Jökull - 01.12.1999, Page 39

Instead they seem to have been active throughout the
burial tilting. This mechanism implies growth faults,
with greater throw as the faults are deeper, and will
necessarily apply to all the faults of this area.
DYKE ORIENTATIONS
The 158 dykes were measured along the same proíiles
as the faults (Figure Ib). They are located outside the
Reykjadalur and Laugardalur central volcanoes and
outside of the perimeter of the associated cone sheet
swarms, and are thus regarded as regional dykes. The
measured dykes are basaltic, mostly made up of fine to
coarse-grained tholeiite, some with small phenocrysts
(mainly plagioclase and pyroxene) and vesicles. In
a vertical section, some dykes show an en échelon
arrangement (Figure 6f), similar to the map view of
eruptive and noneruptive fissures in the neovolcanic
zone (Nakamura, 1970).
Dyke orientations
Regional dykes of Iceland are generally regarded as
being parallel to the rift zones (Sæmundsson, 1978;
Guðmundsson, 1995). In the Tjörnes Fracture Zone,
dyke trends change gradually from N-S to N110°E;
this was interpreted as evidence of block rotation
along the transform fault (Young et al., 1985). In the
Northwest Peninsula and Snæfellsnes, the main dyke
trend swings from N-S to ENE; predominant NW and
NE trends occur to the south and east of Snæfellsnes
(Sigurðsson, 1967).
The present study indicates that in this part of
western Iceland dykes trend mostly N0°-10°E and
N20°-30°E in the Tertiary lava piles, with a disper-
sion about N30°-40°E and N160°-170°E (Figure 3c).
WNW to NW trends are also a part of the dyke pat-
tern, although they are less frequent among measured
dykes than among measured faults (Figures 3a and
3c). ENE dykes such as the swarm of Hafnarfjall-
Skarðsheiði (Franzson, 1978) and E-W dykes com-
mon some 20 kilometres to the northeast (Jóhann-
esson, 1975) are poorly represented in this study.
Strikes of dykes in Borgarfjörður do not reflect a sim-
ple rift-parallel swarm. NNE dykes trend parallel to
the RLRZ and to the NE portion of the SRZ. WNW
dykes are perpendicular to the two rift axes but are
slightly oblique to the SRZ and parallel to the SVZ
(Figure la). NW and N-S dykes are oblique to all
the above regional structures. WNW to NW striking
dykes, although parallel with the trend of SVZ, are
not all young. As described earlier, some of the NW
to WNW dykes are cut by, and are thus older than,
NNE strike-slip faults. In addition, dykes of these
trends appear to be most altered (Jóhannesson, 1975).
In Hafnarfjall-Skarðsheiði to the south, WNW dykes
have been altered by a high temperature geothermal
gradient during volcanic activity between 6 and 4 Ma,
without feeding any later volcanism (Franzson, 1978).
Dip
Planar, sinuous, and stepped dykes are subvertical in
the Borgarfjörðurarea and, like faults, they are steeply
dipping on both flanks of the Borgarnes anticline, with
the peak at 80°-85° (Figure 3d). Dykes steeper than
85° (76% of the measurements) are, however, more
common than faults steeper than 85° (Figure 3b). Few
dykes dip less than 70°, indicating that the bulk of
the magmatic fractures are regional dykes, generated
by the regional stress field during the activity of the
SRZ and the RLRZ. The relationship between dyke
dips and tilting towards the two rift axes is discussed
below.
Relationships between amagmatic and magmatic
fractures
Field examples of dykes injected into fractures, and
dyke and fault cross cuttings were observed. Dykes
were seen either using the columnar joints of the lava
piles (Figure 6f) or, in six cases, injected into nor-
mal faults (Figure 6g). In these cases both dykes and
faults have a NE trend. Field evidence did not con-
firm whether the dykes were injected laterally or ver-
tically. No general rule appears regarding the relation
between dyke injection and the magnitude of fault dis-
placements. The dyke thickness varies from 0.15 to
30 m, and the throw of the faults from 0.3 to >10.5 m.
Cross-cutting relations are complicated. As ex-
plained earlier, a few WNW dykes are cut by NNE
dextral strike-slip faults (Figures 5b and 5c). Other
field examples show, by contrast, NNE and N-S dykes
cut mainly by WNW normal and dextral strike-slip
faults, and secondarily by NW and E-W normal faults.
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