A dynamic atlas of interference patterns in superimposed, opposite sense ductile shear zones

Abstract

Ductile shear zones that reactivate a coplanar shear zone with opposite shear sense are known from a variety of tectonic environments. Recognition of reactivation requires understanding the interference patterns that form when microstructures produced in the first event (D1) are modified by the second event (D2). We use numerical modelling to demonstrate the effect of D1 structures on the development of shear zone interference patterns during a coplanar, opposite sense D2 ductile shearing event. Seven models were generated from increasing D1 dextral simple shear strains (γdextral = 2–14) and we then superimposed D2 sinistral shearing (γsinistral = 10). The interference patterns produced are highly variable with geometric relationships between weak layers and strong lithons determining deformation style. Shear zones with high D1 strain can more easily accommodate D2 strain because more strain is localised into long, weak phase C planes, which are readily inverted and reused during sinistral shear. Interference structures in models and naturally deformed rocks include rotated σ-clasts, folded D1 S planes, disharmonic and hook folds, and cuspate layers of weak phase. We present a dynamic atlas of interference patterns produced due to overprinting shear zones to facilitate identification of these zones in nature.