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

Determining the timing, magnitude, and location of deformation due to the I ndo-Asian collision is widely acknowledged as an important step in understa nding how the lithosphere responds during continental collision. A puzzling result of geological investigations of the Lhasa Block over the past 2 dec ades has been the apparent lack of significant Tertiary deformation there. Perhaps the most important structural feature of the Lhasa Block is the sou th 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 so utheastern Tibet by back thrusts of the younger, north directed Renbu Zedon g Thrust System. The best documented site where both thrusts are exposed oc curs near Zedong (Zedong Window). Systematic geochronologic analyses were c onducted in this area. U-Pb zircon dating of three samples of a synkinemati cally to postkinematically deformed hanging wall granitoid (the Yaja granod iorite) cut by the Gangdese Thrust indicates a crystallization age of 30.4/-0.4 Ma (2 sigma), thus placing an upper bound on the initiation of the th rust, U-Pb zircon dating of granitoid samples structurally higher in the Ga ngdese hanging wall yields emplacement ages of 42.5+/-1.0 Ma and 48.9+/-0.8 Ma (2 sigma), similar to other magmatic complexes within the Gangdese are. Geochemical results are consistent with these plutons forming in the same environment as precollisional intrusions within the Gangdese batholith, sug gesting a significant post-collisional input of juvenile heat. The Ar-40/Ar -39 thermochronology of samples from a vertical section through the hanging wall within the Yaja granodiorite, coupled with results of a numerical the rmal model, indicate an average slip rate along the Gangdese Thrust of 7 mm /yr between 30 and 23 Ma and a minimum displacement of similar to 50 km, Fa rther east in the Zedong: Window, thermal effects produced by later north d irected thrust sheets of the Renbu Zedong system appear to have obscured th ermal history signatures in the Gangdese hanging wall related to earlier so uth directed thrusting. Thermochronological results from this region indica te that thermal overprinting related to the north directed thrusting occurr ed between 25 and 10 Ma, consistent with previous estimates.