly positive ??Nd (500) values (Th-1 = +7.9-+8.6; Th-2 = +5.6-+7.0) and relatively enriched Pb isotopic compositions (206Ph/204Pb = 18.037-19.784; 207/204Pb = 15.531-15.660; 2088Pb/204Pb = 37.810-38.817). The basalts have compositions transitional between recent normal and enriched mid-ocean-ridge basalt, and they were probably derived by partial melting of compositionatly heterogeneous asthenosphenc mantle. Two types of rhyolite also are present. Type R-1 rhyolite, which mostly occurs as tuffs

  6. 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.

  7. 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.

  8. 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.

  9. Guerrero terrane of Mexico: Its role in the Southern, Cordillera from new geochemical data

    NASA Astrophysics Data System (ADS)

    Centeno-García, Elena; Ruíz, Joaquín; Coney, Peter J.; Patchett, P. Jonathan; Ortega-Gutiérrez, Fernando

    1993-05-01

    The Guerrero terrane makes up most of the western part of Mexico, is one of the largest terranes of the North American Cordillera, and is characterized by an Upper Jurassic-Lower Cretaceous volcanic-sedimentary sequence of are affinity. Metamorphic rocks that crop out in the western area of the terrane (Arteaga complex) may represent its basement. They are mostly composed of terrigenous sediments (Varales Formation) with minor basaltic pillow lavas, chert, tuff, and limestone. Initial ɛNd values (+13) and rare earth element (REE) values for pillow lavas of the Arteaga complex are characteristic of mid-ocean ridge basalts (MORB). In contrast, the Varales Formation sedimentary rocks from the Arteaga complex have negative initial ɛNd (-6.2 and -7.2) and are enriched in light REEs. These data indicate that the sediments of the Varales Formation were supplied from an evolved continental crust. The overlying Jurassic(?)-Cretaceous arc-related rocks have initial ɛNd (+7.9 to +3.9) and REE patterns similar to those of evolved intraoceanic island arcs. These data show that the evolution of the Guerrero terrane had an early pre-Cretaceous(?) stage, which consisted of an oceanic crust receiving sediments from a continental source, and a Cretaceous stage, which was the development of an island arc.The oceanic-continental isotopic signature of the Arteaga complex is different from other western North American Cordilleran terranes (e.g., Alexander, Wrangelfia) that are more completely "oceanic" in affinity. Nevertheless, the extensive Jurassic(?).Cretaceous arc represents additions of juvenile material to the western North American Cordillera.

  10. 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.

  11. A mantle plume initiation model for the wrangellia flood basalt and other oceanic plateaus.

    PubMed

    Richards, M A; Jones, D L; Duncan, R A; Depaolo, D J

    1991-10-11

    The vast Wrangellia terrane of Alaska and British Columbia is an accreted oceanic plateau with Triassic strata that contain a 3- to 6-kilometers thick flood basalt, bounded above and below by marine sedimentary rocks. This enormous outpouring of basalt was preceded by rapid uplift and was followed by gradual subsidence of the plateau. The uplift and basalt eruptions occurred in less than approximately 5 million years, and were not accompanied by significant extension or rifting of the lithosphere. This sequence of events is predicted by a mantle plume initiation, or plume head, model that has recently been developed to explain continental flood volcanism. Evidence suggests that other large oceanic basalt plateaus, such as the Ontong-Java, Kerguelen, and Caribbean, were formed as the initial outbursts of the Louisville Ridge, Kerguelen, and Galapagos hot spots, respectively. Such events may play an important role in the creation and development of both oceanic and continental crust.

  12. Metamorphic terranes, isotopic provinces, and implications for crustal growth of the western United States

    NASA Astrophysics Data System (ADS)

    Ernst, W. G.

    1988-07-01

    evidence of post-Proterozoic plutonism and regional metamorphism. Initial Pb, Nd, and Sr isotopic compositions and crystallization ages of igneous rocks, in comparison with times of intrusion, recrystallization, and metamorphic facies distributions are compatible with a geologic scenario involving gradual, proximal growth of the ancient continent chiefly southward from the Archean Wyoming craton during early and mid-Proterozoic time, followed by late Proterozoic-Phanerozoic westward development. Accretion involved the incorporation of some exotic, mostly oceanic, outboard terranes. However, continental growth in the western U.S. Cordillera during both Proterozoic and Phanerozoic time periods appears to have been due chiefly to partial fusion and magma ascent above subducting paleo-Pacific lithospheric plates, combined with metamorphic (+ sedimentary) reworking in the forearc + trench + back arc setting, rather than resulting from the amalgamation of preexisting ancient continental fragments.

  13. 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

  14. Geology of the Gorny Altai subduction accretion complex, southern Siberia: Tectonic evolution of an Ediacaran Cambrian intra-oceanic arc-trench system

    NASA Astrophysics Data System (ADS)

    Ota, Tsutomu; Utsunomiya, Atsushi; Uchio, Yuko; Isozaki, Yukio; Buslov, Mikhail M.; Ishikawa, Akira; Maruyama, Shigenori; Kitajima, Koki; Kaneko, Yoshiyuki; Yamamoto, Hiroshi; Katayama, Ikuo

    2007-07-01

    The Gorny Altai region in southern Siberia is one of the key areas in reconstructing the tectonic evolution of the western segment of the Central Asian Orogenic Belt (CAOB). This region features various orogenic elements of Late Neoproterozoic-Early Paleozoic age, such as an accretionary complex (AC), high- P/ T metamorphic (HP) rocks, and ophiolite (OP), all formed by ancient subduction-accretion processes. This study investigated the detailed geology of the Upper Neoproterozoic to Lower Paleozoic rocks in a traverse between Gorno-Altaisk city and Lake Teletskoy in the northern part of the region, and in the Kurai to Chagan-Uzun area in the southern part. The tectonic units of the studied areas consist of (1) the Ediacaran (=Vendian)-Early Cambrian AC, (2) ca. 630 Ma HP complex, (3) the Ediacaran-Early Cambrian OP complex, (4) the Cryogenian-Cambrian island arc complex, and (5) the Middle Paleozoic fore-arc sedimentary rocks. The AC consists mostly of paleo-atoll limestone and underlying oceanic island basalt with minor amount of chert and serpentinite. The basaltic lavas show petrochemistry similar to modern oceanic plateau basalt. The 630 Ma HP complex records a maximum peak metamorphism at 660 °C and 2.0 GPa that corresponds to 60 km-deep burial in a subduction zone, and exhumation at ca. 570 Ma. The Cryogenian island arc complex includes boninitic rocks that suggest an incipient stage of arc development. The Upper Neoproterozoic-Lower Paleozoic complexes in the Gorno-Altaisk city to Lake Teletskoy and the Kurai to Chagan-Uzun areas are totally involved in a subhorizontal piled-nappe structure, and overprinted by Late Paleozoic strike-slip faulting. The HP complex occurs as a nappe tectonically sandwiched between the non- to weakly metamorphosed AC and the OP complex. These lithologic assemblages and geologic structure newly documented in the Gorny Altai region are essentially similar to those of the circum-Pacific (Miyashiro-type) orogenic belts, such as the

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

    NASA Astrophysics Data System (ADS)

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

    2003-04-01

    Modern Tethyan, Mediterranean, and Pacific analogues are considered for several Appalachian, Caledonian, and Variscan terranes (Carolina, West and East Avalonia, Oaxaquia, Chortis, Maya, Suwannee, and Cadomia) that originated along the northern margin of Neoproterozoic Gondwana. These terranes record a protracted geological history that includes: (1) ˜1 Ga (Carolina, Avalonia, Oaxaquia, Chortis, and Suwannee) or ˜2 Ga (Cadomia) basement; (2) 750-600 Ma arc magmatism that diachronously switched to rift magmatism between 590 and 540 Ma, accompanied by development of rift basins and core complexes, in the absence of collisional orogenesis; (3) latest Neoproterozoic-Cambrian separation of Avalonia and Carolina from Gondwana leading to faunal endemism and the development of bordering passive margins; (4) Ordovician transport of Avalonia and Carolina across Iapetus terminating in Late Ordovician-Early Silurian accretion to the eastern Laurentian margin followed by dispersion along this margin; (5) Siluro-Devonian transfer of Cadomia across the Rheic Ocean; and (6) Permo-Carboniferous transfer of Oaxaquia, Chortis, Maya, and Suwannee during the amalgamation of Pangea. Three potential models are provided by more recent tectonic analogues: (1) an "accordion" model based on the orthogonal opening and closing of Alpine Tethys and the Mediterranean; (2) a "bulldozer" model based on forward-modelling of Australia during which oceanic plateaus are dispersed along the Australian plate margin; and (3) a "Baja" model based on the Pacific margin of North America where the diachronous replacement of subduction by transform faulting as a result of ridge-trench collision has been followed by rifting and the transfer of Baja California to the Pacific Plate. Future transport and accretion along the western Laurentian margin may mimic that of Baja British Columbia. Present geological data for Avalonia and Carolina favour a transition from a "Baja" model to a "bulldozer" model. By

  16. 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

  17. 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

  18. 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.

  19. U-Pb Basement and Detrital Zircon Geochronology of the Lhasa and Qiangtang Terranes in Tibet

    NASA Astrophysics Data System (ADS)

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

    2007-12-01

    U-Pb ages of >3500 zircons from Paleozoic sandstones and basement exposures in the Qiangtang and Lhasa terranes of Tibet provide new constraints on the pre-Mesozoic geological framework and paleogeography of Tibetan terranes. 1. Carboniferous-Permian strata exposed to the north and south of the E-W trending high- pressure central Qiangtang metamorphic belt (CQMB) show no obvious differences in their detrital zircon age spectra. These results, together with stratigraphic and sedimentologic similarities, indicate that the CQMB does not separate strata of Cathaysian affinity in the north from that of Gondwana affinity in the south as previously suggested. 2. Upper Paleozoic strata in the southern Qiangtang terrane are locally exposed structurally above kyanite/silliminate-bearing orthogneisses and metasedimentary assemblages. The orthogneisses yielded crystallization ages in the 470-480 Ma range and metasandstones show minimum ages of ~540 Ma. We suggest that these metamorphic rocks represent early Ordovician and older basement of the Qiangtang terrane which was deformed and metamorphosed during an early- to mid-Paleozoic orogeny and then unconformably overlain by upper Paleozoic strata. 3. In the central Lhasa terrane, an exposure of ~509 Ma granitic basement is unconformably overlain by a straight section of Cambrian (?) through Permian strata. Detrital zircon age spectra determined for the Paleozoic strata are distinct from those of Qiangtang strata, but strikingly similar to Tethyan strata of the Tibetan Himalaya. These results are unexpected because it is generally thought the Lhasa and Qiangtang terranes were separated by a relatively narrow and short-lived Meso-Tethys Ocean whereas the Lhasa terrane and Tethyan Himalaya were separated by the large and long-lived Neo-Tethys Ocean. 4. The youngest ages of detrital zircons in all Paleozoic sandstones are significantly older (>100 Ma) than their depositional ages, consistent with their deposition in passive margin

  20. 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

  1. 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.

  2. 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

  3. Ambient-noise tomography of north Tibet limits geological terrane signature to upper-middle crust

    NASA Astrophysics Data System (ADS)

    Karplus, M. S.; Klemperer, S. L.; Lawrence, J. F.; Zhao, W.; Mechie, J.; Tilmann, F.; Sandvol, E.; Ni, J.

    2013-03-01

    We use ambient-noise tomography to map regional differences in crustal Rayleigh-wave group velocities with periods of 8-40 s across north Tibet using the International Deep Profiling of Tibet and the Himalaya phase IV arrays (132 stations, deployed for 10-24 months). For periods of 8-24 s (sensitive to midcrustal depths of ~5-30 km), we observe striking velocity changes across the Bangong-Nujiang and Jinsha suture zones as well as the Kunlun-Qaidam boundary. From south to north, we see higher velocities beneath the Lhasa terrane, lower velocities beneath the Qiangtang, higher velocities in the Songpan-Ganzi and Kunlun terranes, and the lowest velocities beneath the Qaidam Basin. Maps at periods of 34 and 40 s (sensitive to the middle and lower crust at depths of ~30-60 km) do not show evidence of changes across those boundaries. Any differences between the Tibetan terrane lower crusts that were present at accretion have been erased or displaced by Cenozoic processes and replaced almost ubiquitously by uniformly low velocities.

  4. 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

  5. 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.

  6. Reconstructing the lost eastern Tethys Ocean Basin: Convergence history of the SE Asian margin and marine gateways

    NASA Astrophysics Data System (ADS)

    Heine, Christian; Müller, R. Dietmar; Gaina, Carmen

    Plate tectonic reconstructions for the late Mesozoic-Cenozoic evolution of the eastern Tethyan Ocean Basin, separating eastern Gondwanaland from Proto-Southeast Asia, are usually based on geological data gathered from the different tectonic blocks accreted to Southeast Asia. However, this approach only provides few constraints on the reconstruction of the eastern Tethys Ocean and the drift path of various terranes. We have used marine magnetic anomalies in the Argo and Gascoyne Abyssal Plains off the Australian Northwest Shelf, jointly with published geological data, to reconstruct the seafloor spreading history and plate tectonic evolution of the eastern Tethys and Proto-Indian Ocean basins for the time between 160 Ma and the present. Based on the assumption of symmetrical seafloor spreading and a hotspot-track-based plate reference frame, we have created a relative and absolute plate motion model and a series of oceanic paleo-age grids that show the evolution of Tethyan mid-ocean ridges and the convergence history along the southeast Asian margin through time. A thermal boundary layer model for oceanic lithosphere is used to compute approximate paleo-depths to oceanic basement to predict the opening and closing of oceanic gateways. The proposed model not only provides improved boundary conditions for paleoclimate reconstructions and modelling of oceanic currents through time, but also for understanding stress changes in the overriding plate and the formation of new accretionary crust along the Southeast Asian margin, driven by changing subduction parameters like hinge rollback and slab dip.

  7. Connecting lower crustal rheology with ultrahigh pressure terrane evolution

    NASA Astrophysics Data System (ADS)

    Rodda, C.; Koons, P. O.; Roy, S.; Upton, P.

    2012-12-01

    Continental rocks subducted to ultrahigh pressure (UHP) conditions (> ca. 90km) during continental collision and subsequently exhumed to the surface afford a unique view into the controlling physics of a subduction zone. Rocks now exposed in the Western Gneiss Region of Norway record the transient UHP subduction of strong, dry, lower-crustal granulites from the Baltica margin beneath the leading edge of Laurentia during the Scandian Orogeny at ca. 425Ma. These rocks appear to have experienced limited metamorphic re-equilibration at UHP conditions. Recent tomographic studies of the Hikurangi Plateau indicate that the leading edge of a 25km thick oceanic plateau is currently subducted to 200km depth beneath the North Island of New Zealand. Earlier subduction of the Hikurangi Margin likely produced a strong dehydrated lower crust. A series of 2-D numerical models were produced to test the dependence of UHP terrane creation and decoupling of continental materials from a sinking slab on the presence of a strong lower crust. We present the results of a series of numerical model solutions in which we impose material property changes consistent with UHP-equilibration of continental materials in a continental-collision zone. Variation in the timing of imposed equilibration and geometry of softened regions in presented models mimics both low- and high-degrees of metastable preservation of low-pressure assemblages. We compare the decoupling of continental materials from a sinking slab and the early stages of exhumation observed in the model solutions with natural data from the Western Gneiss Region and the Hikurangi Margin. We find UHP terrane creation and decoupling can occur in a collision zone with either complete or limited UHP-equilibration. In both cases exhumation is initiated by a hammer-and-anvil process in a collapsing subduction channel during continued subduction. Vertical velocities during early exhumation in numerical models are much higher than those inferred

  8. 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.

  9. Gabbro-peridotite Interaction in the Northern Cache Creek Composite Terrane Ophiolite, British Columbia and Yukon

    NASA Astrophysics Data System (ADS)

    Zagorevski, A.

    2015-12-01

    The northern Cache Creek composite terrane comprises a thrust stack of chert, limestone, siltstone, basalt, gabbro and ultramafic complexes ranging in age from Mississippian to Triassic. Fields studies and geochemical investigations indicate that ophiolitic mafic-ultramafic complexes formed in a supra-subduction zone setting. Ophiolitic rocks in the southeast form a structurally disrupted Penrose-type ophiolite; however, northwestern ophiolitic rocks generally lack lower and middle crust in most sections, exhibit a direct contact between supracrustal and mantle sections and locally contain ophicalcites suggesting that supracrustal rocks were structurally emplaced over mantle along extensional detachment(s). Mantle peridotite in the footwall of the detachment is extensively intruded by vari-textured, fine-grained to pegmatitic gabbro sills, dykes and stocks. These gabbro intrusions are locally boudinaged within fresh peridotite suggesting that the host mantle was rapidly exhumed prior to emplacement of the gabbro. Intrusive relationships between gabbro and variably serpentinized mantle peridotite are observed throughout the northern Cache Creek terrane (>300 km) suggesting a presence of a regional-scale Middle Triassic ocean-core complex. Overall, these data indicate that parts of the northern Cache Creek terrane formed in a setting analogous to backarc ocean core complexes such as the Godzilla Megamullion in the Parece Vela backarc basin, western Pacific.

  10. U-Pb Ages From Detrital Zircon in Avalonian Sedimentary Rocks: Temporal Changes in Provenance Tied to Terrane Migration?

    NASA Astrophysics Data System (ADS)

    Samson, S.; Hamilton, M.; Barr, S.; White, C.; Satkoski, A.

    2009-05-01

    The Avalon microcontinent in the northern Appalachian orogen originated near the margin of the supercontinent Gondwana, but its position along that extensive margin, and its timing of separation, remain disputed. Avalonia is characterized by Neoproterozoic - Cambrian clastic sedimentary sequences. Detrital zircon ages from these sedimentary units may provide constraints on the locations of the terrane prior to its accretion to Laurentia. U-Pb ages have been obtained for detrital zircon from units with depositional ages ranging from c. 630 Ma to c. 505 Ma. The oldest sample, from the Hammondvale Metamorphic Suite (HMS) in southern New Brunswick, contains zircon as young as 630 Ma, providing a maximum depositional age. The dominant Neoproterozoic zircon population has an age of 682 Ma, which likely represents the age of the main sediment source, but the sample also contains a few older Neoproterozoic grains (approaching 800 Ma). Importantly, the HMS sample also contains relatively abundant 1.9 - 1.0 Ga zircon, but no zircon with ages between 2.9 and 1.9 Ga. In contrast, published data from quartzite clasts from a conglomerate thought to be deposited at c. 550 Ma in New Brunswick and Nova Scotia show different detrital zircon age patterns: the percentage of Mesoproterozoic grains is lower than in the HMS, and a population of 2.0 - 1.9 Ga grains is present. Thus the latest Precambrian appears to mark the beginning of an important change in sediment sources to Avalonia. A younger (c. 540 Ma) quartzite (Ratcliffe Brook Formation) reinforces this apparent change in provenance in that Mesoproterozoic zircons are even lower in abundance and the abundance of 2.1 - 1.9 Ga zircon is higher. Additionally, a new c. 800 Ma zircon population is noted. This new age peak may also reflect a fundamental shift in provenance, perhaps as a consequence of migration of the terrane along the Gondwanan margin. Two additional (c. 520 Ma) Cambrian samples also have also been investigated; the

  11. 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.

  12. Paleomagnetic Progress in Peri-Gondwanan Terranes of Cape Breton Island, Nova Scotia

    NASA Astrophysics Data System (ADS)

    Grunow, A. M.; Thompson, M. D.; Barr, S. M.; White, C. E.

    2009-05-01

    Paleopoles from primary Ediacaran magnetization directions established the Gondwanan origin of northern Appalachian Avalonian terranes, but magnetic overprints in the same rocks also provide useful tectonic information. Thus, in the Southeastern New England Avalon Zone, virtual geomagnetic poles (VGPs) calculated from magnetic B and C components in both 595 Ma Lynn-Mattapan volcanic rocks and 490-488 Ma Nahant Gabbro track mid- and late Paleozoic segments of the North American apparent polar wander path (APWP), suggesting the influence of Acadian and Neo-Acadian accretionary events. We report here on multi- vectorial magnetizations in pilot samples from Cape Breton Island, Nova Scotia where the Bras d'Or and Mira terranes represent both Ganderian and Avalonian elements transferred from Gondwana. Overprint relationships in these terranes may constrain their amalgamation with each other as well their docking with Laurentia. As in southeastern New England, secondary remanences can be identified in Cape Breton Island as consistent magnetization directions in rocks of differing ages. The S- to SSE-trending and gently downward pointing direction reported in 1985 by Johnson and Van der Voo in Middle Cambrian sedimentary rocks of the Bourinot Group (Bras d'Or terrane), for example, is also present in the 563 Ma Main à Dieu Formation and in 620 Ma Chisholm Brook Granite and East Bay Hill rhyolite (Mira terrane). This magnetization represents the C component already found around Boston, MA. The resulting VGPs in both areas occupy positions on the North American APWP consistent with a Neo-Acadian overprint, possibly related to the docking of the Meguma terrane against previously accreted Avalonia. Other overprint directions encountered in this investigation give rise to VGPs that do not coincide with the North American APWP, hence appear to reflect tectonic events independent of Laurentia. One such cluster comprising both Mira and Bras d'Or VGPs includes the paleopole also

  13. A 2-D tomographic model of the Juan de Fuca plate from accretion at axial seamount to subduction at the Cascadia margin from an active source ocean bottom seismometer survey

    NASA Astrophysics Data System (ADS)

    Horning, G.; Canales, J. P.; Carbotte, S. M.; Han, S.; Carton, H.; Nedimović, M. R.; Keken, P. E.

    2016-08-01

    We report results from a wide-angle controlled source seismic experiment across the Juan de Fuca plate designed to investigate the evolution of the plate from accretion at the Juan de Fuca ridge to subduction at the Cascadia margin. A two-dimensional velocity model of the crust and upper mantle is derived from a joint reflection-refraction traveltime inversion. To interpret our tomography results, we first generate a plausible baseline velocity model, assuming a plate cooling model and realistic oceanic lithologies. We then use an effective medium theory to infer from our tomography results the extent of porosity, alteration, and water content that would be required to explain the departure from the baseline model. In crust of ages >1 Ma and away from propagator wakes and regions of faulting due to plate bending, we obtain estimates of upper crustal hydration of 0.5-2.1 wt % and find mostly dry lower crust and upper mantle. In sections of the crust affected by propagator wakes we find upper estimates of upper crustal, lower crustal, and upper mantle hydration of 3.1, 0.8, and 1.8 wt %, respectively. At the Cascadia deformation front, we find that the amount of water stored at uppermost mantle levels in the downgoing JdF plate is very limited (<0.3 wt %), with most of the water carried into the subduction zone being stored in the oceanic crust.

  14. 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.

  15. 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

  16. Petrochemistry of Mafic Rocks Within the Northern Cache Creek Terrane, NW British Columbia, Canada

    NASA Astrophysics Data System (ADS)

    English, J. M.; Johnston, S. T.; Mihalynuk, M. G.

    2002-12-01

    The Cache Creek terrane is a belt of oceanic rocks that extend the length of the Cordillera in British Columbia. Fossil fauna in this belt are exotic with respect to the remainder of the Canadian Cordillera, as they are of equatorial Tethyan affinity, contrasting with coeval faunas in adjacent terranes that show closer linkages with ancestral North America. Preliminary results reported here from geochemical studies of mafic rocks within the Nakina area of NW British Columbia further constrain the origin of this enigmatic terrane. The terrane is typified by tectonically imbricated slices of chert, argillite, limestone, wacke and volcaniclastic rocks, as well as mafic and ultramafic rocks. These lithologies are believed to represent two separate lithotectonic elements: Upper Triassic to Lower Jurassic, subduction-related accretionary complexes, and dismembered basement assemblages emplaced during the closure of the Cache Creek ocean in the Middle Jurassic. Petrochemical analysis revealed four distinct mafic igneous assemblages that include: magmatic 'knockers' of the Nimbus serpentinite mélange, metabasalts of 'Blackcaps' Mountain, augite-phyric breccias of 'Laughing Moose' Creek, and volcanic pediments to the reef-forming carbonates of the Horsefeed Formation. Major and trace element analysis classifies the 'Laughing Moose' breccias and the carbonate-associated volcanics as alkaline in nature, whereas the rest are subalkaline. Tectonic discrimination diagrams show that the alkaline rocks are of within-plate affinity, while the 'Blackcaps' basalts and 'knockers' from within the mélange typically straddle the island-arc tholeiite and the mid-ocean ridge boundaries. However, primitive mantle normalized multi-element plots indicate that these subalkaline rocks have pronounced negative Nb anomalies, a characteristic arc signature. The spatial association of alkaline volcanic rocks with extensive carbonate domains points to the existence of seamounts within the Cache

  17. Accretion Processes in Astrophysics

    NASA Astrophysics Data System (ADS)

    González Martínez-País, Ignacio; Shahbaz, Tariq; Casares Velázquez, Jorge

    2014-03-01

    List of contributors; List of participants; Preface; Acknowledgments; Abbreviations; 1. Accretion disks Henk Spruit; 2. The evolution of binary systems Philipp Podsiadlowski; 3. Accretion onto white dwarfs Brian Warner; 4. Accretion in X-ray binary systems Robert I. Hynes; 5. X-ray binary populations in galaxies Giuseppina Fabbiano; 6. Observational characteristics of accretion onto black holes I Chris Done; 7. Observational characteristics of accretion onto black holes II Rob Fender; 8. Computing black hole accretion John F. Hawley; Appendix: Piazzi Smyth, the Cape of Good Hope, Tenerife and the siting of large telescopes Brian Warner.

