ALBERT

Notes: Abstract. Assuming that other sources of error can be neglected, the reliability of a land suitability classification depends on the homogeneity of physiographically delineated map units with regard to land qualities. The map unit homogeneity of a small area in France was estimated using 64 observation points, arranged according to a nested sampling scheme, followed by nested analysis of variance.The analysis shows that in this area map units are too heterogeneous to accept the suitability classification as being completely reliable. However, alternative procedures using methods of optimal interpolation to map gradual change within the physiographic units are too expensive at a mapping scale of 1:25000 or smaller. It is not possible to produce completely accurate suitability maps at smaller scales. However, incorporating nested sampling and analysis of variance as standard procedures in land evaluation surveys costs little effort and yields at least an estimate of map accuracy and reliability of the suitability classification.

Notes: The low field susceptibility variations in the Chinese and Tajik loess sequences reflect the palaeoclimatic fluctuations of the Quaternary on the continents in a very direct and complete way. It can be demonstrated that the fine-grained ferromagnetic mineral fraction is largely responsible for the climatically controlled susceptibility enhancement in the loess sediments. In order to gain more information about the origin and quality of the susceptibility enhancement we are scrutinizing the F-factor (FD) which is conventionally used as a measure of the frequency dependence of the susceptibility. For qualitative estimates of the grain sizes present, FD seems to be a rather misleading parameter because it is flawed when variable amounts of minerals with both frequency-independent and frequency-dependent susceptibility are present. We introduce a new concentration-independent F-factor (Fc) which takes the influence of the mineral fractions with frequency-independent susceptibility on the total susceptibility into consideration. Fc is fairly constant in all the investigated sections. This result is explained theoretically following Néel's (1949) fine-particle theory and Stephenson's (1971a) modelling of various single-domain grain distributions. It is found that the grain-size distribution is essentially the same throughout all loess sections investigated and the upper and lower grain-size limits are essentially invariable. In addition, susceptibility and its frequency dependence were measured at low temperatures on selected loess samples in order to determine the maximum grain size present in the fine-grained magnetic fraction.

Notes: Two shear-wave profiles, E and G, collected during the 1977 Reykjanes Ridge Iceland Seismic Experiment have played an important role in models of the Icelandic crust. They were originally interpreted as indicating very low shear-wave velocities and abnormally low shear-wave quality factors in the 10–15 km depth range. These attributes, which are indicative of near-solidus temperatures, were used to support the hypothesis that the crust of Iceland is relatively thin (10–15 km) and underlain by partially molten material. More recent seismic data, however, contradict this hypothesis and suggest that the crust is thicker (20–30 km) and cooler. A re-examination of the RRISP-77 data indicates that the low shear-wave velocities are artefacts arising from source static anomalies (in the case of profile G) and misidentification of a secondary shear phase, SmS, as S (in the case of profile E). Furthermore, the attenuation occurs at ranges when rays from the shots pass near the Askja (profile E) and Katla and Oraefajokull (profile G) volcanoes. It may therefore have a localized source, and not be diagnostic of Icelandic crust as a whole. This new interpretation of the RRISP-77 shear-wave data is consistent with models having a thick, cold crust.

Notes: We modified Dohr's method and were able to establish trends of crustal reflectivity distribution. Four main types of crustal reflectivity-depth histograms could be distinguished. Dohr's method was based on line drawings from which the total length of reflections was calculated for time-distance frames along the profile. In this paper we present a new method to derive such histograms automatically from the final stack tapes. This allows an objective comparison of deep reflection seismograms from different crustal units and from different research groups. It is shown that our energy histograms are consistent with earlier manually produced histograms and that the previous results can be reproduced. First applications to DEKORP and KTB data in Germany are presented.

Notes: It is well known that similar earthquakes, i.e. earthquakes having almost identical waveforms, allow extremely accurate relative timing of the seismic arrivals. This has traditionally been used for achieving accurate relative locations of clusters of similar earthquakes. The arrival time differences between similar events depend not only on their relative location but also on the absolute location of the group. Moving a pair of events 200 m while retaining their relative locations can cause a 1 ms change in the time difference between the first arrivals of the events at a station 6 km distant. A change in time difference of 1ms can easily be estimated by cross-correlating the waveforms of the two earthquakes. We use the accurate relative timings to improve absolute locations of groups of similar events, as well as to obtain extremely accurate relative locations. The absolute locations from relative timings are expected to have errors that are independent of the errors associated with locations based on absolute arrival time observations.We analyse data from five earthquake sequences, comprising a total of 96 earthquakes, recorded by a regional network in southern Iceland. One of the clusters is located within the on-land spreading ridge in south-western Iceland, and the other four are within the South Iceland seismic zone, a transform zone between overlapping branches of the spreading ridge. The events vary in magnitude between Ml−0.3 and 2.8. After determining the absolute and relative locations of each swarm, we estimate the orientation of a best-fitting plane through the hypocenters. The mean distance of events from a best-fitting plane varies between 4 and 15 m for the five swarms. This is comparable to the formal error estimates for the relative locations. Together with (non-unique) fault-plane solutions, the relative locations constrain the fault planes and the type of faulting. Faulting within the nascent transform zone in southern Iceland is predominantly strike slip on near-vertical N-S striking planes, in agreement with the orientation of mapped earthquake fractures in the area. The earthquakes within the spreading zone clearly define a fault plane striking parallel to the ridge and dipping 63°. Each group of similar events probably represents repeated slip on the same fault.

