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.