Late Paleozoic and Mesozoic thermotectonic evolution of the north-eastern margin of the Tianshan (Kazakhstan, NW China), and investigations of apatite detrital provenance methods

Abstract

Intracontinental deformation and orogenesis is an understudied but important aspect of the Earth system. The Central Asian Orogenic Belt is a natural laboratory for the study of intracontinental mountain-building, with the major Tianshan and Altai mountain belts as the overwhelming focus of previous thermochronological studies in the area. However, these studies tend to investigate primarily the most recent phase of deformation that occurred in response to the Cenozoic collision of India with Eurasia. This thesis presents studies of areas in Central Asia that did not experience large amounts of reworking during the Cenozoic, and correspondingly preserve less disturbed thermochronological information about the thermotectonic history of the region prior to the late Cenozoic. The thermal history of late Paleozoic and early Mesozoic deformation in the Eurasian continental interior is well preserved in the West Junggar and Tarbagatai Mountains, which bridge the gap between the Tianshan and Altai. The West Junggar Mountains record an episode of rapid cooling in the Early– Middle Permian, followed by an extended period of tectonic quiescence. The Tarbagatai Mountains, to the west of the West Junggar, record a more complex history. Middle–Late Permian rapid cooling is preserved in some regions of the study area, but is largely overprinted by a Late Triassic cooling event. Permian rapid cooling in both these areas is related to the deformation of Central Asia following the amalgamation of Kazakhstan with the Siberian active margin to the north and the collision of the Tarim block to the south. The development of large strike-slip structures during this widespread Permian deformation had an important effect on the subsequent history of these regions. The spatial extent of Late Triassic exhumation in the Tarbagatai Mountains was controlled by the reactivation of these structures. Limited Jurassic–Cretaceous reactivation was similarly accommodated by the reactivation of late Paleozoic strikeslip structures. The orientation of these structures may be an important control on their later reactivation. Early Cretaceous cooling along the NW–SE striking Chingiz-Tarbagtai Fault is recorded in apatite fission track and (U-Th-Sm)/He data. In constrast, the anomalous NE–SW strike of faults in the West Junggar compared to other major structures in the Kazkhstan Orocline may explain why this area is tectonically stable relative to its surroundings. The easternmost Tianshan experienced significant reworking during the late Mesozoic and Cenozoic. The thermal history of this area is more similar to the other ranges of the Tianshan than to the West Junggar or Tarbagatai Mountains. Triassic rapid cooling signals are limited to areas outside the major range-bounding faults, common Jurassic–Cretaceous ages reflect extended residence in the partial annealing zone before rapid cooling in the Late Cretaceous, and no Cenozoic fission track ages were obtained. The use of single grain apatite U–Pb and trace element analysis to understand the provenance of sediment was also investigated in this thesis, developing capabilities for more representative detrital geochronology studies in the future.