Cenozoic thermo-tectonic evolution of the northeastern Pamir revealed by zircon and apatite fission-track thermochronology

Abstract

The northeastern Pamir is a key location to explore Asian intracontinental tectonic processes during the Cenozoic. New zircon fission-track (ZFT) data show a 20- to 50-km-wide region of partially reset ages on the northeastern margin of the Pamir salient, interpreted as an exhumed and tilted partial annealing zone (PAZ). Widespread ZFT age peaks at ~50Ma within the ZFT PAZ likely date early motion of the Kashgar–Yecheng transfer system (KYTS), but suggest this fault system was narrower in the Early Cenozoic than it is today. Apatite fission-track (AFT) ages of ~10–6Ma, combined with field observations across the KYTS, hint at an episode of strong thrusting-related exhumational cooling, which indicates that the modern fault system probably formed at this time. To the southwest of the KYTS, the combination of new fission-track and existing thermochronology data allows establishing temperature-time trajectories that present diachronous rapid cooling from ~450 to 120°C in the Sares (>13–10Ma), Muztagata (~10–7Ma) and Kongur Shan (~3–1Ma) domes. Rapid cooling in the eastern Sares and southern Muztagata massifs is driven by doming, as supported by kinematic analyses of the Shen-ti fault. Successive rapid cooling of these massifs confirms eastward propagation of doming processes, shortly postdating magma emplacement at ~11Ma. We propose that the synchronicity of regional tectonism, magmatism and metamorphism implies that strong crustal thickening and contraction occurred beneath the northeastern Pamir during the Middle–Late Miocene, possibly associated with initial collision between the Pamir and Tian Shan.