Reykjanes Field Trip: Tectonic Magmatic Interaction at an oblique rift zone Amy Clifton, Nordic Volcanological Institute

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1 Reykjanes Field Trip: Tectonic Magmatic Interaction at an oblique rift zone Amy Clifton, Nordic Volcanological Institute At latitude N, the Reykjanes ridge comes onshore at Reykjanes Peninsula The ridge bends gradually eastward between longitudes W and W until it is oriented approximately 30 oblique to the direction of plate motion (DeMets et al., 1994). Our understanding of how Reykjanes Peninsula fits into the plate tectonic model has evolved considerably, as the model itself has evolved, over the past thirty years. Because of its geometry with respect to adjacent portions of the MAR, early researchers (e.g.ward, 1971, Courtillot et al., 1974) believed the peninsula to be a transform fault, but problems arose with that model when no through-going strike-slip fault could be found. Nakamura (1970) was one of the first researchers to suggest that it is an oblique rift zone. A major ridge jump approximately 6-7 million years ago initiated active spreading on the Reykjanes Peninsula [Saemundsson, 1979; Johannesson, 1980]. The peninsula is characterized by arrays of eruptive fissures, spaced on average approximately 5 km apart, and having an average strike of 040. These have been described in the literature as comprising either five or four distinct volcanic systems (Jakobsson et al., 1978; Sæmundsson, 1979), each with their own magma supply, high temperature geothermal system, and clusters of closely spaced fractures referred to as fissure swarms. The fissure swarms are comprised of shear fractures (mainly normal faults), extension fractures (mainly gaping fissures with no shear displacement) and hybrid fractures which exhibit components of both shear (vertical offset parallel to the fracture plane) and extension (opening perpendicular to the fracture plane). Tectonic map of Reykjanes Peninsula showing fissure swarms (black lines), eruptive fissures (red lines), strike slip faults (green lines), and the approximate location of the plate boundary (dashed line), and geothermal centers (blue stars). Four volcanic systems shown in grey shading and black ellipses. Purple ellipse shows a possible 5 th system, the Grindavík system, proposed by Jakobsson, (1978). (data from Árnason et al., 1986;Einarsson, 1991; Eyólfsson, 1998; Hreinsdóttir et al, 2001; Sigurdsson, 1985; Sæmundsson and Einarsson, 1980; Jakobsson, 1978). Sub-glacial and sub-aerial (post-glacial) fissure eruptions have formed 1

2 prominent NE-trending ridges and crater rows that dominate the topography of the peninsula. A number of table mountains and hyaloclastite cones, products of subglacial eruptions from isolated vents, are also present. Eruption of early post-glacial basaltic (large volume) and picritic (small volume) lava shields have also played a major role in surfacing this ridge segment with voluminous pahoehoe lava flows, which both cover and are covered by the products of fissure eruptions. Lava shields and eruptive fissures have been active on the peninsula during the Holocene, but the last known eruption was in the thirteenth century [Saemundsson, 1995]. DEM of Reykjanes Peninsula. Notice prominent NE-trending ridges resulting from sub-glacial fissure eruptions. Brittle deformation on the peninsula has been accommodated primarily by the extensional features that make up the fissure swarms [Fig. 1; Saemundsson, 1979; Gudmundsson, 1980]. A narrow zone of seismicity 2 to 5 km wide, characterized by predominantly strike-slip focal mechanisms and having an average trend of 075 runs along the length of the peninsula. This zone has been defined as the currently active plate boundary [Klein et al., 1977; Einarsson, 1991]. The zone of seismicity intersects the fissure swarms near regions of maximum volcanic production and seems to control the location of geothermal activity on the peninsula [Einarsson, 1991]. The base of the seismogenic zone is between 8 and 11 km on the peninsula and most seismicity occurs at depths of 1-5 km. In the eastern part of the peninsula, seismicity is characterized by focal mechanisms indicating right-lateral strike-slip faulting on N- S planes or left-lateral strike-slip faulting on E-W planes. However, seismicity on the western part of the peninsula typically occurs in swarms and is principally characterized by normal faulting on NE-striking planes [Einarsson, 1991]. During a large swarm in 1972, focal mechanisms ranged from normal to oblique to strike-slip faulting [Klein et al., 1977]. Geodetic measurements between 1986 and 1998 [Sturkell et al., 1994; Hreinsdottir et al., 2001] show that left-lateral shear is currently accumulating on the peninsula. Data from Synthetic Aperture Radar Interferometry (InSAR) support this and indicate that below a depth of 5 km plate motion is accommodated by continuous ductile deformation [Vadon and Sigmundsson, 1997]. 2

