Journal of Geophysics
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Stratigraphy and paleomagnetism of the Esja, Eyrarfjall and Akrafjall Mountains, SW-Iceland
Get PDFDetailed geological and magnetic mapping in an area of Pliocene and Plio-Pleistocene volcanic rocks in Southwestern Iceland has enabled us to correlate a 2,100-m-thick lava succession with similar dated sequences in Iceland and with the ocean-floor geomagnetic polarity time scale. This correlation, supported by additional K-Ar dating, implies (1) that the succession is between 4.2 and 1.8 Ma in age, (2) that at least 13 glaciations occurred in Western and Southwestern Iceland between 3.1 and 1.8 Ma ago, and (3) that at least two geomagnetic events are present in the Lower Matuyama epoch. Paleomagnetic results from 353 igneous units, mostly basalt lavas, are tabulated. Analysis of directions from 258 of these shows them to possess some serial correlation; their mean is very close to a central axial dipole field value, but explanations are proposed for observed systematic departures from this field in other Icelandic paleomagnetic survey results.
ARK: https://n2t.net/ark:/88439/y042132
Permalink: https://geophysicsjournal.com/article/184
 
A continuum model of crustal generation in Iceland - kinematic aspects
Get PDFA steady-state plate-tectonic kinematic model of crustal accretion in Iceland is presented. It describes quantitatively the overall time-averaged movements of solid crustal elements during the accretion process, and correlates accretion parameters in the axial zone (width of lava deposition zone, total lava production rate, width of horizontal strain zone, spreading velocity, normal faulting) with structural properties in the Tertiary lava pile (lava dips, lava deposition rate, dyke fraction). The model is used, firstly, to predict the accretion parameters of the Tertiary volcanic zone on the basis of observed structural properties in the Tertiary lava pile; secondly, to predict possible structures of the lower crust in terms of a lava/intrusion ratio; thirdly, the model may be used to calculate the crustal temperature field caused by intrusions, but this application is outside the scope of the present paper. The model is essentially a further development of a previous one presented earlier by the author. The analysis, in terms of the model, of various published structural observations indicates that the width of lava deposition and the spreading rate in the Tertiary volcanic zone were consistent with the corresponding properties in the present-day volcanic zone. This may suggest a certain uniformity in the volcanic processes during the last 10-15 Ma. The visible Tertiary lava pile was, according to the model, deposited outside the innermost 50-km-wide central part of the volcanic zone, which may explain the difference in appearance between the two main volcanic regions of Iceland, i.e., the active volcanic zone and the Tertiary flood basalts. Furthermore, an analysis of possible structures of the lower crust, consistent with various surface observations, indicates a gradual rather than a sharp transition from an upper lava-dominated crust to a lower intrusion-dominated crust.
ARK: https://n2t.net/ark:/88439/y050901
Permalink: https://geophysicsjournal.com/article/69
 
Subsidence events in the Krafla area, North Iceland, 1975-1979
Get PDFIn the inflation-deflation sequence of the Krafla magma chamber since its beginning in 1975, 12 deflation or subsidence events have been identified until May, 1979. The tilt and distance measurements relating to these subsidence events are discussed in some detail. All of these subsidence events are associated with horizontal magma flow along the N-S trending fissure zone, which goes through the central part of the Krafla caldera, to form a dike 3 to 5 m wide, 80 km long, and 1.0 to 2.5 km high from the bottom to the top. The total volume of magma, which flowed out of the Krafla magma chamber during these 12 events, is estimated as 481 x 106 m3, whereof 407 x 106 m3 are estimated to have flowed northwards, 72 x 106 m3 southwards and about 2 x 106 m3 came to the surface as basaltic lava.
ARK: https://n2t.net/ark:/88439/y011015
Permalink: https://geophysicsjournal.com/article/76
 
