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Sample records for al bayda terrane

  1. Using remote sensing techniques and field-based structural analysis to explore new gold and associated mineral sites around Al-Hajar mine, Asir terrane, Arabian Shield

    NASA Astrophysics Data System (ADS)

    Sonbul, Abdullah R.; El-Shafei, Mohamed K.; Bishta, Adel Z.

    2016-05-01

    Modern earth resource satellites provide huge amounts of digital imagery at different resolutions. These satellite imageries are considered one of the most significant sources of data for mineral exploration. Image processing techniques were applied to the exposed rocks around the Al-Aqiq area of the Asir terrane in the southern part of the Arabian Shield. The area under study has two sub-parallel N-S trending metamorphic belts of green-schist facies. The first belt is located southeast of Al-Aqiq, where the Al-Hajar Gold Mine is situated. It is essentially composed of metavolcanics and metasedimentary rocks, and it is intruded by different plutonic rocks of primarily diorite, syenite and porphyritic granite. The second belt is located northwest of Al-Aqiq, and it is composed of metavolcanics and metasedimentary rocks and is intruded by granite bodies. The current study aimed to distinguish the lithological units, detect and map the alteration zones, and extract the major fault lineaments around the Al-Hajar gold prospect. Digital satellite imageries, including Landsat 7 ETM + multispectral and panchromatic and SPOT-5 were used in addition to field verification. Areas with similar spectral signatures to the prospect were identified in the nearby metamorphic belt; it was considered as a target area and was inspected in the field. The relationships between the alteration zones, the mineral deposits and the structural elements were used to locate the ore-bearing zones in the subsurface. The metasedimentary units of the target area showed a dextral-ductile shearing top-to-the-north and the presence of dominant mineralized quartz vein-system. The area to the north of the Al-Hajar prospect showed also sub-parallel shear zones along which different types of alterations were detected. Field-based criteria such as hydrothermal breccia, jasper, iron gossans and porphyritic granite strongly indicate the presence of porphyry-type ore deposits in Al-Hajar metamorphic belt that

  2. 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.

  3. Terrane suturing, Mindoro, Philippines

    SciTech Connect

    Wynne, D.B.; McCabe, R.; Mazzullo, J.; Malicse, A.

    1985-01-01

    A middle to late Miocene suture zone (SZ) on Mindoro Island separates the older North Palawan Continental terrane (NPCT) (west) from the younger Central Philippine Arc Terrane (CPAT) (east). The SZ consists of mafic and ultramafic rocks and amphibolites thrust westward against slaty meta-sediments (NPCT). East of the SZ lies the East Mindoro Basin (EMB), separated from the SZ by the East Mindoro Fault Zone (EMFZ). Locally, topography and geology suggest normal motion on the EMFZ. However, in central Mindoro, topographic expression of the EMFZ is very diffuse and geologic map patterns are complex. Lithotectonic units and sequences are sometimes repeated and motion appears to have been multiphase. In the eastern central SZ, westerly thrust CPAT ( ) crystalline rocks are overlain by lower Pliocene shelfal limestone. This limestone contains both serpentinite pebbles and metamorphic, polcrystalline quartz grains near its base, thus constraining thrusting and terrane suturing to pre-Pliocene. 100 km NNW, at the town of Puerto Galera, the same relations are observed, although thrusting appears to have been SSW there. 100 km WNW of Puerto Galera, a northeast-dipping ophiolite on Ambil Island lies several km NE of slaty metasediments (NPCT ) on Luband Island. The authors suggest that these three ultramafic exposures represent western CPAT Basement, thrust westward against portions of the advancing NPCT.

  4. 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

  5. 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.

  6. 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.

  7. Tectonics and terranes of the Southeastern Caribbean

    SciTech Connect

    Speed, R.C. )

    1993-02-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

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

  9. Sinistral Displacement of the Pearya Terrane along the Canadian Arctic Margin

    NASA Astrophysics Data System (ADS)

    McClelland, W.; Malone, S.; von Gosen, W.; Piepjohn, K.; Läufer, A.

    2012-12-01

    Structural and geochronologic observations from the Pearya terrane, northern Ellesmere Island, provide insight into the origin of the Canadian Arctic margin. The composite terrane records amalgamation of various fragments between 475 and 460 Ma and was accreted via sinistral transpression to the northern margin of Laurentia by 425 Ma (Trettin et al., 1991). New U-Pb SHRIMP-RG (USGS/Stanford) data from igneous rocks of the Pearya terrane help refine magmatic components of the composite terrane to include Neoproterozoic arc plutons (ca 960-970 Ma), a Cambrian syenite-monzodiorite complex (Ward Hunt pluton: 539 ± 4 Ma), Ordovician arc magmatism (Ayles Fiord pluton: 475 ± 3 Ma; Cape Richards pluton: 464 ± 3 Ma) and Devonian-Carboniferous magmatism (Petersen Bay plutonic complex: 332 ± 2 Ma). Samples from the Ordovician Taconite River Formation provide unimodal peaks characteristic of arc settings with LA-ICPMS (Arizona LaserChron Center) detrital zircon U/Pb ages ranging from 440 to 480 Ma. The Pearya terrane is cut by the Pearya shear zone, a major ductile structure defined by a thick zone of sub-vertical mylonites that consistently display a sinistral sense of shear. Mylonites exposed at Cape Columbia are cross-cut by leucocratic pegmatite dikes that give a U-Pb zircon age of 453 ± 3.5 Ma. Quartz and feldspar deformation textures coupled with the orientation of a studied dike are consistent with emplacement during late sinistral deformation. Displacement on the Pearya shear zone at or prior to 450 Ma is interpreted to record translation of the Pearya terrane toward or along the north Laurentian margin, outboard of the Franklinian basin, in the early stages of the Baltica-Laurentia Caledonian collision. The Pearya shear zone merges with or is truncated by the Petersen Bay fault, a complex northwest-southeast contractional structure that marks a segment of the Ellesmerian boundary between the Pearya terrane and Franklinian basin. Major structures within the Pearya

  10. 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

  11. 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). 

  12. 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

  13. 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.

  14. Terranes and suture zones in east central Alaska

    NASA Astrophysics Data System (ADS)

    Churkin, M., Jr.; Foster, H. L.; Chapman, R. M.; Weber, F. R.

    1982-05-01

    East central Alaska, with its 17 terranes, forms a part of the mosaic of allochthonous terranes that join the North American and Siberian plates. These terranes range from continental and continental margin, such as the Tatonduk with its thick well-bedded succession of marine shelf rocks, to seamount, arc, and ocean floor terranes. The Yukon crystalline terrane, the largest described here, is a composite of at least four subterranes juxtaposed across the Tintina fault with the Tatonduk terrane, a northwestern extension of the North American plate in Alaska. Inboard of the Yukon crystalline terrane are packets of closely appressed microterranes separated from the Tatonduk and other terranes belonging to North America by major suture zones. These microterranes lie between North America and the mosaic of accretionary terranes that form the more southerly part of Alaska. The most obviously allochthonous microterranes within the suture zones are the Woodchopper Canyon, an Early Devonian basaltic seamount, and the White Mountains, an Ordovician volcanic arc terrane capped by Silurian and Devonian carbonate bank deposits. The nearest counterpart of these terranes is the Alexander terrane in southeastern Alaska. The Tintina fault of Mesozoic and Cenozoic age, like the Denali fault, primarily follows old suture zones that separate terranes. Strike slip faulting developed after collision in places where further convergence was oblique to the terrane margins. Where terranes met head-on, their leading edges lie along a multiple set of high-angle faults that outline microterranes in accretion zones.

  15. 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.

  16. 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. 

  17. 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.

  18. Late Paleozoic orogeny in Alaska's Farewell terrane

    USGS Publications Warehouse

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

    2003-01-01

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

  19. Circum-North Pacific tectonostratigraphic terrane map

    USGS Publications Warehouse

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

    1994-01-01

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

  20. 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

    The Neoproterozoic Ar Rayn terrane is exposed along the eastern margin of the Arabian shield. The terrane is bounded on the west by the Ad Dawadimi terrane across the Al Amar fault zone (AAF), and is nonconformably overlain on the east by Phanerozoic sedimentary rocks. The terrane is composed of a magmatic arc complex and syn- to post-orogenic intrusions. The layered rocks of the arc, the Al Amar group (>689 Ma to ???625 Ma), consist of tholeiitic to calc-alkaline basaltic to rhyolitic volcanic and volcaniclastic rocks with subordinate tuffaceous sedimentary rocks and carbonates, and are divided into an eastern and western sequence. Plutonic rocks of the terrane form three distinct lithogeochemical groups: (1) low-Al trondhjemite-tonalite-granodiorite (TTG) of arc affinity (632-616 Ma) in the western part of the terrane, (2) high-Al TTG/adakite of arc affinity (689-617 Ma) in the central and eastern part of the terrane, and (3) syn- to post-orogenic alkali granite (607-583 Ma). West-dipping subduction along a trench east of the terrane is inferred from high-Al TTG/adakite emplaced east of low-Al TTG. The Ar Rayn terrane contains significant resources in epithermal Au-Ag-Zn-Cu-barite, enigmatic stratiform volcanic-hosted Khnaiguiyah-type Zn-Cu-Fe-Mn, and orogenic Au vein deposits, and the potential for significant resources in Fe-oxide Cu-Au (IOCG), and porphyry Cu deposits. Khnaiguiyah-type deposits formed before or during early deformation of the Al Amar group eastern sequence. Epithermal and porphyry deposits formed proximal to volcanic centers in Al Amar group western sequence. IOCG deposits are largely structurally controlled and hosted by group-1 intrusions and Al Amar group volcanic rocks in the western part of the terrane. Orogenic gold veins are largely associated with north-striking faults, particularly in and near the AAF, and are presumably related to amalgamation of the Ar Rayn and Ad Dawadimi terranes. Geologic, structural, and metallogenic

  1. 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. 

  2. 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.

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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.

  8. Implications of SHRIMP and microstructural data on the age and kinematics of shearing in the Asir terrane, southern Arabian Shield, Saudi Arabia

    USGS Publications Warehouse

    Johnson, P.R.; Kattan, F.H.; Wooden, J.L.

    2001-01-01

    The Asir terrane consists of north-trending belts of variably metamorphosed volcanic, sedimentary, and plutonic rocks that are cut by numerous shear zones (Fig. 1). Previous workers interpreted the shear zones as sutures, structures that modify earlier sutures, or structures that define the margins of tectonic belts across which there are significant lithologic differences and along which there may have been major transposition (Frisch and Al-Shanti, 1977; Greenwood et al., 1982; Brown et al., 1989). SHRIMP data from zircons (Table 1) and sense-of-shear data recently acquired from selected shear zones in the terrane help to constrain the minimum ages and kinematics of these shearing events and lead to an overall model of terrane assembly that is more complex than previously proposed. 

  9. Second look at suspect terranes in southern Mexico

    NASA Astrophysics Data System (ADS)

    Ratschbacher, Lothar; Riller, Ulrich; Meschede, Martin; Herrmann, Uwe; Frisch, Wolfgang

    1991-12-01

    The boundary between the Xolapa and the Guerrero, Mixteca, and Juarez (or Oaxaca) terranes is a zone of normal faulting indicating north-south subhorizontal extension. Stratigraphic and geochronometric evidence dates tectonic uplift of the Xolapa terrane as Late Cretaceous and Tertiary. We propose that the Xolapa terrane represents a late Mesozoic-early Tertiary magmatic arc built near or on North American continental crust, and we discuss, as possible tectonic uplift mechanisms, (1) extension associated with back-arc rifting, (2) extension during gravitational spreading of the upper and middle crust, and (3) transtension within a strike-slip regime established during formation of the Caribbean. Both far- and near-field deformations indicate distributed transtension. Therefore, a single regional tectonic framework can account for the Mesozoic and Cenozoic geologic history of these terranes.

  10. 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.

  11. Tectonostratigraphic terranes of the frontier circum-Pacific region

    SciTech Connect

    Howell, D.G.; Jones, D.L.; Schermer, E.R.

    1983-03-01

    Many major exploration frontiers around the Pacific are in regions where complex geologic relations reflect plate-tectonic processes, crustal mobility, and accretion of exotic terranes. The destruction of the proto-Pacific ocean (Panthalassa) involved accretion of terranes to cratonal regions such as Gondwana and Laurasia. Terranes in southwestern New Zealand and eastern Antarctica were also probably accreted during the Paleozoic. The southern margin of Siberia, extending into China, underwent a protracted period of accretion from the late Precambrian through the early Mesozoic. Mid-Paleozoic accretion is reflected in the Innuitian foldbelt of the Arctic Ocean, the Black Clastic unit of the northern Rocky Mountains, and the Antler orogeny of the western US cordillera. The Mesozoic breakup of Pangaea and the acceleration of subduction aided in the rifting and dispersal of terranes from equatorial paleolatitudes. Fragments of these terranes now compose much of the continental margins of the Pacific basin, including New Zealand, Indochina, southern China, southeast Siberia, the North American cordillera, and South America. Some terranes are presently being further fragmented by post-accretionary dispersion processes such as strike-slip faulting in western North America and Japan. Although the character and distribution of terranes in the western US are fairly well documented, details are needed for other terranes around the Pacific basin. Interpretation of structure and stratigraphy at depth will be aided by more data on the timing of accretion and the nature of deformation associated with accretion and dispersion. Such data are needed for further define specific exploration targets in the circum-Pacific region.

  12. 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

  13. Seismic images of a Grenvillian terrane boundary

    USGS Publications Warehouse

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

    1992-01-01

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

  14. 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

  15. 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

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

    NASA Astrophysics Data System (ADS)

    Petrie, Meredith Blair

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

  17. 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

  18. 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.

  19. 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

  20. 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.

  1. 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 s

  2. 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.

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

    NASA Technical Reports Server (NTRS)

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

    2000-01-01

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

  4. 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.

  5. Moho Depth of the Yakutat Terrane, Southern Alaska

    NASA Astrophysics Data System (ADS)

    Christeson, G. L.; Van Avendonk, H. J.; Gulick, S. P.; Pavlis, G. L.; Hansen, R. A.

    2011-12-01

    The Yakutat terrane, a thickened oceanic plateau, is currently colliding with southern Alaska forming the Chugach-St. Elias orogen. Two-dimensional marine seismic profiles acquired during the STEEP project map the Moho of the terrane at a constant depth of 30-32 km over a distance >300 km from the Bering Glacier east to the Dangerous River Zone, and over a distance of >100 km from the Transition fault north towards Yakutat Bay. However, Moho depth of the terrane is poorly constrained to the north beneath the Chugach-St. Elias mountains. Fortunately, good Moho reflections are observed by land-based seismometers recording the marine shots of the STEEP project over source-receiver offsets up to 200 km. We will use these data to invert for Yakutat terrane Moho depth. We will first use all first-arriving energy to do a three-dimensional tomographic inversion for crustal velocity structure, and then will invert all interpreted Moho reflections for Moho depth. The results should give us new insights into the tectonic processes associated with the Chugach-St. Elias orogen.

  6. Paleomagnetic constraints on the Mesozoic drift of the Lhasa terrane (Tibet) from Gondwana to Eurasia

    NASA Astrophysics Data System (ADS)

    Li, Zhenyu; Lippert, Peter; Ding, Lin; Song, Peiping; Yue, Yahui; van Hinsbergen, Douwe

    2016-04-01

    The Mesozoic plate tectonic history of Gondwana-derived crustal blocks of the Tibetan Plateau is hotly debated, but so far, paleomagnetic constraints quantifying their paleolatitudinal drift history remain sparse. Here, we compile existing data published mainly in Chinese literature and provide a new, high-quality, well-dated paleomagnetic pole from the ˜180 Ma Sangri Group volcanics of the Lhasa terrane. Our Sangri Group pole is calculated from pre-folding characteristic remanent magnetizations carried by thermoremanent magnetizations in low-Ti titanomagnetite and titanohematite in basalts and basaltic andesites that we have dated using zircon U-Pb geochronology. Forty-two lava sites (68%) meet our quality criteria and provide an average direction of D±ΔD = 341.9±3.4° , I±ΔI = -13.3±6.5° , A95 = 3.4, K = 42.9, n=42, corresponding to a paleolatitude of ˜6° S. The A95 value falls within the n-dependent confidence envelope of Deenen et al. (2011) (A95min=2.7; A95max=7.8), indicating that the data scatter can be straightforwardly explained by paleosecular variation of the paleomagnetic field alone. In addition, positive fold tests are consistent with a pre-folding remanence acquisition. Our new pole confirms a trend in existing data of variable quality that suggests the Lhasa terrane rifted from Gondwana in Late Triassic rather than Permian time, as widely perceived. A total northward drift of ˜ 4500 km between ˜220 and ˜130 Ma yields a reasonable average paleolatitudinal plate motion rate of 5 cm/yr. Our results are consistent with both an Indian or an Australian original position of the Lhasa terrane and cannot directly discriminate between these two interpretations. Nonetheless, we show that paleomagnetic data can provide a strong constraint on Mesozoic plate kinematics of the Tethyan realm. Our study also underscores the need for new, high-quality and well-dated paleomagnetic poles from the Paleozoic and Mesozoic of the Tibetan terranes.

  7. 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.

  8. Variations of the SCLM structures and geochemical features of the peridotites in different mantle terranes beneath Siberian craton.

    NASA Astrophysics Data System (ADS)

    Ashchepkov, I. V.; Vladykin, N. V.; Kuligin, S. S.; Smelov, A. P.; Ntaflos, T.; Kostrovitsky, S. I.; Lelyukh, M. I.; Rotman, A. Ya.; Afanasiev, V. P.; Tychkov, N. S.; Malygina, E. V.; Ovchinnikov, Yu. I.; Palessky, S. V.; Nikolaeva, I. V.; Khmelnikova, O. S.; Nigmatulina, E. N.

    2012-04-01

    The kimberlite fields cross several (7) tectonic terranes compiling Siberian craton. According tectonic data (Rosen et al., 2006 ) they are locating within the Paleoproterozoic Accretion Zone which have ~1.8 ma age corresponds to the peak of the Re/Os dates (Ionov et al , 2011; Malkovets et al., 2011) and dating in other isotopic systems (Rosen et al., 2006). The lithospheric mantle beneath seven different tectonic terrains in Siberia is characterized by TRE geochemistry and major elements of peridotitic clinopyroxenes. The mantle in Magan terrain contains more fertile peridotite in the South (Mir pipe) then in North (Alakite) which are metasomatized by subduction-related (LILE Sr-enriched) melts producing Phl and Cpx about 500-800 Ma ago. Daldyn terrain is essentially harburgitic in the west (abyssal peridotite ) but in the east is more differentiated to fertile and depleted varieties and more oxidized in Upper Muna (East Daldyn terrain). The Markha terrain (Nakyn) contains depleted but partly refertilized harzburgites, subducted pelitic material and abundant eclogites. Circum-Anabar mantle is ultradepleted in the lower part but in the upper regions it has been fertilized by fluid-rich melts very enriched in incompatible elements. The SCLM in Magan terrane near Mir pipe contains in upper part fertile and hydrous metasomatic peridotites and eclogite lens in middle part (40 kbas). More depleted lens starts from 50 kbar beneath Mir pipe. But SCLM of Internationalnaya pipe show large amount of eclogites and hybrid peridotite material at the same depth. In SCLM beneath Nakyn the more continuous thick peridotite sections contain abundant various eclogites and Garnet bearing Fe- rich micaceous rocks (metapellites in protolith) (Spetcius et al., 2004 ). The general granulite-gneiss siliceous character of terrane coincides with the rather fertile mantle type. In the Markha terrane the SCLM is essentially metasomatic and contains essentially depleted lenses near Aykhal and

  9. 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.

  10. Continuation, south of Oaxaca City (southern Mexico) of the Oaxaca-Juarez terrane boundary and of the Oaxaca Fault. Based in MT, gravity and magnetic studies

    NASA Astrophysics Data System (ADS)

    Campos-Enriquez, J. O.; Corbo, F.; Arzate-Flores, J.; Belmonte-Jimenez, S.; Arango-Galván, C.

    2010-12-01

    The Oaxaca Fault represents Tertiary extensional reactivation of the Juarez shear zone constituting the boundary-suture between the Oaxaca and Juarez terranes (southern Mexico). South of Oaxaca City, the fault trace disappears and there are not clear evidences for its southward continuation at depth. The crust in southern México has been studied through seismic refraction, and seismological and magnetotelluric (MT) studies. The refraction studies did not image the Oaxaca Fault. However, previous regional MT studies suggest that the Oaxaca-Juarez terrane boundary lies to the east of the Zaachila and Mitla sub-basins, which implies sinistral displacement along the Donaji Fault. Campos-Enriquez et al. (2009) established the shallow structure of the Oaxaca-Juarez terrane boundary based in detailed gravity and magnetic studies. This study enabled: 1) to establish the shallow structure of the composite depression comprising three N-S sub-basins: the northern Etla and southern Zaachila sub-basins separated by the Atzompa sub-basin. According to the Oaxaca-Juarez terrane boundary is displaced sinistrally ca. 20 km along the E-W Donají Fault, which defines the northern boundary of the Zaachila sub-basin. At the same time,, the Oaxaca Fault may either continue unbroken southwards along the western margin of a horst in the Zaachila sub-basin or be offset along with the terrane boundary. This model implies that originally the suture was continuous south of the Donaji Fault. A constraint for the accreation of the Oaxaca and Juarez terranes. Thirty MT soundings were done in the area of the Central Valleys, Oaxaca City (southern Mexico). In particular we wanted to image the possible southward continuation of the Oaxaca Fault. 22 Mt sounding are located along two NE-SW profiles to the northern and to the south of the City of Oaxaca. To the north of Oaxaca City, the electrical resistivity distribution obtained show a clear discontinuity across the superficial trace of the Oaxaca

  11. 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.

  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

    The poorly known, suspect, Uchee terrane occupies a critical tectonic position with regard to how and when peri-Gondwanan (Carolina) and Gondwanan (Suwannee) terranes were sutured to Laurentia. It lies sandwiched between Laurentian(?) continental basement exposed in the Pine Mountain window and adjacent buried Gondwanan crust of the Suwannee terrane. The Uchee terrane has been proposed as both a septum of Piedmont rocks that once was continuous across the erosionally breached Pine Mountain window or part of the Carolina zone. To help resolve this issue, we conducted U-Pb (SHRIMP-RG) (sensitive high-resolution ion microprobe-reverse geometry) zircon studies and whole-rock isotopic analyses of principal metasedimentary and metaplutonic units. U-Pb ages for zircons from the Phenix City Gneiss suggest igneous crystallization at ca. 620 Ma, inheritance ca. 1000 to ca. 1700 Ma, and a ca. 300 Ma (Alleghanian) overprint recorded by zircon rims. Zircons from the metasedimentary/metavolcaniclastic Moffits Mill Schist yield bimodal dates at ca. 620 and 640 Ma. The 620 to 640 Ma dates make these rocks age-equivalent to the oldest parts of the Carolina slate belt (Virgilina and Savannah River) and strongly suggest a Gondwanan (Pan-African and/or Trans-Brasiliano) origin for the Uchee terrane. Alternatively, the Uchee terrane may be correlative with metamorphic basement of the Suwannee terrane. The ca. 300 Ma overgrowths on zircons are compatible with previously reported 295 to 288 Ma 40Ar/39Ar hornblende dates on Uchee terrane rocks, which were interpreted to indicate deep tectonic burial of the Uchee terrane contemporaneous with the Alleghanian orogeny recorded in the foreland. Temperature-time paths for the Uchee terrane are similar to that of the Pine Mountain terrane, indicating a minimum age of ca. 295 Ma for docking. In terms of tectono-metamorphic history of the Uchee terrane, it is important to note that no evidence for intermediate "Appalachian" dates (e.g, Acadian or

  13. Deep Structure and Past Kinematics of Accreted Terranes

    NASA Astrophysics Data System (ADS)

    Hillhouse, John W.

    The concept of "accreted terranes," that continents grow by the addition of displaced fragments of crust, was the central topic of a recent multidisciplinary symposium sponsored by the International Union of Geodesy and Geophysics (IUGG). This volume contains a selection of papers presented in Symposium 12 at XIX General Assembly of the IUGG, held August 15-18, 1987, in Vancouver. Convenors Edward Irving and David Stone focused the program on two major themes. The first was to track the displacement of terranes using paleomagnetism, geologic mapping, and paleontology. The second theme was to explore the deep structure of accreted terranes using seismological, geochemical, and potential-field methods. Given the scope of those topics, it is no surprise that the participants represented many specialties within the field of solid-earth geophysics. The program included case histories from a collection of fold belts spanning much time and long distances, from the Proterozoic sutures of North America to the Cenozoic accretionary complexes of the Pacific rim.

  14. Terrane Definition From Textural Measures of Aeromagnetic Data

    NASA Astrophysics Data System (ADS)

    Gettings, M.

    2007-12-01

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

  15. 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

    2014-05-01

    The Central Asian Orogenic Belt (CAOB) is a typical accretionary orogen divided into numerous lithostratigraphic terranes corresponding to magmatic arcs, back arcs, continental basement blocks, accretionary wedges and metamorphic blocks. These terranes should be in theory characterized by contrasting magnetic and gravity signatures thanks to their different petrophysical properties. To test this hypothesis, the stratigraphically defined terranes in southern Mongolia were compared with potential field data to constrain their boundaries and extent. The existence of terranes in southern Mongolia cannot be attested by the uniform geophysical fabrics due to the lack of systematic correspondence between the high/low amplitude and high/low frequency geophysical domains and major terranes. Processed magnetic and gravity grids show that both gravity and magnetic lineaments are E-W trending in the west and correlate with direction of some geological units. In the east, both magnetic and gravity lineaments are disrupted by NE-SW trending heterogeneities resulting in complete blurring of the geophysical pattern. Correlation of magnetic signal with geological map shows that the magnetic highs coincide with late Carboniferous-early Permian volcanic and plutonic belts. The matched-filtering shows good continuity of signal to the depth located along the boundaries of these high magnetic anomalies which may imply presence of deeply rooted tectono-magmatic zones. The axes of high density bodies in the western and central part of the studied CAOB are characterized by periodic alternations of NW-SE trending high frequency and high amplitude gravity anomalies corresponding to late Permian to Triassic cleavage fronts up to 20 km wide. The matched-filtering analysis shows that the largest deformation zones are deeply rooted down to 20 km depth. Such a gravity signal is explained by the verticalization of high density mantle and lower crustal rocks due to localized vertical shearing

  16. 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.

  17. 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

  18. Polycrystalline Diamonds from the Erzgebirge Ultrahigh-Pressure Metamorphic Terrane, Germany

    NASA Astrophysics Data System (ADS)

    Dobrzhinetskaya, L.; Wirth, R.; Green, H. W.

    2010-12-01

    Previous studies showed that microdiamonds from Erzgebirge terrane of Germany are crystallized from a C-O-H fluid (Stoeckhert et al., 2001, 2009; Dobrzhinetskaya et al., 2003, 2007) due to course of the UHPM. Usually metamorphic diamonds are presented by single crystals of 5 to 80 micron size, which are caracterized by a complicated morphology suggesting that their crystallization took place in a media rich in impurities. Within the microdiamonds population ocurred in the Erzgebirge quartz-feldspathic gneisses, we have recently found polycrystalline diamonds which extend our knowledge related to mechanisms of their formation. These polycrystalline diamonds occur as inclusions in zircons. Several focused ion beam foils were prepared from polished slide containing zircon with diamond inclusions, and studied with transmission electron microscopy. Bright Field images revealed that single diamond inclusions in zircon consist of 5 to 15 microcrystals which are characterized by ’zig-zag’ boundaries. The series of triangle fluid-pockets are situated at the interface diamond-zircon, and ’glue’ together microcrystals. The octahedral voids accountered by (111) crystal faces are observed within polycrystalline sectors of diamond. The octahedral voids are characterized by low density contrast what suggests that the void is a negative nanocrystal of diamond filled by fluid/gas. In many cases the fluid was evaporated during the foil preparation by high energy Ga-ions beam. The fluid consisted of Ti, Cl, S, K, Cr, Ba, Pb, Mo, Co, Al. The presence of the negative crystals of diamonds filled with a fluid, suggest that such a fluid was in equilibrium with the diamond, and represents the diamond-forming media. Triangle pockets of the former fluid situated at the zircon-diamond interface are also penetrated by FIB, the residual fluid composition is characterized by presence of Al, Ti, Ca, F, V, Zn, Si, Cl, and S, or Ca, Al, K, Cl, Fe and Mg, or Al, Co, F, V, Zn, Si, Cl, or

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

    SciTech Connect

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

    1993-04-01

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

  20. 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.

  1. 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.

  2. 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.

  3. 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

  4. Eocene high-Mg adakitic rocks of the northern Qiangtang terrane, central Tibet: possible delamination of lower crust and early uplift in the central plateau?

    NASA Astrophysics Data System (ADS)

    Xu, J.; Chen, J.; Zhao, W.; Dong, Y.; Wang, B.; Kang, Z.

    2011-12-01

    Details of the timing and mechanisms of uplift of the Tibetan Plateau have long been debated, such as whether the earliest uplift occurred at ~45 Ma in northern and northeastern Tibet (Turner et al. 1996; Chung et al. 1998; Guo et al. 2006; Wang et al. 2008), or if the whole plateau was instead formed by one stage or more stages of uplift (Turner et al. 1996; Williams et al. 2001; Tapponnier et al. 2001; Chen et al., 2009). It is generally believed that changes at the surface and in tectonic regime are typically marked by changes in the composition of associated magmatic products. Eocene high-Mg adakitic rocks (45-36 Ma) from the Qiangtang terrane provide important insights into the possible delamination of lower crust and/or underlying lithospheric mantle with a thickened keel, as well as early uplift processes of the central Tibetan plateau. We report new geochemical data for the Luanqinshan, Yuejinla and Dongyuehu Eocene lavas from the northern Qiangtang terrane. Analysis of a Meiriqiecuo lava yielded a 40Ar/39Ar plateau age of 43.35 ± 0.22 Ma, this result together with previously published ages indicate that the Eocene lavas in the northern Qiangtang terrane were mainly erupted from 45 to 36 Ma. The lava from the northern Qiangtang terrane have high Al2O3 and Sr contents, as well as high Sr/Y, La/Yb, and Mg# (43-69), but low Y and Yb concentrations. The same samples have low 87Sr/86Sr(t)(0.7062-0.7075) and low ɛNd(t) (-6.3 to -2.9). These geochemical features, combined with other characteristics of Tibetan Cenozoic lavas, indicate that they did not result from slab melting of a young oceanic crust, assimilation and fractional crystallization (AFC) of a mafic magma, or from either underplated or thickened continental crust. Instead, they were derived from partial melting of delaminated lower continental crust, which subsequently reacted with surrounding mantle peridotites during ascent to crustal depths. The high-Mg and low-Mg adakitic rocks (45-36 Ma), north

  5. 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.

  6. 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.

  7. Reconstructing the protracted P-T-t-d path of a giant ultrahigh-pressure terrane: Linking in-situ techniques with multiple methods of conventional geochronology (Invited)

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

    TIMS analyses of titanites found in in-situ melts throughout the WGR range from ~398 to 390 Ma indicate that melting continued for ~10 Myr. Smaller titanites from gneisses indicate cooling through ~600°C by ~385 Ma in the deepest portions of the terrane. Cooling through ~400°C was achieved by ~380 Ma (Root et al., 2005). LA-ICPMS allows for rapid analyses of multiple samples over a broad region of the WGR to constrain the areas which underwent Caledonian deformation. Zircons from more than 10 undeformed dikes and titanites from ca. 50 gneiss samples show the absence of deformation and recrystallization in large portions of the hinterland and only small pockets of deformation in the largely undeformed foreland. The aggregate data reveal that large UHP terranes can spend more than 40 Myr at depth and that much of the WGR resisted deformation/recrystallization. A protracted genesis for large UHP terranes is supported by studies of other large terranes (e.g., Dabie-Sulu), and emphasizes the necessity for meaningful interpretations of ages individual samples obtained from such terranes.

  8. 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.

  9. Ocmulgee fault: The Piedmont-Avalon terrane boundary in central Georgia

    SciTech Connect

    Hooper, R.J. ); Hatcher, R.D. Jr. )

    1990-08-01

    The Ocmulgee fault is a fundamental boundary within the Piedmont of central Georgia separating North American, Piedmont terrane rocks from exotic rocks of the Avalon (Carolina) terrane. Contrasts in stratigraphy, metamorphic grade, and aeromagnetic signature coincide with the fault trace. The fault comprises a broad zone (ca. 2 km) of foliation dipping steeply southeast and containing localized syn- and post-thermal-peak right-lateral strike-slip shear zones. Oblique dextral convergence between the Avalon and Peidmont terranes, most likely around 350 Ma, produced the steep zone and its attendant dextral shear zones. Steep foliation within the fault zone resulted in its predisposition to later faulting.

  10. Distinct Thermal and Metasomatic Characteristics of Mantle Lithosphere Beneath Two Proterozoic Terranes Bordering the Kaapvaal Craton of Southern Africa

    NASA Astrophysics Data System (ADS)

    Janney, P. E.; Shiimi, E. T.

    2015-12-01

    There is a first order contrast in compositional and thermal properties between cold and infertile Archean cratonic mantle and younger, warmer and more fertile Proterozoic lithosphere, but it has also become apparent that coherent thermal and compositional differences exist between adjacent Proterozoic terranes, even in regions that have been stable for over 1 Ga. We report new thermobarometry and in-situ trace element data for garnet peridotite xenoliths from several late Cretaceous (100-70 Ma) kimberlite localities in the western Namaqua-Natal Belt (NNB) and Rehoboth Province (RP), which bound the Archean Kaapvaal craton to the west and south, respectively. The localities include some for which no data have been reported previously. Re-depletion model ages from Os isotopes indicate that the lithosphere beneath the NNB and RP is mainly Early Proterozoic (Pearson et al., Chem. Geol., 2004; Janney et al. J. Petrol., 2010) and there is no evidence from xenolith modal proportions for significant differences in average fertility between lithospheric terranes. Equilibration pressures for garnet peridotites from both terranes fall in a similar range (2 to 5 GPa). However, peridotites from the RP typically have P and T values that fall on or very close to the Kaapvaal cratonic geotherm (apart from a group of peridotites from the Gibeon kimberlites with pressures > 4 GPa that follow an adiabatic gradient; e.g. Franz et al., J. Geol., 1996) whereas peridotites from the western NNB have temperatures roughly 100°C warmer than the cratonic geotherm over the whole depth range. Peridotites from the 140 Ma Melton Wold kimberlite, also in the western NNB, lack these warmer temperatures and suggest that warming was contemporaneous with Late Cretaceous kimberlite magmatism. Metasomatic enrichment in incompatible elements (consistent with interaction with kimberlitic melts) is more pronounced in NNB as compared to RP peridotites. The association of higher temperatures with a greater

  11. 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.

  12. Boninitic magmatism in a continental margin setting, Yukon- Tanana terrane, southeastern Yukon, Canada

    NASA Astrophysics Data System (ADS)

    Piercey, Stephen J.; Murphy, Donald C.; Mortensen, James K.; Paradis, Suzanne

    2001-08-01

    Mid-Paleozoic mafic rocks in the Finlayson Lake region of the Yukon-Tanana terrane, southeastern Yukon, Canada, have the diagnostic geochemical signatures of boninites: high MgO, Cr, Ni, and Co contents, intermediate SiO2 contents, high Mg#'s (MgO/ (MgO+FeO*), Al2O3/TiO2, and Zr(Hf)/middle rare earth element (REE) ratios; low TiO2, REE, and high-field-strength element contents; and U-shaped primitive mantle normalized trace element patterns. However, unlike most modern and ancient boninitic rocks that are typically associated with intraoceanic realms, those from the Finlayson Lake region are part of a mid-Paleozoic continental margin arc-backarc magmatic system. We propose a model in which the boninitic rocks from the Finlayson Lake region formed as a result of spreading ridge propagation into an arc built on composite basement of oceanic and continental crust. In the oceanic segment, upwelling asthenosphere induced melting of a subducted-slab metasomatized refractory mantle source to form boninitic magmatism. In the continental sector, upwelling asthenospheric mantle, and/or the melts derived thereof, induced crustal melting, which explains the large volume of temporally equivalent felsic volcanic and intrusive rocks.

  13. SHRIMP U-Pb dating of zircon from the Xugou UHP eclogite, Sulu terrane, eastern China

    USGS Publications Warehouse

    Zhao, R.; Liou, J.G.; Zhang, R.Y.; Wooden, J.L.

    2005-01-01

    Eclogites, together with garnet clinopyroxenites, occur as lenses within the Xugou garnet peridotite body in the southern Sulu ultrahigh-pressure (UHP) terrane. Combined cathodoluminescence (CL) imaging and SHRIMP U-Pb dating of zircon from two Xugou mafic eclogites provide added constraints on the timing of UHP metamorphism in this area. Zircons from both samples show subrounded to rounded shapes and patchy CL patterns without inherited igneous cores, indicating that they are metamorphic zircons. SHRIMP U-Pb analyses of these zircons yielded apparent U-Pb ages of 214-280 Ma, with a weighted mean age of 237 ?? 8 Ma, which is consistent with previous reported UHP metamorphic ages from eclogite pods and country-rock gneisses. The Xugou mafic lenses may have formed by partial melting of the enclosing peridotites in the mantle before subduction (Zhang et al., 2003); then these eclogites, together with the host peridotites, were tectonically emplaced into the subduction zone and subjected to UHP metamorphism at 237 ?? 8 Ma. Copyright ?? 2005 by V. H. Winston & Son, Inc. All rights reserved.

  14. Zircon U-Pb and Lu-Hf isotopic and geochemical constraints on the origin of the paragneisses from the Jiaobei terrane, North China Craton

    NASA Astrophysics Data System (ADS)

    Shan, Houxiang; Zhai, Mingguo; Zhu, Xiyan; Santosh, M.; Hong, Tao; Ge, Songsheng

    2016-01-01

    Clastic sedimentary rocks are important tracers to understand the evolution of the continental crust. Whole-rock major and trace element data, zircon U-Pb dating and Hf isotopic data for the paragneisses from the Jiaobei terrane are presented in this study in order to constrain their protoliths, provenance and tectonic setting. The paragneisses are characterized by enrichment in Al2O3 and TiO2, negative DF (DF = 10.44 - 0.21SiO2 - 0.32Fe2O3T - 0.98MgO + 0.55CaO + 1.46Na2O + 0.54K2O) values and the presence of aluminum-rich metamorphic minerals (e.g., garnet and sillimanite). Together with the mineral assemblages and zircon features, it can be inferred that the protoliths of these rocks are of sedimentary origin. The K-A (A = Al2O3/(Al2O3 + CaO + Na2O + K2O), K = K2O/(Na2O + K2O)) and log(Fe2O3/K2O)-log(SiO2/Al2O3) diagrams indicate that they belong principally to clay-silty rocks with some contributions from graywacke. A series of geochemical indexes, such as the widely employed CIA (CIA = [Al2O3/(Al2O3 + CaO∗ + Na2O + K2O)] × 100; molar proportions) and ICV (ICV = (Fe2O3 + MnO + MgO + CaO + Na2O + K2O + TiO2)/Al2O3) values, and the A-CN-K diagram for the paragneisses indicate relatively weak weathering in the source rocks and negligible post-depositional K-metasomatism. In addition, their REE patterns, low Cr/Zr (0.61-1.99), high Zr/Y (4.81-23.59) and Th/U (3.21-40.67) ratios, the low to moderate contents of Cr (197-362 ppm) and Ni (6.68-233 ppm), and source rock discrimination diagrams collectively suggest that the sediments of the protoliths of the paragneisses in the Jiaobei terrane were derived from the source with intermediate-acidic composition, probably granitic-to-tonalitic rocks. In combination with geochronological and isotopic studies on the paragneisses and the basement rocks in the Jiaobei terrane, it is suggested that the Archean-early Paleoproterozoic granitic rocks in the Jiaobei terrane possibly provided the most important source materials. In

  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. 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.

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

    SciTech Connect

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

    1986-12-01

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

  18. 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.

  19. Tectono-stratigraphic terranes in Cape Breton Island, Nova Scotia: Implications for the configuration of the northern Appalachian orogen

    SciTech Connect

    Barr, S.M.; Raeside, R.P. )

    1989-09-01

    Cape Breton Island is divided into four terranes on the basis of contrasts in pre-Carboniferous geology. The Blair River Complex in the north is an exposure of North American Grenvillian basement, analogous to the Humber zone basement in Newfoundland. Ordovician to Devonian metavolcanic, metasedimentary, gneissic, and granitic rocks of the Aspy terrane are correlative with parts of the Gander terrane of Newfoundland and New Brunswick. The Bras d'Or terrane, characterized by low-pressure gneisses, a carbonate-clastic platform sequence, and later Precambrian-Early Cambrian plutons, may be correlative with units previously included in the Gander terrane in southern Newfoundland and the Avalon terrane in southern New Brunswick. The Mira terrane in southeastern Cape Breton Island, including late Precambrian to Early Cambrian volcanic and sedimentary sequences and fossiliferous Cambrian-Ordovician units, is clearly part of the Avalon terrane. Therefore, with the exception of the Dunnage terrane, which is not represented in Cape Breton Island, the terranes of Newfoundland continue through Cape Breton Island. They are offset to the northwest to the mainland part of the Appalachian orogen in New Brunswick.

  20. Paleozoic terranes of eastern Australia and the drift history of Gondwana

    NASA Astrophysics Data System (ADS)

    McElhinny, Michael W.; Powell, Chris McA.; Pisarevsky, Sergei A.

    2003-02-01

    Critical assessment of Paleozoic paleomagnetic results from Australia shows that paleopoles from locations on the main craton and in the various terranes of the Tasman Fold Belt of eastern Australia follow the same path since 400 Ma for the Lachlan and Thomson superterranes, but not until 250 Ma or younger for the New England superterrane. Most of the paleopoles from the Tasman Fold Belt are derived from the Lolworth-Ravenswood terrane of the Thomson superterrane and the Molong-Monaro terrane of the Lachlan superterrane. Consideration of the paleomagnetic data and geological constraints suggests that these terranes were amalgamated with cratonic Australia by the late Early Devonian. The Lolworth-Ravenswood terrane is interpreted to have undergone a 90° clockwise rotation between 425 and 380 Ma. Although the Tamworth terrane of the western New England superterrane is thought to have amalgamated with the Lachlan superterrane by the Late Carboniferous, geological syntheses suggest that movements between these regions may have persisted until the Middle Triassic. This view is supported by the available paleomagnetic data. With these constraints, an apparent polar wander path for Gondwana during the Paleozoic has been constructed after review of the Gondwana paleomagnetic data. The drift history of Gondwana with respect to Laurentia and Baltica during the Paleozoic is shown in a series of paleogeographic maps.

  1. 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

  2. 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.

  3. 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.

  4. Paleomagnetic data from Upper Cretaceous Red Beds, Northwest Vietnam (Song Da Terrane), and Their Bearing on the Extrusion History of Indochina and Deformation Along its Margins

    NASA Astrophysics Data System (ADS)

    Geissman, J. W.; Pho, N.; Burchfiel, B.; Muggleton, S. R.

    2008-12-01

    demagnetization, with full remanence unblocking by about 685°C, and yield characteristic magnetization directions of north-northeast to northeast declination and moderate positive inclination (about 30 to 35°). Our preliminary results are comparable to those of Takemoto et al. (2005, EPSL, 229, 273- 285) and we tentatively conclude that there has been no significant latitudinal translation of the Song Da terrane, since the Early Cretaceous, with respect to the South China Block. We continue to explore the possibility of local scale, vertical axis rotation of parts of the Song Da terrane. Extrusion of the Indochina Block, in association with its own style of internal deformation, appears to have been facilitated by displacement along structures west of the Song Ma fault.

  5. Evolution of the Asir terrane and Phanerozoic rifting in southwestern Saudi Arabia

    SciTech Connect

    Fairer, G.M.

    1985-01-01

    The Asir terrane, defined by D.B. Stoeser and V.E. Camp, in southwestern Saudi Arabia was formed from about 1200 Ma to 560 Ma by the coalescence of the Baish ensimatic island-arch complex and the Andean-type Tarib arc complex. In this period, the Afif and Ar Rayn exotic terranes impinged from the east and were accreted to the Asir terrane along a west-directed subduction zone to form the Nabitah suture. Mapping on the eastern side of the Asir terrane by the author shows that the Ablah sedimentary basin was compressed and tectonically incorporated into west-directed thrust faults as the Asir terrane was accreted to the African craton. A saprolite mantle formed over the Asir terrane during a late-Precambrian-early Paleozoic episode of chemical weathering. Later, the terrane gradually subsided and was covered by Paleozoic seas in which the Wajid Sandstone and the Kholan and Amran Limestones were deposited. Initiation of a thermal cell at the Afar triple junction during the latest Cretaceous or early Tertiary caused crustal attenuation and sagging into a proto-Red Sea rift. Rifting and uplift of the Arabian Shield by as much as 2000 meters since the middle Miocene are indicated by deposition of the Asir group volcaniclastic and clastic rocks, emplacement of shallow, layered-intrusive rocks and basaltic dikes, and displacement of the Wajid Sandstone. Proterozoic thrust faults were reactivated as normal and strike-slip faults by Miocene rifting, and sandstone dikes were injected downward into the faults. Quaternary volcanism, stream incision, and historic earthquake activity are evidence of continued uplift.

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

    USGS Publications Warehouse

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

    1991-01-01

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

  7. 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.

  8. 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.

  9. The western transverse ranges microplate as a native terrane

    SciTech Connect

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

    1994-04-01

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

  10. 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.

  11. Contrasting evolution of low-P rare metal granites from two different terranes in the Hoggar area, Algeria

    NASA Astrophysics Data System (ADS)

    Kesraoui, Mokrane; Nedjari, Samia

    2002-05-01

    Two mineralogically different rare metal granites located in two distinct terranes from the Tuareg area are compared: the Tin-Amzi granite in the north of the Laouni Terrane and the Ebelekan granite in the Assodé-Issalane Terrane. The Tin-Amzi granite is enclosed within Eburnean granulitic gneisses, and consists of albite, quartz, protolithionite, K-feldspar and topaz granite (PG). The accessory minerals include columbite tantalite, U- and Hf-rich zircon, Th-uraninite, wolframoixiolite and wolframite. This facies is characterised by a mineralogical evolution from the bottom to the top underlined by a strong resorption of K-feldspar and albite and the crystalliK-feldspar of more abundant topaz and protolithionite II which is further altered in muscovite and Mn-siderite. It is underlain by an albite, K-feldspar, F-rich topaz, quartz and muscovite granite (MG), with W-Nb-Ta oxides, wolframite, Nb-rutile, zircon and scarce uranothorite as accessories. The Ebelekan granite intrudes into a coarse-grained biotite granite enclosed within upper amphibolite-facies metasediments. It comprises a zinnwaldite, albite, topaz porphyritic granite (ZG) with "snow ball" quartz and K-feldspar. The accessories are zircon, monazite, uranothorite, Ta bearing cassiterite, columbite tantalite and wodginite. It is capped by a banded aplite-pegmatite (AP). The geochemistry of Tin-Amzi and Ebelekan granites is nearly comparable. Both are peraluminous (A/CNK=1.10-1.29; ASI=1.17-1.31), sodolithic and fluorine rich with high SiO 2, Al 2O 3, Na 2O+K 2O, Rb, Ga, Li, Ta, Nb, Sn and low FeO, MgO, TiO 2, Ba, Sr, Y, Zr and REE contents. These rare metal Ta bearing granites belong to the P-poor subclass, relating to their P 2O 5 content ( 0.03-0.15 wt.%). Nevertheless, they are distinguished by their concentration of W, Sn and Ta. The Tin-Amzi granite is W-Ta bearing with high W/Sn ratio whereas the Ebelekan granite is Ta-Sn bearing with insignificant W content. At Tin-Amzi the W-Nb-Ta minerals define

  12. 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.

  13. 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.

  14. 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.

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

    USGS Publications Warehouse

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

    1999-01-01

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

  16. Thermal history of the Pan-African basement under the Jurassic Marib-Shabwa Basin, Yemen

    NASA Astrophysics Data System (ADS)

    Rice, A. Hugh N.; Schneider, David; Veeningen, Resi; Grasemann, Bernhard; Decker, Kurt

    2013-04-01

    Pan-African tectonism within the Arabian Nubian Shield in Yemen is very poorly known. New drill-cores from the Marib-Shabwa Basin (Habban oil field) from central Yemen penetrated 600 m into the pre-Jurassic crystalline basement, providing a unique opportunity to extend our understanding of Pan-African events in Yemen. The cores were obtained some 80 km NE of the exposure limit of the Al Bayda Terrane, which lies SE of Sana'a. This terrane, which has no direct correlative in the ANS further north in Saudi Arabia, comprises deformed greenschist facies acid to basic volcanic rocks later witnessing acid to basic magmatism and has been previously interpreted as a Pan-African island arc complex with a basement component. Ophiolite fragments are common, both within the terrane and at its margins (sutures). To the north lies the Abas Gneiss Terrane and to the south the Al Mahfid Gneiss Terrane; both consist of older pre-Pan-African crystalline basement rocks. Geochemistry of a red, undeformed granite from the drill core indicates an A-type composition. LA-ICPMS U-Pb analysis of granite zircons gave two concordant age populations: 628.3 ± 3.1 Ma (large & small zircons) and 604.9 ± 2.0 Ma (intermediate sized zircons). The former age is interpreted as the time of crystallization, within the range of other A-type Younger Granites in the ANS, and the latter age as constraining lower temperature dissolution-reprecipitation of zircon, due to hydrothermal fluids or melt remobilization. Nd Tdm model ages for two granite samples from the drill core both gave ages of 1.24 Ga, within the range of the Al Bayda Terrane (1.2-2.5 Ga) and outside the range of the adjacent Palaeoproterozoic gneissic terranes (1.7-2.3 Ga, Abas Gneiss Terrane; 1.8-3.0 Ga, Al Mahfid Gneiss Terrane). Thus it seems certain that the Al Bayda Terrane extends at least 80 km to the NE of its present surface exposure. Rb-Sr biotite ages from the granite indicate closure through ~300°C at 593 Ma, indicating fast

  17. 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.

  18. 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.

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

    PubMed

    Bohlen, S R; Mezger, K

    1989-04-21

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

  20. Assessment of Paleozoic terrane accretion along the southern central Andes using detrital zircon geochronology

    NASA Astrophysics Data System (ADS)

    McKenzie, R.; Horton, B. K.; Fuentes, F.; Fosdick, J. C.; Capaldi, T.; Stockli, D. F.; Alvarado, P. M.

    2015-12-01

    Two distinct Paleozoic terranes known as Cuyania and Chilenia occupy the southern central Andes of Argentina and Chile. Because the proposed terrane boundaries coincide with major structural elements of the modern Andean system at 30-36°S, it is important to understand their origins and potential role in guiding later Andean deformation. The Cuyania terrane of western Argentina encompasses the Precordillera (PC) and a thick-skinned thrust block of the western Sierras Pampeanas, persisting southward to the San Rafael Basin (SRB). Although recently challenged, Cuyania has been long considered a piece of southern Laurentia that rifted away during the early Cambrian and collided with the Argentine margin during the Ordovician. Chilenia is situated west of Cuyania and includes the Frontal Cordillera (FC) and Andean magmatic arc. This less-studied terrane was potentially accreted during an enigmatic Devonian orogenic event. We present new detrital zircon U-Pb age data from siliciclastic sedimentary rocks that span the entire Paleozoic to Triassic from the FC, PC, and SRB. Cambrian rocks of the PC exhibit similar zircon age distributions with prominent ~1.4 and subordinate ~1.1 Ga populations, which are distinct from other Paleozoic strata. Plutonic rocks with these ages are common in southern Laurentia, whereas ~1.4 Ga zircons are uncommon in South American age distributions. This supports a Laurentian origin for Cuyania in isolation from Argentina during the Cambrian. Upper Paleozoic strata from the PC, FC, and SRB all yield similar age data suggesting shared provenance across the proposed Cuyania-Chilenia suture. Age distributions also notably lack Devonian-age grains. The regional paucity of Devonian plutonic rocks and detrital zircon casts doubt on a possible arc system between these terranes at this time, a key requisite for the mid-Paleozoic transfer and accretion of Chilenia to the Argentine margin. Collectively, these data question the precise boundaries of the

  1. Pb-Nd Isotopes Indicate the Origin of Island Arc Terranes in the Early Paleozoic Pacific.

    PubMed

    Münker

    2000-03-01

    The Takaka Terrane in New Zealand is one of the best exposed arc fragments of the early Paleozoic Australian-Antarctic convergent margin and constitutes one of the most outboard terranes of this margin in paleogeographic reconstructions. Pb-Nd isotope compositions of clinopyroxenes from the Cambrian Devil River Volcanics of the Takaka Terrane enable identification of the location of the terrane in the Paleo-Pacific Ocean. The Devil River Volcanics, a suite of primitive arc and back-arc rocks, are interbedded with the partly continent-derived Haupiri Group sediments. Extremely radiogenic Pb and unradiogenic Nd compositions in the arc rocks cannot be explained by assimilation of the Haupiri Group sediments or a continental basement of such a composition. Pb isotope compositions of the Takaka Terrane sediments are much less radiogenic and overlap with crustal compositions of the Lachlan Fold Belt in Australia, suggesting that both units are derived from one source, the Australian-Antarctic Pacific margin. Pb-Nd isotope compositions in the Devil River Volcanics reflect contamination of their mantle sources by subducted sediments derived from Archean provinces in either Antarctica or Laurentia. Both provinces show characteristically high 207Pb/204Pb500 and were located at the Pacific rim in the Cambrian. Mixing between mantle and Proterozoic continental material from present western South America or eastern Laurentia cannot explain the high 207Pb/204Pb500 in the New Zealand rocks. As in New Zealand, extreme spreads in Pb-Nd isotope compositions in other Cambrian volcano-sedimentary sequences in southeast Australia and Tasmania can be explained by the same model, suggesting that all these fragments originated along the Australian-Antarctic Gondwana margin. Pb isotope compositions of arc rocks, therefore, provide a new tool for terrane analysis in the early Paleozoic Pacific ocean.

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

    NASA Astrophysics Data System (ADS)

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

    2016-07-01

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

  3. 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.

  4. Magmatism of the Shuteen Complex and Carboniferous subduction of the Gurvansaikhan terrane, South Mongolia

    NASA Astrophysics Data System (ADS)

    Batkhishig, Bayaraa; Noriyoshi, Tsuchiya; Greg, Bignall

    2010-03-01

    The Carboniferous Shuteen Complex, a volcano-plutonic ring complex associated with Cu-Au porphyry mineralization, is located in the Gurvansaikhan island arc terrane of South Mongolia. This paper presents new data on the petrography, major and trace element chemistry, and Sr-Nd isotopic chemistry of the Shuteen Complex. We discuss the relationship between volcanic and plutonic rocks of the complex, and consider their similarity to high-Al 2O 3 trondhjemite-tonalite-granodiorite and adakites. We also consider the origin, magma source, and dynamic processes of the Shuteen Complex; propose a petrogenetic model; and investigate the composition of the subducting slab and the features of arc volcanism at the time. We assess some of the magmatic processes likely to have occurred within the Shuteen Complex, such as Carboniferous slab subduction and partial melting, and examine their influence on magma composition. The Shuteen Complex is geochemically similar to adakite-type rocks. The complex is silica-saturated (SiO 2 ⩾ 56%), rich in Al 2O 3 (⩾15%), MgO (<6%), Y (⩽18 ppm), and Yb (⩽1.9 ppm), depleted in heavy rare earth elements, rich in Sr (>400 ppm), and depleted in high field strength elements. It also has a high Sr/Y value, and ( 87Sr/ 86Sr) I < 0.7040. The Shuteen Pluton yields a well-defined isochron age of 321 ± 9 Ma, whereas the age of the Shuteen andesites is 336 ± 24 Ma. The Shuteen Complex formed within an island arc setting, and partial melting was the dominant process during petrogenesis. The primary Shuteen magma had an adakitic composition and was probably derived from the partial melting of subducting oceanic crust, possibly with minor local interaction with mantle material. The results of quantitative modelling of mass balance and partial melt equilibrium for the magma source indicate that the subducting slab contained oceanic basalt and a minor component of oceanic sediment, which together with a restite eclogite phase formed the source of the

  5. 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.

  6. Detrital zircon provenance of Cambrian Ordovician and Carboniferous strata of the Oaxaca terrane, southern Mexico

    NASA Astrophysics Data System (ADS)

    Gillis, Robert J.; Gehrels, George E.; Ruiz, Joaquin; Flores de Dios Gonzaléz, Luis Antonio

    2005-12-01

    U-Pb geochronologic analyses have been conducted on 135 detrital zircon grains from Paleozoic strata of the Oaxaca terrane of southern Mexico. The grains are mainly mid-Proterozoic in age (age probability peak of 993 Ma), with subordinate clusters of ˜358 and ˜472 Ma. The mid-Proterozoic ages can be used to evaluate three possible paleopositions for the Oaxaca terrane during mid-Proterozoic time. Formation in proximity to northwest South America appears most likely, as southwestern portions of the Amazon craton (and perhaps basement massifs in the northern Andes) contain igneous rocks of the appropriate age. A more specific test of ties to these regions is not yet possible due to the limited number of U-Pb determinations from the region. Formation in proximity to Grenville-age rocks in northeast North America is less likely, as detrital zircons shed from these rocks are mostly older than 993 Ma. Previously proposed links with Grenville-age rocks in southwestern North America are not supported by our data, as there is little overlap of ages from the Oaxaca terrane and from Grenville-derived detrital zircons in southwestern US and northwestern Mexico. The presence of Paleozoic grains in our samples does not help constrain the Late Proterozoic-early Paleozoic displacement of the Oaxaca terrane, as magmatism of this age was apparently widespread within and adjacent to the Iapetus and Rheic ocean basins.

