TOMOGRAPHIC IMAGE OF THE MID-ATLANTIC PLATE BOUNDARY IN SOUTHWESTERN ICELAND

The 170 km South Iceland Seismic Tomography (SIST) profile extends fro m the west and across the Mid-Atlantic Ridge spreading center in the W estern Volcanic Zone and continues obliquely through the transform zon e (the South Iceland Seismic Zone) to the western edge of the Eastern Volcanic Zone. A total of 11 shot points and 210 receiver points were used, allowing precise travel times to be determined for 1050 crustal P wave rays and 180 wide-angle reflections. The large amplitudes of th e wide-angle reflections and an apparent refractor velocity of 7.7 km/ s are interpreted to be from a relatively sharp Moho at a depth of 20- 24 km. This interpretation differs from the earlier models (based on d ata gathered in the 1960s and 19709), of a 10-15 km thick crust underl ain by a upper mantle with very slow velocity of 7.0-7.4 km/s. Neverth eless, these older data do not contradict our new interpretation. Impl ication of the new interpretation is that the lower crust and the crus t-mantle boundary are colder than previously assumed. A two-dimensiona l tomographic inversion of the compressional travel times reveals the following structures in the crust: (1) a sharp increase in thickness o f the upper crust (''layer 2A'') from northwest to southeast and (2) b road updoming of high velocity in the lower crust in the Western Volca nic Zone, (3) depth to the lower crust (''layer 3'') increases gradual ly from 3 km at the northwestern end of the profile to 7 km at the sou theastern end of the profile, (4) a low-velocity perturbation extends throughout the upper crust and midcrust into the lower crust in the ar ea of the transform in south Iceland (South Iceland Seismic Zone), and (5) an upper crustal high-velocity anomaly is associated with extinct central volcanos northwest of the Western Volcanic Zone. The travel t ime data do not support the existence of a large (> 0.5 km thick) crus tal magma chamber in this part of the Western Volcanic Zone but do not exclude the possibility of a smaller one.