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Richard Blewett, Geoscience Australia (Australia)
Karol Czarnota, Geoscience Australia (Australia)
Leonie Jones, Geoscience Australia (Australia)
Paul Henson, Geoscience Australia (Australia)
Ben Goscombe, ITAC (Australia)
Bruce Goleby, Geoscience Australia (Australia)
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Deep seismic reflection profiles provide a unique view of the architecture of the crust and upper mantle, leading to inferences regarding its geodynamic evolution. The Eastern Goldfields Superterrane (EGST) in Western Australia is a classic Archaean granite-greenstone terrane that hosts some of the world's best gold and nickel deposits. Defining the geodynamics and its resultant architecture are two critical components in understanding these major mineral systems.
Two major across strike seismic transects totalling 614 km (91EGF01 and 01AGSNY1) have been acquired, and these are augmented by numerous shorter seismic traverses. The result is a comprehensive grid of traverses that have been integrated with the geology and the potential field geophysics to create a series of 3D models (maps).
The post seismic paradigm for interpreting the folds (domes) and faults was one of thrust duplexing in an overall cyclic contractional-extensional tectonic setting. One of the features of the general map patterns, preservation of stratigraphy, the metamorphic isograds and exhumation of high-grade rocks, and the meso- to macro-scale structures suggests that extension played a more important role in creating the essential architecture of the terrane (and hence the seismic fabric) that initially thought.
A key moment in the importance of interpreting extension in terms of the seismic data became apparent as a result of the reprocessing of the upper 2 seconds of the data. Careful velocity analysis revealed a hitherto unseen extensional fabric in the seismic data, which could be traced into a well-exposed mine. Tracing these seismic events to depth changed the thinking of the entire seismic line, which better matched the emerging new geological understanding of the region.
The result of this geological-geophysical integration (especially the targeted re-processing) has been an improved systems understanding of this most endowed piece of crust. Similar seismic patterns are observed in the Superior Province of Canada, although they are traditionally described in terms of a contractional tectonic mode. Careful reprocessing of the seismic data, together with a reappraisal of the geology may also similarly change paradigms in Canada?
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