--> Thrust Tectonics of the Canning Basin, Western Australia — An Alternative Interpretation

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Thrust Tectonics of the Canning Basin, Western Australia — An Alternative Interpretation

Abstract

Existing extensional tectonic models of the Canning Basin have difficulties explaining some aspects of the basin's evolution, particularly the dynamic relationship between thick sections in the Fitzroy Trough – Gregory Sub-basin and the contemporaneous Alice Springs Orogeny (ASO), and the differences in the timing and thicknesses of sedimentary fill between the northern and southern parts of the basin. A review of geophysical and well data from a crustal perspective, when integrated with a recently acquired Canning deep crustal seismic survey, provides an alternative interpretation of basin formation. Isostatic gravity anomaly imagery of the Canning Basin show a belt of unusual signatures in the centre of the basin, where the shallow basin platforms and deep terraces correspond to low and high gravity anomalies, respectively. These unusual gravity anomalies indicate that basement rocks below the terraces are much denser than those below the platforms thereby overpowering the gravity response from shallow sedimentary rocks. The anomaly boundary is interpreted here as the suture zone between the North Australian Craton to the northeast and the Paterson Orogen to the southwest. Interpretation of the deep crustal seismic profile suggests that the suture was possibly reactivated during the regional late Ordovician to mid-Carboniferous ASO, and the Canning Basin's evolution was related to this reactivation. Following initial subsidence of the Canning Basin in the early Ordovician, the NE margin of the Paterson Orogen beneath the southern part of basin was thrust over the SW margin of the North Australian Craton to the north in the Paleozoic. This prolonged event formed a foreland basin in the north and a restricted marine embayment in the south, separated by a basin high above the cratonic suture zone. The north foreland basin comprises the Fitzroy Trough and Gregory Sub-basin, with a foredeep along the southern side next to the Fenton Fault Zone in places, and terraces corresponding to the wedge top above the thickened crust. In the south of the basin this led to early Ordovician open marine environments becoming progressively more restricted due to the uplift along the convergent boundary, culminating in the precipitation and deposition of thick widespread evaporites during the late Ordovician to early Silurian.