Melt-present shear zones enable intracontinental orogenesis

Abstract

Localized rheological weakening is required to initiate and sustain intracontinental orogenesis, but the reasons for weakening remain debated. The intracontinental Alice Springs orogen dominates the lithospheric architecture of central Australia and involved prolonged (450–300 Ma) but episodic mountain building. The mid-crustal core of the orogen is exposed at its eastern margin, where field relationships and microstructures demonstrate that deformation was accommodated in biotite-rich shear zones. Rheological weakening was caused by localized melt-present deformation coupled with melt-induced reaction softening. This interpretation is supported by the coeval and episodic nature of melt-present deformation, igneous activity, and sediment shed from the developing orogen. This study identifies localized melt availability as an important ingredient enabling intracontinental orogenesis.

Pressure–temperature–time constraints on gneiss dome formation in an intracontinental orogen

“…The Amadeus and Georgina Basins represent fragments of the formerly continental-scale Centralian Superbasin (Figure 1a) that was separated into structural remnants as a consequence of Palaeozoic basement exhumation associated with the c. 450-300 Ma intraplate Alice Springs Orogeny (Buick et al, 2005;Haines et al, 2001;Maidment et al, 2013;Shaw et al, 1991). Similarly, the present arrangement of the HRG is ascribed to the ASO (Haines et al, 2001;Maidment et al, 2013;Piazolo et al, 2020;Raimondo et al, 2011;Silva et al, 2018). This event exhumed the HRG rift complex and underlying Paleoproterozoic Arunta basement, including the EGC, from deep crustal levels (≥20 km), and inverted the north-eastern section of the overlying Centralian Superbasin (Sandiford & Hand, 1998).…”

“…On a regional scale, the EGC is separated by a structurally overlying and rheologically contrasting cover sequence known as the Harts Range Group (HRG; Figures 1b and 2), which experienced deep rift-related metamorphism at 480-460 Ma, immediately prior to the onset of the ASO (Hand, Mawby, Kinny, & Foden, 1999;Tucker et al, 2015). Both the EGC and HRG are dissected by voluminous pegmatite swarms that appear to be episodically emplaced throughout the entire duration of the ASO, matching known intervals of synorogenic sedimentation and high-temperature deformation (Figure 1) recognized in adjacent parts of the orogen (Buick et al, 2008;Haines et al, 2001;Piazolo et al, 2020). However, the vast majority of existing geochronology is derived from the HRG, which appears to have a distinctive temporal evolution to the EGC (Buick et al, 2008;Tucker et al, 2015).…”

“…In the Reynolds Range to the northwest (Figure 1b), it is characterized by aqueous fluid-rock interaction in discrete shear zones at P-T conditions of 5.0-6.0 kbar and 500-600 C (Cartwright & Buick, 1999;Prent et al, 2020;Raimondo et al, 2011;Raimondo et al, 2017). In contrast, the deeper orogenic section in the Strangways and Harts Ranges to the southeast is characterized by pervasive pegmatite swarms (Buick et al, 2008), evidence of melt-rock interaction in shear zones (Piazolo et al, 2020), and higher metamorphic conditions of 7-9 kbar and $700 C (Arnold et al, 1995;Mawby et al, 1999). The Palaeozoic HRG (Figure 1b), which marks the southeasternmost extent of the Arunta Region (Figure 1a), is bound to the south and north by reworked Paleoproterozoic basement and Neoproterozoic-mid-Palaeozoic (c. 900-300 Ma) sediments of the intracratonic Amadeus and Georgina Basins (Edgoose, 2013;Haines et al, 2001).…”

“…The Entia Gneiss Complex (EGC) is located in the Harts Range, central Australia (Figure 1a,b) and represents the exhumation of Paleoproterozoic granulite facies basement that underwent hydration and recrystallization at amphibolite facies conditions during the Palaeozoic intracontinental Alice Springs Orogeny (ASO; 450-300 Ma) (Buick et al, 2008;Fournier et al, 2016;Hand, Mawby, Kinny, & Foden, 1999;Piazolo et al, 2020;Prent et al, 2020;Silva et al, 2018). The EGC crops out in an approximately elliptical domal structure that is 25 Â 15 km in diameter along its major and minor axes, respectively, and contains two antiformal culminations referred to as the Inkamulla and Huckitta sub-domes (Figure 2).…”

Garnet as a monitor for melt–rock interaction: Textural, mineralogical, and compositional evidence of partial melting and melt‐driven metasomatism

“…Partial melting is a fundamental process for differentiating (Brown, 2010) and weakening the deep crust (Piazolo et al, 2020;Rosenberg & Handy, 2005), which in turn exerts a first-order control on deformation style, lateral redistribution of mass, crustal thickness, and surface topography (Beaumont et al, 2004;Hollister & Crawford, 1986;Jamieson et al, 2011;Rabin et al, 2015;Royden et al, 1997;Vanderhaeghe, 2012). Exhumed migmatites preserve, through their mineralogy and structure, a wealth of information that can be used to decipher the thermo-mechanical behaviour of orogenic continental crust.…”