GEOCHRONOLOGICAL EVALUATION OF NEOGENE SYNTECTONIC SEDIMENTS FROM THE KOCHKOR BASIN, KYRGYZSTAN

The Kyrgyz Tien Shan are some of the most rapidly uplifting mountains, resulting in high seismic hazard and unique alpine ecosystems. While the high uplift and shortening rates seen today signal a young mountain range, estimates for the initiation and rate of uplift vary across the published literature. In most regions, this would be the simple, if expensive, question of radiometric dating of volcanic rocks. However, Kyrgyzstan lies thousands of miles from the nearest volcanic center and was last on a continental margin in the Palaeozoic. As a result, previous geochronology is limited to unconstrained paleomagnetostratigraphy and unsubstantiated biostratigraphy. Herein we present not only four new paleomagnetic sections and recalibration of some existing paleomagnetic sections, but the first age control provided by biostratigraphy of newly described Neogene mammal faunas. The faunas associated with the syntectonic basin-filling sequences included in this study are closely related to the Chinese Hipparion faunas of latest Miocene age. Our measured sections span several million years, ranging from the stratigraphically oldest Shamsi Formation, through the finer grained Chu Formation, and into the congolmeratic Sharpyldak Formation. Previous workers have inferred both formational boundaries to be tectonically driven, resulting from increased rates of uplift accommodated along roughly east-west trending thrust faults forming the Tien Shan. We instead infer the formation boundaries to be climatically driven, with the Shamsi/Chu boundary occurring gradationally about 8Ma, resulting from the uplift of the Pamirs changing regional monsoonal patterns, and the Chu/Sharpyldak boundary representing the onset of Pleistocene glaciation 2.6Ma. Fossils occur in productive bone beds, giving biostratigraphic age estimates for narrow stratigraphic ranges. Unfortunately, the limited previous work in the region paleontologically is confounded by a high degree of endemic taxa. Therefore, most geochronological estimates from the fauna alone occur at a generic level. Several key marker taxa exist, with Hipparion not know from Asia till 12Ma, and not widespread till 10Ma. Other key taxa included true rabbits, which are not known till 7Ma, dating the bone beds to the latest Miocene, 10-6Ma.