The gravity field of a dipping plate in Greece

Summary. The surface gravity anomalies for a three-dimensional density model of a dipping lithospheric plate under the Aegean island arc and the Aegean Sea in Greece in the eastern Mediterranean Sea have been calculated. Such a dipping plate has been suggested in geophysical investigations. The calculated gravity maximum over the dipping plate is located in the area of the largest observed Bouguer anomalies inside the island arc. It is suggested that this should be taken into account in studies of crustal structure and gravity in the area.     

Self-consistent treatment of tidal variations in the geocenter for precise orbit determination

We show that the current levels of accuracy being achieved for the precise orbit determination (POD) of low-Earth orbiters demonstrate the need for the self-consistent treatment of tidal variations in the geocenter. Our study uses as an example the POD of the OSTM/Jason-2 satellite altimeter mission based upon Global Positioning System (GPS) tracking data. Current GPS-based POD solutions are demonstrating root-mean-square (RMS) radial orbit accuracy and precision of \({<}1\)  cm and 1 mm, respectively. Meanwhile, we show that the RMS of three-dimensional tidal geocenter variations is \({<}6\)  mm, but can be as large as 15 mm, with the largest component along the Earth’s spin axis. Our results demonstrate that GPS-based POD of Earth orbiters is best performed using GPS satellite orbit positions that are defined in a reference frame whose origin is at the center of mass of the entire Earth system, including the ocean tides. Errors in the GPS-based POD solutions for OSTM/Jason-2 of \({<}4\)  mm (3D RMS) and \({<}2\)  mm (radial RMS) are introduced when tidal geocenter variations are not treated consistently. Nevertheless, inconsistent treatment is measurable in the OSTM/Jason-2 POD solutions and manifests through degraded post-fit tracking data residuals, orbit precision, and relative orbit accuracy. For the latter metric, sea surface height crossover variance is higher by \(6~\hbox {mm}^{2}\) when tidal geocenter variations are treated inconsistently.     

The present rate of uplift of Fennoscandia implies a low-viscosity asthenosphere

The rate of displacement in Fennoscandia has been intensively discussed for many years. It is now widely accepted to be an isostatic response of the glacial history of the area. The Earth's present response to deglaciation in Fennoscandia is simulated using a three-dimensional (3D), viscoelastic model in which the asthenosphere and mantle viscosity are allowed to vary so that the maximum rate of present uplift matches its observed value. The deglaciation history is considered to be known, and the C¹⁴-datings are converted to sidereal years. The pattern of the present uplift gives a firm match with the observed data when a low-viscosity asthenosphere is introduced. Assuming a 15,000 years load cycle, i.e. the glacier was applied to the surface for 15,000 years before the melting started, the best fitting earth viscosity model is a 10²⁴ Nm lithosphere overlying a 75 km-thick 2.0 × 10¹⁹ Pas asthenosphere and a 1.2 × 10²¹ Pas mantle. The simulations suggest a remaining maximum uplift of 40 m.     

Seasonal variations of helium,radon, and uranium in lake waters near the Key Lake uranium deposit,Saskatchewan

To test the usefulness of the He method for the detection of buried U ore, a lake-water survey of the Key Lake area was carried out during March 1977 and repeated in June 1977. A set of 87 lake-water samples from 37 sites in eight different lakes were collected from the same sites in each survey. These samples were analyzed for He, Rn, O₂, Eh, pH, conductivity, alkalinity, and U.In the lakes the highest He concentrations, of nearly four times atmospheric equilibrium concentrations, were obtained during the winter survey. This was under the ice in two bottom lake samples where the sandy overburden extends for some 50 m directly to the ore zone. Radon and U values at these sites were also anomalous, but the highest values of Rn and U were obtained in a shallow lake about 2 km south of the ore zone where concentrations of radioactive boulders occur.The average net He and Rn contents dropped from 21 standard nanolitres/litre (nl/l) and 25 picocuries/litre (pCi/l) in the winter under the ice to 3 nl/l and 8 pCi/l in the summer, respectively. Average U and conductivity dropped from 2.3 ppb and 15 μmho/cm to 1.5 ppb and 11 μmho/cm, respectively, in the same suites of samples. The drop in the ionic species probably reflects the effect of snow and ice meltwater dilution and the much larger drop in the dissolved He and Rn reflects the combined effects of meltwater dilution, wind turbulence over ice-free lakes, and change in thermal gradients.The He results must be viewed with guarded optimism until a detailed investigation is completed to ascertain what fraction of the detected He is from the ore, what fraction from basement He that may have found its way into the lakes via the fracture zone in which the ore is located, and what effect drilling has had on the He flow into the lakes from the ore zone.     

