Jökull - 01.12.1999, Page 29

listric normal faults were also measured, but they are
of local significance only. Rift-parallel fault swarms
are not clearly defined in spite of the high fracture
density. By contrast, the fault pattern is dominated by
an orthogonal network of horsts and grabens, cross-
cutting each other at right angles (Figure 2).
Fault orientations
WNW and NNE are the dominant strikes of the two
main sets of the faults in this part of the area (Figures
2 and 3a). Within the first set, fault strikes range from
N110° to N150°E, with a peak at N110°-130°E. In
the second set, faults have a NNE trend with a peak
at N20°-30°E. E-W striking faults were also mea-
sured, but they are limited here and more frequent
tens of kilometres to the northeast. NNW to N-S
striking faults are minor among these measurements,
but become common to the northeast and north, in
the vicinity of Reykjadalur central volcano (Jóhann-
esson, 1975). The majority of the measured faults
are normal (WNW) to, and others are oblique (N-S
to NNW) to, the NNE trend of the rift zones. The
WNW set coincides with the trend of the young SVZ.
The NNE striking faults are subparallel to the RLRZ
and to the NNE portion of the SRZ. However, further
to the north, the SRZ trends E-W.
Fault dips
Fault dips range from 50° to 90° with a peak at 80°-
85° (Figure 3b). The arithmetic average dip of all
faults measured is 77°, and about 71% of the faults
dip steeperthan 70° (Figure4a). WNW striking faults
(N100°-120°E) are the steepest with a mean dip of
83°. NNE faults (N30°-40°E) and E-W faults (N80°-
90°) have an average dip of 82° and 80°, respectively.
The dip direction of the faults is variable. WNW faults
dip in about equal proportion to the south and to the
north (Figures 4b and 4c). Similarly, NNE faults dip
in about equal proportion, and with roughly equal in-
clinations, to the east and west. Strike-slip faults dip
from 67° to 86°, as do the majority of normal faults
(Figure 4b). There is no significant variation in dip
depending on fault type. Similar results on the dip
of mesoscopic strike-slip and normal faults have been
reported from SW Iceland (Villemin et al., 1994).
Table 1. Fault strike versus slip sense (N=58 striated
planes). — Stefnudreifing skriðráka flokkuð eftir teg-
undum. Tölur sýnafjölda siggengishreyfmga (norm-
al), hœgri (right lateral) og vinstri handar (left lat-
eral) hreyfinga og samgengishreyfinga (reverse).
Fault displacements
The magnitude of vertical displacement (throw) was
positively identified along 112 normal faults, as was
the horizontal offset along five of the 7 major dex-
tral strike-slip faults. Throw and offset were observed
along three of the strike-slip faults, but field evidence
did not clarify their displacement history. Measured
throws range from 0.5 to 38 m (Figure 4d); the largest
value is found along a WNW normal fault in the ign-
imbrite of the Hallarmúli central volcano. Horizontal
offsets vary from 7 to 16 m along WNW and NNW
dextral strike-slip faults, but reach a maximum of 27-
30 m along two NNE dextral strike-slip faults (Fig-
ures 5b and 5c). For comparison, Jóhannesson (1975)
observed 10-75 m displacements along normal faults
in this area, and suspected a maximum value around
250 m on the WNW-trending Dyngjá fault (Figure
5b). He also found similar maximum offsets (20-30
m) along strike-slip faults. Strike-slip faults have con-
tributed less to crustal deformation in Borgarfjörður
JÖKULL, No. 47 27