  18. 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

  19. 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.

  20. 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.; 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.

  1. Petrogenesis and tectonic implications of the Triassic volcanic rocks in the northern Yidun Terrane, Eastern Tibet

    NASA Astrophysics Data System (ADS)

    Wang, Bai-Qiu; Zhou, Mei-Fu; Chen, Wei Terry; Gao, Jian-Feng; Yan, Dan-Ping

    2013-08-01

    Triassic Paleo-Tethyan subduction-related volcanic rocks are extensively distributed in the Xiangcheng and Changtai regions, northern Yidun Terrane, eastern Tibet. The ~ 228 Ma volcanic rocks in the Xiangcheng region have intermediate-felsic composition (60.1 to 64.7 wt.% SiO2) with relatively low Y (14-18 ppm) and high Sr (630-1830 ppm) yielding high Sr/Y ratios (43-94). They have LREE-enriched and HREE-depleted REE patterns (La/Yb = 19-26), typical of adakites. Sr-Nd isotopic compositions of these volcanic rocks are similar to those of the contemporaneous high silica adakitic rocks in the southern Yidun Terrane. The ~ 231-230 Ma volcanic rocks in the Changtai region are composed of basalts, andesites, dacites and rhyolites with SiO2 ranging from 47.7 to 81.0 wt.%. Basalts in the Changtai region have initial 87Sr/86Sr ratios from 0.7052 to 0.7058 and εNd(t) from + 1.1 to + 1.5. They display OIB-like geochemical signatures, and were likely derived from low degrees of partial melting of an OIB-like mantle source with subordinate input of subduction components indicated by increasing Th content. Andesites and dacites show elemental and isotopic compositions that evolved from the basalts, indicative of derivation via crustal contamination and fractional crystallization (AFC) of basaltic magmas. Rhyolites have lower initial 87Sr/86Sr ratios and higher εNd(t) values than andesites/dacites, inconsistent with an AFC process and, are suggested to have been derived from anatexis of crustal materials. Emplacement of volcanic rocks in the Changtai and Xiangcheng regions could be attributed to the subduction of the Ganzi-Litang Ocean, a branch of the Paleo-Tethys Ocean. Volcanic rocks in the Changtai and Xiangchang regions have zircon U-Pb ages about 4-6 myr older than that of arc granites in the eastern Yidun Terrane. The spatio-temporal distribution of the volcanic rocks indicates that the subduction was initiated under the southern Yidun Terrane in the Middle Triassic and

  2. Terrestrial Planets Accreted Dry

    NASA Astrophysics Data System (ADS)

    Albarede, F.; Blichert-Toft, J.

    2007-12-01

    Plate tectonics shaped the Earth, whereas the Moon is a dry and inactive desert. Mars probably came to rest within the first billion years of its history, and Venus, although internally very active, has a dry inferno for its surface. The strong gravity field of a large planet allows for an enormous amount of gravitational energy to be released, causing the outer part of the planetary body to melt (magma ocean), helps retain water on the planet, and increases the pressure gradient. The weak gravity field and anhydrous conditions prevailing on the Moon stabilized, on top of its magma ocean, a thick buoyant plagioclase lithosphere, which insulated the molten interior. On Earth, the buoyant hydrous phases (serpentines) produced by reactions between the terrestrial magma ocean and the wet impactors received from the outer Solar System isolated the magma and kept it molten for some few tens of million years. The elemental distributions and the range of condensation temperatures show that the planets from the inner Solar System accreted dry. The interior of planets that lost up to 95% of their K cannot contain much water. Foundering of their wet surface material softened the terrestrial mantle and set the scene for the onset of plate tectonics. This very same process may have removed all the water from the surface of Venus 500 My ago and added enough water to its mantle to make its internal dynamics very strong and keep the surface very young. Because of a radius smaller than that of the Earth, not enough water could be drawn into the Martian mantle before it was lost to space and Martian plate tectonics never began. The radius of a planet therefore is the key parameter controlling most of its evolutional features.

  3. 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.

  4. Terrane-boundary reactivation: A control on the evolution of the Northern Andes

    NASA Astrophysics Data System (ADS)

    Litherland, M.; Aspden, J. A.

    1992-01-01

    The Andes of northern Ecuador and southern Colombia comprise two post-Oligocene cordilleras, each with its Plio-Pleistocene volcanic chain separated by the fault-bounded, inter-Andean depression or graben. Along the eastern Peltetec-Romeral fault occur Upper Jurassic ophiolitic rocks marking an ancient suture between the allochthonous Chaucah terrane in the west and the South American plate. Along the western Pujilí-Cauca fault are Upper Cretaceous-lower Eocene ophiolites marking the accretion of the Cretaceous-Eocene Westner Cordillera. Post-Oligocne reactivation of these terane boundaries accounts for the origin of the cordilleras and graben and helps to explain the location of the double chain of Plio-Pleistocene volcanic centers. A caldera-graben model is suggested.

  5. 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

  6. Constrains on Crustal Accretion Obtained from Cooling Rate Calculations with a Thermo-Mechanical Model of Fast-Spreading Mid-Ocean Ridges

    NASA Astrophysics Data System (ADS)

    Garrido, C. J.; Machetel, P.

    2012-12-01

    We have used a thermo-mechanical model designed to find steady-state solutions of motion and temperature with variable viscosity, heat diffusion, heat advection, hydrothermal cooling and latent heat release. Cases analogous to the "gabbro glacier" (G accretion structure), "sheeted sills" (S structure) and "mixed shallow and MTZ lenses" (M structure) were computed with and without sheeted dyke level modeling. The results show that thermal patterns near the ridge mainly depend on hydrothermal cooling. Several hydrothermal cooling cracking temperature have been used in order to illustrate the present scientific debate on the penetration depth and efficiency of hydrothermal flows. Second, higher cooling rates are obtained for the G structures. Third, whereas the subsolidus cooling rates, SCR, decrease monotonically with depth, the igneous cooling rates, ICR, display local minima at the merging levels of the upper and lower lenses. It appears that ICR reveal the near-ridge thermal and mechanical structures, whereas the lower value of the initial-to-closure temperature ranges used for SCR cause shifts farther from the ridge that reduces the ability of SCR to discriminate the ridge thermo-mechanical configuration. It also indicates that the common assumption that ICR and SCR should be similar is probably over-simplified. Finally, the cooling rates obtained bears the clear signature of the three intrusion hypothesis. The results show that numerical modeling of the lower crust's thermo-mechanical properties may provide new insights to discriminate among hypotheses related to G, M and S structures for fast-spreading ridges.; Thermal history obtained for the Gabro Glacier (top panels), Mixed shallow and MTZ zone (middle panels) and Sheeted Sills hypothesis (bottom panels)for the magma intrusion at ridge. Columns corresponds to various hydrothermal cooling and viscosity hypothesis.

  7. The buried southern continuation of the Oaxaca-Juarez terrane boundary and Oaxaca Fault, southern Mexico: Magnetotelluric constraints

    NASA Astrophysics Data System (ADS)

    Campos-Enriquez, J. O.; Corbo-Camargo, F.; Arzate-Flores, J.; Keppie, J. D.; Arango-Galván, C.; Unsworth, M.; Belmonte-Jiménez, S. I.

    2013-04-01

    Thirty magnetotelluric soundings were made along two NW-SE profiles to the north and south of Oaxaca City in southern Mexico. The profiles crossed the N-S Oaxaca Fault and the Oaxaca-Juarez terrane boundary defined by the Juarez mylonitic complex. Dimensionality analysis of the MT data showed that the subsurface resistivity structure is 2D or 3D. The Oaxaca and correlative Guichicovi terranes consist of ca. 1-1.4 Ga granulitic continental crust overlain by Phanerozoic sedimentary rocks, characterized by high and low resistivities, respectively. The Juarez terrane consists of oceanic Mesozoic metavolcanic and metasedimentary rocks, characterized by a low to medium resistivity layer, that is approximately 10 km thick. The Oaxaca Fault is a Cenozoic aged, normal fault that reactivated the dextral and thrust Juarez mylonitic complex north of Oaxaca City: its location south of Oaxaca City is uncertain. In the southern profile, the MT data show a ca. 20-50 km wide, west-dipping, relatively low resistivity zone material that extends through the entire crust. This is inferred to be the Juarez terrane bounded on either side by the ca. 1-1.4 Ga granulites. The Oaxaca Fault is imaged only by a major electrical resistivity discontinuity (low to the west, high to the east) along both the western border of the Juarez mylonitic complex (northern profile) and the San Miguel de la Cal mountains (southern profile) suggesting continuity.

  8. Fusuline biotic turnover across the Guadalupian Lopingian (Middle Upper Permian) boundary in mid-oceanic carbonate buildups: Biostratigraphy of accreted limestone in Japan

    NASA Astrophysics Data System (ADS)

    Ota, Ayano; Isozaki, Yukio

    2006-03-01

    Two sections in Upper Middle to Lower Upper Permian shallow-water limestones at Kamura and Akasaka in southwest Japan were analyzed for detailed lithostratigraphy and biostratigraphy. Both sections represent ancient seamount-capping carbonate buildups developed on a basaltic basement in a mid-oceanic environment. The occurrence of abundant Tethyan fusulines allows the recognition of well-defined biostratigraphic zonation in both sections and their mutual correlation. The Upper Guadalupian (Middle Permian) Lepidolina/ Yabeina Zone is overlain conformably by the Lower Lopingian (Upper Permian) Codonofusiella- Reichelina Zone with a 13 m-thick transitional interval barren of index taxa. The Guadalupian-Lopingian (G-L) boundary is marked by the First Appearance Datum (FAD) of the Lopingian Codonofusiella- Reichelina assemblage in both sections. This study recognizes for the first time the G-L boundary horizon in a mid-oceanic shallow-water environment. In addition, the shallow-water carbonates in the study sections record the extinction of the Middle Permian large-sized fusuline family Verbeekinidae at the G-L boundary in mid-Panthalassa, as well as in shallow-water Tethyan shelf areas, demonstrating positively that the G-L boundary mass extinction occurred on a global scale. The abrupt elimination of large-shelled fusulines, followed by the domination of small-shelled fusulines may indicate that environmental stress occurred at the end of Guadalupian. The dying-out of symbiotic algae may have caused the selective extinction of the large-shelled fusulines.

  9. 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.

  10. Deformation processes in an accretionary prism: a study from the Torlesse terrane of New Zealand

    NASA Astrophysics Data System (ADS)

    George, Annette D.

    The style of deformation observed in rocks of the Torlesse (Pahau) terrane, exposed in the Aorangi Range of the North Island, records accretion of thick trench fill by offscraping at the toe of a growing accretionary prism during the early Cretaceous. The relatively coherent nature of the Aorangi Range rocks enables detailed study of the deformation processes produced by offscraping and subsequently within the prism, and many of the structures observed in these rocks are consistent with those described from both modern accreting margins and other ancient accretionary terranes. Overprinting relationships indicate three phases of folding and multiple faulting events. Early deformation involved large-scale sheath-like folding oblique to the overall trend of the margin, and development of an anastomosing axial-planar cleavage. Folding of the sediments promoted dewatering; the subsequent disruption of strata, by shearing parallel to bedding and low-angle to bedding faulting, records the transition to more brittle responses to the deformation. The most widespread folding phase ( D2) produced numerous upright, typically isoclinal folds, with local development of an axial-planar S2 cleavage in macroscopic and some mesascopic fold hinges. The variable plunge of the fold axes to the NNE and SSW within the axial surface indicates progressive rotation of the fold axes after formation. Mesozoic strike-slip faulting most likely produced the open E-W-trending folds and warps of the third folding phase ( D3), in bedding already rotated to moderate dips. Faults which overprint the Mesozoic deformation, were formed in response to renewed subduction along the eastern coast of the North Island during the Cenozoic.

  11. 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.

  12. 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

  13. Eclogite Facies Relicts and Decompression Assemblages; Evidence for the Exhumation of a Large Coherent Metabasite Block From > 40 km Depth; Central Metamorphic Terrane, Eastern Klamath Mountains, Northern California

    NASA Astrophysics Data System (ADS)

    Barrow, W. M.; Fairhurst, R. J.; Metcalf, R. V.

    2007-12-01

    Recent exhumation models for eclogite terranes have focused on the exhumation of sialic rocks. Exhumed high pressure terranes are typically > 85% - 90% sialic material with only minor amounts of mafic and ultramafic rock. Most known metabasitic eclogites are blocks in mélange rather than large coherent bodies. The Central Metamorphic terrane (CMt) is a large (~300 km3) coherent, fault-bounded package of metabasites thought to represent a remnant of a downing plate subducted in an intra-oceanic convergent margin. Thermochronology indicates that the CMt was metamorphosed and later accreted to the base of the Trinity ophiolite along the Trinity fault during Early Permian extension (Hbl and Musc 40Ar/39Ar ages of 275 Ma - 294 Ma). Previous work suggested that the peak metamorphic temperatures and pressures were ~650°C and 0.4 to 0.8 GPa (Peacock and Norris, 1989) which is consistent with the amphibolite facies mineral assemblage. Trace element data confirm the NMORB-like composition of CMt metabasite protoliths. Newly discovered relict textures, however, suggest that CMt amphibolites record much deeper subduction burial with subsequent decompression exhumation. A decompression sequence consisting of rutile cores within ilmenite crystals mantled by titanite is observed in CMt amphibolite samples. Zr-in-rutile thermometry (Watson et al., 2006) combined with experimental data for rutile stability in metabasites (Ernst and Lui, 1998) suggests that relict rutile crystals preserve early P-T conditions of ~600°C and > 1.3 GPa consistent with eclogite facies metamorphism. Transition from eclogite facies is further supported by ilmenite-plagioclase-amphibole symplectites suggesting replacement of garnet (Bhowmik and Roy, 2003) during decompression. Amphibole compositions vary significantly and reflect lower grade (low Na, Al, Ti actinolite) overprint of earlier amphibolite facies compositions (high Na, Al, Ti magnesio- hornblende). Application of the Al-Ti hornblende

  14. 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.

  15. 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.

  16. 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.

  17. Partial accretion regime of accreting millisecond pulsars

    NASA Astrophysics Data System (ADS)

    Eksi, Kazim

    2016-07-01

    The inner parts of the disks around neutron stars in low mass X-ray binaries may become geometrically thick due to inhibition of accretion at the disk mid-plane when the central object is rotating rapidly. In such a case matter inflowing through the disk may keep accreting onto the poles of the neutron star from the parts of the disk away from the disk mid-plane while the matter is propelled at the disk mid-plane. An important ingredient of the evolution of millisecond pulsars is then the fraction of the inflowing matter that can accrete onto the poles in the fast rotation regime depending on the fastness parameter. This ``soft'' propeller regime may be associated with the rapid decay stage observed in the light curves of several accreting millisecond pulsars. To date only a few studies considered the partial accretion regime. By using geometrical arguments we improve the existing studies and test the model by reproducing the lightcurves of millisecond X-ray pulsars via time dependent simulations of disk evolution. We also present analytical solutions that represent disks with partial accretion.

  18. 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

  19. 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.

  20. 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

  1. 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

  2. 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

  3. 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

  4. 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.

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

    PubMed

    Jutzi, M; Asphaug, E

    2011-08-03

    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

  6. 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

  7. Precambrian ophiolites of arabia: geologic settings, UPb geochronology, Pb-isotope characteristics, and implications for continental accretion

    USGS Publications Warehouse

    Pallister, J.S.; Stacey, J.S.; Fischer, L.B.; Premo, W.R.

    1988-01-01

    Disrupted ophiolites occur in linear belts up to 900 km long between microplates that collided during the late Proterozoic to form the Arabian Shield. UPb zircon ages and Pb-isotope data from these ophiolitic rocks help constrain the history of accretion of the Arabian Shield and thereby contribute to the definition of its microplates and terranes. Terranes of the central and western Arabian Shield are generally thought to represent intraoceanic island arcs that range in age from about 900 to 640 Ma; however, a region of the eastern Arabian Shield contains rocks of Early Proterozoic age and may represent an exotic continental fragment entrained between the arc complexes. Ophiolites of the Yanbu suture (northwestern shield), dated by UPb (zircon) and SmNd (mineral isochron) methods, yield model ages of 740-780 Ma. These are among the oldest well-dated rocks in the northwestern Arabian Shield. Ages from the Jabal al Wask complex overlap with ages of adjacent arc rocks. This overlap in age supports geologic and geochemical evidence that the Wask complex represents a fragment of back-arc oceanic lithosphere formed during arc magmatism. Older ages of about 780 Ma for gabbro from the Jabal Ess ophiolite suggest that the ophiolite is either a fragment of fore-arc oceanic crust or oceanic basement on which an arc was built. Gabbro samples from ophiolites of the Bir Umq suture (west-central Arabian Shield) yield zircons with ages of 820-870 Ma and $ ??1250 Ma. The 820-870 Ma dates overlap with ages of the oldest nearby arc rocks; this favors an intra-arc or near-arc paleotectonic setting. The older zircons suggest that middle or early Proterozoic crustal material, possibly derived from the Mozambique belt of Africa, was present during back- or intra-arc magmatism. Plagiogranite from the Bir Tuluhah ophiolitic complex at the nothern end of the 900 km-long Nabitah mobile belt was dated by the zircon UPb method at ??? 830 Ma. This date is in the range of the oldest dated arc

  8. 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.

  9. Interrelationship of the terranes in western and central Cuba

    NASA Astrophysics Data System (ADS)

    Piotrowska, Krystyna

    1993-03-01

    Present-day Cuba is a complex of terranes. In the western and central parts of the island the Sierra de los Organos and the Escambray terranes, which are genetically related, and the volcanosedimentary Cretaceous sequences of the Greater Antilles arc are distinguished. The Escambray and Sierra de los Organos massifs are terranes that were detached from their source areas near the Yucatan. A rift separating the Yucatan from the Florida-Bahama plate was involved. Separation of the Escambray sequence from that of the Sierra de los Organos took place in the Early Cretaceous along a NE-SW-trending transform fault. The Escambray sequence was transported together with the Greater Antilles island arc in front of the Farallon plate. About 80 Ma ago the volcanosedimentary Cretaceous sequence of the Greater Antilles island arc was thrust over the ophiolitic association. In this way the Zaza zone was formed. At that time some overthrusting and metamorphism took place in the Escambray massif, which was then overthrust and pressed into the Zaza zone. At about 45-50 Ma ago detachment and nappe overthrusting took place in the Sierra de los Organos and overthrust units reached the margin of the margin of the Florida-Bahama plate. Various structures are involved in the Zaza zone, including: (1) the volcanosedimentary Cretaceous sequences of the Greater Antilles arc (which is a tectonized and overthrust terrane); and (2) the ophiolitic association (the Greater Antilles arc terrane is thrust over this association). Both of these tectonic units were folded together. The Sierra de los Organos and the Escambray terranes were thrust onto the Zaza zone and then deformed together with it during later phases.

  10. a Possible Ancient Core Complex in the Northern Cache Creek Terrane, British Columbia

    NASA Astrophysics Data System (ADS)

    Zagorevski, A.

    2013-12-01

    The Cache Creek terrane (CCT) in Canadian Cordillera comprises a belt of Mississippian to Jurassic oceanic rocks that include Tethyan carbonates and alkaline basalts that are demonstrably exotic to Laurentia. The exotic Tethyan faunas in the CCT, combined with its inboard position with respect to Stikinia and Yukon-Tanana terranes has led to a variety of tectonic hypotheses including oroclinal enclosure of CCT by Stikinia, Yukon-Tanana and Quesnellia during the Jurassic. Detailed studies have demonstrated that the northern CCT is in fact a composite terrane that includes ophiolitic rocks of both ocean island and island arc origins. The western margin of the CCT is characterized by imbricated harzburgite, island arc tholeiite, sedimentary rocks and locally significant felsic volcanic rocks of the Kutcho arc. Gabbro is volumetrically minor and sheeted dyke complexes are either very rare or not developed. The felsic arc volcanic rocks and the pyroxenite bodies that cut the harzburgite have been previously isotopically dated as Middle Triassic (ca. 245 Ma) suggesting that melt percolation through the mantle was coeval with Kutcho arc magmatism and coincided with a magmatic gap in Stikinia. In general the contact between the mantle and supracrustal rocks is faulted making it difficult to determine the original relationships between the mantle and island arc tholeiites. Locally, the contact appears to be intact and is characterized by mantle tectonites with pyroxenite veins overlain by cumulate plagioclase-orthopyroxene gabbro and fine grained diabase. Elsewhere, volcanic and sedimentary rocks sit in fault contact structurally above the mantle. The absence of voluminous gabbro and sheeted dyke complexes, presence of coeval magmas in the crust and mantle, and low angle extensional faulting in some areas suggests that the western part of the CCT may preserve an ocean core complex similar to the Godzilla Megamullion in the Parece-Vela Basin. Such a hypothesis suggests that

  11. Igneous rocks of the West Sakhalin Terrane of Sakhalin Island

    NASA Astrophysics Data System (ADS)

    Grannik, V. M.

    2016-10-01

    It has been determined that the Rozhdestvenka Formation of the West Sakhalin Terrane composed of Late Mesozoic igneous rocks is a fragment of the accretionary prism of the Rebun-Kabato-Moneron-Samarga island-arc system. Volcanic eruptions, as well as destruction of the Rebun-Kabato-Moneron-Samarga island-arc and the East Sikhote-Alin volcano plutonic marginal continental belt, were the sources of pyroclastic and clastic material entering the sedimentary basin, where the Pobedinsk and Krasnoyarka suites of the West Sakhalin Terrane were formed.

  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. Geologic framework of the Alaska Peninsula, southwest Alaska, and the Alaska Peninsula terrane

    USGS Publications Warehouse

    Wilson, Frederic H.; Detterman, Robert L.; DuBois, Gregory D.

    2015-01-01

    The boundaries separating the Alaska Peninsula terrane from other terranes are commonly indistinct or poorly defined. A few boundaries have been defined at major faults, although the extensions of these faults are speculative through some areas. The west side of the Alaska Peninsula terrane is overlapped by Tertiary sedimentary and volcanic rocks and Quaternary deposits.

  14. Temporal variations of the segmentation of slow to intermediate spreading mid-ocean ridges 2. A three-dimensional model in terms of lithosphere accretion and convection within the partially molten mantle beneath the ridge axis

    NASA Astrophysics Data System (ADS)

    Rabinowicz, Michel; Briais, Anne

    2002-06-01

    We present three-dimensional numerical models of convection within the partially molten mantle beneath the ridge axis. The modeling takes into account the cavity flow driven by plate spreading, the diffuse upwelling due to plate accretion, and the shearing movement generated by large-scale mantle flow. The ridge axis is free to move in the spreading direction to adjust to the maxima of tension at the lithosphere-mantle interface induced by the convective circulation. The melt distribution in the mantle and the crustal production at the ridge axis are estimated using the formalism of McKenzie and Bickle [1988]. During the experiments the record of the ridge axis positions and crustal production is used to compute synthetic maps of the isochrons and oceanic crustal thickness. Close to the ridge, the ascending convective flow consists of 80- to 100-km-long hot sheets oriented either roughly parallel or orthogonal to spreading. Most ridge segments fit with the top of hot upwelling sheets, while transient transform faults coincide with the top of cold downwelling flows. The crustal maps display lineations subparallel or slightily oblique to spreading, a few tens of million years long, and separated by ~60-50 km, resulting from the lithospheric record of the excess crust produced at the junction of hot sheets. When a junction of two hot sheets migrates outside the ridge axial plane, the crustal thickness maximum splits into two maxima along axis, and the induced lineation in the crustal map splits into two branches. The merging of lineations occurs when the ridge plane traps the junction of hot sheets. When the large-scale mantle circulation moves parallel to the ridge crest, it slowly pushs the spreading-parallel convective sheets. The resulting lineations form V shapes pointing in the same direction as the large-scale flow. When the large-scale flow parallels spreading, it slowly pushes the ridge-parallel hot sheets in the upflow direction. Thus the ridge segments

  15. 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

  16. 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.

