TIMING OF AN EXTENSIONAL DETACHMENT DURING CONVERGENT OROGENY - NEW RB-SR GEOCHRONOLOGICAL DATA FROM THE ZANSKAR SHEAR ZONE, NORTHWESTERN HIMALAYA

The Zanskar shear zone of northwest India forms the western segment of the South Tibetan detachment system, a north-dipping normal fault and shear zone that unroofed high-grade metamorphic rocks during contract ion of the Himalayan orogen. New Rb-Sr mineral ages from the shear zon e and its footwall show that ductile deformation was ongoing at 26 Ma, and continued to 16 Ma in some sections. The nature of the deformatio n varies along strike; displacement at 25 Ma or earlier in the western section did not result in a large thermal offset. By contrast, the se ctions to the east display substantial ductile thinning at midcrustal levels that was superseded by brittle detachments as the system contin ued to exhume rocks of the High Himalayan crystalline sequence. The im plication is that the Zanskar shear zone may have had a smaller offset in the west and cut down-section to the east. The time of the bulk of the deformation of this structure is determined to be similar to that of the equivalent structure in most of the eastern and central Himala ya.