LATE PLIOCENE DENUDATION OF THE HIGHER HIMALAYA: FISSION-TRACK EVIDENCE FROM NEPAL AND IMPLICATIONS FOR TECTONIC EVOLUTION OF THE HIMALAYA

Elucidating the thermotectonic, structural, uplift, and exhumation of the Himalaya has significant implications for our understanding of how collisional orogens form, and this task, in turn, requires geochronlogical data obtained by various dating techniques and from various parts of the Himalaya. The Higher Himalayan Crystalline Complex (HHC) in the Mount Everest region (Solu-Khumbu Himal) of Nepal is bounded by the Main Central Thrust (MCT) to the south and the South Tibetan Detachment (STD) to the north. Along the Dudh Koshi valley in Nepal, the HHC is a 20-km thick package of amphibolites-facies metamorphic rocks and anatectic leucogranite that has undergone a progressive ductile-to-brittle deformation with a predominant top-to-south sense of shear movement in line with the north-dipping direction of the MCT. Outliers of the HHC also lie as klippen within the greenschist-facies metasedimentary and sedimentary rocks of the Lesser Himalaya further south. Apatite fission-track (FT) ages determined on a topographic profile of 1600-4000 m in the HHC range from 2.1 Ma through 4.5 Ma, mostly clustering at 2.5 Ma and defining the steep slope of an elevation-cooling age curve. We interpret this as acceleration in the denudation of the MCT hangingwall, probably related to a renewed pulse of uplift and upthrust in the Himalaya. The Late Pliocene-Pleistocene sandstone and conglomerate beds of Upper Siwalik molasse in the Himalayan foreland basin support this interpretation; these sediments are fluvial, not glaciogenic. We also determined apatite FT ages on granitic rocks of the Palung area in Nepal, which is a klippe of the Higher Himalaya, presently exposed in the Lesser Himalaya. In contrast to the MCT root zone, the FT ages of Palung are significantly older (10-12 Ma) even though they come from rocks of lower altitudes. This suggests that the MCT root zone has experienced a remarkably different tectonic, denudation and cooling history in post-Miocene times. These data in conjunction with similar Late Pliocene FT ages from the HHC in other parts of the Himalayan orogen probably indicate a more recent phase of uplift and denudation in addition to the much discussed Early Miocene phase based on the Ar-Ar geochronlogy.