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Mass movement processes in the Southwest Portuguese Continental Margin during the Late Pleistocene-Holocene : a multidisciplinary approach for volume quantification, estimation of recurrence times and hazard implications
Publication . Teixeira, Manuel; Terrinha, Pedro António Gancedo; Roque, Ana Cristina Freixo Pereira
The Alentejo Margin, in the Southwest Portuguese Continental Margin, is a very complex and dynamic geomorphological area, located near the Eurasia-Africa plate boundary and affected by the flow path of the Mediterranean Outflow Water (MOW). This margin comprises the Sines Contourite Drift (SCD), which is the most prominent depositional feature (~2311 km2, 303.9 km perimeter, 98 km length and 35 km width), emplaced in the continental slope of this margin, evolving in four main phases by the action of MOW, since the Late Pleistocene.
The interaction between along-slope and downslope processes forms a mixed morphosedimentary setting, which is greatly affected by mass movement activity in the middle and lower continental slopes, during the Late Pleistocene-Holocene.
This work analyses the occurrence of mass movement processes in the southwest Portuguese margin, in a total extent of ~85 km×82 km and identifies the main triggering and conditioning factors promoting slope instability during the Late Pleistocene-Holocene and assesses the morphosedimentary evolution of the Sines Contourite Drift. This study was performed through geophysical, sedimentological, physical, geochemical, and geotechnical analyses, using multibeam bathymetry, multichannel seismic and sub-bottom profiler, and gravity cores data.
The Alentejo margin is an unstable area, where the presence of a contourite drift significantly contributes for slope instability in consequence of its sediment mechanical properties. The SCD hosts a cluster of dominantly small landslide scars, affecting both steep and smooth slopes. This scar concentration is mainly provided by local intrinsic conditions that favour slope instability in the area. Scars predominantly occur on slope angles steeper than 5º, however sediment properties, especially low consolidation, very low permeability, high pore-pressure, high compressibility and low shear strength greatly promote slope instability in the Alentejo Margin. The inherent instability conditions of the area are increased by frequent seismicity that promotes additional stress, leading to increased slope instability.
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Fundação para a Ciência e a Tecnologia
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OE
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48957