Tectonic evolution of deep crustal structures in the mid-Proterozoic Albany-Fraser Orogen, Western Australia

Abstract

The Albany-Fraser Orogen resulted from the collision of a West Australian continent with a South Australian-East Antarctic continent at 1300 Ma. The northern part of the orogen is well exposed along the south coast of Western Australia, and provides abundant examples of related large- and small-scale structures that formed at deep crustal levels. The orogen was split longitudinally during the rifting of Australia from Antarctica at about 100 Ma, and the southern part of the orogen is exposed along the adjacent coast of Antarctica.

In Australia, the orogen is dominated by thrust slices and duplexes a few kilometres thick and hundreds of kilometres long that formed at deep crustal levels. The structures incorporate Archaean and early Proterozoic granitic rocks together with large volumes of newly generated gabbro and granite, tectonically interleaved with Proterozoic sedimentary rocks. Deformation and contemporaneous metamorphism converted the rocks to gneisses, and tectonic fabrics developed in amphibolite or granulite facies. Early gneissose fabrics were cut by granite, and irregular melt pegmatites formed in low-strain zones during the peak of metamorphism immediately following the major episode of tectonic interleaving. Many of these granites were themselves converted to gneiss and mylonite as slabs of rock continued to be transported over each other at deep crustal levels. This tectonic stacking appears to represent enormous shortening and thickening of the leading edge of the South Australian-East Antarctic continent.

The thrust pile was then elevated over a ramp formed by the southern edge of the West Australian continent, transported on to the edge of the continent, and deeply eroded. Associated tectonic fabrics formed at decreasing metamorphic grades from amphibolite to greenschist facies and ductile structures were increasingly superseded by brittle structures. This all occurred between c. 1300 and 1280 Ma.

Between 1180 and 1130 Ma extensive granite sheets and batholiths were emplaced to the south-east of the ramp that was the former margin of the West Australian continent. These granites may have been derived by melting of the, still deep, thrust-thickened crust that had not been elevated over the ramp to the northwest.

Acknowledgement: Published with permission of the director of the Geological Survey of Western Australia.