Tectonic Framework and Geologic History (North Atlanc and Iceland)

Sarah Slotznick

Enrichment Trip 2014 Ingibjörg Kaldal Iceland Geosurvey Present Day North Atlanc

1, | 5 5a*L~~~~~~I | ____I____-__ I____ I_____ I____I____I_ WIN 58II I 30.W 25 W Fig. 2 (left). The location of Reykjanes Ridge, southwest of Iceland, and the area of Fig. 3. The 1000-fathom submarine contour is shown, together with the 500-fathom contours for Rockall Bank. Fig. 3 (right). Summary diagram of the magnetic anomalies observed over Reykjanes Ridge (see Fig. 2). Straight lines indicate the axis of the ridge and the central positive anomaly (17).

only additional parameter being the (21) in reconstructing the fit of the Ridge (half width, 600 km) is ap- rate of spreading; the scale (Fig. 4) continents around the Atlantic. In this proximately 1 centimeter per annum has recently received striking independ- instance the deep to the southeast of -that is, the rate of "drifting" is ap- ent confirmation from the work of Rockall may represent an initial abor- proximately 2 centimeters per annum. on January 8, 2012 Opdyke et al. (20) on deep-sea sedi- tive split; the oceanic area to the mentary cores. northwest, centered on Reykjanes Ridge, a subsequent and more persistent site Other Ridges of spreading of the ocean floor. There Reykjanes Ridge is every indication from the existing The model proposed by Vine and bathymetry (17) that the ridge crest Matthews (6) and developed by Vine Observed anomaly profiles obtained is linear and not interrupted or off- and Wilson (8) has been applied to during four crossings of the crest of set transverse four areas on by fractures. widely separated the www.sciencemag.org Reykjanes Ridge are compared (Fig. 5) This area therefore, 1200 kilometers midoceanic ridge system (Figs. 6-9) 1, | 5 5a*L~~~~~~I | ____I____-__ I____ I_____ I____I____I_ WIN 58II I 30.W 25 W with simulations obtained by assump- in width, may well record a compara- by assumption of the reversal time Fig. 2 (left). The location of Reykjanes Ridge, southwest of Iceland,tionandof thereversalarea oftimeFig.scalesVine 1966 3. Thefor1000-fathomthe last submarinetively simplecontourand straightforwardVogt 1986 ex- scale shown in Fig. 4 and a rate is shown, together with the 500-fathom contours for Rockall Bank.4 millionFig.years3 (right).and a rateSummaryof spreadingdiagram ampleof theofmagneticdrifting and spreading. The of spreading compatible with the width anomalies observed over Reykjanes Ridge (see Fig. 2). Straight lines indicate the axis of the ridge and the central positive anomaly (17). of 1 centimeter per annum for each oldest rocks in the Thulean or Brito- of the central anomaly. An observed limb of the ridge. The model assumed is Arctic Tertiary Igneous province oc- profile across Juan de Fuca Ridge, analogous to the one I have described cur in northwestern Scotland and east- southwest of Vancouver Island (Fig. only additional parameter being the (21) in reconstructing(8,themodelfit of2),thebut Ridgethe depths(half width,have 600ern Greenland.km) is ap-Preliminary potassium- 1) (8, 19), is compared (Fig. 6) with Downloaded from rate of spreading; the scale (Fig. 4) continents around thebeenAtlantic.madeIncompatiblethis proximatelywith the 1depthcentimeterargon perdatesannumfrom Arran, Mull, and a simulated profile based on a rate has recently received striking independ- instance the deep to tothethesoutheastridge crestof in -thatthis areais, andthe withrate of other"drifting"centersis ap-in the British Isles sug- of spreading of 2.9 centimeters per ent confirmation from the work of Rockall may representthean altitudeinitial abor-at whichproximatelythe survey2 centimeterswas gest anperageannum.of approximately on January 8, 2012 60 inil- annum per limb of the spreading sys- Opdyke et al. (20) on deep-sea sedi- tive split; the oceanicflown.areaIn toperformancethe of the survey, lion years (perhaps slightly greater) tem. A profile across the East Pacific mentary cores. northwest, centered on 58Reykjanesparallel Ridge,courses were flown normal (22). If it is assumed that this igneous Rise, just north of the Eltanin Frac- a subsequent and moreto thepersistentridge axis,site butOtherthe crestRidgeswas not activity indicates the initiation of drift ture Zone (23), is compared (Fig. 7) of spreading of the oceantraversedfloor. byTherethe first four and last in this area, then the implied average with a computed profile based on a Reykjanes Ridge is every indication fromfive courses:the existingthus crossingsThe model15, 25, proposed35, ratebyofVinespreadingand from Reykjanes rate of spreading of 4.4 centimeters bathymetry (17) that andthe45ridgeare showncrest asMatthewsbeing representa-(6) and developed by Vine Observed anomaly profiles obtained is linear and not interruptedtive. The orcorrelationoff- andbetweenWilsonthe(8)ob-has been applied to Jaramillo Olduval during four crossings of the crest of set transverse four areas on by fractures. widely separated the www.sciencemag.org served and computed anomalies is very BRUNHES I 4- GAUSS Reykjanes Ridge are compared (Fig. 5) area system This therefore,encouraging1200 