Structural geology of a ten kilometer scale lower crustal shear zone: Mt. Hay granulites, central Australia

This study reports field and microstructural observations of a well-exposed section of lower continental crust (Mt. Hay block, central Australia). The Mt. Hay granulites (pyroxenitic, anorthositic, intermediate, and quartzofeldspathic) were penetratively deformed at ∼800°, ∼8 kbar during the ∼1780--1720 Ma Strangways event and uplifted during the ∼300--400 Ma Alice Springs orogeny. Throughout the Mt. Hay block granulites, the shear sense is south-side-up (current orientation).; The Mt. Hay block is interpreted to be a ∼10 kilometer thick section of a subhorizontal lower crustal shear zone developed in an obliquely divergent tectonic setting. The Mt. Hay block is composed of two main kilometer-scale structural domains, Mt. Hay and Capricorn ridges, in which strain and kinematics of deformation were partitioned. Mt. Hay ridge records constrictional fabrics and is interpreted as a lower strain domain. The adjacent Capricorn ridge records flattening fabrics and is a high strain domain. Relative strain intensity is inferred from the intensity of foliation and lineation in outcrop and evidence for transposition. Strain localization occurs adjacent to the major lithological boundaries in Capricorn ridge and records competency contrast between major lithological units. In Capricorn ridge, quartz and plagioclase in quartzofeldspathic and anorthositic granulite, respectively, deformed by grain-boundary sliding and/or diffusion creep as inferred from microstructural observations including random crystallographic orientations.; Ductile deformation in this example of lower continental crust has the following characteristics: (1) Strain is partitioned into high strain zones both at the kilometer and 100-meter scales. (2) Lithological domains record different mechanical behaviors. (3) Microstructures indicate that dislocation creep was not the dominant deformation mechanism in quartz and feldspar. These observations suggest that strain localization and deformation mechanisms other than dislocation creep, such as grain-boundary sliding, are important processes during deformation at lower crustal conditions.