Jökull - 01.12.1982, Page 21

Fig- 7. Progressive southward migration of the
locus ofvolcanism in the Austfirðir.
(a) about 12 m.y. ago
(b) about 6.5 m.y. ago
(c) at the present time
Mynd 7. H reyfing eldvirknimiðju Austurlands með tíma.
and Djúpadalsá were probably captured by the
Fagradalsá at an early stage; the Djúpadalsá and
Eyrarteigsá first (wind gaps at 681 and 645 m), and
the Brúðardalsá later (498 m).
The ancestral Grímsá flowing on the 650 m sur-
face is thought to have drained a much larger area
than now; Fig 4; river capture has halved the area,
much of it by the Hamarsá and other parts by the
Fossá, Berufjarðará and other rivers.
REFATIONS BETWEENTHE AUSTFIRÐIR
AND THE INLAND PLATEAU
In the Lagarfljót area there is an apparent time
gap between construction of the Austfirðir 12 to 9
m.y. ago, and construction of the inland plateau
from about 6.5 m.y. (the age of the lavas at the base
of the Bessastaðaá section) to 2.5 m.y. ago. A com-
plex faulted flexure (the Lagarfljót flexure zone)
occurs near the eastern boundary of the inland
plateau, and in this zone the constructional surface
deduced from the zeolite zones drops rapidly in
elevation from 1200 to 700 m. Thick sediments
occur moreover in the lower part of the Bessastaðaá
sequence at the edge of the flexure zone, suggesting
a major change in conditions. It seems likely that
the main uplift of the Austfirðir began at this time,
between about 9 and 6.5 m.y. ago (Fig. 6). Theexact
significance of the Bessastaðaá sediments is uncer-
tain. Possibly they reflect the first establishment of
glaciers in eastern Iceland although the first sedi-
ments which undoubtedly formed in a cold climate
are found at a higher stratigraphic level. It is more
likely that the Bessastaðaá sediments were deposit-
ed in temporary lakes developed inland from the
resulting new mountain mass of the Austfirðir.
The building of the Austfirðir from Gerpir to west
of Djúpivogur over the time period 13 to 5 m.y. ago
is seen as a continuous and steady —state process.
The first major departure from steady - state
conditions came about 6 m.y. ago when the locus of
volcanic activity, which was initially in the Gerpir
and Loðmundarfjörður area (a, Fig. 7) and had
gradually migrated south towards the Hamarsfjörð-
ur area (b, Fig. 7), moved south out of eastem
Iceland into the area of the Vatnajökull (c, Fig. 7).
Along the Lagarfljót the average lava accretion rate
(normalised as explained by Watkins and Walker
1977) in the period 6.5 to 3.5 m.y. ago dropped to
half of its previous value, and significant rhyolitic
volcanism (which in Iceland is a good indicator of
the locus of volcanism) totally ceased in eastern
Iceland. It is thought that the development ofthe
Lagarfljót flexure zone, and the differential uplift of
the Austfirðir which led to the development of the
1100 m and 850 m erosion surfaces, is correlated
with the change from the steady-state conditions of
the Austfirðir to the new lower-accretion-rate
JÖKULL 32. ÁR 19