Accretion and exhumation at a Variscan active margin, recorded in the Saxothuringian flysch

The Saxothuringian flysch basin, on the north flank of the Central European Variscides, was fed and eventually overthrust by the northwestern, active margin of the Tepla-Barrandian terrane. Clast spectra, mineral composition and isotopic ages of detrital mica and zircon have been analyzed in order to constrain accretion and exhumation of rocks in the orogenic wedge. The earliest clastic sediments preserved are of early Famennian age (ca. 370?Ma). They are exposed immediately to the NW of the suture, and belong to the par-autochthon of the foreland. Besides ultramafic (?ophiolite) material, these rocks contain clasts derived from Early Paleozoic continental slope sediments, originally deposited at the NW margin of the Saxothuringian basin. These findings, together with the paleogeographic position of the Famennian clastics debris on the northwestern passive margin, indicate that the Saxothuringian narrow ocean had been closed by that time. Microprobe analyses of detrital hornblendes suggest derivation from the “Randamphibolit” unit, now present in the middle part of the Saxothuringian allochthon (Münchberg nappes). Detrital zircons of metamorphic rocks formed a little earlier (ca. 380?Ma) indicate rapid recycling at the tectonic front. The middle part of the flysch sequence (ca. early to middle Viséan), both in the par-autochthon and in the allochthon, contains abundant clasts of Paleozoic rocks derived from the northwestern slope and rise, together with debris of Cadomian basement, 500-Ma granitoids and 380?Ma (early Variscan) crystalline rocks. All of these source rocks were still available in the youngest part of the flysch (c. middle to late Viséan), but some clasts record, in addition, accretion of the northwestern shelf. Our findings permit deduction of minimum rates of tectonic shortening well in excess of 10–30?mm per year, and rates of exhumation of ca. 3?mm/a, and possibly more.     

Argillite debris converted to bauxite during karst weathering: evidence from immobile element geochemistry at the Olmedo Deposit, Sardinia

The Olmedo bauxite deposit occurs in the Nurra district of northwest Sardinia. It forms a stratiform horizon in Cretaceous limestone and marl. Uplift in mid-Cretaceous had exposed recently deposited limestone to karst weathering, and a layer of argillaceous debris accumulated on its surface and was partly converted to bauxite. Intermediate products were desiccated marl, bauxitic argillite and argillaceous bauxite. Subsidence followed, and the bauxite was preserved by the deposition of late Cretaceous limestone and other sediments. Uplift in Oligocene-Miocene time, with ensuing erosion, exposed the bauxite horizon to its present configuration. Concentrations of normative minerals illustrate chemical processes and the build-up of Al in the bauxite horizon. Plots of chemical data and correlation coefficients show that Al, Ti, Zr, Nb, Th, Cr and V were immobile during the bauxitization process. Mass changes point to large net removal of Si, Mg and K from the system, although some of this material and slightly mobile Al were reprecipitated in the underlying argillite and altered marl. Immobile element ratios trace the source of the bauxite to the underlying argillaceous limestone. Al in the bauxite was accumulated from the degradation of 25 to 50 m of the argillaceous limestone. Received: 10 June 1996 / Accepted: 15 April 1997     

Optimal weighting in the finite difference solution of the convection-dispersion equation

Oscillation and numerical dispersion limit the reliability of numerical solutions of the convection-dispersion equation when finite difference methods are used. To eliminate oscillation and reduce the numerical dispersion, an optimal upstream weighting with finite differences is proposed. The optimal values of upstream weighting coefficients numerically obtained are a function of the mesh Peclet number used. The accuracy of the proposed numerical method is tested against two classical problems for which analytical solutions exist. The comparison of the numerical results obtained with different numerical schemes and those obtained by the analytical solutions demonstrates the possibility of a real gain in precision using the proposed optimal weighting method. This gain in precision is verified by interpreting a tracer experiment performed in a laboratory column.     

The separation of the influence of nutrients and climate on the varve time-series of baldeggersee, Switzerland

The varve data-set from a freeze-core taken in the deepest part of Baldeggersee was subjected to different multivariate statistical analyses in order to estimate the amount of variance in the varve thickness measures explicable by past climate and by the trophic state of the lake. A comparison of two different time-periods (1902 to1992 versus 1920 to 1980) revealed that the lake restoration programme since 1982 has had a significant impact on the formation of the seasonal layers. Results of the partitioning of the variance in the varve thickness measures showed that about two thirds of the variance are unexplained by a climate and trophic state model and that trophic state explains 6%, whereas climate accounts for about 28% of the variance before the effect of lake restoration had a strong impact on the varves. Among the climate parameters the amount of annual precipitation is a strong predictor for explaining the thickness of both dark layer and total couplet thickness, whereas summer precipitation is important for the thickness of the light layer.     

