Switch from thrusting to normal shearing in the Zanskar shear zone, NW Himalaya; implications for channel flow

The Zanskar shear zone is a ductile, normal-sense shear zone that exploited the contact between the High Himalayan Crystalline series and the Tethyan sedimentary series. The Zanskar shear zone is an extension of the South Tibetan detachment system with similar timing and nature, and, in Zanskar, it accommodated 24 km of normal movement. Early thrusting is preserved in the footwall and hanging wall and is overprinted by normal shearing. Thrusting and normal shearing were coplanar and codirectional, with SW-directed thrusting overprinted by NE-directed normal shearing--a simple inversion of movement sense. The telescoped isograds related to normal shearing define a broad pattern of colder rocks on top of hotter. However, we found preserved thrust-related metamorphic series, with hotter rocks on top of colder, severely telescoped by normal shearing. Some determinations of the amount of displacement and thinning on the Zanskar shear zone prior to the current work have assumed a steady-state crustal profile and have disregarded preexisting perturbations of isograds such as those indicated here. Miocene leucogranitic intrusions accumulated within and below the normal Zanskar shear zone. Intrusions were sheared during thrusting and normal movement, and magmatism outlasted normal shearing. We have dated monazites by U-Pb sensitive high-resolution ion microprobe (SHRIMP) from leucogranite samples that were sheared by the thrusting event, by the normal movement event, and those that postdate all shearing. Results constrain the timing of the switch from thrusting to normal movement to between 26 and 24 Ma and ca. 22 Ma. At ca. 20 Ma, normal shearing in the eastern Zanskar shear zone was no longer active, and magmatism was waning, producing late, undeformed leucogranitic dikes. Taking into account the shear zone thickness of 0.83 km, the maximum duration of normal movement of 6 m.y., and the estimated strain of gamma = 28.6, we estimate the lower bound of strain rate for the Zanskar shear zone to be 1.5 X 10 (super -13) s (super -1) . Given the short duration of the normal shearing event and magmatism, we find little support for the hypothesis of channel flow in Zanskar. We propose instead that Miocene anatexis weakened the midcrustal levels and was the cause of the switch from thrusting to normal fault movement, doming, and cooling of the anatectic core of the High Himalayan Crystalline series.