Intrusive large igneous provinces below sedimentary basins: An example from the Exmouth Plateau (NW Australia)

Large igneous provinces (LIPs) are commonly characterized by extrusion of huge outpourings of flood basalts. However, some LIPs associated with thick sedimentary basins display mainly intrusive sill and dike complexes and a relative absence of extrusives as evidenced on the Exmouth Plateau. Here a breakup-related 150 km x 400 km sill complex imaged on seismic reflection data intruded mainly Triassic sedimentary rocks between the Late Jurassic and the Early Cretaceous. The sill complex is most likely sourced by a mafic or an ultramafic magma chamber, seismically imaged as a high-velocity body (HVB) and covering similar to 16 x 10(4) km(2). This magma chamber is located at the base of the crust and did not generate extrusives. Simple hydrostatic calculations suggest that melt became vertically arrested in the basin sediments, primarily owing to a reduction in the magmatic overpressure gradient as a result of the differences between fracture and melt gradients controlled by upward decreasing densities of the basin fill. Furthermore, magma overpressures at the source between 5 and 20 MPa are required to explain the presence of sill complexes at 4-11 km depth, indicating that the HVB is the source of the sill/dike complex on the Exmouth Plateau. The extent and outline of the HVB places constraints on the origin of magmatism and LIP formation. In combination with published data, the results suggest a thermal anomaly (upwelling or plume) source for the observed magmatism.