Geological and geotechnical properties of the “Terres Noires” in southeastern France: Weathering, erosion, solid transport and instability

The geological formation known as the “Terres Noires” covers large areas of southeastern France, between the Rhône valley and the pre-Alpine hills. This formation consists mainly of dark marls of the basal Upper Jurassic. These marls are very susceptibility to weathering and are particularly prone to erosion, resulting in high solid transport. This paper attempts to quantify this transport in experimental catchment areas, and shows that a forest cover can play a major role in preventing this type of phenomenon. Weathering of this formation may also lead to gravity instabilities in fresh marl (although this is rare) and, much more frequently, in the surficial weathered material.     

Origin of the Xigaze ophiolite, Yarlung Zangbo suture zone, southern Tibet

The Xigaze ophiolite, (Tibet) displays unusual lithological, petrological, textural, and structural characteristics. There are no large masses of cumulate gabbros, but dolerite intrusives throughout the whole ophiolite sequence, some of which were intruded into already serpentinized peridotites, only minor residual harzburgites and dunites in dominantly Iherzolitic peridotites equilibrated at low temperatures and pressures, and relatively low-temperature deformation structures in the uppermost peridotites. These features suggest a very low heat flow at the spreading center where the Xigaze ophiolite was formed, in good agreement with a discontinuous and slowly-accreting spreading center origin. However, this ophiolite does not represent a typical mid-oceanic ridge ophiolite; rather it was formed in a small basin located at the southern margin of Eurasia, hence within a preexisting oceanic lithosphere. The opening of the nearly N-S Xigaze paleo-ridge resulted from the W-E drift of Africa relative to Eurasia from 180 to 110 Ma. The N-S emplacement onto the continent of the Xigaze ophiolite, formed 120 to 110 Ma ago, can be correlated to changes in direction of motion of the African and Indian plates: a primary intra-oceanic thrusting event probably occurring at 110 or 85 Ma and the final obduction near 50 Ma during the India-Eurasia collision.     

The vourinos ophiolite complex, Greece: The tectonite suite

Classical structural analysis, in combination with new techniques including pyroxene thermobarometry and recrystallized-grain- and subgrain-size paleopiezometry, has been used to deduce the tectonic history of the Vourinos Ophiolite Complex, Greece. Results can be used to infer the variation with depth of differential stress in the upper mantle and indicate that these rocks were subjected to an initial stress level of 10–100 bar over a depth interval from 100 to 40 km, respectively. Subsequently, they underwent mylonitization, associated with stresses of about 2500 bar at 30 km depth. Rocks equilibrated at the deepest levels commonly have the highest structural positions within the ultramafic tectonites and also show appreciable Al depletion. Lack of annealing recrystallization in the mylonitic rocks, together with low stress levels and an abnormally high pyroxene geotherm for the Complex lead to the interpretation of the Complex being the remnant of a mantle diapir, with an overlying magma chamber, located at a spreading ridge.     

P- and S-velocity images of the lithosphere–asthenosphere system in the Central Andes from local-source tomographic inversion

About 50 000 P and S arrival times and 25 000 values of t * recorded at seismic arrays operated in the Central Andes between 20°S and 25°S in the time period from 1994 to 1997 have been used for locating more than 1500 deep and crustal earthquakes and creating 3-D P , S velocity and Qp models. The study volume in the reference model is subdivided into three domains: slab, continental crust and mantle wedge. A starting velocity distribution in each domain is set from a priori information: in the crust it is based on the controlled sources seismic studies; in slab and mantle wedge it is defined using relations between P and S velocities, temperature and composition given by mineral physics. Each iteration of tomographic inversion consists of the following steps: (1) absolute location of sources in 3-D velocity model using P and S arrival times; (2) double-difference relocation of the sources and (3) simultaneous determination of P and S velocity anomalies, P and S station corrections and source parameters by inverting one matrix. Velocity parameters are computed in a mesh with the density of nodes proportional to the ray density with double-sided nodes at the domain boundaries. The next iteration is repeated with the updated velocity model and source parameters obtained at the previous step. Different tests aimed at checking the reliability of the obtained velocity models are presented. In addition, we present the results of inversion for Vp and Vp/Vs parameters, which appear to be practically equivalent to Vp and Vs inversion. A separate inversion for Qp has been performed using the ray paths and source locations in the final velocity model. The resulting Vp , Vs and Qp distributions show complicated, essentially 3-D structure in the lithosphere and asthenosphere. P and S velocities appear to be well correlated, suggesting the important role of variations of composition, temperature, water content and degree of partial melting.     

