Browsing by Author "Loureiro, Afonso"
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- Deep structure of the North Natal Valley (Mozambique) using combined wide-angle and reflection seismic dataPublication . Lepretre, A.; Schnurle, P.; Evain, Mikael; Verrier, F.; Moorcroft, D.; Clarens, P. de; Corela, C.; Afilhado, Alexandra; Loureiro, Afonso; LEROY, Sylvie; D'Acremont, Elia; Thompson, J.; Aslanian, D.; Moulin, MarylineThe North Natal Valley (NNV) and the Mozambique Coastal Plain (MCP) are key areas for the understanding of the SW Indian Ocean history since the Gondwana break-up. Nevertheless, the deep structures and the nature of the NNV and MCP remain discussed in the absence of deep geophysical data. In 2016, the NNV, MCP and Limpopo margin (LM) have been investigated along seven wide-angle and MCS profiles. The combined wide-angle and reflection seismic interpretation along the N-S MZ7 profile reveals an upper sedimentary sequence characterized by low velocities generally not exceeding 3 km/s, with thicknesses varying from 0.150 km in the central part to similar to 2.8 km in the south. The underlying sequence is formed of a 2.53.0 km thick volcano-sedimentary sequence which presents important lateral and with depth changes and presence of high velocity lenses, indicating inter-bedded volcanic sills and recurrent magmatic episodes. The south of the NNV including the Naude Ridge (NR) presents a disturbed sedimentary cover with structural highs and southward-dipping reflectors and sub-basins. The crust, reaching 35-40 km onshore below the MCP, gently thins below the continental shelf to a regular thickness of similar to 29 km below the NNV. Crustal velocities reveal low velocity gradients, with atypical high velocities. South the ND, the crust thins to 15 km. We interpret the velocity architecture combined with the evidences of volcanism at shallower depths as indicating an intensively intruded continental crust. Contrary to what is proposed in most geodynamic models, the Mozambique Coastal plain and the Natal Valley are both of continental nature, with an abrupt necking zone located south of NR. The Antarctica plate was therefore situated at the eastern limit of these two domains before the Gondwana breakup.
- Gondwana breakup: messages from the North Natal ValleyPublication . Moulin, Maryline; Aslanian, Daniel; Evain, Mikael; Lepretre, Angelique; Schnurle, Philippe; Verrier, Fanny; Thompson, Joseph; De Clarens, Philippe; LEROY, Sylvie; Dias, Nuno; Afilhado, Alexandra; Apprioul, R.; Bronner, A.; Castilla, R.; Corela, Carlos; Crozon, J.; Davy, C.; D'acremont, E.; Droz, Laurence; Duarte, J. L.; Fernagu, P.; Ferrant, A.; Fischer, M.; Franke, D.; Inguane, H.; Jorry, Stephan; Jouet, G.; Loureiro, Afonso; Le Bouteiller, P.; Le Bihan, C.; Mahanjane, S.; Moocroft, D.; Pelleau, P.; Picot, M.; Pierre, D.; Pitel, M.; Rabineau, M.; Rombe, C.; Roudaut, M.; Senkans, A.; Toucanne, SamuelThe Natal Valley, offshore Mozambique, is a key area for understanding the evolution of East Gondwana. Within the scope of the integrated multidisciplinary PAMELA project, we present new wide-angle seismic data and interpretations, which considerably alter Geoscience paradigms. These data reveal the presence of a 30-km-thick crust that we argue to be of continental nature. This falsifies all the most recent palaeo-reconstructions of the Gondwana. This 30-km-thick continental crust 1,000 m below sea level implies a complex history with probable intrusions of mantle-derived melts in the lower crust, connected to several occurrences of magmatism, which seems to evidence the crucial role of the lower continental crust in passive margin genesis.
- Imaging exhumed lower continental crust in the distal Jequitinhonha basin, BrazilPublication . Loureiro, Afonso; Schnürle, P.; Klingelhoefer, Frauke; Afilhado, Alexandra; Pinheiro, J.; Evain, Mikael; Gallais, F.; Dias, Nuno; Rabineau, Marina; Baltzer, A.; Benabdellouahed, M.; Soares, J.; Fuck, R.; Cupertino, J. A.; Viana, Adriano Roessler; Matias, Luis; Moulin, Maryline; Aslanian, D.; Morvan, L.; Mazé, J. P.; Pierre, D.; Roudaut-Pitel, M.; Rio, I.; Alves, D.; Júnior, P. Barros; Biari, Youssef; Corela, C.; Crozon, J.; Duarte, J. L.; Ducatel, C.; Falcão, C.; Fernagu, P.; Lima, M. Vinicius Aparecido Gomes de; Piver, D. Le; Mokeddem, Z.; Pelleau, P.; Rigoti, C.; Roest, W.; Roudaut, M.Twelve combined wide-angle refraction and coincident multi-channel seismic profiles were acquired in the Jequitinhonha-Camamu-Almada, Jacuípe, and Sergipe-Alagoas basins, NE Brazil, during the SALSA experiment in 2014. Profiles SL11 and SL12 image the Jequitinhonha basin, perpendicularly to the coast, with 15 and 11 four-channel ocean-bottom seismometers, respectively. Profile SL10 runs parallel to the coast, crossing profiles SL11 and SL12, imaging the proximal Jequitinhonha and Almada basins with 17 ocean-bottom seismometers. Forward modelling, combined with pre-stack depth migration to increase the horizontal resolution of the velocity models, indicates that sediment thickness varies between 3.3 km and 6.2 km in the distal basin. Crustal thickness at the western edge of the profiles is of around 20 km, with velocity gradients indicating a continental origin. It decreases to less than 5 km in the distal basin, with high seismic velocities and gradients, not compatible with normal oceanic crust nor exhumed upper mantle. Typical oceanic crust is never imaged along these about 200 km-long profiles and we propose that the transitional crust in the Jequitinhonha basin is a made of exhumed lower continental crust.