  7. 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,…

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

    NASA Astrophysics Data System (ADS)

    Ahmadi, Omid; Juhlin, Christopher

    2014-05-01

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

  9. 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. 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.

  11. 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

  12. Slow exhumation of UHP terranes: Titanite and rutile ages of the Western Gneiss Region, Norway

    NASA Astrophysics Data System (ADS)

    Kylander-Clark, A. R. C.; Hacker, B. R.; Mattinson, J. M.

    2008-08-01

    U-Pb ages of titanite and rutile were obtained from the central Western Gneiss Region, Norway, to assess the style and timing of exhumation and cooling of the Western Gneiss UHP terrane. Approximately half of the titanite ages are concordant, the majority of which yield a limited age range from 393 to 390 Ma. The other titanite data are discordant, and define discordia arrays with upper intercept ages of either ˜ 938 Ma or ˜ 1.6 Ga, and a lower intercept of ˜ 389 Ma. Concordant rutile analyses range from 385 to 392 Ma. Both titanite and rutile ages young WNW toward the core of the orogen and are ˜ 4 Ma older than 40Ar/ 39Ar muscovite ages, corresponding to a cooling rate of ˜ 90 °C/Ma. A well-defined boundary between concordant and discordant titanite ages, in combination with the WNW-increasing P- T gradient and the similarity between muscovite cooling ages in the east and eclogite ages in the west, suggests that the WGR remained coherent throughout its exhumation history, and was progressively unroofed from east to west. A 390.2 ± 0.8 Ma titanite in the Sørøyane UHP domain indicates that exhumation occurred at a vertical rate of ˜ 7 mm/yr for ˜ 12 Ma. These rates are slower than estimates from smaller UHP terranes, but similar to other large UHP terranes, suggesting that there may be fundamental differences in the mechanisms controlling the evolution of large UHP terranes that undergo protracted subduction and exhumation, and smaller UHP terranes that undergo rapid subduction and exhumation.

  13. 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.

  14. 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.

  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. 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.

  17. 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.

  18. 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.

  19. 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.

  20. Carbonatitic metasomatism in orogenic dunites from Lijiatun in the Sulu UHP terrane, eastern China

    NASA Astrophysics Data System (ADS)

    Su, Bin; Chen, Yi; Guo, Shun; Chu, Zhu-Yin; Liu, Jing-Bo; Gao, Yi-Jie

    2016-10-01

    Among orogenic peridotites, dunites suffer the weakest crustal metasomatism at the slab-mantle interface and are the best lithology to trace the origins of orogenic peridotites and their initial geodynamic processes. Petrological and geochemical investigations of the Lijiatun dunites from the Sulu ultrahigh-pressure (UHP) terrane indicate a complex petrogenetic history involving melt extraction and multistage metasomatism (carbonatitic melt and slab-derived fluid). The Lijiatun dunites consist mainly of olivine (Fo = 92.0-92.6, Ca = 42-115 ppm), porphyroblastic orthopyroxene (En = 91.8-92.8), Cr-spinel (Cr# = 50.4-73.0, TiO2 < 0.2 wt.%) and serpentine. They are characterized by refractory bulk-rock compositions with high MgO (45.31-47.07 wt.%) and Mg# (91.5-91.9), and low Al2O3 (0.48-0.70 wt.%), CaO (0.25-0.44 wt.%) and TiO2 (< 0.03 wt.%) contents. Whole-rock platinum group elements (PGE) are similar to those of cratonic mantle peridotites and Re-Os isotopic data suggest that dunites formed in the early Proterozoic (~ 2.2 Ga). These data indicate that the Lijiatun dunites were the residues of ~ 30% partial melting and were derived from the subcontinental lithospheric mantle (SCLM) beneath the North China craton (NCC). Subsequent carbonatitic metasomatism is characterized by the formation of olivine-rich (Fo = 91.6-92.6, Ca = 233-311 ppm), clinopyroxene-bearing (Mg# = 95.9-96.7, Ti/Eu = 104-838) veins cutting orthopyroxene porphyroblasts. Based on the occurrence of dolomite, mass-balance calculation and thermodynamic modeling, carbonatitic metasomatism had occurred within the shallow SCLM (low-P and high-T conditions) before dunites were incorporated into the continental subduction channel. These dunites then suffered weak metasomatism by slab-derived fluids, forming pargasitic amphibole after pyroxene. This work indicates that modification of the SCLM beneath the eastern margin of the NCC had already taken place before the Triassic continental subduction. Orogenic

  1. Geochemical and isotopic signatures of Proterozoic granitoids in terranes of the Borborema structural province, northeastern Brazil

    NASA Astrophysics Data System (ADS)

    Ferreira, V. P.; Sial, A. N.; Jardim de Sá, E. F.

    1998-09-01

    Two large E-W trending megastructures, the Patos and Pernambuco shear zones, subdivide the Borborema Province (BP), northeastern Brazil, into three main domains: Northern (ND), Transverse Zone (TZD) and Southern (SD). These domains evolved through reworking and amalgamations, during the Brasiliano cycle (0.70-0.50 Ga), of several tectonostratigraphic terranes and major crustal blocks previously deformed during Meso and Paleoproterozoic orogenic cycles. Petrological and geochemical characteristics of granitoids and syenitoids in these domains allow their classification into nine groups. These groups are enriched in K and Ba and have low Nb (usually <20 ppm), which seems to be a peculiar feature of the lithosphere in the BP. As an isolated criteria, the characteristics of the granitoid groups do not allow a clear individualization of different terranes, in most cases, but point neverthless to important contrasts among the major tectonic domains. Calc-alkalic magmatic epidote (mEp)-bearing granitoids in the ND display low δ18O (+6 to +8‰ SMOW), magnetic susceptibility (MS)>1.0×10 -3SI, ɛNd(0.6 Ga) from -15 to -20, and t DM model ages>2.0 Ga. Within the TZD, in contrast, calc-alkalic and high-K calc-alkalic mEp-bearing granitoids, respectively in the Cachoeirinha-Salgueiro and Alto Pajeúterranes, display lower MS (0.4×10 -3SI), high δ18O (+10 to +13‰ SMOW), lower ɛNd (-1 to -4), and younger t Nd (1.1-1.4 Ga). In high-K mEp-free granitoids, ɛNd values cluster around -12 in the Granjeiro terrrane and from -12 to -15 in the Capibaribe terrane. The Alto Pajeúterrane in the TZD is characterized by intrusions of peralkalic ultrapotassic syenitoids that show negative ɛNd (-15 to -19), t DM from 2.1 to 2.4 Ga, and MS from 0.7 to 1.0×10 -3 SI, and are regarded as derived from a metasomatized lithospheric mantle source. Granitoids in the SD are isotopically more complex. Three t DM Nd model-age intervals (1.0-1.5, 1.8-2.2 and 2.4-2.5 Ga) are found in the

  2. New Geochemical and Geochronological Constrains on the Tectonic Evolution of the Okhotsk Terrane

    NASA Astrophysics Data System (ADS)

    Bakharev, A.; Prokopiev, A.; Hourigan, J.; Toro, J.

    2004-12-01

    The Okhotsk Terrane is a continental block with Proterozoic basement which lies east of the South Verkhoyansk fold-and-thrust belt of eastern Siberia. It has been proposed that thrusting in the South Verkhoyansk occurred as a result of Late Jurassic-Early Cretaceous collision of the Okhotsk Terrane against the North Asia craton followed by plate convergence that produced the Uda-Murgal volcanic arc. Ten zircon grains from a biotite plagiogneiss of the Upper Maya uplift, analyzed using the Stanford/USGS SHRIMP-RG, yielded a weighted mean 207Pb/206Pb age of 2624±13 Ma, corrected for 204Pb. TIMs analysis yielded a statistically undistinguishable upper intercept of 2595±26 Ma. These ages are similar to those reported from the hornblende granulite complex of the Kukhtuy uplift of the central Okhotsk Terrane. 2.6 Ga ages are also common in the North Asia craton. The Neoarchean gneisses of the Upper Maya are intruded by meta-aluminous biotite-hornblende granodiorites and quartz diorites of the Mastakh pluton. Mineralogical composition, REE distribution, and negative Ta, Nb, Zr, and Ti anomalies of the Mastakh pluton are comparable to continental margin, subduction-related plutons in western North America and the Urals. Ten zircons separated from the Mastakh pluton yielded a 207Pb-corrected weighted mean 238U/206Pb age of 376±4 Ma (Late Devonian). This age suggests that the Mastakh pluton is related to Late Devonian calc-alkaline volcanic rocks of the Matiy formation that are widespread in the Okhotsk Terrane. 40Ar/39Ar analysis of biotites from the Mastakh pluton yielded a weighted mean plateau age of 355±1Ma (Early Carboniferous), therefore that the pluton underwent slow cooling after emplacement. During the Middle Devonian to Early Carboniferous the eastern margin of the North Asia craton underwent a regional episode of rifting. Several major rift grabens and aulacogens formed along the Verkhoyansk margin. The best known of these is the Vilyui basin, but the

  3. Constraints From Deep-Imaging Magnetotellurics on the Lithospheric Structure and Evolution of the Enigmatic Okwa Terrane, Botswana

    NASA Astrophysics Data System (ADS)

    Muller, M. R.; Jones, A. G.; Evans, R. L.

    2009-12-01

    The Okwa Terrane, located in central Botswana, is perhaps one of the least understood terranes within the southern African Archean to Palaeoproterozoic tectonic framework. Thick Quaternary Kalahari sand-cover provides minimal crustal exposure with which to define the nature and evolution of the terrane: its potential affiliations and tectonic relationships with adjacent terranes remain speculative and largely unconstrained. The Okwa Terrane, as defined primarily in potential field images, is bounded to the west and north by the Early Proterozoic Rehoboth Terrane, to the south by the Archean Kaapvaal Craton, and to the east, across a poorly defined boundary, by the Palaeoproterozoic Magondi orogenic belt and the Archean Zimbabwe Craton. While the Okwa Terrane is inferred in some interpretations to constitute the northern-most portion of the Kaapvaal Craton, there is no direct evidence to support an Archean lithospheric stabilisation age for the terrane. The oldest recorded crustal ages, for intrusive granites located in the Okwa Inlier, are between 2.1 and 2.0 Ga. Gneissic deformation of the granites is recognised at ~1.8 Ga and, in alternative interpretations, is regarded as the accretion age of the Okwa Terrane with the Kaapvaal Craton along the major east-west trending Palala Shear Zone. A reported diamondiferous kimberlite pipe in the Gope cluster of the Okwa Terrane suggests a lithospheric thickness in excess of the depth of the diamond stability field (~160 km), at least at the time of kimberlite eruption at ~80 Ma. The multinational Southern African Magnetotelluric Experiment (SAMTEX) has acquired, during the period between 2003 and 2008, more than 730 magnetotelluric (MT) sites along 14,000 kilometers of profile length across southern Africa. In acquiring MT data on two orthogonal ~600 km-long profiles across the Okwa Terrane, SAMTEX provides the first deep crustal and lithospheric mantle images of the terrane. MT stations were installed at roughly 20 km

  4. 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.

  5. Fluids Speciation in Deep Subduction Channels: Evidence From Polycrystalline Diamonds From UHPM Terranes

    NASA Astrophysics Data System (ADS)

    Dobrzhinetskaya, L.; Wirth, R.; Green, H. W.

    2011-12-01

    Geochemical characteristic of fluids circulating in deep subduction zones is of a great interest for many directions of geosciences. One of the intriguing processes is a fluid-rock interaction during subduction of the continental slab because the latter is characterized by contrast chemistry in comparison with the rocks of surrounding mantle and its fluids. Diamond due to its chemical inertness is the only mineral which contains "unchanged" fluid trapped during its crystallization at the peak of UHP metamorphism. Polycrystalline diamonds, that we have recently found within both the Kokchetav and the Earzgebirge UHPM terranes, represent an outstanding material to study geochemical diversity of the fluids circulating in deep subduction zones. Earlier studies of diamond monocrystals included in garnet, zircon, pyroxene and mica of the Kokchetav massif showed that the fluid responsible for diamond crystallization contains traces of both crustal and mantle components: Al, K, Ca, Mg, Fe, Si, Ti, V, Zn, Co, Fe F, Cl, S. The finding of polycrystalline diamonds in UHPM rocks of the Kokchetav and the Erzgebirge areas extends our knowledge related to UHP fluid composition. The polycrystalline diamonds consist of 5-15 single crystals of 0.3-5 micron size with a typical "zig-zag" grain boundaries and triangle voids filled with a C-O-H fluid with traces of Al, Co, F, V, Zn, Si, Cl, S, Ca, Mg, Fe, K in different combinations. Occurrences of "spider-like" dislocations, "zig-zag" boundaries, stacking faults and associated fluid pockets indicate presence of an internal stress accompanied diamond nucleation and growth from a fluid media. Observations of the carbonate (CaCO3) suggest an oxidizing environment of the fluids, close to the CCO buffer. Carbon isotopes speciation (delta 13C = -10 to -27 %) indicates organic carbon reservoir. Furthermore, studies by Sumino et al. (2011) showed that the Kokchetav diamonds, which are formed from organic carbon, are characterized by very high 3

  6. 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.

  7. 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

  8. 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

  9. Water in the Elizabethtown area; a study of a limestone terrane in North Central Kentucky

    USGS Publications Warehouse

    Lambert, T.W.

    1979-01-01

    An inventory of the water resources of a 240-square-mile area in north-central Kentucky is reported. It includes water distribution, chemical quality, water use, and principles of water occurrence and availability, and references. Nolin River is the only major stream in the area. Two of its tributaries, Valley Creek and North Fork Nolin River, have water withdrawn for public supply by Elizabethtown and Hodgenville. The quality of the water from the streams is typical of a limestone terrane. Ground water is also used for public supplies by Elizabethtown and Hodgenville and by two industrial plants. Many springs have flows of more than 0.25 cubic foot per second. Except for stock water, only three springs serve as a water source. Water from wells and springs is typical of limestone terrane. (USGS)

  10. Geological investigations of pre-late Jurassic terranes in the southernmost Andes

    NASA Astrophysics Data System (ADS)

    Forsythe, R. D.

    Pre-Late Jurassic terranes of the Patagonian Archipelago were investigated. Their regional stratigraphic and structural characteristics were surveyed. Their significance in the late Paleozoic to early Mesozoic evolution of South America were determined. Pre-Late Jurassic rocks within the archipelago are distributed in two belts. Within the outer belt the Madre de Dios Archipielago was studied in detail. Pre-Late Jurassic rocks of this area are divisible into three mappable units. These three units are interpreted to be part of a late Paleozoic to early Mesozoic accretionary prism that was located along the ancestral Pacific margin of the South American sector of Gondwana. Within the inner belt, the region of Peninsula Staines was studied in detail. In this region greenschist facies metamorphism and pervasive deformation fabrics prevent stratigraphic subdivision of the terrane. However the lithologies present are correlative with the outer belt suggesting that they also were part of the late Paleozoic to early Mesozoic accretionary prism.

  11. Geologic evolution in the Montana Metasedimentary Terrane from a neodymium perspective

    SciTech Connect

    D'Arcy, D.A.; Mueller, P.A. . Dept. of Geology)

    1992-01-01

    The Archean Wyoming Province, located primarily in southwestern Montana and Wyoming, can be subdivided into three distinct lithotectonic terranes defined on the basis of Late Archean assemblages. These include the Montana Metasedimentary Terrane (MMT), the Beartooth-Bighorn Magmatic Terrane (BBMT), and the Wyoming Greenstone Terrane (WGT). The MMT, located in southwestern Montana, contains a distinctive assemblage of metasedimentary and metaigneous lithologies, but is dominated by Late to Middle Archean tonalitic to granitic gneisses. The BBMT adjoins the MMT on its eastern margin and is composed primarily of Late Archean (2.75 Ga) igneous and metaigneous lithologies, although isolated Middle Archean (3.3 Ga) metasedimentary packages are present. The BBMT is exposed in Beartooth and Bighorn Mountains of Wyoming. Neodymium model ages Tchur have been determined on 55 samples and range from 2.05 to 3.78 Ga, with clusters at about 3 and 3.3 Ga. The 3.3 Ga cluster is generally corroborated by U-Pb data from single-grain, detrital zircon studies of Tobacco Root quartzites and has been documented in metasediments of the BBMT. The 2.75 Ga event which affected the BBMT has, so far, not been detected in the MMT, although 2.65 Ga lithologies have been detected in the Madison Range. It is possible that the 3 Ga ages of MMT metasediments reflect mixing of 3.3 Ga and 2.75 and/or 2.65 Ga material or, that they represent the dominant time of crustal formation in the MMT as opposed to 2.75 Ga in the BBMT. Older Tchur's (3.3 to 3.8 Ga) are found in both the Tobacco Root Mountains and the Madison Range and suggest that the MMT contains evidence of even earlier crust-forming events not documented elsewhere in the Wyoming Craton.

  12. 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.

  13. Accreted terranes of northwestern British Columbia, Canada: Lithospheric velocity structure and tectonics

    NASA Astrophysics Data System (ADS)

    Hammer, Philip T. C.; Clowes, Ron M.

    2004-06-01

    Lithospheric structure beneath the accreted terranes of the northwestern Canadian Cordillera is interpreted from analyses of P wave refraction and wide-angle reflection data. The 535 km long profile extends from the deformed and displaced ancestral North American margin across the Omineca belt suture zone and Intermontane superterrane to the Coast belt. This region is interpreted to have grown westward through a progression from thin-skinned to thick-skinned accretionary tectonics. Large lateral variations of velocities (2.0-6.4 km/s) within the upper 15 km correlate with the mapped geological and terrane elements; fault offsets and the imbricated base of an overlap basin are clearly identified. Inboard of the Stikine accreted terrane a zone of slow upper crustal velocities thins to the east, consistent with an interpretation of thin-skinned accretion leaving slivers of terranes overlying a wedge of metamorphosed Proterozoic continental margin sediments. However, in the lower crust the proposed underlying cratonic ramp is not distinguished from outboard or overlying accreted crust by lateral P velocity contrasts or wide-angle reflections. The Moho remains nearly horizontal beneath the Intermontane and Omineca belts, gradually thinning to the east from 36.5 to 35 km. To the southwest below the Coast belt and to the northeast beneath the Tintina fault, the crust thins to 32 km. Upper mantle velocities across the Coast, Intermontane, and Omineca belts are 7.8-7.9 km/s. The slow upper mantle and lower crustal velocities are indicative of high temperatures that may permit the flat Moho or weak lower crust to partially decouple the crust from the mantle. Reflections in the upper mantle between 50-70 km depth may be related to the base of the lithosphere or to heterogeneities introduced through melting or accretionary subduction events.

  14. 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.

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

    SciTech Connect

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

    1987-12-01

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

  16. 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.

  17. 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.

  18. 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.

  19. Cretaceous-Tertiary structural evolution of the north central Lhasa terrane, Tibet

    NASA Astrophysics Data System (ADS)

    Volkmer, John E.; Kapp, Paul; Guynn, Jerome H.; Lai, Qingzhou

    2007-12-01

    In the north central Lhasa terrane of Tibet, two distinct structural levels of an east-west striking thrust system are exposed along the north trending late Cenozoic Xiagangjiang rift. Upper Paleozoic strata deformed by the south directed Langgadong La thrust, and Cretaceous strata involved in variably north and south directed thrusting characterize these lower and upper structural levels, respectively. These two structural levels are separated by the Tagua Ri passive roof thrust. Balanced cross section restoration suggests that the thrust system accommodated ˜103 km (˜53%) shortening. The 40Ar/39Ar results, together with an interpretation of synthrust deposition of Upper Cretaceous strata, suggest that the majority of shortening occurred during the Late Cretaceous-Paleocene. Cretaceous strata lie unconformable on Permian rocks; volcanic tuffs directly above the unconformity yield U-Pb zircon ages of ˜131 Ma. Upper Cretaceous strata record a change from shallow marine to nonmarine deposition, indicating uplift above sea level during this time. The overall south directed vergence of the thrust belt is consistent with substantial crustal thickening in central Tibet by large-scale northward underthrusting of Lhasa terrane basement beneath the Qiantang terrane prior to the Indo-Asian collision. The documented decoupling of contractional deformation at shallow crustal levels appears to be a regional characteristic of Tibet from at least the Bangong suture in the north to the Tethyan Himalaya to the south. This style of deformation explains the absence of basement exposures and major denudation in this region despite substantial crustal shortening.

  20. 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.

  1. 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.

  2. Triassic arc-derived detritus in the Triassic Karakaya accretionary complex was not derived from either the S Eurasian margin (Istanbul terrane) or the N Gondwana margin (Taurides)

    NASA Astrophysics Data System (ADS)

    Ustaömer, Timur; Ayda Ustaömer, Petek; Robertson, Alastair H. F.; Gerdes, Axel; Zulauf, Gernold

    2014-05-01

    We present new U-Pb zircon source age data for Upper Triassic sandstones of the Istanbul Terrane (S Eurasian margin) and also for Triassic sandstones of the Taurides (N Gondwana margin). The main aim is to detect and quantify the contribution of Triassic magmatism as detritus to either of these crustal blocks. This follows the recent discovery of a Triassic magmatic arc source for the Triassic sandstones of the Palaeotethyan Karakaya subduction-accretion complex (Ustaömer et al. 2013; this meeting). Carboniferous (Variscan) zircon grains also form a significant detrital population, plus several more minor populations. Six sandstone samples were studied, two from the İstanbul Terrane (Bakırlıkıran Formation of the Kocaeli Triassic Basin) and four from the Tauride Autochthon (latest Triassic Üzümdere Formation and Mid-Triassic Kasımlar Formations; Beyşehir region). Detrital zircon grains were dated by the laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS) U-Pb method at Goethe University, Frankfurt. Our results do not reveal Triassic detritus in the Üzümdere Formation. The U-Pb age of the analysed zircon grains ranges from 267 Ma to 3.2 Ga. A small fraction of Palaeozoic zircons are Permian (267 to 296 Ma), whereas the remainder are Early Palaeozoic. Ordovician grains (4%) form two age clusters, one at ca. 450 Ma and the other at ca. 474 Ma. Cambrian-aged grains dominate the zircon population, while the second largest population is Ediacaran (576 to 642 Ma). Smaller populations occur at 909-997 Ma, 827-839 Ma, 1.8-2.0 Ga and 2.4-2.6 Ga. The sandstones of the Kasımlar Formation have similar zircon age cluster to those of the somewhat younger Üzümdere Formation, ranging from 239 Ma to 2.9 Ga. A few grains gave Anisian ages. Cambrian zircon grains are less pronounced than in the Kasımlar Formation compared to the Üzümdere Formation. The detrital zircon record of Tauride sandstones, therefore, not indicates significant contribution

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

    PubMed

    Wortman; Samson; Hibbard

    2000-05-01

    The early magmatic and tectonic history of the Carolina terrane and its possible affinities with other Neoproterozoic circum-Atlantic arc terranes have been poorly understood, in large part because of a lack of reliable geochronological data. Precise U-Pb zircon dates for the Virgilina sequence, the oldest exposed part, constrain the timing of the earliest known stage of magmatism in the terrane and of the Virgilina orogeny. A flow-banded rhyolite sampled from a metavolcanic sequence near Chapel Hill, North Carolina, yielded a U-Pb zircon date of 632.9 +2.6/-1.9 Ma. A granitic unit of the Chapel Hill pluton, which intrudes the metavolcanic sequence, yielded a nearly identical U-Pb zircon date of 633 +2/-1.5 Ma, interpreted as its crystallization age. A felsic gneiss and a dacitic tuff from the Hyco Formation yielded U-Pb zircon dates of 619.9 +4.5/-3 Ma and 615.7 +3.7/-1.9 Ma, respectively. Diorite and granite of the Flat River complex have indistinguishable U-Pb upper-intercept dates of 613.9 +1.6/-1.5 Ma and 613.4 +2.8/-2 Ma. The Osmond biotite-granite gneiss, which intruded the Hyco Formation before the Virgilina orogeny, crystallized at 612.4 +5.2/-1.7 Ma. Granite of the Roxboro pluton, an intrusion that postdated the Virgilina orogeny, yielded a U-Pb upper intercept date of 546.5 +3.0/-2.4 Ma, interpreted as the time of its crystallization. These new dates both provide the first reliable estimates of the age of the Virgilina sequence and document that the earliest known stage of magmatism in the Carolina terrane had begun by 633 +2/-1.5 Ma and continued at least until 612.4 +5.2/-1.7 Ma, an interval of approximately 25 m.yr. Timing of the Virgilina orogeny is bracketed between 612.4 +5.2/-1.7 Ma and 586+/-10 Ma (reported age of the upper Uwharrie Formation). The U-Pb systematics of all units studied in the Virgilina sequence are simple and lack any evidence of an older xenocrystic zircon component, which would indicate the presence of a continental

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

    PubMed

    Wortman; Samson; Hibbard

    2000-05-01

    The early magmatic and tectonic history of the Carolina terrane and its possible affinities with other Neoproterozoic circum-Atlantic arc terranes have been poorly understood, in large part because of a lack of reliable geochronological data. Precise U-Pb zircon dates for the Virgilina sequence, the oldest exposed part, constrain the timing of the earliest known stage of magmatism in the terrane and of the Virgilina orogeny. A flow-banded rhyolite sampled from a metavolcanic sequence near Chapel Hill, North Carolina, yielded a U-Pb zircon date of 632.9 +2.6/-1.9 Ma. A granitic unit of the Chapel Hill pluton, which intrudes the metavolcanic sequence, yielded a nearly identical U-Pb zircon date of 633 +2/-1.5 Ma, interpreted as its crystallization age. A felsic gneiss and a dacitic tuff from the Hyco Formation yielded U-Pb zircon dates of 619.9 +4.5/-3 Ma and 615.7 +3.7/-1.9 Ma, respectively. Diorite and granite of the Flat River complex have indistinguishable U-Pb upper-intercept dates of 613.9 +1.6/-1.5 Ma and 613.4 +2.8/-2 Ma. The Osmond biotite-granite gneiss, which intruded the Hyco Formation before the Virgilina orogeny, crystallized at 612.4 +5.2/-1.7 Ma. Granite of the Roxboro pluton, an intrusion that postdated the Virgilina orogeny, yielded a U-Pb upper intercept date of 546.5 +3.0/-2.4 Ma, interpreted as the time of its crystallization. These new dates both provide the first reliable estimates of the age of the Virgilina sequence and document that the earliest known stage of magmatism in the Carolina terrane had begun by 633 +2/-1.5 Ma and continued at least until 612.4 +5.2/-1.7 Ma, an interval of approximately 25 m.yr. Timing of the Virgilina orogeny is bracketed between 612.4 +5.2/-1.7 Ma and 586+/-10 Ma (reported age of the upper Uwharrie Formation). The U-Pb systematics of all units studied in the Virgilina sequence are simple and lack any evidence of an older xenocrystic zircon component, which would indicate the presence of a continental

  5. 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

  6. Raman spectroscopy of detrital garnet from the (U)HP terrane of eastern Papua New Guinea

    NASA Astrophysics Data System (ADS)

    Andò, Sergio; Baldwin, Suzanne L.; Fitzgerald, Paul G.; Malusà, Marco G.; Aliatis, Irene; Vezzoli, Giovanni; Garzanti, Eduardo

    2013-04-01

    adjacent to orogenic systems, Raman-assisted heavy-mineral studies allow us to detect the first arrival of eclogitic garnet, and thus to assess the minimum age of exhumation and final unroofing of high-pressure rocks (Malusà et al., 2011; Malusà and Garzanti, 2012). However, in the (U)HP terrane of southeastern Papua New Guinea, sediments derived from the actively exhuming D'Entrecasteaux Island core complexes are still being deposited offshore, are rarely preserved sub-aerially, and as such stratigraphic constraints are limited. Raman analysis of detrital garnets from placer sand thus provides invaluable constraints to compare with mineral assemblages preserved in exhumed eclogites. REFERENCES Andò S., Bersani D., Vignola P., Garzanti E. 2009. Raman spectroscopy as an effective tool for high-resolution heavy-mineral analysis: Examples from major Himalayan and Alpine fluvio-deltaic systems. Spectrochim. Acta A73:450-455. Andò S., Morton A., Garzanti E. 2013. Metamorphic grade of source rocks revealed by chemical fingerprints of detrital amphibole and garnet. Geol. Soc. London Spec. Publ. Sediment Provenance Studies in Hydrocarbon Exploration & Production. Baldwin S.L., Monteleone B., Webb L.E., Fitzgerald P.G., Grove M., Hill E.J. 2004. Pliocene eclogite exhumation at plate tectonic rates in eastern Papua New Guinea. Nature 431:263-267. Baldwin S.L., Webb L.E., Monteleone B.D. 2008. Late Miocene coesite-eclogite exhumed in the Woodlark Rift. Geology 36:735-738 Bersani D., Andò S., Vignola P., Moltifiori G., Marino I.G., Lottici P.P., Diella V., 2009. Micro-Raman spectroscopy as a routine tool for garnet analysis. Spectrochim. Acta A73:484-491. Hill E.J., Baldwin S.L. 1993. Exhumation of high-pressure metamorphic rocks during crustal extension in the D'Entrecasteaux region, Papua New Guinea. J. Metam. Geol. 11:261-277. Malusà M.G., Faccenna C., Garzanti E., Polino R. 2011. Divergence in subduction zones and exhumation of high-pressure rocks (Eocene Western Alps

  7. 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)

  8. 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.

  9. Soil microbial activity and structure in mineralized terranes of the Western US

    NASA Astrophysics Data System (ADS)

    Blecker, S. W.; Stillings, L. L.; Decrappeo, N.; Ippolito, J.

    2009-12-01

    Mineralized terranes (areas enriched in metal-bearing minerals) occur throughout the Western US, and are characterized by highly variable soil trace metal concentrations across small spatial scales. Assuming that non-lithologic (extrinsic) soil forming factors are relatively constant between mineralized and unmineralized zones, these mineralized areas allowed us to evaluate the effect of lithology on soil microbial activity. We established the following study sites: 1) sage-grassland on a Mo/Cu deposit (Battle Mountain, NV); 2) pine-chaparral on Ni/Cr bearing rocks (Chinese Camp, CA); and 3) two pine woodland sites on acid-sulfate altered rocks (Reno, NV; Bridgeport, CA). Microbial, physical and chemical measurements were performed on soils from undisturbed mineralized areas and adjacent unmineralized areas to determine baseline conditions for comparison to sites disturbed by mining. A host of abiotic soil parameters, along with bioavailable (diethylenetriaminepentaacetic acid (DTPA)-extractable) and total metals, were measured to examine their correlation with the following measures of microbial activity: enzyme assays (arylsulfatase, phosphatase, fluorescein diacetate hydrolysis), C/N mineralization potential, C substrate utilization (Biolog Ecoplate), and microbial biomass and community structure (phospholipid fatty acid analysis). Within the Battle Mountain study area, both microbial activity and structure were statistically similar between mineralized and unmineralized soils. Nutrient and metal concentrations were also similar; the only differences being higher Cu and lower P in the mineralized soils. Within the Chinese Camp study area, soil organic carbon and total nitrogen concentrations were similar between the serpentine (Ni/Cr bearing) and adjacent andesite soils, while differences were noted for other nutrients (S, P, Ca, Mg). For the serpentine soils, Co, Fe, Mn, and Ni showed the strongest correlations with microbial activity, where Cr, Mn showed the

  10. 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

  11. Low palaeoelevation of the northern Lhasa terrane during late Eocene: Fossil foraminifera and stable isotope evidence from the Gerze Basin.

    PubMed

    Wei, Yi; Zhang, Kexin; Garzione, Carmala N; Xu, Yadong; Song, Bowen; Ji, Junliang

    2016-01-01

    The Lhasa terrane is a key region for understanding the paleoelevation of the southern Tibetan Plateau after India-Asia collision. The Gerze Basin, located in the northern part of the Lhasa terrane, is a shortening-related basin. We discovered Lagena laevis (Bandy) fossils in upper Eocene strata of the Gerze Basin. This type of foraminifera is associated with lagoon and estuarine environments, indicating that the northern part of the Lhasa terrane was near sea level during the late Eocene. We speculate that these foraminifera were transported inland by storm surges to low elevation freshwater lakes during times of marine transgressions. This inference is consistent with the relatively positive δ(18)O values in carbonate from the same deposits that indicate low palaeoelevations close to sea level. Considering the palaeoelevation results from the nearby Oligocene basins at a similar latitude and the volcanic history of the Lhasa terrane, we infer that large-magnitude surface uplift of the northern Lhasa terrane occurred between late Eocene and late Oligocene time. PMID:27272610

  12. Low palaeoelevation of the northern Lhasa terrane during late Eocene: Fossil foraminifera and stable isotope evidence from the Gerze Basin

    NASA Astrophysics Data System (ADS)

    Wei, Yi; Zhang, Kexin; Garzione, Carmala N.; Xu, Yadong; Song, Bowen; Ji, Junliang

    2016-06-01

    The Lhasa terrane is a key region for understanding the paleoelevation of the southern Tibetan Plateau after India-Asia collision. The Gerze Basin, located in the northern part of the Lhasa terrane, is a shortening-related basin. We discovered Lagena laevis (Bandy) fossils in upper Eocene strata of the Gerze Basin. This type of foraminifera is associated with lagoon and estuarine environments, indicating that the northern part of the Lhasa terrane was near sea level during the late Eocene. We speculate that these foraminifera were transported inland by storm surges to low elevation freshwater lakes during times of marine transgressions. This inference is consistent with the relatively positive δ18O values in carbonate from the same deposits that indicate low palaeoelevations close to sea level. Considering the palaeoelevation results from the nearby Oligocene basins at a similar latitude and the volcanic history of the Lhasa terrane, we infer that large-magnitude surface uplift of the northern Lhasa terrane occurred between late Eocene and late Oligocene time.

  13. New paleomagnetic constraints on the extrusion of Indochina: Late Cretaceous results from the Song Da terrane, northern Vietnam

    NASA Astrophysics Data System (ADS)

    Takemoto, Kazuhiro; Halim, Nadir; Otofuji, Yo-ichiro; Van Tri, Tran; Van De, Le; Hada, Shigeki

    2005-01-01

    Samples were collected for paleomagnetic investigations from the Upper Cretaceous Yen Chau Formation in the Song Da terrane (21.7°N, 103.9°E) bounded by the Ailao Shan-Red River fault system and Song Ma fault, in an attempt to examine extrusion tectonics of East Asia. Primary nature of the high-temperature (600-690 °C) component of magnetization from 13 sites is supported by a positive fold test. The tectonic corrected data provides the characteristic Late Cretaceous paleomagnetic direction for the Song Da terrane ( D=6.4°, I=32.0° with α95=8.5°, N=13 sites), corresponding to a paleopole lying at 82.9°N, 220.7°E ( A95=6.9°). Comparison with coeval paleomagnetic poles for the neighboring tectonic blocks indicates no latitudinal translation of the Son Da terrane with respect to the South China Block, whereas the Shan-Thai Block and the southern part of the Indochina Block revealed a southward displacement of 10.5±9.5° in latitude with respect to the Song Da terrane. We conclude that the southeastern segment of the Ailao Shan-Red River fault system to the east of the Dien Bien Phu fault is not a demarcation of the extruded Indochina Peninsula. The Indochina Block extruded probably along some faults between the Song Da terrane and the Khorat Plateau.

  14. 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.

  15. Permian and early(?) Triassic radiolarian faunas from the Grindstone Terrane, central Oregon

    USGS Publications Warehouse

    Blome, C.D.; Reed, K.M.

    1992-01-01

    Moderately well preserved Permian and Early(?) Triassic radiolarian faunas from sedimentary melange cherts of the Grindstone terrane in central Oregon are nearly identical to coeval chert faunas in Japan. Although several Oregon taxa have been reported from limestone sequences in the central United States, most of the Oregon forms have only been found in cherty rocks and nearly half have not previously been reported from North America. Forty-two taxa belonging to 19 genera are systematically treated. Co-occurrences of some species in Oregon indicate that their ranges in North America may differ from those in Japan. -from Authors

  16. Major lunar crustal terranes: Surface expressions and crust-mantle origins

    NASA Astrophysics Data System (ADS)

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

    2000-02-01

    In light of global remotely sensed data, the igneous crust of the Moon can no longer be viewed as a simple, globally stratified cumulus structure, composed of a flotation upper crust of anorthosite underlain by progressively more mafic rocks and a residual-melt (KREEP) sandwich horizon near the base of the lower crust. Instead, global geochemical information derived from Clementine multispectral data and Lunar Prospector gamma-ray data reveals at least three distinct provinces whose geochemistry and petrologic history make them geologically unique: (1) the Procellarum KREEP Terrane (PKT), (2) the Feldspathic High-lands Terrane (FHT), and (3) the South Pole-Aitken Terrane (SPAT). The PKT is a mafic province, coincident with the largely resurfaced area in the Procellarum-Imbrium region whose petrogenesis relates to the early differentiation of the Moon. Here, some 40% of the Th in the Moon's crust is concentrated into a region that constitutes only about 10% of the crustal volume. This concentration of Th (average ~5 ppm), and by implication the other heat producing elements, U and K, led to a fundamentally different thermal and igneous evolution within this region compared to other parts of the lunar crust. Lower-crustal materials within the PKT likely interacted with underlying mantle materials to produce hybrid magmatism, leading to the magnesian suite of lunar rocks and possibly KREEP basalt. Although rare in the Apollo sample collection, widespread mare volcanic rocks having substantial Th enrichment are indicated by the remote data and may reflect further interaction between enriched crustal residues and mantle sources. The FHT is characterized by a central anorthositic region that constitutes the remnant of an anorthositic craton resulting from early lunar differentiation. Basin impacts into this region do not excavate significantly more mafic material, suggesting a thickness of tens of kilometers of anorthositic crust. The feldspathic lunar meteorites may

  17. 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

  18. 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

  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. 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.

  1. 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

  2. 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.

  3. Paleomagnetic study of Jurassic and Cretaceous rocks from the Mixteca terrane (Mexico)

    NASA Astrophysics Data System (ADS)

    Böhnel, Harald

    1999-11-01

    Three sites from Cretaceous limestone and Jurassic sandstone in northern Oaxaca, Mexico, were studied paleomagnetically. Thermal demagnetization isolated site-mean remanence directions which differ significantly from the recent geomagnetic field. The paleopole for the Albian-Cenomanian Morelos formation is indistinguishable from the corresponding reference pole for stable North America, indicating tectonic stability of the Mixteca terrane since the Cretaceous. Rock magnetic properties and a positive reversal test for the Bajocian Tecomazuchil sandstone suggest that the remanence could be of primary origin, although no fold test could be applied. The Tecomazuchil paleopole is rotated 10°±5° clockwise and displaced 24°±5° towards the study area, with respect to the reference pole for stable North America. Similar values were found for the Toarcien-Aalenian Rosario Formation, with 35°±6° clockwise rotation and 33°±6° latitudinal translation. These data support a post-Bajocian southward translation of the Mixteca terrane by around 25°, which was completed in mid-Cretaceous time.

  4. Laser Ablation Analyses of Pb Isotopes in Ancient Feldspars: Application to a Polymetamorphic Terrane, West Greenland

    NASA Astrophysics Data System (ADS)

    Krogstad, E. J.; Baker, J. A.; Waight, T. E.

    2001-12-01

    Laser ablation was used to sample the Pb isotopic compositions of various feldspars, as well as isotopic standards. The ablated material was analyzed by MC-ICP-MS. The resulting accuracy and external precision are comparable to conventional (i.e., not double or triple-spiked) feldspar Pb isotope analyses done by TIMS. However, the data can be acquired with no chemical separation and require only a few minutes per sample. A pilot study was made of the feldspars from a polymetamorphic terrane in West Greenland, in which Late Archean gneisses were deformed and metamorphosed during the Early Proterozoic. In this terrane, isotopic contrasts have long been sought to delineate any suture between discrete Archean continental blocks that might mark the site of ocean closure. Previous whole rock Nd and Pb isotopic studies had yielded equivocal results on the presence of such an isotopic discontinuity. The laser ablation feldspar data presented here, combined with existing whole rock Pb data, point to real differences in the sources of gneisses from various parts of the orogen. This indicates that the laser ablation method of sampling feldspar Pb holds real potential for future reconnaissance studies of old continental crust in a manner similar to that of zircon U-Pb geochronology studies.

  5. 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.

  6. 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.

  7. 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.

  8. Chain Lakes massif, west central Maine: northern Appalachian basement or suspect terrane

    SciTech Connect

    Cheatham, M.M.; Olszewski, W.J. Jr.; Gaudette, H.E.

    1985-01-01

    The Chain Lakes massif of west-central Main is a 3 km thick sequence of diamictite and aquagene metavolcanics and metasediments, which contrasts strikingly with its surrounding Paleozoic rocks in lithology, structural style and metamorphic grade. The rocks of the massif are characterized by mineral assemblages developed during two separate metamorphic events. The first, of second sillimanite grade, is reflected by qtz-oligoclase-Kspar-sillimanite-biotite and muscovite. The second metamorphism is a retrograde event of greenschist facies, and chlorite grade. Isotopic Rb-Sr and Sm-Nd whole rock, and Rb-Sr mineral analyses of samples of the diamictite members, now gneiss and granofels, indicate that the first prograde metamorphism occurred at 770 Ma. with the retrograde event at approximately 405 Ma. Due to the restricted range of /sup 147/Sm//sup 144/Nd, no Sm-Nd isochron age could be determined. However, model ages for both Sr and Nd are approximately 1500 Ma for derivation of the Chain Lakes protolith material from depleted mantle. Lithology, bounding formations, complexes and plutons, and the isotopic data support previous contentions that the Chain Lakes massif is a suspect terrane. However, similarities with Proterozoic rocks along the Eastern Margin, as well as recent suggestions of similar rocks underlying the Kearsarge-Central Main synclinorium may suggest the possible widespread occurrence of dismembered masses of a perhaps once coherent, Precambrian terrane underlying the Northern Appalachians.

  9. Gamma-ray spectrometry of granitic suites of the Paranaguá Terrane, Southern Brazil

    NASA Astrophysics Data System (ADS)

    Weihermann, Jessica Derkacz; Ferreira, Francisco José Fonseca; Cury, Leonardo Fadel; da Silveira, Claudinei Taborda

    2016-09-01

    The Paranaguá Terrane, located in the coastal portion of the states of Santa Catarina, Paraná and São Paulo in Southern Brazil is a crustal segment constituted mainly by an igneous complex, with a variety of granitic rocks inserted into the Serra do Mar ridge. The average altitude is approximately 1200 m above sea level, with peaks of up to 1800 m. Due to the difficulty of accessing the area, a shortage of outcrops and the thick weathering mantle, this terrane is understudied. This research aims to evaluate the gamma-ray spectrometry data of the granitic suites of the Paranaguá Terrane, in correspondence with the geological, petrographical, lithogeochemical, relief and mass movement information available in the literature. Aerogeophysical data were acquired along north-south lines spaced at 500 m, with a mean terrain clearance of 100 m. These data cover potassium (K, %), equivalent in thorium (eTh, ppm) and equivalent in uranium (eU, ppm). After performing a critical analysis of the data, basic (K, eU, eTh) and ternary (R-K/G-eTh/B-eU) maps were generated and then superimposed on the digital elevation model (DEM). The investigation of the radionuclide mobility across the relief and weathering mantle consisted of an analysis of the schematic profiles of elevation related with each radionuclide; a comparison of the K, eU and eTh maps with their 3D correspondents; and the study of mass movements registered in the region. A statistical comparison of lithogeochemical (K, U, Th) and geophysical (K, eU, eTh) data showed consistency in all the granitic suites studied (Morro Inglês, Rio do Poço and Canavieiras-Estrela). Through gamma-ray spectrometry, it was possible to establish relationships between scars (from mass movements) and the gamma-ray responses as well as the radionuclide mobility and the relief and to map the granitic bodies.

  10. Abundance and distribution of radioelements in lunar terranes: Results of Chang'E-1 gamma ray spectrometer data

    NASA Astrophysics Data System (ADS)

    Chen, Jian; Ling, Zongcheng; Li, Bo; Zhang, Jiang; Sun, Lingzhi; Liu, Jianzhong

    2016-02-01

    The gamma ray spectrometer (GRS) onboard Chang'E-1 has acquired valuable datasets recording the gamma ray intensities from radioelements (Potassium (K), Thorium (Th) and Uranium (U), etc.) on lunar surface. We extracted the elemental concentrations from the GRS data with spectral fitting techniques and mapped the global absolute abundance of radioelements in terms of the ground truths from lunar samples and meteorites. The obtained global concentration maps of these radioelements indicate heterogeneous distribution among three major lunar crustal terranes (i.e., Procellarum KREEP Terrane (PKT), Feldspathic Highlands Terrane (FHT), and South Pole Aitken Terrane (SPAT)) in relation with their origin and distinct geologic history. The majority of radioelements are restricted in PKT, approving the scenario of KREEP (Potassium (K), rare earth elements (REE), Phosphorus (P)) residua concentrating under the Procellarum region. Moreover, we found the consistency of distribution for radioelements and basalts, concluding that the subsequent volcanism might be associated with local concentrations of radioelements in western Oceanus Procellarum and northwestern South Pole Aitken Basin. The prominent and asymmetric radioactive signatures were confirmed in SPAT comparing to FHT dominated by low level radioactivity, while the magnitudes are much lower than that of PKT, indicating a primary geochemical heterogeneity for the Moon.

  11. 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.

  12. Paleozoic subduction complex and Paleozoic-Mesozoic island-arc volcano-plutonic assemblages in the northern Sierra terrane

    USGS Publications Warehouse

    Hanson, Richard E.; Girty, Gary H.; Harwood, David S.; Schweickert, Richard A.

    2000-01-01

    This field trip provides an overview of the stratigraphic and structural evolution of the northern Sierra terrane, which forms a significant part of the wall rocks on the western side of the later Mesozoic Sierra Nevada batholith in California. The terrane consists of a pre-Late Devonian subduction complex (Shoo Fly Complex) overlain by submarine arc-related deposits that record the evolution of three separate island-arc systems in the Late Sevonian-Early Mississippian, Permian, and Late Triassic-Jurassic. The two Paleozoic are packages and the underlying Shoo Fly Complex have an important bearing on plate-tectonic processes affecting the convergent margin outboard of the Paleozoic Cordilleran miogeocline, although their original paleogeographic relations to North America are controversial. The third arc package represents an overlap assemblage that ties the terrane to North America by the Late Triassic and helps constrain the nature and timing of Mesozoic orogenesis. Several of the field-trip stops examine the record of pre-Late Devonian subduction contained in the Shoo Fly Complex, as well as the paleovolcanology of the overlying Devonian to Jurassic arc rocks. Excellent glaciated exposures provide the opportunity to study a cross section through a tilted Devonian volcano-plutonic association. Additional stops focus on plutonic rocks emplaced during the Middle Jurassic arc magmatism in the terrane, and during the main pulse of Cretaceous magmatism in the Sierra Nevada batholith to the east.

  13. Late Paleozoic intrusive rocks from the southeastern Lhasa terrane, Tibetan Plateau, and their Late Mesozoic metamorphism and tectonic implications

    NASA Astrophysics Data System (ADS)

    Dong, Xin; Zhang, Zeming; Liu, Feng; He, Zhenyu; Lin, Yanhao

    2014-06-01

    The Lhasa terrane in southern Tibet experienced Late Paleozoic and Mesozoic-Cenozoic composite orogenesis. This work reports a study on the petrology, geochemistry, zircon U-Pb chronology and Hf isotopes of Late Paleozoic and Late Mesozoic intrusive rocks from the southeastern Lhasa terrane. The Late Paleozoic intrusive rocks crystallized in the Late Devonian-Early Carboniferous of 371 to 355 Ma, representing a bimodal igneous association formed in the back-arc extensional setting. The mafic end-member originated from the enriched mantle and experienced contamination of crustal materials, characterized by a slight enrichment of LREE, positive anomalies of U, K and Pb and negative anomalies of Th, Nb, Ta and Ti. The felsic end-member was derived from the partial melting of the ancient continental crust, characterized by metaluminous, positive anomalies of Th, Zr and Hf, negative anomalies of Ba, Sr, Nb, Ta and Ti and negative εHf(t) values of zircon with TDM2 ages from 1.90 to 1.40 Ga. The Late Cretaceous (ca. 107 Ma) mafic intrusions, along with the Late Paleozoic intrusive rocks, underwent nearly syn-intrusion amphibolite-facies metamorphism under P-T conditions of 0.56 to 0.69 GPa and 692 to 735 °C during the Andean-type orogeny correlated with the subduction of the Neo-Tethyan oceanic slab beneath the Lhasa terrane. This study provides a new insight into the pre-Cenozoic tectonic evolution of the Lhasa terrane.

  14. P-T-t paths and differential Alleghanian loading and uplift of the Bronson Hill terrane south central New England

    USGS Publications Warehouse

    Wintsch, R.P.; Kunk, M.J.; Boyd, J.L.; Aleinikoff, J.N.

    2003-01-01

    Late Paleozoic U-Pb ages of sphene and 40Ar/39Ar cooling ages of amphibole and muscovite from rocks of the Bronson Hill terrane in Connecticut and central Massachusetts reflect a late Paleozoic (Alleghanian) overprint on Acadian metamorphic rocks. Prograde Alleghanian sphenes crystallized during the Late Pennsylvanian, and eliminate the possibility that amphibole ages reflect delayed Permian cooling from Devonian Acadian metamorphism. Fourteen new amphibole ages from Connecticut form a north-to-south trend of decreasing age from 294 to 245 Ma, while in Massachusetts four new amphibole ages together with three others from the literature produce a random Carboniferous pattern. Seven new muscovite ages support existing data indicating uniform cooling throughout the Bronson Hill terrane through ???350??C in the Early Triassic. The rate of Permian cooling defined by amphibole-muscovite pairs increases from ???4??C/my in northern Connecticut to ???50??C/my near Long Island Sound. Hinged loading and hinged but delayed exhumation in the southern part of the Bronson Hill terrane (with the hinge in central Connecticut) explain these ages and cooling rates as well as a southerly increasing metamorphic field gradient. One-dimensional thermal modeling indicates that loading of Bronson Hill rocks must have begun by the Late Mississippian. The time of peak Alleghanian metamorphic temperature decreases southward from Early Permian in northern Connecticut to Late Permian to the south. These results demonstrate that the metamorphic effects of the Alleghanian orogeny are not restricted to the Avalon terrane of southeastern New England. On the contrary, the Alleghanian orogeny reset 40Ar/39Ar mineral ages, recrystallized minerals, partially melted felsic rocks, and transposed fabrics at least as far west as the Bronson Hill terrane in south-central New England.

  15. Comparison of radon in soil over faulted crystalline terranes: Glaciated versus unglaciated

    SciTech Connect

    Gates, A.E.; Malizzi, L.D. ); Gundersen, L.C.S. )

    1990-05-01

    Radon in soil correlates directly with the bedrock geology in unglaciated terranes. In the Hylas shear zone, Virginia, the rock units exhibit a marked contrast in uranium concentration which is reflected in soil radon. The soils are clay-rich and are derived directly from the underlying bedrock. Even small geologic features such as pegmatite veins and thin shear zones are discernable by differences in soil radon concentrations. This bedrock/soil radon correlation is obscured in areas with thick glacial covers. The Reservoir fault area, New Jersey, is proximal to a terminal moraine. Although the bedrock units exhibit a strong contrast in radioactivity and uranium concentration, soil radon concentrations in the overlying tills reflect the local chemical and physical properties of the till rather than the bedrock.

  16. Regional Geophysical Expression of a Carbonatite Terrane in the Eastern Mojave Desert, California

    NASA Astrophysics Data System (ADS)

    Denton, K. M.; Ponce, D. A.; Miller, D. M.

    2012-12-01

    A world-class, rare earth element carbonatite deposit is located near Mountain Pass, in the eastern Mojave Desert of California and is hosted by Proterozoic rocks that extend along the eastern margins of the Clark Mountain Range, Mescal Range, and Ivanpah Mountains in a north-northwest trending fault-bounded block. This Proterozoic block is generally composed of a complex of 1.7 - 1.6 Ga gneisses and schists that are intruded by ~1.4 Ga carbonatite and ultrapotassic mafic dikes. In the latter suite, common intrusive rock types include shonkinite, syenite, and alkali granites that are associated with carbonatite dikes. Regional geophysical data reveal that the carbonatite deposit itself occurs along the northeast edge of a prominent magnetic high with an amplitude of 200 nanoteslas, which appears to be related to the surrounding Proterozoic block. More than 340 gravity stations and 155 physical property samples were collected to augment existing geophysical data to determine the geophysical and geologic setting of the eastern Mojave Desert carbonatite terrane. Physical properties of representative rock types in the area show that 23 samples of carbonatite ore have an average saturated bulk density of 2,866 with a range of 2,440 to 3,192 kg/m3 and a magnetic susceptibility of 0.22 with a range of 0.03 to 0.61x 10-3 SI units, 17 samples of syenite have an average saturated bulk density of 2,670 with a range of 2,555 to 2,788 kg/m3 and a magnetic susceptibility of 3.50 with a range of 0.19 to 11.46 x 10-3 SI units, 19 samples of shonkinite dike have an average saturated bulk density of 2,800 with a range of 2,603 to 3,000 kg/m3 and a magnetic susceptibility of 0.71 with a range of 0.00 to 4.44 x 10-3 SI units, and 28 samples of Proterozoic gneiss have an average saturated bulk density of 2,734 with a range of 2,574 to 3,086 kg/m3 and a magnetic susceptibility of 1.23 with a range of 0.01 to 7.48 x 10-3 SI units. In general, carbonatites have distinctive gravity

  17. Geology of the Southern Zambales Ophiolite Complex, (Philippines): juxtaposed terranes of diverse origin

    NASA Astrophysics Data System (ADS)

    Yumul, G. P.; Dimalanta, C. B.

    1997-08-01

    The Zambales Ophiolite Complex, which is made up of three massifs—Masinloc, Cabangan and San Antonio, was generated in a subduction-related marginal basin. Combined field geological and geochemical evidences show that the Cabangan and San Antonio massifs are genetically related to the Coto (transitional mid-ocean ridge-island arc) and Acoje (island arc) blocks of the Masinloc massif, respectively. A tectonic contact, the Subic Bay Fault Zone, is believed to separate the San Antonio and Cabangan massifs. The San Antonio massif is a displaced terrane rifted from the Acoje block and translated southward to its present position through the West Luzon Shear (?)/Subic Bay Fault Zone. The Zambales Ophiolite Complex could have not formed through the rifting or folding of the arc-related Acoje block-San Antonio massif to generate the transitional mid-ocean ridge-island arc like Coto block-Cabangan massif!

  18. Regional geophysical expression of a carbonatite terrane in the eastern Mojave Desert, California

    USGS Publications Warehouse

    Ponce, David A.; Denton, Kevin M.; Miller, David M.

    2013-01-01

    A world-class, rare earth element carbonatite deposit is located near Mountain Pass, in the eastern Mojave Desert of California and is hosted by Proterozoic rocks that extend along the eastern margins of the Clark Mountain Range, Mescal Range, and Ivanpah Mountains in a north-northwest trending fault-bounded block. This Proterozoic block is generally composed of a complex of 1.7 - 1.6 Ga gneisses and schists that are intruded by ~1.4 Ga carbonatite and ultrapotassic mafic dikes. In the latter suite, common intrusive rock types include shonkinite, syenite, and alkali granites that are associated with carbonatite dikes. Regional geophysical data reveal that the carbonatite deposit itself occurs along the northeast edge of a prominent magnetic high with an amplitude of 200 nanoteslas, which appears to be related to the surrounding Proterozoic block. More than 340 gravity stations and 155 physical property samples were collected to augment existing geophysical data to determine the geophysical and geologic setting of the eastern Mojave Desert carbonatite terrane. Physical properties of representative rock types in the area show that 23 samples of carbonatite ore have an average saturated bulk density of 2,866 with a range of 2,440 to 3,192 kg/m3 and a magnetic susceptibility of 0.22 with a range of 0.03 to 0.61x 10-3 SI units, 17 samples of syenite have an average saturated bulk density of 2,670 with a range of 2,555 to 2,788 kg/m3 and a magnetic susceptibility of 3.50 with a range of 0.19 to 11.46 x 10-3 SI units, 19 samples of shonkinite dike have an average saturated bulk density of 2,800 with a range of 2,603 to 3,000 kg/m3 and a magnetic susceptibility of 0.71 with a range of 0.00 to 4.44 x 10-3 SI units, and 28 samples of Proterozoic gneiss have an average saturated bulk density of 2,734 with a range of 2,574 to 3,086 kg/m3 and a magnetic susceptibility of 1.23 with a range of 0.01 to 7.48 x 10-3 SI units. In general, carbonatites have distinctive gravity

  19. Origin of deep crystal reflections: seismic profiling across high-grade metamorphic terranes in Canada

    USGS Publications Warehouse

    Green, A.; Milkereit, B.; Percival, J.; Davidson, A.; Parrish, R.; Cook, F.; Geis, W.; Cannon, W.; Hutchinson, D.; West, G.; Clowes, R.

