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

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 alone granite-greenstone contacts, but are centred in strongly foliated granite and granitic gneiss. A wide range of kinematic indicator. 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 deformations 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. (C) 2004 Elsevier B.V. All rights reserved.