The influence of crustal strength on rift geometry and development – Insights from 3D numerical modelling

The lateral distribution of strength within the crust is highly variable. When subject to extension, lithologically and rheologically distinct areas of crust manifest strain differently, influencing the structural style, geometry and evolution of the associated 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. 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, producing a well-developed fault network, whilst 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.