  17. Magnetic Properties of Lunar Geologic Terranes: New Statistical Results

    NASA Technical Reports Server (NTRS)

    Halekas, J. S.; Mitchell, D. L.; Lin, R. P.; Frey, S.; Hood, L. L.; Acuna, M. H.; Binder, A.

    2002-01-01

    We use global magnetic field data and digitized geologic maps to determine the magnetic properties of lunar terranes. Average fields vary by a factor of 100 from demagnetized impact basins and craters to strongly magnetized antipodal regions. Additional information is contained in the original extended abstract.

  18. 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.

  19. 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.

  20. 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.

  1. 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.

  2. 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.

  3. Significance of Jurassic radiolarians from the Cache Creek terrane, British Columbia

    NASA Astrophysics Data System (ADS)

    Cordey, Fabrice; Mortimer, N.; Dewever, Patrick; Monger, J. W. H.

    1987-12-01

    The discovery of new radiolarian localities in the western belt of the Cache Creek terrane in southern British Columbia possibly changes its upper age limit from Late Triassic to Early or Middle Jurassic. It favors a Middle Jurassic, rather than a Late Triassic, age of amalgamation for the Cache Creek terrane and Quesnellia (parts of superterrane I) in southern British Columbia. The new Jurassic ages also mean that the western Cache Creek terrane could be equivalent in age to the Bridge River Hozameen terrane in British Columbia and to terranes containing the Tethyan fusulinid Yabeina in northwest Washington and the Klamath Mountains of California.

  4. Multiple accretion at the eastern margin of the Rio de la Plata craton: the prolonged Brasiliano orogeny in southernmost Brazil

    NASA Astrophysics Data System (ADS)

    Saalmann, K.; Gerdes, A.; Lahaye, Y.; Hartmann, L. A.; Remus, M. V. D.; Läufer, A.

    2011-04-01

    The Neoproterozoic-Eoplalaeozoic Brasiliano orogeny at the eastern margin of the Rio de la Plata craton in southernmost Brazil and Uruguay comprises a complex tectonic history over 300 million years. The southern Brazilian Shield consists of a number of tectono-stratigraphic units and terranes. The São Gabriel block in the west is characterized by c.760-690 Ma supracrustal rocks and calc-alkaline orthogneisses including relics of older, c. 880 Ma old igneous rocks. Both igneous and metasedimentary rocks have positive ɛ Nd(t) values and Neoproterozoic TDM model ages; they formed in magmatic arc settings with only minor input of older crustal sources. A trondhjemite from the São Gabriel block intruding dioritc and tonalitic gneisses during the late stages of deformation (D3) yield an U-Pb zircon age (LA-ICP-MS) of 701 ± 10 Ma giving the approximate minimum age of the São Gabriel accretionary event. The Encantadas block further east, containing the supracrustal Porongos belt and the Pelotas batholith, is in contrast characterized by reworking of Neoarchean to Palaeoproterozoic crust. The 789 ± 7 Ma zircon age of a metarhyolite intercalated with the metasedimentary succession of the Porongos belt provides a time marker for the basin formation. Zircons of a sample from tonalitic gneisses, constituting the Palaeoproterozoic basement of the Porongos belt, form a cluster at 2,234 ± 28 Ma, interpreted as the tonalite crystallization age. Zircon rims show ages of 2,100-2,000 Ma interpreted as related to a Palaeoproterozoic metamorphic event. The Porongos basin formed on thinned continental crust in an extensional or transtensional regime between c. 800-700 Ma. The absence of input from Neoproterozoic juvenile sources into the Porongos basin strongly indicates that the Encantadas and São Gabriel blocks were separated terranes that became juxtaposed next to each other during the Brasiliano accretional events. The tectonic evolution comprises two episodes of magmatic

  5. La structuration de la marge pacifique nord-américaine et du ≪ terrane ≫ Caborca : apports de la découverte d'une faune du Jurassique inférieur et moyen dans la série de Pozos de Serna (Sonora, Mexique)

    NASA Astrophysics Data System (ADS)

    Calmus, Thierry; Pérez Segura, Efrén; Stinnesbeck, Wolfgang

    1997-08-01

    We document an Early and Middle Jurassic age for the lower part of Jurassic rocks of the Pozos de Serna area, on the basis of new datings of ammonites. These rocks are correlated with the Lower Jurassic levels of the Antimonio Formation. The angular unconformity between the Pozos de Serna unit and the Proterozoic and Palaeozoic basement of the Caborca terrane allows us to propose that, in its present position, the whole Caborca terrane, including its Proterozoic basement, is accreted to the North American craton.

  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. Metamorphosed melange terrane in the eastern Piedmont of North Carolina

    NASA Astrophysics Data System (ADS)

    Wright Morton, J., Jr.; Blake, David E.; Wylie, Albert S., Jr.; Stoddard, Edward F.

    1986-07-01

    The Falls Lake melange crops out in the eastern Piedmont of North Carolina between the Carolina slate belt and the Raleigh belt. The melange is composed of mafic and ultramafic blocks and pods of diverse shapes and sizes, dispersed without apparent stratigraphic continuity, in a matrix of pelitic schist and biotite-muscovite-plagioclase-quartz gneiss. Textures and structural relationships suggest formation by a combination of sedimentary and tectonic processes, perhaps in the accretionary wedge of a convergent plate margin. The Falls Lake melange and the overlying late Proterozoic to Early Cambrian volcanic-arc terrane of the Carolina slate belt were thrust upon a probable continental terrane of the Raleigh belt before overprinting by late Paleozoic folding and metamorphism. *Present address: Chevron USA, P.O. Box 1150, Midland, Texas 79701

  8. Impact-induced melting during accretion of the Earth

    NASA Astrophysics Data System (ADS)

    de Vries, Jellie; Nimmo, Francis; Melosh, H. Jay; Jacobson, Seth A.; Morbidelli, Alessandro; Rubie, David C.

    2016-12-01

    Because of the high energies involved, giant impacts that occur during planetary accretion cause large degrees of melting. The depth of melting in the target body after each collision determines the pressure and temperature conditions of metal-silicate equilibration and thus geochemical fractionation that results from core-mantle differentiation. The accretional collisions involved in forming the terrestrial planets of the inner Solar System have been calculated by previous studies using N-body accretion simulations. Here we use the output from such simulations to determine the volumes of melt produced and thus the pressure and temperature conditions of metal-silicate equilibration, after each impact, as Earth-like planets accrete. For these calculations a parameterised melting model is used that takes impact velocity, impact angle and the respective masses of the impacting bodies into account. The evolution of metal-silicate equilibration pressures (as defined by evolving magma ocean depths) during Earth's accretion depends strongly on the lifetime of impact-generated magma oceans compared to the time interval between large impacts. In addition, such results depend on starting parameters in the N-body simulations, such as the number and initial mass of embryos. Thus, there is the potential for combining the results, such as those presented here, with multistage core formation models to better constrain the accretional history of the Earth.

  9. Paleogene high elevations in the Qiangtang Terrane, central Tibetan Plateau

    NASA Astrophysics Data System (ADS)

    Xu, Qiang; Ding, Lin; Zhang, Liyun; Cai, Fulong; Lai, Qingzhou; Yang, Di; Liu-Zeng, Jing

    2013-01-01

    The timing of uplift of the Tibetan Plateau remains controversial, with estimates varying from Eocene to more recent than Pliocene. In particular, the paleoaltimetry of the Qiangtang terrane, on the central Tibetan Plateau, is completely unknown. Here, we present new stable isotope results of fluvial/lacustrine carbonate cement, pedogenic carbonate and marl from the Kangtuo and Suonahu formations deposited between ˜51 and ˜28 Ma in the Heihuling-Bamaoqiongzong area of the northern Qiangtang terrane. The lithofacies associations indicate that the Kangtuo formation was deposited in alluvial fan and fluvial floodplain environments, and the Suonahu formation was deposited in near shore lacustrine, playa-lake and channelized fluvial environments. Carbon and oxygen isotope values, coupled with the sedimentary facies interpretations, point to evaporation and low respiration rates in the Eocene-Oligocene paleosols, suggesting an arid climate in the high Qiangtang area at the time. The δ18Opsw values of paleo-surface water reconstructed from the lowest (i.e. the least evaporated) δ18Oc (PDB) values of the unaltered authigenic carbonates are used to make the minimum estimates of the average paleoelevation of the drainage basin. The paleoelevation of the northern Qiangtang terrane is reconstructed as above 5000 m by at least the middle Oligocene (28 Ma), similar to the present elevation in this area. The aridity and the positive shift in oxygen isotope values of surface waters in our study area may suggest that the high Lhasa terrane established by the middle Oligocene blocked the northward transport of tropical moisture. Maintenance of high elevation (>5000 m) from at least the Oligocene to the present suggests that the Qiangtang crust was already thickened by that time, and that underthrusting of India beneath Asia since then has continued to provide additional material while at the same time driving Asian lower crust eastward by crustal flow.

  10. New U-Pb and Rb-Sr ages from suture zone between Istanbul and Sakarya terranes, northwest Turkey

    NASA Astrophysics Data System (ADS)

    Akbayram, Kenan; Okay, Aral; Satır, Muharrem

    2010-05-01

    We provide new isotopic data from the Intra-Pontide Suture Zone, between the Sakarya and Istanbul terranes at the west of Armutlu Peninsula. Istanbul and Sakarya terranes show different geological histories, as reflected in their stratigraphic record, and are juxtaposed along the Intra-Pontide suture. The new U/Pb zircon and Rb/Sr mica ages come from west of Armutlu Peninsula and Almacık Mountains stretching nearly 160 km E-W direction. The Istanbul terrane has a late Proterozoic basement (Chen et al., 2002; 570 Ma) overlain by a sedimentary sequence of Ordovician to Carboniferous age. The Sakarya terrane is characterized by Carboniferous (330-310 Ma) high temperature metamorphism (Okay et al., 2006), Paleozoic granitic plutonism (Topuz et al., 2007) and by the presence of Palaeo-Tethyan subduction-accretion units. The age of metamorphism of the basement gneisses at the west of Armutlu Peninsula and the age of formation of the basement metagabbros of Almacık Mountains were not constrained. The U/Pb formation age of the zircons from a metagabbro give 1325.3 ± 5.3 Ma. This age shows that mafic rocks of the Istanbul terrane basement is older than its felsic rocks (e.g. 570 Ma). Our U/Pb zircon and Rb/Sr biotite geochronological data shows that the basement gneisses of east of Armutlu Peninsula formed at Late Proterozoic (U/Pb zircon age; 500-685 Ma) and reheated at Late Jurrasic (Rb/Sr biotite age; 154.6 ± 2.7 Ma). This data agrees previously presented Mid Mesozoic (Akbayram et al, 2009; 138 Ma) collision between Istanbul and Sakarya Terranes. REFERENCES Akbayram, K., Okay, A.I., Satır, M., Topuz, G., New U-Pb and Rb-Sr ages from northwest Turkey indicate Early Cretaceous continental collision in the western Pontides, EGU General Assembly 2009 Vienna, Austria. Chen, F., Siebel, W., Satır, M., Terzioğlu, N., Saka, K., 2002. Geochronology of the Karadere basement (NW Turkey) and implications for the geological evolution of the İstanbul Zone. Int. J. Earth Sci

  11. 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.

  12. 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

  13. Early Cretaceous bimodal volcanic rocks in the southern Lhasa terrane, south Tibet: Age, petrogenesis and tectonic implications

    NASA Astrophysics Data System (ADS)

    Wang, Chao; Ding, Lin; Liu, Zhi-Chao; Zhang, Li-Yun; Yue, Ya-Hui

    2017-01-01

    Limited geochronological and geochemical data from Early Cretaceous igneous rocks of the Gangdese Belt have resulted in a dispute regarding the subduction history of Neo-Tethyan Ocean. To approach this issue, we performed detailed in-situ zircon U-Pb and Hf isotopic, whole-rock elemental and Sr-Nd isotopic analyses on Late Mesozoic volcanic rocks exposed in the Liqiongda area, southern Lhasa terrane. These volcanic rocks are calc-alkaline series, dominated by basalts, basaltic andesites, and subordinate rhyolites, with a bimodal suite. The LA-ICPMS zircon U-Pb dating results of the basaltic andesites and rhyolites indicate that these volcanic rocks erupted during the Early Cretaceous (137-130 Ma). The basaltic rocks are high-alumina (average > 17 wt.%), enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depleted in high field strength elements (HFSEs), showing subduction-related characteristics. They display highly positive zircon εHf(t) values (+ 10.0 to + 16.3) and whole-rock εNd(t) values (+ 5.38 to + 7.47). The silicic suite is characterized by low Al2O3 (< 15.4 wt.%), Mg# (< 40), and TiO2 (< 0.3 wt.%) abundances; enriched and variable concentrations of LILEs and REEs; and strongly negative Eu anomalies (Eu/Eu* = 0.08-0.19), as well as depleted Hf isotopic compositions (εHf(t) = + 4.9 to + 16.4) and Nd isotopic compositions (εNd(t) = + 5.26 to + 6.71). Consequently, we envision a process of basaltic magmas similar to that of MORB extracted from a source metasomatized by slab-derived components for the petrogenesis of mafic rocks, whereas the subsequent mafic magma underplating triggered partial melting of the juvenile crust to generate acidic magma. Our results confirm the presence of Early Cretaceous volcanism in the southern Lhasa terrane. Combined with the distribution of the contemporary magmatism, deformation style, and sedimentary characteristics in the Lhasa terrane, we favor the suggestion that the Neo

  14. Revisiting the magnetic anomalies along the West Australian margin identifies a new continental fragment that accreted to Sumatra during the Early Eocene

    NASA Astrophysics Data System (ADS)

    Gibbons, A.; Whittaker, J. M.; Müller, P.

    2010-12-01

    Plate models reconstructing the formation of the West Australian margin differ in their treatment of the section of the Australian margin extending from the Wallaby-Zenith Fracture Zone to the tip of the Exmouth Plateau. Some reconstructions model Greater India as the conjugate, while others do not model any conjugate plate at all. The formation of the passive margin on the Australian plate implies that there must have been a conjugate continental plate that rifted away. Our revised reconstruction that includes all the abyssal plains along the West Australian margin reveals that, apart from Greater India and Argoland, a third continental block (Gascoyneland) must also have rifted from Australia since the Jurassic. From 132 Ma, while initially moving about the same Euler pole as Greater India, it formed the stretched continental crust of the Exmouth Plateau and then the oceanic crust of the Gascoyne and Cuvier abyssal plains. At 115 Ma Gascoyneland began moving in a northerly direction while Greater India continued westward only later moving northward from approximately 95 Ma when it was located entirely west of Gascoyneland. Gascoyneland did not pass west of the Investigator Ridge, a north-south-oriented linear feature at 98°E marking the western limits of the curved fracture zones of the Wharton Basin. Gascoyneland’s change in direction of plate motion would have formed these curved fracture zones and, assuming the N-S orientation of the Investigator Ridge continued into now subducted oceanic crust, would have reached West Sumatra at around 60 Ma. Plate tectonic models indicate that Sumatra was derived from accreted continental fragments originating from Gondwana (Metcalfe, 1996), although the continuity of Triassic sediments in West Sumatra, Sibumasu and East Malaya contradict this (Barber and Crow, 2003). The Woyla Group, consisting of the Sikuleh, Natal and Bengkulu terranes located along the west coast of Sumatra, has been identified as an oceanic arc

  15. 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.

  16. 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

  17. 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.

  18. 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.

  19. Accretion of the Earth.

    PubMed

    Canup, Robin M

    2008-11-28

    The origin of the Earth and its Moon has been the focus of an enormous body of research. In this paper I review some of the current models of terrestrial planet accretion, and discuss assumptions common to most works that may require re-examination. Density-wave interactions between growing planets and the gas nebula may help to explain the current near-circular orbits of the Earth and Venus, and may result in large-scale radial migration of proto-planetary embryos. Migration would weaken the link between the present locations of the planets and the original provenance of the material that formed them. Fragmentation can potentially lead to faster accretion and could also damp final planet orbital eccentricities. The Moon-forming impact is believed to be the final major event in the Earth's accretion. Successful simulations of lunar-forming impacts involve a differentiated impactor containing between 0.1 and 0.2 Earth masses, an impact angle near 45 degrees and an impact speed within 10 per cent of the Earth's escape velocity. All successful impacts-with or without pre-impact rotation-imply that the Moon formed primarily from material originating from the impactor rather than from the proto-Earth. This must ultimately be reconciled with compositional similarities between the Earth and the Moon.

  20. Isotopic Studies of the Guerrero Composite Terrane, West-Central Mexico: Implications for Provenance of Crustal Rocks and Ore Metals

    NASA Astrophysics Data System (ADS)

    Potra, A.; Macfarlane, A. W.; Salters, V. J.; Sachi-Kocher, A.

    2010-12-01

    New Pb, Sr, and Nd isotope analyses of various crustal units and ores from the Guerrero terrane are presented in order to gain insight into their provenance. Mesozoic basement rocks from the Arteaga Complex and Tejupilco metamorphic suite contain radiogenic Pb relative to bulk earth models (206Pb/204Pb between 18.701 and 19.256, 207Pb/204Pb between 15.623 and 15.693, and 208Pb/204Pb between 38.694 and 39.216), plotting to the right of the average Pb crust evolution curve of Stacey and Kramers (1975). The isotopic compositions of Pb in these rocks are substantially more radiogenic than published data on high-grade metamorphic rocks from the Grenvillian-age Oaxaca terrane, but are similar to Paleozoic basement rocks of the Mixteca terrane. Sr and Nd isotope data suggest that the basement rocks of the Guerrero terrane partly originated from ocean-floor rocks which were overlain by sediments derived from a cratonic terrane, possibly represented by the metamorphic complexes of the Oaxaca or Mixteca terranes. Lead isotope ratios of Cretaceous sedimentary rocks of the Zihuatanejo and Huetamo Sequences define two different clusters, with the Zihuatanejo Sequence units shifted to more radiogenic values (206Pb/204 between 18.763 and 19.437, 207Pb/204Pb between 15.580 and 15.643, and 208Pb/204Pb between 38.510 and 38.892). Samples from the Huetamo Sequence are less radiogenic than the metamorphic basement, with Pb isotope ratios between 18.630 and 18.998 for 206Pb/204, 15.563 and 15.641 for 207Pb/204Pb, and 38.369 and 38.610 for 208Pb/204Pb. They plot close to the radiogenic end of the MORB field, suggesting a possible mixing line between the basement rocks and the MORB component. Lead isotope ratios of Tertiary intrusive rocks from La Verde, El Malacate, and La Esmeralda resemble the orogene reservoir in the plumbotectonics model of Zartman and Doe (1981). Plutonic rocks from La Verde show the most radiogenic Pb compositions, suggesting a significant influence of old

  1. Early Jurassic tectonism occurred within the Basu metamorphic complex, eastern central Tibet: Implications for an archipelago-accretion orogenic model

    NASA Astrophysics Data System (ADS)

    Li, Hua-Qi; Xu, Zhi-Qin; Webb, A. Alexander G.; Li, Tian-Fu; Ma, Shi-Wei; Huang, Xue-Meng

    2017-04-01

    The Basu metamorphic complex, surrounded by ophiolitic melanges and intruded by a large volume of undeformed granitoid rocks along the eastern segment of the Bangong-Nujiang suture, holds one of the keys to understanding the pre-Cenozoic tectonic evolution of central Tibet. Zircon U-Pb dating of rocks from the Basu metamorphic complex reveals that meta-igneous rocks yield Early Paleozoic crystallization ages of 500-492 Ma and an Early Jurassic metamorphic age of 173 Ma, and that undeformed granitoid rocks yield crystallization ages of approximately 186-174 Ma. Whole rock geochemical and zircon Lu-Hf isotopic data indicate that the undeformed granitoid rocks originated mainly from partial melting of ancient crustal sources, which may reflect a collisional orogenic setting. 40Ar/39Ar dating of biotite from a sillimanite-garnet-biotite paragneiss shows cooling to 300 ± 50 °C at 165 Ma. These data indicate significant Early Jurassic tectonism, during which most of the Basu metamorphic complex was formed. Furthermore, the age data resemble those of the Amdo metamorphic complex located approximately 500 km to the west along the Bangong-Nujiang suture. Together, these complexes may represent a ;destroyed or unrecognized; block, i.e., the Amdo-Tongka block, which may be the eastern extension of the South Qiangtang terrane. Based on the tectonic outlines of the multiple ophiolitic zones and magmatic belts, we suggest a new archipelago-accretion model that attributes the Early Jurassic tectonism to an arc-continent/micro-continent collision. This model further enables the reconstruction of the eastern Tethyan Ocean and the orogenic processes of central Tibet during the Mesozoic.

  2. Modeling Anomalous Crustal Accretion at Spreading Zones

    NASA Astrophysics Data System (ADS)

    Schmeling, H.; Marquart, G.

    2003-12-01

    The thermal and seismic structure of normal oceanic crust or anomalous crust such as Iceland depends on the mode of melt extraction from the mantle and its emplacement within or on top of the crust. We model crustal accretion by a two fold approach. In a 2D spreading model with anomalous mantle temperature beneath the ridge we solve the Navier-Stokes-, the heat tansport, the mass conservation and the melting equations to determine the enhanced melt production beneath the ridge. This melt is extracted and emplaced on top of the model to form the crust. Two cases are distinguished: a) Extruded crustal material is taken out of the model and is only advected according to the spreading of the plate, b) extruded material is fed back into the model from the top to mimic isostatic subsidence of extruded crust. We find that the feed back of case b) is only moderate. For example, if extruded crustal material as thick as 40 km is fed back into the model, the melting region is depressed downward only by as much as 10km, and the total amount of generated melt is reduced by about 20 %. On the other hand, the upper 30 km of the model is cooled considerably by several 100 degrees. A second set of models focuses on the details of crustal accretion without explicitly solving for the melting and extraction. Knowing the spreading rate, the rate of crustal production can be estimated, but the site of emplacement is not obvious. For an anomalous crust such as Iceland we define four source regions of crustal accretion: surface extrusion, intrusion in fissure swarms at shallow depth connected to volcanic centres, magma chambers at shallow to mid-crustal level, and a deep accretion zone, where crust is produced by widespread dyke and sill emplacement and underplating. We solve the Navier-Stokes-, the heat tansport and the mass conservation equations and prescribe different functions in space and time for crustal production in the four defined regions. The temperature of the imposed

  3. 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.

  4. Kinematic significance of L tectonites in the footwall of a major terrane-bounding thrust fault, Klamath Mountains, California, USA

    NASA Astrophysics Data System (ADS)

    Sullivan, W. A.

    2009-11-01

    Detailed geologic mapping, cross-section reconstructions, strain analyses, and kinematic analyses, enable the reconstruction of a ˜one-kilometer-wide domain of L tectonites in the east-west-striking, subhorizontal to gently south-dipping Pigeon Point high-strain zone (PPHSZ) associated with a major thrust fault separating oceanic- and arc-affinity terranes in the Klamath Mountains, California. L tectonites are associated with: (1) a convex-upward warp of the upper high-strain-zone boundary, (2) a transition from mafic metavolcaniclastic rocks to micaceous quartzites, (3) folds subparallel with mineral lineations, (4) emplacement of synkinematic ultramafic/mafic intrusive bodies, and (5) a local temperature increase from greenschist- to amphibolite-facies conditions. Pure-shear-dominated deformation accommodated zone-normal shortening and transport-parallel elongation coupled with subordinate top-to-the-west-directed, thrust-style simple shear. L tectonite formation was controlled by the shape of the high-strain-zone boundary driving lateral flow into the apex of the lens-shaped zone in response to a favorable kinematic geometry and bulk strain in the constrictional field. Localized magmatic heating best explains the shape of the high-strain-zone boundary, but L tectonites are not partitioned into a single rheological domain. During terrane amalgamation strain-path partitioning occurred with localized top-to-the-west-directed simple shear partitioned into a structurally overlying thrust zone and pure-shear-dominated subvertical shortening and transport-parallel elongation partitioned into the PPHSZ.

  5. 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.

