LATE PLIOCENE TO PLEISTOCENE TECTONIC ACTIVITY IN SW PORTUGAL: NEW DATA FROM FIELD OBSERVATIONS AND TRENCHING

P.M. Figueiredo 1, J. Cabral 1,2, T. Rockwell 3, 1 IDL, Instituto Dom Luis, Lisbon University 2 GEOFCUL, Geology Department, Science Faculty of Lisbon University 3 SDSU, San Diego State University, San Diego, CA 92182 Corresponding author : pmfigueiredo@fc.ul.pt Southwestern Portugal is located close to the Eurasia-Nubia plate boundary, near the Azores-Gibraltar fracture zone. East of the Gloria transform fault, this boundary becomes complex and diffuse, where deformation related to the NW-SE convergence of Iberia and Nubia, at a rate of ca. 4-5 mm/ year, becomes distributed across a few hundred kilometres wide zone. This area corresponds to the inferred seismogenic source zone for the 1755 earthquake and tsunami (estimated ≥ Mw 8), and also for the Mw 7.9 1969 event. Inland, the São Teotónio-Aljezur-Sinceira fault system (STASFS) extends NNE-SSW for 50 km, parallel and close to the southwest Portuguese coast, and controls the development of small Cenozoic tectonic basins. It comprises onshore faults that may accommodate some of the ongoing plate boundary deformation. This fault system expresses primarily sinistral strike-slip with a minor reverse component, and is compatible with NW-SE maximum horizontal compressive stress. Four cenozoic strike-slip basins occur along the STASFS: from north to south, they are the S. Miguel (Odeceixe), Aljezur, Ribeira de Alfambras and Pedralva-Sinceira troughs. These basins generally have lengths of less than 5 km and a maximum width of 1.5 km, and are filled with Miocene to Pleistocene sediments. In some areas, fault-related post-Pliocene vertical displacements of up to 100 m may have occurred, but generally they only reach a few tens of meters. A trench exposure shows that at least on strand of this fault zone has sustained significant motion during the Quaternary. Recent studies in Alfambras basin based on geomorpholy analysis and detail field survey, conducted to the recognition of surface deformation parallel to the main fault trace althought further into the basin. Several trenches were openned showing fluvial sediments underlaying pleistocene(?) coarse colluvium from a nearsource likely to be related with a small axiall drainage. The fluvial package is faulted and folded, deformed by a major sub-vertical structure and the colluvium seems to be tilteld and do not crosses the fault trace. A paleosoil sequence (alfisoil) developed mainly on the colluvium unit was identified and it might be 700 ky in age. This paleosoil is faulted allowing to infer a vertical offset circa 1,5m. The structure identified is N10E on strike and was identified as a left strike-slip fault with a minor reverse component. In order to constrain the sediments ages, OSL samples were collected and samples for micro paleontological studies. Althought we have not identified references to quatify lateral displacements or individualize deformational events, these trenches evidencied pleistocene deformation on Alfambras segment on a site were was not previsiouly recognized.