Crustal thickening leading to exhumation of the Himalayan Metamorphic coreof central Nepal: Insight from U-Pb Geochronology and Ar-40/Ar-39 Thermochronology

New and published U-Pb geochronology and Ar-40/Ar-30 thermochronology from footwall and hanging wall rocks of a segment of the South Tibetan detachmen t system exposed in the Annapurna area of central Nepal Himalaya bring addi tional constraints on the timing of metamorphism, crustal thickening, and n ormal faulting resulting in exhumation of the Himalayan metamorphic core. E arly Oligocene crustal thickening led to Eohimalayan kyanite-grade metamorp hism between 35 Ma and 32 Ma. The resulting thermal event affected the Earl y Ordovician augen gneiss (Formation III) and produced kyanite-bearing leuc osomes in the upper part of the metamorphic core. This event is linked with underthrusting of the Greater Himalayan metamorphic sequence below the Tet hyan sedimentary sequence and the growth of an Oligocene fan structure that has thickened the Tethyan sedimentary sequence to 25 km, thus provoking ky anite-grade melting at deeper structural levels. Early Palcozoic monazite a nd zircon populations indicate that part of the metamorphism affecting the Himalayan metamorphic core could be pre-Cenozoic. Regional correlations ind icate that the Annapurna detachment was active during early Miocene time. A weakly deformed leucogranitic dike intruding into the immediate hanging wa ll yielded reversely discordant monazite ages between 23 and 22.5 Ma, which suggest that the ductile strain in the Annapurna detachment zone terminate d at ca. 22 Ma. On the basis of a Ar-40/Ar-39 muscovite age, renewed southw est verging deformation (D-4) is interpreted to occur at ca. 18 Ma. Rapid e xhumation resulting from extensional faulting cooled the entire metamorphic core through the muscovite Ar closure temperature (330 degrees -430 degree sC) between 15 and 13 Ma. Muscovites from the immediate hanging wall of the Annapurna detachment yielded slightly younger ages, between 13 and 11 Ma, testifying to late hydrothermal activity in the Annapurna detachment zone t hat could be linked with the initiation of brittle faulting associated with the late Neogene Thakkhola graben system.