Thomas Brian Phillips, John Naliboff, Ken McCaffrey, Sophie Pan, Jeroen van Hunen

The lateral distribution of strength within the crust is non-uniform, dictated by crustal lithology and the presence and distribution of heterogeneities within it. During continental extension, areas of crust with distinct lithological and rheological properties manifest strain differently, influencing the structural style, geometry and evolution of the developing rift system. Here, we use 3D thermo-mechanical models of continental extension to explore how pre-rift upper crustal strength variations influence rift physiography. We model a 500x500x100 km volume containing 125 km wide domains of mechanically ‘Strong’ and ‘Weak’ upper crust along with two reference domains, based upon geological observations of the Great South Basin, New Zealand, where extension occurs perpendicular to distinct geological terranes and parallel to terrane boundaries. Crustal strength is represented by varying the initial strength of 5 km3 blocks. Extension is oriented parallel to the domain boundaries such that each domain is subject to the same 5 mm/yr extension rate. Our modelling results show that strain initially localises in the Weak domain, with faults initially following the distribution of Initial Plastic Strain before reorganising to produce a well-established network, all occurring in the initial 100ky timestep. In contrast, little to no localisation occurs in the Strong domain, which is characterised by uniform strain. We find that although faults in the Weak domain are initially inhibited at the terrane boundaries, they eventually propagate through and ‘seed’ faults in the relatively stronger adjacent domains. We show characteristic structural styles associated with ‘strong’ and ‘weak’ crust and relate our observations to rift systems developed across laterally heterogeneous crust worldwide, such as the Great South Basin, NZ, and the Tanganyika rift, East Africa.