Numerical modelling of gravity-driven fold-and-thrust belts at passive continental margins

Continental margins are generally sites of massive material redistribution related to processes that drive continental erosion. This redistribution in form of large sedimentary fluxes (i.e. increased sediment accumulation rates) and a rapidly adapting submarine topography is responsible for major mobilization and re-mobilization of sediments in the sense that they move under their own means, without direct impact of tectonic forces. In basins with deeply-buried fine-grained clastic sediments, excess pore pressure may result in the formation of mobile shale that deforms in a ductile manner at critical state. Such mobile shale horizons can act as major décollements to gravity-driven fold-and-thrust belts that undergo extension in the proximal part of the margin and horizontal shortening farther offshore. In this study, we investigate the effect of variable pore-pressure distribution on the mechanical and structural evolution of gravity-driven fold-and-thrust belts during delta progradation by applying geodynamic numerical modelling.

Numerical experiments are conducted by a two-dimensional finite-difference mechanical model with a visco-elastic-plastic rheology. The model employs a fully staggered Eulerian grid of 500 km width and 25 km height, and a Lagrangian marker field to track deformation. All across-boundary velocities are set to zero. Elastic rigidity of the base allows for lithospheric flexure related to the load of the prescribed prograding delta. Mobile shale forms when material undergoes pore-pressure-dependent brittle failure, following a Bingham-type rheology (i.e., viscous deformation above brittle strength threshold).

Preliminary results reveal that delta progradation and deep shale mobilization lead to the formation of gravity-driven tectonics with three distinct structural domains: landward fault-bounded extensional basins, a transitional zone of shale beneath a mostly undeformed continental slope, and a seaward fold-and-thrust belt at the delta toe. These features are consistent with structural patterns observed in gravitationally unstable Cenozoic deltas, such as the Niger Delta. This study provides insights into the fundamental links between deltaic sedimentation, fluid pressure profiles, and margin-scale gravity spreading, with implications for understanding passive margin tectonics and hydrocarbon exploration.