Jökull - 01.12.1982, Síða 15

Topographic Evolution of Eastern Iceland
G.P.L. WALKER
Department of Geology and Geophysics,
University of Hawaii at Manoa,
2525 Correa Road, Honolulu, Hawaii 96822, U.S.A.
ABSTRACT
This article considers the erosional history of the deeply
dissected Austfirðir and part of the inland plateau to the
west, and includes an estimate of the erosion rate. The
uplifted lava surface on which erosion began was constructed
12 m.y. ago in the extreme east to 2.5. m.y. ago in the west
and the elevation of this surface, as reconstructed from the
Zeolite zones, variesfrom 1500 m to 700 m above sea level. A
steplike uplift of the Austfirðir produced three successive
accordant summit levels, and after uplift of the third, head-
ward erosion by east-flowing rivers caused river capture and
a general westerly migration of the main watershed. The
contrast between the Austfirðir and inlandplateau is attri-
buted to a departure about 5 m.y. ago from steady state
conditions, caused by a significant southward migration of
the locus of volcanism.
INTRODUCTION
I he Austfirðir comprise 4500 km2ofmountainous
country rising to 1200 m above sea level, in which
many of the mountains have the jagged outlines oi
peaks which rose as nunataks above the Quaternary
ice sheets. Inland to the west is an extensive plat-
eau, much of it near 650 m elevation, which was
completely over-run by the ice sheets. It is traversed
by two broad and deep glacial valleys, Jökuldalur
and Fljótsdalur, which carry Vatnajökull meltwat-
ers to the sea. The contrast between the Austfirðir
and the inland plateau is very striking, and the
topographic evolution of this region poses many
geomorphological and volcanological problems.
Some of these problems are considered in this art-
icle which the author contributes in honour of Sig-
urður Thorarinsson, a geomorphologist and geo-
grapher as well as a volcanologist and geologist of
intemational renown.
CONSTRAINTS ON TOPOGRAPHIC
EVOLUTION
There are two major constraints on the origin of
the topographic features in this area. One is set by
zeolite zones within the lavas; these zones are be-
lieved to parallel the original constructional lava
surface, and they give the position of the level on
which erosion was initiated. The other is set by the
age of the rocks; the oldest known have a K-Ar age
of 13.3 million years, and the entire landscape, in-
cluding the creation of the volcanic pile and then its
partial destruction by erosion, is the product of that
time period.
Consider the íirst constraint. Mapping has re-
vealed that the zeolite zones are sub-horizontal and
diachronous, and have been superimposed on the
lava stratigraphy (IValker 1960, 1974). Knowingthe
thickness of each zone and that of the capping zone
of no zeolites, a contour map (Fig. 1) can be drawn
giving the elevation of the original constructional
surface, assuming that each zone is uniform in
thickness. The contours reveal a gently undulating
surface exceeding 1500 m above sea level in two
areas: one in the vicinity of Reyðaríjörður vol-
canic centre north of Reyðaríjörður, and the other
including most of the country south of Beruljörður
and also a northwards extension which embraces
the Thingmúli central volcano. The surface falls in
elevation to 700 m in the inland plateau area.
Consider now the second constraint. At the east-
ern side of the Austfirðir, the Gerpir section con-
tains the oldest dated (13.3 m.y.) rocks, but it is
known from the zeolite zones that several hundreds
of metres of rocks have been stripped olf the top of
the section, and extrapolation suggests that the top-
most of these lavas would have been about 12 m.y.
old. At the western side of the Austfirðir in the same
latitude, rocks dated at 9 m.y. occur at the top ofthe
JÖKULL32. ÁR 13