Harmen Bijwaard and Wim Spakman
Vening Meinesz School of Geodynamics, Utrecht University,
The Netherlands
E-mail: bijwaard@geo.uu.nl
Iceland has long been known to be a center of both hotspot and Mid-Atlantic
Ridge activity.
Hotspots are generally thought to be the surface expression
of mantle plumes:
relatively narrow channels of hot mantle upwellings. Numerical modeling
and laboratory experiments have helped to gain insight into the dynamics
of plumes, but on several aspects no complete consensus exists. Especially
the origin of plumes (the upper-to-lower mantle discontinuity or the D"
layer), the apparent stationarity of hotspots,
and the
size and excess temperature of plumes are subject of ongoing debate.
Further constraints on these issues may come from
seismological investigations. Both regional and global seismological
studies have recently provided evidence for the low seismic wavespeeds and
deflected mantle discontinuities below Iceland, which are expected for a
deep rooted mantle plume. From this evidence estimates of size
and excess temperature
have been derived that agree reasonably well with those from numerical
modeling. Furthermore, the observed deflections point toward a lower mantle
(and hence D") plume origin.
We will show results for the entire
mantle below Iceland
from recent global tomography research (Bijwaard et al., JGR, 1998).
The main advantages of this global study
over previous work are the exploitation of a reprocessed global seismic
data set and the implementation of an irregular cell parameterization. This
has allowed us to construct a detailed solution in well-sampled
mantle volumes. Although many of the well-known hotspots are located
above mantle
volumes that are poorly sampled by seismic rays and hence are not very
well resolved,
one of the few hotspots below which we have reasonably good resolution
for the entire mantle volume seems to be Iceland.
In the mantle below Iceland we observe a wavy, plume-shaped low wavespeed
anomaly with a broad (1000 km wide) root zone, an approximately 500 km
wide central part and a large (1200 km) 'plume head'.
These measures may
be somewhat overestimated since the horizontal resolution
is of the order of 300/500 km in the upper/lower mantle.
At several depths the plume-shaped anomaly seems to be
connected to other low velocity features. For example, at approximately
1300 km depth
it is part of a semi-circular ring of low velocities
below Europe and northern Africa and seems to connect several
hotspot regions (Eifel, Massif Central, Hoggar, Canary Islands).
Apart from that, the plume-shaped anomaly appears to be deflected
with a root zone stretching west toward Greenland
and a central part east from Iceland, approximately 1000 km below
the Faeroe Islands. This wavy shape may contradict the concept
of stationary hotspots. The vertical continuity of the (deflected)
structure is well
resolved (on the order of 120-200 km)
which suggests a D" origin for the Iceland hotspot.
The amplitude of the inferred seismic anomaly can be translated
into an excess
temperature of 200-300 K for the entire mantle which agrees well with
previous and independent estimates.