Mechanisms of intracratonic and rift basin formation: Insights from Canning Basin, northwest Australia

The Canning basin was investigated in order to determine the mechanisms responsible for its initiation and development. The basement morphology, determined using magnetic and gravity inversion techniques, was used to map the distribution, amplitude and subsidence history of the basin. The sag development of the Canning basin is hypothesized to be a consequence of a major late Proterozoic thermal event that induced broad-scale uplift, extrusion of tholeiitic basalt, and substantial crustal erosion. The development of the Canning basin is consistent with removal of up to 11 km of crustal rocks, followed by isostatic re-adjustment during the cooling of the lithosphere. Earlier models that employed both lower crustal metamorphism and erosion are considered inappropriate mechanisms for intracratonic basin formation because this work has shown that their effects are mutually exclusive. The time scale for the metamorphic-related subsidence is typically short (<10 m.y.) and the maximum subsidence is small (<4 km) compared to the long subsidence (ca. 200 m.y.) and maximum depths (6–7 km) recorded in the Canning basin. Observed amplitudes and rates of basement subsidence are compatible with a thermal anomaly that began to dissipate in the early Cambrian and lasted until the Permian. Punctuating the long-lived intracratonic basin subsidence is a series of extensional pulses that in Silurian to Carboniferous/Permian time led to the development of several prominent normal faults in the northeastern portion of the Canning basin (Fitzroy graben). Stratigraphic and structural data and section-balancing techniques have helped to elucidate the geometry and evolution of the basin-bounding fault of the Fitzroy graben. The fault is listric, with a dip that decreases from approximately 50° at the surface to 20° at a depth of 20 km, and with an estimated horizontal offset of 32–41 km. The southern margin of the Fitzroy graben was tilted, truncated, and onlapped from the south, consistent with the flexural rebound of a lithosphere with an elastic thickness of ca. 30 km.