Structural Evolution of a 1.6 Ga Orogeny Related to the Final Assembly of the Supercontinent Nuna: Coupling of Episodic and Progressive Deformation

Abstract

The poly-deformed Georgetown Inlier (GTI) in NE Australia has recently been suggested to record a 1.60 Ga orogenic event related to final Nuna assembly. However, the structural evolution of the inlier has remained poorly constrained at the regional scale, and major tectono-thermal events occurred at circa 1.55 Ga. The GTI is the type region for conceptualization of crenulation cleavage development and where the foliation intersection axes (FIAs) approach has been applied. We reevaluated both concepts by combining a multiscale petrostructural analysis with recent petrological and geochronological data. Three main deformation events (D1–D3) and associated composite fabrics (S1–S3) are identified in the GTI. The original NE orientation of 1.60 Ga D1 compressional structures is preserved in the low-grade western domain, and the associated composite S1 fabric is retained as microstructural relicts within circa 1.55 Ga D2 low-strain domains to the east. Extensional D2 structures, characterized by a pervasive, high-grade, composite S2 foliation throughout the central and eastern domains, are interpreted as the footwall of a regional N-S trending, W dipping crustal-scale detachment zone. Syn-D2 S-type granites formed at 1.55 Ga as the detachment evolved. D1 stage was associated with Nuna assembly, whereas D2 represents postcollisional extension. Progressive foliation development occurred twice in the GTI, at 1.60 (D1) and 1.55 Ga (D2), but the previous FIA analysis only records the 1.60 Ga event and cannot be easily reconciled with the regional structural analysis. This study highlights that a multiscale and multidisciplinary approach is required to unravel the structural history of orogenic belts.