ACTIVE TECTONICS IN SW PORTUGAL - QUATERNARY DEFORMATION ANALYSIS IN A LOW TECTONIC RATE AREA (Invited Presentation)

The SW of Portugal is in Iberia (Western Europe), along the eastern North Atlantic coastline. Despite being considered as a passive margin, this region is tectonically active and capable of generating infrequent large seismicity and tsunamis (Mw ≥8 Lisbon, 1755). This is due to the Africa-Eurasia transpressive plate boundary (4-5 mm/yr) expressed in the Gulf of Cadiz and Southwest Iberian Margin (offshore Portugal) by a complex tectonic framework.

Inland, several Variscan and Alpine inherited structures, are reactived during the Quaternary generally as strike-slip with/or reverse component. Erosion processes are dominant, affecting preservation of Q sediments and landforms and masking deformation evidences. A remarkable and distinct geomorphologic feature in SW Portugal is a ≥80m high coastal cliff with remnants of marine terraces and a wide abrasion platform 70-130m asl, suggesting a regional uplift.

Seismicity is diffuse and low magnitude, despite large intensity historical events. The tectonic rates are low, in the range of a few 0.1 mm/yr. Nonetheless, the active structures dimensions indicate a seismic potential for Mw≥6 events, with a recurrence period of tens of ka.

Long term and short term deformation in the area were investigated combining different tools and approaches. A strike-slip fault system 50 km long and parallel to the western coastline presents evidence of Pleistocene deformation, displacing the wide abrasion platform. Studies were conducted to constrain tectonic rates and incision/erosion rates. To investigate the cumulative long term deformation, a set of morphotectonic indices (stream sinuosity, basin relief ratio, basin elongation and shape ratios, terminal basin shape ratio, valley height-width, basin asymmetry factor, hypsometry, stream-length index) were applied regionally to exorheic basins across SW Portugal and neighboring areas. As expected, different indices show different sensibilities to the landscape and by themselves may not provide sufficient information. However, the cross-correlation of the morphotectonic indices provided support for an adequate interpretation of a tectonic forcing, even in a low tectonic rate environment. Results corroborated SW Portugal undergoing uplift at higher rates than neighboring areas and limited by major active structures. These observations are partially consistent with a GPS velocity pattern based on ~20 years observation that suggests that the SW Portugal is accommodating deformation differently, behaving as a tectonic block.