The Hengduan Mountains (HM) are located on the southeastern edge of the Tibetan Plateau (TP) and feature high mountain ridges (> 6000 m a.s.l.) separated by deep valleys. The HM region also features an exceptionally high biodiversity, believed to have emerged from the topography interacting with the climate. To investigate the role of the HM topography on regional climate, we conduct simulations with the regional climate model COSMO at high horizontal resolutions (at ~12 km and a convection-permitting scale of ~4.4 km) for the present-day climate. We conduct one control simulation with modern topography and two idealised experiments with modified topography, inspired by past geological processes that shaped the mountain range. In the first experiment, we reduce the HM’s elevation by applying a spatially non-uniform scaling to the topography. The results show that, following the uplift of the HM, the local rainy season precipitation increases by ~25%. Precipitation in Indochina and the Bay of Bengal (BoB) also intensifies. Additionally, the cyclonic circulation in the BoB extends eastward, indicating an intensification of the East Asian summer monsoon. In the second experiment, we remove the deep valley by applying an envelope topography to quantify the effects of terrain undulation with high amplitude and frequency on climate. On the western flanks of the HM, precipitation slightly increases, while the remaining fraction of the mountain range experiences ~20% less precipitation. Simulations suggest an overall positive feedback between precipitation, erosion, and valley deepening for this region, which could have influenced the diversification of local organisms.

Mixed siliciclastic and carbonate active orogens are common on Earth’s surface, yet most studies have focused on physical erosion and chemical weathering in silicate-rich landscapes. Relative to purely siliciclastic landscapes, the response of erosion and weathering to uplift may differ in mixed-lithology regions. However, our knowledge of weathering and erosion in mixed carbonate-silicate lithologies is limited and thus our understanding of the mechanistic coupling between uplift, chemical weathering, and the carbon cycle. Here, we partition the denudation fluxes into erosion and weathering fluxes of carbonates and silicates in the Northern Apennine Mountains of Italy—a mixed siliciclastic-carbonate active orogen—using dissolved solutes, the fraction of carbonate sand in sediments, and existing 10Be denudation rates. Erosion fluxes are generally an order of magnitude higher than weathering fluxes and dominate total denudation. The contribution of carbonate and silicate minerals to erosion varies between lithologic units, but weathering fluxes are systematically dominated by carbonates. Silicate weathering may be limited by reaction rates, whereas carbonate weathering may be limited by acidity of the rivers that drain the orogen. Precipitation of secondary calcite from super-saturated streams leads to the loss of up to 90% of dissolved Ca2+ from carbonate-rich catchments. Thus, in the weathering zone, [Ca2+] is exceptionally high, likely driven by high soil pCO2; however, re-equilibration with atmospheric pCO2 in rivers converts solutes back into solid grains that become part of the physical denudation flux. Limits on weathering in this landscape therefore differ between the subsurface weathering zone and what is exported by rivers.

The eastern margin of Madagascar has a prominent relief change from the flat coastal plain to the low-relief high plateau, characterizing a typical great escarpment topography at a passive margin. A quantification of the spatial distribution of erosion rates is necessary to understand the rate of landscape evolution. We present catchment-averaged erosion rates from detrital cosmogenic 10Be concentrations, systematically covering distinct morphological zones of the escarpment. Erosion rates are differentiated across the escarpment, where the high plateau and the coastal plain are slowly eroding with an average rate of 9.7 m/Ma, and the escarpment basins are eroding faster with an average rate of 16.6 m/Ma. The Alaotra-Ankay Graben related basins have the highest erosion rate with an average rate of 27 m/Ma. The spatial pattern of erosion rates indicates a retreating escarpment landscape. Retreat rates calculated from the 10Be concentrations are from 182 m/Ma to 1886 m/Ma. The rates of escarpment retreat on Madagascar are consistent with a model of a steady retreat from the coastline since the time of rifting, similar to the Western Ghats escarpment on its conjugate margin of the India Peninsula.

A great escarpment at a rift margin can correspond directly to a major water divide but at many margins, including in Madagascar, the escarpment often appears as a steep knickzone on rivers that have their main water divides in the interior of the high plateau. We hypothesize that this variability in morphology is a reflection of the frequency and size of drainage area captured from the high plateau over the escarpment. To test this hypothesis, we document morphological features and weathering conditions from river sediment of Madagascar. We propose that the existence of a weathered weak surface layer of crystalline bedrock encourages large river captures from the upper plateau, leading to a dominance of knickpoint type rivers. We demonstrate that this is feasible, using 2-D landscape evolution models and show that an easily-eroded surface layer is prone to fast divide migration through frequent river capture and reversal. A positive scaling relationship between captured area and escarpment retreat rate is found from models. We demonstrate that this scaling is also observed in the great escarpments of Madagascar and India.