Eastern Iceland Research Articles

Crustal modeling of the North Atlantic from spectrally correlated free-air and terrain gravity

We investigate the use of the correlations between free-air and terrain gravity for extending critical, but spatially restricted seismic constraints on the crust of the North Atlantic region. In this tectonically complicated area, we spectrally correlate spherical coordinate free-air gravity anomalies against computed gravity effects of the terrain at 20 km altitude for regional crustal thickness variations. However, at the Mid-Atlantic Ridge, thermal plume, and rift zones of Iceland, our gravity Moho estimates are deeper than the seismic estimates to suggest local reductions of upper mantle density. Invoking thermal expansion, partial melting, and related mantle depletion effectively accounts for the reductions that range up to a few percent of mantle density. The resultant North Atlantic crustal model conforms well to regional seismic constraints. Crustal thickness estimates for Iceland ranging from 25 to 35 km are consistent with seismic predictions of a thickened crust. However, about the plume and rift zones, temperatures approaching the basalt solidus may reach depths up to about 20 km. Continental crust extending from the Jan Mayen Platform to the eastern Iceland Plateau appears to have rifted off eastern Greenland by magnetic chron 7 (ca. 26–28 Ma). A now obscure transform boundary on the Iceland Plateau that may include a rift or fault separating the southeastern Iceland Plateau from the rest of the plateau transformed Reykjanes Ridge spreading from the extinct Aegir Ridge. Thickened crust extends up to 500 km across the Greenland–Scotland Ridge. Prominent necking of our crustal thickness estimates for the Greenland–Iceland Ridge immediately west and east of the Iceland Plateau may reflect the reduction in the strength of the Icelandic Plume at ca. 26–28 Ma. However, our results also map the broad, thickened crust of the Iceland Plateau and prominent Reykjanes Ridge that developed with the resurgence of the Icelandic Plume at ca. 25 Ma.

Asthenospheric channeling of the Icelandic upwelling: Evidence from seismic anisotropy

Two end-member geometries, radial flow and ridge-channeled flow, have been proposed for the dispersion of material upwelling beneath Iceland. Seismic anisotropy provides information on mantle flow, and therefore has the potential to discriminate these two geometries. In this study, we combine the HOTSPOT and SIL datasets (39 stations) and select 28 events for teleseismic shear-wave splitting analysis. Splitting results in central and eastern Iceland show 1–2 s splitting times with an average NNW–SSE orientation of the fast splitting direction and an anti-clockwise rotation of fast axes from east to central Iceland. In western Iceland, smaller splits with more N–S orientations are observed. Since crustal splitting times in Iceland are 0.1 s to 0.3 s, our delays of up to 2 s indicate a mantle source. Both the lack of dependence of the splitting parameters on event back azimuth and the observations of null splits for events where the back azimuth is parallel or perpendicular to the fast splitting directions (observed using other events) suggest that one layer of anisotropy dominates beneath Iceland. While both high stress plus enriched water content and melt-rich layers can result in a 90° rotation of the fast splitting direction with respect to the flow direction, we interpret our fast axis orientation as pointing in the direction of flow as the magnitude of stress is low and the amount and geographical extent of melt are likely small beneath Iceland. The observed anisotropy pattern beneath Iceland is inconsistent with radial flow away from the upwelling. Instead we propose a ridge-channeled flow model in which there is horizontal flow of material away from the upwelling axis beneath southeast Iceland toward the southern end of the Kolbeinsey Ridge and the northern end of the Reykjanes Ridge, both of which are west of the upwelling. This geometry is similar to the ridge perpendicular flow predicted for off-ridge hotspots towards the ridge. We hypothesize that upwelled material then feeds ridge parallel asthenospheric channels beneath the North Atlantic Ridge. Our interpretation is thus consistent with generation of V-shaped ridges by channeling of upwelling material down the Reykjanes and Kolbeinsey ridges.

