Strain partitioning in transpression zones

Abstract

Transprcssional strain acting upon structurally anisotropic rocks can be partitioned into separate deformational domains of pure shear and simple shear. This contrasts with homogeneous transpression in which both the pure shear and the simple shear strain components are uniformly distributed across the zone of deformation. Structural weaknesses capable of partially or fully accommodating one component of deformation include lithological contacts, rheological heterogeneities, and faults or shear zones situated within the deformation zone or lying along its boundaries. Partitioning of transprcssional strain can occur when stress is applied oblique to pre-existing structural weaknesses, or can occur during later stages of progressive strain, when the early deformation of isotropic rocks imparts sufficient anisotropy to allow subsequent strain to be partitioned. Partitioning of transpressional strain into domains lying parallel to the deformation zone boundaries can be distinguished from ‘fault-stepped’ transpression, in which strain is partitioned along the length of a segmented fault zone. Mesofracture analysis of rocks affected by mid-Devonian deformation on both sides of the Highland Boundary Fault Zone (HBFZ) in central Scotland shows that strain was not homogeneous. The mesofracture data suggest that regional north-south compression, orientated oblique to the pre-existing NE-SW-trending HBFZ, was partitioned into separate deformational domains. The HBFZ accommodated most of the simple shear component, whilst the rocks flanking the zone were deformed predominantly by pure shear. A contemporary example is the San Andreas Fault in central California, where analyses of neo-tectonic stresses show that the direction of principal compression is perpendicular to the fault zone. Many examples of the partitioning of transpressional strain have also been recognised at destructive plate margins where the direction of plate motion is oblique to the edge of the over-riding plate.