HR: 13:55h
AN: T23D-02    [Abstracts]
TI: Making sense out of CAOS: Preliminary results on the Paleozoic-Mesozoic deformation of the Central Asian Orogenic System
AU: * Briggs, S M
EM: sbriggs@ess.ucla.edu
AF: University of California-Los Angeles, 595 Charles Young Drive E., Los Angeles, CA 90095-1567 United States
AU: Yin, A
EM: yin@ess.ucla.edu
AF: University of California-Los Angeles, 595 Charles Young Drive E., Los Angeles, CA 90095-1567 United States
AU: Manning, C E
EM: manning@ess.ucla.edu
AF: University of California-Los Angeles, 595 Charles Young Drive E., Los Angeles, CA 90095-1567 United States
AB: There are two contrasting models for the tectonic evolution of accretionary orogens, both based on the Paleozoic geology of the Central Asian Orogenic System (CAOS), which is located between the Siberian, European, and Chinese cratons. One model requires that syn-subduction strike-slip faults with more than 1000 km of displacement control the construction of accretionary orogens, whereas a second model suggests that thrust faults are responsible for shortening during and after arc magmatism. Specifically for the CAOS, previous studies show that igneous activity occurred in two episodes at 408-377 Ma and 344-290 Ma. The early phase is marked by calc-alkaline magmatism and occurred in an island-arc setting while the second phase exhibits geochemical characteristics of post-collisional magmatism. In the context of the existing pluton geochronology and geochemical interpretations, the first tectonic model requires CAOS strike-slip faulting to be Devonian in age, whereas the second requires that thrust faulting is as young as Permian. We completed field mapping, U-Th-Pb geochronology, thermobarometry, and ⁴⁰Ar/³⁹Ar thermochronology to determine the nature of tectonic contacts, timing and extent of metamorphism and deformation, and low temperature exhumation history of the southern Altai Mountains in NW China. Our results show that at least three northeast-dipping thrust faults were responsible for crustal shortening of at least 50%. The Ertix fault (also known as the Irtysh fault) juxtaposes high-grade metamorphic rocks from > 20 km depth against a Devonian-Carboniferous arc sequence. Thermochronologic data suggest that rocks were exhumed during motion on the Ertix fault between 280 Ma and 260 Ma. The Kuerti thrust, a range-bounding thrust fault, was exhuming rocks in its hanging-wall between 270 and 200 Ma. The Barils thrust was responsible for exhumation and cooling of lower amphibolite-grade hanging-wall rocks between 230 and 150 Ma. Minor orogen-parallel sinistral shear occurred after 261 Ma, based on zircon ages from a deformed granitoid. Our data suggest that major deformation, primarily accomplished by thrusting, occurred after the termination of arc magmatism in the CAOS, and does not support tectonic models requiring major syn-subduction strike-slip faulting during accretionary orogen development.
DE: 8104 Continental margins: convergent
SC: Tectonophysics [T]
MN: Fall Meeting 2005