Polyphase Extension at the Southern Margin of the Tibetan Plateau, Ama Drime Range, Tibet

The physiographic transition at the southern margin of the Tibetan Plateau is broadly coincident with the trace of the major detachments of the South Tibetan fault (STF) system. The STF system has been assigned an Early-Middle Miocene initiation age based on its relationship with leucogranites in the Greater Himalaya, but recent geomorphic and structural studies of this transition suggest that extensional deformation may have persisted along the STF system into the Pliocene and Pleistocene. Our recent work in the Ama Drime Range of southern Tibet (ca. 28°30'N-28°50'N; 87°20'E- 87°45'E) adds to this growing body of evidence. The range itself is a N-S transverse feature bound on the E and W by high-angle normal faults that display Quaternary scarps but probably initiated in Early Pliocene time (Jessup et al., 2008). These extensional features cut ca. 16 Ma E-W-striking STF detachments (Hodges et al., 1994), which are preserved in both the hanging wall and footwall of the range-bounding faults. We have found strong evidence for a third major structure, also E-W striking, that occurs SE of the Ama Drime range and is truncated by the high-angle Quaternary normal fault on the east side of the range. The structure in question separates Greater Himalayan sequence rocks on either side of a WNW-ESE-trending valley that is controlled by the fault. Although we have found abundant tectonite fabrics in these rocks consistent with the existence of a major structure in the valley, the fault surface itself is not exposed in the areas we have been able to reach during the early stages of fieldwork. The principal evidence for the character of the fault is that the rocks on either side of the structure have distinctive single crystal (U-Th)/He zircon and apatite cooling ages that are consistent with down to the north normal-sense displacement after ca. 7 Ma. There is some evidence that another E-W-striking structure related to a very young phase of STF deformation occurs at the southern end of the Ama Drime Range: 1) large river knickpoints coinciding with 2) loss of north-south trending boundary fault traces in digital imagery and 3) an E-W trending array of zircon (U-Th)/He ages (< 2.8 Ma) that are younger than many of the apatite (U-Th)/He ages previously reported from the interior of the range. If this feature exists, it would be the youngest STF structure in the region and would extend the known age range of STF deformation to the Late Pliocene or Pleistocene Epoch. Hodges et al. (1994) Contrib Min Pet v117 Jessup et al. (2008) Geology v36