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The Bass, Gippsland and Otway Basins, Southeast Australia: A Branched Rift System Formed by Continental Extension
The Bass, Gippsland and Otway basins are Lower Cretaceous to Recent extensional basins that underlie much of Bass Strait, between mainland Australia and Tasmania. All three basins were initiated by north-northeast-trending extension in the Early Cretaceous. The main evidence for extension is in the form of widespread shallow to moderately steeply dipping, rotational normal faults of domino style, which are considered to sole out on major detachment faults. The normal faults have a strike of between 295° and 300° in all three basins. Analysis of well characterized normal faults in the Bass Basin indicates an extension of about 60 percent (β = 1.6) for that basin. The fault geometry is insufficiently well constrained to determine extension in the other basins, but fault displacements in the Gippsland Basin suggest that it may be somewhat more highly extended, especially at its eastern end. The normal faults are short along-strike, and are terminated or offset on a widespread set of orthogonal, steeply dipping transfer faults. The transfer faults have been interpreted from the offsets of normal faults, from zones of minor structuring and reactivation throughout the sequence, and from the alignment of Tertiary igneous bodies and related magnetic anomalies. Transfer faults divide the rift basins into transverse compartments in which the spacing, position and/or dip of the normal faults differs from adjacent compartments. Two scales of transfer faults have been defined: 1) first-order faults accommodate major changes in spacing and/or reversals of dip of the normal faults, or basin-scale lateral offset of the extension: and 2) second-order faults result in small offsets of the normal faults, with systematic offsets giving rise to obliquity of the basin axis to the fault trend. The three Bass Strait basins form a branching rift system, in which the transfer faults, and probably the detachment faults, play a key role in accommodating shifts in the locus of extension, and connecting the three basins. Transfer faults therefore must cross the interbasin highs, and a range of topographic, bathymetric, satellite and airborne geophysical data supports that interpretation. The primary extensional phase of the Gippsland Basin is shown to be related to the Bass – Otway – Southern Ocean rift system, and not to the opening of the Tasman Sea.
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