3 On this field trip we will stop at several locations where we can observe some of the volcanic and tectonic features typical of Icelandic rift zones and see how their geometry has been influenced by the oblique geometry of the peninsula with respect to the spreading direction. Location map for field trip. Numbers refer to stops described in the guide. Approximately 1 hour drive from Geysir to Hveragerði: Stop 1. Kambar - At this overlook we are standing at a triple junction. To our east lies the South Iceland Seismic Zone, to the north begins the Western Volcanic Zone, and now we are about to begin our drive through the Reykjanes Peninsula Rift Zone. Here we are also standing close to the center of Hengill Nov 1 3 Nesjavellir June 4 Hrómundartindur Ölke ld uhá ls Grensdalur Hveragerð i 4 km the so-called Hengill Volcanic System, which consists of three volcanoes. Below us in the valley that contains the town of Hveragerði, is the now deeply eroded Grensdalur volano and its geothermal system dominated by surface flow and relatively low temperature. Directly behind us is the Hrómundartindur volcano and its high temperature geothermal field Ölkelduháls. The last eruption in this system was about 5,000 years ago (Sæmundsson, 1995), but it has been the locus of high background seismicity and period seismic unrest throughout historic times. The most recent activity occurred in the period 1994 to 1998 when over 80,000 earthquakes occurred, centered around the Ölkeduháls geothermal system. This was associated with uplift of 2 cm per year, detected by geodetic methods and interpreted as resulting from a point source of pressure, probably an injection of magma, at 7 km depth. Activity culminated in a M = 5.1 earthquake in June 1998 and another of M = 5.0 in November 1998, after which uplift appears to have ceased (Fiegl et al., 2000; Clifton et al., 2002). The two earthquakes occurred along the same 10km-long, N10 E striking, right-lateral strike-slip fault. The Hengill volcano, just to the west, is the largest of the 3

4 three in this system. Its last eruption occurred about 2000 ybp, and its high temperature geothermal field, Nesjavellir, provides most of the hot water for the city of Reykjavík. Our route takes us west via Bláfjöll to Krísuvík. We will pass through two historic lava flows from the Brennisteinsfjöll volcanic system, the Svínahraun or Christianity lava erupted in the year 1000 AD, and the Hellnahraun, erupted in the year 950 AD Historic lava flows on Reykjanes Peninsula. Each color represents lavas of a certain age, but are made up of more than one flow erupted in several events. Early post-glacial tholeittic shields are shown in green. Stop 2- Selvogsgata From here we can see Grindaskorð, beyond which is the source area for the historic lavas of the Brennisteinsfjöll system. If the weather is clear, you can see the distance these flows traveled to Hafnarfjörður where they entered the sea. Stop 3 Syðri-stapi, west shore of Kleifarvatn. From here we can get an overview of the area surrounding Lake Kleifarvatn. We can see the juxtaposition of sub-glacial and post-glacial sub-aerial lavas. This area exemplifies the interplay of structures that form in an oblique rift zone. The ridges of Sveifluháls and Núpshlíðarháls to our west trend approximately N40 E and are a result of sub-glacial fissure eruptions. The Vatnshlíð scarp along the eastern shore of the lake, on the other hand, is cut by a fault zone which trends approximately N10 E. This is the trend of strike-slip faults in the South Iceland Seismic Zone (SISZ). Lake shore, November 2000 Lake shore, July

