Kinematic nature and origin of regional-scale ductile shear zones in the central Yilgarn Craton, Western Australia

Abstract

Regional-scale, northwest- and northeast-trending ductile shear zones, 1- to 6-km wide and over 100-km long, are the most conspicuous structures in the central Yilgarn Craton, Western Australia. They are commonly developed along granite–greenstone contacts, but are centred in strongly foliated granite and granitic gneiss. A wide range of kinematic indicators, including well-developed S–C fabrics, C′-type shear band cleavage, and abundant asymmetric porphyroclasts, consistently indicate sinistral movement on northwest-trending shear zones and dextral movement on northeast-trending shear zones. Major planar fabrics within the ductile shear zones include (1) a northwest- or northeast-trending foliation parallel to the shear zones, (2) a northerly trending gneissic banding, and (3) a northerly trending foliation. The northwest- and northeast-trending foliations with associated asymmetric structural elements and a shallowly plunging mineral lineation were developed during a progressive, inhomogeneous east–west shortening event (D 3) characterized by low-temperature, noncoaxial deformation. Formation of the northerly trending gneissic banding and associated symmetric structural elements is attributed mainly to east–west coaxial shortening (D 2) in a high-temperature state during, or shortly after, granite intrusion. The northerly trending foliation within the ductile shear zones commonly forms the S-fabric of S–C fabrics that was either developed during D 3, same time as the C-fabric, or was reactivated from S 2 foliations. Therefore, major planar fabrics in the regional-scale ductile shear zones of the central Yilgarn Craton can be ascribed to two tectonic events (D 2 and D 3), and deformation within the shear zones includes both coaxial and noncoaxial components.