Tectonic framework of the Southwestern area of the Iberian Peninsula

Seismic information source from the National Geographic Institute (IGN) (https://doi.org/10.7419/162.03.2022). Raster cartography of Spain from the IGN CC BY 4.0 ign.es. Geological data source: Thematic map of the National Atlas of Spain (ANE) CC BY 4.0 ign.es (2020), synthesis based on the IGME-SGE Geological Map of Spain 2M (2004) and the IGME-LNEG Geological Map of Spain and Portugal 1M (2015). 50% transparency. Main Quaternary active faults compiled from the QAFI data base (García-Mayordomo et al., 2012; IGME, 2022). ). FAST: Alzejur-San Teotorio fault, FAZ: Falla Azambuja fault, FC: Carcavai fault, FLTV: Lower Tagus Valley fault, FMM: Madre de las Marismas fault, FOT: Ota fault, FPA: Porto Alto fault, FPLA: Plasencia fault, FPNS: Pinhal Novo-Setúbal fault, FSE: San Estevao fault, FSMQU: Flla San Marcos-Quarteira fault, FTCM: Flla Torre Carbonero-Marilópez fault, FTM: Torre Marilópez fault, FVFX: Vila Franca de Xira fault, FVM: Vidigueira-Moura fault, LTVFZ: Lower Tagus Valley fault zone.

The current state of the southwestern portion of the Iberian Peninsula is the result of a complex tectonic evolution in which three different phases are distinguished.

The first tectonic phase is compressive and linked to the Hercynian orogeny, origin of the Iberian Massif. This orogeny shows different phases and was active throughout most parts of the mid and late Paleozoic. Its origin is the collision between the Euroamerican and Gondwana landmasses that ended up with the formation of the Pangea supercontinent.

The second tectonic phase is distensive and active along the Mesozoic. During this phase the Atlantic Ocean opened and therefore the Pangea supercontinent got fractured. In this stage, the Algarve Basin was formed and filled with continental and marine sediments that reflect the Atlantic Ocean aperture and show a low deformation degree.

The third tectonic phase is compressive and active along the Cenozoic. It is related to the Alpine orogeny caused by the collision between the Eurasian and the African plate. In this stage, the Betic System got uplifted and the Guadalquivir basin was formed and infilled with sediments. The other Cenozoic basins in the region were developed on the Iberian Massif in an extensive intraplate tectonic regime.

Despite its complex and variable tectonic evolution, there are few documented active faults outcropping in this region of the Iberian Massif, the most notable ones are, the southern part of the left-lateral strike-slip fault of Plasencia (FPLA), and the reverse with left-lateral strike-slip component fault (on debate) of Vidigueira-Moura (FVM). The active faults in the sedimentary basins of the region are the most relevant ones, and mostly, they present very low slip rates (below 0.02 cm/yr). In the Algarve Basin the main faults are the left-lateral strike-slip with reverse component of Aljezur-S. Teotónio (FAST) and Carvacai (FC), the reverse with right-lateral strike-slip component fault of San Marcos-Quarteira (FSMQU), and the right-lateral strike-slip with normal component of Estevao (FSE), besides, in this basin there are more faults whose trace and activity is under debate. In the Bajo Tajo River basin, in the surrounding area of Lisboa and forming a N-S lineament, there are some active faults identified, the reverse faults of Ota (FOT), Azambuja (FAZ), Vilafranca de Xira (FVFX) and the fault zone of Lower Tagus Valley (LTVFZ, on debate), the normal fault of Porto Alto (FPA), and the left-lateral strike-slip faults of Pinhal Novo-Setubal (FPNS) and Lower Tagus Valley (FLTV, on debate). Lastly, in the Guadalquivir River Basin and close to the mouth river, there are some normal faults, Torre Carbonero-Marilópez (FTCM), Madre de las Marismas (FMM) and Torre Marilópez (FTM).