Contrasting exhumation histories and relief development within the Three Rivers Region (south-east Tibet)

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.

Farmland change and its implications in the Three River Region of Tibet during recent 20 years

“…The TRR has an area of 67,949 km 2 , involving three prefecture-level cities of Lhasa, Xigaze and Shannan, and 18 county-level administrative units. The region is acknowledged as the granary of Tibet [ 27 ], occupying 46.84% of the farmland area and producing 52.32% of the total grain output in 2018. The terrain comprises river valleys or basins with the altitude mostly at 2700–4600 m a.s.l.…”

“…in the middle, and high mountains in the north and south ( Fig 1 ). It has a semi-arid temperate monsoon climate, characterized by warm summer, and clod and dry winter [ 27 ]. The mean temperature for annual, July, and January in the valley areas is 4–9°C, 10–16°C, and -12–0°C, respectively.…”

Drivers of landscape evolution in eastern Tibet

“…Some authors have argued that this morphology is caused by incision into a uniform elevated pre-existing low-relief landscape (Clark et al, 2006;Whipple et al, 2017a;Yuan et al, 2022), whereas others claim its origin is due to river network reorganization driven by tectonic deformation, a process that might leave remnants of the original network isolated and starved of drainage area (Yang et al, 2015). Towards the Three Rivers region, active thrust faulting is thought to drive channel incision (Clark et al, 2004;Clark et al, 2005;Ou et al, 2021), but to the east, the contrasting gorges and low relief surfaces are not co-located with major faults (Whipple et al, 2017). One hypothesis is that this morphology is driven by channel incision signals propagating upstream from the tectonically active region (Harkins et al, 2007;Fan et al, 2021), which we aim to test in this study, as detailed below.…”

Multiple Effects of Topographic Factors on Spatio-temporal Variations of Vegetation Patterns in the Three Parallel Rivers Region, Southeast Tibet

“…Existing studies from the TPRR mainly focus on geology, dynamic changes of alpine vegetation, land use/cover changes, and biodiversity conservation [36][37][38][39], and tourism resources. Although few scholars have studied vegetation changes and its driving factors analysis in the whole basin of the TPRR, studies in various sub-basins (e.g., Lancang river and Jinsha river basins and etc.)…”