Abstract Twenty magnetostratigraphic profiles from IODP (International Oceanic Drilling Project) sediment cores distributed on the Earth's surface allowed the scatter of virtual geomagnetic poles (VGPs) during Brunhes–Matuyama times to be examined. We identified two groups of recording sites which give different paths for the VGPs during a time interval of about 1.1 myr. Calculations of the VGP velocities and accelerations, as well as the corresponding azimuths, resulted in mean/median values like those observed for recent times. No significant differences were observed during the ‘stable’ and transitional fields. The acceleration azimuths show variations from north–south to east–west depending on the field state: normal/reversed or transitional. Despite the uncertainties in the magnetization of the sediments (overprints and/or low-resolution records), we demonstrate that the use of this database is valid for obtaining kinematic parameters of the geomagnetic field when analysed on a statistical basis.

Abstract We present a synthesis based on the interpretation of two pairs of deep seismic reflection crustal sections within the Southern Rift System (SRS) separating Australia and Antarctica. One pair of sections is from the conjugate margins between the Great Australian Bight (GAB) and Wilkes Land, in the central sector of the SRS, which broke up in the Campanian. The second pair of conjugate sections is located approximately 400 km further east, between the Otway Basin and Terre Adélie, which probably broke up in Maastrichtian time. Interpretations are based on an integrated synthesis of deep multi-channel seismic, gravity and magnetic data, together with sparse sonobuoy and dredging information, and the conjugate sections are presented with the oceanic crust removed beyond the continent–ocean boundary (COB). At first order, both conjugate pairs show a transition from thinned continental crust, through a wide and internally complex continent–ocean transition zone (COTZ), which shows features in common with magma-poor rifted margins worldwide, such as basement ridges interpreted as exhumed subcontinental mantle. In the central GAB sector, the COTZ is symmetric around the point of break-up and displays a pair of mantle ridges, one on each margin, outboard of which lies a deep-water rift basin. Break-up has occurred in the centre of this basin in this sector of the SRS. In contrast, the Terre Adélie margin is nearly 600 km wide and shows an abandoned crustal megaboudin, the Adélie Rift Block. This block is underlain by interpreted middle crust, and appears to have a mantle ridge structure inboard, as well as an outboard exhumed mantle complex from which mylonitized harzburgite has been dredged. The conjugate margin of the Beachport Sub-basin is relatively narrow ( c. 100 km wide) and does not appear to contain an exhumed mantle ridge, as observed along strike in the GAB. These observations from a single rift spreading compartment show that radically different break-up symmetries and margin architectures can result from an essentially symmetric rifting process involving multiple, paired detachment systems. This indicates the need for caution in interpreting causative mechanisms of rifting from limited conjugate sections in other rifts. We speculate that the underlying crustal composition, rheology and structural preconditioning play a significant role in partitioning strain during the transition to break-up.

The Nullarbor Plain of southeastern Australia, ∼200,000 km 2 in area, is flat and mostly treeless. It contains widely scattered collapse dolines and a few hundred caves, some of which are large and extensive. Initial karst development probably occurred during the warm, seasonally wet climatic conditions of the Oligocene, when the withdrawal of the sea exposed the recently deposited Eocene Wilson Bluff Limestone for over ∼10 m.y. Several major conduits probably developed at this time. These were flooded by the return of the sea, which finally retreated in the late Miocene followed by regional uplift. Cave formation in the Pliocene and Quaternary was inhibited by the semiarid climate, which became increasingly arid ca. 1 Ma. The overall dryness caused crystallization of evaporite minerals in cracks and pore spaces within the limestone walls of the caves, and they suffered extensive collapse, producing large passages, dome chambers, and dolines. However, during a wet phase 5–3 Ma, rivers extended across the karst plain, and caves formed where they sank into the limestone. Shallower caves probably also formed at this time, perhaps associated with perched water tables. The Nullarbor Plain did not develop extensive surface and underground karst features, even during the wetter climate of the Oligocene. It appears that the flatness of the plain and the particular characteristics of the limestone (primary porosity and lack of jointing and inception horizons) resulted in relatively uniform downwasting and little cave formation. Climate played a relatively minor role in restricting karst development.