A well-preserved Cambrian impact exposed in Central Sweden

The Lockne impact crater south of Östersund formed in the early Middle Cambrian with a diameter of 7 km. It is identified by its rim wall of crushed Precambrian basement granite, by fragments of impact melt, and by grains of shocked quartz. The exceptional preservation, in particular of the rim wall, is due to a complicated geological history, the first stage of which consisted of burial by marine sediments. This stage lasted until the Middle Ordovician, or over 50 million years. An early Caradoc lowering of the sea-level may have induced debris flows that stripped the rim wall of much of its sedimentary cover. Because normal marine sedimentation recommenced soon after this event, the structure was not seriously damaged, as it was buried again. The Caledonian orogeny emplaced an overthrust nappe as ultimate protection, which was removed by a recent erosion episode from all but the center of the structure.Structures formed by the impact of extraterrestrial bodies are very rare throughout much of Europe, because such structures are neither well preserved nor displayed in young mountain belts or sedimentary basins. However, northern Europe has several ascertained structures of this kind (Svensson &Wickman, 1965;Svensson, 1968;Bruun &Dahlman, 1982;Kala et al., 1984;Flodén et al., 1986;Wickman, 1988). Unfortunately, the hitherto known structures are either poorly preserved or hidden by younger deposits. We are reporting the discovery of a well-preserved exposed and accessible impact structure that has escaped the notice of geologists although important features of it have been described and puzzled over by generations of researchers (Wiman, 1900;Hadding, 1927;Thorslund, 1940;Lindström et al., 1983).The structure is located in the Lockne area to the south of Östersund in central Sweden. It has a diameter of 7 km and its center is near Tramsta on the northwest shore of Lake Locknesjön (Fig. 1). Its middle is covered by folded Lower to Middle Ordovician Orthoceratite Limestone with the sheared Lower Ordovician Töyen Shale at its base. This local expanse of deformed rock is the remainder after erosion of an extensive nappe of overthrust rocks emplaced during the Caledonian orogeny. The good preservation of the impact structure is due to the nappe cover, which had to be removed before erosion could attack the underlying structures.The rim of the impact crater is outlined by a wall of strongly shattered fragments of Proterozoic crystalline rocks (Fig. 2), which formed the local bedrock (Strömberg et al., 1984) at the time of impact. The rim wall is best preserved along the western part of the structure. It was referred to as »arkose-like breccia« byThorslund (1940), who interpreted it as the result of continental weathering, but the components rather show evidence of intense crushing than of weathering (Simon, 1987a). The »arkose-like breccia« does not contain components derived from the lower Palaeozoic deposits of the area.     

Hydrodynamics of Lake Lugano

The hydrodynamic processes in the Lake of Lugano (Lago di Lugano) have been the object of three major measuring campaigns. The first during summer 1979 in the North Basin, the second in the autumn of 1984 in the South Basin and the last in 1989 again in the North Basin. It is expected that analysis of the collected data in conjunction with theoretical concepts will yield conclusive findings on the driving forces and hydrodynamics of the lake. Numerical models have been applied to the lake and the results have been compared to the measurements. Some models calculate the periods and distributions of amplitude, as well as surface velocities and internal seiches. Other models calculate wind-induced water circulation. Evidence is given to the peculiarities of the North Basin and the difficulties related with the depth-width ratio and with its deep chemical stratification.     

Deterministic and stochastic parameters in inertial survey adjustment

In the adjustment of inertial position surveys the additional parameters describing the systematic errors of individual traverses can be considered as deterministic or stochastic. The paper deals with various aspects of the deterministic or stochastic approach by way of a standard functional model. If purely deterministic parameters are set up, the solvability of the least squares problem depends on redundant observations like coordinate discrepancies of forward and backward runs or coordinate differences at cross-over points of traverse networks. Inequalities are presented to handle the configuration problem for any net and for several ways of introducing parameter sets. Also condition equations being geometrically explainable are developed solving the datum problem in free adjustment applications. Based on the Ebersberger Forst campaigns with a large amount of Ferranti, Honeywell and Litton data, numerical investigations into the stochastic properties of the additional parameters and the observations follow. It turns out that additional parameters for Honeywell and Litton data can be considered as stochastic parameters while for Ferranti data significant azimuth and time dependent effects can be found. The investigations of true errors show that in case of the deterministic adjustment approach a diagonal covariance matrix can be introduced and in case of stochastic additional parameters a first order Gauss-Markov process serves as a good approximation for the stochastic behaviour of the observations.     

Frontbildung während einer Phase extremer Schichtung in der Deutschen Bucht

Zusammenfassung Es wird eine Phase sehr ausgeprägter Temperaturschichtung in der Deutschen Bucht beschrieben, die bei einer sehr stabilen Hochdruck-Wetterlage aufgetreten ist. Sie war mit deutlichen Frontbildungen und Auftriebserscheinungen verbunden. Rechnungen mit einem hydrodynamischen Modell unterstützen diese Auffassung.Eine besonders ausgeprägte Temperaturfront folgt klar der 20 m-Tiefenlinie. Daraus wird im Hinblick auf die Theorie von Pingree und Griffith [1978]) geschlossen, daß bei der herrschenden Wetterlage die Gezeitenstromturbulenz die Hauptursache für die Vermischung der Wasserschichten ist: Auftrieb spielt eine untergeordnete Rolle.

