FAROE-ICELAND RIDGE EXPERIMENT .2. CRUSTAL STRUCTURE OF THE KRAFLA CENTRAL VOLCANO

The seismic velocity structure of the Krafla central volcano is charac terized by large variations in compressional velocity. A 40 km wide hi gh-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 an d shear wave shadowing to be 2-3 km N-S, 8-10 km E-W, and 0.7-1.8 km t hick. The near-surface structure (uppermost 2.5 km) of the Krafla cald era is approximately flat-lying, with only minor lateral heterogeneiti es. The crust beneath the magma chamber has low shear wave attenuation and anomalously high compressional and shear wave velocities. Shear w aves, reflected from a 19 km deep Moho, are clearly visible for some p aths through the crustal volume below the magma chamber, even though t he more shallow diving S waves are severely attenuated. The midcrust b eneath the shallow magma chamber cannot contain partial melt or even b e at near-solidus temperatures. The Krafla central volcano plays a maj or 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.