DESCRIPTION OF A NEW EIMERIA SPECIES AND REDESCRIPTION OF EIMERIA MAYERI (PROTOZOA: EIMERIIDAE) FROM WILD REINDEER RANGIFER TARANDUS IN ICELAND

Altogether, 195 fecal samples of reindeer calves (Rangifer tarandus) were collected from the ground in 3 distinct areas in eastern Iceland, where geographically isolated reindeer populations graze during the summer months. The samples were examined for coccidian parasites. Two species were found, and all infections were monospecific. Eimeria mayeri was found in calves in all sampling areas, with 1-4% prevalence and 450-167,700 oocysts per gram (opg). The sporulated oocyst lacks a polar granule but has an inconspicuous micropyle, and a small Stieda body is present on sporocysts, which are ovoid but not pointed as reported in the original description. The other coccidian, found in single calves in 2 of the 3 areas (prevalence 1 and 4%, 150 and 500 opg, respectively) is described here as a new species. The oocysts are ovoid, average 34.9 x 27.6 microm, and have 2 distinct walls. Wall thickness is approximately 1.9 microm, and the outer wall, approximately 3/4 of total thickness, is generally smooth and appears bicolored. The outermost portion is pale red and the innermost portion yellow to pale brown. The inner wall is grey to dark brown and separated from the outer wall by a dark brown line. Oocysts contain a prominent micropyle, approximately 5 microm, and enclose 4 spindle-shaped sporocysts, slightly pointed at the end opposite the Stieda body. Average size of sporocysts is 18.6 x 9.2 microm. Sporocysts contain granular sporocyst residuum and usually 2, sometimes 1, large refractile bodies in each sporozoite.

Crust–mantle transition and Moho model for Iceland and surroundings from seismic, topography, and gravity data

An increasing data set exists on the nature and thickness of the Iceland crust. This paper relates topography, i.e., elevation and bathymetry (TOP), Bouguer gravity anomalies (BA) and Moho depths to each other to assess the consequences of the “thick crust model” for Iceland in the context of the North Atlantic. Results of regression of TOP and BA vs. Moho depth are converted into “Airy densities” Δ ρ* (mantle crust density contrasts in the case of ideal Airy isostasy). For Iceland, Δ ρ* is very low (105±10 kg/m 3); for the adjacent continental margins and relicts, it is high and intermediate for the Jan Mayen Ridge. The values are affected by lithosphere cooling and systematic variations of internal crustal structure, and reductions for these effects are applied. For Iceland, the thermal reduction leads to slightly enhanced value of Δ ρ* (112±8 kg/m 3), thus part of the observed topography is due to the cooling effect, while the upper crust reduction reduces the value, implying that part of topography is compensated by internal crustal structure. With both reductions combined, Δ ρ* is 103±10 kg/m 3. The differences between gravity- and topography-derived values are compatible with undulation widths similar to Moho depths (half-wavelengths of order 30 km). Regional variations of Δ ρ* values in Iceland suggest differences in crustal generation and evolution. “Modern” Iceland (MI; surface lava ages ≤3.1 Ma) produced from the currently active and recent axes is remarkably homogeneous while the extinct Skagi–Snaefellsnes zone has extremely low values and the older Westfjords and East Iceland regions have slightly enhanced values. The Iceland–Faeroe Ridge (IFR) resembles Iceland (after thermal reduction) while the Greenland–Iceland Ridge has extremely high values. The Iceland “Moho” and the Iceland “lower crust” are special features of ridge–plume interaction and differ from continental Moho and lower crust. Lower crust, only 100 kg/m 3 less dense than uppermost mantle, is assumed to be a plume-generated transitional layer which, directly above the plume centre, belongs rheologically to the asthenosphere. The regionalized regression results are used to construct a new Moho depth map from the gridded topography file. It compares reasonably to other recent maps. A notable feature is a band of crustal thickening along the SE Iceland shelf edge continuing northward across eastern Iceland to the Jan Mayen Ridge. This and other features suggest an evolution of the North Atlantic where a continental edge splinter carried into eastern and perhaps northern parts of Iceland, beside the Jan Mayen Ridge.

