Evidence of slope failure in the Sines Contourite Drift area (SW Portuguese Continental Margin) - preliminary results

Abstract

Slope instability, expressed by landslide activity, is an important natural hazard both onshore as well as offshore.Offshore processes create great concern on coastal areas constituting one of the major and most prominenthazards, directly by the damages they generate and indirectly by the possibility of generating tsunamis, which mayaffect the coast line. The Southwest Portuguese Continental Margin has been identified as an area where severalmass movements occurred from Late Pleistocene to Present. Recently, an area of∼52 km long by 34 km wide,affected by slope failure has been recognized in the Sines contourite drift located off the Alentejo.SWIM and CONDRIBER multibeam swath bathymetry has been used for the geomorphologic analysis and forrecognition of mass movement scars on the seabed. Scars’ areas and volumes were calculated by reconstructingpaleo-bathymetry. The net gain and net loss were calculated using both paleo and present day bathymetry.Geomorphologically, the study area presents 4 morphologic domains with landslide scars: I) Shelf and upper slopedisplay an irregular boundary with domain II with a sharp step (∼150m – 600m); II) Smooth area with gentleslope angles making the transition from smoother area to the continental slope (scarp), with large scars, suggestingslow rate and distributed mass wasting processes over this area (∼600 – 1200m); III) Scarp with high rates ofretrograding instability, where faster processes are verified and a great number of gullies is feeding downslopearea (1200m – 3200m); IV) Lebre Basin where mass movements deposits accumulate (> 3200m).A total of 51 landslide scars were identified with a total affected area of 137.67 km2, with 80.9 km2 being locatedin the continental slope with about 59% of the disrupted area, between 1200 and 3200m, and 41% (56.6 km2) liesin the continental shelf and upper slope, on a range of depths between 150 and 800m. The mean scar area is 2.7km2 and the maximum area recorded on a scar is 7.63 km2, while the minimum is 0.14 km2. About 43% of thescars present areas below 2 km2 and 63% below 3 km2. Only 3.9% of the scars present areas higher than 7 km2.There is a total volume of displaced material of 4.46 km3 with a mean volume of 0.1 km3. The maximum volumerecorded on a scar is 0.45 km3, while the minimum is 0.01 km3. The volume of material removed is quite variable,although the major part of the scars corresponds to a very small volume of removed material, with 69% of thescars presenting less than 0.1 km3. About 55% of the scars are located in slopes <=7º and 20% in slopes between0 and 2º, while 24% of the scars belong to the class of slope gradient between 1.5 and 3º. We may conclude thatthere is, apparently, a reverse relationship between slope angle and scar area, meaning that slope is not the maincondition for big landslide scars and that the areas with steep slopes, such as fault escarpments, favour continuousfast retrograde erosion