Measurements of the extension required for crustal breakup on the magma-poor Iberia-Newfoundland conjugate margins

Abstract

Rifted margins have been extensively studied, however there are still many important questions related to their formation processes. We use the Iberia-Newfoundland conjugate rifted margins as a natural laboratory to investigate how much extension is required to produce crustal rupture and separation. To achieve our aim we use; (i) gravity anomaly inversion to measure continental crustal thinning, (ii) subsidence analysis to measure continental lithosphere thinning and (iii) fault heave summation from seismic observations to obtain brittle continental crust extension. Integration of thinning from gravity anomaly inversion and subsidence analysis is used to determine continental crust extension and continental lithosphere extension respectively. These measurements have been made between the proximal continental crust and the distal Limit of Contiguous Continental Crust (LCCC) on the northern (SCREECH1 and ISE01) and the southern (SCREECH2 and TGS/LG12) conjugate seismic profiles. For the Iberia-Newfoundland conjugate rifted margins, extension values determined from the integration of crustal and lithosphere thinning are similar and suggest that on average 172 km of extension is required to produce crustal breakup. In contrast, measured extension from fault heave summation is on average 128 km which indicates an apparent extension discrepancy at the scale of the whole conjugate margin system when observed brittle continental crust extension is compared with lithosphere and crust extension. Fault population analysis shows that sub-seismic resolution faulting significantly contributes to an under-estimate of brittle crustal extension by up to 25%. Allowing for fault extension at sub-seismic resolution indicates that an extension discrepancy at the scale of the whole conjugate margin system does not exist. Fault geometries and the location of the distalmost contiguous continental crust are also important issues that should be carefully considered when comparing fault heave summation and extension from crust and lithosphere thinning. The absence of an extension discrepancy between fault extension, crustal extension and lithosphere extension at the scale of a whole conjugate margin system should not be confused with the presence of depth-dependent stretching and thinning at a given location on the margin which is both expected and observed at rifted continental margins. • Continental breakup and formation of the conjugate Iberia-Newfoundland rifted margins. • Extension required for rupture and separation of continental crust during breakup. • Measure extension from continental crust and lithosphere thinning and fault heaves. • No overall extension discrepancy but local depth-dependent stretching and thinning.