Research Article| July 01, 2004 Precise temperature estimation in the Tibetan crust from seismic detection of the α-β quartz transition J. Mechie; J. Mechie 1GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany Search for other works by this author on: GSW Google Scholar S.V. Sobolev; S.V. Sobolev 2GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany, and Institute of Physics of the Earth, Moscow, Russia Search for other works by this author on: GSW Google Scholar L. Ratschbacher; L. Ratschbacher 3Institut für Geologie, Technische Universität Freiberg, 09599 Freiberg, Germany Search for other works by this author on: GSW Google Scholar A. Y. Babeyko; A. Y. Babeyko 4GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany Search for other works by this author on: GSW Google Scholar G. Bock; G. Bock 4GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany Search for other works by this author on: GSW Google Scholar A.G. Jones; A.G. Jones 5Geological Survey of Canada, Ottawa, Ontario, Canada, and Department of Earth Sciences, Syracuse University, Syracuse, New York 13244, USA Search for other works by this author on: GSW Google Scholar K.D. Nelson; K.D. Nelson 6Department of Earth Sciences, Syracuse University, Syracuse, New York 13244, USA Search for other works by this author on: GSW Google Scholar K.D. Solon; K.D. Solon 6Department of Earth Sciences, Syracuse University, Syracuse, New York 13244, USA Search for other works by this author on: GSW Google Scholar L.D. Brown; L.D. Brown 7Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York 14853, USA Search for other works by this author on: GSW Google Scholar W. Zhao W. Zhao 8Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, Beijing 100037, China Search for other works by this author on: GSW Google Scholar Author and Article Information J. Mechie 1GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany S.V. Sobolev 2GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany, and Institute of Physics of the Earth, Moscow, Russia L. Ratschbacher 3Institut für Geologie, Technische Universität Freiberg, 09599 Freiberg, Germany A. Y. Babeyko 4GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany G. Bock 4GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany A.G. Jones 5Geological Survey of Canada, Ottawa, Ontario, Canada, and Department of Earth Sciences, Syracuse University, Syracuse, New York 13244, USA K.D. Nelson 6Department of Earth Sciences, Syracuse University, Syracuse, New York 13244, USA K.D. Solon 6Department of Earth Sciences, Syracuse University, Syracuse, New York 13244, USA L.D. Brown 7Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York 14853, USA W. Zhao 8Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, Beijing 100037, China Publisher: Geological Society of America Received: 02 Dec 2003 Revision Received: 29 Mar 2004 Accepted: 01 Apr 2004 First Online: 02 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2004) 32 (7): 601–604. https://doi.org/10.1130/G20367.1 Article history Received: 02 Dec 2003 Revision Received: 29 Mar 2004 Accepted: 01 Apr 2004 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation J. Mechie, S.V. Sobolev, L. Ratschbacher, A. Y. Babeyko, G. Bock, A.G. Jones, K.D. Nelson, K.D. Solon, L.D. Brown, W. Zhao; Precise temperature estimation in the Tibetan crust from seismic detection of the α-β quartz transition. Geology 2004;; 32 (7): 601–604. doi: https://doi.org/10.1130/G20367.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract In the deep crust, temperature, which is among the key parameters controlling lithospheric dynamics, is inferred by extrapolation from the surface using several assumptions that may fail in regions of active tectonics and fluid migration. In the rare case that temperatures of 700 °C or higher are exceeded in the upper and middle continental crust composed of quartz-rich felsic rocks, the α-β quartz transition (ABQT) will occur, generating a measurable seismic signature and offering the possibility for precisely estimating temperature from the known ABQT phase diagram. Here it is shown that all expected seismic features of the ABQT are met by the boundary between the upper and middle crust below the INDEPTH III profile in central Tibet. This finding implies that a temperature of 700 °C is achieved at a depth of 18 km under the southern Qiangtang block, which agrees with the depth to the top of a high electrical conductivity anomaly, likely representing partially melted crust. To the south in the northern Lhasa block, the ABQT is at 32 km depth, corresponding to a temperature of 800 °C. It thus appears that this seismic boundary representing the ABQT is the result of recent geologic processes rather than being a lithologic boundary. