Seismic and tectono‐metamorphic characters of the lower continental crust in Phanerozoic areas: A consequence of post‐thickening extension

Deep seismic profiles of Phanerozoic continental crust commonly show a highly reflective lower crust. Rheological considerations suggest that the seismic fabric of the lower crust can be attributed to the tectonic transposition of various petrological heterogeneities in the main flow plane. The Variscan provinces of western Europe have been affected, during Phanerozoic times, by several extensional and compressional events. The geometrical relationships between seismic and geological structures indicate that the layering of the lower crust was acquired during the Late Carboniferous to Permian when the thickened Variscan crust was affected by gravitational collapse. Petrological and geochronological analyses of deep crustal rocks (xenoliths and exposed sections) indicate that the lower crust has recorded a major high‐T / medium‐P granulite facies metamorphism during the late Variscan extension, whereas on the surface Upper Carboniferous to Permian basins were being deposited. A similar scenario characterizes other Phanerozoic orogenic belts. In the Caledonian provinces of the British Islands, the lower crust is seismically reflective; it has undergone medium‐P granulitic metamorphism during the deposition of Devonian sedimentary basins, at the end of the Caledonian orogeny. In the same way, collapse of the Mesozoic belt in the western part of North America is responsible, during the Cenozoic, for pervasive crustal extension whose consequence is a seismic layering of the lower crust accompanied by a low‐P granulite grade metamorphic event, while in the mid‐ and upper crusts, low‐angle, ductile, normal faults give rise to the Basin and Range Province. Therefore, it is proposed that there is a genetic relationship between (1) post‐thickening crustal extension, (2) low‐ to medium‐P granulite facies metamorphism of the lower crust and (3) seismic layering of the lower crust.