A MODEL FOR THE ORIGIN OF HIMALAYAN ANATEXIS AND INVERTED METAMORPHISM

The origin of the paired granite belts and inverted metamorphic sequen ces of the Himalaya has generally been ascribed to development of the Main Central Thrust (MCT). Although a variety of models have been prop osed that link early Miocene anatexis with inverted metamorphism, rece nt dating studies indicate that recrystallization of elements of the M CT footwall occurred in the central Himalaya as recently as similar to 6 Ma. The recognition that hanging wall magmatism and footwall metamo rphism are not spatially and temporally related renders unnecessary th e need for exceptional physical conditions to explain generation of th e High Himalayan leucogranites and North Himalayan granites, which dif fer in age, petrogenesis, and emplacement style. We suggest that their origin is linked to shear heating on a continuously active thrust tha t cuts through Indian supracrustal rocks that had previously experienc ed low degrees of partial melting. Numerical simulations assuming a sh ear stress of 30 MPa indicate that continuous slip on the Himalayan de collement beginning at 25 Ma could trigger partial melting reactions l eading to formation of the High Himalayan granite chain between 25 and 20 Ma and the North Himalayan belt between 17 and 8 Ma. The ramp-flat geometry we apply to model the Himalayan thrust system requires that the presently exposed rocks of the hanging wall resided at middle crus tal levels above the decollement throughout the early and middle Mioce ne. Late Miocene, out-of-sequence thrusting within the broad shear zon e beneath the MCT provides a mechanism to bring these rocks to the sur face in their present location (i.e., well to the north of the present tectonic front) and has the additional benefit of explaining how the inverted metamorphic sequences formed beneath the MCT. We envision tha t formation of the MCT Zone involved successive accretion of tectonic slivers of the Lesser Himalayan Formations to the hanging wall and inc orporate these effects into the model. The model predicts continued an atexis up to 400 km north of the Himalayan range, consistent with the timing and geochemistry of leucogranites exhumed on the flank of a sou th Tibetan rift.