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Ocean Drilling Program Leg 210 was devoted to studying the history of rifting and postrift sedimentation in the Newfoundland–Iberia rift. 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 known 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 transition zone crust is exhumed mantle that is extensively serpentinized in its upper part. Transition zone 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, basin-wide reflection (U) that is poorly developed on the conjugate Iberia margin, together with the intervening section.

Site 1276 was cored from 800 to 1739 m below seafloor with excellent recovery (average = 85%). Before drilling was terminated because of unstable conditions in the uncased hole, drilling reached sills >10 m thick that are estimated to be 100–200 m above basement. The sills are alkaline diabases, they have sedimentary contacts that show extensive hydrothermal metamorphism, and associated sediment structural features indicate that they were intruded at shallow levels below the seafloor. The top of the upper sill is approximately coincident with U, which correlates with lower Albian fine- to coarse-grained sedimentary gravity flows. The nature of basement at this site remains uncertain, but the presence of the deep sills indicates that there was a significant postrift magmatic event that may have affected much of the basin. This feature of the basin could help to explain the asymmetry in basement depth and basement roughness on the conjugate Newfoundland and Iberia margins.

The overlying Albian–lower Oligocene sediments record paleoceanographic conditions similar to those in the main North Atlantic Basin and on the Iberia margin, including deposition of Cretaceous "black shales," but they show an extensive component of gravity flow deposits throughout. Major paleoceanographic events including a number of Ocean Anoxic Events, the Cretaceous/Tertiary boundary, and the recovery from the Paleocene/Eocene Thermal Maximum are well represented in the cored section. A prominent seismic marker that is correlated with the initiation of deep circulation in the North Atlantic was cored, and preliminary biostratigraphic data indicate that it is a hiatus dating to the middle Eocene, perhaps several million years older than proposed in previous interpretations.

Site 1277 was drilled 80 m into a shallow basement high ~40 km southeast of Site 1276. This crust, presumed to be oceanic, is on the young side of a magnetic anomaly interpreted as M1. Cores from the upper part of basement at this site recovered a remarkable assemblage of basalt flows interleaved with gravity flows containing slivers of gabbro, serpentinized peridotite, and sediments (e.g., fine- to coarse-grained sandstones). Below these largely allochthonous rocks, basement is serpentinized peridotite with veins of gabbro and this rock is interpreted as being in situ. These rocks were emplaced in a magma-limited, highly extensional environment which we interpret as very slow spreading ocean crust.

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