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Reykjanes Field Trip: Tectonic – Magmatic Interaction at an oblique rift zone
Amy Clifton, Nordic Volcanological Institute
At latitude 63°48’N, the Reykjanes ridge comes onshore at Reykjanes
Peninsula The ridge bends gradually eastward between longitudes 24°30’W and
23°30’ 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 5th 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
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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 NS 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].
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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
Ne sjavellir
the so-called Hengill Volcanic System,
which consists of three volcanoes.
Below us in the valley that contains the
Hrómundartindur
Hengil l
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
Grensdalur
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
4 km
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
Ölke ld uhá ls
June 4
Hv erage rð i
Nov 1 3
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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 2002
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On June 17, 2000 an earthquake with M > 5 was triggered within this fault zone by a
Ms = 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
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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.
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
?
?
D
F 1 F2
UD
U D UD
U
D
lavas dating approximately 2000
UD
UD
U
UD
D
F5
ybp (Sæmundsson, 1995). From
U
F3
F4
?
D
D
this same vantage point we can see
U
U
?
several faults to our south which
cut most but not all of the lavas.
The Háleyabunga fault has an
D U
D
?
U
1 km
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 1967. 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.
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?
F3
F2
F5
F4
F2
F3
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.
Stop 8 – Valahnúkur/ Younger
Stampar lava –
At this location we can
Stóraliterally see the Mid-Atlantic
Sandvík
Younger Stampar lava
Ridge rise out of the sea.
(1211 AD)
Directly to our east, the
lava >2000 ybp
Háleyabunga-Valbjargagjá
shield lavas
(early post-glacial)
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
Karl
Sandvík
cone
hyaloclastite hills directly to
the north, formed as a result of
Vatnsfell
cone
Valahnúkur
submarine fissure eruptions.
The present lighthouse sits on
one of these hills, while the
(Sæmundsson, 1995)
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 (1211-1240 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
Kinn
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along a 4 km long NE trending fissure which built a row of spatter cones and
pahoehoe lava fields covering approximately 4 km2. 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
StóraSandvík
x
2
1
current site of
“Bridge across the continents”
0
100
meters
N
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/JHD02. 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, 1994 – 1998, Journal of Volcanology
and Geothermal Research 115, 233-255, 2002.
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, 317-329.
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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, 2191-2194.
Einarsson, P., 1991, Earthquakes and present-day tectonism in Iceland:
Tectonophysics 189, 261-279.
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.61-80
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, 1998.
Feigl, K., Gasperi., J., Sigmundsson, F., Rigo, A., 2000, Crustal deformation near
Hengill volcano, Iceland 1993-1998: 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, 2001.
Jakobsson, S.P., Honsson, J. and Shido, F., Petrology of the western Reykjanes
Peninsula, Iceland, Journal of Petrology 19, 669-705, 1978.
Johannesson, H., 1980, Jardlagaskipan og thróun rekbelta á Versturlandi (Evolution
of rift zones in western Iceland, in Icelandic with English summary).
Náttúrufraedingurinn 50, 13-31.
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, 5084-5099.
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, 7-28.
Saemundsson, K., 1995, Svartsengi geological map (bedrock) 1:25000. Orkustofnun,
Hitaveita Sudurnesja and Landmaelingar Islands. Reykjavik.
Saemundsson, K. and Einarsson, S.,1980, Geological map of Iceland, sheet 3, SWIceland, 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.211-230.
Sturkell, E., Sigmundsson, F., Einarsson, P., and Bilham, R., 1994, Strain
accumulation 1986-1992 across the Reykjanes Peninsula plate coundary,
Iceland, determined from GPS measurements: Geophysical Research Letters
21, 125-128.
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.193-197.
References used:
Thordarson, Th. and Hoskuldsson, A., 2002, Classical Geology in Europe 3: Iceland,
Terra Publishing, Hertfordshire, England. 200 pp.
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