kilometersand suggestsmidoceanica rateridgeof NORMAL(Figs.FIELD6-9) ZZA W] VA rz //////------r i i A A with simulations obtained by assump- in width, may well recordspreadinga compara-of rather bylessassumptionthan 1 centi-of the reversal time N I r//, 1 ----. tion of reversal time scales for the last tively simple and straightforwardmeter per annum.ex- scale shown in Fig. 4 K-Arand AGEa rate0 1I0 2-0 3-0 4.0 M. YRS 4 million years and a rate of spreading ample of drifting and spreading.When oneTheappliesof spreadingthe conceptcompatibleof with the width REVERSED FIELD | / of 1 centimeter per annum for each oldest rocks in the Thuleancontinentalor Brito-drift to thisof theregion,centralit seemsanomaly. An observed ////r A /Z - limb of the ridge. The model assumed is Arctic Tertiary Igneousreasonableprovincetooc-assumeprofilethatacrossRockallJuan de Fuca Ridge, < MATUYAMA > | eGILBERT analogous to the one I have described cur in northwestern ScotlandBank, southeastand east- of southwestthe ridge (Fig.of Vancouver2), Island (Fig. Mammoth (8, model 2), but the depths have ern Greenland. Preliminaryis a continentalpotassium- fragment,1) (8, 19),as wasis comparedas- Fig.(Fig.4. Geomagnetic-polarity6) with epochsDownloaded from deduced from paleomagnetic results and potassium- been made compatible with the depth argon dates from Arran,sumed Mull,by Bullard,and Everett,a simulatedand Smithprofile basedargon ondating.a rate[Based on Cox, Doell, and Dalrymple, and Doell and Dalrymple (7)1 to the ridge crest in this area and with other centers in the British16 DECEMBERIsles sug-1966 of spreading of 2.9 centimeters per 1407 the altitude at which the survey was gest an age of approximately 60 inil- annum per limb of the spreading sys- flown. In performance of the survey, lion years (perhaps slightly greater) tem. A profile across the East Pacific 58 parallel courses were flown normal (22). If it is assumed that this igneous Rise, just north of the Eltanin Frac- to the ridge axis, but the crest was not activity indicates the initiation of drift ture Zone (23), is compared (Fig. 7) traversed by the first four and last in this area, then the implied average with a computed profile based on a five courses: thus crossings 15, 25, 35, rate of spreading from Reykjanes rate of spreading of 4.4 centimeters and 45 are shown as being representa- tive. The correlation between the ob- Jaramillo Olduval served and computed anomalies is very BRUNHES I 4- GAUSS encouraging and suggests a rate of NORMAL FIELD ZZA W] VA rz //////------r i i A A spreading of rather less than 1 centi- N I r//, 1 ----. meter per annum. K-Ar AGE 0 1I0 2-0 3-0 4.0 M. YRS When one applies the concept of REVERSED FIELD | continental drift to this region, it seems ////r A //Z - reasonable to assume that Rockall < MATUYAMA > | eGILBERT Bank, southeast of the ridge (Fig. 2), Mammoth is a continental fragment, as was as- Fig. 4. Geomagnetic-polarity epochs deduced from paleomagnetic results and potassium- sumed by Bullard, Everett, and Smith argon dating. [Based on Cox, Doell, and Dalrymple, and Doell and Dalrymple (7)1 16 DECEMBER 1966 1407 T.H. Torsvik et al. / Earth and Planetary Science Letters 291 (2010) 106–112 107 we describe a single important example, namely the calculation of net Geochem. Geophys. Geosyst.). In Section 3 we explore NR, not only for lithosphere rotation (NR). If mantle convection is the principal driving present times but for the past 150 Ma. mechanism for plate motions, NR should be zero unless individual lithospheric plates have different couplings to the underlying mantle 2. Global plate polygons flow. A proper reference frame with appropriate NR is important for discussions of poloidal/toroidal partitioning of plate motions (Lith- Building global plate polygons through Earth history (Fig. 1; gow-Bertelloni et al. 1993). Most plate models predict westward drift Supplementary data) requires knowledge of relative plate motions of the lithosphere with respect to the deep mantle, which has been from both continental and oceanic areas. The uncertainty in ascribed to lateral viscosity variations (Ricard et al. 1991; O'Connell constraining these motions increases for older times, due to the et al., 1991). Westward drift estimates vary considerably (1.5–9 cm/ destruction (through subduction) or distortion (such as collision) of year) and are usually larger than those calculated from geodynamic relative motion. For example, more than half of the oceanic crust models (Becker 2006). However, comparison of westward drift created since the Jurassic has been consumed by subduction, therefore estimates with geodynamic models is problematic, since all geody- past plate boundary configuration has to be restored by making namic models are based on simplifying assumptions. Recently, seismic assumptions based on limited geological constraints (like the age of anisotropy has emerged as a further tool to estimate NR for recent preserved ophiolites or slab-window related volcanism) and the rules times (Becker 2008;Plate Reconstrucons Kreemer 2009; Conrad and Behn, submitted of plate tectonics. World uncertainty — the fraction of the Earth's