  6. Paleobiogeographic affinities of emsian (late early devonian) gastropods from farewell terrane (west-central Alaska)

    USGS Publications Warehouse

    Fryda, J.; Blodgett, R.B.

    2008-01-01

    The vast majority of Emsian gastropods from Limestone Mountain, Medfra B-4 quadrangle, west-central Alaska (Farewell terrane) belong to species with lecithotrophic larval strategy. The present data show that there is no significant difference in the paleobiogeo-graphic distribution of Emsian gastropod genera with lecithotrophic and planktotrophic larval strategies. Numerical analysis of the faunal affinities of the Emsian gastropod fauna from the Farewell terrane reveals that this terrane has much stronger faunal connections to regions like Variscan Europe, eastern Australia, and the Alexander terrane of southeast Alaska than to cratonic North America (Laurentia). The Canadian Arctic Islands is the only region of cratonic North America (Laurentia) that shows significant faunal affinities to the Emsian gastropod faunas of the Farewell terrane. The analysis also indicates a close faunal link between the Farewell and Alexander terranes. Published paleontological and geological data suggest that the Farewell and Alexander terranes represents tectonic entities that have been rifted away from the Siberia, Baltica, or the paleo-Pacific margin of Australia. The results of the present numerical analysis are not in conflict with any of these possibilities. However, the principle of spatial continuity of the wandering path prefers Siberia as the most probable "parental" paleocontinent for the derivation of both the Farewell and Alexander terranes. ?? 2008 The Geological Society of America.

  7. Tectonic evolution of high-grade metamorphic terranes in central Vietnam: Constraints from large-scale monazite geochronology

    NASA Astrophysics Data System (ADS)

    Nakano, Nobuhiko; Osanai, Yasuhito; Owada, Masaaki; Nam, Tran Ngoc; Charusiri, Punya; Khamphavong, Keo

    2013-09-01

    on the pressure-temperature-time-protolith evolutions, metamorphic rocks from central Vietnam provide a continuous record of subduction-accretion-collision tectonics between the South China and Indochina blocks: in the Ordovician-Silurian, the region was characterized by active continental margin tectonics, followed by continental collision during the Late Permian to Early Triassic and subsequent exhumation during the Late Triassic. The results also suggest that the timing of metamorphism and protolith formation as well as the geochemical features in other Southeast Asian terranes should be verified to achieve a better understanding of the Precambrian to Early Mesozoic tectonic history in Asia.

  8. Compositional terranes on Mercury: Information from fast neutrons

    NASA Astrophysics Data System (ADS)

    Lawrence, David J.; Peplowski, Patrick N.; Beck, Andrew W.; Feldman, William C.; Frank, Elizabeth A.; McCoy, Timothy J.; Nittler, Larry R.; Solomon, Sean C.

    2017-01-01

    We report measurements of the flux of fast neutrons at Mercury from 20ºS to the north pole. On the basis of neutron transport simulations and remotely sensed elemental compositions, cosmic-ray-induced fast neutrons are shown to provide a measure of average atomic mass, , a result consistent with earlier studies of the Moon and Vesta. The dynamic range of fast neutron flux at Mercury is 3%, which is smaller than the fast-neutron dynamic ranges of 30% and 6% at the Moon and Vesta, respectively. Fast-neutron data delineate compositional terranes on Mercury that are complementary to those identified with X-ray, gamma-ray, and slow-neutron data. Fast neutron measurements confirm the presence of a region with high , relative to the mean for the planet, that coincides with the previously identified high-Mg region and reveal the existence of at least two additional compositional terranes: a low- region within the northern smooth plains and a high- region near the equator centered near 90ºE longitude. Comparison of the fast-neutron map with elemental composition maps show that variations predicted from the combined element maps are not consistent with the measured variations in fast-neutron flux. This lack of consistency could be due to incomplete coverage for some elements or uncertainties in the interpretations of compositional and neutron data. Currently available data and analyses do not provide sufficient constraints to resolve these differences.

  9. 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.

  10. Lunar Accretion and the Moon’s Initial Thermal State

    NASA Astrophysics Data System (ADS)

    Salmon, Julien; Canup, R. M.

    2012-10-01

    Previous models of lunar formation from an impact-generated disk predict the Moon accretes in less than a year (Ida et al. 1997, Kokubo et al. 2000). Such a rapid accretion implies a fully molten initial Moon (e.g., Pritchard & Stevenson 2000). However, the lack of global faults on the Moon has been interpreted as constraining the depth of initial melting to the Moon’s outer few hundred kilometers (Solomon & Chaiken 1976). Depth estimates for the lunar magma ocean (250 to 1000 km; e.g., Shearer et al. 2006) are also typically less than the Moon’s full radius (1738 km). Taken at face value, such observations appear most consistent with a partially molten initial Moon. We have developed a new lunar accretion model that includes a more accurate description of the inner protolunar disk, taking into account thermal processes that limit the disk’s evolution rate (Salmon and Canup 2012). Our model predicts a 3-phase accretion of the Moon, in which material initially orbiting in the outer disk accumulates rapidly, followed by a much slower evolution of the vapor/fluid inner disk and a final phase of accretion of inner disk material onto the Moon. The Moon’s total accretion then occurs over about 100 years, which could be compatible with partial cooling of protolunar material. We have coupled our accretion model to a simple model for the Moon’s initial thermal state. We use a 3D spherical grid to model the forming Moon. We estimate heating due to each accretionary event using the impactor properties predicted by our accretion model, assuming that a fixed fraction ("h") of each impactor’s kinetic energy is retained locally within the Moon’s interior, and include cooling from the Moon’s surface. We use this model to estimate the Moon’s initial thermal state as a function of h and specific accretionary histories.

  11. 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.

  12. The nature of Archean terrane boundaries: an example from the northern Wyoming Province

    USGS Publications Warehouse

    Mogk, D.W.; Mueller, P.A.; Wooden, J.L.

    1992-01-01

    The Archean northern Wyoming Province can be subdivided into two geologically distinct terranes, the Beartooth-Bighorn magmatic terrane (BBMT) and the Montana metasedimentary terrane (MMT). The BBMT is characterized by voluminous Late Archean (2.90-2.74 Ga) magmatic rocks (primarily tonalite, trondhjemite, and granite); metasedimentary rocks are preserved only as small, rare enclaves in this magmatic terrane. The magmatic rocks typically have geochemical and isotopic signatures that suggest petrogenesis in a continental magmatic arc environment. The MMT, as exposed in the northern Gallatin and Madison Ranges, is dominated by Middle Archean trondhjemitic gneisses (3.2-3.0 Ga); metasedimentary rocks, however, are significantly more abundant than in the BBMT. Each terrane has experienced a separate and distinct geologic history since at least 3.6 Ga ago based on differences in metamorphic and structural styles, composition of magmatic and metasupracrustal rocks, and isotopic ages; consequently, these may be described as discrete terranes in the Cordilleran sense. Nonetheless, highly radiogenic and distinctive Pb-Pb isotopic signatures in rocks of all ages in both terranes indicate that the two terranes share a significant aspect of their history. This suggests that these two Early to Middle Archean crustal blocks, that initially evolved as part of a larger crustal province, experienced different geologic histories from at least 3.6 Ga until their juxtaposition in the Late Archean (between 2.75 to 2.55 Ga ago). Consequently, the boundary between the BBMT and MMT appears to separate terranes that are not likely to be exotic in the sense of their Phanerozoic counterparts. Other Archean provinces do appear to contain crustal blocks with different isotopic signatures (e.g. West Greenland, India, South Africa). The use of the term exotic, therefore, must be cautious in situations where geographic indicators such as paleontologic and/or paleomagnetic data are not available

  13. 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.

  14. Paleoclimatic Evidence for Cenozoic Migration of Alaskan Terranes

    NASA Astrophysics Data System (ADS)

    Keller, G.; von Huene, R.; McDougall, K.; Bruns, T. R.

    1984-08-01

    Chronostratigraphic and paleoclimatic comparisons of microfossils from deep-sea cores, from samples of an exploratory drill hole, and from dredged rock of the Gulf of Alaska with coeval microfossil assemblages on the North American continent provide constraints on the northward migration of the Yakutat block, the Prince William terrane and the Pacific plate during Tertiary time. The comparative paleolatitudes of microfauna and flora provide three main constraints. (1) The Prince William terrane was in its present position with respect to North America (at high latitudes, 50° ± 5°N) by middle Eocene time (40-42 Ma), consistent with models derived from paleomagnetic data. (2) The adjacent Yakutat block was 30° ± 5° south of its present position in early Eocene (50 Ma), 20° ± 5° south in middle Eocene (40-44 Ma), and 15° ± 5° south in late Eocene time (37-40 Ma), thus requiring a northward motion of about 30° since 50 Ma. Moreover, the Yakutat block was at least 10° south of the Prince William terrane during Eocene time. These data are consistent with migration of the Yakutat block with the Pacific and Kula plates for at least the last 50 Ma. (3) site 192 on the Pacific plate was at about 15° ± 5°N latitude in the late Cretaceous (68 Ma), at 30° ± 5°N in early Eocene (50 Ma), at 40° ± 5°N in middle Eocene (40-44 Ma), at 45° ± 5°N in late Eocene (37-40 Ma), and north of 50° ± 5°N in latest Eocene to early Oligocene time (34-37 Ma). These paleolatitudes, based on planktonic foraminiferal assemblages, indicate northward drift consistent with the North America-Pacific plate reconstructions from about 68 Ma to 40 Ma (Engebretson, 1982). However, from Cretaceous to early Eocené time, faunal data indicate significantly lower latitudinal positions, and from Oligocwne to early Miocene time, significantly higher latitudinal positions. These discrepancies can be explained by the northward expansion of tropical faunas during the globally warm early

  15. 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.

  16. The pressure-temperature-time evolution of the Antarctic Peninsula - magmatic arc and/or terrane tectonics?

    NASA Astrophysics Data System (ADS)

    Wendt, A. S.; Vidal, O.; Vaughan, A.

    2003-04-01

    The tectonic mobility in orogenic systems requires that the geologic history of each rock unit must be evaluated on the merits of the information gleaned more from individual outcrops than from regional generalisation. Continental margins affected by tectonic processes commonly have a region where the stratigraphic elements should be considered suspect in regard to palaeogeographic linkages both among the elements and between each element and the adjoining continent. Such occurrences might be considered as a natural consequence of the mobility and transient state of oceanic crust so that exotic far-travelled crustal fragments can be expected. The collision of those fragments and their distribution patterns reflect in general a combination of several tectonic phases such as overthrusting, stitching of plutons along the contact and welding metamorphism. The Antarctic Peninsula is an example "par excellence" for testing those tectonic processes occurring along continental margins. Prior to Mid-Jurassic times, the peninsula in its entity is thought to have formed a part of the palaeo-Pacific margin. East-directed subduction along the margin occurred during Mesozoic-Tertiary times producing a magmatic arc complex, in which volcanic and plutonic rocks are distributed widely along the length of the peninsula. However, recent discoveries suggest also that the Antarctic Peninsula is composed of at least two terranes in transpressional contact with para-autochthonous continental Gondwana margin. The reconstruction of the geological history becomes a challenging task in the hostile environment of the Antarctic where individual outcrops are scattered over large geographical distances, and structural relationships are obscured by thick layers of ice. In this work, we are attempting to correlate for the first time the pressure-temperature-time evolution of metamorphic rocks parallel to the spine of the peninsula and their structural relationship to the volcanic and plutonic

  17. 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.

  18. 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.

  19. 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.

  20. Dynamics of core accretion

    DOE PAGES

    Nelson, Andrew F.; Ruffert, Maximilian

    2012-12-21

    In this paper, we perform three-dimensional hydrodynamic simulations of gas flowing around a planetary core of mass Mpl = 10M⊕ embedded in a near Keplerian background flow, using a modified shearing box approximation. We assume an ideal gas behaviour following an equation of state with a fixed ratio of the specific heats, γ = 1.42, consistent with the conditions of a moderate-temperature background disc with solar composition. No radiative heating or cooling is included in the models. We employ a nested grid hydrodynamic code implementing the ‘Piecewise Parabolic Method’ with as many as six fixed nested grids, providing spatial resolutionmore » on the finest grid comparable to the present-day diameters of Neptune and Uranus. We find that a strongly dynamically active flow develops such that no static envelope can form. The activity is not sensitive to plausible variations in the rotation curve of the underlying disc. It is sensitive to the thermodynamic treatment of the gas, as modelled by prescribed equations of state (either ‘locally isothermal’ or ‘locally isentropic’) and the temperature of the background disc material. The activity is also sensitive to the shape and depth of the core's gravitational potential, through its mass and gravitational softening coefficient. Each of these factors influences the magnitude and character of hydrodynamic feedback of the small-scale flow on the background, and we conclude that accurate modelling of such feedback is critical to a complete understanding of the core accretion process. The varying flow pattern gives rise to large, irregular eruptions of matter from the region around the core which return matter to the background flow: mass in the envelope at one time may not be found in the envelope at any later time. No net mass accretion into the envelope is observed over the course of the simulation and none is expected, due to our neglect of cooling. Except in cases of very rapid cooling however, as

  1. Dynamics of core accretion

    SciTech Connect

    Nelson, Andrew F.; Ruffert, Maximilian

    2012-12-21

    In this paper, we perform three-dimensional hydrodynamic simulations of gas flowing around a planetary core of mass Mpl = 10M embedded in a near Keplerian background flow, using a modified shearing box approximation. We assume an ideal gas behaviour following an equation of state with a fixed ratio of the specific heats, γ = 1.42, consistent with the conditions of a moderate-temperature background disc with solar composition. No radiative heating or cooling is included in the models. We employ a nested grid hydrodynamic code implementing the ‘Piecewise Parabolic Method’ with as many as six fixed nested grids, providing spatial resolution on the finest grid comparable to the present-day diameters of Neptune and Uranus. We find that a strongly dynamically active flow develops such that no static envelope can form. The activity is not sensitive to plausible variations in the rotation curve of the underlying disc. It is sensitive to the thermodynamic treatment of the gas, as modelled by prescribed equations of state (either ‘locally isothermal’ or ‘locally isentropic’) and the temperature of the background disc material. The activity is also sensitive to the shape and depth of the core's gravitational potential, through its mass and gravitational softening coefficient. Each of these factors influences the magnitude and character of hydrodynamic feedback of the small-scale flow on the background, and we conclude that accurate modelling of such feedback is critical to a complete understanding of the core accretion process. The varying flow pattern gives rise to large, irregular eruptions of matter from the region around the core which return matter to the background flow: mass in the envelope at one time may not be found in the envelope at any later time. No net mass accretion into the envelope is observed over the course of the simulation and none is expected, due to our neglect of cooling. Except in cases of very rapid cooling

  2. 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.

  3. 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

  4. 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.

  5. Internal Differentiation in Early-Accreting Planetesimals

    NASA Astrophysics Data System (ADS)

    Elkins-Tanton, Linda; Weiss, B. P.; Zuber, M. T.

    2009-09-01

    Chondritic meteorites are unmelted, variably metamorphosed samples of the earliest solids of the solar system. A recent paleomagnetic study of CV chondrites suggests that though the chondrites themselves were not melted, their parent body was internally differentiated and produced a core magnetic dynamo. Bodies that accreted before 1.3 to 3 Ma after CAIs likely contained sufficient 26Al to melt internally from the insulated cumulative effects of radiogenic heating. These bodies would melt from the interior out, sometimes forming an interior magma ocean under a solid, conductive, undifferentiated shell. This shell would consist of the same chondritic material that made up the bulk accreting body before melting began. Simulations indicate that this model of chondrite meteorite genesis requires specific but reasonable conditions: The chondrites experienced metamorphism in the undifferentiated crust of an internally differentiated body of radius 250 km to 450 km, which likely accreted no later than 2.0 Ma after CAIs. This body could have produced a core dynamo lasting more than 10 Ma that left a thermoremanent magnetization in Allende. Because of the limited lifetime of 26Al and the longer apparent period over which chondrite parent bodies were forming, many other parent bodies likely heated without significant melting. However, bodies that are internally differentiated may well exist undetected in the asteroid belt. The shapes and masses of 1 Ceres and 2 Pallas are consistent with differentiated interiors. Other internally differentiated asteroids may have lost their hydrostatic shapes through later impacts, but their surfaces may be irregular, space-weathered primitive material, perhaps with altered or differentiated material at the bottoms of the largest craters. This scenario can explain the mismatch between the enormous diversity (more than 130) of parent bodies represented by achondrites and the paucity (fewer than 10) of basaltic asteroids. Funded by NSF

  6. 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.

  7. Dynamics of continental accretion

    NASA Astrophysics Data System (ADS)

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

    2014-04-01

    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.

  8. Late Triassic Porphyritic Intrusions And Associated Volcanic Rocks From The Shangri-La Region, Yidun Terrane, Eastern Tibetan Plateau: Implications For Adakitic Magmatism And Porphyry Copper Mineralization

    NASA Astrophysics Data System (ADS)

    Wang, B.; Zhou, M.; Li, J.; Yan, D.

    2011-12-01

    The Yidun terrane, located on the eastern margin of the Tibetan plateau, has been commonly considered to be a Triassic volcanic arc produced by subduction of the Ganzi-Litang oceanic lithosphere. The Yidun terrane is characterized by numerous arc-affinity granitic intrusions located along a 500-km-long, north-south-trending belt. Among these granitic bodies, several small porphyritic intrusions in the southern segment of the terrane (Shangri-La region) are associated with large porphyry copper deposits. These porphyritc intrusions are composed of diorite and quartz diorite, and spatially associated with andesites and dacites. LA-ICP-MS zircon U-Pb ages of the intrusions range from 230 to 215 Ma. The andesites and dacites are intercalated with slates and sandstones and have ages of around 220 Ma. The intrusive and volcanic rocks have SiO2 contents from 56.6 to 67.1 wt.%, Al2O3 from 14.2 to 17.4 wt.% and MgO from 1.9 to 4.2 wt.%. They show significant negative Nb-Ta anomalies on primitive mantle-normalized spidergrams. They have high La/Yb (13-49) ratios with no prominent Eu anomalies. All the rocks have high Sr (258-1980 ppm), and low Y (13-21 ppm) with high Sr/Y ratios (29-102). The geochemical features indicate that both the volcanic rocks and porphyritic intrusions were derived from adakitic magmas. They have similar initial 87Sr/86Sr ratios (0.7058 to 0.7077) and ɛNd (-1.88 to -4.93) values, but can be further divided into two groups: high silica (HSA) and low silica adakitic rocks (LSA). The HSA, representing an early stage of magmatism (230 to 215 Ma), were derived from oceanic slab melts with limited interaction with the overlying mantle wedge. At 215 Ma, more extensive interaction resulted in the formation of LSA. We propose that HSA were produced by flat subduction leading to melting of oceanic slab, whereas subsequent slab break-off caused the significant interaction between slab melts and the mantle wedge and thus the generation of the LSA. Compared with

  9. Rethinking Black Hole Accretion Discs

    NASA Astrophysics Data System (ADS)

    Salvesen, Greg

    Accretion discs are staples of astrophysics. Tapping into the gravitational potential energy of the accreting material, these discs are highly efficient machines that produce copious radiation and extreme outflows. While interesting in their own right, accretion discs also act as tools to study black holes and directly influence the properties of the Universe. Black hole X-ray binaries are fantastic natural laboratories for studying accretion disc physics and black hole phenomena. Among many of the curious behaviors exhibited by these systems are black hole state transitions -- complicated cycles of dramatic brightening and dimming. Using X-ray observations with high temporal cadence, we show that the evolution of the accretion disc spectrum during black hole state transitions can be described by a variable disc atmospheric structure without invoking a radially truncated disc geometry. The accretion disc spectrum can be a powerful diagnostic for measuring black hole spin if the effects of the disc atmosphere on the emergent spectrum are well-understood; however, properties of the disc atmosphere are largely unconstrained. Using statistical methods, we decompose this black hole spin measurement technique and show that modest uncertainties regarding the disc atmosphere can lead to erroneous spin measurements. The vertical structure of the disc is difficult to constrain due to our ignorance of the contribution to hydrostatic balance by magnetic fields, which are fundamental to the accretion process. Observations of black hole X-ray binaries and the accretion environments near supermassive black holes provide mounting evidence for strong magnetization. Performing numerical simulations of accretion discs in the shearing box approximation, we impose a net vertical magnetic flux that allows us to effectively control the level of disc magnetization. We study how dynamo activity and the properties of turbulence driven by the magnetorotational instability depend on the

  10. 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.

  11. Migration of accreting giant planets

    NASA Astrophysics Data System (ADS)

    Crida, A.; Bitsch, B.; Raibaldi, A.

    2016-12-01

    We present the results of 2D hydro simulations of giant planets in proto-planetary discs, which accrete gas at a more or less high rate. First, starting from a solid core of 20 Earth masses, we show that as soon as the runaway accretion of gas turns on, the planet is saved from type I migration : the gap opening mass is reached before the planet is lost into its host star. Furthermore, gas accretion helps opening the gap in low mass discs. Consequently, if the accretion rate is limited to the disc supply, then the planet is already inside a gap and in type II migration. We further show that the type II migration of a Jupiter mass planet actually depends on its accretion rate. Only when the accretion is high do we retrieve the classical picture where no gas crosses the gap and the planet follows the disc spreading. These results impact our understanding of planet migration and planet population synthesis models. The e-poster presenting these results in French can be found here: L'e-poster présentant ces résultats en français est disponible à cette adresse: http://sf2a.eu/semaine-sf2a/2016/posterpdfs/156_179_49.pdf.

  12. 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.

  13. The trajectory of India towards Eurasia recorded by subducted slabs: evidence for southward subduction of the Tethys Ocean under India after 130 Ma

    NASA Astrophysics Data System (ADS)

    Suppe, John; Wu, Jonny; Lin, Chris D. J.; Kanda, Ravi V. S.

    2014-05-01

    At the ~130 Ma breakup of India from Australia-Antarctica, published paleomagnetic data estimate that India was located south of 30°S. As India began its northward journey that continues today, the 5000 km of Tethys Ocean that stood in the expanse between India and Eurasia gradually disappeared, leaving only traces of its existence in ophiolite sutures and accreted terranes at the Himalayas, the largest orogeny on Earth today. Here we present newly mapped, sub-horizontal slabs under the Indian Ocean at depths of 1600 to 2100 km and at latitudes between 35°S to 15°N. These slabs are further south than any published paleolatitudinal estimates of initial India-Asia collision and thus, the existence of these slabs cannot be explained by the popular idea of northward Tethys subduction under Eurasia. Instead, our slab constraints show for the first time that the majority of the Tethys Ocean was subducted southward after 130 Ma, overrun by a northward-moving India. When restored to the surface of a spherical model Earth, the slabs closely correspond to the well-known track of India from Eastern Gondwanaland to Eurasia that began at ~130 Ma, viewed in a mantle reference. We present a plate reconstruction that includes other restored slabs from the India-Eurasia collision zone, which are at shallower depths and ubiquitously located north of the equator. The reconstruction implies that the Tethys Ocean was subducted under Greater India, India and the post-breakup ocean at the eastern margin of the Indian plate - now identified as the enigmatic 'Burma slab'. Slab geometries were mapped from global P- and S-wave models and restored to the surface of a spherical Earth model. Gplates software was used to reconstruct the mapped slabs.

  14. 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.

  15. 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.

  16. 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

  17. 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

  18. An exotic terrane in the Sulu UHP region, China

    NASA Astrophysics Data System (ADS)

    Chu, W.; Zhang, R.; Tsujimori, T.; Liou, J. G.

    2004-12-01

    The Haiyangsuo region of about 15 km2 along the coast in the NE part of the Triassic Sulu UHP terrane occurs three major rock types: amphibolitized metagabbro, gneiss and granitic dikes. Three different gneisses were observed in the field: A) Light color felsic gneiss is the dominant country rock and contains Qtz, Pl, Ms and Bi. B) Dark color plagioclase-amphibole gneiss occurs as thin layers within country rock; C) Granulite facies rock occurs as discontinuous lens. The amphibolitized metagabbros intrude into the gneisses as massive bodies (several m to hundreds of m in size) and thin dikes. Both metamorphic intrusives and gneisses are cross-cut by granitic dikes. The amphibolitized metagabbro was divided into three types: coronal metagabbro, transitional rock and garnet amphibolite: 1) Coronal metagabbro preserves gabbroic texture and primary assemblage of Opx+Cpx+Pl+Amp+Ilm. Most pyroxene grains are partially rimmed by thin corona of Amp+Ab+Qtz. Garnet occurs as fine-grained coronas at interface between plagioclase, pyroxene or ilmenite. 2) Transitional rocks contain similar assemblage and texture but most orthopyroxenes were partially or totally replaced by Amp+Qtz; garnet increases in content and size. Some gabbroic textures are preserved, but calcic plagioclase was replaced by zoisite, albite and muscovite. 3) Garnet amphibolite occurs at the margins of intrusive bodies and boudins where only minor relict clinopyroxenes preserve. Garnet coronal chains are not clear any more. Granitic dikes show pronounced deformation with mylonitic texture and contain 40-50% quartz porphyroclasts. Zircon separates from 2 metagabbros, 4 gneisses and 1 granitic rock were dated by using Stanford SHRIMP-RG. Metagabbroic zircons are angular and fractured shapes. The upper-intercept ages of gneisses rang from 1730 to about 2400 Ma, indicating variable protoith age. The 2 garnet amphibolites have upper-intercept ages 1734±5Ma and 1735±21Ma respectively. They are much older than

  19. 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.

