Crustal conductivity and distribution of melt beneath the Krafla caldera, N-Iceland inferred from magnetotelluric data

Abstract

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