MAKING SENSE OF CAOS: PRELIMINARY RESULTS FROM AN INTEGRATED GEOLOGIC INVESTIGATION OF THE CENTRAL ASIAN OROGENIC SYSTEM (= ALTAIDS)

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.