Neogene exhumation and relief evolution in the eastern Betics (SE Spain): Insights from the Sierra de Gador

The Betics are a key area to study an orogenic landscape disrupted by late‐orogenic extension. New low‐temperature thermochronology (LTT) data (AHe and AFT) coupled with geomorphic constraints in the Sierra de Gador (Alpujarride complex) are used to reconstruct the cooling history and evolution of relief during the Neogene. We document three stages: (1) a fast cooling event between 23 and 16 Ma associated with the well‐known extensive tectonic exhumation of the Alpujarride unit, (2) a period of slow cooling between 16 and 7.2 Ma related to a planation event and (3) a post‐7.2 Ma surface uplift associated with the inversion of the Alboran domain undetected by LTT. The planation event followed by this late uplift can explain the occurrence of inherited low‐relief surfaces overlain by Tortonian–Messinian platform deposits at the top of the range. Finally, we propose that the Sierra de Gador is a more transient landscape than the nearby Sierra Nevada.     

Évolution temporelle et architecture interne d'un banc sableux estuarien : la Longe de Boyard (littoral atlantique,France)

The evolution and the internal architecture of an estuary type sand ridge has been studied with a set of bathymetric data recorded during the last two centuries and with a dense grid of recent very high resolution seismic profiles. Bathymetric data of the so-called, Longe de Boyard sand ridge, displays sand losts due to wave and tide erosion. Internal geometry, through seismic profile analysis, indicates two main phases of deposition recording both, a recent high energy environment and an older low energy one, respectively. Such an evolution is believed to record changes in sedimentation processes mainly related to the end of the Holocene transgression (8 000–5 000 yr BP). To cite this article: É. Chaumillon et al., C. R. Geoscience 334 (2002) 119–126.     

Geological aspects of deformation in continental shear zones

A method of kinematic analysis of structures, microstructuresand mineral preferred orientations, initially devised in the study of peridotites, has been applied to crustal rocks bearing evidence of large strains produced in metamorphic environments. Three tectonic lineaments (Angers-Lanvaux, Montagne Noire and Maydan) were selected. They illustrate a general situation arising in continental crusts when they are deformed by ductile transcurrent fault systems.The Angers-Lanvaux structure is bilaterally symmetric; its dominant feature is the horizontal stretching lineation which is parallel to the fold axes. The foliation and slaty cleavage in the most surficial formations wrap around the axis of the whole structure. The folds in the slates away from the axis also exhibit axes parallel to the general trend, but no stretching lineations. These folds are attributed to crustal shortening in a direction normal to the ductile fault. In the Montagne Noire recumbent folds are thrusted away from the axis of the structure over at least 25 km. The metamorphism is also centered on the structure and symmetrically reduced away from it. The core of the structure is occupied by a strongly lineated orthogneiss, cut by a late intrusive granite. The Maydan axial zone displays clear evidence of partial melting at various scales within the deformed gneisses: (1) in gashes perpendicular to the stretching lineation which in these anatectic formations tends to plunge at more than 45°; (2) in bands of deformed pegmatites; and (3) possibly in granites which on the one hand intrude the surrounding formations and on the other converge with increasing deformation on the fault zone. The quartz preferred orientations and microstructures in quartzite layers from Angers indicate that the plastic flow plane and direction lie, respectively, close to the foliation and lineation, the slight departure is ascribed to a flow with a rotational shear component.All this suggests a general model for the origin of such ductile zones. The horizontal relative displacement of crustal blocks along a ductile band is responsible for its overall steeply dipping foliation and horizontal lineations. Viscous heating progressively tends to concentrate the plastic flow along its axis. It is also responsible for the development of metamorphism and of anatexis at depth; the partially melted rocks tend to rise, building at shallower depth the arched structure in the axis of the ductile zone, with a continuing flowage parallel to this axis probably now in the solid state; they can also intrude the surrounding terrain as undeformed batholiths. The folds parallel to the stretching lineation in the axial zone are explained by the fact that, due to the escape of anatectic melts, the formations at depth flow in a narrowing channel. The upwelling of the axial structure induces a compression with folding in the surrounding sedimentary formations and gravity nappe sliding away from the axis.     

