Jökull - 01.12.1999, Side 28

system and to the south of it. The sections are outside
of the two major central volcanoes, Reykjadalur and
Laugardalur, to minimize the effect of the stress field
related to collapse calderas and inclined sheet injec-
tions. Only one profile (number 6) lies north of the
Hallarmúli central volcano, which lacks a caldera and
cone sheet swarm. The majority of the profiles trend
NNE (favouring WNW structures), but topographic
irregularities allowed observations of structures strik-
ing differently.
Field measurements consisted of orientations and
dips of faults, dykes, and fault striae; slip sense and
magnitude of fault displacements; thickness of fault
breccia; and thickness of dykes. The strike of all
the faults and dykes were measured, but either dip
or displacement of a few faults, and thickness of a
few dykes, could not be positively identified. Cross-
Figure 3. (a) Rose diagram
of the strike distribution of
122 faults; (b) Histogram
of the dip distribution of
109 faults; (c) Rose dia-
gram of the strike distribu-
tion of 158 dykes; (d) His-
togram of the dip distribu-
tion of 157 dykes. Note that
the dip could not be esti-
mated for all the faults and
dykes. — Stefnurósir 122
misgengja (a) og 158 berg-
ganga (c) ásamt stólpalínu-
ritum sem sýna halladreif-
ingu 109 misgengja (b) og
157 bergganga (d). Mis-
gengin hafa flest VNV-ASA
stefnu en berggangarnir
liggjaflestir NNA-SSV. Mis-
gengin liggja þvíflest þvert
á stefnu gömlu gosbeltanna
en gangarnir samsíða þeim.
Ekki reyndist mögulegt að
ákvarða halla allra mis-
gengja og bergganga sem
mœld voru.
cutting relations were observed and are discussed be-
low. Fault type was mainly inferred from the displace-
ment of marker horizons, and in a few cases from
analysis of striae. Estimates of fault-displacement
magnitude used markers comprising the facies of the
lava, interbedded sedimentary layers, and dyke con-
tacts. In a few cases the sense of the movement could
be determined from striae but the magnitude could not
be estimated because of a lack of good marker hori-
zons. Field data were complemented by local obser-
vations of aerial photographs.
FAULT ORIENTATIONS AND SLIP
Faults and striated planes were analysed in the Ter-
tiary lava pile, in the field. They are mostly dip-slip
faults, but 16 dextral and sinistral strike-slip motions
were identified as well. A few small reverse faults and
26 JÖKULL, No. 47