British Institutions Reflection Profiling Syndicate (BIRPS) deep seismic reflection surveys show that the upper mantle around the British Isles is reflective, exhibiting discrete, isolated reflecting zones within an otherwise seismically transparent mantle. There are 20 known occurrences of reflectors within the upper mantle across the continental shelf of the British Isles, many delimited by intersecting or closely spaced profiles. Most of the mantle reflectors have similar reflection characteristics: they dip 10° to 30° and are thin (∼0.5 s [∼2 km]) and discrete. The geologic history of the continental shelf around the British Isles (an area 1000 x 1000 km) is complex, and the tectonic settings of these reflectors vary. Each mantle reflector is associated with a major zone of deformation in the crust. This association, the dips of the reflectors, and their high reflectivity all indicate a structural origin. The BIRPS mantle reflectors include probable early Paleozoic thrusts and Mesozoic normal faults. In contrast to BIRPS data, deep reflection profiles in other areas of the world, in a variety of geologic settings, have only rarely image reflectors within the upper mantle

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