Lithospheric-Scale Transtension between Vertical, Non-parallel, Planar Zone Boundaries

Abstract

Strain distribution in crustal-scale transtensional zones with non-parallel zone boundaries is heterogeneous, with higher strain gradients in the narrower parts of the zone. Kinematic modeling of such zones shows that, for a constant displacement vector and finite displacement of the zone boundary, the finite strain achieved at a point is dependent on its initial position within the zone. These heterogeneities are governed only by the geometry of the deforming zone; rheologic heterogeneity adds to the departure from homogeneous finite and instantaneous strain. The finite strain distribution can be deduced by determining the instantaneous strain and how it changes both spatially and temporally. In zone geometries where there is a prolate (bouncing) point, marking a change from a horizontal principal finite shortening direction (and therefore vertical foliation) to a vertical principal finite shortening direction (horizontal foliation), it separates areas of wrench- and extension-dominated transtension. This causes spatial partitioning of the strain without any necessity for inherited fabric anisotropies. Migration of this point results in polyphase deformation and, if initial foliation development results in sufficient mechanical anisotropy, overprinting of fabric orientation. Whereas the formation of non-parallel-walled shear zones is promoted by existing structures such as conjugate basement faults and shear zones, the complications predicted in this model result from considering only kinematic arguments and are made more complex by existing heterogeneities found in real rocks.