Jökull - 01.12.1999, Side 41

Figure 8. Dyke thickness (histograms), strikes (rose diagram), and poles to dyke planes (Schmidt's lower
hemisphere) separated into groups below (a) and above (b) the Hreðavatn unconformity. — Þykktardreifing
(stólparit) og stefnurósir bergganga sem liggja sitt hvorum megin Hreðavatnsmislœgisins.
Hreðavatn unconformity, other studies describe the
tectonic pattern as being much younger, dating from
the Quaternary to present.
Based on observations of aerial photographs,
faults in Tertiary rocks around the Langavatn and
Hreðavatn lakes have been interpreted as oblique-slip
faults forming horsts and grabens (Schafer, 1972).
The pattern consists of WNW striking faults, many
with right-normal components and a few with left-
normal, and N-S and NW-trending faults respectively
with right-normal and left-normal slips. This pattem,
oblique to the E-W trend of the SVZ, was interpreted
as monogenetic Riedel faults, generated by the dex-
tral movement along the volcanic zone since 2 Ma.
Sæmundsson (1978) described conjugate WNW dex-
tral and N-S sinistral strike-slip faults in the western
and central parts of the SVZ, and explained the two
sets by a NE extension and NW compression. East
of the Ljósufjöll system, E-W and NE-trending nor-
mal faults were active during the 1974 Borgarfjörður
earthquakes (Einarsson etal., 1977), indicating a NS-
trending extension (03 horizontal).
Bergerat et al. (1990) analysed mesoscopic fault-
slip data in the Tertiary (3.1-13 Ma) and Plio-
Pleistocen (0.7-3.1 Ma) rocks of Iceland, includ-
ing nearby Borgarfjörður, and suggested a general
JÖKULL, No. 47 39