Ocean Drilling Program, United States

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 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 ( approximately 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. 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 lower part of the hole, we cored sills >10 m thick that are estimated to be 100-200 m above basement. The sills are diabases of probable alkaline composition. They have sedimentary contacts that show extensive hydrothermal alteration. Associated sedimentary and structural features suggest that the sills were intruded at shallow levels below the seafloor. The top of the upper sill is approximately coincident with the U reflection, within uppermost Aptian(?) to 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.