The seismic velocity structure and tectonic features of the Africa-Europe plate boundary region in the Atlantic: new high-quality geophysical data

The LISA and ATLANTIS geophysical cruises were conducted in 2021 onboard the Spanish R/V Sarmiento de Gamboa. We collected wide-angle seismic reflection and refraction (WAS) data (ATLANTIS) and coincident streamer data (FRAME) to constrain the seismic velocity structure on the region to the west of the Gorringe Bank (West Iberia Margin), where the nature and structure of the lithosphere are mostly unknown. The LISA profile runs across different geological domains: from the Ampère seamount at SE, the West Horseshoe Abyssal Plain, the region of the Josephine seamount (at the intersection of the Tore-Madeira Rise with the Gloria fault) and the undisputed North Atlantic oceanic domain to the NW.

WAS data were acquired with 21 ocean bottom hydrophones (OBH) spaced at ~15 km, along a NNW-SSE oriented, ~400 km long profile, at 250 Hz sampling frequency. The seismic source was designed to provide high penetration and map the entire crust and the upper mantle structure and consisted of two sub-arrays of 16 airguns with total volume of 5200 c.i., towed at 15 m depth. Multichannel seismic reflection (MCS) streamer data were also acquired with a 6 km long streamer towed at 23 m and using the same seismic source.

OBH sections were analyzed for picking of Ps, Pg, PmP and Pn phases, and show high variability along the profile. Joint inversion of refraction and reflection travel times of key boundaries observed in the WAS and MCS data were used to build a final P-wave velocity (Vp) model, following a layer stripping strategy, and Vp uncertainty was evaluated using Monte Carlo analysis.

In this work we focus on the northern part of the profile, sampled by the 14 northernmost OBH, to present the velocity structure and a seismic image across the region of the contact of the plate boundary and the Tore-Madeira Rise, near the Josephine seamount. A vertically and laterally complex velocity distribution is observed. An apparently low velocity (<4 km/s) from the top of the basement extends ~5 km underneath. There is a significant lateral variation of velocity within the lower crust and upper mantle. Wide-angle Moho reflections could be identified and modeled, in some places marking the transition to 8 km/s mantle, and in others to lower velocity mantle, implying the occurrence of serpentinization. We discuss the role of magmatic intrusion and tectonic deformation processes in the crustal structure, as well as implications for plate boundary activity, and for the isostatic equilibrium of this important bathymetric feature.

This work was funded by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) –  UIDB/50019/2020 (https://doi.org/10.54499/UIDB/50019/2020), LA/P/0068/2020 (https://doi.org/10.54499/LA/P/0068/2020), and projects LISA (https://doi.org/10.54499/PTDC/CTA-GEF/1666/2020) and RESTLESS (http://doi.org/10.54499/PTDC/CTA-GEF/6674/2020). Support from the Spanish Ministry of Science and Innovation (CTM2015-71766-R, PID2019-109559RB-I00) and Spanish Research Agency (CEX2019-000928-S) is also acknowledged.