Geology and structure of the Reykjanes volcanic system, Iceland

Kristján Sæmundsson

, Magnús Á. Sigurgeirsson

^a,

⁎ , Guðmundur Ómar Friðleifsson

aÍSOR, Grensásvegur 9, 108 Reykjavík, Iceland

bHS Orka, Svartsengi, 240 Grindavík, Iceland

a b s t r a c t a r t i c l e i n f o

Article history:

Received 11 September 2017 Received in revised form 6 July 2018 Accepted 23 November 2018 Available online xxxx

The Reykjanes Peninsula is a trans-tensional plate boundary with several volcanic systems from the centers of whichfissure swarms extend to the NE and SW. Thefissure swarms accommodate the extensional component, whereas north-south trending strike-slip faults accomodate the transform component. Thefissure swarms re-lease stress during volcano-tectonic episodes that occur at intervals of several hundred years. Stress is rere-leased by the strike-slip faults by microearthquake swarms at intervals of a few decades.

Theﬁssure swarms are segmented along their trend. The segments include a volcanic center with a clustering of ﬁssure eruptions and extensional faults. A proximal zone of 20–40 km follows were faults become dominant with distance from the center. Linear anomalies of a high geothermal gradient extend further still, into the marginal area of Early Quaternary to Miocene rocks indicating dyke propagation for another 30–40 km beyond visible faults.

The Reykjanes volcanic center is nested in a 5–6 km wide rift zone with boundary faults of just over 20 m visible throw. Volcanic production keeps pace with extension and subsidence toﬁll the rift. The last three volcano-tectonic episodes occurred at Reykjanes in the 13th century, about 2000 years ago and about 3200 years ago.

The lavas from these threefissure eruptions coverN2/3 of Reykjanes by area, largely smoothing over faults of the riftfloor. During the Weichselian glacial maximum the ice margin may have reached 75–100 km beyond Reykjanes. It had become ice free 14,500 years ago. There is evidence of at least 10 eruptions on the Reykjanes volcanic system since, of which thefirst ones were of lava shield type. A similar eruption frequency may have prevailed at Reykjanes during its postglacial time interval.

There are three volcanicﬁssure zones within the Reykjanes center. The middle zone of 1.5 km²hosts the main geo-thermal resource of the system. Recently the western zone proved to host an exploitable resource also. It may be restricted to a narrow zone of dykes. The reservoir temperatures of these two lie in the range 280–310 °C. The east-ern zone has not proved productive so far. It has erupted olivine rich shield lavas on most of its trace whereas the others have erupted slightly evolved tholeiite. Feed zones in the main production zone have been correlated partly with the axis of the eastern of two tindars. They cluster also in near horizontal intervals which may suggest that density controlled offshots also play a role.

Keywords:

Reykjanes is the name of the southwestern tip of Reykjanes Penin-sula, a lava covered area of 25 km². Its position is transitional between the Reykjanes Peninsula's trans-tensional plate boundary and that of the Reykjanes Ridge of primarily extensional structure. The geology of Iceland is controlled by a varied set of rift and transform structures (Fig. 1). There are four main types; 1) spreading zones of rifting and vol-canism producing the Iceland tholeiitic crust; 2) fracture zones connecting offset branches of the spreading zones; 3) trans-tensional zones including transform faulting and spreading of which the

Reykjanes Peninsula is the only on-land example and is mirrored by the Tjörnes Fracture Zone (TFZ), off the north cost; and 4)ﬂank zones which are superimposed on the eroded tholeiitic crust produce alkalic to transitional rocks from stratovolcanoes and are associated with very minor rifting.

We prefer to use postglacial rather than Holocene which refers to a ﬁxed beginning. Reykjanes became ice-free 4000–5000 years before the Holocene.

Short term GPS measurements of spreading rates in the southwest of the WVZ and EVZ show a very insigniﬁcant part of the spreading is oc-curring on the WVZ. One documented rifting event of 1789 CE which af-fected the NE Hengillﬁssure swarm involved widening of the gjár of Thingvellir and a subsidence of 2.5 m in the center of the rift zone (Sæmundsson, 1992). The WVZ thus may rift episodically rather than by latent creep.