    1990-01-01

    In an attempt to better understand the origin of deep crustal reflections LITHOPROBE has sponsored or co-sponsored Seismic reflection surveys across tracts of high-grade metamorphic rock in the Archean Superior craton, the Proterozoic Grenville orogen and the Phanerozoic Cordilleran orogen. Common to these three diverse terranes are near-surface zones of prominent Seismic reflectivity that are typically associated with velocity discontinuities at highly strained contacts between gneissic rocks of varying lithology. At some locations the reflective layering resulted from transposition and rearrangement of previously layered rocks (stratified assemblages, sills, etc.), whereas in other regions it was generated by extreme attenuation, stretching and ductile flow of weakly layered or irregularly organized rocks. It seems likely that compositionally layered gneissic rock is a common source of reflections in the deep crust, with reflections originating at lithological boundaries and zones of mylonite. ?? 1990.

  20. 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.

  1. Yield of bedrock wells in the Nashoba terrane, central and eastern Massachusetts

    USGS Publications Warehouse

    DeSimone, Leslie A.; Barbaro, Jeffrey R.

    2012-01-01

    The yield of bedrock wells in the fractured-bedrock aquifers of the Nashoba terrane and surrounding area, central and eastern Massachusetts, was investigated with analyses of existing data. Reported well yield was compiled for 7,287 wells from Massachusetts Department of Environmental Protection and U.S. Geological Survey databases. Yield of these wells ranged from 0.04 to 625 gallons per minute. In a comparison with data from 103 supply wells, yield and specific capacity from aquifer tests were well correlated, indicating that reported well yield was a reasonable measure of aquifer characteristics in the study area. Statistically significant relations were determined between well yield and a number of cultural and hydrogeologic factors. Cultural variables included intended water use, well depth, year of construction, and method of yield measurement. Bedrock geology, topography, surficial geology, and proximity to surface waters were statistically significant hydrogeologic factors. Yield of wells was higher in areas of granites, mafic intrusive rocks, and amphibolites than in areas of schists and gneisses or pelitic rocks; higher in valleys and low-slope areas than on hills, ridges, or high slopes; higher in areas overlain by stratified glacial deposits than in areas overlain by till; and higher in close proximity to streams, ponds, and wetlands than at greater distances from these surface-water features. Proximity to mapped faults and to lineaments from aerial photographs also were related to well yield by some measures in three quadrangles in the study area. Although the statistical significance of these relations was high, their predictive power was low, and these relations explained little of the variability in the well-yield data. Similar results were determined from a multivariate regression analysis. Multivariate regression models for the Nashoba terrane and for a three-quadrangle subarea included, as significant variables, many of the cultural and

  2. Comparison and analysis of linear alignments from Landsat imagery of diverse geologic terrane

    SciTech Connect

    Frederking, R.L.; Allenbach, R.T.; Jackson, R.B.; Keefer, W.D.; Motamedi, A.R.; Von Der Heyde, W.S.; Wingo, R.D.

    1985-01-01

    Geometric alignments referred to as linears, linear features or lineaments are routinely observed on Landsat imagery. Alignments that cannot be accounted for by cultural, known geologic or other recognized sources are classed as speculative and of dubious geologic significance. These features may in fact be imaginary. Speculative alignments are however, important to exploration because of their frequent association with known hydrocarbon accumulations and/or areas of mineralization. Data pertaining to speculative linear alignments in diverse geologic terrane have been obtained. Alignment frequency rose diagrams are prepared for each area and statistical profiles are developed. Replication of linear alignments suggest that these features are real elements of the imagery and not imaginary lines. Observed orientation modal frequencies are consistent with alignment trends of known structural features in the different areas. Variation in spatial density of alignments appears to be related to lithologic factors.

  3. 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.

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

    SciTech Connect

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

    1993-04-01

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

  5. Carbonatite: A Geophysical investigation of a rare earth element terrane, eastern Mojave Desert, California

    NASA Astrophysics Data System (ADS)

    Denton, K. M.; Ponce, D. A.; Miller, D. M.; MacPherson-Krutsky, C. C.

    2013-12-01

    Geophysical investigations reveal gravity and magnetic anomalies related to a Proterozoic carbonatite terrane in the eastern Mojave Desert, host to one of the largest rare earth element carbonatite deposits in the world. The deposit is located near Mountain Pass, California and occurs in a north-northwest trending fault-bounded block that extends along the eastern parts of the Clarke Mountain Range, Mescal Range, and Ivanpah Mountains. This Early to Middle Proterozoic block is composed of a 1.7 Ga metamorphic complex of gneiss and schist intruded by a 1.4 Ga suite of ultrapotassic alkaline intrusive rocks that includes carbonatite. The intrusive suite (oldest to youngest) includes shonkinite, mesosyenite, syenite, quartz syenite, potassic granite, carbonatite, and late shonkinite dikes which are spatially and temporally associated with carbonatite intrusions and dikes. Regional geophysical data reveal that the carbonatite deposit occurs along a gravity high and the northeast edge of a prominent magnetic high with an amplitude of about 200 nanoteslas. More than 1400 gravity stations and over 200 physical property samples were collected to augment existing geophysical data and will be used to determine the geophysical and geologic setting that provide an improved structural interpretation of the eastern Mojave Desert carbonatite terrane. Physical properties of representative rock types in the area include carbonatite ore, syenite, shonkinite, gneiss, granite, and dolomite. Carbonatite intrusions typically have distinctive gravity, magnetic, and radiometric signatures because these deposits are relatively dense, contain magnetite, and are enriched in thorium or uranium. However, our results show that the main carbonatite body is essentially nonmagnetic. Thus, it is unlikely that carbonatite rocks are the source of the magnetic high associated with the Clark Mountain and Mescal Ranges. Instead, we suggest that weakly to moderately magnetic intrusive rocks or

  6. A comparison of the seismic structure of oceanic island arc crust and continental accreted arc terranes

    NASA Astrophysics Data System (ADS)

    Calvert, A. J.

    2015-12-01

    Amalgamation of island arcs and their accretion to pre-existing continents is considered to have been one of the primary mechanisms of continental growth over the last 3 Ga, with arc terranes identified within Late Archean, Proterozoic, and Phanerozoic continental crust. Crustal-scale seismic refraction surveys can provide P wave velocity models that can be used as a proxy for crustal composition, and although they indicate some velocity variation in accreted arcs, these terranes have significantly lower velocities, and are hence significantly more felsic, than modern island arcs. Modern oceanic arcs exhibit significant variations in crustal thickness, from as little as 10 km in the Bonin arc to 35 km in the Aleutian and northern Izu arcs. Although globally island arcs appear to have a mafic composition, intermediate composition crust is inferred in central America and parts of the Izu arc. The absence of a sharp velocity contrast at the Moho appears to be a first order characteristic of island arc crust, and indicates the existence of a broad crust-mantle transition zone. Multichannel seismic reflection surveys complement refraction surveys by revealing structures associated with variations in density and seismic velocity at the scale of a few hundred meters or less to depths of 60 km or more. Surveys from the Mariana and Aleutian arcs show that modern middle and lower arc crust is mostly non-reflective, but reflections are observed from depths 5-25 km below the refraction Moho suggesting the localized presence of arc roots that may comprise gabbro, garnet gabbro, and pyroxenite within a broad transition from mafic lower crust to ultramafic mantle. Such reflective, high velocity roots are likely separated from the overlying arc crust prior to, or during arc-continent collision, and seismic reflections within accreted arc crust document the collisional process and final crustal architecture.

  7. 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.

  8. Terranes in the foreland of the East European Craton imaged by the CELEBRATION 2000 experiment

    NASA Astrophysics Data System (ADS)

    Celebration 2000 Working Group; Malinowski, M.

    2003-04-01

    The results of the CELEBRATION 2000 seismic experiment help us to resolve the question of the western extent of the East European Craton (EEC) crust in SE Poland, as well as to delineate main paleozoic terranes in this region. Profiles CEL01 and CEL02 cross each other in the Holy Cross Mts. area and end in the EEC. Profile CEL03 runs through the TESZ along the border of the EEC. 2-D P-wave velocity models along those lines were obtained by 2-D ray tracing technique using package SEIS83 (Červeny and Pšencík, 1983). The model along CEL02 profile shows great contrast in the crustal structure between Upper Silesian and Malopolska terranes, separated by the Krakow - Lubliniec Zone. The Malopolska Block is characterised by Vp values similar to the rest of the TESZ (profiles CEL03, TTZ, and lines from POLONAISE'97 experiment): 5.4-5.8 km/s down to 20 km depth. We have found an unusually high velocity body (HVB) (6.8 - 7.2 km/s at 15-17 km depth) beneath CEL01 and CEL02 profiles in the Lublin Trough region, which coincides with positive gravity anomalies. Further towards the West, such velocities have been imaged on CEL03 profile - in the axis of the TESZ we have modelled a thin and elongated HVB, which might be a result of the magmatic activity along the EEC border and probably is an westward extension of the HVB from CEL01 and CEL02 lines. The Moho depth varies from 45 km in the EEC part to 35-32 km in the Malopolska Block and the Upper Silesian Block. It comes from our models that EEC lower crust seems to extend as far as Holy Cross Mts. Fault, however this view is still debated.

  9. Paleomagnetic evidence for Post-Jurassic stability of southeastern Mexico: Maya Terrane

    NASA Astrophysics Data System (ADS)

    Guerrero, Jose C.; Herrero-Bervera, Emilio; Helsley, Charles E.

    1990-05-01

    The tectonic evolution of southeastern Mexico has been a subject of major controversy, not only in regard to past geometry but also in the timing of proposed geological events as well. For the past 10 years, most, if not all, investigators agree that the Gulf of Mexico Basin was formed by Late Jurassic time and that the Maya Terrane was in its current location prior to the Cretaceous. In order to gain further insight into the drift history of the Maya Terrane we have undertaken a paleomagnetic study of the uppermost Jurassic-Lower Cretaceous (Tithonian-lower Neocomian?) San Ricardo Formation in southeastern Mexico, at 93.7°W, 16.8°N. The sampling site is located east of the Isthmus of Tehuantepec, on the southwest side of the Maya block, at the base of the Yucatan Peninsula. A suite of 133 samples was collected in stratigraphic succession from a 114-m-thick sequence of red shales and sandstones near Cintalapa, Chiapas, Mexico. After progressive thermal demagnetization of all samples at six steps from 350°C to 630°C, 89 samples were selected for final paleopole analysis on the basis of their magnetic stability. Four different polarity intervals were observed, the sequence being from bottom to top: N, R, N, R which assists in the assessment of the reliability of the observations. The mean pole position obtained, 160.0°E, 69.8°N, agrees with the mean pole position of the upper part of the Morrison Formation of Colorado, a unit of virtually identical age. These results indicate that no discernible rotation or displacement of the Maya block has occurred since at least early Neocomian times.

  10. Sedimentology and tectonics of Devonian Nation River Formation, Alaska, part of yet another allochthonous terrane

    SciTech Connect

    Howell, D.G.; Murray, R.W.; Wiley, T.J.; Boundy-Sanders, S.; Kauffman-Linam, L.; Jones, D.L.

    1987-05-01

    Sandwiched between terra incognito of the Yukon Flats, Alaska, and the disrupted cratonal sequences of Yukon Territory, Canada, is a complex array of Proterozoic and Phanerozoic rock units composing a poorly defined group of tectonostratigraphic terranes. The Nation River formation (NRF) is a conspicuous siliciclastic submarine fan complex interbedded in a Paleozoic sequence characterized by deep-water cherts, siliceous shales, and platform to basin-plain carbonates. The NRF ranges from 500 to 2000 m thick. Where the basal part is exposed, NRF overlies the Devonian McCann Hill Chert, a deep-water radiolarian chert sequence. Above the NRF is either another radiolarian chert sequence, the Mississippian Ford Lake Shale, or Permian shallow-water Tahkandit Limestone or Step Conglomerate. NRF lithologies include fine-grained to pebbly turbidites assembled in both thinning- and fining-upward and thickening- and coarsening-upward cycles typical of middle to outer fan settings. Compositionally the grains are principally chert (green, gray, white, black, and rarely red) with minor amounts of vein quartz and quartz sandstone. Most of the chert seems to be replacement chert from a carbonate terrane, though some pebbles yield an Ordovician radiolarian assemblage. Paleocurrent flow directions based on thousands of bottom features (flutes, prods, and grooves) indicate, in present-day coordinates, flow toward the east. Individual azimuth directions are throughout the two easterly quadrants, by 60% of these data indicate flow between 045 and 150/sup 0/. This spread of data is consistent from outcrop to outcrop, indicating that there are no localized block rotations. Easterly flow has also been determined for the overlying Cretaceous units of the Kandik basin (Biederman Argillite and Kathul Graywacke).

  11. Age and duration of eclogite-facies metamorphism, North Qaidam HP/UHP terrane, Western China

    USGS Publications Warehouse

    Mattinson, C.G.; Wooden, J.L.; Liou, J.G.; Bird, D.K.; Wu, C.L.

    2006-01-01

    Amphibolite-facies para-and orthogneisses near Dulan, at the southeast end of the North Qaidam terrane, enclose minor eclogite and peridotite which record ultra-high pressure (UHP) metamorphism associated with the Early Paleozoic continental collision of the Qilian and Qaidam microplates. Field relations and coesite inclusions in zircons from paragneiss suggest that felsic, mafic, and ultramafic rocks all experienced UHP metamorphism and a common amphibolite-facies retrogression. SHRIMP-RG U-Pb and REE analyses of zircons from four eclogites yield weighted mean ages of 449 to 422 Ma, and REE patterns (flat HREE, no Eu anomaly) and inclusions of garnet, omphacite, and rutile indicate these ages record eclogite-facies metamorphism. The coherent field relations of these samples, and the similar range of individual ages in each sample suggests that the ???25 m.y. age range reflects the duration of eclogite-facies conditions in the studied samples. Analyses from zircon cores in one sample yield scattered 433 to 474 Ma ages, reflecting partial overlap on rims, and constrain the minimum age of eclogite protolith crystallization. Inclusions of Th + REE-rich epidote, and zircon REE patterns are consistent with prograde metamorphic growth. In the Lu??liang Shan, approximately 350 km northwest in the North Qaidam terrane, ages interpreted to record eclogite-facies metamorphism of eclogite and garnet peridotite are as old as 495 Ma and as young as 414 Ma, which suggests that processes responsible for extended high-pressure residence are not restricted to the Dulan region. Evidence of prolonged eclogite-facies metamorphism in HP/UHP localities in the Northeast Greenland eclogite province, the Western Gneiss Region of Norway, and the western Alps suggests that long eclogite-facies residence may be globally significant in continental subduction/collision zones.

  12. Stratigraphic and structural implications of conodont and detrital zircon U-Pb ages from metamorphic rocks of the Coldfoot terrane, Brooks Range, Alaska

    USGS Publications Warehouse

    Moore, T.E.; Aleinikoff, J.N.; Harris, A.G.

    1997-01-01

    New paleontologic and isotopic data from the Emma Creek and Marion Creek schists of the Coldfoot terrane, Arctic Alaska superterrane, central Brooks Range, suggest Devonian and possibly younger ages of deposition for their sedimentary protoliths. Conodonts from marble of the Emma Creek schist, intruded by a roughly 392 Ma orthogneiss, are late Lochkovian (early Early Devonian, between about 408 and 396 Ma) and Silurian to Devonian at two other locations. Spherical to oblong detrital zircons from quartz-mica schist of the overlying Marion Creek schist yield mostly discordant U-Pb data suggestive of provenance ages of 3.0, 2.0-1.8, and 1.5-1.4 Ga; however, several euhedral grains of zircon from Marion Creek quartz-mica schist have concordant U-Pb ages from 370 to 360 Ma. The Marion Creek schist in our study area therefore is at least 26 m.y. younger than the Emma Creek schist. The age data imply that the protolith of the Emma Creek schist is age correlative with Devonian carbonate rocks in the Hammond and North Slope terranes, whereas the Marion Creek schist is age correlative with Upper Devonian and Lower Mississippian clastic sedimentary rocks of the Endicott Group in the Endicott Mountains terrane and shale and carbonate units in the De Long Mountains and Sheenjek River terranes. Consequently, tectonic models restoring the entire Coldfoot terrane beneath partly or wholly coeval rocks of the Hammond, Endicott Mountains, De Long Mountains, and Sheenjek River terranes of the Arctic Alaska superterrane require revision. Alternative reconstructions, including restoration of the Coldfoot terrane inboard of the Endicott Mountains terrane or outboard of the De Long Mountains and Sheenjek River terranes are plausible but require either larger amounts of shortening than previously suggested or indicate problematic facies relations. copyright. Published in 1997 by the American Geophysical Union.

  13. 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

  14. 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

  15. 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

  16. 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

  17. 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

  18. 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.

  19. Preliminary evaluation of the petroleum potential of the Tertiary accretionary terrane, west side of the Olympic Peninsula, Washington

    SciTech Connect

    Not Available

    1989-01-01

    Convergence between the Pacific and North American plates during late Eocene and late middle Miocene times produced two principal terranes of melange and broken formation (Eocene, Ozette Melange and Miocene, Hoh Melange) exposed onshore along the west side of the Olympic Peninsula. Organic geochemical analyses of 150 samples collected from these two accretionary terranes indicate that they are marginally mature and have a low content of type III organic matter, therefore, they are gas prone rather than oil prone. Geochemical analyses, using molecular markers, indicate that the oil in the Sunshine Mining Co. Medina No. 1 is related to oil extracted from middle Eocene siltstone of the Ozette Melange located as much as 140 km north of the well. The stable carbon and hydrogen isotopic abundance of methane in natural gas seeps and gas in the melange along the west side of the Olympic Peninsula indicate that the gas is mainly thermogenic; however, the relation between these two sources of gas is uncertain.

  20. Geochemistry, Metamorphic Assemblages, and Microstructures in Small Ultramafic Bodies from the Northern Nason Terrane, Washington

    NASA Astrophysics Data System (ADS)

    Magloughlin, J. F.

    2014-12-01

    Ultramafic bodies ranging from <1 to 2500 m in length occur in multiple settings across the northern part of the Nason Terrane in the North Cascade Mountains of Washington State. Within the Wenatchee Ridge Orthogneiss (WRO) the bodies are approximately equidimensional, ranging from dm-scale metasomatized lenses up to an exposed diameter of (typically) approximately 40 m. Some bodies are completely serpentinized, but others include dunite, harzburgite, and rare seams of pyroxenite. Many are rimmed by blackwall (talc, phlogopite, tremolite, chlorite, serpentine) coinciding with the Late Cretaceous metamorphism. The Napeequa Ultramafic Body (NUB), cut through and well exposed by the Napeequa River west of Lake Wenatchee, is within the White River Shear Zone (WRSZ, Magloughlin & McEwan, 1988). Though highly variable, it consists of dunite and peridotite and is variably serpentinized with common Mg-amphibole. Assemblages include serp+carb+chl+talc+opq and suggest upper greenschist to low amphibolite facies overprinting. Rare high-strain zones resemble relict pseudotachylyte veins. More common are ultramylonitic zones with olivine grain sizes of <5 microns, suggesting terrane-boundary paleostresses of >250 MPa. The Nine Mile Creek Ultramafic Body (NMCUB) and Grave Ultramafic Body (GUB) are the largest bodies outside of the White River Shear Zone, and are approximately 300 m and 800 long, respectively. Both are characterized by ol+talc+amph along with chlorite pseudomorphs, commonly cut by <40 micron thick ribbons of calcite, and rarely containing Si defined by chromite. These tectonites contain fine-grained olivine, but post-deformational, metamorphic cummingtonite and tremolite. In both bodies, a moderate to strong foliation is developed. It is suggested both bodies are retrogressed garnet peridotites. An interesting problem is why the possible retrogressed garnet peridotite bodies are present south of the WRSZ and surrounded by the metatonalites of the WRO, but none

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

    NASA Astrophysics Data System (ADS)

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

    2015-11-01

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

  2. Baltica from the late Precambrian to mid-Palaeozoic times: The gain and loss of a terrane's identity

    NASA Astrophysics Data System (ADS)

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

    2005-09-01

    The old terrane of Baltica occupies the mass of northern Europe eastwards to the Urals and lies mostly to the north of the Trans-European Suture Zone. The core, the East European Craton, is thick and formed of rocks dating back to well over 3 billion yr, and Protobaltica can be identified as forming part of the supercontinent of Rodinia at about 1 billion yr ago. Following Rodinia's break up at about 800 Ma, Protobaltica remained attached to Laurentia until it became the newly independent Baltica at between 570 and 550 Ma, with the inauguration of plate spreading to form the northern part of the Iapetus Ocean. To the south, during the Early Cambrian, Baltica was separated from Gondwana by the relatively narrow Ran Ocean. Baltica remained a separate terrane until its docking, firstly with Avalonia at the very end of the Ordovician (443 Ma), and then with Laurentia during the Silurian in the Scandian part of the Caledonide Orogeny. The terrane was much enlarged in the Vendian to include the areas such as Timan-Pechora now lying to the north as they became accreted to Baltica during the late Precambrian Timanide Orogeny. During the Cambrian and Ordovician, Baltica firstly rotated through more than 120° and then drifted northwards from high to low palaeolatitudes. New maps present Baltica's outline and progressive positioning, its late Precambrian and Lower Palaeozoic history, and the Cambrian, Ordovician and Silurian land, basins and biofacies belts within and around it. Some of the Lower Palaeozoic faunas are reviewed briefly: the oceans surrounding Baltica were so wide during the Early Ordovician that a substantial part of the benthic fauna of trilobites, brachiopods and other phyla were endemic. As those oceans narrowed, so the faunas of Baltica became progressively more similar to those of adjacent terranes. Some plankton distributions augment the palaeomagnetic data in latitudinal positioning.

  3. 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.

  4. A review of tectonic aspects of the Limpopo belt and other Archean high-grade gneissic terranes

    NASA Technical Reports Server (NTRS)

    Kidd, W. S. F.

    1985-01-01

    Published information on the Archean high grade terrains varies a great deal in the detail available. Such information as exists indicates marked differences in the lithic types and proportions present in the central Limpopo belt compared with the better studies of the other Archean high grade terrains. These differences may be important because they are expressed by the presence in the Limpopo belt of subordinate, but significant quantities (about 5% each) of two rock suites likely to have formed on a shallow marine platform of significant size (Eriksson and Kidd, in prep.). These suites consist of thick sections dominantly consisting of either carbonate and calc-silicate, or of pure metaquartzites, often fuchsite bearing, whose lithic characters are unlike those expected for metacherts but are very like those expected for platform arenites. Isotopic ages suggest these sediments are probably older than 3.3 Ga and younger than 3.5 Ga. Studies lead to the conclusions that (1) continental fragments large enough to provide a substrate for significant platform arenite and carbonate sedimentation existed by 3.3 to 3.5 Ga ago; (2) Wilson cycle tectonics seems to adequately explain most major features of the Archean gneissic terranes; and (3) Tibetan-Himalayan style collisional tectonics 2.6 Ga and older accounts for the large scale relationships between the Limpopo belt and the adjacent Archean greenstone granitoid terrane cratons. By inference, other more fragmentary Archean gneissic terranes may have once been part of such collisional zones.

  5. 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

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

    NASA Technical Reports Server (NTRS)

    Mart, Y.

    1988-01-01

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

  7. Mapping of tectonic corridors through hidden parts of the Greater Dharwar Terrane, Southern India

    NASA Astrophysics Data System (ADS)

    Raval, U.; Veeraswamy, K.

    2011-11-01

    The Dharwar craton (India) has been divided into western and eastern domains by a tectonic boundary between the Closepet granite and Chitradurga schist belt. It has been noted that only the central portion of the Dharwar craton is exposed; its northern projection is buried beneath the Deccan traps and its southern margin merges with a high grade metamorphic transition of late Archean age. Resolution of structure and extent of the Greater Dharwar Terrane (GDT) beneath its northern and southern end, the Deccan traps, metamorphic transition zone, and Cuddapah basin thus remains unsolved. Results of deep geophysical probings have been examined along with associated geological and tectonic data. The analysis leads to: (a) delineation of palaeo-accretionary corridors extending under the Deccan trap cover, along which, two damaging stable continental region earthquakes (Koyna and Latur) occurred ( Veeraswamy and Raval, 2005), (b) it is seen that the Latur earthquake seem to lies within an unsuspected tectonic corridor which bifurcates the eastern Dharwar craton (EDC) into eastern Dharwar craton-1 and eastern Dharwar craton-2 passing through the Cuddapah basin, (c) possibility of the Palghat-Cauvery shear zone connecting to the 2.5 Ga metamorphic Karimnagar granulite belt through the interface between Napier (NC) and Rayner (RC) complexes of East Antarctica, and (d) the indication that the Bavali-Moyar, Bhavani-Mettur and Palghat-Cauvery shear zones form the southern margins of the western Dharwar craton, eastern Dharwar craton-1 and eastern Dharwar craton-2 respectively.

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

    SciTech Connect

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

    1985-01-01

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

  9. Significance of exchanging SSURGO and STATSGO data when modeling hydrology in diverse physiographic terranes

    USGS Publications Warehouse

    Williamson, Tanja N.; Taylor, Charles J.; Newson, Jeremy K.

    2013-01-01

    The Water Availability Tool for Environmental Resources (WATER) is a TOPMODEL-based hydrologic model that depends on spatially accurate soils data to function in diverse terranes. In Kentucky, this includes mountainous regions, karstic plateau, and alluvial plains. Soils data are critical because they quantify the space to store water, as well as how water moves through the soil to the stream during storm events. We compared how the model performs using two different sources of soils data--Soil Survey Geographic Database (SSURGO) and State Soil Geographic Database laboratory data (STATSGO)--for 21 basins ranging in size from 17 to 1564 km2. Model results were consistently better when SSURGO data were used, likely due to the higher field capacity, porosity, and available-water holding capacity, which cause the model to store more soil-water in the landscape and improve streamflow estimates for both low- and high-flow conditions. In addition, there were significant differences in the conductivity multiplier and scaling parameter values that describe how water moves vertically and laterally, respectively, as quantified by TOPMODEL. We also evaluated whether partitioning areas that drain to streams via sinkholes in karstic basins as separate hydrologic modeling units (HMUs) improved model performance. There were significant differences between HMUs in properties that control soil-water storage in the model, although the effect of partitioning these HMUs on streamflow simulation was inconclusive.

  10. New insights into typical Archaean structures in greenstone terranes of western Ontario

    NASA Technical Reports Server (NTRS)

    Schwerdtner, W. M.

    1986-01-01

    Ongoing detailed field work in selected granitoid complexes of the western Wabigoon and Wawa Subprovinces, southern Canadian Shield, has led to several new conclusions: (1) Prominent gneiss domes are composed of prestrained tonalite-granodiorite and represent dense hoods of magmatic granitoid diapirs; (2) the deformation history of the prestrained gneiss remains to be unraveled; (3) the gneiss lacked a thick cover of mafic metavolcanics or other dense rocks at the time of magmatic diaprisim; (4) the synclinoral structure of large greenstone belts is older than the late gneiss domes and may have been initiated by volcano-tectonic processes; (5) small greenstone masses within the gneiss are complexly deformed, together with the gneiss; and, (6) no compelling evidence has been found of ductile early thrusting in the gneiss terranes. Zones of greenstone enclaves occur in hornblende-rich contaminated tonalite and are apt to be deformed magmatic septa. Elsewhere, the tonalite gneiss is biotite-rich and hornblende-poor. These conclusions rest on several new pieces of structural evidence; (1) oval plutons of syenite-diorite have magmatic strain fabrics and sharp contacts that are parallel to an axial-plane foliation in the surrounding refolded gneiss; (2) gneiss domes are lithologically composite and contain large sheath-like structures which are deformed early plutons, distorted earlier gneiss domes, or early ductile nappes produced by folding of planar plutonic septa, and (3) the predomal attitudes of gneissosity varied from point to point.

  11. Paragneiss zircon geochronology and trace element geochemistry, North Qaidam HP/UHP terrane, western China

    USGS Publications Warehouse

    Mattinson, C.G.; Wooden, J.L.; Zhang, J.X.; Bird, D.K.

    2009-01-01

    In the southeastern part of the North Qaidam terrane, near Dulan, paragneiss hosts minor peridotite and UHP eclogite. Zircon geochronology and trace element geochemistry of three paragneiss samples (located within a ???3 km transect) indicates that eclogite-facies metamorphism resulted in variable degrees of zircon growth and recrystallization in the three samples. Inherited zircon core age groups at 1.8 and 2.5 Ga suggest that the protoliths of these rocks may have received sediments from the Yangtze or North China cratons. Mineral inclusions, depletion in HREE, and absence of negative Eu anomalies indicate that zircon U-Pb ages of 431 ?? 5 Ma and 426 ?? 4 Ma reflect eclogite-facies zircon growth in two of the samples. Ti-in-zircon thermometry results are tightly grouped at ???660 and ???600 ??C, respectively. Inclusions of metamorphic minerals, scarcity of inherited cores, and lack of isotopic or trace element inheritance demonstrate that significant new metamorphic zircon growth must have occurred. In contrast, zircon in the third sample is dominated by inherited grains, and rims show isotopic and trace element inheritance, suggesting solid-state recrystallization of detrital zircon with only minor new growth. ?? 2009 Elsevier Ltd.

  12. Crustal structure of the northeastern margin of the Tibetan plateau from the Songpan-Ganzi terrane to the Ordos basin

    USGS Publications Warehouse

    Liu, M.; Mooney, W.D.; Li, S.; Okaya, N.; Detweiler, S.

    2006-01-01

    The 1000-km-long Darlag-Lanzhou-Jingbian seismic refraction profile is located in the NE margin of the Tibetan plateau. This profile crosses the northern Songpan-Ganzi terrane, the Qinling-Qilian fold system, the Haiyuan arcuate tectonic region, and the stable Ordos basin. The P-wave and S-wave velocity structure and Poisson's ratios reveal many significant characteristics in the profile. The crustal thickness increases from northeast to southwest. The average crustal thickness observed increases from 42??km in the Ordos basin to 63??km in the Songpan-Ganzi terrane. The crust becomes obviously thicker south of the Haiyuan fault and beneath the West-Qinlin Shan. The crustal velocities have significant variations along the profile. The average P-wave velocities for the crystalline crust vary between 6.3 and 6.4??km/s. Beneath the Songpan-Ganzi terrane, West-Qinling Shan, and Haiyuan arcuate tectonic region P-wave velocities of 6.3??km/s are 0.15??km/s lower than the worldwide average of 6.45??km/s. North of the Kunlun fault, with exclusion of the Haiyuan arcuate tectonic region, the average P-wave velocity is 6.4??km/s and only 0.5??km/s lower than the worldwide average. A combination of the P-wave velocity and Poisson's ratio suggests that the crust is dominantly felsic in composition with an intermediate composition at the base. A mafic lower crust is absent in the NE margin of the Tibetan plateau from the Songpan-Ganzi terrane to the Ordos basin. There are low velocity zones in the West-Qinling Shan and the Haiyuan arcuate tectonic region. The low velocity zones have low S-wave velocities and high Poisson's ratios, so it is possible these zones are due to partial melting. The crust is divided into two layers, the upper and the lower crust, with crustal thickening mainly in the lower crust as the NE Tibetan plateau is approached. The results in the study show that the thickness of the lower crust increases from 22 to 38??km as the crustal thickness increases from

  13. Geochemistry of Devonian and Carboniferous of sedimentary rocks in the Tsetserleg terrane, Hangay-Hentey basin, central Mongolia

    NASA Astrophysics Data System (ADS)

    Purevjav, N.; Roser, B.

    2011-12-01

    The Hangay-Hentey basin forms part of the Central Asian Orogen Belt in central Mongolia. Contrasting hypotheses have been proposed explain its origin, and its tectonic evolution and provenance are controversial. Many studies to date have been based on geological evidence and only limited petrographic and geochemical analysis is available. The Hangay-Hentey basin is divided into seven terranes, of which the Tsetserleg terrane is one. The Tsetserleg terrane consists of siliceous- clastic sediments deposited in a turbidite environment. It is divided into the Middle Devonian Erdenetsogt Formation (sandstone, siltstone, shale and jasper), Middle-Upper Devonian Tsetserleg Formation (sandstone and siltstone), and the Lower Carboniferous Jargalant Formation (finer grained sandstone and mudstone) Sandstone petrography and major and trace element geochemistry of mudstones and sandstones has been used in attempt to constrain some aspects of provenance, tectonic setting and weathering of these Devonian and Carboniferous sediments. The Devonian sandstones are moderately sorted, and composed of sub-angular to sub-rounded quartz, plagioclase, microcline and rock fragments, and a few grains of chlorite and mica. Volcanic (dacite and rhyolite) lithics dominant the lithic population (Lv/L >0.90), and QFL values suggest deposition in an undissected to transitional arc environment. Geochemically the sandstones immature and are classed as wackes, while the mudstones are classed as shales. Major and trace element concentrations of 94 sandstones and mudstones indicate both the Devonian and Carboniferous sediments in the Tsetserleg terrane were mainly derived from felsic sources, although more intermediate detritus dominates in some samples. Major element data suggests deposition probably occurred in an Active Continental Margin setting (ACM), but scatter into the ARC field means an evolved continental island arc (CIA) setting or back arc environment is also possible. Chemical Index of

  14. U-Pb Geochronologic, Nd Isotopic, and Geochemical Evidence for the Correlation of the Chopawamsic and Milton Terranes, Piedmont Zone, Southern Appalachian Orogen.

    PubMed

    Coler; Wortman; Samson; Hibbard; Stern

    2000-07-01

    We report U-Pb crystallization ages from four metavolcanic rocks and two granitic gneiss samples as well as whole-rock chemical analyses and Sm-Nd isotopic ratios from 25 metaigneous and metasedimentary rocks from the Chopawamsic and Milton terranes, southern Appalachian Orogen. A metarhyolite sample from the Chopawamsic Formation and a metabasalt sample from the Ta River Formation in the Chopawamsic terrane have indistinguishable U-Pb crystallization ages of 471.4+/-1.3 Ma and 470.0+1.3/-1.5 Ma, respectively. A sample from the Prospect granite that intruded metavolcanic rocks of the Ta River Formation yields a younger U-Pb date of 458.0+/-1 Ma. Metarhyolite and granitic gneiss samples from the northern part of the Milton terrane yield U-Pb dates of 458.5+3.8/-1.0 Ma and 450+/-1.8 Ma, respectively. Metavolcanic and metaplutonic rocks from both terranes span a range in major element composition from basalt to rhyolite. Trace element concentrations in these samples show enrichment in large-ion lithophile elements K, Ba, and Rb and depletion in high field strength elements Ti and Nb, similar to those from island arc volcanic rocks. Initial epsilon(Nd) values and T(DM) ages of the metaigneous and metasedimentary samples range from 0.2 to -7.2 and from 1200 to 1700 Ma for the Chopawamsic terrane and from 3.7 to -7.2 and from 850 to 1650 Ma for the Milton terrane. The crystallization ages for the metavolcanic and metaplutonic samples from both terranes indicate that Ordovician magmatism occurred in both. Similar epsilon(Nd) values from representative samples from both terranes suggest that both were generated from an isotopically similar source. Xenocrystic zircons from metavolcanic rocks in the Chopawamsic terrane have predominately Mesoproterozoic (207)Pb/(206)Pb ages (600-1300 Ma), but a single Archean (2.56 Ga) core was also present. The xenocrystic zircons and the generally negative epsilon(Nd) values indicate that both terranes are composed of isotopically evolved

  15. U-Pb Geochronologic, Nd Isotopic, and Geochemical Evidence for the Correlation of the Chopawamsic and Milton Terranes, Piedmont Zone, Southern Appalachian Orogen.

    PubMed

    Coler; Wortman; Samson; Hibbard; Stern

    2000-07-01

    We report U-Pb crystallization ages from four metavolcanic rocks and two granitic gneiss samples as well as whole-rock chemical analyses and Sm-Nd isotopic ratios from 25 metaigneous and metasedimentary rocks from the Chopawamsic and Milton terranes, southern Appalachian Orogen. A metarhyolite sample from the Chopawamsic Formation and a metabasalt sample from the Ta River Formation in the Chopawamsic terrane have indistinguishable U-Pb crystallization ages of 471.4+/-1.3 Ma and 470.0+1.3/-1.5 Ma, respectively. A sample from the Prospect granite that intruded metavolcanic rocks of the Ta River Formation yields a younger U-Pb date of 458.0+/-1 Ma. Metarhyolite and granitic gneiss samples from the northern part of the Milton terrane yield U-Pb dates of 458.5+3.8/-1.0 Ma and 450+/-1.8 Ma, respectively. Metavolcanic and metaplutonic rocks from both terranes span a range in major element composition from basalt to rhyolite. Trace element concentrations in these samples show enrichment in large-ion lithophile elements K, Ba, and Rb and depletion in high field strength elements Ti and Nb, similar to those from island arc volcanic rocks. Initial epsilon(Nd) values and T(DM) ages of the metaigneous and metasedimentary samples range from 0.2 to -7.2 and from 1200 to 1700 Ma for the Chopawamsic terrane and from 3.7 to -7.2 and from 850 to 1650 Ma for the Milton terrane. The crystallization ages for the metavolcanic and metaplutonic samples from both terranes indicate that Ordovician magmatism occurred in both. Similar epsilon(Nd) values from representative samples from both terranes suggest that both were generated from an isotopically similar source. Xenocrystic zircons from metavolcanic rocks in the Chopawamsic terrane have predominately Mesoproterozoic (207)Pb/(206)Pb ages (600-1300 Ma), but a single Archean (2.56 Ga) core was also present. The xenocrystic zircons and the generally negative epsilon(Nd) values indicate that both terranes are composed of isotopically evolved

  16. Detrital Zircon U-Pb Age Populations in Time and Space in the Arctic Alaska Terrane

    NASA Astrophysics Data System (ADS)

    Moore, T. E.

    2010-12-01

    The Arctic Alaska Terrane (ATT) occupies the only margin of the Ameriasia Basin whose origin and position since Paleozoic time is incompletely known. To better understand its tectonic history, detrital zircon (DZ) U-Pb ages from about 75 samples of clastic strata were obtained from representative parts of the ATT in northern Alaska. The oldest known strata of the AAT are Neoproterozoic clastic rocks exposed in the northeastern Brooks Range. DZ dating of these rocks show that they contain abundant ~1.8 Ga zircons and subordinate populations that indicate derivation from the northwest part of Laurentia. Upper Neoproterozoic strata in the Brooks Range, in contrast, contain populations dominated by ~600 Ma zircons. The latter ages are similar to those in parts of the ATT outside of northern Alaska, including the Seward Peninsula, Chukotka, and Wrangel Island that are thought to have been derived from the Timanian orogen of northern Baltica. Similar DZ populations have also been obtained from Silurian sandstones of the Lisburne Peninsula, suggesting that much of the western and southern parts of AAT may have formed in or near northern Baltica. A third group of DZ ages were found in deformed clastic rocks that were deposited across large parts of the North Slope in the Silurian and/or Devonian and are also present in parautochthonous settings in the Brooks Range. These rocks typically are dominated by DZ ages of 390-470 Ma, and sometimes contain subordinate non-Laurentian populations of ~1.5 Ga. These DZ ages, the underlying rocks of probable Baltic and Laurentian affinity, and evidence of significant deformation indicate that the ATT may have been constructed by Caledonian tectonism in the Silurian and Devonian. Following Devonian deformation, Mississippian to Triassic platform strata of the Ellesmerian Sequence were deposited on a regional unconformity. DZ ages from these rocks appear to reflect the compositions of the sub-unconformity units and indicate that Timanian

  17. 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

  18. The Greater Caucasus - A Galatian or Hanseatic terrane? Comment on “The formation of Pangea” by G.M. Stampfli, C. Hochard, C. Vérard, C. Wilhem and J. von Raumer [Tectonophysics 593 (2013) 1-19

    NASA Astrophysics Data System (ADS)

    Ruban, Dmitry A.

    2013-11-01

    A set of new global plate tectonic reconstructions sheds light on the evolution of the Earth's oceans, continents, and individual terranes in the Palaeozoic-Triassic. The Greater Caucasus is interpreted as a Hanseatic terrane that separated from Laurussia in the Devonian and then collided with the Gondwana-derived Galatian Superterrane. However, this contrasts with the hypothesis according to which the Greater Caucasus Terrane is Galatian in its nature. The reasons for the interpretation of the Greater Caucasus as a Hanseatic block are unclear, and this interpretation does not explain some observations. Further research is necessary in order to understand the plate tectonic position of the Greater Caucasus Terrane in the Palaeozoic.

  19. Variations in deformation styles within the central Maine terrane: An example from the Presidential Range, NH

    SciTech Connect

    Eusden, J.D. Jr.; Garesche, J.; Johnson, A.; Maconochie, J.M. . Dept. of Geology)

    1993-03-01

    The Silurian and Devonian cover rocks of the Central Maine Terrane in the northern Appalachians are a complex, highly metamorphosed, stratigraphy that has been equally, if not more complexly, deformed. Correlations of regional deformation sequences (be they Acadian and/or Alleghanian) have been difficult, controversial, and elusive. This stems from the common observation that across-strike and strike-parallel variations in deformation sequences appear to be the norm rather than the exception in this belt. Enormous variations in structural style have been routinely observed in a detailed, five-year study of the well-exposed outcrops above treeline in the Presidential Range. Domains of different structural styles have been mapped in the northern, central and southern portions of the range. The northern domain, including Mts. Madison, Adams, and Jefferson, is composed of various members of the Devonian Littleton Formation. The map pattern is dominated by three macroscopic F1 synclines that face and are interpreted to verge northeasterly. The central domain, which includes Mt. Clay and much of Great Gulf, is interpreted to be a klippe of presumably Silurian gneisses probably correlative to the Rangeley, Smalls Falls, and Madrid Formations. A stratigraphic discontinuity, interpreted as the Greenough Spring thrust fault, separates this domain from the others and truncates the early fold structures of all domains. Within the Klippe there is evidence for an early phase of deformation but no folds related to it have been mapped. The map pattern is dominated by second phase folds that deform the early schistosity. The Greenough Spring thrust fault, and the second phase folds within it are refolded by a late phase of deformation which is characterized by a macroscopic, overturned synform.

  20. Legal aspects of sinkhole development and flooding in karst terranes: 1. Review and synthesis

    NASA Astrophysics Data System (ADS)

    Quinlan, James F.

    1986-03-01

    Structures built within the area of influence of a sinkhole can be affected by collapse, subsidence, or flooding. Unanticipated property losses may be involved, and litigation commonly ensues. Insurance compensation for damages that result from sinkhole collapse or subsidence in a karst terrane are covered by statute only in Florida and by voluntary agreement of companies operating in Tennessee Liability or insurance compensation for damages resulting from sinkhole flooding is not specifically covered by any state or federal statute. Regulations of the National Flood Insurance Program have been interpreted to allow coverage by this program for homes affected by sinkhole flooding in Bowling Green, Kentucky In the present article, case law, legal concepts of groundwater and surface water, liability, and law review articles relevant to sinkhole litigation are summarized The rationales of plaintiffs and defendants are reviewed Liability for damages have been based on allegations of negligence, breach of various water law doctrines, trespass, nuisance, loss of support, breach of contract, and implied warranty of habitability Defenses against these allegations have been based on the merits of each of them and on caveat emptor Several alternative rationales for claiming liability for losses incurred because of sinkhole development or flooding are proposed and discussed. The little-known Henderson v Wade Sand and Gravel is highly recommended as an alternative leading case that clearly and justifiably gives protection to adjacent landowners, and ties liability for damages caused by groundwater pumpage to nuisance law and related interference with property rights. Several little-known litigated cases of sinkhole development in response to groundwater pumpage will be summarized in a second article at a later date. Concepts of liability are evolving It can be expected that the professional geologist or engineer will have an increasing number of claims made against him or her

  1. Noble metal and graphite formation in metamorphic rocks of the Khanka terrane, Far East Russia

    NASA Astrophysics Data System (ADS)

    Khanchuk, A. I.; Plyusnina, L. P.; Berdnikov, N. V.

    2015-03-01

    Noble metal-graphite mineralization has been identified in the Riphean-Cambrian metamorphic complexes of the northern Khanka terrane, Russia. The graphite mineralization is hosted in magmatic and sedimentary rocks metamorphosed under greenschist to granulite facies conditions. This paper provides the results of our study of the Turgenevo-Tamga graphite deposits. This study analyzes the geochemistry of the noble metals with the aim of determining the spatial relationships between noble metals and graphite. The graphitized rocks, analyzed by various geochemical methods, show a wide range of noble metal concentrations (ppm): Pt (0.02-62.13), Au (0.02-26), Ag (0.56-4.41), Pd (0.003-5.67), Ru (0.007-0.2), Rh (0.001-0.74), Ir (0.002-0.55), and Os (0.011-0.09). Crystallization from gas-condensates is indicated by the relationships between the noble metal mineralization and the graphite, and in particular the inhomogeneous distribution of graphite in the rocks, the inhomogeneous distribution of metals in the graphite, the microglobular graphite structures, and the carbon isotopic compositions. Thermal analysis and Raman spectroscopy indicate that some of the graphite formed from the metamorphism of sedimentary biogenic carbonaceous matter. The uneven distribution of noble metals in the rocks, and the compositional variability of the mineralization, implies that the origin of the metals was largely related to endogenic processes involving reduced fluids derived from depth. Our conclusion is that the noble metals and graphite mainly originated from magmatic fluids, but that some material was derived from exogenic and metamorphic sources.

  2. Bathymetric gradients within a Paleozoic Island Arc, southeastern Alaska (Alexander Terrane)

    SciTech Connect

    Soja, C.M. )

    1990-05-01

    Early to Late Silurian (Wenlock-Ludlow) limestones belonging to the Heceta Formation reflect bathymetric gradients within the ancient island arc exposed in the Alexander terrane of southeastern Alaska. These rocks record the earliest occurrence of widespread carbonate deposition in the region and represent the earliest foundation for shallow-water platform development within the arc. The excellent preservation of platform, platform margin, and slope deposits contrasts with the poor preservation of many marine sediments that originated within other island arcs. Hence, these limestones provide important insights into the styles, processes, and bathymetry of carbonate deposition in island arcs. Carbonate depositional sites within the arc extended laterally from nearshore intertidal and relatively shallow subtidal zones of a marine platform, to the seaward margins of a rimmed shelf, and into deeper subtidal areas of a slope environment. Fossiliferous deposits that originated on the platform comprise a diversity of shelly benthos, including corals and stromatoporoids in growth position. Dasycladacean algae, oncoids, and Amphipora also indicate shallow-water conditions. Organic buildups and reefs were constructed by cyanobacteria, massive stromatoporoids, corals, and algae at the platform margin. Deposition beyond the seaward edge of the shelf is evident from the carbonate turbidites that consist of skeletal debris of shallow-water derivation and an absence of coarse siliciclastic detritus. Sedimentation and resedimentation along a bathymetric gradient within the arc is especially well illustrated by the carbonate breccias that are enclosed within these deep subtidal sediments. They comprise detached stromatolites and clasts of shallow-water origin that were derived from the platform and its margin during periodic slumping of the shelf edge.

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

    NASA Astrophysics Data System (ADS)

    Tan, Jieqing; Aitchison, Jonathan

    2014-05-01

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

  5. The Mud Hills, Mojave Desert, California: Structure, stratigraphy and sedimentology of a rapidly extended terrane

    SciTech Connect

    Ingersoll, R.V.; Devaney, K.A.; Geslin, J.K.; Cavazza, W.; Diamond, D.S.; Jagiello, K.J.; Marsaglia, K.M.; Paylor, E.D. II; Short, P.F. . Dept. of Earth and Space Sciences)

    1993-04-01

    The Mud Hills exposes synorogenic breccia (Mud Hills Fm.) deposited during the final stages of crustal extension of the upper plate above the Waterman Hills detachment (20--18 Ma). Previous workers have misinterpreted fault contacts as stratigraphic contacts, and have developed intricate pseudostratigraphy to explain their observations. The authors' detailed mapping, combined with stratigraphic and sedimentologic data, documents that the volcaniclastic Pickhandle Fm. is conformably overlain by the plutoniclastic Mud Hills Fm., with no interfingering. Repetition of these south-dipping lithologic units is due to imbricate, north-dipping listric faults. These relations are demonstrated by the systematic northward v''ing of fault contacts and southward v''ing of stratigraphic contacts. Stratigraphic dip decreases upsection, which is consistent with incremental rotation of basinal strata simultaneously with deposition. Most of the Mud Hills Fm. consists of rock-avalanche breccia and megabreccia derived from granodiorite, which is identical to basement exposed beneath the Pickhandle and Jackhammer Fms. to the north. The Mud Hills Fm. was derived from now-buried granodiorite of a stranded upper-plate block to the south, as demonstrated by northward paleocurrents, facies relations and the presence of fine-grained units close to the presumed master fault (as is typical of half-graben sedimentation). Unconformably overlying the Mud Hills Fm. is the Owl Conglomerate (Barstow Fm.), which has mixed provenance with southward paleocurrents; the Owl Conglomerate was derived from residual highlands after extension ceased. Integration of structural, stratigraphic and sedimentologic information is essential for correct reconstruction of highly extended terranes.

  6. Quinn River Formation, Black Rock terrane, northern Nevada: New Permian and Triassic radiolarian data

    SciTech Connect

    Blome, C.D. . Federal Center); Reed, K.M. )

    1993-04-01

    The Quinn River Formation near Quinn River Crossing contains in ascending order: a basal tuff overlain by limestone and ferruginous dolomite, 25 m of dark interbedded radiolarian-bearing chert and argillite, an unconformably overlying 22 m of siltstone and carbonaceous shale, and 110 m of partly volcaniclastic rocks that include siltstone, shale, and minor sandstone and radiolarian-bearing argillite. Previous workers reported the occurrence of early Guadalupian radiolarians and the late Wordian conodonts Mesogondolella phosphoriensis and M. aff. M. prolongata from near the top of the overlying chert. Re-collection of the chert (35 samples) shows that all but the uppermost samples contain radiolarians characteristic of Kozur and Mostler's Capitanian (late Guadalupian) Follicucullus charveti-Imotoella triangularis Assemblage Zone. Similar faunas have been described from the Dekkas Formation in the Klamath Mountains and in cherts from the Grindstone terrane of east-central Oregon. Even younger radiolarian forms belonging to Neoalbaillella were found near the top of the chert section. Occurrence of Wordian conodonts at the same stratigraphic level as Capitanian or younger radiolarians suggests that the conodonts are reworked. In the overlying volcaniclastic section, previous workers reported Early Triassic radiolarians from the lower part, Middle Triassic (Ladinian) ammonites from the middle part, and an early Anisian ammonite and Middle Triassic conodonts and radiolarians from near the top. However, argillite approximately 25 m below the top of the section yielded a Late Triassic (Carnian) radiolarian fauna that includes taxa belonging to Castrum, Corum, Poulpus, Pseudostylosphaera, Triassocampe, and Xipha. This fauna is similar to that from the Fields Creek Fm in east-central Oregon. These anomalies suggest either that the previous collections from this part of the section are misdated or that the upper part of the volcaniclastic section is structurally disrupted.

  7. Epistemological issues in the study of microbial life: alternative terran biospheres?

    PubMed

    Cleland, Carol E

    2007-12-01

    The assumption that all life on Earth today shares the same basic molecular architecture and biochemistry is part of the paradigm of modern biology. This paper argues that there is little theoretical or empirical support for this widely held assumption. Scientists know that life could have been at least modestly different at the molecular level and it is clear that alternative molecular building blocks for life were available on the early Earth. If the emergence of life is, like other natural phenomena, highly probable given the right chemical and physical conditions then it seems likely that the early Earth hosted multiple origins of life, some of which produced chemical variations on life as we know it. While these points are often conceded, it is nevertheless maintained that any primitive alternatives to familiar life would have been eliminated long ago, either amalgamated into a single form of life through lateral gene transfer (LGT) or alternatively out-competed by our putatively more evolutionarily robust form of life. Besides, the argument continues, if such life forms still existed, we surely would have encountered telling signs of them by now. These arguments do not hold up well under close scrutiny. They reflect a host of assumptions that are grounded in our experience with large multicellular organisms and, most importantly, do not apply to microbial forms of life, which cannot be easily studied without the aid of sophisticated technologies. Significantly, the most powerful molecular biology techniques available-polymerase chain reaction (PCR) amplification of rRNA genes augmented by metagenomic analysis-could not detect such microbes if they existed. Given the profound philosophical and scientific importance that such a discovery would represent, a dedicated search for 'shadow microbes' (heretofore unrecognized 'alien' forms of terran microbial life) seems in order. The best place to start such a search is with puzzling (anomalous) phenomena, such as

  8. Epistemological issues in the study of microbial life: alternative terran biospheres?

    PubMed

    Cleland, Carol E

    2007-12-01

    The assumption that all life on Earth today shares the same basic molecular architecture and biochemistry is part of the paradigm of modern biology. This paper argues that there is little theoretical or empirical support for this widely held assumption. Scientists know that life could have been at least modestly different at the molecular level and it is clear that alternative molecular building blocks for life were available on the early Earth. If the emergence of life is, like other natural phenomena, highly probable given the right chemical and physical conditions then it seems likely that the early Earth hosted multiple origins of life, some of which produced chemical variations on life as we know it. While these points are often conceded, it is nevertheless maintained that any primitive alternatives to familiar life would have been eliminated long ago, either amalgamated into a single form of life through lateral gene transfer (LGT) or alternatively out-competed by our putatively more evolutionarily robust form of life. Besides, the argument continues, if such life forms still existed, we surely would have encountered telling signs of them by now. These arguments do not hold up well under close scrutiny. They reflect a host of assumptions that are grounded in our experience with large multicellular organisms and, most importantly, do not apply to microbial forms of life, which cannot be easily studied without the aid of sophisticated technologies. Significantly, the most powerful molecular biology techniques available-polymerase chain reaction (PCR) amplification of rRNA genes augmented by metagenomic analysis-could not detect such microbes if they existed. Given the profound philosophical and scientific importance that such a discovery would represent, a dedicated search for 'shadow microbes' (heretofore unrecognized 'alien' forms of terran microbial life) seems in order. The best place to start such a search is with puzzling (anomalous) phenomena, such as

  9. Crustal structure and tectonics of the northern part of the Southern Granulite Terrane, India

    USGS Publications Warehouse

    Rao, V.V.; Sain, K.; Reddy, P.R.; Mooney, W.D.

    2006-01-01

    Deep seismic reflection studies investigating the exposed Archean lower continental crust of the Southern Granulite Terrane, India, yield important constraints on the nature and evolution of the deep crust, including the formation and exhumation of granulites. Seismic reflection images along the Kuppam-Bhavani profile reveal a band of reflections that dip southward from 10.5 to 15.0??s two-way-time (TWT), across a distance of 50??km. The bottom of these reflections beneath the Dharwar craton is interpreted as the Moho. Further south, another reflection band dipping northward is observed. These bands of reflectivity constitute a divergent reflection fabric that converges at the Moho boundary observed at the Mettur shear zone. Reflection fabrics that intersect at a steep angle are interpreted as a collisional signature due to the convergence of crustal blocks, which we infer resulted in crustal thickening and the formation of granulites. Anomalous gravity and magnetic signatures are also observed across the Mettur shear zone. The gravity model derived from the Bouguer gravity data corroborates seismic results. The tectonic regime and seismic reflection profiles are combined in a 3-D representation that illustrates our evidence for paleo-subduction at a collision zone. The structural dissimilarities and geophysical anomalies suggest that the Mettur shear zone is a suture between the Dharwar craton in the north and another crustal block in the south. This study contributes significantly to our understanding of the operation of Archean plate tectonics, here inferred to involve collision and subduction. Furthermore, it provides an important link between the Gondwanaland and global granulite evolution occurring throughout the late Archean. ?? 2006 Elsevier B.V. All rights reserved.

  10. Lethality of Terrestrial Impacts may BE Strongly Influenced by Impacted Terrane Composition

    NASA Astrophysics Data System (ADS)

    Hildebrand, Alan

    The magnitude of environmental perturbations associated with impacts on Earth remains the greatest uncertainty in assessing the statistical risk of the impact hazard. Many large craters, including some young examples, such as the 100 km diameter Popigai crater, are not associated with environmental perturbations sufficient to have an obvious signature recorded in the sedimentary record, nor to have caused a mass extinction. The outstanding example in the Phanerozoic remains the Chicxulub impact which resulted in a crater 180 km across. While Chicxulub would have resulted in dissipated energy approximately an order of magnitude greater than Popigai, the latter did also distribute impact products across the globe. This could be explained by global lethality being strongly threshold dependent. However, Chicxulub-sized impacts are expected to occur with a recurrence interval of 100 million years, and other impact-induced mass extinctions are not known from the Phanerozoic. Understanding which of the environmental perturbations associated with Chicxulub were globally lethal remains the obvious way to resolve this apparent puzzle. Currently, a global dust layer (darkness for 3 months), an acid rain pulse, or long enduring ( one decade) shroud of sulphur-based stratigraphic aerosols seem the most likely lethal agents. The suitability of all three of these mechanisms has been challenged based upon the observed paleontological record (aerosol's decade-long effect) or plausibility that the Chicxulub impact would have generated a sufficient quantity of t necessary impact products (dust or acid rain). All three of these mechanisms are potentially strongly influenced by the impacted terrane on the Yucatan Peninsula having been covered by a 3 to 4 km-thick sequence of carbonate and evaporate sediments. Aside from these lithologies increasing the potential yield of aerosols and acid rain, the occurrence of a multi-component impact fireball may have substantially modified

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

    NASA Astrophysics Data System (ADS)

    Palandzhyan, S. A.

