Paulo E. Fonseca

40Ar/39Ar mineral age constraints for the tectonothermal evolution of a Variscan suture in southwest Iberia

Abstract Dallmeyer, R.D., Fonseca, P.E., Quesada, C. and Ribeiro, A., 1993. 40Ar/39Ar mineral age constraints for the tectonothermal evolution of a Variscan suture in southwest Iberia. Tectonophysics, 222: 177–194. Mafic units interpreted to represent an internally disrupted ophiolite succession are exposed along a Variscan suture situated between the South Portuguese and Ossa-Morena zones in southern Iberia. Structural characteristics of various units adjacent to the suture zone suggest maintainence of an oblique (transpressional), sinistral tectonic regime during late Paleozoic plate convergence. This involved initial, limited, north-directed subduction beneath the Ossa-Morena zone, and was followed by oblique collision of the South Portuguese Terrane. Metamorphic evolution of rocks adjacent to the suture occurred during maintainence of a moderately high geothermal regime. Five samples of amphibolite were collected within the hanging wall of the suture. These included samples from the ophiolitic terrane and from the Ossa-Morena zone. Two samples were also collected within the post-kinematic Beja Gabbro which intruded the other two tectonic units following their structural juxtaposition and penetrative ductile deformation. Hornblende concentrates from these seven samples record well-defined 40Ar/39Ar plateau ages which range between c. 342 and 335 Ma. These results imply that the regional tectonostratigraphic units were maintained at moderate crustal levels until the Visean, and that their juxtaposition and internal ductile deformation occurred in the late Devonian. Subsequent cooling was probably effected by transpressional uplift during oblique collision following closure of intervening oceanic elements.

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.

The emergence of volcanic oceanic islands on a slow‐moving plate: The example of Madeira Island, NE Atlantic

The transition from seamount to oceanic island typically involves surtseyan volcanism. However, the geological record at many islands in the NE Atlantic—all located within the slow-moving Nubian plate—does not exhibit evidence for an emergent surtseyan phase but rather an erosive unconformity between the submarine basement and the overlying subaerial shield sequences. This suggests that the transition between seamount and island may frequently occur by a relative fall of sea level through uplift, eustatic changes, or a combination of both, and may not involve summit volcanism. In this study, we explore the consequences for island evolutionary models using Madeira Island (Portugal) as a case study. We have examined the geologic record at Madeira using a combination of detailed fieldwork, biostratigraphy, and 40Ar/39Ar geochronology in order to document the mode, timing, and duration of edifice emergence above sea level. Our study confirms that Madeiras subaerial shield volcano was built upon the eroded remains of an uplifted seamount, with shallow marine sediments found between the two eruptive sequences and presently located at 320–430 m above sea level. This study reveals that Madeira emerged around 7.0–5.6 Ma essentially through an uplift process and before volcanic activity resumed to form the subaerial shield volcano. Basal intrusions are a likely uplift mechanism, and their emplacement is possibly enhanced by the slow motion of the Nubian plate relative to the source of partial melting. Alternating uplift and subsidence episodes suggest that island edifice growth may be governed by competing dominantly volcanic and dominantly intrusive processes.

Geodynamic evolution of the South Variscan Iberian Suture as recorded by mineral transformations