Late Holocene solifluction history reconstructed using tephrochronology

Abstract Phases of activity of four solifluction lobes at an altitude of 750–800 m are dated by tephrochronology at Snaefell, central eastern Iceland (64°48′ N 15°33′ E). The sample includes sorted lobes with tread gradients of 3–11° and unsorted (turf-banked terraces) in slope-foot locations. Trenches through lobe fronts reveal detailed internal structures picked out by multiple tephra layers. The tephras V1717, V1477, Ö1362, V870, Hekla-3 (2900 years bp and Hekla-4 (3800 years bp ) provide isochronous surfaces of known age whose deformation and disturbance indicate mass movement and/or cryoturbation of the soil cover. Undisturbed soil including the Hekla-3 tephra indicates an absence of solifluction prior to 2900 years bp . Several centuries after Hekla-3, gravel-rich horizons mark wide-spread frost heave and solifluction of hillslopes. Later stabilization of these lobes allowed the accumulation of aprons of aeolian sediment below lee-side risers. These aprons contain in situ mediaeval tephras, dating the inception of solifluction to a considerable time prior to Norse settlement. The likely period of this first phase of solifluction is the Later Bog Period of the Subatlantic, c. 2500-1000 years bp . The aprons are currently being overridden and deformed by solifluction lobes reactivated in the Little Ice Age.

東部アイスランドの火山岩の地球化学的特徴と1-13 Maの時間変化

東部アイスランドの火山岩の地球化学的特徴と1-13 Maの時間変化

Reproductive cycles of Mytilus edulis L. on the west and east coasts of Iceland

Studies evaluating the reproductive pattern of Mytilus edulis L. were conducted in western and eastern Iceland at two sites at about the same latitude but with different temperature regimes. Mussels were sampled once or sometimes twice a month during two years in Breidifjordur, western Iceland and one year at Mjoifjordur, eastern Iceland. Gonad development was monitored by microscopic observation of thin sections of the gonads. The initiation of gonad development was observed in January in Breidifjordur while in Mjoifjordur some of the animals started developing gonads in October, a month before spawning was over in the population. Spawning started in late June or July, peaked in August and continued until November at both sites. Nutrient reserve stores seemed to be limited and used for initiation of gonad development in winter but were not sufficient for maturation of the gonads. The main growth of the gonads occured in spring in conjunction with phytoplankton blooming and renewal of food resources.

Icelandic-type crust

SUMMARY Numerous seismic studies, in particular using receiver functions and explosion seismology, have provided a detailed picture of the structure and thickness of the crust beneath the Iceland transverse ridge. We review the results and propose a structural model that is consistent with all the observations. The upper crust is typically 7 ± 1 km thick, heterogeneous and has high velocity gradients. The lower crust is typically 15‐30 ± 5 km thick and begins where the velocity gradient decreases radically. This generally occurs at the V p ∼ 6.5 km s −1 level. A low-velocity zone ∼10 000 km 2 in area and up to ∼15 km thick occupies the lower crust beneath central Iceland, and may represent a submerged, trapped oceanic microplate. The crust‐mantle boundary is a transition zone ∼5 ± 3 km thick throughout which V p increases progressively from ∼7.2 to ∼8.0 km s −1 . It may be gradational or a zone of alternating high- and low-velocity layers. There is no seismic evidence for melt or exceptionally high temperatures in or near this zone. Isostasy indicates that the density contrast between the lower crust and the mantle is only ∼90 kg m −3 compared with ∼300 kg m −3 for normal oceanic crust, indicating compositional anomalies that are as yet not understood. The seismological crust is ∼30 km thick beneath the Greenland‐Iceland and Iceland‐Faeroe ridges, and eastern Iceland, ∼20 km beneath western Iceland, and ∼40 km thick beneath central Iceland. This pattern is not what is predicted for an eastward-migrating plume. Low attenuation and normal V p/V s ratios in the lower crust beneath central and southwestern Iceland, and normal uppermost mantle velocities in general, suggest that the crust and uppermost mantle are subsolidus and cooler than at equivalent depths beneath the East Pacific Rise. Seismic data from Iceland have historically been interpreted both in terms of thin‐hot and thick‐cold crust models, both of which have been cited as supporting the plume hypothesis. This suggests that the plume model for Iceland is an a priori assumption rather than a hypothesis subject to testing. The long-extinct Ontong‐Java Plateau, northwest India and )

Azimuthal anisotropy and phase velocity beneath Iceland: implication for plume–ridge interaction