⁎Corresponding author.

E-mail address:magnus.a.sigurgeirsson@isor.is(M.Á. Sigurgeirsson).

Contents lists available atScienceDirect

Journal of Volcanology and Geothermal Research

j o u r n a l h o m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / j v o l g e o r e s

2. Reykjanes in the context of the geology of Iceland

Volcanic systems are the building blocks of spreading zones. The rate of spreading has been defined at 18.6 mm/y for the Iceland area from the ocean floor reversal pattern (DeMets et al., 1994), and at 18–20 mm/y for a 9 year period from GPS measurements of extension across Iceland's combined Western and Eastern Volcanic Zones (LaFemina et al., 2005). That rate of spreading viewed against the nic production rate (Jakobsson, 1972) and a lifetime of individual volca-nic systems of at least 500 ky, creates a pile of extrusive rocks. Such piles are at least 5 km thick forming units of elongated, lenticular shape (Gibson and Piper, 1972). A preferred zone of magmatic upwelling pro-duces crustal temporal or long term magma storage from which dykes are expelled upwards and laterally, perpendicular to minimum com-pression to form dyke swarms and crater rows. The latter extend rarely over 50 km from the magmatic center, but faults and openfissures–the surface marks of dykes (Walker, 1965)–continue far beyond. Observed and documented rift zone examples are the Krafla Fires of 1975–1984 (Einarsson, 1991) and Bárðarbunga 2014–2015 (Sigmundsson et al., 2015). Most volcanic systems run subparallel with the plate boundary to form volcanic rift zones, however trans-tensional zones such as the Reykjanes Peninsula are different. Northeast of the Peninsula the dykes of its volcanic systems invade into theflank of Early Quaternary and Tertiary age. This has proved of a major economic benefit in produc-ing low to medium temperature geothermal systems on their distal seg-ments.Walker (1974)recognized this kind of relationship in East

Iceland where dyke swarms extend across lava isochrones up to 5 mil-lion years older than the central volcano where they originated.

Transform faulting offsets 8 million year old rocks by at least 100 km laterally along the Tjörnes Fracture Zone (TFZ) in North-Iceland (Hjartarson and Sæmundsson, 2014). A second apparently much youn-ger transform fault zone occurs in South Iceland (SISZ). There a true transform fault has not developed, but rather a zone of conjugate N–S and ENE–WSW strike-slip faults indicating left lateral slip. This system continues along the Reykjanes Peninsula with only the N–S component well developed at the surface (Hreinsdóttir et al., 2001). A narrow epi-central zone is manifested at 4–5 km depth along the Peninsula, connecting to the SISZ in the east.

3. The Reykjanes Peninsula

The Reykjanes Peninsula connects towards the east to the Hengill volcanic system, the southernmost volcanic system in Iceland's Western volcanic spreading zone. Magnetic anomalies zigzag along the Reykjanes Peninsula, with a reasonably well defined boundary between rocks of Matuyama and Brunhes age (Fig. 2). A large part of the reverse Matuyama epoch series is overlapped by Brunhes rocks, most of them non-tilted lava shields and theirflow foot breccias of postglacial and in-terglacial age. Six boreholes have been drilled into such rocks in the northwest of the Peninsula. Some of these passed through a minimum thickness of 100–200 m fresh rock. Shells dated from sediment immedi-ately belowflow foot breccias at 70–80 m depth in the boreholes Fig. 1.Main tectonic elements of Iceland shown schematically. Volcanic spreading zones (black) and non-volcanic fracture zones (red) dominate in the structural pattern. Volcanic trans-tensional rift zones (pink) join offset spreading zones. Volcanicflank zones of transitional rock composition (blue) built up stratovolcanoes on tholeiitic crust. Theflank zone of Snæfellsnes (SN) translates towards east into a WNW volcanic rift zone and furthest east into a non-volcanic fault zone (A) (Sæmundsson, 2010). Arrows show spreading direction. Abbreviations: EÁ = Eyjafjörður Deep. TFZ = Tjörnes Fracture Zone. SISZ = South Iceland seismic zone. A = Arnarvatn fault zone. NVZ, EVZ, WVZ = Northern, Eastern and Western Volcanic Zones. H = Hofsjökull volcanic system. SH = South Icelandflank zone. GK = Grímsvötn–Kverkfjöll volcanic systems. RR = Reykjanes Ridge. ÖS = Öræfajökull–Snæfellflank zone. RPTZ = Reykjanes Peninsula trans-tensional zone. (For interpretation of the references to color in thisfigure legend, the reader is referred to the web version of this article.)

