Shear zones as a new type of palaeostress indicator

Abstract

This paper proposes that shear zones act as palaeostress indicators in much the same way as brittle faults. Several hundred shear zones, from both the limbs of the Variscan Langland-Mumblcs anticline (South Wales), are treated using the graphical and numerical methods of fault slip analysis. Estimated palaeostress directions derived from shear zones are characterized by greater scatter than where brittle faults are used as stress indicators. An important reason for this is the fact that on some shear zones the direction of maximum shear stress (slip lineation) is not perpendicular to the line of S/C intersection. As a consequence, the use of slip lineations produces better solutions than cases where the slip direction is inferred as a line perpendicular to the S/C intersection. The filtering of data to reduce the scatter in the results produces insignificant improvement and is considered unnecessary.