Large Igneous Provinces – Intrusive Beneath Sedimentary Basins, an Example from Exmouth Plateau, NW Australia

Australia: The Land Where Time Began

Large Igneous Provinces – Intrusive Beneath Sedimentary Basins, an Example from Exmouth Plateau, NW Australia

Very large LIPs are often characterised by very large outpourings of flood basalts. Some LIPs, however, that are associated with thick sedimentary basins form mainly intrusive sill and dyke complexes and a relative absence of extrusives, as evidenced by the Exmouth Plateau. In this paper Rohrman¹ reports a sill complex of 150 x 400 km related to breakup that is imaged on seismic reflection data that intruded mainly Triassic rocks at some time between the Late Jurassic and the Early Cretaceous. Rohrman¹ suggests the sill complex is most likely sourced by a mafic or ultramafic magma chamber, which has been imaged seismically as a high-velocity body (HVB) that covers ~16 x 10⁴ km². Situated at the base of the crust this magma chamber did not generate intrusives. It is suggested by simple hydrostat calculations that melt became vertically arrested in the sediments of the basin, primarily because of a reduction of the magmatic overpressure gradient that resulted from the differences between fracture and melt gradients that are controlled by densities, that are decreasing upwards, of the basin fill. Also, overpressures between 5 and 20 MPa at the source are required to explain the presence of sill complexes at depths of 4-11 km, which indicates that the HVB is the source of the sill/dyke complex on the Exmouth Plateau. Constraints are placed on the origin of the magmatism and LIP formation by the extent and outline of the HVB. When combined with published data the results suggest a thermal anomaly (upwelling or plume) source for the magmatism that is observed.

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