Magnetotelluric measurements across the eastern neovolcanic zone in south Iceland

Abstract

A magnetotelluric survey consisting of 18 separate soundings was performed in 1982 along a 125‐km profile crossing the eastern neovolcanic zone in south Iceland. Although the three‐dimensional ocean effect may be significant at the longest periods which previous workers have analyzed to study features in the upper mantle beneath Iceland, our studies suggest that this effect is minimal for periods less than 1000 s at distances greater than 50 km from the coast. Therefore we are confident that two‐dimensional interpretations are valid at periods suitable for interpreting crustal resistivities. Such a two‐dimensional interpretation presented here indicates the presence of a layer (∼5 km thick) of low resistivity (5 ohm m) at the crust‐mantle interface (10–20 km depth). The depth to this layer increases from the center of the neovolcanic zone. This feature has been seen elsewhere in Iceland by other workers. However, the present study suggests that east of the neovolcanic zone the low‐resistivity layer is not present, which we argue may be associated with a recent eastward shift of the axial rift zone to a new location within older crust. A striking feature of our two‐dimensional crustal interpretation is that pockets of low‐resistivity material appear to be present at approximately 8 km depth at the base of the crust beneath the two volcanic centers that we have studied, Torfajokull and Hekla. These may be manifestations of local magma chambers.

Two-dimensional resistivity modeling based on regional magnetotelluric survey in the northern Tohoku district, northeastern Japan.

“…Other than Japan, as for the resistivity structure beneath the active Quaternary volcanic region, existence of conductive layer in the lower crust has also recently been reported by EYNSTEINSSON and HERMANCE (1985) in Iceland and by ANDER et al (1984) in New Mexico. EYNSTEINSSON and HERMANCE (1985) interpreted the crustal conductor beneath the active volcano as `magma pocket'.…”

“…EYNSTEINSSON and HERMANCE (1985) interpreted the crustal conductor beneath the active volcano as `magma pocket'.…”

Timescales of magma differentiation from basalt to andesite beneath Hekla Volcano, Iceland: Constraints from U-series disequilibria in lavas from the last quarter-millennium flows

“…This in turn could be large enough to be detected by seismic studies, but have not been successful (Soosalu and Einarsson, 2004). On the other hand, Eysteinsson and Hermance (1985) inferred the presence of a partially molten layer at the mantle-crust boundary from magnetotelluric soundings. Hence, as an alternative model, there is the possibility of a larger basaltic magma chamber at a deeper location (near-Moho) that could supply heat to the smaller chambers above to prevent solidification throughout the course of their differentiation processes.…”

Seismic constraints on magma chambers at Hekla and Torfaj�kull volcanoes, Iceland

“…Kjartansson and Grönvold (1983) concluded from geodetic measurements that the top of the magma reservoir of Hekla is roughly at 8 km depth. Eysteinsson and Hermance (1985) found a 'pocket' of low-resistivity material at approximately 8 km depth beneath Hekla in a magnetotelluric survey. They suggest it to be a manifestation of a local magma chamber.…”

“…In their magnetotelluric survey, Eysteinsson and Hermance (1985) found a low-resistivity area under Torfajökull, similarly to that beneath Hekla. Gumundsson (1988) discusses the existence of the Torfajökull magma chamber, and suggests the top of the chamber is at about 3 km depth, based on the measurements of Eysteinsson and Hermance, and the existence of hightemperature geothermal activity.…”