Correlation of magnetotelluric, seismic, and temperature data from southwest Iceland

Regional enhancement of temperatures in the crust and upper mantle are revealed by magnetotelluric measurements performed during 1969 in Thingvellir graben in southwest Iceland. An electrical interface at a depth of 15 to 20 km apparently coincides with the seismic boundary between layers having P-wave velocities of 6.5 km/sec and 7.4-km/sec, respectively. Based on the interpretation that the 6.5-km/sec material is intrusive basalt and the 7.4-km/sec material is partially fused upper-mantle material consisting of peridotite and 10% basalt magma, the electrical data have been inverted to determine a temperature range of 820°C to 1120°C at a depth of approximately 15 km. These temperatures are significantly above Green and Ringwood's oceanic geotherm but below their solidus. To be consistent with the seismic interpretation, hydrous conditions may persist to depths of 15 km, causing he solidus to be depressed below the observed temperature, so that a 10% liquid fraction of basalt can form. The data suggest that a surface geothermal gradient of 60°C/km can be linearly extrapolated to depths of 15 km, indicating that the major source of heat is below this depth. Effective viscosities based on power-law creep mechanisms may be on the order of 1022 poise at depths between 15 km and 20 km.