Blue Road Research Articles

The Blue Road Geotraverse: Investigations of local earthquakes in the land-uplift area of Fennoscandia by mobile seismograph stations

Abstract In studies of micro-earthquake activity in the area with the highest rate of uplift in Fennoscandia, mobile earthquake stations of high sensitivity were operated for 3 weeks in connection with the Blue Road seismic-field experiments in 1972.

The Blue Road Geotraverse: Some comments

Abstract In four notes in the September issue of the Geologiska Föreningens i Stockholm Förhandlingar, Vol. 98, pp. 251–254 and 257–274, Professor A. Vogel, of the Department of Geophysics at the Free University in Berlin, presented some crustal-model interpretations of gravity, magnetic and seismic measurements along the Blue Road Geotraverse. Since some of the statements made in these notes need more distinct explanations and since Vogel's method of obtaining agreement between the seismic and the gravity interpretations is not correct, a few comments have to be made.

The Blue Road Geotraverse: Future research on the basis of new hypotheses

Abstract Further research on the structure and dynamics of the lithosphere along the Blue Road Geotraverse requires the detailed interpretation of available data and the execution of additional field surveys. New field studies, however, should be carried out systematically on the basis of earlier results and working hypotheses. The interrelationships between the parameters of a complex static and dynamic model of the lithospere, as it can be constructed from the simultaneous interpretation of geophysical data and interdisciplinary research, are here discussed.

The Blue Road Geotraverse: The relative residual of the teleseismicP-wave: Application to the study of deep structure in Fennoscandia

Abstract Studies of the mean travel-time residual of seismic P-waves propagated from distant sources to stations in Fennoscandia indicate that the mean station residual increases from the centre of the Baltic Shield towards the Atlantic coast. This is interpreted as a decrease of the mean vertical velocity in the upper mantle towards the Atlantic coast. Assuming the usual proportionality between seismic-wave velocity and density, the associated change in the mean upper-mantle density is more than sufficient to support isostatically the height difference between the Scandinavian mountain chain and the lowlands of the central Shield, even assuming an unchanging crustal structure.

The Blue Road Geotraverse: Studies of short-period crustal dynamics, using continuously recording instruments

Abstract Short-period crustal dynamics concern the spectrum of processes between long-period seismic waves and those long-period processes which can be surveyed by repeated geodetic measurements. The instruments used are recording gravity meters, tiltmeters, and strain-meters. These instruments have conventionally been used for earth tidal research on a global scale. The gradual increase of sensitivity and resolution, however, has caused disturbances like oceanic tides, individual tidal movements of lithospheric blocks, stress accumulation, crustal loading by atmospheric pressure changes, precipitation, and tectonic processes to become a main problem. On the other hand, these “dirty effects” are an important source of information on the structure and dynamics of the lithosphere. In order to obtain this kind of information, a network of geodynamic stations is planned along the Blue Road Geotraverse, in addition to further interdisciplinary research.

The Blue Road Geotraverse: The determination of the earth's crustal and upper-mantle structure from sea-load tilts

Abstract The comparison of primary sea-load tilts observed by recording borehole pendulums with tilts calculated on the basis of theoretical models and marine tidal charts yields information on the elastic behaviour of the lithosphere and asthenosphere. The results of tilt investigations in northern Germany are presented, and the difficulties of similar studies along the Blue Road Geotraverse are emphasized.

The Blue Road Geotraverse: Mapping of intercrustal boundaries from P to S converted waves

Abstract P to S converted waves were observed in connection with a seismic-refraction profile in southern Norway. The observations are consistent with a four-layer crustal model with the following mean depths to the discontinuities: 9 km, 16 km (Conrad), 26 km, and 37 km (Moho).

The Blue Road Geotraverse: Heat flow dynamic model

Abstract Basic ideas for the computation of a heat flow dynamic model of the lithosphere in the area of Fennoscandian land uplift are presented.

The Blue Road Geotraverse: The Bouguer field and optimized density models

Abstract On the basis of national gravity surveys in Sweden and Norway and additional gravity measurements within a zone ∼50 km wide along the Blue Road Geotraverse, a Bouguer map was produced for the geotraverse area. In order to explain the large, negative, Bouguer anomaly within the mountain area, two-dimensional density models limited by polygons were used, and the geometrical parameters optimized by a least-squares procedure. Mass deficiencies from relicts of a downgoing slab during the Caledonian orogeny seem to be unlikely from the isostatic point of view. Mass deficiencies caused by a depression of the Moho seem reasonable when isostacy is considered. Clear seismic evidence, however, is still lacking.

