Jökull - 01.12.1999, Page 27

and modelled by Pálmason (1973), is due to accumu-
lation of lava and extension causing loading and sub-
sidence along the rift axis. Thus, the tilted lava dips
southeast in the northwestem flank of the Snæfells-
nes syncline, and northwest in the southeastern flank
which is also the western flank of the Borgarnes anti-
cline.
Around 6-7 Ma, the southem part of the SRZ be-
came extinct and a new spreading axis, the predeces-
sor of the RLRZ, started some 70 km to the east (Jó-
hannesson, 1975). The volcano-tectonic activity of
this new rift zone has been virtually continuous from
6 Ma to present (McDougall et al., 1977). A period
of erosion which separated the two rifting phases is
marked by the Hreðavatn unconformity (Figure la),
west of Hallarmúli (Jóhannesson, 1975) and its coun-
terpart, the Höfn unconformity, west of Hafnarfjall-
Skarðsheiði (Franzson, 1978). Sedimentary layers,
such as plant-bearing beds and lake sediments of silt-
stone and sandstone, lie unconformably on tilted and
eroded basement. The age gap across the Hreðavatn
unconformity is around 6 m.y. and becomes smaller
in Hítardalur farther to the northwest (Jóhannesson,
1975). The old basement and the sedimentary beds
were subsequently covered by the younger lava pile,
and due to sagging along the new rift axis, the eastern
part of the area changed its dip direction from NW to
SE (Jóhannesson, 1975). This local retilting towards
the new rift zone gave rise to the Borgarnes anticline,
which has a NE trend (Figure la). Lava covering the
eastern flank of the anticline become progressively
younger towards the active rift zone. The main central
volcanoes of the new lava pile are Reykjadalur (5.8
Ma), Laugardalur (age obscure, but slightly younger
than Reykjadalur) and an acidic intrusion (Mt. Baula)
at 3.4 Ma (Jóhannesson, 1975). Farther to the south,
the Hafnarfjall-Skarðsheiði central volcano was active
from 6 to 4 m.y. ago (Franzson, 1978).
The SVZ became active 2 Ma (Sigurðsson, 1970)
and erupted the alkali basalts of the western flank zone
(Jakobsson, 1972). Bathymetric-magnetic investiga-
tions indicate the volcanic zone continues at least by
100 km west of the Snæfellsnes peninsula (Thors and
Kristjánsson, 1974). Within the SVZ on land, Sig-
urðsson (1970) described three WNW en échelon vol-
canic systems: Snæfellsjökull, Lýsuskarð and Ljósu-
fjöll (Figure la). These systems each include rows
of basaltic crater cones (Jóhannesson, 1982) and a
central volcano which differs from those in the rift
zone by shape and height (Sæmundsson, 1978) and
their high proportion of siliceous rocks (Jakobsson,
1972). The Ljósufjöll system ends to the ESE by the
recent crater row Grábrók which is about 3000 years
old (Schwarzbach, 1956).
No obvious transform or transcurrent fault coin-
cides with the SVZ at the surface, but dextral shear-
ing along it has been suggested from mapping of the
late Pleistocene to Recent volcanism and from fault
analysis (Sigurðsson, 1970; Scháfer, 1972). Exten-
sional tectonic features, which are common in the rift
zones, are poorly developed in these swarms. The
fault pattern consists of sets of conjugate WNW and
N-S strike-slip faults in the west and the central part of
the SVZ (Sæmundsson, 1978). This pattern changes
by the eastern end of Ljósufjöll, where NE-SW and
E-W normal faults were found to be active during the
Borgarfjörður earthquakes in 1974 (Einarsson et al.,
1977). The SVZ also has been interpreted as prop-
agating southeastwards and approaching the RLRZ
(Guðmundsson, 1996).
From observations of the infrastructure of the
SVZ, Jóhannesson (1980) suggested that WNW to
NW-SE, NE-SW and N-S fractures were already ac-
tive before 6.5-7.0 Ma, as their density decreases
across the Hreðavatn unconformity. NW-SE trend-
ing dykes, apparently related to the young SVZ, are
more altered than other dykes in the same area (Jó-
hannesson, 1975). These dykes, along with NW-SE
to WNW-trending faults, indicate the SVZ was an ac-
tive zone of crustal weakness (Sigurðsson, 1967; Jó-
hannesson, 1975), probably already at the beginning
of crustal generation (Jancin et al., 1985).
FIELD METHODS
A total of 122 faults, 158 dykes and 58 striated planes
were measured along six profiles located on both
flanks of the Borgarnes anticline (Figure lb) in flows
as old as 12-13 Ma (profiles 1, 2 and 3) and 5.5-6 Ma
(profiles 4, 5 and 6). In relation to the young SVZ, the
sections are within the eastern part of the Ljósufjöll
JÖKULL, No. 47 25