Continental breakup is usually marked by a sharp contact between continental and oceanic crust. However, in settings with low magma supply, this transition zone is much wider1,2 and exhibits a gradual change from continental crust, to exhumed blocks of continental mantle, to oceanic crust. Traditionally, the timing and location of continental breakup is defined by the first observation of a magnetic anomaly that is generated by magma erupted from the newly formed mid-ocean ridge. As the magma cools to form rock, it preserves a record of the Earth’s magnetic field at that time. However, in the broad transitional zone at magma-poor rifted margins, the seafloor-spreading origin of magnetic anomalies is debated3,4. Here we compare seismic5,6,7,8,9,10 and drill-hole data11,12,13 with measurements of the first magnetic anomaly—the J anomaly—formed in the Newfoundland–Iberia rift system. We find that the J anomaly is associated with locally high topography and thickened crust, probably resulting from voluminous magma both erupted at the surface and added beneath blocks of exhumed continental mantle. We therefore argue that the J anomaly did not form during seafloor spreading, but instead represents a pulse of magmatism that may have triggered continental breakup before seafloor spreading. Our findings imply that opening of this part of the North Atlantic may have occurred later than previously thought and may explain inconsistencies in some previous kinematic reconstructions.
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We thank T. Minshull, P. Miles, S. Dean, B. Whitmarsh and D. Shillington for providing published and unpublished data. We acknowledge Othmar Müntener for discussions on this work. The work was supported by the Action-Marges, the French margin program (CNRS-INSU), and is a contribution of the ANR project ‘Rift-to-Ridge’.
The authors declare no competing financial interests.
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Bronner, A., Sauter, D., Manatschal, G. et al. Magmatic breakup as an explanation for magnetic anomalies at magma-poor rifted margins. Nature Geosci 4, 549–553 (2011) doi:10.1038/ngeo1201
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