Jökull - 01.12.1985, Page 58

belong to the easternmost part of the Snaefellsnes Frac-
ture Zone. These tectonic features seem to fade out east
of the research area, and have usually small or no
vertical displacement. Some of the NW-SE trending
faults control, to a certain extent, the distribution of the
hot springs, especially in the outer part of the
Reykholtsdalur valley (Georgsson et al. 1978).
Only one NW-SE trending fault passes through the
Vellir thermal field, the Sturlu-Reykir fault (SRF). It
runs from the Hurdarbak farm to Sturlu-Reykir where it
disappears underneath the sediment cover. It may
appear again at Logaland, south of the valley. It has a
throw of 5—6 m towards SW.
Secondly, there are NE-SW trending normal faults.
Georgsson et al. (1981a and 1984) have suggested that
they control the regional flow of hot water at depth.
Two NE-SW trending faults run across the research
area. The Snaeldubeinsstadir fault (SSF) has a throw of
about 10 m towards SE in Kroppsmúli but north of the
Reykholtsdalur valley it has a throw in the opposite
direction. The Logaland fault (LF) has been traced
from Geirshlíd in Flókadalur to Logaland where it
disappears underneath the sediments to reappear north
of the valley. The fault descends the mountain slope
south of Logaland with an unusual northwestward trend
suggesting that the faultplane is not as steep as com-
monly observed for these faults in Borgarfjördur.
As mentioned above the Logaland and Snaeldubeins-
stadir faults are not exposed at the bottom of the
Reykholtsdalur valley. However, recent joints and frac-
tures transect the sediment cover and many hot springs
are related to these joints. Most of the joints have a
trend close to NIO—30°W (Fig. 2). They are best
exposed in the Hveragil gully where the hot springs and
related joints seem to form an en echelon pattern. A
similar pattern is observed at the Deildartunga —
Kleppjárnsreykir thermal field (Georgsson et al. 1978).
It is believed to be caused by shear movements on an
underlying fault or fracture.
Relatively few dykes have been found in the research
area (Fig. 4). Three of them, the Snaeldubeinsstadir
(ss), the Logaland (1) and the Sturlu-Reykir (sr) dykes
pass through the Vellir thermal field. Einarsson (1937)
suggested that the Snaeldubeinsstadir and Logaland
dykes controlled the ascent of thermal water at Sturlu-
Reykir and along the southern part of the Vellir line,
respectively. The dykes transect the thermal alignments
Fig. 4. The main tectonic features of the western part of
Reykholtsdalur.
4. mynd. Yfirlitskort af jarðfræði utanverðs
Reykholtsdals.
56 JÖKULL 35. ÁR