Jaume Pous

A modified Archie's law for two conducting phases

Abstract Many types of mixing model are used widely within the earth sciences to determine the electrical properties of porous media consisting of solid and fluid phases of known conductivities, volume fractions and distributions (i.e. phase connectivities). Most models are valid for two or more conducting phases. However, of the simple models only Archie’s law includes a variable term, the Archie cementation exponent m, that describes the connectivity of the phases. Unfortunately, Archie’s law is only valid for one conducting phase distributed within a non-conducting phase, which makes it inapplicable in instances where the rock matrix has a significant conductivity such as for clay-rich rocks and in calculations involving partial melting. More complex models exist which account for multiple conducting phases and control over phase conductivity. We have adapted the conventional Archie’s law to produce a simple modified Archie’s law that can be used with two conducting phases of any conductivity and any volume fraction, while retaining the ability to model variable connectivities within those phases that result from variations in their distribution. The modified model has two exponents (m and p) that describe the connectivity of each of the two phases. The exponents are related through an equation that depends also on the volume fractions of the two phases. The modified and the conventional versions of Archie’s law have been tested on a granular analogue porous medium with a conducting matrix and a pore space saturated with a range of saline fluids with different salinities and conductivities. The new model describes the experimentally determined electrical behaviour of the system extremely well, improving greatly on the conventional Archie’s law.

Electromagnetic imaging of Variscan crustal structures in SW Iberia: the role of interconnected graphite

Abstract The western part of the Iberian Peninsula (Iberian Massif) is the best exposed fragment of the Variscan orogen in Europe. Its southern half was generated by an oblique collision between three continental terranes belonging to the margins of Laurassia (Avalonia) – the South Portuguese Zone (SPZ) – and Gondwana – the Ossa Morena Zone (OMZ) and the Central Iberian Zone (CIZ). The boundaries between them are considered to be suture zones. A 200 km long magnetotelluric profile across the three Variscan terranes was done in a NNE direction, approximately perpendicular to the main tectonic features. The results of two-dimensional inversion of the MT dataset reveal high-conductivity zones coinciding with the transitions SPZ/OMZ and OMZ/CIZ. These conductive bodies related to the sutures at depth were interpreted as graphite enrichments along shear planes formed due to the overall transpressive regime. A high-conductivity layer extending along the whole OMZ was found at a depth of 15–25 km, the top of which spatially correlates with a broad reflector detected by a recently acquired deep seismic reflection profile. The high conductivity was interpreted as caused by the Precambrian Serie Negra graphite-rich rocks. Carbon and oxygen X-ray mapping with electron microprobe on polished sections of Serie Negra samples from OMZ revealed the presence of interconnected graphite, which supports the hypothesis that graphite is determinant for the high conductivity. Two graphite types, which help to record the geological evolution, were identified: graphite accumulations in the schistosity surfaces produced by folding and metamorphism, and metallic films of graphite developed along late faults. The conductive layer shows blobs of higher conductivity suggesting macro-anisotropy. Additional mylonitisation and shearing produced by thrusting at depth can be the origin of these zones of enhanced conductivity, given that the detachment level is located within the Serie Negra. Several high-resistivity features were found in the upper crust, related to Devonian and Carboniferous successions and probably to some unexposed plutons in the SPZ and the Palaeozoic series of OMZ plus some granitic intrusions. In the CIZ, a high-resistivity zone extending to the whole crust is correlated with extensive late Variscan granite intrusions.

