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- Preliminary results of a study of magnetic properties in the Foum-Zguid dyke (Morocco)Publication . Silva, P. F.; Marques, F. O.; Henry, B.; Mateus, A.; Lourenço, Nuno; Miranda, J. M.This work focuses on the study of flow and propagation of magma using rock magnetic analyses along sections across the thick Jurassic dyke of Foum-Zguid (Southern Morocco). Thermomagnetic data show that Ti-poor titanomagnetite is the main magnetic carrier. Petrographic analysis shows that the main Ti phase (ilmenite) occurs either as lamellae within spinel (center of the dyke) or as isolated grains (dyke margin). Bulk magnetic properties display distinct behavior according to the distance to the dyke margin; grain size of the main magnetic carrier decreases towards the center of the dyke, while the natural remanent magnetization and the bulk magnetic susceptibility increase. Only the magnetic susceptibility ellipsoid close to the dyke margin corresponds to that usually found in thin dykes, with the magnetic foliation sub parallel to dyke margins. Maximum principal axis is in most cases either parallel or perpendicular to the intersection between the planes of magnetic foliation and dyke wall. Moreover, when this axis is perpendicular to the intersection it is associated with a more oblate magnetic susceptibility ellipsoid shape, indicating the presence of complex magnetic fabrics. The studied magnetic properties show that, in this 100 m wide thick dyke, flow structures related with dyke propagation are only preserved close to the quickly cooled dyke margins. (C) 2004 Elsevier Ltd. All rights reserved.
- Long-term seismicity of the Reykjanes Ridge (North Atlantic) recorded by a regional hydrophone arrayPublication . Goslin, J.; Lourenço, Nuno; Dziak, R. P.; Bohnenstiehl, D. R.; Haxel, J.; Luis, JoaquimThe seismicity of the northern Mid-Atlantic Ridge was recorded by two hydrophone networks moored in the sound fixing and ranging (SOFAR) channel, on the flanks of the Mid-Atlantic Ridge, north and south of the Azores. During its period of operation (05/2002-09/2003), the northern 'SIRENA' network, deployed between latitudes 40 degrees 20'N and 50 degrees 30'N, recorded acoustic signals generated by 809 earthquakes on the hotspot-influenced Reykjanes Ridge. This activity was distributed between five spatio-temporal event clusters, each initiated by a moderate-to-large magnitude (4.0-5.6 M) earthquake. The rate of earthquake occurrence within the initial portion of the largest sequence (which began on 2002 October 6) is described adequately by a modified Omori law aftershock model. Although this is consistent with triggering by tectonic processes, none of the Reykjanes Ridge sequences are dominated by a single large-magnitude earthquake, and they appear to be of relatively short duration (0.35-4.5 d) when compared to previously described mid-ocean ridge aftershock sequences. The occurrence of several near-equal magnitude events distributed throughout each sequence is inconsistent with the simple relaxation of main shock-induced stresses and may reflect the involvement of magmatic or fluid processes along this deep (>2000 m) section of the Reykjanes Ridge.
- Variation of magnetic properties in sedimentary rocks hosting the Foum Zguid dyke (southern Morocco): combined effects of re-crystallization and Fe-metasomatismPublication . Silva, P. F.; Henry, B.; Marques, F. O.; Mateus, A.; Madureira, P.; Lourenco, N.; Miranda, J. M.The effects of dyke intrusion on the magnetic properties of host sedimentary rocks are still poorly understood. Therefore, we have evaluated bulk magnetic parameters of standard palaeomagnetic samples collected along several sections across the sediments hosting the Foum Zguid dyke in southern Morocco. The study has been completed with the evaluation of the magnetic fabric after laboratory application of sequential heating experiments. The present study shows that: (1) close to Fourn Zguid dykes, the variations of the bulk magnetic parameters and of the magnetic fabric is strongly related with re-crystallization and Fe-metasomatism intensity. (2) The thermal experiments on AMS of samples collected farther from the dyke and, thus, less affected by heating during dyke emplacement, indicate that 300-400 degrees C is the minimum experimental temperature necessary to trigger appreciable transformations of the pre-existing magnetic fabrics. For temperatures higher than ca. 580 degrees C, the magnetic fabric transformations are fully realized, with complete transposition of the initial fabric to a fabric similar to that of samples collected close to the dyke. Therefore, measured variations of the magnetic fabric can be used to evaluate re-crystallization temperatures experienced by the host sedimentary rock during dyke emplacement. The distinct magnetic behaviour observed along the cross-sections strongly suggests that samples collected farther from the dyke margins did not experience thermal episodes with temperatures higher than 300 degrees C after dyke emplacement. (3) AMS data shows a gradual variation of the magnetic fabric with distance from the dyke margin, from sub-horizontal K-3 away from the dyke to vertical K3 close to the dyke. Experimental heating shows that heat alone can be responsible for this strong variation. Therefore, such orientation changes should not be unequivocally interpreted as the result of a stress field (resulting from the emplacement of the dyke, for instance). (4) Magnetic studies prove to be a very sensitive tool to assess rock magnetic transformations, thermally and chemically induced by dyke intrusion in hosting sediments. (c) 2005 Elsevier B.V. All rights reserved.
