Faroe-Iceland Ridge Experiment .2. Crustal structure of the Krafla central volcano

The seismic velocity structure of the Krafla central volcano is characterized by large variations in compressional velocity. A 40 km wide high-velocity dome extends from the lower crust (11-14 km depth) beneath the volcano narrowing upward. A magma chamber sits at its top near 3 km depth. It is defined by both 0.2-0.3 s compressional wave delays and shear wave shadowing to be 2-3 km N-S, 8-10 km E-W, and 0.7-1.8 km thick. The near-surface structure (uppermost 2.5 km) of the Krafla caldera is approximately flat-lying, with only minor lateral heterogeneities. The crust beneath the magma chamber has low shear wave attenuation and anomalously high compressional and shear wave velocities. Shear waves, reflected from a 19 km deep Moho, are clearly visible for some paths through the crustal volume below the magma chamber, even though the more shallow diving S waves are severely attenuated. The midcrust beneath the shallow magma chamber cannot contain partial melt or even be at near-solidus temperatures. The Krafla central volcano plays a major role in crustal genesis along the plate boundary. The high-velocity dome, in our view, represents crust generated in and around the magma chamber, which has subsequently been advected to greater depths.