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Title: Break-up history and evolution of the southern passive margin of Australia
Author: Ball, Philip Joseph.
ISNI:       0000 0001 3441 4189
Awarding Body: Royal Holloway, University of London
Current Institution: Royal Holloway, University of London
Date of Award: 2005
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Despite intensive studies, the process of continental break-up remains enigmatic, in large part because the record of break-up is located in deepwater regions. Existing models are based on a limited academic data set. This study aims to add to our data base of weakly magmatic margins worldwide through a study of the continental breakup between Australia and Antarctica. Earlier studies did not examine the conjugate margins in tandem, and the deep water setting hampered attempts to establish a regional chrono-stratigraphic framework. Furthermore, existing reconnaissance seismic reflection and sparse refraction data provided only a 2-D view of small areas of the >1800 km-long margin. Thus, there was little consensus in interpretations of the evolution of the southern margin of Australia, as well as models for break-up. This study aims to (i) establish the structural variability and framework of the Australian and Antarctic margins; (ii) determine the along-axis segmentation of the margin and its relation to faulting and seafloor spreading processes; (iii) determine the sediment thickness and crustal thickness in 3-D; (iv) reconcile the position of the ocean-continent boundary derived from magnetic anomaly and seismic interpretations. These results are then interpreted in light of existing data from the conjugate Antarctic margin to propose a new model for continental break-up between the Australo-Antarctic conjugate margins. I analyse a new compilation of merged onshore, offshore, and satellite gravity data, as well as seismic reflection, magnetic, and well data from the Great Australian Bight, southern Australia. Estimates of the 3-D crustal thickness, depth to basement, the position and nature of the continent-ocean-boundary, and interpretations of major basement-involved structures were derived from analyses of terrain-corrected Bouguer gravity data. The gravity interpretations are constrained with seismic reflection, refraction and magnetic data. The potential field modelling within this thesis has been conducted using Geosoft and in-house software. Continental break-up between Australia and Antarctica occurred oblique to the identified major basement terrains and pre-existing structures. Two spatially and temporally discrete rift events are identified along the Australian southern margin. Integrated structural patterns are used to suggest that the first rift phase (165-115 Ma)developed within two discrete overlapping rift systems; the W-E trending Bight-Wilkes rift (165-135 Ma) and the NW-SE trending Otway-Adelie (145-115 Ma) rift. Each rift system is interpreted to terminate at or near to the boundary of the mechanically strong Gawler craton, suggesting that the craton served as an obstacle to rift propagation The second identified rift phase (92-50 Ma) corresponds with the localization of strain to a narrow rift zone within the interpreted COT zone. Within the transitional rift basement faults are observed to young both oceanwards and to the SE indicating that rifting processes were diachronous within the transitional rift itself. Concomitant with the transitional rift, complex and anomalously high density, high susceptibility structural highs are observed. The basement highs either have a close relationship to basement-involved faults or some appear to be intrusive within the transitional crust. These anomalous basement features are tentatively interpreted to be mafic bodies. By analogy to other non-volcanic margins (e.g. Whitmarsh et al. 2000; Beslier et al. 2004) it is likely that they represent either exhumed upper mantle, serpentinite, or lower crustal rocks or partial melt products derived from the underlying upper mantle. Plate reconstructions of the continent-ocean-transition [COB] indicate that break-up initiated within the centre of the Australo-Antarctic plate at -83 to 79 Ma. The reconstructions of the newly defined, gravity and seismic, COB's across the Australian and Antarctic margins reveal large degrees of overlap casting doubts upon the validity of the existing poles of rotation to replicate the early kinematics of the Australian and Antarctic margins. For the Otway-Adelie Sector the asymmetry of break-up, the delayed onset of oceanic accretion and the large overlaps in reconstructions may be explained with a two phase oblique-shear model. It is proposed, therefore, that rifting and break-up between Australia and Antarctica occurred initially within a NW-SE regional plate divergence which switched to N-S at -61 Ma (post-c27y). Break-up or rather the onset of rapid oceanic accretion processes offshore the northern sector ofthe Otway-Adelie rift is inferred to have initiated by -53 Ma. The integration of the results presented within this thesis has been used to develop a new break-up model for Australia and Antarctica. These results provide new insights into continental break-up within weakly magmatic rift zones, as well as constraints for heat flow modeling and hydrocarbon prospectivity studies.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available