Die Bedeutung des Magmas in der Orogenese der südamerikanischen Kordillere

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Soil geochemistry of Mother Lode-type gold deposits in the Hodson mining district, central California, U.S.A.

The Hodson mining district is in the westernmost foothills of the Sierra Nevada in California, about 17 km west of the town of Angels Camp. This district is part of the West Gold Belt, which lies about 12–16 km west of, and generally parallel to, the better known Mother Lode Gold Belt in central California. The district produced several million dollars worth of Au between about 1890 and 1940.     

Isotope composition of Medieval lead glasses reflecting early silver production in Central Europe

The lead isotope composition of 32 lead glasses excavated from strata of the twelfth to early fifteenth century in six countries of NW Europe made the predominance of the Harz Mountains in this period of the Medieval European lead and silver production highly probable. Post-Variscan vein type galena, Devonian syngenetic hydrothermal ore (Rammelsberg) and blended ore from both deposits in the Harz were used. Our evaluation of 200 mining sites in Germany, Britain and Ireland also demonstrated that minor lead sources for lead glass were located in Bavaria/Bohemia and England. Lead ores from the Rhenohercynian orogenic belt in Germany are derived from rock sources close to the upper continental crust composition in ²³⁸U/²⁰⁴Pb of about 10 and Th/U of about 4. The ores in Central England originated from rocks with ²³⁸U/²⁰⁴Pb of about 11. Received: 24 September 1996 / Accepted: 17 January 1997     

Fold development in the Jura

The term “folding” encompasses a wide range of processes, most of which are poorly understood. Jura folds, though comparatively simple, have developed by a superposition of different types of instabilities both in space and time. They are never periodic and sinusoidal and are more realistically approximated by kink bands with rounded hinges. Thrusting and kinking instabilities had closely similar thresholds, with kinks usually following and deforming thrusts. An analysis of embryonic folds shows that instabilities in the sedimentary cover were initiated primarily at inherited flaws of the basal décollement layer. They thence spread upward, often following stratigraphically higher incompetent layers in secondary décollement and there nucleating secondary instabilities before reaching the surface (disharmonic folding). Embryonic folds thus are usually narrow, emanating from secondary décollement layers that are connected with the basal décollement zone by thrusts nucleated at inherited obstacles. These are eventually overcome, permitting basal décollement to coalesce with kinking instabilities that grow downward from nuclei in higher décollement intervals. In this way folds centered in the basal décollement layer, and consequently of normal width, may be superposed on the narrow embryonic folds. The sequence and importance of the different elements may vary from place to place to result in a vast catalog of fold shapes.     

Geochemical processes in the saltwater–freshwater transition zone: comparing results of a sand tank experiment with field data

Geochemical processes, occurring in a stable transition zone between saltwater and freshwater, were simulated in a 2D, multi-layer flow chamber experiment. Mixing, calcite dissolution, and oxidative degradation of organic matter were identified as the main controlling factors. The results of the chamber experiment were compared to field data and verified by thermodynamic modeling. Similarity in most ion distributions suggests the general applicability of the experimental method. Differences in the redox conditions between field and experiment were reflected by the oxidants involved in the mineralization of organic carbon; while field data show evidence of sulfate reduction, the presence of oxygen in the laboratory experiment resulted in the reoxidation of sulfides.     

A statistical analysis applied to the Rf/φ method

A statistical analysis for the R_f/φ method is developed in the paper. The technique provides estimates of R_s, R_i and θ according to R_f and φ. Moreover, we can restore the pattern of initial orientations of marker objects by means of their frequency histogram, provided the strain is homogeneous. It is unnecessary to postulate random initial orientations if the principal directions of strain ellipsoid are known.