SW Iceland Research Articles

Orientation Cage Experiments with Dunlins during Autumn Migration in Iceland

Migratory orientation of Dunlins Calidris alpina at sunset was studied by using orientation cages, during autumn migration in SW Iceland. The aim of the experiments was to examine whether orientation funnels can be used to record the directional behaviour of shorebirds. The results clearly demonstrate that Dunlins are capable of expressing their preferred migratory direction in funnels. Unmanipulated Dunlins showed a well-oriented mean direction towards geographical SSE which is in excellent agreement with available ringing recovery data on Dunlins linked to Iceland. This opens up the possibility to perform carefully controlled experiments to investigate the orientation and navigation mechanisms of migrating waders.

Crustal shear‐wave splitting from local earthquakes in the Hengill Triple Junction, southwest Iceland

The Hengill region in SW Iceland is an unstable ridge‐ridge‐transform triple junction between an active and a waning segment of the mid‐Atlantic spreading center and a transform that is transgressing southward. The triple junction contains active and extinct spreading segments and a widespread geothermal area. We evaluated shear‐wave birefringence for locally recorded upper‐crustal earthquakes using an array of 30 three‐component digital seismographs. Fast‐polarization directions, ϕ, are mostly NE to NNE, subparallel to the spreading axis and probably caused by fissures and microcracks related to spreading. However, there is significant variability in ϕ throughout the array. The lag from fast to slow S is not proportional to earthquake depth (ray length), being scattered at all depths. The average wave‐speed difference between qS1 and qS2 in the upper 2–5 km of the crust is 2–5%. Our results suggest considerable heterogeneity or strong S scattering.

First epoch GPS survey of the Hengill triple jungtion, SW Iceland, and the effect of ocean loading

First epoch GPS survey of the Hengill triple jungtion, SW Iceland, and the effect of ocean loading

Brittle deformation and fracture patterns on oceanic rift shoulders: the esja peninsula, SW Iceland

This paper provides detailed structural data on the crustal deformation of the Esja peninsula, at the southwestern margin of the rift zone in SW Iceland. Forty percent of the faults are strike-slip with steep dips, 35% are normal dip-slip with dips ranging from 50° to 90°. For normal faults, E-W, NE-SW and NNE-SSW are the three major trends. The paleostress tensors determined using the fault data, indicate three main states of stress. Two extensional regimes ( σ 3 N180°E ±10° and N110°E ±10°) are related to normal slip faults. The third is a compressive stress regime with σ 1 trending N10°E-N30°E. Associations between strike-slip and dip-slip faults are often observed, resulting from both the geometrical requirements of interactions between faulted blocks and from local-regional permutations between σ 1 and σ 2. We note a constant obliquity of 10–20° between the trend perpendicular to the regional structures and the computed direction of extension. This conclusion is also supported by the major trends of dykes and tensional fractures. There are two mechanisms of volcanic injection (local sheets and regional dykes) resulting in power-law cumulative thickness distributions with exponents of 1.5 and 3.0, respectively.

Strain accumulation 1986–1992 across the Reykjanes Peninsula Plate Boundary, Iceland, determined from GPS measurements

GPS geodetic observations 1986–1992 spanning the oblique plate boundary on the Reykjanes Peninsula, SW Iceland, show that left‐lateral shear strain is accumulating in the area. The principal strain rates are: έ1=0.255 ± 0.055 µstrain/yr (extension) at N121 ± 6°E, and έ2=‐0.190 ± 0.053 µstrain/yr (contraction) at N31 ± 6°E. The principal axis of extension is perpendicular to the strike of fractures within volcanic systems on the Peninsula. The direction of maximum left‐lateral shear strain is N76 ±6°E, the same as the trend of the seismic zone on the Peninsula. Tensor shear strain in this direction is −0.222±0.054 µstrain/yr. Continuous aseismic slip of the North American‐Eurasian plates below a 5–11 km locking depth on the Peninsula can explain the observed depth of earthquakes, the width of the deformation zone determined from recent fissuring and faulting, and the GPS‐measurements of strain.

The Fossvogur marine sediments in SW Iceland ‐ confined to the Allerød/Younger Dryas transition by AMS 14 C dating

An exhaustive 14C dating programme of molluscs from the Fossvogur sediments in Reykjavik. Iceland is presented. For the first time all the fossiliferous units of the sediments are dated. The results confirm earlier conclusions of a widespread occurrence of marine sediments of Allerød age in Reykjavik. The set of dates from the Fossvogur sediments shows a narrow 14C age distribution (standard deviation of ±235 years) of molluscs from all localities and from successive marine units in vertical sections. The weighted mean conventional 14C age is 11,400 BP. Assuming a reservoir effect of 400 years. this corresponds to a reservoir‐corrected age of I1,000 BP. i.e. the Allerød‐ Younger Dryas transition for the sampled units, These new 14C dates from Fossvogur confirm the need for a revision of the Upper Pleistocene chronology of the Reykjavik region. They also have a bearing on the Late Weichselian record of glacier readvances and sea‐level changes in the area. The dates suggest that the marine units in Fossvogur accumulated within a restricted time‐span of a few hundred years. The sediments in Fossvogur are of volcaniclastic origin and are extremely lithified, indicating local geothermal activity soon after their deposition. This may explain anomalously high D/L amino acid ratios measured in molluscs from the Fossvogur sediments. δ13C and δ18O results suggest that temperatures may have ranged up to 60°C.