    2015-03-01

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

  12. Titanite-scale insights into multi-stage magma mixing in Early Cretaceous of NW Jiaodong terrane, North China Craton

    NASA Astrophysics Data System (ADS)

    Jiang, Peng; Yang, Kui-Feng; Fan, Hong-Rui; Liu, Xuan; Cai, Ya-Chun; Yang, Yue-Heng

    2016-08-01

    The Early Cretaceous Guojialing-type granodiorites in northwestern Jiaodong terrane carry significant records for strong mantle-crust interaction during the destruction of North China Craton (NCC); however, the definite petrogenetic mechanism and detailed magmatic process remain an enigma. Titanite in igneous rocks can serve as an effective petrogenetic indicator. Here, we present integrated geochronological and geochemical studies on titanites from Guojialing-type granodiorites and their dioritic enclaves to constrain their petrogenesis. Titanites from granodiorites (G-type) and plagioclase-rich dioritic enclaves (E-type-I) present an identical U-Pb age (~ 130 Ma) and an indistinguishable wide range of Zr and total REEs contents, and Th/U ratios. However, these two types of titanites exhibit distinct micro-scale textures and geochemical compositions. G-type titanites are characterized by oscillatory zonings with two Light BSE zones (LBZ) and two or three dark BSE zones, whereas E-type-I titanites are marked by core-mantle-rim zonings. Drastic increase of LREEs, Zr, Hf, and Fe and decrease of Nb, Ta, Al, and F contents are observed in LBZ of G-type titanites, whereas remarkable reduction of LREEs, Zr, and Hf and elevation of F contents are observed from the cores to the mantles of E-type-I titanites. Based on Zr-in-titanite thermometry, G-type titanites are interpreted to have experienced twice notable temperature increase, while E-type-I titanites are inferred to have undergone a rapid cooling process. Furthermore, we suggest that the drastic chemical changes in G-type and E-type-I titanites are ascribed to early-stage magma mixing between a colder felsic magma and a Fe-, REE-rich hotter dioritic magma. Compared to G-type and E-type-I titanites, titanites from plagioclase-poor dioritic enclaves (E-type-II) are characterized by their occurrence in interstitial space and present a relatively younger U-Pb age (~ 128 Ma) and much narrower and lower range of Zr, total

  13. 78 FR 79078 - Designation of 2 individuals Pursuant to Executive Order 13224 of September 23, 2001, “Blocking...

    Federal Register 2010, 2011, 2012, 2013, 2014

    2013-12-27

    ... AYED''; a.k.a. ``ABU AYID''), Yemen; DOB 04 Aug 1972; POB al-Zahir, al-Bayda', Yemen; Passport 03902409 (Yemen) issued 13 Jun 2010 expires 13 Jun 2016; alt. Passport 01772281 (Yemen); Personal ID Card 1987853 (Yemen) (individual) . 2. AL-NU'AYMI, 'Abd al-Rahman bin 'Umayr (a.k.a. AL NAIMEH, Abdelrahman Imer...

  14. ALS Association

    MedlinePlus

    ... toward a world without ALS! Walk to Defeat ALS® Walk to Defeat ALS® draws people of all ... We need your help. I Will Advocate National ALS Registry The National ALS Registry is a congressionally ...

  15. 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

  16. Disruption and translation of an orogenic wedge by exhumation of large continental ultrahigh pressure terranes: Examples from the Scandinavian Caledonides

    NASA Astrophysics Data System (ADS)

    Cuthbert, S.; Brueckner, H.

    2012-04-01

    Many collisional orogens are cored by extensive metamorphic terranes composed of reworked continental crust that developed high pressure/ultrahigh pressure (HP/UHP) metamorphic assemblages during subduction into the mantle. The return of these large, buoyant masses to shallow crustal levels has a major effect on orogenic architecture. A model is proposed where thrust-dominated accretion of an orogenic wedge during continental subduction is succeeded by stretching and passive transport of the wedge on top of an exhuming UHP terrane. Initial thrusting occurs when cratons collide and one subducts beneath the other into the mantle. The subducted portion of the craton undergoes HP/UHP metamorphism while an accretionary orogenic wedge develops at its junction with the overlying craton. Subsequent exhumation of the HP/UHP portion occurs either by true extension, which pulls it out of the mantle, and/or by buoyancy-driven extrusion, which inserts it along faults between the lower craton and the base of the wedge. In either case, shearing along the top of the exhuming terrane will reverse from foreland-directed thrusting during subduction to hinterlandward normal displacement during exhumation. The latter shear traction stretches the frontal part of the orogenic wedge away from the rearward part and may even detach it, allowing a fragment to be carried passively towards the foreland on the exhuming plate. The length of displacement would be a function of the amount of exhumation of the UHP terrane, and the total displacement of the leading wedge taper could be considerably further than indicated by palinspastic restorations of thrust allochthons alone. The Jotun and Trondheim Nappe Complexes form major allochthon elements of the Caledonide orogenic wedge in southern and central Scandinavia, respectively. We propose the late-stage behaviour of these allochthons was a response to the rise and lateral transport of the underlying HP/UHP Western Gneiss Complex (WGC). During the

  17. 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

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

    NASA Astrophysics Data System (ADS)

    Musgrave, Robert J.

    2013-06-01

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

  19. Crustal electrical structures and deep processes of the eastern Lhasa terrane in the south Tibetan plateau as revealed by magnetotelluric data

    NASA Astrophysics Data System (ADS)

    Xie, Chengliang; Jin, Sheng; Wei, Wenbo; Ye, Gaofeng; Jing, Jianen; Zhang, Letian; Dong, Hao; Yin, Yaotian; Wang, Gang; Xia, Ruixue

    2016-04-01

    A 3D Magnetotelluric (MT) inversion for a 2D broadband MT profile along 92°E in the eastern Lhasa terrane was applied to understand the crustal electrical structures and deep processes in the India-Tibet continental collision zone. The middle and lower (- 20 to - 50 km) crust is distributed with conductors that are primarily concentrated north of the Yarlung-Zangbo sutures (YZS). The results imply that the hypothesis of middle (and/or lower) crustal flow between the Tethyan Himalaya and Lhasa terrane are not supported by the MT data within the profile area. We suggest that given the possibility of the existence of channel flow in the middle (and/or lower) crust extruding southward from Tibet, the southernmost portion should be limited in the northern YZS. The electric model also indicates that the primarily conductive region in the middle to lower crust can be imaged from the YZS to ~ 30.8°N, while the crust of the northern Lhasa terrane north of ~ 30.8°N has a higher resistivity. From this result, it can be inferred that the northern Lhasa terrane might have a cold and strong middle to lower crust and that the front of the India crust might be halted in the northern Lhasa terrane (~ 30.8°N) along 92°E.

  20. 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.

  1. New Paleomagnetic Results From Late Paleozoic and Mesozoic Rocks of Tibet: Implications for the Paleogeography of the Eastern Qiangtang Terrane

    NASA Astrophysics Data System (ADS)

    Lippert, P. C.; Zhao, X.

    2005-12-01

    We present new paleomagnetic, rock magnetic, and geochronologic data from our ongoing study of late Paleozoic and Mesozoic paleomagnetic poles from the Eastern Qiangtang Terrane in Central Tibet. This study consists of four sampling localities of the Eastern Qiangtang Terrane, from south to north: 1) the Kaixingling basalts and andesites, with underlying marls and overlying limestones of late Permian age (11 sites, 81 samples); 2) a basalt flow and overlying quartzose sandstones exposed in the southern TuoTuo He basin, also of suggested late Permian age (4 sites, 41 samples); 3) basalts and andesites from the late Triassic (Norian) Batang Group (7 sites, 55 samples); and a mid-Cretaceous (?) tonolite 15 km south of Wudaoliang (2 sites, 18 samples). These locations have previously been dated by biostratigraphic ages of overlying and underlying strata. Here we present new geochronologic data from the volcanic rocks themselves. High NRM moment values and weak-field susceptibility vs. temperature measurements suggest that magnetite is the dominant magnetic mineral. Resistance to alternating field demagnetization in many samples, in addition to data from magnetic hysteresis measurements, however, indicate a significant amount of hematite exists in these samples as well. All samples were subjected to progressive thermal demagnetization to isolate the characteristic, primary, and secondary magnetizations. Preliminary results show useful paleomagnetic poles may be forthcoming from our collections. Our data may have significant bearing on the geodynamic history of Tibet, including paleogeography of the Eastern Qiangtang Terrane, block rotations, and the nature of the Tertiary low-paleolatitude anomaly in Central Asia.

  2. Plate rotation during continental collision and its relationship with the exhumation of UHP metamorphic terranes: Application to the Norwegian Caledonides

    NASA Astrophysics Data System (ADS)

    Bottrill, A. D.; van Hunen, J.; Cuthbert, S. J.; Brueckner, H. K.; Allen, M. B.

    2014-05-01

    variation and asynchronous onset of collision during the convergence of continents can significantly affect the burial and exhumation of subducted continental crust. Here we use 3-D numerical models for continental collision to discuss how deep burial and exhumation of high and ultrahigh pressure metamorphic (HP/UHP) rocks are enhanced by diachronous collision and the resulting rotation of the colliding plates. Rotation during collision locally favors eduction, the inversion of the subduction, and may explain the discontinuous distribution of ultra-high pressure (UHP) terranes along collision zones. For example, the terminal (Scandian) collision of Baltica and Laurentia, which formed the Scandinavian Caledonides, resulted in the exhumation of only one large HP/UHP terrane, the Western Gneiss Complex (WGC), near the southern end of the collision zone. Rotation of the subducting Baltica plate during collision may provide an explanation for this distribution. We explore this hypothesis by comparing orthogonal and diachronous collision models and conclude that a diachronous collision can transport continental material up to 60 km deeper, and heat material up to 300°C hotter, than an orthogonal collision. Our diachronous collision model predicts that subducted continental margin material returns to the surface only in the region where collision initiated. The diachronous collision model is consistent with petrological and geochonological observations from the WGC and makes predictions for the general evolution of the Scandinavian Caledonides. We propose the collision between Laurentia and Baltica started at the southern end of the collisional zone, and propagated northward. This asymmetric geometry resulted in the counter clockwise rotation of Baltica with respect to Laurentia, consistent with paleomagnetic data from other studies. Our model may have applications to other orogens with regional UHP terranes, such as the Dabie Shan and Papua New Guinea cases, where block

  3. Plate rotation during continental collision and its relationship with the exhumation of UHP metamorphic terranes: application to the Norwegian Caledonides

    NASA Astrophysics Data System (ADS)

    Bottrill, Andrew; van Hunen, Jeroan; Cuthbert, Simon; Allen, Mark; Brueckner, Hannes

    2014-05-01

    Lateral variation and asynchronous onset of collision during the convergence of continents can significantly affect the burial and exhumation of subducting material. We use 3D numerical models for continental collision to discuss how deep burial and exhumation of ultra-high pressure metamorphic rocks are enhanced by oblique convergence and resulting rotation of the colliding plates. Rotation during collision locally favours eduction, the inversion of the subduction process following ocean slab break-off, and may relate to the discontinuous distribution of ultra-high pressure (UHP) terranes along collision zones. For example the terminal (Scandian) collision of Baltica and Laurentia, which formed the Scandinavian Caledonides resulted in the exhumation of only one large high pressure/ultra-high pressure (HP/UHP) terrane, the Western Gneiss Complex (WGC), near the southern end of the collision zone. Rotation of the subducting Baltica plate during collision may provide a likely explanation for this distribution. We explore this hypothesis by comparing orthogonal and oblique collision models and conclude that an oblique collision can transport continental material up to 60km deeper, and heat material up to 300°C hotter, than an orthogonal collision. Our oblique collision model predicts that subducted continental margin material returns to the surface only in the region where collision initiated. The oblique collision model is consistent with petrological and geochonological observations from the Western Gneiss Complex and makes predictions for the general evolution of the Scandinavian Caledonides. We propose the collision between Laurentia and Baltica started at the southern end of the collisional zone, and propagated northward. This asymmetric geometry resulted in the counter clockwise rotation of Baltica and the northwards movement of Baltica's rotational pole with respect to Laurentia, consistent with paleomagnetic data from other studies. Our model has applications

  4. Paleomagnetic and geologic data indicating 2500 km of northward displacement for the Salinian and related terranes, California

    NASA Astrophysics Data System (ADS)

    Champion, D. E.; Howell, D. G.; Gromme, C. S.

    1984-09-01

    Results of a pair of paleomagnetic studies of sedimentary rocks in the Salinian terrane of westernmost California indicate a northward displacement of about 2500 km since Cretaceous time. Stratigraphic relations suggest that the Salinian terrane became amalgamated with several others by the Late Cretaceous and all underwent the same northward translation. This composite terrane (allochthon) became sutured to cratonal North America by early Tertiary time. One paleomagnetic study involved Upper Cretaceous turbidites of the Pigeon Point Formation. Fine-grained, thin-bedded turbidites yielded the most reliable results, whereas medium- to coarse-grained sandstones are not always stably magnetized. Contorted layers resulting from soft sediment deformation usually gave directional results parallel to those of undeformed sedimentary layers, thereby implying a postdepositional remanence process. There is no indication of inclination error or other processes affecting the fidelity of the remanent magnetization. Paleomagnetic results from four separate locations in the Pigeon Point Formation gave positive fold tests and mean inclination values near 36°. A mean geomagnetic latitude calculated from these results is 21.2°±5.3°, much more southerly than the value of 46.5°±2.3° calculated for Pigeon Point using paleomagnetic pole data from Late Cretaceous rocks from cratonic North America. Declination values are not antipodal for normal and reversed polarity sites. Analysis of depositional current directions from sandstone beds of the same outcrops shows that the difference is the result of Neogene rotations of small crustal blocks adjacent to the San Gregorio fault. In a sequence of Paleocene turbidites that crop out at Point San Pedro, four sample localities yielded positive fold tests with mean inclination values near -40°. Declination values, however, were spread over a range between 230° and 320°. Comparison with depositional current directions from the same

  5. 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.

  6. Accretionary history of the Altai-Mongolian terrane: perspectives from granitic zircon U-Pb and Hf-isotope data

    NASA Astrophysics Data System (ADS)

    Cai, Keda; Sun, Min; Xiao, Wenjiao

    2014-05-01

    The Central Asian Orogenic Belt (CAOB) consists of many tectonic terranes with distinct origin and complicated evolutionary history. Understanding of individual block is crucial to reconstruct the geodynamic history of the gigantic accetionary collage. This study presents zircon U-Pb ages and Hf isotopes for the granitoid rocks in the Russian Altai mountain range (including Gorny Altai, Altai-Mongolian terrane and CTUS suture zone between them), in order to clarify the timing of granitic magmatism, source nature, continental crustal growth and tectonic evolution. Our dating results suggest that granitic magmatism of the Russian Altai mountain range occurred in three major episodes including 445~429 Ma, 410~360 Ma and ~241 Ma. Most of the zircons within the Paleozoic granitoids present comparable positive ɛHf(t) values and Neoproterozoic crustal model ages, which favor the interpretation that the juvenile crustal materials produced in the early stage of CAOB were probably dominant sources for the Paleozoic magmatism in the region. The inference is also supported by widespread occurrence of short-lived juvenile materials including ophiolites, seamount relics and arc assemblages in the north CAOB. Consequently, the Paleozoic massive granitic rocks maybe not represent continental crustal growth at the time when they were emplaced, but rather record reworking of relatively juvenile Proterozoic crustal rocks although mantle-derived mafic magma was possibly involved to sever as heat engine during granitic magma generation. The Early Triassic granitic intrusion may be product in an intra-plate environment, as the case of same type rocks in the adjacent areas. The positive ɛHf(t) values (1.81~7.47) and corresponding Hf model ages (0.80~1.16 Ga) together with evidence of petrology are consistent with the interpretation that the parental magma of the Triassic granitic intrusion was produced from enriched mantle-derived sources under an usually high temperature condition

  7. U-Pb SHRIMP geochronology of zircon in garnet peridotite from the Sulu UHP terrane, China: Implications for mantle metasomatism and subduction-zone UHP metamorphism

    USGS Publications Warehouse

    Zhang, R.Y.; Yang, J.S.; Wooden, J.L.; Liou, J.G.; Li, T.F.

    2005-01-01

    We studied the Zhimafang ultrahigh-pressure metamorphic (UHP) peridotite from pre-pilot drill hole PP-1 of Chinese Continental Scientific Drilling project in the Sulu UHP terrane, eastern China. The peridotite occurs as lens within quartofeldspathic gneiss, and has an assemblage of Ol + Opx + Cpx + Phl + Ti-clinohumite (Ti-Chu) + Grt (or chromite) ?? magnesite (Mgs). Zircons were separated from cores at depths of 152 m (C24, garnet lhezolite), 160 m (C27, strongly retrograded phlogopite-rich peridotite) and 225 m (C50, banded peridotite), and were dated by SHRIMP mass spectrometer. Isometric zircons without inherited cores contain inclusions of olivine (Fo91-92), enstatite (En91-92), Ti-clinohumite, diopside, phlogopite and apatite. The enstatite inclusions have low Al2O3 contents of only 0.04-0.13 wt.%, indicating a UHP metamorphic origin. The weighted mean 206Pb/238U zircon age for garnet lherzolite (C24) is 221 ?? 3 Ma, and a discordia lower intercept age for peridotite (C50) is 220 ?? 2 Ma. These ages are within error and represent the time of subduction-zone UHP metamorphism. A younger lower intercept age of 212 ?? 3 Ma for a foliated wehrlite (C27) was probably caused by Pb loss during retrograde metamorphism. The source of zirconium may be partially attributed to melt/fluid metasomatism within the mantle wedge. Geochronological and geochemical data confirm that the mantle-derived Zhimafang garnet peridotites (probably the most representative type of Sulu garnet peridotites) were tectonically inserted into a subducting crustal slab and subjected to in situ Triassic subduction-zone UHP metamorphism. ?? 2005 Elsevier B.V. All rights reserved.

  8. 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.

  9. Cenozoic exhumation history of Sulu terrane: Implications from (U-Th)/He thermochrology

    NASA Astrophysics Data System (ADS)

    Wu, Lin; Monié, Patrick; Wang, Fei; Lin, Wei; Ji, Wenbin; Bonno, Michael; Münch, Philippe; Wang, Qingchen

    2016-03-01

    The Qinling-Dabie-Sulu orogen is the most prominent Phanerozoic orogenic belt in China. The discovery of ultra-high pressure (UHP) minerals in zircon inclusions suggests that the crust was subducted to deeper than 120 km into the mantle and then exhumed to shallow crustal. Recently, low temperature thermochronology has been applied to constrain the final exhumation of Dabie Shan, while there are few studies describing the Cenozoic exhumation history of the Sulu belt. Here we report some (U-Th)/He ages for various lithologies from Sulu Orogenic belt and its northern part-Jiaobei terrane. The single grain He ages range between 18 and 154 Ma, and most of the samples having large intra-sample age scattering. Several reasons such as invisible U/Th-rich inclusions, grain size effect, slow cooling rate, and zonation of parent nuclide or radiation damage effect may account for this dispersion. For all samples, the pattern of the single grain age data exhibits a peak at ~ 45 Ma which is consistent with the borehole fission-track age pattern in adjacent Hefei Basin. Both (U-Th)/He and fission track ages of the Sulu area suggest an enhanced exhumation/cooling in Early-Middle Eocene in the southern part of Tan-Lu fault zone. This enhanced cooling event coincides with rapid subsidence of North China Basin and rapid uplift of its surrounding reliefs, which indicates basin-mountain coupling. This Eocene event is widespread in central China and could be far-field consequence of India-Asia collision. The convergence rate between Pacific Plate and Eurasia decreased substantially during early Tertiary and reached a minimum in Eocene (~ 30-40 mm/yr) while at the same time the collision between India and Asia was completed. Therefore, the Cenozoic exhumation history of the Sulu Orogenic Belt was a combined result of far-field effect of India-Asia collision and declined subduction rate of the Pacific Plate under Eurasia.

  10. Mesoarchean sanukitoid rocks of the Rio Maria Granite-Greenstone Terrane, Amazonian craton, Brazil

    NASA Astrophysics Data System (ADS)

    de Oliveira, Marcelo Augusto; Dall'Agnol, Roberto; Althoff, Fernando Jacques; da Silva Leite, Albano Antonio

    2009-02-01

    The Archean sanukitoid Rio Maria Granodiorite yielded zircon ages of ˜2.87 Ga and is exposed in large domains of the Rio Maria Granite-Greenstone Terrane, southeastern Amazonian craton. It is intrusive in the greenstone belts of the Andorinhas Supergroup, in the Arco Verde Tonalite and Caracol Tonalitic Complex (older TTGs). Archean potassic leucogranites, younger TTGs and the Paleoproterozoic granites of Jamon Suite are intrusive in the Rio Maria Granodiorite. The more abundant rocks of the Rio Maria Granodiorite have granodioritic composition and display medium to coarse even-grained textures. These rocks show generally a gray color with greenish shades due to strongly saussuritized plagioclase, and weak WNW-ESE striking foliation. The significant geochemical contrasts between the occurrences of Rio Maria Granodiorite in different areas suggest that this unit corresponds in fact to a granodioritic suite of rocks derived from similar but distinct magmas. Mingling processes involving the Rio Maria Granodiorite and similar mafic to intermediate magmas are able to explain the constant occurrence of mafic enclaves in the granodiorite. The associated intermediate rocks occur mainly near Bannach, where mostly quartz diorite and quartz monzodiorite are exposed. The dominant rocks are mesocratic, dark-green rocks, with fine to coarse even-grained texture. The Rio Maria Granodiorite and associated intermediate rocks show similar textural and mineralogical aspects. They follow the calc-alkaline series trend in some diagrams. However, they have high-Mg#, Cr, and Ni conjugate with high contents of large ion lithophile elements (LILEs), typical of sanukitoids series. The patterns of rare earth elements of different rocks are similar, with pronounced enrichment in light rare earth elements (LREEs) and strong to moderate fractionation of heavy rare earth elements (HREEs). Field aspects and petrographic and geochemical characteristics denote that the granodiorites and

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

    USGS Publications Warehouse

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

    1989-01-01

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

  12. Detrital zircon record of the early Paleozoic meta-sedimentary rocks in Russian Altai: Implications on their provenance and the tectonic nature of the Altai-Mongolian terrane

    NASA Astrophysics Data System (ADS)

    Chen, Ming; Sun, Min; Cai, Keda; Buslov, Mikhail M.; Zhao, Guochun; Rubanova, Elena S.; Voytishek, Elena E.

    2015-09-01

    An integrated U-Pb and Hf-isotope study on detrital zircons from the early Paleozoic meta-sedimentary rocks along the Charysh-Terekta-Ulagan-Sayan suture zone in Russian Altai was conducted in order to trace their provenance and tectonic setting. Most of the zircons possess oscillatory zoning and high Th/U ratios (> 0.1), indicating their magmatic origin. The investigated samples yield similar zircon populations, i.e., dominant groups with late Neoproterozoic to early Paleozoic ages, followed by those from Mesoproterozoic to late Neoproterozoic and minor ones from Archean to middle Mesoproterozoic, indicating multiple tectono-thermal events in the source area. Comparison with surrounding tectonic units shows that the Tuva-Mongolian terrane and its adjacent island arcs possibly provided substantial materials to the sedimentary basin. These rocks show detrital zircon age patterns and Hf-isotope compositions similar to their counterparts in the Chinese Altai and Tseel terrane in western Mongolia, but different from those in the Gorny Altai terrane. Therefore, the investigated meta-sedimentary units possibly represented the northernmost segment of the Altai-Mongolian terrane. With combination of previous studies in the Chinese Altai and Tseel terrane, our data suggest that the Altai-Mongolian terrane possibly represents a coherent continental arc-accretionary prism system built upon the active margin of the western Mongolia during the Cambrian to Ordovician and thus does not support the micro-continent model with a passive margin. A compilation of U-Pb and Hf-isotope data of detrital zircons from the whole Altai-Mongolian terrane shows that the source area (i.e., the western Mongolia) underwent two most extensive magmatic activities at ca. 1.02-0.67 Ga and 0.67-0.43 Ga. These zircons possess both positive and negative εHf(t) values, suggesting significant crustal growth and reworking during the magmatic activities. Our study underlines a crucial role of Precambrian

  13. Paleomagnetism of the Mesozoic Asik Mountain mafic complex in northern Alaska: implications for the tectonic history of the Arctic composite terrane

    USGS Publications Warehouse

    Lewchuk, Michael T.; Foucher, Jamie; Elmore, R.D.

    2004-01-01

    At least three mutually exclusive hypotheses exist for the origin of the Arctic composite terrane and its Mesozoic location relative to the stable craton of North America. The most widely accepted hypothesis calls for counterclockwise rotation of the Arctic composite terrane as it rifted from the Arctic Archipelago. A second hypothesis calls for no relative movement, and a third places the Arctic composite terrane on the Kula plate as a part of a separate ribbon-shaped microcontinent. All three hypotheses predict unique positions for the Arctic composite terrane with respect to rotation and translation since the middle of the Mesozoic. Paleomagnetic and susceptibility studies were conducted on rocks from 15 sites in the ~160 Ma (K-Ar cooling age) Asik Mountain mafic to ultramafic complex in the western part of the Arctic composite terrane. Coherent data from 11 sites yielded a direction of dec = 255.1°, inc = 82.1° κ = 19.3, α95 = 9.6°, α63 = 5.6°. Contact and fold tests were not possible but the direction differs distinctly from the modern magnetic direction. The anisotropy of magnetic susceptibility revealed a well-developed oblate fabric of variable orientation. The orientation of the fabric was not related to the regional stress regime, so we conclude that the rocks were not deformed and metamorphosed during thrusting, and thus the magnetic remanence direction obtained is most likely primary. The direction yields a pole position at long = 166.8°E, lat = 59.8°N, A95 = 18.4°, A63 = 10.7° that is discordant to the expected 160 Ma reference direction for North America. Counterclockwise rotation of the Arctic composite terrane would yield a perfect fit to the 160 Ma reference pole with an allowance for up to 5° of northward translation. This result, combined with previous paleomagnetic data, makes a convincing argument that the Arctic composite terrane has not remained fixed in its current orientation with respect to North America. However, the data are

  14. Direction and shear sense during suturing of the Seven Devils-Wallowa terrane against North America in western Idaho

    SciTech Connect

    Strayer, L.M. IV.; Hyndman, D.W.; Sears, J.W. ); Myers, P.E. )

    1989-11-01

    A northeast-dipping 1.5-km-thick mylonite near Dworshak Dam marks the suture zone between Precambrian North America and the Seven Devils-Wallowa terrane in western Idaho. The mylonite formed under amphibolite facies conditions from quartz diorite containing apparently synplutonic mafic and synkinematic pegmatite dikes of the Kamiah plutonic complex. Mylonitic lineations and fold axes have a mean plunge of 48{degree} toward 056{degree}, nearly down the dip of the mylonitic foliation. Shear sense, given by offset of late-stage crosscutting pegmatites, is consistently top-to-the-southwest, reverse-slip, parallel to the mylonitic lineation. Folds that formed by progressive folding of the mylonitic foliation approach sheath-fold geometry. Axial planes and fold limbs are nearly parallel to the mylonitic foliation. Mafic dikes that are apparently synplutonic in the undeformed quartz diorite immediately south of the mylonite zone and north of Kamiah have variable dips and azimuths. In the shear zone, however, these dikes lie nearly in the mylonitic foliation. Transposition of the dikes into near concordance with the foliation by simple shear requires high values of shear strain and suggests that cumulative top-to-the-southwest, reverse-slip displacement across the mylonite zone is at least 27 km, and likely more than 80 km. This displacement involves underthrusting of the Kamiah plutonic complex, emplaced within the Seven Devils-Wallowa terrane, beneath North America during Late Cretaceous docking with continental North American.

  15. Translation and docking of an arc terrane: geological and geochemical evidence from the southern Zambales Ophiolite Complex, Philippines

    NASA Astrophysics Data System (ADS)

    Yumul, G. P.; Dimalanta, C. B.; Faustino, D. V.; De Jesus, J. V.

    1998-08-01

    The Zambales Ophiolite Complex is made up of three massifs: the Masinloc, Cabangan and San Antonio Massifs. Field, petrographic and geochemical analyses show that the Cabangan and San Antonio Massifs are genetically related to the Coto (transitional mid-ocean ridge-island arc) and Acoje (island arc) blocks of the Masinloc Massif, respectively. The Subic Bay Fault Zone, a left-lateral fault zone, separates the San Antonio Massif island arc terrane from the transitional mid-ocean ridge-island arc-like sheeted diabase dikes-pillow basalts of the Cabangan Massif. The San Antonio Massif is a rifted terrane from the Acoje block which was translated southward to its present position through the West Luzon Shear-Subic Bay Fault Zone. Tectonized clinopyroxenite and gabbronorite hills, which mimic the physical and geochemical characteristics of the Acoje block and the San Antonio Massif ultramafic-mafic cumulate rocks, were left behind along the western side of the Cabangan Massif during the translation of the arc massif southward. This scenario can account for the present-day configuration of the Zambales Ophiolite Complex.

  16. 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.

  17. Crustal thickness variation from a continental to an island arc terrane: Clues from the gravity signatures of the Central Philippines

    NASA Astrophysics Data System (ADS)

    Manalo, Pearlyn C.; Dimalanta, Carla B.; Faustino-Eslava, Decibel V.; Ramos, Noelynna T.; Queaño, Karlo L.; Yumul, Graciano P.

    2015-05-01

    Offshore and ground gravity data were utilized to estimate crustal thickness across the Central Philippines where a transition from continental to island arc terrane occurs. Significant differences in gravity anomalies were observed between the Palawan Microcontinental Block (PCB) and the Philippine Mobile Belt (PMB), two major terranes that came together through arc-continent collision. Islands of the PCB (Mindoro, Tablas, Romblon, Sibuyan and western Panay), made up of an assortment of continent-derived sedimentary and igneous rocks and slivers of ophiolitic bodies, register lower Bouguer anomalies compared to that displayed by Masbate Island in the PMB. The calculated crustal thickness of this region exhibits a complex Moho topography of non-uniform depth across the collision zone with the thickest parts (∼32 km) corresponding with ophiolitic units emplaced consequent to arc-continent collision. On the other hand, relatively thinner crust (∼21 km) within the collision zone coincides with areas surmised to have undergone attenuation following intra-arc rifting. The same characteristics are observed offshore of western Mindoro and within the Marinduque Basin, areas known to have experienced crustal thinning following regional tectonic rearrangements that triggered riftings and intra-basin openings.

  18. Strontium and oxygen isotopic evidence for strike/slip movement of accreted terranes in the Idaho Batholith

    NASA Astrophysics Data System (ADS)

    King, Elizabeth M.; Beard, Brian L.; Valley, John W.

    2007-07-01

    The oxygen and strontium isotope compositions of granitic rocks of the Idaho Batholith provide insight into the magma source, assimilation processes, and nature of the suture zone between the Precambrian craton and accreted arc terranes. Granitic rocks of the Idaho Batholith intrude basement rocks of different age: Triassic/Jurassic accreted terranes to the west of the Salmon River suture zone and the Precambrian craton to the east. The age difference in the host rocks is reflected in the abrupt increase in the initial 87Sr/ 86Sr ratios of granitic rocks in the batholith across the previously defined 0.706 line. Initial 87Sr/ 86Sr ratios of granitic rocks along Slate Creek on the western edge of the batholith jump from less than 0.704 to greater than 0.707 along an approximately 700 m transect normal to the Salmon River suture. Initial 87Sr/ 86Sr ratios along the Slate Creek transect do not identify a transition zone between accreted arcs and the craton and suggest a unique tectonic history during or after suturing that is not documented along other transects on the west side of the Idaho Batholith. The lack of transition zone along Slate Creek may be a primary structure due to transcurrent/transpressional movement rather than by contractional thrust faulting during suturing or be the result of post-imbrication modification.

  19. 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.

  20. Geochronology and tectonic significance of Middle Proterozoic granitic orthogneiss, North Qaidam HP/UHP terrane, Western China

    USGS Publications Warehouse

    Mattinson, C.G.; Wooden, J.L.; Liou, J.G.; Bird, D.K.; Wu, C.L.

    2006-01-01

    Amphibolite-facies para- and orthogneisses near Dulan, in the southeast part of the North Qaidam terrane, enclose minor ultra-high pressure (UHP) eclogite and peridotite. Field relations and coesite inclusions in zircons from paragneiss suggest that felsic, mafic, and ultramafic rocks all experienced UHP metamorphism and a common amphibolite-facies retrogression. Ion microprobe U-Pb and REE analyses of zircons from two granitic orthogneisses indicate magmatic crystallization at 927 ?? Ma and 921 ?? 7 Ma. Zircon rims in one of these samples yield younger ages (397-618 Ma) compatible with partial zircon recrystallization during in-situ Ordovician-Silurian eclogite-facies metamorphism previously determined from eclogite and paragneiss in this area. The similarity between a 2496 ?? 18 Ma xenocrystic core and 2.4-2.5 Ga zircon cores in the surrounding paragneiss suggests that the granites intruded the sediments or that the granite is a melt of the older basement which supplied detritus to the sediments. The magmatic ages of the granitic orthogneisses are similar to 920-930 Ma ages of (meta)granitoids described further northwest in the North Qaidam terrane and its correlative west of the Altyn Tagh fault, suggesting that these areas formed a coherent block prior to widespread Mid Proterozoic granitic magmatism. ?? Springer-Verlag 2006.

  1. 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

  2. SinoProbe-02: Deep Seismic Reflection Profiling of the Bangong Suture and Qiangtang terrane in central Tibet

    NASA Astrophysics Data System (ADS)

    Lu, Z.; Chen, C.; Gao, R.; Brown, L.; Xiong, X.; Li, W.; Deng, G.

    2010-12-01

    The Bangong Suture represents the tectonic junction between the two primary terranes of the Tibetan Plateau- the Lhasa block and the Qiangtang terrane.This suture was originally formed during a Jurassic collision of these two terranes,but has retained considerably significance as the suspect surface position of the buried mantle suture between Indian and Asian lithosphere that formed during Himalayan collision.As a corollary,these terranes have been associated with very different styles of mantle tectonics,perhaps as a result of mantle delamination beneath the Qiangtang.There have been a number of attempts to probe the deep structure of both the suture and its flanking terranes with refraction and teleseismic methods.Here we report the results of the first multichannel seismic reflection profile across this critical region.Deep seismic reflection method is internationally recognized as a pioneering technology for imaging crustal details,and it has been successfully applied in Southern Tibetan plateau in early 1990s.From October 2009 to May 2010,SinoProbe collected 310 km of a deep seismic reflection profile crossing BNS,successfully revealing structural details down to the Moho and possible deeper.The profile starts west of Silin Co in the northern Lhasa block,crosses the Bangong-Nujiang suture west of Lunpola,skirts the eastern extension of the central Qiantang anticline and ends at Dogai Coring just of south of Jinsha suture.The survey used explosive sources with variable shot size to insure adequate imaging of both the upper and lower crust.In the southern part of the profile,small shots of 50kg explosive were placed at 30m depth at 250m spacing,augmented by larger shots of 200kg t 50m depth spaced every 1km.In addition,large shots of 1000kg were placed every 50km.In the middle and north part,only 200 kg shots at 500m spacing along with the big shots.A linear array of receivers was used with a group interval of 50 m.The data was acquired by Sercel 408 XL using

  3. ALS - resources

    MedlinePlus

    Resources - ALS ... The following organizations are good resources for information on amyotrophic lateral sclerosis : Muscular Dystrophy Association -- mda.org/disease/amyotrophic-lateral-sclerosis National Amyotrophic Lateral Sclerosis (ALS) Registry -- ...

  4. 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.

  5. 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.

  6. 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.

  7. 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.

  8. Latest Paleozoic early Mesozoic structures in the central Oaxaca Terrane of southern Mexico: deformation near a triple junction

    NASA Astrophysics Data System (ADS)

    Centeno-Garcia, E.; Keppie, J. Duncan

    1999-01-01

    Paleozoic rocks in the Oaxaca Terrane of southern Mexico occur as two outliers (Rio Salinas and Santiago Ixtaltepec) unconformably overlying the 1-Ga Oaxaca Complex. They consist of the Upper Cambrian-Lower Ordovician Tiñu Formation, Mississippian Santiago Formation, lower-middle Pennsylvanian Ixtaltepec Formation and the unfossiliferous Yododeñe Formation of presumed Permian or younger age. These Paleozoic rocks have been deformed by several sets of structures. Three moderately westerly-dipping, bedding-parallel shear zones displaying dextral kinematic indicators (C-S fabrics, curvilinear isoclinal folds, en-echelon boudinage, en-echelon veins, and strained amygdales) occur along the boundaries between the Tiñu and Santiago formations, between the Santiago and Ixtaltepec formations, and within the Ixtaltepec Formation in the Santiago Ixtaltepec outlier. The bedding and these bedding-parallel shear zones are deformed by N-S, upright-asymmetric, subhorizontal folds accompanied by a slaty or spaced cleavage, that are in turn deformed by several sets of kink bands: subhorizontal and steeply dipping E-W, NW-SE and NE-SW. In the northern part of the Santiago Ixtaltepec outlier, the stratigraphy, bedding-parallel shear zones and slaty cleavage are displaced by NW-SE normal faults, all of which are truncated by the angular unconformity at the base of the Cretaceous, which brackets their age as post-Early Permian and pre-Cretaceous. Geometric correlation of the major N-S folds with E-vergent thrusting of the Oaxacan Complex over the Juarez Terrane suggests that they are older than Middle Jurassic. No age constraints are available for the kink and chevron folds; however, most may be related to Laramide structures in the overlying Cretaceous rocks. Unfolding the major structures in the Paleozoic rocks reorients the bedding-parallel shear zones to subhorizontal detachment shear zones or faults with a top-to-the-north sense of displacement. They may be related to either

  9. 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

  10. GPR Imaging of Fault Related Folds in a Gold-Bearing Metasedimentary Sequence, Carolina Terrane, Southern Appalachian Mountains

    NASA Astrophysics Data System (ADS)

    Diemer, J. A.; Bobyarchick, A. R.

    2015-12-01

    The Carolina terrane comprises Ediacaran to earliest Paleozoic mixed magmatic and sedimentary assemblages in the central and eastern Piedmont of the Southern Appalachian Mountains. The terrane was primarily deformed during the Late Ordovician Cherokee orogeny, that reached greenschist facies metamorphism. The Albemarle arc, a younger component of the Carolina terrane, contains volcanogenic metasedimentary rocks with intercalated mainly rhyolitic volcanic rocks. Regional inclined to overturned folds with axial planar cleavage verge southeast. At mesoscopic scales (exposures of a few square meters), folds sympathetic with regional folds are attenuated or truncated by ductile shear zones or contractional faults. Shear and fault zones are most abundant near highly silicified strataform zones in metagraywacke of the Tillery Formation; these zones are also auriferous. GPR profiles were collected across strike of two silicified, gold-bearing zones and enclosing metagraywacke to characterize the scale and extent of folding in the vicinity of ore horizons. Several GSSI SIR-3000 / 100 MHz monostatic GPR profiles were collected in profiles up to 260 meters long. In pre-migration lines processed for time zero and background removal, several clusters of shallow, rolling sigmoidal reflectors appeared separated by sets of parallel, northwest-dipping reflective discontinuities. These features are inferred to be reverse faults carrying contractional folds. After migration with an average velocity of 0.105 m/ns, vertical heights of the inferred folds became attenuated but not removed, and contractional fault reflections remained prominent. After migration, a highly convex-up cluster of reflections initially assumed to be a fold culmination resolved to an elliptical patch of high amplitudes. The patch is likely an undisclosed shaft or covered trench left by earlier gold prospecting. In this survey, useful detail appeared to a depth of 7.5 meters, and only a few gently inclined

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

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

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

  12. Grenville age of basement rocks in Cape May NJ well: New evidence for Laurentian crust in U.S. Atlantic Coastal Plain basement Chesapeake terrane

    USGS Publications Warehouse

    Sheridan, R.E.; Maguire, T.J.; Feigenson, M.D.; Patino, L.C.; Volkert, R.A.

    1999-01-01

    The Chesapeake terrane of the U.S. mid-Atlantic Coastal Plain basement is bounded on the northwest by the Salisbury positive gravity and magnetic anomaly and extends to the southeast as far as the Atlantic coast. It underlies the Coastal Plain of Virginia, Maryland, Delaware and southern New Jersey. Rubidium/Strontium dating of the Chesapeake terrane basement yields an age of 1.025 ?? 0.036 Ga. This age is typical of Grenville province rocks of the Middle to Late Proterozoic Laurentian continent. The basement lithologies are similar to some exposed Grenville-age rocks of the Appalachians. The TiO2 and Zr/P2O5 composition of the metagabbro from the Chesapeake terrane basement is overlapped by those of the Proterozoic mafic dikes in the New Jersey Highlands. These new findings support the interpretation that Laurentian basement extends southeast as far as the continental shelf in the U.S. mid-Atlantic region. The subcrop of Laurentian crust under the mid-Atlantic Coastal Plain implies unroofing by erosion of the younger Carolina (Avalon) supracrustal terrane. Dextral-transpression fault duplexes may have caused excessive uplift in the Salisbury Embayment area during the Alleghanian orogeny. This extra uplift in the Salisbury area may have caused the subsequent greater subsidence of the Coastal Plain basement in the embayment.

  13. Development, description, and application of a geographic information system data base for water resources in karst terrane in Greene County, Missouri

    USGS Publications Warehouse

    Waite, L.A.; Thomson, Kenneth C.

    1993-01-01

    A geographic information system data base was developed for Greene County, Missouri, to provide data for use in the protection of water resources. The geographic information system data base contains the following map layers: geology, cave entrances and passages, county and quadrangle boundary, dye traces, faults, geographic names, hypsography, hydrography, lineaments, Ozark aquifer potentio- metric surface, public land survey system, sink- holes, soils, springs, and transportation. Several serious incidents of ground-water contamination have been reported in the karst terrane developed in soluble carbonate rocks in Greene County. Karst terranes are environmentally sensitive because any contaminant carried by surface runoff has the potential for rapid transport through solution enlarged fractures to the ground-water system. In the karst terrane in Greene County, about 2,500 sinkholes have been located; these sinkholes are potential access points for contamination to the ground-water system. Recent examples of ground-water contamination by sewage, fertilizers, and hydrocarbon chemicals have demonstrated the sensitivity of ground water in the Greene County karst terrane to degradation. The ground-water system is a major source of drinking water for Greene County. The population in Greene County, which includes Springfield, the third largest city in Missouri, is rapidly increasing and the protection of the water resources of Greene County is an increasing concern.

  14. SHRIMP U-Pb evidence for a Late Silurian age of metasedimentary rocks in the Merrimack and Putnam-Nashoba terranes, eastern New England

    USGS Publications Warehouse

    Wintsch, R.P.; Aleinikoff, J.N.; Walsh, G.J.; Bothner, W.A.; Hussey, A.M.; Fanning, C.M.

    2007-01-01

    U-Pb ages of detrital, metamorphic, and magmatic zircon and metamorphic monazite and titanite provide evidence for the ages of deposition and metamorphism of metasedimentary rocks from the Merrimack and Putnam-Nashoba terranes of eastern New England. Rocks from these terranes are interpreted here as having been deposited in the middle Paleozoic above Neoproterozoic basement of the Gander terrane and juxtaposed by Late Paleozoic thrusting in thin, fault-bounded slices. The correlative Hebron and Berwick formations (Merrimack terrane) and Tatnic Hill Formation (Putnam-Nashoba terrane), contain detrital zircons with Mesoproterozoic, Ordovician, and Silurian age populations. On the basis of the age of the youngest detrital zircon population (???425 Ma), the Hebron, Berwick and Tatnic Hill formations are no older than Late Silurian (Wenlockian). The minimum deposition ages of the Hebron and Berwick are constrained by ages of cross-cutting plutons (414 ?? 3 and 418 ?? 2 Ma, respectively). The Tatnic Hill Formation must be older than the oldest metamorphic monazite and zircon (???407 Ma). Thus, all three of these units were deposited between ???425 and 418 Ma, probably in the Ludlovian. Age populations of detrital zircons suggest Laurentian and Ordovician arc provenance to the west. High grade metamorphism of the Tatnic Hill Formation soon after deposition probably requires that sedimentation and burial occurred in a fore-arc environment, whereas time-equivalent calcareous sediments of the Hebron and Berwick formations probably originated in a back-arc setting. In contrast to age data from the Berwick Formation, the Kittery Formation contains primarily Mesoproterozoic detrital zircons; only 2 younger grains were identified. The absence of a significant Ordovician population, in addition to paleocurrent directions from the east and structural data indicating thrusting, suggest that the Kittery was derived from peri-Gondwanan sources and deposited in the Fredericton Sea

  15. 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.

  16. Contrasting tectonothermal domains and faulting in the Potomac terrane, Virginia-Maryland - Discrimination by 40Ar/39Ar and fission-track thermochronology

    USGS Publications Warehouse

    Kunk, M.J.; Wintsch, R.P.; Naeser, C.W.; Naeser, N.D.; Southworth, C.S.; Drake, A.A.; Becker, J.L.

    2005-01-01

    New 40Ar/39Ar data reveal ages and thermal discontinuities that identify mapped and unmapped fault boundaries in the Potomac terrane in northern Virginia, thus confirming previous interpretations that it is a composite terrane. The rocks of the Potomac terrane were examined along the Potomac River, where it has been previously subdivided into three units: the Mather Gorge, Sykesville, and Laurel Formations. In the Mather Gorge Formation, at least two metamorphic thermal domains were identified, the Blockhouse Point and Bear Island domains, separated by a fault active in the late Devonian. Early Ordovician (ca. 475 Ma) cooling ages of amphibole in the Bear Island domain reflect cooling from Taconic metamorphism, whereas the Blockhouse Point domain was first metamorphosed in the Devonian. The 40Ar/39Ar data from muscovites in a third (eastern) domain within the Mather Gorge Formation, the Stubblefield Falls domain, record thrusting of the Sykesville Formation over the Mather Gorge Formation on the Plummers Island fault in the Devonian. The existence of two distinctly different thermal domains separated by a tectonic boundary within the Mather Gorge argues against its status as a formation. Hornblende cooling ages in the Sykesville Formation are Early Devonian (ca. 400 Ma), reflecting cooling from Taconic and Acadian metamorphism. The ages of retrograde and overprinting muscovite in phyllonites from domain-bounding faults are late Devonian (Acadian) and late Pennsylvanian (Alleghanian), marking the time of assembly of these domains and subsequent movement on the Plummers Island fault. Our data indicate that net vertical motion between the Bear Island domain of the Mather Gorge complex and the Sykesville Formation across the Plummers Island fault is east-side-up. Zircon fission-track cooling ages demonstrate thermal equillbrium across the Potomac terrane in the early Permian, and apatite fission-track cooling ages record tilting of the Potomac terrane in the Cretaceous

  17. Crystallochemical and structural evolution of tourmaline in auriferous quartz veins of the Iskel terrane prospect (western Hoggar, Tamanrasset, South Algeria)

    NASA Astrophysics Data System (ADS)

    Talbi, Mohamed; Chaouche, Ismahane; Fuchs, Yves

    2016-04-01

    A mylonite zone limits the Iskane Terrane tectonic unit (Western Hoggar). This zone is intruded by granitic units belonging to the Taourirt cycle. North -South and North East-South West trending auriferous quartz veins are hosted in the cataclased areas. Visible gold can be observed but gold is also present in sulfides (pyrite, chalcopyrite). Tourmaline is abundant in these veins. Mossbauer spectrometry as well as FTIR spectrometry shows that in some sectors tourmaline underwent an oxidation process posterior to its formation. The general structure of tourmaline studied, shows the coexistence of ferric iron Fe3+ with ferrous iron Fe2+ in the Y site. This represents a tourmaline "deprotonated". This oxidation induced a partial transformation of Fe2+ in Fe3+ that is charge compensated by a deshydroxylation of the central OH group. The relationship of the gold deposition with the oxidation of tourmaline is discussed. Key words: Tourmaline, oxidation, "deprotonation-deshydroxylation", sulfides, gold.

  18. Delineating the major KREEP-bearing terranes on the moon with global measurements of absolute thorium abundances

    SciTech Connect

    Lawrence, D.J.; Feldman, W.C.; Barraclough, B.L.; Elphic, R.C.; Prettyman, T.H.; Binder, A.B.; Maurice, S.; Miller, M.C.

    1999-03-01

    The Lunar Prospector (LP) Gamma-Ray Spectrometer (GRS) has been used to map the global composition of thorium on the lunar surface. Previous LP results of relative thorium abundances demonstrated that thorium is highly concentrated in and around the nearside western maria and less so in the South Pole Aitken (SPA) basin. Using new detector modeling results and a larger data set, the authors present here a global map of absolute thorium abundances on a 2{degree} by 2{degree} equal-area pixel scale. Because thorium is a tracer of KREEP-rich material, these data provide fundamental information regarding the locations and importance of terranes that are rich in KREEP bearing materials.

  19. Carbonate bank sedimentation in a volcaniclastic arc setting: Lower Carboniferous limestones of the eastern Klamath terrane, California

    SciTech Connect

    Watkins, R. . Dept. of Geology)

    1993-09-01

    Carboniferous volcaniclastic-arc deposits of the eastern Klamath terrane, California, include Late Visean/Namurian limestone lenses that formed as small carbonate banks. The limestone lenses, within the Bragdon and Baird formations, reach 17 m in thickness and 1.2 km in length. Slope deposits consist of argillaceous spiculitic wackestone, and bank-edge deposits include ooid grainstone, Striatifera packstone, argillaceous phylloid algal packstone, and argillaceous skeletal packstone, Bank-interior deposits include skeletal wackestone/packstone and argillaceous sandy mudstone. The limestone lenses overlie proximal deltaic deposits of thick-bedded volcaniclastic sandstone and conglomerate. Carbonate banks developed on delta lobes during intervals of minimal clastic sedimentation, possibly related to sea-level rise and volcanic quiescence. The carbonate banks were short-lived depositional systems, and they were covered by prograding deposits of younger volcaniclastic sands.

  20. A high-resolution seismic reflection/refraction study of the Chugach- Peninsular terrane boundary, southern Alaska

    USGS Publications Warehouse

    Brocher, T.M.; Fisher, M.A.; Geist, E.L.; Christensen, N.I.

    1989-01-01

    We present results from a high-resolution seismic refraction analysis of the shallow (approximately 2 km) crustal structure along the 107-km-long Trans-Alaska Crustal Transect Chugach reflection line in southern Alaska and a comparison with laboratory measurements of field samples. The refraction analysis includes the two-dimensional interpretation of several thousand first- and secondary-arrival travel times digitized from 1024-channel split-spread common shot gathers. The velocity model derived from this analysis better defines the location and geometry of terrane boundaries than does the normal incidence reflection section and agrees well with surface mapping of lithologies. Furthermore, the model predicts travel times within 100 ms of the reflection times recorded from the base of the Quaternary on the Chugach reflection section. -from Authors

  1. Application of dye-tracing techniques for determining solute-transport characteristics of ground water in karst terranes

    SciTech Connect

    Mull, D.S.; Liebermann, T.D.; Smoot, J.L.; Woosley, L.H.

    1988-10-01

    Approximately 20% of the United States is underlain by karst aquifers. This approximation includes roughly 50% of both Kentucky and Tennessee, substantial portions of northern Georgia and Alabama, and parts of other Region IV states. The prevalence of karst aquifers in the southeast, the common use of karst aquifers as drinking water sources and the vulnerability of these aquifers to contamination highlighted the need to provide a mechanism to assist in ground-water management and protection in karst terranes. In an attempt to meet this need, the U.S. Environmental Protection Agency (EPA)--Region IV and the Kentucky District of the U.S. Geological Survey (USGS), have been cooperating to document the application of dye tracing techniques and concepts to ground-water protection in karst aquifers. These efforts have resulted in the preparation of the manual. The information presented herein should be viewed as another analytical 'tool' to assist in the management and protection of karst water supplies.

  2. Crustal deformation in the south-central Andes backarc terranes as viewed from regional broad-band seismic waveform modelling

    NASA Astrophysics Data System (ADS)

    Alvarado, Patricia; Beck, Susan; Zandt, George; Araujo, Mario; Triep, Enrique

    2005-11-01

    The convergence between the Nazca and South America tectonic plates generates a seismically active backarc region near 31°S. Earthquake locations define the subhorizontal subducted oceanic Nazca plate at depths of 90-120 km. Another seismic region is located within the continental upper plate with events at depths <35 km. This seismicity is related to the Precordillera and Sierras Pampeanas and is responsible for the large earthquakes that have caused major human and economic losses in Argentina. South of 33°S, the intense shallow continental seismicity is more restricted to the main cordillera over a region where the subducted Nazca plate starts to incline more steeply, and there is an active volcanic arc. We operated a portable broad-band seismic network as part of the Chile-Argentina Geophysical Experiment (CHARGE) from 2000 December to 2002 May. We have studied crustal earthquakes that occurred in the back arc and under the main cordillera in the south-central Andes (29°S-36°S) recorded by the CHARGE network. We obtained the focal mechanisms and source depths for 27 (3.5 < Mw < 5.3) crustal earthquakes using a moment tensor inversion method. Our results indicate mainly reverse focal mechanism solutions in the region during the CHARGE recording period. 88 per cent of the earthquakes are located north of 33°S and at middle-to-lower crustal depths. The region around San Juan, located in the western Sierras Pampeanas, over the flat-slab segment is dominated by reverse and thrust fault-plane solutions located at an average source depth of 20 km. One moderate-sized earthquake (event 02-117) is very likely related to the northern part of the Precordillera and the Sierras Pampeanas terrane boundary. Another event located near Mendoza at a greater depth (~26 km) (event 02-005) could also be associated with the same ancient suture. We found strike-slip focal mechanisms in the eastern Sierras Pampeanas and under the main cordillera with shallower focal depths of ~5

  3. Gravity and Magnetic Survey of the Oaxaca-Juarez Terrane Boundary (Oaxaca Fault), Southern Mexico: Evidence for three Half Grabens

    NASA Astrophysics Data System (ADS)

    Campos-Enriquez, J. O.; Belmonte-Jimenez, S. I.; Ortega-Gutierrez, F.; Keppie-Moorhouse, J. D.; Martinez-Silva, J.; Martinez-Serrano, R.

    2007-05-01

    A geophysical survey of the Oaxaca Fault boundary between the Oaxaca (Oaxaquia) (Zapoteco) and Juarez (Cuicateco) terranes along the Etla and Zaachila valleys area, southern Mexico shows a series of NW-SE Bouguer and magnetic anomalies with stronger gradients towards the east. The basement from the Oaxaca terrane has a high density (2.8 gr/cm3 ) and magnetic susceptibility of up to 0.0051 cgs units, which contrast with the Juarez basement that has a lower density (2.67 gr/cm3) and a higher magnetic susceptibility (values ranging between 0.0025 to 0.0045 cgs units). The magnetic susceptibility is similar south of the Donaji fault. Interpretation of six combined gravity and magnetic NE-SW profiles perpendicular to the valleys indicates the presence of a composite depression comprising three N-S sub-basins with the Etla and Zachila sub-basins located at the northern and southern portions, respectively, separated by a third sub-basin relatively displaced westwards. They are bounded on the east by the steeply W-dipping Oaxaca master fault, and on the west by the gently E-dipping Huitzo-Zimatlan fault. Two interpretations are suggested for the southward continuation of the Oaxaca Fault: 1) it continues southwards at depth with the same strike. Together the Bouguer and total field magnetic anomalies suggest that the Oaxaca fault is continuous from Etla via Oaxaca City and Ocotlán de Morelos probably to Miahuatlán de Porfirio Díaz, and 2) it continues with the same strike but is displaced eastwards ~20 km along a sinistral transfer fault, which forms the northern boundary of the Zaachila sub-basin.

  4. Precambrian Field Camp at the University of Minnesota Duluth - Teaching Skills Applicable to Mapping Glaciated Terranes of the Canadian Shield

    NASA Astrophysics Data System (ADS)

    Miller, J. D.; Hudak, G. J.; Peterson, D.

    2011-12-01

    Since 2007, the central program of the Precambrian Research Center (PRC) at the University of Minnesota Duluth has been a six-week geology field camp focused on the Precambrian geology of the Canadian Shield. This field camp has two main purposes. First and foremost is to teach students specialized field skills and field mapping techniques that can be utilized to map and interpret Precambrian shield terranes characterized by sparse outcrop and abundant glacial cover. In addition to teaching basic outcrop mapping technique , students are introduced to geophysical surveying (gravity, magnetics), glacial drift prospecting, and drill core logging techniques in several of our geological mapping exercises. These mapping methodologies are particularly applicable to minerals exploration in shield terranes. The second and equally important goal of the PRC field camp is to teach students modern map-making and map production skills. During the fifth and sixth weeks of field camp, students conduct "capstone" mapping projects. These projects encompass one week of detailed bedrock mapping in remote regions of northern Minnesota that have not been mapped in detail (e.g. scales greater than 1:24,000) and a second week of map-making and map generation utilizing geographic information systems (currently ArcGIS10), graphics software packages (Adobe Illustrator CS4), and various imaging software for geophysical and topographic data. Over the past five years, PRC students and faculty have collaboratively published 21 geologic maps through the Precambrian Research Center Map Series. These maps are currently being utilized in a variety of ways by industry, academia, and government for mineral exploration programs, development of undergraduate, graduate, and faculty research projects, and for planning, archeological studies, and public education programs in Minnesota's state parks. Acquisition of specialized Precambrian geological mapping skills and geologic map-making proficiencies has

  5. Accretion of Grenvillian terranes to the southwestern border of the Río de la Plata craton, western Argentina

    NASA Astrophysics Data System (ADS)

    Varela, Ricardo; Basei, Miguel A. S.; González, Pablo D.; Sato, Ana M.; Naipauer, Maximiliano; Campos Neto, Mario; Cingolani, Carlos A.; Meira, Vinicius T.

