Abstract
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.
Highlights
Despite its high mean elevation of ∼ 5000 m, Tibet is mainly characterized by low relief (< 1 km), with an average slope of ∼ 5◦ (Fielding et al, 1994)
About 1 km of river incision is added to 2.5 km of regional uplift-driven exhumation, resulting in a total exhumation of ∼ 3.5 km since 10 Ma (Fig. 7). Using such a simplified topographic evolution scenario mimicking the incision of a plateau, we show that differential erosion between the lowrelief, mean-elevation Baima Xueshan massif and the deeply incised Mekong River valley is necessary to reproduce the break in slope in the apatite (U–Th) / He (AHe) age–elevation profile, between samples on the plateau and samples from the Mekong River valley (Fig. 3c)
Moraines, and U-shaped valleys are observed across the Baima Xueshan massif, providing evidence for significant glacial erosion (Fig. 8). Based on this observation as well as the maximum elevation of the massif (5400 m), which is close to the present-day equilibrium line altitude (ELA) (5400 m), we suggest that buzzsaw-like glacial processes could be active in the Baima Xueshan massif, such that any tectonic uplift bringing the elevation of the plateau above the ELA could trigger glacial processes that would erode and smooth these highlands, as previously proposed by Zhang et al (2016)
Summary
Despite its high mean elevation of ∼ 5000 m, Tibet is mainly characterized by low relief (< 1 km), with an average slope of ∼ 5◦ (Fielding et al, 1994). The narrow spacing between the rivers has been interpreted as resulting from strong lateral shortening in response to indentation of the Indian-plate corner (Hallet and Molnar, 2001; Yang et al, 2015) This transition region, separating areas without low-relief surfaces to the west and south-west from areas with extensive low-relief surfaces to the east and north, is key to better understanding plateau-growth mechanisms and the geodynamic processes operating both within the high strain zone around the syntaxis and within the lower strain zone of the Yunnan and Sichuan margin (Fig. 1b). We quantify the exhumation history of the low-relief Baima Xueshan massif during the collision period to identify the different roles played by regional rock uplift and river incision, and we compare it with the exhumation history of the high-relief Kawagebo massif, while constraining the geometry of the crustal fault responsible for uplifting the latter
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