RIFT DEVELOPMENT IN REGIONS OF HOT, OVERTHICKENED CRUST: INSIGHT FROM TIBET

Active, N-trending rifts in Tibet provide a unique opportunity to investigate how extensional systems develop during orogenic collapse. Most Tibetan rifts have accommodated minimal extension and are characterized by graben or half-graben basins, high-angle range-bounding normal faults, and internally drained lacustrine depocenters in their central parts. In contrast, the most evolved rifts in Tibet (Yadong-Gulu Rift near Lhasa and newly documented Lunggar Rift in central Tibet) show the following characteristics. 1. The central parts of the rifts are bound by low-angle normal (detachment) faults with mylonitic shear zones in the footwall. The detachments and their footwalls are not corrugated like those of metamorphic core complexes where the bounding detachments have accommodated more displacement. This suggests that corrugations develop during progressive extension and are not related to the arcuate geometry of breakaway normal faults. 2. Geological, neotectonic, thermochronometric, and seismic reflection data are consistent with active slip on the detachment faults in the uppermost crust. It is unlikely that the detachments reactivated preexisting faults of similar orientation because they cut obliquely across the dominant E-W structural grain in the region. 3. The supradetachment basins show intrabasin topographic highs and are actively being incised in areas of maximum extension. We propose a kinematic model in which Tibetan rifts initiate as high-angle normal faults and half-graben basin systems and evolve in response to increasing extension and footwall isostatic rebound into detachment fault systems that are active at uppermost crustal levels and above which rift basin fill is being uplifted and eroded. For these Tibetan-style systems, the potential for the supradetachment basins (and primary breakaway zones) to be preserved in the geological record is extremely poor