Torsvik et al. 2010

Fig. 1. Global plate reconstructions and plate polygons (red lines) at 10, 60, 100 and 150 Ma. Dominantly Oceanic plates are shaded blue. Absolute velocity fields are projected 5 My forward from the re-constructed age. Exaggerated (brown) arrows show the generalized velocity pattern. WU=world uncertainty. We also show as black lines the continental part of the plates, mostly present coastlines and intra-plate boundaries that were active at various times through the Phanerozoic. Extended continental margins are not distinguished. NAM=North America, EUR=Europe (Eurasia), IND=India, AFR=South Africa, NWA=Northwest Africa, NEA=Northeast Africa, SAM=South America, PAT=Patagonia, MB=Marie Byrd Land, AUS=Australia, ANT=East Antarctica, GRE=Greenland, PAC=Pacific, FAR=Farallon, COC=Cocos, PHO=Phoenix, KUL=Kula, CAR=Caribbean, BUR=Burma, PHI=Philippine, LHR=Lord Howe Rise. Mollweide projection. NA- EU Spreading Rates

Eysteinsson and Gunnarsson 1995 6 P. Mihalffy et al. / Tectonophysics 447 (2008) 5–18

This model succeeded to predict the orientation and age pro- land–Iceland ridge (Fig. 1) from crustal isostacy studies by Kaban gression of the Faroe-Greenland and the Vøring plateaus, as- et al. (2002). Under the Northern Volcanic Zone of Iceland the suming an interaction radius of at least 700 km. The lithospheric FIRE experiment constrained a crust of 19 km and at the older thickness increases away from ridges due to lithospheric cool- Tertiary areas of north-eastern Iceland a 35 km thick crust. The ing. This forms a negative drainage pattern for buoyant material pronounced thickened crust of the Greenland–Iceland–Faroe under the lithosphere, which could be of importance for con- ridge is suspected to be a geologic record of the interaction of the trolling plume head motion (Kincaid et al., 1996). Ribe and mid-Atlantic ridge with the Iceland plume (White, 1997). Using Delattre (1998) find that interaction of plume and ridge is locally-recorded earthquake data Menke and Levin (1994) have stronger when they move away from each other than towards investigated the crustal structure of central Iceland. The each other. experiment shows a crustal thickness of 22 km. As the Iceland plume might have affected the region north of present-day 1.2. Geophysical indication of the Iceland plume Iceland, it is of interest to assess crustal thickness anomalies in this region. While the crust around the Kolbeinsey ridge (Fig. 1) has Location of the Bouguer anomaly minimum and the thickest almost normal oceanic thickness (Kodaira et al., 1997), the crust from receiver function analysis (Thorbergsson et al., 1993; aseismic south Jan Mayen ridge (Fig. 1) between Iceland and Jan Shen et al., 2002) indicate a present-day location of the Iceland Mayen has an anomalous crust of 20 km (Evans and Sacks, 1979), plume at 18° W, 64.4° N. The crust of Iceland and the aseismic and is generally regarded as a continental fragment. Here, the total Faroe–Iceland ridge (Fig. 1) has been extensively studied by lithospheric thickness is 50 km, which compensates isostatically seismic methods (Menke and Levin, 1994; Staples et al., 1997; the thick crust (Evans and Sacks, 1979). On the mid-Norwegian Brandsdottir et al., 1997, Smallwood, 1999). passive margin the oceanic crust in the southern area is thicker The FIRE experiment (Faroe–Iceland Ridge Experiment; than in the northern part, possibly because it was generated by Staples et al., 1997) has investigated the crust and the upper hotter material (Kodaira et al., 1995). mantle along the aseismic Faroe–Iceland Ridge between Faroe Islands and the present-day spreading center. They found that 1.3. Mantle flow in the North Atlantic the crust along the Faroe–Iceland ridge is 25–30 km thick (Smallwood, 1999), which indicates, that the mantle potential Given appropriate rheological assumptions, the flow struc- temperature is elevated by 200–250° C above normal. Similar ture of the mantle can be calculated based on density anomalies crustal thickness (25–35 km) has been inferred forHot Spot! the Green- derived from seismic tomography and surface plate velocities

Fig. 1. Bathymetry and ridges in the North Atlantic. Mihalffy et al. 2008 Hot Spot Tracks

S.M. Howell et al. / Earth and Planetary Science Letters 392 (2014) 143–153 145

Fig. 2. Plate reconstructions and paleo-basement depth (Müller et al., 2008)showingthetectonicevolutionofthestudyarea.Filledcirclesmarkestimatedcenterofhotspot relative to Greenland by Lawver and Müller (1994) (red) and Mihalffy et al. (2008) (purple). Dashed circles show corresponding (like colors) areas of influence of the Iceland plume for perfectly circular plume pancakes when the Aegir Ridge became extinct 25 Ma, based on the distance to the rough–smooth boundary in seafloor fabric created ∼ at the Reykjanes Ridge at 25 Ma. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Howell et al. 2014 hotspot influence. (2) Variations in lithospheric thickness, includ- ing the conduit-like, “inverted troughs” which form beneath the ridge axes, and thick lithosphere of the JMMC, promote plume expansion SW along the RR and impede plume expansion NE to the AR. To test the above hypotheses about the comparatively restricted hotspot influence in the AR basin and to address the cause of asymmetric Iceland hotspot influence overall, we use 3-D numer- ical models that simulate a plume interacting with rifting conti- nents and spreading ridges. The models simulate ridge geometries and spreading rates based on geological estimates from the time of continental breakup until present-day. Varied model parameters are plume volume flux, mantle viscosity, and rheology of the litho- Fig. 3. Modeled spreading rate evolution for the North Atlantic. Geological estimates of the spreading rates at MR (Breivik et al., 2009; Voss et al., 2009), RR (Smallwood sphere, which controls the structure of the lithosphere. In one set and White, 2002), AR (red, Breivik et al., 2006), and KR (yellow, Mosar et al., 2002) of models, the lithosphere corresponds to the cool thermal bound- were averaged to create a mean North Atlantic spreading rate through time (black). ary layer near the surface. In another set of models, the rheology Since 33 Ma, spreading rates by Mosar et al. (2002) for all four ridges are incorpo- is controlled by water content, and partial melting removes water rated. The mean spreading rate (black) was used to model all of the active ridges from the solid leaving a stiff, dehydrated lithosphere, independent at times when the geological estimates of their spreading rates were very simi- lar (deviating by <2mm/yr). However, during times marked by shaded bands, the of the thermal boundary layer. We quantify the effects of the above model Aegir and Kolbeinsey Ridges were assigned the low rates defined by the variables on the asymmetry of a plume interacting with the ridge. geological estimates, and the model Reykjanes and Mohns Ridges shared the same Finally, we compare model predictions with observations to infer (faster) spreading rate, determined by the average (black) of their individual geolog- the volume flux of the Iceland plume and rheology of the litho- ical rates. The blue line shows the time evolution of seismically measured crustal thickness across the AR (Profile 1-03, Breivik et al., 2006). (For interpretation of the sphere. references to color in this figure legend, the reader is referred to the web version of this article.) 2. Methods in the north without any prominent transform faults, in con- 2.1. Model setup trast to the rougher seafloor to the south, created by orthogonal spreading along segments separated by transform faults (Vogt We employ Citcom, a finite element code widely used to sim- and Avery, 1974; White, 1997). This “rough–smooth” boundary ulate mantle convection (e.g. Moresi and Gurnis, 1996; Zhong et is interpreted to delineate the maximum extent of plume in- al., 2000). Citcom solves the equations describing conservation of fluence along the RR, and was 600–1200 km SSW of the Ice- mass, conservation of momentum, and conservation of energy in a land hotspot. The same distance northeast from the projected Cartesian coordinate system for a fluid with zero-Reynolds number hotspot center would encompass a large portion, if not all, of and infinite Prandlt number (see supplementary material). The ex- the AR (Fig. 2). The lack of thickened crust along most of the tended Boussinesq approximation is used to simulate the adiabatic AR during all but the first 2–3 Myr of its spreading history in- temperature gradient and latent heat loss due to melting (Bianco dicates that the plume influence was asymmetric starting near et al., 2011). Model dimensions are 2400 2800 400 km, with × × 49–47 Ma, influencing the RR further SSW than the AR to 289 257 65 elements of size 8 11 6 km in the x, y,andz ∼ × × × × the NE. directions, respectively (Fig. 4). When considering the tectonic evolution, two hypotheses can The structure of the stiff part of the plate, or lithosphere, beformulatedastothecauseofthelong-termasymmetryinthe is controlled by the rheology. One set of models simulates a Iceland hotspot. (1) The asymmetric geometry of the ridges rela- “thermal lithosphere”, which develops because viscosity varies tive to each other and to the hotspot center leads to asymmetric as a standard Arrhenius function of temperature and pressure Hotspot Tracks (Method) 120 T. Thordarson, G. Larsen / Journal of Geodynamics 43 (2007) 118–152

