Bernard Robineau

Deposits related to degradation processes on Piton des Neiges Volcano (Reunion Island): overview and geological hazard

Abstract Piton des Neiges (PN) Volcano on Reunion Island offers a rare opportunity to study deposits related to degradation processes in a deeply eroded oceanic shield volcano. Both the inner parts and flanks reveal a large amount of resedimented volcaniclastic material, including extensive debris avalanche deposits. PN litho–structural units, first studied by Upton and Wadsworth [1965, Philos. Trans. R. Soc. Lond., A 271, pp. 105–130], are re-examined. This review highlights the importance of long volcanic repose periods and erosion processes during PN history. volcaniclastic deposits have been studied in the field in order to evaluate the spatial and temporal distribution of the three main types of PN degradation processes. The deposits of these processes have been classified into: (1) talus, (2) mudflow and debris flow, and (3) debris avalanche. Lithology, frequency and estimated volumes of each deposit type imply that the structural evolution of PN can be considered in terms of the competition between the volcanic productivity and the degradation and erosion processes. The occurrence of huge catastrophic avalanches produced by flank failure is convincingly linked to the basaltic activity of PN, which implies a very low risk at present. On the contrary, mudflows and debris flows pose an important risk due to the high population density focussed around the basin outlets. Moreover, if smaller debris avalanches can occur in the cirques of PN, another major risk must be evaluated.

Electrical structure beneath the eastern collapsed flank of Piton de la Fournaise volcano, Reunion Island: Implications for the quest for groundwater

Time domain electromagnetic (TDEM) and tensor audiomagnetotelluric (AMT) data have been acquired at several locations on the eastern flank of Piton de la Fournaise volcano (Reunion Island) within a depressed area called Grand Brule, interpreted as a collapse structure. The survey objectives were (1) to provide a geophysical estimate of the subsurface structure and (2) to evaluate the possibility of detecting aquifers in a volcanic environment not very known. The TDEM and the AMT data collected along two E-W traverses orthogonal to coastline on the northern and southern edges of Grand Brule were interpreted with one-dimensional layered models. From the surface downward, the geoelectrical sections reveal two major units: very resistive, young lava flows (dry) and a shallow conductor (<500 m) which is probably primarily attributable to a clayey, poorly permeable base. A notable exception to this pattern is seen at sites close to the coast, where we found three-layered structures. There is an intermediate layer of resistivity of about 100–200 ohm m between the top resistive layer and bottom conductive layer that represents a probable freshwater lens in the southern part and an alluvial fan with resistivities substantially higher (200 ohm m) in the northern part of Grand Brule. It is suggested that the 200 ohm m layer, interpreted as a buried paleoriver, corresponds to a drainage structure.

CRUSTAL AND UPPER MANTLE ELECTRICAL CONDUCTIVITY STRUCTURES IN WEST AFRICA GEODYNAMIC IMPLICATIONS

Abstract Measurements of the electrical and magnetic fields at the surface of the earth have been carried out in West Africa in the period range 10–10,000 s. In this area, the method of magnetotelluric (MT) sounding finds its application in a variety of geological problems such as the deep shape of sedimentary basins, the geometry of orogenic belts and craton deep structures. The present article is an attempt to correlate the conductivity model with large scale tectonics. MT surveys have revealed the presence of several areas where electrical resistivity is abnormally low at crustal and upper mantle depths and the existence of lateral electrical conductivity inhomogeneities within the lithosphere. Subsurface electrical conductivity distribution is in good relation with the geological features, and its variation associated with lithological and structural changes. A major discontinuity found at a depth of several hundred kilometers in the lithosphere separates two structural blocks, the eastern one, with higher resistivities, being the West African craton. The lithosphere derived from this work has a maximum thickness of 300 km under the Senegalo-Mauritanian basin and more than 400 km beneath the craton. The electrical asthenosphere is certainly not “well-developed” under the craton. A sketch of electric structures across West Africa is presented and a geodynamic interpretation is proposed to explain the present structural pattern of the southern segment of the Mauritanides belt.

Debris avalanches on the western flank of Piton des Neiges shield volcano (Reunion Island)

The Saint-Gilles breccias, on the western flank of Piton des Neiges volcano, are clearly identified as debris avalanche deposits. A petrographic, textural and structural analysis of the breccias and inter-bedded autochthonous lava flows enables us to distinguish at least four successive flank slides. The oldest deposit sampled the hydrothermally-altered inner parts of the volcano, and has a large volume. Failure was favored by the presence of a deep intensely-weathered layer. The younger deposits are from superficial sources, as their products are rarely hydrothermalized and are more vesicular. The breccia formation, and especially the progressive breaking up occurring during the debris avalanche displacement, indicates the existence of high speed transport. In the Cap La Houssaye coastal area, abrasion and striation of the underlying lava formation, as well as the packing features observed in the breccia, are considered to be deceleration structures.

Étude de la zone sommitale du volcan Karthala (Grande Comore) par polarisation spontanée

Abstract A study of Karthala caldera using the self-potential (SP) method reveals that the main hydrothermal activity of the volcano does not coincide with its central crater, the Choungou-Chahale. but with the northern and western parts of the caldera. A secondary focus exists south of Choungou-Chahale. These zones are well correlated with recent seismic activity and with historic volcano-tectonic (collapses) and eruptive events. In this context, the heat sources of the hydrothermal systems are magma bodies corresponding to storage and intrusions. The strategy for the monitoring of Karthala should take into account these new results.

New geological and hydrogeological implications of the resistivity distribution inferred from audiomagnetotellurics over La Fournaise young shield volcano (Reunion Island)

Abstract An audiomagnetotelluric survey has been performed along an inactive flank of La Fournaise volcano massif in Reunion island, to study the subsurface resistivity structure. One-dimensional modelling of the AMT data at each site revealed an extensive low-resistivity (less than 10 ω m) zone at a few hundred meters below the surface. The significance of this unexpected conductive substratum is discussed in relation with the proposed impact of volcano-tectonic processes (caldera and landslides collapses), and a new interpretation of the geological structure is proposed. Moreover, it is likely that these conductors are poorly permeable argilaceous materials; then they coincide with a limit in permeability and determine groundwater behavior.

Audiomagnetotelluric prospecting for groundwater in the Baril coastal area, Piton de la Fournaise Volcano, Reunion Island

Surface geophysics has been used for a number of years to solve a variety of major groundwater exploration problems in coastal and island regions. The electrical properties of near‐surface rocks are highly dependent on porosity, degree of saturation, and pore fluid resistivity (Keller and Frischknecht, 1966). Because the resistivity of the formations decreases with increasing salinity of the water content, electrical methods, such as vertical electrical soundings, time‐domain electromagnetic soundings, and audiomagnetotellurics (AMT), are well‐suited for mapping changes in groundwater salinity and, in particular, for detecting a fresh water‐salt water interface.