Compressional and shear velocity structure of the lithosphere in northern Iceland

Seismic data from the B96 array in northern Iceland are used to constrain t he compressional and shear velocity structure of the crust and uppermost ma ntle in northern Iceland, at the western flank of the northern volcanic zon e (NVZ) (the mid-Atlantic plate boundary in northern Iceland). Travel times from P and S waves from shots and microearthquakes north and south of the array are used. Two dome structures, with velocities 6% faster than average , are detected at depths down to 7 km. They may be the fossil roots of exti nct central volcanoes. P, S, PmP, and SmS wave travel times indicate that t he compressional-to-shear-wave velocity ratio in the crust is 1.75 to 1.76, with no significant variation detected between the mid and lower crust. Ne ar-solidus lower crustal temperatures, previously predicted on the basis of high surface heat flow, are ruled out. The crust is 25 to 31 km thick, wit h the southward thickening occurring in an abrupt step. The relatively high Pn and Sn wave apparent velocities of 8.00 +/- 0.1 km/sec and 4.31 +/- 0.0 4 km/sec, respectively, from an earthquake in southern Iceland, are consist ent with a mantle lid, that is, a layer of subsolidus mantle separating the Moho from a deeper, partial melt zone. Travel times from P waves crossing the NVZ from a fan shot in eastern Iceland cross the previously identified Krafla high-velocity dome at a depth of 8 to 10 km. They provide new suppor t for the existence of this mid-crustal feature and indicate that its base extends 40 to 50 km along the strike of the NVZ. The neighboring Peistareyk ir central volcano has no dome at 8 to 10 km depth, suggesting that it has had a more subsidiary role in the formation of the lower crust.