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2 GRC Transactions, Vol. 33, 2009 Volcano-Tectonic-Geothermal Interaction at the Hengill Triple Junction, SW Iceland B. S. Hardarson 1, G. M. Einarsson 1, H. Franzson 1 and E. Gunnlaugsson 2 1 Iceland GeoSurvey, Grensasvegur 9. IS-108 Reykjavik, Iceland (bsh@isor.is) 2 Reykjavik Energy, Bæjarhals 1. IS-110 Reykjavik, Iceland (einarg@or.is) Keywords Iceland, Hengill, Hellisheidi, central volcano, volcanic, intrusion, triple junction, geothermal, high-temperature field, alteration, aquifer, fault, tectonic Abstract The active volcanic zones in Iceland are characterized by high heat flow and extensive geothermal activity. The high-temperature geothermal areas are mainly confined to volcanic systems, in particular central volcanoes, and are subject to strong tectonic control. The heat source is considered to be magmatic associated with shallow level crustal magma chambers or dyke swarms. The prevalent permeability, in general, seems to be affiliated with intrusions and sub-vertical faults/fractures. The Hengill central volcano, which is seismically very active, hosts one of the most powerful geothermal fields in Iceland. Structurally it is dominated by NE-SW striking faults which are, however, in places intersected by easterly striking features which may play a role in the permeability of the geothermal reservoir. The Hengill low resistivity area covers about 112 km 2 and is located at a triple junction where two active rift zones meet a seismically active transform zone. At present Reykjavik Energy is constructing 300 MWe and 400 MWt power stations at the Hellisheidi high-temperature field, which is situated SW of the Hengill central volcano. Reykjavik Energy has to date drilled 46 deep ( m) exploration and production wells and 12 reinjection wells into the Hellisheiði field. Here we present new well data (cutting analyses and geophysical borehole logs) and regional geophysical surveys from the Hellisheidi geothermal field focusing on the western margin of the field. The data are used in conjunction with apparent aquifers to evaluate the relationship between permeability and geological structures. Introduction Iceland is unique for its location astride the diverging Mid- Atlantic Ridge and, furthermore, on top of a mantle plume. These 49 two dynamic systems combine fundamental factors that promote magmatism and tectonics. Today the Mid-Atlantic Ridge is represented on land by the Western and the Northern Volcanic Zones (WVZ and NVZ, respectively, Figure 1). The WVZ and NVZ are offset along a region known as the Mid-Iceland Volcanic Zone (MVZ) which may be viewed as a leaky transform fault. The NVZ is connected to the Kolbeinsey Ridge (KR) in the north by the Tjörnes Fracture Zone (TFZ). The Eastern Volcanic Zone (EVZ) is currently propagating to the south with the Vestmanna Islands (VI) representing the tip of the propagator. The EVZ is connected to the WVZ by the South Iceland Seismic Zone (SISZ) and the WVZ is connected to the Reykjanes Ridge (RR) in the south by the Reykjanes Peninsula (RP). Eventually, a ridge-jump is expected Figure 1. Geological map of Iceland showing the location of the active volcanic zones and transforms discussed in this paper. RR = Reykjanes R i d g e ; R P = Reykjanes Peninsula; WVZ = Western Volcanic Zone; MVZ = Mid-Iceland Volcanic Zone; NVZ = Northern Volcanic Zone; EVZ = Eastern Volcanic Zone; VI = Vestmanna Islands; SISZ = South Iceland Seismic Zone; TFZ = Tjörnes Fracture Zone. Red dots indicate high-temperature areas. Orange circle represents the approximate location of the Hengill volcanic system (modified from Johannesson and Sæmundsson, 1999).

