Jökull


Jökull - 01.12.1982, Side 15

Jökull - 01.12.1982, Side 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
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