We have determined the pattern of azimuthal anisotropy beneath Iceland from shear-wave splitting and Rayleigh wave tomography using seismic data recorded during the ICEMELT and HOTSPOT experiments. The fast directions of shear-wave splitting are roughly N–S in western Iceland and NNW–SSE in eastern Iceland. In western Iceland azimuthal variations in Rayleigh wave phase velocity show that fast directions are close to the plate spreading direction at short periods of 25–40 s and roughly parallel to the Mid-Atlantic Ridge at longer periods of 50–67 s. Beneath the rift zones in central Iceland, we find a ridge-parallel fast direction at periods of 25–40 s and significantly weaker azimuthal anisotropy at periods of 50–67 s. The 2-D variation of isotropic phase velocity at periods of 33–67 s indicates that the lowest velocities are beneath the rift zones in central Iceland rather than above the plume conduit in southeast Iceland. While ridge-parallel alignment of melt films might contribute to anisotropy above 50 km depth beneath the rift zones, the overall observations are consistent with a model in which plume-influenced, hot, buoyant mantle rises beneath the Mid-Atlantic Ridge at depths greater than 50 km and is preferentially channeled along the ridge axis at the base of the lithosphere beneath Iceland. The shear-wave splitting results are attributed primarily to a N–S mantle flow at depths greater than 100 km.

Shear wave splitting across the Iceland hot spot: Results from the ICEMELT experiment

We report on observations of upper mantle anisotropy from the splitting of teleseismic shear waves (SKS, SKKS, and PKS) recorded by the ICEMELT broadband seismometer network in Iceland. In a ridge‐centered hot spot locale, mantle anisotropy may be generated by flow‐induced lattice‐preferred orientation of olivine grains or the anisotropic distribution of magma. Splitting measurements of teleseismic shear waves may thus provide diagnostic information on upper mantle flow and/or the distribution of retained melt associated with the Iceland mantle plume. In eastern Iceland, fast polarization directions lie between N10°W and N45°W and average N24°W; delay times between the fast and slow shear waves are generally 0.7–1.35 s. In western Iceland, in contrast, the fast polarization directions, while less well constrained, yield an average value of N23°E and delay times are smaller (0.2–0.95 s). We propose that splitting in eastern Iceland is caused by a 100‐ to 200‐km‐thick anisotropic layer in the upper mantle. The observed fast directions in eastern Iceland, however, do not correspond either to the plate spreading direction or to a pattern of radial mantle flow from the center of the Iceland hot spot. We suggest that the relatively uniform direction and magnitude of splitting in eastern Iceland, situated on the Eurasian plate, may therefore reflect the large‐scale flow field of the North Atlantic upper mantle. We hypothesize that the different pattern of anisotropy beneath western Iceland, part of the North American plate, is due to the different absolute motions of the two plates. By this view, splitting in eastern and western Iceland is the consequence of shear by North American and Eurasian plate motion relative to the background mantle flow. From absolute plate motion models, in which the Eurasian plate is approximately stationary and the North American plate is moving approximately westward, the splitting observations in both eastern and western Iceland can be satisfied by a background upper mantle flow in the direction N34°W and a velocity of 3 cm/yr in a hot spot reference frame. This inference can be used to test mantle flow models. In particular, it is inconsistent with kinematic flow models, which predict southward flow, or models where flow is dominated by subduction‐related sources of mantle buoyancy, which predict westward flow. Our observations are more compatible with the flow field predicted from global seismic tomography models, which in particular include the influence of the large‐scale lower mantle upwelling beneath southern Africa. While the hypothesized association between our observations and this upwelling is presently speculative, it makes a very specific and testable prediction about the flow field and hence anisotropy beneath the rest of the Atlantic basin.

Redwing Turdus iliacus migration and wintering areas as shown by recoveries of birds ringed in the breeding season in Fennoscandia, Poland, the Baltic Republics, Russia, Siberia and Iceland

