A Precise Linear Sampler of Convective Circulation: New Images of V-shaped Ridges South of Iceland

The V-shaped ridges, which straddle the mid-oceanic ridges on either side of Iceland, are an important window into transient convective circulation. We have constructed an accurate chronology of Icelandic plume activity using a set of regional seismic reflection profiles located along flowlines. These flowlines straddle the Reykjanes Ridge, traversing the Iceland and Irminger basins between 60^o and 62^o N. We used a single generator-injector airgun operating at 3,000 p.s.i. and a 2 km streamer with a group spacing of 12.5 m and an average fold of 20. The seismic images are excellent: sediment-basement interfaces can be accurately mapped and fine details of the sedimentary cover are resolved. Either side of the ridge axis, V-shaped ridges and normal faulting can easily be distinguished. Many V-shaped ridges are simple linear features but several have a composite multi-stranded and asymmetric structure. Further away from the ridge axis at a time when the plume was cooler, V-shaped ridges are less obvious and fracture zones predominate. The oldest oceanic crust lacks fracture zones and is acoustically smooth with occasional V-shaped ridges. Where oceanic crust abuts the continental margin, seaward dipping reflections are observed which indicate sub-aerial accretion. We have carefully mapped the sediment-basement interface, transformed each profile onto an astronomical timescale, and removed the effects of long wavelength plate cooling. The resulting chronology of V-shaped ridges yields important insights into the fluid dynamics of a large plume. The mass flux of the Icelandic plume has evidently waxed and waned since its inception. From a regional point of view, this transient behavior manifests itself in several important ways. Within sedimentary basins which fringe the North Atlantic Ocean, short-lived regional uplift events periodically interrupt thermal subsidence from Eocene times to the present day. From a paleoceanographic perspective, there is an excellent correlation between V-shaped ridge activity and changes in overflow of the ancient precursor of North Atlantic Deep Water. On the seismic profiles, changes in deep-water overflow are indirectly recorded by spectacularly imaged contourite drifts.