Microcontinent formation around Australia

Abstract

Microcontinents are common in the accreted continental geological record, but relatively rare in modern settings. Many of today's microcontinents are found in the Tasman Sea and Indian Ocean. These include the East TasmanRise, the Gilbert Seamount Complex, the Seychelles, Elan Bank (Kerguelen Plateau), and possibly fragments of the Lord Howe Rise and Norfolk Ridge, and the Wallaby Plateau. We review their history of formation, and propose that the mechanisms that led to their isolation were mostly plume-related. Tasman Sea continental fragments formed by ridge jumps onto adjacent continental margins after sea-floor spreading in the southern Tasman Sea commenced. The East Tasman Plateau was separated from the Lord Howe Rise at about chron 34 (83 Ma) and the Gilbert Seamount Complex rifted off the South Tasman Rise at roughly 77 Ma, by ridge jumps in opposing directions. Evidence for thermal anomalies under the central Lord Howe Rise, Ross Sea and possible eastern Australian margin explain ridge jumps that led to the isolation of the East Tasman Plateau, Gilbert Seamount and possibly the northern Lord Howe Rise and Dampier Ridge. In the central Indian Ocean, spectacular exposures of granite make the Seychelles a type example of a microcontinent. As in the Tasman Sea, ridge-plume interactions have been responsible for separating a thinned continental sliver from a large continent (India). Elan Bank, as part of the Kerguelen Plateau, represents another example of a continental fragment in the Indian Ocean. Newly identified M-sequence anomalies in the Enderby Basin, off Antarctica, suggest that this microcontinent was detached from India no earlier than 124 Ma when a northward ridge jump towards the Kerguelen plume may have isolated Elan Bank. This interpretation implies that a mantle thermal anomaly due to the incipient Kerguelen plume pre-dated the Rajmahal Traps, emplaced at about 118 Ma, by ∼6 million years. An early Kerguelen hot-spot position north of Elan Bank, with subsequent southward migration, is also supported by palaeomagnetic data and mantle convection models. The Wallaby Plateau off Western Australia, located between the Perth and Cuvier Abyssal Plains, may also include slivers of continental crust, but unequivocal evidence is not available, as it has never been drilled. Here it is suggested that most of these microcontinents formed by re-rifting of a young continental margin in the vicinity of a mantle plume stem. The weak inner flank of a rifted margin weakens further when passing over a mantle plume, causing a nearby spreading ridge to jump onto this zone of weakness. This process Isolates a passive margin segment, and leaves a narrow passive margin behind. After the onset of subduction and ocean basin destruction, such microcontinents may be accreted again to an active plate margin.