  20. 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

  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. 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

  6. Proterozoic sutures and terranes in the southeastern Baltic Shield interpreted from BABEL deep seismic data

    NASA Astrophysics Data System (ADS)

    Abramovitz, T.; Berthelsen, A.; Thybo, H.

    1997-03-01

    A hitherto unknown terrane and its bounding sutures have been revealed by a combined study of normal-incidence and wide-angle seismic data along the BABEL profile in the Baltic Sea. This Intermediate Terrane is situated between a Northern Terrane of Svecofennian age and a Southwestern Terrane of Gothian age. It is delimited upwards by two low-angle and oppositely dipping sutures and occupies mainly middle and lower crustal levels with a width of ˜ 300 km at Moho level. The ˜ 1.86 Ga suture against the Northern Terrane is imaged by a prominent almost continuous NE-dipping crustal reflection from 3.5 to 14 s twt over 175 km. Where it downlaps on the Moho, sub-Moho velocities change from 8.2 to 7.8 km/s (±0.2) over less than 25 km. A relatively strong, NE-dipping normal-incidence and wide-angle reflection at 19-23 s twt indicates that the suture extends into the upper mantle. The pervasive NE-dipping reflection fabric of the Intermediate Terrane is interpreted as shear zones that developed during collision and possibly were reactivated by later events. High Poisson's ratios suggest a mafic composition or high fluid content. The ˜ 1.86 Ga collision was probably succeeded by continental break-up and removal of an unknown continent, except for the Intermediate Terrane. Subsequent formation of an east-dipping subduction zone further to the west led to the emplacement of 1.81-1.77-Ga-old granitoids in the southern part of the Transscandinavian Igneous Belt. The ˜ 1.65-1.60 Ga suture against the Southwestern Terrane is defined by a semi-continuous band of strong SW-dipping reflections between 3 and 8 s twt over 65 km, which are interpreted as a low-angle thrust zone along which Gothian crust overrode the Intermediate Terrane. The identification of three individual seismic terranes in the southeastern part of the Baltic Shield provides new evidence for Palaeoproterozoic plate tectonic processes.

  7. Active slivering of oceanic crust along the Molucca ridge (Indonesia-Philippine): Implication for ophiolite incorporation in a subduction wedge?

    NASA Astrophysics Data System (ADS)

    Bader, Anne GaëLle; Pubellier, Manuel; Rangin, Claude; Deplus, Christine; Louat, RéMy

    1999-08-01

    A recent marine geophysical survey in the northern Molucca Sea revealed the structure to be that of a classical active convergent margin. We observe from west to east a volcanic arc (Sangihe), a forearc basin resting on an outer ridge (the Molucca ridge), which serves as a buttress for an accretionary wedge, and a composite downgoing plate (Snellius Ridge and Philippine Sea Basin). Gravity modeling indicates a strong negative anomaly above the wedge, which cannot be explained with reasonable density values. Modeling imposes a basement deepening and a rupture of the 700-km-long subducting lithosphere. This process individualized the lithospheric slab from the Snellius Ridge, which in turn was separated recently from the south Philippine Basin by the incipient Philippine Trench. This induces a deformation of the forearc region with backthrusting of the outer ridge and forearc basin, visible on bathymetry and seismic data. We extrapolate the tectonic emplacement of such oceanic blocks to the Oligocene times in order to explain the origin of the Pujada Miangas outer ridge as a sliver previously incorporated to the margin, and we discuss the possibility of this deformation process being fabric for terrane accretion.

  8. Swept wing ice accretion modeling

    NASA Technical Reports Server (NTRS)

    Potapczuk, M. G.; Bidwell, C. S.

    1990-01-01

    An effort to develop a three-dimensional ice accretion modeling method is initiated. This first step toward creation of a complete aircraft icing simulation code builds on previously developed methods for calculating three-dimensional flowfields and particle trajectories combined with a two-dimensional ice accretion calculation along coordinate locations corresponding to streamlines. This work is intended as a demonstration of the types of calculations necessary to predict a three-dimensional ice accretion. Results of calculations using the 3D method for a MS-317 swept wing geometry are projected onto a 2D plane normal to the wing leading edge and compared to 2D results for the same geometry. These results indicate that the flowfield 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 3D calculation.

  9. Eclipse Mapping of Accretion Discs

    NASA Astrophysics Data System (ADS)

    Baptista, R.

    The eclipse mapping method is an inversion technique that makes use of the information contained in eclipse light curves to probe the structure, the spectrum and the time evolution of accretion discs. In this review I present the basics of the method and discuss its different implementations. I summarize the most important results obtained to date and discuss how they have helped to improve our understanding of accretion physics, from testing the theoretical radial brightness temperature distribution and measuring mass accretion rates to showing the evolution of the structure of a dwarf novae disc throughout its outburst cycle, from isolating the spectrum of a disc wind to revealing the geometry of disc spiral shocks. I end with an outline of the future prospects.

  10. 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

  11. 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.

  12. 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.

  13. 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.

  14. Sedimentology of Hirnantian glaciomarine deposits in the Balkan Terrane, western Bulgaria: Fixing a piece of the north peri-Gondwana jigsaw puzzle

    NASA Astrophysics Data System (ADS)

    Chatalov, Athanas

    2017-04-01

    Glaciomarine deposits of late Hirnantian age in the western part of the Palaeozoic Balkan Terrane have persistent thickness ( 7 m) and lateral uniformity in rock colour, bedding pattern, lithology, and sedimentary structures. Four lithofacies are distinguished from base to top: lonestone-bearing diamictites, interbedded structureless mudstones, crudely laminated diamictites, and finely laminated mudstones. The diamictites are clast-poor to clast-rich comprising muddy to sandy varieties. Their compositional maturity is evidenced by the very high amount of detrital quartz compared to the paucity of feldspar and unstable lithic grains. Other textural components include extraclasts derived from the local Ordovician basement, mudstone intraclasts, and sediment aggregates. Turbate structures, grain lineations, and soft sediment deformation of the matrix below larger grains are locally observed. Sedimentological analysis reveals that deposition occurred in an ice-intermediate to ice-distal, poorly agitated shelf environment by material supplied from meltwater buoyant plumes and rain-out from ice-rafted debris. Remobilization by mass-flow processes (cohesive debris flows and slumps) was an important mechanism particularly for the formation of massive diamictites. The glaciomarine deposits represent a typical deglaciation sequence reflecting retreat of the ice front (grounded or floating ice sheet), relative sea-level rise and gradually reduced sedimentation rate with increasing contribution from suspension fallout. This sequence was deposited on the non-glaciated shelf of the intracratonic North Gondwana platform along the southern margin of the Rheic Ocean. The Hirnantian strata of the Balkan Terrane can be correlated with similar glaciomarine deposits known from peri-Gondwana terranes elsewhere in Europe showing clear 'Armorican affinity'. Several lines of evidence suggest that the provenance of siliciclastic material was associated mainly with sedimentary recycling of

  15. The Orocopia-Pelona Schist Terrane, southern California and southwest Arizona: A Calibrated Target for Crustal Seismic Anisotropy Studies

    NASA Astrophysics Data System (ADS)

    Haxel, G.; Christensen, N.; Okaya, D.; Jacobsen, C.

    2003-12-01

    The Orocopia-Pelona Schist belt represents an enigmatic element within the Late Cretaceous to early Tertiary tectonic framework of the southern Cordillera. This oceanic supracrustal terrane is widely believed to underlie much of southern California and the southwest corner of Arizona. Alternative interpretations of geologic and geophysical data suggest that the schist may have a considerably more limited subsurface distribution, restricted to the vicinity of the curvilinear belt defined by the pre-San Andreas distribution of outcrops of the Orocopia and Pelona Schists. Regional depth and thickness, breadth, and large-scale internal configuration of the schist differ according to the current evolutionary models: (1) low-angle, eastward subduction, (2) trapping of marine rocks along a transpressional continental margin, or (3) tectonic burial of Great Valley-like forearc strata beneath overriding Mesozoic batholithic arc crust. The three-dimensional distribution of the schist could help to constrain these models, but is poorly known. The Orocopia-Pelona Schist is thus a promising candidate for mapping via crustal seismic anisotropy. The Orocopia-Pelona Schist terrane is exposed in some 15 tectonic windows, stretching from the central Transverse Ranges, California, to the Kofa region, southwest Arizona. Along the eastern two-thirds of its extent, exposures of the schist are tightly aligned along the Chocolate Mountains anticlinorium. The schist terrane comprises predominantly homogeneous quartzofeldspathic metagraywacke, with subordinate to minor interlayered metabasalt, metachert, siliceous marble, and ultramafic rock. The schist exhibits strong foliation and pronounced lineation, with regionally uniform orientation. Metamorphic mineral assemblages indicate tectonic burial to depths approx. 25 - 35 km. We have performed petrophysical laboratory measurements on samples of Orocopia Schist from the Chocolate and Trigo Mountains, determining seismic velocities and

  16. A Paleomagnetic Traverse of the Frontenac, Sharbot Lake, Mazinaw and Elsevir Terranes, Grenville Province, Ontario

    NASA Astrophysics Data System (ADS)

    Brett, J. S.; Dunlop, D. J.

    2004-05-01

    A paleomagnetic traverse has been carried out across the Ontario section of the Central Metasedimentary Belt (CMB) of the Precambrian Grenville Province, between the St. Lawrence and Ottawa Rivers in the east and extending west as far as the Bancroft Terrane, where it links with an earlier traverse of the Central Gneiss Belt (CGB) and the CMB Boundary Zone (CMBBZ). 54 sites were sampled, 11 in the Frontenac (F) Terrane, 12 in the Sharbot Lake (SL) Terrane, 12 in the Mazinaw (M) Terrane, 3 in the Bancroft (B) Terrane, and 16 in the Elsevir Terrane, mainly in the Belmont Domain. Ar/Ar thermochronometric work across the CMB by Cosca et al. (Tectonics 10, 959-77, 1991; Contrib. Min. Petr. 110, 211-25, 1992) documented differential unroofing of different terranes and slow cooling (1.0-1.5 oC /Ma) along parallel T-t paths offset by 50-100 Ma for the Frontenac, the western and eastern Elsevir (Ew, Ee) and the CMBBZ. Around 1000 Ma, inferred burial depths were 20 km for the CMBBZ, 15 km for Ew, >20 km for Ee (or M), 10 km for F, and transitional (12-16 km) in SL between F and Ee/M. The very different hornblende, phlogopite, muscovite and biotite Ar/Ar ages between neighboring terranes should have a counterpart in the paleomagnetic results of the terranes because the interval 1100-850 Ma was a time of rapid paleolatitude shift. However our results and previous data from B, CMBBZ and the adjacent CGB (Costanzo-Alvarez & Dunlop, Earth Planet. Sci. Lett. 157, 89-103, 1998) do not bear this out. F, SL and M have very similar R-polarity poles in the central part of the Grenville Track, while F and SL N-polarity poles are typical 980 Ma Grenville A poles. Cooling through 500oC occurred around 1100 Ma for F but the F poles do not resemble Keweenawan poles of this age. Cooling through 500oC occurred around 950 Ma for Ee/M but the M pole does not match this age. Our Ew mean pole, like that of the Tudor Gabbro, falls off the Grenville Track on a possible pre-Grenvillian track.

  17. Holocene reef accretion: southwest Molokai, Hawaii, U.S.A.

    USGS Publications Warehouse

    Engels, Mary S.; Fletcher, Charles H.; Field, Michael E.; Storlazzi, Curt D.; Grossman, Eric E.; Rooney, John J.B.; Conger, Christopher L.; Glenn, Craig

    2004-01-01

    extension across Hawaii in general, is controlled by wave-induced near-bed shear stress related to refracted North Pacific swell. Holocene accretion patterns here also reflect the long-term influence of wave-induced near-bed shear stress from north swell during late Holocene time. This finding is consistent with other studies (e.g., Grigg 1998; Cabioch et al. 1999) that reflect the dominance of swell energy and sea level in controlling modern and late Holocene accretion elsewhere in Hawaii and across the Pacific and Indian oceans. Notably, however, this result is refined and clarified for Hawaii in the hypothesis of Rooney et al. (2003) stating that enhancement of the El Niño Southern Oscillation beginning approximately 5000 years ago led to increased north swell energy and signaled the end to net accretion along exposed coastlines in Hawaii. The exposure of Hale O Lono to north swell and the age of sea floor there (ca. 4,800 cal yr BP), coupled with the lack of north swell incidence at Hikauhi and the continuous accretion that has occurred there over the last millennium, strongly supports the ENSO reef hypothesis as outlined by Rooney et al. (2003). Other factors controlling Holocene reef accretion at the study site are relative sea-level position and rate of rise, and wave sheltering by Laau Point. Habitat suitable for reef accretion on the southwest shore of Molokai has shrunk throughout the Holocene.

  18. Accretion physics: It's not U, it's B

    NASA Astrophysics Data System (ADS)

    Miller, Jon

    2017-03-01

    Black holes grow by accreting mass, but the process is messy and redistributes gas and energy into their environments. New evidence shows that magnetic processes mediate both the accretion and ejection of matter.

  19. 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.

  20. 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.

  1. Neutrinos from Accreting Neutron Stars

    NASA Astrophysics Data System (ADS)

    Anchordoqui, Luis A.; Torres, Diego F.; McCauley, Thomas P.; Romero, Gustavo E.; Aharonian, Felix A.

    2003-05-01

    The magnetospheres of accreting neutron stars develop electrostatic gaps with huge potential drops. Protons and ions, accelerated in these gaps along the dipolar magnetic field lines to energies greater than 100 TeV, can impact onto the surrounding accretion disk. A proton-induced cascade develops, and charged pion decays produce ν emission. With extensive disk shower simulations using DPMJET and GEANT4, we have calculated the resulting ν spectrum. We show that the spectrum produced out of the proton beam is a power law. We use this result to propose accretion-powered X-ray binaries (with highly magnetized neutron stars) as a new population of pointlike ν sources for kilometer-scale detectors such as ICECUBE. As a particular example, we discuss the case of A0535+26. We show that ICECUBE should find A0535+26 to be a periodic ν source, one for which the formation and loss of its accretion disk can be fully detected. Finally, we comment briefly on the possibility that smaller telescopes such as AMANDA could also detect A0535+26 by folding observations with the orbital period.

  2. Strongly Magnetized Accretion Disks Around Black Holes

    NASA Astrophysics Data System (ADS)

    Salvesen, Greg; Armitage, Philip J.; Simon, Jacob B.; Begelman, Mitchell C.

    2017-01-01

    Recent observations are suggestive of strongly magnetized accretion disks around black holes. Performing local (shearing box) simulations of accretion disks, we investigate how a strong magnetization state can develop and persist. We demonstrate that poloidal flux is a necessary prerequisite for the sustainability of strongly magnetized accretion disks. We also show that black hole spin measurements can become unconstrained if magnetic fields provide a significant contribution to the vertical pressure support of the accretion disk atmosphere.

  3. Plastic Deformation of Accreted Planetesimals

    NASA Astrophysics Data System (ADS)

    Kadish, J.

    2005-08-01

    The early stages of planetesimal growth follow an accretion model (Weidenschilling, Icarus 2000), which influences the intrinsic strength of a body and may control how its shape evolves after growth. In previous work we have determined the stress field of an accreted planetesimal accounting for possible variation in the object's spin as it accretes (Kadish et al., IJSS In Press) At the end of growth, these objects are subject to transport mechanisms that can distribute them throughout the solar system. As they are transported these objects can be spun-up by tidal forces (Scheeres et al, Icarus 2000), YORP (Bottke et al., Asteroids III 2002), and collisions (Binzel et al., Asteroids II 1989). Such an increase of spin will cause perturbations to the initial stress field and may lead to failure. We are able to show analytically that failure is initiated on the object's surface and a plastic zone propagates inward as the object's spin is increased. If we model an accreted body as a conglomeration of rocks similar to a gravel or sand, the deformation in the region of failure is characterized using a Mohr-Coulomb failure criterion with negligible cohesion and zero hardening(e.g. Holsapple, Icarus 2001). Such a response is highly non-linear and must be solved using finite elements and iterative methods (Simo and Hughes, Computational Inelasticity 1998). Using the commercial finite element code ABAQUS, we present the shape deformation resulting from an elasto-plastic analysis of a spinning, self-gravitating accreted sphere that is spun-up after growth is complete. The methodology can be extended to model plastic deformation due to local failure for more complex planetesimal shapes, such as for the asteroid Kleopatra. This work has implications for the evolution of planetesimal shapes, the creation of binary and contact binary asteroids, and for the maximum spin rate of small planetary bodies.

  4. 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

  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.

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

    USGS Publications Warehouse

    Bohlen, S.R.; Mezger, K.

    1989-01-01

    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 crystalliztion 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.

  7. Diachronous cleavage development in the Robertson Bay Terrane, Northern Victoria Land, Antarctica: Tectonic implications

    NASA Astrophysics Data System (ADS)

    Dallmeyer, R. D.; Wright, Thomas O.

    1992-04-01

    The Robertson Bay terrane is the easternmost of three regional lithotectonic units comprising Northern Victoria Land. It is largely represented by a thick succession of Cambrian-Ordovician distal turbidites (Robertson Bay Group), which has been folded into upright structures with development of an associated, regionally penetrative cleavage during maintenance of regionally uniform, low-grade regional metamorphic conditions. Whole rock 40Ar/39Ar incremental-release analysis of slate/phyllite documents diachronous cleavage development between ˜500 Ma (west) and 460 Ma (east) during the Ross orogeny. This systematic regional variation in age is consistent with tectonic models for the Ross orogeny that involve southwest directed subduction of the outboard Robertson Bay terrane beneath the Bowers terrane. Convergence rates of ˜0.4-1.0 cm/yr are suggested.

  8. Pre-Mid-Mesozoic tectonic evolution of the Yukon-Tanana Terrane, Yukon and Alaska

    NASA Astrophysics Data System (ADS)

    Mortensen, J. K.

    1992-08-01

    Yukon-Tanana Terrane (YTT) underlies much of central and western Yukon and east central Alaska. Its history and tectonic evolution, particularly prior to mid-Mesozoic time, has been largely obscured by younger magmatism and tectonism. The application of geochronological and isotopic techniques over the past decade, together with detailed field studies in certain critical areas of the terrane, has shed new light on the early history of YTT. Much of YTT is a product of episodic continental arc magmatism, with three main pulses in Late Devonian-Early Mississippian, mid-Permian, and Late Triassic-Early Jurassic time. From Late Devonian to mid-Mississippian time, subduction was north or northeast dipping, but arc polarity was apparently reversed by mid-Permian time. The main, subhorizontal structural fabric characterizing much of YTT was produced between mid-Permian time and the onset of renewed magmatism in Late Triassic time and probably reflects a major continent-continent collision. Although the Triassic-Jurassic magmatism is also considered to be arc related, it occurred over a very broad area of not only YTT, but also Quesnellia, and the Stikine, Nisling, Cache Creek, and Slide Mountain terranes. This magmatism appears to have coincided with final amalgamation of the Intermontane Superterrane, and the arc polarity and the position and orientation of the associated subduction zone is still controversial. Available evidence suggests that Nisling Terrane is closely related to YTT and mainly consists of older strata that underlie the Devonian and younger units generally considered to be more typical of YTT. There are close similarities between YTT and a number of other "pericratonic" terranes in the central and eastern parts of the Cordillera, and it is likely that these terranes originally formed a single arc and arc basement assemblage which has now been fragmented and dispersed by transcurrent faulting.

  9. Tectonics and geochronology of the northern margin of the Zhongba terrane, Southern Tibet: implications for the closing processes of the western Neo-Tethys

    NASA Astrophysics Data System (ADS)

    Liu, Q.; Li, S.; DONG, Y.; Han, G.

    2015-12-01

    belts yielded 40Ar/39Ar age of 71.6 ±1.1 Ma and 56.5±2.5 Ma, respectively. The former represents the collisional time between the Zhongba terrane and the Gangdese arc, while the latter indicates the final closing time of the Neo-Tethys ocean.

  10. 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

  11. 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

  12. Planetary accretion in circumstellar disks

    NASA Technical Reports Server (NTRS)

    Lissauer, Jack J.; Stewart, Glen R.

    1993-01-01

    The formation of terrestrial planets and the cores of Jovian planets is reviewed in the framework of the planetesimal hypothesis, wherein planets are assumed to grow via the pairwise accumulation of small solid bodies. Emphasis is placed on the dynamics of solid body accretion from kilometer size planetesimals to terrestrial type planets. This stage of planetary growth is least dependent on the characteristics of the evolutionary state of the central star. It is concluded that the evolution of the planetesimal size distribution is determined by the gravitationally enhanced collision cross-section, which favors collisions between planetesimals with smaller velocities. Runaway growth of the largest planetesimal in each accretion zone appears to be a likely outcome. The subsequent accumulation of the resulting protoplanets leads to a large degree of radial mixing in the terrestrial planet region, and giant impacts are probable.

  13. 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.

  14. Hydrodynamic Viscosity in Accretion Disks

    NASA Astrophysics Data System (ADS)

    Duschl, Wolfgang J.; Strittmatter, Peter A.; Biermann, Peter L.

    We propose a generalized accretion disk viscosity prescription based on hydrodynamically driven turbulence at the critical effective Reynolds number. This approach is consistent with recent re-analysis by Richard & Zahn (1999) of experimental results on turbulent Couette-Taylor flows. This new β-viscosity formulation applies to both selfgravitating and non-selfgravitating disks and is shown to yield the standard α-disk prescription in the case of shock dissipation limited, non-selfgravitating disks.

  15. 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.

  16. 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.

  17. 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.

  18. 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.

  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. 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.

  1. 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

  2. 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

  3. A Hands-On Approach to Teaching the Terrane Concept in Historical Geology.

    ERIC Educational Resources Information Center

    Bykerk-Kauffman, Ann

    1989-01-01

    Describes an exercise in which students convert lithostratigraphic columns into chronostratigraphic columns, infer paleolatitude using paleomagnetic data, interpret depositional environments, determine the timing of deformation and terrane collision, construct models, and synthesize the results into a geologic history. Background data, procedures,…

  4. Crustal anatexis and melt migrations in the Higo metamorphic terrane, west-central Kyushu, Kumamoto, Japan

    NASA Astrophysics Data System (ADS)

    Obata, M.; Yoshimura, Y.; Nagakawa, K.; Odawara, S.; Osanai, Y.

    1994-03-01

    This paper deals with the anatectic migmatites and leucogranites developed in a high- T, low- P metamorphic terrane, Kyushu, Japan. The migmatites in this terrane are divided into metatexites and diatexites and they occur at and above the garnet-cordierite grade. Layers, lenses and pods of leucogranite (a few centimeters up to 6 m in thickness) are wide-spread in the metamorphic terrane and are most abundant, not in the highest grade zone, but in intermediate grade zones, in which migmatites do not develop. The migmatite leucosomes are typically depleted in orthoclase component, while many leucogranites contain abundant orthoclase. It is suggested that the leucogranites are crystal cumulates precipitated from peraluminous melts, that had been generated and ascended from deeper levels in the crust and that the zone of metatectic migmatites represents both the source region and passage region for the anatectic melts. It is thus clear that the anatectic melts had segregated and ascended on the order of 5 km in the crust of this region. Diatexites on the other hand represent solid-melt mixtures at more advanced stages of partial melting, dominated by biotite breakdown. Implications of the observations made in the Higo terrane are discussed in reference to an evolutionary scheme for crustal anatexis and granite magma genesis.

  5. Ternary feldspar thermometry of Paleoproterozoic granulites from In-Ouzzal terrane (Western Hoggar, southern Algeria)

    NASA Astrophysics Data System (ADS)

    Benbatta, A.; Bendaoud, A.; Cenki-Tok, B.; Adjerid, Z.; Lacène, K.; Ouzegane, K.

