What drives the normal faults at the northern piedmont of the West Kunlun range?

The Tibetan plateau rose in response to the convergency of the India and Asia continent over the past 55 Ma.  Geomorphic observations, broadband seismic imaging, tomography, wide-angle seismic sounding, Bouguer gravity results and tectonic analyses all indicate that the continental lithosphere of northern Tibet overthrusts the Tarim Block to the north. Cenozoic foreland basin deposits accumulated along the southern Tarim Block because of the loading of the overthrusting Tibetan Plateau and formed the piedmont of the West Kunlun Rang. However, a system of right-stepping normal faults appearing in the West Kunlun range front is observed by earth-surface observation and Landsat images.  Why and how the normal fault was formed in the West Kunlun collision mountain belts front?  High-quality seismic reflection data across the fault in the foothills of the West Kunlun Range is presented to understand the behavior of the normal fault system.

Three high resolution time-migrate seismic profile across the normal fault system for the piedmont of the joint between the West Kunlun and Altyn Ranges were collected for oil exploration. They reveal a series of south-dipping overthrusting faults beneath the large ramp-flat structure.  The fault-bend fold, ramp-flat fault and a series of south-dipping thrust faults exist on the top, middle and bottom, respectively, which indicates the overthrusting northward occurs here.  Above the ramp-flat fault, the strata were folded, lugged and broken down by the pull of the thrusting sheet.  The gypsum-mud rock of the Cambrian’s bottom and thicken mudstones of the Permian top 400-700 meters work as the detachment surface to detach the conterminous stacking thrust sheets.  The breaking down appears where the fault direction and the stress direction turn from ramp to flat. 

The geophysical images reveal that the Tarim lithosphere subducts beneath the Tibet along its northwestern margin, the West Kunlun range.  Though the subducting distance should not be longer, a compressional stress field in the NNE direction between the West Kunlun range front and southwestern Tarim was formed.  Eastward is the Altyn Tagh Fault, a famous sinistral strike-slip fault.  It results in a compressional stress field in the SW direction between the Altyn Tagh front and southwestern Tarim.  The compressional stress field in the NNW direction is the result of the integrated effect from the above two regions.  The engulfed Neogene strata of the fold on three sections suggest that the thrusting here occurred during the late Neogene, after West Kunlun and Altyn range thrusting.  Furthermore, this congregate force makes the basement overthrusting in the NNW direction and lower crust and mantle underthrust in the SSE direction. The overthrusting sheet over the ramp-flat fault should move further along the flat fault. The ENE trend direction of the basement overthrusting, which is the trend of the normal fault system that occurred at this site, is the integrated result of the NNE West Kunlun and SW Altyn overthrusting.