RAPID MIDDLE TO LATE MIOCENE SLIP ALONG THE ZANSKAR NORMAL FAULT, GREATER HIMALAYAN RANGE, NW, INDIA: CONSTRAINTS FROM LOW-TEMPERATURE THERMOCHRONOMETRY

The Zanskar fault (ZF) is a moderately NE-dipping, NW-striking normal fault that bounds the northern flank of the Greater Himalaya Range, NW Himalaya and is the far west continuation of the South Tibetan Detachment System (STDS), a major arc-parallel normal sense shear zone spanning the length of the Himalayan orogen. The exhumed ZF fault-plane near Padum, India, displays well-developed triangular facets with an average dip of ~33° and juxtaposes unmetamorphosed to weakly metamorphosed Tethyan sedimentary rocks in the hanging wall against high-grade metamorphic (amphibolite to migmatite) rocks of the Greater Himalayan Sequence in the footwall. The ZF is unique in that this extensional fault developed within a region of compression during the ongoing continent-continent collision between India and Asia. To constrain better the young exhumation and normal slip histories along the ZF, we completed new detailed zircon and apatite (U-Th)/He (ZHe and AHe) and apatite fission-track (AFT) thermochronometry along three vertical transects of High Himalayan crystalline rocks exposed in the footwall of the ZF. Thermochronometric age vs. elevation plots over vertical distances of ~570 to 850 m, show that ZHe and AHe yield the same mean ages, within error, of ~14 Ma and ~10 Ma, respectively; AFT ages, from one transect decrease down the escarpment from ~15.5 Ma to ~10.5 Ma. Inverse modeling of the ZHe and AHe data yield t-T cooling histories that are interpreted as indicating: (1) initiation of normal fault slip at ~14-13 Ma and rapid exhumation of the footwall between ~14 and 10 Ma at rates of 0.5-2.5 mm/yr, (2) rapid thermal re-equilibration between ~11 and 9 Ma, and (3) either slow exhumation and/or quiescence between ~9 Ma to the present day or slow exhumation and/or quiescence punctuated by a second episode of rapid normal slip between ~9 and 6 Ma. These results suggest that brittle normal slip along the ZF is 1-3 million years younger than normal slip along the central and eastern segments of the STDS. The periods of normal slip along the ZF at ~14 and ~9-6 Ma, coincide broadly with the timing of postulated continental slab break-off episodes to ~600 km beneath the western Himalaya, a mechanism that can result in an increase in gravitational potential energy and extensional deformation along crustal-scale fault structures.