HR: 11:05h
AN: T22C-04    [Abstracts]
TI: New Paleomagnetic Results From the Alxa Region of China: Implications for the Long-Term Slip History of the Altyn Tagh Fault and Strain Accommodation During the Indo-Asian Collision
AU: * Lippert, P C
EM: plippert@pmc.ucsc.edu
AF: Department of Earth and Planetary Sciences, University of California 1156 High Street, Santa Cruz, CA 95064, United States
AU: Ritts, B
EM: BradleyRitts@chevron.com
AF: Chevron Energy Technology Company, 600 Bollinger Canyon Rd., Room D1232, San Ramon, CA 94583, United States
AU: Darby, B J
EM: brianjdarby@yahoo.com
AF: ExxonMobile Upstream Research Company, PO Box 2189, Houston, TX 77252, United States
AU: Coe, R S
EM: rcoe@pmc.ucsc.edu
AF: Department of Earth and Planetary Sciences, University of California 1156 High Street, Santa Cruz, CA 95064, United States
AB: The Altyn Tagh Fault (ATF) on the northern margin of the Tibetan Plateau is a key structural feature in all tectonic models of Asia. Mechanical models for strain accommodation within the Asian lithosphere largely fall into two end members: distributed crustal thickening and lateral extrusion. Geodetic and structural studies indicate that the northern margin of the Tibetan Plateau currently deforms via distributed crustal thickening, but recent piercing point studies demonstrate two distinct phases of slip on the ATF: fast, high-magnitude Late Oligocene-Early Miocene slip and slow, low-magnitude post-Early Miocene slip. These long-term slip-rate results suggest a coherent model that reconciles the two opposing mechanical models. First, the ATF accommodated plate-like lateral extrusion during the Oligocene and Early Miocene with ~310 km of left-lateral displacement and slip rates in excess of 20 mm/year. Second, since the Early Miocene, slip on the ATF has slowed to <10 mm/yr (~the present rate), accumulated only ~65 km of offset, and has accommodated distributed shortening within the northern Tibetan Plateau. An Oligocene-Early Miocene phase of lateral extrusion requires structures for accommodating lateral extrusion north of the Qilian Shan, suggesting transfer of left-lateral slip into the Alxa-East Mongolia fault system (AEMFS), a system of strike-slip faults posited to extend from the northeast end of the ATF to the Sea of Okhostk. This proposed fault system is supported by geological data from southern Mongolia and a few geophysical studies. Furthermore, existing paleomagnetic results from Tarim and the North China Block are compatible with the existence of a major post-Cretaceous tectonic discontinuity between these two lithospheric units. Recent field mapping in the Alxa region confirms the existence of a system of E-NE striking strike-slip faults with clear post-Cretaceous offsets of 10s to possibly ~150 km but limited post- Miocene offsets. Here we present new paleomagnetic results from Mesozoic and Cenozoic sedimentary rocks from the Alxa region (Yabrai Shan in the NE: 5 sites, 59 samples; Hei Shan in the SW: 10 sites, 121 samples). All samples were thermally demagnetized to isolate characteristic remanent magnetizations (ChRM). Similar ChRM directions carried by magnetite and hematite, and positive conglomerate tests suggest the ChRMs are ancient. Preliminary differential vertical axis rotation studies from fault-bound blocks and from coeval rocks from opposing sides of the mapped Alxa faults suggest significant (>15° of cw or ccw rotation) post-Mesozoic shear, with limited-to-negligible post-Miocene shear. These results are compatible with the hypothesis of a two-phase evolution of the ATF and the northern margin of the Tibetan plateau, in which the Asian lithosphere deformed via lateral extrusion along the ATF and AEMFS from Late Oligocene to Early Miocene time and subsequently by distributed crustal thickening within the northern margin of the Tibetan Plateau.
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 1525 Paleomagnetism applied to tectonics: regional, global
DE: 8110 Continental tectonics: general (0905)
DE: 8111 Continental tectonics: strike-slip and transform
SC: Tectonophysics [T]
MN: 2007 Fall Meeting