Modelling Groundwater Fluctuations and the Frequency of Movement of a Landslide in the Terres Noires Region of Barcellonnette (France)

Onn the basis of measurements of hydrological parameters and field monitoring of a landslide in the Terres Noires in the basin of Barcelonnette (France), a hydrological model was developed, describing groundwater fluctuations in relation to precipitation. These groundwater fluctuations can be used as input to a stability model in order to assess the temporal frequency of instability of the landslide. The calculated groundwater fluctuations, which can forecast years with landslide incidents, were roughly calibrated against dated movements obtained by dendrochronological research. The hydrological system of the landslide can be understood through a three-layer sequence: a rather permeable colluvial top layer underlain by a less permeable colluvial second layer, both overlying the nearly impermeable in situ non-weathered black marls (Terres Noires). The mean Ksat value for the matrix flow in the top layer is 15·7 cm/day and in the underlying layer 0·7 cm/day. However, water fluxes in these layers occur by two types of groundwater flow: matrix flow obeying Darcy's law, and more rapid gravitational flow through preferential flow paths, increasing the conductivity by a factor of 10 to 100, as cube method Ksat measurements revealed. The model shows long-term yearly fluctuations of the phreatic surface, with peaks at the end of winter, as well as at the beginning of spring, and minimum values during the dry summer period. These long-term fluctuations are explained by the high drainage capacity of the top colluvial layer and the relatively low vertical water fluxes within the underlying colluvial layer. The model shows that maximum critical peak height conditions of the groundwater, causing instability, occur in wet seasons, with at least six consecutive months with high amounts (more than 60 mm) of precipitation. © 1997 by John Wiley & Sons, Ltd.     

Prehistoric settlement and landscape change on alluvial fans in the upper Mississippi river valley

Geoarchaeological studies of alluvial fans in the northern Sny Bottom (Upper Mississippi River valley) focused on distal fan lobe processes, the effects of small-scale landscape change on prehistoric locational decisions, and the resulting structure of the archaeological record. Aerial imagery together with coring and trenching of paleochannels on distal lobes found that frequent channel avulsion (1) produced abrupt landform/habitat changes and (2) shifted the primary loci of sediment deposition on fans through time. Analysis of wood charcoal from prehistoric occupations indicates that vegetation succession during post-avulsion overbank aggradation along a new channel was characterized by a shift from Fraxinus-dominated to mesophytic forest. Results also indicate that use of fans by Early Woodland (ca. 2550–2100 B.P.) and other foragers centered on portions along active channels. Shifting depositional loci on fans led to surfaces and depositional units of varying age, and hence a complex stratigraphic record of Holocene occupations.     

Rheology of the upper mantle: Inferences from peridotite xenoliths

Stress estimates as a function of depth are obtained for peridotite xenoliths from the upper mantle of three types of tectonic environments by applying revised recrystallizedgrain-size paleopiezometry and pyroxene thermobarometry. The general increase in grain size with depth and hence decrease in deviatoric stress, observed previously, is confirmed but reversals in these trends are now established and remain enigmatic. Stresses and temperatures obtained are combined with a representative creep-flow law to calculate strainrate and viscosity profiles that appear to be physically reasonable. Profiles for the highthermal-gradient rift/ridge environments show a complexity that is interpreted as.a rheological discontinuity resulting from the emplacement of asthenospheric diapirs during late stages of continental rifting. Profiles for broad continental extension zones (C.E.Z.), believed to be most representative of oceanic upper mantle, fluctuate between 50 and 80 km, with a general small increase in strain rate and decrease in viscosity with depth; deepest samples apparently come from the base of the lithosphere. Profiles for the infracratonic mantle of southern Africa show nearly a uniform increase in strain rate to values greater than 10⁻¹⁴/sec, and a decrease in viscosity to lower than 10²¹ poise, at a depth of 230 km. These profiles may transect the mechanically defined lithosphere—asthenosphere transition at about 200 km and, if so, there is no evidence for a mechanical discontinuity at the boundary. This observation, coupled with evidence that the sense of shear is homogeneous for all mantle profiles constructed, clearly favors a model whereby lithospheric plates are dragged by thermal convection of the asthenosphere below. Sea-floor spreading rates and relative plate-velocity estimates are consistent with this interpretation but do not independently permit a definitive choice between the two favored models advanced to explain the driving force for plate motions.     

