Crustal structure of the Rhenish Massif: results of deep seismic reflection lines Dekorp 2-North and 2-North-Q

Abstract

The reflection seismic line DEKORP 2-N reveals an almost complete cross section through the Rhenohercynian Zone, the most external part of the Variscan orogen in Europe. The northern part of DEKORP 2-N and a NE-directed branch (2-N-Q) reveal the Cretaceous of the Munsterland basin and the underlying folded Palaeozoic rocks. The northward decreasing intensity of folding is depicted in great detail by the highly reflective Late Carboniferous coal-measures and deeper reflections down to the level of the Givetian/Frasnian shallow-water carbonates. In the Devonian and older rocks of the Rhenish Massif, bedding is only represented by relatively weak, short and irregular reflections. These are truncated by stronger, southward dipping reflections, which exhibit the listric curvature and flat/ramp geometry characteristic of faults. In the northern part of the section, the thrusts appear to be blind. From the Ebbe Anticline southwards, prominent reflections can be correlated with important thrust faults known from the surface, such as the Ebbe-, Siegen-, Musen- and Sackpfeife- Thrusts, as well as further important thrust faults in the Lahn- and Dill Synclines. The basal thrust of the extremely thin-skinned Giessen Nappe is only recognizable for a very short distance. At depth, the thrusts flatten out in a relatively transparent zone between 3–5 s TWT, with strongly reflective bands at its bottom and top. The transparent zone might correlate with a high-conductivity layer detected in a magnetotelluric survey; it represents either graphitic metapelites or a zone with an interconnected, brine-filled pore space. The seismic record relates either to lithological differences, or to rheological boundaries. The lower crust in the north is characterized by a relatively transparent zone, which wedges out towards south under the northern margin of the Siegen Anticline. Comparisons with a similar feature in the ECORS profile »Nord de la France« suggest that the transparent zones in both sections correspond to a pre-Palaeozoic basement, such as it underlies the Brabant Massif. Further south, the lower crust is increasingly reflective. The curvilinear, thrust-related reflections are cut by a conjugate set of much weaker, N- and S-dipping reflectors indicating a later deformation with pure shear. Displacement of some marker reflections suggests late- or post-Variscan compression. In an alternative interpretation, these straight and weak reflections represent the only thrust faults, while the curvilinear elements might relate to bedding. A southward rise of the Moho from approx. 11 to 8.5 s TWT is probably due to Tertiary rifting.