2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 1
Presentation Time: 8:00 AM


YIN, An1, BRIGGS, Stephanie M.1, MANNING, Craig E.1, KELTY, T.K.2, CHEN, Z.L.3 and DASH, Batulzii4, (1)Earth and Space Sciences, University of California, Los Angeles, Los Angeles, CA 90095, (2)Department of Geological Sciences, California State University, Long Beach, Long Beach, CA 90840, (3)Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing, 100081, China, (4)Department of Geology and Mineralogy, Mongolian University of Science and Technology, Ulaanbaatar, 46520, Mongolia, yin@ess.ucla.edu

The Central Asian Orogenic System (CAOS), about 5-6 times larger than the North America Cordillera and consisting of widesperad late Proterozoic-Paleozoic oceanic assemblages and arc complexes, is a major site of Phanerozoic continental growth. Debate for its development has been centered on whether development of strike-slip duplexes has accommodated >2000-km trench-parallel right-slip motion between 510 Ma and 310 Ma during oceanic subduction (Cambrian to Carboniferous). To address this controversy, we conducted field mapping, U-Pb-Th geochronology and 40Ar/39Ar thermochronology across the southern limb of the Altai-Sayan sector of the CAOS in the central Chinese-Mongolia Altai Range. The results of our fieldwork indicate that the 300-km wide Chinese-Mongol Altai is dominated by NE-dipping thrusts, consistently placing older or higher-grade rocks over younger and lower-grade rocks. Preliminary kinematic analysis across some of these faults, in particular the Ertix (Irtysh) fault which figures as the central element of the CAOS, is a major thrust placing high-grade rocks from >25 km depth over a shallow crustal volcanic arc sequence. Structural reconstruction and our preliminary geochronological data across the central Chinese-Mongol Altai suggest that >500 km of post-subduction crustal shortening occurred between ~280 Ma and ~180 Ma and NW-striking left-slip shearing was locally active in the orogen between ~261 Ma and ~231 Ma. Previous studies show that igneous activity in the CAOS 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 this tectonic framework, our age data suggest that major deformation, which included both thrusting and strike-slip faulting, occurred after the termination of arc development.