Impact of river incision on lower crustal flow: insights from thermo-mechanical models

Crustal deformation is highly influenced by surface processes, such as erosion and sedimentation, particularly in tectonically active regions. While these processes have been intensively studied in large-scale erosive settings and tectonically active areas, the specific effect of river incision on valley morphology and crustal deformation remains poorly constrained. In this study, we show that valley incision can have a significant impact on the morphological and tectonic evolution of orogenic systems. Using a two-dimensional thermo-mechanical model and inspired by the case study of the Nanga Parbat Haramosh Massif (NPHM), we investigated the effects of varying incision rates and topographic diffusion coefficient on crustal deformation in the absence of imposed tectonic boundary forces. Our results indicate that with the lowest incision rates (between 10 and 70mm.yr^-1), surface processes predominantly govern the morphology of the valley, with limited tectonic feedback. Conversely, at higher incision rates (over 90mm.yr^-1), the tectonic response becomes increasingly significant, impacting the long-term regional deformation and the morphology of the valley. Over a timescale of 10 million years, this dynamic interplay can lead to substantial crustal deformation involving the exhumation of the lower crust (at rates up to 3mm.yr^-1) . Our reference model is in very good agreement with natural observations from the NPHM, suggesting that valley incision alone can drive significant crustal deformation, even in the absence of far field stresses (shortening). These results offer valuable insights into the interplay between surface processes and crustal deformation, highlighting the critical role of river incision in shaping mountainous landscapes and promoting the exhumation of deep crustal materials in actively deforming orogenic areas.