Structural-metamorphic evolution of the Palaeoproterozoic Bryah and Padbury Groups during the Capricorn orogeny, Western Australia

Volcano-sedimentary successions of the Bryah and Padbury Groups were deposited and deformed along the northern margin of Archaean Yilgarn Craton, Western Australia, during the ca 2.0-1.8 Ga Capricorn Orogeny. This orogeny is interpreted to record convergence and collision of the Yilgarn Craton with the Pilbara Craton to the north. The Bryah Group with its extensive mafic and ultramafic volcanic rocks represents a short-lived proto-oceanic back-arc basin whereas the sedimentary Padbury Group records the filling in of a retro-arc foreland basin. The two groups were deformed as a single package and are now everywhere in faulted contact with older Palaeoproterozoic rocks or underlying Archaean granite-greenstone. Four events are recognized in a progressive compressional deformation regime. During D1 subhorizontal high strain shear zones and tectonic interleaving developed between the volcano-sedimentary succession and reworked Archaean crustal fragments that were underthrust or underplated during initial basin closure. Movement directions were mainly north-to-south. D2 N-S shortening resulted in upright E-W folds dominated by two regional, basement-cored, anticlines, which are interpreted as fault bend folds above a crustal scale frontal thrust ramp. Uplifted deeper crustal rocks are now represented by zones of higher grade metamorphic assemblages in the core of these anticlines. D3 folds are upright, N-S striking and restricted to a zone (depression) between the two basement-cored anticlines (or domes). D2 and D3 folds are largely mutually exclusive with little or no evidence for overprinting, and are regarded as contemporaneous. The D3 structures are interpreted to have formed above lateral ramps in the crustal scale thrust fault during continued N-S shortening. D4 is a minor event of NNE-SSW compression and did not influence the regional geometry. This deformation history records the closure and deformation of marginal back-arc and foreland basins.