The structure of the crust and uppermost mantle offshore Britain: deep seismic reflection profiling and crustal cross-sections

BIRPS deep seismic reflection profiles, recorded in the waters around the British Isles show dominantly extensional faults in the upper crust, a highly reflective lower crust and probable extensional shear zones in the mantle. As the reflective lower crust is best developed in regions of extension, where it seems to act as a divide between the extensional tectonics of the upper crust and the upper mantle, the reflectivity of the lower crust is probably related to extensional processes. Comparisons with exposed crustal cross-sections are used to provide constraints on the cause of lower crustal reflectivity, suggesting that the reflections could come from intrusions or from shear zones, but are less likely to be caused by fluids. Fundamental to the typical BIRPS profile is the observation that the shear zones in the mantle are not collinear with the faults in the upper crust. Simple analysis is used to show that this requires the lower crust to be strongly sheared during extension, as it transfers deformation between the upper crust and the mantle. -from Author