THE SOUTH LUNGGAR RIFT: A JUVENILE DETACHMENT IN WESTERN TIBET?

Here we present the results of recent structural and neotectonic mapping and preliminary zircon (U-Th)/He thermochronology from the recently discovered South Lunggar Rift in the western Lhasa terrane, Tibet. The NNE-trending rift is ~50 km long in the N-S direction and up to 12 km wide in the E-W direction. Maximum elevations exceed 6600 m in the rift-flanking range and maximum basin elevations are less than 5000 meters. The South Lunggar Rift valley bounds the western side of the range. The South Lunggar Rift is seismically active, as a Mw 6.8 earthquake occurred below the range on 25 August 2008. The range is composed of gneisses and volcanic to hypabyssal rocks, all of which are intruded by granites. Much of the South Lunggar range is bounded to the west by a >100 m thick mylonitic shear zone dipping ~20° west. Ductile S-C fabrics and asymmetric porphyroclasts in the shear zone indicate top to the west, normal sense of shear. The mylonitic shear zone is cut by minor low-angle antithetic and synthetic brittle faults. Fault scarps located up to 2 km into the hanging wall basin cut Quaternary moraines and have a cumulative throw of ~100 m, indicating the recent activity of the fault system, though the moraines are uncut where they directly overlie the range-bounding mylonitic shear zone, suggesting basinward migration of active surficial faulting. These observations raise the possibility that the South Lunggar Rift is a west-dipping detachment system.