The Influence of the Nature of Brittle Failure in Numerical Models of Extension

Differing styles of extension, such as horst-and-graben, half-graben, metamorphic core complexes and areas of distributed crustal thinning, are observed in rift basins on Earth. The morphology of the rift basin may be asymmetric or symmetric, with the large majority of rifts being classified as asymmetric. Faulting zones can range from either being distributed, where individual faults are characterized by a relatively steep angle and small offset, to highly localized, where individual faults exhibit a low angle and very large displacement. Observations indicate that rheological properties exert control over the extensional deformation style, symmetry, and fault spacing in extension zones. We present time-dependent numerical models of extension that focus on the influence of the nature of brittle failure, specifically differences between (a) a Mohr-Coulomb yield criterion implemented through a transition to a non-linear, transversely isotropic viscous flow rule, and (b) a Drucker-Prager yield criterion implemented as an non-linear, isotropic, viscous flow rule. Unlike the classical implementation of the Mohr Coloumb plasticity law, which limits both the shear and normal stresses along a plane of failure, our transversely isotropic plasticity law limits the shear stress only and the normal stress remains unchanged. The use of a transversely isotropic rheology in the extension models produces an upper crust strength profile that is a closer match to predicted upper crust strength profiles from laboratory experiments, slightly smaller fault spacing, increased asymmetry and increased fault interaction in comparison to the models using an isotropic yielding mechanism. These outcomes can be attributed to the larger portion of normal stress being transferred across the faults. In addition these models are more likely to have three dimensional deformation patterns, due to increased interaction between faults, The results imply that employment of a transversely isotropic rheology to generate rift zones produce more Earth-like results, as stresses and forces are transmitted across simulated faults in the same manner as stresses and forces are transmitted across faults on Earth. An accurate stress field is essential to understanding rifting phenomena such as rift initiation, interaction between normal faults, and the role of stress transfer in earthquake occurrence.