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Frontogenesis during a phase of extreme stratification in the German Bight

Summary This paper describes a phase of pronounced thermal stratification in the German Bight which developed during a period of very stable high pressure. Calculations using a hydrodynamic model support the view that it was accompanied by strong frontogenesis and upwelling.A very distinct temperature front is found along the 20 m depth contour. With regard to Pingree and Griffith's [1978] theory, we conclude from this that in such weather conditions, turbulence due to tidal currents mainly causes the water strata to mix and that upwelling is only of secondary importance.

     

A prototype earthquake warning system for strike-slip earthquakes

A prototype expert system has been developed to provide rapid warning of earthquakes while they are occurring. Warning times of up to 100 seconds will be possible. In the complete system, several accelerometers are distributed at intervals within a few kilometers of a known fault; data are telemetered to a central computer which implements the expert system. The expert system incorporates specific information about the type of fault to be monitored, and includes simple rules for estimating the fault slip, rupture length, and seismic moment, all in real time. If the seismic moment exceeds a preset value, an alarm may be issued. The prototype is designed for deployment on near-surface strike-slip faults such as the San Andreas and has been successfully tested with data from the 1979 Imperial Valley and 1984 Morgan Hill earthquakes. Crucial concepts have also been tested using synthetic data calculated for a model of the 1857 Fort Tejon earthquake. Parkfield, California, could be used as a test site.     

Miscellaneous information

Miscellaneous information     

Provenance and depositional environment of Rhenohercynian synorogenic greywackes from the Giessen Nappe,Germany

The Giessen Nappe of the south-eastern Rhenish Massif can be subdivided into a northern group and southern group of Frasnian-?Lower Carboniferous greywackes that differ slightly in their sedimentology, clast petrography, heavy mineral assemblages and bulk geochemistry. Petrographic and geochemical tectonic setting discrimination diagrams indicate that both the northern and the southern Giessen Greywacke groups were derived from a predominantly acidic continental arc source. However, due to the nature of basin development, generation of significant volumes of acidic arc material as a result of southwards directed subduction of Rhenohercynian oceanic crust is considered unlikely. A large part of the arc source may have been contributed by earlier, predominantly acidic, Precambrian terranes together with minor basic igneous and sedimentary components.     

Parallelisierung des norddeutschen oberen Muschelkalks mit dem süddeutschen Hauptmuschelkalk anhand von Sedimentationszyklen

The interpretation of microfacial and geochemical sediment features resulted in a detailed sequence analysis of Upper Muschelkalk strata in the northern and middle parts of West-Germany. It is now possible to correlate columnar sections at far distances. The method allows to make a synthesis of basin evolution. In this way it also succeed to reconstruct the dynamic activities in an epicontinental sedimentation area. The Upper Muschelkalk strata shows a hierarchically nested development of stratification cycles. There are minor cycles of probably paleotectonic origin, which are only locally significant. The major cycles are basing probably on sea level changes and allow supraregional parallelizations of sections. Indicators of the sea level maximums are clay horizons. They are called »Tonhorizonte« in Southern Germany and could be proved also in the Upper Muschelkalk strata of Northern Germany. So the stratigraphic correlation of the two sedimentation areas, which were so different in paleotectonic views, is possible.     

Age and evolution of scheelite-hosting rocks in the Felbertal deposit (Eastern Alps): U-Pb geochronology of zircon and titanite

U-Pb isotope analyses of zircon and titanite extracted from different rocks of the Felbertal scheelite deposit yield the following information: (1) An age of 593±22 Ma (2) is obtained for zircon crystallization in the scheelite-bearing matrix of an eruption breccia in the western ore field. (2) Discordant zircons from an elongated, up to 8 m thick scheelite-rich quartzite body in the eastern ore field give an upper intercept age of 544±5 Ma. This quartzite contains a laminated, fine-grained scheelite mineralization. (3) Zircons from a small granitoid intrusion of the western ore field reveal an age of 336±16 Ma, and concordant titanites document an age of 282±2 Ma for Variscan amphibolite facies metamorphism. Both events, granitoid intrusion and later metamorphism caused ore re-mobilization, including the formation of yellowish fluorescent (molybdo-) scheelite porphyroblasts. (4) For a narrow lamprop-1hyric dike in the western ore field, a concordant titanite age of 283±7 Ma is obtained. This age is identical with the titanites from the amphibolite facies metamorphic intrusion. Tiny scheelite grains were tapped by the dike from pre-existing scheelite mineralizations in the truncated host rocks. (5) Alpine metamorphism at 31±4 Ma did not exceed lowermost amphibolite facies conditions, and it caused scheelite re-mobilization on a minor scale only, producing bluish fluorescent porphyroblasts in quartz veinlets and veins, as well as bluish fluorescent scheelite rims around older scheelite grains. Moreover, crosscutting Alpine fissure fillings show bluish fluorescent, inclusion-free scheelite. (6) The preservation of Variscan titanites, the absence of Alpine titanite growth, and the large degree of Variscan scheelite re-mobilization demonstrate that amphibolite facies metamorphism in the Felbertal area has a Variscan age. This result clearly documents Variscan tectono-metamorphism to be the dominant event, instead of the hitherto surmised Alpine metamorphism. This multi-stage evolution of the Felbertal ore bodies corroborates the view that tungsten deposits are conditioned by several succeeding thermal events, leading to a series of stages that ultimately produce high-grade scheelite concentrations. These high-grade ores predominately occur along shear zones of different age, accompanied by the formation of large volumes of low-grade scheelite mineralizations along host rock foliations and quartz veinlets and veins.