Résultat de la recherche
Multiple caldera collapses inferred from the shallow electrical resistivity signature of the Las Cañadas caldera, Tenerife, Canary Islands
The Las Cañadas caldera of Tenerife (LCC) is a well exposed caldera depression filled with pyroclastic deposits and lava flows from the active Teide–Pico Viejo complex (TPVC). The caldera's origin is controversial as both the formation by huge lateral flank collapse(s) and multiple vertical collapses have been proposed. Although vertical collapses may have facilitated lateral slope failures and thus jointly contribute to the exposed morphology, their joint contribution has not been clearly demonstrated. Using results from 185 audiomagnetotelluric (AMT) soundings carried out between 2004 and 2006 inside the LCC, our study provides consistent geophysical constraints in favour of multiple vertical caldera collapse. One-dimensional modelling reveals a conductive layer at shallow depth (30–1000 m), presumably resulting from hydrothermal alteration and weathering, underlying the infilling resistive top layer. We present the resistivity distribution of both layers (resistivity images), the topography of the conductive layer across the LCC, as well as a cross-section in order to highlight the caldera's evolution, including the distribution of earlier volcanic edifices. The AMT phase anisotropy reveals the structural and radial characteristics of the LCC.
Structural pattern of the western Las Cañadas caldera (Tenerife, Canary Islands) revealed by audiomagnetotellurics
The local and regional structural pattern of volcanic edifices strongly controls the space distribution of electrical resistivity. Here we report on the structural context of the western part of the Las Cañadas caldera of Tenerife (LCC) thought to have initiated the formation of the caldera. Using a new dataset of 11 audiomagnetotelluric tensors we emphasize the resistivity distribution of Ucanca caldera and propose a major revision of its extension. We find that Ucanca caldera has a limited westwards extent and that El Cedro sector is a depression margin of the caldera. According to the extent of hydrothermalized rocks at the base of the LCC wall and the distribution of Pico Teide – Pico Viejo vents, we constrain the location and size of Ucanca caldera. The interpretation of these results also constrains the extension of the Icod Valley and proposes a headwall located below the Pico Teide – Pico Viejo Complex.
Conductive structures around Las Cañadas caldera, Tenerife (Canary Islands, Spain): A structural control
External eastern areas of the Las Cañadas caldera (LCC) of Tenerife (Canary Islands, Spain) have been investigated using the audiomagnetotelluric (AMT) method with the aim to characterize the physical rock properties at shallow depth and the thickness of a first resistive layer. Using the results of 50 AMT tensors carried out in the period range of 0.001 s to 0.3 s, this study provides six unpublished AMT profiles distributed in the upper Orotava valley and data from the Pedro Gil caldera (Dorsal Ridge). Showing obvious 1-D behaviour, soundings have been processed through 1-D modeling and gathered to form profiles. Underlying a resistive cover (150-2000 Ωm), a conductive layer at shallow depth (18-140 Ωm, 250-1100 m b.g.l.) which is characterized by a “wavy-like” structure, often parallel to the topography, appears in all profiles. This paper points out the ubiquitous existence in Tenerife of such a conductive layer, which is the consequence of two different processes: a) according to geological data, the enhanced conductivity of the flanks is interpreted as a plastic breccia within a clayish matrix generated during huge lateral collapse; and b) along main tectonic structures and inside calderas, this layer is formed by hydrothermal alteration processes. In both areas, the conductive layer is thought to be related to major structural volcanic events (flank or caldera collapse) and can be seen as a temporal marker of the island evolution. Moreover, its slope suggests possible headwall locations of the giant landslides that affected the flanks of Tenerife.
Identification of zones of preferential groundwater tracer transport using a mobile downhole fluorometer
Un fluorimètre a été utilisé pour détecter les zones découlement préférentiel dun traceur dans un puits dobservation. Les identifications de telles zones nest pas possible si les échantillons individuels sont collectés sur toute les longueurs des parties crépinées du puits. Les tests de laboratoire utilisant un fluorimètre et un dispositif adéquat ont montré que les fluorimètres pouvaient servir à définir les régimes découlement de leau dans les puits. Durant les investigations de terrain dans un aquifère poreux, le fluorimètre a enregistré les concentrations du traceur dans un puits dobservation avec une crépine de 12 m. de long, 10 m sous le gradient hydraulique dans un puits à pénétration totale. Les résultats des tests ont montré que le traceur apparaissait dans un interval discret de 2.5 m de long. Un test de dilution en puits unique et des données découlement vertical ont indiqué que leau rentrait dans le puits à dautres profondeurs, mais le traceur na pas été détecté à ces niveaux. Un modèle numérique reproduisant le test de dilution et le profil de concentration a indiqué que leau entrait dans le puits à ces niveaux à des vitesses comparables de celle du traceur. Ces données suggèrent que lécoulement des eaux souterraines varie avec la profondeur dans laquifère sous la zone dinvestigation. Dailleurs, les simulations de larrivée du traceur ont montré que la distribution des concentrations du traceur dans le puits ne pouvaient pas être dues à une couche plus fine que 0.5 m., A mobile downhole fluorometer was used to detect zones of preferential groundwater tracer transport into an observation well. Identification of such zones is not possible if individual samples are collected over the wells entire screened interval. Laboratory-based tests using the fluorometer, and a purpose-built apparatus demonstrated that the fluorometer could be used with tracers to characterise well water flow regimes. During field investigations in a porous aquifer, the fluorometer monitored tracer concentrations in an observation well with a 12-m-long screen, 10 m down the hydraulic gradient from a fully penetrating injection well. Test results showed that the tracer occurred in the observation well over a discrete 2.5-m-thick interval. Single-well dilution test and vertical-flow data indicated that water entered the well at additional depths, but no tracer was detected at these levels. A numerical model reproducing dilution test concentration profiles indicated that water entered the well in many of these horizons at comparable velocities to those in the tracer-bearing zone. These data suggest that groundwater flow direction varied with depth in the aquifer under investigation. Moreover, simulations of tracer arrival indicated that the tracer distribution observed in the observation well was derived from a horizon that may be no thicker than 0.5 m., Se utilizó un fluorómetro móvil descendente para detectar zonas de transporte preferencial de trazadores de agua subterránea en un pozo de observación. La identificación de tales zonas no es posible si se colectan muestras individuales en todo el intervalo enmallado del pozo. En base a pruebas de laboratorio utilizando el fluorómetro y un aparato especial construido se demostró que el fluorómetro podría utilizarse con trazadores para caracterizar ambientes de flujo de agua en los pozos. Durante investigaciones de campo en un acuífero poroso, el fluorómetro monitoreó concentraciones de trazadores en un pozo de observación con una malla de 12 m de largo, 10 m abajo del gradiente hidráulico de un pozo de inyección que penetra totalmente el acuífero. Los resultados de las pruebas muestran que el trazador se presentó en el pozo de observación en un intervalo discreto de 2.5 m de espesor. Pruebas de dilución en un solo pozo y datos de flujo vertical indicaron que el agua entró al. pozo en profundidades adicionales, pero que no se detectó el trazador en esos niveles. Un modelo numérico que reproduce los perfiles de concentración de las pruebas de dilución indica que el agua entró al. pozo en muchos de estos horizontes en velocidades comparables a las existentes en la zona portadora de trazadores. Estos datos sugieren que la dirección de flujo de agua subterránea varió con la profundidad en el acuífero bajo investigación. Por otra parte, las simulaciones de llegada del trazador indicaron que la distribución del trazador observada en el pozo de observación se derivó de un horizonte cuyo espesor puede no ser mayor de 0.5 m.