    2011-04-01

    A comprehensive review of the geological, geochronological, and isotopic features of the Mesoproterozoic Grenvillian terranes attached to the southwest of the Río de la Plata craton in Early Paleozoic times is presented in this paper. They are grouped into the northern (sierras de Umango, Maz and del Espinal and surroundings), central (Sierra de Pie de Palo, southern Precordillera and Frontal Cordillera), and southern (San Rafael and Las Matras Blocks) segments. The Mesoproterozoic basement consists mainly of arc related, intermediate to acidic and mafic-ultramafic rocks of 1,244-1,027 Ma, with juvenile, Laurentian affinity. Exception to it is the Maz Group, with a protracted history and reworked character. They are affected by 846-570 Ma, extensional magmatism in the northern and central segments, which represents the Neoproterozoic breakup of the Rodinia supercontinent. Successive passive margin sedimentation is registered in Late Neoproterozoic (~640-580 Ma) and Cambro-Ordovician (~550-470 Ma) times. The southern segment is noted for the younger sedimentation alone, and for showing the exclusive primary unconformable relationship between the Mesoproterozoic basement and Early Ordovician cover. The effects of Early Paleozoic Famatinian orogeny, associated with the collisions of Cuyania and Chilenia terranes, are recorded as main phase (480-450 Ma), late phase (440-420 Ma), and Chanic phase (400-360 Ma). Among them, the tectonothermal climax is the Ordovician main phase, to which klippe and nappe structures typical of collisional orogens are related in the northern and central segments. Preliminary data allow us to suggest a set of paired metamorphic belts, with an outboard high-P/T belt, and an inboard Barrowian P/T belt.

  6. Thermochronology and geochemistry of the Pan-African basement below the Sab'atayn Basin, Yemen

    NASA Astrophysics Data System (ADS)

    Veeningen, Resi; Rice, A. Hugh N.; Schneider, David A.; Grasemann, Bernhard

    2015-02-01

    Three important lithologies occur in two drill wells from the Pan-African basement underlying the Mesozoic Sab'atayn Basin, in a previously undocumented area of the Pan-African, 83 and 90 km NE of known exposures in Yemen. Cores from well 1 include amphibolite, with basaltic to andesitic compositions, affected by crustal contamination during emplacement into a thickened crust. Deeper in the well, an unfoliated dark red monzogranite has a U-Pb zircon age of 628.8 ± 3.1 Ma and a Rb-Sr biotite cooling age of 591.6 ± 5.8 Ma (∼300 °C). Regional constraints suggest emplacement in a transitional tectonic setting with compressional terrane amalgamation followed by extensional collapse. Sm-Nd isotope analysis yields a TDM model age of 1.24 Ga with negative εNd values, suggesting the monzogranite is part of the Al Bayda island arc terrane. Cores from well 2 contains a weakly deformed, massive (unbedded) medium grey meta-arkose exhibiting essentially no geochemical signature of weathering and with an almost pure dacitic composition. This rock may have been directly derived from an (extrusive) granitoid that was emplaced prior to, or during terrane amalgamation. A (U-Th-Sm)/He zircon age of 156 ± 14 Ma constrains the time of basement cooling to ∼180 °C, synchronous with basin formation. These lithologies provide new insights in the development of the Pan-African basement of Yemen, extending our knowledge of the nearby surface geology to the subsurface.

  7. The Palu Metamorphic Complex, NW Sulawesi, Indonesia: Origin and evolution of a young metamorphic terrane with links to Gondwana and Sundaland

    NASA Astrophysics Data System (ADS)

    van Leeuwen, Theo; Allen, Charlotte M.; Elburg, Marlina; Massonne, Hans-Joachim; Palin, J. Michael; Hennig, Juliane

    2016-01-01

    The Palu Metamorphic Complex (PMC) is exposed in a late Cenozoic orogenic belt in NW Sulawesi, Indonesia. It is a composite terrane comprising a gneiss unit of Gondwana origin, a schist unit composed of meta-sediments deposited along the SE Sundaland margin in the Late Cretaceous and Early Tertiary, and one or more slivers of amphibolite with oceanic crust characteristics. The gneiss unit forms part of the West Sulawesi block underlying the northern and central sections of the Western Sulawesi Province. The presence of Late Triassic granitoids and recycled Proterozoic zircons in this unit combined with its isotopic signature suggests that the West Sulawesi block has its origin in the New Guinea margin from which it rifted in the late Mesozoic. It docked with Sundaland sometime during the Late Cretaceous. U-Th-Pb dating results for monazite suggest that another continental fragment may have collided with the Sundaland margin in the earliest Miocene. High-pressure (HP) and ultrahigh-pressure (UHP) rocks (granulite, peridotite, eclogite) are found as tectonic slices within the PMC, mostly along the Palu-Koro Fault Zone, a major strike-slip fault that cuts the complex. Mineralogical and textural features suggest that some of these rocks resided at depths of 60-120 km during a part of their histories. Thermochronological data (U-Th-Pb zircon and 40Ar/39Ar) from the metamorphic rocks indicate a latest Miocene to mid-Pliocene metamorphic event, which was accompanied by widespread granitoid magmatism and took place in an extensional tectonic setting. It caused recrystallization of, and new overgrowths on, pre-existing zircon crystals, and produced andalusite-cordierite-sillimanite-staurolite assemblages in pelitic protoliths, indicating HT-LP (Buchan-type) metamorphism. The PMC was exhumed as a core complex at moderate rates (c. 0.7-1.0 mm/yr) accompanied by rapid cooling in the Plio-Pleistocene. Some of the UHP rocks were transported to the surface at significantly higher

  8. Basaltic Clasts in Y-86032 Feldspathic Lunar Meteorite: Ancient Volcanism far from the Procellarum Kreep Terrane

    NASA Technical Reports Server (NTRS)

    Yamaguchi, A.; Takeda, H.; Nyquist, L. E.; Bogard, D.; Karouji, Y.; Ebihara, M.

    2008-01-01

    Lunar meteorite, Y-86032 is a fragmental or regolith breccia enriched in Al2O3 (28-31 wt%) and having very low concentrations of REEs and Th, U [e.g., 1]. Nyquist et al. [2] suggested that Y- 86032 contains a variety of lithologies not represented by the Apollo samples. They found clasts with old Ar-Ar ages and an ancient Sm-Nd age, and negative Nd indicating a direct link to the primordial magma ocean. Importantly, the final lithification of the Y-86032 breccia was likely >3.8-4.1 Ga ago. Therefore, any lithic components in the breccia formed prior to 3.8 Ga, and lithic components in breccia clasts in the parent breccia formed even earlier. Here we report textures and mineralogy of basaltic and gabbroic clasts in Y- 86032 to better understand the nature of ancient lunar volcanism far from the Procellarum KREEP Terrain (PKT) [3] and the central nearside.

  9. New insights into regional tectonics of the Indochina Peninsula inferred from Lower-Middle Jurassic paleomagnetic data of the Sibumasu Terrane

    NASA Astrophysics Data System (ADS)

    Fujiwara, Katsuya P.; Zaman, Haider; Surinkum, Adichat; Chaiwong, Nikhom; Fujihara, Makoto; Ahn, Hyeon-Seon; Otofuji, Yo-ichiro

    2014-11-01

    The post-Jurassic occurrence of differential tectonic rotation between the Indochina and South Sundaland blocks remains an issue to be properly investigated. New paleomagnetic study is used here to find the role of Sibumasu Terrane in this rotation, which is located between a clockwise rotated Indochina Block and a counter-clockwise rotated South Sundaland Block. For this purpose, lower to middle Jurassic red sandstones of the Umphang Group in the Sibumasu Terrane were sampled at 21 sites in the Ratchaburi area (13.6°E, 99.6°E), Thailand. Stepwise thermal demagnetization by 680 °C unblocked a pre-folding characteristic remanent magnetization. A mean direction of this component at 100% unfolding is Ds = 348.5°, Is = 24.7°, α95 = 10.5°, k = 10.7, N = 20, corresponding to an Early-Middle Jurassic pole of λ = 78.6°N, ϕ = 10.6°E (A95 = 9.3). Comparison of this direction with those reported from other localities of the Umphang Group (Kalaw, Mae Sot and North Trang Syncline localities) reveal variable declinations (between 348.5° and 44.7°) for the Sibumasu Terrane. We ascribe this variation to differential tectonic deformation in the Sibumasu Terrane, as reflected from sinusoidal shaped structural features in the study area. The presence of such features in the granitic rocks indicates the occurrences of deformational activities after their intrusion, which took place in the period between 130 Ma and 51 Ma. The Sibumasu Terrane behaved as an independent fragment at a time when Indochina was undergoing a clockwise rotation and southward displacement, as a result of extrusion tectonics after the gigantic India-Asia collision. Taking into consideration a westerly deflected declination (D = 342.8°) from the West Trang area in Peninsular Thailand, a counterclockwise rotation of 15° is estimated for the Sibumasu Terrane, as a result of continuous northward indentation of the Australian Plate into South Sundaland Block.

  10. 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,

  11. Geochemical study of the Cambrian-Ordovician meta-sedimentary rocks from the northern Altai-Mongolian terrane, northwestern Central Asian Orogenic Belt: Implications on the provenance and tectonic setting

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

    The Altai-Mongolian terrane (AM) is a key component of the Central Asian Orogenic Belt (CAOB), but its tectonic nature has been poorly constrained. This paper reports geochemical compositions of Cambrian-Ordovician meta-sedimentary rocks from the northern AM to trace their source nature and depositional setting, which in turn place constraints on the geodynamic evolution of the AM. The Cambrian-Ordovician meta-sedimentary rocks from the northern AM show variable major-element compositions, with negative correlation between SiO2 and TiO2, Al2O3, Fe2O3T, MgO and K2O. Their high ICV values (1.18-2.53) and relatively low CIA values (37.9-76.3) indicate that the sediments were immature and probably underwent mild to moderate chemical weathering. The low-SiO2 samples are characterized by relatively restricted SiO2/Al2O3 (mostly 2.60-6.07) and low Rb/Sr ratios (0.02-1.89), implying their proximal deposition without obvious sedimentary sorting and recycling. In contrast, the high-SiO2 samples show much higher SiO2/Al2O3 ratios (15.4-19.9) possibly due to sedimentary sorting and/or silicification. All these samples yield relatively high Al2O3/TiO2 ratios (15.6-22.8), strong LREEs/HREEs differentiation ((La/Yb)N = 4.86-10.7) and obvious negative Eu anomalies (δEu = 0.61-0.83). Combined with their Th/Sc, Zr/Sc, La/Th and Co/Th ratios comparable with intermediate-acidic magmatic rocks, we infer that these kinds of magmatic rocks served as a major source for the investigated meta-sedimentary rocks. The TiO2, Al2O3 and Fe2O3T + MgO concentrations are mostly higher than typical sediments from passive margin, and the Th/U, La/Sc, Th/Sc, Eu/Eu∗, Zr/Hf, Zr/Th and La/Th ratios are quite similar to sediments from continental arcs. These data suggest that the Cambrian-Ordovician meta-sedimentary rocks from the northern AM were most likely deposited in an environment related to a continental arc setting rather than a passive regime. These rocks show strong similarities to their

  12. Late Proterozoic-Paleozoic evolution of the Arctic Alaska-Chukotka terrane based on U-Pb igneous and detrital zircon ages: Implications for Neoproterozoic paleogeographic reconstructions

    USGS Publications Warehouse

    Amato, J.M.; Toro, J.; Miller, E.L.; Gehrels, G.E.; Farmer, G.L.; Gottlieb, E.S.; Till, A.B.

    2009-01-01

    The Seward Peninsula of northwestern Alaska is part of the Arctic Alaska-Chukotka terrane, a crustal fragment exotic to western Laurentia with an uncertain origin and pre-Mesozoic evolution. U-Pb zircon geochronology on deformed igneous rocks reveals a previously unknown intermediate-felsic volcanic event at 870 Ma, coeval with rift-related magmatism associated with early breakup of eastern Rodinia. Orthogneiss bodies on Seward Peninsula yielded numerous 680 Ma U-Pb ages. The Arctic Alaska-Chukotka terrane has pre-Neoproterozoic basement based on Mesoproterozoic Nd model ages from both 870 Ma and 680 Ma igneous rocks, and detrital zircon ages between 2.0 and 1.0 Ga in overlying cover rocks. Small-volume magmatism occurred in Devonian time, based on U-Pb dating of granitic rocks. U-Pb dating of detrital zircons in 12 samples of metamorphosed Paleozoic siliciclastic cover rocks to this basement indicates that the dominant zircon age populations in the 934 zircons analyzed are found in the range 700-540 Ma, with prominent peaks at 720-660 Ma, 620-590 Ma, 560-510 Ma, 485 Ma, and 440-400 Ma. Devonian- and Pennsylvanian-age peaks are present in the samples with the youngest detrital zircons. These data show that the Seward Peninsula is exotic to western Laurentia because of the abundance of Neoproterozoic detrital zircons, which are rare or absent in Lower Paleozoic Cordilleran continental shelf rocks. Maximum depositional ages inferred from the youngest detrital age peaks include latest Proterozoic-Early Cambrian, Cambrian, Ordovician, Silurian, Devonian, and Pennsylvanian. These maximum depositional ages overlap with conodont ages reported from fossiliferous carbonate rocks on Seward Peninsula. The distinctive features of the Arctic Alaska-Chukotka terrane include Neoproterozoic felsic magmatic rocks intruding 2.0-1.1 Ga crust overlain by Paleozoic carbonate rocks and Paleozoic siliciclastic rocks with Neoproterozoic detrital zircons. The Neoproterozoic ages are

  13. 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

  14. Ion Probe U-Pb dating of the Central Sakarya basement: a peri-Gondwana terrane cut by late Lower Carboniferous subduction/collision related granitic magmatism

    NASA Astrophysics Data System (ADS)

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

    2010-05-01

    Our aim here is to better understand the age and tectonic history of crystalline basement units in the Sakarya Zone of N Turkey, north of the Neotethyan İzmir-Ankara-Erzincan Suture Zone, utilising field, petrographic and ion probe dating, the latter carried out at the University of Edinburgh. One of the largest basement units, Central Sakarya, is dominated by paragneisses and schists that are best exposed between Bilecik and Sarıcakaya, forming a belt ~15 km wide x 100 km long. Smaller outcrops of this basement are exposed further north, for instance in the Geyve area. High-grade metamorphic basement is unconformably overlain by Lower Jurassic-Upper Cretaceous cover sediments of the Sakarya Zone and is in tectonic contact with the Late Palaeozoic-Early Mesozoic Karakaya Complex to the south. Ion-probe U-Pb dating of 89 detrital zircons, separated from one garnet micaschist sample, range from 551 Ma (Ediacaran) to 2738 Ma (Neoarchean). 85% of the ages are > 90 % concordant. Zircon populations cluster at ~550-750 Ma (28 grains), ~950-1050 Ma (27 grains) and ~2000 Ma (5 grains), with smaller groupings at ~800 Ma and ~1850 Ma. The first, prominent population (Neoproterozoic) reflects derivation from a source area related to a Cadomian-Avalonian magmatic arc, likely to be associated with a Cadomian/NE African terrane rather than Baltica (Baltica is known to be magmatically inactive during this period), or Avalonia/Amazonia (in view of the absence of Mesoproterozoic ages in Avalonian-Amazonian terranes). The early Neoproterozoic ages (0.9-1 Ga) deviate significantly from the known age spectra of Cadomian terranes (i.e. Armorican Terrane Assemblage) and instead suggest derivation from an original part of NE Africa. The detrital zircon age spectrum of Cambrian-Ordovician sandstones deposited at the northern periphery of the Arabian-Nubian Shield (i.e. the Elat sandstone) is notably similar to that of the Sakarya basement. The Central Sakarya terrane may have rifted in

  15. Post-Triassic para-autochthoneity of the Yukon-Tanana Terrane: paleomagnetism of the Early Cretaceous Quiet Lake batholith

    NASA Astrophysics Data System (ADS)

    Symons, D. T. A.; McCausland, P. J. A.; Kawasaki, K.; Hart, C. J. R.

    2015-10-01

    Was the Yukon-Tanana Terrane (YTT), a California-sized part of south-central Yukon, an autochthonous or para-autochthonous part of northern British Columbia in the Early Cretaceous or was it part of a proposed allochthonous `Baja B.C.' continent offshore of southern California? To answer this fundamental question, a paleomagnetic study has been completed on 347 specimens from 24 sites in the 114.7 ± 1.1 Ma Quiet Lake batholith. This 1300 km2 pluton is composed mostly of massive medium-to-coarse grained biotite quartz monzonite that exhibits no evidence of either deformation or metamorphism, and that intrudes metamorphosed pre-Cretaceous basement rocks of the YTT in southern Yukon. The paleomagnetic analysis utilized thermal and alternating field step demagnetization, and saturation isothermal remanence methods. A well-defined characteristic remanent magnetization (ChRM) direction was isolated throughout the 500-585 °C temperature range at Decl. = 340.6°, Incl. = 77.4° (N = 14 sites, k = 51.2, A95 = 5.6°). The ChRM resides in magnetite with a low titanium content and is interpreted to be a primary thermoremanent magnetization. After correction for 490 km of geologically demonstrable dextral displacement on the inboard Tintina fault zone, the Quiet Lake batholith's paleopole is not significantly different at 95 per cent confidence from the co-eval 115 Ma reference paleopole for North America, giving non-significant translation and rotation estimates of 1.4° ± 5.1° (1σ) northwestwards and 10° ± 13° (1σ) clockwise, respectively. Thus, this is the first Early Cretaceous paleopole to show clearly that the YTT in Yukon is a para-autochthon that was part of North America's continental margin at that time. Further, after correction for Tintina fault displacement, the eight available Mesozoic YTT paleopoles agree closely with the North American apparent polar wander path (APWP). In contrast, the 22 paleopoles from the Intermontane Belt show the expected

  16. Late Triassic alkaline complex in Sulu UHP terrane: Implications for post-collisional magmatism along the continental subduction zone

    NASA Astrophysics Data System (ADS)

    Xu, H.; Song, Y.; Liu, Q.

    2014-12-01

    In order to insight into crust-mantle interaction triggered by partial melting of the subudcted continental crust during its exhumation, we carried out a combined study on Shidao alkaline complex in the Sulu ultrahigh pressure (UHP) terrane. The alkaline complex is composed of shoshonitic to ultrapotassic gabbro, pyroxene syenite, amphibole syenite, quartz syenite, and granite. Field researches suggest that the mafic rocks are earlier than the felsic ones in sequence. LA-ICPMS zircon U-Pb dating on them gives Late Triassic ages of 214 ± 2 to 200 ± 3 Ma from mafic to felsic rocks. These ages are a bit younger than Late Triassic ages for partial melting of the Sulu UHP terrane during exhumation, indicating syn-exhumation magmatism during continental collision. The alkaline rocks have wide ranges of SiO2 (49.7 - 76.7 wt.%), MgO (8.25 - 0.03 wt.%),total Fe2O3 (9.23 - 0.47 wt.%), CaO (8.39 - 0.39 wt.%), Ni (126.0 - 0.07 ppm), and Cr (182.0 - 0.45 ppm) contents. Other major oxides are regularly changed with SiO2. The alkaline rocks have characteristics of arc-like patterns in the trace element distribution, e.g., enrichment of LREE and LILE (Rb, Ba, Th and U), depletion of HFSE (Nb, Ta, P and Ti), and positive Pb anomalies. From the mafic to felsic rocks, (La/Yb)N ratios and contents of the total REE, Sr and Ba are decreased but Rb contents are increased. The alkaline rocks also display features of A2-type granitoids, suggesting a post-collisional magmatism. They have high initial 87Sr/86Sr ratios (0.70575 and 0.70927) and negative ɛNd(t) values (-18.6 to -15.0) for whole-rock. The homogeneous initial 87Sr/86Sr ratios and ɛNd(t) values of the alkaline rocks are almost unchanged with SiO2 and MgO contents, suggesting a fractional crystallization (FC) process from a same parental magma. Our studies suggest a series of crust-mantle interaction processes along the continental subduction interface as follows: (1) melts from partial melting of the subducted continental

  17. Regional gravity and magnetic anomalies related to a Proterozoic carbonatite terrane in the eastern Mojave Desert, California

    NASA Astrophysics Data System (ADS)

    Denton, K. M.; Ponce, D. A.; Miller, D. M.; Jernigan, C. T.

    2014-12-01

    One of the world's largest rare earth element carbonatite deposits is located at Mountain Pass in the eastern Mojave Desert, California. The 1.4 Ga carbonatite deposit is hosted by and intruded into 1.7 Ga gneiss and schist that occurs in a narrow north-northwest trending belt along the eastern parts of Clark Mountain Range, Mescal Range, and Ivanpah Mountains. The carbonatite is associated with an ultrapotassic intrusive suite that ranges from shonkinite through syenite and granite. Regional geophysical data reveal that the eastern Mojave carbonatite terrane occurs along the northeast edge of a prominent magnetic high and the western margin of a gravity high along the eastern Clark Mountain Range. To improve our understanding of the geophysical and structural framework of the eastern Mojave carbonatite terrane, we collected over 1900 gravity stations and over 600 physical rock property samples to augment existing geophysical data. Carbonatite intrusions typically have distinct gravity, magnetic, and radiometric signatures because these deposits are relatively dense, contain magnetite, and are enriched in thorium or uranium. However, our results show that the carbonatite is essentially nonmagnetic with an average susceptibility of 0.18 x 10-3 SI (n=31) and the associated ultrapotassic intrusive suite is very weakly magnetic with an average susceptibility of 2.0 x 10-3 SI (n=36). Although the carbonatite body is nonmagnetic, it occurs along a steep gradient of a prominent aeromagnetic anomaly. This anomaly may reflect moderately magnetic mafic intrusive rocks at depth. East of the ultrapotassic intrusive rocks, a prominent north trending magnetic anomaly occurs in the central part of Ivanpah Valley. Based on geologic mapping in the Ivanpah Mountains, this magnetic anomaly may reflect Paleoproterozoic mafic intrusive rocks related to the 1.7 Ga Ivanpah Orogeny. Physical property measurements indicate that exposed amphibolite along the eastern Ivanpah Mountains are

  18. 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

  19. 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

  20. Gravity and magnetic survey of the Oaxaca city region: Cenozoic horst-and-graben structure superimposed on the Oaxaca-Juarez terrane boundary, southern Mexico

    NASA Astrophysics Data System (ADS)

    Campos-Enríquez, J. O.; Belmonte-Jiménez, S. I.; Keppie, J. D.; Ortega-Gutiérrez, F.; Arzate, J. A.; Martínez-Silva, J.; Martínez-Serrano, R. G.

    2010-04-01

    A geophysical survey of the Oaxaca Fault along the north-trending Etla and Zaachila valleys area, southern Mexico, shows a series of NNW-SSE Bouguer and magnetic anomalies with steeper gradients towards the east. The Oaxaca Fault represents Tertiary extensional reactivation of the Juarez shear zone that constitutes the boundary between the Oaxaca and Juárez terranes. Cooperative interpretation of six combined gravity and magnetic NE-SW profiles perpendicular to the valleys indicates the presence of a composite depression comprising three N-S sub-basins: the northern Etla and southern Zaachila sub-basins separated by the Atzompa sub-basin. The Etla sub-basin is bounded by the moderately E-dipping, Etla Fault and the more steeply W-dipping Oaxaca Fault, which together constitute a graben that continues southwards into the Atzompa graben. The deeper Zaachila sub-basin, south of Oaxaca city, is a wide V-shaped graben with a horst in the middle. The new geophysical data suggest that the Oaxaca-Juarez terrane boundary is displaced sinistrally ca. 20 km along the E-W Donají Fault, which defines the northern boundary of the Zaachila sub-basin. On the other hand, the Oaxaca Fault may either continue unbroken southwards along the western margin of the horst in the Zaachila sub-basin or be offset along with the terrane boundary. The sinistral movement may have taken place either during the Late Mesozoic-Early Cenozoic, Laramide Orogeny as a lateral ramp in the thrust plane or under Miocene-Pliocene, NE-SW extension. The former suggests that the Donají Fault is a transcurrent fault, whereas the latter implies that it is a transfer fault. The models imply that originally the suture was continuous south of the Donaji Fault and provide a constraint for the accretion of the Oaxaca and Juarez terranes.

  1. Proterozoic ophiolites and mafic ultramafic complexes marginal to the İstanbul Block: An exotic terrane of Avalonian affinity in NW Turkey

    NASA Astrophysics Data System (ADS)

    Bozkurt, Erdin; Winchester, John A.; Yiğitbaş, Erdinç; Ottley, Christopher J.

    2008-12-01

    Among the Proterozoic inliers in the İstanbul Block, the lowest structural levels are exposed in the Sünnice Massif, north of Bolu. Amphibolite-facies mafic and subordinate ultramafic rocks of the Çele meta-ophiolite underlie the greenschist-facies Ediacaran calc-alkaline Yellice metavolcanics, which are intruded by the ˜ 565-576 Ma Dirgine granitoids. Hornblende gneisses of the Çele meta-ophiolite comprise island arc meta-tholeiites and transitional to calc-alkaline metabasalts which, together with minor serpentinite are disposed in a broadly antiformal structure. The meta-ophiolitic rocks are therefore the oldest ophiolites in NW Turkey, and are themselves thrust on to a putative pre-existing continental margin, now represented by the metasedimentary migmatites of the Demirci gneisses, which may thus be the oldest rocks of the complex. The İstanbul Block is an exotic terrane. Unlike other western Turkish terranes, it lacks Variscan metamorphism: its different provenance, indicated by its geological record, faunal affinities, and inherited mid-Proterozoic isotopic dates, suggests a former link with Avalonian basement in England, NW Europe and the Maritime Provinces of Canada. Hence, together with other terranes now situated further east than the Avalonian terranes of NW Europe, the İstanbul Block may represent the easternmost extremities of Avalonia, which were detached during its end-Ordovician collision with the Bruno-Silesian Promontory on the SW margin of Baltica. Subsequent migration of the İstanbul Block to its present location occurred by eastward displacement by sinistral transpression along the southern margin of Baltica to a point east of the Dobrogea and south of the Scythian Platform, followed by collision with the Sakarya Block in the Mesozoic and Late Cretaceous southward displacement with the opening of the Black Sea basin.

  2. Protolith and metamorphic ages of the Haiyangsuo Complex, eastern China: A non-UHP exotic tectonic slab in the Sulu ultrahigh-pressure terrane

    USGS Publications Warehouse

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

    2006-01-01

    The Haiyangsuo Complex in the NE Sulu ultrahigh-pressure (UHP) terrane has discontinuous, coastal exposures of Late Archean gneiss with amphibolitized granulite, amphibolite, Paleoproterozoic metagabbroic intrusives, and Cretaceous granitic dikes over an area of about 15 km2. The U-Pb SHRIMP dating of zircons indicates that theprotolith age of a garnet-biotite gneiss is >2500 Ma, whereas the granulite-facie metamorphism occurred at around 1800 Ma. A gabbroic intrusion was dated at ???1730 Ma, and the formation of amphibolite-facies assemblages in both metagabbro and granulite occurred at ???340-460 Ma. Petrologic and geochronological data indicate that these various rocks show no evidence of Triassic eclogite-facies metamorphism and Neoproterozoic protolith ages that are characteristics of Sulu-Dabie HP-UHP rocks, except Neoproterozoic inherited ages from post-collisional Jurassic granitic dikes. Haiyangsuo retrograde granulites with amphibolite-facies assemblages within the gneiss preserve relict garnet formed during granulite-facies metamorphism at ???1.85 Ga. The Paleoproterozoic metamorphic events are almost coeval with gabbroic intrusions. The granulite-bearing gneiss unit and gabbro-dominated unit of the Haiyangsuo Complex were intruded by thin granitic dikes at about 160 Ma, which is coeval with post-collisional granitic intrusions in the Sulu terrane. We suggest that the Haiyangsuo Complex may represent a fragment of the Jiao-Liao-Ji Paleoproterozoic terrane developed at the eastern margin of the Sino-Korean basement, which was juxtaposed with the Sulu terrane prior to Jurassic granitic activity and regional deformation. ?? Springer-Verlag 2006.

  3. Generation of new continental crust and terrane accretion in Southeastern Alaska and Western British Columbia: constraints from P- and S-wave wide-angle seismic data (ACCRETE)

    NASA Astrophysics Data System (ADS)

    Morozov, Igor B.; Smithson, Scott B.; Chen, Jingru; Hollister, Lincoln S.

    2001-11-01

    The ACCRETE study addresses the question of continental assemblage in southeastern Alaska and western British Columbia through accretion of exotic terranes and generation of new crust by magmatic addition in a former continental arc. We present results of wide-angle P- and S-wave seismic interpretation of a 300-km long marine-land seismic line across the contacts between accreted terranes and Coast Mountains. Additional constraints on the model are obtained from correlation with geologic mapping. Our results indicate that the Coast Shear Zone (CSZ) is a nearly vertical fault zone probably related to a transpressive regime. West of the CSZ, the mid-Cretaceous (90 Ma) thrust belt is rooted in the deep crust and is truncated by the CSZ. From the interpretation of the imaged sub-vertical reflecting zones, we infer the positions of the Alexander-Wrangellia terrane boundary (AWB) and of Tertiary extensional grabens within Dixon Entrance near its intersection with the profile. The observed values of Vp and Vp/Vs in the lower crust of the Alexander terrane are similar to those of oceanic crust and distinctly different from the Coast Mountains Batholith (CMB) to the northeast. The crust under the CMB (32 km) is thinner than the average continental crust, and the Moho is sharp (˜200 m) and highly reflective. The low-velocity mantle (7.9 km/s) suggests high temperature consistent with the stability of garnet in mafic rocks in the lower crust. The lower crustal velocity of 6.9 km/s supports a lower crust composed of interlayered garnet pyroxene granulite and quartzofeldspathic-restite related to batholith generation. The crustal section under the CMB is seismically identical to the lower two thirds of normal crust, heated and inflated by intrusions of tonalite, and gabbro interlayered with restites from batholith generation and uplifted during exhumation.

  4. Metamorphism in the Tlikakila Complex, Lake Clark National Park, Alaska: Does it Record the Collision of the Peninsular Terrane With Alaska?

    NASA Astrophysics Data System (ADS)

    Amato, J. M.; Bogar, M. J.; Calvert, A. T.

    2001-12-01

    The Tlikakila complex is a ~80 km x ~5 km belt of variably metamorphosed and deformed rocks thought to be part of the Peninsular terrane of southern Alaska. This project uses detailed mapping, structural analysis, and thermochronology to address the tectonic evolution of rocks thought to be part of the Peninsular terrane in southern Alaska. Both meta-igneous and metasedimentary rocks of Triassic (?) age are exposed. Meta-igneous protoliths include mafic (gabbro, basalt) and ultramafic rocks. Metasedimentary protoliths include limestone, chert, and other siliceous sediments. Metapelites are rare. Metamorphic rocks in the study area include two distinct occurrences. Smaller outcrops, appear to be roof pendants in Tertiary plutons. At Kasna Creek, near Kontrashibuna Lake, limestone beds were contact metamorphosed with copper sulfide mineralization within a mafic pluton. Larger outcrops in the Tlikakila complex are more continuous, more pervasively deformed, and more recrystallized. A new 40Ar/39Ar analysis of white mica from a metasedimentary rock in the Tlikakila complex located just southwest of Saddle Lake yielded a monotonically increasing age spectrum, with the oldest high-temperature step giving a date of around 160 Ma, and the low-temperature step giving a date of 60.5 Ma. The oldest date could represent the timing of greenschist facies metamorphism of the Tlikakila complex. It is interesting that this 160 Ma date is similar to the youngest of the Middle to Late Jurassic plutons (174-158 Ma) in the Alaska-Aleutian Range batholith, considered to be part of the Peninsular terrane. Metamorphism in the Tlikakila complex could be related to the onset of the collision of the Peninsular terrane with Alaska, which also resulted in the cessation of arc magmatism. The youngest date from this sample overlaps with existing 59-63 Ma K-Ar dates from Tertiary volcanic and plutonic rocks in the area and records new mica growth associated with Tertiary magmatism.

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

  6. Microbes in Pliocene paleosols in volcanic terrane on Earth correlated with similar exposures on Mars

    NASA Astrophysics Data System (ADS)

    Mahaney, W. C.; Dohm, J.; Barendregt, R. W.; Kim, K. J.; Milner, M. W.

    2009-12-01

    processes have produced exposures reachable by a roving vehicle. [1] Dohm, J.M., et al., 2008. GRS evidence and the possibility of paleooceans on Mars. Journal of Planetary and Space Sciences, doi: 101016/j.pss.2008.10-08. [2] Murchie SL, et al. 2009. A synthesis of Martian aqueous mineralogy after one Mars year of observations from the Mars Reconnaissance Orbiter. Journal of Geophysical Research. Geophys. Res., in press. [2] Mahaney, W.C., et al., 2001. Morphogenesis of Antarctic paleosols: martian analogue, Icarus, 154: 113-130.

  7. Influence of tectonic terranes adjacent to Precambrian Wyoming province of petroleum source and reservoir rock stratigraphy in northern Rocky Mountain region

    SciTech Connect

    Tonnsen, J.J.

    1984-07-01

    The perimeter of the Archean Precambrian Wyoming province can be generally defined. A Proterozoic suture belt separates the province from the Archean Superior province to the east. The western margin of the Precambrian rocks lies under the western Overthrust belt, but the Precambrian province extends at least as far west as southwest Montana and southeast Idaho. The province is bounded on the north and south by more regionally extensive Proterozoic mobile belts. In the northern belt, Archean rocks have been remobilized by Proterozoic tectonic events, but the southern belt does not appear to contain rocks as old as Archean. The tectonic response of these Precambrian terranes to cratonic and continental margin vertical and horizontal forces has exerted a profound influence on Phanerozoic sedimentation and stratigraphic facies distributions. Petroleum source rock and reservoir rock stratigraphy of the Northern Rocky Mountain region has been correlated with this structural history. In particular, the Devonian, Permian, and Jurassic sedimentation patterns can be shown to have been influenced by articulation among the different terranes comprising the ancient substructure. Depositional patterns in the Chester-Morrow carbonate and clastic sequence in the Central Montana trough are also related to this substructure. Further, a correlation between these tectonic terranes and the localization of regional hydrocarbon accumulations has been observed and has been useful in basin analyses for exploration planning.

  8. Crustal structure of Precambrian terranes in the southern African subcontinent with implications for secular variation in crustal genesis

    NASA Astrophysics Data System (ADS)

    Kachingwe, Marsella; Nyblade, Andrew; Julià, Jordi

    2015-07-01

    New estimates of crustal thickness, Poisson's ratio and crustal shear wave velocity have been obtained for 39 stations in Angola, Botswana, the Democratic Republic of Congo, Malawi, Mozambique, Namibia, Rwanda, Tanzania and Zambia by modelling P-wave receiver functions using the H-κ stacking method and jointly inverting the receiver functions with Rayleigh-wave phase and group velocities. These estimates, combined with similar results from previous studies, have been examined for secular trends in Precambrian crustal structure within the southern African subcontinent. In both Archean and Proterozoic terranes we find similar Moho depths [38-39 ± 3 km SD (standard deviation)], crustal Poisson's ratio (0.26 ± 0.01 SD), mean crustal shear wave velocity (3.7 ± 0.1 km s-1 SD), and amounts of heterogeneity in the thickness of the mafic lower crust, as defined by shear wave velocities ≥4.0 km s-1. In addition, the amount of variability in these crustal parameters is similar within each individual age grouping as between age groupings. Thus, the results provide little evidence for secular variation in Precambrian crustal structure, including between Meso- and Neoarchean crust. This finding suggests that (1) continental crustal has been generated by similar processes since the Mesoarchean or (2) plate tectonic processes have reworked and modified the crust through time, erasing variations in structure resulting from crustal genesis.

  9. Rhyolitic calderas of the Yukon-Tanana Terrane, east central Alaska: volcanic remnants of a mid-Cretaceous magmatic arc

    USGS Publications Warehouse

    Bacon, C.R.; Foster, H.L.; Smith, James G.

    1990-01-01

    Four large but poorly exposed rhyolitic calderas are present in the Yukon-Tanana terrane (YTT) in east central Alaska. At least two are mid-Cretaceous in age (~93 Ma). Similar volcanic rocks, the South Fork Volcanics, occur northeast of the Tintina fault in Yukon Territory. Evidence for the calderas consists of thick deposits of devitrified crystal- and lithic-rich densely welded tuff, interpreted as caldera fill, associated with lava domes or shallow intrusive rocks. Coeval outflow sheets have been largely stripped by erosion. The calderas are preserved within a northeast trending depression extending across the axis of the elongate mid-Cretaceous plutonic province. Trace element abundances in andesites and rhyolites associated with the caldera structures are similar to those of volcanic and plutonic rocks of subduction-related magmatic arcs developed on continental crust and thus are suggestive of formation in such an environment. Late Cretaceous and early Tertiary igneous rocks in the YTT near the calderas are interpreted to have been emplaced in a more extensional setting when the subduction-related magmatic front was farther oceanward. -Authors

  10. 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.

  11. Lithospheric bending of the Tengchong Terrane from late Eocene to early Miocene: New extrusion mechanism of SE Tibet during the Indo-Asian collision

    NASA Astrophysics Data System (ADS)

    Xu, Zhiqin; Wang, Qin; Cai, Zhihui; Dong, Hanwen; Li, Huaqi; Chen, Xijie; Duan, Xiangdong; Cao, Hui; Li, Jing; Burg, Jean-Pierre

    2016-04-01

    It is generally believed that the extrusion of SE Tibet was bounded by the dextral Gaoligong and the sinistral Ailaoshan-Red River strike-slip shear zones from the Oligocene to early Miocene. This study integrates field mapping, structural analysis and geochronology in western Yunnan (China), where foliated Precambrian basement rocks and late Cretaceous to early Eocene plutons are exposed to the west of the Gaoligong shear zone. We found that the Tengchong Terrane was neither rigid nor vertically coherent during its southward extrusion and clockwise rotation. The Tengchong Terrane consists of four elongated gneiss domes (Donghe, Guyong, Yingjiang and Sudian) that are cored by high-grade metamorphic rocks and pre-kinematic granite plutons, and bounded by top-to-NE detachments and NE-trending dextral strike-slip shear zones. Zircon U-Pb ages from LA-ICP-MS analysis and 40Ar/39Ar ages of micas and hornblende demonstrate that the flat-lying Donghe Detachment (>35-15 Ma) and the Nabang dextral strike-slip shear zone (41-19 Ma) were sites of prolonged, mostly coeval ductile deformation from amphibolite to greenschist facies metamorphism. The Gaoligong shear zone experienced dextral shearing under similar metamorphic conditions between 32 and 10 Ma. Coeval activation of the flat-lying detachments and strike-slip shear zones resulted in fast exhumation and SW-ward extrusion of the basement rocks and granite plutons of the Tengchong Terrane. The Tengchong Terrane can be regarded as vertically plunging folds formed by lithospheric bending around the proto-Eastern Himalayan syntaxis since 41 Ma. The intense clockwise rotation of the Tengchong Terrane was accommodated by strike slip along the Sudian, Yingjiang, Lianghe and Nabang shear zones before 35 Ma, and the subsequent localized movement along the Nabang and Gaoligong strike-slip shear zones until the early Miocene. This deformation geometry indicates the importance of mid-crustal detachments in accommodating the large

  12. Provenance of Early Paleozoic metasediments in the central Chinese Altai: Implications for tectonic affinity of the Altai-Mongolia terrane in the Central Asian Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Wang, Yujing; Long, Xiaoping; Wilde, Simon A.; Xu, Huilong; Sun, Min; Xiao, Wenjiao; Yuan, Chao; Cai, Keda

    2014-12-01

    The Chinese Altai is one of the most typical segments of the Altai-Mongolia terrane, of which the tectonic evolution and affinity are hotly disputed and still not well constrained. Early Paleozoic metasedimentary rocks are extensive in the Altai-Mongolia terrane and their provenance is a key to unravelling the tectonic history of this terrane. Metasediments from Kulumuti Group were collected from the central Chinese Altai for geochemical study. They have low Chemical Index of Alteration (CIA = 52-54) and high Index of Compositional Variability (ICV = 0.81-1.19) values, different from the mature post-Archean Australian average shale (PAAS), indicating relatively weak chemical weathering and a source compositionally dominated by immature material that lacks alumina-rich minerals. These rocks are moderately enriched in light rare earth element (LREE) and show relatively flat heavy rare earth element (HREE) patterns (LaN/YbN = 2.36-9.80, GdN/YbN = 1.31-2.45). Compared with PAAS, they mostly have lower large ion lithophile element concentrations (e.g. Rb, Sr, Ba, Th, U and Pb) and similar contents of high field strength elements (e.g. Zr, Hf and Y), but with lower Nb and Ta. The metasediments have high Rb concentrations (> 50 ppm), relatively high K2O (> 1.1 wt.%) and low REE contents with negative Eu/Eu* anomalies, indicating that these rocks were derived from an acid-intermediate igneous source. The rocks have slightly enriched Nd isotopic compositions with calculated initial εNd(t) values mostly ranging from - 4.3 to - 0.2 and TDM2 model ages between 1.22 and 1.56 Ga. Detrital zircons from two samples of the Kulumuti metasediments have similar age spectra, dominated by Early Paleozoic to latest Neoproterozoic zircons (465-576 Ma) with a few older grains formed between 766-972 Ma and 1321-2572 Ma, remarkably consistent with those from the low-grade metamorphic Habahe Group in the western Chinese Altai. In combination with previous published data, the detrital zircons

  13. Old mantle beneath the Avalon terrane: evidence from Osmium isotopes in spinel peridotite xenoliths

    NASA Astrophysics Data System (ADS)

    Minarik, W. G.; Hermes, O. D.; Walker, R. J.

    2002-05-01

    Spinel lherzolite xenoliths from mid-Jurassic lamprophyre dikes near Ashaway, R.I. have been analyzed for Os isotopic ratios and Re and Os concentrations. The xenoliths were collected from freshly exposed outcrops, and are minimally serpentinized. Most contain evidence of carbonate metasomatism. Two grams of powdered peridotite was dissolved in aqua regia in sealed Carius tubes, purified using solvent extraction (Os) and anion resin exchange (Re) and analyzed using NTIMS. The bulk xenoliths contain approx. 4 ppb Os, and have 187Re/188Os ratios of 0.04 to 0.2. These low Re concentrations result in minimal correction back to an initial Jurassic 187Os/188Os ratio of 0.1165, which corresponds to a sub-chondritic initial γ Os of -7.4. The minimum model age (TRD) of melt extraction and separation of these lherzolites from the convecting upper mantle using these 187Os/188Os ratios is 1.5 to 1.7 Ga (after Meisel et al., 2001). Major and trace element compositions will be used to estimate the degree of melt depletion in order to refine the constraints on formation of this subcontinental lithospheric mantle. Avalonian supercrustal rocks range from the Neoproterozoic to Permian in age. These preliminary model ages imply that Mesoproterozoic (or older) mantle remains under the Hope Valley subterrane of Avalonia in Rhode Island. This supports evidence from Archean-age zircon cores found in the surrounding Permian-age granites that significantly older lithosphere exists at depth. This lithosphere could be either an old lithosphere core to the Avalonian arc or underthrust West African craton. Models that call for the introduction of asthenospheric mantle during the Alleghanian orogeny or the initiation of Atlantic rifting are not supported by these data.

  14. The Granite Aqueduct and Advection of Water and Heat Through Plutonic Terranes

    NASA Astrophysics Data System (ADS)

    Glazner, A. F.; Bartley, J. M.; Law, B.; Coleman, D. S.

    2011-12-01

    invoke large-volume, long-lived areas of interconnected melt in an attempt to keep alive traditional ideas regarding processes such as magma flow, stoping, and crystal fractionation. However, thermal modeling consistently demonstrates that without continual input of new magma, such volumes cannot be maintained for times greater than a few hundred ka. Furthermore, advective heat loss via the granite aqueduct, coupled with fluid convection in wall rocks, will cool plutons far faster than conductive cooling alone. Models demonstrating long-lived interconnected melt without continued magma input require highly unrealistic and contrived assumptions, such as instantaneous emplacement of huge volumes of magma with no vertical heat transport (Memeti et al., 2010).

  15. Al Composites

    NASA Astrophysics Data System (ADS)

    Chandanayaka, Tharaka; Azarmi, Fardad

    2014-05-01

    In the present study, cold spraying technique was used to fabricate a metal matrix composite (MMC) that consists of Ni matrix and 20 vol.% Ni3Al particles at two different particle sizes as reinforcement. This study intends to investigate the effect of reinforcement particle size on microstructural and mechanical properties of cold sprayed MMCs. Two different Ni3Al powders with nominal particle size of -45 to +5 and +45 to 100 μm were used as reinforcement in this study. Cold sprayed Ni-Ni3Al samples were subjected to the microstructural observation and characterization prior to any mechanical testing. Then, samples were tested using nano-indentation, Knoop hardness, Vickers hardness, and Resonance frequency to evaluate their mechanical properties. No significant changes were observed in microstructural characteristics due to different particle sizes. The results obtained from a variety of mechanical testings indicated that the increasing reinforcement particle size resulted in the slight reduction of mechanical properties such as elastic modulus and hardness in cold sprayed MMCs. The mechanical interlock between deposited particles defines the bonding strength in cold sprayed samples. Small size particles have a higher velocity and impact resulting in stronger interlock between deformed particles.

  16. High-precision UPb ages of metamorphic rutile: application to the cooling history of high-grade terranes

    USGS Publications Warehouse

    Mezger, K.; Hanson, G.N.; Bohlen, S.R.

    1989-01-01

    Metamorphic rutiles occurring in granulite and upper amphibolite facies metapelitic rocks of the Archean Pikwitonei granulite domain (Manitoba) and the Proterozoic Adirondack terrane (New York) give concordant and near concordant UPb ages. The Pb concentrations in rutile range from 2.85 to 168 ppm, U concentrations range from 10.9 to 390 ppm and the measured 206Pb 204Pb ratios range from 182 to 22,100 corresponding to 238U 204Pb ratios of 398-75,100. The proportions of radiogenic 208Pb are very low, ranging from 0.0 to 6.9% of total radiogenic Pb. The habits of the rutile crystals range from stubby to acicular, the physical properties vary from opaque/black to transparent/reddish-brown. Separate batches of black and reddish-brown rutile grains from the same samples have similar U and Pb concentrations, Pb-isotope ratios, and yield the same U Pb ages within analytical uncertainty. No correlation of U concentration and 206Pb 204Pb ratios with morphology or color of the rutiles was observed among the samples analyzed. Most rutiles yield concordant UPb ages which are reproducible within analytical uncertainty, i.e. generally ??2 Ma. The UPb ages for prograde rutile are younger than the time of peak metamorphism given by UPb ages for garnet and zircon, and also younger than UPb ages for sphene and monazite, and 40Ar 39Ar and KAr ages for hornblende but older than 40Ar 39Ar and KAr ages for biotite from the same area. This suggests that the rutile ages reflect cooling below closure temperatures. Within a single hand-specimen, and thus for an identical thermal history, larger rutile grains give older ages than do smaller grains. This suggests that volume diffusion is the most probable mechanism responsible for the ages being younger than the time of peak metamorphism. It also suggests that the dimensions for such diffusion are directly related to the dimensions of the rutile crystal and not to the dimensions of sub-grain domains, as is the case for Ar diffusion in

  17. Nutrients, Select Pesticides, and Suspended Sediment in the Karst Terrane of the Sinking Creek Basin, Kentucky, 2004-06

    USGS Publications Warehouse

    Crain, Angela S.

    2010-01-01

    This report presents the results of a study by the U.S. Geological Survey, in cooperation with the Kentucky Department of Agriculture, on nutrients, select pesticides, and suspended sediment in the karst terrane of the Sinking Creek Basin. Streamflow, nutrient, select pesticide, and suspended-sediment data were collected at seven sampling stations from 2004 through 2006. Concentrations of nitrite plus nitrate ranged from 0.21 to 4.9 milligrams per liter (mg/L) at the seven stations. The median concentration of nitrite plus nitrate for all stations sampled was 1.6 mg/L. Total phosphorus concentrations were greater than 0.1 mg/L, the U.S. Environmental Protection Agency's recommended maximum concentration, in 45 percent of the samples. Concentrations of orthophosphates ranged from less than 0.006 to 0.46 mg/L. Concentrations of nutrients generally were larger during spring and summer months, corresponding to periods of increased fertilizer application on agricultural lands. Concentrations of suspended sediment ranged from 1.0 to 1,490 mg/L at the seven stations. Of the 47 pesticides analyzed, 14 were detected above the adjusted method reporting level of 0.01 micrograms per liter (mug/L). Although these pesticides were detected in water-quality samples, they generally were found at less than part-per-billion concentrations. Atrazine was the only pesticide detected at concentrations greater than U.S. Environmental Protection Agency drinking water standard of 3 mug/L, and the maximum detected concentration was 24.6 mug/L. Loads and yields of nutrients, selected pesticides, and suspended sediment were estimated at two mainstream stations on Sinking Creek, a headwater station (Sinking Creek at Rosetta) and a station at the basin outlet (Sinking Creek near Lodiburg). Mean daily streamflow data were available for the estimation of loads and yields from a stream gage at the basin outlet station; however, only periodic instantaneous flow measurements were available for the

  18. 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

    A belt of unusual volcanogenic massive sulfide (VMS) occurrences is located along the eastern margin of the Alexander terrane throughout southeastern Alaska and northwestern British Columbia and exhibits a range of characteristics consistent with a variety of syngenetic to epigenetic deposit types. Deposits within this belt include Greens Creek and Windy Craggy, the economically most significant VMS deposit in Alaska and the largest in North America, respectively. The occurrences are hosted by a discontinuously exposed, 800-km-long belt of rocks that consist of a 200- to 800-m-thick sequence of conglomerate, limestone, marine elastic sedimentary rocks, and tuff intercalated with and overlain by a distinctive unit of mafic pyroclastic rocks and pillowed flows. Faunal data bracket the age of the host rocks between Anisian (Middle Triassic) and late Norian (late Late Triassic). This metallogenic belt is herein referred to as the Alexander Triassic metallogenic belt. The VMS occurrences show systematic differences in degree of structural control, chemistry, and stratigraphic setting along the Alexander Triassic metallogenic belt that suggest important spatial or temporal changes in the tectonic environment of formation. At the southern end of the belt, felsic volcanic rocks overlain by shallow-water limestones characterize the lower part of the sequence. In the southern and middle portion of the belt, a distinctive pebble conglomerate marks the base of the section and is indicative of high-energy deposition in a near slope or basin margin setting. At the northern end of the belt the conglomerates, limestones, and felsic volcanic rocks are absent and the belt is composed of deep-water sedimentary and mafic volcanic rocks. This northward change in depositional environment and lithofacies is accompanied by a northward transition from epithermal-like structurally controlled, discontinuous, vein- and pod-shaped, Pb-Zn-Ag-Ba-(Cu) occurrences with relatively simple mineralogy

  19. Concentrations of nutrients, pesticides, and suspended sediment in the karst terrane of the Sinking Creek basin, Kentucky, 2004

    USGS Publications Warehouse

    Crain, Angela S.

    2006-01-01

    Water samples were collected in streams and springs in the karst terrane of the Sinking Creek Basin in 2004 as part of study in cooperation with the Kentucky Department of Agriculture. A total of 48 water samples were collected at 7 sites (4 springs, 2 streams, and 1 karst window) from April through November 2004. The karst terrane of the Sinking Creek Basin (also known as Boiling Spring Basin) encompasses about 125 square miles in Breckinridge County and portions of Meade and Hardin Counties in Kentucky. Fourteen pesticides were detected of the 52 pesticides analyzed in the stream and spring samples. Of the 14 detected pesticides, 12 were herbicides and 2 were insecticides. The most commonly detected pesticides?atrazine, simazine, metolachlor, and acetochlor?were those most heavily used on crops during the study. Atrazine was detected in 100 percent of all samples; simazine, metolachlor, and acetochlor were detected in more than 35 percent of all samples. The pesticide-transformation compound, deethylatrazine, was detected in 98 percent of the samples. Only one nonagricultural herbicide, prometon, was detected in more than 30 percent of the samples. Malathion, the most commonly detected insecticide, was found in 4 percent of the samples, which was followed by carbofuran (2 percent). Most of the pesticides were present in low concentrations; however, atrazine was found in springs exceeding the U.S. Environmental Protection Agency?s (USEPA) standards for drinking water. Atrazine exceeded the USEPA?s maximum contaminant level 2 times in 48 detections. Concentrations of nitrate greater than 10 milligrams per liter (mg/L) were not found in water samples from any of the sites. Concentrations of nitrite plus nitrate ranged from 0.21 to 3.9 mg/L at the seven sites. The median concentration of nitrite plus nitrate for all sites sampled was 1.5 mg/L. Concentrations of nitrite plus nitrate generally were higher in the springs than in the main stem of Sinking Creek. Forty

  20. Detrital rutile geochemistry and thermometry from the Dabie orogen: Implications for source-sediment links in a UHPM terrane

    NASA Astrophysics Data System (ADS)

    Liu, Lei; Xiao, Yilin; Wörner, G.; Kronz, A.; Simon, K.; Hou, Zhenhui

    2014-08-01

    This study explores the potential of detrital rutile geochemistry and thermometry as a provenance tracer in rocks from the Central Dabie ultrahigh-pressure metamorphic (UHPM) zone in east-central China that formed during Triassic continental collision. Trace element data of 176 detrital rutile grains selected from local river sediments and 91 rutile grains from distinct bedrocks in the Shuanghe and Bixiling areas, obtained by both electron microprobe (EMP) and in situ LA-ICP-MS analyses, suggest that geochemical compositions and thermometry of detrital rutiles are comparable to those from their potential source rocks. After certification of the Cr-Nb discrimination method for the Central Dabie UHPM zone, we show that 29% of the detrital rutiles in the Shuanghe area were derived from metamafic sources whereas in the Bixiling area that it is up to 76%. Furthermore, the proportion of distinct types of detrital rutiles combined with modal abundances of rutile in metapelites and metamafic bedrocks can be used to estimate the proportion of different source lithologies. Based on this method the proportion of mafic source rocks was estimated to ∼10% at Shuanghe and >60% at Bixiling, respectively, which is consistent with the proportions of eclogite (the major rutile-bearing metamafic rock) distribution in the field. Therefore, the investigation of detrital rutiles is a potential way to evaluate the proportion of metamafic rocks and even to prospect for metamafic bodies in UHPM terranes. Zr-in-rutile temperatures were calculated at different pressures and compared with temperatures derived from rock-in rutiles and garnet-clinopyroxene Fe-Mg thermometers. Temperatures calculated for detrital rutiles range from 606 °C to 707 °C and 566 °C to 752 °C in Shuanghe and Bixiling, respectively, at P = 3 GPa with an average temperatures of ca. 630 °C for both areas. These temperature averages and ranges are similar to those calculated for rutiles from surrounding source rocks

  1. Early Mesozoic granitoid and rhyolite magmatism of the Bureya Terrane of the Central Asian Orogenic Belt: Age and geodynamic setting

    NASA Astrophysics Data System (ADS)

    Sorokin, A. A.; Kotov, A. B.; Kudryashov, N. M.; Kovach, V. P.

    2016-09-01

    Early Mesozoic granitoids and volcanic rocks are widespread throughout the structures of all of the continental massifs in the eastern part of the Central Asian Orogenic Belt, although its tectonic setting is not yet clear. Generally, they are associated with subduction and plume processes or rifting. Such uncertainty is mostly explained by the unequal investigation of Early Mesozoic magmatism. This paper presents the results of geochemical, Sm-Nd isotope, and U-Pb geochronologic (ID-TIMS) studies of "key-type" Early Mesozoic magmatic rock complexes of the Bureya Terrane. This is one of the largest continental massifs in the eastern Central Asian Orogenic Belt and knowledge of its geological structure is of fundamental importance in understanding the history of its formation. It has been established that the leucogranites of the Altakhtinsky Complex and the trachyrhyolites of the Talovsky Complex are practically coeval (~ 209-208 Ma). The subalkaline leucogranites of the Kharinsky Complex have a slightly younger age of ~ 199 Ma. These data correspond to the general stage of Early Mesozoic magmatic and metamorphic events (236-180 Ma) in most continental massifs in the eastern Central Asian Orogenic Belt. We believe that large-scale Early Mesozoic events were related to the amalgamation of the continental massifs of the eastern Central Asian Orogenic Belt into a single continental structure (the Amur superterrane or microcontinent Amuria) and collision with the North Asian Craton. It should be noted that the collision processes were followed by crustal thickening, thus creating the conditions for metamorphism and formation of magmatic rock complexes of various geochemical types.

