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Australia: The Land Where Time Began |
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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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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |