Stratigraphic record of an Atlantic‐type global tectonic cycle in the Palaeoproterozoic Ashburton Province of Western Australia

Abstract

New descriptive and interpretive stratigraphic frameworks for the Ashburton (and Gascoyne) Province of the Capricorn Orogen are based on sequence stratigraphy. Two megasequences are recognised. A lower megasequence (Megasequence VIIIL) is interpreted to record the opening, to the northwest, of an Atlantic‐type ocean. Four supersequences record rift and thermal‐subsidence stages of a divergent continental margin, with north‐ or northeast‐trending ocean‐margin and northwest‐trending failed‐rift geometries. There is no evidence for a southern oceanic regime. Rather, the Gawler Craton of South Australia is interpreted to have lain to the southwest of the Pilbara Craton, such that the cratons were a contiguous divergent continental margin. An upper megasequence (Megasequence VIIIU) is interpreted to record the conversion of the divergent margin to a convergent (hinterland) margin during southeast‐dipping subduction. Eight supersequences are recognised. These record backarc, remnant‐ocean, retroarc‐satellite and indent‐linked hinterland basins. All the basins preserve the failed‐rift geometry. To the southwest, collision‐indentation of the hinterland (Pilbara‐Gawler) continent by an indenter (Yilgarn Craton) is interpreted to have begun during the remnant‐ocean stage. An east‐ to east‐southeast‐trending megashear between the Pilbara and Yilgarn Cratons is interpreted to have been the boundary transform fault on the northern margin of that indenter.