Closure of a hyperextended system in an orogenic lithospheric pop‐up,Western Pyrenees: The role of mantle buttressing and rift structural inheritance

The Early Cretaceous hyperextended Mauléon rift is localized in the north‐western Pyrenean orogen. We infer the Tertiary evolution of the Mauléon basin through the restoration of a 153‐km‐long crustal‐scale balanced cross‐section of the Pyrenean belt, which documents at least 67 km (31%) of orogenic shortening in the Western Pyrenees. Initial shortening, accommodated through inversion of inherited crustal structures, led to formation of a pop‐up structure, in which the opposite edges underwent similar shortening with different tectonic reactivation styles, localized versus. distributed. Underthrusting of the Iberian margin accommodated further convergence, forming the Axial Zone antiformal stack of crustal nappes within a lithospheric pop‐up. Thin‐skinned and thick‐skinned structures propagated outward from the heart of this pop‐up, a block of strong mantle acting as a buttress inhibiting complete inversion of the Mauléon rift basin.     

Case history of one of the few successful Superfund remediation sites: A site at Salinas, California, USA

In 1985, a former tire manufacturing plant surrounded by agricultural fields in the Salinas Valley was designated a Superfund Site by the US Environmental Protection Agency. The plant had been operating for seventeen years, from 1963 until 1980. When dismantling of the plant was started, it was determined that toxic hydrocarbon solvents and oils from the plant had contaminated soil and groundwater in alluvial deposits alongside the plant. It was determined later that the groundwater contamination also lay beneath the agricultural fields in a narrow groundwater plume that extends about 4.3 km downgradient from the plant. Because of the complex architecture of the aquifer system, the gradient, and extensive pumping of agricultural wells, the contaminants migrated northwestward and downward to deeper levels away from the plant.

The agricultural fields are underlain by an unconfined shallow aquifer and by a system of confined aquifers that extend to more than 180 m below surface. Aquifers are discontinuous beds of channel sand and gravel; confining beds are overbank clay and silt, and estuarine clay.

Geophysical data, logs of existing agricultural and other wells, and careful consideration of the stratigraphic architecture of the depositional environment provided the basis for a conceptual hydrogeologic model and for locating characterization wells for detailed visual and geophysical logging and hydrologic testing. Successive refinements of the characterization by sequential installation of wells indicated optimal locations for installation of extraction and monitoring wells. Validity of the concept of the hydrogeologic regime was verified by close match of predictions made by modeling with the later results of pumping from the extraction wells in a pump-and-treat system.

Successful remediation was accomplished by analyzing data from 110 agricultural wells, the few domestic water wells, nearly 200 sequentially installed stratigraphic-characterization and monitoring wells, 25 extraction wells, and by close cooperation among federal, state and local agencies, and the ranchers and growers.

Total contaminants recovered from activated-charcoal strippers of the treatment system totalled < 230 kg. Large quantities were harmlessly volatilized and dispersed into the atmosphere by air strippers and by agricultural sprinkling systems spraying water onto the fields. Crop testing showed no contamination of food crops. The activity has taken seven years and has cost more than US$22 million.     

Enhanced bioremediation of crude oil utilizing lipophilic fertilizers combined with biosurfactants and molasses

Many research studies have demonstrated the feasibility and efficacy of fertilization with nitrogen and phosphorus to combat oil spills in marine environments. Rapid dilution of water-soluble nutrients can be overcome by oleophilic formulations that retain optimal nutrient concentrations at the oil-water interface where biodegradation occurs. Previous work has demonstrated that biodegradation processes are enhanced by the addition of lipophilic fertilizers of natural origin (uric acid and lecithin). In this work, we examined the effectiveness of these nutrients in combination with biosurfactants (rhamnolipids) and molasses (source of C and vitamins) to enhance the biodegradation by naturally occurring microorganisms. It was found that the use of biosurfactants resulted in an increased removal of petroleum hydrocarbons (96% removal of C19-C34 n-alkanes within a period of 18 days) as well as in a reduction of the lag phase (almost 80% removal was achieved within the first week of biosurfactant application).     