Fig. 1. Iceland is an elevated plateau in the middle of the North Atlantic, situated at the junction between the Reykjanes and Kolbeinsey Ridge segments. Also shown: the axis of the Mid-Atlantic Ridge (heavy solid line), the North Atlantic basalt plateau (black) and their submarine equivalents (dark grey). The line with the dots shows the positionThordarson of the Iceland mantle and Larson 2007 plume from 65 million years to the present day. Modified after Saunders et al. (1997).

Gudmundsson, 2000). Of those the most prominent belt is the axial volcanic zone, the loci of active spreading and plate growth that follows the plate boundary across Iceland from Reykjanes in the southwest to Oxarfj¨ or¨ ður in the north (Fig. 2). The axial zone is typified by tholeiitic magmatism and its core structures are the West (WVZ) and the North (NVZ) Volcanic Zones, which are joined by Mid-Iceland Belt (MIB) and linked to the Mid-Atlantic Ridge system by the Reykjanes Volcanic Zone (RVZ) in the south and the Tjornes¨ Fracture Zone (TFZ) in the North. The East Volcanic Zone (EVZ) is an axial rift in the making that eventually will take over from the West Volcanic Zone. It is dominated by tholeiitic magmatism in the northeast (Fig. 3a), whereas mildly alkalic magmatism characterizes the currently propagating southwest segment of the zone (e.g. Jakobsson, 1979a; Sæmundsson, 1979; Gudmundsson, 1995a). Its construction has taken place by southwest propagation of volcanism through pre-existing crust and at present it is the most volcanically active region in Iceland (Fig. 2). There are also two active intraplate volcanic belts of mildly alkalic magmatism in Iceland. The Oræfi¨ Volcanic Belt situated to the east of the current plate margins, which may represent an embryonic rift (Thordarson and Hoskuldsson,¨ 2002). The Snæfellsnes Volcanic Belt in west Iceland, an old rift zone reactivated about 2 Ma and is currently propagating to the east-southeast letters to nature