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Research Article| March 01, 1987 Overview of the COCORP 40°N Transect, western United States: The fabric of an orogenic belt R. W. ALLMENDINGER; R. W. ALLMENDINGER 1Institute for the Study of the Continents and Department of Geological Sciences, Cornell University, Ithaca, New York 14853 Search for other works by this author on: GSW Google Scholar T. A. HAUGE; T. A. HAUGE 2Institute for the Study of the Continents, Cornell University, Ithaca, New York 14853 Search for other works by this author on: GSW Google Scholar E. C. HAUSER; E. C. HAUSER 2Institute for the Study of the Continents, Cornell University, Ithaca, New York 14853 Search for other works by this author on: GSW Google Scholar C. J. POTTER; C. J. POTTER 2Institute for the Study of the Continents, Cornell University, Ithaca, New York 14853 Search for other works by this author on: GSW Google Scholar S. L. KLEMPERER; S. L. KLEMPERER 3Institute for the Study of the Continents and Department of Geological Sciences, Cornell University, Ithaca, New York 14853 Search for other works by this author on: GSW Google Scholar K. D. NELSON; K. D. NELSON 4Institute for the Study of the Continents, Cornell University, Ithaca, New York 14853 Search for other works by this author on: GSW Google Scholar P. KNUEPFER; P. KNUEPFER 4Institute for the Study of the Continents, Cornell University, Ithaca, New York 14853 Search for other works by this author on: GSW Google Scholar J. OLIVER J. OLIVER 5Institute for the Study of the Continents and Department of Geological Sciences, Cornell University, Ithaca, New York 14853 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1987) 98 (3): 308–319. https://doi.org/10.1130/0016-7606(1987)98<308:OOTCNT>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation R. W. ALLMENDINGER, T. A. HAUGE, E. C. HAUSER, C. J. POTTER, S. L. KLEMPERER, K. D. NELSON, P. KNUEPFER, J. OLIVER; Overview of the COCORP 40°N Transect, western United States: The fabric of an orogenic belt. GSA Bulletin 1987;; 98 (3): 308–319. doi: https://doi.org/10.1130/0016-7606(1987)98<308:OOTCNT>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The COCORP 40°N Transect of the Cordillera of the western United States crosses tectonic features ranging in age from Proterozoic to Recent and provides an acoustic cross-section of a complex orogen affected by extension, compression, magmatism, and terrane accretion. The key features of the transect, centered on the Basin and Range Province, include (1) asymmetric seismic fabrics in the Basin and Range, including west-dipping reflections in the eastern part of the province and predominantly subhorizontal ones in the west; (2) a pronounced reflection Moho at 30 ± 2 km and locally as deep as 34 km in the Basin and Range with no clear sub-Mono reflections; and (3) complex-dipping reflections and diffractions locally as deep as 48 km in the Colorado Plateau and Sierra Nevada. The eastern part of the transect, shot above known and inferred Precambrian crystalline basement, probably records features related to the entire history of the orogen, locally perhaps as old as 1800 Ma. In this region, major paleotectonic features probably controlled subsequent structural development. In title western half of the transect, however, most reflectors are probably no older than Mesozoic. Within the Basin and Range Province, there appears to be a strong Cenozoic overprint that is characterized by asymmetric half-grabens, low-angle normal faults, and a pervasive subhorizontal system of reflections in the lower crust; no one model of intracontinental extension is universally applicable. Processes that produce or are accompanied by thermal anomalies (magmatism, enhanced ductility, and extension) appear to be essential in developing a highly layered lower crust. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
International Deep Profiling of Tibet and the Himalaya (INDEPTH) deep seismic reflection profiles show that the Indian lithosphere is underthrusting the central Himalaya along a gently north dipping decollement that is traceable northward beneath the Tethyan belt to ∼28.6°N and to a depth of about 45 km. The decollement carries in its hanging wall a near‐crustal‐thickness slice of internally deformed Indian continental basement and cover represented in outcrop by the Greater Himalayan belt and structurally overlying Tethyan belt. Geometric relationships suggest that the decollement probably ramps downward beneath the northern Tethyan belt to near the base of the crust and that the hanging wall crustal