Thesis

Detrital zircon geochronology of the Southern Tian Shan foreland and implications for Late Cenozoic tectonics of regional intracontinental orogenic systems, Northwestern China

The western Tarim Basin contains the sedimentary record of the adjacent Tian Shan and Pamir orogens. Diverse basement lithologies and mixing of sources here complicates the sedimentary signature of orogenic growth. To understand the southern Tian Shan foreland response to tectonic changes, we present new U/Pb detrital zircon (DZ) data and sandstone compositions from 9 samples collected within a ~4 km-thick, Miocene to Quaternary stratigraphic section to establish a paleogeographic reconstruction of the Southwestern Tian Shan, Eastern Pamir, and the Northern Tarim Basin during the Late Cenozoic. Bulk petrographic analysis of the nine sandstones reveals a recycled orogen signature for the section, with dominant sedimentary-metamorphic lithic compositions that exhibit little to no stratigraphic or depositional trends. Although the present position of this stratigraphic section implies the strata resulted from Miocene present uplift and erosion of the Tian Shan, the DZ data reveal a different history with a strong Pamir signal in the middle Miocene. Overall, three distinctive DZ spectra are observed that represent a change from dominantly Pamir sourced sediment at 14.3 Ma to exclusively Tian Shan sourced by 1.5 Ma. The apparent influx of Pamir sourced grains decreases up-section, except for the eolian strata at 11.5 Ma that contain anomalous DZ peak ages at ca. 12 Ma and 100 Ma, which are unique to the Pamir and must have been blown in from the southwest. Although it is difficult to completely distinguish Paleozoic DZ age-populations between the Pamir and Tian Shan, the presence of Cenozoic, Cretaceous, and Triassic grains in the lower part our section are distinctively Pamir sourced, revealing that sediment from the Pamir was actively being deposited in the Tian Shan Foreland from ~14 Ma until at least 7.6 Ma. Samples from 5.8, 3.6, and 1.5 Ma do not have any DZ peaks <200 Ma, suggesting a lack of Pamir influence. Our study reveals the complicated provenance signal from the amalgamated Tian Shan - Pamir foreland, highlighting the complexity and importance of resolving provenance from mixed, heterogeneous sources in intracontinental basin settings.

Relationships

Items