Untitled

Funder

Organizational Unit

Publications

The 2014-15 eruption and the short-term geochemical evolution of the Fogo volcano (Cape Verde): evidence for small-scale mantle heterogeneity

Publication . Mata, J.; Martins, S.; Mattielli, N.; Madeira, JFA; Faria, B.; Ramalho, R. S.; Silva, P.; Moreira, M; Caldeira, R.; Moreira, Mário; Rodrigues, J.; Martins, L.

Recurrent eruptions at very active ocean island volcanoes provide the ideal means to gain insight on the scale of spatial variations at the mantle source and on temporal changes of magma genesis and evolution processes. In 2014, after 19 years of quiescence, Fogo volcano (Cape Verde Archipelago) experienced a new eruption, with the vents located 200 m from those of the 1995 eruption, and less than 2000 m from those of the 1951 event. This offered a unique opportunity to investigate the existence of small-scale mantle heterogeneities and the short-term compositional evolution of magmas erupted by a very active oceanic volcano like Fogo. Here we present petrological and geochemical data from the early stages of the Fogo's most recent eruption - started on November 23, 2014 - and compare them with the signature of previous eruptions (particularly those of 1995 and 1951). The magmas erupted in 2014 are alkaline (up to 23.4% and 0.94% of normative ne and lc, respectively) with somewhat evolved compositions (Mg # < 56), ranging from tephrites to phonotephrites. The eruption of phonotephritic lavas preceded the effusion of tephritic ones. Lavas carried to the surface clinopyroxene and kaersutite phenocrysts and cognate megacrysts, which indicate that the main stages of magma evolution occurred in magma chambers most probably located at mantle depths (25.6 5.5 km below sea level). This was followed by a shallower (<1.5 km below sea level) and shorter (approximate to 50 days) magma stagnation before the eruption. 2014 magmas have more unradiogenic Sr and more radiogenic Nd compositions than those of the previous 1951 and 1995 eruptions, which generally have less radiogenic Pb ratios. These isotopic differences- coming from quasi-coeval materials erupted almost in the same place- are remarkable and reflect the small-scale heterogeneity of the underlying mantle source. Moreover, they reflect the limited isotopic averaging of the source composition during partial melting events as well as the inefficient homogenization within the plumbing system when on route to the surface. The lid effect of an old and thick lithosphere is considered of utmost importance to the preservation of a significant part of source heterogeneity by erupted magmas. The decrease in the contribution of an enriched component to the Fogo magmas in the 2014 eruption marks a change on the volcano shortterm evolution that was characterized by a progressive increase of the importance of such a component. Nb/U ratios of the 2014 lavas are similar, within 2 sigma, to the mean value of OIB, but significantly lower than those reported for the 1995 and 1951 eruptions. This is considered to reflect the lack of significant mixing of the 2014 magmas with lithospheric melts, as opposed to what is here hypothesised for the two previous eruptions.

2017-09Journal article

Restricted access

Crustal seismic structure beneath Portugal and southern Galicia (Western Iberia) and the role of Variscan inheritance

Publication . Veludo, Idalina; Dias, Nuno; Fonseca, Paulo E.; Matias, Luís; Carrilho, Fernando; Haberland, Christian; Villasenor, António

The crustal structure in Western Iberia, is the result of a complex geodynamic history. Most of the surface is covered by rocks dating to the Variscan orogeny, the coastal ranges dominated by Mesozoic structures and Mesocenozoic basins covering partially the mainland. In this study we present the results of a local earthquake tomographic study, performed to image in depth this complex crustal structure down to 24 km depth.& para;& para;The obtained tomograms present a good correlation with the surface geology, in general with higher Vp velocities in the north, and lower Vp velocities in the south. The heterogeneity observed on the surface geology of the Galicia-Tras-os-Montes Zone is well marked, being a relatively thin layer over the smoother structure of the Centro Iberia Zone CIZ. The CIZ crustal block confined between the Porto-Tomar-Ferreira do Alentejo and the Manteigas-Braganqa faults having higher Vp values, enhancing the contrast to the Lusitanian Basin to the west. The Ossa-Morena Zone corresponds to the unit presenting the greater heterogeneity in both Vp and Vp/Vs models, showing also with a complex transition to the South Portuguese Zone and a relatively smooth transition to the Centro Iberian Zone. Unexpectedly, the South Portuguese Zone present an east-west velocity variation with no apparent correspondence with the surface geology. The transition to the coastal Mesocenozoic basins is clearly marked, the Lower Tagus Valley corresponding to a similar to 4 km thick low velocity region.& para;& para;The relocation inland recorded seismicity in the period 2000-2014 allows cleansing some of the alignments and establish their correlation with some of the active structures in Portugal. The model features and seismicity pattern reveal the strong role played by the Variscan heritage.

2017-10Journal article

Restricted access

Volcano-tectonic evolution of a linear volcanic ridge (Pico-Faial Ridge, Azores Triple Junction) assessed by paleomagnetic studies

Publication . Silva, Pedro; Henry, Bernard; Marques, Fernando Ornelas; Hildenbrand, Anthony; Lopes, Ana; Madureira, Pedro Miguel; Madeira, JFA; Nunes, João C.; Roxerová, Zuzana

The morphology of volcanic oceanic islands results from the interplay between constructive and destructive processes, and tectonics. In this study, the analysis of the paleomagnetic directions obtained on well-dated volcanic rocks is used as a tool to assess tilting related to tectonics and large-scale volcano instability along the Pico-Faial linear volcanic ridge (Azores Triple Junction, Central-North Atlantic). For this purpose, 530 specimens from 46 lava flows and one dyke from Pico and Faial islands were submitted to thermal and alternating magnetic fields demagnetizations. Detailed rock magnetic analyses, including thermomagnetic analyses and classical high magnetic field experiments revealed titanomagnetites with different Ti-content as the primary magnetic carrier, capable of recording stable remanent magnetizations. In both islands, the paleomagnetic analysis yields a Characteristic Remanent Magnetization, which presents island mean direction with normal and reversed polarities in agreement with the islands location and the age of the studied lava flows, indicating a primary thermo-remanent magnetization. Field observations and paleomagnetic data show that lava flows were emplaced on pre-existing slopes and were later affected by significant tilting. In Faial Island, magmatic inflation and normal faults making up an island-scale graben, can be responsible for the tilting. In Pico Island, inflation related to magma intrusion during flow emplacement can be at the origin of the inferred tilting, whereas gradual downward movement of the SE flank by slumping processes appears mostly translational.

2018-02-15Journal article

Restricted access

Educating for earthquake science and risk in a tectonically slowly deforming region

Publication . Custódio, Susana; Silveira, Graça; Matias, Luís; Mata, Isabel; Matos, Catarina; Palma-Oliveira, José Manuel; Rocha, Francisco; Lopes, Fernando C.

Over the past decade, scientists have been called to participate more actively in public education and outreach (E&O). This is particularly true in fields of significant societal impact, such as earthquake science. Local earthquake risk culture plays a role in the way that the public engages in educational efforts. In this article, we describe an adapted E&O program for earthquake science and risk. The program is tailored for a region of slow tectonic deformation, where large earthquakes are extreme events that occur with long return periods. The adapted program has two main goals: (1) to increase the awareness and preparedness of the population to earthquake and related risks (tsunami, liquefaction, fires, etc.), and (2) to increase the quality of earthquake science education, so as to attract talented students to geosciences. Our integrated program relies on activities tuned for different population groups who have diferente interests and abilities, namely young children, teenagers, young adults, and professionals.

2016-05Journal article

Restricted access