Der nacheiszeitliche Bergsturz im Kandertal (Schweiz): Alter und Auswirkungen auf die damalige Umwelt

Zusammenfassung. Beim Bau des neuen AlpTransit Lötschberg Basistunnels wurden unter murgangartig verschwemmten Ablagerungen der alten Bergsturzmasse des Kandertals Stillwasserablagerungen mit zahlreichen organischen Resten und Torflagen gefunden. Die ¹⁴C-datierten Resultate der Pollen, Makrorest-, Holz- und Holzkohleanalysen ermöglichten eine Rekonstruktion der lokalen bis regionalen Umweltgeschichte. Ein Gewässer, vermutlich ein kleiner See, begann beim Tellenfeld in Frutigen um 8800 kal. Jahre v. Chr. zu verlanden. In der näheren Umgebung wuchs von 8800 v. Chr. bis 8000 v. Chr. ein Föhrenwald (Pinus silvestris), der reichlich mit Hasel (Corylus avellana) und anderen wärmeliebenden Gehölzen (Ulmen, Linden, Eichen; Ulmus, Tilia, Quercus) und Birken (Betula) durchsetzt war. Diese für die Nordalpen sehr frühe Bedeutung der Hasel ist durch ¹⁴C-datierte Corylus-Nussfragmente (9310±50 ¹⁴C BP, 8722–8337 v. Chr.) belegt. Nach 8500 v. Chr. drängte die Hasel die Waldföhre allmählich zurück. Auf Grund der paläoökologischen Resultate muss angenommen werden, dass die Wälder um 7600 v. Chr. durch ein katastrophales Ereignis stark gestört wurden. Als Reaktion darauf kam es zu einer starken Zunahme der Waldbrände und es breiteten sich zuerst Farne und Gräser sowie wenig später Waldföhren aus. Das Gewässer wurde um 7100 v. Chr. durch verschwemmtes Bergsturzmaterial zerstört. Der geomorphologische Befund deutet darauf hin, dass diese Ereignisse in engem Zusammenhang mit dem Hauptbergsturz im Kandertal stehen, der aussergewöhnliche Ausmasse hatte (ca. 800 Millionen m³). Die Zerstörung der lokalen ökosysteme als Folge des Bergsturzes um 7600–7100 v. Chr. fiel in ein frühes holozänes Wärme- und Sonneneinstrahlungsmaximum, in dem es, wie vorgängige Untersuchungen in den Alpen und in anderen Gebirgen belegen, zu überdurchschnittlich vielen Hanginstabilitäten kam.

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During the construction of the new AlpTransit railway line wetland sediments containing numerous fossils and peat layers were found below rockfall masses transported by debris flows. Radiocarbon-dated results of pollen, macrofossils, wood, and charcoal along with radiocarbon dating analysis were used to reconstruct the environmental history of the site. The wetland, originally probably a small lake, started to accumulate sediments at about 8800 cal. yr BC at Frutigen Tellenfeld. A pine forest (Pinus silvestris) admixed with hazel (Corylus avellana), other thermophilous arboreal taxa (Ulmus, Tilia, Quercus) and birch (Betula) grew in the surroundings of the lake. This very early importance of hazel is documented by ¹⁴C-dated Corylus nut fragments (9310±50 ¹⁴C yr BP, 8722–8337 yr BC). After 8500 BC hazel expanded on the costs of pine. The palaeo records suggest that the forests were severely disturbed by a catastrophic event at around 7600 BC. In response, forest fires strongly increased and ferns and grasses expanded and then pine stands established. At ca. 7100 BC the lake was abruptly destroyed by rockfall masses transported by a debris flow. The geomorphic situation suggests that these events were closely related with the main Kander valley rockfall, which had an exceptional size (800 millions m³). Local environmental catastrophes as a consequence of the rockfall at 7600–7100 BC occurred during an early Holocene thermal and solar irradiation maximum. As documented by previous investigations, this period was characterised by pronounced slope instabilities in the Alps and elsewhere.

Manuskript eingegangen 9. Februar 2004 Revidierte Fassung angenommen 17. Januar 2005     

Propagation pathways and fluid transport of hydrofractures in jointed and layered rocks in geothermal fields

