On thrusting, regional unconformities and exhumation of high-grade greenstones in Neoarchean orogens. The case of the Waroonga Shear Zone, Yilgarn Craton

During the Neoarchean, the dominant tectonic style progressively changed from an episodic-overturn/stagnant-lid regime to modern-style plate tectonics. The Neoarchean strengthening of continental lithosphere changed the style of deformation of orogenic belts. The case study presented here provides insights into how such transition in tectonic style occurred, a matter that is generally controversial. We present structural and metamorphic data from the c. 2660 Ma Waroonga Shear Zone (WSZ) in the Neoarchean Yilgarn orogen (Western Australia). The WSZ contains a syntectonic pluton and older, high-grade greenstones. The tectonic fabric in the pluton developed during melt-present thrusting, followed by syn-cooling wrench-dominated transpression. Mafic greenstones preserve three metamorphic assemblages. The M1 assemblage (Grt–Cpx–Qtz) records peak P–T conditions of 12 ± 1 kbar and 800 ± 50 °C, followed by isothermal decompression to ~ 9 kbar (M2). These anhydrous assemblages might predate the WSZ. Greenstones then underwent decompression at c. 2660 Ma (3–4 kbar; 600–650 °C), defined by the amphibole-rich M3 assemblage, synkinematic with the tectonic fabric in the WSZ. We show that shearing along the WSZ exhumed these greenstones by at least 10 km, inducing major uplift and erosion. Archean accretionary orogens developed on weak lithosphere, where deformation suppressed crustal thickening, orogenic relief and synorogenic exhumation of orogenic roots. However, our study indicates a genetic link between (i) strain localization along contractional structures, inducing large-scale uplift; (ii) exhumation of high-grade greenstones; (iii) development of inverse metamorphic gradients; (iv) establishment of a regional unconformity, with clastic sediments fed by the uplifted terrane; (v) incorporation of portions of the newly-formed orogenic basins into the footwall of the WSZ. These features imply that the Yilgarn orogenic lithosphere at c. 2660 Ma was stiff enough to allow tectonic processes analogous to those that dominate modern-style orogenic belts, representing an intermediate stage between Archean- and modern-type orogens.