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

Abstract

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 G 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 s 4 km, p s 10 ohm-m) at the base of the crust (8-15 km) 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 Jceland, the vertical gradient of temperature is of the order of only a few "C km-', or less. 424

Formation and mechanics of magma reservoirs in Iceland

“…The high heat-flow, low seismic velocity, low density, high P-wave/S-wave ratio (Palmason 1971) and magnetotelluric measurements (Beblo & Bjornsson 1980;Thayer et al 1981;Eysteinsson & Hermance 1985) indicate that the upper mantle beneath Iceland is in a state of partial fusion. A 5-14 km (average 10 km) thick layer of exceptionally low resistivity (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15) is at the depth of 8-10 km within the neovolcanic zones ( Fig.…”

Dynamics of crustal rifting in NE Iceland

“…corresponds to a volume of 0.6 km 3 (calculations based on the Mogi model). On the other hand, the dike-forming mechanism at large distances from the caldera may be different and follow the classical model of dikes formed by magma ascending vertically in fissures Thayer et al [1981]. If the roof of the magma chamber or the magma itself is to some extent compressible, as is indicated by gravity measurements [Johnsen et al, 1980], this figure could be an underestimate.…”

“…They have revealed information on the lower crustal and upper mantle conditions at some 70 sites in Iceland[Hermance, 1973;Hermance et al, 1976;Thayer et al, 1981; Bj6rnsson, 1978, 1980;Beblo et al, 1983]. The Theistareykir and Fremrinfimur central volcanoes have no developed calderas, which indicates the absence of shallow crustal magma chambers.…”

Electromagnetic studies in geothermal regions

“…Hermance and his coworkers have done telluric-magnetotelluric measurements in geothermal regions of Iceland and in other areas (Hermance et al, 1976;Thayer et al, 1981). Measurements of a similar kind have also been made by Bodvarsson et al, (1974) in geothermal areas of Oregon.…”