Tectonic evolution of the São Vicente area and tsunamigenic potential of the active structures in the SW Iberian margin: Implications for hazard assessment

Abstract

The southwestern margin of the Iberian Peninsula, which includes the Gulf of Cadiz, is characterized by a present-day active deformation mainly driven by the NW-SE trending convergence between the African and Eurasia plates. In addition, this area hosts some of the largest earthquakes occurred in Western Europe. For this reason, in this PhD Thesis, tectonic and sedimentary processes in the Cape of São Vicente area are identified, important faults of the SW Iberian margin are analysed with high-resolution data, and 3D fault-plane models of the main active structures in the study area, to carry on tsunami simulations, are done. Related to the first issue, in the Cape of São Vicente area the largest submarine canyon in the external part of the Gulf of Cadiz is developed, the São Vicente Canyon. Their dynamics are highly conditioned by the convergence between the Eurasian and African plates and it is controlled by three current-active thrust faults: the Marquês de Pombal Fault, the São Vicente Fault and the Horseshoe Fault. The origin of the canyon is fully tectonic, as it shows a strong structural control. The main sediment source of the canyon is related to the disintegration of canyon flanks and the Mediterranean Outflow Water sedimentary deposits. Due to the canyon retrogressive erosion, submarine slides and scars are the main seafloor morphologies. The São Vicente Canyon is a clear example of a diachronic and segmented canyon developed since the Late Miocene in an area of active plate tectonics. Linked to the second issue, high-resolution data (autonomous underwater vehicle bathymetry and high-resolution 2D multichannel seismic profiles) allow to characterize, in detail, the Lineament South and the Marquês de Pombal faults. The Lineament South strike-slip fault is the longest active structure in the study area that may generate one of the most powerful earthquakes in the SW Iberian margin. Both ends of the Lineament South fault raise the seafloor demonstrating its current activity. The main motion component of the fault is right-lateral, as the dislocated structures in both extremes of the fault suggest. Although, a vertical component cannot be excluded. The Marquês de Pombal fault is an active left-lateral reverse fault. As isochore maps and the distribution of the vertical slip

evidenced, this structure was especially active from the Middle-Miocene until nowadays where its uplift control the incision and the activity of the São Vicente Canyon. Finally, to identify the most vulnerable areas to be affected by a tsunami in SW Iberia and NW Morocco, realistic tsunami models have been built modelling in detail the main active faults in the region, including: the Gorringe Bank, Marquês de Pombal, Horseshoe, North Coral Patch and South Coral Patch thrust faults, and the Lineament South strike-slip fault. Based on a large dataset of 2D multi-channel seismic profiles, 3D sub-surface models that involves dimensional complexities of the fault planes have been generated. Additionally, with the aim to know the influence of the slip distribution along the fault plane in tsunami simulations, various heterogeneous slip-distributions have been considered in the Horseshoe fault tsunami scenario. The results show that using more complex fault geometries and slip distributions, the peak wave height (about the mean sea level) at the coastline can double compared to simpler tsunami source scenarios from planar fault geometries. Therefore, complex fault geometries and non-uniform slip distribution should be considered in future tsunami hazard updates. Tsunami simulations also show that the presence of submarine canyons attenuates the wave heights that reach the coastline, while the submarine ridges and the shallow shelf increase it. The tsunami simulations done, reveal that the most dangerous faults for the Moroccan coast are the Horseshoe, Gorringe Bank and South Coral Patch; for the Spanish and the Algarve coasts is the Lineament South fault, while the Gorringe Bank is the fault that most affects the west coast of Portugal.

Aquesta tesis doctoral s’enfoca en caracteritzar les estructures actives del Golf de Cadis, la relació d’aquestes amb els processos sedimentaris de la zona i el seu potencial tsunamigènic. En el Golf de Cadis es desenvolupa un dels canyons més importants del marge atlàntic, el canyó de São Vicente (SVC). Aquest canyó i la seva activitat estan fortament condicionats per tres falles actives: la falla de Marquês de Pombal (MPF), la falla de Horseshoe (HF) i la falla de São Vicente. El SVC presenta un origen tectònic i un fort control estructural. La principal font de sediment, que alimenten el canyó, està relacionada amb la desintegració dels seus flancs i els dipòsits sedimentaris de les corrents submarines de la zona. El SVC és un clar exemple de canyó diacrònic i segmentat desenvolupat des de finals del Miocè fins l’actualitat en una zona de tectònica activa de plaques. Per fer un estudi detallat de la perillositat tsunamigènica de les falles de MPF, HF, NCPF (Falla de North Coral Patch), SCPF (Falla de South Coral Patch), GBF (Falla de Gorringe Bank) i LSF (Falla del Lineament South) s’ha avaluat la sensibilitat dels escenaris de tsunamis a la geometria dels plans de falla i la distribució del lliscament. Primer, s’han considerant plans de falla simples amb lliscament uniforme per a totes les falles. Seguidament, s’han realitzat models complexos en 3D dels plans de falla de MPF, NCPF, SCPF i HF en base a un gran conjunt de dades de perfils de sísmica multicanal 2D. Finalment, s’utilitzen diverses distribucions de lliscament heterogeni pel cas de HF. Els models de tsunami desenvolupats suggereixen que utilitzant geometries de falla complexes i distribucions de lliscament heterogènies, l'alçada màxima de l'ona a la costa es pot duplicar en comparació amb escenaris simples, on el tsunami és generat per una falla amb geometria planar. Les simulacions de tsunami realitzades revelen que les falles més perilloses per a la costa marroquina són HF, GBF i SCPF, mentre que per a les costes espanyoles i de l'Algarve és el LSF. GBF és la falla que més afecta la costa oest de Portugal.