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Architecture of volcanic rifted margins: new insights from the Exmouth - Gascoyne margin, Western Australia


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Direen, NG, Stagg, HMJ, Symonds, PA and Colwell, JB 2008 , 'Architecture of volcanic rifted margins: new insights from the Exmouth - Gascoyne margin, Western Australia' , Australian Journal of Earth Sciences, vol. 55, no. 3 , pp. 341-363 , doi: 10.1080/08120090701769472.

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Australia’s western continental margin formed during Gondwana breakup between Australia –
Antarctica and India, leaving behind an evolving plate boundary with significant new magmatic
crust accreted to the continental fragments. In this context, the margin segment from the outer
Exmouth Plateau to the Gascoyne Abyssal Plain has been repeatedly studied as an example of
volcanic margin formation. However, many recent analyses based on selective interpretation of
geophysical datasets have come to conflicting conclusions regarding both the tectonic architecture
and the age of breakup on this margin. These differences profoundly impact on models for both the
global plate circuit and for the timing and extent of magmatism implicated in volcanic breakup
processes. We present new interpretations of the distribution of magmatic and pre-rift rock packages in
this margin, based on the integrated interpretation of two deep crustal transects with existing seismicreflection,
refraction, gravity, magnetic and geochemical data. Interpretations are constrained by
data from sparse Ocean Drilling Program and petroleum-exploration drilling, and dredging. We find
evidence for significant accumulation of magmatic rocks and their clastic derivatives infilling
extensional fault-controlled basins developed in a broad volcanic-margin transition zone between
the outer Exmouth Plateau and true oceanic crust. These rocks have distinctive seismic facies in the
form of seaward-dipping reflector sequences, and are dense and magnetised. Most significantly, these
packages give rise to potential-field anomalies that have previously been interpreted as due to seafloor
spreading. Recognition of these packages in a volcanic-margin transition zone has implications for the
recognition of the inboard edge of unequivocal oceanic crust, the oceanic – volcanic-margin
boundary. The main locus of igneous activity in the volcanic-margin transition zone off the Exmouth
Plateau is spatially offset from a previously recognised high-velocity zone, suggesting that these two
phenomena may not be temporally related. Seismically imaged differences in total thinning and
partitioning of thinning, between upper and lower crust, provide support for models of depthdependent
thinning previously proposed for this margin.

Item Type: Article
Authors/Creators:Direen, NG and Stagg, HMJ and Symonds, PA and Colwell, JB
Keywords: continental breakup, Exmouth, geophysics, seaward-dipping reflector sequences, volcanic-margin transition, volcanic rifted margin, Western Australia.
Journal or Publication Title: Australian Journal of Earth Sciences
ISSN: 0812-0099
DOI / ID Number: 10.1080/08120090701769472
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The definitive version is available online at

Copyright © 2008 Taylor & Francis

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