Investigation of recharge dynamics and flow paths in a fractured crystalline aquifer in semi-arid India using borehole logs: implications for managed aquifer recharge

The recharge flow paths in a typical weathered hard-rock aquifer in a semi-arid area of southern India were investigated in relation to structures associated with a managed aquifer recharge (MAR) scheme. Despite the large number of MAR structures, the mechanisms of recharge in their vicinity are still unclear. The study uses a percolation tank as a tool to identify the input signal of the recharge and uses multiple measurements (piezometric time series, electrical conductivity profiles in boreholes) compared against heat-pulse flowmeter measurements and geochemical data (major ions and stable isotopes) to examine recharge flow paths. The recharge process is a combination of diffuse piston flow and preferential flow paths. Direct vertical percolation appears to be very limited, in contradiction to the conceptual model generally admitted where vertical flow through saprolite is considered as the main recharge process. The horizontal component of the flow leads to a strong geochemical stratification of the water column. The complex recharge pattern, presented in a conceptual model, leads to varied impacts on groundwater quality and availability in both time and space, inducing strong implications for water management, water quality evolution, MAR monitoring and longer-term socio-economic costs.     

Slant Column Measurements of O3 and NO2 During the NDSC Intercomparison of Zenith-Sky UV-Visible Spectrometers in June 1996

In June 1996, 16 UV-visible sensors from 11 institutes measured spectra of the zenith sky for more than 10 days. Spectra were analysed in real-time to determine slant column amounts of O3 and NO2. Spectra of Hg lamps and lasers were measured, and the amount of NO2 in a cell was determined by each spectrometer. Some spectra were re-analysed after obvious errors were found. Slant columns were compared in two ways: by examining regression analyses against comparison instruments over the whole range of solar zenith angles; and by taking fractional differences from a comparison instrument at solar zenith angles between 85° and 91°. Regression identified which pairs of instruments were most consistent, and so which could be used as universal comparison instruments. For O3, regression slopes for the whole campaign agreed within 5% for most instruments despite the use of different cross-sections and wavelength intervals, whereas similar agreement was only achieved for NO2 when the same cross-sections and wavelength intervals were used and only one half-day's data was analysed. Mean fractional differences in NO2 from a comparison instrument fall within ±7% (1-sigma) for most instruments, with standard deviations of the mean differences averaging 4.5%. Mean differences in O3 fall within ±2.5% (1- sigma) for most instruments, with standard deviations of the mean differences averaging 2%. Measurements of NO2 in the cell had similar agreement to measurements of NO2 in the atmosphere, but for some instruments measurements with cell and atmosphere relative to a comparison instrument disagreed by more than the error bars.     

The slope movements within the Mondorès graben (Drôme, France); the interaction between geology, hydrology and typology

The Mondorès graben in the south-eastern French Alps is an uncommon structural feature, which originates in a complex polyphasic tectonic evolution. In contrast with its immediate surroundings, with hardly any huge landslides, the Mondorès graben is characterised by various types of landslides. A huge sagging caused part of the limestone cliff to subside some 50 m within 50 years. Two recent mud flows that occurred were considered a potential threat to some inhabited places downstream. The hydrologic aspects of the Boulc-Mondorès landslide resemble the geology: infiltration (and karst input?) in the rock-sliding zone and exfiltration in the marls resulting in slumps and mud flows. The mass movement interactions could be explained by a structural geology analysis with geodetic monitoring using different techniques. It is also shown that hydrochemistry as well as geophysical surveys are of importance in unravelling the hydrologic systems and the geological subsurface structures. The present paper aims at explaining the geological control of the different slides in view of estimating their potential danger. Understanding the geological structure and its evolution therefore is a necessary prerequisite.