Jökull - 01.12.1999, Page 25

Figure lb. Location of the
studied profiles; (a) Pleisto-
cene-Holocene volcanics;
(b) Tertiary acidic intrusion;
(c) Tertiary ignimbrite; (d)
Strike and dip direction of
the Tertiary lava pile; (e)
Profiles: 1) Grímsstaðamúli;
2) Beilá river; 3) west of
Hreðavatn; 4) Hraunsnefs-
öxl/Hvassafell; 5) Bjarna-
dalsá/Kálfagil; 6) north of
Hallarmúli. - Jarðfrœði og
staðsetning rannsóknarsnið-
anna. (a) Gosbergfrá ísöld
og nútíma; (b) súrt innskot
frá tertíer; (c) strik oghalli
tertíerjarðlaga; (d) strik og
halli jarðlaga; (e) staðsetn-
ing sniðanna.
either located on, or very near, large scale faults. A
throw of 0.5 m to 1 m can be due to irregularities at the
base of lava beds along a joint. Small scale (minor)
faults have a throws of 1-5 m, large scale (major) may
have throws more than 5-10 m.
Rift zone and flank zone have a different tec-
tonism and chemistry. According to Sæmundsson
(1974, 1978) rift zones trace the boundary of diverg-
ing plates. Rift zones undergo extension parallel with
the direction of spreading. Subvertical fractures (nor-
mal faults, joints, dykes, open fissures and eruptive
fissures) and central volcanoes are the main structures
of the rift zones. Volcanic products of the neovol-
canic zone in the rift zone are tholeiitic to transitional,
in contrast to alkali rocks of the neovolcanic zone in
thzflank zone (Jakobsson, 1972). Afiank zone, also
called a lateral rift zone or an off-rift, is located away
from an active plate boundary. Horizontal shear stress
dominates and the fault pattern consists of conjugate
set of faults (Sæmundsson, 1978). Volcanism in the
lateral rift zones accounts for about 15% of the total
volcanic production during postglacial time (Jakobs-
son, 1972).
GEOLOGICAL SETTING
The oldest crust in Borgarfjörður formed by magma-
tism of the SRZ which was active between at least 15
and 6 Ma (Jóhannesson, 1975, 1980). K/Ar dating
suggests an age of 13.2 ± 2 m.y. (Moorbath et al.,
1968) and 9.4 ± 0.7 m.y. (Aronson and Sæmunds-
son, 1975) for the oldest rocks found in the crestal
area of the Borgarnes anticline. The first age better
fitting the distribution of anomaly 5 in the western
part of the Borgarfjörðurregion, according to Jóhann-
esson (1980). Rocks in the core of the Snæfellsnes
syncline have an age of 6.7 ± 0.4 m.y. (Jóhannes-
son, 1980). The bulk of the lava consist of basalts,
commonly with secondary minerals such as zeolites,
quartz, chalcedony and calcite. Acidic and interme-
diate rocks are confined to the Hallarmúli central vol-
cano dated from 6.7 Ma (Jóhannesson, 1975).
The Snæfellsnes syncline is the structural ex-
pression of this extinct rift zone (Sigurðsson, 1967;
Sæmundsson, 1967). It shows a kink shape at the
scale of western Iceland (Jóhannesson, 1980), where
it cuts obliquely across the Snæfellsnes peninsula,
JÖKULL, No. 47 23