    2017-03-01

    The In Ouzzal terrane in western Hoggar (Southern Algeria) preserves evidence of ultrahigh temperature (UHT) crustal metamorphism. It consists in Archean crustal units, composed of orthogneissic domes and greenstone belts, strongly remobilized during the Paleoproterozoic orogeny which was recognized as an UHT event (peak T > 1000 °C and P ≈ 9-12 kbar). This metamorphism was essentially defined locally in Al-Mg granulites, Al-Fe granulites and quartzites outcropping in the Northern part of the In Ouzzal terrane (IOT). In order to test and verify the regional spread of the UHT metamorphism in this terrane, ternary feldspar thermometry on varied rock types (Metanorite, Granulite Al-Mg and Orthogneiss) and samples that crop out in different zones of the In Ouzzal terrane. These rocks contain either perthitic, antiperthitic or mesoperthitic parageneses. Ternary feldspars used in this study have clearly a metamorphic origin. The obtained results combined with previous works show that this UHT metamorphism (>900 °C) affected the whole In Ouzzal crustal block. This is of major importance as for future discussion on the geodynamic context responsible for this regional UHT metamorphism.

  6. Accretion Flow in the inner Accretion Discs of Cataclysmic Variables

    NASA Astrophysics Data System (ADS)

    Balman, Solen; Revnivtsev, Mikhail

    2012-07-01

    We study nature of time variability of brightness of non-magnetic cataclysmic variables. We show that lightcurtves of all analyzed DN systems in UV and X-ray energy bands demonstrate band limited noise, which can be adequately described in the framework of the model of propagating fluctuations. The frequency of the break indicates the inner disc truncation with a range of radii (10-3)e+9 cm. We analyse the RXTE and optical (RTT150) data of SS Cyg in outburst and quiescence which show that during the outburst the inner disk radius moves towards the white dwarf and receeds as the outburst declines to quiescence. Cross-correlations between the simultaneous UV and X-ray light curves find time lags in the X-rays of 90-180 sec consistent with travel time of matter from a truncated inner disc to the white dwarf surface. This suggests that DN and other plausible nonmagnetic systems have truncated accretion discs indicating that the accretion may occur through coronal flows in the disc. We compare and contrast magnetic and nonmagnetic systems in terms of their aperiodic noise characteristics and the model of propagating fluctuations. The comparison of the X-ray/UV time lag observed by us in the case of non-magnetic CVs with those, detected for magnetic CVs allows us to make an rough estimate of the viscosity parameter. Multi band simultaneous observations of coming observattories like ASTROSAT will give us the opportunity to study time variability of brightness variations of accretion disks in cataclysmic variables in quiescence and outburst using LAXPC and UVIT/OPT instruments. We will elaborate on the nature and possible outcomes of such research.

  7. 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

  8. 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

  9. Lead isotope studies of the Guerrero composite terrane, west-central Mexico: implications for ore genesis

    NASA Astrophysics Data System (ADS)

    Potra, Adriana; Macfarlane, Andrew W.

    2014-01-01

    New thermal ionization mass spectrometry and multi-collector inductively coupled plasma mass spectrometry Pb isotope analyses of three Cenozoic ores from the La Verde porphyry copper deposit located in the Zihuatanejo-Huetamo subterrane of the Guerrero composite terrane are presented and the metal sources are evaluated. Lead isotope ratios of 3 Cenozoic ores from the El Malacate and La Esmeralda porphyry copper deposits located in the Zihuatanejo-Huetamo subterrane and of 14 ores from the Zimapan and La Negra skarn deposits from the adjoining Sierra Madre terrane are also presented to look for systematic differences in the lead isotope trends and ore metal sources among the proposed exotic tectonostratigraphic terranes of southern Mexico. Comparison among the isotopic signatures of ores from the Sierra Madre terrane and distinct subterranes of the Guerrero terrane supports the idea that there is no direct correlation between the distinct suspect terranes of Mexico and the isotopic signatures of the associated Cenozoic ores. Rather, these Pb isotope patterns are interpreted to reflect increasing crustal contribution to mantle-derived magmas as the arc advanced eastward onto a progressively thicker continental crust. The lead isotope trend observed in Cenozoic ores is not recognized in the ores from Mesozoic volcanogenic massive sulfide and sedimentary exhalative deposits. The Mesozoic ores formed prior to the amalgamation of the Guerrero composite terrane to the continental margin, which took place during the Late Cretaceous, in intraoceanic island arc and intracontinental marginal basin settings, while the Tertiary deposits formed after this event in a continental arc setting. Lead isotope ratios of the Mesozoic and Cenozoic ores appear to reflect these differences in tectonic setting of ore formation. Most Pb isotope values of ores from the La Verde deposit (206Pb/204Pb = 18.674-18.719) are less radiogenic than those of the host igneous rocks, but plot within the

  10. Angular momentum transport in thin accretion disks and intermittent accretion.

    PubMed

    Coppi, B; Coppi, P S

    2001-07-30

    The plasma modes, transporting angular momentum in accretion disks, under minimally restrictive conditions when the magnetic energy density is significant relative to the thermal energy density, are shown to be singular if the ideal MHD approximation is adopted. A similarity with the modes producing magnetic reconnection in current carrying plasmas is established. The combined effects of finite plasma temperature, of plasma compressibility, of the gradient of the rotation frequency, and of appropriate transport processes (outside ideal MHD) are involved in the onset of these nonaxisymmetric and locally corotating modes.

  11. 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

  12. 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.

  13. Chondrule Accretion with a Growing Protoplanet

    NASA Astrophysics Data System (ADS)

    Matsumoto, Yuji; Oshino, Shoichi; Hasegawa, Yasuhiro; Wakita, Shigeru

    2017-03-01

    Chondrules are primitive materials in the solar system. They were formed in about the first 3 Myr of the solar system’s history. This timescale is longer than that of Mars formation, and it is conceivable that protoplanets, planetesimals, and chondrules might have existed simultaneously in the solar nebula. Due to protoplanets’ perturbation on the planetesimal dynamics and chondrule accretion on them, all the formed chondrules are unlikely to be accreted by the planetesimals. We investigate the amount of chondrules accreted by planetesimals in such a condition. We assume that a protoplanet is in oligarchic growth, and we perform analytical calculations of chondrule accretion by both a protoplanet and planetesimals. Through the oligarchic growth stage, planetesimals accrete about half of the formed chondrules. The smallest planetesimals get the largest amount of chondrules, compared with the amount accreted by more massive planetesimals. We perform a parameter study and find that this fraction is not greatly changed for a wide range of parameter sets.

  14. Paleomagnetic results from the Upper Permian of the eastern Qiangtang Terrane of Tibet and their tectonic implications

    NASA Astrophysics Data System (ADS)

    Huang, Kainian; Opdyke, Neil D.; Peng, Xingjie; Li, Jiguang, Li

    1992-06-01

    Paleomagnetic samples were collected from red intercalations within the Upper Permian Tuoba Formation from the eastern Qiangtang Terrane of Tibet. Progressive thermal demagnetization has revealed a prefolding, possibly primary, characteristic remanent magnetization (ChRM), which yields a tilt-corrected mean direction ofD/I=25.2°6.7° (α 95 = 8.6°). The equatorial paleolatitude for the Qiangtang Terrane indicated by this result and the similarity of Late Permian fauna and flora shared by the Qiangtang Terrane and South China Block (SCB) suggest that the two terranes were in close proximity during the Late Permian. A Late Permian paleogeographic reconstruction involving the major terranes of the present-day East Asia is constructed, in which the Songpan-Garze Sea is surrounded by Laurasia, the North China Block (NCB), the SCB and the Qiangtang Terrane. The Permian development of the Songpan-Garze Sea appears to be related to the rifting of the Qiangtang Terrane off the SCB. Its Triassic evolution may resemble that of the Solomon Sea today in the southwestern Pacific.

  15. 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.

  16. Crustal structure of norther Oaxaca terrane; The Oaxaca and caltepec faults, and the Tehuacan Valley. A gravity study.

    NASA Astrophysics Data System (ADS)

    Campos-Enriquez, J. O.; Alatorre-Zamora, M. A.; Ramón, V. M.; Belmonte, S.

    2014-12-01

    Northern Oaxaca terrane, southern Mexico, is bound by the Caltepec and Oaxaca faults to the west and east, respectively. These faults juxtapose the Oaxaca terrane against the Mixteca and Juarez terranes, respectively. The Oaxaca Fault also forms the eastern boundary of the Cenozoic Tehuacan depression. Several gravity profiles across these faults and the Oaxaca terrane (including the Tehuacan Valley) enables us to establish the upper crustal structure of this region. Accordingly, the Oaxaca terrane is downward displaced to the east in two steps. First the Santa Lucia Fault puts into contact the granulitic basamental rocks with Phanerozoic volcanic and sedimentary rocks. Finally, the Gavilan Fault puts into contact the Oaxaca terrane basement (Oaxaca Complex) into contact with the volcano-sedimentary infill of the valley. This gravity study reveals that the Oaxaca Fault system gives rise to a series of east tilted basamental blocks (Oaxaca Complex?). A structural high at the western Tehuacan depression accomadates the east dipping faults (Santa Lucia and Gavilan faults) and the west dipping faults of the Oaxaca Fault System. To the west of this high structural we have the depper depocenters. The Oaxaca Complex, the Caltepec and Santa Lucia faults continue northwestwards beneath Phanerozoic rocks. The faults are regional tectonic structures. They seem to continue northwards below the Trans-Mexican Volcanic Belt. A major E-W to NE-SW discontinuity on the Oaxaca terrane is inferred to exist between profiles 1 and 2. The Tehuacan Valley posses a large groundwater potential.

  17. The Cannery Formation--Devonian to Early Permian arc-marginal deposits within the Alexander Terrane, Southeastern Alaska

    USGS Publications Warehouse

    Karl, Susan M.; Layer, Paul W.; Harris, Anita G.; Haeussler, Peter J.; Murchey, Benita L.

    2011-01-01

    cherts on both Admiralty and Kupreanof Islands contain radiolarians as young as Permian, the age of the Cannery Formation is herein extended to Late Devonian through early Permian, to include the early Permian rocks exposed in its type locality. The Cannery Formation is folded and faulted, and its stratigraphic thickness is unknown but inferred to be several hundred meters. The Cannery Formation represents an extended period of marine deposition in moderately deep water, with slow rates of deposition and limited clastic input during Devonian through Pennsylvanian time and increasing argillaceous, volcaniclastic, and bioclastic input during the Permian. The Cannery Formation comprises upper Paleozoic rocks in the Alexander terrane of southeastern Alaska. In the pre-Permian upper Paleozoic, the tectonic setting of the Alexander terrane consisted of two or more evolved oceanic arcs. The lower Permian section is represented by a distinctive suite of rocks in the Alexander terrane, which includes sedimentary and volcanic rocks containing early Permian fossils, metamorphosed rocks with early Permian cooling ages, and intrusive rocks with early Permian cooling ages, that form discrete northwest-trending belts. After restoration of 180 km of dextral displacement of the Chilkat-Chichagof block on the Chatham Strait Fault, these belts consist, from northeast to southwest, of (1) bedded chert, siliceous argillite, volcaniclastic turbidites, pillow basalt, and limestone of the Cannery Formation and the Porcupine Slate of Gilbert and others (1987); (2) greenschist-facies Paleozoic metasedimentary and metavolcanic rocks that have Permian cooling ages; (3) silty limestone and calcareous argillite interbedded with pillow basalt and volcaniclastic rocks of the Halleck Formation and the William Henry Bay area; and (4) intermediate-composition and syenitic plutons. These belts correspond to components of an accretionary complex, contemporary metamorphic rocks, forearc-basin deposits,

  18. Theory of protostellar accretion disks

    NASA Technical Reports Server (NTRS)

    Ruden, S.

    1994-01-01

    I will present an overview of the current paradigm for the theory of gaseous accretion disks around young stars. Protostellar disks form from the collapse of rotating molecular cloud cores. The disks evolve via outward angular momentum transport provided by several mechanisms: gravitational instabilities, thermal convective turbulence, and magnetic stresses. I will review the conditions under which these mechanisms are efficient and consistent with the observed disk evolutionary timescales of several million years. Time permitting, I will discuss outbursts in protostellar disks (FU Orionis variables), the effect of planet formation on disk structure, and the dispersal of remnant gas.

  19. Accretion Onto Magnetic Degenerate Stars

    NASA Technical Reports Server (NTRS)

    Frank, Juhan

    2000-01-01

    While the original objectives of this research program included the study of radiative processes in cataclysmic variables and the evolution of neutron star magnetic fields, the scope of the reported research expanded to other related topics as this project developed. This final report therefore describes the results of our research in the following areas: 1) Irradiation-driven mass transfer cycles in cataclysmic variables and low-mass X-ray binaries; 2) Propeller effect and magnetic field decay in isolated old neutron stars; 3) Decay of surface magnetic fields in accreting neutron stars and pulsars; 4) Finite-Difference Hydrodynamic simulations of mass transfer in binary stars.

  20. Disk tides and accretion runaway

    NASA Technical Reports Server (NTRS)

    Ward, William R.; Hahn, Joseph M.

    1995-01-01

    It is suggested that tidal interaction of an accreting planetary embryo with the gaseous preplanetary disk may provide a mechanism to breach the so-called runaway limit during the formation of the giant planet cores. The disk tidal torque converts a would-be shepherding object into a 'predator,' which can continue to cannibalize the planetesimal disk. This is more likely to occur in the giant planet region than in the terrestrial zone, providing a natural cause for Jupiter to predate the inner planets and form within the O(10(exp 7) yr) lifetime of the nebula.

  1. 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.

  2. 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.

  3. 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.

  4. 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). 

  5. Plate Interface Rheology, Mechanical Coupling and Accretion during Subduction Infancy

    NASA Astrophysics Data System (ADS)

    Agard, P.; Yamato, P.; Mathieu, S.; Prigent, C.; Guillot, S.; Plunder, A.; Dubacq, B.; Monie, P.; Chauvet, A.

    2015-12-01

    Understanding subduction rheology in both space and time has been a challenge since the advent of plate tectonics. We herein focus on "subduction infancy", that is the first ~1-5 My immediately following subduction nucleation, when a newly born slab penetrates into the upper plate mantle and heats up. The only remnants of this critical yet elusive geodynamic step are thin metamorphic soles, commonly found beneath pristine, 100-1000 km long portions of oceanic lithosphere emplaced on continents (i.e., ophiolites). Through the (i) worldwide compilation of pressure-temperature conditions of metamorphic sole formation augmented by pseudosection thermodynamic modeling, (ii) calculations of the viscosity of materials along the plate interface and (iii) generic numerical thermal models, we provide a conceptual model of dynamic plate interface processes during subduction infancy (and initiation s.l.). We show in particular how major rheological switches across the subduction interface control slab penetration, and the formation of metamorphic soles. Due to the downward progression of hydration and weakening of the mantle wedge with cooling, the lower plate (basalt, sediment) and the upper plate (mantle wedge) rheologies equalize and switch over a restricted temperature-time-depth interval (e.g., at ~800°C and ~1 GPa, during 0.1-2 My, for high-temperature metamorphic sole formation). These switches result in episodes of maximum interplate mechanical coupling, thereby slicing the top of the slab and welding pieces (basalt, sediment) to the base of the mantle wedge. Similar mechanical processes likely apply for the later, deeper accretion and exhumation of high-temperature oceanic eclogites in serpentinite mélanges, or for the accretion of larger tectonic slices. This model provides constraints on the effective rheologies of the crust and mantle and general understanding, at both rock and plate scale, for accretion processes and early slab dynamics.

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

    USGS Publications Warehouse

    Churkin, M.; Trexler, J.H.

    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.

  7. RINGED ACCRETION DISKS: EQUILIBRIUM CONFIGURATIONS

    SciTech Connect

    Pugliese, D.; Stuchlík, Z. E-mail: zdenek.stuchlik@physics.cz

    2015-12-15

    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.

  8. Nonlinear variations in axisymmetric accretion

    NASA Astrophysics Data System (ADS)

    Bose, Soumyajit; Sengupta, Anindya; Ray, Arnab K.

    2014-05-01

    We subject the stationary solutions of inviscid and axially symmetric rotational accretion to a time-dependent radial perturbation, which includes nonlinearity to any arbitrary order. Regardless of the order of nonlinearity, the equation of the perturbation bears a form that is similar to the metric equation of an analogue acoustic black hole. We bring out the time dependence of the perturbation in the form of a Liénard system by requiring the perturbation to be a standing wave under the second order of nonlinearity. We perform a dynamical systems analysis of the Liénard system to reveal a saddle point in real time, whose implication is that instabilities will develop in the accreting system when the perturbation is extended into the nonlinear regime. We also model the perturbation as a high-frequency traveling wave and carry out a Wentzel-Kramers-Brillouin analysis, treating nonlinearity iteratively as a very feeble effect. Under this approach, both the amplitude and the energy flux of the perturbation exhibit growth, with the acoustic horizon segregating the regions of stability and instability.

  9. PROPERTIES OF GRAVITOTURBULENT ACCRETION DISKS

    SciTech Connect

    Rafikov, Roman R.

    2009-10-10

    We explore the properties of cold gravitoturbulent accretion disks-non-fragmenting disks hovering on the verge of gravitational instability (GI)-using a realistic prescription for the effective viscosity caused by gravitational torques. This prescription is based on a direct relationship between the angular momentum transport in a thin accretion disk and the disk cooling in a steady state. Assuming that opacity is dominated by dust we are able to self-consistently derive disk properties for a given M-dot assuming marginal gravitational stability. We also allow external irradiation of the disk and account for a non-zero background viscosity, which can be due to the magneto-rotational instability. Spatial transitions between different co-existing disk states (e.g., between irradiated and self-luminous or between gravitoturbulent and viscous) are described and the location of the boundary at which the disk must fragment is determined in a variety of situations. We demonstrate in particular that at low enough M-dot external irradiation stabilizes the gravitoturbulent disk against fragmentation to very large distances thus providing means of steady mass transport to the central object. Implications of our results for the possibility of planet formation by GI in protoplanetary disks and star formation in the Galactic center and for the problem of feeding supermassive black holes in galactic nuclei are discussed.

  10. 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.

  11. 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, Timothy M.; Bradley, Dwight C.; Haeussler, Peter J.; Karl, Susan M.

    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

  12. 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

  13. Foundations of Black Hole Accretion Disk Theory.

    PubMed

    Abramowicz, Marek A; Fragile, P Chris

    2013-01-01

    This review covers the main aspects of black hole accretion disk theory. We begin with the view that one of the main goals of the theory is to better understand the nature of black holes themselves. In this light we discuss how accretion disks might reveal some of the unique signatures of strong gravity: the event horizon, the innermost stable circular orbit, and the ergosphere. We then review, from a first-principles perspective, the physical processes at play in accretion disks. This leads us to the four primary accretion disk models that we review: Polish doughnuts (thick disks), Shakura-Sunyaev (thin) disks, slim disks, and advection-dominated accretion flows (ADAFs). After presenting the models we discuss issues of stability, oscillations, and jets. Following our review of the analytic work, we take a parallel approach in reviewing numerical studies of black hole accretion disks. We finish with a few select applications that highlight particular astrophysical applications: measurements of black hole mass and spin, black hole vs. neutron star accretion disks, black hole accretion disk spectral states, and quasi-periodic oscillations (QPOs).

  14. Accretion Disks in Algols: Progenitors and Evolution

    NASA Astrophysics Data System (ADS)

    van Rensbergen, W.; de Greve, J. P.

    2017-02-01

    There are only a few Algols with derived accretion disk parameters. These measurements provide additional constraints for tracing the origin of individual systems. With a modified binary evolution code, series of close binary evolution were calculated. For six Algols with accretion disks we found initial systems that evolve closely into the presently observed system parameters and disk characteristics.

  15. Pulsed accretion in a variable protostar

    NASA Astrophysics Data System (ADS)

    Muzerolle, James; Furlan, Elise; Flaherty, Kevin; Balog, Zoltan; Gutermuth, Robert

    2013-01-01

    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 105 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.

  16. 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.

  17. 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.

  18. 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

  19. Accretion flows onto supermassive black holes

    NASA Technical Reports Server (NTRS)

    Begelman, Mitchell C.

    1988-01-01

    The radiative and hydrodynamic properties of an angular momentum-dominated accretion flow onto a supermassive black hole depend largely on the ratio of the accretion rate to the Eddington accretion rate. High values of this ratio favor optically thick flows which produce largely thermal radiation, while optically thin 'two-temperature' flows may be present in systems with small values of this ratio. Observations of some AGN suggest that thermal and nonthermal sources of radiation may be of comparable importance in the 'central engine'. Consideration is given to the possibilities for coexistence of different modes of accretion in a single flow. One intriguing possibility is that runaway pair production may cause an optically thick 'accretion annulus' to form at the center of a two-temperature inflow.

  20. Early Proterozoic ties between two suspect terranes and the Mojave crustal block of the Southwestern U.S

    USGS Publications Warehouse

    Bender, E. Erik; Morrison, Jean; Anderson, J. Lawford; Wooden, Joseph L.

    1993-01-01

    Southern California and adjacent areas contain two suspect or exotic terranes comprised largely of ancient continental crust, namely the Tujunga (San Gabriel) and Joshua Tree terranes, that have been considered part of a larger displaced terrane, the Santa Lucia-Orocopia allochthon. Paleomagnetic data for the allochthon indicate northward transport in excess of 2000 km and, thus, an origin extraneous to North America. However, Early Proterozoic plutons of the Mojave crustal block and the Joshua Tree and Tujunga terranes have strikingly comparable features, including: (1) crystallization ages of 1.63 to 1.68 Ga; (2) biotite + sphene + magnetite hornblende garnet mineralogy; (3) high LIL and enriched HFS elemental composition; (4) WPG (within-plate granite) trace element chemistry; (5) similar and unique oxygen isotopic compositions; and (6) distinct Pb and Nd isotopic signatures. These features of the Mojave block, which clearly originated as part of native North America, nevertheless distinguish it from crust elsewhere in North America. On the basis of data presented here, we conclude that the Tujunga terrane is a disrupted portion of the Mojave crustal block and is neither far-traveled nor exotic to North America. Its apparent "exotic" nature stems from derivation out of the middle crust. We also conclude that the Joshua Tree terrane is correlative to the Mojave block. We have found no significant evidence for its displacement and consider Joshua Tree to be contiguous with the Mojave block and thus not a valid terrane. The Tujunga (San Gabriel) and Joshua Tree terranes should not be considered as part of, or having shared the same transport as, the Santa Lucia-Orocopia allocthon.

  1. Classification of lunar terranes using neutron and thorium gamma-ray data

    SciTech Connect

    Feldman, W.C.; Lawrence, D.J.; Elphic, R.C.; Barraclough, B.L.; Maurice, S.; Binder, A.B.; Lucey, P.G.

    1999-04-01

    A major scientific goal of the Lunar Prospector (LP) gamma-ray and neutron spectrometers is to classify all lunar terranes according to composition. A preliminary analysis of early data indicates this goal will be met for the major rock-forming elements on a spatial scale of about 200 km. The low-altitude phase of LP now in progress should allow reduction of this scale by about a factor of 10 for those elements that have sufficiently high measurable fluxes relative to their backgrounds. Most promising are the flux intensities of thermal, epithermal, and fast neutrons (which each average about 300 counts per 50 km of ground track) and 2.6 MeV gamma rays from thorium (which averages about 50 counts per 50 km of ground track). The authors therefore explore the information content of these measurables to classify the various lunar terrane types.

  2. Chondrule formation during planetesimal accretion

    NASA Astrophysics Data System (ADS)

    Asphaug, Erik; Jutzi, Martin; Movshovitz, Naor

    2011-08-01

    We explore the idea that most chondrules formed as a consequence of inefficient pairwise accretion, when molten or partly molten planetesimals ~ 30-100 km diameter, similar in size, collided at velocities comparable to their two-body escape velocity ~ 100 m/s. Although too slow to produce shocks or disrupt targets, these collisions were messy, especially after ~ 1 Ma of dynamical excitation. In SPH simulations we find that the innermost portion of the projectile decelerates into the target, while the rest continues downrange in massive sheets. Unloading from pre-collision hydrostatic pressure P0 ~ 1-100 bar into the nebula, the melt achieves equilibrium with the surface energy of chondrule-sized droplets. Cooling is regulated post collision by the expansion of the optically thick sheets. on a timescale of hours-days. Much of the sheet rains back down onto the target to be reprocessed; the rest is dispersed.