Commercial production of salt from geothermal brine at reykjanes, iceland

In the Reykjanes geothermal area in SW Iceland, the ground water is infiltrated by seawater. Due to interactions with basaltic volcanic rocks, the chemical composition of the geothermal brine differs somewhat from pure seawater. The most striking change is the increase of potassium and depletion of magnesium. A number of geothermal wells have been drilled in the area. Two of these wells are extremely powerful and have been utilized for commercial production of common salt for several years. The salt plant has recently been rebuilt and expanded considerably in order to produce common salt for domestic use and a new mineral salt product, “light salt” for export. The process is based on a vacuum evaporation technique where the geothermal well is the source of both salt-brine and process steam.

Multi-stage evolution of the picritic M�lifell rocks, SW Iceland: constraints from mineralogy and inclusions of glass and fluid in olivine

The picritic Maelifell pillow lava series contains olivine macrocrysts (Fo 83.0–91.7) and microphenocrysts (Fo 86.8–88.5), resorbed Cr−Al endiopside, ± plagioclase, and microphenocrysts of Cr-spinel. The most primitive olivine cores (Fo 90–91.7) are probably derived from a peridotitic mantle. Gabbroic adcumulus xenoliths in the lavas contain plagioclase, Cr−Al endiopside and olivine (Fo 85.5–87.5) which overlap compositionally with lava minerals, ± Cr-spinel. This suggests that all pyroxene and much of the olivine ± feldspar in the lavas are xenocrysts. Olivines from the pillow lavas and from the gabbroic xenoliths contain inclusions of Cr-spinel, silicate glass and pure or nearly pure CO2. Early (type 1) silicate melt inclusions which occur in lava-olivine only, have crystalized 0.1 to 4 vol.% daughter spinel and unknown amounts of olivine during pre-eruptive cooling. Later (type 2) glass inclusions in olivine from the lavas do not contain daughter minerals; similar type 2 inclusions also occur in the xenoliths. High-temperature microthermometry at buffered oxygen fugacity (fO2) gives a plagioclaseout temperature of about 1230°C for both types of silicate melt inclusions; this was interpreted as the liquidus temperature for type 2 inclusions. Molar volumes of undisturbed CO2 inclusions in olivine from both lavas and xenoliths correspond to a depth of trapping of 7–10 km (2.2–3.0 kbar) at 1230°C. This is interpreted as a minimum depth to a partially molten layer near the crust/mantle boundary in the rift zone. The xenoliths are thus probably derived from a layered olivine-gabbro complex similar to those occurring in ophiolite complexes.

A study of the palaeomagnetism of subglacial basalts, SW Iceland: a comparison with oceanic crust

SUMMARY Subglacial volcanic sequences in Iceland were used as analogues of oceanic crust to get an indication of the degree of primary heterogeneity of magnetic directions that can be expected within subaqueous volcanic sequences erupted in both deep and shallow water. In the Icelandic sequences the degree 6f tilt, brecciation and the amount of hyaloclastite are known so it is possible to test for primary magnetic heterogeneity. We measured magnetic directions of 119 oriented core samples (2.5 cm diameter) collected from five sites in SW Iceland where there is good exposure in Quaternary subglacial volcanic sequences. Sample sites vary from compact pillowed flows (most analogous to deep water volcanic sequences) to pillow breccia with up to 40 per cent interpillow hyaloclastite (more analogous to shallow water sequences). Magnetic directions of multiple samples from 16 individual pillows have angular differences of 2-66 with most being less than lo. Of the five sites sampled, two sites have inclinations that approximate that of the centred axial dipole, one site has steeper inclinations and two sites have shallower inclinations. When the site averages of magnetic direction with their circles of ag5 are plotted, there is little overlap. There is little correlation between either the scatter of magnetic measurements or the percentage of non-dipolar inclinations and the occurrence of pillow breccias, hyaloclastite or depositional dips. We doubt the tectonic significance of small amounts of tilt (< 30) estimated from drill cores on the basis of palaeomagnetic inclination as it is not uncommon for rocks to rotate by as much as 30 after they cool through the Curie temperature or for such a scatter to be the result of secular variation. With this in mind, the same caution must be applied when orienting seafloor basalts for palaeomagnetic or structural studies using geopetal features such as drain-away ledges or segregation vesicles, because the rocks may rotate after these features form (at 1200-1000 C) but before the rocks cool through the Curie temperature.

