OROCLINAL BENDING SOUTH OF THE ALTYN TAGH FAULT SYSTEM: CONSTRAINTS FOR THE TOTAL OFFSET

The Altyn Tagh Fault (ATF) is a 1200km long left-lateral strike-slip fault bounding the Tarim basin to the north from the Tibetan Plateau to the south. The ATF is a key feature in the debate between distributed shortening versus localized deformation models describing the formation the Tibetan Plateau. Systematic arcuate structures (oroclines?) are observed directly south of the ATF. The geometry of these structures may result from distributed deformation leading to bending (by drag folding) within a shear zone 50-100 km from the fault trace. We test the feasibility of this mechanism by comparing paleomagnetic directions along the arcuate trend of the Tula basin. Results from a preliminary paleomagnetic sampling (64 sites) in the Jurassic to Tertiary red beds of this basin supports the bending hypothesis. Three sections in the bent segment close to the ATF show 21.7±18.7° of counterclockwise rotation relative to three sections away from the fault. Additional sampling in key areas (45 sites) has been undertaken to further quantify bending. Preliminary results from this second sampling indicate that bending may have occurred in combination with thrusting over a preexisting arcuate ramp. Forward modeling of the observed bending through a thin viscous sheet model with power-law rheology (Nelson and Jones 1987) shows that the dataset is not compatible with shear related to the total length and offset of the ATF but may correspond to a smaller splay running 15 km off the main ATF indicating that the deformation is not confined to the ATF. Estimates of offset on the ATF (Gehrels et al. 2000) based on pre-Tertiary piercing points indicate 205 km of discrete slip. If oroclinal bending occurs systematically along the fault, up to 100 km of distributed shear must be added to this slip to account for the total displacement across the ATF and associated distributed shear system separating the Tibetan Plateau from the Tarim Basin.