The Blue Road Geotraverse: The aim of the interdisciplinary research

Abstract The Caledonides and the Baltic Shield, with its area of Fennoscandian land uplift, are tectonic features of high significance, as regards the structure, dynamic history and present dynamics of the earth's crust in Scandinavia. The Caledonides represent the remnants of an old orogenesis, which most probably took place in the collision zone between drifting lithospheric plates 700–400 m.y. ago. The Baltic Shield is an ancient crustal block, indicating folding older than 1500 m.y. The Fennoscandian land uplift is regarded as an isostatic re-adjustment after the last ice age, but there are evidences of a continuous but much smaller land uplift since the Caledonian. In order to gain a better understanding of the Caledonian orogenesis, the origin of the Baltic Shield and the process of land uplift, the static and dynamic properties of the crust and upper mantle are being investigated along a zone which runs from the Norwegian Sea in a south-easterly direction through Sweden, the Bothnian Sea and furthe...

The Blue Road Geotraverse: The tentative synthesis of a lithosperic model

Abstract An attempt is made to synthesize the results of different investigations along the Traverse. For comparison, a map shows the mean values of the topography, the geological cross-section, a seismic-velocity model, the Bouguer field and its interpretation, as well as the magnetic field and a model. The interpretation of seismic-wave travel times does not show a Moho depression below the Caledonides, compared with the Baltic Shield. The interpretation of gravity results by a Moho depression requires a very steep seismic-wave-velocity gradient in the lower crust and almost no velocity jump at the Moho, in order to satisfy the seismic-wave travel times. Because of the difficulty of explaining isostatic compensation of the mountains by such a model, light masses below the lithosphere, such as the relicts of a downgoing slab during the orogeny, should be taken into account to explain both a part of the negative Bouguer field and a part of the isostatic equilibrium.

The Blue Road Geotraverse: Deep seismic sounding in 1972

Abstract Deep-seismic-sounding experiments along the Blue Road Geotraverse were carried out in 1972. Seismic waves from five equally spaced shot points were recorded at altogether 88 recording sites. The seismic project was initiated and plans for the field procedure were made by the Department of Solid Earth Physics at Uppsala. The field measurements were carried out in co-operation with Swedish, German, Norwegian, and Finnish institutes. As the result of a first travel-time interpretation of P-wave arrivals, velocity models of the lithosphere can be presented.

The Blue Road Geotraverse: A magnetic ground survey and the interpretation of magnetic anomalies

Abstract Ground measurements of the magnetic field were carried out along a 20-km-wide zone along the Blue Road Geotraverse. In addition, a profile of high precision was measured. A large positive anomaly within the mountain area cannot be explained by anomalous magnetization of surface rocks. Optimization of theoretical models shows that it may have its origin at a depth of 30 km, indicating a rise of the Curie isothermal surface from the mountain area to the Baltic shield. The rise of the isotherm may be connected with the land uplift. Interpretation of smaller anomalies reveals the thickness of sedimentary layers.

A tentative description of the geology along the Blue Road Geotraverse

Abstract A greatly generalized section along the Blue Road Geotraverse in Scandinavia is presented. The traverse transects the western part of the Baltic Shield and the Caledonides. Major tectonic features have been indicated according to earlier regional investigations, but some consideration has been given to later research.

Seismic wave velocities in the Swedish crust

Records from Swedish seismograph stations of underwater explosions in northern Scandinavia on 1 and 3 August, 1972-the so-called ‘Blue Road’ experiment-are used to calculate the average structure of an assumed two-layered crust. This model can serve as a basis in the process of locating earthquakes and explosions in and near the Scandinavian area. The values of the wave velocities with their standard deviations are (km/s):Pn 7.87±0.02,P* 6.69,Pgl 6.27±0.03,Sn 4.55±0.03,S* 3.69±0.03,Sgl 3.54±0.02,Sg2 3.40±0.01 andRg 2.81±0.02. Except forRg, these values, like those of the thicknesses of the two horizontal layers-the granitic layer 10 km and the basaltic layer 26 km-are similar to the values found byBath [1] in 1971 for the same area.

Pollen analysis of late Otiran and Aranuian sediments at Blue Spur Road (S51), North Westland

Abstract In a section exposed along the Blue Spur Road, Terangi Interglacial marine gravel is overlain by Waimea Glacial outwash gravel beneath the outwash aggradational surface. Colluvial gravel and silt in a channel on that surface are interpreted as representing the early stadial in the Otira Glacial, and, judged by radiocarbon dates and pollen analysis, peat and loess represent an interstadial, the later Otiran stadials and the post-glacial (Aranuian). Only the later part of the interstadial is recorded in the pollen-analysed section. Dacrydium cupressinum forest is replaced by forest of the Nothofagus fusca type which in turn was followed by N. menziesii forest. The N. menziesii was accompanied by an increase of grassland and dicotyledonous shrubland and was succeeded by Dacrydium cupressinum forest in the post-glacial. The interstadial is inferred to have begun some thousands of years before 30,300 B.P. and to have lasted until about 25,000 B.P.