An estimate of the influence of loading effects on tectonic velocities in the Pyrenees

Surface displacements due to temporal changes in environmental mass redistributions are observable in the coordinate time series of many Global Navigation Satellite System (GNSS) sites. In this study, we investigated the effect of loading on estimates of tectonic velocity computed from campaign-style GNSS observations. The study region is in the Pyrenees mountain range between France and Spain (ResPyr campaigns). In this area, seismic activity is continuous and moderate and the expected amplitude of the horizontal tectonic velocity is less than 0.5 mm/yr. In order to determine the velocity, 4 sparse GNSS campaigns were carried out from 1995 to 2010. Considering this small rate of deformation, loading phenomena can contribute a non-negligible artifact to the velocity computation that could affect our geodynamical interpretation. In this investigation, we specifically considered the atmospheric, hydrological, and non-tidal ocean loading phenomena. The computed loading deformations for this region show the horizontal displacements are dominated by the non-tidal ocean loading (maximum 4 mm for the North and 3.1 mm for the East components); the main vertical contributions are due to the atmospheric and continental water storage loading (maximum 14.3 for the atmosphere and 8.1 mm for the hydrology, respectively). We have found that the dominant loading effect on the horizontal velocity is the non-tidal ocean loading (mean of 0.11 mm/yr), whereas the vertical component is dominated by the hydrological loading (mean of 0.21 mm/yr). Since the study area is in a mountainous region, we also analyzed the difference between the atmospheric and the topography dependent atmospheric loading models at our GNSS campaign sites. We did not find any significant difference between the two atmospheric loading models in terms of horizontal velocity. Finally, we performed simulations to identify the optimum timing and frequency of future GNSS campaigns in this area that would minimize the loading effects on tectonic velocity estimates.     

TEM study of a silicate-carbonate-microbe interface prepared by focused ion beam milling

The biogeochemical alteration of an Mg-Fe orthopyroxene, reacted for 70 yr under arid conditions in a desert environment, was studied by transmission electron microscopy. For this purpose, an electron transparent cross-section of the interface between a single microorganism, an orthopyroxene and nanometer-sized calcite crystals, was prepared with a focused ion beam system. X-ray energy dispersive spectrometry and electron energy loss spectroscopy allowed one to clearly distinguish the microorganism en route to fossilization from the nanometer-sized calcite crystals, showing the usefulness of such a protocol for identifying unambiguously traces of life in rocks. A 100-nm-deep depression was observed in the orthopyroxene close to the microorganism, suggesting an enhanced dissolution mediated by the microbe. However, an Al- and Si-rich amorphous altered layer restricted to the area just below the microorganism could be associated with decreased silicate dissolution rates at this location, suggesting complex effects of the microorganism on the silicate dissolution process. The close association observed between silicate dissolution and carbonate formation at the micrometer scale suggests that Urey-type CO₂ sequestration reactions could be mediated by microorganisms under arid conditions.     

Precipitation Changes in High Southern Latitudes from Global Reanalyses: A Cautionary Tale

The temporal consistency of the moisture fields (precipitation, evaporation and total precipitable water) from five global reanalyses is examined over Antarctica and the Southern Ocean during 1989?C2009. This concern is important given that (1) global reanalyses are known to be prone to inhomogeneities and artificial trends caused by changes in the observing system, and (2) the period of study has seen a dramatic increase in the volume of satellite observations available for data assimilation. In particular, the study aims to determine whether the recent reanalyses are suitable for investigating changes in Antarctic surface mass balance. The datasets investigated consist of NCEP-2, JRA-25, ERA-Interim, MERRA and CFSR. Strong evidence of spurious changes is found in NCEP-2, JRA-25, MERRA and CFSR, although the magnitude, spatial patterns and timing of these artifacts vary between the reanalyses. MERRA exhibits a jump in Antarctic precipitation-minus-evaporation (P?CE) and in Southern Ocean precipitation in the late 1990s. This jump is related to the introduction of sounding radiances from the Advanced Microwave Sounding Unit (AMSU). The impact of AMSU is also discernible, albeit less pronounced, in CFSR data. It is shown that ERA-Interim likely provides the most realistic depiction of the interannual variability and overall change in Antarctic P?CE since 1989. We conclude that the presence of spurious changes is not a solved problem in recent global reanalyses. Caution should continue to be exercised when using these datasets for trend analyses in general, particularly in high southern latitudes.