Mesozoic – Cenozoic tectonic evolution of southwestern Tian Shan: Evidence from detrital zircon U/Pb and apatite fission track ages of the Ulugqat area, Northwest China

The Late Tertiary tectonic and topographic evolution of the Tian Shan Range has been widely studied as it represents a key example of active intra-continental mountain belts. Recent studies have shown that both the general tectonic framework of Tian Shan and some of its actual topographic features were inherited from the still poorly constrained Late Paleozoic–Mesozoic evolution of the range. In addition, better understanding of the tectonic and topographic evolution of the area before the last phases of late Cenozoic deformation is required to constrain the unresolved climatic and paleogeographic reconstructions. We present here U/Pb (LA-ICP-MS) dating of detrital zircons and apatite fission track analysis on detrital apatites from the exceptionally well-exposed Jurassic to Cenozoic sediment series of the still poorly constrained southwestern Tian Shan piedmont to investigate changes in sediment provenance through time. The U/Pb detrital zircon ages range widely from 222 to 3179 Ma and can be statistically separated in four main groups: 240–320 Ma, 400–540 Ma, 550–1600 Ma and 1640–2800 Ma. These zircons were derived from the Tian Shan area to the north and from recycling of the Paleozoic North Tarim margin. The detrital apatite fission track ages encompass sources with preserved Mesozoic ages as well as much younger sources exhumed during middle Miocene times. Combined together those data show a general planation of the range from Middle Jurassic to Late Cretaceous associated to a wide drainage system. The progressive decrease in the variety of sources through the Mesozoic is consistent with burying of the basement exposures by sediments. Detrital zircon U/Pb data indicate an initial Tertiary uplift of the southern Tian Shan piedmont around Eocene times and a possible activation of the Talas Fergana Fault between 18 and 16 Ma.