| Summary: | The crustal structure of two ~150 km long segments across two main tectonic plate boundaries in the North Atlantic, the Gloria Fault and the Terceira Rift are presented. The Gloria Fault stands as a seismogenic fracture zone that generates high magnitude earthquakes, such as the M8.4 event in 1941. Vp and Vs waves were recorded during an active seismic refraction experiment using 18 Ocean Bottom Stations. The velocity model allows discrimination of five layers (L1 to L5), L1 for sediments, L2 for upper crust, L3 for lower crust, L4 for a layer of unknown origin and L5 for mantle. Poisson coefficient from Vp/Vs ratio allowed estimation of layers’ densities. We speculate on L4 origin and nature from velocities and densities. Two possible models, L4 corresponds to a mixture of gabbro and peridotite or to hydrated mantle (serpentinized mantle). The Terceira Rift seismic refraction line comprehends data from the S. Miguel Island. Velocities of S waves were not recorded. The model is based in Vp only. A five layer model is proposed, L1 for volcano-sedimentary layer, L2 for upper crust, L3 for lower crust and L5 for the mantle. L4 between L3 and L5 has a lensoid shape and its velocities suggest either a cumulate gabbro underplated layer or hydrated mantle. A south dipping extensional shear zone aligned with the Monaco Graben was identified from the brittle upper crust and across the lower crust, L4 and mantle. This shear zone coincides with a cluster of seismicity located to the south of S. Miguel Island. To the north of S. Miguel seismicity is barely inexistent and a 20 km long recent landslide with a toe thrust is clearly imaged, suggesting northward tilting of the island caused by the extensional shear zone, the south flank of the Terceira Rift. German Research Foundation, DFG, grant Hu698/19-1
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