Study of the crystalline crust from a two-ship normal-incidence and wide-angle experiment

Abstract

In 1992, British Institutions' Reflection Profiling Syndicate conducted a two-ship coincident near-vertical and wide-angle experiment to determine the detailed velocity structure of the continental crust in the Central North Sea. The results show a near-surface high velocity gradient layer (P-wave velocity reaches 6.0 km/s at ∼5 km depth), a high-velocity zone (6.5 km/s) at 10–15 km depth, a 19-km-thick upper crust and the Moho at a depth of 32–34 km along the 135-km profile. The travel-time and amplitude of wide-angle data require the presence of a dipping reflector originating at the Moho (32 km depth) and continuing down to 38 km in the mantle. The velocity between 19 and 24 km depth is slightly lower (6.1 km/s) than that in the upper crust. The waveform modelling of the combined normal-incidence and wide-angle data requires a number of alternating high- and low-velocity layers in the lower crust (19–33 km). The velocity contrast in the lower part of the lower crust (29–33 km) is large, of the order of 1 km/s (6.5–7.5 km/s). There are a few alternating high- (8.2 km/s) and low-velocity (7.5 km/s) layers in the upper part of the mantle (33–36 km). The high velocity (6.5 km/s) in the upper crust may be due to a high grade of metamorphism, and it may have been formed during the subduction process. The low-velocity (6.1 km/s) in the upper part of the lower crust might be caused by the presence of meta-sediments. The large velocity variations (6.5–7.5 km/s) in the lower part of the lower crust suggest the presence of mafic materials, and may result from under-plating of mafic materials from the mantle. The fractionation of melt can also produce the layering in the upper part of the mantle that is observed from seismic analyses.