DOES THE GREAT-GLEN FAULT REALLY DISRUPT MOHO AND UPPER-MANTLE STRUCTURE

Many previous interpretations of deep seismic reflection profiles acro ss the Great Glen strike-slip fault system have postulated that the fa ult penetrates the crust and upper mantle as a vertical plane that sha rply offsets the Moho discontinuity. Such an interpretation has become a general feature of more recent regional syntheses of the deep geolo gy of the British Isles. Reprocessing of portions of four profiles acr oss the fault improves the resolution of lower crustal and upper mantl e structure and calls into question the ''Moho step'' interpretation. Diffraction analysis and seismic migration applied to reflection data north of Shetland across the northward continuation of the Great Glen fault (Walls Boundary fault) indicate a narrow synform or ''keel'' dev eloped on the Moho directly beneath the fault. This keel is itself und erlain by a possible mantle reflection that is cut off by the downward projection of the fault. North and west of Ireland, analyses of ampli tudes, frequencies, and the geometrical behavior of reflections upon m igration show that structures previously interpreted as Moho steps may be better explained as distinct packages of reflectivity that are upp ermost mantle or possibly out of the plane in origin. In one location north of Ireland where some three-dimensional control exists, the vert ically downward projection of the Great Glen fault intersects, without disrupting, dipping structure in the upper mantle. This observation l eads to a model for displacement on the fault system in which motion i s laterally transferred along a dipping ramp (or blind thrust) in the uppermost mantle, somewhat analogous to models developed for the San A ndreas fault that indicate displacement along the fault to be laterall y offset within the middle crust. One of the principal conclusions of this study, that major vertical steps on the Moho beneath the Great Gl en fault are difficult to support from the available seismic data, is consistent with rheologically based studies which predict that Moho '' topography'' such as vertical steps is unlikely to be preserved over l ong periods of geologic time.