Oligo-Miocene extensional tectonics and fluid flow across the Northern Snake Range detachment system, Nevada

The Northern Snake Range (Nevada) represents a spectacular example of a metamorphic core complex and exposes a complete section from the mylonitic footwall into the hanging wall of a fossil detachment system. Paired geochronological and stable isotopic data of mylonitic quartzite within the detachment footwall reveal that ductile deformation and infiltration of meteoric fluids occurred between 27 and 23 Ma. Ar-40/Ar-39 ages display complex recrystallization-cooling relationships but decrease systematically from 26.9 +/- 0.2 Ma at the top to 21.3 +/- 0.2 Ma at the bottom of footwall mylonite. Hydrogen isotope (delta D) values in white mica are very low (-150 to -145 %) within the top 80-90 m of detachment footwall, in contrast to values obtained from the deeper part of the section where values range from -77 to -64 %, suggesting that time-integrated interaction between rock and meteoric fluid was restricted to the uppermost part of the mylonitic footwall. Pervasive mica-water hydrogen isotope exchange is difficult to reconcile with models of Ar-40 loss during mylonitization solely by volume diffusion. Rather, we interpret the Ar-40/Ar-39 ages of white mica with low-delta D values to date syn-mylonitic hydrogen and argon isotope exchange, and we conclude that the hydrothermal system of the Northern Snake Range was active during late Oligocene (27-23 Ma) and has been exhumed by the combined effects of ductile strain, extensional detachment faulting, and erosion.

High-resolution ammonite and carbon isotope stratigraphy across the Triassic-Jurassic boundary at New York Canyon (Nevada)

The Triassic-Jurassic boundary is generally considered as one of the major extinctions in the history of Phanerozoic. The high-resolution ammonite correlations and carbon isotope marine record in the New York Canyon area allow to distinguish two negative carbon excursions across this boundary with different paleoenvironmental meanings. The Late Rhaetian negative excursion is related to the extinction and regressive phase. The Early Hettangian delta(13)C(org) negative excursion is associated with a major floristic turnover and major ammonite and radiolarian radiation. The end-Triassic extinction-Early Jurassic recovery is fully compatible with a volcanism-triggered crisis, probably related to the Central Atlantic Magmatic Province. The main environmental stress might have been generated by repeated release of SO(2) gas, heavy metals emissions, darkening, and subsequent cooling. This phase was followed by a major long-term CO(2) accumulation during the Early Hettangian with development of nutrient-rich marine waters favouring the recovery of productivity and deposition of black shales. (C) 2004 Elsevier B.V. All rights reserved.

The conditions for the reproduction of the SAGUAPAC water cooperative in the city of Santa Cruz de la Sierra, Bolivia : discourse analysis

The SAGUAPAC cooperative in the city of Santa Cruz de la Sierra (Eastern Bolivia) is regularly presented as an example of cooperative successes regarding water supply and sanitation. Its efficiency, both economic and technical, is widely considered as the main reason for its attractiveness. However, without denying its importance, we show, through a discourse analysis from and about SAGUAPAC in local media, that moral and non-instrumental factors are crucial in the reproduction of the cooperative. These factors create attachment and affection toward the cooperative, through a storytelling using a four-dimensional rhetoric (mythification, identification, emotionalisation and personification). This storytelling technique, internalized in the local media discourse and materializing the so-called new spirit of capitalism, exploits the affects and instrumentalisation of local myths and legends, as well as the 'camba' ethnic identity. In that, it tends to retain SAGUAPAC members and to canvass new ones, by providing them with recognition in their quality of local community members. However, the mobilisation of social norms and power hierarchies might end up reinforcing the social exclusion of Andean non-camba immigrants, inspite of an a priori inclusive and democratic organisation.