THE CRUSTAL MAGMA CHAMBER OF THE KATLA VOLCANO IN SOUTH ICELAND REVEALED BY 2-D SEISMIC UNDERSHOOTING

Results of a 2-D, seismic undershooting experiment on the Katla centra l volcano in south Iceland are reported. Large localized traveltime an omalies (0.4 s) are observed on an array within the Katla caldera. The traveltimes are forward modelled using a wavefront tracker developed in Appendix A. Thus, non-linear effects encountered in traveltime tomo graphy are avoided as well as common problems with ray tracing in the presence of strong lateral heterogeneity. An extreme variation in comp ressional velocity is required to extend over a significant volume in order to model the data. The resulting model is not unique, but constr aints on the allowable range of velocities (2.5-6.0 km s-1) render the basic features well constrained. A clear S-wave shadow is closely ass ociated with delays in traveltime due to a shallow slow anomaly. Low-a mplitude P waves go hand in hand with early arrivals due to thin struc tural features flanking the slow anomaly. The model is interpreted in terms of a magma chamber containing extensively molten rock. The magma chamber is shallow, with a bottom at a depth of about 1.5 km below se a-level (3.0 km below surface), and measures about 5 km across. The de pth of the chamber is roughly at the level of buoyant equilibrium for basaltic melt in the crust. Owing to poor vertical resolution at shall ow depths in the undershooting geometry the top of this shallow magma chamber is not well resolved. On the other hand, the bottom of the cha mber is well resolved. The chamber is underlain by rocks of average or high velocity for that depth. The magma chamber is a persistent featu re, big enough (10 km3) to supply magma for large eruptions and to sup ply heat to permit remelting of hydrated basaltic crust to produce sil icic magmas at shallow levels. The chamber is fed by magma fracturing from below. The model agrees with phenomenological models of magma cha mbers in Iceland based on geological observations and provides a quant ification of those models in terms of depth and size. On the other han d, it is fundamentally different from recent models of magma chambers at mid-ocean ridges which may be more akin to the pervasive region of partial melt at depth beneath Iceland. This underlines the important e ffect of the Icelandic hotspot on tectonics and volcanism in Iceland a nd implies a substantially different crustal and thermal structure in Iceland from that of 'normal' mid-ocean ridges.