slice was detached from Indian mantle lithosphere that presently underlies southern Tibet to the north. The North Himalayan anticlinorium (Kangmar dome) appears to be a large duplex ramp anticline in the hanging wall of the decollement. The South Tibetan Detachment appears to have been imaged in two areas. In the Wagye La area it appears to follow the Tethyan belt/Greater Himalayan belt contact northward in the subsurface and to have been folded over the Kangmar dome. In the Zherger La area it appears to have been active relatively recently as a ∼30° north dipping normal fault that cuts deeply into the Greater Himalayan belt allochthon. Palinspastic reconstruction indicates a minimum of about 326 km of shortening of the Indian basement across the Himalaya since the initiation of the Main Central Thrust. Hence Indian continental lithosphere, largely stripped of its overlying crust, can extend northward beneath the Tibetan plateau to at least 32°N, where earthquake seismological observations show that the properties of the upper mantle change markedly [e.g., McNamara et al ., 1995]. Taken together, the seismic reflection and earthquake seismological observations lend support for the view that the Indian mantle lid mechanically underplates roughly the southern half of the Tibetan plateau. The INDEPTH reflection data do not show that Indian crust attached to this lithosphere extends north of southernmost Tibet (Kangmar dome), nor do they negate this possibility. Similarly, the reflection data do not yield direct evidence for or against lower‐crustal subduction beneath the Himalaya.
COCORP recently collected 1100 km of new deep reflection data along two transects crossing the portion of the Appalachian orogen buried beneath the Georgia/Florida coastal plain, the superjacent Triassic/Early Jurassic South Georgia basin, and the southernmost Piedmont. On all profiles the base of the crust is marked by discontinuous patches of reflections at 11 to 12 s, which probably originate from Moho, implying a crustal thickness of about 35 to 38 km. Three crossings of the Brunswick magnetic low show that this feature coincides with a major crustal penetrating zone of dipping reflections and diffractions. Based on regional relations the authors argue that the dipping zone marks the Alleghenian terrane boundary (suture), flanking the N side of the African terrane underlying northern Florida. Continuation of the Brunswick anomaly offshore implies that this boundary continues NE across the Georgia/South Carolina continental shelf. In western Georgia deep-crustal reflections are sparse within Suwannee terrane basement and within Greenville basement to the N; however, in eastern GA numerous mid- and lower-crustal reflections are observed in the region bounded approximately by the Brunswick anomaly on the S and the Appalachian gravity gradient on the NW. It is not clear whether this regional deep-reflection fabric ismore » a manifestation of Paleozoic accretionary structure, or Mesozoic extension. Data across the Pine Mountain belt and Inner Piedmont immediately to the N indicate approximately 9 km of structural relief across the Towaliga fault, much of which may be due to Mesozoic normal slip.« less
INDEPTH seismic reflection profiling shows that the decollement beneath which Indian lithosphere underthrusts the Himalaya extends at least 225 kilometers north of the Himalayan deformation front to a depth of approximately 50 kilometers. Prominent reflections appear at depths of 15 to 18 kilometers near where the decollement reflector apparently terminates. These reflections extend north of the Zangbo suture to the Damxung graben of the Tibet Plateau. Some of these reflections have locally anomalous amplitudes (bright spots) and coincident negative polarities implying that they are produced by fluids in the crust. The presence of geothermal activity and high heat flow in the regions of these reflections and the tectonic setting suggest that the bright spots mark granitic magmas derived by partial melting of the tectonically thickened crust.
Project INDEPTH (International Deep Profiling of Tibet and the Himalaya) has collected over 300 km of multichannel, deep seismic reflection data using explosive sources as part of a multidisciplinary effort to image the structure of the crust and uppermost mantle of the Tibetan plateau. The reflection profiles lie within the Yadong‐Gulu rift and were acquired in the summers of 1992 and 1994. Data processing utilized typical industry tools, and a new method was used to migrate the data. Both unmigrated and migrated sections are presented here in large format to facilitate further interpretations.