...... The ¯ux variations generate pulses in the plume that rise up the stem and expand outward beneath the boundary at which viscosity Reykjanes `V'-shaped ridges increases due to dehydration. Figure 2 shows temperature and ¯ow at three time steps of a calculation in which the plume ¯ux varies originating from a pulsing 2 between 6:5 6 4:9† 3 106 km3 Myr 1. At a time of 2 Myr after the and dehydrating mantle plume initiation of pulsing, the plume has reached its maximum radius. Because the stem is wider than the steady-state model, a greater Garrett Ito volume of hot material ¯ows more rapidly up the plume stem. At positions near the outer portions of the stem, material is hotter than Department of Geology, University of California, Davis, California 95616, USA in the steady-state model because of the greater plume width. Temperatures at the centre of the stem are unchanged. The leading ...... portion of the pulse has already struck the dehydration boundary Prominent crustal lineations straddle the Reykjanes ridge, south and has begun to expand away from the plum stem as an annulus of of Iceland (Fig. 1). These giant V-shaped features are thought to hotter-than-normal material. record temporal variations in magma production at the Reykjanes At 4 Myr the basal radius of the plume stem has returned to the ridge axis, associated with along-axis ¯ow of Icelandic plume steady-state value and the pulse is now expanding away from the material1. It has been proposed that this ¯ow is channelled plume stem beneath the dehydration viscosity boundary (Fig. 2c). preferentially along the ridge axis2±4, and that temporal variability Compared to the steady-state condition, the 1,450 and 1,500 8C is induced by ¯uctuations of the Iceland plume itself1,4±7 or, isotherms extend a greater radial distance from the centre of the alternatively, by relocations of the ridge axis on Iceland8. Here I plume, indicating hotter material in these regions. Temperature present a geodynamic model that predicts the formation of crustal perturbations are greatest (the maximum is +56 8C) at radial V-shaped ridges from a pulsing and radially ¯owing mantle distances of ,75±375 km. A cross-section at a depth of 135 km plume. In this model, plume pulses produce mantle temperature shows that temperature perturbations diminish with distance away perturbations that expand away from the plume in all directions from the ridge axis. This is a result of conductive cooling beneath a beneath the zone of partial melting. The melting zone has a high plate of increasing age. viscosity owing to mantle dehydration at the onset of partial Finally, at 7 Myr the plume stem is in a contracted form (Fig. 2d). melting9. This high-viscosity region allows for reasonable varia- Material surrounding this thinner plume stem is cooler than in the tions in crustal thickness, produces crustal Vs that extend hun- steady-state model and an annulus of cooler-than-normal material dreds of kilometres along the axis, and prevents the plume is expanding away from the stem beneath the dehydration bound- material from being preferentially channelled along the ridge ary. The original, high-¯ux pulse is now 300±500 km away from the axis. The angle of the crustal V-shaped features relative to the plum stem, though the magnitude of temperature perturbations has Geochemistry ridge axis re¯ects the rate of lateral plume ¯ow, which remains Geophysics 3 BENEDIKTSDÓTTIR ET AL.: TECTONICS OF THE RR PAST 15 MA 10.1029/2011GC003948 several times greater than the ridge half-spreading rate over the Geosystems G length of a crustal V. Consequently, this radially expanding plume V-shaped Ridges produces lineations in crustal thickness and free-air gravity anomalies that appear to be nearly straight. This model builds on previous three-dimensional numerical simulations of a ridge-centred plume10,11. Mantle convection is simulated by solving the equations describing conservation of mass, momentum and energy in a ¯uid of zero Reynolds number and moderately temperature-dependent viscosity12. An essential feature of this model is an increase in viscosity by a factor of 100 near the base of the primary melt production zone. Such an increase in viscosity is likely to occur as water is extracted from the mantle at the very early stages of decompression partial melting9. The mantle ¯ow and temperature distribution of a steady-state, ridge-centred plume is shown in Fig. 2a. Low-viscosity and ther- mally buoyant plume material rises beneath the ridge axis, spreads laterally beneath the high-viscosity melting zone, and attains a steady-state, along-axis width10,11,13,14. In the overlying melting zone, high viscosities minimize along-axis and vertical ¯ow, and upwelling occurs primarily to accommodate the spreading of the two plates. This slow, passive upwelling allows a plume of relatively high excess temperature and narrow radius15,16 to produce crustal thicknesses11 that are consistent with seismic observations on Iceland (for example, see refs 17 and 18). I now vary the volume ¯ux of upwelling plume material as a periodic function in time. This is done by varying the imposed radius of the plume stem about the steady-state value of 100 km. The period of variation of 8 Myr is in the range of 5±10 Myr estimated for the age contrasts between adjacent V-shaped ridges 1,4,19 in the North Atlantic Ocean . A possible cause for such ¯ux Figure 13. Satellite gravity and tectonic boundaries near Iceland [Sandwell and Smith, 2009]. Oblique Mercator perturbations is the surfacing of solitary waves in the Iceland mantle projection. Pseudofaults and failed rifts predictedBenediktsdor by our magnetic et al 2012 models are shown; solid lines connect the pseudo- Figure 1 Satellite-derived free-air gravityIto 2001 anomalies in region surroundingfaults Iceland of southward27. propagating rifts, dashed lines connect pseudofaults of northward propagators, and red dots are plume. Solitary waves have been shown to be stable features that failed rifts. Heavy dashed line is the Reykjanes Ridge and its extension up to Iceland; dash-dotted lines are the locations initiate from perturbations in ascending plumes of buoyant viscous Reykjanes ridge is the linear high (bold line) that bisects V-shaped featuresof pointing the paleo-spreading centers in Iceland and dotted lines are an attempt to trail the paleo-spreading centers down to 20,21 southward from Iceland. Similar though less regular lineations are also evidentour survey near area. the Numbers indicate the location of our profiles (17–25). V, Vestfirðir; S, Snæfellsnes; R, Reykjanes ¯uid . The maximum temperature anomaly of the plume stem Peninsula; L, Loki; F, Fenrir; S, Sleipnir; H, Hel. remains constant. Kolbeinsey ridge to the north.

area to the south where the A-scarp is much more observation is that the pattern of the free air gravity NATURE | VOL 411 | 7 JUNE 2001 | www.nature.com © 2001 Macmillan Magazines Ltd linear. Secondly,681 a circular structure interpreted on the ridge flanks changes drastically on profiles as a central volcano [Höskuldsson et al., 2010] 17 and 18. A long gravity low along profile 18 on (centered on 63° 10′ N and 25° 30′ W), is apparent North America is present and the gravity ridges on from the free air gravity at the ridge axis in profiles the Eurasia side are not detectable. These observa- 17 and 18 (Figure 13) which differs considerably tions suggest a different and a more complex crustal from the southern profiles where the free air gravity accretion process along profiles 17 and 18 com- shows lineations subparallel to the ridge. A third pared to the profiles to the south.