3 Figure 2. Satellite image of the Hengill central volcano. White lines show faults and fissures. Yellow lines show the location of cross sections A-A and B-B. whereupon the focus of extension in S Iceland will transfer from the WVZ to the EVZ (e.g. Sæmundsson, 1980; Hardarson et al., 1997 and refs. therein). From the time when the Mid-Atlantic ridge system migrated WNW over the Iceland plume about 24 m.y. ago (Vink, 1984), the plume has repeatedly refocused the location of spreading with the necessary adjustments being accommodated by transform displacements of the ridge. Relocation of the spreading axis through ridge jumping is a prominent process in the evolution of Iceland and is the primary cause for the tectonic configuration as seen on the island and for the arrangement of high- and lowtemperature geothermal areas (Sæmundsson, 1980). The active volcanic zones in Iceland are characterized by high heat flow and extensive geothermal activity. The high-temperature reservoirs (>200 C at 1 km depth) are mainly confined to volcanic systems (Figure 1), in particular the central volcanoes, and are subject to strong tectonic control. The heat source is considered to be magmatic associated with shallow level crustal magma chambers or dyke swarms. The prevalent permeability, in general, seems to be affiliated with intrusive bodies and sub-vertical faults and fractures. Seismic activity in the active volcanic zones is primarily related to volcanoes and magmatic movement resulting in rather small earthquakes, whilst all major earthquakes in Iceland have originated within the TFZ and SISZ. The transform motion is commonly achieved by strike-slip on faults that are transverse to the zone (e.g. Einarsson 2008; LaFemina et al., 2005). The Hengill region (Figures 1 and 2) covers about 112 km 2 and is one of the most extensive geothermal areas in Iceland. It is located at a triple junction where two active rift zones meet a seismically active transform zone (Figure 1). The Hengill triple junction is a complex of fissure swarms and volcanoes located between the southern part of the WVZ (the Reykjanes Peninsula, RP), the WVZ and the SISZ. The Reykjanes peninsula is a highly oblique, en echelon extensional rift zone about 70 km in length. The WVZ north of Hengill system extends about a 100 km NE to the Langjökull glacier. Normal faulting is prominent throughout the zone. The fissure swarms are almost parallel to the trend of the zone itself, indicating a spreading direction perpendicular to the zone. The SISZ is oriented E-W, is about km wide and km long and takes up the transform motion between the RR and the EVZ (Einarsson, 1991). The overall left-lateral transform motion is accommodated by right-lateral faulting on many parallel transverse faults and counterclockwise rotation of the blocks between the faults, namely bookshelf faulting (Einarsson, 2008). As all of these tectonically different zones meet at the Hengill triple-junction, the tectonic scenario of the area is very complicated and enigmatic. The main geothermal utilization in Iceland until recently was for direct uses, with space heating being by far the most important. In recent years there has been a growing interest in electrical energy production from geothermal energy, and currently (2008) about 25% of the electricity generated in Iceland is of geothermal origin, the rest being from hydro resources. However, roughly 82% of primary energy used in Iceland is derived from indigenous renewable sources (62% geothermal, 20% hydropower). The rest of Iceland s energy sources come from imported fossil fuel used for fishing and transportation (e.g. Gunnlaugsson & Gislason, 2005; Björnsson, 2006; National Energy Authority, 2009). Reykjavik Energy already operates a geothermal power plant at Nesjavellir, north of the Hengill volcano (Figure 2), with installed capacity of 120 MWe and a 300 MWt for a hot water plant. Further power plants are being constructed at Hellisheidi, SW of the Hengill volcano. Eventual production of these is estimated about 300 MWe and 400 MWt but current production capacity is 213 MWe. At present 46 deep ( m) exploration and production wells have been drilled at Hellisheidi, 12 reinjection wells, numerous cold water wells and several shallow exploration wells. Exploration wells have also been drilled at locations near Bitra and Hverahlid (Figure 2). Strong evidence suggests that geothermal systems associated with central volcanoes like Hengill are fracture controlled and thereby highly dependent on the local stress field around the volcanoes (e.g. Franzson et al., 2005). In this paper we present preliminary well data (cutting analyses and geophysical borehole logs) and regional geophysical surveys from the Hellisheidi geothermal field, with special reference to the western margin of the field. The data are used in conjunction with apparent aquifers to evaluate the relationship between permeability and geological structures. Geological Structures The Hengill volcanic system is currently active while its predecessor, the Hveragerdi system, is now extinct in terms of 50

