114 research outputs found
Neogene to recent contraction and basin inversion along the Nubia-Iberia boundary in SW Iberia
The SW of Iberia is currently undergoing compression related to the convergence between Nubia and Iberia. Multiple compressive structures, and their related seismic activity, have been documented along the diffuse Nubia-Iberia plate boundary, including the Gorringe bank west of the Gulf of Cadiz, and the Betic-Rif orogen to the east. Despite seismic activity indicating a dominant compressive stress along the Algarve margin in the Gulf of Cadiz, the structures at the origin of this seismicity remain elusive. This paper documents the contractional structures that provide linkage across the Gulf of Cadiz and play a major role in defining the present-day seismicity and bathymetry of this area. The structures described in this paper caused the Neogene inversion of the Jurassic oblique passive margin that formed between the central Atlantic and the Ligurian Tethys. This example of a partially inverted margin provides insights into the factors that condition the inversion of passive margins
Improving the Knowledge on Seismogenic Sources in the Lower Tagus Valley for Seismic Hazard Purposes
The Lower Tagus Valley, that includes the metropolitan area of Lisbon, has been struck by several earthquakes which produced significant material damage and loss of lives. Their exact location remains unknown. Our goal is to shed some light into the seismogenic sources in the area using seismic reflection and geological data. In areas with no seismic coverage, potential-field data interpretation was carried out. Seismicity was overlaid to the potential seismogenic structures and high-resolution data was acquired in order to confirm which structures have been active into the Quaternary. Three major fault-zones affecting the Neogene were identified: V. F. Xira, Samora-Alcochete and Pinhal Novo. For the first fault, strong evidences suggest it is active. The other two fault-zones and other structures previously unknown can be correlated with several epicentres. Empirical relationships between maximum moment magnitude and fault area indicate that MW > 6.5 earthquakes can be expected for the larger structures
FLUVIAL LIMESTONE TUFFS FROM THE S. PORTUGAL (LOULE-TAVIRA) AND N. MOROCCO (TETUAN): COMPARISON AND ENVIRONMENTAL IMPLICATIONS
Limestone tuff deposits precipitated in fluvial environments are widespread in the northern Morocco and the S Portugal. These deposits represent environmental conditions prone for the carbonate precipitation mainly in areas of Mesozoic limestone substratum with active water seepage from the aquifers.
The mesoscopic and microscopic characteristics of tuff deposits from Tetouan (Morocco) and Loule and Tavira (Portugal) are very alike; namely on deposit types (e.g. phytoherm barriers; banded limestones with abundant vegetation macro-remains; carbonates precipitation as cement of some early alluvial conglomerates or interbedded alluvial deposits). The common textural features and macro-facies (e.g. banded calcite in regular layers ; moldic porosity; textures resulting from carbonate precipitation mediated by bacteriological activity) point to a similar origin of these limestone tuffs on both sides of Gibraltar Strait despite their slightly different climatic conditions that are influenced by the active inflow of surface Atlantic water towards the Mediterranean (for the Tetouan region) and by the Azores anticyclone (for the Loulé-Tavira region). The field observations show that the limestone tuffs precipitation is nowadays residual and radiocarbon dating of Portuguese deposits points to a chronological framework for their formation starting in the beginning of the post-Younger Dryas climatic recovery and ending around 2.5 ky BP. The oxygen stable isotopes data show a gradual change in the tuffs isotopic signature through the time interval during which the system remained active, without any particular record of climate events such as the 8.2 ka.
The available data and those from monitored modern pluvial characteristics allow the speculation on the parameters controlling the limestone tuffs deposition on both northern and southern sides of the Gibraltar Strait during the Holocene. The carbonate deposition on fluvial systems, such as the studied ones, probably depends on the precipitation / evaporation balance leading to the availability and turbulence of the flowing water along the streams.
