Active crustal deformation in the Tian Shan region, central Asia

Abstract

The Tian Shan mountain ranges in central Asia are one of the largest and most active intracontinental orogenic belts in the world. Its Cenozoic reactivation and deformation manifest the far-field impact of the Indian-Eurasian continental collision. How the collision-induced crustal shortening is accomondated in the Tian Shan region, however, remains debated. Some of the shortening is clearly accommodated by the piedmont fold-and-thrust belts; but the role of deformation in the interior of the orogen, and the overall pattern of crustal deformation in the Tian Shan region, differs in previous studies. We have used NeoKinema, a kinematic finite-element computer code, to analyze the long-term average rates of strain and its partitioning in the Tian Shan region. The model is constrained by updated kinematic data sets, including fault traces, geological fault slip rates, GPS site velocities, principal stress directions, and kinematic boundary conditions. Our results indicate that, in addition to shortening in the piedmont fold-and-thrust belts, significant shortening and strike-slip faulting have occurred in the interior of the Tian Shan orogen. The intra-orogen strain is concentrated north of the Nalati Fault, around the intramontane basins. The overall pattern of crustal deformation in the Tian Shan region is pure shear shortening, facilitated by NEE-trending sinistral and NW-NWW trending dextral strike-slip faults that cut across the mountain ranges. We also calculated the long-term potential of seismicity in the region and compared it with earthquake records.