Development of active low-angle normal fault systems during orogenic collapse: Insight from Tibet

Active north-trending rifts in Tibet vary significantly in their character as a function of extension magnitude. Most rifts are characterized by internally drained basins bounded by high-angle normal faults with Paleogene or older rocks in the footwall. However, the central part of the Yadong-Gulu Rift (near Lhasa) and the newly documented Lunggar Rift in west-central Tibet are bounded by low-angle normal faults (detachments) with mylonitic rocks and Miocene granites in the footwall, and exhibit active basin incision and intrabasin topographic highs in areas of inferred maximum extension. We suggest that Tibetan rifts initiate as high-angle normal fault and half-graben or graben basin systems and evolve in response to increasing extension and footwall isostatic rebound into detachment systems that are active at uppermost crustal levels and above which rift basin fill is being uplifted and eroded.