A high-resolution geochemical record from the Kuche depression: Constraints on early Miocene uplift of South Tian Shan

The building of Tian Shan is often simplified to a Cenozoic reactivation of a Paleozoic fold belt. During the past decades extensive case studies have arrived at the conclusion that Tian Shan experienced diachronous uplift during the Cenozoic, however, understanding of the specific location and duration of the mountain building is still hindered by complex tectonic history of the orogenic belt. Considering the geochemical differences of various tectonic units within the mountains, we studied the chemical composition variability for the carbonate-free fine-grained samples collected from the longest terrestrial outcrop in the Kuche depression, southern Tian Shan, at a high resolution in order to better constrain the orogenic process. Our data show significant increases in mobile elements during the interval of 23-17 Ma, departing substantially from the stable long-term weathering trends and limited sedimentary sorting effects. We interpret the trend as provenance changes, which provide crucial evidence to support that the South Tian Shan experienced significant surface uplift during the early Miocene. This argument is supported by other studies conducted on both local and regional scales. Mineralogical analyses for the study section have shown that, since the early Neogene, a marked increase of detrital limeclasts derived from the South Tian Shan is accompanied by the decreases of the abundance of garnets derived from an ultrahigh-pressure metamorphic fold belt in the central Tian Shan. Moreover, low-temperature thermochronological results revealed that the central Tian Shan merely experienced pre-Neogene cooling, contrasting to the southern piedmont of the mountain where Neogene cooling dominated. Taken all together, we conclude that the uplift of the South Tian Shan initiated at ~ 23 Ma and the paleoelevation may exceed the height of the central Tian Shan at ~ 17 Ma.