The crustal structure and regional development of the Irish Atlantic margin region

Abstract

During the past decade, a suite of wide-angle seismic reflection/refraction profiles has been shot in the Porcupine, Rockall and Hatton basins, as well as across the Hatton continental margin. Integration of the wide-angle seismic data with normal-incidence reflection profiles and with gravity and magnetic data reveals a clear picture of the regional crustal and upper mantle structure, and of the large-scale sedimentary geometry of the Late Palaeozoic–Cenozoic basins in the area. The region contains a set of large sedimentary basins resting upon variably thinned continental crust. The crust beneath the basin-bounding structural highs is typically around 25km thick, with the crust beneath the basins being as thin as 6km. The Porcupine Basin contains up to 10 km of sediments. Up to 7km of sedimentary strata are indicated in the Rockall Basin, with 4km of sediment in the Hatton Basin. Several seismically distinct layers, with P-wave velocities of 1.8–5.3 kms−1, were identified above the basement in the sedimentary basins. A significant variability in thickness of the sedimentary layers is seen on the wide-angle data. These reflect the presence of a number of older Mesozoic rift sedimentary basins. The wide-angle data reveal marked topographic irregularity at the base of the sedimentary succession in the Rockall Basin, interpreted as the result of extensive multi-phase Mesozoic rifting. The Moho beneath the Rockall Basin shows a clear asymmetry, with a steeper gradient along the eastern margin of the basin. The continent–ocean boundary west of the Hatton High has a significant underplated and volcanic component, while that at the southern margin of the Rockall Basin shows evidence of a greater transpressive component. The nature of the lower crust appears to exert a controlling influence on the location of crustal separation. In areas where thinning is accommodated by upper and middle crustal attenuation, the strength of the upper crust (and underlying brittle mantle) has served to preserve the integrity of the continental crust and inhibit seafloor spreading. However, where the lower crust is thinned significantly, as beneath the Hatton continental margin, crustal separation has taken place even though the upper and middle crust remains relatively thick.