Geometric constraints on the development of shear bands in rocks

Abstract

Shear bands in ductile shear zones have been used to determine sense of shear, but they also contain information on the flow pattern and flow history in shear zones. A simple geometric analysis of two types of shear bands, S-C fabrics and extensional crenulation cleavage (ECC) fabrics, is used to delimit possible flow patterns and flow history in shear zones where such structures develop. S-C fabrics can form in bulk simple shear during the entire active period of a shear zone. Development of ECC-fabrics as observed in nature, however, is favoured by bulk extension of the shear zone as a whole. This means that such fabrics preferentially develop in an extensional tectonic setting, or in a constrictional tectonic setting during late stages of activity on ductile shear zones, when flow in the zone develops from bulk simple shear to bulk non-coaxial extension.