LEG 152

East Greenland Margin

Leg 152 represents the second ODP leg to investigate the processes of continental rifting. The Greenland margin is believed to have been generated above a mantle plume, producing a "volcanic rifted margin", and the nearby Iceland hotspot supports some plume involvement in the area. These mantle plumes may be responsible for generating the large pulse of basaltic magma comprising the observed seaward-dipping reflector sequence (SDRS). During Leg 152, a total of six sites were investigated (Site 914 to Site 919).

During Leg 152, full penetration of the landward featheredge of the SDRS was achieved. The trace element chemistry of the basalts suggest that normal, depleted, upper mantle was underlying the rift during the early eruptive phase, unlike the chemically undepleted mantle source of Icelandic basalts. Similar depleted sources probably produced the basalts on the modern Mid Atlantic Ridge. However, the greater thickness of the basaltic crust of the SDRS compared to normal oceanic crust shows that greater volumes of melt were being generated at the time of breakup than along the modern Mid Atlantic Ridge, implying that the mantle was hotter then. The Iceland hot spot, which is diminishing with time, is apparently a broad thermal plume, ~ 2000 km across. Within this, is a small undepleted chemical plume, originally sited close to Kangerdlugssuaq in East Greenland and now directly under Iceland. Greater amounts of elements such as Ba and K in the basalts suggest contamination of the asthenosphere-derived melts by melting of the continental lithosphere. The basalts were erupted during the latest Paleocene to early Eocene under subaerial conditions and subsequently subsided to their present positions due to thermal cooling of the lithosphere and possibly a thermal collapse of the buoyant asthenosphere/plume mantle.

Since the marine transgression, the sediments on the margin have recorded the paleoceanographic, tectonic and eustatic evolution of the area. A systematic increase in the degree of clastic input and the development of glauconitic hardgrounds, dated at 11-13 Ma, marks the onset of flow of oxygenated North Atlantic Deep Water, significantly predating the start of glaciation. Within deep-marine silts, dropstones of high-grade metamorphic rocks derived from the Precambrian craton indicate that southern Greenland had a permanent icecap at this time. These glacial sediments are dated at 6-7 Ma, far older than any other known glacial sediment from the North Atlantic. Southern Greenland would thus seem to be the first area to develop a permanent icecap. Prior to the renewed strong glacial sedimentation after 5.0 Ma, a long period of non-glacial sedimentation occurred which is similar to, but longer than, a normal warm interglacial.