A symmetry-based method for kinematic analysis of large-slip brittle fault zones

Abstract

A symmetry-based, statistical method for kinematic analysis of mesoscale fault-zone structures is applied to resolve the transport directions for the Rosario fault zone and Lopez Structural Complex in the San Juan Islands. Individual asymmetric folds and Riedel composite structures consisting of arrays of fault-related planar shear fractures and foliations are represented by an internal rotation axis, which describes the sense of shear and orientation of each structure. The overall deformation in the zone is interpreted as a general non-coaxial shear with monoclinic symmetry. A post-faulting, solution-mass-transfer cleavage superimposed on the fault zones and nappes alike also records bulk shortening in a northeast-southwest direction. These compatible results support the hypothesis that the San Juan thrusts and nappes, and the larger Cascade orogen encompassing them, developed during northeast-southwest regional shortening in Late Cretaceous time. -from Authors