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- 3D-ambient noise surface wave tomography of Fogo volcano, Cape VerdePublication . Carvalho, J.; Silveira, Graça; Dumont, Stéphanie; Ramalho, Ricardo Dos SantosFogo volcano belongs to the Cape Verde Archipelago, and it is one of the most active volcanoes in the Atlantic Ocean, which most recent eruption occurred from November 2014 to February 2015. We analyzed ambient seismic noise recordings of three different networks deployed in the island, totalizing 14 seismic stations, to derive a crustal 3D shear-wave crustal velocity model of the volcano. Through the phase cross-correlation technique followed by a time-domain phase weighted stack, we were able to measure Rayleigh wave group-velocity dispersion measurements in the period range from 1.0 to 10 s. These dispersion measurements were used to invert for 2D group velocity maps at selected periods, and then inverted to produce a 3D shear-wave velocity model of the island. The tomographic model shows three velocity domains. First, an asymmetric upper layer, above 5–6 km of depth, with lower velocities concentrated in the northeastern sector of the island and a clear higher-velocity horizontal body at 3–4 km of depth in the southwestern sector of the island; the spatial correlation between these two velocity zones and the Galinheiros normal fault suggests a genetic link between the high velocities and long-term surface deformation, which we related to sill intrusions between 3 and 4.5 km depth, beneath the southwestern sector of the island. Second, a marked higher-velocity horizontal layer in between 5 and 6 km and 8–9 km, interpreted as the seismic expression of pervasive sill and laccolith intrusions, now cooled, beneath the volcanic edifice and within the underlying oceanic crust. Third, a lower velocity layer below 8–9 km of depth, more pronounced beneath the northeastern sector, which could be explained by a hotter and possibly melt-rich zone beneath the volcano or a significantly altered/serpentinized crust. Finally, our study also confirms that Fogo lacks any sizable magma chambers (ancient or recent) within the volcanic edifice, in agreement with other geophysical and petrological studies. These observations demonstrate that 3D-ambient noise Rayleigh wave tomography is a powerful tool to image the crustal and upper mantle structure beneath volcanic islands, as shown here for Fogo volcano.
- Heterogeneous seismic anisotropy beneath Madeira and Canary archipelagos revealed by local and teleseismic shear wave splittingPublication . Schlaphorst, David; Silveira, Graça; Mata, João; Krüger, Frank; Dahm, Torsten; Ferreira, Ana M GMid-plate upward mantle flow is a key component of global mantle convection, but its patterns are poorly constrained. Seismic anisotropy is the most direct way to infer mantle flow as well as melt distribution, yet the convection patterns associated with plume-like mantle upwelling are understudied due to limited seismic data coverage. Here, we investigate seismic anisotropy beneath the Madeira and Canary hotspots using a dense set of shear wave splitting observations and combining teleseismic and local events recorded by three-component broad-band and short-period seismic stations. Using a total of 26 stations in the Madeira archipelago and 43 stations around the Canary Islands, we obtain 655 high-quality measurements that reveal heterogeneous flow patterns. Although local event results are sparse around most islands, we can observe a small average of S-wave splitting times of 0.16 ± 0.01 s, which significantly increase with source depth beneath El Hierro (>20 km) and Tenerife (>38 km) up to 0.58 ± 0.01 and 0.47 ± 0.05 s. This suggests an influence of melt pocket orientation in magma reservoirs developed at uppermost-mantle depths. Likewise, anisotropy increases significantly beneath the islands with shield stage volcanism (up to 9.81 ± 1.78 per cent at El Hierro, western Canaries, against values up to 1.76 ± 0.73 per cent at Lanzarote, eastern Canaries). On average, teleseismic SKS-wave splitting delay times are large (2.19 ± 0.05 s), indicating sublithospheric mantle flow as the primary source for anisotropy in the region. In the Canaries, the western islands show significantly smaller average SKS delay times (1.93 ± 0.07 s) than the eastern ones (2.25 ± 0.11 s), which could be explained by destructive interference above the mantle upwelling. Despite complex patterns of fast polarization directions throughout both regions, some azimuthal pattern across close stations can be observed and related to present- day mantle flow and anisotropy frozen in the lithosphere since before 60 Ma. Additionally, we infer that the current presence of a mantle plume beneath the archipelagos leads to the associated complex, small-scale heterogeneous anisotropy observations.
