| Summary: | The Rockall Trough is the most southerly and widest part of the pre-Tertiary "proto North Atlantic' rift zone running from the Porcupine Bank region west of Ireland to the Voring Plateau off central Norway. Although the northerly part of this rift is intra-continental, the southern part is quasi-oceanic in origin, but of uncertain age. The many arguments for Cretaceous sea floor spreading are evaluated in turn; the few apparently valid ones remaining are shown to relate not directly to the Rockall Trough, but to a newly-identified spreading phase of approximately mid-Cretaceous age which partially opened up the Hatton-Rockall Basin. The opening of the Rockall Trough predates this. Stratigraphic evidence suggests that the proto North Atlantic is pre-Cretaceous, and probably even pre-Jurassic, in age. The rival arguments for late Palaeozoic (late Carboniferous-early Permian) sea floor spreading are all consistent with an important phase of rifting at that time, but they provide no direct evidence for opening. A tentative conclusion is that the Rockall Trough was initiated as a rift, and then opened by a quasi- sea floor spreading mechanism, during the late Carboniferous to early Permian.
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