Changing rift physiography across blocks of laterally variable crustal strength – Insights from 3D numerical modelling

Abstract:

The evolution of deformation in continental rifts is inherently non-unique. Rift physiography may be influenced by external factors such as the orientation and magnitude of the applied stress, and internal factors such as crustal rheology and pre-existing structural heterogeneities. The characteristic structural styles of fault networks associated with these different factors, and how they may vary spatially across a rift, remain largely unknown. In this contribution we use 3D thermal-mechanical models of continental extension to constrain the relationship between rift physiography and pre-rift crustal strength variations. We examine how rift physiography varies across a 500x500 km area containing 125 km wide ’blocks’ of ‘Strong’, ‘Weak’ and ‘Normal’ crust. The different crustal strengths are represented in the model by varying amounts of initial plastic strain, intended to replicate pre-existing heterogeneities and weaknesses within the crust. Extension is oriented parallel to the block boundaries, such that all blocks undergo equal amounts of finite strain, expressed via different structural styles of the resultant fault network. We examine the characteristic rift physiography within each block as well as the interactions at the boundaries.

Our numerical modelling is based on geological observations from the Great South Basin, offshore New Zealand, where rifting occurs across distinct terranes including a granitic batholith (representing a relatively homogeneous ‘Strong’ terrane) and a fore-arc sedimentary basin (representing a ‘Weak’ Terrane containing numerous pre-rift heterogeneities), producing variable physiography along the rift.

Our preliminary numerical simulations reveal that the strong terrane is characterised by the development of closely-spaced, low-displacement faults, with strain eventually localising along isolated structures. In the weak terrane, strain rapidly localises onto widely spaced, high displacement faults. Faults segment and abruptly terminate at the boundaries between the different strength blocks. This study highlights how rifts and fault networks develop across areas of variable crustal strength; our observations may be applicable to other rifts developed across heterogeneous crust globally, including the Tanganyika and Corinth rifts.