Hyperextension of continental crust at the Deep Galicia rifted margin in the North Atlantic has been accommodated by the rotation of continental fault blocks, which are underlain by the S reflector, an interpreted detachment fault, along which exhumed and serpentinized mantle peridotite is observed. West of these features, the enigmatic Peridotite Ridge has been inferred to delimit the western extent of the continent‐ocean transition. An outstanding question at this margin is where oceanic crust begins, with little existing data to constrain this boundary and a lack of clear seafloor spreading magnetic anomalies. Here we present results from a 160 km long wide‐angle seismic profile (Western Extension 1). Travel time tomography models of the crustal compressional velocity structure reveal highly thinned and rotated crustal blocks separated from the underlying mantle by the S reflector. The S reflector correlates with the 6.0–7.0 km s−1 velocity contours, corresponding to peridotite serpentinization of 60–30%, respectively. West of the Peridotite Ridge, shallow and sparse Moho reflections indicate the earliest formation of an anomalously thin oceanic crustal layer, which increases in thickness from ~0.5 km at ~20 km west of the Peridotite Ridge to ~1.5 km, 35 km further west. P wave velocities increase smoothly and rapidly below top basement, to a depth of 2.8–3.5 km, with an average velocity gradient of 1.0 s−1. Below this, velocities slowly increase toward typical mantle velocities. Such a downward increase into mantle velocities is interpreted as decreasing serpentinization of mantle rock with depth.

COMM3D (POS453 & 458): Crustal Structure and Ocean Mixing observed with 3D Seismic Measurements.Galicia 3D Seismic (MGL1307): Seismic Study of the Galicia S Detachment.OBS/OBH were deployed during POSEIDON cruise POS453 and recovered during POSEIDON cruise POS458. Seismic profiles were aquired during expedition MGL1307 (doi: 10.1594/IEDA/320627).

Supplement to: Davy, Richard G; Minshull, Tim A; Bayrakci, Gaye; Bull, Jonathan M; Klaeschen, Dirk; Papenberg, Cord; Reston, Timothy J; Sawyer, Dale S; Zelt, CA (2016): Continental hyperextension, mantle exhumation, and thin oceanic crust at the continent-ocean transition, West Iberia: New insights from wide-angle seismic. Journal of Geophysical Research: Solid Earth, 121(5), 3177-3199