Post‐nappe brittle extension in the inner Western Alps (Schistes Lustrés) following late ductile exhumation: a record of synextension block rotation?

Fault data collected from the Schistes Lustrés domain point to the existence of successive steps of deformation and indicate that extension is not multidirectional. This study underlines the continuity between the patterns of late brittle/ductile exhumation tectonics and brittle deformation, and strenghtens the view that extensional movements dominate in shallow levels of the inner Western Alps since at least 35–30 Ma. The progressive clockwise rotation of the earliest directions of extension with time is compatible with the amount of anticlockwise rotation from c. 35 Ma determined by recent palaeomagnetic studies, whereas the last documented N–S extension may reflect a short‐lived stage of orogen‐parallel extension.     

Using turbidity dynamics and geochemical variability as a tool for understanding the behavior and vulnerability of a karst aquifer

In a karst system, the characterization of transport properties is based on the comparison of natural tracers observed at the inlet (a swallow hole on the karst plateau) and the outlets of the system (a spring and a well). At Norville, northwest France, electrical conductivity (EC) and turbidity (T) were used as natural tracers for characterizing dissolved elements (surface water geochemistry) and particulate matter transport, respectively. Two methods were used for this study: (1) a comparison of the relations between EC, T and spring discharge (Q) by means of normalized EC–T–Q curves, and (2) a principal component analysis (PCA) including water geochemistry data in addition to EC, T and Q. Three different characteristic flood events have been chosen for the analyses. EC–T–Q curves highlighted the direct transfer, resuspension and deposition of particles during their transport in the karst network. Transport from the swallow hole to both the spring and the well appeared to be dominated by karst-conduit flow. On the other hand, PCA results showed a diffuse source of contamination of groundwaters by nitrate and a point-source contamination of groundwaters by streaming/runoff surface waters with high turbidity and phosphate concentration infiltrated at the swallow hole.     

From rifting to spreading in the eastern Gulf of Aden: a geophysical survey of a young oceanic basin from margin to margin

A geophysical survey in the eastern Gulf of Aden, between the Alula–Fartak (52°E) and the Socotra (55°E) transform faults, was carried out during the Encens–Sheba cruise. The conjugate margins of the Gulf are steep, narrow and asymmetric. Asymmetry of the rifting process is highlighted by the conjugate margins (horst and graben in the north and deep basin in the south). Two transfer fault zones separate the margins into three segments, whereas the present‐day Sheba Ridge is divided into two segments by a transform discontinuity. Therefore segmentation of the Sheba Ridge and that of the conjugate margins did coincide during the early stages of oceanic spreading. Extensive magma production is evidenced in the central part of the western segment. Anomaly 5d was identified in the northern and southern parts of the oceanic basin, thus confirming that seafloor spreading in this part of Gulf of Aden started at least 17.6 Ma ago.     

The deposition of silica in hot springs

Natural hot spring waters ascending rapidly to the surface become supersaturated with respect to quartz because of rapid cooling, separation of steam and sluggish deposition of quartz and other crystallineSiO ₂ phases. Large amounts of silica are likely to be deposited in hot spring systems only after the solubility of amorphous silica has been exceeded. Cristobalite and chaleedony probably form in hot spring systems only by the crystallization of previously deposited silica gel rather than by direct deposition from solution. Experimental data indicate that the solubilit of quartz in water rises with increasing temperature along the vapor pressure curve to a maximum value of 725 ppm at 330°C. However, the maximum amount of silica likely, to occur in hot spring systems where quartz precipitates at depth is appreciably greater. Steam formation during adiabatic cooling of a water quickly brought to the surface from 330°C at depth might leave the silica in the remaining liquid concentrated to about 1150 to 1400 ppm. Under such conditions, amorphous silica might precipitate (probably as a colloidal suspension) after the water cooled below about 200°C to 250°C. Waters initially in equilibrium with quartz at a temperature less than 210°C probably will precipitate amorphous silica in channelways underground only when and where large quantities of steam separate from the waters as a result of sudden decreases in pressure or hydrostatic head. Above 150° to 200°C amorphous silica and volcanic glass can contribute very large quantities of silica to the solution. However, at these temperatures in natural systems they are eventually converted to crystalline phases. Thus, control of dissolved silica at depth is likely to be relatively short lived in respect to the ages of most hot spring systems. The dissolved-silica content of 90 hot spring waters from Yellowstone National Park was measured colorimetrically in the field immediately after collection. Comparison with laboratory studies on the solubility of amorphous silica indicates that many waters in «alkaline» springs are markedly undersaturated with silica with respect to amorphous silica at the temperatures of the pools. Thus, the dissolved silica content of these waters cannot be accounted for by equilibria with amorphous silica. Rather, silica appears to be controlled by the dissolution, deposition, or alteration of other silica-bearing phases at depth. Furthermore, many springs now have compositions essentially identical (with respect to all components) to those determined in 1888, indicating that either equilibrium or steady state conditions have prevailed at depth for a long time. Veins of fine-grained quartz were found in drill core from the Upper Basin, and it is reasonable to assume that quartz controlled the quantity of silica in solution in those places of deposition. Possibly the silica content of the surface waters might allow an estimate of the temperatures at which these waters were last in equilibrium with quartz at depth. Assuming adiabatic cooling along the vapor pressure curve and correcting for steam formation, quartz solubility data compared with natural water analyses suggests that underground temperatures approach 205°C in the Upper Geyser Basin of Yellowstone. In the Norris Geyser Basin, underground temperatures of 245°C are suggested.     