Recovery data from nominate Redwings Turdus iliacus ringed in four breeding zones in Europe and Asia are analysed in 10 wintering watershed areas in Europe, North Africa and Asia. Icelandic Redwing data are analysed separately. Nominate Redwing data are heavily biased, with almost 60% originating in Finland (4% of the world breeding range), but less than 1% from Siberia (60% of the breeding range). Large numbers of autumn passage migrants visit “Schelde” (Belgium/northeast France), and “Po” (North Italy), but few winter in these areas. 78% of recoveries are in “West France”, “South France/West Italy” or “Iberia”. Hunting and habitat adjusted recovery rates indicate that “Gironde” (southwest France) holds the heaviest concentrations of the nominate race. Adult birds are more likely to winter in the more distant areas, Iberia and northwest Africa, Cyprus, the Levant and Transcaucasus. Few ringed nominate Redwings are recovered in Britain or Ireland but adjusted recovery rates suggest these are moderately important wintering areas, where juveniles are more likely to be in England or Wales and adults in Scotland or Ireland. Icelandic Redwings winter almost entirely in Scotland, Ireland, western France and northwest Iberia. Adjusted recovery rates indicate that Ireland is much the most important wintering area for this race. Adults and birds from eastern Iceland are more likely to be recovered in Scotland and Ireland, while first winter birds and those from western Iceland are more likely to be found in France and Iberia. The origin of the large waves of autumn migrants seen in eastern Britain and eastern Spain remains obscure. More ringing is needed to establish the migration and wintering areas of the large Siberian populations.

Crustal structure beneath western and eastern Iceland from surface waves and receiver functions

We determine the crustal structures beneath 14 broad-band seismic stations, deployed in western, eastern, central and southern Iceland, using surface wave dispersion curves and receiver functions. We implement a method to invert receiver functions using constraints obtained from genetic algorithm inversion of surface waves. Our final models satisfy both data sets. The thickness of the upper crust, as defined by the velocity horizon Vs = 3.7 km s−1, is fairly uniform at ∼6.5–9 km beneath the Tertiary intraplate areas of western and eastern Iceland, and unusually thick at 11 km beneath station HOT22 in the far south of Iceland. The depth to the base of the lower crust, as defined by the velocity horizon Vs = 4.1 km s−1 is ∼20–26 km in western Iceland and ∼27–33 km in eastern Iceland. These results agree with those of explosion profiles that detect a thinner crust beneath western Iceland than beneath eastern Iceland. An earlier report of a substantial low-velocity zone beneath the Middle Volcanic Zone in the lower crust is confirmed by a similar observation beneath an additional station there. As was found in previous receiver function studies, the most reliable feature of the results is the clear division into an upper sequence that is a few kilometres thick where velocity gradients are high, and a lower, thicker sequence where velocity gradients are low. The transition to typical mantle velocities is variable, and may range from being very gradational to being relatively sharp and clear. A clear Moho, by any definition, is rarely seen, and there is thus uncertainty in estimates of the thickness of the crust in many areas. Although a great deal of seismic data are now available constraining the structures of the crust and upper mantle beneath Iceland, their geological nature is not well understood.

Concurrent changes of relative sea-level and glacier extent at the Weichselian–Holocene boundary in Berufjordur, Eastern Iceland

A detailed study of raised shorelines and radiocarbon dated sublittoral sediments in the Berufjordur area, Eastern Iceland, revealed a complex relationship between relative sea-level changes and variations in extent of glaciers. The marine limit shoreline at 58 m a.s.l. is correlated with a glacier advance and formation of distinct moraines in late Younger Dryas time. After a considerable retreat of the glaciers and concurrent lowering of relative sea-level, the glaciers readvanced in early Preboreal time and relative sea-level rose and culminated at about 9900 14C yr BP. An extrapolation of a tentative minimum shoreline displacement curve indicates a minimum position of relative sea-level at about −30 m around 8600 14C yr BP. This suggests a total relative sea-level change of about 90 m and a total glacio-isostatic rebound of 120 m, yielding 7.3 cm 14C yr −1 as a minimum rate of uplift between 10,300 and 9900 14C yr BP. The glacier advances in Eastern Iceland are correlated with Younger Dryas and Preboreal expansions of the Icelandic inland ice radiocarbon dated elsewhere to about 10,300 and 9800 14C yr BP, respectively. These glacier advances and concurrent rise of relative sea-level in Iceland coincide in time with changes in North Atlantic oceanic and atmospheric circulation, revealed e.g. from studies of Greenland ice-cores and North Atlantic deep-sea and shelf cores.

