Miocene-aged extension within the main mantle thrust zone, Pakistan Himalaya
During the early stages of the Himalayan orogeny, rocks of the Kohistan Island Arc were thrust southward onto the Indian Plate along the Main Mantle Thrust (MMT), a N-dipping crustal-scale fault zone developed within the NW Himalaya. During thrusting the basement gneisses and granitic lithologies of the Indian Plate developed a strong south-vergent S-C fabric and, in zones of very high shear strains, were mylonitised. However, the presence of high-grade metamorphic rocks on the footwall and low-grade rocks on the hangingwall of the MMT, coupled with discordances in fission track ages across the MMT, suggest that the fault zone was later reactivated as a zone of extension. Field and laboratory investigations of fault rock samples collected from the Kohistan arc and Indian Plate identify a series of N-dipping, N-side down, ductile to brittle, normal shear fabrics and normal faults that post-date peak metamorphism and all south-vergent thrust fabrics. Considerable amounts of extensional slip may have been accommodated along localised, large-scale normal faults on the MMT hangingwall. Distributed, finite, extensional strains were accommodated on the MMT footwall. On the immediate footwall, N-dipping, ductile, normal shears, defined by very fine-grained cataclastic biotites and syn-tectonic chlorite, displace early Himalayan fabrics. Deeper into the MMT footwall, extension was accommodated along larger, more brittle, N-dipping structures some of which, such as the Banna Shear, are of regional extent. As ductile extensional features all occur within the upper 400m of the footwall, the late-stage extension must either have been partitioned into a very narrow zone immediately below the fault, or the zone of ductile shearing was initially much thicker but has been thinned or cut-out by the later brittle faulting. Fault zone products indicate that extensional deformation occurred during decreasing temperature under greenschist-facies conditions. Differences in fission track ages across the MMT suggest that extension was early Miocene in age, synchronous with that within the main Himalayan chain where it is believed to have been driven by uplift along the Main Central Thrust (MCT). However, there is no obvious MCT analogue within the Pakistan Himalaya. The question thus arises as to whether late orogenic extension is a function solely of relatively shallow level thrusting or a result of isostatic adjustment following overthickening or delamination of the deep crust.