The regional geophysical response of the Stuart Shelf, South Australia

Abstract

Analysis of regional aeromagnetic and gravity data sets shows that the Stuart Shelf is host to a major volcano-plutonic complex consisting of large volumes of felsic-dominated volcanic rocks and shallow-crustal level granitic bodies. The complex is interpreted as a major eruptive centre associated with the Gawler Range volcano-plutonic event of 1600 Ma to 1580 Ma. The mid-Proterozoic basement surface in the northern and central areas of the Stuart Shelf lies beneath a thick cover of younger sediments and is interpreted as representing a section through the lower-middle levels of the volcano-plutonic complex. Volcano-plutonic elements present at this stratigraphic level include collapsed cauldrons, epizonal granites, and ring faults associated with pluton emplacement. Some of these structures acted as fluid pathways during an extensive period of Fe-metasomatism associated with the volcano-plutonic event, and consequently are evident in the geophysical data as gravity and/or magnetic highs resulting from localised deposits of magnetite and hematite. The Stuart Shelf is a prime example of a terrane where the geophysical response is dominated by secondary alteration assemblages rather than primary lithologies. The presence of a thick cover sequence over the entire Stuart Shelf restricts direct examination of the volcano-plutonic complex. However, comparison of the geophysical data from the Stuart Shelf with data from well-exposed, well-documented terranes of a similar tectonic setting provides a useful insight into the possible structural and tectonic history of the Stuart Shelf. Such terranes include the mid-Proterozoic St. Francois Mountains of south-east Missouri and the Quaternary volcanic provinces of the mid-western United States of America.