Structure of the crust and uppermost mantle of Iceland from a combined seismic and gravity study

We present a map of the depth to the base of the upper crust and the total crustal thickness across Iceland constrained by seismic refraction results, receiver function analysis and gravity modelling. Upper crustal thicknesses (as defined by Ó.G. Flóvenz, J. Geophys. 47 (1980) 211–220) lie in the range of approximately 2–11 km, with the thinnest upper crust below active and extinct central volcanoes and the thickest upper crust close to the flanks of the rift zones. The thickest crust (40–41 km) lies above the centre of the Iceland mantle plume, where active upwelling and high mantle temperatures enhance melt production. Thick crust (∼35 km) is also found in eastern Iceland, between the current plume centre and the Faroe–Iceland Ridge. Elsewhere, the crust thins away from the plume centre. The thinnest crust (≤20 km) is found in the active rift in the northern part of the Northern Volcanic Zone, where melt production has been affected by a ridge jump, and in the far southwest of Iceland. The uppermost mantle below Iceland is characterised by reduced densities below the rift zones, suggesting higher mantle temperatures and the possible presence of partial melt in these regions.

Crust generated above the Iceland mantle plume: From continental rift to oceanic spreading center

Since the North Atlantic continental breakup in the early Tertiary, the process of rifting above a mantle plume has produced large thicknesses of igneous crust. We report results of an integrated offshore‐onshore seismic study of the crust and upper mantle along a transect of the aseismic Faroe‐Iceland Ridge, between the continental fragment beneath the Faroe Islands and the present‐day spreading center in northeast Iceland. Normal‐incidence seismic data provide an image of the uppermost crust, which is complemented by a velocity model from streamer refraction analyses. These data together image four sedimentary basins, up to 400 m deep, along the crest of the ridge. The streamer refraction velocity model in turn forms the uppermost section of a full crustal velocity model derived from travel time modeling of air gun and explosive data. The compressional to shear wave velocity ratio in the crust of the Faroe‐Iceland Ridge is 1.77 ± 0.02, similar to that of eastern Iceland. The Moho beneath the Faroe‐Iceland Ridge lies at a depth of 25–30 km. Crust of 25–30 km thickness requires an upper mantle potential temperature elevated 200–250°C above normal if formed by passive adiabatic decompression melting of the mantle or a somewhat less elevated temperature if, as is likely, there is a component of active convection in the underlying mantle plume core.

Detailed paleomagnetic study of two volcanic polarity transitions recorded in eastern Iceland

Two sequences of 45 and 49 individual lava flows respectively have been sampled in eastern Iceland. The two sections range in age from 12.09 to 10.21 Ma as reported by Watkins and Walker [Watkins, N., Walker, G.P.L., 1977. Magnetostratigraphy of eastern Iceland. Am. J. Sci. 277, 513–584.] and are labelled as profiles C and D. Stepwise alternating field (AF) and thermal demagnetizations accompanied by investigations of rock magnetic properties indicate that the magnetization is primarily carried by titanomagnetite. Demagnetization experiments have identified eight and six transitional lavas, respectively, for these profiles. The transitional virtual geomagnetic poles (VGPs) of the (R-N) profile C reversal are located mainly between Patagonia and Antarctica with a tendency of the virtual poles to move towards west coast of South America, subsequently traveling to the northern hemisphere through several discrete steps located in the middle part of South America, then to west Africa and on to central Asia before the poles settle into normal polarity. The second and younger VGP path corresponding to profile D is a path characterized by a reverse-to-normal-to-reverse (R-N-R) motion of the virtual poles. This path is characterized by poles located in west Antarctica, Patagonia and the western and eastern part of South America. The passage from the southern to the northern hemisphere is also through a discrete sets of steps along the southwestern Pacific followed by a rapid motion to the northern Siberian region, continuing to the western equatorial part of South America before moving on to the central region of Asia, followed by a motion to the western Pacific prior to the final move to the eastern part of Antarctica. The data from eastern Iceland based on highly reliable transitional results, as indicated by the demagnetizations and rock magnetic experiments derived from relatively spaced sites and different ages, seem to indicate that the eastern Icelandic high latitude (66°N) profiles studied have several persistent transitional paleofield features that are uniquely observed at those sites particularly when compared to other volcanic records located at lower latitudes.

