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SUMMARY
Formation and development
of the fractures
at Thingvellir,
SW Iceland
by
Ágúst Guðmundsson
Nordic Volcanological Institute
University of Iceland, Reykjavík
The Thingvellir fractures dissect a 9,000
year-old basaltic lava (pahoehoe lava)
north of lake Thingvallavatn and form a
part of the Hengill fissure swarm (Fig. 1).
The main conclusions of the paper may
be summarized as follows.
1) The average strike of the fractures,
referring to the linear orientation between
the fracture ends, is N30°E.
2) The average length of 101 measured
fractures is 620 m, referring to the linear
length between the fracture ends. The
lengths range from 57 m to about 7.7 km
(the Almannagjá fault). Most of the frac-
tures are relatively short; 44 fractures are
less than 500 m long.
3) Width and throw of many fractures
were measured in the field at intervals of
25 or 50 m (Figs. 3 and 9). The maximum
measured width is 68 m on the Hrafnagjá
fault and 64 m on the Almannagjá fault.
Both these measurements were made from
aerial photographs. The maximum mea-
sured width in the field is 60 m, on Hrafna-
gjá. The maximum measured throw on a
single fault is 28 m, in one point of
measurement on Almannagjá (Fig. 3).
Because the east fault wall of Almannagjá
stands 10-12 m above the area immedia-
tely to the east (Figs. 6 and 7) the total
maximum throw on Almannagjá is about
40 m. On the fault Gildruholtsgjá the
throw reaches 25 m in many points of
measurement.
4) All the fractures are vertical at the sur-
face. The estimated depth of the vertical
part of the normal faults, which is the same
as the depth of the pure extension frac-
tures, is of the order several hundred
meters. The depth is thus similar to the
length of the fractures and also similar to
the mean spacing between the major frac-
tures (Fig. 2).
5) All the major fractures seem to have
grown by coalescence of smaller fractures
that initially were offset and, occasionally,
had an en echelon arrangement (Fig. 8).
6) Two hypotheses are advanced to
explain the formation of the fractures.
One hypothesis suggests that the major
faults, such as Almannagjá and Hrafnagjá,
are the result of regional gravity gliding
combined with plate movements, whereas
the small fissures and faults are caused by
dykes that fail to reach the surface. The
other hypothesis suggests that the frac-
tures are the result of magmatic pressure
changes in the magma layer beneath the
Hengill fissure swarm.
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