Paleomagnetic data from the New England Orogen (eastern Australia) and implications for oroclinal bending

Abstract

Orogenic curvatures (oroclines) are common in modern and ancient orogens, but the geodynamic driving forces of many oroclines remain controversial. Here we focus on the New England oroclines of eastern Australia, the formation of which had been previously broadly constrained to the Early–Middle Permian. This time interval encompasses periods of both back-arc extension (at ~300–280Ma) and subsequent contractional deformation (Hunter–Bowen Orogeny) that commenced at ~270Ma along the paleo-Pacific and Gondwanan subduction plate boundary. We present new paleomagnetic data from volcanic rocks that were extruded during the transition from extension to contraction (at ~272Ma), and we show that the oroclinal structure must have formed prior to the emplacement of the volcanic rocks. Our results thus indicate that oroclinal bending in the southernmost New England Orogen has been completed prior to the onset of Middle Permian contractional deformation. It is therefore concluded that the oroclines have likely formed during back-arc extension, and that a major contribution to the orogenic curvature was driven by trench retreat.