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Naturaliste Plateau, offshore Western Australia: A submarine window into Gondwana assembly and breakup

Jacqueline A. Halpin^*,1, Anthony J. Crawford¹, Nicholas G. Direen^,2, Millard F. Coffin³, Caroline J. Forbes^,4 and Irina Borissova⁵

^{1 1} ARC Centre of Excellence in Ore Deposits, University of Tasmania, Private Bag 79, Hobart, TAS 7001, Australia
^{2 2} Continental Evolution Research Group, University of Adelaide, DP 313, Adelaide, SA 5005, Australia
^{3 3} National Oceanography Centre, Southampton, University of Southampton, Waterfront Campus European Way, Southampton SO14 3ZH, UK
^{4 4} Geological Survey of Western Australia, 100 Plain Road, Perth, Western Australia 6004, Australia
^{5 5} Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia

Correspondence: *E-mail: jahalpin{at}utas.edu.au.

The origin of the submarine Naturaliste Plateau off the southwestern coast of Australia is controversial; previous work supports both oceanic and continental affinities for the basement to volcanic and sedimentary sequences. We report the first evidence of reworked Mesoproterozoic (ca. 1230–1190 Ma) continental crust, based on laser ablation–inductively coupled plasma–mass spectrometry analysis of zircons from granite and orthogneiss samples dredged from the southern margin of the plateau. Thermobarometry of peak metamorphic minerals and electron microprobe chemical dating of monazite reveal that these igneous rocks were metamorphosed to ~700 °C and ~6.5 kbar during the Cambrian Pinjarra Orogeny at ca. 515 Ma. These data confirm a continental origin for a significant swathe of the southern Naturaliste Plateau, and suggest that the protoliths may have affinities to Mesoproterozoic crust within the Albany-Fraser-Wilkes Orogen (Australia-Antarctica). The present Naturaliste Plateau basement beneath its volcanic carapace probably represents a middle-to lower-crustal extensional allochthon exhumed during Cretaceous hyperextensional breakup between Australia and Antarctica.

Key Words: extensional allochthon • Naturaliste Plateau • Pinjarra Orogen • U-Pb zircon ages • U-Th-Pb monazite ages