Transition from symmetry to asymmetry during continental rifting: an example from the Bight Basin–Terre Adélie (Australian and Antarctic conjugate margins)

Abstract

Terra Nova, 24, 167–180, 2012AbstractThe rifting history of the magma‐poor conjugate margins of Australia and Antarctica is still a controversial issue. In this article, we present a model for lithosphere‐scale rifting and deformation history from initial Jurassic rifting to Late Cretaceous breakup for the conjugate Bight Basin–Terre Adélie section of the margin, based on the interpretation of two regional conjugate seismic profiles of the margins, and the construction of a lithosphere‐scale, balanced cross‐section, sequentially restored through time. The model scenario highlights the symmetric pattern of initial stretching resulting from pure shear at lithospheric‐scale accompanied by the development of four conjugate detachments and crustal half‐graben systems. This system progressively evolves to completely asymmetric shearing along a single south‐dipping detachment at the scale of the lithosphere. Antarctica plays the role of the upper plate and Australia, the lower plate. The detachment accounts for the exhumation of the mantle part of the Australian lithosphere, and the isolation of a crustal klippe separated from the margin by a serpentinized peridotite ridge. The total elongation amount of the Australian–Antarctic conjugate system reaches ∼473 km (178%). Elongation was partitioned through time: ∼189 and ∼284 km during symmetric and asymmetric stages respectively. During the symmetric stage, both margins underwent approximately the same degree of crustal stretching [∼105 km (75%) and ∼84 km (67%) for Australia and Antarctica respectively]. Again, both margins accommodated relatively the same elongation during the asymmetric stage: the Antarctic upper plate records an elongation amount of ∼284 km (88%) as crustal/mantle stretching, above the inferred low‐angle south‐dipping detachment zone, whereas the Australian lower plate underwent ∼270 km (206%) of elongation through mantle exhumation. Although the restoration process does not allow reconstruction of the precise geometry before deformation, we propose that the Jurassic early geometric evolution of the margins may have been controlled by the inherited structure or rheological heterogeneities of the continental crust; its later evolution is thought to relate to the mechanical evolution of the crustal and mantle material during exhumation, with a strong increase in localization of shear in the lower crust and mantle part of the Australian margin. The geometry of the rifted margins is comparable to other magma‐poor rifted margin such as the Newfoundland–Iberia margins or the exhumed Alpine Tethys margin exposed in the Central Alps.