DEEP-CRUSTAL REFLECTORS UNDER REYDARFJORDUR, EASTERN ICELAND - CRUSTAL ACCRETION ABOVE THE ICELAND MANTLE PLUME

Deep seismic reflection profiling on the Faroe-Iceland Ridge Experimen t (FIRE) imaged a sequence of dipping events in the upper to mid-crust beneath the Tertiary lava pile in Reydarfjordur, eastern Iceland. The se events dip towards the spreading centre at which the crust was form ed, and continue down to similar to 6 s two-way time, corresponding to depths down to 15-18 km. Similar crustal reflections have been observ ed previously within the crust in other areas of Iceland, although the reflective events located by previous surveys have not been interpret ed rigorously, In this study we calculate synthetic seismograms from m odelled sequences of lava flows to examine the reflective character of a lava pile, The synthesized lava pile sections are based on held mea surements made in the Reydarfjordur area: the 1 km vertical exposures on the sides of the fjord are composed of lava flows within stratigrap hic units which have a range of compositions, and some intercalated tu ffaceous elastic horizons. We compare the synthetic seismograms with o ur seismic reflection data to test the interpretation that the deep di pping reflections are caused by a gently dipping sequence of subaerial ly erupted lavas. We conclude that the upper and mid-crustal reflectio ns that we observe can be interpreted as lava flow sequences similar t o those seen at the surface, with the main reflections resulting from packets of flows of similar thicknesses. The reflectors that we have i maged beneath Iceland are similar to those defining the seaward-dippin g reflectors on the volcanic continental margins, and we therefore sug gest that the seaward-dipping reflections commonly result from packets of lavas with similar distributions of thicknesses. The occurrence of lavas down to 15-18 km beneath Reydarfjordur can be explained partly by multiple episodes of accretion of the upper crust, although the vol canic processes forming the crust in Iceland generally lead to advecti on of extrusive material to mid-crustal levels, The production of an a verage extrusive layer totalling 15 km thickness is within reasonable estimates of the current volcanic productivity. The advection of froze n lava flows to mid-crustal levels contributes to the maintenance of t he subsolidus crust that is observed on Iceland, and the zeolite miner als within the extrusive sequence may be responsible for the crustal l ow resistivity zone that has been observed using magnetotelluric techn iques.