Jökull - 01.12.1982, Page 16

Fig. 1. The Austfirðir and part of the inland plat-
eau west of it showing the height above sea level(in
hundreds of metres) of the original constructional
surface of the lava pile, taken to lie 600 m above the
top of the analcite zone (IValker 1974).
Mynd I. Austfirðir og hluti hálendisim vestur af þeim.
Sýnd er hœð upprunalegsyfirborðs hraunlagastaflans.
uppermost zeolite zone in the Eyvindará area (the
topofprofile F; IVatkins and IValker 1977).
The part of the Austfirðir farther south is dis-
tinctly younger, reflecting a southward migration
with time of the main locus of volcanism. Thus at
the eastern side in the Djúpivogur area, the extra-
polated age of the topmost lavas is about 7 m.y.,
falling to about 5 m.y. at the westem side in the
same latitude.
These dates, varying from 12 m.y. at Gerpir to 5
m.y. inland from Djúpivogur, relate to the original
Austfirðir constructional surface on which erosion
was initiated; they thus set the time limits for uplift
and erosion of the land to its present form. It is
supposed, although not proved, that significant up-
lift and erosion proceeded in the older eastem and
northern parts while the younger western and
southern parts were still being constructed.
Similar constraints apply to the inland plateau
west of the Austfirðir. Along the west of Lagaríljót
the zeolite zones indicate tliat the constructional
surface stood at about 900 m above sea level, falling
in level to 700 m westwards: only 250 to 50 metres of
erosion has therefore occurred to cut down to the
650 m plateau level. The rocks on the plateau at the
top of the Bessastaðaá section (McDougall et al.
1976) are dated at 4.6 m.y. and farther west are 2.5
m.y. old.
THE 1100 M EROSION SURFACE
It seems possible to recognise several distinct
erosion surfaces in the Austfirðir, such as could arise
from a steplike uplift of the region, as three different
summit levels on many of the major ridges between
the fjords. The highest of these surfaces is around
1100 m high and is referred to here as the 1100 m
surface. Mountain summits approximating to this
level are distributed over a broad belt averaging 25
km wide and covering 2600 km2, as stippled on Fig.
2 (only 4% of the area of this belt at present however
exceeds 1000 m in altitude). Mountains in the
middle of the belt are often fiat-topped and repre-
sent little-degraded remnants of this surface, while
mountains near the margins of this belt tend to be
sharp-sided and knife-edged ridges where the plat-
eau has been sapped by erosion (notably by the
convergence of cirques) and has often been reduced
to 1000 m or less.
The level of the 1100 m surface exceeds 1200 m in
thc Thingmúli area, which is also one of the areas
where the zeolite zones are highest. It is uncertain
whether this is a culmination of the original erosion
surface, or is due tosubsequent difTerential upwarp-
ing such as could have resulted from difi'erential
isostatic uplift ofthe relatively low-density rhyolitic
pile in the Thingmúli volcano.
The 1100 m accordant summit level is clearly an
erosion surface which truncates the stratigraphy of
the lava pile. The thickness of rocks eroded from
14 JÖKULL 32. ÁR