22 of 27 Plate Boundaries in Iceland:

Volcanoes and Faults Iceland rifts and transforms

Einarsson 2008

Figure 2. Earthquake epicenters 1994–2007 and volcanic systems of Iceland. Volcanic systems and active faults are from Einarsson and Sæmundsson (1987). Epicenters are from the data bank of the Icelandic Meteorolog- ical Office. Individual plate boundary segments are indicated: RPR Reykjanes Peninsula Rift, WVZ Western Volcanic Zone, SISZ South Iceland Seismic Zone, EVZ Ea stern Volcanic Zone, CIVZ Central Iceland Volcanic Zone, NVZ Northern Volcanic Zone, GOR Grímsey Oblique Rift, HFZ Húsavík-Flatey Zone, ER Eyjafjarðaráll Rift, DZ Dalvík Zone. SIVZ South Iceland Volcanic Zone. Kr, Ka, H, L, V mark the central volcanoes of Krafla, Katla, Hengill, Langjökull, and Vestmannaeyjar. – Upptök jarðskjálfta 1994–2007, misgengi og eldstöðvakerfi áÍslandi.Skjálftaupptökerufengin frá Veðurstofu Íslands.

determined by the length of the plate velocity vec- of fracturing and volcanism is pervasive as seen in tor and its degree of obliqueness along the segment. the Northern Volcanic Zone (NVZ) in North Iceland Some segments are purely divergent. In these seg- and the two sub-parallel rift zones in South Iceland, ments normal faulting and fissuring are the main types the Western and the Eastern Volcanic Zones (WVZ

JÖKULL No. 58, 2008 37 Plate Boundaries in Iceland T. Thordarson, G. Larsen / Journal of Geodynamics 43 (2007) 118–152 121

Fig. 2. The principal elements of the geology in Iceland, outlining the distribution of the major geological subdivisions, including the main fault structures and volcanic zones and belts. RR, Reykjanes Ridge; RVB, Reykjanes Volcanic Belt; SISZ, South Iceland Seismic Zone; WVZ, West Volcanic Zone; MIB, Mid-Iceland Belt; EVZ, East Volcanic Zone; NVZ, North Volcanic Zone; TFZ, Tjornes¨ Fracture Zone; KR, Kolbeinsey Ridge; OVB,¨ Oræfi¨ Volcanic Belt; SVB, Snæfellsnes Volcanic Belt. Modified from Thordarson and Hoskuldsson¨ (2002). Thordarson and Larson 2007

(e.g. Gudmundsson, 2000). Collectively, the regions of active volcanism cover 30,000 km3 or about one third of Iceland. ∼

2.3. Volcanic structures in Iceland

The volcanic system can be viewed as the principal geological structure in Iceland (Fig. 3). It is characterized by conspicuous volcanotectonic architecture that features a fissure (dyke) swarm or a central volcano or both and has a typical lifetime of 0.5–1.5 million years (e.g. Jakobsson et al., 1978; Jakobsson, 1979a; Sæmundsson, 1978, 1979). The fissure swarms of each system are elongate structures that normally are aligned sub-parallel to the axis of the hosting volcanic zone. The central volcano, when present, is the focal point of eruptive activity and typically the largest edifice within each system (Fig. 3). The first to identify the clustering of volcanic and tectonic surface structures that now are 696 T. Arnad´ ottir´ Acve Plate Boundary in Iceland et al. Downloaded from http://gji.oxfordjournals.org/

Figure 3. Horizontal GPS station velocities with 95 per cent confidenceÁrnadór et al. 2009 ellipses, relative to stable North America, determined from the ISNET measurements spanning the time interval 1993–2004 (black arrows) and CGPS stations in Iceland for the time interval 1999–2004 (red arrows). The predicted velocityof Eurasia relative to stable North America from the NUVEL-1A plate motion model (DeMets et al. 1994) is shown with green arrows.

In southern Iceland, a significant increase in horizontal veloci- dications of local deformation processes are observed near active ties is evident across the Reykjanes Peninsula, the SISZ and the volcanoes. The southward motions of the CGPS stations south of EVZ. This confirms conclusions from prior studies, indicating that Myrdalsj´ okull¨ have been taken as a sign of magma accumulation in the plate motion is primarily accommodated across these parts of Katla (e.g. Sturkell et al. 2008). The ISNET station located between at California Institute of Technology on June 27, 2014 the plate boundary in southern Iceland (e.g. Sigmundsson et al. the two CGPS stations south of Katla, however, does not show a sim- 1995; Hreinsdottir´ et al. 2001; LaFemina et al. 2005; Arnad´ ottir´ ilar trend. This velocity difference between the ISNET and CGPS et al. 2006). The stations near the western edge of the active rift stations could be attributed to surface deformation caused by pro- zone move towards the zone, relative to the North American plate, posed magma accumulation at shallow depth at Katla in the time because these stations are located within the plate boundary zone, interval spanned by the CGPS observations (1999–2004), as the and hence do not move at the full plate rate. A gradual increase in CGPS stations are located closer to the volcano than the ISNET sta- the eastward component of the velocity is apparent across southern tions, and their time series cover the short time interval suggested central Iceland, in the proposed Hreppar block, i.e. the area between for the Katla inflation (Pinel et al. 2007; Sturkell et al. 2008). The the two overlapping rift zones (the WVZ and the EVZ). We do not large eastward velocity at station 0500 (at Gr´ımsvotn)¨ is most likely estimate the Euler vector for the Hreppar block as there are only caused by magma accumulation at shallow depth below the volcano, five ISNET stations located on the block, and their velocities are as the ISNET measurements span the 1998 eruption and inflation potentially affected by post-seismic deformation following the 2000 periods before and after the eruption (Sturkell et al. 2003). A small June earthquake sequence. Further studies are needed to determine eastward component of motion is also observed at station 7384 lo- whether the area of the proposed Hreppar block is truly a rigid mi- cated NE of Krafla, which may be an indication of deep magma croplate and, if so, the nature of the MIB, that forms the northern accumulation (de Zeeuw-van Dalfsen et al. 2004), or post-rifting boundary of that plate, or whether the area is a sliver of crust that deformation. is presently caught between the two overlapping rift zones in south Iceland, and will eventually become part of the North American plate as the WVZ becomes extinct. To examine possible local deformation in the eastern part of Ice- 3.2 Vertical velocities land, we estimate the horizontal ISNET velocities relative to stable We estimate the average vertical velocities for the 1993–2004 time Eurasia by transforming the station velocities using the absolute interval, in the ITRF2005 reference frame as described above. Fig. 5 rotation pole for the motion of Eurasia in the ITRF2005 determined shows a broad zone of uplift in central Iceland, with a maximum 1 by Altamimi et al. (2007). In this reference frame stations moving of about 23 mm yr− at the CGPS station SKRO. The vertical rates with the Eurasian plate should have small velocities. Fig. 4, how- decay towards southwest and northwest, while they remain high ever, shows that several stations have significant residual velocity along the southeast coast. The broad pattern of uplift in central relative to the Eurasian plate. In particular, an interesting, nearly ra- and southeast Iceland as well as the subsidence on the Reykjanes dial pattern of horizontal motion decreasing away from Vatnajokull¨ PeninsulahavebeennotedinpreviousGPSstudies(e.g.Hreinsdottir´ is evident in the eastern part of Iceland. This pattern indicates de- et al. 2001; Sjoberg¨ et al. 2004; Arnad´ ottir´ et al. 2006; Geirsson formation that is not predicted by plate motion models, and hence, et al. 2006; Pagli et al. 2007a), although those observations cover a most likely due to processes other than plate spreading. Other in- smaller area than the ISNET network. The uplift around Vatnajokull¨