- Crustal and uppermost mantle structure of Cape Verde from ambient noise tomographyPublication . Carvalho, J.; Silveira, Graça; Kiselev, S; Custódio, Susana; Dos Santos Ramalho, Ricardo; Stutzmann, E; Schimmel, MWe present a seismic ambient noise tomography of the Cape Verde archipelago, located in the Atlantic Ocean, approximately 600 km west of Senegal. We used 38 seismic broad-band stations that continuously recorded for 10 months, in order to construct the first 3-D model of Sv-wave velocities for the crust and uppermost mantle beneath the Cape Verde region. We started by computing phase cross-correlations for vertical component recordings using all possible inter-island station pairs. Next, a time–frequency phase-weighted stack was applied to obtain robust Rayleigh-wave group-velocity dispersion curves in the period band between 10 and 24 s. Group-velocity maps at different periods are obtained by inverting the dispersion curves. We then inverted the group-velocity maps to obtain the 3-D shear wave velocity structure of the crust and uppermost mantle beneath Cape Verde. The final 3-D model extends from 8 km down to 23 km and has a lateral resolution of about 50 km. The crust in the southwestern sector, encompassing Fogo, presents lower S-wave velocities that may be caused by the presence of melt pockets and/or hydrothermal fluids circulation. The uppermost mantle beneath the northwestern sector is characterized by higher S-wave velocities in agreement with previous results obtained from Ps and Sp receiver functions. Those high-velocity anomalies can reflect non-altered crust or remnants of magma chambers or solidified basaltic intrusions, which fed the volcanism in these islands. Our maps revealed the presence of crustal underplating across the entire archipelago, yet stronger beneath the groups Santo Antão—São Vicente—São Nicolau and Fogo—Santiago—Maio.
- The Role of the Seismically Slow Central‐East Atlantic Anomaly in the Genesis of the Canary and Madeira Volcanic ProvincesPublication . Civiero, Chiara; Custódio, Susana; Neres, Marta; Schlaphorst, David; Mata, João; Silveira, GraçaThe Canary and Madeira provinces in the Central-East Atlantic Ocean are characterized by an irregular spatio-temporal distribution of volcanism along the hotspot tracks, and several alternative scenarios have been suggested to explain it. Here, we combine results from seismic tomography, shear-wave splitting and gravity along with plate reconstruction constraints to investigate the mantle structure and dynamics beneath those provinces. We find that the Central-East Atlantic Anomaly (CEAA), which rises from the core-mantle boundary and stalls in the topmost lower mantle, is the deep source of distinct upper-mantle upwellings beneath the region. The upwellings detach intermittently from the top of the CEAA and appear to be at different evolutionary stages. We argue that the accumulation of plume material in the topmost lower mantle can play a key role in governing the first-order spatio-temporal irregularities in the distribution of hotspot volcanism.