  2. Crust and lithosphere structure of the northwestern U.S. with ambient noise tomography: Terrane accretion and Cascade arc development

    NASA Astrophysics Data System (ADS)

    Gao, Haiying; Humphreys, Eugene D.; Yao, Huajian; van der Hilst, Robert D.

    2011-04-01

    To address the tectonic and magmatic modifications of the Pacific Northwest lithosphere, including transformation of the Farallon oceanic terrane "Siletzia" into continent, we study the crust and uppermost mantle of the Pacific Northwest with fundamental-mode Rayleigh-wave ambient noise tomography using periods 6-40 s, resolving isotropic shear-wave velocity structure from the surface to 70 km depth (3 crustal layers and 2 upper mantle layers). We optimize this estimate with the aid of a neighborhood search algorithm, which we also use with receiver functions to estimate Moho depth. Horizontal node spacing is 0.25°. The EarthScope Transportable Array, the Wallowa array, a portion of the High Lava Plains array, and seven permanent stations are joined to achieve high resolution. Very slow western Columbia Basin upper crust above very fast lower crust expresses the large Eocene sedimentary basins above a magmatically underplated crust of extended Siletzia lithosphere. High-velocity lower crust in adjacent areas to the east and south represents Siletzia thrust under the pre-accretion North America forearc. This interpretation is supported by an anomalous absence of post-accretion magmatism in these areas, implying an absence of slab removal. The southeast termination of the fast lower crust is especially strong and sharp about 35 km southeast of the Klamath-Blue Mountains gravity lineament, suggesting the Farallon slab to the southeast was torn away. The Columbia River Flood Basalts erupted at ~ 16 Ma, apparently creating a hole of diameter ~ 150 km in the edge of the underthrust Siletzia lithosphere. The magmatically active Oregon Cascade arc is slow at all depths, and the much less active Washington Cascades tend to have a volcano-centered structure that is slow in the lower crust but fast in the upper crust and upper mantle. This structure suggests that magmatic intrusion has increased upper crustal velocity, but that the higher temperatures beneath the active

  3. The timing of Jurassic orogenesis in the continental arc terrane of the western US Cordillera: Jackston Mountains, northwestern Nevada

    SciTech Connect

    Quinn, M.J.; Wright, J.E. . Dept. of Geology and Geophysics)

    1993-04-01

    Pre-Nevadan, Jurassic orogenesis appears to be characteristic of many Mesozoic arc assemblages of the western US Cordillera. In most places this deformation is Middle Jurassic ([approximately]170--160 Ma). The authors recent work in the Black Rock Desert (BRD) documents an episode of Early Jurassic metamorphism and tectonism in the Jackson Mountains (JM). Here, an extensive arc section, including the Norian to Middle ( ) Jurassic Happy Creek Volcanics and Early to Middle Jurassic plutons, indicate vigorous arc magmatism in the arly Mesozoic. Among the southwest flank of the range the lower Mesozoic section is overridden by an east vergent thrust sheet carrying upper Paleozoic rocks of the McGill Canyon Unit. Another thrust package is located on the same side of the range, but further to the north, and contains variably metamorphosed clastic rocks, carbonate, and mafic volcanics. There is also an abrupt drop in metamorphic grade across the main thrust fault where greenschist facies rocks are in contact with virtually unmetamorphosed Happy Creek Volcanics. Two plutons with U/Pb zircon age of 188 [+-] 2 Ma and 193 [+-] 3 Ma crosscut the northern thrust fault. These relations suggest than an episode of regional metamorphism followed by E-W compressional tectonism was completed during the Early Jurassic in this portion of the arc terrane. These results differ significantly from previous studies that have interpreted all compressive deformation in the JM to have taken place between the Middle Jurassic and Cretaceous. The authors new data indicate that pre-Nevada Jurassic deformation is somewhat older in the BRD than that documented in other parts of the Mesozoic arc. Because voluminous Jurassic magmatism also began at an earlier time (Early Jurassic) in the BRD than elsewhere, they suggest that deformation and metamorphis within the arc may be facilitated and localized during periods of high heat flux related to magmatic input.

  4. 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

  5. Exhumation of high-P marbles of the Samaná Terrane (Northern Hispaniola): Insights from paleostress and microstructural imprints

    NASA Astrophysics Data System (ADS)

    Fernández, Francisco José; Rodríguez, Indira; Escuder-Viruete, Javier; Pérez-Estaún, Andrés

    2016-08-01

    Paleostress variations and microstructural imprints of a subducted carbonate slab record changes in mechanical strength during its exhumation. The slab studied here forms part of the high-P Samaná Terrane located on the north-eastern margin of the Hispaniola Island. Cold-cathodoluminescence images reveal relict cataclastic fabrics within the highest-pressure marbles of the Punta Balandra and Santa Bárbara Schists structural units, formed in the early stages of exhumation at P-T conditions ca. 2.0 GPa - 500 °C. Cataclastic flow was triggered after a moderate increase of water content (1.2% < w.t. H2O < 1.8%). Accordingly, grain sizes larger than equivalent radius ri = 40 μm preserve distribution of power law type with fractal dimensions D2 = 2.43 in Punta Balandra unit and D2 = 2.72 in Santa Bárbara unit. After cataclastic flow, the stress dropped and grain comminution conducted the marbles to the dissolution-precipitation domain. Then, as exhumation progressed, the effective stress increased and calcite intracrystalline plasticity process dominated. Calcite-twinning incidence and recrystallized grain-size indicate maximum paleostress ca. 350 MPa and mean flow paleostress ≈ 130 MPa. SEM-EBSD analyses show similar weak type-c calcite fabrics in all high-P carbonate units, even though they record different metamorphic P peak. Therefore, intracrystalline plasticity was probably dominant during the development of the final tectonic fabric. Finer grain-size distributions are out of fractal range, with D1 < 1, because of the further superposed deformation. Most of the data are consistent with an initial forced exhumation model of the carbonate slab in a brittle-ductile rheology of the confined plate interface.

  6. Slab window migration and terrane accretion preserved by low-temperature thermochronology of a magmatic arc, northern Antarctic Peninsula

    NASA Astrophysics Data System (ADS)

    Guenthner, William R.; Barbeau, David L.; Reiners, Peter W.; Thomson, Stuart N.

    2010-03-01

    Existing paleogeographic reconstructions indicate that the northern Antarctic Peninsula was central to several Mesozoic and Cenozoic tectonic events that have implications for ocean circulation and continental margin evolution. To evaluate the exhumational record of these processes, we collected new samples and measured fission track and (U-Th)/He cooling ages of apatite and zircon from 13 Jurassic and Cretaceous granitoids in western Graham Land between the northern tip of the peninsula and the Antarctic Circle. Apatite He data reveal distinct ages and systematic age patterns north and south of Anvers Island, near the midpoint of the study area: To the south, apatite He ages range from 16 to 8 Ma and young northward, whereas to the north they range between 65 and 24 Ma (with one exception at 11 Ma) and young southward. Thermal histories inferred from the ages and closure temperatures of multiple thermochronometers in single samples indicate distinct histories for northern and southern Graham Land. Northern sites reveal a Late Cretaceous pulse of rapid cooling (>7°C/Myr) followed by very slow cooling (˜1°C/Myr) to the Recent, whereas southern sites record either a pulse of rapid mid-Miocene cooling (˜8°C/Myr) or steady and moderate cooling (˜3°C/Myr) from the Late Cretaceous to the Recent. We interpret the Late Cretaceous rapid cooling in the northern part of the study area as a possible manifestation of terrane accretion associated with the Palmer Land event. We interpret the systematic spatial trends in apatite He ages and contrasting thermal histories along the peninsula as recording progressive Late Cenozoic northward opening of a slab window south of Anvers Island. This is consistent with a time transgressive pulse of ˜2-3 km of rock uplift and exhumation in the upper plate following ridge-trench collision, cessation of subduction, and opening of the slab window, presumably caused by increased asthenospheric upwelling beneath the overriding plate.

  7. In situ Sr isotopic analyses of epidote: tracing the sources of multi-stage fluids in ultrahigh-pressure eclogite (Ganghe, Dabie terrane)

    NASA Astrophysics Data System (ADS)

    Guo, Shun; Ye, Kai; Yang, Yueheng; Chen, Yi; Zhang, Lingmin; Liu, Jingbo; Mao, Qian; Ma, Yuguang

    2014-02-01

    This study presents in situ strontium (Sr) isotope and Sr content data on multi-stage epidote crystals from ultrahigh-pressure (UHP) eclogites and omphacite-epidote veins therein at Ganghe (Dabie terrane, China), determined using LA-MC-ICP-MS. The Ganghe eclogites occur as lenses in mainly leucocratic UHP gneisses, and therefore, our data provide insights into the origin, composition, and transport scale of the discrete multi-stage fluids in UHP eclogites during the subduction and exhumation of a continental crust. Four textural types of epidote that record compositional and isotopic signatures of fluid at various metamorphic P- T conditions have been distinguished based on petrographic observations and compositional analyses. They are (1) fine-grained high-pressure (HP) epidote inclusions (Ep-In) in omphacite that define the earliest stage of epidote formation in the eclogite; (2) coarse-grained UHP epidote porphyroblasts (Ep-P) that contain omphacite with Ep-In inclusions in the eclogite; (3) fine-grained HP epidote in omphacite-epidote veins (Ep-V) as well as (4) the latest-stage epidote in disseminated amphibolite-facies veinlets (Ep-A), which crosscut the Ep-P or matrix minerals in the eclogite and HP vein. Both Ep-P and Ep-V crystals exhibit significant and complex chemical zonations with respect to the XFe (= Fe/(Fe + Al)) ratio and Sr content. In contrast to the varying Sr contents, Ep-In, Ep-P, and Ep-V have similar and narrow ranges of initial 87Sr/86Sr ratios (from 0.70692 to 0.70720 for Ep-In, from 0.70698 to 0.70721 for Ep-P, and from 0.70668 to 0.70723 for Ep-V), which are significantly different from those in Ep-A (from 0.70894 to 0.71172). The initial 87Sr/86Sr ratio of Ep-A is closer in value to the initial Sr isotopic composition of the gneisses (from 0.710790 to 0.712069) which enclose the UHP eclogite. These data indicate different sources of the eclogite-facies fluids and retrograde amphibolite-facies fluid in the Ganghe eclogites. The HP

  8. 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

  9. 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

  10. U-Pb Geochronology of Devonian Granites in the Meguma Terrane of Nova Scotia, Canada: Evidence for Hotspot Melting of a Neoproterozoic Source.

    PubMed

    Keppie; Krogh

    1999-09-01

    U-Pb isotopic analyses of monazite and zircon from six granitic plutons in the Meguma Terrane yield nearly concordant ages of 373+/-3 Ma, interpreted as the time of intrusion. U-Pb analyses of euhedral zircons with thick rims overgrowing cores, which were abraded to remove all or most of the rim, plot on chords between 370+/-3 and 628+/-33 Ma (Larrys River and Halfway Cove plutons), 372+/-3 and approximately 660 Ma (Shelburne pluton), and 373+/-2 and approximately 732 Ma (Barrington Passage pluton). The upper intercepts are interpreted as the age of magma source, correlatives of which are present in the Avalon Composite Terrane to the north. This basement may be either in depositional or tectonic contact with the overlying Cambro-Ordovician Meguma Group. Other zircons in the granites are generally irregular-euhedral with thin rims, and most U-Pb isotopic analyses fall between two chords from 373-2040 and 373-2300 Ma, with a few lying outside this field. These zircons are probably derived from the country rock (Goldenville Formation), which a previous study has shown contains detrital zircons with concordant U-Pb ages of 3000, 2000, and 600 Ma, and numerous intermediate discordant ages. These new ages, along with published data, document a relatively short (5-10 m.yr.) but voluminous period of magmatism. This age is approximately synchronous with intrusion of mafic rocks and lamprophyre dikes and regional low-pressure metamorphism and was followed by rapid denudation of 5-12 km. These observations may be interpreted in terms of shallowly dipping subduction and overriding of a mantle plume that eventually penetrates through the subducting plate to melt the overriding continental plate. Subsequent northward migration of the plume could explain both the approximately 360 Ma magmatism in the Cobequid Highlands (Avalon Composite Terrane) and the mid-Carboniferous plume-related intrusions around the Magdalen Basin.

  11. 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.

  12. Evidence for pre-Taconic metamorphism in the Potomac terrane, Maryland and Virginia: Hornblende and Muscovite [sup 40]Ar/[sup 39]Ar results

    SciTech Connect

    Becker, J.L.; Wintsch, R.P. . Dept. of Geological Sciences); Kunk, M.J.; Drake, A.A. Jr. )

    1993-03-01

    New [sup 40]Ar/[sup 39]Ar age spectra of hornblende and white mica from the Great Falls area of the Potomac terrane of Maryland and Virginia indicate pre-Taconic metamorphism. Age spectra of hornblende samples are interpreted to represent cooling from peak metamorphic conditions through their closure temperatures for argon diffusion ([approximately]500C) at about 490 Ma. These older Ordovician postmetamorphic cooling ages strongly contrast with younger post-Ordovician metamorphic cooling ages now being reported in the Blue Ridge and Goochland terranes to the west and east respectively. A late phyllitic sheen observed on rocks in the field and petrographic observations of undulose plagioclase and amphibole, and older muscovite, and kinked primary muscovite in the Bear Island Granodiorite reflect a younger retrogressive metamorphism involving the growth of secondary muscovite (Fisher's S4 ). [sup 40]Ar/[sup 39]Ar Age spectra of white micas from the Bear Island Granodiorite are complex and probably indicate both primary and secondary white mica, the latter apparently growing below the closure temperature for retention of argon in muscovite ([approximately]350C). The age spectra permit an estimate of a minimum age of 420 Ma for cooling through closure of the older generation of white mica. The above ages of hornblende and muscovite closure imply a minimum cooling rate of [approximately]2C/m.y., and exhumation rate of about 1 mm/yr. The projected time of peak metamorphism at upper amphibolite facies for the Great Falls area clearly predates the Ordovician Taconic orogeny and suggests that these rocks escaped this event and largely escaped younger Paleozoic metamorphic events, which are well documented in adjacent terranes.

  13. U-Pb Geochronology of Devonian Granites in the Meguma Terrane of Nova Scotia, Canada: Evidence for Hotspot Melting of a Neoproterozoic Source.

    PubMed

    Keppie; Krogh

    1999-09-01

    U-Pb isotopic analyses of monazite and zircon from six granitic plutons in the Meguma Terrane yield nearly concordant ages of 373+/-3 Ma, interpreted as the time of intrusion. U-Pb analyses of euhedral zircons with thick rims overgrowing cores, which were abraded to remove all or most of the rim, plot on chords between 370+/-3 and 628+/-33 Ma (Larrys River and Halfway Cove plutons), 372+/-3 and approximately 660 Ma (Shelburne pluton), and 373+/-2 and approximately 732 Ma (Barrington Passage pluton). The upper intercepts are interpreted as the age of magma source, correlatives of which are present in the Avalon Composite Terrane to the north. This basement may be either in depositional or tectonic contact with the overlying Cambro-Ordovician Meguma Group. Other zircons in the granites are generally irregular-euhedral with thin rims, and most U-Pb isotopic analyses fall between two chords from 373-2040 and 373-2300 Ma, with a few lying outside this field. These zircons are probably derived from the country rock (Goldenville Formation), which a previous study has shown contains detrital zircons with concordant U-Pb ages of 3000, 2000, and 600 Ma, and numerous intermediate discordant ages. These new ages, along with published data, document a relatively short (5-10 m.yr.) but voluminous period of magmatism. This age is approximately synchronous with intrusion of mafic rocks and lamprophyre dikes and regional low-pressure metamorphism and was followed by rapid denudation of 5-12 km. These observations may be interpreted in terms of shallowly dipping subduction and overriding of a mantle plume that eventually penetrates through the subducting plate to melt the overriding continental plate. Subsequent northward migration of the plume could explain both the approximately 360 Ma magmatism in the Cobequid Highlands (Avalon Composite Terrane) and the mid-Carboniferous plume-related intrusions around the Magdalen Basin. PMID:10504135

  14. Paleomagnetism in northern Alaska (and the career of David Symons), from displaced terranes on the west coast of North America to the age dating of base metal ores

    NASA Astrophysics Data System (ADS)

    Lewchuk, M. T.; Leach, D.; Kelley, K.; Symons, D. T.; Elmore, R. D.; Foucher, J.

    2004-05-01

    Paleomagnetism of barren and mineralized Paleozoic sedimentary rocks of the Red Dog Zn-Pb deposit in the Brooks Range Mountains of northern Alaska isolated several components. Mineralized and barren rocks with quartz alteration have a steep west-southwesterly magnetization retained by magnetite (N=16, D=247, I=73, k=73). Fluid inclusions indicate that the quartz formed during deep burial and Ar/Ar dating yielded an age of 126 Ma. Heavily mineralized rocks plus mineralized shales lacking quartz replacement have a shallower southwesterly magnetization carried by pyrrhotite (N=11, D=220, I=51, k=28). Geological features and Re/Os dating indicate that the ore formed in the late Paleozoic. A Mississippian igneous sill (344 Ma, Ar/Ar) in the mine has a southwest and shallow magnetization carried by magnetite (1 site). Ultramafic igneous intrusive rocks from Asik Mountain ~100 kms south of Red Dog, have been indirectly dated at about 150 Ma (K/Ar) and have a west-southwesterly magnetization carried by magnetite (N=11, D=255, I=82, k=19). Tectonic models for the Mesozoic origin of northern Alaska can be grouped into three categories: 1) contiguous to ancestral of North America; 2) peri-autochthonous with angular displacements; and, 3) allochthonous terranes accreted to ancestral North America. The combination of geologic, radiometric and paleomagnetic data from Red Dog allows for both testing of the models and paleomagnetic dating of the Red Dog ores. The data can only be collectively explained by Mississippian syngenetic mineralization and pyrrhotite magnetization, Mesozoic northward translation and counterclockwise rotation of a displaced terrane (aka "The Alaskan Terrane Wreck") and, finally, deep burial resulting in remagnetization of some of the ores associated with quartz replacement.

  15. 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.

  16. The age and composition of the pre-Cenozoic basement of the Jalisco Block: implications for and relation to the Guerrero composite terrane

    NASA Astrophysics Data System (ADS)

    Valencia, Victor A.; Righter, Kevin; Rosas-Elguera, Jose; López-Martínez, Margarita; Grove, Marty

    2013-09-01

    The Jalisco Block is thought to be part of the Guerrero terrane, but the nature and age of the underlying crystalline basement are largely unknown. We have collected a suite of schists, granitoids, and weakly metamorphosed marine sediments from various parts of the Jalisco Block including Atenguillo and Ameca, Mascota and San Sebastián, Cuale, Puerto Vallarta, Punta Mita, Yelapa, and Tomatlán. The schists range in age from 135 to 161 Ma, with many exhibiting Proterozoic and Phanerozoic zircon ages. The granitoids range in age from 65 to 90 Ma, and are calc-alkaline compositionally—similar to granitoids from the Puerto Vallarta and Los Cabos batholiths. The Jalisco granitoids also experienced similar uplift rates to granitoids from the regions to the north and south of the Jalisco Block. The marine sediments yield a maximum depositional age of 131 Ma, and also contain a significant zircon population with ages extending back to the Archean. Granitoids from this study define two age groups, even after the effects of thermal resetting and different closure temperatures are considered. The 66.8-Ma silicic ash flow tuff near Union de Tula significantly expands the extent of this Cretaceous-Paleocene age ash flow tuff unit within the Jalisco Block, and we propose calling the unit "Carmichael silicic ash flow tuff volcanic succession" in honor of Ian Carmichael. The ages of the basement schists in the Jalisco Block fully overlap with the ages of terranes of continental Mexico, and other parts of the Guerrero terrane in the south, confirming the autochthonous origin of the Jalisco Block rather than exotic arc or allochthonous origin. Geologic data, in combination with geochronologic and oxygen isotopic data, suggest the evolution of SW Mexico with an early 200-1,200-Ma passive margin, followed by steep subduction in a continental arc setting at 160-165 Ma, then shallower subduction by 135 Ma, and finally, emplacement of granitoids at 65-90 Ma.

  17. Isotopic and trace element variations in the Ruby Batholith, Alaska, and the nature of the deep crust beneath the Ruby and Angayucham Terranes

    USGS Publications Warehouse

    Arth, Joseph G.; Zmuda, Clara C.; Foley, Nora K.; Criss, Robert E.; Patton, W.W.; Miller, T.P.

    1989-01-01

    Thirty-six samples from plutons of the Ruby batholith of central Alaska were collected and analyzed for 22 trace elements, and many were analyzed for the isotopic compositions of Sr, Nd, O, and Pb in order to delimit the processes that produced the diversity of granodioritic to granitic compositions, to deduce the nature of the source of magmas at about 110 Ma, and to characterize the deep crust beneath the Ruby and Angayucham terranes. Plutons of the batholith show a substantial range in initial 87Sr/86Sr (SIR) of 0.7055–0.7235 and a general decrease from southwest to northeast. Initial 143Nd/144Nd (NIR) have a range of 0.51150–0.51232 and generally increase from southwest to northeast. The δ18O values for most whole rocks have a range of +8.4 to +11.8 and an average of +10.3‰. Rb, Cs, U, and Th show large ranges of concentration, generally increase as SiO2 increases, and are higher in southwest than in northeast plutons. Sr, Ba, Zr, Hf, Ta, Sc, Cr, Co, and Zr show large ranges of concentration and generally decrease as SiO2 increases. Rare earth elements (REE) show fractionated patterns and negative Eu anomalies. REE concentrations and anomalies are larger in the southwest than in the northeast plutons. Uniformity of SIR and NIR in Sithylemenkat and Jim River plutons suggests a strong role for fractional crystallization or melting of uniform magma sources at depth. Isotopic variability in Melozitna, Ray Mountains, Hot Springs, and Kanuti plutons suggests complex magmatic processes such as magma mixing and assimilation, probably combined with fractional crystallization, or melting of a complex source at depth. The large variations in SIR and NIR in the batholith require a variation in source materials at depth. The southwestern plutons probably had dominantly siliceous sources composed of metamorphosed Proterozoic and Paleozoic upper crustal rocks. The northeastern plutons probably had Paleozoic sources that were mixtures of siliceous and intermediate

  18. Advanced Land Observing Satellite (ALOS) Phased Array Type L-Band Synthetic Aperture Radar (PALSAR) mosaic for the Kahiltna terrane, Alaska, 2007-2010

    USGS Publications Warehouse

    Cole, Christopher J.; Johnson, Michaela R.; Graham, Garth E.

    2015-01-01

    The USGS has compiled a continuous, cloud-free 12.5-meter resolution radar mosaic of SAR data of approximately 212,000 square kilometers to examine the suitability of this technology for geologic mapping. This mosaic was created from Advanced Land Observing Satellite (ALOS) Phased Array type L-band Synthetic Aperture Radar (PALSAR) data collected from 2007 to 2010 spanning the Kahiltna terrane and the surrounding area. Interpretation of these data may help geologists understand past geologic processes and identify areas with potential for near-surface mineral resources for further ground-based geological and geochemical investigations.

  19. Kinematics of the Tengchong Terrane in SE Tibet from the late Eocene to early Miocene: Insights from coeval mid-crustal detachments and strike-slip shear zones

    NASA Astrophysics Data System (ADS)

    Xu, Zhiqin; Wang, Qin; Cai, Zhihui; Dong, Hanwen; Li, Huaqi; Chen, Xijie; Duan, Xiangdong; Cao, Hui; Li, Jing; Burg, Jean-Pierre

    2015-12-01

    It is generally believed that the extrusion of SE Tibet was bounded by the dextral Gaoligong and the sinistral Ailaoshan-Red River strike-slip shear zones from the Oligocene to early Miocene. This study integrates field mapping, structural analysis and geochronology in western Yunnan (China), where foliated Precambrian basement rocks and late Cretaceous to early Eocene plutons are exposed to the west of the Gaoligong shear zone. We found that late Eocene to early Miocene flat-lying ductile shear zones were kinematically related to steeply dipping strike-slip shear zones. Four elongated gneiss domes (Donghe, Guyong, Yingjiang and Sudian) are cored by high-grade metamorphic rocks and pre-kinematic granite plutons, and bounded by top-to-NE detachments and NE-trending dextral strike-slip shear zones. Zircon U-Pb ages from LA-ICP-MS analysis and 40Ar/39Ar ages of micas and hornblende demonstrate that the flat-lying Donghe Detachment (> 35-15 Ma) and the Nabang dextral strike-slip shear zone (41-19 Ma) were sites of prolonged, mostly coeval ductile deformation from amphibolite to greenschist facies metamorphism. The Gaoligong shear zone experienced dextral shearing under similar metamorphic conditions between 32 and 10 Ma. Consistent 40Ar/39Ar ages of hornblende from the three shear zones indicate their contemporaneity at mid-crustal depth, causing the rapid exhumation and SW-ward extrusion of the Tengchong Terrane. The strain geometry and shear zone kinematics in the Tengchong Terrane are interpreted with folding of the anisotropic lithosphere around a vertical axis, i.e., the northeast corner of the Indian Plate since 41 Ma. The newly discovered NE-trending Sudian, Yingjiang, and Lianghe strike-slip shear zones are subordinate ductile faults accommodating the initially rapid clockwise rotation of the Tengchong Terrane. The detachments caused mid-crustal decoupling and faster SW-ward extrusion below the sedimentary cover, whereas the strike-slip shear zones accommodated

  20. 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

    The Zhongba terrane represents a special tectonic unit sitting between the northern and the southern ophiolitic sub-belt in the western segment of the Yarlung Zangbo Suture Zone (YZSZ), which separates the Indian and the Eurasian continental plates. The central-east section of the Zhongba terrane can be divided into fault-fold belt, fold belt and fault belt. The fault-fold belt, composed of northward-dipping thrust faults and asymmetric folds in the Carboniferous and late Devonian strata, mainly occurs at the northern margin of the Zhongba terrane. Four stages of deformation (D1-D4) can be recognized in this belt. Stage D1 is recorded by the rolling folds in the Devonian strata in which the residual S0 can be observed in asymmetric small folds. Quartz c-axis fabrics show that the dominant slip system of quartz is basal under transpression and low temperature (~ 400℃). In the stage D2, many small folds with axial plane inclined at high angle to the north were formed. The highly inclined C foliation in mylonitic limestones suggests that this belt represents the strain concentration zone during the northward subduction of the Neo-Tethyan lithosphere beneath the Gangdese arc belt. Coaxial progressive deformation is demonstrated by strongly shortened layers and abundant cleavages that are parallel to the axial plane of pre-existing folds in the stage D3. Mylonitic foliation is replaced by the cleavages plane (S2) associated with southward thrusts. In the stage D4, some northward thrusts and relevant cleavage with medium angle replaced early folds. Zircon U-Pb ages found in nearby quartz diorites at Gangdese arc belt yielded 91.8 ± 1.3 Ma. Since these quartz diorites are compositionally similar to island-arc magma from active continental margins, we infer that the northward subduction of the north sub-belt of the YZSZ started in the upper Cretaceous. Muscovite separates from representative tectonites within the strain concentration zone and the southward thrust

  1. 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

    Accretion along continental or island arcs at cratonic margins was responsible for most Paleoproterozoic crustal growth. For the development of the Krishna orogen, India, at the southeastern margin of the Eastern Dharwar craton (EDC), two contrasting models, one by long-lived accretion between ~ 1.85 Ga and 1.33 Ga terminating in continental collision with the Napier Complex and the other involving continental collision with the Napier Complex at ~ 1.6 Ga have been proposed. Here we report the geology and geochemistry of the granitoid rocks grouping them into the Vinjamuru arc terrane. These comprise biotite ± hornblende high-silica granite which are mostly calc-alkaline, weakly metaluminous to peraluminous with normalized trace and rare earth element plots resembling those derived from arc sources as seen by negative Nb, Ti, Zr anomalies, enriched LREE and moderate Eu anomalies. On (La/Yb)CN vs YbCN and Sr/Y vs Y discrimination diagrams these rocks plot in the field of liquids from mantle-derived melts resembling Cordilleran type granitoids. Petrography, major oxide and trace element concentrations suggest formation in an arc tectonic setting during convergent tectonics at the active continental margin of the EDC with evidence for crustal assimilation. To generate the observed high-silica granite, using selected trace and REE, we modeled 10% aggregate continuous melting of a lower crustal hydrous, high K2O-bearing gabbro yielding a granodiorite magma that underwent fractional crystallization at mid-to lower crust followed by mixing with country rock tonalite and minor assimilation with metasedimentary crustal rocks resulting in the observed heterogeneity in trace elements from the granite. We interpret Paleoproterozoic paleopostions of component Indian cratons leading to their Mesoproterozoic assembly and in that context relate the crustal growth along the southeastern margin of the EDC. In contrast to the existing two models, we propose an alternative

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

    NASA Astrophysics Data System (ADS)

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

    2016-05-01

    Granitoids and their hosted mafic enclaves may retain important information on crust-mantle interaction, and thus are significant for study of crustal growth and differentiation. An integrated petrological, geochronological and geochemical study on the granitoid plutons of the Yaloman intrusive complex from the Gorny Altai terrane, northwestern Central Asian Orogenic Belt, was conducted to determine their source nature, petrogenesis and geodynamics. Mafic enclaves are common in the plutons, and a zircon U-Pb age (389 Ma ± 4 Ma) indicates that they are coeval with their granitoid hosts (ca. 393-387 Ma). Petrographic observations reveal that these mafic enclaves probably represent magmatic globules commingled with their host magmas. The relatively low SiO2 contents (46.0-60.7 wt.%) and high Mg# (38.9-56.5) further suggest that mantle-derived mafic melts served as a crucial component in the formation of these mafic enclaves. The granitoid hosts, including quartz diorites and granodiorites, are I-type in origin, possessing higher SiO2 contents (60.2-69.9 wt.%) and lower Mg# (32.0-44.2). Their zircon Hf and whole-rock Nd isotopic compositions indicate that the magmas were dominated by remelting of Neoproterozoic (0.79-1.07 Ga) crustal materials. Meanwhile, the geochemical modeling, together with the common occurrence of igneous mafic enclaves and the observation of reversely zoned plagioclases, suggests that magma mixing possibly contributed significantly to the geochemical variation of the granitoid hosts. Our results imply that mafic magmas from the mantle not only provided substantial heat to melt the lower crust, but also mixed with the crust-derived melts to form the diverse granitoids. The oxidizing and water-enriched properties inferred from the mineral assemblages and compositions imply that the granitoid plutons of the Yaloman intrusive complex were possibly formed in a continental arc-related setting, which is also supported by their geochemistry. The

  3. Crustal development of the eastern Tibetan Plateau preceding and post India-Asia continent-continent collision: Insights from terrestrial 'redbed' basins in the Yidun Arc Terrane

    NASA Astrophysics Data System (ADS)

    Robinson, D. M.; Jackson, W. T., Jr.; Weislogel, A. L.; Shang, F.; Jian, X.

    2015-12-01

    Understanding the spatial crustal configuration and temporal evolution of the pre-Cenozoic ancestral Tibetan Plateau is essential to accurately model the Himalayan-Tibetan orogenic system. The eastern Tibetan Plateau contains several terrestrial 'redbed' basins within the Yidun Arc Terrane (YAT) that preserve a Mesozoic sedimentary record of YAT post-accretionary tectonism, and a Cenozoic sedimentary record influenced by terrane collisions to the south, including arc magmatism and re-activation of inherited structures in response to strike-slip translation and far-field deformation. The West and East Ganzi basins in the northern YAT formed in a compressional tectonic setting and basin fill records a detrital zircon signature indicative of provenance from Precambrian and Paleozoic rocks of the Zhongza Massif and local Mesozoic Yidun arc plutons. The West and East Ganzi basins may be isolated flexural basins associated with the collision of the YAT and the Gondwana-derived Qiangtang Terrane with the Lhasa block during the Jurassic-Early Cretaceous time. Structural analysis shows that the West and East Ganzi basin strata have deformational phases associated with additional Mesozoic terrane collisions, far-field reactivation deformation associated with the India-Asia collision, and Late Cenozoic strike-slip deformation associated with growth of Tibetan Plateau. The Jawa and Mula basins located in the YAT to the southeast of the West and East Ganzi basins contain similar basin fill and structural features; however, both the Jawa and Mula basins contain a Cenozoic zircon population, establishing a maximum depositional age of ~41 Ma for both basins. The Jawa and Mula basins formed as small isolated basins associated with Early Cenozoic far-field deformation related to the India-Asia collision. Analyses of these four basins, along with recent study of Mesozoic deposits in the Qamdo basin of the eastern Qiangtang area to the west, shows that the southeastern Tibet plateau

  4. The Proterozoic Mount Isa Fault Zone, northeastern Australia: is it really a ca. 1.9 Ga terrane-bounding suture?

    NASA Astrophysics Data System (ADS)

    Bierlein, Frank P.; Betts, Peter G.

    2004-09-01

    In marked contrast to Palaeoproterozoic Laurentia, the location of sutures and boundaries of discrete crustal fragments amalgamated during Palaeoproterozoic formation of the North Australian Craton remain highly speculative. Interpretations of suture locations have relied heavily on the analysis of regional geophysical datasets because of sparse exposure of rocks of the appropriate age. The Mount Isa Fault Zone has been interpreted as one such Palaeoproterozoic terrane-bounding suture. Furthermore, the coincidence of this fault zone with major shale-hosted massive sulphide Pb-Zn-Ag orebodies has led to speculations that trans-lithospheric faults may be an important ingredient for the development of this deposit type. This study has integrated geophysical and geochemical data to test the statute of the Mount Isa Fault as a terrane-bounding suture. Forward modelling of gravity data shows that basement rocks on either side of the Mount Isa Fault have similar densities. These interpretations are consistent with geochemical observations and Sm-Nd data that suggest that basement lithologies on either side of the Mount Isa Fault are geochemically and isotopically indistinguishable from each other, and that the Mount Isa Fault is unlikely to represent a suture zone that separates different Palaeoproterozoic terranes. Our data indicate that the crustal blocks on both sides of the Mount Isa Fault Zone must have been in within close proximity of each other since the Palaeoproterozoic, and that the Western Fold Belt was part of the (ancestral) North Australian Craton well before the ˜1.89-1.87 Ga Barramundi Orogeny. It appears that deep crustal variations in density may be related to the boundary between a shallowly west-dipping high-density mafic to ultramafic plate and low-density basement rocks. This interpretation in turn impacts on crustal-scale models for the development of shale-hosted massive sulphide Pb-Zn mineralisation, which do not require trans

  5. Structural geology and kinematics associated with the collision of the Wrangellia composite terrane and North America, south-central Alaska

    NASA Astrophysics Data System (ADS)

    Bier, Sara Elizabeth

    The collision of the Insular superterrane, and thus, the Wrangellia composite terrane (WCT), with the Mesozoic margin of North America is one of the most important, yet enigmatic events in the tectonic history of North American cordillera. The location and therefore the nature of the collision of the Insular superterrane with North America remains controversial. In southern Alaska, the suture zone between the WCT and North America consists of the Kahiltna assemblage, Jurassic-Cretaceous submarine fan deposits. Structural investigation of the Kahiltna assemblage provides additional data on the kinematics of the collision and suggests an oblique collision with a significant component of right-lateral shearing. The first study of the dissertation presents the results across a transect at the northern end of Broad Pass where the depositional and deformational history of three tectonostratigraphic units enables determination of the tectonic evolution of the suture zone. The Reindeer Hills exposes melange units that include oceanic lithologies and represent a remnant of an accretionary complex that formed during subduction prior to the collision of the WCT. Structures within the Kahiltna assemblage in the Talkeetna Mountains indicate oblique northwest-directed thrusting and right-lateral shear during the collision of the WCT. The Jack River conglomerate, a fluvial clast-supported conglomerate unconformably overlies the Reindeers Hills melange and represents uplift, erosion, and deposition late in the collision. The second study is on the other side of Broad Pass, in the southern Alaska Range, and consists of a composite transect across the Peters and Dutch Hills and Chelatna Lake. Horizontal stretching lineations and steeply-dipping foliation indicate that deformation occurred during transpression as a result of an oblique collision. Strain analysis of pressure shadows indicate a counterclockwise rotation of the extension direction and thus, right-lateral shearing during

  6. Magnetic Anomalies and Rock Magnetic Properties Related to Deep Crustal Rocks of the Athabasca Granulite Terrane, Northern Canada

    NASA Astrophysics Data System (ADS)

    Brown, L. L.; Williams, M. L.

    2010-12-01

    The Athabasca granulite terrane in northernmost Saskatchewan, Canada is an exceptional exposure of lower crustal rocks having experienced several high temperature events (ca 800C) during a prolonged period of deep-crustal residence (ca 1.0 GPa) followed by uplift and exhumation. With little alteration since 1.8 Ga these rocks allow us to study ancient lower crustal lithologies. Aeromagnetic anomalies over this region are distinct and complex, and along with other geophysical measurements, define the Snowbird Tectonic zone, stretching NE-SW across northwestern Canada, separating the Churchill province into the Hearne (mid-crustal rocks, amphibolite facies) from the Rae (lower crust rocks, granulite facies). Distinct magnetic highs and lows appear to relate roughly to specific rock units, and are cut by mapped shear zones. Over fifty samples from this region, collected from the major rock types, mafic granulites, felsic granulites, granites, and dike swarms, as well as from regions of both high and low magnetic anomalies, are being used to investigate magnetic properties. The intention is to investigate what is magnetic in the lower crust and how it produces the anomalies observed from satellite measurements. The samples studied reveal a wide range of magnetic properties with natural remanent magnetization ranging from an isolated high of 38 A/m to lows of 1 mA/m. Susceptibilities also range over several orders of magnitude, from 1 to 1 x10-4 SI. Magnetite is identified in nearly all samples using both low and high temperature measurements, but concentrations are generally very low. Hysteresis properties on 41 samples reveal nearly equal numbers of samples represented by PSD and MD grains, with a few samples (N=6) plotting in or close to the SD region. Low temperature measurements indicate that most samples contain magnetite, showing a marked Verway transition around 120K. Also identified in nearly half of the samples is pyrrhotite, noted by low temperature

  7. Magnetotelluric evidence of the tectonic boundary between the Río de La Plata Craton and the Pampean terrane (Chaco-Pampean Plain, Argentina): The extension of the Transbrasiliano Lineament

    NASA Astrophysics Data System (ADS)

    Peri, Verónica Gisel; Pomposiello, María Cristina; Favetto, Alicia; Barcelona, Hernan; Rossello, Eduardo Antonio

    2013-11-01

    The Transbrasiliano Lineament represents a continental shear belt that transversely intersects the South American Platform from NNE to SSW. Much evidence of this lineament exists to the north, but it remains uncharacterized in the distal Andean foreland to the south, where it is associated with the tectonic boundary between the Río de la Plata Craton and the Pampean terrane. This tectonic boundary is mostly unexposed in the Chaco-Pampean Plain (Argentina). Debate continues about the existence of an east- or west-dipping subduction zone and a later collisional event between the terranes. Here, we report the results of a magnetotelluric survey along a W-E profile at 27° S between 63° 45‧ and 60° 30‧ W. To characterize the geoelectric structure, the magnetotelluric data were processed, the dimensionality and distortion of the geoelectrical medium were analyzed, and a 2-D inversion model was developed. The distribution of the resistivities at the lithospheric scale indicates a highly resistive crust (20,000 Ω m) on the east side of the profile that is correlated with the Río de La Plata Craton. On the west side of the profile, a less resistive but still highly resistive crust (12,000 Ω m) is correlated with the Pampean terrane. The highly resistive blocks are separated by an east-dipping conductive anomaly (150-250 Ω m) that is correlated with the Transbrasiliano Lineament. This conductive feature is consistent with the east-dipping subduction model and can be explained by the presence of graphite in paleosutures in long-stable geological terranes. Oblique convergence between the terranes may have developed the transpressional shear belt. The results provide new geophysical evidence of the tectonic boundary between the Río de La Plata Craton and the Pampean terrane beneath the Chaco-Pampean Plain that extends southward into the Transbrasiliano Lineament. The study also improves the knowledge of the amalgamation of Western Gondwana.

  8. The Cuenca de Oro, a Pull-Apart Basin Hosting Precious Metal Deposits Along the Re- Activated Seri-Tahue Terrane Boundary

    NASA Astrophysics Data System (ADS)

    Feinstein, M. N.; Goodell, P. C.

    2007-05-01

    At the intersection of Chihuahua, Sonora, and Sinaloa a boundary between the Seri and Tahue terranes has been hypothesized, and further refined as the Sinforosa Lineament. Near the western termination of the Sinforosa Lineament lies a topographic basin. Part of this study will be to better define this pull-apart basin, informally named the Cuenca de Oro due to its numerous precious metal deposits. The intention of this study is to test that the Seri-Tahue terrane boundary was re-energized during the beginning of extension related to the opening of the Sea of Cortez (~30ma). It is probable that the precious metal occurrences are related to the initiation of extension(alunite at El Sauzal has been dated at ~30ma). Five field excursions totaling sixty days of field work have been completed and a first draft of a regional geologic map has been made. Large shear zones support the hypothesis of a pull-apart basin. A study of the alteration and lineament intersections determine the location of many known precious metal deposits. By creating multiple cross-sections the basin can be modeled in three dimensions and a tectonic history can be interpreted. This study will present a structural analysis of the Cuenca de Oro and develop a tectonic history related temporally with the epithermal mineralization events.

  9. 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.

  10. 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

  11. Episodic granitoid emplacement in the western Kaapvaal Craton: evidence from the Archæan Kraaipan granite-greenstone terrane, South Africa

    NASA Astrophysics Data System (ADS)

    Anhaeusser, Carl R.; Walraven, Feo

    1999-02-01

    , the Mosita Adamellite, yielded a Pb evaporation age of 2749±3 Ma and is the youngest intrusive body recorded in the Kraaipan granite-greenstone terrane. Its presence beneath Kalahari sand cover is defined by Bouguer gravity data. The Kraaipan granite-greenstone terrane, with a prominent north-south trend, appears to represent an Archæan crustal segment that may have accreted episodically on to the western edge of the Kaapvaal Craton. In a manner similar to the Murchison granite-greenstone terrane in the northeastern part of the craton, the region may also have constituted an important potential source of placer Au mineralisation found in the Witwatersrand Basin.

  12. Pressure-temperature-time paths, prograde garnet growth, and protolith of tectonites from a polydeformational, polymetamorphic terrane: Salmon River suture zone, west-central Idaho

    NASA Astrophysics Data System (ADS)

    McKay, Matthew Paul

    The metamorphic rocks of the Salmon River suture zone (SRSZ) in west-central Idaho provide a unique glimpse into mid-lower crustal processes during continental growth by island arc accretion. The SRSZ, which separates island arc terranes of the Blue Mountains Province (BMP) from the Mesozoic margin of North America, contains medium to high grade tectonites that record multiple metamorphic and deformation events. The SRSZ is divided by the Pollock Mountain thrust fault (PMtf) into two structural blocks: the higher-grade Pollock Mountain plate (PMp), and the lower-grade, underlying Rapid River plate (RRp). Previous studies interpreted pre-144 Ma metamorphism within the SRSZ related to assembly of the BMP. Counter-clockwise P-T paths for metamorphism within the RRp [peak=8--9 kbar ˜600°C, retrograde=5--7 kbar, 450--525°C] were inferred to include prograde garnet growth during pre-144 Ma loading followed by garnet growth during rapid cooling due to lithospheric delamination. The PMp was interpreted to have subsequently been buried to increasing depth and metamorphosed again at 128 Ma as a result of the BMP docking with North America. New P-T-t paths for the RRp and PMp constructed from geochronology, geothermobarometry, pseudosections, and petrography suggest that after loading, slow cooling rates caused diffusion in garnet rims, which produced counter-clockwise P-T paths. Garnet Sm-Nd ages of 112.5+/-1.5 Ma from the RRp, and 141--124 Ma from the PMp suggest that metamorphism within the SRSZ is diachronous and that crustal thickening was protracted occurring between 141--112 Ma. P-T-t paths between both plates indicate that the PMp reached peak metamorphism prior to peak metamorphism of the RRp. This suggests that the PMp was buried prior to the development of the PMtf. The RRp was subsequently buried along the PMtf, which was followed by development of the Rapid River thrust fault, which juxtaposed RRp schists onto the Wallowa terrane of the BMP. This model

  13. Metaconglomerate preserves evidence for kimberlite, diamondiferous root and medium grade terrane of a pre-2.7 Ga Southern Superior protocraton

    NASA Astrophysics Data System (ADS)

    Kopylova, M. G.; Afanasiev, V. P.; Bruce, L. F.; Thurston, P. C.; Ryder, J.

    2011-12-01

    We studied heavy minerals extracted from a diamondiferous metaconglomerate that formed 2697-2701 Ma in a successor basin within the Michipicoten Greenstone Belt (MGB) of the Wawa-Abitibi Terrane (Southern Superior Craton). The conglomerate is metamorphosed in the greenschist facies and contains mainly locally derived igneous mafic to felsic detritus, but also very minor components of medium grade metamorphic minerals, diamonds and paragenetic diamond indicator minerals. Comparison of the size distribution, resorbtion and N aggregation of diamonds in nearby Wawa lamprophyres and the metaconglomerate diamonds confirms that the latter were not derived from the proximal lamprophyric source. The heavy minerals in the metaconglomerate include diopside, olivine, corundum, chromite, almandine, pyrope with kelyphitic rims, picroilmenite, amphibole and anorthite. Low abundances of the heavy minerals (several grains per 4-70 tons of the metaconglomerate) are, in part, explained by their complete or partial replacement by the greenschist mineral assemblage. Detrital almandine and amphibole are inferred to originate in amphibolite facies rocks. Cr-diopside, olivine, chromite and anorthite were sourced from mafic-ultramafic anorthosite- and chromitite-bearing layered complexes mapped in the MGB. The presence of pyrope with more than 6 wt.% Cr 2O 3 suggests derivation from a cratonic root. Picroilmenite has compositions typical of kimberlite and unlike that of ultramafic lamprophyres and other unconventional diamondiferous volcanics. The Wawa metaconglomerate, therefore, should be considered analogous to the Witwatersrand successor basin conglomerate in recording indirect evidence for Archean kimberlites. The tight localization of the diamondiferous conglomerate in time and space was controlled by a quick (~ 3 Ma) erosion of the source kimberlite body. The location of the kimberlite-bearing > 2.7 Ga Superior protocraton was inferred from the provenance of the metaconglomerate

  14. A Sm-Nd eclogite and U-Pb detrital zircon study of a probable Baltic HP-UHP metamorphic terrane in the Greenland Caledonides

    NASA Astrophysics Data System (ADS)

    Johnston, S. M.; Brueckner, H. K.; Belousova, E. A.; Medaris, L. G., Jr.; Griffin, W. L.; Hartz, E. H.; Hemming, S. R.; Bubbico, R.

    2015-12-01

    Liverpool Land, at the southern tip of the Greenland Caledonides, exposes the small eclogite-facies, peridotite-bearing Tvaerdal complex tectonically juxtaposed against the mid-crustal, high-pressure granulite facies Jaettedal complex. Recent literature supports a Laurentian origin for the Jættedal complex, but the structurally lower Tværdal complex has been tentatively correlated with Baltica. Their juxtaposition raises the possibility of lower plate to upper plate terrane transfer during continental subduction. Pressure-temperature estimates from Tvaerdal eclogites indicate ultrahigh pressure (UHP) metamorphic conditions during recrystallization. Sm-Nd mineral isochrons from the eclogites indicate UHP recrystallization occured ≈400 millions years ago, the same time HP/UHP metamorphism occurred in the Western Gneiss Complex of the Scandinavian Caledonides. Almost half of detrital zircons collected from a modern stream channel within the Tvaerdal complex give LA-ICPMS U-Pb ages of 1.68 and 1.3-0.95 Ma ages, which are dates characteristic of both Baltic and Laurentia. However, none of the detrital zircons give the Archean or ≈1.8 Ga Proterozoic ages that are also characteristic of Laurentia. Most of the remaining detrital zircons from the Tvaerdal Complex give younger U-Pb ages that range between 411-375 Ma which contrast with the older 450-410 Ma ages obtained from the Jaettedal as well as from other gneiss terranes in the southern Greenland Caledonides. The different age patterns provide compelling evidence that the Tvaerdal Complex is indeed an orphaned Baltic Terrane. The Jaettedal complex took part in the lengthy evolution of a compressional continental arc complex along the eastern Laurentian margin during the closure of Iapetus while the Tvaerdal complex was a fragment of the approaching Baltic passive margin. Eclogite-facies metamorphism of the Tvaerdal Complex occurred when Iapetus closed and the edge of Baltica subducted to UHP mantle conditions

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

    NASA Astrophysics Data System (ADS)

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

    2015-11-01

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

  16. 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

  17. 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

  18. 40Ar/39Ar Data for White Mica, Biotite, and K-Feldspar Samples from Low-Grade Metamorphic Rocks in the Westminster Terrane and Adjacent Rocks, Maryland

    USGS Publications Warehouse

    Kunk, Michael J.; McAleer, Ryan

    2008-01-01

    This report contains reduced 40Ar/39Ar data of white mica and K-feldspar mineral separates and matrix of a whole rock phyllite, all from low-grade metamorphic rocks of the Westminster terrane and adjacent strata in central Maryland. This report presents these data in a preliminary form, but in more detail than can be accommodated in todays professional journals. Also included in this report is information on the location of the samples and a brief description of the samples. The data contained herein are not interpreted in a geological context, and care should be taken by readers unfamiliar with argon isotopic data in the use of these results; many of the individual apparent ages are not geologically meaningful. This report is primarily a detailed source document for subsequent publications that will integrate these data into a geological context.

  19. Eastern boundary of the Siletz terrane in the Puget Lowland from gravity and magnetic modeling with implications for seismic hazard analysis

    NASA Astrophysics Data System (ADS)

    Anderson, M. L.; Blakely, R. J.; Wells, R. E.; Dragovich, J.

    2011-12-01

    The forearc of the Cascadia subduction zone in coastal Oregon and Washington is largely composed of a 15-30 km-thick stack of basalt flows comprising the Crescent Formation (WA) and Siletz River Volcanics (OR), and collectively termed the Siletz terrane. We are developing 3-D structural maps of the Puget Lowland to distinguish older and currently active structures for seismic hazard analysis. The boundaries of the Siletz terrane in particular may strongly influence crustal rheology and neotectonic structures of the region. Careful analysis of the areal extent of this terrane will also facilitate more accurate interpretation of seismic data and gravity anomalies, which will help define the extent and shape of overlying basins. Absence of extensive outcrop in the Lowland and a widespread veneer of Quaternary deposits require extensive subsurface geophysical studies to establish Lowland-wide crustal structure. Previous studies have used active seismic surveys and interpretation of existing industry seismic data, with several studies using gravity and magnetic data or passive-source tomography support. However, steeply dipping boundaries in the mid-crust are difficult targets for seismic study. We need to independently discriminate between potential models established by seismic data using gravity and magnetic datasets. In the Puget Lowland the Siletz is a region of high seismic wave speed, density, and magnetic susceptibility, and therefore its mid-crustal boundaries are good targets for definition by gravity and magnetic data. We present interpretations of gravity and magnetic anomalies for the Puget Lowland region that together establish the most likely position and structure of the Crescent Formation boundary in the mid-upper crust. Well-constrained physical properties of Crescent basalts inform our aeromagnetic map interpretation and give us baseline values for constructing three two-dimensional models by simultaneous forward modeling of aeromagnetic and isostatic

  20. Similar crustal evolution in the western units of the Adrar Souttouf Massif (Moroccan Sahara) and the Avalonian terranes: Insights from Hf isotope data

    NASA Astrophysics Data System (ADS)

    Gärtner, Andreas; Villeneuve, Michel; Linnemann, Ulf; Gerdes, Axel; Youbi, Nasrrddine; Hofmann, Mandy

    2016-06-01

    The Adrar Souttouf Massif is located at the western margin of the West African Craton and consists of several NNE-SSW trending units. Of them, the two westernmost have been interpreted to be linked with the Avalonian terrane assemblage and Meguma, respectively. New Hf isotopic data corroborates the Avalon correlation but has no impact one way or another on the possible Meguma connection, as there is no Hf data available from the latter. The obtained pattern of εHf(t) values vs. zircon age of the likely Avalonia related Oued Togba unit is similar to published data from Avalonia. Zircons of this unit show characteristic patterns of crustal mixing at 0.7 to 1.3 Ga and 1.75 to 2.25 Ga, while juvenile crust was likely formed around 0.6 to 0.75 Ga, from 1.2 to 2.2 Ga, and between 2.5 and 3.2 Ga. The zircons of the Sebkha Gezmayet unit reveal crustal mixing for the entire Palaeozoic and Neoproterozoic, from 2.05 to 2.11 Ga, and 2.8 to 2.9 Ga. Juvenile crust formation is interpreted to have occurred from 0.5 to 0.7 Ga, at around 2.1 Ga, and at ca. 2.9 Ga. As Mesoproterozoic zircons are abundant in the likely Avalonia-like Oued Togba unit, but uncommon at the West African Craton, their origin has to be found elsewhere. A comparison of available Hf data from Amazonia and Baltica, the two potential source cratons of Avalonia, shows similarities but is hampered by the lack of available data from Amazonia. Finally, a few grains from both units have Eoarchaean model ages. Among similar grains from other peri-Gondwanan terranes, they give indication of partial recycling of Eoarchaean crust in the vicinity of the northwestern West African Craton.

  1. 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.

  2. U-Pb sensitive high-resolution ion microprobe (SHRIMP) zircon geochronology of granitoid rocks in eastern Zambia: Terrane subdivision of the Mesoproterozoic Southern Irumide Belt

    NASA Astrophysics Data System (ADS)

    Johnson, S. P.; de Waele, B.; Liyungu, K. A.

    2006-12-01

    The Southern Irumide Belt (SIB) is a structurally and metamorphically complex region of mainly Mesoproterozoic igneous rocks in southern and eastern Zambia, northern Mozambique and northern Malawi that was strongly overprinted in the Neoproterozoic to Cambrian Damara-Lufilian-Zambezi (DLZ) orogeny. Because of the scarcity of geological data from this region, little is known about the timing of tectonomagmatic events; however, this belt has traditionally been considered to be a southerly continuation of the adjacent Irumide Belt (IB). Here we provide 27 new U-Pb sensitive high-resolution ion microprobe (SHRIMP) zircon ages that constrain the Paleoproterozoic to Cambrian tectonomagmatic history of this belt and which, for the first time, allow for direct comparison with the adjoining IB. The SIB is floored by a predominantly late Paleoproterozoic basement, which was intruded by voluminous continental margin arc-related magmas between 1.09 and 1.04 Ga and accompanied by high-temperature/low-pressure metamorphism. In contrast, the IB is floored by a late Paleoproterozoic basement that is generally older than 2.0 Ga, contains significant mid-Mesoproterozoic plutonic rocks that are not present within the SIB, and underwent moderate-pressure/moderate-temperature compressional metamorphism and S-type granitoid magmatism at circa 1.02 Ga. These data indicate that the crust underlying the SIB is not a continuation of that underlying the IB but represents an allocthonous continental margin arc terrane juxtaposed against the Congo-Tanzania-Bangweulu Craton during the late Mesoproterozoic Irumide orogeny. Reworking and shearing of the SIB occurred during the DLZ orogen, resulting in the present-day architecture as a series of stacked terranes which have been exploited by voluminous posttectonic granitoid batholiths.

  3. 76 FR 20450 - Designation of Nine Individuals Pursuant to Executive Order 13566

    Federal Register 2010, 2011, 2012, 2013, 2014

    2011-04-12

    ..., Safia); DOB 1952; POB Al Bayda, Libya (individual) 4. GADDAFI, Hannibal (a.k.a. AL-GADDAFI, Hannibal; a.k.a. AL-QADHAFI, Hannibal; a.k.a. ELKADDAFI, Hannibal; a.k.a. EL-QADDAFI, Hannibal; a.k.a. GADDAFI, Hannibal Muammar; a.k.a. GHADAFFI, Hannibal; a.k.a. GHATHAFI, Hannibal; a.k.a. QADDAFI, Hannibal;...

  4. The mineral resource potential of the Wadi al Jarir and Al Jurdhawiyah quadrangles, sheets 25/42C and 25/42D, Kingdom of Saudi Arabia

    USGS Publications Warehouse

    Fenton, Michael D.

    1983-01-01

    Areas with potential for metallic mineral deposits in the Wadi al Jarir and Al Jurdhawiyah quadrangles, northeastern Arabian Shield, have been identified by reconnaissance rock geochemistry, inspection of ancient prospects, and interpretation of previous work. The ancient prospects of Abraq Shawfan, Abraq Shawfan South, Ad Du'ibi, Ad Du'ibi West, and Ad Dirabi are not recommended for further study. The Bald al Jimalah East ancient lead-silver mine should be drilled to investigate its mineral potential at depth and to.determine its apparent relationship to the nearby Baid al Jimalah West tungsten-tin prospect. High precious metal and copper contents confirmed at the Jarrar ancient prospect suggest additional study. Preliminary results of core and percussion drilling at the Bald al Jimalah West tungsten-tin prospect indicate that the mineralized rocks decrease in grade with depth and are not suitable for current economic exploitation. Geochemically anomalous areas in both plutonic and layered volcanic and clastic terrane are possible sites of significant base metal, molybdenum, tin, tungsten, and rare-earth element mineralization.

