Complex continental growth along the proto-Pacific margin of East Gondwana

Abstract

We use ambient noise recordings from the largest transportable seismic array in the Southern Hemisphere to image azimuthal variations in Rayleigh wave phase anisotropy in the crust beneath southeast Australia. This region incorporates a transition from the Precambrian shield region of Australia in the west to younger Phanerozoic terranes in the east, which are thought to have been formed by subduction-accretion processes. Our results, which span the shallow to lower crust, show a strong and consistent pattern of anisotropy that is oriented north-south, approximately parallel to the former margin of East Gondwana. However, significant deviations from this trend persist through the period range 2.5 s to >10 s. One of the most notable deviations occurs along the edge of cratonic Australia, where the Curnamona Province forms a salient into the younger accretionary terrane, here, the fast axis of anisotropy follows the boundary almost exactly, and is virtually coincident with magnetic lineations extracted from aeromagnetic data. To the east of this boundary beneath the Lachlan orogen, a region masked by the Cenozoic Murray Basin, the fast axis of anisotropy becomes strongly curved and traces out a semicircular pattern with a radius of 200-250 km. Farther east, the fast axis of anisotropy returns to a dominantly north-south orientation. These new findings provide strong observational support to recent geodynamic modeling results that demonstrate how large-scale oroclinal structures can become embedded in accretionary mountain belts.