Microcontinents offshore Western Australia: insights into the make-up and break-up of East Gondwana

The seafloor offshore Western Australia records the spreading history between Greater India and Australia–Antarctica during the Cretaceous breakup of East Gondwana. During 2011, we retrieved the first dredge samples from prominent bathymetric highs, Gulden Draak and Batavia knolls – two submarine plateaux that lie > 1000 km west of the Perth Basin, at the boundary between the Perth Abyssal Plain and Wharton Basin, Indian Ocean. We recovered continental rocks from the steep western slopes that rise more than 3000m above the surrounding seafloor including granite, gneiss, schist and sandstone. Isotopic analysis of zircon and paleontology has constrained the age and affinity of basement and sedimentary rocks, confirming that these knolls are microcontinents rifted from the Indian plate during the Mesozoic breakup of East Gondwana.

Gulden Draak Knoll comprises a high-grade basement complex, including pelitic paragneiss (deposited < 1.1 Ga) and mafic orthogneiss (emplaced > 600 Ma) intruded by Cambrian granite (~540 Ma). Boulders and cobbles of felsic orthogneiss with Archean (~2.85 Ga), Mesoproterozoic (~1.3-1.2 Ga) and Cambrian (~530-510 Ma) zircon ages were likely locally sourced. Sampled basement from Batavia Knoll comprises granite and felsic orthogneiss emplaced at ~540-530 Ma. Dating of metamorphic zircon constrains the timing of granulite facies metamorphism to ~530-510 Ma. Fossil assemblages and detrital zircon ages constrain sampled sandstones to the late Early Cretaceous (Albian). The newly discovered microcontinents represent the Indian conjugate to both the Antarctic Wilkes-Queen Mary Land margin, and the Australian Naturaliste Plateau.

Our new data provide important constraints on the pre-rift configuration of East Gondwana. Incorporation of the newly discovered microcontinents in a ‘Leeuwin’ full-fit model satisfies key geological and geophysical constraints. Onshore in East Antarctica, rare geological outcrops recording latest Neoproterozoic to Cambrian-aged magmatism and metamorphism in this sector have been interpreted to represent part of a Pan-African-aged orogenic belt. The path of this belt into the Antarctic interior as well as the tectonic context is controversial, but the prevailing model is that a Gondwana-forming collision zone between blocks of broadly Indo–Antarctic and Australo–Antarctic affinity. There is strong evidence for Cambrian metamorphism and magmatism in the dredge samples from the knolls. However, our isotopic data do not easily fit within a convergent plate boundary model, as clear evidence of Cambrian subduction and arc magmatism during ocean closure is lacking. Instead we explore alternative models whereby the Gulden Draak and Batavia basement rocks were located at a transpressive plate margin or major shear zone at ~540-510 Ma. Exhumation some 350-400 Myrs later occurred near the nexus of East Gondwanan Mesozoic breakup, with final rifting from India at ~102 Ma.