Magma genesis and crustal spreading in the northern neovolcanic zone of Iceland: telluric-magnetotelluric constraints

• Published in: Geophysical Journal International
• Main Authors: Thayer, R. E., Bjornsson, A., Alvarez, L., Hermance, J. F.
• Format: Text
• Language: English
• Published: Oxford University Press 1981
• Subjects: Articles; Iceland
• Online Access: http://gji.oxfordjournals.org/cgi/content/short/65/2/423; https://doi.org/10.1111/j.1365-246X.1981.tb02720.x

Regional telluric-magnetotelluric measurements performed as a cooperative research project between Brown University and the National Energy Authority of Iceland are interpreted in terms of physical processes in the crust and upper mantle associated with magma genesis and crustal spreading. First, these results confirm, over a larger region than was previously studied, the presence of a relatively conducting crust (25–30 ohm-m) at shallow depth ( d ≦ 4.5 km) reflecting electrolytic conduction in hydrothermal pore fluids on a regional scale. Secondly, and perhaps of greatest interest to current work on processes of accretion along plate boundaries, is the presence of an anomalous conducting layer ( T ≦ 4 km, ρ ≦ 10 ohm-m) at the base of the crust (8–15km) beneath the neovolcanic zone. This layer we feel represents an accumulation of molten magma. There is some suggestion that it is at shallower depth beneath the most active portion of the neovolcanic zone, and that its depth increases with age of the crust. The presence of this feature suggests that the crust is being underplated by the accretion of molten material to its base which, as it solidifies, contributes a significant component of crustal thickening. We feel that the molten phase residing in this basal zone represents an available source of magma for high level intrusions. Finally, our results confirm that, beneath the zone of magma accumulation, the resistivity changes little with depth in the mantle. Although there may be an indication that the upper few tens of kilometres of the mantle is somewhat more conducting, reflecting the presence of a minor amount of melt, this is not a firmly resolved feature of our analysis. Nevertheless, the mantle has almost a constant resistivity to a depth exceeding 100 km. Assuming a homogenous composition for the mantle beneath Iceland, the vertical gradient of temperature is of the order of only a few °Ckm‐ or less.