- Mantle dynamics and characteristics of the Azores plateauPublication . Adam, C.; Madureira, P.; Miranda, M; Lourenço, Nuno; Yoshida, M.; Fitzenz, D.Situated in the middle of the Atlantic Ocean, the Azores plateau is a region of elevated topography encompassing the triple junction between the Eurasian, Nubian and North American plates. The plateau is crossed by the Mid-Atlantic Ridge, and the Terceira Rift is generally thought of as its northern boundary. The origin of the plateau and of the Terceira Rift is still under debate. This region is associated with active volcanism. Geophysical data describe complex tectonic and seismic patterns. The mantle under this region is characterized by anomalously slow seismic velocities. However, this mantle structure has not yet been used to quantitatively assess the influence of the mantle dynamics on the surface tectonics. In this study, we use a highly resolved tomography model to model the convection occurring in the mantle beneath the Azores region. The convection pattern points out two distinct upwelling, thus proving that the volcanism emplacement is created by a buoyant mantle upwelling. The modeled dynamic topography recovers well the characteristics of the depth anomaly associated with the Azores plateau, except for the south-eastern most part, thus proving that most of the depth anomaly associated with the Azores plateau is created by the present-day mantle dynamics. The stresses induced by the mantle convection can account for the lifting regime observed over the Azores plateau and the Terceira Rift, and its consequences in terms of surface morphology and seismicity. (C) 2012 Elsevier B.V. All rights reserved.
- A subida do nível do mar durante o Holocénico no Golfo de Cadiz – tendência regional e diferenças locaisPublication . Moura, Delminda; Boski, T.; Duarte, Duarte; Veiga-Pires, C.; Pedro, Paulo; Lourenço, Nuno; Diniz, FilomenaIn Guadiana River Estuary, the Holocene sequence is underlain by pebbly delta fan formed in two stages, prior to 5E and 3 highstands. The Holocene sedimentary sequence was lain either directly on gravels or on top of fluvial sands deposited between 11000 and 10000 yr. BP. An accelerated phase of infilling of the estuary by clayey sediments, containing marsh foraminifer Trochammina, began at ca. 9800 yr. BP when the sea level was about 39 m below the present.At ca. 7000 BP the central part of the estuary started to accommodate coarser sediments, partially introduced from the shelf. The first phase of the Holocene sea level rise at a rate of 0.85 m/century terminated at ca 6500yr. BP. Since then, the lagoonal bodies in the vicinity of estuary started to enclose behind sand spits and predominantly sandy sedimentation initiated within the estuary. After the second phase of a slower rise at the rate of 0.3 m/century, which lasted until ca. 5000 yr.BP, the sea approached the present level. Data from Guadalete Estuary published in Spain set the beginning of Holocene transgression at shallower level (ca 10m)than in Guadiana Estuary.This discrepancy may be explained by isostatic uplift of Betic Cordillera or slumping of Guadiana terrace on which the Holocene sediments are lain. Since 7500 yr. BP the data from Odiel-Tinto, Guadalete and Guadiana are in a relatively good agreement.
- The postglacial sea level rise within the Guadiana Estuary- a contribution to a regional record in the Gulf of CadizPublication . Boski, T.; Moura, Delminda; Lourenço, Nuno; Veiga-Pires, C.In the Guadiana River Estuary, the Holocene estuarine infill is underlain by coarse delta fan sediments enclosing saltmarh deposits, dating back to isotope stage 5e or a stage 3 interstadial highstands.