We (I’Anson et. al, 2018, forthcoming) use Underworld, a particle-in-cell finite element code, to solve equations of momentum, mass and energy in incompressible flow (Moresi et al., 2003). We model the extension of a previously deformed continental/cratonic margin. At its margin, the modelled continental crust layer has an imposed, low density strain zone with a 45 degree dip that extends to 2 km above the Moho. The 2D numerical experiment is 128 km long and 64 km deep with a vertical and horizontal resolution of 250 m. This high resolution allows for detailed analysis of extension related faulting and patterns of accommodation space creation through time.

Since the nature of the deep crust on margins is commonly unknown, we vary the viscosity of the lower crust across four orders of magnitude: weak (5e19 Pa.s) to intermediate (5e20 Pa.s), strong (5e21 Pa.s) and very strong (5e22 Pa.s) and run the models at three constant extension velocities: slow (1 cm/yr), intermediate (3 cm/yr) and fast (5 cm/yr). Extension is imposed through a velocity boundary condition on the left vertical wall down to the base of the model. The left wall is moving and all other walls are fixed. A function at the base of the model allows the inflow of asthenospheric mantle to isostatically balance the model. All models have a 4 km ‘sticky air’ layer with a viscosity of 1e18 Pa.s that emulates a free surface.

The models show that depocentres which form proximal to a continental/cratonic margins can form from the reactivation of high angle zones of weakness producing basin bounding faults that range from low-angle listric, high angle planar and ‘ramp-flat’. Distributed geometries (wide rift of Buck, 1991; Brun, 1999), occur when the lower crust and mantle are coupled, while focused deformation (narrow rift of Buck, 1991; Brun, 1999) occurs when the lithosphere is decoupled by a weak lower crust. The models produce additional diversity of crustal architectures and extensional structures which differ from classical models of seaward dipping-normal faults including lower crustal boudinage, continent and seaward dipping normal faults and intra-depocentre highs.

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