The color of pumice: case study on a trachytic fall deposit, Meidob volcanic field, Sudan

 This paper examines the cause of color variations of trachytic pumices which are essentially uniform in chemical composition and proposes a geological model for their formation. A pyroclastic sequence of distinct subunits with brown, buff, and black pumices was deposited during the 5000-B.P. eruption of a tuff ring in the central Meidob volcanic field (Sudan). Subunits of buff pumices locally contain minor amounts of streaky pumice with pale-gray and dark-gray domains. The combined evidence of petrographic studies, chemical analyses of whole pumices and groundmass separates, electron microprobe analyses, optical spectroscopy, transmission electron microscopy, and magnetic susceptibility measurements show that color variations of the pumice clasts are related to the size and distribution of Fe³⁺-rich oxide microcrysts. Buff pumice and light-gray domains of streaky pumice have a colorless, transparent groundmass with very few microcrysts. Dark-gray domains of streaky pumice contain abundant hematite and/or magnetite microcrysts visible in thin section within a transparent, colorless glass groundmass. The groundmass of the black pumice clasts is brown in thin section which is most likely caused by submicroscopic magnetite microcrysts. Brown pumice clasts have a mixed groundmass consisting of brown domains and domains with opaque microcrysts in transparent glass. Variations in the eruption dynamics have been inferred from lithological observations. Subunits of black pumices are related to eruption pulses with low magma discharge and high water/magma mass ratio, whereas subunits of buff pumice were deposited during eruption pulses with high magma discharge and low water/magma mass ratio. Brown pumices represent the top part of the magma body, and the initial stage of the eruption probably had a low magma discharge. Streaky pumices are interpreted as the product of syn-eruptive mixing of Fe³⁺-rich oxide microcryst-bearing magma and microcryst-free magma. Received: 3 February 1997 / Accepted: 28 July 1997     

Testing an earthquake prediction algorithm

A test to evaluate earthquake prediction algorithms is being applied to a Russian algorithm known asM8 TheM8 algorithm makes intermediate term predictions for earthquakes to occur in a large circle, based on integral counts of transient seismicity in the circle. In a retroactive prediction for the period January 1, 1985 to July 1, 1991 the algorithm as configured for the forward test would have predicted eight of ten strong earthquakes in the test area. A null hypothesis, based on random assignment of predictions, predicts eight earthquakes in 2.87% of the trials. The forward test began July 1, 1991 and will run through December 31, 1997. As of July 1, 1995, the algorithm had forward predicted five out of nine earthquakes in the test area, which success ratio would have been achieved in 53% of random trials with the null hypothesis.     

Artificial periodic irregularities in the auroral ionosphere

Artificial periodic irregularities (API) are produced in the ionospheric plasma by a powerful standing electromagnetic wave reflected off the F region. The resulting electron-density irregularities can scatter other high-frequency waves if the Bragg scattering condition is met. Such measurements have been performed at mid-latitudes for two decades and have been developed into a useful ionospheric diagnostic technique. We report here the first measurements from a high-latitude station, using the EISCAT heating facility near Troms0, Norway. Both F-region and lower-altitude ionospheric echoes have been obtained, but the bulk of the data has been in the E and D regions with echoes extending down to 52-km altitude. Examples of API are shown, mainly from the D region, together with simultaneous VHP incoherent-scatter-radar (ISR) data. Vertical velocities derived from the rate of phase change during the irregularity decay are shown and compared with velocities derived from the ISR. Some of the API-derived velocities in the 75–115-km height range appear consistent with vertical neutral winds as shown by their magnitudes and by evidence of gravity waves, while other data in the 50–70-km range show an unrealistically large bias. For a comparison with ISR data it has proved difficult to get good quality data sets overlapping in height and time. The initial comparisons show some agreement, but discrepancies of several metres per second do not yet allow us to conclude that the two techniques are measuring the same quantity. The irregularity decay time-constants between about 53 and 70 km are compared with the results of an advanced ion-chemistry model, and height profiles of recorded signal power are compared with model estimates in the same altitude range. The calculated amplitude shows good agreement with the data in that the maximum occurs at about the same height as that of the measured amplitude. The calculated time-constant agrees very well with the data below 60 km but is larger above 60 km by a factor of up to 2 at 64 km. The comparisons with the model are considered to be a good basis for more refined comparisons.     

ASPI experiment: measurements of fields and waves on board the INTERBALL-1 spacecraft

The plasma-wave experiment ASPI (analysis of spectra of plasma waves and instabilities) on board the INTERBALL spacecraft is a combined wave diagnostics experiment. It performs measurements of the DC and AC magnetic field vector by flux-gate and searchcoil sensors, the DC and AC electric field vector by Langmuir double probes and the plasma current by Langmuir split probe. Preliminary data analysis shows the low noise levels of the sensors and the compatibility of new data with the results of previous missions. During several months of in-orbit operation a rich collection of data was acquired, examples of which at the magnetopause and plasma sheet are presented in second part of the paper.     

EISCAT/CRRES observations: nightside ionospheric ion outflow and oxygen-rich substorm injections

We present combined observations made near midnight by the EISCAT radar, all-sky cameras and the combined released and radiation efects satellite (CRRES) shortly before and during a substorm. In particular, we study a discrete, equatorward-drifting auroral arc, seen several degrees poleward of the onset region. The arc passes through the field-aligned beam of the EISCAT radar and is seen to be associated with a considerable upflow of ionospheric plasma. During the substorm, the CRRES satellite observed two major injections, 17 min apart, the second of which was dominated by O⁺ ions. We show that the observed are was in a suitable location in both latitude and MLT to have fed O⁺ ions into the second injection and that the upward flux of ions associated with it was sufficient to explain the observed injection. We interpret these data as showing that arcs in the nightside plasma-sheet boundary layer could be the source of O⁺ ions energised by a dipolarisation of the mid- and near-Earth tail, as opposed to ions ejected from the dayside ionosphere in the cleft ion fountain.