Hugo Matias

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 (

Salt tectonics in the western Gulf of Cadiz, southwest Iberia

This study presents the results from the interpretation of an extensive and recent regional two-dimensional seismic survey focused on the understanding of the salt tectonics in the western Gulf of Cadiz (GoC). Two different salt units were identified: an autochthonous salt unit of the Late Triassic or the Early Jurassic (Hettangian) and an allochthonous unit that originated from the Hettangian salt. Interpretation of the pattern of distribution of the salt in the basin allowed subdivision of the area of study into three distinct salt domains: the eastern domain characterized by the presence of a conspicuous allochthonous salt nappe (Esperanca Salt), the central domain dominated by salt diapirs with mild deformation of Miocene strata and wide salt-withdrawal minibasins, and the southwestern domain where present-day tectonics induces impressive salt deformation affecting the sea floor. This complex pattern is mainly the result of the interaction of inherited basement structure, complex tectonic history, and stress regime of the basin. The intense halokinesis observed has created several salt-related trap geometries and fluid migration pathways. As the focus of worldwide exploration along passive margins is gradually shifting to deep-water regions, the western GoC has the potential to become a deep-water petroleum province in the near future.

Magnetotelluric study of a plio-quaternary tectonic depression: / the Vilarica basin NE Portugal

The Vilarica basin, located northeast Portugal astride a major late-Variscan NNE-SSW reactivated strike-slip fault, is an excellent example of interplate neotectonic activity whose development has been mainly interpreted as a result of left-lateral displacement. Thirty magnetotelluric soundings were carried out in the Sta Comba da VilaricarSampaio region northern of . the tectonic basin in order to investigate the internal structure of the basin and its relationship with the main tectonic faults. Distortions of the impedance tensors were studied using Groom-Bailey decomposition technique. The predominant regional . strike N26E is in good accordance with exposed geology and can be explained for the reactivation of previous structures. Using two-dimensional inversion, three resistivity cross sections were obtained at north, center and south of the studied area. . The graben is revealed as a low resistivity 20-100 V m structure due to the sedimentary filling. The increasing electrical . resistivity from 400 to 4000 V m at depths greater than 2 km is related to the Hesperian basement rocks. The main faults, which controlled the formation and evolution of the basin, are revealed by resistivity gradients within the upper crust. q 2000 Elsevier Science B.V. All rights reserved.

Three-dimensional magnetotelluric modelling of the Vilariça depression (NE Portugal)

Abstract The Vilarica depression is located astride the northern branch of a Late Variscan (NNE–SSW) left-handed strike–slip fault known as the Vilarica Fault Zone (NE Portugal), whose reactivation in Pliocene–Quaternary times is well documented. The magnetotelluric method was applied in order to evaluate the electrical structure of the northern part of this tectonic depression. Thirty magnetotelluric soundings were carried out and a three-dimensional model was constructed. Different segments of the Vilarica Fault Zone, as well as the earlier Variscan thrusts preserved in the studied area, are well displayed in the model. Low-resistivity (30–200 Ω m) structures were revealed in the central part of the depression up to a depth of 1 km. These shallow conductors are mainly associated with the sedimentary infillings of the basin and are bordered and/or interrupted by strongly weathered and fractured blocks of metasediments displaying variable electrical resistivity. Deeper domains show a relatively homogeneous high resistivity (3500 Ω m), being difficult to distinguish the nonfractured and nonweathered metamorphosed autochthonous metasediments (at depths above 2 km) from the gneissic basement. A conductive layer (500 Ω m) was detected at a depth ranging from 8 to 12 km; this feature is tentatively associated with an important crustal detachment that may correspond to the root domain of the major active strike–slip faults in NW Iberia.

Understanding the naturalization of Eucalyptus globulus in Portugal: a comparison with Australian plantations

Despite the potential utility of a biogeographical approach to understanding the naturalization of exotic species, studies using this approach are scarce. Eucalyptus globulus is an economically important Australian tree species that has become naturalized in a number of countries where it was introduced. Portugal is an ideal territory to study the naturalization of E. globulus owing to: a long introduction history, the antipodal location compared to Australia and the large cultivated area. Wildling density was assessed in 116 E. globulus plantations in central Portugal through 213 transects established along plantation borders. Boosted regression trees were used to model the influence of plantation-scale variables. Results from this survey were compared with data obtained in plantations from seven Australian regions, where a similar sampling protocol had been used. In Portugal, wildlings were more abundant in plantations that were: located in moist aspects, coppiced, with older tree stems and corresponding to intermediate site growth indexes. The overall density (127 plants ha−1) was 14.9 times higher than in the Australian estate, but this ratio was reduced to 3.1 in a more comparable subset of unburnt, first rotation plantations. A generalized linear model fitted using a dataset combining the two surveys showed that country influenced wildling density, together with plantation rotation and stem age. These results provide insights into the naturalization of a widely cultivated tree species, pointing to a fundamental role of the introduction history, possibly acting along with the biogeographical characteristics of the introduced range.