Research Article| April 01, 1986 Comment and Reply on “New COCORP profiling in the southeastern United States”: REPLY K. D. Nelson; K. D. Nelson 1Institute for the Study of the Continents, Snee Hall, Cornell University, Ithaca, New York 14853 Search for other works by this author on: GSW Google Scholar J. H. McBride; J. H. McBride 1Institute for the Study of the Continents, Snee Hall, Cornell University, Ithaca, New York 14853 Search for other works by this author on: GSW Google Scholar J. A. Arnow; J. A. Arnow 1Institute for the Study of the Continents, Snee Hall, Cornell University, Ithaca, New York 14853 Search for other works by this author on: GSW Google Scholar J. E. Oliver; J. E. Oliver 1Institute for the Study of the Continents, Snee Hall, Cornell University, Ithaca, New York 14853 Search for other works by this author on: GSW Google Scholar L. D. Brown; L. D. Brown 1Institute for the Study of the Continents, Snee Hall, Cornell University, Ithaca, New York 14853 Search for other works by this author on: GSW Google Scholar S. Kaufman S. Kaufman 1Institute for the Study of the Continents, Snee Hall, Cornell University, Ithaca, New York 14853 Search for other works by this author on: GSW Google Scholar Geology (1986) 14 (4): 363. https://doi.org/10.1130/0091-7613(1986)14<363:CARONC>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation K. D. Nelson, J. H. McBride, J. A. Arnow, J. E. Oliver, L. D. Brown, S. Kaufman; Comment and Reply on “New COCORP profiling in the southeastern United States”: REPLY. Geology 1986;; 14 (4): 363. doi: https://doi.org/10.1130/0091-7613(1986)14<363:CARONC>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract No Abstract Available. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
In the summer of 1998, project INDEPTH recorded a 400 km long NNW–SSE wide-angle seismic profile in central Tibet, from the Lhasa terrane across the Banggong-Nujiang suture (BNS) at about 89.5°E and into the Qiangtang terrane. Analysis of the P-wave data reveals that (1) the crustal thickness is 65 ± 5 km beneath the line; (2) there is no 20 km step in the Moho in the vicinity of the BNS, as has been suggested to exist along-strike to the east based on prior fan profiling; (3) a thick high-velocity lower crustal layer is evident along the length of the profile (20–35 km thick, 6.5–7.3 km s−1); and (4) in contrast to the southern Lhasa terrane, there is no obvious evidence of a mid-crustal low-velocity layer in the P-wave data, although the data do not negate the possibility of such a layer of modest proportions.
Research Article| September 01, 1987 Normal-fault boundary of an Appalachian basement massif?: Results of COCORP profiling across the Pine Mountain belt in western Georgia K. D. Nelson; K. D. Nelson 1Institute for the Study of the Continents (INSTOC), Snee Hall, Cornell University, Ithaca, New York 14853 Search for other works by this author on: GSW Google Scholar J. A. Arnow; J. A. Arnow 1Institute for the Study of the Continents (INSTOC), Snee Hall, Cornell University, Ithaca, New York 14853 Search for other works by this author on: GSW Google Scholar M. Giguere; M. Giguere 1Institute for the Study of the Continents (INSTOC), Snee Hall, Cornell University, Ithaca, New York 14853 Search for other works by this author on: GSW Google Scholar S. Schamel S. Schamel 2Earth Sciences & Resources Institute, University of South Carolina, Columbia, South Carolina 29208 Search for other works by this author on: GSW Google Scholar Author and Article Information K. D. Nelson 1Institute for the Study of the Continents (INSTOC), Snee Hall, Cornell University, Ithaca, New York 14853 J. A. Arnow 1Institute for the Study of the Continents (INSTOC), Snee Hall, Cornell University, Ithaca, New York 14853 M. Giguere 1Institute for the Study of the Continents (INSTOC), Snee Hall, Cornell University, Ithaca, New York 14853 S. Schamel 2Earth Sciences & Resources Institute, University of South Carolina, Columbia, South Carolina 29208 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1987) 15 (9): 832–836. https://doi.org/10.1130/0091-7613(1987)15<832:NBOAAB>2.0.