TECTONIC CONTROL OF OFFSHORE SEDIMENTARY BASINS TO THE NORTH AND WEST OF IRELAND

Abstract

A newly compiled tectonic map of Ireland and part of the offshore area can be used to show that a pattern of rejuvenated Caledonian structures was superseded by newly imposed structures at about the time of the opening of the northern North Atlantic Ocean. There are three distinct patterns of structural control: 1. Older Caledonian structures, 2. Younger Caledonian structures and 3. ‘Oceanic’ structures. Younger structural patterns are each superimposed on older, so that locally interference patterns are developed. Each of the structural regimes has controlled sedimentation in particular zones, and where superimpostion has occurred, mixed shelf‐continental and ‘oceanic’ sedimentation can be expected at different structural levels.One of the major control structures in and immediately off NW Ireland is the Great Glen Fault system, which has been repeatedly rejuvenated since its development in Caledonian times. Other major control structures are associated with the Porcupine Seabight, which probably developed during the initiation of the immediately adjacent North Atlantic Ocean basin. Where these structural systems intersect at the head of the Seabight, sedimentational provinces can be defined.A poorly defined E‐W structural line at about 53° 25′ across the northern part of the Seabight may reflect deep wrench faulting related to a subjacent transform. Major structures along this line would have influenced sedimentation and consequent growth structures. This line may be an eastward analogue of the Gibbs Fracture Zone against which the axial zone of ‘quasi‐oceanic’ or attenuated continental crust in the Porcupine Seabight could terminate.