Crustal structure of the Iceland-Faeroe Ridge
Get PDFKnowledge of the crustal structure of the Iceland-Faeroe Ridge is based on results from the North Atlantic Seismic Project of 1972 supplemented by earlier short refraction lines and reflection, gravity and magnetic surveys. The main 5. 7 km/s upper crustal layer is locally overlain by lower velocity layers of variable thickness. The upper crust is interpreted as being predominantly basaltic, comprising lavas, regions of pyroclastic rock and intrusives including ring complexes. A 6.7 km/s lower crustal layer underlies the upper crust at a depth of between about 4 and 8 km along the Ridge; this layer is present also beneath the Icelandic shelf but not beneath the Faeroe shelf. A deeper 7.8 km/s refractor interpreted as the Moho occurs at about 30-35 km depth beneath the south-eastern part of the Ridge, shallowing to about 28 km towards the north-western end of it. A significant increase in velocity with depth within the main 6.7 km/s layer has not been detected but may occur, in which case the Moho would be somewhat deeper. The seismic crustal results are consistent with a gravity profile across the Ridge, which indicates approximate Airy isostatic equilibrium. The crust beneath the Ridge, which is of a thickness more typical of the continents than the oceans, is believed to have been formed by sea-floor spreading during the period 55 to 40 Ma ago.
ARK: https://n2t.net/ark:/88439/y051071
Permalink: https://geophysicsjournal.com/article/80
 
Seismological evidence for lateral magma intrusion during the July 1978 deflation of the Krafla volcano in NE-Iceland
Get PDFThe July 1978 deflation of Krafla volcano in the volcanic rift zone of NE-Iceland was in most respects typical of the many deflation events that have occurred at Krafla since December 1975. Separated by periods of slow inflation, the deflation events are characterized both by rapid subsidence and volcanic tremor in the caldera region, as well as extensive rifting in the fault swarm that transects the volcano. Earthquakes increase in the caldera region shortly after deflation starts and propagate along the fault swarm away from the central part of the volcano, sometimes as far as 65 km. The deflation events are interpreted as the result of subsurface magmatic movements, when magma from the Krafla reservoir is injected laterally into the fault swarm to form a dyke. In the July 1978 event, magma was injected a total distance of 30 km into the northern fault swarm. The dyke tip propagated with a velocity of 0.4-0.5 m/s during the first 9 h, but the velocity decreased as the length of the dyke increased. Combined with surface deformation data, these data can be used to estimate the cross-sectional area of the dyke and the driving pressure of the magma. The cross-sectional area is variable along the dyke and is largest in the regions of maximum seismic energy release. The average value is about 1,200 m2. The pressure difference between the magma reservoir and the dyke tip was of the order of 10-40 bars and did not change much during the injection.
ARK: https://n2t.net/ark:/88439/y057911
Permalink: https://geophysicsjournal.com/article/134
 
Magnetometer array observations of a giant pulsation event
Get PDFOn 19 November 1976 a rather well developed giant pulsation event occurred at about 0600 MLT over northern Scandinavia. Results of observations obtained with a dense network of magnetometer stations are reported. The method of analysis is based on the concept of the analytical signal. The disturbance region was strongly localised. The amplitudes of the horizontal components show a pronounced difference: the amplitude maximum of the N-S component is clearly moving to the west while that of the E-W component is nearly fixed relative to the array. The frequency of the event decreased with time. Both the frequency decrease and the drift of the amplitude maximum are interpreted in terms of bounce - resonance instability. The spatial polarisation pattern found is in agreement with the fieldline-resonance theory in that the sense of rotation of the horizontal disturbance vector is as predicted by the theory. At the region of maximum amplitudes the pulsation is linearly polarised in E-W direction, as observed by other workers. This does not agree with the fieldline-resonance model. Apparent azimuthal wave numbers of the horizontal components show that the event analysed is a westward-travelling wave. The wave numbers are as large as predicted by Olson and Rostoker's (1978) empirically derived relation between the E-W wave number and the frequency of pc 4-5 pulsations.
ARK: https://n2t.net/ark:/88439/y020914
Permalink: https://geophysicsjournal.com/article/172
 
Gravity and height variations during the present rifting episode in Northern Iceland
Get PDFIn 1975, a rifting episode started in the neovolcanic zone of northern Iceland, consisting of a succession of slow inflation periods and rapid subsidence events, which is still going on. The center of activity is situated below the Krafla caldera, and the rifting process is affecting the 80-km-long fissure swarm associated with this central volcano. Gravity and height variations associated with this process have been investigated by re-observing profiles earlier established in the Namafjall and in the Gjastikki area, situated nearly 10 km south and north of Krafla respectively, as well as by the re-observation of a number of gravity stations in the northern part of the fissure zone, in 1976, 1977, and 1978. By repeated observations with 2 or 3 LaCoste-Romberg gravity meters, the accuracy obtained in each gravity survey is of the order of ± 10 x 10-8 ms-2. In the profiles crossing the fissure zones, a rate of gravity increase of more than 100 x 10-8 ms-2/a has been found in the central part, while gravity at the flanks decreases at the same order. These variations are correlated with subsidence and elevation rates of the order of 0.5 m/a.
ARK: https://n2t.net/ark:/88439/y060150
Permalink: https://geophysicsjournal.com/article/268
 
