Abstract Mafic and ultramafic rocks intercalated with metamorphosed deep-marine sediments in the Glenelg River Complex of SE Australia comprise variably tectonized fragments of an interpreted late Neoproterozoic–earliest Cambrian hyper-extended continental margin that was dismembered and thrust westwards over the adjacent continental margin during the Cambro-Ordovician Delamerian Orogeny. Ultramafic rocks include serpentinized harzburgite of inferred subcontinental lithospheric origin that had already been exhumed at the seafloor before sedimentation commenced, whereas mafic rocks exhibit mainly enriched- and normal-type mid-ocean ridge basalt (E- and N-MORB) compositions consistent with emplacement in an oceanic setting. These lithologies and their metasedimentary host rocks predate deposition of the Cambrian Kanmantoo Group and are more likely to represent temporal equivalents of the older Normanville Group or underlying Neoproterozoic Adelaide Supergroup. The Kanmantoo Group is host to basaltic rocks with higher degrees of crustal contamination and yields detrital zircon populations dominated by 600–500 Ma ages. Except for quartz greywacke confined to the uppermost part of the sequence, metasedimentary rocks in the Glenelg River Complex are devoid of detrital zircon, and are interstratified with subordinate amounts of metachert and carbonaceous dolomitic slate suggestive of deposition in a deep-marine environment far removed from any continental margin. Seismic reflection data support the idea that the Glenelg River Complex is underlain by mafic and ultramafic rocks, and preclude earlier interpretations based on aeromagnetic data that the continental margin incorporates a thick pile of seawards-dipping basaltic flows analogous to those of volcanic margins in the North Atlantic. Correlative hyper-extended continental rift margins to the Glenelg River Complex occur along strike in formerly contiguous parts of Antarctica. Supplementary material: Geochemical data for mafic and ultramafic rocks in the Glenelg River Complex and correlative terranes, and U–Th–Pb data for western Victoria gabbros are available at http://www.geolsoc.org.uk/SUP18821

Deformation in accretionary orogens, such as the eastern Australian Tasmanides, is clearly partitioned either as thin-skinned thrusting or thick-skinned faulting, with structural style dependent on the nature and stratal thicknesses of the sequences involved. The thin-skinned thrust systems consist of either detachment-related folds and thrust sheets within attenuated passive margin sequences or thrust sheets of chevron-folded turbidites with leading imbricate-fan geometry that are developed within former submarine fans overlying back-arc basin oceanic lithosphere. Thick-skinned belts consist of major thrust faults that root into the seismic reflection Moho with no apparent common décollement and cause crustal-scale imbrication of former arc, forearc, submarine fan, and accretionary complex elements. The Tasmanides are a composite orogenic system made up of three distinct orogenic belts whose character and structural style have resulted from the deformation of different tectonic components; the former rifted passive margin to make the Delamerian Orogen, a turbidite fan system(s) in a back-arc setting to make the Lachlan Orogen, and an arc-subduction complex that includes some older accreted components to make the New England Orogen. The inboard Delamerian Orogen consists of an external, craton-vergent thrust belt with foreland-style, detachment-related folds and thrusts linked to a high-T/low-P metamorphic complex. The centrally located Lachlan Orogen is made up of three separate thrust systems largely developed in submarine turbidite fans and incorporates a shear-zone-bounded high-T/low-P metamorphic belt. The outermost New England Orogen is constructed from craton-vergent, fore-arc and magmatic arc sequences, subduction complexes, and ophiolite fragments.