Quaternary tectonic activity of the São Marcos–Quarteira fault (Algarve, southern Portugal): a case study for the characterization of the active geodynamic setting of SW Iberia

Tectonic deformation along the Sao Marcos–Quarteira fault zone affecting lithostratigraphic units ascribed to the Pliocene (the Falesia Sands) and to the Middle to Upper Pleistocene (the Boliqueime Sands and the Barrancosa Sands) is studied at the Boliqueime area, Algarve (southern mainland Portugal). Evidence for this tectonic activity includes brittle (faults and joints) and ductile (drag folds) deformation structures, as well as other structures interpreted as generated by liquefaction and fluidization phenomena. The biostratigraphical study of samples collected from the Boliqueime Sands unit allows to propose a Pleistocene age based upon its content of ostracod species, pointing to a probable Calabrian to Middle Pleistocene (ca. 1.8 Ma to 130 ka) time constraint. This age is now also constrained by optically stimulated luminescence, the K-feldspar post-IRIR290 dating of a sediment sample collected from a faulted sand layer of the Boliqueime Sands having provided a minimum age of 250 ka. This study thus supports the Quaternary activity of that major regional structure, which has the potential to generate an Mw 7+ earthquake with surface rupture, a conclusion that has high relevance for the seismic hazard assessment of the very populated area of influence.