Jökull - 01.12.1984, Blaðsíða 114

Figure 5. The tectonic features of the Reykholt thermal system.
5. mynd. Höggun og hverir í Reykholtsdal og nágrenni.
northeasterly trending faults operate as main
channels for regional flow from the highlands
towards the thermal fields. Where these aquifers
are intersected in the lowlands by open fractures
the water flows to the surface.
The best example is the Deildartunga hot
spring (Georgsson et al. 1978). The spring is
located at the intersection of a fracture trending
N20°W and a dyke trending N30°E (Fig. 5). The
dyke was traced up to the hills by a ground
magnetic survey, showing a combined anomaly of
a dyke and a fault. About 1 km northwards the
anomaly from the dyke disappeared. The fault
was traced further north where it is exposed at
the surface.
The close relation between active faults and
the flow of hot water was emphasized in the
Borgarfjördur earthquakes in 1974. The active
seismic zone was 15-20 km north of Reykholtsdal-
ur. The main epicentral zone showed a westerly
trend, but was intersected by a secondary zone
with a northeasterly trend (Einarsson et al. 1977).
The latter coincides with the northern part of the
20 Qm low resistivity anomaly of the Reykholt
thermal system. The Deildartunga fault stretches
to northeast towards the same area. The effect of
the earthquakes on thermal activity was most
notable at Helgávatn. The thermal water disap-
peared for three weeks, reappearing with sub-
stantially increased discharge and slightly higher
temperature. Increased discharge was also noted
at the Lundahver hot spring, and at Varmaland
flow measurements indicate an increase of
around 50%. In numerous springs the water
turned milky, gradually returning to normal in a
few weeks time.
The active tectonics play an important role in
the existence of the Reykholt thermal system.
112 JÖKULL 34. ÁR