Acknowledgments: This works was done with the financial support of the FCT Proc. 441.00 CNRST- MARROCOS and PTDC/CTE-GIX/117608/2010 projects and the SFRH/BD/62323/2009 grant
Anatomy and tectonic significance of WNW-ESE and NE-SW lineaments at a transpressive plate boundary (Nubia-Iberia)
Recent mapping of the Gulf of Cadiz seafloor permitted to identify major tectonic lineaments: the SWIM lineaments (Zitellini et al., 2009) and Cadiz lineament, striking WNW-ESE and NE-SW, respectively. Multibeam swath bathymetry and interpretation of multi-channel seismic data indicate that these features can be interpreted to correspond to the seafloor morphological expression of active dextral strike-slip faults. Based on the interpreted data and recently published GPS plate kinematic velocity vectors of Nubia with respect to Iberia and the Alboran block (e.g. Fernandes et al.; 2003 Stich et al., 2006) we propose that the SWIM Faults are related to the general NW-SE convergence of Nubia with respect to Iberia, and the Cadiz fault is related to the westward movement of the Gibraltar orogenic arc
Occurrence of marine litter along abyssal areas of the Gloria Fracture Zone (NE Atlantic).
Marine litter pollution is a recognized form of anthropogenic
disturbance that affects widely the marine environment,
particularly near the continental margins, although also
present at abyssal and bathyal depths. This study reports the
occurrence of marine litter distribution and abundance in four
abyssal basins along the Gloria fracture zone in the NE
Atlantic. Litter items occurrences were analysed using TV-CTD
video surveys carried out during the multidisciplinary
activities of the R/V Meteor M162 cruise. The surveys reached
depths between 3500-4500m and covered 16 km of seafloor,
between the Terceira ridge and the Madeira-Tore Rise. Litter
items were annotated and categorized by type (i.e., plastic,
fishing gear, metal, glass, other unknown items). Results
revealed that litter was exclusively found on soft sediment
habitats across all areas, i.e. depositional areas, with the
overwhelming dominance of plastics items (71%), such as
plastic containers, cups and bag fragments. Although less
common (6-8%), metal, glass and lost fishing gear were also
observed. Litter density was on average 9 times higher in the
easternmost area near the Madeira-Tore ridge, when
compared to the other areas. Higher litter densities are likely
explained due to the proximity to the Madeira-Tore
seamount complex targeted by the fishing industry and
nearimportant corridors of marine traffic between various
Atlantic and Mediterranean locations
Seismic characterization of fluid migration and Pockmarks formation in the Estremadura Spur, Western Iberian Margin
: A field with more than 30 pockmarks was recently discovered in the Estremadura Spur outer shelf (Lourinhã Monocline). The processing of high-resolution seismic lines permitted the characterization of this pockmark field and of the seismic sequence of the Plio-Quaternary basin where they occur. Results show that this region has been affected by several episodes of fluid migration and fluid escape during the Plio-Quaternary, expressed by the presence of the pockmarks that at the present-time seem to be mainly inactive
Megafauna communities from abyssal sites along the Gloria Fracture Zone (NE Atlantic)
Fracture zones are areas of poorly known abyssal landscapes
and benthic faunal communities. During the R/V Meteor
M162 cruise several imagery surveys were performed using a
TV-CTD guided camera system along four main areas of the
Gloria fault system, between the Terceira ridge and the
Madeira Tore Rise. Based on these records, we were able to
characterize - for the first time - the megafaunal composition,
their abundance and diversity along the sub-basin of the
Gloria fault at depths between 3500 and 4500m. Quantitative
annotations of the observed fauna, as well as evidence of
animal traces on the seafloor were explored in relation to
topography, substrate type, and geochemical data acquired
during surveys. Preliminary observations revealed a total of
121 morphospecies, of which Holothuroidea is the most
diverse group, with 12 morphospecies. Deepest transects
carried out along the main trace of Gloria Fault system
showed the highest similarity among dives, predominantly
characterised by soft sediment areas with Elpidiidae
holothurians frequently observed. In contrast, the survey
carried out at the Terceira ridge showed more diverse
communities, which is likely due to a larger variability in
substrate and topography. Specifically, soft-sediment
sections showed a higher proportion of holothurians,
ophiuroids and acorn worms, while unique Anthozoan and
Porifera morphospecies were observed in areas with
presence of hard substrates. The findings of this study provide
unique knowledge of abyssal fauna associated with the Gloria
Fault System, including areas inside of national jurisdiction
and of relevance for management and conservation actions
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