Tectonic significance of fault-slip data

Abstract

The statistical analysis of populations of minor faults is commonly used by structural geologists working in areas of brittle rock. It is based on measurements of fault attitude, direction and sense of slip. At individual sampling localities, results are classically interpreted as indicators of stress or strain rate fields, assuming a homogeneous stress or strain rate tensor, respectively. However, fault patterns are expected to vary with time because of displacements, rigid rotations, and internal strains, which generally occur along boundaries of fault-bounded blocks as deformation proceeds. Thus, in the general situation where early faults accumulate displacements and rigid rotations, and where new faults develop during progressive deformation, fault-slip data can be rather complex and variable in space, and reflect neither local stress or strain rate tensors, nor finite strains and finite rotations in a simple way. We use two examples of faulted regions to illustrate the spatial variability of fault-slip data. This can be due to local complications at the edges of fault blocks, or to complex kinematic conditions at regional boundaries. Such complications may make it difficult to deduce a consistent and simple pattern of either stresses or strain rates. Instead, our results suggest that information contained in fault-slip data can be pertinent to finite deformation. In particular, the principal axes that we have deduced from the analysis of fault-slip data are consistent with the finite displacements at block boundaries that we have calculated by numerical restoration.