THE NATURE AND CONSEQUENCES OF STRAIN PARTITIONING DURING OBLIQUE EXTENSION

Using examples from the UK and Brazil, we demonstrate that oblique extension leads to the development of 3-D transtensional strains with very different styles of faulting compared to the plane strain patterns associated with orthogonal extensional and strike-slip tectonics. Both theoretical models and field studies have shown how oblique divergence can be either accommodated by homogeneous or heterogeneous partitioned transtension. Homogeneous transtension is an inherently unstable deformation state since the non-coaxial component of deformation will tend to continuously rotate and re-orient structures within the deformation zone. By contrast, partitioned transtension is more stable since the non-coaxial deformation tends to be localised within strike-slip-dominated shear zones of finite width oriented parallel to the deformation zone boundaries, with the coaxial deformation accommodated in a more distributed manner throughout the deformation zone.

Strain partitioning at basin- to plate margin-scales is likely to occur within natural systems due to the presence of pre-existing structures lying at an oblique angle to imposed opening vectors. This may occur because the pre-existing anisotropies are zones of long-lived weakness that lie in an orientation particularly favourable to the preferential accommodation of either strike-slip or dip-slip displacements. During basin- to reservoir-scale transtension, an additional level of complexity and strain partitioning can be triggered by significant lithologically-controlled variations in the value of Poisson's ratio in adjacent rock units. Strain partitioning and kinematically heterogeneous fault patterns can also develop as a function of finite strain intensity which controls the switch between “wrench-“ and “extension-dominated” transtension for low values (up to 30°) of the angle of oblique divergence.