5 On June 17, 2000 an earthquake with M > 5 was triggered within this fault zone by a M s = 6.6 earthquake in the SISZ. The quake caused primary rupture along the southeastern shore of the lake and shook the ridges and hills surrounding the epicenter generating rock falls and small scale block slides. In addition a series of fissures opened at the north end of the lake, allowing water to drain downward into a lower level of the water table and probably into the fault zone itself. Within 9 months of the earthquake, the lake level lowered by 4 meters. At the present time (September 2003) the lake level appears to be rising and the fissures are no longer visibly open at the surface. Stop 4 Seltún This was the site of a research borehole drilled in 1949 which, although rather robust, was never used for energy exploitation. Rather it was turned into a tourist attraction, complete with coffee shop and restrooms. The borehole was left open and continually erupting to the delight of tourists, until October 1999 when the erupting stopped suddenly, most likely due to a buildup of precipitated minerals in the borehole casing. Two weeks later, a pressure buildup caused a large explosion at the borehole, blasting out a water-filled hole 43 m across and tossing a 100 kg stone onto the roof of the coffee shop. Although borehole explosions similar to this have occurred in other parts of Iceland, it is interesting that this explosion occurred in an area containing many explosion craters, now mostly water-filled. If we walk to the top of the path above the hot springs we can easily see one of the largest of these craters, Grænavatn, just to the south. Stop 5 Ögmundarhraun This lava erupted during the Krísuvík Fires in 1151 AD. The eruption originated from fissures within the Móhalsdalur valley which lies between Núpshlíðarháls and Sveifluháls ridges. From the top of one of the hyaloclastite islands we can see the N45 E trending eruptive fissure stepping across the valley to the north. Here it breaks into short segments with an overall northward trend, until further to the north it once again takes on a trend of N45 E. This north trending segment coincides in space with the location of the seismic zone that marks the current plate boundary on Reykjanes Peninsula (Einarsson et al, 1991). The Ögmundarhraun flowed south to the sea, while another flow, Kapelluhraun, flowed northward into the present town of Hafnarfjörður. The Krísuvík fires destroyed the Old Krýsuvík settlement, one of the oldest on Reykjanes Peninsula. Stop 6 Festarfjall Festarfjall is a remnant of a Surtseyan type eruption that occurred as sea level was rapidly rising towards the end of the Weichselian glaciation. It is the highest seacliff on Reykjanes Peninsula and has been dissected by the 5

6 battering of waves so that we can see an excellent cross-section of an Icelandic table mountain (stapi). Hyaloclastite breccia is capped by a lava flow, and a number of feeder dikes are visible in the cliff face. Younger cinder cones can be seen onlapping the eastern flank of the mountain. Drive through Grindavík, continuing west on Road 425. Notice the large open fissures that cross the road west of the experimental fish farm on the outskirts of Grindavík. We then pass through the historic Eldvörp lavas which erupted during the Reykjanes Fires of the 13th century. The Eldvörp crater row, which can be seen in the distance to the north, sits atop the high-temperature geothermal field of Svartsengi. D U F1 F2 D U D U U D F3 U D U D U D U D U D U D F5 F4 D U 1km D U Stop 7 Háleyabunga fault/melur lava channel Here we see the complex interplay of volcanism, geothermal activity and different styles of faulting that occur at oblique rift zones. We are standing at an eruptive crater approximately 8000 years old whose lavas flowed eastward along a well-defined channel. These lavas partially cover the earliest post-glacial lavas of the Háleyabunga picrite lava shield, directly to our south. Behind us to the north are fissure lavas dating approximately 2000 ybp (Sæmundsson, 1995). From this same vantage point we can see several faults to our south which cut most but not all of the lavas. The Háleyabunga fault has an average strike of N48 E and is made up of several right-stepping segments which cross the Háleyabunga lava shield. Due to the segmented nature of the fault, there are many displacement minima and maxima along its trace. Maximum vertical displacement is about 20 meters down to the northwest. Many short north-striking fault segments splay off from the Háleyabunga fault and cut the lava channel in a right lateral sense. However, vertical offset is more dominant along these faults. They step down to the east by about 15 meters between the Valbjargagjá and Háleyabungu Faults. The Valbjargagjá fault emerges from the sea at our next stop. It has a generally ENE strike and is highly segmented. Historical movements along this fault were probably frequent, with the last episode occurring in A swarm of M>4 earthquakes here caused 10 cm of subsidence and significant changes to the geothermal system which emerges along parts of this fault. Precipitates from this episode are evident directly to our west. Valbjargagjá and the short splay faults all disappear under the 2000 year old lavas to the north. However, these same lavas are cut by the Háleyabunga fault at its northeastern end, several kilometers away. D U 6

