Subtle evidence of Quaternary fault reactivation in Southwest Iberia, Portugal

Southwest Portugal is the most seismically active region in Portugal mainland. Historical and instrumental seismicity, transpressive deformation accommodated by brittle structures (from which NNE-trending reverse left-lateral faults are the prominent ones), and uplifted marine landforms attest for the ongoing Quaternary crustal deformation. Geophysics highlights a positive gravimetric anomaly, consistent with the uplifted area. Simultaneously, geodesy suggests this region to be limited northward by a likely continuous right-lateral NW-trending structure, inferred to be 90 km long. However, this inferred structure is poorly understood, and southward partially overlaps a known active fault (São Marcos Quarteira) for ~50 km. The northward remaining length of 40 km lacks recognition of Quaternary deformation, despite a noticeable 50-100 m height scarp in the landscape along a ~10 km segment. This geomorphic feature has not been interpreted as an active fault, but as an inherent Variscan structure, possibly reactivated during the Miocene and since, evolved as a scarp retreated due to differential erosion promoted by the presence of Paleozoic quartzites, which are more resistant to erosion.

We present a preliminary analysis based on recently available 50 cm high-resolution lidar and revisited Plio-Quaternary data, together indicating evidence of likely subtle geomorphic deformation, which is expressed by small linear features NW-trending, some associated with changes of topography across a ~2km wide area. We propose these features to possibly correspond to subtle evidence of a cryptic fault system, likely to correspond to an inherited fabric, that has been reactivated. The newly discovered features will be investigated through combining geology, geophysics, and geochronology methods. Fault reactivation will be investigated through a detailed analysis of the damage zone and fault gouge, applying trapped-charges dating methods, namely OSL (Optically Stimulated Luminescence) and ESR (Electron Spin Resonance).

This work is supported by FCT, I.P./MCTES through national funds (PIDDAC): LA/P/0068/2020-https://doi.org/10.54499/LA/P/0068/2020, UID/50019/2025, https://doi.org/10.54499/UID/PRR/50019/2025 ,UID/PRR2/50019/2025 and by Marie Skłodowska-Curie Actions, European Union (project SEISMO-REACT, GA101211167).

Keywords: Quaternary activity, seismogenic sources, low strain deformation, cryptic structures, SW Iberia