Miocene exhumation of the Pamir revealed by detrital geothermochronology of Tajik rivers

Abstract

The Pamir mountains are the western continuation of the Tibetan‐Himalayan system, the largest and highest orogenic system on Earth. Detrital geothermochronology applied to modern river sands from the western Pamir of Tajikistan records the history of sediment source crystallization, cooling, and exhumation. This provides important information on the timing of tectonic processes, relief formation, and erosion during orogenesis. U‐Pb geochronology of detrital zircons and 40Ar/39Ar thermochronology of white micas from five rivers draining distinct tectonic terranes in the western Pamir document Paleozoic through Cenozoic crystallization ages and a Miocene (13–21 Ma) cooling signal. Detrital zircon U‐Pb ages show Proterozoic through Cenozoic ages and affinity with Asian rocks in Tibet. The detrital 40Ar/39Ar data set documents deep and regional exhumation of the Pamir mountains >30 Myr after Indo‐Asia collision, which is best explained with widespread erosion of metamorphic domes. This exhumation signal coincides with deposition of over 6 km of conglomerates in the adjacent foreland, documenting high subsidence, sedimentation, and regional exhumation in the region. Our data are consistent with a high relief landscape and orogen‐wide exhumation at ∼13–21 Ma and correlate with the timing of exhumation of the Pamir gneiss domes. This exhumation is younger in the Pamir than that observed in neighboring Tibet and is consistent with higher magnitude Cenozoic deformation and shortening in this part of the orogenic system.