7 F3 F5 F4 F3 F2 F2 F1 Melur eruptive vent 3-d views of fault interaction within sight of stop 7. Photo on left shows splay faults from Háleyabunga fault cutting the Melur lava channel. Photo on right shows right-lateral strike-slip faults crossing the Valbjargagjá fault. Karl cone Younger Stampar lava (1211 AD) lava >2000 ybp shield lavas (early post-glacial) Vatnsfell cone Valahnúkur Kinn Stóra- Sandvík Sandvík (Sæmundsson, 1995) Stop 8 Valahnúkur/ Younger Stampar lava At this location we can literally see the Mid-Atlantic Ridge rise out of the sea. Directly to our east, the Háleyabunga-Valbjargagjá fault system forms the southeastern rift margin. The northwestern margin will be seen at our last stop, about 5 km to the north. Submarine (Surtseyan type) eruptions have occurred at this location probably several times. Valahnúkur, like Festarfjall, formed during the end of the Weichselian glaciation. It and the small hyaloclastite hills directly to the north, formed as a result of submarine fissure eruptions. The present lighthouse sits on one of these hills, while the foundation of the former lighthouse can be seen at the top of Valahnúkur. It was severly damaged by shaking during a period of intense earthquake activity here at the end of the 19th century. Surrounding Valahnúkur we can see the Younger Stampar lava which was erupted at the beginning of the period known as the Reykjanes Fires ( AD). Just off shore we see Karl, the remnants of one of two tuff cones which formed during that eruption. Detailed mapping (Sigurgeirsson, 1995) has revealed the sequence of events. The event began with a Surtseyan eruption which produced the Vatnsfell cone. The eruptive center then jumped 500 m seaward to form the Karl cone. These two tuff cones overlap and can be seen along the shore. After the Surtseyan activity ended, fire fountaining began 7

8 along a 4 km long NE trending fissure which built a row of spatter cones and pahoehoe lava fields covering approximately 4 km 2. The feeder dikes for this flow are visible along the coast, cutting through the tephra of the Karl cone. Drive 4 km northward along Road 425 and stop in parking area on right side of the road. Stop 9 Kinn/Stóra-Sandvík Kinn 2 x Stóra- Sandvík 1 current site of Bridge across the continents N meters Here, although the sign tells us we are crossing a bridge between two continents, we are really only standing on the northwestern margin of the rift zone. The southern margin is 5 km to our southeast, at our previous location. The normal faults we see here exemplify the inhomogenous distribution of stresses that occurs in an oblique rift zone. The bridge goes across one of many narrow grabens which all trend approximately N40 E. In contrast, the large normal fault which emerges out of the sea and bounds the rift zone has a trend of N70 E. The most likely explanation for this difference in strike is that the narrow grabens form above dikes during periods of magmatism, therefore having a trend perpendicular to the direction of maximum horizontal extension, whereas the rift-margin normal fault responds to a different stress system during periods when magma is not present. If we walk along either of these structures towards northeast, we will come to their point of intersection and can discuss a possible sequence of events. End of trip. References cited: Árnason, K., G.I. Haralsddon, G.V. Johnsen, G. Thorbergsson, G.P. Hersir, K. Sæmundsson, L.S. Georgsson, and S.P. Snorrason, 1986, Nesjavellir, Geological and geophysical investigations 1985 (in Icelandic), OS-86014/JHD- 02. National Energy Authority, Reykjavík, Iceland Clifton, A.E., Sigmundsson, F., Feigl, K. L., Gudmundsson, G., and Árnadóttir, Th., Surface effects of faulting and deformation resulting from magma accumulation at the Hengill triple junction, SW Iceland, , Journal of Volcanology and Geothermal Research 115, , Courtillot, V. Tapponier, P. and Varet, J., 1974, Surface features associated with transform faults: a comparison between observed examples and an experimental model: Tectonophysics 24,

