Seismic signature and tectonic cross section of the Variscan Crust in western France

Abstract

Deep seismic reflection profiling (ECORS program vertical and wide‐angle survey) coupled with field and subsurface geology are used to propose a complete NNE–SSW section of the Variscan Belt in western France from Belgium to southern Brittany. On this 800 km long profile, which probably crosses two Paleozoic sutures, the Variscan Belt appears as a broad fanlike orogen characterized by large (100 km) northward and southward overthrusts with polyphase deformation and metamorphism. The central part of the Belt (Central Armorican zone) suffered a simpler dextral shearing parallel to the strike of the belt with only green schist metamorphism. Deep reflection profiling shows that all the large thrusts visible at the surface root deeply in the lower crust or even crosscut the Moho. In the same way, all the large strike‐slip faults parallel to the belt appear at depth as narrow discontinuities which crosscut the whole crust and, in some cases, the Moho. The seismically well‐layered crust develops essentially at depth in the most deformed and metamorphic internal parts of the belt, in contrast with the relatively transparent Brabant foreland basement, which escaped the Variscan deformation. The Moho appears as a relatively flat boundary at an average depth of 35 km with different characters: It is sharp and strongly reflective both by wide angle and vertical seismics below the internal thrust zones, much less distinct in the central Armorican zone, and only defined by low frequency signals at wide angle below the Brabant foreland. All these characteristics led us to consider the layering of the lower crust and the Moho itself as initially tectonometamorphic features essentially produced by shearing and crust mantle decollement during the Variscan stacking of the crust by intracontinental lithospheric subduction.