Tectonic and Climatic Controls on Himalayan Topographic Evolution: Numerical modelling of tectonics-erosion-precipitation interactions

Topography and erosion in active convergent mountain belts arise from coupled feedbacks betweentectonics, climate, and surface processes. Tectonic deformation generates topography through crustalshortening and thickening, which modifies precipitation via orographic effects. Enhanced precipitationdrives river incision, mass wasting, and sediment transport that erode the landscape, feeding back intotopography and precipitation patterns over geological timescales.In the Himalaya, crustal shortening produces an orogenic wedge above the Main Himalayan Thrust, thebasal décollement with a flat-ramp-flat geometry where sub-horizontal flats at different crustal levels areconnected by inclined mid-crustal ramps. Wedge growth occurs primarily through basal accretion, wherebymaterial from the subducting Indian plate is scraped off and emplaced beneath the wedge as thrust-boundedrock slices (horses) between a floor thrust and roof thrust, forming a mid-crustal duplex. As convergencecontinues, this process operates episodically: new horses are sequentially accreted through footwallimbrication, punctuated by phases when breakthrough ramps form to transfer slip between décollementlevels. This temporal cyclicity in basal accretion creates alternating phases of duplex thickening and rampactivation. However, how this cyclic process modulates climate-tectonic feedbacks—specifically, howepisodic duplex growth and ramp activation influence topographic evolution, precipitation distribution, anderosion rates across the wedge—remains poorly constrained over tens of millions of years.To investigate these feedbacks, we employ a 2D coupled lithosphere-scale numerical framework thatcaptures the physics of climate–tectonic–surface interactions, building on the coupled modelling approachdeveloped by Yuan et al. (2024). This framework integrates a thermomechanical geodynamic model(ASPECT) to account for tectonic deformation and uplift, a landscape evolution model (FastScape) tosimulate surface processes and an orographic precipitation model (LFPM) to evaluate climate–topographyfeedbacks. We reproduce first-order geometries of the India-Eurasia collision zone by introducing crustaldécollements as pre-defined horizontal weaknesses in the Indian pate.Preliminary results indicate that variations in basal décollement strength modulate tectonic style and rampcyclicity, controlling mountain-belt width and, in turn, precipitation patterns and surface erosion acrossdifferent ramp phases. A stronger basal décollement relative to an intermediate décollement leads to thedevelopment of distinct inner and outer wedges. The outer wedge thereby grows laterally by frontalaccretion while uplift of the inner wedge occurs via duplex formation. Uplift of the inner wedge produces ahighly elevated, low-relief landscape, suggesting a transient geomorphic response to ongoing duplex uplift,as observed in parts of the Himalaya. In these zones, two distinct rainfall maxima are observed, associatedwith the inner and outer wedges, along with corresponding dual bands of high relief and enhanced channelsteepness. We find that variations in erosional parameters, together with crustal rheology, can substantiallyinfluence the geometry of the Himalayan wedge, thereby modulating crustal deformation, topographychanges and the climate.Reference: Yuan, X., Li, Y., Brune, S. et al. Coordination between deformation, precipitation, and erosionduring orogenic growth. Nat Commun 15, 10362 (2024). https://doi.org/10.1038/s41467-024-54690-4