HR: 09:35h
AN: T31E-07    [Abstracts]
TI: Mesozoic and Cenozoic Intracontinental Deformation in Southeastern Mongolia
AU: * Webb, L E
EM: lewebb@syr.edu
AF: Dept of Earth Sciences, Syracuse University, Syracuse, NY 13244 United States
AU: Johnson, C L
AF: Dept of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112 United States
AU: Minjin, C
AF: School of Geology, Mongolian University of Science and Technology, Ulaanbaatar, 210646 Mongolia
AU: Sersmaa, G
AF: School of Geology, Mongolian University of Science and Technology, Ulaanbaatar, 210646 Mongolia
AU: Affolter, M D
AF: School of Geology, Mongolian University of Science and Technology, Ulaanbaatar, 210646 Mongolia
AU: Manchuk, N
AF: School of Geology, Mongolian University of Science and Technology, Ulaanbaatar, 210646 Mongolia
AB: Structural and basin analysis studies of the NE-trending East Gobi Fault Zone (EGFZ) reveal distinct deformation events postdating Late Paleozoic arc accretion and continental amalgamation. This presentation, a companion abstract to Johnson et al. (2004), focuses on the kinematic signatures of these deformation events and their relationship to the larger Asian tectonic framework and its evolution. Metamorphic tectonites of basement blocks in the EGFZ, previously mapped as Precambrian basement, belong to a Late Triassic sinistral shear zone that comprises a suite of synkinematic intrusions, amphibolite-facies gneisses and greenschist-facies mylonites. The shear zone is dominated by steeply dipping foliations and subhorizontal ENE or WSW-plunging stretching lineations. At higher structural levels, variably metamorphosed Paleozoic sedimentary sequences are caught up in discrete zones of mylonitic deformation. We have documented the shear zone at several localities over a distance of 250 km along strike in the EGFZ. High-angle fabrics associated with the Late Triassic sinistral shear zone were exploited by Early Cretaceous basin-bounding faults that formed in the brittle-ductile to brittle regime. A NW-SE horizontal extension direction indicated by this oldest generation of brittle faults is similar to that implied by stretching lineations at the Yagan-Onch Hayrhan metamorphic core complex located to the SW along the Chinese-Mongolia border. Early Cretaceous rift basins were inverted by mid Cretaceous dextral transpression along mainly WNW-trending faults. All Mesozoic rocks are cut by Cenozoic conjugate strike-slip and thrust faults that accommodate NNW-SSE compression. These youngest faults are associated with gouge zones, cut unconsolidated alluvium, and control the present day topographic expression of basement blocks within the EGFZ. Our data suggest that extrusion tectonics have been active in Asia since the Early Mesozoic, and bear on the viability of tectonic models that postulate linkages between the Altyn Tagh and EGFZ. Though a Cenozoic linkage between the two faults is permissible, major sinistral offset on the EGFZ likely occurred in the Late Triassic.
DE: 8000 STRUCTURAL GEOLOGY (New field, replaces single entry 8165)
DE: 8100 TECTONOPHYSICS
DE: 8110 Continental tectonics--general (0905)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting