AN EARLY PLIOCENE THERMAL DISTURBANCE OF THE MAIN CENTRAL THRUST, CENTRAL NEPAL - IMPLICATIONS FOR HIMALAYAN TECTONICS

Since the beginning of the collision between India and Asia at about 50 Ma, the convergence in the Himalaya has largely been taken up along major thrust zones. In this study, samples of the Lesser Himalaya Formations, up to 10 km below the Main Central Thrust (MCT), and the Greater Himalaya Sequence, up to 12 km above the MCT, have been analyzed by the 40Ar/39Ar and U-Pb methods to investigate the thermal history of the MCT. The ages can be summarized as follows: (1) The youngest ages from muscovites (3.1 Ma), biotites (3.4 Ma), and hornblendes (4.1 Ma) all come from within 1 km of the MCT, (2) there is a marked asymmetry of ages between the footwall and the hanging wall of the MCT; the maximum mica age in the hanging wall (Greater Himalaya Sequence) is 13 Ma, whereas a muscovite 5 km below the MCT, in the footwall, has an age spectrum with a gradient from 400 to 1400 Ma, (3) five K-feldspars from -6.2 to 11.9 km above the MCT all give minimum ages in the range 3.0-6.4 Ma, and (4) a Pb-206/U-238 age on a brannerite from the Greater Himalayan Sequence is 4.8 Ma. Structural and petrologic observations preclude the possibility that this age-distance distribution reflects faulting within the Greater Himalaya Sequence and production of the necessary thermal energy by shear heating requires unrealistically high shear stresses. Infiltration of hot fluids through the MCT zone appears to be the best hypothesis to explain these data. Simple numerical simulations, which account for heat transfer by advection within the fluid flow zone and by conduction outside it, indicate that the observed age distribution could have been produced by infiltration of hot fluids through the MCT zone at circa 5 to 4 Ma within the following range of conditions: the fluids heated rocks to temperatures in the range 470 to 510-degrees-C for less than 1 million years in a region narrower than the entire MCT zone. The temperature of the thermal disturbance inferred from the 40Ar/39Ar data is consistent with petrologic data from albitic alteration assemblages in the Greater Himalaya Sequence and the U-Pb age of the brannerite is similar to the youngest 40Ar/39Ar hornblende age. This scenario, hydrothermal heating culminating at about 4 Ma, is similar to the model of Le Fort (1981) for the generation of the High Himalayan leucogranites in the Late Oligocene - Early Miocene by dehydration of the footwall rocks of an active thrust. In the younger instance, we interpret that fluids derived from the footwall rocks of the Main Boundary Thrust migrated upward through the Lesser Himalaya Formations and were subsequently channeled along the MCT, producing the thermal disturbance.