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The rift-to-drift transition in the North Atlantic: A stuttering start of the MORB machine?

¹ Institut für Geologie, Universität Bern, CH-3012 Bern, Switzerland
² Centre géochimie surface-École et observatoire des sciences de la Terre, Université Louis Pasteur, F-67084 Strasbourg, France
³ Research School of Earth Sciences, Australian National University, Canberra 0200 ACT, Australia
⁴ Centre géochimie surface- École et observatoire des sciences de la Terre, Université Louis Pasteur, F-67084 Strasbourg, France
⁵ Department of Geological Sciences, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854, USA
⁶ Institut für Geologie, Universität Bern, CH-3012 Bern, Switzerland, and Universita di Milano Bicocca, 20126 Milano, Italy

We report U-Pb and ³⁹Ar-⁴⁰Ar measurements on plutonic rocks recovered from the Ocean Drilling Program (ODP) Legs 173 and 210. Drilling revealed continental crust (Sites 1067 and 1069) and exhumed mantle (Sites 1070 and 1068) along the Iberia margin and exhumed mantle (Site 1277) on the conjugate Newfoundland margin. Our data record a complex igneous and thermal history related to the transition from rifting to seafloor spreading. The results show that the rift-to-drift transition is marked by a stuttering start of MORB-type magmatic activity. Subsequent to initial alkaline magmatism, localized mid-oceanic ridge basalts (MORB) magmatism was again replaced by basin-wide alkaline events, caused by a low degree of decompression melting due to tectonic delocalization of deformation. Such "off-axis" magmatism might be a common process in (ultra-) slow oceanic spreading systems, where "magmatic" and "tectonic" spreading varies in both space and time.

Key Words: Ocean-Continent transition • MORB • rifting • magma-poor margins • age determinations

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