Elastic stress control on the pattern of tensile fracturing around a small fault network at Nash Point, UK

Abstract

Complex trajectories of fault-related opening-mode fractures at Nash Point in South Wales, UK are consistent with elastic stress fields computed for the observed fault network subject to north–south compression. The linear elasticity theory is used to calculate local stress increments produced by slip on the fault network. Faults are assumed to be free of shear stress and to slip in response to some remote stress load. Brittle failure of rock within this stress field is determined according to a combined Griffith–Coulomb criterion. Discrete fracture networks are simulated according to simple growth rules where fractures initiate according to the state of brittle failure and propagate along a surface normal to the direction of most tensile local stress. Results demonstrate the dominant opening-mode fracture were caused by a uniform body force (e.g. unloading) and are just locally perturbed by the pre-existing fault network. Fault heights of 2–3 times the mean fault block width, and regional contraction along an azimuth of 180±10° are required to reproduce the observed fracture pattern. Independent kinematic analysis of nearby faults yield estimates of the azimuth of regional contraction of 170–180°. Some features in the observed fracture pattern are not well reproduced but these are small-scale and related to small-scale irregularities on the fault planes.