GEOLOGY, December 2007 1087 INTRODUCTION It is generally accepted that rupturing of conti- nents is followed by localized seafloor spreading at mid-ocean ridges (MOR), which are consid- ered, on geological time scales, to be symmetric and steady state. The continuity of this process is documented by the correspondence of crustal accretion ages (dated by magnetic anomalies) and isotopic ages. While these processes are well studied at present-day MOR, little is known about how stable these systems are during their embryonic stage. Even though our understand- ing of the mechanisms associated with extension and rifting of continents has improved in the last decade (Whitmarsh et al., 2001), the processes that ultimately start the MOR basalt (MORB) machine and the switch from rifting (delocalized deformation) to spreading (localized deforma- tion and accretion) are still poorly constrained. A striking discovery of ODP drilling along the Iberia-Newfoundland conjugate margins was the scarcity of effusive magmatism and only minor volumes of intrusive rocks in areas with weak magnetic anomalies (Sibuet et al., 2007). In this paper, we present new U-Pb and 40 Ar- 39 Ar measurements across the conjugate Iberia- Newfoundland margins to document the mag- matic and retrograde history of igneous rocks related to the final rifting and onset of seafloor spreading in the North Atlantic. The data show that the initiation of magmatic seafloor spreading is more complex than previously anticipated and is transitional in time and space. THE IBERIA-NEWFOUNDLAND RIFTED MARGINS SYSTEM The conjugate, magma-poor Iberia-Newfound- land rifted margins resulted from polyphase Late Triassic to Early Cretaceous rifting and separa- tion of the North America and Iberia plates. Rifting migrated from proximal to distal parts of the future margin, leading to fault-bounded basins and local thinning of the crust to less than 10 km before final breakup (Whitmarsh et al., 2001). The Iberia-Newfoundland margins are the only example of rifted margins where scientific drilling has sampled basement rocks from the Ocean-Continent Transition (OCT). The 18 sites drilled during Deep Sea Drilling Project (DSDP) Leg 47B and ODP Legs 103, 149, 173, and 210, combined with a relatively dense geophysical data set, document the existence of a Zone of Exhumed Continental Mantle (ZECM) up to 200 km wide, which separates thinned continen- tal crust from oceanic crust. Based on magnetic analyses and refraction studies (e.g., Russell and Whitmarsh, 2003; Lau et al., 2006), crust ocean- ward of the first unambiguous magnetic anomaly (M3, ca. 128 Ma; Gradstein et al., 2004) is gen- erally referred to as oceanic. However, based on its proximity to the continental margins and its highly variable basement structure (Tucholke et al., 2007), we refer to it in this paper as embryonic oceanic crust, to emphasize the difference from “normal” slow-spreading oceanic crust. Due to the polyphase evolution of rifting, subdued magmatic activity, and poorly con- strained magnetic anomalies, the age of final breakup and the onset of seafloor spreading between Iberia and Newfoundland are debated. Based on the first undisputed oceanic magnetic anomaly, M3, Whitmarsh and Miles (1995) suggested a Barremian age (128 Ma), whereas Tucholke et al. (2007) and Péron-Pinvidic et al. (2007) proposed a late Aptian to earliest Albian age (ca. 112 Ma), based on drilling results and seismic stratigraphic arguments. EMBRYONIC OCEANIC CRUST DRILLED AT ODP SITES 1070 AND 1277 Embryonic oceanic crust was drilled at a base- ment high near magnetic anomaly M1 at Site 1070 on the Iberia margin and at a basement high a few kilometers oceanward of magnetic anomaly M1 at Site 1277 on the Newfoundland margin. The locations of both sites, relative to seafloor spreading magnetic anomalies, suggest a similar crustal accretion age of ca. 127 Ma (Fig. 1). At Site 1070, drilling penetrated tectonic breccias with gouge horizons grading downhole into massive serpentinized peridotite intruded by an E(enriched)-MORB–derived pegmatitic hornblende gabbro (Hébert et al., 2001; Beard et al., 2002) (see Fig. DR1 in the GSA Data Repository 1 ). A breccia overlying the pegma- tite consists of serpentinite and rare gabbro and albitite clasts. Sediments immediately on top are Late Aptian (Whitmarsh et al., 1998). At Site 1277, drilling penetrated a 40-m-thick, volcano-sedimentary unit composed of three basalt flows with a T-(transitional) to N(normal)-MORB composition (Tucholke et al. 2004) interlayered with debris flows and sand- stones that are separated from the underly- ing basement by a tectonic contact (see Fig. DR1). Clasts in the sediments are composed of serpentinized peridotites and gabbros identi- cal to rocks forming the underlying basement. Geology, December 2007; v. 35; no. 12; p. 1087–1090; doi: 10.1130/G23613A.1; 3 figures; Data Repository item 2007270. © 2007 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. 1 GSA Data Repository item 2007270, analytical methods, tabulated and illustrated results, and a stratigraphic column indicating sample locations of Sites 1277 and 1070, is available online at www. geosociety.org/pubs/ft2007.htm, or on request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA. The rift-to-drift transition in the North Atlantic: A stuttering start of the MORB machine? Oliver Jagoutz Othmar Müntener Institut für Geologie, Universität Bern, CH-3012 Bern, Switzerland Gianreto Manatschal Centre géochimie surface- École et observatoire des sciences de la Terre, Université Louis Pasteur, F-67084 Strasbourg, France Daniela Rubatto Research School of Earth Sciences, Australian National University, Canberra 0200 ACT, Australia Gwenn Péron-Pinvidic Centre géochimie surface- École et observatoire des sciences de la Terre, Université Louis Pasteur, F-67084 Strasbourg, France Brent D. Turrin Department of Geological Sciences, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854, USA Igor M. Villa Institut für Geologie, Universität Bern, CH-3012 Bern, Switzerland, and Universita di Milano Bicocca, 20126 Milano, Italy ABSTRACT We report U-Pb and 39 Ar- 40 Ar 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 igne- ous 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. Keywords: Ocean-Continent transition, MORB, rifting, magma-poor margins, age determinations.