Notes: Reflection seismics; in compressional belts has revealed the structure of crustal shortening and thickening processes, showing complex patterns of indentation and interfingering of colliding crusts and subcrustal lithospheres. Generally in the upper crust large zones of detachments develop, often showing duplexes and ‘crocodile’ structures. The lower crust from zones of active collision (e.g. Alps, Pyrenees) is characterized by strongly dipping reflections. The base of the crust with the Moho must be continuously equilibrating after orogenic collapse as areas of former continental collision exhibit flat Mohos and subhorizontal reflections. The depth to the Moho increases during collision and decreases after the onset of post-orogenic extension, until finally the crustal root disappears completely together with the erosion of the mountains.Processes, active during continental collisions and orogenic collapse, create distinct structures which are imaged by reflection seismic profiling. Examples are shown and discussed.

Notes: Unsaturated solute transport experiments were conducted using several small and one large column packed with the same sandy soil material.Chloride and tritium were used as tracers for water movement. Soil solution samples were collected as a function of time in the small columns and versus time and depth in the large column. The data were compared with solutions of the classical two-parameter convection-dispersion equation and also with solutions of a more elaborate four- parameter transport model that assumes a fraction of the water to be immobile. Fairly symmetrical effluent curves resulted from both the small and large columns, suggesting small transfer coefficients between mobile and immobile regions and small amounts of immobile water. Dispersivity values ranged from 1 cm for the small columns to about 5 cm for the 6-m-long column; retardation factors for chloride were nearly the same (about 0.8) for both column sizes. Thus, while the dispersivity measured in the small columns did not apply to the large columns, the same retardation factors could be applied to the different scale columns.

Notes: Laboratory experiments involving both homogeneous and heterogeneous porous media are used to demonstrate that fluid flow and solute transport will occur regularly in the capillary fringe (CF), including both vertical (upward as well as downward) and horizontal flow velocities. Horizontal flow above the water table appears to be limited primarily to the region of high water saturation (i.e., the CF), an observation supported by numerical modeling and consistent with the literature. Beyond observations presented in prior literature, it was observed that exchange of water within the CF with water below the water table is active, with flux both from the CF downward across the water table and from the region below the water table, upward into the CF. This flux is enhanced by the presence of physical heterogeneity. These findings strongly contrast the common conceptualization of predominantly downward vertical fluid flow through the unsaturated zone, with transition to fully three-dimensional flow only below the water table. Based on these observations, it is suggested that the CF may affect, far more significantly than is usually assumed, the natural geochemical and microbial conditions present in the region of transition from unsaturated to saturated ground water flow.

Notes: Abstract The North Shore Volcanic Group in northern Minnesota is part of the Middle Proterozoic Keweenawan sequence, one of the largest plateau lava provinces in the world. The primary geochemistry of the basalts suggests that volcanism occurred in an intracontinental rift environment. The subaerial lava flows, mainly amygdaloidal olivine tholeiites and tholeiites, have undergone low-grade metamorphism from zeolite to lower greenschist facies. On the basis of alteration phases replacing the primary magmatic minerals, infilling amygdales and veins, and replacing secondary minerals, the following zones have been distinguished: (1) thomsonite-scolecite-smectite, (2) heulandite-stilbite-smectite, (3) laumontitechlorite-albite, (4) laumontite-chlorite-albite ± prehnite ± pumpellyite and (5) epidote-chlorite-albite ± actinolite zone.In addition to the overall zonation based on mineral parageneses, zonations in the composition of the Ab content of the newly formed albite replacing primary Ca-rich plagioclase and of the newly formed mafic phyllosilicates are observed within the sequence and within single flows. Mafic phyllosilicates in the upper part of the sequence (mainly smectites and mixed-layer smectite/chlorites) display high Si and Ca + Na + K contents, whereas in the lower part of the sequence the amounts of Si and Ca + Na + K are markedly lower (mainly chlorites and mixed-layer chlorite/smectites). Similar zonations are observed within the individual flows. The albite content of the newly formed plagioclase is highest, and the Si and Ca + Na + K content of the phyllosilicates lowest in the amygdaloidal flow top while the opposite is true for the massive flow interior.The above features suggest that the overall pattern is one of burial-type metamorphism associated with extension in the rift setting. In detail, the mineral assemblages are controlled not only by the stratigraphic position but also by the flow morphology controlling permeability whose effect on the assemblages is most pronounced in the stratigraphically upper parts. This suggests that at the first stages of alteration (lowest grade) the patterns of fluid flow were important effects in controlling the assemblages. At greater burial depth, assemblages are more homogeneous, perhaps representative of a more even and pervasive flow pattern.Using the observed assemblages at face value to define grade and/or facies, different conditions would be assigned within the different morphological flow portions. Thus at low-grade metamorphic conditions it is essential to integrate assemblages from different morphological flow portions in order to define satisfactorily the overall metamorphic conditions.