A rift-drift-delamination model of continental evolution: Palaeozoic tectonic development of eastern Australia

Abstract

The Adelaide, Lachlan and New England fold belts of eastern Australia record the continental growth of eastern Gondwanaland during the Palaeozoic. The New England Fold Belt (NEFB) represents a tectonic collage formed by subduction/accretion during the Late Paleozoic, but contains remnants of subduction-related rocks that date from the Cambrian. The Lachlan Fold Belt (LFB) developed inboard from the NEFB, initially as an amalgam of Proterozoic continental and Cambrian oceanic fragments that formed by rift and drift at the leading edge of eastern Gondwanaland. Convergent tectonism at ~500 Ma welded the fragments to the craton and formed the Adelaide fold belt. Oceanward dispersal of detritus across the LFB produced an overlap assemblage of quartzose Ordovician turbidites, and a new subduction zone developed at the eastern margin. The tectonic setting was similar to the modern Philippines plate of the western Pacific. Behind-the-arc, Silurian-Devonian convergent tectonics converted the 1700–2000-km-wide continental margin of the Proto-LFB into a ~ 750-km-wide, thin-skinned, fold-magmatic belt within 60 Ma. The driving force was delamination, which produced a ubiquitous basaltic underplate that generated: (1) regional low- P metamorphism; (2) local anticlockwise P- T- t paths and crustal-scale isobaric cooling; (3) voluminous syn- to late-tectonic granitoids, emplaced at rates approximately twice that of modern arcs; and (4) normal thickness crust, despite an average of ~60% shortening. Delamination occurred in two separate areas originally ~ 1000 km apart, first in the eastern, then in the western part of the LFB. The LFB and its northern counterpart, the Thomson fold belt, represent approximately one-fifth of the Australian continent, and evolved from dispersed fragments into stable continental crust in ~300 Ma. Such rapid growth rates are typical of the 2.7–2.5 Ga and 1.9–1.7 Ga periods, when voluminous, widespread granitoids and large turbidite-dominated, low- P metamorphic belts were produced in settings that are not obviously subduction-related. Therefore, a similar process of rift-drift-delamination (RIDDEL tectonics) may have periodically operated throughout Earth history.