Dynamics of crustal rifting in NE Iceland

Abstract

Magnetotelluric measurements have revealed a crustal thickness of 8–10 km in the axial rift zone of NE Iceland and above the proposed hot spot in central east and north Iceland. The crust thickens with age and is 20–30 km thick in the older Tertiary areas to the east and west of the axial rift zone. It also thickens toward north with increasing distance from the hot spot. The crust is underlain by a diapiric updoming asthenosphere over large parts of the country. At the crust asthenosphere interface there is a partially molten basaltic layer a few kilometers thick. This layer partially decouples the crust from the anomalous mantle. Its low viscosity may explain the spatial and temporal instability of the rift axis and the high mobility of the crust during technically active periods. A major volcano‐tectonic rifting episode started in the Krafla area in 1975 involving an 80‐km‐long segment of the plate boundary. Several meters of opening accompanied by subsidence was observed in the center of the axial fissure swarm and contraction and uplift occurred on the sides. The rifting occurred in several short events. Magma flowed horizontally from an upper crustal magma chamber and mixed with magma ascending directly from the semifluid layer at the base of the crust. Crustal accretion was mainly engineered by dike intrusions. Only a minor part of the magma transported reached the surface. During technically quiet epochs, plate‐driving forces, caused by the upwelling asthenosphere, result in long‐term stretching and accumulation of tensile stress in the thinnest part of the crust. The vertical and horizontal displacements during rifting episodes can be explained by elastic rebound, where fissure opening accompanied by magma flow causes the release of tension.

Volcanism in Iceland in historical time: Volcano types, eruption styles and eruptive history

“…Initially the earthquake activity was confined to the region below the caldera, but quickly spread out along the fissure swarm to the north coast atÖxarfjörður and south towards Lake Mývatn (e.g. Björnsson et al, 1979;Brandsdóttir and Einarsson, 1979;Björnsson, 1985). This rifting was associated with a 1.5 m widening of the fissure swarm and a 2 m deep graben was formed along the central part of the fissure system.…”

“…3b and in some instances surfaced to produce an eruption. In total 21 rifting events occurred during the 1975-1984 Krafla volcanotectonic episode (Björnsson, 1985). The first four rifting events occurred in October 1976, January, April, and September 1977, but only the last two resulted in an eruption at the surface.…”

“…The Krafla and Mývatn Fires show that the rifting and spreading along the plate boundary is not a continuous process, but is periodic and confined to relatively short volcanotectonic episodes that make up for previous periods of tectonic quiescence (e.g. Björnsson, 1985).…”

“…Sigurdsson and Sparks, 1978;Björnsson et al, 1979;Brandsdóttir and Einarsson, 1979;Saemundsson, 1979;Björnsson, 1985;Einarsson, 1991). Jóhannesson and Saemundsson (1998 Wolfe et al (1997).…”

“…On the NVZ sector of the axial zone only three systems have produced eruptions in historical time, namely Theistareykir, Krafla and Askja ( Fig. 3; Thorarinsson, 1963Thorarinsson, , 1965aBjörnsson et al, 1979;Björnsson, 1985;Larsen et al, 1979;Grönvold, 1984;Einarsson, 1991;Saemundsson, 1991;Thorarinsson and Sigvaldason, 1962;Sigurdsson and Sparks, 1978;Sigvaldason, 1979Sigvaldason, , 1982Sparks et al, 1981). In contrast, of the eight systems on the much younger EVZ, with the exception of Tindfjöll, have all recorded historical eruptions and four of these systems, Grímsvötn, Veidivötn, Hekla and Katla, have been profoundly active since the settlement of Iceland (e.g.…”