Remarks on Crustal Structure in Iceland

In connection with a recent theory of crustal drift and crustal structure in Iceland (Bodvarsson & Walker 1964) it seems desirable to discuss further some facts concerning the gravity field, seismic boundaries, and the geological history of Iceland. The Bouguer anomaly map is shown in Fig. 1 (after Einarsson 1954 with later additions of gravity stations by the same author, in the northern and northwestern parts). The main feature is a slightly oval bowl, about 75milligals deep. This bowl may be said to be a mirror picture of the topography and a good first criterion for isostatic equilibrium. Isostatic reduction on the basis of the Airy-Heiskanen crustal model indicates that the Bouguer bowl is best explained by central thickening under the country of a crust of original thickness T -= 20 km (Einarsson 1954). Bodvarsson & Walker (1964, Fig. 7) have made a new reduction, using seismically indicated crustal layers, and find that the facts can be explained by a thickening of a 6 km thick ‘D-layer’ to a 15-20km thick ‘blister’. Both reductions thus give a similar overall picture. Going further into details, a main aspect of the gravity field is the fact that the contours of the Bouguer bowl have essentially the same shape as the island-both are slightly oval with an E-W major axis. Moreover, deviations from a regular oval shape are often identical in both. This is for instance clear in the case of the Northwestern Peninsula (Figs. 1 and 2). This fact shows that there is a connection between the gravity anomaly and the present (and geologically late) extent and shape of the island. This is most significant in face of the geological knowledge that the present island is a fragment of an older, much more extensive land area. Consider first the Northwestern Peninsula. It is built up of Tertiary basalts the total thickness of which is at least several kilometres. The flat-lying basalts are truncated along the coast, and the present peninsula cannot be but a small remnant of an original basalt plateau. Yet, the Bouguer anomaly and, hence, the deeper structure, is closely connected with the shape of this remnant. There is hardly any other explanation of this fact than the assumption that the peninsula was formed by faulting. On the basis of geomorphologic studies it has been concluded (Einarsson 1963) that marine abrasion of the order of 10 km can be reconstructed on coastal sections in this area exposed to the open sea. Before this (differential) abrasion the coast was still more characterized by straight lines than now. On this basis, and the fact that older base levels for valley erosion have been uplifted to about 300m above sealevel, it may be concluded that the peninsula was formed by faulting after formation of the basalt plateau and that the position of the main faultlines, bordering the peninsula, is approximately as shown in Fig. 2. The gravimetric and geological interpretations are, therefore, in good agreement with each other.