Jökull - 01.12.1984, Page 83

Tectonic Aspects of Dykes in Northwestern Iceland
ÁGÚST GUÐMUNDSSON
Menntaskólinn við Sund, 104 Reykjavík, Iceland
ABSTRACT
About 400 basaltic dykes were studied in 15
profiles in northwestern Iceland. About 94% of
the dykes dip within 10° of the vertical. There are
three main directions of dykes in the area, but
about 57% ofthe dykes strike between N50°E and
E. The average thickness ofthe dykes is 4.3 m, but
the average thickness in individual profiles is from
5.5 m to 2.2 m. The length/width ratio of 12 well-
defined dykes is from about 0.3-103 to about
1.5-103. Eleven dykes wereseen to end in a vertical
section, but no dykes were seen to continue as lava
flows. Most of the dykes are multiple in the sense
that they are composed of up to tens of columnar
rows. It is suggested that the dykesform in magma
phases which give rise to the columnar rows. Each
phase ofmagma is subsequently split in two by the
next magma phase, and so on. For a typical dyke,
the time between successive magma phases is of
the order of several hundred days. The maximum
dilation in about one hundred 185 m long sections,
fromfour widely separated profiles, is about 17%,
but the average dilation in the profiles is 5—6%.
The average dip of the 68 normal faults observed
in the area is about 69|0. The throw is 0.5-25.0 m,
with an average of 5.3 m. No dykes were seen to
occupy ordinary normal faults, that is, normal
faults that dip 60T—7CT. In comparison with east-
ern Iceland one finds, for northwestern Iceland,
that normal faults are more common and tend to
have larger throws, and that thin dykes are
relatively more common.
INTRODUCTION
This paper presents some of the results
obtained in a detailed study of about 400 dykes in
northwestern Iceland. Apart from general obser-
vations and measurements of the dykes, the main
objective of the study was to estimate the regio-
nal dilation of the crust, and to observe how
dykes end vertically to see if they were the feed-
ers of the lava flows.
Fig. 1 shows the main research areas in relation
to the general geology of northwestern Iceland.
The general geology of a part of northwestern
Iceland is described by Kristjánsson et al. (1975)
and by McDougall et al. (1984). The research
areas are mainly made up of basaltic lavas toget-
her with a few, mostly clastic, sedimentary hori-
zons. The age of the rocks that make up the
research areas is approximately 12—14 m.y.
(Saemundsson 1979). The dip of the lavas
throughout most of the areas is 2°—5° to SSE, but
increases towards the south, reaching a maximum
of 9° in Kjálkafjördur. Minimum dip, 0°—1°, is at
the top of the mountains near Bíldudalur on the
south coast of Arnarfjördur.
The relevant published works on dykes in
northwestern Iceland are Thoroddsen (1906),
Friedrich (1966), Sigurdsson (1967), Kristjánsson
et al. (1975) and McDougall et al. (1984). Thor-
oddsen (1906) measured the direction of several
tens of dykes in northwestern Iceland and shows
the directions on a dyke-map of the northwestern
peninsula. Friedrich (1966) measured the direc-
tion of dykes near Brjánslaekur, on the west
coast of Vatnsfjördur. Using air photographs,
Sigurdsson (1967) mapped tectonic lineaments,
mainly dykes, in many areas in northwestern
Iceland. Kristjánsson et al. (1975) show some of
the thickest dykes on their maps of the area
Patreksfjördur-Arnarfjördur. McDougall et al.
(1984, Fig.l) indicate dominant dyke trends in
parts of northwestern Iceland.
METHODS
The dykes were studied in well-exposed sec-
tions along cliffs, the coast or in river channels.
The main profiles are located in Fig. 2. Special
attention was given to the following factors: (1)
dyke strike and dip; (2) dyke thickness; (3) litho-
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