BIOSTRATIGRAPHICALLY CALIBRATED PALEOMAGNETOSTRATIGRAPHY IN THE TIEN SHAN OF KYRGYZSTAN

The intermontane basins of the high Tien Shan of Central Asia hold clues to the timeline and processes of continental collisions better than possibly anywhere in the world... if you can date them. With basin-filling sequences spanning the Eocene to present, incredible rates of uplift and structural shortening, and fantastic exposure, the record of deformation lies in the basins. However, Central Asia has maintained its landlocked status, lacking any Cenozoic or late Mesozoic sediments, placing it far from convenient sources of volcanism on continental margins. To alleviate this lack of radiometrically datable material, both thermochronology and magnetostratigraphy are applied. Problematically the geochronology proposed from these methods over the past 70 years varies from an initiation of deformation from the late Eocene to the Middle Miocene, with published dates on the same section on the southern shores of lake Issyk Kul varying by greater than 30 million years. Validity or interpretation of some thermochronology data is questioned owing to possibly dating collision-related magmatic events. Magnetostratigraphy is also not without issue, as previous biostratigraphic constraints were limited. While hardly a new technique, we are utilizing biostratigraphy as calibration points for newly collected and newly reinterpreted paleomagnetostratigraphy across three large intermontane basins in Kyrgyzstan. While much of this biostratigraphic data currently rests on generic-level identification owing to a high degree of endemism, this still serves to reduce uncertainty by tens of millions of years in some cases. Furthermore, newly measured stratigraphic section allow for detailed estimates of sedimentation rates as another check. Overall, we find that uplift originates younger than previously proposed, with the oldest portions of the uplift-driven basin-filling sequences falling around 12 Ma. Our rate of shortening matches that of present times, with Neogene sedimentation rates also matching current rates. Faunal turnover seems driven by tectonically caused climate change, as topographic remodeling of the region changes local ecosystems.