C " 2009 The Authors, GJI, 177, 691–716 C Journal compilation " 2009 RAS Past Plate Boundaries in Iceland

Ivarsson 1992 Fig. 2. Crustal accretion, relocation and propagation of the Icelandic rift zones in the last 12 Ma (numbers in Ma). The panels show map views for 8, 6, 4, 2 and 0 Ma. The 8 Ma panel shows the spreading along the Snæfellsnes and Skagi rift zones. The 6 and 4 Ma panels demonstrate the incipient propagation and mature development of the Western and Northern Rift Zones after the new rift initiation at about 7 Ma. The 2 and 0 Ma panels show the southward propagation of the Eastern Rift Zone, initiated at about 3 Ma. Based on data from Sæmundsson (1979) and Jóhannesson (1980) and a synthesis by Ivarsson (1992).

The Icelandic volcanic systems, rift zones and off-rift volcanic zones The currently active volcanic systems in Iceland are shown in Fig. 3 (e.g. Sæmundsson, 1979; Einarsson, 1991; Jóhannesson and Sæmundsson, 1998). The 40-50 km wide rift zones (Reykjanes, Western, Eastern and Northern Rift Zones) comprise en echelon arrays of volcanic fissure swarms, with 3-4 semi-parallel swarms across the rift zone width. The swarms are 5-15 km wide and up to 200 km in length. With time, they develop a volcanic centre with maximum volcanic production somewhere along their length. The volcanic centres will often develop into central volcanoes with high-temperature geothermal systems, sometimes also with caldera structures produced by large ash-flow eruptions of silicic magma. Each fissure swarm, with or without a central volcano, constitutes a volcanic system. In the non-rifting volcanic flank zones (Snæfellsnes, Eastern and Southern Flank Zones) most of the volcanic centres lack well-developed fissure swarms. The geothermal activity is also generally lower in the off-rift volcanic systems. Past Plate Boundaries in Iceland Older crust underlies Iceland 673 -32˚ -28˚ -24˚ -20˚ -16˚ -12˚ -8˚

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-1000 -1800 ? 12.92 Snaefellsnes ± 0.14 Ma 13 7 Vatnajokull

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23 (54) at California Institute of Technology on June 27, 2014 Ridge

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Figure 1. Map of the Iceland region showing bathymetric contours and tectonic features. Oceanic magnetic anomalies (Nunns 1983) are labelled with anomaly number. Approximate ages in Ma are shown in parentheses after theFoulger 2006 anomaly number on the eastern flank of the Reykjanes ridge. Thick black lines: axes of Reykjanes and Kolbeinsey ridges, thin lines on land: outlines of neovolcanic zones, grey: spreading segments, white: glaciers. WVZ, EVZ, NVZ: Western, Eastern, Northern Volcanic Zones, TFZ: Tj¨ornes Fracture Zone. Individual faults are shown by lines, dotted where uncertain. Dashed lines: extinct rift zones (two in west Iceland and two in east Iceland), WFU: Western Fjords Unconformity. Lavas northwest of this unconformity formed at an extinct rift that lies offshore. Black dots: locations of rocks dated at 16 0.3 Ma and 12.92 0.14 Ma (Moorbath et al. 1968; Ross & Mussett 1976; Hardarson et al. 1997). Line ± ± with arrowheads: the width of oceanic crust predicted to separate the 16 and 13 Ma isochrons, given a 2cma 1 full spreading rate. This is much less than ∼ − the distance between the outcrops, measured in the spreading direction. JMM: Jan Mayen microcontinent.

in the spreading direction and the most easterly and westerly regions exposed rocks in Iceland, but younger than the minimum age of are covered by young lavas and sediment. However, an estimate of 26 Ma deduced above for submerged crust beneath north Iceland. the maximum age of crust there may be made from the width of The EW width of the older, submerged crust thus probably reduces the island. For example, the distance from the WVZ to the volcano to the south. Oraefaj¨okull,measured¨ in the present-day spreading direction, is 200 km (Fig. 1). Part of this crust must have been created at the ∼ EVZ, which formed at 2 Ma. Assuming that subsequent to 2 Ma 2DISTRIBUTIONANDNATURE ∼ half the spreading occurred along the EVZ and half along the WVZ, OF THE OLDER CRUST then 20 km of crust would have formed at each. Of this, the 20 km∼ that formed along the EVZ, plus the 10 km that formed There are two end-member possibilities for the spatial distribution on the eastern flank of the WVZ, will currently contribute to the of the older crust: ¨ crust between the WVZ and Oraefaj¨okull.It then follows that, at (a) It forms a coherent oceanic microplate, analogous to the this latitude, 200 30 170 km formed prior to 2 Ma. If this crust − = Easter microplate, underlying central Iceland. This possibility is formed on the eastern flank of the WVZ and/or its predecessors in suggested by the plate boundary reconstruction of Bott (1985) western Iceland (i.e. on one plate) it would have taken 17 Myr to ∼ (Fig. 2). On the basis of ocean-floor magnetic isochrons and struc- form. This suggests that crust at least as old as 17 2 19 Ma tural arguments, Bott (1985) suggested that at 26 Ma, crustal ¨ + = underlies the Oraefaj¨okullarea. This crust is older than the oldest accretion in the region changed from spreading along∼ a single ridge

