Greinar (Vísindafélag Íslendinga) - 01.01.1976, Page 127

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can be made at the SW-end of the high Bláfjöll-ridge. The ridge
drops here to the level of about 550 m where, on a length of 1.5 km,
it has been overflowed by the lavas from the large shieldvolcano
Heidin há. This postglacial shield has the usual signs of early post-
glacial age: heavy weathering, and a thick cover of soil on the
lower flanks. A minimum estimate of its age is 5000 years. These
lavas, then, tell us what tectonic movement has taken place on the
prolonged Bláfjöll fracture line during a rather long time interval,
and the answer is: none. Only at one place on the mentioned stretch
is there a few metres long narrow fracture, which by a stretch of
the imagination might be postulated as “spreading” by 10 cm or so.
This means less than 1/500 cm/yr on the average. For such a pro-
nounced and significantly placed fracture line as that of Bláfjöll-
ridge, this is a non-trivial result. We shall find a quite similar
upper limit for a “spreading rate” in the Thingvellir graben for
a time interval covering at least the last several hundred thousand
years. Reversely magnetized rock at a stratigraphically high level
in the graben rocks demonstrates an age of more than 0.7. My for
such strata within the zone which fail to show any signs of “spread-
ing’.
B. The Thingvellir-Langjökull section of tlie Median Zone
(“B-section”)
a) General remarks and the latest activitv.
As already indicated, the volcanic and tectonic activity in this
Section can be traced much farther back in time than in the penin-
sula, and is spread over a broad zone. Craters have not only been
destroyed, so that instead we find coarse plugs (e.g. Lyngdalsheidi,
Ingólfsfjall), the erosion has in many cases gone much further. Yet,
we only consider here such ruins which are undoubtedly those of
volcanoes.
The postglacial activity follows a very narrow zone, no more
than 1 km wide, that falls naturally into two parts. The southern
part “Sandey line”) lies 5-6 km east of the Bláfjöll axis and
stretches from Litli Meitill NE to the island Sandey, a length of
26 km. This part lies both in section A and B and demonstrates a