ACTIVE FAULTING IN THE TIEN SHAN, A CONSISTENT STORY OVER 12 MILLION YEARS

The Tien Shan are young and tectonically active mountains, dominating the landscape of Kyrgyzstan. Faults bounding and dissecting the Tien Shan produce significant modern seismic hazard from active faulting, with a historic record including M >8.0 earthquakes. While the modern rate of shortening, as shown by geodetic measurements, is quite high, debates exists across the literature if this rate of shortening is consistent throughout the tectonic history of the range. We present new paleomagnetic data, which we calibrated with the first systematic vertebrate biostratigraphy for the Cenozoic of Kyrgyzstan, and reevaluate the conclusions of several previous studies in the context of the newer, higher resolution geochronology. Our findings suggest a young initiation of uplift, at maximum 12 Ma, with ~3 mm/yr rate of shortening in the Kochkor Basin, similar to the ~3-5mm/yr seen in modern rates across the Tien Shan. Both the new paleomagnetic data, and the vertebrate fossils are from the Kochkor Basin, where previous studies estimated 5.5-7.9 km of shortening. We found four mass-death assemblages of Neogene fossil vertebrates, which added some vital age control. All fossil sites contain Hipparion horse fossils, a taxon not found in Eurasia until 12 Ma. Additionally, several sites contain Paleolagus, a fossil rabbit, further limiting the maximum age those fossiliferous strata to <7 Ma. These taxa, and many additional ones such as giraffids, hyenas, and modern-like deer, allowing for few possible correlations to a global standard for the paleomagnetic data. Our new interpretation from the expanded exiting paleomagnetic data and new samples, the major faults in the Kochkor Basin appear to have shortened at a rate of ~2-3mm/yr throughout their tectonic history, with uplift initiating approximately 11 Ma. Preliminary fossil collection in the Issyk Kul basin, as well as our reinterpretation of previous studies given the age new constraints from the recently connected Kochkor basin, also indicate a similar, high rate of shortening. We conclude that the Tien Shan initiated uplift well into the Neogene, and the rate of shortening has been temporally and geographically consistent across the range.