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Sequential faulting explains the asymmetry and extension discrepancy of conjugate margins

Abstract

During early extension, cold continental lithosphere thins and subsides, creating rift basins. If extension continues to final break-up, the split and greatly thinned plates subside deep below sea level to form a conjugate pair of rifted margins. Although basins and margins are ubiquitous structures, the deformation processes leading from moderately extended basins to highly stretched margins are unclear, as studies consistently report that crustal thinning is greater than extension caused by brittle faulting1,2,3,4. This extension discrepancy might arise from differential stretching of brittle and ductile crustal layers2, but that does not readily explain the typical asymmetric structure of conjugate margins5,6—in cross-section, one margin displays gradual thinning accompanied by large faults, and the conjugate margin displays abrupt thinning but smaller-scale faulting5. Whole-crust detachments, active from early in the rifting, could in theory create both thinning and asymmetry1, but are mechanically problematical. Furthermore, the extension discrepancy occurs at both conjugate margins, leading to the apparent contradiction that both seem to be upper plates to a detachment fault7,8. Alternative models propose that much brittle extension is undetected because of seismic imaging limitations caused either by subseismic-resolution faulting9, invisible deformation along top-basement 100-km-scale detachments8 or the structural complexity of cross-cutting arrays of faults3. Here we use depth-migrated seismic images to accurately measure fault extension and compare it with crustal thinning. The observations are used to create a balanced kinematic model of rifting that resolves the extension discrepancy by producing both fault-controlled crustal thinning which progresses from a rift basin to the asymmetric structure, and extreme thinning of conjugate rifted margins. Contrary to current wisdom, the observations support the idea that thinning is to a first degree explained by simple Andersonian faulting that is unambiguously visible in seismic data.

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Figure 1: WIM and NFM.
Figure 2: Pre-stack depth-migrated line IAM11.
Figure 3: Interpretation of a pre-stack depth-migrated segment of line SCREECH-1.
Figure 4: Selected steps of the balanced cross-section of the kinematic model showing development of crustal thinning and margin asymmetry.
Figure 5: Extension factors from model in Fig. 4

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Acknowledgements

The ideas presented in this work have benefited from discussions with J. Adam, A. Amilibia, E. Casciello, T. Cunha, M. R. Fowler, S. Hardy, J. García-Pintado, G. Manatschal, K. McClay, M. Menzies, E. Saura and F. Storti, and from numerous discussions and previous collaborations with T. J. Reston. The early work that led to the ideas presented here was carried out when C.R.R. worked at IFM-GEOMAR and M.P.-G. at IFM-GEOMAR and later at ICTJA-CSIC. We are grateful to J. Collier and C. Beaumont for their reviews, which helped improved this article. This is a publication of the Department of Earth Sciences of the Royal Holloway, University of London. C.R.R. has been supported by the Kaleidoscope project, funded by Repsol, and by the Spanish National Project Medoc of the Ministry of Science and Innovation.

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C.R.R. processed the seismic data up to pre-stack depth migration and interpreted them. M.P.-G. built the tectonic model. Both authors contributed equally to writing the manuscript and to developing the ideas behind the tectonic model.

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Correspondence to César R. Ranero or Marta Pérez-Gussinyé.

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The authors declare no competing financial interests.

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Ranero, C., Pérez-Gussinyé, M. Sequential faulting explains the asymmetry and extension discrepancy of conjugate margins. Nature 468, 294–299 (2010). https://doi.org/10.1038/nature09520

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