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

Abstract

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.

Strain Localization and Migration During the Pulsed Lateral Propagation of the Shire Rift Zone, East Africa

“…This interpretation is consistent with observations of normal fault splaying across misoriented crustal terrane boundaries along the path of lateral propagation of rift zones in the Great South Basin, New Zealand (Phillips et al, 2019b). Numerical models also demonstrate the temporary stagnation of propagating rift tips at terrane boundaries that are rift-orthogonal and bound terranes of contrasting crustal strength (Phillips et al, 2021). In addition, observations in other areas of early-stage continental rifting show that rift zones and their bounding faults terminate at major riftoblique/orthogonal basement shear zones, for example, the termination of the Okavango Rift at the Sekaka Shear Zone , and the termination of the Rhino Rift at the Aswa Shear Zone (Figure 1a; Katumwehe et al, 2015;Kolawole et al, 2021b).…”

A review of structural inheritance in rift basin formation

“…Modelling shows that rifting of heterogeneous lithosphere results in varying styles of deformation above different lithospheric or crustal blocks. Extension of two laterally juxtaposed crustal domains of different integrated strengths results in earlier strain localisation and more widely spaced and higher displacement faults in the weaker domain (Phillips et al, 2021b;Samsu et al, 2021). When the domain boundary strikes perpendicular to the extension direction, greater strain localisation above the weaker domain leads to rift margin asymmetry (Corti et al, 2013a;Beniest et al, 2018).…”

Flexural strike-slip basins

“…Fault trends suggest the region near the ABCF experienced ENE-WSW extension in the Oligocene that shifted to NNW-SSE strike-slip motion during the early to mid-Miocene (lasting ~5 Myr; Morley, 2017). The ABCF formed in a previous necking zone of hyperextended continental crust (7-10 km thick; Morley, 2017;, likely because mantle heterogeneities influence fault location (Phillips et al, 2021). During strike-slip motion, the easterly Mergui Ridge (MR) was partially subaerial and acted as an asymmetric clastic sediment source for the East Andaman Basin (EAB) located along the ABCF .…”