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Seismic images of continental lithosphere

President's anniversary address 1989

Geology Department, Royal Holloway and Bedford New College, Egham, Surrey TW20 0EX, UK

Seismic reflection profiles in USA and West Germany over thirty years ago showed strong reflections from the lower crust but technical advances through the 1970s were needed for the method to image the continental lithosphere. The British Institutions Reflection Profiling Syndicate, BIRPS, found advantage in working offshore and has obtained over 12,000 km of profiles around Britain during the past decade, giving the densest regional coverage anywhere. Reflections are observed from strata in sedimentary basins, from low angle faults that can be followed from surface to mid-crust, from a laterally variable reflective zone in the lower crust, from the Moho and from fault-like features in the upper mantle. There is evidence around Britain that faults bounding sedimentary basins are older structures which have been reactivated. It is argued that lower crustal reflectivity around Britain is mainly due to anastomosing ductile shears which accommodate extension in the lower crust commensurate with extension in the upper crust by faulting and block rotation. There is also evidence of reflections preserved from older orogenic features, so that reflections interpreted as extensional ductile shears in the lower crust may also derive from older features that have been reactivated. Most reflections in the lower crust arise from rocks that have been highly strained. Evidence of lateral variability in the reflective character of the Moho suggests that the Moho has been actively involved in the structural evolution of the lithosphere. Upper mantle reflections are interpreted as shear zones which transfer crustal deformation through the lithosphere.

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