Jökull - 01.12.1979, Page 51

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TERTIARY FLOOD BASALTS
ACTIVE ZONESOF RIFTING
AND VOLCANISM
PLIO-PLEISTOCENE BASALTS
AND HYALOCLASTITES
----- FLOW DIRECTIONS OF
THERMAL WATER TRACED BY
DEUTERIUM
A HIGH TEMPERATURE AREA
Fig. 3. The general flow
pattem of thermal ground-
water systems in Iceland
according to deuterium
measurements (Árnason,
1976) superimposed on a
simplified geological map of
Iceland. The arrows join the
thermal areas in the lowlands
with possible recharge areas in
the highlands, and are
modified such that they are
almost perpendicular to the
isolines of average topo-
graphic heights based on rect-
angular areas of 520 km2. The
arrows were drawn indepen-
dently of the geological map.
Note how closely the flow
direction arrows generally fall
with the geological strike.
(Slightly modified from Frid-
leifsson, 1978).
geological map and the flow pattern arrows, how-
ever, shows a remarkably good correlation between
the flow directions and the geological strike. This
suggests that the water may flow along the same
pervious horizons in the strata and/or dykes and
faults along the strike all the way from the high-
lands to the lowlands.
Further indication of the preferential flow of the
water along stratiform horizons and/or dyke
swarms (which generally have a direction deviating
0°—30° from the strike) is seen in the distribution
of hot springs with regard to erosional features.
Although hot springs are very widely distributed in
Iceland (Fig. 2) there are certain areas, particularly
in the eastern part of the country, that are almost
devoid of thermal activity. A comparison of the
distribution of hot springs and geological strikes
with the direction of major erosional features, such
as fjords and valleys, shows that all the major low
temperature areas of the country are characterized
by the erosional directions being approximately
parallel to the strike. This implies that water can
flow undisturbed along the same permeable hori-
zons from the recharge areas in the mountains to
the outflow areas in the lowlands. The regions that
are devoid of hot springs, such as the Eastern
Fjords, are on the other hand characterized by the
erosional directions being nearly perpendicular to
the strike directions. This can be interpreted in the
way, that water that seeps into the bedrock in the
mountains in these areas cannot flow for but a few
kilometers along strike, as the permeable horizons
and the transecting dykes and faults are intersected
by erosional features (valleys and fjords) at
relatively short intervals. As the flow distance is so
short and the hydrostatic gradient much disturbed,
the water does not get the same opportunity to
withdraw heat from the regional heat flow as water
that flows undisturbed for tens of kilometers. In-
deed the few hot spring localities in eastern Iceland
are in areas where the erosional directions are
nearly parallel to the strike of the bedrock (Fig. 2).
The total natural flow from hot springs in the
low temperature areas has been estimated 1200 1/s.
The bulk of the hot springs has a flow rate of less
than 5 1/s, but both in the Plio-Pleistocene and
Tertiary strata individual springs may have flow
rates of several tens of 1/s. The largest springs are
commonly accompanied by a localized cluster of
smaller springs. The main upflow zones are gener-
ally controlled by dykes and/or faults. Many hot
spring localities are associated with regional dyke
and fault swarms. Similarly hot springs are com-
mon on the outskirts of extinct and eroded central
volcanoes characterized by abundant intrusives
and faulting. Intrusive activity, faulting and tilting
clearly play an important role in creating secon-
dary permeability and in directing the flow of
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JÖKULL 29. ÁR 49