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation K. D. Nelson, J. A. Arnow, M. Giguere, S. Schamel; Normal-fault boundary of an Appalachian basement massif?: Results of COCORP profiling across the Pine Mountain belt in western Georgia. Geology 1987;; 15 (9): 832–836. doi: https://doi.org/10.1130/0091-7613(1987)15<832:NBOAAB>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Recent seismic reflection profiling in western Georgia and adjacent eastern Alabama, conducted by the Consortium for Continental Reflection Profiling (COCORP), shows that reflections commonly associated with the Appalachian detachment do not continue southeastward beneath the Pine Mountain belt. Rather, these reflections terminate abruptly on the north side of the belt, along the downdip projection of the Towaliga fault. This observation is difficult to reconcile with the basement duplex interpretation traditionally applied to the Pine Mountain belt and to all other Appalachian interior basement massifs. An alternative interpretation, consistent with the reflection data and with local surface geologic relations, is that the Towaliga fault is, at least in its later evolution, a large northwest-dipping normal fault that cuts the Piedmont allochthon, Appalachian detachment, and Grenville basement. Where crossed by the COCORP profile, this fault has an inferred average dip of about 54° and offsets Grenville basement about 9 km. This interpretation is consistent with the view that the Pine Mountain belt is a structural window through the Piedmont allochthon. However, it implies that much of the structural relief on the basement exposed in the window is due to late normal faulting rather than to thrust imbrication alone. If correct, this has several important implications for Appalachian geology: (1) It implies that normal faulting of late Paleozoic and/or Mesozoic age has played a much more important role in the development of the exposed southern Appalachians than has generally been considered to date. (2) Grenville basement exposed in the Pine Mountain belt has been attached to North America since Precambrian time; it does not represent a Paleozoic accreted terrane. (3) The Appalachian detachment may be exposed around the periphery of the Pine Mountain belt and hence may be available for direct observation at this locality. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
COCORP seismic reflection profiling in the Ouachita Mountains of western Arkansas indicates that (1) Carboniferous foreland basin deposits within the Arkoma Basin thicken dramatically toward the south, reaching an aggregate thickness in excess of 12 km (4.5 s two‐way travel time) beneath the southern edge of the Frontal Thrust Zone, (2) evidence for northward directed low‐angle thrusting within this clastic sequence is prominent, with a probable decollement surface lying at or near the contact with underlying Early Paleozoic shelf carbonates, (3) beneath the Benton Uplift a sequence of discontinuous events defines a broad antiform cresting at approximately 7 km (2.8 s); this structure appears to mimic the anticlinorial shape of the Benton Uplift indicated by the surface outcrop pattern, (4) beneath the Southern Ouachitas, south dipping stratified events are observed to depths in excess of 14 km (5.0 s), (5) deeper in the section a prominent, gently north dipping reflection occurs at approximately 22 km depth (7.6 s) beneath the northern Coastal Plain/Southern Ouachitas. Extrapolation of data along strike in the Ouachita belt, and consideration of large‐scale structure observed in other collisional orogenic belts, suggest that the reflection events observed beneath the Benton Uplift represent Early Paleozoic shelf carbonates correlative with those that floor the Arkoma Basin to the north. This interpretation requires that the Early Paleozoic deep‐water sediments exposed in the core of the Ouachitas be allochthonous, and also implies significant basement uplift beneath the core zone. To the south, the prominent package of layered reflections occurring beneath the Southern Ouachitas and northern Coastal Plain indicates that a significant portion of the crust in that region is composed of imbricate sedimentary and/or metasedimentary strata. At relatively ‘shallow’ levels these strata are correlative with the Carboniferous flysch cropping out in the Southern Ouachitas, and at somewhat deeper levels, they are correlative with the Early to Middle Paleozoic deep‐water sediments exposed on the Benton Uplift.
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