Abstract

Structural, thermobarometric, and geochronological data place limits on the age and tectonic displacement along the Zanskar shear zone, a major north-dipping synorogenic extensional structure separating the high-grade metamorphic sequence of the High Himalayan Crystalline Sequence from the overlying low-grade sedimentary rocks of the Tethyan Himalaya. A complete Barrovian metamorphic succession, from kyanite to biotite zone mineral assemblages, occurs within the 1-km-thick Zanskar shear zone. Thermobarometric data indicate a difference in equilibration depths of 12 ± 3 km between the lower kyanite zone and the garnet zone, which is interpreted as a minimum estimate for the finite vertical displacement accommodated by the Zanskar shear zone. For the present-day dip of the structure (20°), a simple geometrical model shows that a net slip of 35 ± 9 km is required to regroup these samples to the same structural level. Because the kyanite to garnet zone rocks represent only part of the Zanskar shear zone, and because its original dip may have been less than the present-day dip, these estimates for the finite displacement represent minimum values. Field relations and petrographic data suggest that migmatization and associated leucogranite intrusion in the footwall of the Zanskar shear zone occurred as a continuous process starting at the Barrovian metamorphic peak and lasting throughout the subsequent extension-induced exhumation. Geochronological dating of various leucogranitic plutons and dikes in the Zanskar shear zone footwall indicates that the main ductile shearing along the structure ended by 19.8 Ma and that extension most likely initiated shortly before 22.2 Ma.

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