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 between
tectonics, climate, and surface processes. Tectonic deformation generates topography through crustal
shortening and thickening, which modifies precipitation via orographic effects. Enhanced precipitation
drives river incision, mass wasting, and sediment transport that erode the landscape, feeding back into
topography and precipitation patterns over geological timescales.
In the Himalaya, crustal shortening produces an orogenic wedge above the Main Himalayan Thrust, the
basal décollement with a flat-ramp-flat geometry where sub-horizontal flats at different crustal levels are
connected by inclined mid-crustal ramps. Wedge growth occurs primarily through basal accretion, whereby
material from the subducting Indian plate is scraped off and emplaced beneath the wedge as thrust-bounded
rock slices (horses) between a floor thrust and roof thrust, forming a mid-crustal duplex. As convergence
continues, this process operates episodically: new horses are sequentially accreted through footwall
imbrication, punctuated by phases when breakthrough ramps form to transfer slip between décollement
levels. This temporal cyclicity in basal accretion creates alternating phases of duplex thickening and ramp
activation. However, how this cyclic process modulates climate-tectonic feedbacks—specifically, how
episodic duplex growth and ramp activation influence topographic evolution, precipitation distribution, and
erosion 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 that
captures the physics of climate–tectonic–surface interactions, building on the coupled modelling approach
developed 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) to
simulate surface processes and an orographic precipitation model (LFPM) to evaluate climate–topography
feedbacks. We reproduce first-order geometries of the India-Eurasia collision zone by introducing crustal
décollements as pre-defined horizontal weaknesses in the Indian pate.
Preliminary results indicate that variations in basal décollement strength modulate tectonic style and ramp
cyclicity, controlling mountain-belt width and, in turn, precipitation patterns and surface erosion across
different ramp phases. A stronger basal décollement relative to an intermediate décollement leads to the
development of distinct inner and outer wedges. The outer wedge thereby grows laterally by frontal
accretion while uplift of the inner wedge occurs via duplex formation. Uplift of the inner wedge produces a
highly 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, associated
with the inner and outer wedges, along with corresponding dual bands of high relief and enhanced channel
steepness. We find that variations in erosional parameters, together with crustal rheology, can substantially
influence the geometry of the Himalayan wedge, thereby modulating crustal deformation, topography
changes and the climate.

Reference: Yuan, X., Li, Y., Brune, S. et al. Coordination between deformation, precipitation, and erosion
during orogenic growth. Nat Commun 15, 10362 (2024). https://doi.org/10.1038/s41467-024-54690-4