The Zedong Window: A record of superposed Tertiary convergence in southeastern Tibet

Determining the timing, magnitude, and location of deformation due to the Indo-Asian collision is widely acknowledged as an important step in understanding how the lithosphere responds during continental collision. A puzzling result of geological investigations of the Lhasa Block over the past 2 decades has been the apparent lack of significant Tertiary deformation there. Perhaps the most important structural feature of the Lhasa Block is the south directed Gangdese Thrust System, which developed along its southern edge. The thrust system, which separates the Andean-type batholith of southern Asia from rocks of Indian affinity, is obscured at most locations across southeastern Tibet by back thrusts of the younger, north directed Renbu Zedong Thrust System. The best documented site where both thrusts are exposed occurs near Zedong (Zedong Window). Systematic geochronologic analyses were conducted in this area. UPb zircon dating of three samples of a synkinematically to postkinematically deformed hanging wall granitoid (the Yaja granodiorite) cut by the Gangdese Thrust indicates a crystallization age of 30.4±0.4 Ma (2σ), thus placing an upper bound on the initiation of the thrust. U-Pb zircon dating of granitoid samples structurally higher in the Gangdese hanging wall yields emplacement ages of 42.5±1.0 Ma and 48.9±0.8 Ma (2σ), similar to other magmatic complexes within the Gangdese arc. Geochemical results are consistent with these plutons forming in the same environment as precollisional intrusions within the Gangdese batholith, suggesting a significant postcollisional input of juvenile heat. The Ar/Ar thermochronology of samples from a vertical section through the hanging wall within the Yaja granodiorite, coupled with results of a numerical thermal model, indicate an average slip rate along the Gangdese Thrust of 7 mm/yr between 30 and 23 Ma and a minimum displacement of ~50 km. Farther east in the Zedong Window, thermal effects produced by later north directed thrust sheets of the Renbu Zedong system appear to have obscured thermal history signatures in the Gangdese hanging wall related to earlier south directed thrusting. Thermochronological results from this region indicate that thermal overprinting related to the north directed thrusting occurred between 25 and 10 Ma, consistent with previous estimates.