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- Investigating collapse structures in oceanic islands using magnetotelluric surveys: The case of Fogo Island in Cape VerdePublication . Martínez-Moreno, F.J.; Monteiro Santos, Fernando Acácio; Madeira, José; Pous, J.; Bernardo, I.; Soares, A.; Esteves, M.; Adão, F.; Ribeiro, J.; Mata, João; Brum da Silveira, A.One of the most remarkable natural events on Earth are the large lateral flank collapses of oceanic volcanoes, involving volumes of rock exceeding tens of km3. These collapses are relatively frequent in recent geological times as supported by evidence found in the geomorphology of volcanic island edifices and associated debris flows deposited on the proximal ocean floor. The Island of Fogo in the Cape Verde archipelago is one of the most active and prominent oceanic volcanoes on Earth. The island has an average diameter of 25 km and reaches a maximum elevation of 2829 m above sea level (m a.s.l.) at Pico do Fogo, a young stratovolcano located within a summit depression open eastward due to a large lateral flank collapse. The sudden collapse of the eastern flank of Fogo Island produced a megatsunami ~73 ky ago. The limits of the flank collapse were deduced as well from geomorphologic markers within the island. The headwall of the collapse scar is interpreted as either being located beneath the post-collapse volcanic infill of the summit depression or located further west, corresponding to the Bordeira wall that partially surrounds it. The magnetotelluric (MT) method provides a depth distribution of the ground resistivity obtained by the simultaneous measurement of the natural variations of the electric and magnetic field of the Earth. Two N-S magnetotelluric profiles were acquired across the collapsed area to determine its geometry and boundaries. The acquired MT data allowed the determination of the limits of the collapsed area more accurately as well as its morphology at depth and thickness of the post-collapse infill. According to the newly obtained MT data and the bathymetry of the eastern submarine flank of Fogo, the volume involved in the flank collapse is estimated in ~110 km3. This volume –the first calculated onshore– stands between the previously published more conservative and excessive calculations –offshore– that were exclusively based in geomorphic evidence. The model for the summit depression proposing two caldera collapses preceding the collapse of the eastern flank of Fogo is supported by the MT data.
- A geological record of multiple Pleistocene tsunami inundations in an oceanic island: The case of Maio, Cape VerdePublication . Madeira, José; Ramalho, Ricardo Dos Santos; Hoffmann, Dirk L.; Mata, João; Moreira, Mário Augusto de AndradeIn the Central Atlantic archipelagos – the Canaries, Cape Verde, Madeira and the Azores – tsunami hazard is often regarded as low, when compared with other extreme wave events such as hurricanes and storms. The geological record of many of these islands, however, suggests that tsunami hazard may be underestimated, notwithstanding being lower than in areas adjacent to subduction zones, such as the margins of the Pacific and Indian oceans. Moreover, tsunamis in oceanic islands are generally triggered by local large-scale volcanic flank collapses, for which little is known about their frequency, making it difficult to estimate the probability of a new occurrence. Part of the problem lies in the fact that tsunami deposits are usually difficult to date, and few islands in the world exhibit evidence for repeated tsunami inundation on a protracted timescale. This study reports on the presence of abundant tsunami deposits (conglomerates and sandstones) on Maio Island (Cape Verde) and discusses their stratigraphy, sedimentological characteristics, probable age and tsunamigenic source. Observations indicate that four distinct inundation events of variable magnitude took place during the Pleistocene. One of the tsunami deposits yielded a high-confidence U/Th age of 78.8 +- 0.9 ka, which overlaps within error with the 73 +- 7 ka age proposed for Fogo volcano’s flank collapse, an event known to have had a significant tsunami impact on nearby Santiago Island. This shows that the Fogo tsunami also impacted Maio, resulting in runups in excess of 60 m above coeval sea-level at ca 120 km from the source. Two older deposits, possibly linked to recurrent flank collapses of the Tope de Coroa volcano in Santo Antão Island, yielded lower-confidence ages of 479 to 390 ka and 360 to 304 ka. A younger deposit (<78 ka) remains undated. In summary, the geological record of Maio exhibits well-preserved evidence of repeated tsunami inundation, reinforcing the notion that tsunami hazard is not so low at volcanic archipelagos featuring prominent and highly-active volcanoes such as in Cape Verde.
- 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.
- 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.
- Physical volcanology and emplacement mechanism of the Central Atlantic Magmatic Province (CAMP) lava flows from the Central High Atlas, MoroccoPublication . El Hachimi, Hind; Youbi, Nasrrddine; Madeira, José; Marzoli, Andrea; Mata, João; Bertrand, Hervé; Bensalah, Mohamed Khalil; Boumehdi, Moulay Ahmed; Doblas, Miguel; Medina, Fida; Ben Abbou, Mohamed; Martins, LíniaThe best preserved and most complete lava flow sequences of the Central Atlantic Magmatic Province (CAMP) in Morocco are exposed in the Central High Atlas and can reach up to 300 m in thickness. Four distinct formations, emplaced in subaerial environments, are classically recognized: the Lower, Intermediate, Upper and Recurrent formations. These formations are separated by paleosoils and sedimentary sequences (mudstones, siltstones, sandstones, limestones), that are in general less than two meter-thick and may exceptionally reach a thickness of 80 m, representing minor periods of volcanic quiescence. CAMP lava flows of the Central High Atlas can be grouped into two main categories: subaerial compound pahoehoe flows and simple flows. The former type is exclusively confined to the Lower and Intermediate Formations, while simple flows occur in the Upper and Recurrent Formations. The dominance of compound flows in the two lowermost units of the CAMP suggests a slow emplacement during successive sustained eruptive episodes. Instead the thick single flows characterizing the Upper and Recurrent units indicate higher effusive rates. Basaltic pillow lavas (always of short lateral extent: 10 to 100 m), showing radial jointing and vitreous rinds, identical to those found in the Western Meseta, are occasionally associated with hyaloclastites in the base of the Intermediate Formation, immediately above clastic sediments, or in the Upper Formation. The occurrence of pillow lavas does not imply a generalized subaqueous environment at the time of the lava emission. Instead, they represent subaerial flows that entered small lakes occupying depressions on the volcanic topography of the Lower and Intermediate Formations. The short lateral extent of the pillow lavas and their constant stratigraphic position, the existence of lava flows with unequivocal subaerial characteristics associated to sediments containing fossilized wood, clearly indicate onshore emplacement.