Tectonics drives rapid exhumation of the western Himalayan syntaxis: Evidence from low-temperature thermochronometry of the Neelum valley region, Pakistan

Reconstructing the geometry of exhumation in the western Himalayan syntaxis region around Nanga Parbat is of critical importance to understanding how tectonics and surface processes interact during orogenesis. In order to determine the role played by the main Himalayan thrusts, we combine apatite U-Pb dating with apatite and zircon (U-Th-[Sm])/He analyses as well as apatite fission track dating from the Neelum valley region of Azad Jammu and Kashmir, Pakistan. Apatite U-Pb ages range from Proterozoic to mid-Miocene and can be separated into three age groups depending on the degree to which they have been affected by Himalayan tectonothermal events. Pooled fission track ages range from 7.0 +/- 0.4 to 2.2 +/- 0.4 Ma (1 sigma), apatite He ages range from 8.7 +/- 0.2 to 2.0 +/- 0.2 Ma, and zircon He ages range from 20.0 +/- 0.4 to 6.1 +/- 0.1 Ma. The data support rapid regional cooling at 10-8 Ma caused by the removal of 5-6 km of crust. This is synchronous with the initiation of movement along the Main Boundary thrust. Stream power analysis of the Neelum River catchment indicates a high normalized steepness index (k(sn)) of > 500 m(0.9) along the major river and lower k(sn) in the headwaters. The boundary between the different apatite U-Pb age groups and the transition from high k(sn) to lower k(sn) values coincide with the mapped trace of the Main Central thrust, corroborating the presence of the thrust in the southwestern part of the region. Compiling all apatite fission track cooling ages from the Nanga Parbat syntaxis region shows that cooling age contours are parallel to the major thrusts. Collectively these data provide convincing support for the contention that the well-established pattern of exhumation known from much of the Himalayan front continues around to the western syntaxis of the Himalayas.