New heat flow observations on the Reykjanes Ridge
Get PDFDuring METEOR cruise 45 in August 1977 14 heat flow measurements were obtained along a profile east of the Reykjanes Ridge and perpendicular to the ridge axis covering a distance range from 30 to 240 km. Closely spaced measurements were grouped together. The mean heat flow of all groups amounts to 109±20 mW m-2. The values do not reveal a distinct increase with decreasing distance from the ridge axis as may be expected from the theoretical heat flow distribution based upon a cooling plate model. Including earlier measurements a high and very uniform heat flow in the distance range from 170 to 340 km was observed with a mean value of about 100 mW m-2. In order to explain this high heat flow a temperature of 930° C is required at the lower boundary of the lithosphere at a depth of 50 km, assuming a purely conductive heat transport. Compared with the results obtained from previous measurements west of the Reykjanes Ridge, the data reveal an asymmetric thermal behaviour of the ridge area. The average heat flow east of the ridge amounts to 93 mW m-2 being nearly twice the heat flow of the region west of the ridge.
ARK: https://n2t.net/ark:/88439/y073309
Permalink: https://geophysicsjournal.com/article/37
 
Seismic structure of Iceland along RRISP-profile I
Get PDFAs part of the RRISP 77 combined land-sea refraction seismic experiment, observations were carried out on Iceland itself with special emphasis on resolving the deep structure beneath Iceland and its transition towards the eastern flank of Reykjanes Ridge. The data, interpretational procedures, and results for the land part are described in this paper. A structural model of Iceland is presented which is characterized by a generalized two-layered crust of variable thickness underlain by anomalous mantle with P-wave velocities of 7.0 km/s at the base of the crust increasing to 7.4 km/s at 30 km depth. Two regions of relatively low velocity have been identified in the lower crust, possibly indicating zones of high melt concentration. A normal P- to S-wave velocity ratio of 1.76 is found within the crust, whereas this ratio reaches unusually high values of up to 2.2 in the anomalous mantle. From this and the P-wave velocity distribution the amount of partial melt is calculated. The melt content is highest (17%-23%) at the top of the mantle and decreases with increasing depth indicating differentiation processes in the upper mantle. The anomalous mantle is confined to Iceland and a sharp transition exists in the area of the shelf edge where normal oceanic lithosphere replaces the updoming asthenosphere.
ARK: https://n2t.net/ark:/88439/y083228
Permalink: https://geophysicsjournal.com/article/72
 
Morphology and magnetic anomalies north of Iceland
Get PDFDetailed low-level aeromagnetic data between Iceland and 70° N are combined with published bathymetric, seismic reflection, and other data to yield a new tectonic synthesis of this region of anomalously shallow sea floor. On Kolbeinsey Ridge short transform faults have repeatedly formed and disappeared over the last 7-8 Ma. Spreading from Kolbeinsey Ridge began about anomaly 6C time (24 Ma); total opening rates increased from 1.5 cm/a to 2 cm/a about 12-13 Ma ago. The Intermediate-Iceland- Plateau extinct axis thus does not exist, and oceanic crust must underlie much of the Greenland margin, perhaps up to the coast itself. This in turn implies locally over 100 km prograding, much of which probably occurred during Plio-Pleistocene glacial periods. The Iceland shelf has been prograded locally 25 km or more. The plate acceleration a 12-13 Ma ago correlates with time-transgressive basement ridges or escarpments previously found on Reykjanes Ridge and here identified north of Iceland as well. The features are proposed to reflect an abrupt mid-Miocene increase in discharge from the Iceland plume. Other time-transgressive basement structures are found on younger Kolbeinsey Ridge crust. Lower Tertiary anomalies 13 to at least 22 are identified in the Denmark Straits, ruling out the hypothesis that the Iceland platform resulted from a westward jump of the spreading center at anomaly 7 time. The magnetic smooth zones being formed where the Kolbeinsey and Reykjanes Ridges enter Iceland have a multiple origin: degassing at depths less than 500 m, coupled with crustal reheating as a result of burial by sediment may be the most important processes.
ARK: https://n2t.net/ark:/88439/y093477
Permalink: https://geophysicsjournal.com/article/74