Extension in granulite facies orthogneisses of the Biranup Complex (Albany-Fraser Orogen), Bremer Bay, Western Australia

Granulite-facies rocks from the Bremer Bay area of the Grenville-aged Albany-Fraser orogen, Western Australia, show widespread, pervasive bidirectional extension during overall orogenic-wide contraction. Crustal extension occurred ca. 1180 Ma, based on SHRIMP U–Pb zircon geochronology of melts generated during deformation, which coincides with Stage II (1210-1140 Ma), a period of NW-directed contraction. Contractional and/or shear-related structures alternate with extensional structures; melt generation was concurrent with all stages of deformation.

Granulites in the Bremer Bay area are dominantly stromatic migmatites comprising tonalitic, granodioritic and dioritic orthogneisses. The deformation recorded by these rocks, from oldest to youngest, consists of: (1) formation of localized isoclinal folds of cm-wide melt bands; (2) formation of prevalent migmatitic fabric, defined by alternating leucosomes and melanosomes, parallel to the main compositional layering; (3) simultaneous NE-SW and NW-SE extension, which formed cm-scale square boudins of mafic/restitic layers parallel to the main migmatitic fabric; (4) formation of km-scale, N-NW verging overturned folds, and associated outcrop scale (centimeters to a few meters) folds, which are upright to overturned, isoclinal to open, and both N-NW and S-SE vergent; (5) renewed coeval NE-SW and NW-SE extension that produced intermediate (< 1 meter to a few meters) to large (decameter) scale boudins of the migmatitic fabric. Granitic melts segregated in boudin necks of intermediate NE-SW and NW-SE boudins have given SHRIMP U-Pb zircon ages of 1187±5 Ma and 1178±4 Ma respectively.

Widespread bidirectional extension of the Bremer Bay orthogneisses coupled with extensive melting and granulite facies metamorphism is most likely related to upwelling of asthenospheric mantle. The complex history of contraction and extension during overall NW-directed collisional orogenesis is consistent with either “breakoff” of a subducting oceanic slab and/or delamination of a thickened lithosphere, or accelerated slab rollback during subduction. With current tectonic models for the Albany-Fraser orogen, slab-breakoff or delamination is favored.