Fig. 2.Aeromagnetic map of Reykjanes Peninsula (Kristjánsson and Jónsson, 1989). The spreading direction has been added and the volcanic systems are outlined. They show up with normal magnetization about as far as their eruptive segments reach. Red dots mark boreholes in the dominantly reversely magnetized Matuyama border zone north of Reykjanes.

Rocks of normal magnetization of Brunhes epoch age overlap a substantial part of the reversely magnetized Matuyama anomaly. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

Fig. 3.Three volcanic and rifting episodes have occurred on the Reykjanes Peninsula in the past 4000 years. The Hengill volcanic system is included for comparison. A non-volcanic rifting episode occurred there in 1789 CE. This system is usually considered as belonging to Iceland's Western volcanic zone. The numbers between the shaded segments represent the time in-tervals between the volcanic inin-tervals (Sæmundsson and Sigurgeirsson, 2013). The up-slope time trend from east to west underlines the time sequence of the volcanic episodes jumping successively at intervals of 100–200 years.

farthest east and southwest yielded an age of about 14,500 years, i.e.

Bölling-Alleröd interstadial (Sæmundsson, 2019).

The plate boundary as deﬁned from earthquake epicenters runs N80°E along the western part of the Peninsula passing through three central volcanic complexes. From west to east these are:

Reykjanes, Svartsengi, and Krýsuvík. Krýsuvík is the only one of those on the Peninsula with features common to most central vol-canoes, such as segments of arcuate crater rows and tindars, circu-lar geophysical anomalies (resistivity and gravity), alternating inflation-deflation periods (Michalczewska et al., 2012) and gabbroic ejecta from volcanic-hydrothermal explosion craters (Johnston-Lavis, 1895;Jónsson, 1978). Acid rocks have not been found in any of the Peninsula‘s centers but Hengill nearest to it in the east has them (Sæmundsson et al., 2016). Only Krýsuvík has erupted large volumes of evolved tholeiite. As regards to volcanic production fromfissure eruptions, the Brennisteinsfjöll system next east to Krýsuvík has been most productive considering the areal extent of lavas produced during the past 7000 years. Compar-ing further back in time is problematic. As for age assessment,

tephra layers older than 6100 y BP are poorly preserved in the soils of the western part of the Reykjanes Peninsula except locally.

A number of radiocarbon ages have been acquired. For the oldest postglacial members of the lava succession the poorly deﬁned time transgressive character of ice melt poses a problem.

On the Reykjanes Peninsula periods of rifting and volcanism occur at intervals of 800–1000 years alternating with periods of only transcurrent movement manifested as earthquake episodes occurring at intervals of a few tens of years (Einarsson, 1991). The volcanic activity affects one sys-tem at a time jumping successively from east to west with the jumps spaced 100–200 years apart (Fig. 3).

Postglacial volcanism started with the eruption of small volume picritic lavas. Voluminous olivine tholeiite lava shields followed, each about 150 km²by area, of which the two westernmost were dated to about 14,500 y BP (Sæmundsson, 2019). Fissure volcanism became dominant from then on. Shield volcanism fell into two periods, the ﬁrst occurred during the deglaciation and includes four major shields.

