Focused Pliocene–Quaternary exhumation of the Eastern Pamir domes, western China

Abstract

The Kongur Shan and Muztagata domes are the most prominent topographic features in the Eastern Pamir indenter corner. In spite of several previous bedrock thermochronology studies, the extent, processes and mechanisms of dome exhumation remain unclear. We present 1158 detrital zircon fission-track (DZFT) ages from modern glacier and river sediments along the domes to better constrain the spatial distribution of exhumation. Our thermochronologic dataset from the Chakragil massif shows a sharp increase in the proportion of DZFT ages >10Ma from basins that drain the peak region to those further to the north, which suggests that the Kalagile fault forms the northern boundary of the rapidly exhuming dome structure. A cluster of Middle Miocene DZFT ages in these samples documents pre-doming cooling, possibly associated with motion on the Main Pamir thrust and/or Oytag fault. Large fractions of DZFT grain ages <10Ma in samples from catchments east of the divide likely originate from the Chakragil, Kongur Shan and Muztagata massifs, implying focused exhumation consistent with DZFT age distributions in samples from catchments draining the western flank of the range and bedrock geo- and thermochronologic data from the core of the domes. Furthermore, the majority of the samples contain two distinct DZFT age groups at ∼6–4Ma and ∼3–1Ma respectively. These ages are synchronous with major fluvial sedimentation phases at the outlets of the rivers draining northeastward into the Tarim Basin and suggest rapid Pliocene–Quaternary exhumation of the domes. We interpret the ∼6–4Ma age group to record the onset of extension along the entire Kongur Shan fault; its timing with respect to compressional deformation supports a model of gravitational collapse of over-thickened crust. The fraction of ∼3–1Ma DZFT ages is larger than what would be inferred from the bedrock data alone. We interpret this to reflect recent focused denudation in response to glaciation and fluvial incision of the domes and their possible feedback with extensional faulting.