Jökull - 01.12.1999, Qupperneq 43

Figure 9. Sets of dykes, faults, and inferred paleostress fields; (a and b, respectively) Dextral and sinistral
strike-slip faults; (c) Normal faults; (d) Dykes; (e) Direction of extension (a3); (f) Direction of compression
The order of these sets does not reflect their relative ages. - Skipting bergganga, misgengja og fomra
spennusviða. Hœgri handar (a) og vinstri handar (b) sniðgengi; (c) siggengi; (d) berggangar; stefna minnstu
(e) og mestu (f) þrýstispennu.
trends of dykes, or normal and strike-slip faults; as
well as by NW-WNW dykes and normal faults cut by
NNE strike-slip faults. N-S, and NW to WNW dykes
are thus among the oldest fractures; they are also more
common below the unconformity (Figure 8a). Since
NNE fractures are involved, the complexity results
from the rifting periods and not from the activity of
the SVZ. Cross-cuttings, along with the frequency of
non-rift parallel fractures in the basement, indicate
that N-S, NNE, and NW to WNW fractures date likely
from early tectonic stages and that the stress field and
fracture formation have evolved with time.
SUMMARY AND CONCLUDING
REMARKS
Field data were collected during a preliminary cam-
paign in 1995. At regional and mesoscopic scales,
structural analysis in the Tertiary lava piles shows
that normal faulting and extension predominate on
both flanks of the Borgarnes anticline. Strike-slip and
oblique-slip faults also have occurred. A few reverse-
slip motions occur near irregularities and bends of
steeply dipping normal faults, or due to dyke in-
jection. The fault pattern is dominated by NNE
and WNW faults; both populations show normal and
strike-slip movements with similar length, displace-
ment and frequency. The dyke pattern is dominated
by N-S and NNE dykes; oblique trending dykes have
the same thickness and frequency as the rift parallel
dykes. N-S and NW-trending dykes are more com-
mon below the unconformity. NNE fractures are par-
allel to the NE portion of the SRZ (15-6 Ma) and the
RLRZ (6 Ma-present). WNW fractures trend paral-
lel to SVZ (2 Ma-present) and partly overlap with
the EW-trending portion of the SRZ. Strike-slip faults
contributed less to crustal deformation than normal
faults, due to their low number and low magnitude
of displacements. The horizontal offset of strike-slip
faults in Borgarfjörður (20-30 m) forms a separate
class between those (1-2 m) found in the South Ice-
land Seismic Zone (Einarsson and Eiríksson, 1982;
Bjarnason et al., 1993), and those (order of kilome-
tres) found along the Tjörnes Fracture Zone (Sæm-
undsson, 1974; Young et al., 1985). It is unclear
whether the strike-slip faults of Borgarfjörður were
initially formed along either rifts or transforms and re-
activated later, or if they formed outside of any plate
boundary. Faults and dykes are steeply dipping (80° -
85°) on both flanks of the Borgarnes anticline, and
strike-slip faults dip as steeply as normal faults. Faults
consistently dip steeply at all erosional levels down
to an inferred paleo-depth of 1.3 to 1.5 km. If large-
scale listric normal faults exist on land (Guðmunds-
son, 1992), they must be curved below the paleo-
JÖKULL, No. 47 41