Rit (Vísindafélag Íslendinga) - 01.06.1984, Side 309

PETROGENETIC RELATIONSHIPS
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drift in the North Atlantic is relatively slow, usually taken as a total of ~2
cm per year on a long term basis (Wyllie, 1976), i.e. 1 cm per year in each
direction from the spreading axis. It is known that the drift does not leave
the spreading axis with a constant speed, but rather in a series of small
bursts (Björnsson et al., 1979). Where the speed evens out and becomes
constant is not exactly known. Apparently this happens to a relatively great
extent already within the spreading zone (Tryggvason, 1978). Outside the
active volcanic zones of the rift systems, the drift rate may thus apparently
be taken as constant. The size of the thermal anomaly may be given the
limits, set by the temperature surface, below which magmas can be pro-
duced in the crust. To identify these limits in nature is difficult, as not all
magmas produced in the crust are likely to be erupted and surface lavas or
other surface features do thus not demarcate the anomaly. The surface of the
anomaly (isotherm surface) has the shape of a bisected elongated dome and
may be expected to be roughtly symmetrical about a southwest plane. The
eastward drifting plate may carry with it some heat obtained inside the
anomaly and thus somewhat deform the symmetrical shape. Apparently it
extends further towards the southwest from the fracture zone than it does
towards nothwest or southeast along the fracture zone, from the shallowest
top. This is indicated by the shape of the volcanic island and the general rift
zone widths. The island extends over 50 km southwest from the fracture
zone but is quite narrow. The rift and eruption zones in the oceanic ridges
are usually much less than 50 km. Apparently the island is situated
somewhat west of the top of the thermal anomaly. The oldest lavas exposed
on the island are less than 0.7 m.y. according to their magnetic polarity, but
volcanism started prior to the emergence of an island from the ocean
(Imsland, 1978a). If we assume the thermal anomaly to have a total width
of 40 km and take the eastward drift rate of the plate as being 1 cm per year,
4 million years pass from the generation of the first magma within the
anomaly (in its western extreme) to the time the locus of this magma
production leaves the anomaly in the east. As long as the drift continues and
the temperature anomaly is maintained magmas will be produced in the Jan
Mayen magma system. Only for the 4 million years, though, will each rock
portion be located within the magma generating field. How fast magma can
be produced from this rock portion, or how much magma it will donate,
depends highly on the composition of the rocks. As time passes, the
previously mentioned highly fractionated portion of the ocean bottom,
produced during the continental break-up, will drift through the thermal
anomaly, and following it, into the anomaly, will be a more typical tholeiitic
ocean bottom segment, a less evolved crust. This would result in a decrease
ofthe alkalinity of the futurejan Mayen rocks.
This simplified model of crustal movements will be somewhat compli-
cated by the subsidence caused by accumulation of the volcanic pile. The