ACTIVE METAMORPHIC CORE-COMPLEX DEVELOPMENT IN HINTERLAND REGIONS OF HOT, THICK CRUST: A NEW VIEW OF EAST-WEST EXTENSION IN TIBET

We present new field observations augmented with satellite remote-sensing data from several north-trending rift systems in the Lhasa and Qiangtang terranes, Tibet. The fault geometry and kinematics observed from several rifts in eastern, central, and western Tibet provide new insight into the geometry, kinematics, and the dynamics of crustal deformation within the hinterland regions of the active Indo-Asian continental collision zone. Our new data bears on extension of the continental crust, and the mechanics associated with likely active, low-angle normal faults. An example is the Lunggar Shan which is a north-trending mountain range that parallels a rift valley in the northern Lhasa block of southern Tibet. The Lunggar Shan is bounded by a <40° east-dipping mylonitic shear zone that juxtaposes variably deformed biotite granite and mylonitic gneiss in the footwall, against Paleozoic marbles and Neogene alluvial fan, fluvial and lacustrine rocks in the hanging wall. Neogene gravels in the immediate proximal hanging wall record the unroofing of the Lunggar Shan as indicated by growth strata and clasts of cobbles and boulders composed of both footwall and hanging wall lithologies. The gravels are cut by numerous north-striking, range-bounding brittle normal faults. The normal faults are both west and east-dipping, and fault striae and sense-of-shear indicators show dominantly top-to-the-east displacement, with minor top-to-the-west slip on antithetic normal faults. The map relationships of the Lunggar Shan detachment, inactive range front, and the active range-parallel normal faults within the largely denuding rift basin are consistent with high angle normal faults that sole at depth into an active and isostatically rebounding low-angle detachment. Additionally, detailed analysis of Shuttle Radar Topography Mission (SRTM) elevation data for nascent rifts, the Lunggar Shan, and the highly evolved Nyainqentanglha Shan range reveal temporal “snapshots” of rift development. In particular, the SRTM elevation data are consistent with active isostatic rebound of the detachment footwall, which makes specific predictions for the temporal development of rift basin architecture and associated sedimentary dispersal patterns.