  3. Coastal Mudflat Accretion under Energetic Conditions, Louisiana Chenier-Plain Coast, USA

    NASA Technical Reports Server (NTRS)

    Draut, Amy E.; Kineke, Gail C.; Huh, Oscar K.; Grymes, John M., III; Westphal, Karen A.; Moeller, Christopher C.

    2005-01-01

    Mudflat accretion on the chenier-plain coast of Louisiana, northern Gulf of Mexico, is anomalous in an area that otherwise experiences widespread land loss due to rapid relative sea level rise. Accretion is shown to be related to energetic events (Winta cold fronts and occasional tropical-dcprrssion srmms) using a 17-year record of meteorological conditions and aerial surveys The results indicate substantial differences between the behavior of sand- and mud-dominated coastal systems under energetic conditions. Comparison of the Louisiana chenier plain to other mud-rich coasts indicates that certain conditions are necessary for mudflat accretion to occur during energetic atmospheric activity. These include an abundant supply of fine-grained fluvial sediment and resuspension events that maintain an unconsolidated sea floor, dominant onshore wind direction during energetic conditions, particularly when onshore winds coincide with high fluvial sediment input to the coastal ocean, and a low tidal range.

  4. 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.

  5. 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.

  6. Southeast Directed Thrusting Associated With the Leading Edge of the Wrangellia Composite Terrane: The Chulitna Block South Central Alaska.

    NASA Astrophysics Data System (ADS)

    Gilman, T. L.; Fisher, D.

    2005-12-01

    Structural mapping of the Chulitna block in south central Alaska reveals a regional scale southeast-vergent, anticline-syncline pair and a series of related southeast-directed thrusts that record the imbrication of the leading edge of the Wrangellia Composite Terrane (WCT). The Chulitna block has been recognized as an unusual Paleozoic through Mesozoic collage of oceanic and clastic rocks that are not found anywhere else in Alaska or the North American Cordilleran. The distribution of these rocks varies across strike from southeast to northwest. In the southeast portion of the field area the oceanic rocks are a Late Devonian (Famennian) ophiolitic sequence including highly sheared serpentinite that is typically altered to magnesite and quartz, basalt (occasionally pillowed), gabbro (with possible cumulate texture), and red radiolarian chert. In the northwest of the map area, the marine rocks are composed of pillow basalts, basalts, and weakly metamorphosed limestones with Norian age fossils. Trace element chemistry of the basalts has been likened to the Late Triassic Nikolai greenstone of the WCT (Clautice et al., 2001). Conformably overlying the oceanic rocks in the northwest of the map area are Triassic red colored conglomerates, sandstones, and siltstones. In the southeast of the field area, these Triassic red rocks unconformably overlie the ophiolitic sequence. The clasts of the Triassic red beds are primarily composed of the underlying red radiolarian chert, basalt, and serpentinite. The main structure in the map area is an anticline-syncline pair that folds all the strata within the Chulitna block. The structure has a steep overturned limb, with a wavelength of ~2 km and an amplitude of ~1 km. To the southeast, there are a pair of out-of-syncline thrusts that juxtapose the serpentinites with overlying units. Large asymmetric folds in the area show a reversal in vergence across the hinge of the larger scale anticline and syncline pair, suggesting they are

  7. 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.

  8. The structure of the Kohistan-Arc terrane in northern Pakistan as inferred from gravity data

    NASA Astrophysics Data System (ADS)

    Malinconico, Lawrence L.

    1986-04-01

    Modelling of gravity data taken across the Kohistan Island-Arc terrane in northern Pakistan can be used to constrain the shape and thickness of the Arc. Over 600 new gravity measurements were made across the Kohistan Island-Arc terrane in northern Pakistan. These data were taken along traverses normal to the structures bounding the Arc and were reduced to terrain-corrected Bouguer values. The reduced data were then modelled using standard two-dimensional modelling techniques. The southern margin of the Arc, the Main Mantle Thrust (MMT), dips to the north at approximately 45° and gradually flattens out at a depth of 7-9 km. The northern margin of the Arc, the Main Karkoram Thrust (MKT), also dips towards the north, but at a shallower initial angle (15°). From the models, the Arc terrane now appears to be around 7-9 km thick with the thicker sections occurring closer to the southern margin. The proposed model, in particular the angle of the MMT and the MKT, may have been significantly affected by the recent and rapid uplift that is occurring along the Nanga Parbat-Haramosh Massif.

  9. 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.

  10. 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

    2016-10-01

    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.

  11. 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.

  12. Formation of a hot proto-atmosphere on the accreting giant icy satellite: Implications for the origin and evolution of Titan, Ganymede, and Callisto

    NASA Astrophysics Data System (ADS)

    Kuramoto, Kiyoshi; Matsui, Takafumi

    1994-10-01

    Judging from accretion energy and accretion time for the giant icy satellites it is suggested that a proto-atmosphere is formed by the evaporation of icy materials during accretion of these bodies around the proto-gaseous giant planets. The blanketing effect of proto-atmosphere during accretion of these satellites in the gas-free environment is studied. We use a gray atmosphere model in which the condensation of H2O in a convective atmosphere is taken into account. The numerical results strongly suggest that the accretion energy flux is large enough to increase the surface temperature higher than approximately 500 K during accretion due to the blanketing effect of proto-atmosphere as long as the accretion time is shorter than 105 years. Such a high surface temperature causes the formation of a deep water-rich ocean due to the melting of icy materials. Also, a rocky core should be eventually formed by sinking of rocky materials through the water-rich ocean during accretion. Therefore, the apparent difference in the surface geologic features between Ganymede and Callisto can hardly be explained by whether or not these bodies have experienced the formation of rocky core. Stability of hydrostatic structure of the proto-atmosphere is also studied. Vigorous escape of the proto-atmosphere is likely to occur under high surface temperature. A large portion of accretional energy is possibly consumed by the vigorous escape during accretion. Thus, the giant icy satellites may lose a significant amount of icy materials during their accretions.

  13. The metallogeny of Late Triassic rifting of the Alexander terrane in southeastern Alaska and northwestern British Columbia

    USGS Publications Warehouse

    Taylor, C.D.; Premo, W.R.; Meier, A.L.; Taggart, J.E.

    2008-01-01

    , to sulfosalt-enriched VMS occurrences exhibiting characteristics of vein, diagenetic replacement, and exhalative styles of mineralization, and finally to Cu-Zn-(Co-Au) occurrences with larger and more clearly stratiform orebody morphologies. Occurrences in the middle of the belt are transitional in nature between structurally controlled types of mineralization that formed in a shallow-water, near-arc setting, to those having a more stratiform appearance, formed in a deeper water, rift-basin setting. The geologic setting in the south is consistent with shallow subaqueous emplacement on the flanks of the Alexander terrane. Northward, the setting changes to an increasingly deeper back- or intra-arc rift basin. Igneous activity in the Alexander Triassic metallogenic belt is characterized by a bimodal suite of volcanic rocks and a previously unrecognized association with mafic-ultramafic hypabyssal intrusions. Immobile trace and rare earth element (BEE) geochemical data indicate that felsic rocks in the southern portion of the belt are typical calc-alkaline rhyolites, which give way in the middle of the belt to peralkaline rhyolites. Rhyolites are largely absent in the northern part of the belt. Throughout the belt, the capping basaltic rocks have transitional geochemical signatures. Radiogenic isotope data for these rocks are also transitional (basalts and gabbros: ??-Nd = 4-9 and 87Sr/86Sr initial at 215 Ma = 0.7037-0.7074). Together these data are interpreted to reflect variable assimilation of mature island-arc crust by more primitive melts having the characteristics of either mid-ocean ridge (MORB) or intraplate (within-plate) basalts (WPB). The ore and host-rock geochemistry and the sulfosalt-rich mineralogy of the deposits are strikingly similar to recent descriptions of active sea-floor hydrothermal (white smoker) systems in back arcs of the southwest Pacific Ocean. These data, in concert with existing faunal ages, record the formation of a belt of VMS deposits

  14. 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

  15. 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

  16. 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

  17. Accretion Rate: An Axis Of Agn Unification

    NASA Astrophysics Data System (ADS)

    Trump, Jonathan R.; Impey, C. D.; Kelly, B. C.

    2011-01-01

    We show how accretion rate governs the physical properties of broad-line, narrow-line, and lineless active galactic nuclei (AGNs). We avoid the systematic errors plaguing previous studies of AGN accretion rate by using accurate accretion luminosities from well-sampled multiwavelength SEDs from the Cosmic Evolution Survey (COSMOS), and accurate black hole masses derived from virial scaling relations (for broad-line AGNs) or host-AGN relations (for narrow-line and lineless AGNs). In general, broad emission lines are present only at the highest accretion rates (L/L_Edd>0.01), and these rapidly accreting AGNs are observed as broad-line AGNs or possibly as obscured narrow-line AGNs. Narrow-line and lineless AGNs at lower specific accretion rates (L/L_Edd<0.01) are unobscured and yet lack a broad line region. The disappearance of the broad emission lines is caused by an expanding radiatively inefficient accretion flow (RIAF) at the inner radius of the accretion disk. The presence of the RIAF also drives L/L_Edd<0.01 narrow-line and lineless AGNs to be 10-100 times more radio-luminous than broad-line AGNs, since the unbound nature of the RIAF means it is easier to form a radio outflow. The IR torus signature also tends to become weaker or disappear from L/L_Edd<0.01 AGNs, although there may be additional mid-IR synchrotron emission associated with the RIAF. Together these results suggest that specific accretion rate is an important physical "axis" of AGN unification, described by a simple model.

  18. 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.

  19. 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.

  20. 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.

  1. Accretion in Saturn's F Ring

    NASA Astrophysics Data System (ADS)

    Meinke, B. K.; Esposito, L. W.; Stewart, G.

    2012-12-01

    Saturn's F ring is the solar system's principal natural laboratory for direct observation of accretion and disruption processes. The ring resides in the Roche zone, where tidal disruption competes with self-gravity, which allows us to observe the lifecycle of moonlets. Just as nearby moons create structure at the B ring edge (Esposito et al. 2012) and the Keeler gap (Murray 2007), the F ring "shepherding" moons Prometheus and Pandora stir up ring material and create observably changing structures on timescales of days to decades. In fact, Beurle et al (2010) show that Prometheus makes it possible for "distended, yet gravitationally coherent clumps" to form in the F ring, and Barbara and Esposito (2002) predicted a population of ~1 km bodies in the ring. In addition to the observations over the last three decades, the Cassini Ultraviolet Imaging Spectrograph (UVIS) has detected 27 statistically significant features in 101 occultations by Saturn's F ring since July 2004. Seventeen of those 27 features are associated with clumps of ring material. Two features are opaque in occultation, which makes them candidates for solid objects, which we refer to as Moonlets. The 15 other features partially block stellar signal for 22 m to just over 3.7 km along the radial expanse of the occultation. Upon visual inspection of the occultation profile, these features resemble Icicles, thus we will refer to them as such here. The density enhancements responsible for such signal attenuations are likely due to transient clumping of material, evidence that aggregations of material are ubiquitous in the F ring. Our lengthy observing campaign reveals that Icicles are likely transient clumps, while Moonlets are possible solid objects. Optical depth is an indicator of clumping because more-densely aggregated material blocks more light; therefore, it is natural to imagine moonlets as later evolutionary stage of icicle, when looser clumps of material compact to form a feature that appears

  2. Diskoseismology: Probing relativistic accretion disks

    NASA Astrophysics Data System (ADS)

    Nowak, Michael Allen

    1992-08-01

    Helioseismology has provided a wealth of information about the structure of the solar atmosphere. Little is known, however, about the structure of accretion disks that are thought to exist around black holes and neutron stars. In this thesis we present calculations of modes that are trapped in thin Keplerian accretion disks. We hope to use observations of thes modes to elucidate the structure of the inner relativistic regions of accretion disks. Our calculations assume that the thin disk is terminated by an innermost stable orbit, as would occur around a slowly rotating black hole or weakly magnetized compact neutron star. The dominant relativistic effects, which allow modes to be trapped within the inner region of the disk, are approximated via a modified Newtonian potential. Using the Lagrangian formulation of Friedman and Schutz, we develop a general formalism for investigating the adiabatic oscillations of arbitrary unperturbed disk models. First we consider the special case of acoustic waves in disks with isothermal atmospheres. Next we describe the Lagrangian perturbation vectors in terms of the derivatives of a scalar potential, as has been done by Ipser and Lindblom. Using this potential, we derive a single partial differential equation governing the oscillations of a disk. The eigenfunctions and eigenfrequencies of a variety of disk models are found to fall into two main classes which are analogous to the p-modes and g-modes in the sun. Specifically we use the potential formalism to compute the g-modes for disks with isothermal atmospheres. Physical arguments show that both the p-modes and g-modes belong to the same family of modes as the p-modes and g-modes in the sun, just viewed in a different parameter regime. With the aid of the Lagrangian formalism we consider possible growth or damping mechanisms and compute the (assumed) relatively small rates of growth or damping of the modes. Specifically, we consider gravitational radiation reaction and

  3. 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

  4. Galaxies in Clusters : Gas Stripping and Accretion

    NASA Astrophysics Data System (ADS)

    O'Dea, Chris; Balsara, Dinshaw; Livio, Mario

    1994-05-01

    We study the process of a galaxy moving through the intercluster gas in a cluster of galaxies, using a high quality hydrocode run at high resolutions. We find that ram pressure stripping occurs in the form of individual events that are separated by about ten million years. In addition we find that the galaxy accretes gas from the downstream side into the core. This accretion process exhibits a radial "pumping" mode, similar to the one found previously in simulations of wind accretion onto compact objects. Some implications of our results for the understanding of a few recent observations are discussed.

  5. Cyclotron Lines in Accreting Neutron Star Spectra

    NASA Astrophysics Data System (ADS)

    Wilms, Jörn; Schönherr, Gabriele; Schmid, Julia; Dauser, Thomas; Kreykenbohm, Ingo

    2009-05-01

    Cyclotron lines are formed through transitions of electrons between discrete Landau levels in the accretion columns of accreting neutron stars with strong (1012 G) magnetic fields. We summarize recent results on the formation of the spectral continuum of such systems, describe recent advances in the modeling of the lines based on a modification of the commonly used Monte Carlo approach, and discuss new results on the dependence of the measured cyclotron line energy from the luminosity of transient neutron star systems. Finally, we show that Simbol-X will be ideally suited to build and improve the observational database of accreting and strongly magnetized neutron stars.

  6. 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).

  7. 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.

  8. The beaming of subhalo accretion

    NASA Astrophysics Data System (ADS)

    Libeskind, Noam I.

    2016-10-01

    We examine the infall pattern of subhaloes onto hosts in the context of the large-scale structure. We find that the infall pattern is essentially driven by the shear tensor of the ambient velocity field. Dark matter subhaloes are preferentially accreted along the principal axis of the shear tensor which corresponds to the direction of weakest collapse. We examine the dependence of this preferential infall on subhalo mass, host halo mass and redshift. Although strongest for the most massive hosts and the most massive subhaloes at high redshift, the preferential infall of subhaloes is effectively universal in the sense that its always aligned with the axis of weakest collapse of the velocity shear tensor. It is the same shear tensor that dictates the structure of the cosmic web and hence the shear field emerges as the key factor that governs the local anisotropic pattern of structure formation. Since the small (sub-Mpc) scale is strongly correlated with the mid-range (~ 10 Mpc) scale - a scale accessible by current surveys of peculiar velocities - it follows that findings presented here open a new window into the relation between the observed large scale structure unveiled by current surveys of peculiar velocities and the preferential infall direction of the Local Group. This may shed light on the unexpected alignments of dwarf galaxies seen in the Local Group.

  9. 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.

  10. 1900-Ma ocean crust in Canada

    NASA Astrophysics Data System (ADS)

    Maggs, William Ward

    The oldest known occurrence in North America of an ophiolite, considered to be a piece of ancient ocean crust, has been reported in the Cape Smith Belt in northern Quebec, Canada.The recognition last summer of a key structural component of the characteristic ophiolite suite has buttressed confidence in the theory that the 1900-Ma fragments of an ocean basin were accreted to an early Proterozoic Canadian continent. The tectonic mixing of oceanic and continental crust is strong evidence for the operation of plate tectonics early in Earth's history.

  11. Accretion-ejection models for AGN jets

    NASA Astrophysics Data System (ADS)

    Zanni, C.

    2008-10-01

    It is likely that jets from Active Galactic Nuclei derive their energy from accretion onto the central black hole. It is actually possible to fuel the jets by extracting energy and angular momentum from the accretion disk and/or the rotating black hole via the action of large-scale magnetic fields. In this talk I will first present results of analytical and numerical models of the launching process of jets from magnetized accretion disks: I will show that, although a sizeable fraction of the accretion power goes into the jets, these outflows are presumably only mildly relativistic. In the second place, I will therefore suggest that the strongly relativistic components observed at the VLBI scales are accelerated in the innermost parts of the AGNs by Blandford-Znajek and/or Compton-rocket processes. Nonetheless, the non-relativistic disk-wind is needed to collimate the relativistic component and to reproduce the total power of extragalactic jets.

  12. Planetary science: Iron fog of accretion

    DOE PAGES

    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.

  13. Time lag in transient cosmic accreting sources

    NASA Astrophysics Data System (ADS)

    Bisnovatyi-Kogan, G. S.; Giovannelli, F.

    2017-02-01

    Context. We develop models for time lag between the maxima of the source brightness in different wavelengths during a transient flash of luminosity that is connected with a short-period increase of the mass flux onto the central compact object. Aims: We derive a simple formula for finding the time delay among events in different wavelengths which is valid in general for all disk-accreting cosmic sources. We quantitatively also discuss a model for time-lag formation in active galactic nuclei (AGNs). Methods: In close binaries with accretion disks, the time lag is connected with effects of viscosity that define a radial motion of matter in the accretion disk. In AGN flashes, the infalling matter has a low angular momentum, and the time lag is defined by the free-fall time to the gravitating center. Results: We show the validity of these models by means of several examples of galactic and extragalactic accreting sources.

  14. The basement of the Punta del Este Terrane (Uruguay): an African Mesoproterozoic fragment at the eastern border of the South American Río de La Plata craton

    NASA Astrophysics Data System (ADS)

    Basei, Miguel A. S.; Peel, Elena; Sánchez Bettucci, Leda; Preciozzi, Fernando; Nutman, Allen P.

    2011-04-01

    The Punta del Este Terrane (eastern Uruguay) lies in a complex Neoproterozoic (Brasiliano/Pan-African) orogenic zone considered to contain a suture between South American terranes to the west of Major Gercino-Sierra Ballena Suture Zone and eastern African affinities terranes. Zircon cores from Punta del Este Terrane basement orthogneisses have U-Pb ages of ca. 1,000 Ma, which indicate an lineage with the Namaqua Belt in Southwestern Africa. U-Pb zircon ages also provide the following information on the Punta del Este terrane: the orthogneisses containing the ca. 1,000 Ma inheritance formed at ca. 750 Ma; in contrast to the related terranes now in Africa, reworking of the Punta del Este Terrane during Brasiliano/Pan-African orogenesis was very intense, reaching granulite facies at ca. 640 Ma. The termination of the Brasiliano/Pan-African orogeny is marked by formation of acid volcanic and volcanoclastic rocks at ca. 570 Ma (Sierra de Aguirre Formation), formation of late sedimentary basins (San Carlos Formation) and then intrusion at ca. 535 Ma of post-tectonic granitoids (Santa Teresa and José Ignacio batholiths). The Punta del Este Terrane and unrelated western terranes represented by the Dom Feliciano Belt and the Río de La Plata Craton were in their present positions by ca. 535 Ma.

  15. 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

  16. 43 CFR 3110.5-4 - Accreted lands.

    Code of Federal Regulations, 2013 CFR

    2013-10-01

    ... 43 Public Lands: Interior 2 2013-10-01 2013-10-01 false Accreted lands. 3110.5-4 Section 3110.5-4 Public Lands: Interior Regulations Relating to Public Lands (Continued) BUREAU OF LAND MANAGEMENT... Accreted lands. Where an offer includes any accreted lands, the accreted lands shall be described by...

  17. 43 CFR 3110.5-4 - Accreted lands.

    Code of Federal Regulations, 2014 CFR

    2014-10-01

    ... 43 Public Lands: Interior 2 2014-10-01 2014-10-01 false Accreted lands. 3110.5-4 Section 3110.5-4 Public Lands: Interior Regulations Relating to Public Lands (Continued) BUREAU OF LAND MANAGEMENT... Accreted lands. Where an offer includes any accreted lands, the accreted lands shall be described by...

  18. 43 CFR 3110.5-4 - Accreted lands.

    Code of Federal Regulations, 2011 CFR

    2011-10-01

    ... 43 Public Lands: Interior 2 2011-10-01 2011-10-01 false Accreted lands. 3110.5-4 Section 3110.5-4 Public Lands: Interior Regulations Relating to Public Lands (Continued) BUREAU OF LAND MANAGEMENT... Accreted lands. Where an offer includes any accreted lands, the accreted lands shall be described by...

  19. 43 CFR 3110.5-4 - Accreted lands.

    Code of Federal Regulations, 2012 CFR

    2012-10-01

    ... 43 Public Lands: Interior 2 2012-10-01 2012-10-01 false Accreted lands. 3110.5-4 Section 3110.5-4 Public Lands: Interior Regulations Relating to Public Lands (Continued) BUREAU OF LAND MANAGEMENT... Accreted lands. Where an offer includes any accreted lands, the accreted lands shall be described by...

  20. Gas dynamics for accretion disk simulations

    NASA Technical Reports Server (NTRS)

    Whitehurst, R.

    1994-01-01

    The behavior of accretion disks can largely be understood in terms of the basic physical processes of mass, energy, and momentum conservation. Despite this, detailed modeling of these systems using modern computational techniques is challenging and controversial. Disturbing differences exist between methods used widely in astrophysics, namely Eulerian finite-difference techniques and particle codes such as SPH. Therefore neither technique is fully satisfactory for accretion disk simulations. This paper describes a new fully Lagrangian method designed to resolve these difficulties.

  1. ASYMMETRIC ACCRETION FLOWS WITHIN A COMMON ENVELOPE

    SciTech Connect

    MacLeod, Morgan; Ramirez-Ruiz, Enrico

    2015-04-10

    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.

  2. Transonic disk accretion onto black holes

    NASA Technical Reports Server (NTRS)

    Liang, E. P. T.; Thompson, K. A.

    1980-01-01

    The solution for the radial drift velocity of thin disk accretion onto black holes must be transonic, and is analogous to the critical solution in spherical Bondi accretion, except for the presence of angular momentum. The transonic requirement yields a correct treatment of the inner region of the disk not found in the conventional Keplerian models and may lead to significantly different overall disk structures. Possible observational consequences, relevant to the black hole hypothesis for Cyg X-1 and other candidates, are discussed.

  3. Ultraslow Ridges through Binoculars: Teleseismic Earthquake Characteristics Illuminate Accretion Processes

    NASA Astrophysics Data System (ADS)

    Schlindwein, V.; Laederach, C.; Korger, E.

    2011-12-01

    Ultraslow spreading ridges with full spreading rates < 20 mm/y constitute the largest portion of the global mid-ocean ridge system, yet 85% of these ridges are still unexplored. Understanding the structure and dynamics of crustal production and the associated hydrothermal systems including their biota has become a major challenge of modern mid-ocean ridge research. The complex interplay between tectonic, magmatic and hydrothermal processes that governs lithospheric accretion at ultraslow-spreading ridges is so poorly investigated because their main representatives, the Arctic ridge system and the Southwest Indian Ridge (SWIR), are situated in remote areas with difficult working conditions. While local seismicity studies with ocean bottom seismometers on slow and fast spreading ridges have greatly contributed to our understanding of active accretion processes, comparable studies are lacking for ultraslow spreading ridges forcing to fall back on studies of larger earthquakes recorded on land. Using teleseismic data from the Bulletin of the International Seismological Centre between the years 1976 and 2010, we performed a systematic analysis of the ridge related seismicity (M > 4) of the ultraslow spreading Arctic ridge system and the SWIR. These ridges were divided in 11 sections of uniform seismological, topographic and geological characteristics, totalling a length of 7200 km with the rift axis defined as a multisegment line along the topographic low of the rift valley. Only events within 30 km of the rift axis were included in our study. We found that magmatic and amagmatic accretion sections cannot be distinguished neither by event rate, moment release rate, maximum earthquake magnitude, nor by the b-value. Yet using single link cluster analysis for identification of swarms of 8 or more earthquakes, small clusters of 2-7 earthquakes and single events, we found that sections with amagmatic accretion lack swarms and show consistently a high percentage of single

  4. Promises and Problems of Pebble Accretion

    NASA Astrophysics Data System (ADS)

    Kretke, Katherine A.; Levison, H. F.

