TWO BILLION YEARS OF FAULT REACTIVATION IN THE CAPRICORN OROGEN, WESTERN AUSTRALIA

The Proterozoic Capricorn Orogen is a 1000 x 500 km belt of deformed meta-igneous and metasedimentary rocks between the Archean Pilbara and Yilgarn Cratons in Western Australia. The orogen records the collision of these two cratons (during the 2215–2145 Ma Ophthalmian and 2005–1950 Ma Glenburgh Orogenies), and over two billion years of coaxial intracratonic re-working. Detailed geological mapping, as well as high-precision geochronology and a recent deep crustal seismic reflection survey across the orogen, have documented at least five tectonic re-working events, the style and orientation of which, were controlled by the pre-existing collisional architecture. High-grade metamorphic, structural and magmatic re-working within discrete tectonic corridors is recorded during the Capricorn (1820–1770 Ma) and Mangaroon (1680–1620 Ma) Orogenies. However, although subsequent re-working events were coaxial to the pre-existing structure, they were magma-poor and of lower metamorphic grade. Between 1620 and 1465 Ma, up to 7% of NE–SW-directed extension, opened the Edmund Basin, into which 4–5 km of siliciclastic sedimentary rocks (the Edmund Group) were deposited. The basin architecture was controlled by the extensional reactivation of former compressional faults and shears zones. The Edmund Basin was subsequently deformed and faulted during NE–SW-oriented transpressional shortening (up to 24%) during the Mutherbukin Tectonic Event (1385–1170 Ma). The structural controls on sediment deposition in the Collier Basin are largely unknown, but sediment was supplied by the uplift and erosion of the Edmund Group sedimentary rocks at ~1070 Ma. Both basins were folded during NE-SW shortening (4–11 %) of the Edmundian Orogeny (1030–950 Ma). During the Mulka Tectonic Event at ~570 Ma, the major structures were reactivated under a dextral strike slip regime. Present-day reactivation is recorded by seismic activity on the major fault structures. The style and orientation of over two billion years of crustal re-working in the Capricorn Orogen, including deformation, metamorphism, magmatism and sedimentation, has ultimately been governed by a pre-existing crustal architecture which formed during Paleoproterozoic collisional orogenesis.