4 volcanic activity but still active seismically and hosts geothermal reservoirs (Figure 2). Three well-fields have been developed within the greater Hengill area, Nesjavellir, Hellisheidi, where a resource utilization is well underway, and Hveragerdi where the geothermal resource is utilized by the local community (Figure 2). Furthermore, exploration drilling has started at Bitra and Hverahlid adjacent to the Hengill system (Figure 2). Structurally the Hengill system is dominated by a large NE-SW striking fault/fissure swarm which is, however, in places intersected by easterly striking features which may play a role in the permeability of the geothermal field (e.g. Arnason and Magnusson; 2001, Hardarson et al., 2007, 2008). The volcano is mainly built up of hyaloclastite formations erupted underneath the ice sheet of the last glacials, forming a mountain complex rising up to some 800 m at Hengill (Figure 2). Interglacial lavas on the other hand flow down and accumulate in the surrounding lowlands. The age of the volcano has been estimated to be around 400,000 years (Franzson et al., 2005). Postglacial volcanism includes three fissure eruptions of ~9, ~5 and ~2 thousand years (Franzson et al., 2005). The volcanic fissures of the latter two can be traced to the north, through the Nesjavellir field (Figure 2) and into Lake Thingvallavatn (Sæmundson, 1995). At Nesjavellir these volcanic fissures act as the main outflow channel of the geothermal system and the fissures are also believed to act as major outflow zones in the Hellisheidi field (e.g. Franzson et al., 2005). Extensive geological mapping, fluid geochemistry and geophysical surveys have shown the existence of a large geothermal high temperature anomaly in the whole area (e.g. Arnason and Magnusson 2001; Gunnlaugsson and Gislason, 2005). Volcanic Succession A simplified volcanic succession at Hellisheidi from east to west is shown on Figure 3 (Franzson et al., 1995), but the profile is located slightly north of profile B-B shown on Figure 2. The succession is mainly composed of two rock types; hyaloclastites and lava series. The former is dominant and is formed in sub-glacial eruptions, while lava series form during interglacials. Hyaloclastite form when magma quenches during eruption into the base of the glacier, and piles up into a heap above the orifice, mostly as pillow basalts, breccias and tuffs. Although of relatively high porosity, these formations tend to have low permeability, especially when they have been hydrothermally altered. Most of the rock formations are basaltic in composition, ranging from picrite to tholeiite (Larsson et al., 2002; Alfredsson et al., 2008), but occasionally evolved rocks are seen, especially as intrusive bodies. The Hellisheiði field is within the Hengill central volcano where volcanism is most intense, and where hyaloclastites have formed highlands. Interglacial lavas, however, when erupting in the highlands will flow downhill and accumulate in the lowlands surrounding the volcano (Franzson et al., 2005). The top of the thick lava series found at m b.s.l. is interpreted as representing the base of the Hengill central volcano. These lavas were not formed in the Figure 3. Geological cross section along a line slightly north of B-B` on Figure 2. Blue formations are lava series, and formations of all other colors are individual hyaloclastite formations. Large fault displacements are seen to the left in the figure. The trace of the wells are shown as black lines. Thin orange colored lines between wells 6 and 3 are traces of volcanic fissures of 2 and 5 thousand years. Note that the thick lava series at the bottom (blue) were not formed in the Hengill volcanic system but probably belong to its predecessor, the Hveragerdi volcanic system (Figure 2). From Franzson et al., Hengill volcanic system but probably belong to its predecessor, the Hveragerdi volcanic system to the east (Figure 2). Intrusive rocks are identified by their compact