No abstract

The high-velocity dome, in our view, represents crust generated in and around the magma chamber, which has subsequently been advected to greater depths.

Abstract.Since the North Atla, ntic continenta,1 breakup in the early Tertiary, the process of rifting above a. mantle plume has produced large thicknesses of igneous crust. We report results of an integrated offshore-onshore seismic study of the crust and upper mantle along a transect of the aseismic Faroe-Iceland Ridge, between the continental fragment beneath the Faroe Islands and the present-day spreading center in northeast Iceland. Norma.l-incidence seismic data provide an image of the uppermost crust, which is complemented by a velocity model from streamer refra, ction a, nalyses.

S U M M A RYDeep seismic re£ection pro¢ling on the Faroe^Iceland Ridge Experiment (FIRE) imaged a sequence of dipping events in the upper to mid-crust beneath the Tertiary lava pile in ReydarfjÎrdur, eastern Iceland. These events dip towards the spreading centre at which the crust was formed, and continue down to *6 s two-way time, corresponding to depths down to 15^18 km. Similar crustal re£ections have been observed previously within the crust in other areas of Iceland, although the re£ective events located by previous surveys have not been interpreted rigorously. In this study we calculate synthetic seismograms from modelled sequences of lava £ows to examine the re£ective character of a lava pile. The synthesized lava pile sections are based on ¢eld measurements made in the ReydarfjÎrdur area: the 1 km vertical exposures on the sides of the fjord are composed of lava £ows within stratigraphic units which have a range of compositions, and some intercalated tu¡aceous clastic horizons.We compare the synthetic seismograms with our seismic re£ection data to test the interpretation that the deep dipping re£ections are caused by a gently dipping sequence of subaerially erupted lavas. We conclude that the upper and mid-crustal re£ections that we observe can be interpreted as lava £ow sequences similar to those seen at the surface, with the main re£ections resulting from packets of £ows of similar thicknesses. The re£ectors that we have imaged beneath Iceland are similar to those de¢ning the seaward-dipping re£ectors on the volcanic continental margins, and we therefore suggest that the seaward-dipping re£ections commonly result from packets of lavas with similar distributions of thicknesses.The occurrence of lavas down to 15^18 km beneath ReydarfjÎrdur can be explained partly by multiple episodes of accretion of the upper crust, although the volcanic processes forming the crust in Iceland generally lead to advection of extrusive material to mid-crustal levels. The production of an average extrusive layer totalling 15 km thickness is within reasonable estimates of the current volcanic productivity. The advection of frozen lava £ows to mid-crustal levels contributes to the maintenance of the subsolidus crust that is observed on Iceland, and the zeolite minerals within the extrusive sequence may be responsible for the crustal low resistivity zone that has been observed using magnetotelluric techniques.

Since the 1970s there has been an ongoing debate about whether the crust beneath Iceland is relatively thin (<15 km) and hot, or thicker (>20 km) and cooler. New results from the Faeroe‐Iceland Ridge Experiment (FIRE) conducted in the summer of 1994 suggest that it is thicker and cooler. This major land‐sea study investigated the crust generated where the North Atlantic spreading center intersects the Iceland mantle plume and mapped the transition from thickened oceanic crust to the continental fragment on which the Faeroe Islands sit. Seismic techniques were used to obtain a 600‐km profile of the area (Figure 1). Preliminary results suggest that a 20‐km‐thick crust is being generated beneath the northern Neovolcanic Zone of Iceland, with a high‐level crustal magma chamber beneath Krafla. In northeast Iceland, the igneous crust is considerably thicker, reaching 35 km, while along the Faeroe‐Iceland Ridge it varies between 25–30 km.