Sedimentology, physical properties and ages of mass-transport deposits associated to the Marquês de Pombal Fault, Southwest Portuguese Margin

The SW Iberian Margin is located at the convergence of the European and African Plates, where the largest magnitude earthquakes in Western Europe occur. Several active structures, such as the Marquês de Pombal fault, are potential sources of large magnitude earthquakes and tsunamis. Associated with faulting, submarine landslides are also commonly observed. A large area (~260 km2) of high acoustic backscatter in the central part of the Marquês de Pombal escarpment corresponds to a complex translational slide and debris flow. Detailed lithological description, physical properties and dating of four sediment cores sampled on the toe of the slide allow us to investigate the sediment facies, age and triggering mechanism of the Marquês de Pombal slides. The maximum age of the Marquês de Pombal landslide is 3270 ± 60 Cal yr BP. Radiocarbon dating of previous and subsequent Holocene mass wasting deposits gives an estimated recurrence rate of < 2 kyr. Although a number of mechanisms may be invoked to account for landslide triggering, earthquakes are the most likely triggering mechanism for the observed slope instabilities in the Marquês de Pombal area, at least during the Holocene.