nature, relatively low alteration, and sometimes by oxidation found at their margins. Geophysical logs often show them to have relatively high neutron-neutron and resistivity values. Low porosity intrusions are mainly of two types, fine grained basalt and fine grained andesites to rhyolites indicating that they are dikes and/or sills. The intrusions are relatively rare down to about 800 m b.s.l. but become more numerous at deeper levels (Franzson et al., 2005). Faults The geological cross section in Figure 3 shows the presence of two major NE-SW faults in the west part of the field with a total throw of about 260 m. These large faults can be traced about km to the northeast, where throw of the faults approach some 300 m towards the SE. They are believed to represent the western margin of the Hengill fissure/fault zone (e.g. Sæmundsson, 1995; Franzson et al., 2005). Other major faults in the area have not been found from borehole data, and minor faults are more difficult to identify due to the lack of reliable horizontal marker horizons, such as lava series (Franzson et al., 2005). Aquifers (feed points) in the wells are located using, for example, circulation losses, temperature logs and hydrothermal alteration. A detailed analysis of these data and their exact relation to the geological factors is ongoing. Faults and feed points at the western margin of the Hengill system are shown in Figure 4, overleaf. The faults tilt to the east, except the fault labeled E which tilts west and we estimate the throw on the surface at least 30 m. However,

5 preliminary analysis of drill cuttings and drilling data from wells that intersect these faults indicate a throw of at least 300 m at 1300 m b.s.l. Consequently a substantial, previously unrecorded, graben (the Reykjafell graben) is found between faults W and E (Figure 4). It is of interest that aquifers often appear to be largest at locations of highest temperatures (Figures 5 and 6) and production wells that cut the Reykjafell graben are amongst the most powerful at Hellisheidi. Permeability along the faults and the two postglacial eruptive fissures at the western margin of the Hengill system (Figure 3) is believed to be high causing a strong outflow towards the south from the Skardsmyrarfjall hyaloclastite mountain and north from the Reykjafell mountain (Figure 2). At least some of the aquifers encountered in the wells can be directly related to margins of intrusions indicating the importance of fracture permeability in the geothermal reservoir (Franzson et al., 2005). However, our preliminary results indicate that permeability at the western margin of the Hengill system is primarily related to the major faults. The heat source at Hengill is magmatic, intrusions and magma chambers. Figure 5a. Alteration minerals in cross sections A-A and B-B (FIgure 5b, next page) showing the appearance of quartz, epidote and amphibole. Black lines indicate the appearance (upper) and disappearance (lower) of laumontite. The top formations are in the zeolite zone. Red line shows the base of the Hengill formations. Black dots on well paths show the main aquifers. Production wells are red and reinjection wells blue. Figure 4. Faults at the western margin of the Hengill system (Figure 2). Drill holes which cut the faults are shown and black dots represent feed points. Production wells are shown in red and reinjection holes blue. The faults dip to the east, except fault labeled E which dips west. Consequently a graben is found between faults W and E. The view is from SW to NE. 52 It has been suggested that E-W trending tectonic faults, related to the SISZ, may influence permeability where these faults intersect the NE-SW trending faults (c.f. Figure 7) and that these areas show greater intrusive activity (Arnason and Magnusson, 2001). Such activity was observed in 1994 when intensive swarm activity commenced at the eastern margin of the Hengill system and geodetic measurements between 1986 and 1995 detected uplift and expansion in the area (Sigmundsson et al., 1997). The inferred rate of maximum uplift was fairly constant at 19.2 ± 2 mm/yr and the source depth was estimated 7 ± 2 km. It was proposed that magmatic injection into the crust was the cause of the pressure source (Sigmundsson et al., 1997; Feigl et al., 2000; Sens-Schönfelder et al., 2006). It is also possible that E-W striking faults (c.f. Figure 7) may affect and increase permeability where they intersect the main NE-SW structures (e,g, Arnason and Magnusson, 2001, Hardarson et al., 2007).

