Authors / CoAuthors
Gibson, G.M. | Bernardel, G. | Hall, L.S. | Nicholson, C.J. | Rollet, N. | Totterdell, J.M. | Stacey, A.R. | Mitchell, C.H.
Abstract
Australia is bounded on three sides by passive continental margins, a legacy of Gondwana breakup as first India and then Zealandia, followed by Antarctica, separated from Australia during the Late Jurassic-Early Cretaceous through to earliest Oligocene. As with most other rifted continental margins, breakup along each of these three margins occurred episodically, controlled by a number of factors including mantle rheology, pre-existing lithospheric and basement structure, and the direction of crustal extension prevailing at any one time during successive stages of continental rifting. Resulting post-rift passive margin geometries are consequently highly segmented and characterised by abrupt changes in orientation along strike that commonly coincide with pre-existing basement structures or crustal-scale heterogeneities across which there is a commensurate change in offshore basin architecture and normal fault patterns. Mapping of these heterogeneities in geological and geophysical datasets combined with a growing realisation that many of these basement features extend all the way to the ocean-continent boundary has focussed attention on the extent to which these same crustal structures may also have influenced the distribution and pattern of ocean floor fracture zone development. A prominent re-entrant along Australia's 4000-km long southern rifted margin marks the site of an early Paleozoic crustal-scale basement structure whose N-S orientation was optimal for reactivation during a switch in the direction of extension from NW-SE to N-S during the closing stages of continental rifting from about 55-47 Ma onward. This structure evolved from a continental transform boundary into the Tasman Fracture Zone with consequent development of a sheared continental margin along the western margin of the South Tasman Rise analogous to that formed off the Ghanaian coast during the separation of Africa from South America. As with its West African counterpart, seismic reflection profiles point to a strong strike-slip influence on basin geometry with en echelon development of elongate, narrow depocentres bounded by discontinuous steep to subvertical faults. Equally spectacular pull-apart basins associated with the 1500km-long Wallaby-Zenith Fracture Zone off Western Australia are similarly developed in thinned continental crust but, unlike the basins associated with the South Tasman Rise, they have been better seismically imaged and contain a substantially greater thickness of sediment (up to 5 seconds TWT). Interpreted seismic sections across the Zeewyck Sub-basin beneath the Valanginian breakup unconformity show a complex network of deep sedimentary basins bounded by steep faults and blocks of elevated older basement (positive flower structures) across which there is only limited lateral continuity in stratigraphy. Sedimentary sequences immediately above the breakup unconformity thicken into the basin axis and exhibit wedge-like geometries consistent with detritus shed from the adjacent basement highs as the sheared continental margin evolved and the associated spreading axis migrated oceanward. A period of basin-wide folding and faulting accompanied by uplift and erosion brought this phase of basin formation to a close and possibly occurred in response to transpression immediately prior to the onset of full drift. Fabrics in the adjacent N-S striking Pinjarra Orogen and related Darling Fault played an important role in localising extensional strain during formation of the Zeewyck Sub-basin and greater Perth Basin.
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78441
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- External PublicationAbstract
- Australian and New Zealand Standard Research Classification (ANZSRC)
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- Earth Sciences
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2014-01-01T00:00:00
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