Stress patterns in an interplate shear zone: an effective anisotropic model and implications for the transverse ranges, California

Abstract

Strong lateral variations in geological structure within a transcurrent interplate deformation boundary have a substantial influence upon the way in which ambient stress is related to the relief of regional stress within the boundary zone. Much of the crustal deformational structure in southern California and environs consists of a conjugate wrench fault system. The Quaternary fault system consists of a series of parallel and sub-parallel strike-slip faults that are causally related to the horizontal interplate shearing. A prominent crustal structural inhomogeneity is the Transverse Ranges, where fault orientation is east-west, transverse to the dominant northwesterly trend. We investigate some of the consequences of this transverse inhomogeneity on the overall stress and strain field in the southern California region. The activity of the strike-slip (or wrench) system to the south and north of the Transverse Ranges suggests a mechanical model consisting of weak zones with a relatively strong degree of orientation. An effective anisotropy model is constructed based on: (1) a two-component laminate model consisting of competent unfaulted rock adjacent to incompetent faulted rock; (2) theoretical results for the weakening of a plate due to a doubly periodic array of cracks; and (3) finite element treatment of a checkerboard array of cracks. The fundamental parameter for weakening is = 1 - where is a non-dimensional form of Biot’s slide modulus. In the limit of 1 the crust becomes extremely weak and anisotropic, and as A -> 0 the condition of a strong, isotropic crust is recovered. The components of the stiffness (or compliance) matrix are directly related to the mechanical properties of a finite width fault zone, or to the average fault spacing and asperity density within a particular geological province, or both. An elastic plate model that incorporates the stress-strain channelling caused by multiple, oriented fault systems is constructed. The plate is assumed to be stressed by pure shearing forces maintained at infinity. The ambient field then corresponds to the north-south compressions, east-west extensions tectonic regime that dominates North-American-Pacific interplate shear along the San Andreas fault, California. Embedded within the plate is an elliptical inclusion in which multiple fault stress channelling also occurs. The inclusion thus mimics the misaligned structure of the Transverse Ranges in southern California. The boundary value problem associated with the model is treated both analytically and with finite element computations. The simple model predicts (i) the enhanced seismic energy release associated with the Transverse Ranges; and (ii) the clockwise rigid rotation indicated by a palaeomagnetic studies. The relatively simple nature of the model helps to isolate those features of the southern California tectonic stress regime that might be attributed to the transverse orientation of the Transverse Ranges. Stress channelled into the crosscutting tectonic structure from the ambient interplate field is significant. Contradirectionality alone cannot provide an explanation for the enhanced north-south compressive stress relative to east-west extension.