Paper No. 21
Presentation Time: 2:00 PM


SHURTLEFF, Brett, Department of Geology, Central Washington University, 400 E. University Way, Ellensburg, WA 98926, LEE, Jeffrey, Central Washington University, 400 East University Way, Ellensburg, WA 98926, HAGER, Christian, Chevron U.S.A. Inc, Bakersfield, CA 93305, STOCKLI, Daniel F., Department of Geological Sciences, University of Texas at Austin, Austin, TX 78712, VAN VLECK, Drew, Department of Geology, Occidental College, 1600 Campus Road, Los Angeles, CA 90041 and BLYTHE, Ann E., Dept. of Geology, Occidental College, Los Angeles, CA 90041,

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.