Cenozoic growth of the Tibetan Plateau: insights from exhumation and geomorphology along the eastern margin

In the nearly 50 years since Paul Tapponnier first recognized that eastward motion of Asian lithosphere played a key role in accommodating ongoing convergence between India and Eurasia (Molnar and Tapponnier, 1975, Science), the debate over the mechanical processes governing plateau growth have been a source of both inspiration and controversy.  Along the eastern margin of the Tibetan Plateau, adjacent to and north of the Sichuan Basin, a robust debate continues as to whether massifs substantially elevated above the plateau interior developed largely along upper-crustal faults or, alternatively, were built by flow and thickening in the lower crust.  We explore constraints on the timing, rates, and patterns of mountain building along the eastern margin of the Tibetan Plateau provided by over two decades of thermochronologic and geomorphologic studies.  Early models attributed mountain building along the plateau margin to extrusion along the left-lateral Kunlun fault; however, recent work has shown that slip along the Kunlun fault dies out eastward and is absorbed by deformation and rotation about the fault tip.  Rates of shortening across the plateau margin in the Longmen Shan region are low (< 1-2 mm/yr), but multi-thermochronometer relief transects (age-elevation) from three separate localities across the plateau margin imply that moderate to high rates of rock uplift and exhumation have been sustained along the plateau margin since ~30 Ma.  Forward modeling of the thermal response to exhumation reveals details of spatial differences in the exhumation history.  In the Pengguan Massif, immediately adjacent to the Sichuan Basin, these data require a two-phase exhumation history, separated by a hiatus or significant reduction in exhumation rate (Wang et al., 2012).  To the west, however, in the Xuelongbao Massif, new thermochronologic data require continuous (but temporally variable) exhumation rates >500 m/Myr during the entire late Cenozoic (Furlong et al., 2021).  Such rapid, localized exhumation coincident with high relief along the plateau margin requires a sustained influx of crustal mass at depth.

North of the Sichuan Basin, the topographic margin of the plateau is defined by the Min Shan.  The lack of a direct association between topography and upper crustal faults affords an opportunity to evaluate the patterns of differential rock uplift in the absence of inherited crustal anisotropy.  Here, correlations among topography, channel steepness, and erosion rate indicate a locus of moderate (300-500 m/Myr) erosion rate coincident with the Min Shan.  Fluvial incision rates inferred from dated strath terraces along the Bailong Jiang confirm spatial gradients in fluvial incision, with the highest incision rates (1000-2000 m/Myr) localized along the axis of the range.  This locus of incision has been sustained for 80-100 ka, and we interpret it to reflect differential rock uplift along the plateau margin.  The wavelength of rock uplift is consistent with thickening in the deep crust.  Collectively, the spatial patterns and rates of exhumation and erosion along the eastern margin of the plateau suggest that crustal thickening in the deep crust is ongoing today and may have been sustained since the late Oligocene.