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Short period magnetotelluric measurements (15 s–1 h) were made at 19 sites in NE-Iceland, distributed over the neovolcanic zone and the adjoining older Tertiary flood basalt areas. With model-calculations of one- and two-dimensional resistivity distributions a characteristic model was found for the lower crust and upper mantle. Beneath a thin surface layer the resistivity is 100 Ωm except within the active neovolcanic zone where it is 50 Ωm. This layer extends to a layer with low resistivity of 15 Ωm. The low-resistivity layer is about 5 km thick. The depth of its upper boundary increases from 10 km to about 20 km with increasing distance from the rift axis. The resistivity beneath the low-resistivity layer is about 100 Ωm down to at least 100 km. Comparison of field data with laboratory measurements on conductivity at high temperatures indicates that the low-resistivity layer consists of partially molten basalt at a temperature of 1,000° –1,100° C. The underlying layer very probably consists of partially molten ultramafic rocks and is presumably the uppermost part of the mantle beneath Iceland. The basaltic low-resistivity layer is interpreted as the base of the crust formed by upward movement of the basaltic melt fraction from the mantle.
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