INTERPRETING COMPLEX DETRITAL ZIRCON SPECTRA FROM INTRACONTINENTAL FORELAND BASINS: AN EXAMPLE FROM THE ACTIVE TIAN SHAN, WESTERN CHINA

The western Tarim Basin contains the sedimentary record of the adjacent Tian Shan and Pamir orogens, which are ideal modern analogs for the Ancestral Rocky Mountains. Orogenic growth, as recorded in foreland basin strata, is complicated by the diversity of basement lithologies in source areas and by the mixing of sources. To understand the southern Tian Shan foreland response to climatic and tectonic changes, we present new U/Pb detrital zircon (DZ) data and bulk-rock sandstone compositions from 9 samples collected within a ~4 km, Miocene to Quaternary stratigraphic section. Although the present position of the 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, six distinct DZ spectra are recognized 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 newly-recognized 11.5 Ma eolian strata that contain anomalous DZ peak ages at ca. 12 Ma and 100 Ma that have no Tian Shan analog and thus are interpreted to have been blown in from the Pamir region. Although it is difficult to distinguish Paleozoic DZ age-populations between the Pamir and Tian Shan, the presence of Cretaceous and Cenozoic grains in our section are distinctively Pamir-sourced, and reveal that sediment from the Pamir was actively being deposited in the Tian Shan foreland until at least 7.6 Ma. Samples collected from Plio-Quaternary sandstones (5.8 – 1.5 Ma) do not have any DZ peaks <200 Ma, suggesting a lack of Pamir influence. During Plio-Quaternary time, there is documented deformation in the Tian Shan foreland basin, implying that the depo-center may have moved southward and inhibited northward migrating, Pamir-sourced sediment from reaching these locations. 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.