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The deliberate search for the subtle fault at magma-poor rifted margins

Author(s): Tim Reston University of Birmingham, United Kingdom
Ken McDermott University of Birmingham, United Kingdom
Derren Cresswell University of Birmingham, United Kingdom
Gael Lymer University of Birmingham, United Kingdom
the Galicia 3D working group Rice University (U.S.A.), Lamont-Doherty Earth Observatory (U.S.A.), Southampton Oceanographic Centre (UK), Barcelona (ESP), GEOMAR Kiel (GER)

Magma-poor rifted margins form by the breakup of continents and the opening of the ocean basins in the absence of voluminous magmatism. As a result they are characterized by thinning of the crust to nothing, unusual paleogeographies associated with the formation of a deep rift within a continent, and the eventual onset of seafloor spreading. We investigate the process of rifting to breakup at these margins through a combination of seismic interpretation of both industry and bespoke seismic reflection data, including a large academic 3D survey collected in 2013 across the Galicia margin, analysis of the distribution of crustal thinning from wide-angle seismic profiles, and through seismic modeling of the sort of structures that might be expected to form during progressive extension.

Even magma-poor margins, where extension should be close to constant volume and not obscured by lavas, exhibit a pronounced extension discrepancy: the amount of extension that can be measured from the geometry of faults on seismic images is far too little to explain the observed crustal thinning and subsidence. Either the crust has been thinned in some other way or the amount of extension has been severely underestimated. Examination of the crustal structure of all margins well constrained by wide-angle data reveals no evidence for depth-dependent thinning on the scale required to explain the extension discrepancy: within error, stretching appears close to depth uniform. We conclude that the extension discrepancy must be caused by the failure to recognise all the faulting. Although sub-seismic faulting and distributed deformation contribute, much of the remaining extension that should be recognized is not. To investigate this, we created a model structural section across a rifted margin by focusing extension in the center of a rift, producing successive phases of crosscutting faults. From one side of this section, a synthetic seismic image is generated (see below) and interpreted as if it were a real profile. Just as for real margins, apparent listric faults and eroded fault block crests are seen, but these are not present in the model and instead represent intersecting fault surfaces, and are thus diagnostic of polyphase faulting. Just as for real margins, the amount of extension measured from the seismic is only a fraction of the true extension. Just as for real margins, this extension discrepancy increases markedly oceanward. Demonstrably for the synthetic margin, and by implication for real margins, the extension discrepancy is the failure of the seismic method to image unambiguously the polyphase faulting required to accommodate increasing extension, combined with a general lack of awareness of the features, diagnostic of such faulting. We conclude by showing some of these diagnostic features from the Galicia margin, showing that this margin formed by multiple phases of cross-cutting faults.


Title:
The deliberate search for the subtle fault at magma-poor rifted margins
Type:
Oral
Origin:
Academia
Day:
2
Session:
2
Daily sequence no.:
16
Lead author last name:
Reston
Lead author first name:
Tim
Affiliation(s):
University of Birmingham, UK
Country:
UK
Abstract status:
ok
UID:
61