Age and magnetism of lavas in Jökuldalur area, Eastern Iceland: Gilsá event revisited

We present results of paleomagnetic measurements and K–Ar age determinations of 38 lava flows collected in five separate sections in the Jökuldalur area of Eastern Iceland, including one section previously studied by Watkins et al. [Watkins, N.D., Kristjansson, L., McDougall, I., 1975. A detailed paleomagnetic survey of the type location for the Gilsá geomagnetic polarity event. Earth Planet. Sci. Lett. 27, 436–444]. These sites are close to the type locality of the normal “Gilsá event” in the Matuyama chron first identified by McDougall and Wensink [McDougall, I., Wensink, H., 1966. Paleomagnetism and geochronology of the Pliocene–Pleistocene lavas in Iceland. Earth Planet. Sci. Lett. 1, 232–236]. Using new experimental results as well as field observations, we could correlate the sequence of lava flows in the Jökuldalur area spanning the time interval between 1.8 and 0.5 Ma. The magnetic polarities and ages obtained in this study are quite consistent with the standard time scale for Brunhes–Matuyama ages given by Cande and Kent [Cande, S.C., Kent, D.V., 1995. Revised calibration of the geomagnetic polarity timescale for the Late Cretaceous and Cenozoic. J. Geophys. Res. 100, 6093–6095] based on marine magnetic anomalies, except that the Gilsá event needs to be added. Existence of Olduvai age lavas under Gilsá was inferred but not certain. Our results are fairly in good agreement with the former studies by Wensink [Wensink, H., 1964a. Secular variation of Earth magnetism in Plio–Pleistocene basalts of eastern Iceland. Geol. Mijnbouw 43, 403–413; Wensink, H., 1964b. Paleomagnetic stratigraphy of younger basalts and intercalated Plio–Pleistocene tillites in Iceland. Geol. Rund. 54, 364–384] and Watkins et al. [Watkins, N.D., Kristjansson, L., McDougall, I., 1975. A detailed paleomagnetic survey of the type location for the Gilsá geomagnetic polarity event. Earth Planet. Sci. Lett. 27, 436–444], but the assignment to the polarity zones is different because of the newly obtained K–Ar ages. Based on the present study, we propose that the Gilsá event is a short normal subchron in the Matuyama chron distinct from and above the more well-established Oldvai subchron.

Porosity evolution and mineral paragenesis during low-grade metamorphism of basaltic lavas at Teigarhorn, eastern Iceland

Low-grade alteration of basaltic lavas at Teigarhorn, eastern Iceland, resulted in three distinct stages of mineral paragenesis that correlate to events in the burial and intrusive history of Icelandic crust. Metasomatism and brittle deformation during the paragenetic stages dramatically affected the paleo- hydrology of the lavas and formed temporally distinct mineral assemblages. The lavas initially contained up to 22 percent total porosity concentrated near the tops and bottoms of individual lava flows. Celadonite and silica (Stage I) precipi- tated along the walls of primary pores prior to deep burial of the lavas and occluded 8 percent of the initial porosity. During burial (Stage II), hydrolysis of olivine and basaltic glass led to the formation of mixed-layer chlorite/smectite clays in the matrix of the lavas and as rims filling 40 percent of the volume of primary pores. Chlorite contents in Stage II mixed-layer clay rims increased from 20 to 80 percent during the infilling of individual vesicles, reflecting increas- ing temperatures with time as the lavas were buried. The end of Stage II occurred after burial and is represented by filling of 40 percent of total primary porosity by zeolites (scolecite or heulandite stilbite mordenite epistilbite) and re- placement of plagioclase by zeolites and albite. The Stage II zeolite assemblages are indicative of two regional metamorphic mineral zones in eastern Iceland, the mesolite scolecite and heulandite stilbite zones. The presence of the bound- ary between the mesolite scolecite and heulandite stilbite zones indicates that the maximum temperature during burial metamorphism was 90° 10°C. Localized areas of intense hydrothermal alteration associated with intrusion of basaltic dikes (Stage III) overprint Stages I and II. Extensive fracturing and hydrothermal brecciation during Stage III added 3 to 11 percent total porosity in which mm- to cm-scale museum-grade crystals of quartz, calcite (Iceland spar), stilbite, scolecite, heulandite, and laumontite precipitated. Estimates of the tem- perature during Stage III (based on fluid inclusion homogenization temperatures in calcite, chlorite geothermometry, and the zeolite assemblage) range from 120° to 180°C. Although the thermobarometric conditions during Stages II and III led to similar mineral assemblages, careful attention to textural and geologic rela- tions permits seemingly complex, multi-stage parageneses in metabasalts to be interpreted in a petrotectonic context.