- Imaging the crust and uppermost mantle structure of Portugal (West Iberia) with seismic ambient noisePublication . Silveira, Graça; Dias, Nuno Afonso; Kiselev, Sergey; Stutzmann, Eleonore; Custódio, Susana; Schimmel, MartinWe present a new high-resolution 3-D shear wave velocity (Vs) model of the crust and uppermost mantle beneath Portugal, inferred from ambient seismic noise tomography. We use broad-band seismic data from a dense temporary deployment covering the entire Portuguese mainland between 2010 and 2012 in the scope of the WILAS project. Vertical component data are processed using phase correlation and phase weighted stack to obtain empirical Green functions (EGFs) for 2016 station pairs. Further, we use a random sampling and subset stacking strategy to measure robust Rayleigh-wave group velocities in the period range 7–30 s and associated uncertainties. The tomographic inversion is performed in two steps: First, we determine group-velocity lateral variations for each period. Next, we invert them at each grid point using a new trans-dimensional inversion scheme to obtain the 3-D shear wave velocity model. The final 3-D model extends from the upper crust (5 km) down to the uppermost mantle (60 km) and has a lateral resolution of ∼50 km. In the upper and middle crusts, the Vs anomaly pattern matches the tectonic units of the Variscan Massif and Alpine basins. The transition between the Lusitanian Basin and the Ossa Morena Zone is marked by a contrast between moderate- and high-velocity anomalies, in addition to two arched earthquake lineations. Some faults, namely, the Manteigas–Vilariça–Bragança fault and the Porto–Tomar–Ferreira do Alentejo fault, have a clear signature from the upper crust down to the uppermost mantle (60 km). Our 3-D shear wave velocity model offers new insights into the continuation of the main tectonic units at depth and contributes to better understanding the seismicity of Portugal.
- Critical analysis of CMIPs past climate model projections in a regional context: The Iberian climatePublication . Soares, Pedro M.M.; Lemos, Gil; André Lima, Daniela CatarinaThe Iberian Peninsula is a known climate change hotspot. In the last decades, the Coupled Model Intercomparison Project (CMIP) has allowed for thousands of Global Climate Model (GCM) simulations to be conducted, an important tool to assess and understand future changes in Earth's climate. The comparison of past future projections with observations provides a robust evaluation of the models’ skill according to different emission scenarios. Here, a comprehensive performance assessment of GCM-simulated mean, maximum, and minimum temperatures, and accumulated precipitation is conducted for Iberia, by retrospectively comparing historical simulations and past future projections from CMIPs 1 to 6 with reference datasets. From means to extremes, and multi-year intra-annual cycles to inter-annual trends, GCM simulations are compared with Iberia0.1 and E-OBS observational gridded datasets, and ERA5 reanalysis. The matching between the variables’ distributions is assessed through the distribution added values (DAVs), a measure of gain or loss in performance between CMIPs. Results show relevant improvements in the description of the Iberian climate throughout the CMIP effort, for historical and past future periods. While the representation of intra-annual cycles (inter-annual trends) is enhanced after CMIP3 (CMIP2), GCMs from all CMIPs are generally able to depict the observed warming trend. Nevertheless, until 2021, a slight detrimental effect in the performance of CMIP6 models is found, in comparison with CMIP5 ones, with positive DAVs obtained only for past future temperature projections (less than 2%). A continuous monitorization of modelling accuracy for Iberia is needed, considering the increasing relevance of climate change information for adaptation strategies.
- Strain partitioning in transpressive shears zones in the southern branch of the Variscan Ibero-Armorican arcPublication . Dias, R; A., Mateus; Ribeiro, AntonioThe Torre de Moncorvo region (NE Portugal) is a key-sector of the Autochthon Domain of the Iberian Terrane. The region experienced Variscan deformation in the southern branch of the Ibero-Armorican Arc wherein the early structures (of Upper Devonian age—D1) denotes the establishment of a heterogeneous sinistral transpressive regime. This regime was also responsible for the development of large-scale left-lateral shear zones whose direction is subparallel to major folds. Finite strains analyses were carried out in the Torre de Moncorvo region using the normalised Fry method on different strain markers: (1) distribution of detrital quartz grains in quartzite rocks of Arenigian–Lanvirnian age; (2) arrangement of oolites in discontinuous Ordovician–Silurian ironstone horizons; (3) the rotation experienced by Skolithos preserved in Lower Ordovician metasedimentary clastic rocks. The results obtained indicate the predominance of slightly prolate strain ellipsoids. Nevertheless, the variation of their orientation around mesoscopic folds emphasises the role of strain partitioning in a transpressive regime, suggesting that different folding mechanisms were active in the course of the same deformation phase. For the studied cases, tangential longitudinal strain and flexural shear combined with regional sinistral shear seem to be the most common mechanisms of folding. Some of these three-dimensional theoretical models for strain patterns in folds could be used in other regions, where a transpressive regime is inferred.
