Modelling and interpretation of crustal reflections

Deep seismic reflection profiling has revealed reflective upper crustal structures, often interpreted as faults or shear zones, and sometimes a strongly reflective lower crust. This thesis investigates what crustal reflections represent, and to what extent seismic sections can be used to investigate a three-dimensional Earth. The lower crust often consists of reflective bands and transparent zones. An analogy is drawn between the geometry of reflective bands seen in the lower crust and the orientation of shear zones likely to develop in a plastic lower crust during both bulk pure and simple shear. These results suggest that lower crust reflections come from shear zones. Crustal reflections are often cross-cutting and shorter than the Fresnel Zone diameter. Simple 2-D seismic modelling demonstrates that these reflection segments often do not directly represent real structure, but are rather the product of complex spatial interference, which is itself controlled by the geometrical packing of the reflecting surfaces. Intersections of orthogonal profiles and limited three-dimensional modelling are combined with these results to provide constraints on the three-dimensional shape of lower crustal reflectors. The results are consistent with shear pods and boudins within shear zones, supporting the hypothesis that lower crust reflections are from shear zones. These results are then integrated with other data to provide a model for lower crustal reflectivity and structure. The modelling results show that 3-D control is vital in interpreting crustal reflections, which must be viewed as an interference pattern, and so can only crudely define structure. These conclusions are emphasised in structural interpretations of crustal reflections observed on grids of seismic data, from the southern North Sea and the Oberpfalz region of West Germany.