Detrital zircon U-Pb provenance study in Cenozoic strata of the Pamir-Tianshan collision zone
Abstract:Sediments in the foreland basin of the Pamir-Tianshan collision zone provide a rich record of the Cenozoic collision between the Pamir and the Tianshan. Recently, several authors studied the provenance of sediments in the foreland of the Pamir and Tian Shan using detrital zircon U-Pb dating (Bershaw et al., 2012；Sun et al., 2012; Yang et al., 2013). Statistical analysis of bedrock U-Pb ages from the Pamir and Tianshan (e.g. Schwab et al., 2004; Lukens et al., 2012) yield distinct age spectra and indicate that almost all the ages from Pamir are no more than 250 Ma, whereas almost all the ages from Tianshan are more than 250 Ma. Additional provenance studies from different sedimentary sections will allow a more detailed description of collision processes.
We collected 13 sediment samples from major streams draining the Pamir and the Tianshan on the western margin of the Tarim Basin to analyze the modern detrital zircon signature of the two orogens. We also collected 35 samples for provenance analysis from the Cenozoic sediments of Bieertuokuoyi section which crops out in the hanging wall of Pamir Frontal Thrust (PFT).
Preliminary results from 4 modern streams draining the Tianshan show two prominent age peaks at 240-300 Ma and 400-490 Ma. Two modern samples from rivers draining the Pamir have prominent peaks at 230-270 Ma and 280-330 Ma.
Each of the 10 samples from the Bieertuokuoyi section that has been analyzed yields U-Pb ages that are consistent with a mixed provenance from the Tianshan and the Pamir, but the relative contribution of two orogens changes throughout the section. In the late Paleocene a higher contribution comes from the Tianshan followed by a dominant middle Pamir signature during the early-middle Eocene. Minimum U-Pb ages constrain the depositional age of this part of the section to less than~47 Ma. We interpret contrasting provenance signatures as representing changes in the relative rate of rock uplift and denudation of these orogens. In the Late Eocene and the Middle Miocene, age spectra are suggestive of increased activity in the northern Pamir with an intervening quiet period from Oligocene through Early Miocene. In the Late Miocene, thrusting in the hanging wall of PFT begins and the Tianshan starts to deform rapidly.