Deep reflection data across the UK continental margin are used to test theories concerning the origin of a pervasive reflective fabric in the lower continental crust seen on many deep reflection profiles worldwide. The origin of the reflections is at present little understood, but clearly represents an important crustal process. The Western Approaches Margin (WAM) line was designed to determine the behaviour of the lower crustal reflections in crust which has undergone significant extension. The WAM profile across the continental shelf is characterized by bright, continuous lower crustal reflections, some of which reach 30 km in length and have minimum reflection coefficients of 0.1. Deep water multiples in the area of the continental slope partially obscure reflections from the lower crust, but detailed analyses show that reflectors are present in the lower crust on the continental slope. Adjacent to the continent-ocean boundary, the lower crust on the WAM profile is well-imaged and free of noise and multiples. This very thin crust exhibits the same reflectivity pattern as is observed on the continental shelf, i.e. the upper half of the crust is blank and the lower half contains sub-horizontal reflections. The lower crustal reflections adjacent to the continent-ocean boundary are less continuous than the reflections seen on the shelf, and are interpreted as being the remnants after extension of the lower crustal reflectors observed on the continental shelf. The lower crustal reflectors are thus interpreted to pre-date the extensional episode that led to the opening of the Atlantic. The preservation of thinned sections of both transparent upper crust and reflective lower crust adjacent to the continent-ocean boundary suggests that extension by pure shear was the mechanism by which the crust was thinned in the Goban Spur area.

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