The geophysical signatures and exploration potential of Australia's meteorite impact structures

Philip Hawke

Research output: ThesisDoctoral Thesis

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Abstract

[Truncated abstract. Please see the pdf version of the abstract for the complete text.] Thirty impact structures of confirmed or possible status are currently identified in Australia. Twenty-two of these structures are confirmed by the presence of meteorite fragments or shock metamorphic features that are diagnostic of meteorite impact. The remainder have an impact origin supported by strong secondary evidence. New impact structures are being discovered in Australia at a rate of about one every year, with geophysics a key tool in the identification of candidate structures for further investigation. It is estimated that between two and five times the current number of impact structures are yet to be discovered on the Australian continent. Past compilations of the geophysical signatures of impact structures, particularly of their potential field responses, have been focused on structures formed in mainly crystalline targets. From these studies the expected gravity response is an overall low due to fracturing of the target rocks, with a local gravity high common over the centre of large complex structures, due to the structural uplift of denser material. An overall demagnetisation of the target rocks by the high shock pressures generated by the impact is also expected, although central magnetic highs may also be produced by remanently magnetised melt or the uplift of magnetic rocks from depth. The geophysical signatures of fifteen Australian impact structures are discussed, including individual case studies on nine structures and a detailed study of the Yallalie structure. Only one of the structures discussed here was formed in crystalline rocks, with a further two in mixed sedimentary / crystalline targets. The other structures that were studied were formed in either Phanerozoic basins or mildly-deformed Proterozoic sedimentary rocks. The potential field responses of these structures show a greater variability than was expected, particularly between structures that were formed in different types of target rock. A positive gravity response is found over four structures formed in clastic sedimentary rocks deposited in a Phanerozoic basin. These anomalies are due to the emplacement of denser rock into the central uplift. A decrease in density due to brecciation is not apparent in this target rock type. Furthermore, it is suggested that by collapsing pore space and removing water, the density of wet sedimentary rocks may be locally increased by impact. Circular magnetic anomalies are found outside the central uplift of six impact structures formed in either Phanerozoic or weakly-metamorphosed Proterozoic sedimentary basins.
Original languageEnglish
QualificationDoctor of Philosophy
Publication statusUnpublished - 2004

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