Magnetotelluric study of the Las Cañadas caldera (Tenerife, Canary Islands) : structural and hydrogeological implications

The Las Can ‹ adas caldera in Tenerife (Canary Islands) is a well-exposed caldera depression in which the active Teide^Pico Viejo complex stands. In addition to its volcanological interest, the Las Can ‹ adas caldera also holds the main groundwater reservoir of Tenerife. An audiomagnetotelluric and magnetotelluric survey was carried out in order to image the interior of the caldera depression. The field campaign consisted of 33 audiomagnetotelluric sites in the period range from 0.001 to 0.3 s and 11 magnetotelluric sites from 0.004 to 200 s. A detailed mapping of the electrical conductivity of the subsurface was obtained. For the long periods a three-dimensional modelling of the island ^ including the bathymetry ^ was carried out to study the effect of the ocean. This effect starts to be important at periods longer than 10 s. Accordingly, the sites were arranged into six profiles and a two-dimensional joint inversion of all data until 10 s was performed for each profile. The geometry of the high conductive zones found indicates that the caldera includes two closed depressions in the western (Ucanca) and central (Guajara) sectors, whereas in the

Partial melting of subducted continental lower crust in the Pyrenees

A magnetotelluric profile through the Central Pyrenees indicates the présence of very high conductive zones at lower crustal and upper mantle depths. High conductivity at upper mantle depths is interpreted as partial melts within subducted lower crust, whereas high conductivity at lower crustal depths is interpreted as rising magmas derived from the melting crustal slab. Such melts suggest that the continental lower crust, together with its lithospheric mantle may have been subducted into the mantle during the Pyrenean continental collision. Thus, magma generation can be related to thermal reequilibration of a subducted lower crust, a scenario that may serve as a model for understanding the late evolution of other collisional orogens.

Are some of the deep crustal conductive features found in SW Iberia caused by graphite

Abstract Recent results obtained from two-dimensional inversion of magnetotelluric (MT) data acquired in SW Iberia reveal high-conductive features at the middle–lower levels of the crust. The top of these anomalous structures correlates very well with the depth (10–13 km) of an important seismic interface that has been interpreted as a regional detachment horizon. Very shallow and relatively narrow conductors in the Ossa Morena Zone appear to correspond to small-scale fluid-deposited graphite systems in the preorogenic metasedimentary sequences. Some of the midcrustal conductors can be ascribed to graphite-bearing thrust zones, the formation of graphite with variable crystallinity being a consequence of Variscan shearing processes. Deep-seated conductors are tentatively interpreted as a result of relatively continuous, highly crystalline grain-boundary graphite films presumably preserved in basement, granulite(?) rocks. Assuming that graphite occurs as interconnected films, calculations indicate that a fraction of 0.006–0.02% of this accessory mineral is enough to explain the range of the electrical resistivity estimated on the basis of MT models. The role of graphite on the thermal behaviour of the crust is also discussed. The results show that low contents of graphite do not significantly change the thermal behaviour of earth materials.

New geophysical constraints on the deep structure of the Pyrenees

A 2D modeling of geoid/quasigeoid data across the Pyrenees is consistent with electromagnetic images of the Pyrenean lithospheric structure, showing a subducted Iberian lower crust. Using new magnetotelluric data acquired through the orogen, a 3D electrical conductivity model of the Pyrenean lithospheric structure is obtained. The most outstanding feature is the presence of a high conductivity zone at lower crustal depths along the orogen in the contact zone between the Iberian and European plates. The integration of various geophysical parameters suggests that the simplest and most reasonable mechanism to explain the observed data is partial melting of the subducted Iberian lower crust.

A multidisciplinary geophysical study in the Betic chain (southern Iberia Peninsula)

Abstract The combined analysis of magnetotelluric measurements, tomographic velocity models and deep seismic reflection images confirms that the Betics orogen consists of the juxtaposition of two crustal domains characterized by distinctive physical properties. At depth these data sets show evidence for a non-coincidence of the petrological and the seismic Moho beneath the Betics chain. The data sets reveal the geophysical properties of the Alboran domain (Internal Betics) and the Iberian Massif (External Betics). According to this, the Iberian crust features a relatively high seismic velocity, is seismically transparent in the seismic reflection images and is electrically resistive. The Alboran domain crust is characterized by a low average velocity, displays high reflectivity in the seismic reflection images and is electrically conductive. The outcrops of the metamorphic complexes (Alpujarride and Nevado Filabride), showing relatively high velocities coupled with low V p /V s values (1.67) derived from the Wadati slopes, suggest the existence of rocks rich in silica beneath the Alboran domain crust. An interpreted detachment at 12 km depth imaged by deep seismic reflection suggests that these rocks could be related to the Iberian upper crust. Partial melts and fluids are proposed to explain the high conductivity observed at deep crustal levels. These would account also for the reflectivity and the low V p /V s ratios mapped beneath the Alboran domain.

