Structural evolution of the Neogene Gar Basin, western Tibet: Implications for releasing bend development and drainage patterns

We investigated the interaction among basin-bounding faults, basin fill, and geomorphic features of the southern Gar Basin, one of only two known releasing double-bend basins along the Karakoram fault, to better understand their structural evolution and role in basin development. The southern Gar Basin is bounded by an similar to 44-km-long, N20 degrees W-striking central fault segment flanked by two N40 degrees W-striking segments that parallel the regional strike of the Karakoram fault system. The central fault segment is composed of a system of strike-slip and normal faults that young basinward and incorporate basin fill in their uplifted footwalls. The oldest faults along the extensional portion of the bend are dominantly strike-slip, and they strike similar to 15 degrees W from the main strike of the Karakoram fault. Basin fill is broadly folded about a NNW-trending axis and can be explained by E-SE-directed slip along a listric normal fault. Cross sections across the basin and associated faults suggest the geometry is best described as an extensional flower structure. Forward structural modeling of the intrabasinal faults shows that the system has accommodated similar to 8 km of east-west extension. We interpret the bend to have formed from linking of R and P shears into a through-going principal displacement zone. At shallow levels in the crust (low confining pressures), R shears are exploited; at deeper levels, these faults merge with the principal displacement zone, forming the extensional flower structure geometry. We estimate that the shear zone is 50-35 km wide based on the aerial distribution of P and R shears. Restoration of R shears on the west and east sides of the Gar Valley indicates similar to 55 km of right-lateral separation along the Karakoram fault, which is a minimum slip estimate for the Karakoram fault system.