Contrasting exhumation histories and relief development within the Three Rivers Region (Southeast Tibet)

Abstract. The Three Rivers Region in Southeast Tibet represents a transition between the strongly deformed zone around Eastern Himalayan Syntaxis and the less deformed southeast Tibetan plateau margin in Yunnan and Sichuan. In this study, we compile and model published thermochronologic ages for two massifs facing each other across the Mekong River in the core of the Three Rivers Region, by using the thermo-kinematic code Pecube to constrain their exhumation and relief development history. Modelling results for the low-relief, mean-elevation BaimaXueshan massif, east of the Mekong River, suggest regional rock uplift at a rate of 0.25 km/Myr since ~ 10 Ma, following slow exhumation at a rate of 0.01 km/Myr since at least 22 Ma. River incision accounts for only 15 % of the total exhumation in the BaimaXueshan. Exhumation since ~ 10 Ma is significantly higher (2.5 km) than that estimated (~ 0.23 km) for the most emblematic low-relief or relict surfaces of Eastern Tibet, which are characterized by apatite (U-Th)/He ages older than the collision age (> 50 Ma). We conclude that the BaimaXueshan massif, which shows younger ages ( relict surface despite its low relief. Modelling results for the high-relief, high-elevation Kawagebo massif, to the west of the Mekong, imply a similar contribution of Mekong River incision (20 %) to exhumation, but much stronger local rock uplift at a rate of 0.45 km/Myr since at least 10 Ma, accelerating to 1.86 km/Myr since 1.6 Ma. We show that the age-elevation profiles for three thermochronometers are best modeled by rock uplift on a kinked westward-dipping thrust striking roughly parallel to the Mekong River, with a steep shallow segment flattening out at depth. Thus, the strong differences in elevation and relief that characterize these massifs are linked to variable exhumation histories due to a strongly differing tectonic imprint.