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  • Geological Society of America (GSA)  (2)
  • Cambridge University Press
  • 1
    Publication Date: 2013-07-31
    Description: The Longmen Shan, located at the boundary between the Tibetan Plateau and the Sichuan Basin, has received considerable attention following the 2008 Wenchuan earthquake. However, the tectonic history of the southwestern segment of the range has remained poorly constrained. We present zircon fission-track, zircon (U-Th)/He, and apatite (U-Th)/He data from the Baoxing region in the southwestern Longmen Shan that provide the first constraints on the cooling and exhumation history of the region. All of the measured ages are Cenozoic, and the data suggest that exhumation of the Baoxing region was ongoing by ca. 15 Ma. Zircon (U-Th)/He ages from several samples appear to be affected by radiation damage, suggesting that damage may be a concern even in samples with Cenozoic cooling ages. Samples were collected from two bodies of Precambrian crystalline rocks separated by the Wulong fault, and for all three thermochronometers, ages west of the Wulong fault are systematically younger than ages to the east, indicating that the fault has accommodated differential exhumation since 8–10 Ma. The regions east and west of the Wulong fault have experienced 7–13 km and at least 7–10 km of exhumation, respectively. The magnitude of exhumation in the southwestern Longmen Shan is similar to that reported in the central Longmen Shan, indicating consistency along strike. The thermochronology data also suggest that the Erwangmiao fault in the southwestern Longmen Shan is analogous to the Beichuan fault in the central Longmen Shan, and therefore may represent a source of seismic hazard.
    Print ISSN: 1941-8264
    Electronic ISSN: 1947-4253
    Topics: Geosciences
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  • 2
    Publication Date: 2015-03-31
    Description: The generally east-west–trending Balkan orogen (eastern Europe) consists of a northern belt of folded and thrusted Mesozoic and Cenozoic strata that forms the external fold-thrust belt of late Mesozoic and early Cenozoic age, and a southern belt that consists of deformed igneous and metamorphic rocks overprinted by Cenozoic extensional basins. Unlike most foreland fold-thrust belts, wherein deformation commonly migrates toward the foreland, the fold-thrust belt within the Balkan orogen is marginal to the Moesian Platform to the north, but was deformed in at least three events related to three different dynamic systems caused by changes in plate interactions. The earliest event of late-Early to early-Late Cretaceous deformed strata deposited within the Moesian continental margin and within a continental rifted belt containing deep-water flysch of Late Jurassic–Early Cretaceous age, a probable eastward extension of oceanic troughs from the Southern Carpathians. The shortening was a consequence of south or southwest synthetic subduction within the Vardar zone along the southern margin of the Balkan orogen. In Late Cretaceous time a backarc and/or intraarc rift zone developed along the southern margin of the fold belt, terminating shortening. The backarc and/or intraarc basin closed in Late Cretaceous–early Paleocene time, deforming the fold-thrust belt for a second time, but antithetically to north or northeast subduction in the Vardar zone. North- and northwest-vergent subduction within the Vardar zone caused magmatism, metamorphism, and deformation within the Rhodope area of southern Bulgaria south of the foreland thrust belt. In Paleogene time the southern part of the Balkan orogen became extensional with development of extensional basins and abundant magmatism due to trench rollback. The time of the final foreland fold-thrust belt deformation was late Eocene extending into Oligocene or early Miocene, contemporaneous with the extension to the south. The deformation within the fold-thrust belt was caused by a transfer of transpressional right shear within north Bulgaria and the Southern Carpathians as crustal units were translated northward west of the Moesian foreland crust and moved northeast and eastward into the eastern Carpathian west-dipping subduction zone. During the third event of deformation crustal units were molded around the Moesian foreland crust. The shortening ceased by early Miocene time and the right shear west of Moesian foreland crust was manifested by discrete right-slip faults to the present. During this third event southern Bulgaria was in an extensional regime that dominated the south- to southwest-vergent Hellenide orogen throughout the Cenozoic, thus dividing the Balkan orogen into two different deformational regions.
    Electronic ISSN: 1553-040X
    Topics: Geosciences
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