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Abstract

Orogenic wedges evolve by different mass transfer modes which control different styles of deformation, metamorphic pattern, and bulk reflectivity. Structural patterns and reflectivity strongly vary depending mainly on the length/thickness ratio of thrust sheets and on their displacement/length ratio. Reevaluation of DEKORP seismic sections across the central European Variscides (lines DEKORP 1A-C, 9N, 2N-2S, 3B) underlines the usefulness of the integrated interpretation strategy in establishing orogenic growth mode and evolution. The results are in contrast with reflectivity patterns predicted from critical taper theory. The northern flank of the Mid-European Variscides is formed by a doubly vergent orogenic wedge system which evolved from collision of a passive margin (Rhenohercynian zone) with a continental arc (Mid-German Crystalline Rise, MGCR, of the Saxothuringian zone). The two fold and thrust wedges show inward-dipping shear zone-related reflections above subhorizontal mid-crustal reflection bands, evidence of dominantly frontal accretion of upper crustal material in both wedges. In contrast, the sections crossing the axial zone (MGCR) either show arched reflection stacks related to underplated amphibolite-facies rocks in exposed antiformal nappe stacks, or a transparent wedge-shaped unit related to low-grade calcalkaline magmatics and steeply dipping Devono-Carboniferous sediments of the former upper-plate magmatic arc. Analysis suggests that part of this arc was tectonically eroded prior to collision. In combination with isograd and structural patterns the observed features indicate that heterogeneous basal accretion was the dominant accretion mode in the axial zone during collision. The discussed patterns are suggested to reflect the material transfer site into the orogenic wedge and therefore also growth mode and material provenance in collisional belts.