Characterization of sills associated with the U reflection on the Newfoundland margin: evidence for widespread early post‐rift magmatism on a magma‐poor rifted margin

Summary Drilling during ODP Leg 210 penetrated two post‐rift sills (dated as ∼105.3 and ∼97.8 Ma) in the deep sediments overlying basement of the continent–ocean transition zone on the magma‐poor Newfoundland margin. The sill emplacement post‐dated the onset of seafloor spreading by at least 7–15 Myr. The shallower of the two sills coincides with the high‐amplitude U reflection observed throughout the deep Newfoundland Basin, and strong reflectivity in the sub‐U sequence suggests that a number of other sills are present there. In this paper, we use multichannel seismic reflection data and synthetic seismograms to investigate the nature, magnitude and extent of this post‐rift magmatism in the deep basin. Features observed in seismic profiles that we attribute to sill injection include high‐amplitude reflections with geometries characteristic of intrusions such as step‐like aspect; abrupt endings, disruptions and junctions of reflections; finger‐like forms; differential compaction around possible loci of magma injection and disruption of overlying sediments by apparent fluid venting. Interpreted sills occur only over transitional basement that probably consists of a mixture of serpentinized peridotite and highly thinned continental crust, and they cover an area of ∼80 000 km 2 . From analysis of synthetic seismograms, we estimate that sill intrusions may comprise ∼26 per cent of the sub‐U high‐reflectivity sequence, which yields a crude estimate of ∼5800 km 3 for the total volume of sills emplaced by post‐rift magmatism. This is significant for a margin usually described as ‘non‐volcanic’. We discuss competing hypotheses about the source of the magmatism, which is still uncertain.