Dynamics of Orogenic Collapse Controlled by Coupled Brittle–Ductile Deformation

The life cycle of orogenic belts is governed by the competition between compressional tectonic forces that build topography and gravitational forces that destroy it through extension. In mature orogens, extension is commonly thought to involve viscous flow within a weak crustal channel (WCC), driven by topographic gradients between mountain belts and their margins. This process is expressed in the upper crust as normal faulting atop high mountain belts, such as the Tibetan Plateau and the Apennines. However, the mechanical link by which flow within the WCC drives extension in the brittle upper crust remains poorly understood. In previous work (Dawood et al., 2025 EGU), we designed an analytical model predicting the instantaneous, characteristic rate of brittle extension enabled by WCC flow. Here, we extend and test this framework by coupling it with two-dimensional numerical simulations to investigate the time-dependent dynamics of orogenic collapse. While the analytical model captures the static force balance and provides a snapshot estimate of extension rates for a given orogenic state, the numerical approach resolves the temporal evolution of topography, crustal-channel flow, and fault activity. Our simulations show that topographic gradients drive viscous flow within the WCC, which generates basal shear tractions that promote extension along upper-crustal normal faults. We find that sustained orogenic extension requires both a sufficiently weak WCC (η_wcc  ≤ 10²¹ Pa.s) and an orogenic elevation exceeding a critical threshold height, h_min. This threshold is controlled by the frictional strength of the brittle crust and the magnitude of basal shear stress transmitted from the WCC. Extension rates scale systematically with fault strength, orogenic height, and WCC viscosity and thickness: high extension rates occur for weak faults and high topography (h >>h_min), especially in the presence of a thick, low-viscosity WCC. In contrast, stronger faults, lower elevations, or thinner and more viscous channels suppress extension. Together, these results validate our analytical scaling laws, indicating that while a static force-balance description predicts the instantaneous extensional behavior, numerical models capture the longer-term, time-dependent, self-limiting evolution of collapsing orogens.