| Summary: | Subsurface resistivity structure across the divergent plate boundary of Iceland is characterized by thin, intermittent near-surface high conductivity layers associated with geothermal alteration and a thick, mid-crustal conductor deepening away from the plate boundary. In addition, one-dimensional magnetotelluric (MT) models of the Krafla central volcano have revealed two anomalous, updoming zones of high conductivity beneath the Krafla caldera overlapping partly shear wave shadow zones interpreted as areas of melt accumulation during the 1974-1989 rifting episode. To further constrain the extent of the updoming conductors and elucidate their relationship with the lower crustal conductor as well as near surface anomalies two-dimensional magnetotelluric inversions were conducted along two east-west profiles across the Krafla caldera. The northern profile which crosses both shear-wave shadow zones reveals a single updoming conductor within the western zone suggesting that the northern boundary of the eastern zone is located just south of the profile. Sensitivity tests indicate that the dimensions of the updoming conductor (0.5 – 2 km wide and 4 – 5 km high) are in a good agreement with seismic and geodetic data. The conductive dome connects with a near-surface conductive layer, at less than 500 m, which correlates with surface geothermal manifestations and the mid crustal connector, at 6-16 km depth. The top of the deeper conductor correlates fairly well with the brittle-ductile crustal boundary. Although joint interpretation of magnetotelluric data with other geophysical data further illuminates the geometrics of the shallow magma system of Krafla the percentage of partial melt within the mid-crustal conductor remains unknown.
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