Jökull - 01.12.1999, Page 40

Offset and throw of these faults range from 7 to 16 m
and from 1 to 28 m, respectively. Although an age
relationship cannot be easily established from the ob-
served cross-cuttings, dextral movements of WNW
and NNE strike-slip faults are younger than WNW
and NNE normal faults and dykes.
WNW and NNE dykes alternatively cross-cut
each other, and in three cases, ENE dykes cut pre-
existing N-S and NNE dykes. These dykes have a
thickness between 0.55 to 5.5 m. As with the normal
faults, the mutual cross-cutting of WNW and NNE
dykes indicates magma injection simultaneously into
the orthogonal fracture system. Since NNE dykes are
involved, this activity may date from the rifting pe-
riods. Data from Pleistocene and Tertiary rocks of
southwest Iceland show it is rather common to find
rift-perpendicular fractures (joint sets and veins) as-
sociated with the rifting process (Jefferis and Voight,
1981).
DYKES AND THE HREÐAVATN
UNCONFORMITY
Figure 8 shows the orientations, dips and thickness
of the dykes below and above the erosional Hreða-
vatn unconformity. N0°-10°E striking dykes pre-
dominate below the unconformity, and NNE-trending
dykes are secondary. A peak of NW dykes is also
evident (Figure 8a). By contrast, N20°-30°E is the
dominant strike of dykes above the unconformity, and
NS-trending dykes are secondary. Another peak ap-
pears with WNW trend (Figure 8b).
The thickness of dykes ranges from 0.2 to 30 m,
with a minimum of 1 columnar row, and a maxi-
mum of 6 often separated by chilled margins. The
thickness of individual dykes reaches a maximum of
30 m below the unconformity, where about 80% of
dykes have thickness of 5 m or less; for 82 mea-
sured dykes, the mean thickness is 4.40 m (Figure
8a). Dykes are not thicker than 10 m above the un-
conformity, with 80% less than 3 m; for 73 dykes, the
mean thickness is 2.70 m (Figure 8b). Observations
confirm that thin dykes are more common than thick
dykes (Jóhannesson, 1975; Torfason, 1979; Friðleifs-
son, 1983; Guðmundsson, 1995), and that Icelandic
dykes become generally thicker with increasing depth
(Walker, 1960; Helgason and Zentilli, 1985). In addi-
tion the number of thin dykes is always greater than
thick dykes at any erosional levels (Figure 8). Below
and above the unconformity, the thickest dykes strike
along both N-S and NNE trends. However, N-S dykes
are a separate orientation class and should not be re-
garded as a subgroup of the NNE dykes. They trend
obliquely to the rift zone but have the same frequency
and the same thickness as the rift-parallel dykes.
The profiles are located on both sides of the ero-
sional unconformity and also on both flanks of the
Borgames anticline. These regional structures are the
main indicators of the shift in activity from the SRZ
to the RLRZ. As with the faults, no major changes
appear in the dip angles of dykes, neither dipping
towards and away from the rift axis, nor below and
above the unconformity (Figure 8). Dips are roughly
equal towards west and east, but not in equal propor-
tions. Below the unconformity, 64% of the dykes dip
towards east, while by contrast 57% dip towards west
above the unconformity. Measured dykes may thus
be of different ages and reflect the diachronous tilt-
ing towards the two rift axes. Part of the N-S and
NNE striking east-dipping dykes below the unconfor-
mity are the oldest dykes, emplaced during the Snæ-
fellsnes rifting. Their dip has changed from initially
subvertical to eastwards because of the northwest tilt
of the lava pile towards the SRZ. That part of the N-S
and NNE dykes which dip westwards above the un-
conformity were emplaced during the RLRZ activity,
and were tilted later. Their dip has changed from ini-
tially subvertical to westwards, conformably with the
SE tilt of the lava piles towards the RLRZ.
SIGNIFICANCE OF THE FAULT AND
THE DYKE PATTERNS
Faults in Borgarfjörður have anomalous trends and
polarities. Previous studies have led to varied con-
clusions regarding fault genesis and stress conditions,
reflecting the complexity of the tectonics in this area.
Whereas Jóhannesson (1980) suggested that WNW
to NW-SE, NE-SW and N-S fractures are older than
6.5-7 Ma, because their density decreases across the
38 JÖKULL, No. 47