Anatomy of a 10 km scale sheath fold, Mount Hay ridge, Arunta Region, central Australia: The structural record of deep crustal flow

Abstract

Mount Hay ridge, Arunta Region, central Australia, exposes a 10 km scale, antiformal sheath fold formed in the deep crust (P ≈ 0.6–0.9 GPa) in Proterozoic time. Tilting and exhumation during the Devonian Alice Springs orogeny resulted in an oblique cross section, such that the once subhorizontal sheath fold now has a moderately dipping, gently folded axial plane and overturned southern limb. Fabric types (e.g., S = L, L > S) were mapped in the field, and finite strain data were collected from a widely distributed gabbroic granulite unit. Fabric and finite strain data sets exhibit remarkable correspondence and confirm the common assumption that fabric type is directly related to finite strain type for deep crustal rocks. Both data sets reveal a distinct pattern of oblate strain on fold limbs grading into prolate strain in the fold hinge. There is no field evidence for significant strain localization, and measured strain magnitudes are consistent across the fold. Thus, we infer that different structural domains record different though coeval strain paths and that strain compatibility was maintained across domain boundaries. The uniformity of foliation, lineation, and mesoscale fold orientations across Mount Hay and the consistent magnitude of maximum stretch across structural domains indicate that the sheath fold and subsidiary structures formed during a single, progressive deformational event. Based on its geometry, the inferred scale of the enclosing shear zone (≥8 km in thickness), and regional tectonic correlations, we infer that the Mount Hay sheath fold formed in a subhorizontal, crustal‐scale flow zone during the Paleoproterozoic Strangways orogeny (1740–1690 Ma). The record of deformation is consistent with fold formation either on one side of a channel or through detachment flow.