Morphostructure, tectono-sedimentary evolution and seismic potential of the Horseshoe Fault, SW Iberian Margin

Martínez-Loriente, Sara, Gràcia, Eulàlia, Bartolome, Rafael, Perea, Hector, Klaeschen, Dirk, Dañobeitia, Juan José, Zitellini, Nevio, Wynn, Russell B. and Masson, Douglas G. (2018) Morphostructure, tectono-sedimentary evolution and seismic potential of the Horseshoe Fault, SW Iberian Margin. Basin Research, 30 (Suppl 1), 382–400. (doi:10.1111/bre.12225).

Abstract

High-resolution acoustic and seismic data acquired 100 km offshore Cape São Vicente, image with unprecedented detail one of the largest active reverse faults of the SW Iberian Margin, the Horseshoe Fault (HF). The HF region is an area seismogenically active, source of the largest magnitude instrumental and historical earthquake (Mw > 6) occurred in the SW Iberian Margin. The HF corresponds to a N40 trending, 110 km long, and NW-verging active thrust that affects the whole sedimentary sequence and reaches up to the seafloor, generating a relief of more than 1 km. The along-strike structural variability as well as fault trend suggests that the HF is composed by three main sub-segments: North (N25), Central (N50) and South (N45). Swath-bathymetry, TOBI sidescan sonar backscatter and parametric echosounder TOPAS profiles reveal the surface morphology of the HF block, characterized by several, steep (20°) small scarps located on the hangingwall, and a succession of mass transport deposits (i.e. turbidites) on its footwall, located in the Horseshoe Abyssal Plain. A succession of pre-stack depth-migrated multichannel seismic reflection profiles across the HF and neighbouring areas allowed us to constrain their seismo-stratigraphy, structural geometry, tectono-sedimentary evolution from Upper Jurassic to present-day, and to calculate their fault parameters. Finally, on the basis of segment length, surface fault area and seismogenic depth we evaluated the seismic potential of the HF, which in the worst-case scenario may generate an earthquake of magnitude Mw 7.8 ± 0.1. Thus, considering the tectonic behaviour and near-shore location, the HF should be recognized in seismic and tsunami hazard assessment models of Western Europe and North Africa.

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