--> Igneous Intrusions and Volcanic Complexes in the North West Shelf Passive Margin

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Igneous Intrusions and Volcanic Complexes in the North West Shelf Passive Margin

Abstract

Igneous intrusions and volcanic complexes are well developed in the Exmouth sub-basin and Exmouth Plateau of the North-West Shelf. Detailed interpretation of 2D and 3D seismic data particularly using multi-attribute analyses has enabled documentation of their distributions, forms, timings and interactions with the surrounding sedimentary sections. The dykes and associated sill complexes occur in the Triassic and Jurassic strata of both the Exmouth sub-basin and the Exmouth plateau. Some sills and dykes extend into the Early Cretaceous units and are linked to volcanic complexes and extrusives. The sill and dykes extend for many kilometres and form spectacular linked networks in cross-section. They are typically planar systems with the sills forming wide bed-parallel bodies whereas the linked dykes as typically stepped and fault controlled. Wing-like apotheoses are commonly found at the tips of the dykes. In some 3D surveys well-developed vertical hydrothermal vent systems are associated with the fault-controlled sill / dyke complexes. Large sills and dykes are particularly well-developed in the Triassic Mungaroo delta sequences. Volcanic complexes form large mound structures probably formed mainly by hyaloclastics and some formed by extrusive flows. Growth strata onlap these complexes indicating mainly Early Cretaceous ages. In many instances the tops of these complexes are strongly eroded with planar truncated tops. The distribution of these systems and their control by the passive margin extensional faults indicate an intimate relationship to the rifting and eventual break-up of this part of the NW Shelf passive margin. A new tectonostratigraphic model for this part of the NW Australian margin is discussed.