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 d eposited and deformed along the northern margin of Archaean Yilgarn Cr aton, Western Australia, during the ca 2.0-1.8 Ga Capricorn Orogeny. T his orogeny is interpreted to record convergence and collision of the Yilgarn Craton with the Pilbara Craton to the north. The Bryah Group w ith its extensive mafic and ultramafic volcanic rocks represents a sho rt-lived proto-oceanic backarc basin whereas the sedimentary Padbury G roup records the filling in of a retro-arc foreland basin. The two gro ups were deformed as a single package and are now everywhere in faulte d contact with older Palaeoproterozoic rocks or underlying Archaean gr anite-greenstone. Four events are recognized in a progressive compress ional deformation regime. During D1 subhorizontal high strain shear zo nes and tectonic interleaving developed between the volcanosedimentary succession and reworked Archaean crustal fragments that were underthr ust or underplated during initial basin closure. Movement directions w ere mainly north-to-south. D2 N-S shortening resulted in upright E-W f olds dominated by two regional, basement-cored, anticlines, which are interpreted as fault bend folds above a crustal scale frontal thrust r amp. Uplifted deeper crustal rocks are now represented by zones of hig her 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 overprin ting, and are regarded as contemporaneous. The D3 structures are inter preted 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 deformat ion history records the closure and deformation of marginal back-are a nd foreland basins. (C) 1998 Elsevier Science B.V.