Jökull - 01.12.1979, Side 29

structural trends may have been inherited from an
early stage of plume activity. Plumes are supposed
to cause the overlying lithosphere to be uplifted
and ruptured along ideally three rifts, two of which
may subsequently become spreading axes. The rifts
typically meet at an angle of 120°. The Green-
land-Faeroes aseismic ridge may have grown out of
such a plume generated uplift of which the East
Greenland and Faeroes basalts are remnants. Re-
arrangements of spreading axes in the area between
the Jan Mayen Fracture Zone and Iceland about
27 m.y. ago caused a switch to a more northerly
trend of the axis north of Iceland as seen in the
trends of the extinct Aegir Ridge and the active
Kolbeinsey Ridge. Northerly trends of linear tec-
tonics in the northern part of Iceland and south-
westerly trends in the southern part were possibly
initiated during the last major reorganization of
the plume and spreading axes across the Iceland
area some 27 m.y. ago.
From the trend of linear tectonic features the
centre of the plume would be expected to lie under
the eastern part of Iceland which is also the topo-
graphically highest part of the country with
maximum volcanic production along the axial rift
zones. The decrease in volcanic activity towards the
north and southwest is thought to be directly
related to the diminishing effects of the plume. The
decrease in volcanic activity results in less volcanic
emission per unit time, narrowing of the axial rift
zone and disappearance of the central volcanoes.
The eastward offset of the neovolcanic zone
relative to the submerged ridges could be inter-
preted as a consequence of the mantle plume being
located east of the Mid-Atlantic Ridge axis. The
axial rift zones would tend to locate themselves
above the plume. Migration of the mantle plume
relative to the sub-oceanic ridge axes could explain
the eastward and southward shift of the volcanic
zones in Iceland which has left extinct axial rift
zones in the western part of Iceland that have been
identified from the synclinal structures in that area.
The eastward shift of the axial rift zones with time
probably causes the development of fracture zones
and flank zones; the latter seem to be recent
features of Icelandic geology.
SELECTED REFERENCES
Albertsson, K. J., 1978: Um aldur jardlaga á Tjör-
nesi (Some notes on the age of the Tjörnes strata
sequence, northern Iceland). Náttúrufraeding-
urinn 48: 1—8.
Aronson, J. L. and K. Saemundsson, 1975:
Relatively old basalts from structurally high
areas in Central Iceland. Earth Planet. Sci.
Lett. 28: 83—97.
Björnsson A., K. Saemundsson, P. Einarsson, E.
Tryggvason, and K. Grönvold, 1977: Current
rifting episode in North Iceland. Nature 266:
318—323.
Björnsson, A., G. Johnsen, S. Sigurdsson, G. Thor-
bergsson and E. Tryggvason, 1979: Rifting of
the plate boundary in North Iceland 1975—
1978. J. Geophys. Res. 84, B6: 3029-
3038.
Björnsson, S., and P. Einarsson, 1974: Seismicity of
Iceland. In Kristjansson, L. (Ed.): Geodynamics
of Iceland and the North Atlantic Area: 225—
329. D. Reidel, Publ. Co.
Bödvarsson, G. and G. P. L. Walker, 1964: Crustal
drift in Iceland. Geophys. J. R. astron. Soc. 8:
285—300.
Brander, J. and G. Wadge, 1973: Distance
measurements across the Heimaey eruptive
fissure. Nature 244: 496— 498.
Burke, K. and J. F. Dewey, 1973: Plume generated
triple junctions: Key indicators in applying
plate tectonics to old rocks. J. Geol. 81: 406—
433.
Einarsson, P., F. W. Klein, and S. Björnsson, 1977:
The Borgarfjördur earthq"akes in West Iceland
1974. Bull. Seismol. Soc. Am. 67: 187 — 208.
Einarsson,Th., 1971: Jardfraedi (Geology): 254 pp.
Heimskringla, Reykjavík.
Einarsson, Th., D. M. Hopkins and R. R. Doell,
1967: The stratigraphy of Tjörnes, northern
Iceland, and the history of the Bering Land
Bridge. In Hopkins, D. M. (Ed.): The Bering
Land Bridge: 312—325. Stanford Univ. Press.
Everts, P., 1975: Die Geologie von Skagi und der
Ost-Kiiste des Skagafjords (Nord-Island). Son-
derveröff. Geol. Inst. Univ. Köln 25: 114 pp.
Fridleifsson, I. B., 1977: Distribution of large
basaltic intrusions in the Icelandic crust and the
nature of the layer 2 — layer 3 boundary. Geol.
Soc. Am. Bull. 88: 1689—1693.
Gibson, I. L. and J. D. A. Piper, 1972: Structure of
the Icelandic basalt plateau and the process of
drift. Phil. Trans. R. Soc. Lond. A. 271: 141 —
150.
Haimson, B. C., 1979: New stress measurements in
Iceland reinforce previous hydrofracturing
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