THERMOCHRONOLOGIC AND GEOCHRONOLOGIC CONSTRAINTS ON THE TIMING OF CRUSTAL SHORTENING AND THE INITIATION OF LEFT-LATERAL SHEAR WITHIN THE CENTRAL KUNLUN SHAN, NORTHERN TIBET: IMPLICATIONS FOR THE UPLIFT HISTORY OF THE TIBETAN PLATEAU

The transition from crustal shortening to strike-slip faulting along the Kunlun Shan may specifically constrain the time of high elevation attainment of the Tibetan Plateau. However, attributing episodes of crustal exhumation to either compressional or translational tectonics along the northern plateau margin has remained enigmatic. To address this outstanding question, we investigated structures along the active Xidatan fault strand of the Kunlun strike-slip fault. In this work, we present new apatite (U-Th)/He, apatite fission-track, zircon (U-Th)/He data and QTQt thermal modeling results, as well as new 40Ar/39Ar fault gouge ages. Paired with structural observations, these data constrains the tectonic history along the central Kunlun Shan.

We measured thermochronologic ages throughout the Dongdatan Valley region, east of Kunlun Pass. Thermochronologic data most germane to understanding the Cenozoic history of this region are from terrestrial sedimentary strata of presumed mid-late Cretaceous or early Cenozoic age and from transpressional knots along the Kunlun strike-slip faults. The terrestrial strata were progressively buried until 51–45 Ma to depths capable of resetting AFT ages and were subsequently exhumed. Fault gouge ages of 47 Ma data indicate active deformation at that time. Similar AHe and AFT ages on either side of observed thrust faults suggest that the faults were no longer active by 43 Ma. Together, these data indicate crustal shortening ceased between 47 and 43 Ma, followed by regional exhumation to present levels of exposure. Conversely, samples collected in transpressional portions of the Xidatan strike-slip fault reveal rapid exhumation at 22–19 Ma.

Combined with published data, these results suggest early Eocene shortening in the Dongdatan Valley that transitioned to the northern Kunlun Shan by 35 Ma. The onset of left-lateral shear along the Xidatan fault by 22–19 Ma, along with extension in central and southern Tibet by 18 Ma and lower crustal flow in eastern Tibet by 13 Ma, suggests that east-west oriented extension and extrusion initiated in the mid Miocene across the orogen. The plateau-wide shift in stress accommodation indicates that high elevation and gravitational potential energy may have been attained by middle Miocene time.