    2013-10-01

    Despite the large number of exoplanets indicating that giant planets are a common outcome of the star formation process, theoretical models still struggle to explain how ~10 Earth mass rocky/icy embryos can form within the lifetimes of gaseous circumstellar disks. In recent years, aerodynamic-aided accretion of ``pebbles,'' particles ranging from millimeters to decimeters in size, has been suggested as a potential solution to this long-standing problem. Local simulations, simulations which look at the detailed behavior of these pebbles in the vicinity of a planetary embryo, have shown that the potential planetary growth rates can be surprisingly fast. If one assumes that most of the mass in a protoplanetary disk resides in these pebble-sized particles, a Mars mass core could grow to 10 Earth masses in only a few thousand years. However, these local studies cannot investigate how this accretion process behaves in the more complicated, multi-planet environment. We have incorporated the local accretion physics into LIPAD, a Lagrangian code which can follow the collisional / accretional / dynamical evolution of a planetary system, to investigate the how this pebble accretion will manifest itself in the larger planet formation picture. We present how these more comprehensive models raise challenges to using pebble accretion to form observed planetary systems.

  5. Problems and Promises of Pebble Accretion

    NASA Astrophysics Data System (ADS)

    Kretke, Katherine A.; Levison, H. F.

    2013-05-01

    Abstract (2,250 Maximum Characters): Despite the large number of exoplanets indicating that planets are a common outcome of the star formation process, theoretical models still struggle to explain how ~10 Earth mass rocky/icy embryos can form within the lifetimes of gaseous circumstellar disks. Recently, aerodynamic-aided accretion of ``pebbles,'' particles ranging from millimeters to decimeters in size, has been suggested as a potential solution to this long-standing problem. Local simulations, simulations which look at the detailed behavior of these pebbles in the vicinity of a planetary embryo, have shown that the potential planetary growth rates can be surprisingly fast. If one assumes that most of the mass in a protoplanetary disk resides in these pebble-sized particles, a Mars mass core could grow to 10 Earth masses in only a few thousand years. However, these local studies cannot investigate how this accretion process behaves in the more complicated, multi-planet environment. We have incorporated a prescription of this pebble accretion into LIPAD, a Lagrangian code which can follow the collisional/accretional/dynamical evolution of a planetary system, to investigate the how this pebble accretion will manifest itself in the larger planet formation picture. We discuss how these more comprehensive models present challenges for using pebble accretion to form observed planetary systems.

  6. Giant planet formation via pebble accretion

    NASA Astrophysics Data System (ADS)

    Guilera, O. M.

    2016-08-01

    In the standard model of core accretion, the formation of giant planets occurs by two main processes: first, a massive core is formed by the accretion of solid material; then, when this core exceeds a critical value (typically greater than ) a gaseous runaway growth is triggered and the planet accretes big quantities of gas in a short period of time until the planet achieves its final mass. Thus, the formation of a massive core has to occur when the nebular gas is still available in the disk. This phenomenon imposes a strong time-scale constraint in the giant planet formation due to the fact that the lifetimes of the observed protoplanetary disks are in general lower than 10 Myr. The formation of massive cores before 10 Myr by accretion of big planetesimals (with radii 10 km) in the oligarchic growth regime is only possible in massive disks. However, planetesimal accretion rates significantly increase for small bodies, especially for pebbles, particles of sizes between mm and cm, which are strongly coupled with the gas. In this work, we study the formation of giant planets incorporating pebble accretion rates in our global model of planet formation.

  7. 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

  8. 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.

  9. 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

  10. Paleomagnetic Reconstruction of Post-Paleozoic Tectonic Motions for the Intermontane and Yukon-Tanana Terranes of Baja British Columbia

    NASA Astrophysics Data System (ADS)

    Symons, D. T.; Harris, M. J.; McCausland, P. J.; Blackburn, W. H.; Hart, C. J.

    2004-05-01

    Paleopoles from 42 Mesozoic and Cenozoic rock units in the Intermontane Belt (IMB) and Yukon-Tanana (YT) terranes are deemed to be sufficiently represented paleomagnetically and well dated radiometrically to merit consideration. Unlike previous analyses based on paleoinclination only that have led to estimates up to ˜3500 Km for northward (poleward) displacement of Baja BC relative to the North American craton, the pattern of both paleoinclination and paleodeclination are used to assess the reliability of each paleopole. The analysis indicates that: 1) the YT terrane is autochthonous or parautochthonous; 2) the IMB terranes rotated steadily atop the craton by 16° ±6° between 54 Ma and the present, accomodated by extension in the south and compression in the north; 3) the IMB terranes were rotated a further 35° ±14° and translated northward by 8° ±7° (900 Km) from 102 to 54 Ma, consistent with geological estimates for the northward displacement of interior Baja BC; and, 4) the Cache Creek, Quesnel and probably Stikine terranes were part of Baja BC since Early Jurassic.

  11. Accretion Disks around Young Stars

    NASA Astrophysics Data System (ADS)

    D'Alessio, Paola

    1996-04-01

    A method to calculate the structure and brightness distribution of accretion disks surrounding low and intermediate mass young stars is introduced and discussed. The method includes a realistic treatment of the energy transport mechanisms and disk heating by radiation from external sources. The disk is assumed steady, geometrically thin and in vertical hydrostatic equilibrium. The turbulent viscosity coefficient is expressed using the α prescription and the α parameter and the mass accretion rate are assumed to be constant through the disk. Energy is transported in the vertical direction by: (a) a turbulent flux, computed self-consistently with the viscosity coefficient used to describe the viscous energy dissipation, (b) radiation, using the first moments of the transfer equation, the Eddington approximation, and the Rosseland and Planck Mean Opacities, and (c) convection, taking into account that the convective elements, not necessarily optically thick, lose energy by radiation and turbulent flux. This treatment of the energy transport mechanisms differs from previous work in this field, allowing one to extend, with confidence, the calculation of the disk structure to optically thin regimes. The heating mechanisms considered, which affect the disk's structure and emission, are stellar radiation and a circumstellar envelope which reprocesses and scatters radiation from the star and from the disk itself. In addition to a detailed numerical calculation, an analytical self-consistent formulation of the irradiation of the disk is given. This analytical formulation allows one to understand and extend the numerical results. To evaluate the potential of the method presented in this thesis, a set of models of viscous non-irradiated and irradiated disks are computed. Their predictions are compared with observations of young stellar sources likely to have disks. Given the disk structure and specifying its orientation with respect to the line of sight, the specific

  12. Asymmetric extension of the Middle Proterozoic lithosphere, Mount Isa terrane, Queensland, Australia

    NASA Astrophysics Data System (ADS)

    Betts, P. G.; Lister, G. S.; O'Dea, M. G.

    1998-11-01

    The Middle Proterozoic Mount Isa Basin, of the Mount Isa terrane, hosts several large Pb-Zn-Ag deposits and is arguably one of the richest mineral provinces in the world. The deformed remnants of this basin extend from the eastern margin of the Leichhardt River Fault Trough through to the Murphy Tectonic Ridge in the far north of the terrane. The Mount Isa Basin initially evolved in response to NW-SE-directed extension during the Mount Isa Rift Event. This event began before ˜1708 Ma and had ceased by ˜1653 Ma. A sag basin continued to evolve thereafter until ˜1595 Ma. Regional analysis of the highest level cover rocks of the Mount Isa Basin reveals a notable difference in the locus of syn-rift sedimentation, syn-rift magmatism, and post-rift subsidence. Although crustal extension was widespread across the Mount Isa Basin, tectonic subsidence was focussed along the ˜N-S-oriented Mount Isa Rift. Approximately 3-5 km of fluvial to shallow marine clastic sediments were deposited into isolated rift basins. Bimodal volcanism and emplacement of shallow level plutons occurred along the western and northwestern margins of the Mount Isa Rift. Magmatic provinces mark the locus of significant subcrustal lithospheric thinning, asthenospheric upwelling, and mafic underplating. Within these magmatic provinces the syn-rift sequences are fewer and thinner (750-2000 m) and were dominantly deposited in subaerial environments, suggesting a relatively stable uplift and subsidence history. The position of maximum subcrustal lithospheric extension is determined by the position of greatest post-rift subsidence. This occurred beneath the northern Mount Isa terrane where the thickest post-rift sequences are preserved and the depositional history is more protracted. We propose that the evolution of the Mount Isa Basin is a consequence of asymmetric extension of the Middle Proterozoic lithosphere.

  13. The PISA Pre-Main Sequence accreting models

    NASA Astrophysics Data System (ADS)

    Tognelli, Emanuele; Prada Moroni, Pier Giorgio; Degl'Innocenti, Scilla

    2013-07-01

    The poster investigates the effect of accretion processes on the evolution of stellar models computed by means of the well tested and updated PROSECCO evolutionary code, under the hypothesis of thin-disk accretion. We analysed the effect on the evolution of the adoption of different parameters as the accretion rate, accretion history, seed mass, and the fraction of the infalling matter energy (alpha_acc) deposed in to the star (accretion energy). We confirm that the most critical parameter is the accretion energy. We show that, depending on alpha_acc the evolution of accreting and non-accreting objects can be completely different, confirming that the adoption of small alpha_acc value (i.e. small accretion energy, cold accretion) produces fainter and more compact models with respect to the ones predicted from non-accreting structures at the same mass and age, models that can not be reconciled with the data available for young objects (i.e. position in the HR diagram, lithium abundances). On the contrary, if a large part of the accretion luminosity is deposed into the star (alpha_acc = 1, hot accretion), at least during the fisrt stages of the accretion phase or during bursts episodes, large radii and luminosities are achievable, with resulting structures much more similar to the non-accreting ones.

  14. 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

  15. 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

  16. 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

  17. Accretion onto Fast X-Ray Pulsars

    NASA Technical Reports Server (NTRS)

    Rappaport, S. A.; Fregeau, J. M.; Spruit, H.

    2004-01-01

    The recent emergence of a new class of accretion-powered, transient, millisecond X-ray pulsars presents some difficulties for the conventional picture of accretion onto rapidly rotating magnetized neutron stars and their spin behavior during outbursts. In particular, it is not clear that the standard paradigm can accommodate the wide range in M(i.e., approx. greater than a factor of 50) over which these systems manage to accrete and the high rate of spindown that the neutron stars exhibit in at least a number of cases. When the accretion rate drops sufficiently, the X-ray pulsar is said to become a "fast rotator," and in the conventional view, this is accompanied by a transition from accretion to "propellering," in which accretion ceases and the matter is ejected from the system. On the theoretical side, we note that this scenario for the onset of propellering cannot be entirely correct because it is not energetically self-consistent. We show that, instead, the transition is likely to take place through disks that combine accretion with spindown and terminate at the corotation radius. We demonstrate the existence of such disk solutions by modifying the Shakura-Sunyaev equations with a simple magnetic torque prescription. The solutions are completely analytic and have the same dependence on M and a (the viscosity parameter) as the original Shakura-Sunyaev solutions, but the radial profiles can be considerably modified, depending on the degree of fastness. We apply these results to compute the torques expected during the outbursts of the transient millisecond pulsars and find that we can explain the large spin-down rates that are observed for quite plausible surface magnetic fields of approx. 10(exp 90 G.

  18. Accretion onto Pre-Main-Sequence Stars

    NASA Astrophysics Data System (ADS)

    Hartmann, Lee; Herczeg, Gregory; Calvet, Nuria

    2016-09-01

    Accretion through circumstellar disks plays an important role in star formation and in establishing the properties of the regions in which planets form and migrate. The mechanisms by which protostellar and protoplanetary disks accrete onto low-mass stars are not clear; angular momentum transport by magnetic fields is thought to be involved, but the low-ionization conditions in major regions of protoplanetary disks lead to a variety of complex nonideal magnetohydrodynamic effects whose implications are not fully understood. Accretion in pre-main-sequence stars of masses ≲1M⊙ (and in at least some 2-3-M⊙ systems) is generally funneled by the stellar magnetic field, which disrupts the disk at scales typically of order a few stellar radii. Matter moving at near free-fall velocities shocks at the stellar surface; the resulting accretion luminosities from the dissipation of kinetic energy indicate that mass addition during the T Tauri phase over the typical disk lifetime ˜3 Myr is modest in terms of stellar evolution, but is comparable to total disk reservoirs as estimated from millimeter-wave dust emission (˜10-2 M⊙). Pre-main-sequence accretion is not steady, encompassing timescales ranging from approximately hours to a century, with longer-timescale variations tending to be the largest. Accretion during the protostellar phase—while the protostellar envelope is still falling onto the disk—is much less well understood, mostly because the properties of the central obscured protostar are difficult to estimate. Kinematic measurements of protostellar masses with new interfometric facilities should improve estimates of accretion rates during the earliest phases of star formation.

  19. Interpretation of gravity profiles across the northern Oaxaca terrane, its boundaries and the Tehuacán Valley, southern Mexico

    NASA Astrophysics Data System (ADS)

    Campos-Enríquez, J. O.; Alatorre-Zamora, M. A.; Keppie, J. D.; Belmonte-Jiménez, S. I.; Ramón-Márquez, V. M.

    2014-12-01

    A gravity study was conducted across the northern Oaxaca terrane and its bounding faults: the Caltepec and Oaxaca Faults to the west and east, respectively. These faults juxtapose the Oaxaca terrane against the Mixteca and Juarez terranes, respectively. The Oaxaca Fault also forms the eastern boundary of the Cenozoic Tehuacán depression. On the west, at depth, the Tehuacán valley is limited by the normal buried Tehuacán Fault. This gravity study reveals that the Oaxaca Fault system gives rise to a series of east tilted basamental blocks (Oaxaca Complex). The tectonic depression is filled with Phanerozoic rocks and has a deeper depocenter to the west. The gravity data also indicate that on the west, the Oaxaca Complex, the Caltepec and Santa Lucia faults continue northwestwards beneath Phanerozoic rocks. A major E-W to NE-SW discontinuity is inferred to exist between profiles 1 and 2.

  20. Coupled mechanical and hydrothermal modelling of crustal accretion at fast spreading ridges

    NASA Astrophysics Data System (ADS)

    Theissen, Sonja; Iyer, Karthik; Rüpke, Lars H.; Phipps Morgan, Jason

    2010-05-01

    Several geophysical studies imaged a melt lens beneath intermediate to fast spreading ridges, with the depth to the melt lens depending on spreading velocity. It is also widely accepted that the heat released during cooling and crystallisation during the accretion/formation of new oceanic crust is removed by hydrothermal circulation. Two competing end member models explain the formation of the oceanic crust: In the gabbro glacier model the lower crust crystallises in a shallow melt lens and the solidified material is advected to its final position, whereas in the many sill model the crust crystallises in situ from multiple sills at different levels in the lower crust. Many numerical models of crustal accretion and hydrothermal cooling have been developed in the last years, but regardless of whether the models simulate the gabbro glacier or sheeted sill assumption, the previous models focus mainly on one of the processes. They solve either for hydrothermal circulation and create the lithospheric material continuously with spreading velocity [e.g.Cherkaoui et al., 2003] or for viscous advection but parameterise the hydrothermal cooling with an enhanced thermal conductivity/diffusivity as described by Morgan and Chen [1993]. Our new approach couples both processes in one model. The formation of new oceanic crust is implemented as in the gabbro glacier assumption, where all the lower crust crystallises in a shallow melt lens. It is a two dimensional model which uses the finite element method to solve simultaneously for crustal accretion and hydrothermal cooling. The solid velocities in crust and mantle are described by viscous flow of incompressible fluids. Magma injection is implemented by a dilation term and hydrothermal circulation is described by Darcy fluid flow for water. Although the time scales for accretion of the crust and cooling due to hydrothermal circulation are different it was possible to couple the processes in one model and to solve for a steady state

  1. Geochemistry and isotopic composition of the Guerrero Terrane (western Mexico): implications for the tectono-magmatic evolution of southwestern North America during the Late Mesozoic

    NASA Astrophysics Data System (ADS)

    Mendoza, O. T.; Suastegui, M. G.

    2000-10-01

    The composite Guerrero Terrane of western Mexico records much of the magmatic evolution of southwestern North America during Late Mesozoic time. The Guerrero includes three distinctive subterranes characterized by unique stratigraphic records, structural evolutions, and geochemical and isotopic features that strongly suggest they evolved independently. The eastern Teloloapan Subterrane represents an evolved intra-oceanic island arc of Hauterivian to Cenomanian age, which includes a high-K calc-alkaline magmatic suite. The central Arcelia-Palmar Chico Subterrane represents a primitive island arc-marginal basin system of Albian to Cenomanian age, consisting of an oceanic suite and a tholeiitic arc suite. The western Zihuatanejo-Huetamo Subterrane comprises three components that represent an evolved island arc-marginal basin-subduction complex system of Late Jurassic (?) -Early Cretaceous age built on a previously deformed basement. The Zihuatanejo Sequence includes a thick high-K calc-alkaline magmatic suite. The Las Ollas Complex consists of tectonic slices containing exotic blocks of arc affinity affected by high-pressure/low-temperature metamorphism included in a sheared matrix. The Huetamo Sequence consists mainly of volcanic-arc derived sedimentary rocks, including large pebbles of tholeiitic, calc-alkaline, and shoshonitic lavas. These sequences are unconformably underlain by the Arteaga Complex, which represents the subvolcanic basement. On the basis of available geology, geochemistry, geochronology, and isotopic data, we suggest that Late Mesozoic volcanism along the western margin of southern North America developed in broadly contemporaneous but different intra-oceanic island arcs that constitute a complex fossil arc-trench system similar to the present-day western Pacific island arc system.

  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. 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

  4. 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).

  5. The Final Fates of Accreting Supermassive Stars

    NASA Astrophysics Data System (ADS)

    Umeda, Hideyuki; Hosokawa, Takashi; Omukai, Kazuyuki; Yoshida, Naoki

    2016-10-01

    The formation of supermassive stars (SMSs) via rapid mass accretion and their direct collapse into black holes (BHs) is a promising pathway for sowing seeds of supermassive BHs in the early universe. We calculate the evolution of rapidly accreting SMSs by solving the stellar structure equations including nuclear burning as well as general relativistic (GR) effects up to the onset of the collapse. We find that such SMSs have a less concentrated structure than a fully convective counterpart, which is often postulated for non-accreting ones. This effect stabilizes the stars against GR instability even above the classical upper mass limit ≳105 M ⊙ derived for the fully convective stars. The accreting SMS begins to collapse at the higher mass with the higher accretion rate. The collapse occurs when the nuclear fuel is exhausted only for cases with \\dot{M}≲ 0.1 {M}⊙ {{{yr}}}-1. With \\dot{M}≃ 0.3{--}1 {M}⊙ {{{yr}}}-1, the star becomes GR unstable during the helium-burning stage at M ≃ 2-3.5 × 105 M ⊙. In an extreme case with 10 {M}⊙ {{{yr}}}-1, the star does not collapse until the mass reaches ≃8.0 × 105 M ⊙, where it is still in the hydrogen-burning stage. We expect that BHs with roughly the same mass will be left behind after the collapse in all the cases.

  6. 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.

  7. The Burst Mode of Protostellar Accretion

    NASA Astrophysics Data System (ADS)

    Vorobyov, E. I.; Basu, Shantanu

    2006-10-01

    We present new numerical simulations in the thin disk approximation that characterize the burst mode of protostellar accretion. The burst mode begins upon the formation of a centrifugally balanced disk around a newly formed protostar. It comprises prolonged quiescent periods of low accretion rate (typically <~10-7 Msolar yr-1) that are punctuated by intense bursts of accretion (typically >~10-4 Msolar yr-1, with duration <~100 yr) during which most of the protostellar mass is accumulated. The accretion bursts are associated with the formation of dense protostellar/protoplanetary embryos, which are later driven onto the protostar by the gravitational torques that develop in the disk. Gravitational instability in the disk, driven by continuing infall from the envelope, is shown to be an effective means of transporting angular momentum outward and mass inward to the protostar. We show that the disk mass always remains significantly less than the central protostar's mass throughout this process. The burst phenomenon is robust enough to occur for a variety of initial values of rotation rate and frozen-in (supercritical) magnetic field and a variety of density-temperature relations. Even in cases where the bursts are nearly entirely suppressed, a moderate increase in cloud size or rotation rate can lead to vigorous burst activity. We conclude that most (if not all) protostars undergo a burst mode of evolution during their early accretion history, as inferred empirically from observations of FU Orionis variables.

  8. 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.

  9. 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.

  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. Pine Mountain terrane, a complex window in the Georgia and Alabama Piedmont; evidence from the eastern termination

    NASA Astrophysics Data System (ADS)

    Hooper, Robert J.; Hatcher, Robert 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.

  12. 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.

  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. 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.

  15. Ice Accretions on Modern Airfoils Investigated

    NASA Technical Reports Server (NTRS)

    Addy, Harold E., Jr.

    2000-01-01

    The Icing Branch at the NASA Glenn Research Center at Lewis Field initiated and conducted the Modern Airfoils Ice Accretions project to identify ice shapes and determine their effects on the aerodynamic performance of aircraft, particularly on lift and drag. Previous aircraft ice shape and performance documentation focused on a few, older airfoils. This permitted more basic studies of the ice accretion process to be undertaken. However, having established both a working data base of ice shapes and the capability to predict these shapes for basic airfoils, questions arose about how ice might accrete differently on airfoils more representative of those being designed and flown on various aircraft today. Similarly, information about how these ice shapes would affect aerodynamic performance was needed.

  16. Galaxies in clusters: Gas stripping and accretion

    NASA Astrophysics Data System (ADS)

    Balsara, Dinshaw; Livio, Mario; O'Dea, Christopher P.

    1994-12-01

    We study the process of a galaxy moving through the intracluster gas in a cluster of galaxies, using two-dimensional hydrodynamic simulations at high resolution. We find that ram pressure stripping occurs in the form of individual events which are separated by a few times 107 yr. In addition, we find that the galaxy accretes gas from the downstream side into the core. This accretion process exhibits a radial 'pumping' mode, similar to the one found previously in simulations of wind accretion onto compact objects. The flow is found to exhibit a complex shock structure around the core. Some implications of our results for the understanding of a few recent observations are discussed.

  17. Reverberation Mapping of AGN Accretion Disks

    NASA Astrophysics Data System (ADS)

    Fausnaugh, Michael; AGN STORM Collaboration

    2017-01-01

    I will discuss new reverberation mapping results that allow us to investigate the temperature structure of AGN accretion disks. By measuring time-delays between broad-band continuum light curves, we can determine the size of the disk as a function of wavelength. I will discuss the detection of continuum lags in NGC 5548 reported by the AGN STORM project and implications for the accretion disk. I will also present evidence for continuum lags in two other AGN for which we recently measured black hole masses from continuum-Hbeta reverberations. The mass measurements allow us to compare the continuum lags to predictions from standard thin disk theory, and our results indicate that the accretion disks are larger than the simplest expectations.

  18. SPECTRAL ENERGY DISTRIBUTIONS OF ACCRETING PROTOPLANETS

    SciTech Connect

    Eisner, J. A.

    2015-04-10

    Planets are often invoked as the cause of inferred gaps or inner clearings in transition disks. These putative planets would interact with the remnant circumstellar disk, accreting gas and generating substantial luminosity. Here I explore the expected appearance of accreting protoplanets at a range of evolutionary states. I compare synthetic spectral energy distributions with the handful of claimed detections of substellar-mass companions in transition disks. While observed fluxes of candidate companions are generally compatible with accreting protoplanets, challenges remain in reconciling the extended structure inferred in observed objects with the compact emission expected from protoplanets or circumplanetary disks. I argue that a large fraction of transition disks should harbor bright protoplanets, and that more may be detected as larger telescopes open up additional parameter space.

  19. 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.

  20. Roche Potential with Luminous Accretion Disks

    NASA Astrophysics Data System (ADS)

    Fukue, Jun; Hanamoto, Keishi

    2002-12-01

    The radiative environments of an interacting binary, which contains a luminous accretion disk, are investigated. The disk radiation field has no effect in the direction of the orbital plane, while it significantly affects in the polar direction. As the disk luminosity increases, the Roche potential around the compact star becomes hollow in the polar direction. It further crashes toward the pole, and a cone of avoidance appears at the normalized luminosity Γd ≡ Ld/LE = 0.5, where Ld is the disk luminosity and LE the Eddington luminosity of the compact star. The disk corona, the wind-fed accretion flow, and the common envelope suffer a remarkable influence by the luminous disk in active binaries. Of these, the wind-fed accretion is briefly discussed.

  1. 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 Roch