HOW USEFUL ARE FORELAND BASIN STRATA FOR INTERPRETING TECTONICS?– AN EXAMPLE FROM THE SOUTHERN TIAN SHAN FORELAND, CHINA

Foreland basin strata are commonly used to infer the geologic history of an adjacent orogen, but this view is limited, as they can include sediment derived from distal sources. To understand the significance of stratigraphic changes within the tectonically active southern Tian Shan foreland, we present new detrital zircon data (n=839), carbonate oxygen (δ¹⁸O_c) and carbon (δ¹³C_c) stable isotope compositions (n=368), structural modeling, and stratigraphic observations from a 3800 m-thick section that is dated between 17.5-0.8 Ma. Dramatic changes in depositional environments occurred at 13.6, 13.3, 12.2, 7.0, and 2.2 Ma, although detrital zircon (DZ) data define only one obvious change in provenance from a mixed orogen source (Kunlun Shan, Pamir, and Tian Shan) to a uniquely Tian Shan source at ca. 6 Ma. δ¹⁸O_c from carbonates indicate a negative 1‰ shift (-7.8±0.4‰ to -8.7±0.7‰) at 12 Ma, and return to more positive values (-7.2±0.8‰) after 4.2 Ma. δ¹³C_c values remain relatively stable (-0.8‰) until 5 Ma, when values increase to ~0‰. These three independent datasets suggest that from 15-12 Ma, the area was dominated by low gradient meandering rivers and saline lakes, with mixed DZ provenance from the Pamir, Kunlun Shan and interior Tian Shan north delivered via large, trans-continental river systems. From 12-7 Ma, dune deposition resulted from aridification in the rainshadow of the Pamir-Tian Shan nexus, which blocked westerly systems into the Tarim Basin and caused dune deposition against the northernmost foreland basin structures. The ~1‰ decrease in δ¹⁸O_c. resulted from the addition of depleted waters on the eastern and southern margins of the Pamir and Tian Shan, respectively, consistent with DZ provenance data. After 7 Ma, dune deposition was replaced by southward fluvial systems as deformation within the southern Tian Shan foreland migrated southward. Shortening along two frontal foreland faults initiated between 6.0-3.5 Ma, which isolated our study area from the Tarim Basin and resulted in positive δ¹³C_c excursions to values close to 0‰ and unique DZ spectra. Both δ¹³C_c and DZ data after 3.5 Ma reflect exhumation in the Tian Shan and recycling of Paleozoic carbonates rocks. This study highlights the complex relationship between climate, tectonics, and sedimentary transport within foreland basins.