Quantifying drainage divide migration in active orogens: Insights from the eastern margin of the Tibetan Plateau

Abstract

Landscape of active orogens results from a competition balance between tectonic rock uplift and erosion. Drainage divide migration is a useful approach for quantifying the landscape evolution and the underpinning drivers, because the fluvial networks adjust their geometry in response to spatiotemporal changes in rock uplift, climate and lithology. Many quantitative geomorphological metrics, such as χ-anomaly, Gilbert metrics, have been proposed to analyze the drainage divide migration. However, their interpretations often yield inconsistent results, especially in tectonically active orogens with spatial changes in rock uplift. The highly elevated and tectonically active eastern margin of the Tibetan Plateau has been studied by multiple methods of different time-scales, producing abundant structural and erosion rates determinations needed for evaluating the landscape stability and evolution. Here, we evaluated the effectiveness of χ- and Gilbert metrics in quantifying the stability of the Longmenshan-Minshan divide parallel with the plateau margin, by comparing with erosion rate data of different time scales, rock uplift predicted by upper crustal structures, bedrock lithology, precipitation. Our analyses reveal that the divide has reached dynamic steady state, as shown by the consistent cross-divide Gilbert metrics and consistent spatial changes of short, medium and long-term erosion rates, indicating the overall balance between the erosion and rock uplift. The spatial changes in rock uplift underpinned by reverse faulting along a listric upper crustal fault also explains the development of cross-divide χ-anomaly that cannot be used as an index for divide migration in regions with spatially nonuniform rock uplift. The formation of the long-term stable drainage divide in the EMTP serves as a natural laboratory for studying the divide formation and evolution in response to a phase of non-uniform rock uplift. We infer that in regions of this kind, drainage divide forms and evolves from uplift-driven to erosion-driven migration, via an intervening stable state.