From basal shear to left-slip: two-stage evolution of the Cenozoic Altyn Tagh Fault evidenced by sedimentary and structural features of the Qaidam Basin, NW China

The ~1600km long active Altyn Tagh Fault (ATF) defines the northern edge of the Tibetan plateau, NW China. As one of the most important boundary factors constraining the timing, mechanism and stress dissipation of the Tibetan uplift, aspects of its Cenozoic activities including its inception time, total offset and slip rates have been attracting hot debates. In this contribution, however, we concern a topic less discussed but crucially important in deciphering the Cenozoic evolution of the ATF: whether the ATF has been all along a left-slip strike-slip fault since its initiation during late Eocene? Based on systematically study on the sedimentary and structural features of the NW Qaidam Basin located at the southern side of the central segment of the ATF, together with geomorphological analysis of the Altyn Mountain, we find that: 1. The Altyn Mountain started to uplift at ca. 36Ma, while left-slip along the ATF commenced 21 million years later, at ca. 15Ma. 2. Uplifting effect of the Altyn Mountain during ca. 36 - 15Ma reached as far as 100km into the Qaidam Basin interior and was featured by a set of en echelon EW-trending uplifts; while that since ca. 15Ma reached only less than 20km following a local unconformity along the southern side of the ATF. 3. The uplifting rates of the Altyn Mountain accelerated through time during ca. 36 - 15Ma, but fluctuated since ca. 15Ma. 4. Northwestern Qaidam Basin accumulated coarse sediments from the Altyn Mountain since ca. 36Ma. However, their provenance changed from a uniform Mesozoic source to a mixed and changeable Mesozoic - Proterozoic source at ca. 15Ma. 5. The Altyn Mountain is made up of 3 types of uplifts: NEE-trending uplift parallel to the ATF, NE-trending uplifts situated mainly in the southwestern Altyn Mountain and EW-trending uplifts en-echelon configured at the both side of the ATF. These different uplifts formed in different time and tectonic settings, and are indicative for different types of activities of the ATF during the Cenozoic. We thus propose a two-stage evolution model for the Cenozoic ATF. During the first stage of ca. 36 - 15Ma, the ATF was a basal shear zone confined in the middle - lower crust, creating obvious surface uplift of the Altyn Mountain that was characterized by EW-trending en-echelon folds and faults. The extent of the Altyn Mountain might be much larger than the present as the shear stress was mostly absorbed by the uplift alone. During the second stage since ca. 15Ma, the crust beneath the Altyn Mountain was eventually split due to persisting push from the south, and left-slip motion along the ATF initiated successively, cutting the early folds and faults and shaping the Altyn Mountain to its present fabric. Extent of the Altyn Mountain greatly reduced as left-slip along the fault absorbed most of the stress from the south at the time.