  5. Thorium Enrichment within the Procellarum KREEP Terrane: The Record in Surface Deposits and Significance for Thermal Evolution

    NASA Technical Reports Server (NTRS)

    Jolliff, B. L.; Gillis, J. J.; Haskin, L. A.

    1999-01-01

    The nearside-farside structural and compositional asymmetry of the Moon was recognized during the early days of Apollo and the suggestion was made that the migration of mantle melts to the nearside would have been favored by early Earth-Moon orbital dynamics and nonuniform planetesimal bombardment. Recent global geochemical mapping by Lunar Prospector has provided additional data, particularly in the Th distribution, that strongly supports the notion of global, preferential melt migration, which led in part to the development of the Procellarum KREEP Terrane (PKT) [2-5]. The surface distribution of Th was then reshaped by basin-forming impacts into the PKT, especially the Imbrium impact, which was the last and largest to strike in that region. The Imbrium event probably excavated material from a partially molten zone deep in the crust and delivered Th-rich ejecta Moon-wide. A fundamentally important but poorly understood aspect of the global Th distribution is the concentration of Th in the subsurface rocks of the PKT crustal section. For example, depending on what assumptions are made, the PKT crustal section, which is about 12% of the crust and only about 1.2% of the whole Moon, may contain as much as 40% of the Moon's entire Th budget. Such a distribution of Th and related heat-producing elements would have had a profound effect on melting, mixing, and the thermal evolution of the PKT and the underlying mantle. In this abstract, we examine the compositions of terra formations within the PKT and relate them to some of the Th-bearing rock types known from the Apollo samples. It appears that the existence of the PKT may be a unifying concept for a number of petrologic and geochemical observations. From the initial Lunar Prospector gamma-ray spectrometer data(-about 5 deg. resolution) and from the preliminary low-orbit data, there appears to be a number of relatively hotter "spots" within the PKT in terms of Th concentration. Some of the hotter spots correspond to

  6. Thorium Enrichment within the Procellarum KREEP Terrane: The Record in Surface Deposits and Significance for Thermal Evolution

    NASA Astrophysics Data System (ADS)

    Jolliff, B. L.; Gillis, J. J.; Haskin, L. A.

    1999-01-01

    The nearside-farside structural and compositional asymmetry of the Moon was recognized during the early days of Apollo and the suggestion was made that the migration of mantle melts to the nearside would have been favored by early Earth-Moon orbital dynamics and nonuniform planetesimal bombardment. Recent global geochemical mapping by Lunar Prospector has provided additional data, particularly in the Th distribution, that strongly supports the notion of global, preferential melt migration, which led in part to the development of the Procellarum KREEP Terrane (PKT) [2-5]. The surface distribution of Th was then reshaped by basin-forming impacts into the PKT, especially the Imbrium impact, which was the last and largest to strike in that region. The Imbrium event probably excavated material from a partially molten zone deep in the crust and delivered Th-rich ejecta Moon-wide. A fundamentally important but poorly understood aspect of the global Th distribution is the concentration of Th in the subsurface rocks of the PKT crustal section. For example, depending on what assumptions are made, the PKT crustal section, which is about 12% of the crust and only about 1.2% of the whole Moon, may contain as much as 40% of the Moon's entire Th budget. Such a distribution of Th and related heat-producing elements would have had a profound effect on melting, mixing, and the thermal evolution of the PKT and the underlying mantle. In this abstract, we examine the compositions of terra formations within the PKT and relate them to some of the Th-bearing rock types known from the Apollo samples. It appears that the existence of the PKT may be a unifying concept for a number of petrologic and geochemical observations. From the initial Lunar Prospector gamma-ray spectrometer data(-about 5 deg. resolution) and from the preliminary low-orbit data, there appears to be a number of relatively hotter "spots" within the PKT in terms of Th concentration. Some of the hotter spots correspond to

  7. 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

  8. 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

    The W-edge of the Caribbean Plate is characterized by two major basement domains, separated today by a SW-NE trending diffuse fault zone located SE of the Nicoya Peninsula (Costa Rica) and possibly connecting with the Hess Escarpment. To the NW, in the area originally called "Chortis Block", oceanic island/arc basements range in age from Late Triassic to Early Cretaceous and form a complicated puzzle of geodynamic units. To the SE of this fault line, no age older than Turonian-Santonian (90-85 Ma) is known. This area only represents the trailing edge of the Caribbean Large Igensous Procince (CLIP). The Mesquito Composite Oceanic Terrane (MCOT) comprises the southern half of the "Chortis Block", classically considered as a continental fragment of N-America. The MCOT is defined by isolated outcrops of ultramafic, mafic oceanic/arc rocks, and radiolarites of Late Triassic, Jurassic and Early Cretaceous age: Rhaetian (latest Triassic) radiolarites found in the El Castillo Mélange (S-MCOT: S-Nicaragua/N- Costa Rica). They are associated with blocks of OIB-metabasalts. These rocks document the presence of a Late Triassic oceanic basement that must have been the substrate of the 174 -177 Ma old (Early/Middle Jurassic) Petit-Spot-like alkaline volcanics that intruded Early Jurassic radiolarites. These rocks form tectonic slivers in the middle Cretaceous Santa Rosa Accretionary Complex (relative autochthonous of the Santa Elena ultramafic unit, N-Costa Rica). The oldest rocks of the Nicoya Complex s. str. (NW-Nicoya Peninsula, Costa Rica) are Bajocian (Middle Jurassic) radiolarites, that occur as blocks magmatically engulfed in plateau-type basalts and intrusives that range in age thoughout the pre-Campanian Cretaceous (130-83 Ma). Middle and Late Jurassic metaradiolarites occur as blocks in the Siuna Serpentinite Médange (NE-Nicaragua), along with High-p, arc-related mafics. We envision an oceanic arc that collided in the latest Jurassic with the Agua Fria arc system

  9. A Lower Carboniferous two-stage extensional basin along the Avalon-Meguma terrane boundary: Evidence from southeastern Isle Madame, Nova Scotia

    USGS Publications Warehouse

    Force, E.R.; Barr, S.M.

    2006-01-01

    Anomalously thick and coarse clastic sedimentary successions, including over 5000 m of conglomerate, are exposed on Isle Madame off the southern coast of Cape Breton Island. Two steeply to moderately dipping stratigraphic packages are recognized: one involving Horton and lower Windsor groups (Tournasian-Visean); the other involving upper Windsor and Mabou (Visean-Namurian) groups. Also anomalous on Isle Madame are three long narrow belts of "basement" rocks, together with voluminous chloritic microbreccia and minor semi-ductile mylonite, which are separated from the conglomerate-dominated successions by faults. The angular relations between the cataclastic rocks and the conglomerate units, combined with the presence of cataclasite clasts in the conglomerate units and evidence of dip-slip faults within the basin, suggest an extensional setting, where listric normal faults outline detachment allochthons. Allochthon geometry requires two stages of extension, the older stage completed in early Windsor Group time and including most of the island, and the more local younger stage completed in Mabou Group time. Domino-style upper-plate faulting in the younger stage locally repeated the older detachment relation of basement and conglomerate to form the observed narrow belts. Re-rotation of older successions in the younger stage also locally overturned the Horton Group. These features developed within a broad zone of Carboniferous dextral transcurrent faulting between already-docked Avalon and Meguma terranes. Sites of transpression and transtension alternated along the Cobequid-Chedabucto fault zone that separated these terranes. The earlier extensional features in Isle Madame likely represent the northern headwall and associated clastic debris of a pull-apart or other type of transtensional basin developed along part of this fault zone that had become listric; they were repeated and exposed by being up-ended in the second stage of extension, also on listric faults. The

  10. Geochemical, oxygen, and neodymium isotope compositions of metasediments from the Abitibi greenstone belt and Pontiac Subprovince, Canada: Evidence for ancient crust and Archean terrane juxtaposition

    SciTech Connect

    Feng, R.; Kerrich, R. ); Maas, R. )

    1993-02-01

    The Abitibi greenstone belt (AGB) and Pontiac Subprovince (PS) in the southwestern Superior Province are adjacent greenstone-plutonic and metasedimentary-dominated terranes, respectively, separated by a major fault zone. Metasediments from these two contrasting terranes are compared in terms of major- and trace-element and O- and Nd-isotope compositions, and detrital zircon ages. The following two compositional populations of metasediments are present in the low-grade, Abitibi southern volcanic zone: (1) a mafic-element-enriched population (MEP) characterized by flat, depleted REE patterns; enhanced Mg, Cr, Co, Ni, and Sc; low-incompatible-element contents; and minor or absent normalized negative troughs at Nb, Ta, and Ti; and (2) a low-mafic-element population (LMEP) featuring LREE-enriched patterns; enhanced Rb, Cs, Ba, Th, and U contents; and pronounced normalized negative troughs at Nb, Ta, and Ti. These geochemical features are interpreted to indicate that the MEP sediments were derived from an ultramafic- and mafic-dominated oceanic provenance, whereas the LMEP sediments represent mixtures of mafic and felsic are source rocks. The PS metasediments are essentially indistinguishable from Abitibi LMEP on the basis of major-element and transition metal abundances, suggesting comparable types of source rocks and degrees of maturity, but are distinct in terms of some trace elements and O-isotope compositions. The Pontiac metasediments are depleted in [sup 18]O and enriched in Cs, Ba, Pb, Th, U, Nb, Ta, Hf, Zr, and total REE and also have higher ratios of Rb/K, Cs/Rb, Ba/Rb, Ta/Nb, Th/La, and Ba/La relative to the Abitibi LMEP. Two subtypes of REE patterns have been identified in PS metasediments. The first subtype is interpreted to be derived from provenances of mixed mafic and felsic volcanic rocks, whereas the Eu-depleted type has features that are typical of post-Archean sediments or Archean K-rich granites and volcanic equivalents. 100 refs., 9 figs., 4 tabs.

  11. Discovery of the base of the Pinal Schist and the Bear Canyon sequence below it in the eastern Metamorphic terrane of the Dos Cabezas mountains, Cochise County, Arizona

    SciTech Connect

    Erickson, R.C. . Geology Dept.)

    1993-04-01

    The eastern metamorphic terrane of the Dos Cabezas mountains is underlain by Pinal Schist (PS). Recent detailed mapping along the western edge of this terrane shows that a flat planar regional unconformity (UC) bounds this Pinal body at its western stratigraphic base. Below the UC lies 8 km[sup 2] of greenschist facies metapelites and metafelsites here given the provisional name of the Bear Canyon sequence (BCS). The PS above the UC is over 10 stratigraphic km, grading from 1 km conglomerate at the base (clasts are metafelsites and metasediments with very minor quartz and granite) up into 3 km of sandstones and then pelite, all with greenschist facies overprint. Relict bedding with abundant crossbedding is common; the sequence is upright. Foliation parallels bedding. Lineation is absent. The UC itself is well exposed in two places, on both sides of Happy Camp Canyon (HCC). On the west of HCC, the surface is exposed at the mouth of Bear Canyon (NW1/4 Sec. 31, T13S R28E) striking south and then southwest about 4 km, ending in the N center of Sec. 12, T14S R27E. The unconformity and the Pinal above it lie NS 30E in the north and smoothly swing to NE 50 SE in the south. Below the UC are 6 km[sup 2] of metasandstone and metafelsites of the BCS. The metasandstone is quartz-sericite schist with strong lineation ([minus]50 S70E) in its northern km of exposure. Bedding and foliation are NS 60 E in N, and NE 70 SE in S. This schist body is intruded on the E by a dike of quartz-phyric metafelsite to 1 km wide which underlies the UC along its entire length. The felsite shows strong relict flow layering and no foliation. Flow layering is NS 90 in the N, and strikes and dips variably in the S. A 1/4 km[sup 2] metafelsite plug intrudes the schist on the W, with flow layering NE 90. The two exposures of the UC are separated by a Precambrian normal fault striking N45E and dipping moderately SE under HCC alluvial fill; it has about a km of dip slip.

  12. Two Cenozoic tectonic events of N-S and E-W extension in the Lhasa Terrane: Evidence from geology and geochronology

    NASA Astrophysics Data System (ADS)

    Huang, Feng; Xu, Ji-Feng; Chen, Jian-Lin; Wu, Jian-Bin; Zeng, Yun-Chuan; Xiong, Qiu-Wei; Chen, Xue-Feng; Yu, Hong-Xia

    2016-02-01

    Cenozoic active structures in the Tibetan Plateau are mainly regional N-S trending extensional faults and grabens, and E-W trending extensional tracks that are related to the transition from syn- to post-collision between India and Asia. E-W trending tracks are parallel to the direction of Neo-Tethyan oceanic convergence and consist of extensional volcanic-sedimentary basins and magmatic dykes in the southern Lhasa Terrane, Tibet. N-S trending tracks comprise faults and grabens, which are widely developed in Tibet. It remains unknown how and when the geodynamic transition from E-W to N-S trending tectonic tracks occurred. This study describes both E-W and N-S trending tectonic tracks identified at Dazi area of southern Lhasa Terrane, where E-W trending mafic dykes intruded a granitoid and late-stage N-S trending felsic dykes cut across E-W trending mafic dykes. Zircons from four granitoid samples yield consistent crystallization ages of ca. 60 Ma and positive εHf(t) values (~+ 9). An altered dioritic vein, which cuts the mafic dykes, yields an age of ca. 53 Ma. These new dating results indicate that E-W trending dykes, which formed due to regional N-S extension, were emplaced between 60 and 53 Ma. In addition, two N-S trending monzonitic porphyritic dykes, which cut the mafic dykes, yield U-Pb zircon ages of ca. 17 Ma with moderate positive εHf(t) values (+ 3 to + 9.6), as well as a NNE-SSW trending quartz monzonitic dyke, which cuts all other types of dykes, yields U-Pb ages of ca. 13 Ma. This suggests that E-W extension took place between 17 and 13 Ma. These results, in combination with existing age data for Gangdese granitoids and mafic magmatism, indicate the occurrence of two major extensional events at 60-53 Ma and 17-13 Ma. In turn, this implies that the transition from E-W to N-S trending tectonic and the onset of E-W extension occurred at ca. 17 Ma or slightly earlier. Paleocene granitoids have geochemical characteristics that are indicative of both

  13. U-Pb and K-Ar Geochronological Constraints on the Tectonothermal Evolution of the Precambrian Terranes of the SE São Paulo State, Brazil

    NASA Astrophysics Data System (ADS)

    Passarelli, C. R.; Wemmer, K.; Siga, O.; Basei, M. A.

    2008-12-01

    The Ribeira belt, the major tectonic unit of the Mantiqueira Province, southeastern Brazil, records the interface between the Congo-São Francisco, Kalahari, La Plata cratons and the Paranapanema cratonic block. The belt is made up of many terrains of different nature, and was generated and affected by the tectonomagmatic episodes of the Brasiliano orogenic cycle, the South American equivalent of the Pan- African Orogeny. Aiming at the characterization of the thermal history of Pre-Cambrian terranes of the southeastern of São Paulo state, specific geochronological studies were carried out on mylonitic and granitic rocks. This sector of Ribeira Belt consists of four major tectonic domains limited by significant shear zones, associated with Neoproterozoic events. The Embu Domain, north of the Cubatão Shear Zone (CSZ), is composed of metasedimentary rocks, and peraluminous granites, that yielded U-Pb ages of ca.790 Ma, 620 Ma and 600 Ma. Gneiss-migmatite rocks (640-620 Ma) and intrusive granites (580 Ma) predominate in the Mongaguá Domain, which is limited by Cubatão and Itariri shear zones. The Registro Domain, between Cubatão - Itariri Shear System (CISS) and the Serrinha Shear Zone (SSZ), is formed by metasediments and granitic rocks with migmatitic features represents a Paleoproterozoic terrane (1.9-2.2 Ga) strongly deformed during the Neoproterozoic (750-580 Ma). Rocks of the Iguape Domain, limited to the north by the SSZ, include granites (ca. 600 Ma) and low grade metasediments. The Itariri and Cubatão mylonites occur as high- and low-temperature varieties, formed in amphibolite and greenschist facies conditions, respectively, and the Serrinha mylonites developed under amphibolite facies. U-Pb zircon and monazite geochronological evidence indicates a short time interval at the end of Neoproterozoic for the blocks juxtaposition. From the compilation of geochronological data and apatite fission-track thermochronology, a temperature vs time correlation

  14. Rb-Sr whole-rock and mineral ages, K-Ar, 40Ar/39Ar, and U-Pb mineral ages, and strontium, lead, neodymium, and oxygen isotopic compositions for granitic rocks from the Salinian Composite Terrane, California:

    USGS Publications Warehouse

    Kistler, R.W.; Champion, D.E.

    2001-01-01

    This report summarizes new and published age and isotopic data for whole-rocks and minerals from granitic rocks in the Salinian composite terrane, California. Rubidium-strontium whole-rock ages of plutons are in two groups, Early Cretaceous (122 to 100 Ma) and Late Cretaceous (95 to 82 Ma). Early Cretaceous plutons occur in all granitic rock exposures from Bodega Head in the north to those from the Santa Lucia and Gabilan Ranges in the central part of the terrane. Late Cretaceous plutons have been identified in the Point Reyes Peninsula, the Santa Lucia and the Gabilan Ranges, and in the La Panza Range in the southern part of the terrane. Ranges of initial values of isotopic compositions are 87Sr/86Sr, 0.7046-0.7147, δ18O, +8.5 to +12.5 per mil, 206Pb/204Pb, 18.901-19.860, 207Pb/204Pb, 15.618-15.814, 208Pb/204Pb, 38.569- 39.493, and εNd, +0.9 to -8.6. The initial 87Sr/86Sr=0.706 isopleth is identified in the northern Gabilan Range and in the Ben Lomond area of the Santa Cruz Mountains, in Montara Mountain, in Bodega Head, and to the west of the Farallon Islands on the Cordell Bank. This isotopic boundary is offset about 95 miles (160km) by right-lateral displacements along the San Gregorio-Hosgri and San Andreas fault systems.

  15. A review of Lower and Middle Palaeozoic biostratigraphy in west peninsular Malaysia and southern Thailand in its context within the Sibumasu Terrane

    NASA Astrophysics Data System (ADS)

    Cocks, L. R. M.; Fortey, R. A.; Lee, C. P.

    2005-03-01

    Fossils from the Cambrian to Devonian rocks of southern Thailand, the Langkawi Islands, mainland Kedah, Perlis, north Perak and central West Peninsular Malaysia are listed and reviewed, and their stratigraphy and correlation reassessed. The hitherto anomalous record of the trilobite Dalmanitina from Malaysia is reviewed and found to be of latest Ordovician (Hirnantian) age, rather than Lower Silurian age as previously reported, and is considered a probable synonym of the widespread Mucronaspis mucronata. A new stratigraphical nomenclature is erected for part of the Langkawi, mainland Kedah and Perlis area successions, in which the term Setul Limestone (which stretched from the Ordovician to the Devonian) is abandoned and replaced by the Middle Ordovician Kaki Bukit Limestone, the late Ordovician and early Silurian Tanjong Dendang Formation, the Silurian Mempelam Limestone, and the early Devonian Timah Tasoh Formation, all underlying the paraconformity with the late Devonian Langgun Red Beds. There was a single depositional basin in the generally shallow-water and cratonic areas of southern Thailand, Langkawi, and mainland Kedah and Perlis, in contrast to the deeper-water basin of north Perak. Only Silurian rocks are dated with certainty within another basin in central West Malaysia, near Kuala Lumpur, which were also cratonic and shallow-water, although to the east in west Pahang there are basal Devonian deeper-water sediments with graptolites. The area is reviewed in its position within the Sibumasu Terrane, which, in the Palaeozoic, also included central and northern Thailand, Burma (Myanmar) and southwest China (part of Yunnan Province).

  16. U-Pb ages and Sr, Pb and Nd isotope data for gneisses near the Kolar Schist Belt: Evidence for the juxtaposition of discrete Archean terranes

    NASA Technical Reports Server (NTRS)

    Krogstad, E. J.; Hanson, G. N.; Rajamani, V.

    1988-01-01

    Uranium-lead ages and Sr, Pb, and Nd isotopic data for gneisses near the Kolar Schist Belt and their interpretation as evidence for the juxtaposition of discrete Archean terranes were presented. The granodioritic Kambha gneiss east of the schist belt has a zircon age of 2532 + or - 3 Ma and mantle-like initial Sr, Pb, and Nd isotopic ratios. Therefore these gneisses are thought to represent new crust added to the craton in the latest Archean. By contrast, more mafic Dod gneisses and leucocratic Dosa gneisses west of the schist belt (2632 + or - 7 and 2610 + or - 10 Ma) show evidence for contamination of their magmatic precursors (LREE-enriched mantle-derived for the Dod gneisses) by older (greater than 3.2 Ga) continental crust. Fragments of this older crust may be present as granitic and tonalitic inclusions in the 2.6-Ga gneisses and in shear zones. The antiquity of these fragments is supported by their Nd, Sr, and Pb isotopic compositions and by 2.8 to greater than 3.2 Ga zircon cores.

  17. U-Pb ages and Sr, Pb and Nd isotope data for gneisses near the Kolar Schist Belt: Evidence for the juxtaposition of discrete Archean terranes

    NASA Astrophysics Data System (ADS)

    Krogstad, E. J.; Hanson, G. N.; Rajamani, V.

    Uranium-lead ages and Sr, Pb, and Nd isotopic data for gneisses near the Kolar Schist Belt and their interpretation as evidence for the juxtaposition of discrete Archean terranes were presented. The granodioritic Kambha gneiss east of the schist belt has a zircon age of 2532 + or - 3 Ma and mantle-like initial Sr, Pb, and Nd isotopic ratios. Therefore these gneisses are thought to represent new crust added to the craton in the latest Archean. By contrast, more mafic Dod gneisses and leucocratic Dosa gneisses west of the schist belt (2632 + or - 7 and 2610 + or - 10 Ma) show evidence for contamination of their magmatic precursors (LREE-enriched mantle-derived for the Dod gneisses) by older (greater than 3.2 Ga) continental crust. Fragments of this older crust may be present as granitic and tonalitic inclusions in the 2.6-Ga gneisses and in shear zones. The antiquity of these fragments is supported by their Nd, Sr, and Pb isotopic compositions and by 2.8 to greater than 3.2 Ga zircon cores.

  18. Isotopic evidence for the sources of Cretaceous and tertiary granitic rocks, east-central Alaska: Implications for the tectonic evolution of the Yukon-Tanana terrane

    USGS Publications Warehouse

    Aleinikoff, J.N.; Farmer, G.L.; Rye, R.O.; Nokleberg, W.J.

    2000-01-01

    Magnetotelluric traverses across the southern Yukon-Tanana terrane (YTT) reveal the presence of a thick conductive layer (or layers) beneath Paleozoic crystalline rocks. These rocks have been interpreted to be flysch of probable Mesozoic age, on the basis of the occurrence of Jurassic-Cretaceous flysch in the Kahiltna assemblage and Gravina-Nutzotin belt flanking the YTT to the southwest and southeast, respectively. The Pb, Nd, Sr, and O isotopes in Cretaceous and Tertiary granitic rocks that crop out throughout the YTT were measured to determine if these rocks do in fact contain a component of flysch. Previous limited analyses indicated that the Pb isotopes of the granitic rocks could be a mixture of radiogenic Pb derived from Paleozoic crystalline rocks of the YTT with an increasing component of relatively nonradiogenic Pb with decreasing age. Our Nd, Sr, and O data, along with additional Pb isotope data, eliminate flysch as a likely source and strongly suggest that the nonradiogenic end-member was derived from mafic rocks, either directly from mantle magma or by melting of mafic crust. The lack of a sedimentary component in the granitic plutons suggests either that the plutons did not incorporate significant amounts of flysch during intrusion or that the conductive layer beneath the YTT crystalline rocks is not flysch.

  19. Late-Proterozoic to Paleozoic history of the peri-Gondwana Calabria-Peloritani Terrane inferred from a review of zircon chronology.

    PubMed

    Fornelli, Annamaria; Micheletti, Francesca; Piccarreta, Giuseppe

    2016-01-01

    U-Pb analyses of zircon from ten samples of augen gneisses, eight mafic and intermediate metaigneous rocks and six metasediments from some tectonic domains along the Calabria-Peloritani Terrane (Southern Italy) contribute to knowledge of peri-Gondwanan evolution from Late-Proterozoic to Paleozoic times. All samples were equilibrated under amphibolite to granulite facies metamorphism during the Variscan orogeny. The zircon grains of all considered samples preserve a Proterozoic memory suggestive of detrital, metamorphic and igneous origin. The available data fit a frame involving: (1) Neoproterozoic detrital input from cratonic areas of Gondwana; (2) Pan-African/Cadomian assemblage of blocks derived from East and West African Craton; (3) metamorphism and bimodal magmatism between 535 and 579 Ma, within an active margin setting; (4) rifting and opening of Ordovician basins fed by detrital input from the assembled Cadomian blocks. The Paleozoic basins evolved through sedimentation, metamorphism and magmatism during the Variscan orogeny involving Palaeozoic and pre-Paleozoic blocks. The Proterozoic zircon records decidedly decrease in the high grade metamorphic rocks affected by Variscan pervasive partial melting. PMID:27026906

  20. Late-Proterozoic to Paleozoic history of the peri-Gondwana Calabria-Peloritani Terrane inferred from a review of zircon chronology.

    PubMed

    Fornelli, Annamaria; Micheletti, Francesca; Piccarreta, Giuseppe

    2016-01-01

    U-Pb analyses of zircon from ten samples of augen gneisses, eight mafic and intermediate metaigneous rocks and six metasediments from some tectonic domains along the Calabria-Peloritani Terrane (Southern Italy) contribute to knowledge of peri-Gondwanan evolution from Late-Proterozoic to Paleozoic times. All samples were equilibrated under amphibolite to granulite facies metamorphism during the Variscan orogeny. The zircon grains of all considered samples preserve a Proterozoic memory suggestive of detrital, metamorphic and igneous origin. The available data fit a frame involving: (1) Neoproterozoic detrital input from cratonic areas of Gondwana; (2) Pan-African/Cadomian assemblage of blocks derived from East and West African Craton; (3) metamorphism and bimodal magmatism between 535 and 579 Ma, within an active margin setting; (4) rifting and opening of Ordovician basins fed by detrital input from the assembled Cadomian blocks. The Paleozoic basins evolved through sedimentation, metamorphism and magmatism during the Variscan orogeny involving Palaeozoic and pre-Paleozoic blocks. The Proterozoic zircon records decidedly decrease in the high grade metamorphic rocks affected by Variscan pervasive partial melting.

  1. Young mare volcanism in the Orientale region contemporary with the Procellarum KREEP Terrane (PKT) volcanism peak period ˜2 billion years ago

    NASA Astrophysics Data System (ADS)

    Cho, Yuichiro; Morota, Tomokatsu; Haruyama, Junichi; Yasui, Minami; Hirata, Naru; Sugita, Seiji

    2012-06-01

    The crater retention ages of the mare deposits within the Orientale multi-ring impact basin are investigated using 10-m resolution images obtained by the SELENE (Kaguya) spacecraft, in order to constrain the volcanic history of the Moon around the nearside-farside boundary. Precise crater-counting analyses reveal that mare deposits in the Orientale region are much younger than previously estimated: ˜2.9 Ga mare basalt in the eastern part of Mare Orientale and ˜1.8-2.2 Ga mare deposits in Lacus Veris and Lacus Autumni, maria along the northeastern rings of the basin. The latter age estimates indicate that the Orientale region experienced volcanic activities ˜2 billion years after the basin-formation impact. The dominance of a uniform surface age across the mare deposits in the peripheral regions strongly suggests that these volcanic eruptions are contemporary with the elevated volcanic activity episode proposed for the Procellarum KREEP Terrane on the lunar nearside at ˜2 Ga and that this activity peak is much more widespread than previously estimated. The longevity of mare volcanism in the Orientale region further suggests high initial temperatures and/or high content of heat-producing elements in the underlying mantle of this region.

  2. Geochemical evidence for the tectonic setting of the Coast Range ophiolite: A composite island arc oceanic crust terrane in western California

    NASA Astrophysics Data System (ADS)

    Shervais, John W.; Kimbrough, David L.

    1985-01-01

    The Middle to Late Jurassic age Coast Range ophiolite (CRO) of California contains two geochemically distinct volcanic rock associations that formed in different tectonic settings. Volcanic rocks from the southern CRO (Point Sal, Cuesta Ridge, Stanley Mountain, Llanada, Quinto Creek, and Del Puerto) and parts of the northern CRO (Healdsburg, Elder Creek) are similar to low-K tholeiites and calc-alkaline rocks of the island-arc suite. The thin volcanic sections of these ophiolite remnants suggest formation by intra-arc rifting. In contrast, volcanic rocks from Stonyford seamount and Paskenta in the northern CRO are transitional subalkaline metabasalts with geochemical characteristics similar to enriched mid-ocean ridge basalts or ocean-island tholeiites. These rocks are associated with Tithonian radiolarian cherts and may be part of the Franciscan Complex. Alternatively, they may represent a change in tectonic setting within the CRO during the Late Jurassic. Regardless, the CRO as currently conceived cannot be considered a single terrane with one mode of origin.

  3. Tectonic conditions of sedimentation and source areas of Upper Proterozoic and Lower Paleozoic terrigenous deposits of the Lesser Khingan Terrane of the Central Asian Fold Belt

    NASA Astrophysics Data System (ADS)

    Smirnova, Yu. N.; Sorokin, A. A.; Kotov, A. B.; Kovach, V. P.

    2016-05-01

    This work presents the results of geological, geochemical, and Sm-Nd isotopic and geochemical studies of Late Riphean-Cambrian terrigenous rocks of the Khingan Group of the Lesser Khingan Terrane of the Central Asian Fold Belt, as well as the results of U-Pb geochronological (LA-ICP-MS) studies of detrital zircons from these deposits. These deposits are the most ancient in the structure of the terrain. It was found that the deposits of Iginchi and underlying Murandavi formations are attributed to the Late Riphean-Vendian age interval, and the Kimkan sequence, to the Late Cambrian-Early Ordovician. The periods of formation of the Murandavi and Iginchi formations, on one hand, and the Kimkan sequence, on the other hand, are separated by the stage of granitoid magmatism at the turn of the Vendian-Cambrian. Because of this, they cannot be attributed to a unified sedimentary sequence. It is the most probable that the sedimentation of the Iginchi and Murandavi formations and the Kimkan sequence occurred under subduction conditions against the backdrop of magmatic activity.

  4. Crustal evolution in a cratonic nucleus: Granitoids and felsic volcanic rocks of the North Caribou Terrane, Superior Province Canada

    NASA Astrophysics Data System (ADS)

    Wyman, Derek A.; Hollings, Pete; Biczok, John

    2011-04-01

    Attempts to decipher the petrogenesis of Archean Trondhjemite-Tonalite-Granodiorite (TTG) and establish trends in their compositions over time typically do not assess the possible role of assimilation-crystal fractionation processes. Studies of TTG suites worldwide, however, demonstrate that reprocessing of older crust is common. Felsic volcanic and intrusive rocks from the North Caribou Lake area, located in the nucleus of the Superior Province, exhibit a range of compositions that are not accounted for by variable degrees of amphibolite melting alone. Characteristic of Archean TTG, the rocks mainly fall in the lower right hand quadrant of a Nb/Ta vs Zr/Sm plot but they also show clear trends between ɛNd T and ratios such as Al 2O 3/TiO 2, Th/Nb, [Tb/Yb] N and Eu/Eu*. The lowest Nb/Ta values in the TTG likely result from crystal fractionation including titanite, which strongly partitions and fractionates these elements.

  5. Partial melting and rapid exhumation of a Pliocene UHP terrane: CA-TIMS zircon results from Normanby Island, Papau New Guinea

    NASA Astrophysics Data System (ADS)

    DesOrmeau, J. W.; Gordon, S. M.; Little, T. A.; Bowring, S. A.

    2012-12-01

    Some ultra-high pressure (UHP) terranes contain eclogite embedded within migmatitic gneisses that preserve evidence for multiple episodes of partial melting. Understanding the mechanisms by which crustal rocks are subducted to mantle depths and subsequently exhumed requires understanding the timing of metamorphism and the timing and role of partial melting. The D'Entrecasteaux Islands of southeastern Papua New Guinea (PNG) expose Pliocene UHP eclogites contained within migmatitic host gneisses. Garnet-whole rock Lu-Hf geochronology suggests UHP metamorphism may have begun by ca. 7 Ma, and Ar-Ar thermochronology indicates that the rocks were exhumed to the surface by ca. 2 Ma; the history between 7 and 2 Ma is crucial for tectonic models of PNG. We have applied U-Pb chemical-abrasion thermal-ionization mass spectrometry (CA-TIMS) geochronology to zircons separated from granitoid intrusions within the host gneisses to better understand the timing and role of melting in the exhumation of the PNG UHP rocks. Three granodiorite intrusions were collected from Normanby Island as it has received far less study in previous investigations than the other D'Entrecasteaux Islands (Goodenough and Fergusson) and exposes the contact of the UHP-HP rocks against the structurally overlying, weakly metamorphosed rocks of the Papuan ultramafic belt (PUB). From the west side of Normanby dome in contact with the PUB, zircons from a foliated protomylonitic granodiorite sill yielded 206Pb/238U dates of ca. 4.1 Ma. In comparison, on the eastern side of Normanby dome, zircons from a weakly foliated, discordant pegmatitic dike yielded a complex array of zircon analyses and the youngest dates are ca. 4.1 Ma. The zircons extracted from these deformed intrusions are interpreted to date melt emplacement, crystallization, and zircon growth, which may be coeval with amphibolite facies (570-730 °C and ~7-12 Kbar) metamorphism recorded from Goodenough and Fergusson Islands. Zircons from a non

  6. Devonian granitoids and their hosted mafic enclaves in the Gorny Altai terrane, northwestern Central Asian Orogenic Belt: crust-mantle interaction in a continental arc setting

    NASA Astrophysics Data System (ADS)

    Chen, Ming; Sun, Min

    2016-04-01

    Granitoids are a major component in the upper continental crust and hold key information on how did the continental crust grow and differentiate. This study focuses on the Yaloman intrusive complex from the Gorny Altai terrane, northwestern Central Asian Orogenic Belt (CAOB). The association of granitoids and mafic enclaves can provide important clues on the source nature, petrogenetic processes and geodynamic setting of the Yaloman intrusive complex, which in turn will shed light on the crustal evolution in the northwestern CAOB. Zircon U-Pb dating shows that the granitoids, including quartz diorites and granodiorites, were emplaced in ca. 389-387 Ma. The moderate Na2O + K2O contents and low A/CNK values indicate that these rocks belong to the sub-alkaline series with metaluminous to weakly peraluminous compositions. The granitoids yield two-stage zircon Hf model ages of ca. 0.79-1.07 Ga and whole-rock Nd model ages of ca. 0.90-0.99 Ga, respectively, implying that they were mainly sourced from Neoproterozoic juvenile crustal materials. The mafic enclaves show an almost identical crystallization age of ca. 389 Ma. The identification of coarse-grained xenocrysts and acicular apatites, together with the fine-grained texture, makes us infer that these enclaves are likely to represent magmatic globules commingled with the host magmas. The low SiO2 and high MgO contents of the mafic enclaves further suggest that substantial mantle-derived mafic melts were probably involved in their formation. Importantly, the SiO2 contents of the granitoids and mafic enclaves are well correlated with other major elements and most of the trace elements. Also a broadly negative correlation exists between the SiO2 contents and whole-rock epsilon Nd (390 Ma) values of the granitoids. Given the observation of reversely zoned plagioclases within the granitoids and the common occurrence of igneous mafic enclaves, we propose that magma mixing probably played an important role in the formation

  7. Tracking the exhumation of a Pliocene (U)HP terrane: U-Pb and trace-element constraints from zircon, D'Entrecasteaux Islands, Papua New Guinea

    NASA Astrophysics Data System (ADS)

    DesOrmeau, Joel W.; Gordon, Stacia M.; Little, Timothy A.; Bowring, Samuel A.

    2014-10-01

    structures within the D'Entrecasteaux Islands of eastern Papua New Guinea expose ultrahigh-pressure (UHP) Pliocene (5.6-4.6 Ma) eclogites and evidence for partial melting. To better interpret the (U)HP exhumation history, U-Pb geochronology and trace-element abundances were determined in zircon from variably deformed host gneiss and crystallized melt (leucosomes, sills, dikes, and plutons) from the Goodenough and Normanby Domes by ID-TIMS (isotope-dilution thermal ionization mass spectrometry) and ICP-MS (inductively coupled plasma mass spectrometry), respectively, to constrain the timing of melt crystallization and deformation relative to UHP metamorphism. Zircons extracted from orthogneiss and deformed granodiorite sills of Normanby Dome, located ˜40 km southeast of the UHP eclogite, record HP metamorphism from 5.66 ± 0.02 to 5.04 ± 0.07 Ma, and melt crystallization at ˜4.1 Ma. Strongly deformed, layer-parallel leucosomes from Goodenough Dome, ˜20 km northwest of the UHP eclogite, began to crystallize by 3.85 ± 0.02 Ma. These dates indicate that melt crystallization began in the Goodenough and Normanby Domes within ˜0.75 m.y. of (U)HP metamorphism. The ID-TIMS dates from the orthogneiss and crystallized melt show that exhumation and cooling of the (U)HP rocks in the PNG terrane began first in the east, within Normanby Dome, then to the west, in the Goodenough Dome ˜1 m.y. later, and finally the middle dome rocks, exposed within the Mailolo Dome, cooled ˜2 m.y. after exhumation of Normanby Dome. All domes reveal synchronous crystallization of late, nondeformed melts, and final extension-driven exhumation by 1.82 ± 0.03 Ma.

  8. Late Permian basalts in the northwestern margin of the Emeishan Large Igneous Province: Implications for the origin of the Songpan-Ganzi terrane

    NASA Astrophysics Data System (ADS)

    Li, Hongbo; Zhang, Zhaochong; Santosh, M.; LÜ, Linsu; Han, Liu; Liu, Wei; Cheng, Zhiguo

    2016-07-01

    SHRIMP zircon U-Pb ages, geochemical and Sr-Nd isotopic data are reported for two types of basalts (Type I and Type II) from a Permian volcanic-pyroclastic succession in the Tubagou section, Baoxing area along the southeastern margin of the Songpan-Ganzi terrane (SGT) in the Sichuan province of SW China. Zircons from the uppermost basaltic flows yield crystallization age of 257.3 ± 2.0 Ma, which may represent the time of culmination the basaltic eruption. Type I shows alkaline affinity with εNd(t) values of + 2.4 to + 2.9, and is characterized by oceanic island basalt (OIB)-type light rare earth element (LREE) and trace-element patterns. In contrast, Type II rocks are tholeiitic, and close to initial rift tholeiite (IRT)-like REE and trace element patterns, and are relatively depleted in highly incompatible elements with slightly negative Nb-Ta anomaly. The εNd(t) values of Type II are between + 1.8 to + 2.2. The geochemical characteristics suggest the Type I has not been significantly crustally contaminated, whereas Type II maybe have experienced minor crustal contamination. Clinopyroxene crystallization temperature is ~ 80-120°C higher than that of the normal asthenospheric mantle, implying anomalous thermal input from mantle source and a possible plume-head origin for the Tubagou lava. The geochemical and isotopic fes, reflecting progressive lithosphere thinning probably through plume-lithosphere interaction. The spatial and temporal coincidence between the Dashibao basalt eruptions, reflecting progressive lithosphere thinning probably through plume-lithosphere interaction. The spatial and temporal coincidence between the Dashibao basalt eruption and continental rifting suggest that continental break-up and the opening of an extensional basin was probably related to the Late Permian Emeishan plume, which triggered the breakup between the SGT and the Yangtze craton.

  9. Integrating geologic and satellite radar data for mapping dome-and-basin patterns in the In Ouzzal Terrane, Western Hoggar, Algeria

    NASA Astrophysics Data System (ADS)

    Deroin, Jean-Paul; Djemai, Safouane; Bendaoud, Abderrahmane; Brahmi, Boualem; Ouzegane, Khadidja; Kienast, Jean-Robert

    2014-11-01

    The In Ouzzal Terrane (IOT) located in the north-western part of the Tuareg Shield forms an elongated N-S trending block, more than 400 km long and 80 km wide. It involves an Archaean crust remobilized during a very high-temperature metamorphic event related to the Palaeoproterozoic orogeny. The IOT largely crops out in the rocky and sandy desert of Western Hoggar. It corresponds mainly to a flat area with some reliefs composed of Late Panafrican granites, dyke networks or Cambrian volcanic rocks. These flat areas are generally covered by thin sand veneers. They are favorable for discriminating bedrock geological units using imaging radar, backscattering measurements, and field checking, because the stony desert is particularly sensitive to the radar parameters such as wavelength or polarization. The main radar data used are those obtained with the ALOS-PALSAR sensor (L-band), in ScanSAR mode (large swath) and Fine Beam modes. The PALSAR sensor has been also compared to ENVISAT-ASAR and to optical imagery. Detailed mapping of some key areas indicates extensive Archaean dome-and-basin patterns. In certain parts, the supracrustal synforms and orthogneiss domes exhibit linear or circular features corresponding to shear zones or rolling structures, respectively. The geological mapping of these dome-and-basin structures, and more generally of the Archaean and Proterozoic lithological units, is more accurate with the SAR imagery, particularly when using the L-band, than with the optical imagery. A quantitative approach is carried out in order to estimate the backscatter properties of the main rock types. Due to the large variety of configurations, radar satellite imagery such as ALOS PALSAR represents a key tool for geological mapping in arid region at different scales from the largest (e.g., 1:500,000) to the smallest (e.g., 1:50,000).

  10. Petrogenesis of Paleocene-Eocene porphyry deposit-related granitic rocks in the Yaguila-Sharang ore district, central Lhasa terrane, Tibet

    NASA Astrophysics Data System (ADS)

    Zhao, Junxing; Li, Guangming; Evans, Noreen J.; Qin, Kezhang; Li, Jinxiang; Zhang, Xia'nan

    2016-11-01

    The Paleocene-Eocene ore deposits in the Gangdese Metallogenic Belt, Tibet, are thought to have been formed during the main period of India-Asia continental collision. This paper reports the whole-rock major element, trace element, and Sr-Nd-Hf isotopic compositions and zircon trace element contents of volcanic and intrusive rocks from the Paleocene Yaguila skarn Pb-Zn-Ag deposit and adjacent Eocene Sharang porphyry Mo deposit in the central Lhasa terrane, Tibet. Geochemical signatures and Nd-Hf isotopic compositions indicate that the Yaguila Cretaceous rhyolitic rocks were formed by the melting of ancient continental crust, whereas the Paleocene causative granite porphyry may have resulted from the interaction between mantle-derived and crustal-derived materials when continental collision was initiated. The dramatic increase of εNd(t) values between emplacement of the granite porphyry and later porphyritic biotite granite suggests a greater involvement of mantle materials during the crystallization of the barren biotite granite stock. The post-ore Miocene granodiorite porphyry has a similar geochemical signature to the Sharang Miocene dykes, suggesting they were both generated from melting of enriched lithospheric mantle. Nd-Hf mixing calculations indicate an increasing contribution of mantle materials in Paleocene to Eocene intrusions, consistent with the regional tectonic model of Neo-Tethyan oceanic slab roll-back and break-off. Zircons from both the Yaguila and Sharang ore-related porphyries have higher Ce anomalies than those from the barren granitoids, suggesting that Mo mineralization was closely related to highly oxidized and differentiated magma. The fertile intrusions in the Yaguila-Sharang district contain EuN/EuN∗ values from 0.3 to 0.6, higher than the non-mineralized intrusions. The processes of early crystallization of plagioclase and/or SO2-degassing from underlying magma can explain the observed negative Eu anomalies in zircon.

  11. The Bossoroca Complex, São Gabriel Terrane, Dom Feliciano Belt, southernmost Brazil: Usbnd Pb geochronology and tectonic implications for the neoproterozoic São Gabriel Arc

    NASA Astrophysics Data System (ADS)

    Gubert, Mauricio Lemos; Philipp, Ruy Paulo; Stipp Basei, Miguel Angelo

    2016-10-01

    Usbnd Pb LA-ICPMS geochronological analyses were carried out on zircon grains from metavolcanic rocks of the Bossoroca Complex and for one ash tuff of the Acampamento Velho Formation of the Camaquã Basin, in order to understand the evolution of the Neoproterozoic São Gabriel magmatic arc. A total of 42 analyses of igneous zircon grains were performed in three samples. The results yielded Usbnd Pb ages of 767.2 ± 2.9 Ma for the metavolcanic agglomerate (BOS-02); 765 ± 10 Ma for the metacrystal tuff (BOS-03) and 565.8 ± 4.8 Ma for the ash tuff (BOS-04). The Orogenic Cycle in Brazil is characterized by a set of orogenic belts consisting of petrotectonic associations juxtaposed by two collisional events that occurred at the end of the Neoproterozoic. In southern Brazil this orogeny formed the Dom Feliciano Belt, a unit composed of associations of rocks developed during two major orogenic events called São Gabriel (900-680 Ma) and Dom Feliciano (650-540 Ma). The main São Gabriel associations are tectonically juxtaposed as elongated strips according to the N20-30°E direction, bounded by ductile shear zones. The Bossoroca Complex comprises predominantly metavolcano-sedimentary rocks, characterized by medium-K calc-alkaline association generated in a cordillera-type magmatic arc. The volcanism occurred in sub-aerial environment, developing deposits generated by flow, resurgence and fall, sporadically interrupted by subaqueous epiclastic deposits, suggesting an arc related basin. The São Gabriel Terrane contains the petrotectonic units that represent the closure of the Charrua Ocean associated to the subduction period of the Brasiliano Orogenic Cycle in the Sul-rio-grandense Shield.

  12. Extending the time range of apatite (U-Th)/He thermochronometry in slowly cooled terranes: Palaeozoic to Cenozoic exhumation history of southeast Sweden [rapid communication

    NASA Astrophysics Data System (ADS)

    Söderlund, Pia; Juez-Larré, Joaquim; Page, Laurence M.; Dunai, Tibor J.

    2005-11-01

    The use of (U-Th)/He thermochronometry in old slowly cooled terranes has long been debated. Hence, studies of natural setting are needed to verify the expected diffusivity behaviour of helium. Our results demonstrate that the (U-Th)/He method can be applicable to samples older than 200 Ma, and that available helium diffusion models can reproduce the results obtained from the age-vs.-depth trends. In addition, the results shed some light on the cross-validation with the apatite fission-track thermochronometer. (U-Th)/He thermochronometry on apatites has been applied to samples taken from drill cores in the Precambrian basement in southeast Sweden to unravel the time of exhumation of the uppermost 1-3 km of the crust. Much of the Phanerozoic exhumation history in Sweden is poorly constrained due to a large hiatus in the sediment record. Results from the drill cores yield He ages decreasing with increasing depth from ˜270 Ma at the surface down to ˜120 Ma at 1700 m depth. The age-vs.-depth trend shows an inflection point at ˜1400 m depth, which coincides with the upper boundary of the present Helium Partial Retention Zone. A period of slow exhumation (˜17 m/my), probably linked to the isostatic uplift of the Caledonian foreland basin, is revealed by the upper section of the trend in the data. The results demonstrate that the area has not been reheated since the mid-Permian. This argues against the existence of a thick Mesozoic sedimentary cover in this region, which, if present at all, cannot have exceeded 100 m in thickness. Furthermore, the age-vs.-depth trend below the inflection point allows dating the cessation of exhumation at approximately 100 Ma. The area has had no significant amount of sedimentation, erosion or uplift since this time.

  13. Cretaceous to Cenozoic evolution of the northern Lhasa Terrane and the Early Paleogene development of peneplains at Nam Co, Tibetan Plateau

    NASA Astrophysics Data System (ADS)

    Haider, Vicky L.; Dunkl, István; von Eynatten, Hilmar; Ding, Lin; Frei, Dirk; Zhang, Liyun

    2013-07-01

    Highly elevated and well-preserved peneplains are characteristic geomorphic features of the Tibetan plateau in the northern Lhasa Terrane, north-northwest of Nam Co. The peneplains were carved in granitoids and in their metasedimentary host formations. We use multi-method geochronology (zircon U-Pb and [U-Th]/He dating and apatite fission track and [U-Th]/He dating) to constrain the post-emplacement thermal history of the granitoids and the timing and rate of final exhumation of the peneplain areas. LA-ICP-MS U-Pb geochronology of zircons yields two narrow age groups for the intrusions at around 118 Ma and 85 Ma, and a third group records Paleocene volcanic activity (63-58 Ma) in the Nam Co area. The low-temperature thermochronometers indicate common age groups for the entire Nam Co area: zircon (U-Th)/He ages cluster around 75 Ma, apatite fission track ages around 60 Ma and apatite (U-Th)/He ages around 50 Ma. Modelling of the thermochronological data indicates that exhumation of the basement blocks took place in latest Cretaceous to earliest Paleogene time. By Middle Eocene time the relief was already flat, documented by a thin alluvial sediment sequence covering a part of the planated area. The present-day horst and graben structure of the peneplains is a Late Cenozoic feature triggered by E-W extension of the Tibetan Plateau. The new thermochronological data precisely bracket the age of the planation to Early Eocene, i.e. between ca. 55 and 45 Ma. The erosional base level can be deduced from the presence of Early Cretaceous zircon grains in Eocene strata of Bengal Basin. The sediment generated during exhumation of the Nam Co area was transported by an Early Cenozoic river system into the ocean, suggesting that planation occurred at low elevation.

  14. The Alashan Terrane did not amalgamate with North China block by the Late Permian: Evidence from Carboniferous and Permian paleomagnetic results

    NASA Astrophysics Data System (ADS)

    Yuan, Wei; Yang, Zhenyu

    2015-05-01

    Rock magnetic and paleomagnetic studies have been carried out on the early Carboniferous limestones and the Late Permian purple sandstones sampled in the eastern Alashan Terrane (ALT), northwest of China. Two components were isolated from the Early Carboniferous limestone by thermal progressive demagnetisation: a low unblocking temperature component (LTC) of recent origin; a pre-folding medium temperature component (MTC) (the paleomagnetic pole is λ = 13.1°N, Ф = 11.0°E, A95 = 7.0°) that is probably the result of the hydrothermal fluids from the Qilian Orgenic Belt acquired during the Late Carboniferous-Early Permian. Also, two components were separated from the Late Permian purple sandstone by thermal progressive demagnetisation: the LTC with the recent viscous remanent magnetisation, and the higher temperature component (HTC) revealed from three sections which has passed a regional fold test at the 95% probability level and reversal test, suggesting a primary characteristic magnetisation. The corresponding paleomagnetic pole is λ = 27.2° N, Ф = 18.8° E, A95 = 12.0°. The apparent polar wander path (including early Carboniferous, late Carboniferous-Early Permian, Late Permian and Early-middle Triassic poles) of the ALT is significantly different with those of the NCB. Comparison of the APWPs between the ALT and NCB shows a strong similarity. If the APWP of Hexi Corridor-Alashan rotated counterclockwise around an Euler pole at 44°N, 84°E by 32°, then the coeval APW path of the ALT overlaps to that of the NCB. This result indicates that the ALT migrated to the NCB after the Early-Middle Triassic along a tectonic boundary located between Helanshan Mountain and Zhuozishan Mountain, and finally amalgamated to the NCB before the Early Cretaceous.

  15. 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).

  16. The E-MORB like geochemical features of the Early Paleozoic mafic-ultramafic belt of the Cuyania terrane, western Argentina

    NASA Astrophysics Data System (ADS)

    Boedo, F. L.; Vujovich, G. I.; Kay, S. M.; Ariza, J. P.; Pérez Luján, S. B.

    2013-12-01

    The Argentine Precordillera is located in the central western region of Argentina, within the Central Andes. Throughout its westernmost sector, mafic and ultramafic bodies including serpentinites, mafic granulites, basaltic dikes/sills and pillow lavas are associated with metasedimentary rocks deposited in a deep marine and slope environment. These magmatic units, which are known as the Precordillera ultramafic-mafic belt, are considered to have a range of Early Paleozoic age based on published U-Pb zircon ages and fossil fauna. The entire sequence shows the effects of complex polyphase Paleozoic deformation and was subjected to a low grade metamorphism considered to be of middle-late Devonian age. The chemistry of the Peñasco and Cortaderas mafic dikes and sills in the southern part of this belt, which are largely plagioclase + clinopyroxene-bearing tholeiitic basalts, is the focus of this study. These volcanic rocks all have E-MORB-like major and trace element and ɛNd (+6.0 to +9.3) signatures with similarities to those previously reported throughout the belt. The new descriptions and major and trace-element analyses presented here confirm the similarity of the E-MORB-like chemistry of the Early Paleozoic mafic rocks along the entire belt, which spans some 500 km in length. There is a general consensus that these units are exposed as a consequence of the collision of the Chilenia terrane against the Gondwana margin during the middle to late Devonian, but the details of timing, the origins of the continental blocks and the nature of the collision are still debated. The results presented support the western Precordillera basaltic dikes/sills as having formed in the early stages of oceanic rifting along the Gondwana (Precordillera) continental margin with their E-MORB-like character reflecting mixing of depleted and enriched mantle and continental lithospheric sources.

  17. 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.

  18. Two-stage Triassic exhumation of HP-UHP terranes in the western Dabie orogen of China: Constraints from structural geology

    NASA Astrophysics Data System (ADS)

    Li, Sanzhong; Kusky, Timothy M.; Zhao, Guochun; Liu, Xiaochun; Zhang, Guowei; Kopp, Heidrun; Wang, Lu

    2010-07-01

    Abundant exposures of widely-distributed HP-UHP metamorphic rocks in the western part of the Dabie orogen enable us to study the tectonic evolution of HP-UHP terranes associated with the world's largest preserved continental subduction zone. Previous tectonic models for the Dabie orogen were based largely on metamorphic studies, most of them lacking significant structural constraints. We present a comprehensive structural analysis based on detailed structural geology. The results suggest that syn-UHP (D0 at 241-231 Ma) and syn-HP (D1 at 225-215 Ma) southeast-vergent thrusting formed a series of stacked structural slices. This was followed by southeast-vergent folding under amphibolite facies conditions (D2 at 215-205 Ma); then a third generation of flexural folding occurred at shallow levels (D3 at 200-184 Ma). This leads us to proposes a two-stage Triassic exhumation model in which initially rapid vertical extrusion (D0-D1) from UHP to HP conditions to lower crustal levels is followed by slow southeastward extrusion (D3) from lower crustal levels to the Earth's surface. The tectonic model combines the early southeastward vertical extrusion with the later southeastward lateral extrusion, revealing two different stages and thus different types of Triassic extrusion for the exhumation of HP-UHP rocks in the Dabie orogen. The first stage extrusion occurred in the Middle Triassic, whereas the second stage extrusion lasted from the Late Triassic to Early Jurassic. These two extrusion episodes correlate with the two stages of Triassic exhumation of the Dabie HP-UHP rocks, respectively, during continental collision.

  19. Geochronology of plutonic rocks and their tectonic terranes in Glacier Bay National Park and Preserve, southeast Alaska: Chapter E in Studies by the U.S. Geological Survey in Alaska, 2008-2009

    USGS Publications Warehouse

    Brew, David A.; Tellier, Kathleen E.; Lanphere, Marvin A.; Nielsen, Diane C.; Smith, James G.; Sonnevil, Ronald A.

    2014-01-01

    We have identified six major belts and two nonbelt occurrences of plutonic rocks in Glacier Bay National Park and Preserve and characterized them on the basis of geologic mapping, igneous petrology, geochemistry, and isotopic dating. The six plutonic belts and two other occurrences are, from oldest to youngest: (1) Jurassic (201.6–145.5 Ma) diorite and gabbro of the Lituya belt; (2) Late Jurassic (161.0–145.5 Ma) leucotonalite in Johns Hopkins Inlet; (3) Early Cretaceous (145.5–99.6 Ma) granodiorite and tonalite of the Muir-Chichagof belt; (4) Paleocene tonalite in Johns Hopkins Inlet (65.5–55.8 Ma); (5) Eocene granodiorite of the Sanak-Baranof belt; (6) Eocene and Oligocene (55.8–23.0 Ma) granodiorite, quartz diorite, and granite of the Muir-Fairweather felsic-intermediate belt; (7) Eocene and Oligocene (55.8–23.0 Ma) layered gabbros of the Crillon-La Perouse mafic belt; and (8) Oligocene (33.9–23.0 Ma) quartz monzonite and quartz syenite of the Tkope belt. The rocks are further classified into 17 different combination age-compositional units; some younger belts are superimposed on older ones. Almost all these plutonic rocks are related to Cretaceous and Tertiary subduction events. The six major plutonic belts intrude the three southeast Alaska geographic subregions in Glacier Bay National Park and Preserve, from west to east: (1) the Coastal Islands, (2) the Tarr Inlet Suture Zone (which contains the Border Ranges Fault Zone), and (3) the Central Alexander Archipelago. Each subregion includes rocks assigned to one or more tectonic terranes. The various plutonic belts intrude different terranes in different subregions. In general, the Early Cretaceous plutons intrude rocks of the Alexander and Wrangellia terranes in the Central Alexander Archipelago subregion, and the Paleogene plutons intrude rocks of the Chugach, Alexander, and Wrangellia terranes in the Coastal Islands, Tarr Inlet Suture Zone, and Central Alexander Archipelago subregions.

  20. [Environmental factors in ALS].