Palaeofluid-transporting systems, observed as networks of mineral-filled veins in deeply eroded parts of extinct geothermal fields, indicate that hydrofractures commonly supply fluids to geothermal fields. Here we examine well-exposed vein networks that occur at crustal depths of around 1.5 km below the initial surface of the Tertiary lava pile in North Iceland. The veins are located in the damage zone of a major fault zone that dissects basaltic lava flows, the most common host rocks of geothermal fields in Iceland. The lava flows contain numerous weaknesses, particularly columnar (cooling) joints and contacts. For hydrofractures to supply fluids to geothermal fields, the fractures must be able to propagate, and transport fluids, to the surface. We explore hydrofracture pathway formation using boundary-element models of hydrofractures with fluid overpressure varying linearly from 10 MPa at the fracture centre to 0 MPa at the fracture tip (or the fluid front). The hydrofractures propagate through a vertically jointed and horizontally layered pile of lava flows with a general rock-matrix Young’s modulus of 1×10¹⁰ Pa and a Poisson’s ratio of 0.25. The joints and contacts between layers are modelled as internal springs, each with a stiffness (‘strength’) of 6 MPa/m. The location and sizes of discontinuities, as well as the location of the hydrofracture tip, vary between the models. The results indicate that tensile stresses generated at the tip of an overpressured hydrofracture can open up horizontal and vertical discontinuities out to a considerable distance from the tip, and that these discontinuities eventually link up to form the hydrofracture pathway. Analytical models indicate that for a hot spring of a given yield associated with a fault, the dimensions of the fluid-transporting part of the fault are likely to be similar for a typical normal fault and a strike-slip fault. Also, a hot spring of yield 180 l/s (the maximum in the low-temperature fields of Iceland) can be supplied through a hydrofracture of aperture 3 mm and trace length 1.2 m. These dimensions are very similar to those of typical veins in the studied networks. Buoyancy, rather than excess pressure in the fluid source, appears to be the primary driving force of hydrofractures in the geothermal fields of Iceland.     

Formation of Asteroid Families by Catastrophic Disruption: Simulations with Fragmentation and Gravitational Reaccumulation

This paper builds on preliminary work in which numerical simulations of the collisional disruption of large asteroids (represented by the Eunomia and Koronis family parent bodies) were performed and which accounted not only for the fragmentation of the solid body through crack propagation, but also for the mutual gravitational interaction of the resulting fragments. It was found that the parent body is first completely shattered at the end of the fragmentation phase, and then subsequent gravitational reaccumulations lead to the formation of an entire family of large and small objects with dynamical properties similar to those of the parent body. In this work, we present new and improved numerical simulations in detail. As before, we use the same numerical procedure, i.e., a 3D SPH hydrocode to compute the fragmentation phase and the parallel N-body code pkdgrav to compute the subsequent gravitational reaccumulation phase. However, this reaccumulation phase is now treated more realistically by using a merging criterion based on energy and angular momentum and by allowing dissipation to occur during fragment collisions. We also extend our previous studies to the as yet unexplored intermediate impact energy regime (represented by the Flora family formation) for which the largest fragment's mass is about half that of the parent body. Finally, we examine the robustness of the results by changing various assumptions, the numerical resolution, and different numerical parameters. We find that in the lowest impact energy regime the more realistic physical approach of reaccumulation leads to results that are statistically identical to those obtained with our previous simplistic approach. Some quantitative changes arise only as the impact energy increases such that higher relative velocities are reached during fragment collisions, but they do not modify the global outcome qualitatively. As a consequence, these new simulations confirm previous main results and still lead to the conclusion that: (1) all large family members must be made of gravitationally reaccumulated fragments; (2) the original fragment size distribution and their orbital dispersion are respectively steeper and smaller than currently observed for the real families, supporting recent studies on subsequent evolution and diffusion of family members; and (3) the formation of satellites around family members is a frequent and natural outcome of collisional processes.     

The use of helium in mineral exploration

A literature review of the source and occurrence of helium shows that it can, under favorable conditions, lead to the discovery of radioactive ore deposits, oil and gas pools, and fracture zones associated with mineral occurrences. Analytical results show that anomalous helium is present in groundwaters and near uranium occurrences and thus can aid in the identification of uranium occurrences or prospective target areas for uranium exploration.     

Subsurface hydrology of layered colluvium mantles in unchannelled valleys—south-Eastern Brazil

The effects of subsurface hydrology on the evolution of erosive processes inside layered colluvium mantles in unchannelled valleys are investigated. Tensiometer nests are installed in different morphological conditions with their specific depths controlled by the subsurface structure of the colluvium mantle. Daily readings (during one year) and physical and mechanical analyses of the different colluvium layers are carried out. In spite of a highly discontinuous subsurface structure, present topography is the major factor controlling subsurface flow in this landscape. Paleotopographic features, usually resulting from the lateral mobilization of the previous topographic hollow axis, can locally control the preferential expansion of the saturated zone towards areas other than the present hollow axis. During rainy periods, pressure head at the base of the hollow axis increases towards a maximum value which is not increased by subsequent precipitation inputs. New water input is dissipated by direct precipitation onto the saturated zone and exfiltration by seepage at the gully head walls, precluding the development of the excess pore pressures required to trigger failure by landsliding.