The second period began 7000–8000 y BP with a shield of about 100 km², the only postglacial shield of the Krýsuvík volcanic system. It

Fig. 4.Volcanic systems of the Reykjanes Peninsula have a core area (1) from which extend volcanic and openfissures (2). Further away, non-volcanicfissures continue (3). The underlying dyke swarms continue into Early Quaternary (green) and Pliocene to Miocene (blue) formations. There they bring about geothermal convection systems where hot springs and still further away tepid springs occur (4), but there is clear evidence from linear anomalies of ultrahigh near surface geothermal gradient that the dykes propagate much further to the NE producing vertical permeability (5). A number of geothermal systems have been detected in the (5) segment settings lacking visible surface manifestations. The distribution and trend of those is shown as red lines on the map. A volcanic system, Fagradalsfjall, is indicated between Svartsengi and Krýsuvík. It is made up of tuyas, short tindars and a number of small postglacial lavas, all of picrite and olivine tholeiite composition (Sæmundsson et al., 2016). We name it Fagradalsfjall from the dominant tuya. A NE-SWfissure swarm and a geothermalfield are lacking. This and the primitive rocks suggest an embryonic stage for it.

was followed by six more shields at about 1000 year intervals, the last about 900 CE. These shields were small and occurred on the Brennisteinsfjöll volcanic system (Sæmundsson et al., 2016).

Tephrochronology based on eight distinctive ash layers and a number of C-14 datings allow a fairly detailed record of volcanic ac-tivity on the Peninsula for the last 4500 years (Fig. 3). It occurred ep-isodically during active periods lasting a few hundred years. The active periods were separated by commonly 800–1000 volcanically quiescent years (Fig. 3). For the western half of the Peninsula a tephra fall from a near shore eruption off Reykjanes dated at 6100 BP might provide a marker for viewing the volcanic history approxi-mately back to that datum. This tephra most likely had its origin on then a near shore platform southwest of Skálafell where tephra mounds project through the last Skálafell lavas (Fig. 7). For theﬁrst 7000 years of postglacial timeﬁve C-14 dates and local tephra falls from maars in Krýsuvík provide for time constraints for volcanic ac-tivity during early postglacial time (Sæmundsson and Friðleifsson, 2018, in prep.).

The volcanic systems of the Reykjanes Peninsula are segmented along their trend (Fig. 4). The central area is marked by maximum vol-canic production. It hosts a high temperature geothermal system that is kept active as volcanic high temperature geothermal systems generally are, by repeated magma intrusions nesting in their roots. This has been demonstrated repeatedly in deeply eroded central volcanic complexes.

An impressive example (much used for demonstration purposes) with twelve separable intrusive phases was studied byFriðleifsson (1983) in SE-Iceland. In the center and on thefirst 10–15 km of the associated fissure swarms faults, openfissures, crater rows and tindars abound.

Tectonic faults andfissures do not show on the surface of the youngest lavaflows (Fig. 7). The faults andfissures continue far beyond the erup-tive segment with variable throws and widths depending on the age of

the surface rock but volcanic manifestations are rare if any. The dykes apparently continue at depth even farther than the segment of faults andﬁssures. This is concluded from a surface expression, which is at best visible as lines of hot springs but otherwise lacking surface expres-sions, but show up as linear anomalies of ultrahigh geothermal gradient (Sæmundsson, 2011, 2013). The maxima commonly show values 3–4 times higher than the regional average.

On this basis the farthest type of segment has been traced for about 50 km in case of the Krýsuvík and Svartsengi dyke swarms (Sæmundsson, 2013). They create vertical permeability in otherwise low permeability rocks of the plateau-basalt series, and allow for con-vective geothermal systems (85°–140 °C) to form. These provide a sig-niﬁcant proportion of SW-Iceland's population with hot water for domestic use.

The transform tectonics are represented by N–S right lateral strike-slip fractures, irregularly distributed along the Reykjanes Peninsula.