New structural, petrographic, mineralogical and geochemical data from the Beja-Acebuches Ophiolite Complex (BAOC) are presented, and reviewed together with data published elsewhere. The new data obtained shed light on questions such as: 1) the relative importance of the obduction event; 2) its geological record in the deep levels of BAOC; 3) the nature and intensity of the Variscan metamorphism and deformation during subsequent continental (arc) collision; 4) the age relationships between BAOC and the Beja Igneous Complex; and 5) by means of numerical modelling, the thermal metamorphism of the Ossa-Morena autochthonous terranes induced by the ophiolite obduction. The emerging picture is that of a fairly simple overall geological evolution for BAOC, seamlessly integrated within the evolution of the southern branch of the Iberian Variscides. Obduction of BAOC is a relatively minor early event in the general NE–SW convergence that gave rise to the orogen as seen regionally and is recorded by an anisotropic, high-temperature, metamorphic fabric at the gabbro levels and by subtle features of the chemical composition of primary minerals at the underlying peridotite level; it caused chilling of the obducted ophiolitic slab and no significant metamorphism on the autochtonous rocks of the Ossa-Morena Zone. BAOC underwent most of its deformation and (amphibolite facies) metamorphism during a later collisional event, that took place as the most primitive rocks of the Beja Igneous complex were being intruded, and whose waning stages are responsible for extensive serpentinisation of peridotites and for important aquocarbonic fluid discharges along the semibrittle–brittle shear zones meanwhile developed.

Prograde epizonal clay mineral assemblages and retrograde alteration in tectonic basins controlled by major strike-slip zones (W Iberian Variscan chain)

Abstract We have carried out optical microscopy, X-ray diffraction (XRD) and scanning and transmission electron microscopy (SEM and TEM) studies of phyllosilicates from black slates of very low to low-grade metamorphism. Such slates belong to a Middle/Late Devonian basin and an Early Carboniferous basin associated with the Porto-Tomar-Ferreira do Alentejo strike-slip shear zone (Ossa-Morena Zone, Portuguese Iberian Variscan Massif). These black slates are imbricated in an Upper Proterozoic substratum of higher metamorphic grade. Kübler Index values of white micas and mineral assemblages deduced from the XRD, SEM and TEM data (muscovite, chlorite and pyrophyllite) indicate high anchizonal and epizonal metamorphic conditions for slates from these basins. The b parameter and the phengitic contents of mica suggest the occurrence of low pressures (1-2 kbar) related to an extensional geotectonic setting. The dense fracture network shown by SEM images and the high density of crystal defects revealed by the TEM study in the eastern basin, adjacent to faults produced by shearing, suggest that their epizonal phyllosilicates were more affected during deformation than those belonging to the western basin, favouring the development of a retrograde association (siderite, kaolin group minerals and Al-smectite) on the epizonal paragenesis. Microcavities formed along phyllosilicate cleavage acted as channels for fluid transport favouring alteration under low-temperature conditions.

A comprehensive analysis of groundwater resources using GIS and multicriteria tools (Caldas da Cavaca, Central Portugal): environmental issues

Hard-rock watersheds are essentially confined to fractured and weathered horizons, but they are a source of valuable water resources at a regional level, namely for domestic, industrial and agricultural purposes, and public supply. They commonly exhibit complex geological bedrock and morphological features as well as distinctive gradients in rainfall and temperature. Hydromineral and geothermal resources have relevant economic value both for the bottled water/thermal spas industry and for energy supply. A comprehensive evaluation and integrated groundwater resources study has been carried out for the Caldas da Cavaca hydromineral system in Central Portugal, using hydrogeomorphology and GIS mapping techniques. Thematic maps were organised from a geodatabase comprising several layers, namely lithology, tectonic lineaments density, slope, drainage density, rainfall, net groundwater recharge and water quality. Normalised weights were assigned to all these categories according to their relative importance to groundwater potential, based on their effectiveness factors. Hydrogeochemistry, natural radioactivity and intrinsic vulnerability assessment (GOD-S, DRASTIC-Fm, SINTACS, SI indexes) issues were also cross-checked. Based on all the compiled information, a hydrogeomorphological map was produced. This multidisciplinary approach highlights the importance of hydrogeomorphological mapping as a tool to support hydrological conceptualisation, contributing to groundwater decision-making process in different stages, like water resources management and territory planning, and thus, to environmental sustainability.

