Synorogenic extension: Quantitative constraints on the age and displacement of the Zanskar shear zone (northwest Himalaya)

subsequent extension-induced exhumation. Geochronological dating of various Structural, thermobarometric, and geochronological data place limits on th e age and tectonic displacement along the Zanskar shear zone, a major north -dipping synorogenic extensional structure separating the high-grade metamo rphic sequence of the High Himalayan Crystalline Sequence from the overlyin g low-grade sedimentary rocks of the Tethyan Himalaya, A complete Barrovian metamorphic succession, from kyanite to biotite zone mineral assemblages, occurs within the I-km-thick Zanskar shear zone. Thermobarometric data indi cate a difference In equilibration depths of 12 +/- 3 km between the lower kyanite zone and the garnet zone, which is Interpreted as a minimum estimat e for the finite vertical displacement accommodated by the Zanskar shear zo ne. For the present-day dip of the structure (20 degrees), a simple geometr ical model shows that a net slip of 35 +/- 9 km is required to regroup thes e samples to the same structural level. Because the kyanite to garnet zone rocks represent only part of the Zanskar shear zone, and because its origin al dip may have been less than the present-day dip, these estimates fur the finite displacement represent minimum values. Field relations and petrogra phic data suggest that migmatization and associated leucogranite intrusion in the footwall of the Zanskar shear zone occurred as a continuous profess starting at the Barrovian metamorphic peak and lasting throughout the subse quent extension-induced exhumation. Geochronological dataing of various leu cogranitic plutons and dikes in the Zanskar shear zone footwall indicates t hat the main ductile shearing along the structure ended by 19.8 Ma and that extension most likely initiated shortly before 22.2 Ma.