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Variations in rift symmetry: cautionary examples from the Southern Rift System (Australia–Antarctica)

Direen, NG, Stagg, HMJ, Symonds, PA and Norton, IO 2012 , 'Variations in rift symmetry: cautionary examples from the Southern Rift System (Australia–Antarctica)' , Special Publication of the Geological Society (London), vol. 369 , doi:

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We present a synthesis based on the interpretation of two pairs of deep seismic reflection
crustal sections within the Southern Rift System (SRS) separating Australia and Antarctica.
One pair of sections is from the conjugate margins between the Great Australian Bight (GAB)
and Wilkes Land, in the central sector of the SRS, which broke up in the Campanian. The
second pair of conjugate sections is located approximately 400 km further east, between the
Otway Basin and Terre Adelie, which probably broke up in Maastrichtian time. Interpretations
are based on an integrated synthesis of deep multi-channel seismic, gravity and magnetic data,
together with sparse sonobuoy and dredging information, and the conjugate sections are presented
with the oceanic crust removed beyond the continent–ocean boundary (COB).
At first order, both conjugate pairs show a transition from thinned continental crust, through a
wide and internally complex continent–ocean transition zone (COTZ), which shows features in
common with magma-poor rifted margins worldwide, such as basement ridges interpreted as
exhumed subcontinental mantle. In the central GAB sector, the COTZ is symmetric around the
point of break-up and displays a pair of mantle ridges, one on each margin, outboard of which
lies a deep-water rift basin. Break-up has occurred in the centre of this basin in this sector of the
SRS. In contrast, the Terre Ade´lie margin is nearly 600 km wide and shows an abandoned
crustal megaboudin, the Ade´lie Rift Block. This block is underlain by interpreted middle crust,
and appears to have a mantle ridge structure inboard, as well as an outboard exhumed mantle
complex from which mylonitized harzburgite has been dredged. The conjugate margin of the
Beachport Sub-basin is relatively narrow (c. 100 km wide) and does not appear to contain an
exhumed mantle ridge, as observed along strike in the GAB.
These observations from a single rift spreading compartment show that radically different breakup
symmetries and margin architectures can result from an essentially symmetric rifting process
involving multiple, paired detachment systems. This indicates the need for caution in interpreting
causative mechanisms of rifting from limited conjugate sections in other rifts. We speculate that the
underlying crustal composition, rheology and structural preconditioning play a significant role in
partitioning strain during the transition to break-up.

Item Type: Article
Authors/Creators:Direen, NG and Stagg, HMJ and Symonds, PA and Norton, IO
Journal or Publication Title: Special Publication of the Geological Society (London)
DOI / ID Number:
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Copyright © 2012 The Geological Society of London

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