JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 94, NO. B12, PAGES 17,511-17,522, DECEMBER 10, 1989 Structure andEvolution of the Hengill-Grensdalur VolcanicComplex, Iceland: Geology, Geophysics, andSeismic Tomography G. R. FOULGER Department ofGeological Sciences, University ofDurham, Science Laboratories, Durham, England D. R. TOOMEY Department ofEarth, Atmospheric, and Planetary Sciences, Massachusetts Institute ofTechnology, Cambridge Recent geological and geophysical research indicates thepresence of three volcanic systems in the Hengill- Grensdalur areaandprogressive westerly migration of the accretionary plateboundary. The Hengill system comprises the activeHengill central volcano and a NNE trending fissure swarm that is the present locus of crustal accretion in thearea. Extinct volcanic systems areidentified with themountain Hromundartindur and the Grensdalur area and associated NNE trending zones. TheGrensdalur system was formerly thelocus of accretion in the area. A widespread geothermal area encompasses l!:e whole area and is fueled by atleast three distinct heat sources associated withthe three volcanic systems. A tomographic study of theupper 5 km of crust, using local earthquakes, imaged three bodies with velocities up to 15% higher than the average background velocities and volumes of several tens of cubic kilometers. One of these underlies the Grensdalur volcano, and a second underlies the Olkelduhals area withinthe Hromundartindur system. These are interpreted as intrusions thatare the solidified magma reservoirs of theirrespective volcanic systems andtheheatsources of those parts of the geothermal areaabove them. The thirdhigh-velocity bodyunderlies the extinct basalt shield Husmuli, whichis not associated with geothermal resources. That body is interpreted asa coldintrusion that is thefrozen magma conduit thatfed the surface eruptive site. A low-velocity bodywith a volume of a few cubic kilometers was imaged in thedepth range 2-4 km beneath the northern part of the presently active Hengill central volcano. This volume may contain partial melt andrepresent the heat source, or part of the heatsource, fueling the Hengill field. Heatbalance calculations show that subsurface magmatism continued in the extinct Grensdalur volcano longafter volcanic activity and crustal accretion migrated fromit to theHengillsystem. 1. INTRODUCTION Kilauea [Thurber, 1984], Long Valley [Kissling et al., 1984], the Geysers area[Eberhart-Phillips, 1986],and the CosoHot Springs During the last decade, seismic tomography has been applied [Walck and Clayton, 1987]. Regional explosions have recently been extensively toresearch the three-dimensional structure of the crustused to image Newberry Volcano •chauer et al., 1988],and andupper mantle on regional andlocalscales. Since central Medicine Lake Volcano [Evans andZucca, 1988]. volcanoes and geothermal areas in particular are associated with It is interesting tonote that with the exception of Kilauea and extreme lateral crustal and mantle structural inhomogeneities, Medicine Lake Volcano, and possibly Newberry Volcano, none of several ofthese have been targeted using either teleseisms or localthese small-scale studies revealed clearly defined low-velocity earthquakes [e.g., Iyer, 1984] . bodies that could be identified as crustal level magma chambers. In Ofgreat interest is the possibility of delineating magma chambers the cases ofLong Valley and the Coso Hot Springs, broad regions or volumes of partial melt within the crust orupper mantle that of slightly low-velocity were identified that were interpreted as would be manifest byzones ofrelatively low-velocity [Mavko, partial melt, but inthe cases of Mount Hood and the Geysers, high- 1980]. A number of studies using teleseisms have successfully velocity volumes were imaged that were interpreted asigneous achieved this, for example, Yellowstone [Iyer, 1979], Kilauea intrusive bodies. High velocities were also detected beneath the [Ellsworth and Koyanagi, 1977], Etna [Sharp et al., 1980], Coso Kilauea riftzones and surrounding the small, low-velocity magma [Reasenberg etal., 1980], the Roosevelt HotSprings [Robinson chamber, in the neighborhood of ringfractures at Newberry andIyer, 1981], andthe Geysers-Clear Lake area[Iyer et al., 1979; Volcano and atMedicine Lake Volcano. Oppenheimer and Herkenhoff, 1981]. In all these cases, low- A tomographic study ofthe Hengill-Grensdalur volcanic complex velocity volumes were imaged, which were interpreted as zones of was performed using local earthquakes and adds another case 5-30% partial melt. These results were in concord with surface history. In the case of this study, most notable are the three high- geology and also laboratory experiments which show that velocity velocity bodies imaged, which correlate wellwithextinct surface reductions may exceed 50%. In several other cases, however, no eruptive sites. A low-velocity zone underlying the presently active evidence was found forlow-velocity bodies using teleseisms, for Hengill central volcano and the accretionary plate boundary was example, Mount Hood [Weaver et aL, 1982] and Newberry Volcano observed. However, its volume was small in comparison with the [Stauber et al., 1988]. three imaged anomalously high-velocity volumes. Whereas thelong wavelengths of teleseisms limitthe image The high-velocity bodies are interpreted as intrusives that represent resolution to several kilometers, the higher frequencies of regional solidified shallow crustal magma reservoirs or conduits to the or local earthquake or explosive phases enable crustal velocity surface. Two of these are still hot and fuel geothermal fields atthe anomalies tobeimaged onthe scale of a single kilometer. Again, surface, but the third isnot cooling rapidly. The small, low-velocity several central volcanoes and geothermal areas have been targeted body beneath the northern part of the presently active Hengill using this methodology. McNutt and Jacob [1986] imaged the roots central volcano may represent partial melt. This paper synthesizes of Pavlof Volcano, Alaska, using regional events, and local events recent geological and geophysical work conducted in the area and were used to study the structures of Mount Hood [Leaver, 1984], interprets the results of the tomographic study in thecontext of other studies. Copyright 1989 by the American Geophysical Union. Paper number 89JB01217. 0148-0227/89/89JB-01217505.00 2. SYNTHESIS OF PREVIOUS WOl•K The Hengillridge-ridge-transform triple junction is the meeting point of the Reykjanes Peninsula Volcanic Zone, the Western 17,511