Volume 111 (2024) Fascículo I
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- Reappraisal of active tectonics of the Porto Alto buried fault zone (Portugal) considering new geophysical shallow studiesPublication . Carvalho, João; Cabral, João; Ghose, Ranajit; Borges, José Fernando; Dias, RubenABSTRACT: The Lower Tagus Valley area (LTV), where Lisbon is located, has been affected by several destructive, M 6+ earthquakes whose sources remain to be determined. The identification of expectable surface or near surface ruptures in the area is a challenging task that requires a multidisciplinary analysis that includes geophysical techniques, as the source faults are mainly buried despite likely to deform Upper Pleistocene to Holocene alluvial cover of the Tagus River. This paper focuses on the characterization of the Porto Alto fault zone for seismic hazard mitigation purposes. The Porto Alto fault zone was recognized in oil-industry P-wave 1980’s seismic reflection data as an important, Miocene reactivated, deep structure in the LTV. Highresolution P-wave seismic reflection data were later acquired in the early 2000’s to investigate related Holocene fault activity, leading to the identification of a shallow fault zone near the surface. However, the vertical resolution of the acquired P-wave seismic reflection data was considered insufficient to corroborate any presumably small vertical offset related to fault rupture in the ca. 50 m thick alluvium cover. Trenching for the recognition and characterization of surface faulting was previously tested in the study region but it proved to be a challenging and poorly efficient methodology due to the very shallow water table (at ~1 m) and low cohesion of the sediments. Due to these constraints, we revisited the former fault study site to acquire higher resolution S-wave seismic and ground penetrating radar (GPR) data. The new seismic profiles show interruption of the reflectors in the stacked sections. Diffracted energy, changes in amplitude/shape of the reflection hyperbolae in the shot gathers and spatially coincident low velocity anomalies, also indicate the presence of several shallow fault strands deeper than 10 m. The GPR profile, overlapping and extending the seismic profiles in 30 m reaches a maximum investigation depth of about 15 m and shows the presence of deformation at three locations, one of which matches with one of the fault strands detected in the high resolution S-wave seismic data. In this profile, sediment disruption was detected extending upwards to a depth as shallow as ca. 3.5 m, corresponding to alluvium with a poorly constrained age of ca. 2,300 yrs. Slip rate, maximum earthquake magnitude and recurrence, and other parameters are also estimated for the Porto Alto fault zone. These recently acquired seismic and GPR datasets indicate that there were at most three to five maximum earthquakes generated by the fault in the last 13,100 years, with an average recurrence of approximately 4,400 to 2,600 years respectively. However, the data show a grouping of these earthquakes in time, the first two in the period 13,100-12,300 years, separated by about 800 years, and the third or the last grouped three having occurred in the past 2,300 years with a similar average recurrence time of ca. 800 years. However, the regional historical and instrumental seismicity does not show an obvious link of any known major earthquake with the Porto Alto fault zone.
- Opportunities and challenges in a world-class mining district, the Iberian Pyrite BeltPublication . Castelo Branco, José M.ABSTRACT: A summary is made on the exploration work carried out in the Iberian Pyrite Belt (IPB) and an attempt is made to describe the evolution of the exploration methods and strategies applied in the field which led to the discovery of the main deposits. Finally, a reference is made to the good potential for new discoveries in the IPB, giving clues about new exploration models to be pursued. Being undoubtedly a world-class metallogenic province, the IPB naturally deserves a special mention regarding the mining history and on the evolution of the exploration techniques, adjusted to the new paradigms, namely in the need to search at ever greater depths. The most recent cases of successful new discoveries in the IPB are described, largely resulting from the refining of geological, geophysical and geochemical prospecting methodologies, and clues that are expected to lead to the discovery of new concealed deposits are equated.
- Is the Rosario Antiform, Iberian Pyrite Belt, a Variscan tectonic window?: a tectono-stratigraphic interpretation based on the geology of the Neves-Corvo mine regionPublication . Oliveira, José TomasABSTRACT: This work suggests a new structural interpretation for the Rosario Antiform, Portuguese Pyrite Belt. This is based on the following successive developments: 1 - Deposition of an upper Visean flysch succession, designated by CMt2, which conformably overlaid the autochthon represented in ascending order by the clastic succession of the Phyllite-Quartzite Formation (PQ) followed by the lower Volcano-Sedimentary Complex (VSC, CVS in the geological map annex) dominated by felsic volcanism and black shale sediments, all of late Devonian age, and the upper VSC composed by shales, volcanoclastic sediments, including cherts and jaspers, and mafic volcanic and igneous rocks of late Tournaisian – late Visean ages; 2 - the reinterpretation of fifteen selected drill hole logs previously carried out by Somincor and LNEG geologists and recovered from the region west of the Neves-Corvo mine proved that below the CMt2 flysch succession the upper VSC is absent in many places and the unit is in direct fault contact with the lower VSC, in a position similar to that recognized in the mine. This abnormal contact is inferred to represent the trace of an extensional fault that locally erased the upper VSC lithologies. A sub-autochthon block composed of remnants of the lower VSC and the overlying CMt2 sediments was then formed above the extensional fault, as discussed below; 3 - a SW directed tangential transport, well documented in the Neves-Corvo mine region, placed a distinct flysch succession (CMt1) and the underlying clastic dominated upper VSC units, all of late Visean age, both forming the allochthon, upon the CMt2 succession. The allochthon is organized as a thinskinned package moving southwestward upon the subautochthon developing a large overthrust. All the package units were folded by a second episode of pervasive regional tectonic compression. 4 – a second tectonic reverse fault at the west limb of the antiform caused the uplifting of the autochthon and the subautochthon which after the regional erosion became a true tectonic window according to this interpretation.