6 Figure 5b. White broken lines in the B-B section indicate the approximate location of the faults forming the Reykjafell graben. Hydrothermal Alteration and Formation Temperature Hydrothermal alteration has been studied in some detail in the first eleven wells drilled in the Hellisheidi area (Franzson et al., 2005), and preliminary data are now available for additional 35 wells. In general the hydrothermal alteration spans all the typical hydrothermal alteration zones from totally fresh rocks to the epidote-amphibole zone. The topography of the hydrothermal system is exemplified here by the first occurrence of zeolites, quartz, Figure 6. A comparison of alteration and estimated formation temperatures in cross section A-A (Figure 2). Black lines show the zeolite-, quartz-, epidote- and amphibole zones. Black dots on well paths show the main aquifers. 53 epidote and amphibole as shown in Figure 5. Elevated hydrothermal alteration to shallower levels is seen at four locations in the NE-SW cross section A-A and it becomes progressively shallower to the east in cross section B-B (Figure 5). These highs and lows correlate well with the estimated formation temperatures in the wells as seen in Figure 6 and may indicate separate upflow zones. Hydrothermal alteration and formation temperatures also correlate with resistivity anomalies in the Hellisheidi area (Figure 7, overleaf). It is of interest that variation in the depth of alteration also correlates well with landscape topography which may reflect higher temperatures in areas of higher volcanic accumulation rates. Furthermore, there are indications that the largest aquifers seem to be located in areas of high alteration. Figure 5 shows the upper and lower limits of laumontite and it is apparent that most of the main aquifers are located below the laumontite zone. Laumontite forms at higher temperatures than any other zeolite ( C), excluding wairakite, and often seems to indicate cooling and reduced permeability (Franzson et al., 2005). It is apparent that the lower limit of laumontite is at deeper level furthest towards the south and west. Reinjection holes west of the main NE-SW faults of the Hengill system show indeed a reverse temperature gradient and reverse alteration where alteration minerals of lower formation temperature overprint those of higher temperatures indicating that the geothermal system in that area is cooling. Conclusions The exploration and production drilling so far at the Hellisheiði field have revealed several features relevant to the hydrothermal system. Our results from the western margin of the Hengill system indicate that the prevalent permeability is associated with major sub-vertical faults rather than intrusions. A substantial, previously unrecorded, graben (the Reykjafell graben) striking NE-SW has been identified and production wells which penetrate the Reykjafell graben are among the most powerful at Hellisheidi. Elevated hydrothermal alteration (amphibole) to shallower levels is seen at four locations in the NE-SW cross section A-A (Figure 5) and these highs correlate well with the estimated formation temperatures in the wells (Figure 6). These four highs in alteration and temperature may indicate separate up-flow zones.

7 Figure 7. Resistivity at 850 m below sea level according to recent TEM surveys. High resistivity below low resistivity (< 10 Ωm) is shown as red, crossed lines. Surface geothermal springs as red dots. Surface fissures and faults as blue lines. Green lines are fissures and faults defined by earthquake locations and yellow lines are post glacial (< 12 ka) fissures. White lines show the location of cross sections A-A and B-B in Figure 2 (Arnason, 2006). Extensive geophysical surveys have been carried out at Hengill in conjunction with the Hellisheidi drilling activities. It is of interest that resistivity maps from the Hellisheidi area show that resistivity anomalies (Figure 7) seem to correlate with hydrothermal alteration and formation temperatures presented in this paper (Figures 5 and 6). Acknowledgements The data presented in