Numerous multichannel seismic data acquired on the starved continental margin off Galicia show that the prominent intra-crustal S reflector is observed only below the oceanward portion of the continental margin, close to the ocean–continent boundary. Superficial extension has been computed from the geometry of the well-imaged tilted fault blocks of the entire margin by using a series of parallel seismic lines specifically designed for this purpose. The pure shear model clearly accounts for the lithospheric extension of the continentward portion of the continental margin but not for the oceanward portion of the continental margin where the S reflector is present. The S reflector is a detachment zone which presents a decreasing offset in the direction of the continent, and this detachment zone has been active since the beginning of rifting. Recent kinematic reconstructions show that the conjugate margin to Galicia is the southeastern Flemish Cap margin. From the few seismic profiles available on this Canadian margin, a major master fault is observed beneath the continental slope and is interpreted as the outcropping portion of a main detachment zone identified on the Galicia margin as the S reflector. The proposed mechanism of extension for these two conjugate margins is pure shear which affects the whole lithosphere, but in the upper part of the crust, the rifting mechanism is either pure shear for the continentward portion of the Galicia margin or simple shear for the oceanward portion of the Galicia and the whole southeastern Flemish Cap margins where the detachment zone is active. I consider that pure shear is the basic lithospheric mechanism and that simple shear is a mechanism which affects only the upper crust.

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