- Gemological analysis of ruby origins: a case studyPublication . Oliveira, Isaac Gomes de; Cardoso, Jessica de Sousa; Franco, Julia; Santos, Breno Ravielly dos; Silva, Suely Andrade da; Santos, Lucilene; Mattos, Irani Clezar; Neri, Tereza Falcão de OliveiraABSTRACT: Gemstones are substances of natural, artificial or synthetic origin, which due to their hardness, rarity, uniqueness, diaphaneity and beauty are used as adornments. In the market, ruby is the second most expensive type of gemstone. Depending on its size, it can exceed the value of the diamond, besides being the most valuable colored gem in the market. It is a variety of the mineral corundum, just like sapphire, and it is a form of crystallized alumina (Al2O3) and the responsible for its red color is chromium. There are deposits in many different places around the world, being common the practice of speculating the origin of the ruby based on the shade of red. Provenance strongly impacts the commercial value of the gem. Several faceted rubies were donated to the Federal University of Ceará. This project aims to discover from what regions (countries) the donated rubies are coming, using gemological characteristics. To establish the origin of the specimen collection, the following methods and equipment were used: bibliographic research about rubies characteristics, refractometer to measure refractive indexes, dichroscope for pleochroism analysis, ultraviolet lamp (UV) for determination of fluorescence, and especially the gemological (stereoscopic) microscope to study the types of inclusions in transparent to translucent rubies. Using the methods cited, it was possible to produce, successfully, the gemological characterization of the specimens and deduce, based on strong evidence, what was the geographic origin of the studied specimens. Through microscopy it was deduced that all the faceted stones are natural and the whole collection include stones from the following countries: Burma, Thailand, Pakistan, Sri Lanka and Tanzania. Results are gathered systematizing bibliographic data and analytical results in order to identify the country of origin of the rubies.
- Domingos Vandelli (1735-1816) e a mina carvão do Cabo MondegoPublication . Brandão, José Manuel Moraes ValeRESUMO: No presente estudo contextualizam-se e abordam-se alguns aspetos da contribuição de Domingos Vandelli para o conhecimento e exploração do jazigo de carvão do Cabo Mondego no último quartel do século XVIII. Elege como principais fontes, dois manuscritos de sua autoria, um deles inédito, onde se explicam os contornos da sua aproximação e afastamento dos trabalhos mineiros, e as orientações técnicas fornecidas ao oficial do Exército português nomeado, em 1773, pelo Secretário de Estado Martinho de Melo e Castro para conduzir as pesquisas e a exploração da mina. Sublinha-se o facto de que a escolha de um homem de Ciência para acompanhar a mina, denota o reconhecimento da importância estratégica do carvão no almejado desenvolvimento industrial do país.
- Catalogue of Cenozoic echinoid type specimens in the Geological Museum (Lisbon, Portugal)Publication . Pereira, PedroABSTRACT: The present catalogue lists the Cenozoic echinoid type specimens housed in the collections of the Geological Museum (Lisbon, Portugal). This material comprises a total of 69 type specimens (seven holotypes, one paratype, 52 syntypes, two lectotypes, and seven paralectotypes) of fifteen Miocene species from Angola and Portugal. Additionally, it is considered that Echinolampas (Echinolampas) maxima de Loriol, 1896 is a valid species and E. (E.) barcinensis Lambert, 1907 is its junior synonym.
- Levelling geochemical datasets as a tool to overcome boundary features in data applied to mineral explorationPublication . Batista, Maria Joao; de Oliveira, Daniel Pipa Soares; Salgueiro, Rute; Inverno, CarlosABSTRACT: The existence of a large number of chemical analyses resulting from decades of geochemical exploration campaigns, yields huge numbers of analytical results produced with different analytical methods, sampling protocols and sampling means. Therefore, it is necessary to level these analytical results to make them comparable and thus take advantage of the potential of geochemical information from contiguous areas. In this study, a levelling technique was used on geochemical datasets of tested bands between the 1:25 000 scale maps, 293/294 and 305/306 sheets, respectively, from the Beira Baixa region where stream sediments were sampled during a project for rare earth elements exploration. These samples were analysed for lanthanum by the same analytical methods and in the same laboratory, but in different periods and the border effects were observed. These levelling techniques must be adapted to the data to be compared and in the present case the disappearance of the border effect of the analytical results of 293/294 and 305/306 topographic sheets was observed.