How Coupled Brittle-Ductile Deformation Controls the Rates and Temporal Evolution of Orogenic Collapse

The collapse of orogenic belts is commonly thought to involve viscous flow in a mid-crustal channel, and manifests as extensional faulting in the upper crust. Recent observations in some orogenic belts have indicated a power-law relationship between local elevation and extensional strain rates. Simple mechanical considerations predict that the flow of the weak crustal layer beneath these belts is driven by topographic gradients, suggesting that the observed extension is linked to this flow. To test this hypothesis and examine the temporal evolution of collapsing orogenic belts, we developed a 2-D numerical model simulating how topography-driven viscous flow in the weak mid-lower crust induces, and is affected by, orogenic belt extension. Our results show that flow of a weak mid-lower crust triggers orogenic collapse via normal faulting, provided mountain height exceeds a critical threshold (h_min​). The simulated faults form within the highest regions of the orogen, where the weak crustal layer flow originates. Once the mountain collapses so much that its height falls below h_min, extension ceases, where h_min​ depends on both the thickness of the weak layer and the strength of the upper crust.  Additionally, we find that collapse rates increase with hotter and thicker weak channels, taller orogens, and weaker upper crustal faults, while stronger upper crust restricts fault distribution, concentrating deformation within smaller areas, leading to a core complex extension mode. Finally, a strong agreement between our numerical and analytical (detailed in companion abstract: Dawood et al. 2025 EGU General Assembly 2025) models demonstrates that orogenic collapse rates and their temporal evolution are jointly controlled by the brittle and ductile properties of the continental crust.