The effects of extensional inheritance on transtensional deformation patterns

Abstract:

Continental rifting and breakup frequently occurs through multiple phases of extension in which margin architecture is governed by distinct modes of brittle and viscous deformation, as well as surface processes. Transitions between such phases of extension may in some cases reflect plate-scale changes in extension obliquity and velocity. For example, recent studies indicate that localization of transtensional deformation onto a narrow shear zone in the Gulf of California Rift corresponds temporally with an increase in rift obliquity. Here, we use high-resolution 3-D models (run with pTatin3D) to examine how transtensional deformation patterns evolve with such increases in extension obliquity.

Our models build on previous crustal-scale analogue and numerical studies examining oblique reactivation of normal faults. These studies show that the orientation and strength of inherited normal faults strongly control the style of slip and deformation partitioning between new and reactivated faults. While highly insightful, these studies examine deformation over limited spatial extents and do not account for key processes such as isostasy and temperature-dependent rock strength.

Here, we model the thermal-mechanical evolution of the lithosphere and asthenosphere to a depth of 200 km. Deformation occurs through an initial phase of orthogonal, Basin and Range style extension, followed by 1-2 phases of extension at different obliquities. The initial lithospheric structure and time-dependent boundary conditions follow geologic observations and plate-reconstructions within the Gulf of California. Our experiments highlight the strong control of inheritance on transtensional crustal deformation. By comparing fault development, crustal thickness evolution and block rotations directly with observations from the Gulf of California and other extensional and transtensional regions, we can better evaluate the relative control of structural inheritance on passive margin architecture.