Basaltic melt evolution of the Hengill Volcanic System, SW Iceland, and evidence for clinopyroxene assimilation in primitive tholeiitic magmas

The thick oceanic crust of Iceland is formed by tholeiitic central volcanoes arranged in en echelon patterns along the 40–50 km wide rift zones. The Hengill central volcano in the southwestern rift zone has produced 25–30 km3 of hyaloclastites and lava during the last 0.11 m.y., with maximum productivity during the isostatic rebound following the degalciations 0.13 and 0.01 m.y. ago. The eruption units cover a compositional spectrum from picrite to rhyolite, but the volume of andesite, dacite, and rhyolite is small. The petrographic relations of pillow rim and hyaloclastite glass indicate that the basaltic melts were saturated with olivine and plagioclase, except for the most primitive ones that were undersaturated with plagioclase. Saturation with clinopyroxene was reached in some of the intermediate and evolved basaltic melts. Corroded and partly resorbed crystals of clinopyroxene and partly disintegrated gabbro nodules with resorbed clinopyroxene indicate that selective assimilation contributed to the evolution of the most primitive melts. The intermediate and evolved basaltic glass compositions fall along the low‐pressure cotectic for mid‐ocean ridge basalt (MORB) compositions saturated with olivine, plagioclase, and clinopyroxene, but the primitive glasses (9–9.7 wt % MgO) fall well inside the low‐pressure olivine + plagioclase primary phase volume. The primitive Hengill glasses have significantly higher CaO and lower Al2O3 than primitive glasses from oceanic spreading centers. Their low pressure undersaturation with respect to clinopyroxene and the absence of clinopyroxene phenocrysts indicate that they are not parental to the intermediate Hengill basalts, since fractionation modelling requires a large proportion of clinopyroxene in the fractionating assemblage. The most primitive melts could be produced by fractionation of olivine and plagioclase combined with 5–30 % assimilation of clinopyroxene, and the intermediate melts could be derived by mainly olivine and plagioclase fractionation, from melts equilibrated with peridotitic residues at pressures of 1–2 GPa. The further evolution of the basaltic spectrum can be explained by fractionation of olivine, plagioclase, and clinopyroxene combined with minor contamination by anatectic crustal melts. The rate of magma supply from the mantle to the crust is controlled by the isostatic conditions and is very low in periods of glacial loading of the crust. This leads to infrequent and small eruptions of dominantly evolved magmas. The dense picritic magmas (9–9.7 wt % MgO in the glass phase) were driven to the surface by magmatic overpressure in the mantle at an early deglaciation stage characterized by the absence of large, trapping magma chambers in the lower crust. The assimilation of clinopyroxene in these melts could proceed by direct contact with the solidified cumulate sequences and gabbro intrusions. Clinopyroxene assimilation in combination with olivine fractionation may also contribute to the chemical evolution of some of the most primitive MORB magmas.

HENGILL TRIPLE JUNCTION, SW ICELAND 1. TECTONIC STRUCTURE AND THE SPATIAL AND TEMPORAL DISTRIBUTION OF LOCAL EARTHQUAKES

The Hengill area is an unstable ridge‐ridge‐transform triple junction in SW Iceland. It contains the active central volcano Hengill, associated with the present accretionary zone, and the extinct Grensdalur central volcano, associated with an accretionary zone that became inactive at about −0.7 m.y. when the locus of spreading migrated about 5 km W to the Hengill zone. The dominant tectonic trend of the area is N25°E, parallel to the accretionary zones, but transverse tectonic/topographic features have also developed in the neighborhood of the two central volcanoes. These result from local modification of the regional stress field associated with the spreading by radially symmetric stress fields associated with the volcanoes, and the migration of volcanism. A double‐high temperature geothermal system mirrors the double volcanic system. The transform branch of the triple junction is represented by an EW striking zone of historic destructive earthquakes in the S of the area. A seismological study was conducted with the aims of studying the tectonic structure and evaluating the passive seismic method as a geothermal prospecting tool. The seismicity may be broadly divided into two groups. First, infrequent intense episodes of crustal movement occur that are associated with the release of tectonic stress along the plate boundary. Second, continuous small‐magnitude activity occurs on a day to day basis. This activity is mostly associated with the extinct Grensdalur central volcano, not the present plate boundary. Statistically significant temporal variations in b accompanied a swarm in the transform zone. The continuous small‐magnitude activity may be geothermal in origin, releasing, thermal stress, and not associated with plate boundary tectonics.

Trace metals in mussels (Mytilus edulis) from southwest Iceland

The concentrations of Hg, Cd, Pb, Cu and Zn were determined inMytilus edulis from 48 locations in SW Iceland, as well as in moored mussles. The purpose was to establish baseline conditions and to evaluate anthropogenic influences. As reference stations, 34 locations away from population centres, were selected. Data on the metal content of soft tissues were examined in relation to both tissue weight and shell length. The data reveal Pb and Hg contamination in mussels from the vicinity of Reykjavik. Leaded petrol, paints and diffuse urban uses are considered as sources of these anomalies. A comparison is made with published mussel watch results.