Electromagnetic imaging of a transpressional tectonics in SW Iberia

Forty-one magnetotelluric soundings were carried out along a 200 km-long profile (approximately NNE-SSW) across the three major geotectonic units in SW Iberia. A model obtained from two-dimensional inversion of the magnetotelluric data set reveals high conductivity zones in the middle-lower crust (10–30 km). Two of these zones correspond to the transition between the main geotectonic units: one between the South Portuguese Zone and the Ossa Morena Zone, interpreted as having been caused by metasediments, and the other between the Ossa Morena Zone and the Central Iberia Zone associated with a shear zone and metasediments. Another high conductivity anomaly related to black shales with major graphite impregnation was detected within the Ossa Morena Zone. The resistive features, located preferentially in the upper crust (1–10 km), coincide with gabbroic and granitic complexes.

Magnetotelluric survey of the electrical conductivity of the crust across the Ossa Morena Zone and South Portuguese Zone suture

Abstract The transition between the South Portuguese Zone (SPZ) and the Ossa Morena Zone (OMZ) is made up of a major geosuture, which is indicated by the Beja-Acebuches ophiolite. Several geological data suggest that this suture is the result of an oblique collision, with a northward propagation, between the SPZ and OMZ. From the structural point of view the OMZ is much more complex than the adjacent Central Iberian Zone and the SPZ. The frequently complicated internal structure is the result of superposition of various structural elements developed during different tectonic events. With the aim of providing new constraints to this complex deep structure a magnetotelluric (MT) survey was designed by the universities of Lisbon and Barcelona in the scope of the bilateral cooperation and the Europrobe programs. A first NNE–SSW profile across the SPZ and OMZ was carried out in September 1997. The profile is 40 km long and consists of nine deep MT soundings with periods ranging from 0.0039 to 4000 s. The time series were processed using a robust algorithm, after visual inspection. The study of the dimensionality and directionality of the regional electrical structures is based on the analysis of the induction arrows and the Groom–Bailey decomposition. A strike of N125E was determined, in accordance with regional geological features. 2-D inversion was undertaken on both TE and TM modes using the RRI method. The main feature of the model is a large conductive body (

Integrated two-dimensional lithospheric conductivity modelling in the pyrenees using field-scale and laboratory measurements

Abstract Recent magnetotelluric (MT) studies have shown that the lower crust in the Pyrenees contains a high conductivity zone consistent with a subducting continental slab, whose conductivity is 0.33 S/m. Partial melting has been interpreted to be the most plausible explanation for this high conductivity. Here we report a two-dimensional conductivity model of the lithosphere by integrating field-scale and laboratory determinations of the conductivity of continental crustal and mantle rocks. The laboratory data provide empirical formulas which allow us to determine the fluid saturated rock and melt conductivity when temperature, pressure and lithology are known. Consequently, we have also calculated the density, lithostatic pressure, and several alternative temperature profiles for use in the model from gravity, seismic and thermal field data. These can be used with a prescribed melt fraction to predict the electrical conductivity at depth, which can be compared with the MT conductivity data. Alternatively, the laboratory data can be combined with the MT conductivity data to predict the melt fraction at depth. The primary outputs of the modelling are conductivity and melt fraction prediction profiles for six mixing models; (i) Waff’s model/Hashin–Shtrikman (HS) upper bound, (ii) HS lower bound, (iii) parallel layers, (iv) perpendicular layers, (v) random melt areas, and (vi) a modified Archie’s law that takes account of the presence of two conducting phases. The modelling results indicate that a good match to the MT data can be obtained along the whole profile by the influence of pressure, temperature and the fluid phase with the only exception being the subducted slab, where a minimum of 4.7% melt fraction is necessary to explain the data.