THE PROBLEM WITH EAST GONDWANA; A MID-PERMIAN GLIMPSE FROM THE SYDNEY BASIN, AUSTRALIA

During the Late Carboniferous, the southern continent of Gondwana and the northern continent of Laurasia assembled into the massive supercontinent Pangaea. Paleogeographic reconstructions can be made with high precision through the post-Triassic paleomagnetic correlation with marine magnetic anomalies. A number of recent studies have focused on the configuration of the Permian-Triassic Pangaea, however, there is an unresolved controversy for reconstructions preceding this time. The problem may be a combination of the low reliability of paleomagnetic data, issues such as inclination shallowing in the sedimentary record, or alternative geometries for the supercontinent. It is therefore necessary to help complete the paleomagnetic record through the analysis of Early-Middle Permian igneous rocks.

Along the easternmost segment of Gondwana lies the Sydney Basin of eastern Australia, an intracratonic basin that records several episodes of back-arc extension, volcanism, and foreland basin deposition. Reliable paleomagnetic Apparent Polar Wander Paths (APWP) from Laurasia and West Gondwana display a unified Pangaea from the Late Carboniferous to Jurassic times, with a unified Gondwana from the Cambrian to the Jurassic. However, when paleomagnetic data from East Gondwana are considered, the APWPs of East and West Gondwana contain significant longitudinal displacement.

Here we present new paleomagnetic and geochronologic data from the flat lying rocks of the southern Sydney Basin, which is considered to be a cratonic and unrotated piece of Australia, and can be used reliably for Gondwana reconstructions for the mid-Permian. The Gerringong Volcanics of the southern Sydney basin form a series of 10 andesitic, basaltic, to shoshonitic basalts, which formed from the eruption of an offshore volcanic arc. Further north, however, volcanism had ceased by the Early Permian in the New England Orogen during the formation of a complex orocline system. The timing between these two volcanic events necessitates a modified tectonic model for the subduction zone. More importantly, results from the volcanics will help provide some resolution for the specious longitudinal discrepancy between the East and West Gondwanan APWPs.