Geodetic measurements of surface deformation on the Reykjanes Pen-insula indicate dominantly left lateral strike slip (Árnadóttir et al., 2008). The seismic imprint of the fracture zone at depth is discontinu-ous. Microearthquake swarms registered over the last 40 years gradu-ally extend the trace of the plate boundary andﬁll into the gap. The focal depths deﬁne a narrow zone above 5 km depth (Fig. 5) which seems to be a steady state feature. The length of the en échelon strike-slip fracture arrays is commonly 3–5 km. The strike-slip is mostly horizontal and opening of the individual cracks is rarely over 1 m. Conjugate left lateral ENE–WSW faults are lacking on the Reykjanes Peninsula. They do occur within the South Iceland seismic zone (SISZ) east of it, but are rare (Einarsson, 2010). The N–S fractures host an exploitable geo-thermal convection system above 1000 m depth in the SISZ. At Reykjanes, where tested, the same have proved poor producers below the casing depth of about 1000 m.

Fig. 5.Seismic image of the plate boundary above 5 km depth in the transition zone between the Reykjanes Ridge and the Reykjanes Peninsula. Focal mechanism solutions show extension on Reykjanes but strike-slip east of it (Klein et al., 1977). Faulting at Reykjanes conforms with the focal mechanism as N–S strike-slip faults are lacking apart from two oblique-slip faults which occur in its SE trending N20°E with vertical and horizontal slip of a few meters in a lava of about 6000 BP.

4. The Reykjanes volcanic system

The Reykjanes volcanic system is at least 45 km in length of which 30 km are on land. The core area is nested within a 5–6 km broad rift structure at the very SW tip of the Peninsula. Its border faults are well defined with a visible throw of 20 m on the NW in a 14,500 BP lava shield and over 20 m visible in lavas ofN8000 BP on the SE. The SE mar-gin of the system is defined by a row of lava shields and tuyas. Among the shields in the row there is the aforementioned 14,500 year old shield of about 150 km². Its lava and crater bound Reykjanes on the north (Fig. 7). In the center of the rift the shield lava has subsided about 80 m (Sigurgeirsson, 2014). This rate of subsidence is of the same order as found for the period 1992–1995 at 6.5 mm/y byVadon and Sigmundsson (1997), and byFriðleifsson and Richter (2010)on the basis of depth of burial of cored supramarine strata. Direct distance measurements across the Reykjanesfissure swarm for a four year pe-riod (1968–1972) (Fig. 5) showed that a combination of left-lateral and extensional movement of 9 mm/y occurred (Brander et al., 1976).

Hreinsdóttir et al. (2001)andSigmundsson (2006)reported horizontal mainly strike-slip movement of 8 mm/y from GPS measurements for the Reykjanes Peninsula over a 5 year period (1993–1998). The Reykjanes volcanic system has well definedfissure swarms towards the NE and SW. Crater rows and tindars reach 15 km NE from the core area and faults and openfissures another 10 km before disappearing below sea level. The topographic expression of the system extends

15 km SW from the shore. It is clearly offset east relative to the“Eldey volcanic system”next to it (Fig. 6).

The others farther east do not show up as clearly in the seafloor to-pography beyond the shore as does Reykjanes (Fig. 2). A broad shoal southwest of Svartsengi which is next to the east probably marks a fis-sure swarm (Fig. 6, east border) and Brennisteinsfjöll may possess an offshorefissure swarm also, with a vague expression on the magnetic map (Fig. 2). Only faults of the one next to Reykjanes, i.e. the Svartsengi-system, extend visibly down to the shore (Sæmundsson et al., 2016). Faults and non-eruptivefissures are likely to occur offshore.

Considering the spreading direction we estimate that of the total spreading of 1.8 cm/y for the Iceland area, about half may be taken up by Reykjanes and its southwesternfissure swarm, with the other half by thefissure swarm of the Eldey volcanic system to the west and by in-distinctive offshorefissure swarms to the east. This would mean that during the 14,500 years of postglacial volcanism of Reykjanes it might have widened about 120 m, including perhapsN10 volcano-tectonic ep-isodes. Extension would be accommodated by dykes, normal border faults and intra-riftfissures and faults with their near vertical down-ward continuation until substituted by dykes. A common feature of

Dans le document Lettre de la Recherche n°12 : Transition énergétique | BRGM (Page 47-60)