THE CRUSTAL STRUCTURE OF THE ROCKALL TROUGH - DIFFERENTIAL STRETCHINGWITHOUT UNDERPLATING

The crustal structure along the axis of the Rockall Trough, in the Nor th Atlantic, has been studied along a 600-km refraction/wide-angle ref lection transect, containing three lines each 200 to 250 km long, usin g explosives and ocean bottom seismometers. One-dimensional inversions of each section were made using the tau - p method and forward modeli ng of the observed travel times. In the next stage, travel times and a mplitudes were modeled using ray tracing techniques through two-dimens ional heterogeneous structures. The results indicate that there are th ree 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 t o 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 k m/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 zo ne increase from 6.9 km/s up to 7.8 km/s along the profile. The underl ying 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 transiti on zone, which is about 3 km thick. The two upper layers in the onshor e region are interpreted as corresponding to the upper crust in the Ro ckall Trough and indicate a stretching factor (beta) of 8 - 10. The ve locity pattern in the lower crust in the Rockall Trough and under Irel and are similar, suggesting significantly less stretching (beta = 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 i n the range of 4 - 6. If this represents the lithospheric stretching f actor, significant underplating would be expected. However, if the str etching factor for the lower crust in the differential stretching mode l is more representative of overall lithospheric stretching, little or no underplating is predicted. The velocity patterns observed in the R ockall 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.