Resumen de Tectonic evolution of the São Vicente area and tsunamigenic potential of the active structures in the SW Iberian margin: implications for hazard assessment
The southwestern margin of the Iberian Peninsula, which includes the Gulf of Cadiz, is characterized by present-day active deformation mainly driven by the NW-SE trending convergence between the African and Eurasian plates. This area hosts some of the largest earthquakes that have occurred in Western Europe leading to major disasters such as the 1755 Lisbon earthquake and tsunami. In the SW Iberian margin there is not yet a complete understanding of the active tectonics and related contemporary sedimentary processes. Further to that, there is a lack of accurate parameters to properly characterize the main active faults, and produce high-quality models of fault rupture and subsequent tsunami generation and propagation.
To fill in, at least partially, these gaps of knowledge, this PhD Thesis develops the following research: 1) An analysis using a multi-scale dataset consisting of multi-beam echo-sounder, 2D multi-channel seismic reflection profiles and high-resolution sidescan sonar (TOBI) data of the Cape São Vicente area to identify the sedimentary and tectonic processes that developed in the region, the interaction between the two and how those gave origin to one of the largest canyons in the Atlantic Ocean.
2) A study of two of the most important faults of the SW Iberian margin, the Marquês de Pombal fault and the Lineament South fault to infer their current activity using the most advanced technologies in marine geophysical surveying covering different scales of resolution, such as: multibeam echosounder, sub-bottom profiler, Autonomous Underwater Vehicle (AUV) “Abyss” microbathymetry and high-resolution 2D multichannel seismic profiles. 3) A series of tsunami simulations using 3D fault-plane models of the main active structures in the study area to evaluate the sensitivity of the tsunami impact on the coast of SW Iberia and NW Morocco to the fault geometry and slip distribution for local earthquakes.
The main findings of my PhD Thesis related with the previous points are the follow: 1) In the Cape São Vicente area a large variety of morphologies, as well as sedimentary and tectonic structures are identified along the São Vicente Canyon, the largest submarine canyon developed in the external part of the Gulf of Cadiz. We find that the São Vicente Canyon and its sedimentary dynamics are highly controlled by the convergence between the Eurasian and African plates. The origin of the canyon is fully tectonic, as it shows a strong structural control by three currently active thrust faults: the Marquês de Pombal Fault, the São Vicente Fault and the Horseshoe Fault. No major rivers feed sediment to the canyon head, and therefore, the main sediment source is related to erosion of the canyon flanks by mas-wasting and sediment supply by the Mediterranean Overflow Water. The multiple submarine landslides and associated scars are the main seafloor morphologies attesting to retrogressive erosion and active tectonics of the adjacent faults. The São Vicente Canyon is a clear example of a diachronic and segmented canyon developed since the Late Miocene (mainly in the Plio-Quaternary) in an area of active plate tectonics.
2) It has been possible to characterize, in detail, the Lineament South and the Marquês de Pombal faults. The Lineament South strike-slip fault is the longest active structure (377 km) 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 and generate positive and negative floor-like structures, 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 evidence, this structure was especially active from the Middle-Miocene until nowadays: relative uplift of the fault hanging-wall block control the incision and activity of the São Vicente Canyon.
3) Realistic tsunami models, built using detailed parameters for 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, identify the most vulnerable areas to be affected by a tsunami in SW Iberia and NW Morocco. The models are based on a large dataset of 2D multi-channel seismic profiles and 3D sub-surface models that provide the dimensional complexities of the fault planes. 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; while for the Spanish and the Algarve coasts it is the Lineament South fault. The Gorringe Bank is the fault that most affects the west coast of Portugal.
