The crustal structure of the Rockall Trough: Differential stretching without underplating

Abstract

The crustal structure along the axis of the Rockall Trough, in the North Atlantic, has been studied along a 600‐km refraction/wide‐angle reflection transect, containing three lines each 200 to 250 km long, using explosives and ocean bottom seismometers. One‐dimensional inversions of each section were made using the τ – p method and forward modeling of the observed travel times. In the next stage, travel times and amplitudes were modeled using ray tracing techniques through two‐dimensional heterogeneous structures. The results indicate that there are three sedimentary layers with velocities ranging from 2 km/s to 4.5 km/s. The whole sedimentary section is up to 6 km thick and interpreted as late Paleozoic to Tertiary in age. A two‐layer continental crust, 5 to 7 km thick, occurs along the length of the profile. The upper crust (6.0–6.3 km/s), is circa 2 km thick and the lower crust (6.6–6.9 km/s), is circa 3 km thick. A Moho transition zone, approximately 1 km thick, lies at the base of the crust. Velocities in this transition zone increase from 6.9 km/s up to 7.8 km/s along the profile. The underlying upper mantle has a laterally variable velocity between 7.6 and 7.8 km/s. Unstretched crust onshore in Ireland comprises a three‐layered crust, with each layer approximately 10 km thick, and a Moho transition zone, which is about 3 km thick. The two upper layers in the onshore region are interpreted as corresponding to the upper crust in the Rockall Trough and indicate a stretching factor (β) of 8–10. The velocity pattern in the lower crust in the Rockall Trough and under Ireland are similar, suggesting significantly less stretching (β = 2 ‐ 3). The differential stretching model is supported by the presence of the Moho transition zone which is stretched by a similar factor to the lower crust. The bulk stretching factor for the crust as a whole is in the range of 4–6. If this represents the lithospheric stretching factor, significant underplating would be expected. However, if the stretching factor for the lower crust in the differential stretching model is more representative of overall lithospheric stretching, little or no underplating is predicted. The velocity patterns observed in the Rockall Trough indicate the absence of any significant underplating at the base of the crust, such as that observed at the continental margin west of the Hatton Basin.