Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 306

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PÁLL IMSLAND
the areas north and south of the Jan Mayen fracture zone, in spite of the
apparently somewhat irregular structure at the extreme southern end, as
indicated by the topography in the Mohns ridge area, just north of the
fracture zone.
The reason for the Jan Mayen volcanism has been claimed to be a hot
spot or mantle plume under the island (Wilson, 1973; Vogt, 1974). These
features are supposed to be the locus of upwelling of deep rooted material
and the thermal donors (Morgan, 1971) in the area, and as such the source
of the ocean bottom generation. Highly productive spreading ridges, a
global scale phenomenon, are present in the North Atlantic area. A local
feature, like a low productivity oceanic island, then seems quite unlikely to
be the surface manifestation of the primus motor of the ocean bottom creation.
The fading out of the naturally high thermal gradient of the Mohns ridge,
the spreading center, was postulated by the author (Imsland, 1978a) to be
the reason for the Jan Mayen volcanism. This view is still held. It is in
agreement with the geological and evolutionary features of the area. It does
not mix into the local geology a new and otherwise unnecessary element of
unknown characters in most respects. It is thus both preferential and also
apparently more natural as an explanation for the volcanism. The same
pattern of elevated temperatures behind fracture zones in prolongation of
rift zones, is used by Óskarsson et al. (1979) to account for the volcanism in
some of the non-rifting volcanic zones of Iceland.
The Jan Mayen volcanism is, according to this, a prolongation of the
Mohns ridge volcanism, which behind the fracture zone takes up a new
style. It leaves the oceanic rift zone mode, marked by relatively high
productivity and tholeiitic compositions, and takes on the non-rifting
oceanic island mode, marked by relatively low productivity and alkaline
compositions. Two reasons for this change apparently play the major role;
in the first place the thermal conditions, and in the second place the
structural and compositional contrast of the crust and most probably the
upper mantle.
In the Mohns ridge the high thermal gradient, as in other oceanic
speading ridges (Oxburg & Turcotte, 1968), is a necessary partisan to a
longtime productive volcanic zone. This high thermal gradient falls off
evenly towards the flanks of the ridge, where the cooling plates are drifting
away (Pálmason, 1973). Beyond the southern end of the ridge the thermal
gradient must similarily fall off, across the fracture zone, but here it happens
in the plate slowly drifting past, south of the fracture zone. Leaving the
Kolbeinsey ridge in the west, this southern plate cools off, but enters this
high temperature field south of the Mohns ridge again, and then finally
leaves it towards the east. This plate thus drifts through a thermal anomaly
for a geologically short time.
The crustal plate drifting eastward along the Jan Mayen fracture zone (on
its southern side), as mentioned above, is anomalously thick for an oceanic