Numerical modelling of coupled tectonics and surface processes in continental collision

Abstract

Topographic evolution results from the coupled action of mantle convection, lithospheric tectonics, and surface processes, including erosion and deposition. Despite substantial progress, the relative roles and interactions of these factors, especially in collisional orogens, remain debated. This study investigates the tectonic-scale interplay between lithospheric dynamics and fluvial-driven erosion, with a focus on drainage systems as critical modifiers of mountain range morphology in collisional settings.

This thesis researches the Three Rivers Region of southeastern Tibet, a tectonically active domain characterized by complex drainage networks and rapid topographic change, to elucidate how coupled tectonics and surface processes influence topographic responses and drainage patterns. To achieve this, two-way, fully coupled numerical models are developed and employed to simultaneously simulate tectonic deformation and landscape evolution. The modelling framework integrates particle-in-cell finite element methods (PIC-FEM) within an Eulerian formulation and leverages the Sticky Air method to approximate a free surface. The models are built on UWGeodynamics, which unites Underworld 2 with surface processes code such as Badlands, thereby enabling coherent coupling between lithospheric dynamics and surface processes.

A central methodological contribution concerns the treatment of the free surface. Although the Sticky Air method offers a pragmatic route to free-surface evolution, it imposes limitations in accuracy and resolution of topographic features. To address this, an innovative scheme is proposed and implemented in Underworld 2 and Underworld 3, embedding the Sticky Air approach within an Arbitrary Lagrangian-Eulerian (ALE) framework, using an internal boundary to enhance free-surface representation and coupling fidelity (ALE-IB). This scheme is designed to improve surface boundary conditions, reduce spurious stresses near the air-land interface, and more accurately capture channel incision, sediment transport, and drainage network evolution. In parallel, a novel coupling framework is developed and tested within this scheme, leveraging the latest generation of geodynamic software (Underworld 3) to enable robust, scalable two-way interactions between mantle-lithosphere dynamics and surface processes. The study cross-validates results from the proposed approach against traditional free-surface treatments and alternative coupling strategies, through a suite of numerical experiments calibrated to tectonics and fluvial geomorphology.

Expected outcomes include (i) quantitative assessment of how drainage-network evolution is modulated by coupled tectonics and fluvial erosion process in collisional orogens; (ii) evaluation of the sensitivity of topographic response and drainage systems patterns to viscosity, erosion-deposition rules, and river incision laws; and (iii) a rigorously tested, openly documented computational framework that advances two-way tectonic-surface-evolution modelling. The anticipated contributions encompass methodological innovation (ALE-IB enhanced free-surface simulation with PIC-FEM in Underworld 2 and Underworld 3) and geophysical insights into the coupled tectonics-surface processes system. Show more