Authors / CoAuthors
Gibson, G.M.
Abstract
Situated just inboard of the late Neoproterozoic Australian rift margin (Tasman Line), the Broken Hill region occupies a critical position in reconstructions of Rodinia, combining an older basement (Willyama Supergroup) deformed by Paleoproterozoic-Mesoproterozoic events with a subsequent record of crustal extension, dyke intrusion and syn-rift sedimentation commencing around 827 Ma. These events not only constrain the timing and initial direction of late Neoproterozoic continental extension but provide a critical test of competing reconstructions for Rodinia in which south-central Australia is juxtaposed against western Laurentia. Contrary to some reconstructions there is no continuation of 1100-1300 Ma Grenville-age rocks into Broken Hill (SWEAT) and alternative restorations based on juxtaposition of the Broken Hill and Mojave-Oaxaca terranes along the Sonora-Mojave mega-shear (southern USA) result in misalignment of this major palaeo-transform fault with late Neoproterozoic normal faults in south-central Australia. Differences in deformational history and tectonic setting also preclude simple matching of 1.7-1.60 Ga orogenic belts in Australia and Laurentia (AUSWUS). In contrast to the southwest margin of Laurentia which was dominated by plate convergence, terrane assembly and arc magmatism throughout much of the Late Proterozoic (Yavapai and Mazatzal orogenies), the Willyama Supergroup preserves a record of 1.72-1.67 Ga intracontinental rifting and crustal extension (D1) followed by nappe emplacement and crustal thickening after 1640 Ma, culminating in the 1600 Ma Olarian orogeny (D2). Crustal thickening produced a second generation of granulite-grade mineral assemblages in the Willyama Supergroup and was superimposed on rocks initially metamorphosed under low P ? high T conditions as a result of D1 crustal thinning and associated bimodal magmatism. The resulting counterclockwise P-T-time path is evident only in the structurally higher parts of the Willyama Supergroup whereas the underlying and once more deeply buried parts of the sequence reveal evidence of decompression and metamorphism under progressively lower pressures as might be expected to occur during emplacement of a metamorphic core complex. A major mylonite zone of D1 age separates upper and lower structural levels. Validation of existing reconstructions for Rodinia requires a greater range of temporally equivalent events be present in western Laurentia than is presently recognised.
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nonGeographicDataset
eCat Id
61020
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Cnr Jerrabomberra Ave and Hindmarsh Dr GPO Box 378
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- External Publication
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- AU-NT
- Australian and New Zealand Standard Research Classification (ANZSRC)
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- Earth Sciences
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- Published_Internal
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2004-01-01T00:00:00
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