Preservation of a fragmented late Neoproterozoic–earliest Cambrian hyper-extended continental-margin sequence in the Australian Delamerian Orogen

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

The formation of non-volcanic rifted margins by the progressive extension of the lithosphere: the example of the West Iberian margin

Non-volcanic margins such as the West Iberian margin exhibit certain characteristics, such as a deficit of synrift igneous rock, a zone of exhumed subcontinental mantle in the continent–ocean transition and an apparent extension discrepancy. These observations can be explained as a consequence of the progressive extension of the lithosphere above relatively cool mantle. The evolving rheological stratification of the lithosphere controls the style of extension at different lithospheric levels at different times; extension is probably heterogeneous at all stages, with lower crustal and upper mantle boudinage controlling the patterns of thinning and mantle upwelling early in the rift history, and complete crustal embrittlement and mantle serpentinization controlling the formation of late-stage detachment faults. Extension in the brittle crust is via multiple phases of faulting, with a general focusing of extension towards the incipient ocean.

The lack of melt is explained by a combination of heterogeneous extension of the lower lithosphere and a cool subcontinental geotherm. The extension discrepancy may in places be controlled by depth-dependent stretching of the crust through lower crustal boudinage, but may also simply be the result of incomplete recognition of the entire polyphase faulting history. The latter seems to be the case for West Iberia.

Evidence for all these processes can be found at the West Iberian rifted margins as well as those preserved and partially exposed in the Alps.