C # 2006 The Author, GJI, 165, 672–676 C Journal compilation # 2006 RAS Geology of Iceland

Sigmundsson 2006 Geologic History 1. Terary—Basalc Lava Pile, 10m thick flows, regionally lted 2. Plio-Pleistocene—Subaerial lavas plus hyaloclastes, fluvioglacial/morainic deposits 3. Upper Pleistocene—More extensive hyaloclastes and pillow lavas, lile erosion 4. Postglacial—Fresh flows, pyroclascs and sediments Sigmundsson 2006 Iceland GPS 697 Downloaded from http://gji.oxfordjournals.org/

Figure 4. Horizontal GPS station velocities with 95 per cent confidence ellipses, relative to stable Eurasia, estimated from the ISNET observations for the time interval 1993–2004 (black arrows) and CGPS stations in Iceland for the time interval 1999–2004 (red arrows). The predicted velocity of North America relative to stable Eurasia from the NUVEL-1A plateGlacial Rebound motion model (DeMets et al. 1994) is shown with green arrows. at California Institute of Technology on June 27, 2014

Figure 5. Vertical velocities in the ITRF2005 from ISNET (1993–2004)Árnadór et al. 2009 and the CGPS network in Iceland (1999–2004). Positive numbers indicate uplift and 1 negative are subsidence. Contour lines are drawn every 4 mm yr− .ThereddotsshowtheGPSstationlocations. has been explained by models of glacial rebound due to the thinning Azoneofupliftextendsnorthwards,fromeastcentralIceland of Vatnajokull¨ (e.g. Thoma & Wolf 2001; Sjoberg¨ et al. 2004; along the NVZ. This pattern of uplift in the NVZ is perturbed by Fleming et al. 2007; Pagli et al. 2007a). The extensive area of uplift local subsidence around Askja volcano, appearing as less rapid up- in central Iceland indicates that the rebound due to the melting of lift in Fig. 5. This is in general agreement with previous geodetic the smaller ice caps also needs to be considered when interpreting studies, indicating that the vertical deformation in the NVZ is con- the ISNET measurements. centrated in the areas around Krafla and Askja. Levelling, GPS and

C " 2009 The Authors, GJI, 177, 691–716 C Journal compilation " 2009 RAS Works Cited 1. Kaldal, I. From Þingvellir. hp://www.geothermal.is/30-open-fissures-diverng-plate-margin-thingvellir 2. Vine, F. J. (1966). Spreading of the ocean floor: new evidence. Science,154(3755), 1405-1415. 3. Vogt, P. R. (1986). The present plate boundary configuraon, in: P.R. Vogt and B. E. Tucholke (eds), The Geology of North America, Volume M: The Western North Atlanc Region, Geological Society of America, Boulder, CO, 189-204. 4. Torsvik, T. H., Steinberger, B., Gurnis, M., & Gaina, C. (2010). Plate tectonics and net lithosphere rotaon over the past 150My. Earth and Planetary Science Leers, 291(1), 106-112. 5. Eysteinsson, H., & Gunnarsson, K. (1995). Maps of gravity, bathymetry and magnecs for Iceland and surroundings. Orkustofnun. 6. Mihalffy, P., Steinberger, B., & Schmeling, H. (2008). The effect of the large-scale mantle flow field on the Iceland hotspot track. Tectonophysics, 447(1), 5-18. 7. Howell, S. M., Ito, G., Breivik, A. J., Rai, A., Mjelde, R., Hanan, B., ... & Vogt, P. (2014). The origin of the asymmetry in the Iceland hotspot along the Mid-Atlanc Ridge from connental breakup to present-day. Earth and Planetary Science Leers, 392, 143-153. 8. Thordarson, T., & Larsen, G. (2007). Volcanism in Iceland in historical me: Volcano types, erupon styles and erupve history. Journal of Geodynamics,43(1), 118-152. 9. Ito, G. (2001). Reykjanes' V'-shaped ridges originang from a pulsing and dehydrang mantle plume. Nature, 411(6838), 681-684. 10. Benediktsdór, A., Hey, R., Marnez, F., & Höskuldsson, Á. (2012). Detailed tectonic evoluon of the Reykjanes Ridge during the past 15 Ma.Geochemistry, Geophysics, Geosystems, 13(2). 11. Einarsson, P. (2008). Plate boundaries, ris and transforms in Iceland. Jökull,58, 35-58. 12. Árnadór, T., Lund, B., Jiang, W., Geirsson, H., Björnsson, H., Einarsson, P., & Sigurdsson, T. (2009). Glacial rebound and plate spreading: results from the first countrywide GPS observaons in Iceland. Geophysical Journal Internaonal, 177(2), 691-716. 13. Ivarsson, G. (1992). Geology and petrochemistry of the Torfajokull central volcano in central south Iceland, in associaon with the Icelandic hot spot and ri zones (Doctoral dissertaon, University of Hawaii). 14. Foulger, G. R. (2006). Older crust underlies Iceland. Geophysical Journal Internaonal, 165(2), 672-676. 15. Sigmundsson, F. (2006). Iceland geodynamics: Crustal Deformaon and Divergent Plate Tectonics. Praxis Publishing Limited, Chichester, UK.