- Lithospheric structuration onshore-offshore of the Sergipe-Alagoas passive margin, NE Brazil, based on wide-angle seismic dataPublication . Pinheiro, J. M.; Schnurle, P.; Evain, Mikael; Afilhado, Alexandra; Gallais, F.; Klingelhoefer, Frauke; Loureiro, Afonso; Fuck, R.; Soares, J.; Cupertino, J. A.; Viana, Adriano Roessler; Rabineau, Marina; Baltzer, A.; Benabdellouahed, M.; Dias, Nuno; Moulin, Maryline; Aslani, D.; Morvan, L.; Maze, J. P.; Pierre, D.; Roudaut-Pitel, M.; Rio, I.; Alves, D.; Barros, P.; Biari, Youssef; Corela, Carlos; Crozon, J.; Duarte, J. L.; Ducatel, C.; Falcão, C.; Fernagu, P.; Le Piver, D.; Mokeddem, Z.; Pelleau, P.; Rigoti, C.; Roest, W.; Roudaut, M.The structure and nature of the crust underlying the Camamu-Almada-Jequitinhonha-Sergipe-Alagoas basins System, in the NE Brazilian margin, were investigated based on the interpretation of 12 wide-angle seismic profiles acquired during the SALSA (Sergipe ALagoas Seismic Acquisition) experiment in 2014. In this work, we present two 220-km-long NW-SE combined wide-angle and reflection seismic profiles, SL 01 and SL 02, that have been acquired using 15 ocean-bottom-seismometers along each profile, offshore the southern part of the Sergipe Alagoas Basin (SAB), north of the Vaza-Barris Transfer zone. The SL 02 has a 150-km long inland continuation with 20 land-seismic-stations until the Sergipano Fold Belt (SFB). Wide-angle seismic forward modeling allows us to precisely divide the crust in three domains: beneath the continental shelf, a similar to 100 km wide necking zone is imaged where the continental crust thins from similar to 35 km on the Unthinned Continental Domain, which displays a three-layered crust structure, to less than 8 km on the Oceanic Crust Domain. In the necking zone, the upper and the middle layers thin dramatically and almost disappear, while the Moho discontinuity shows clear PmPs. The Continental-Oceanic Crust Boundary (COB) is located at similar to 80 km from the coastline and is marked by intracrustal seismic reflectors and changes in the seismic velocity, showing a sharp transition. On profile SL02, the oceanic crust is perturbed by a volcanic edifice together with an anomalous velocity zone underneath the area.
- Monte Carlo approach to assess the uncertainty of wide-angle layered models: Application to the Santos Basin, BrazilPublication . Loureiro, Afonso; Afilhado, Alexandra; Matias, Luis; Moulin, Maryline; Aslanian, DanielIn the Santos Basin (Brazil), two parallel wide-angle refraction profiles show different crustal structures. One shows moderate crustal velocity gradient, and a clear Moho with topography. The other has an anomalous velocity zone, and no clear Moho reflections. This has large implications on the geological and geodynamical interpretation of the basin. Model uncertainties must be excluded as a source of t hese differences. We developed VMONTECARLO, a tool to assess model uncertainty of layered velocity models using a Monte Carlo approach and simultaneous parameter perturbation using all picked refracted and reflected arrivals. It gives insights into the acceptable geological interpretations allowed by data and model uncertainty through velocitydepth plots that provide: a) the velocity-depth profile range that is consistent with the travel times; b) the random model that provides the best fit, keeping most of the observations covered by ray-tracing; c) insight into valid models dispersion; d) main model features unequivocally required by the travel times, e.g., first-order versus second-order discontinuities, and velocity gradient magnitudes; e) parameter value probability distribution histograms. VMONTECARLO is seamlessly integrated into a RAYINVR-based modelling work-flow, and can be used to assess final models or sound the solution space for alternate models, and is also capable of evaluating forward models without the need for inversion, thus avoiding local minima that may trap the inversion algorithms and providing information for models still not well-parametrised. Results for the Brazilian models show that the imaged structures are indeed geologically different and are not due to different interpretations of the same features within the model uncertainty bounds. These differences highlight the strong heterogeneity of the crust in the middle of the Santos Basin, where the rift is supposed to have failed.
- The Portuguese national seismic network: products and servicesPublication . Carrilho, Fernando; Custodio, Susana; Bezzeghoud, Mourad; Oliveira, Carlos; Marreiros, Célia; Vales, Dina; Alves, Paulo; Pena, Areosa; Madureira, Guilherme; Escuer, Maria; Silveira, Graça; Corela, Carlos; Matias, Luis; Silva, Matilde; Veludo, Idalina; Dias, Nuno; Loureiro, Afonso; Borges, J. F.; Caldeira, Bento; Wachilala, Piedade; Fontiela, JoaoPortugal, located in the southwest region of the Eurasian plate, has been affected by several destructive earthquakes throughout its history, the most well-known being the 1755 Great Lisbon earthquake. The seismicity of the territory, both in the mainland and in the Azores and Madeira islands, has prompted the continuous development of seismic monitoring, from the first known macroseismic inquiry, following the 1755 Great Lisbon earthquake, to the current state-of-the-art seismic network. Once scattered in separate efforts, at present, most seismic stations in Portugal relay its data to a common data center, at Instituto Português do Mar e da Atmosfera, where data are automatically processed for the downstream generation of both manually revised and automatically generated products and services. In this article, we summarize the evolution of the permanent seismic network, its current status, the products and services that are publicly available, a recent effort of rapid deployment of a dense network following a mainshock, and state-of-the-art ocean-bottom seismometer developments.