Abstract. The Three Rivers Region in south-east Tibet represents a transition
between the strongly deformed zone around the Eastern Himalayan Syntaxis (EHS)
and the less deformed south-east Tibetan Plateau margin in Yunnan and
Sichuan. In this study, we compile and model published thermochronometric
ages for two massifs facing each other across the Mekong River in the core
of the Three Rivers Region (TRR), using the thermo-kinematic code Pecube to
constrain their exhumation and relief history. Modelling results for the
low-relief (< 600 m), moderate-elevation (∼ 4500 m)
Baima Xueshan 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. Estimated Mekong
River incision accounts for 30 % of the total exhumation since 10 Ma. We
interpret exhumation of the massif as a response to regional uplift around
the EHS and conclude that the low relief of the massif was acquired at high
elevation (> 4500 m), probably in part due to glacial
“buzzsaw-like” processes active at such high elevation and particularly
efficient during Quaternary glaciations. Exhumation of the Baima Xueshan is
significantly higher (2.5 km since ∼ 10 Ma) than that
estimated for the most emblematic low-relief “relict” surfaces of eastern
Tibet, where apatite (U–Th) / He (AHe) ages > 50 Ma imply only a
few hundreds of metres of exhumation since the onset of the India–Asia
collision. The low-relief Baima Xueshan massif, with its younger AHe ages
(< 50 Ma) that record significant rock uplift and exhumation, thus
cannot be classified as a relict surface. Modelling results for the
high-relief, high-elevation Kawagebo massif, to the west of the Mekong,
imply a similar contribution of Mekong River incision (25 %) 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 thermochronometric ages are best reproduced by a model of 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 of two massifs are linked to
variable exhumation histories due to strongly differing tectonic imprint.