The Jack Hills greenstone belt, Western Australia: Part 1: Structural and tectonic evolution over >1.5 Ga

Abstract

The Jack Hills greenstone belt is situated within the Narryer Terrane, Western Australia, and is an approximately 70 km long greenstone and metasedimentary belt surrounded by granitic gneisses and various granitic rocks. Detailed fieldwork, supported by previous geochronological data shows that the Jack Hills greenstone belt has undergone a long-lived, complex deformation history, spanning at least 1.5 billion years. This includes, from oldest to youngest: (1) an early stage of recumbent and chevron folding probably associated with thrust faulting; (2) east-trending, major transpressional shearing including jog formation; (3) followed by kink and conjugate-style folding, brittle faulting, and fault reactivation. Transpressional shearing is interpreted to have taken place during the Paleoproterozoic Capricorn Orogeny. Strong overprinting and reworking with coaxial geometry gives the belt an apparent simplistic structural style that in places appears to be dominated by a single foliation. Pronounced static recrystallisation and fluid flow also mask structural complexity. Satellite and aeromagnetic imagery illustrate the large-scale geometry and show that the belt has a pronounced sigmoidal curvature, suggestive of dextral kinematics associated with transpression along major, east-trending structures. The majority of kinematic indicators in the field support this interpretation. The various structures are not developed uniformly across all lithological units within and outside the belt. Structural analysis, and an increased understanding of the lithological relationships between the various units, have allowed distinctions to be made between successive tectonic events. This has provided key criteria towards understanding the depositional timing and character of the units, and understanding and relating formation of structural elements such as mineral lineations and dominant foliations to specific events in time and space. On a regional scale, the Jack Hills greenstone belt shares some structural similarities with the nearby Errabiddy Shear Zone, which marks the northern margin of the Yilgarn Craton. This, and relationships in the Jack Hills greenstone belt shows that the Paleoproterozoic Capricorn Orogeny has affected a greater portion of the Yilgarn Craton than previously thought. Unravelling these complex structural relationships is a fundamental step towards understanding granite-greenstone formation, and the early evolution of one of Earth's oldest crustal fragments.