Rit (Vísindafélag Íslendinga) - 01.06.1971, Síða 129

CRUSTAL STRUCTURE OF ICELAND
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sent an orthogonal fracture system, probably deriving from horizontal
shear stresses in the oceanic crust.
It is of interest to ask whether the seismic structure of the crust
as found in Iceland can contribute any significant information relat-
ing to processes at the mid-ocean ridges. If this is to he the case, the
nature of the seismic boundaries must be known. In the present work
arguments have been provided supporting the interpretation of the
deeper boundaries, in particular the 2—3 boundary, as boimdaries be-
tween metamorphic facies of basaltic rocks. It will therefore be as-
sumed in the present discussion that the 2—3 boundary is formed in
the crust at a temperature of 350-400 °C, and that it remains meta-
stable if the crust cools down. The 3—4 boundary will be assumed to he
at a temperature close to the melting range of hasalts, as is indicated
by subsurface temperature data in Iceland. The uppermost boundary
between layers 0 and 1 will be assumed to he a stratigraphic boundary
between major volcanic cycles.
The seismic structure of southern and western Iceland indicates
a certain symmetry about the Reykjanes Ridge where it enters the
Reykjanes peninsula (Figs. 38 and 40). The seismic boundaries are
shallower in the crestal zone, which is here about 230 km wide, than
outside this zone. The melting range of basalts may be reached at a
depth of 8-9 km under the axial zone. These data are consistent with
the hypothesis of ocean-f loor spreading and the model of upper mantle
convection as proposed by Oxburgh and Turcotte (1968). The low-
velocity surface layer is in general thickest in the zone of volcanism,
where new material is being extruded at the surface. The deeper
seismic boundaries do not show an anomalous behaviour in the central
volcanic zone. This is probably to be expected if they are essentially
controlled b.y isotherms.
The fact that the crust in northern and eastem Iceland appears
to have cooled down since the creation of the 2—3 boundary suggests
that an ocean-floor spreading process may not be as effectively in
operation in northern Iceland as in SW-Iceland. The Reykjanes
Ridge structure is certainly disturbed and perhaps terminated under
Iceland. The bowl-shaped Bouguer anomaly in Iceland may indicate
where to look for a change in the Reykjanes Ridge structure. Al-
though considerable bathymetric and aeromagnetic data are available
from the Norwegian Sea (Johnson et al., 1967; Avery et al., 1968),
very little is known about crustal and upper mantle structure on the
Iceland-Jan Mayen Ridge.
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