- Geodynamic evolution of the South Variscan Iberian Suture as recorded by mineral transformationsPublication . Figueiras, Jorge; A., Mateus; A. Gonçalves, Mário; Waerenborgh, João; Fonseca, PauloNew 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.
- 57Fe Mossbauer spectroscopy study of the correlation between the Fe3+ content and the magnetic properties of natural Cr-spinelsPublication . Waerenborgh, João C.; Figueiras, Jorge; A., Mateus; A. Gonçalves, MárioBoth the Fe oxidation degree and the magnetic properties determined by 57Fe Mössbauer spectroscopy were used to characterize the Cr-spinels from the Beja-Acebuches Ophiolite Complex (SE Portugal). Two different types of Cr-spinel were observed. The first one has an average Fe3+/(Fe total) ≈ 25%, and corresponds to unaltered Cr-spinel grains which remain paramagnetic down to 6K. The second one results from the alteration of the first type, has an average Fe3+/(Fe total) ≈40% and order magnetically between 222 K and 78 K. Fe3+/(total Fe) increases with the magnetic ordering temperature. The grain population ordering at the highest temperatures, in the range 202–222K, has the highest Fe3+/(total Fe) ≈ 58%. Quantitative Fe site distributions can be obtained from room-temperature Mössbauer data if the different recoilless factors for tetrahedral Fe2+ and octahedral Fe3+ are considered. The observed second-order Doppler shifts are consistent with Mössbauer temperatures of 330K and 605K, reported in the literature for tetrahedral Fe2+ and octahedral Fe3+ in other oxide spinels. The differences in the magnetic ordering temperatures allowed the characterization of both types of Cr-spinels using a single Mössbauer absorber, thus avoiding the need to separate them physically, which, in this case, would be very difficult. The results are compared to those obtained for other Cr-spinels and, for the first time in the case of natural Cr-spinels, the increase of their magnetic ordering temperatures with their oxidation degree, resulting from natural processes, is discussed in terms of the magnetic exchange interactions between the Fe cations.
- Variscan overthrusting, fluid flow and the genesis of magnetite ore-bodies at Azenhas area (Pedrógão, Ossa-Morena Zone, SE Portugal)Publication . A., Mateus; Araújo, A; A. Gonçalves, Mário; Matos, JIn the Azenhas area (SE Portugal), an important segment of a regional WNW-ESE Variscan thrust can be observed. The hanging wall felsic metavolcanics carried from ENE are evolved rhyolite tuffs. The underlying sequence is mainly composed of variably metasomatised allochthonous amphibolites that are believed to represent an ophiolite slice tectonically overlying the autochthonous Moura-Ficalho Complex. Within this latter sequence, numerous WNW-ESE thrusts with prevailing displacement towards W-SW cut prior sub-parallel structures with northwards thrust shear. According to the observed crosscutting relationships and to the available petrographic data, the metasomatism experienced by the lower amphibolite sequence took place mainly before the emplacement of the upper slices of amphibolites, preceding therefore the installation of the felsic metatuffs. Magnetite ore-bodies are found within strongly metasomatised amphibolites immediately below the major WNW-ESE thrust zone or its subsidiary thrust structures. Their genesis is envisaged as a result of a complex chemical reaction path that involves the ascent of aqueous oxidising fluids under a reverse temperature gradient generated during the tectonic emplacement of amphibolites. In order to test the geological plausibility of the thermal evolution and the time span needed for ore genesis a one-dimensional numerical model was developed. This model enables the tracing of vertical thermal profiles at different times and accounts for the thrust emplacement history, incorporating variable erosion rates of the rock sequence. The results obtained show that an inverted thermal gradient in the thrusted sequence is kept for over 1 Ma and a regular temperature increase (from 400ºC to 500ºC) is observed in the underlying rocks lasting for 4.5 to 5.5 Ma.