THROUGH A GLASS, DARKLY: SYMMETRY AND ASYMMETRY IN THE MAGMA POOR RIFTED MARGINS OF SOUTHERN AUSTRALIA AND EAST ANTARCTICA

Integrated interpretation of deep-seismic and potential field data from the conjugate, magma-poor (or hyper-extended), rifted margins of the Great Australian Bight, southern Australia, and central Wilkes Land, East Antarctica, shows that there is pronounced symmetry of structures in a 300 km-wide zone straddling the axis of final breakup. This symmetry is observed consistently for a distance of some hundreds of kilometres along-strike. From inboard to outboard, both margins comprise: a zone of rapid attenuation of the crystalline continental crust; a high-relief basement ridge, interpreted as unroofed mantle peridotites, at the location of maximum necking of the continental crust; and a 100–120 km-wide continent–ocean transition zone that contains a sedimentary basin that is probably underlain by altered exhumed mantle and possible extensional allochthons of crystalline continental crust. The strong symmetry described here is in contrast to the asymmetry of the Iberia–Newfoundland margin (and elsewhere on the southern Australian margin) and is consistent with the operation of a symmetrical extensional detachment system deforming the whole crust in the centre of the rift.

The evidence from this margin is consistent with some mechanisms for rifting by depth-dependent thinning –that is it shows strong differential extension between the upper crust, and the lower and middle crust, with partitioning of strain by a series of stacked detachments. Movements on these detachments allow removal of the entire middle and lower crust in some areas, leaving mantle and upper crust in contact. However, the evidence suggests an alternative mechanism to asymmetric models of depth dependent thinning, in that this rift segment does not contain convex-upward extensional faults, or mantle exposed at the seafloor. Such variety in the outcomes from an extensional process warrant caution in the use of published models based on limited areas (such as Iberia and the Alps) to interpret all occurrences of non-volcanic / hyperextended margins in the geological record.