Abstract Low‐angle detachments are fundamental crustal structures found in many extensional systems and plate tectonic boundaries, including onshore extensional basins, rifted margins and mid‐oceanic ridges. Direct observations of the complete geometry of extensional detachments are rare so that aspects of our understanding of their development and evolution rely mainly on proxy observations and numerical simulations. A high‐resolution 3D seismic reflection survey Offshore West of Ireland images a complete corrugated extensional detachment, from its steep oceanward breakaway faults to its back‐rotated domal crest. The detachment surface, the P reflector, developed during the Jurassic hyperextension of the Porcupine Basin and is preserved in its slip position. It covers 95 × 35 km area and has a N‐S elongate domal shape, at right angles to the prevailing extension direction, with a crest at ca. 6.3 s two‐way travel time. It is overlain by a syn‐rift sequence offset by steep frontal faults that pass eastward into shallower, predominantly west‐dipping highly listric faults that merge downwards with the detachment. The detachment has pronounced E‐W corrugations parallel to the basin opening direction, and N‐S lineaments that correspond with the footwall cutoff lines of overlying Jurassic faults. The most significant N‐S lineaments correspond with changes in dip of the detachment. We propose that the geometry of P can be explained by a conceptual model in which the detachment was assembled from initially steep faults that developed at the front of P and back‐rotated to form listric faults during extension, with punctuated oceanward propagation of the segments of the detachment. Comparisons with 2D profiles and 3D surfaces of published detachments suggest that our conceptual model may be applicable to other detachments that accommodate extreme extension at other rifted margins and at mid‐oceanic slow and ultra‐slow spreading ridges.
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