    PubMed

    Juntas-Morales, Raul; Pageot, Nicolas; Corcia, Philippe; Camu, William

    2014-05-01

    ALS is likely to be a disorder of multifactorial origin. Among all the factors that may increase the risk of ALS, environmental ones are being studied for many years, but in the recent years, several advances have pointed to a new interest in their potential involvement in the disease process, especially for the cyanotoxin BMAA. Food containing BMAA has been found on Guam, a well-known focus of ALS/parkinsonism/dementia and high levels of BMAA have been identified into the brain of these patients. The BMAA cyanotoxin is potentially ubiquitous and have also been found into the food of patients who died from ALS both in Europe and USA. BMAA can be wrongly integrated into the protein structure during mRNA traduction, competing with serine. This may induce abnormal protein folding and a subsequent cell death. Heavy metals, such as lead or mercury may be directly toxic for neuronal cells. Several works have suggested an increased risk of ALS in individuals chronically exposed to these metals. Exposure to pesticides has been suggested to be linked to an increased risk of developing ALS. The mechanism of their toxicity is likely to be mediated by paraoxonases. These proteins are in charge of detoxifying the organism from toxins, and particularly organophosphates. To date, there are insufficient scientific data to suggest that exposure to electromagnetic fields may increase the risk of having ALS. We are particularly missing longitudinal cohorts to demonstrate that risk.

  1. Magma associations in Ediacaran granitoids of the Cachoeirinha‒Salgueiro and Alto Pajeú terranes, northeastern Brazil: Forty years of studies

    NASA Astrophysics Data System (ADS)

    Sial, Alcides N.; Ferreira, Valderez P.

    2016-07-01

    Granitic magmatism in the Cachoeirinha‒Salgueiro and Alto Pajeú terranes in the Transversal Zone Domain of the Borborema Province, northeastern Brazil, occurred in three main time intervals: 650-620 Ma, 590-560 Ma and 545-520 Ma. The oldest one is characterized by intrusions of magmatic-epidote (mEp) bearing calc-alkalic (some with trondhjemitic affinities) and high-K calc-alkalic plutons, synkinematic to the main regional foliation, under contractional tectonic regime, and exhibits TDM < 2.0 Ga and ƐNd (0.6 Ga) from -1 to -4, and δ18O (zircon) values from 7.1 to 10‰VSMOW. O- and Nd-isotope data for the 650‒620 Ma group of plutons is compatible with partial fusion of subducted oceanic basaltic crust (mEp-bearing calc-alkalic tonalites/granodiorites, equivalent to adakites). Voluminous intrusions in the 590-560 Ma interval are represented by abundant mEp-free high-K calc-alkalic, peralkalic, ultrapotassic, mEp-bearing high-K calc-alkalic, and less abundant shoshonitic magmas. Nd-model ages for this group of plutons vary from 1.5 to 2.5 Ga and ƐNd (0.6 Ga) ranges from -8 to -20; δ18O (zircon) varies from 6.4 to 7.9‰VSMOW. Values of δ18O (zircon) for the 590‒560 Ma old group of plutons coupled with Nd isotope data are compatible with remelting of crustal (negative ƐNd, 1.6 to 2.0 Ga old) source rocks. O- and Nd-isotope data for this group of plutons are compatible with underplating of basaltic magma in the base of the lower crust for the high-K calc-alkalic granitoids, coeval to transcurrent movements along sigmoidal shear zones. Intrusion of one shoshonitic (Serrote do Arapuá), one calc-alkalic (Riacho do Icó) plutons besides the peralkalic Manaíra-Princeza Isabel dike set have witnessed this transition from contractional to transcurrent movements along shear zones, around 610‒600 Ma.

  2. U-Pb zircon and geochemical evidence for bimodal mid-Paleozoic magmatism and syngenetic base-metal mineralization in the Yukon-Tanana terrane, Alaska

    USGS Publications Warehouse

    Dusel-Bacon, C.; Wooden, J.L.; Hopkins, M.J.

    2004-01-01

    New SHRIMP (sensitive, high-resolution ion microprobe) U-Pb zircon ages and trace element geochemical data for mafic and felsic metaigneous rocks of the pericratonic Yukon-Tanana terrane in east-central Alaska help define the tectonic setting of mid-Paleozoic magmatism and syngenetic hydrothermal Zn-Pb-Ag mineralization along the ancient Pacific margin of North America. We compare data from similar greenschist-facies sequences of bimodal volcanic and subvolcanic rocks associated with carbonaceous and siliciclastic marine sedimentary rocks, in the Wood River area of the Alaska Range and the Salcha River area of the Yukon-Tanana Upland, and from amphibolite-facies augen gneiss and mafic gneiss (amphibolite) in the Goodpaster River area of the upland. Allowing for analytical uncertainties, igneous crystallization age ranges of 376-353 Ma, 378-346 Ma, and 374-358 Ma are indicated by 13 new SHRIMP U-Pb dates for the Wood River, Salcha River, and Goodpaster River areas, respectively. Bimodal magmatism is indicated by Late Devonian crystallization ages for both augen gneiss (371 ?? 3 and 362 ?? 4 Ma) and associated orthoamphibolite (369 ?? 3 Ma) in the upland and by stratigraphic interleaving of mafic and felsic rocks in the Alaska Range. Metabasites in all three study areas have elevated HFSE (high field strength element) and REE (rare earth element) contents indicative of generation in a within-plate (extensional) tectonic setting. Within-plate trace element signatures also are indicated for peralkaline metarhyolites that host the largest volcanogenic massive sulfide deposits of the Bonnifield district in the Wood River area and for metarhyolite tuff interlayered with the carbonaceous Nasina assemblage, which hosts sedimentary exhalative sulfide occurrences in the Salcha River area. Most of the other felsic metaigneous samples from the Alaska Range and the Yukon-Tanana Upland have geochemical signatures that are similar to those of both average upper continental crust

  3. Origin of unique coesite-bearing zircon with low-U and high HREE contents from allanite-bearing gneisses in the Sulu UHP terrane, eastern China

    NASA Astrophysics Data System (ADS)

    Liu, F.; Gerdes, A.; Liou, J. G.; Liu, P.

    2009-12-01

    Zircon from allanite-bearing para- and ortho-gneisses provides a unique insight into the geologic evolution of the Sulu ultrahigh-pressure (UHP) terrane in eastern China. Laser Raman, cathodoluminescence (CL) imaging combined with trace element, U-Pb, and Lu-Hf isotope data indicate that zircon grains consist of three distinct zones. Inherited magmatic core records a Neoproterozoic protolith age of 800 Ma, the coesite-bearing mantle reveals a UHP metamorphic age of 230 +/-7 Ma, and the low-P mineral-bearing rim retains an age of 210 +/-3 Ma for the amphibolite-facies retrogression. Positive Hf(t) values for the inherited cores reflect involvement of juvenile materials in the protolith. Low 176Lu/177Hf ratios (0.0002-0.0003) of coesite-bearing mantles indicate their formation during /after garnet crystallization. Their 176Hf/177Hf ratios (0.28252-0.28257) are homogenous and more radiogenic than those of the inherited cores implying fractionation of the Lu-Hf system during isotope equilibration within the metamorphic mineral assemblage. Moreover, coesite-bearing mantles show geochemical characteristics that are distinctly different from those of previous studies: (1) The chondrite-normalized pattern shows a steep slope from HREE to LREE with a moderately negative Eu anomaly (mean Eu/Eu* = 0.53). Such a pattern requires that phases enriched in LREE (e.g., allanite) and europium (e.g., K-feldspar) were stable during UHP conditions. (2) The positive Ce anomaly is very pronounced (Ce/Ce* of 612-657), e.g., about seven to eight times higher than that of the inherited core; (3) The uranium content is very low (6-25 ppm) and the Th/U ratio elevated (0.52-2.83). These data for the coesite-bearing mantle are distinctly different from those of previous studies. These geochemical data suggest high O fugacity during the UHP and retrograde metamorphic evolution, which also played a critical role in restraining diamond growth in Sulu-Dabie UHP rocks.

  4. Brittle deformation in Southern Granulite Terrane (SGT): A study of pseudotachylyte bearing fractures along Gangavalli Shear Zone (GSZ), Tamil Nadu, India.

    NASA Astrophysics Data System (ADS)

    mohan Behera, Bhuban; Thirukumaran, Venugopal; Biswal, Tapas kumar

    2016-04-01

    High grade metamorphism and intense deformation have given a well recognition to the Southern Granulite Terrane (SGT) in India. TTG-Charnockite and basic granulites constitute the dominant lithoassociation of the area. Dunite-peridotite-anorthosite-shonkinite and syenites are the intrusives. TTG-charnockite-basic granulite have undergone F1 (isoclinal recumbent), F2 (NE-SW) and F3 (NW-SE) folds producing several interference pattern. E-W trending Neoarchean and Palaeoproterozoic Salem-Attur Shear Zone exhibits a low angle ductile thrust as well as some foot print of late stage brittle deformation near Gangavalli area of Tamil Nadu. The thrust causes exhumation of basic granulites to upper crust. Thrusting along the decollement has retrograded the granulite into amphibolite rock. Subsequently, deformation pattern of Gangavalli area has distinctly marked by numerous vertical to sub-vertical fractures mostly dominating along 0-15 and 270-300 degree within charnockite hills that creates a maximum stress (σ1) along NNW and minimum stress (σ3) along ENE. However, emplacement of pseudotachylyte vein along N-S dominating fracture indicates a post deformational seismic event. Extensive fractures produce anastomose vein with varying thickness from few millimeters to 10 centimeters on the outcrop. ICP-AES study results an isochemical composition of pseudotachylyte vein that derived from the host charnockitic rock where it occurs. But still some noticeable variation in FeO-MgO and Na2O-CaO are obtained from different parts within the single vein showing heterogeneity melt. Electron probe micro analysis of thin sections reveals the existence of melt immiscibility during its solidification. Under dry melting condition, albitic rich melts are considered to be the most favorable composition for microlites (e.g. sheaf and acicular micro crystal) re-crystallization. Especially, acicular microlites preserved tachylite texture that suggest its formation before the final coagulation

  5. Roles of the Mendocino Transform, Vizcaino Block, and Onshore King Range Terrane in Evolution of the Northern San Andreas Fault System and Its Associated Slab Windows

    NASA Astrophysics Data System (ADS)

    McLaughlin, R. J.; Barth, G. A.; Scheirer, D. S.; Hoover, S. M.; Trehu, A. M.; Jencks, J.

    2014-12-01

    We integrate recent seismic reflection, geochemical and radiometric age data from basalts and sedimentary rocks along the Mendocino Transform (MT) and Gorda Escarpment, with basalt ages and biostratigraphy from the Miocene King Range terrane (KRT) of the Franciscan Complex, to better link the onshore and offshore geology and clarify how the northernmost San Andreas Fault (SAF) evolved. The MT extends eastward from the Gorda Ridge spreading center, along the S side of the Gorda Plate, to the edge of the North American plate (NAP) and separates the Cascadia subduction zone to the north, from the modern SAF to the south. Between 127.5º W and the shoreline, the MT and Mendocino Ridge (MR) align with the N side of the S-tilted Vizcaino structural block (VB), a remnant of NAP captured by the Pacific plate ~12 Ma, when the MT was 480 km S of its present location. The modern SAF bounds the NE-side of the VB. The SW side of the VB is bounded at the base of the continental slope by the proto-San Andreas fault (PSAF), where extinct remnants of the Pacific-Farallon ridge (PFR) interacted with the paleosubduction margin to form an incipient transform and several microplates, now part of the Pacific plate. Capture of the VB resulted from inboard breaking of the MT with a jump of the PSAF to the modern SAF. Dated ~20-12 Ma basaltic rocks from the MR between ~125º-128º W may be partly exhumed slab window underplating that formed beneath the VB during breakup of the PFR along the PSAF. High Fe and Ti relative to Mg in MR and KRT basalts, suggest eruption near ridge-transform intersections and perhaps, intratransform spreading.Onshore, high KRT relief aligns with the MR offshore. The KRT was assembled ~16-15 Ma (basalt K-Ar age; biostratigraphy); followed by its complex deformation and zeolitic metamorphism, indicating subduction to 5-8 km depth ~15-14 Ma and thermal metamorphism ~13.8 Ma (K-Ar age; vitrinite reflectance). The thermal overprint sets the KRT apart from adjacent

  6. U-Pb SHRIMP zircon dating of high-grade rocks from the Upper Allochthonous Terrane of Bragança and Morais Massifs (NE Portugal); geodynamic consequences

    NASA Astrophysics Data System (ADS)

    Mateus, A.; Munhá, J.; Ribeiro, A.; Tassinari, C. C. G.; Sato, K.; Pereira, E.; Santos, J. F.

    2016-04-01

    Bragança and Morais Massifs are part of the mega-klippen ensemble of NW Iberia, comprising a tectonic pile of four allochthonous units stacked above the Central-Iberian Zone autochthon. On top of this pile, the Upper Allochthonous Terrane (UAT) includes different high-grade metamorphic series whose age and geodynamic meaning are controversial. Mafic granulites provided U-Pb zircon ages at 399 ± 7 Ma, dating the Variscan emplacement of UAT. In contrast, U-Pb zircon ages of ky- and hb-eclogites, felsic/intermediate HP/HT-granulites and orthogneisses (ca. 500-480 Ma) are identical to those of gabbros (488 ± 10 Ma) and Grt-pyroxenites (495 ± 8 Ma) belonging to a mafic/ultramafic igneous suite that records upper mantle melting and mafic magma crustal underplating at these times. Gabbros intrude the high-grade units of UAT and did not underwent the HP metamorphic event experienced by eclogites and granulites. These features and the zircon dates resemblance among different lithologies, suggest that extensive age resetting of older events may have been correlative with the igneous suite emplacement/crystallisation. Accordingly, reconciliation of structural, petrological and geochronological evidence implies that the development and early deformation of UAT high-grade rocks should be ascribed to an orogenic cycle prior to ≈ 500 Ma. Undisputable dating of this cycle is impossible, but the sporadic vestiges of Cadomian ages cannot be disregarded. The ca. 500-480 Ma time-window harmonises well with the Lower Palaeozoic continental rifting that trace the Variscan Wilson Cycle onset and the Rheic Ocean opening. Subsequent preservation of the high heat-flow regime, possibly related to the Palaeotethys back-arc basin development (ca. 450-420 Ma), would explain the 461 ± 10 Ma age yielded by some zircon domains in felsic granulites, conceivably reflecting zircon dissolution/recrystallisation till Ordovician times, long before the Variscan paroxysm (ca. 400-390 Ma). This

  7. Sources of granite magmatism in the Embu Terrane (Ribeira Belt, Brazil): Neoproterozoic crust recycling constrained by elemental and isotope (Sr-Nd-Pb) geochemistry

    NASA Astrophysics Data System (ADS)

    Alves, Adriana; Janasi, Valdecir de Assis; Campos Neto, Mario da Costa

    2016-07-01

    Whole rock elemental and Sr-Nd isotope geochemistry and in situ K-feldspar Pb isotope geochemistry were used to identify the sources involved in the genesis of Neoproterozoic granites from the Embu Terrane, Ribeira Belt, SE Brazil. Granite magmatism spanned over 200 Ma (810-580 Ma), and is dominated by crust-derived relatively low-T (850-750 °C, zircon saturation) biotite granites to biotite-muscovite granites. Two Cryogenian plutons show the least negative εNdt (-8 to -10) and highest mg# (30-40) of the whole set. Their compositions are strongly contrasted, implying distinct sources for the peraluminous (ASI ∼ 1.2) ∼660 Ma Serra do Quebra-Cangalha batholith (metasedimentary rocks from relatively young upper crust with high Rb/Sr and low Th/U) and the metaluminous (ASI = 0.96-1.00) ∼ 630 Ma Santa Catarina Granite. Although not typical, the geochemical signature of these granites may reflect a continental margin arc environment, and they could be products of a prolonged period of oceanic plate consumption started at ∼810 Ma. The predominant Ediacaran (595-580 Ma) plutons have a spread of compositions from biotite granites with SiO2 as low as ∼65% (e.g., Itapeti, Mauá, Sabaúna and Lagoinha granites) to fractionated muscovite granites (Mogi das Cruzes, Santa Branca and Guacuri granites; up to ∼75% SiO2). εNdT are characteristically negative (-12 to -18), with corresponding Nd TDM indicating sources with Paleoproterozoic mean crustal ages (2.0-2.5 Ga). The Guacuri and Santa Branca muscovite granites have the more negative εNdt, highest 87Sr/86Srt (0.714-0.717) and lowest 208Pb/206Pb and 207Pb/206Pb, consistent with an old metasedimentary source with low time-integrated Rb/Sr. However, a positive Nd-Sr isotope correlation is suggested by data from the other granites, and would be consistent with mixing between an older source predominant in the Mauá granite and a younger, high Rb/Sr source that is more abundant in the Lagoinha granite sample. The

  8. Mesoproterozoic continental arc magmatism and crustal growth in the eastern Central Tianshan Arc Terrane of the southern Central Asian Orogenic Belt: Geochronological and geochemical evidence

    NASA Astrophysics Data System (ADS)

    He, Zhen-Yu; Klemd, Reiner; Zhang, Ze-Ming; Zong, Ke-Qing; Sun, Li-Xin; Tian, Zuo-Lin; Huang, Bo-Tao

    2015-11-01

    Numerous microcontinents are known to occur in the Central Asian Orogenic Belt (CAOB), one of the largest accretionary orogens and the most significant area of Paleozoic crustal growth in the world. The evolution of the Precambrian crust in these microcontinents is central to understanding the accretionary and collisional tectonics of the CAOB. Here, we present systematic zircon U-Pb dating and Hf isotope studies of Mesoproterozoic gneissic granitoids from the eastern Central Tianshan Arc Terrane (CTA) of the southern CAOB. The investigated intermediate to felsic (SiO2 = 60.48-78.92 wt.%) granitoids belong to the calcic- to calc-alkaline series and usually have pronounced negative Nb, Ta and Ti anomalies, relative enrichments of light rare earth elements (LREEs) and large ion lithophile elements (LILEs) while heavy rare earth elements (HREEs) and high field strength elements (HFSEs) are depleted, revealing typical active continental margin magmatic arc geochemical characteristics. These spatially-distant rocks show consistent zircon U-Pb crystallization ages from ca. 1.45 to 1.40 Ga and thus constitute a previously unknown Mesoproterozoic continental magmatic arc covering hundreds of kilometers in the eastern segment of the CTA. Furthermore the high and mainly positive zircon εHf(t) values between - 1.0 and + 8.6 and the zircon Hf model ages of 1.95 to 1.55 Ga, which are slightly older than their crystallization ages, suggest that they were mainly derived from rapid reworking of juvenile material with a limited input of an ancient crustal component. Therefore, the formation of these granitoids defines an extensive Mesoproterozoic intermediate to felsic, subduction-related intrusive magmatic arc activity that was active from at least 1.45 to 1.40 Ga, involving significant juvenile continental growth in the eastern segment of the CTA. Furthermore the zircon U-Pb and Hf isotopic data challenge the common belief that the CTA was part of the Tarim Craton during Paleo

  9. Late Jurassic sodium-rich adakitic intrusive rocks in the southern Qiangtang terrane, central Tibet, and their implications for the Bangong-Nujiang Ocean subduction

    NASA Astrophysics Data System (ADS)

    Li, Yalin; He, Juan; Han, Zhongpeng; Wang, Chengshan; Ma, Pengfei; Zhou, Aorigele; Liu, Sheng-Ao; Xu, Ming

    2016-02-01

    The lack of magmatic records with high-quality geochronological and geochemical data in the central segment of the southern Qiangtang subterrane in central Tibet inhibits a complete understanding of the subduction polarity of the Bangong-Nujiang Ocean lithosphere during the Mesozoic. In this study, we present the zircon U-Pb age as well as geochemical and Sr-Nd-Pb isotopic data for the Late Jurassic pluton from the Kangqiong area in the central segment of the southern Qiangtang subterrane. The Kangqiong pluton primarily consists of granodiorites (SiO2 = 62.87-65.17 wt.%) and was emplaced in the Late Jurassic (147.6 ± 2.4-149.9 ± 2.1 Ma). The granodiorites display high Na2O numbers (Na2O/K2O = 1.75-2.24) as well as high MgO (2.21-3.14 wt.%) and Mg-numbers (53-58), are characterized by a low abundance of heavy rare earth elements (e.g., Yb = 1.05-1.92 ppm) and Y (12.63-17.52 ppm), and high Sr/Y (29-61) and La/Yb (14-18) ratios, which are comparable in composition to those of slab-derived adakitic rocks. The Kangqiong adakitic granodiorites have initial (87Sr/86Sr)i ratios of 0.70611 to 0.70669, negative εNd(t) values (- 1.06 to - 0.25), (206Pb/204Pb)t ratios of 18.42 to 18.47, (207Pb/204Pb)t ratios of 15.62 to 15.63, and (208Pb/204Pb)t ratios of 38.50 to 38.60. These geochemical signatures indicate that the magmas were most likely derived from the partial melting of the subducted Bangong-Nujiang oceanic crust and minor contaminants from the accretionary complex. Our results, in combination with the coeval magmatism in the western segment of the southern Qiangtang subterrane, indicate that the Bangong-Nujiang oceanic lithosphere was subducted northward beneath the Qiangtang Terrane, forming a west-east magmatic arc over 800 km during the Late Jurassic.

  10. Amyotrophic Lateral Sclerosis (ALS)

    MedlinePlus

    ... Pictures of ALS Mutant Proteins Support Two Major Theories About How the Disease is Caused May 2003 ... All NINDS-prepared information is in the public domain and may be freely copied. Credit to the ...

  11. Genetic Testing for ALS

    MedlinePlus

    ... Involved Donate Familial Amyotrophic Lateral Sclerosis (FALS) and Genetic Testing By Deborah Hartzfeld, MS, CGC, Certified Genetic ... guarantee a person will develop symptoms of ALS. Genetic Counseling If there is more than one person ...

  12. All About ALS

    MedlinePlus

    ... External link, please review our exit disclaimer . Subscribe All About ALS Understanding a Devastating Disorder In the ... a coffee pot, or button a shirt. Eventually, all muscles under voluntary control are affected, and people ...

  13. What Is ALS?

    MedlinePlus

    ... scarring or hardening ("sclerosis") in the region. Motor neurons reach from the brain to the spinal cord ... the body. The progressive degeneration of the motor neurons in ALS eventually leads to their demise. When ...

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

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

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

  15. Lateral and Vertical Heterogeneity of Thorium in the Procellarum KREEP Terrane: As Reflected in the Ejecta Deposits of Post-Imbrium Craters

    NASA Technical Reports Server (NTRS)

    Gillis, J. J.; Jolliff, B. L.

    1999-01-01

    The Procellarum KREEP Terrane displays the highest concentrations of Th on the Moon. However, locations of elevated Th in this region appear to be random. As observed in the 5 deg per pixel equal-area Th data, and made more evident in the preliminary 2 deg data, Th is enhanced around the craters Aristillus, Aristarchus, Kepler, Mairan, the Apennine Bench formation, and the Fra Mauro region, while noticeably and unexpectedly lower in other locations (e.g., Archimedes, Copernicus, Eratosthenes, and Plato). We have examined the composition of the materials present in these regions with the goal of understanding the patchy nature to the distribution of Th and ultimately to decipher the geologic processes that have concentrated the Th. At present time, the published resolution of the Lunar Prospector Th gamma-ray data is low (5 deg per pixel), but this will soon be superceded by significantly higher-resolution data (2 deg per pixel). Even at this improved resolution, however, it is difficult to resolve the units that are the major source of Th. In an attempt to circumvent this problem, we employ the higher-resolution Clementine multispectral data for those regions mentioned above. We use the UV-VIS-derived compositional information and the spectral properties of craters, and their ejecta as drill holes through the mare-basalt surface to investigate the thickness and composition of underlying material. With this information we attempt to piece together the stratigraphy and geologic history of the Imbrium-Procellanim region. We processed the five-band multispectral data from the Clementine Mission (415, 750,900,950, and 1000nm) using ISIS software and calibration parameters developed by the USGS, Flagstaff, Arizona. Final image mosaics are in equal-area sinusoidal projection, and have a resolution of 250 m/pixel. Using the method of we produced maps of FeO and Ti02 composition. Here we examine the Th, FeO, and Ti02 composition and spectral properties of the craters

  16. Extensive genetics of ALS

    PubMed Central

    Calvo, Andrea; Mazzini, Letizia; Cantello, Roberto; Mora, Gabriele; Moglia, Cristina; Corrado, Lucia; D'Alfonso, Sandra; Majounie, Elisa; Renton, Alan; Pisano, Fabrizio; Ossola, Irene; Brunetti, Maura; Traynor, Bryan J.; Restagno, Gabriella

    2012-01-01

    Objective: To assess the frequency and clinical characteristics of patients with mutations of major amyotrophic lateral sclerosis (ALS) genes in a prospectively ascertained, population-based epidemiologic series of cases. Methods: The study population includes all ALS cases diagnosed in Piemonte, Italy, from January 2007 to June 2011. Mutations of SOD1, TARDBP, ANG, FUS, OPTN, and C9ORF72 have been assessed. Results: Out of the 475 patients included in the study, 51 (10.7%) carried a mutation of an ALS-related gene (C9ORF72, 32; SOD1, 10; TARDBP, 7; FUS, 1; OPTN, 1; ANG, none). A positive family history for ALS or frontotemporal dementia (FTD) was found in 46 (9.7%) patients. Thirty-one (67.4%) of the 46 familial cases and 20 (4.7%) of the 429 sporadic cases had a genetic mutation. According to logistic regression modeling, besides a positive family history for ALS or FTD, the chance to carry a genetic mutation was related to the presence of comorbid FTD (odds ratio 3.5; p = 0.001), and age at onset ≤54 years (odds ratio 1.79; p = 0.012). Conclusions: We have found that ∼11% of patients with ALS carry a genetic mutation, with C9ORF72 being the commonest genetic alteration. Comorbid FTD or a young age at onset are strong indicators of a possible genetic origin of the disease. PMID:23100398

  17. Geochemistry, petrofabrics and seismic properties of eclogites from the Chinese Continental Scientific Drilling boreholes in the Sulu UHP terrane, eastern China

    NASA Astrophysics Data System (ADS)

    Wang, Qin; Burlini, Luigi; Mainprice, David; Xu, Zhiqin

    2009-09-01

    We present an integrated study of geochemistry, petrofabrics and seismic properties of strongly sheared eclogites from the Chinese Continental Scientific Drilling (CCSD) project in the Sulu ultrahigh-pressure (UHP) metamorphic terrane, eastern China. First, geochemical data characterize diverse protoliths of the studied eclogites. The positive Eu- and Sr-anomalies, negative Nb anomaly and flat portion of heavy rare earth elements in coarse-grained rutile eclogites (samples B270 and B295) suggest a cumulate origin in the continental crust, whereas the negative Nb anomaly and enrichment of light rare earth elements in retrograde eclogites (samples B504, B15 and B19) imply an origin of continental basalts or island arc basalts. Second, P-wave velocities ( Vp) of three typical eclogite samples were measured under confining pressures up to 500 MPa and temperatures to 700 °C. At 500 MPa and room temperature, the mean Vp reaches 8.50-8.53 km/s in samples B270 and B295 but drops to 7.86 km/s in sample B504, and the P-wave anisotropy changes from 1.7-2.7% to 5.5%, respectively. The pressure and temperature derivatives of Vp are larger in the retrograde eclogite than in fresh ones. Third, the electron backscatter diffraction (EBSD) measurements of the eclogites reveal random crystal preferred orientation (CPO) of garnet and pronounced CPO of omphacite, which is characterized by a strong concentration of [001]-axes sub-parallel to the lineation and of (010)-poles perpendicular to the foliation. The asymmetric CPO of omphacite in sample B270 recorded a top-to-the-south shear event during subduction of the Yangtze plate. The calculated fastest Vp is generally sub-parallel to the lineation, but a different deformation environment during exhumation could form second-order variations in omphacite CPO and affect the Vp distribution in eclogites (e.g., the fastest Vp is at ~ 35° from the foliation in sample B295). Comparison between measured and calculated seismic properties

  18. Cryogenian alkaline magmatism in the Southern Granulite Terrane, India: Petrology, geochemistry, zircon U-Pb ages and Lu-Hf isotopes

    NASA Astrophysics Data System (ADS)

    Santosh, M.; Yang, Qiong-Yan; Ram Mohan, M.; Tsunogae, T.; Shaji, E.; Satyanarayanan, M.

    2014-11-01

    The Southern Granulite Terrane (SGT) in India preserves the records of the formation and recycling of continental crust from Mesoarchean through Paleoproterozoic to Neoproterozoic and Cambrian, involving multiple subduction-accretion-collision associated with major orogenic cycles. A chain of unmetamorphosed and undeformed alkaline magmatic intrusions occurs along the northern margin of the SGT aligned along paleo-suture zones. Here we investigate two representative plutons from this suite, the Angadimogar syenite (AM) and the Peralimala alkali granite (PM) through field, petrological, geochemical, zircon U-Pb and Lu-Hf studies. Magma mixing and mingling textures and mineral assemblages typical of alkaline rocks are displayed by these plutons. The whole-rock major and trace element data characterize their alkaline nature. In trace element discrimination diagrams, the AM rocks straddle between the VAG (volcanic-arc granites) and WPG (within plate granites) fields with most of the samples confined to the VAG field, whereas the PM rocks are essentially confined to the WPG field. The diversity in some of the geochemical features between the two plutons is interpreted to be the reflection of source heterogeneities. Most zircon grains from the AM and PM plutons display oscillatory zoning typical of magmatic crystallization although some grains, particularly those from the PM pluton, show core-rim structures with dark patchy zoned cores surrounded by irregular thin rims resulting from fluid alteration. The weighted mean 206Pb/238U ages of the magmatic zircons from three samples of the AM syenite are in the range of 781.8 ± 3.8 Ma to 798 ± 3.6 Ma and those from two samples of the PM alkali granite yield ages of 797.5 ± 3.7 Ma and 799 ± 6.2 Ma. A mafic magmatic enclave from the AM pluton shows weighted mean 206Pb/238U age of 795 ± 3.3 Ma. The AM and PM plutons also carry rare xeneocrystic zircons which define upper intercept concordia ages of 3293 ± 13 Ma and 2530

  19. Syn-exhumation partial melting and melt segregation in the Sulu UHP terrane: Evidences from leucosome and pegmatitic vein of migmatite

    NASA Astrophysics Data System (ADS)

    Song, Yanru; Xu, Haijin; Zhang, Junfeng; Wang, Deyuan; Liu, Endong

    2014-08-01

    In order to investigate partial melting and melt evolution during exhumation of deeply subducted continental crust, we carried out a combined study on layered leucosomes and pegmatitic veins from the Weihai-Rongcheng migmatites in the Sulu ultra-high pressure (UHP) metamorphic terrane, eastern China. The leucosomes are millimetric to centimetric in thickness and mainly consists of K-feldspar + quartz + plagioclase. The pegmatitic veins emplace along the compositional layers or crosscut them and are mainly composed of K-feldspar + quartz. CL images show that most zircon grains of the leucosomes have a core-rim zoning structure, whereas zircons of the pegmatitic veins are mostly new growth grains with rarely preserved relict domains. The inherited zircon domains (ca. 750 Ma) are of a magmatic origin with zircon εHf(t) values between - 5.7 and 5.3 (mean = - 0.8 ± 1.5), suggesting that the protolith rock is a Mid-Neoproterozoic juvenile crust. The zircon overgrowth rims (223 ± 3 Ma) of the leucosomes and the new zircon grains (217 ± 2 Ma) of the pegmatitic veins formed in equilibrium with melt and are similar to magmatic zircons in terms of their CL images and trace element compositions, e.g., heavy-enriched REE patterns (i.e., very low (Gd/Lu)N ratios) with positive Ce and variably negative Eu anomalies, high Y contents and low Hf/Y ratios. Compared with the inherited protolith zircon domains, the Triassic zircon domains of the leucosomes and the pegmatitic veins have obviously high U but low Th contents, resulting in very low Th/U ratios (< 0.09). The low Th/U ratios (< 0.1) of zircons suggest that Th/U ratio cannot be used to discriminate igneous/metamorphic zircons. The zircons of the pegmatitic veins have remarkably lower Th/U ratios (0.00008-0.01, mostly < 0.006) than those of zircon rims of the leucosomes (0.006-0.09), suggesting that the melt segregation process can also result in low Th/U ratios of zircons. Calculated growth temperature for the zircon

  20. Heterogeneity within a deep crustal strike-slip shear zone with implications for lower crustal flow, Athabasca granulite terrane, western Canadian Shield

    NASA Astrophysics Data System (ADS)

    Leslie, S. R.; Mahan, K. H.; Regan, S.; Williams, M. L.

    2011-12-01

    Deep crustal strike-slip shear zones play a fundamental role in lower crustal flow. Although commonly modeled in two-dimensions, regional considerations suggest that large-scale crustal flow is a heterogeneous, three-dimensional process. The Athabasca granulite terrane, western Canadian Shield, exposes a large region of high-pressure tectonite (>20,000 km2) that provides a natural example of ancient lower crustal flow and an analog for similar processes active today in other regions. Regional heterogeneous deformation permits preservation of Neoarchean deformation fabrics and metamorphic textures. The Cora Lake shear zone (CLsz) is a NW-dipping km-scale mylonite to ultramylonite zone that forms a discrete tectonic discontinuity between two rheologically distinct Neoarchean lower-crustal domains. Northwest of the CLsz, the domain is primarily underlain by ~2.6 Ga felsic to mafic metaplutonic gneisses and interlayered ~2.55 Ga felsic granulite. Lithologies here preserve Neoarchean granulite-facies metamorphism coupled with partial melting and synkinematic melt-enhanced ESE-directed subhorizontal flow at ~0.9 GPa (~30 km paleodepths). Southeast of the CLsz, the Chipman domain is underlain by ~3.2 Ga metatonalite gneiss, an extensive ~1.9 Ga mafic dike swarm, and generally minor ~2.55 Ga mafic and felsic granulite. In contrast to the northwest, lithologies of the western Chipman domain document higher pressures at ~1.3 GPa (~40 km paleodepths) synchronous with development of a gently dipping Neoarchean gneissic fabric. Strong, anhydrous Chipman domain lithologies and melt-weakened lithologies to the northwest are juxtaposed by sinistral to sinistral-normal oblique shear along the CLsz, consistent with higher pressures (deeper paleodepths) documented in the footwall Chipman domain. A notable and pervasive feature along strike of the CLsz in the western Chipman domain is the marked increase in abundance of m-scale layers of mafic and felsic granulite westward with

  1. On the Paleotectonic Evolution of the Pacific Margin of Southern Mexico, the Maya and Juchatengo Terranes and Chochal Formation Guatemala:Insights from Paleomagnetic and Isotopic Studies

    NASA Astrophysics Data System (ADS)

    Guerrero Garcia, J. C.; Herrero-Bervera, E.

    2009-05-01

    In the paleogeographic reconstruction of Mexico and northern Central America, evidence shows that the entire region is a collage of suspect terranes transported from abroad, whose timing and sense of motion are now beginning to be understood. Among these, the Chortis block and the Baja California Peninsula have been proposed as pieces of continent separated from the Pacific coast of southwestern Mexico, that have moved either southeastward by the Farallon plate or northwestward by the Kula plate. Isotopic mineral ages from coastal granites along the coast from Puerto Vallarta, Jalisco (80 Ma) to Puerto Angel, Oaxaca (11 Ma) record systematic decrease of cooling ages from NW to SE. These results also constrain the position of the Kula- Farallon spreading axis north of Puerto Vallarta. Previous studies mainly confined to the northern margin of the Chortis block, confirmed a left-lateral displacement of 130 km in Neogene time. Further studies suggested times of detachment increased to 30 Ma, 40 Ma, and 66 Ma. We conclude that several indicators, namely: (a) the truncated nature of the Pacific coast of SW Mexico; (b) the genesis of the Kula-Farallon ridge at 85 Ma; (c) the 2,600 km of northward transport of Baja British Columbia from the present-day latitude of the Baja California Peninsula, beginning at 85 Ma; (d) the paleomagnetic counterclockwise rotations of areas both in the Chortis block and along the Mexican coast, during Late Cretaceous-Paleogene time, and (e) the systematic NW-SE decrease of radiometric dates beginning at 85 Ma in Puerto Vallarta and ending at approximately 11 Ma in Puerto Angel, Oaxaca , point to this time and region for the onset of strike-slip drifting of the Chortis block toward its current position. On the other hand, in the reconstruction of past movements of tectonic plates, the determination of reliable paleomagnetic poles is of utmost importance. To achieve accurate results, a full knowledge of the rock magnetic properties of the

  2. Magnetic Characteristics of the Lower Crust: Examples from the Chipman Tonalite, Chipman Dikes, and Fehr Granite, Athabasca Granulite Terrane, Northern Canada

    NASA Astrophysics Data System (ADS)

    Brown, L. L.; Koteas, C.; Seaman, S. J.; Williams, M. L.

    2011-12-01

    The Athabasca granulite terrane (AGT) in northernmost Saskatchewan, Canada is an outstanding exposure of lower crustal rocks having experienced high temperature (~800°C) and high pressure (>1.0 GPa) conditions followed by uplift and exhumation to the surface. With little alteration since 1.9 Ga these rocks allow us to study ancient lower crustal lithologies. Aeromagnetic anomalies over this region are distinct and complex, and along with other geophysical measurements, define the Snowbird Tectonic zone, stretching NE-SW across the Canadian Shield and separating the Churchill province into the Hearne domain (mid-crustal rocks, lower metamorphism) from the Rae domain (lower crust rocks, higher metamorphism). The eastern part of the AGT is dominated by the Chipman tonalite batholith (3.3 Ga), and on the far east the Fehr Granite (2.4 Ga). Both units were intruded by the extensive mafic Chipman dike swarm at ~1.9 Ga. On-going magnetic studies of these three units are aimed at characterizing the rock magnetism and remanence of each group as well as relating magnetic properties to the observed aeromagnetic signatures. The Fehr granite is weakly magnetic, with susceptibilities ranging from 9.4 x 10-6 to 2.1 x 10-4 with an average of 9 x 10-5 SI. The remanence held by many Fehr granite samples is weak, but stronger than expected at ~1 mA/m. The bland aeromagnetic signature over the Fehr granite reflects low susceptibility and low remanence. Chipman tonalite samples show a wide range of magnetic properties including distinct oxide zones with susceptibilities of 0.3 SI and remanence values greater than 10 A/m to relatively non-magnetic areas with susceptibilities of 1x 10-4 SI and magnetization of .01 A/m. Hysteresis properties indicate a range of behavior from single-domain to multi-domain magnetite with a majority of samples indicating pseudo-single-domain behavior (average Mr/Ms = 0.13, Hcr/Hc = 3.3). Low temperature experiments confirm the presence of magnetite, and

  3. ALS superbend magnet system

    SciTech Connect

    Zbasnik, J.; Wang, S.T.; Chen, J.Y.; DeVries, G.J.; DeMarco, R.; Fahmie, M.; Geyer, A.; Green, M.A.; Harkins, J.; Henderson, T.; Hinkson, J.; Hoyer, E.H.; Krupnick, J.; Marks, S.; Ottens, F.; Paterson, J.A.; Pipersky, P.; Portmann, G.; Robin, D.A.; Schlueter, R.D.; Steier, C.; Taylor, C.E.; Wahrer, R.

    2000-09-15

    The Lawrence Berkeley National Laboratory is preparing to upgrade the Advanced Light Source (ALS) with three superconducting dipoles (Superbends). In this paper we present the final magnet system design which incorporates R&D test results and addresses the ALS operational concerns of alignment, availability, and economy. The design incorporates conduction-cooled Nb-Ti windings and HTS current leads, epoxy-glass suspension straps, and a Gifford-McMahon cryocooler to supply steady state refrigeration. We also present the current status of fabrication and testing.

  4. Lateral and Vertical Heterogeneity of Thorium in the Procellarum KREEP Terrane: As Reflected in the Ejecta Deposits of Post-Imbrium Craters

    NASA Technical Reports Server (NTRS)

    Gillis, J. J.; Jolliff, B. L.

    1999-01-01

    The Procellarum KREEP Terrane displays the highest concentrations of Th on the Moon. However, locations of elevated Th in this region appear to be random. As observed in the 5 deg per pixel equal-area Th data, and made more evident in the preliminary 2 deg data, Th is enhanced around the craters Aristillus, Aristarchus, Kepler, Mairan, the Apennine Bench formation, and the Fra Mauro region, while noticeably and unexpectedly lower in other locations (e.g., Archimedes, Copernicus, Eratosthenes, and Plato). We have examined the composition of the materials present in these regions with the goal of understanding the patchy nature to the distribution of Th and ultimately to decipher the geologic processes that have concentrated the Th. At present time, the published resolution of the Lunar Prospector Th gamma-ray data is low (5 deg per pixel), but this will soon be superceded by significantly higher-resolution data (2 deg per pixel). Even at this improved resolution, however, it is difficult to resolve the units that are the major source of Th. In an attempt to circumvent this problem, we employ the higher-resolution Clementine multispectral data for those regions mentioned above. We use the UV-VIS-derived compositional information and the spectral properties of craters, and their ejecta as drill holes through the mare-basalt surface to investigate the thickness and composition of underlying material. With this information we attempt to piece together the stratigraphy and geologic history of the Imbrium-Procellanim region. We processed the five-band multispectral data from the Clementine Mission (415, 750,900,950, and 1000nm) using ISIS software and calibration parameters developed by the USGS, Flagstaff, Arizona. Final image mosaics are in equal-area sinusoidal projection, and have a resolution of 250 m/pixel. Using the method of we produced maps of FeO and Ti02 composition. Here we examine the Th, FeO, and Ti02 composition and spectral properties of the craters

  5. Modulus measurements in ordered Co-Al, Fe-Al, and Ni-Al alloys

    NASA Technical Reports Server (NTRS)

    Harmouche, M. R.; Wolfenden, A.

    1985-01-01

    The composition and/or temperature dependence of the dynamic Young's modulus for the ordered B2 Co-Al, Fe-Al, and Ni-Al aluminides has been investigated using the piezoelectric ultrasonic composite oscillator technique (PUCOT). The modulus has been measured in the composition interval 48.49 to 52.58 at. pct Co, 50.87 to 60.2 at. pct Fe, and 49.22 to 55.95 at. pct Ni for Co-Al, Fe-Al, and Ni-Al, respectively. The measured values for Co-Al are in the temperature interval 300 to 1300 K, while those for the other systems are for ambient temperature only. The data points show that Co-Al is stiffer than Fe-Al, which is stiffer than Ni-Al. The data points for Fe-Al and Ni-Al are slightly higher than those reported in the literature.

  6. New geochronological ages (U-Pb/Lu-Hf) from high-pressure rocks of the Escambray terrane and Santa Clara serpentinite mélange, central Cuba. Regional correlations and geodynamic implications

    NASA Astrophysics Data System (ADS)

    Rojas-Agramonte, Y.; García-Casco, A.; Kröner, A.; Herwartz, D.; Ibis Despaigne, A.; Wilde, S.

    2012-04-01

    Petrological and geochronological data of high pressure (HP) rocks from Cuba and Dominican Republic indicate continuous subduction in the northern edge of the Caribbean since ca. 120 Ma. However, expected correlation of the associated subduction zone towards the west in Guatemala is uncertain, for similar HP rocks in this region have metamorphic ages as old as 150 Ma. In this contribution we present new geochemical and U-Pb/Lu-Hf ages of HP rocks from the Escambray terrane and the central Cuba serpentinite mélange that allow geodynamic correlations between the Greater Antilles and Guatemala during the early and late Cretaceous. The Escambray composite terrane constitutes a metamorphic accretionary complex containing oceanic and platform-derived metasedimentary and metaigneous rocks that were tectonically assembled in the Caribbean subduction environment during the latest Cretaceous. The complex crops out as two domes, named Trinidad to the west and Sancti Spiritus to the east, forming a tectonic window below the arc-related Mabujina amphibolite complex and the allochthonous Cretaceous volcanic arc terrane. The latter overrides the central Cuba mélange, which contains low-pressure ophiolitic blocks and HP blocks of eclogite, garnet amphibolite and blueschist facies rocks within a serpentinitic matrix. This serpentinite mélange and similar melange bodies within the Escambray complex have been interpreted as fragments of the Caribbean subduction channel. The studied HP samples are of basaltic composition, poor in K2O (<0.34 wt %) and Rb (< 3.05 ppm), relatively rich in Ta (0.07-0.77 ppm) and Hf (0.74-4.17 ppm), and rich in Nb (0.68- 13.53 ppm). Most samples show a REE chondrite-normalised patterns characterized by a subtle LREE depletion, except a few samples which are characterized by slight enrichment in LREE. Based on their distinctive trace-element contents, most of the basaltic protholiths of the samples are identified as E- to N-MORB signature. Some samples

  7. Ecosystem Health in Mineralized Terrane-Data from Podiform Chromite (Chinese Camp Mining District, California), Quartz Alunite (Castle Peak and Masonic Mining Districts, Nevada/California), and Mo/Cu Porphyry (Battle Mountain Mining District, Nevada) Deposits

    USGS Publications Warehouse

    Blecker, Steve W.; Stillings, Lisa L.; Amacher, Michael C.; Ippolito, James A.; DeCrappeo, Nicole M.

    2010-01-01

    various mineralized terranes. We were also interested in examining these relations in the context of determining appropriate reference conditions with which to compare reclamation efforts. The purpose of this report is to present the data used to develop indices of soil and ecosystem quality associated with mineralized terranes (areas enriched in metal-bearing minerals), specifically podiform chromite, quartz alunite, and Mo/Cu porphyry systems. Within each of these mineralized terranes, a nearby unmineralized counterpart was chosen for comparison. The data consist of soil biological, chemical, and physical parameters, along with vegetation measurements for each of the sites described below. Synthesis of these data and index development will be the subject of future publications.

  8. Magnetotelluric characterization through the Ambargasta-Sumampa Range: The connection between the northern and southern trace of the Río de La Plata Craton - Pampean Terrane tectonic boundary

    NASA Astrophysics Data System (ADS)

    Peri, V. Gisel; Barcelona, Hernan; Pomposiello, M. Cristina; Favetto, Alicia

    2015-04-01

    The South American Platform was part of the Western Gondwana, a collage of plates of different ages assembled in late Neoproterozoic to Cambrian times. The Transbrasiliano Lineament, a continental shear belt that transversely intersects this platform from NE to SW, has its southern expression in the tectonic boundary between the Río de La Plata Craton and the Pampean Terrane. Magnetotelluric long-period data in a W-E profile (29°30‧ S) that crosses the Ambargasta-Sumampa Range and the Chaco-Pampean Plain were obtained to connect information of this mostly inferred tectonic boundary. A 2-D inversion model shows the Chacoparanense basin, Río Dulce lineament, Ambargasta-Sumampa Range and Salina de Ambargasta in the upper crust. At mid-to-lower crust and 40 km to the east of the Ambargasta-Sumampa Range, a discontinuity (500-2000 Ω m) of 20-km-wide separates two highly resistive blocks, the Río de La Plata Craton (6000-20,000 Ω m) in the east, and the Pampean Terrane (5000-20,000 Ω m) in the west. This discontinuity represents the tectonic boundary between both cratons and could be explained by the presence of graphite. The geometry of the Pampean Terrane suggests an east-dipping paleo-subduction. Our results are consistent with gravimetric and seismicity data of the study area. A more conductive feature beneath the range and the tectonic boundary was associated with the NE-SW dextral transpressive system evidenced by the mylonitic belts exposed in the Eastern Pampean Ranges. This belt represents a conjugate of the mega-shear Transbrasiliano Lineament and could be explained by fluid-rock interaction by shearing during hundreds of years. The eastern border of the Ambargasta-Sumampa Range extends the trace of the Transbrasiliano Lineament. The electrical Moho depth (40 km to the west and 35 km to the east) was identified by a high electrical contrast between the crust and upper mantle. The upper mantle shows a resistive structure beneath the Río de La Plata

  9. Al Shanker Remembers.

    ERIC Educational Resources Information Center

    American Educator, 2000

    2000-01-01

    In a 1996 interview shortly before his death, Al Shanker, longtime president of the American Federation of Teachers, discussed such topics as: his own educational experiences; how he learned about political fighting in the Boy Scouts; the appeal of socialism; multinational corporations and the nation state; teaching tough students; and John Dewey…

  10. Fosetyl-al

    Integrated Risk Information System (IRIS)

    Fosetyl - al ; CASRN 39148 - 24 - 8 Human health assessment information on a chemical substance is included in the IRIS database only after a comprehensive review of toxicity data , as outlined in the IRIS assessment development process . Sections I ( Health Hazard Assessments for Noncarcinogenic Ef

  11. ALS renewal moves forward

    NASA Astrophysics Data System (ADS)

    Falcone, R. W.; Feinberg, B.; Hussain, Z.; Kirz, J.; Krebs, G. F.; Padmore, H. A.; Robin, D. S.; Robinson, A. L.

    2007-11-01

    As the result of an extensive long-term planning process involving all its stakeholders—management, staff, and users—the ALS has seen its future and is aggressively moving ahead to implement its vision for keeping the facility at the cutting edge for the next 2-3 decades. The evolving strategic plan now in place aims to renew the ALS so it can address a new generation of fundamental questions about size dependent and dimensional-confinement phenomena at the nanoscale; correlation and complexity in physical, biological, and environmental systems; and temporal evolution, assembly, dynamics and ultrafast phenomena. The renewal spans three areas: (1) increased staffing at beamlines to support the growing user community and safety professionals to keep an increasingly complex facility hazard free; (2) implementing advances in accelerator, insertion device, beamline, and detector technology that will make it possible for ALS users to address emerging grand scientific and technological challenges with incisive world-class tools; and (3) construction of a user support building and guest housing that will increase the safety and user friendliness of the ALS by providing users office, meeting, experiment staging, and laboratory space for their work and on-site accommodations at reasonable rates.

  12. Geochemistry, petrography, and zircon U-Pb geochronology of Paleozoic metaigneous rocks in the Mount Veta area of east-central Alaska: implications for the evolution of the westernmost part of the Yukon-Tanana terrane

    USGS Publications Warehouse

    Dusel-Bacon, Cynthia; Day, Warren C.; Aleinikoff, John N.

    2013-01-01

    We report the results of new mapping, whole-rock major, minor, and trace-element geochemistry, and petrography for metaigneous rocks from the Mount Veta area in the westernmost part of the allochthonous Yukon–Tanana terrane (YTT) in east-central Alaska. These rocks include tonalitic mylonite gneiss and mafic metaigneous rocks from the Chicken metamorphic complex and the Nasina and Fortymile River assemblages. Whole-rock trace-element data from the tonalitic gneiss, whose igneous protolith was dated by SHRIMP U–Pb zircon geochronology at 332.6 ± 5.6 Ma, indicate derivation from tholeiitic arc basalt. Whole-rock analyses of the mafic rocks suggest that greenschist-facies rocks from the Chicken metamorphic complex, a mafic metavolcanic rock from the Nasina assemblage, and an amphibolite from the Fortymile River assemblage formed as island-arc tholeiite in a back-arc setting; another Nasina assemblage greenschist has MORB geochemical characteristics, and another mafic metaigneous rock from the Fortymile River assemblage has geochemical characteristics of calc-alkaline basalt. Our geochemical results imply derivation in an arc and back-arc spreading region within the allochthonous YTT crustal fragment, as previously proposed for correlative units in other parts of the terrane. We also describe the petrography and geochemistry of a newly discovered tectonic lens of Alpine-type metaharzburgite. The metaharzburgite is interpreted to be a sliver of lithospheric mantle from beneath the Seventymile ocean basin or from sub-continental mantle lithosphere of the allochthonous YTT or the western margin of Laurentia that was tectonically emplaced within crustal rocks during closure of the Seventymile ocean basin and subsequently displaced and fragmented by faults.

  13. Electron-microprobe Th-U-Pb monazite dating in Early-Palaeozoic high-grade gneisses as a completion of U-Pb isotopic ages (Wilson Terrane, Antarctica)

    NASA Astrophysics Data System (ADS)

    Schulz, B.; Schüssler, U.

    2013-08-01

    The electron microprobe (EMP) Th-U-Pb monazite bulk chemical dating method was applied to granulite-facies rocks of the Wilson Terrane in Antarctica. A combination of this method to isotopic U-Pb-SHRIMP ages for the evaluation of metamorphic processes required the analysis of reference monazites. These can be subdivided into three groups: a) Monazite with variable total Pb at constant Th (e.g. VK-1) is unsuitable for EMP data evaluation; b) Monazite with highly variable total Pb and Th, but with at least some Th/Pb approximating an apparent isochrone (e.g. MPN) is partly useful; and c) Monazite with constant Th/Pb at high Th (e.g. Madmon monazite) is best suitable for the combined approach and can be additionally used to improve the Th calibration for EMP. Study of monazite in grain mounts and in thin sections led to partly different but complementary results: Older monazites with EMP ages up to 680 Ma occur mainly in a grain mount from diatexite and metatexite and are interpreted as detrital relics. Some of these monazites show structures and mineral-chemical zonation trends resembling metasomatism by alkali-bearing fluids. A marked mobility of Th, P, Ce, Si and U is observed. The age of the metasomatic event can be bracketed between 510 and 450 Ma. Furthermore, in the grain mount and in numerous petrographic thin sections of migmatites and gneisses, the EMP Th-U-Pb and SHRIMP U-Pb monazite data uniformly signal a major metamorphic event with a medium-pressure granulite facies peak between 512 and 496 Ma. Subsequent isothermal uplift and then amphibolite-facies conditions between 488 and 466 Ma led to crystallisation of pristine monazite. The high-grade metamorphic event, related to the Ross Orogeny, can be uniformly traced more than 600 km along strike in the Wilson Terrane.

  14. 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.

  15. Ausbildung als zentrale Aufgabe

    NASA Astrophysics Data System (ADS)

    Krämer, Walter; Schmerbach, Sibylle

    Anders als Lesen und Schreiben zählen Grundkenntnisse in Statistik heute noch nicht zu den Voraussetzungen einer sinnvollen Teilhabe am Sozialgeschehen. Und auch in der akademischen Statistik-Ausbildung gibt es noch einiges zu tun. Das vorliegende Kapitel zeichnet die Geschichte dieser akademischen Ausbildung an deutschen Universitäten nach dem zweiten Weltkrieg nach, stellt aktuelle Defizite vor und weist auf mögliche Verbesserungen hin.

  16. Searching the Sinus Amoris: Using profiles of geological units, impact and volcanic features to characterize a major terrane interface on the Moon

    NASA Technical Reports Server (NTRS)

    Clark, P.; Joerg, S.; Dehon, R.

    1994-01-01

    Geochemical profiles of surface units, impact, and volcanic features are studied in detail to determine the underlying structure in an area of extensive mare/highland interface, Sinus Amoris. This study region includes and surrounds the northeastern embayment of Mare Tranquillitatis. The concentrations of two major rock-forming elements (Mg and Al), which were derived from the Apollo 15 orbital geochemical measurements, were used in this study. Mapped units and deposits associated with craters in the northwestern part of the region tend to have correlated low Mg and Al concentrations, indicating the presence of Potassium (K)-Rare Earth Elements (REE)-Phosphorus (P) (KREEP)-enriched basalt. Found along the northeastern rim of Tranquillitatis were areas with correlated high Mg and Al concentration, indicating the presence of troctolite. Distinctive west/east and north/south trends were observed in the concentrations of Mg and Al, and, by implication, in the distribution of major rock components on the surface. Evidence for a systematic geochemical transition in highland or basin-forming units may be observed here in the form of distinctive differences in chemistry in otherwise similar units in the western and eastern portions of the study region.

  17. AL Amyloidosis and Agent Orange

    MedlinePlus

    ... for survivors' benefits . Research on AL amyloidosis and herbicides The Health and Medicine Division (formally known as ... to the compounds of interest found in the herbicide Agent Orange and AL amyloidosis." VA made a ...

  18. Modeling neuronal vulnerability in ALS.

    PubMed

    Roselli, Francesco; Caroni, Pico

    2014-08-20

    Using computational models of motor neuron ion fluxes, firing properties, and energy requirements, Le Masson et al. (2014) reveal how local imbalances in energy homeostasis may self-amplify and contribute to neurodegeneration in ALS.

  19. Al Jazirah, Sudan

    NASA Technical Reports Server (NTRS)

    2008-01-01

    Al Jazirah (also Gezira) is one of the 26 states of Sudan. The state lies between the Blue Nile and the White Nile in the east-central region of the country. It is a well populated area suitable for agriculture. The area was at the southern end of Nubia and little is known about its ancient history and only limited archaeological work has been conducted in this area. The region has benefited from the Gezira Scheme, a program to foster cotton farming begun in 1925. At that time the Sennar Dam and numerous irrigation canals were built. Al Jazirah became the Sudan's major agricultural region with more than 2.5 million acres (10,000 km) under cultivation. The initial development project was semi-private, but the government nationalized it in 1950. Cotton production increased in the 1970s but by the 1990s increased wheat production has supplanted a third of the land formerly seeded with cotton.

    The image was acquired December 25, 2006, covers an area of 56 x 36.4 km, and is located near 14.5 degrees north latitude, 33.1 degrees east longitude.

    The U.S. science team is located at NASA's Jet Propulsion Laboratory, Pasadena, Calif. The Terra mission is part of NASA's Science Mission Directorate.