this paper is a part of the exploration work of the Hellisheidi high-temperature system. Reykjavik Energy is thanked for the permission to publish the data. Patrick Dobson is thanked for a thoughtful and constructive review. Comments by B. Steingrimsson and I.S. Macdonald are much appreciated. References Alfredsson H.A., B.S. Hardarson, H. Franzson and S.R. Gislason, CO2 sequestration in basaltic rock at the Hellisheidi site in SW Iceland: Stratigraphy and chemical composition of the rocks at the injection site. Min. Mag., 72, 1-5. Arnason K., TEM resistivity surveys at the Hengill area 2006 and proposed experimental drilling by Eldborg. In Icelandic, Report, ISOR Arnason K. and I.Th. Magnusson, Geothermal activity in the Hengill area. Results from resistivity mapping. Orkustofnun report, in Icelandic with English abstract, OS-2001/091, 250 p. Björnsson G., Reservoir conditions at 3-6 km depth in the Hellisheidi geothermal field, SW-Iceland, estimated by deep drilling, cold water injection and seismic monitoring. Proceedings, Twenty-Ninth Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, California, January 26-28, 2004, SGP-TR-175. Björnsson S., Geothermal development and research in Iceland. National Energy Authority and Ministries of Industry and Commerce. April ISBN: Einarsson P., Earthquakes and present-day tectonics in Iceland. Tectonophys., 189, Einarsson P., Plate boundaries, rifts and transforms in Iceland. Jökull, 58, Feigl K, G. Gasperi, F. Sigmundsson and A. Rigo, Crustal defomation near Hengill volcano, Iceland : Coupling between magmatic activity and faulting inferred from elastic modeling of satellite radar interferrograms. J. Geophys. Res., 105, Franzson H., B.R Kristjansson, G. Gunnarsson, G. Björnsson, A. Hjartarson, B. Steingrimsson, E. Gunnlaugsson and G. Gislason, The Hengill-Hellisheiði Geothermal Field. Development of a Conceptual Geothermal Model. Proceedings World Geothermal Congress, Antalya, Turkey, April Gunnlaugsson E. and G. Gislason, Preparation for a New Power Plant in the Hengill Geothermal Area, Iceland. Proceedings World Geothermal Congress 2005, Antalya, Turkey, April Hardarson B.S., J.G Fitton., R.M. Ellam and M.S Pringle., Rift relocation - a geochemical and geochronological investigation of a palaeo-rift in northwest Iceland. Earth Planet. Sci. Lett. 153, Hardarson B.S., H.M. Helgadottir and H. Franzson, The Hellisheidi power plant. The injection area by Grauhnukar. In Icelandic, Report, ISOR-2007/001. Hardarson B.S., A.K. Mortensen, G.M. Einarsson and H. Franzson, Hole HN-9 and the disposal of waste-water from the Hellisheidi power plant by Kolvidarhol. In Icelandic, Report, ISOR-2008/006. Johannesson, H. and K. Sæmundsson, Geological map of Iceland, 1: Icelandic Institute of Natural History, Reykjavik. LaFemina P.C, T. H. Dixon., R. Malservisi., T. Arnadottir, E. Sturkell, F. Sigmundsson and P. Einarsson, Strain partitioning and accumulation in a propagating ridge system: Geodetic GPS measurements in south Iceland. J. Geophys. Res. 110, B Larsson D., K. Grönvold, N. Oskarsson and E. Gunnlaugsson, Hydrothermal alteration of plagioclase and growth of secondary feldspar in the Hengill Volcanic Centre, SW Iceland. J. Volcanol. Geotherm. Res., 114, National Energy Authority of Iceland, Annual Report National Energy Authority of Iceland, 30 pp. Sens-Schönfelder C., M. Korn and R. Stefansson, Joint interpretation of the size and time distributions of seismic activity around the Hengill triple junction (SW Iceland) between 1993 and J. Seismol., 10, Sigmundsson F, P. Einarsson, S. Rögnvaldsson, G. Foulger, K. Hodgkinson and G. Thorbergsson, The seismicity and deformation at the Hengill triple junction, Iceland: Triggering of earthquakes by minor magma injection in a zone of horizontal shear stress. J. Geophys. Res., 102, Sæmundsson, K., Outline of the geology of Iceland. Jökull, 29, Sæmundsson K., Hengill Geological Map (bedrock), 1: National Energy Authority, Reykjavík Municipal Heating and Iceland Geodetic Survey. Vink G.E., A hotspot model for Iceland and the Vöring Plateau. J. Geophys. Res., 89,