Compressional and shear velocity structure of the lithosphere in northern Iceland

AbstractSeismic data from the B96 array in northern Iceland are used to constrain the compressional and shear velocity structure of the crust and uppermost mantle in northern Iceland, at the western flank of the northern volcanic zone (NVZ) (the mid-Atlantic plate boundary in northern Iceland). Travel times from P and S waves from shots and microearthquakes north and south of the array are used. Two dome structures, with velocities 6% faster than average, are detected at depths down to 7 km. They may be the fossil roots of extinct central volcanoes. P, S, PmP, and SmS wave travel times indicate that the compressional-to-shear-wave velocity ratio in the crust is 1.75 to 1.76, with no significant variation detected between the mid and lower crust. Near-solidus lower crustal temperatures, previously predicted on the basis of high surface heat flow, are ruled out. The crust is 25 to 31 km thick, with the southward thickening occurring in an abrupt step. The relatively high Pn and Sn wave apparent velocities of 8.00 ± 0.1 km/sec and 4.31 ± 0.04 km/sec, respectively, from an earthquake in southern Iceland, are consistent with a mantle lid, that is, a layer of subsolidus mantle separating the Moho from a deeper, partial melt zone. Travel times from P waves crossing the NVZ from a fan shot in eastern Iceland cross the previously identified Krafla high-velocity dome at a depth of 8 to 10 km. They provide new support for the existence of this mid-crustal feature and indicate that its base extends 40 to 50 km along the strike of the NVZ. The neighboring Þeistareykir central volcano has no dome at 8 to 10 km depth, suggesting that it has had a more subsidiary role in the formation of the lower crust.

Deep crustal reflectors under Reydarfjördur, eastern Iceland:crustal accretion above the Iceland mantle plume

Summary Deep seismic reflection profiling on the Faroe–Iceland Ridge Experiment (FIRE) imaged a sequence of dipping events in the upper to |-crust beneath the Tertiary lava pile in Reydarfjordur, eastern Iceland. These events dip towards the spreading centre at which the crust was formed, and continue down to ∼ 6 s two-way time, corresponding to depths down to 15–18 km. Similar crustal reflections have been observed previously within the crust in other areas of Iceland, although the reflective events located by previous surveys have not been interpreted rigorously. In this study we calculate synthetic seismograms from modelled sequences of lava flows to examine the reflective character of a lava πle. The synthesized lava pile sections are based on field measurements made in the Reydarfjordur area: the 1 km vertical exposures on the sides of the fjord are composed of lava flows within stratigraphic units which have a range of compositions, and some intercalated tuffaceous clastic horizons. We compare the synthetic seismograms with our seismic reflection data to test the interpretation that the deep dipping reflections are caused by a gently dipping sequence of subaerially erupted lavas. We conclude that the upper and mid-crustal reflections that we observe can be interpreted as lava flow sequences similar to those seen at the surface, with the main reflections resulting from packets of flows of similar thicknesses. The reflectors that we have imaged beneath Iceland are similar to those defining the seaward-dipping reflectors on the volcanic continental margins, and we therefore suggest that the seaward-dipping reflections commonly result from packets of lavas with similar distributions of thicknesses. The occurrence of lavas down to 15–18 km beneath Reydarfjordur can be explained partly by multiple episodes of accretion of the upper crust, although the volcanic processes forming the crust in Iceland generally lead to advection of extrusive material to mid-crustal levels. The production of an average extrusive layer totalling 15 km thickness is within reasonable estimates of the current volcanic productivity. The advection of frozen lava flows to mid-crustal levels contributes to the maintenance of the subsolidus crust that is observed on Iceland, and the zeolite minerals within the extrusive sequence may be responsible for the crustal low resistivity zone that has been observed using magnetotelluric techniques.