World's Largest In Situ Thermal Desorption Project: Challenges and Solutions

This paper presents the largest In Situ Thermal Desorption (ISTD) project completed to date. The redevelopment of a former aerospace manufacturing facility adjacent to a commercial airport was the main driver, requiring relatively rapid reduction of several chlorinated volatile organic compounds (CVOC) in a 3.2‐acre source zone. The source zone was divided into four quadrants with differing treatment depths, heated simultaneously using a total of 907 thermal conduction heater wells. Five different depths were selected across the area, according to the depth of contaminant impact. Prior to implementation, a risk and optimization study led to placement of a vertical sheet‐pile wall around the treatment zone to minimize groundwater flow, and a pilot test of a novel direct‐drive method for installation of the heater casings. Because of a shallow water table, a layer of clean fill was placed over the treatment zone, and partial dewatering was necessary prior to heating. A network of vertical multiphase extraction wells and horizontal vapor extraction wells was used to establish hydraulic and pneumatic control and to capture the contaminants. The site was split into four decision units, each with a rigorous soil sampling program which included collecting a total of 270 confirmatory soil samples from locations with the highest pretreatment CVOC concentrations requiring reduction to below 1 mg/kg for each contaminant. Temperature monitoring and mass removal trends were used to trigger the sampling events. Eventually, a small area near the center of the site required the installation of four additional heaters before the soil goals were reached after 238 days of heating. The total energy usage for heating and treating the source area was 23 million kWh—slightly lower than the estimated 26.5 million kWh. Actual energy losses and the energy removal associated with the extracted steam were lower than anticipated. An estimated 13,400 kg (29,800 lbs) of CVOC mass was removed, and all soil goals were met. This paper presents the challenges associated with a project of this scale and describes the solutions to successfully complete the ISTD remedy.     

A model for assessing iceberg hazard

Natural Hazards - With the polar regions opening up to more marine activities but iceberg numbers more likely to increase than decline as a result of global warming, the risk from icebergs to...     

Electrical resistivity tomography (ERT) of a coastal carbonate aquifer (Port-Miou, SE France)

As part of a larger regional research program “KarstEAU”, the authors have applied electrical resistivity tomography (ERT) techniques to characterize heterogeneities in the Port-Miou coastal karst aquifer (Cassis, SE France). Field surveys were carried out on intensely fractured and karstified Urgonian carbonates. Extensive research has characterized macro- and micro-scale geology of the Port-Miou area and particularly underground water-filled conduits and fault/fracture and karst systems within a former quarry. The authors applied 2D ERT along two surface profiles of length 420 and 595 m to test capability for delineating subsurface conduits and possibly relationship between conduit and fault/fracture/karst orientation; and 3D ERT with a dense 120 electrode array at 1 m spacing (11 × 10 m) was applied over an area of the quarry that had been profiled using 3D georadar and which has had intensive nearby structural geological interpretation. The 2D profiling imaged several underground conduits at depths to >50 m below ground surface and below sea level, including possibly the main Port Miou submarine spring and smaller springs. The 2D profiling within the quarry provided a better understanding of the connectivity between major fractures and faults on the quarry walls and secondary springs along the coast supporting flow of the secondary springs along interpreted fracture orientations. In addition, 2D inversion-derived conductivity models indicate that high resistivity zones above sea-level are associated with non-saturated zones and low resistivity anomalies in the non-saturated zone are associated with residual clays in paleokarsts. A partitioned lower resistivity zone below sea-level can be associated with a higher porosity/permeability zone with fractures and karstic features. Inversion models of the dense 3D ERT data indicate a higher resistivity volume within the larger surveyed block. The survey characterized the non-saturated zone and the ERT resistivities are correlated with karst features interpreted by 3D georadar and visible in the inferior wall of the quarry.