In situ development of high-elevation, low-relief landscapes via duplex deformation in the Eastern Himalayan hinterland, Bhutan

Prior studies have proposed tectonic and climatic mechanisms to explain surface uplift throughout the Bhutan Himalaya. While the resulting enigmatic, low-relief landscapes, elevated above deeply incised canyons, are a popular setting to test ideas of interacting tectonic and climatic forces, when and why these landscapes formed is still debated. We test the idea that these landscapes were created by a spatially variable and recent increase in rock uplift rate associated with the formation of structural duplexes at depth. We utilize a new suite of erosion rates derived from detrital cosmogenic nuclide techniques, geomorphic observations, and a landscape evolution model to demonstrate the viability of this hypothesis. Low-relief landscapes in Bhutan are eroding at a rate of ∼70 m/Ma, while basins from surrounding steep landscapes yield erosion rates of ∼950 m/Ma, demonstrating that this portion of the range is in a transient period of increasing relief. Applying insights from our erosion rates, we explore the influence of an active duplex on overlying topography using a landscape evolution model by imposing a high rock uplift rate in the middle of a mountain range. Our simulations show that low-relief landscapes with thick alluvial fills form upstream of convex knickpoints as rivers adjust to higher uplift rates downstream, a pattern consistent with geologic, geomorphic, and thermochronometric data from Bhutan. With our new erosion rates, reconstructed paleo-river profiles, and landscape evolution simulations, we show that the low-relief landscapes were formed in situ as they were uplifted ∼ 800 m in the past ∼ 0.8-1 Ma.