9 DeMets, C., Gordon, R., Argus, D. and Stein, S., 1994, Effect of recent revisions to the geomagnetic reversal time scalel on estimates of current plate motions: Geophysical Research Letters 21, Einarsson, P., 1991, Earthquakes and present-day tectonism in Iceland: Tectonophysics 189, Einarsson, S., Jóhannesson, H. and Sveibjörnsdóttir, Á.E., 1991, Krísuvíkureldar II. Kapelluhraun og gátan um aldur Hellnahrauns (in Icelandic, with English summary). Jökull, no. 41, p Eyolfsson, V., Kortlagning sprungna og nútíma eldvarpa í Fagradalsfjalli á vestanverðum Reykjanesskaga (Mapping of fractures and Holocene volcanic vents in Fagradalsfjall, Western Reykjanes Peninsula. In Icelandic). B.Sc. thesis, University of Iceland, Reykjavik, 70 pp, Feigl, K., Gasperi., J., Sigmundsson, F., Rigo, A., 2000, Crustal deformation near Hengill volcano, Iceland : coupling between magmatic activity and faulting inferred from elastic modelling of satellite radar interferograms. Journal of Geophysical Research 105, 25,655-25,670. Hreinsdottir, S., Einarsson, P., and Sigmundsson, F., Crustal deformation at the oblique spreading Reykjanes Peninsula, SW-Iceland: GPS measurements from 1993 to1998, Journal of Geophysical Research 106, 13,803-13,816, Jakobsson, S.P., Honsson, J. and Shido, F., Petrology of the western Reykjanes Peninsula, Iceland, Journal of Petrology 19, , Johannesson, H., 1980, Jardlagaskipan og thróun rekbelta á Versturlandi (Evolution of rift zones in western Iceland, in Icelandic with English summary). Náttúrufraedingurinn 50, Jonsson, J.,1978, Jardfraedikort af Reykjanesskaga (Report on the geology of Reykjanes Peninsula) in Icelandic. Orkustofnun report OS-JHD-7831, Reykjavik. Klein, F.W:, Einarsson, P. and Wyss, M., 1977, The Reykjanes Peninsula, Iceland, earthquake swarm of September 1972 and its tectonic significance: Journal of Geophysical Research 78, Nakamura, K., 1970, En echelon features of Icelandic ground fissures: Acta Naturalia Islandica, Vol. II, no.8, Reykjavik. Saemundsson, K., 1979, Outline of the geology of Iceland: Jökull 29, Saemundsson, K., 1995, Svartsengi geological map (bedrock) 1: Orkustofnun, Hitaveita Sudurnesja and Landmaelingar Islands. Reykjavik. Saemundsson, K. and Einarsson, S.,1980, Geological map of Iceland, sheet 3, SW- Iceland, second edition. Museum of Natural History and the Iceland Geodetic Survey, Reykjavik, Sigurgeirsson, M.Á., 1995, The Younger-Stampar eruption at Reykjanes, SW-Iceland (in Icelandic with English Summary), Náttúrufræðingurinn 64, p Sturkell, E., Sigmundsson, F., Einarsson, P., and Bilham, R., 1994, Strain accumulation across the Reykjanes Peninsula plate coundary, Iceland, determined from GPS measurements: Geophysical Research Letters 21, Vadon, H. and Sigmundsson, F., 1997, Crustal deformation from 1992 to 1995 at the Mid-Atlantic Ridge, Southwest Iceland, mapped by satellite radar interferometry: Science v 275, p References used: Thordarson, Th. and Hoskuldsson, A., 2002, Classical Geology in Europe 3: Iceland, Terra Publishing, Hertfordshire, England. 200 pp. 9