Proceedings of the Ocean Drilling Program Volume 210 Initial Reports: Drilling The Newfoundland Half of the Newfoundland–Iberia Transect: The First Conjugate Margin Drilling in a Nonvolcanic Rift, Sites 1276 and 1277

This Initial Reports volume covers Leg 210 of the cruises of the Drilling Vessel JOIDES Resolution, St. Georges, Bermuda, to St. John’s, Newfoundland, Sites 1276 and 1277, 6 July–6 September 2003. Ocean Drilling Program Leg 210 was devoted to studying the history of rifting and postrift sedimentation in the Newfoundland–Iberia rift. The primary objective was to sample the deep sedimentary structure and basement in order to investigate early rift development. A related objective was to study the shallower stratigraphy and to elucidate the postrift sedimentation processes and paleoceanographic history of this gateway between the North Atlantic and the sub-Arctic seas. Drilling was conducted in the Newfoundland Basin along a transect conjugate to previous drilling on the Iberia margin (Legs 149 and 173). This was the first time that deep-sea drilling has been conducted on both sides of a nonvolcanic rift in order to understand the structural and sedimentary evolution of the complete rift system. The prime site during Leg 210 (Site 1276) was drilled in transitional crust between known continental crust and oceanic crust identified by magnetic Anomalies M3 to M0 (Barremian–Aptian). On the conjugate Iberia margin, extensive geophysical work and deep-sea drilling have shown that the transitional crust is exhumed mantle that is extensively serpentinized in its upper part. Transitional crust on the Newfoundland side, however, is typically a kilometer or more shallower and has much smoother topography, and seismic refraction data suggest that the crust may be thin (~4 km) oceanic crust. These features indicate that the rift may have developed asymmetrically. A major goal at Site 1276 was to investigate these differences by sampling basement and the facies responsible for a strong overlying basinwide reflection (U) that is poorly developed on the conjugate Iberia margin, together with the intervening lithologic section. Drilling on both sides of the Newfoundland-Iberia rift has produced surprising insights into nonvolcanic rifting but at the same time has raised important new questions. Perhaps the single most important observation has been that extensive tracts of mantle ("transition zones") were exhumed seaward of the edges of thinned continental crust and landward of oceanic crust formed by seafloor spreading (we use the term seafloor spreading here to mean generation of new seafloor at a discrete plate boundary and with a major, if not dominant, component of magmatic accretion). Drilling at Site 1277 seaward of anomaly M1 on the Newfoundland margin, and at Sites 897, 899, and 1070 in the area of anomalies M5 to M1 on the Iberia margin, demonstrated that basement there is serpentinized mantle with only very minor indications of magmatism, despite the fact that apparently normal magnetic anomalies are developed in this young part of the M-series reversals. Thus there are major questions not only about the source of the magnetic anomalies but also about whether the prolonged mantle exhumation should be considered to be "seafloor spreading" or just continued rifting, for example, of subcontinental mantle lithosphere. In addition, there are important questions about how expected tectonic events such as separation of continental crust or initiation of seafloor spreading might be manifested in the structural and stratigraphic record along the margins. Finally, drilling at Site 1276 provided the surprising result that substantial and possibly widespread diabase sills were intruded into postrift sediments of the Newfoundland Basin, whereas there is no evidence of similar magmatism on the Iberia margin. Was this magmatic pattern "inherited" because of asymmetries in mantle fertility that developed during rifting, or was it caused by another process, such as plate migration across a mantle plume?

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