INTERACTIONS BETWEEN DEFORMATION, MAGMATISM AND HYDROTHERMAL ACTIVITYDURING ACTIVE CRUSTAL THICKENING - A FIELD EXAMPLE FROM NANGA-PARBAT,PAKISTAN HIMALAYAS

The Nanga Parbat massif is a rapidly eroding, thrust-related antiform that is distinct from other regions of the Himalayan orogen in being b oth intruded by Late Miocene-Pliocene anatectic granites and permeated by a vigorous hydrothermal system. Exhumation is achieved by erosion during thrusting along the Liachar thrust in the apparent absence of e xtensional tectonics. At depths in excess of 20 km, small batches of l eucogranitic melt have been generated by fluid-absent breakdown of mus covite from metapelitic lithologies. These melts ascend several kilome tres prior to emplacement, aided by low geothermal gradients at depth and by interaction with meteoric water as they reach shallow levels. A t intermediate depths (similar to 15 km) limited fluid infiltration is restricted to shear zones resulting in localised anatexis. Within the upper 8 km of crust, magmatic and meteoric fluid fluxes are channelis ed by active structures providing a feedback mechanism for focusing de formation. Leucogranite sheets show a range of pre-full crystallizatio n and high-temperature crystal-plastic textures indicative of strain l ocalisation onto these sheets and away from the country rocks. At subs olidus temperatures meteoric fluids promote strain localisation and ma y trigger cataclastic deformation. Since nearsurface geothermal gradie nts are unusually steep, the macroscopic transition between distribute d shearing and substantial, but localised, cataclastic deformation occ urred at amphibolite-facies conditions (similar to 600 degrees C). Eve n with the greatest topographic relief in the world, the meteoric syst em of Nanga Parbat is effectively restricted to the upper 8 km of the crust, strongly controlled by active structures.