Conceptualizing a mountain hydrogeologic system by using an integrated groundwater assessment (Serra da Estrela, Central Portugal): a review

Mountains are often considered as the world’s water towers. This paper presents a critical review on the research concerning the integrated assessment of groundwater resources of the mountain hydrogeologic system of Serra da Estrela Natural Park (central Portugal). The study area is the Zêzere river basin upstream of Manteigas village located at the Serra da Estrela Mountain in Central Portugal. It provides the source of strategic water resources for the Portuguese mainland, including normal groundwaters, thermomineral waters and surface waters. An integrated approach has been used to formulate a conceptual model for this complex mountain hydrogeological system by integrating the geological, morphotectonic, hydroclimatic, unsaturated soil zone, hydrogeological, hydrogeophysical, hydrogeochemical and isotopic data. This model has been useful to: i) evaluate the water resources; ii) provide the basis for a sustainable management of water resources, iii) design measures for groundwater exploitation and contamination control; and iv) set up land-use policies.

The Odivelas Limestone: evidence for a Middle Devonian reef system in western Ossa-Morena Zone (Portugal)

The Odivelas Limestone: evidence for a Middle Devonian reef system in western Ossa-Morena Zone (Portugal) The Odivelas Limestone constitutes one of the few records of Middle Devonian sedimentation in the western Ossa-Morena Zone. Although deformed and metamorphosed the limestones have an abundant fossil content which allows their positioning as late Eifelian/early Givetian in age and to relate the reef fauna with the typical Rhenish facies for the same time period. Magnetic susceptibility analysis was attempted and is in agreement with the biostratigraphy, but the limited extent of sections and the metamorphism precludes firm correlations. The field evidence, petrographic and geochemical analysis point to a close paleogeographical relation and dependence of the reef system on volcanic structures which are included in the Beja Igneous Complex. The age of part of the volcanic and sub-volcanic suite of this complex is thus constrained.

Role of high mountain areas in catchment hydromineral resources - Northern/Central Portugal: environmental issues.

This study summarizes the results of geological, geomorphological, tectonic, geochemical, geophysical, hydrogeological and isotopic techniques in hydromineral resources assessment (issue temperature between 27oC and 45oC). Two case studies are presented: i) Serra da Estrela mountain region, the highest mountain in Portuguese mainland and ii) Serra do Marao mountain region. A special emphasis is dedicated to the recharge and discharge processes and the role of snowmelt as a source of hydromineral resources. Since local Spas are particularly dependent on water quality, the existence of mixing between hydromineral waters and local shallow groundwaters is also considered.

Transected folds with opposite patterns in Terena Formation (Ossa Morena Zone, Portugal): anomalous structures resulting from sedimentary basin anisotropies

The Terena Formation is located in the central part of the Ossa-Morena Zone (OMZ) and outcrops in the core of a latter (D3) first order syncline. This Formation is a Lower Devonian flysch and shows an unusual “Z” shape, with a central sector trending nearly N-S, and the tips trending NW-SE. This central sector is crossed by the cleavage (NW-SE) showing an apparent dextral (clockwise) transection pattern, anomalous and opposite to the regional widespread sinistral (anti-clockwise) transpression. The same sector with cartographic dextral transection, shows at outcrop scale, mesoscopic folds with a sinistral transection. During the Lower Devonian a N-S trending basin was developed as an effect of an early tectonic deformation phase. This trough was filled with turbidites and its elongated geometry determined the shape of the main syncline. We propose that the dextral transection pattern, at cartographic scale, result from the superposition of the NW-SE upright S3 cleavage on this major regional structure controlled by a sedimentary trough. The mesoscopic folds, observed on the upper levels of the sedimentary sequence were not influenced by the topographic anisotropy of the basin, and therefore they developed a left transection, according to the regional deformation mechanisms. The “Z” shape of the syncline could be explained as a consequence of two major tectonic shear zones situated along the north and south boundaries of the OMZ, respectively the Tomar-Badajoz-Cordoba Shear Zone and the South Iberian Suture, lined by the Beja-Acebuches Ophiolitic Complex. Both shear zones have a sinistral transpressive character and were active during late Variscan tectonic events.