Structure of the crust/mantle transition beneath the Variscan foreland in SW Poland from coincident wide-angle and near-vertical reflection data

Abstract

Lower crustal and Moho reflectivity vary significantly across the transition from Trans-European Suture Zone to the Variscan Orogen of Central Europe along the SUDETES 2003 S01 profile. One 50-km segment of the 270-km profile was a detailed reflection–refraction survey called GRUNDY 2003, in which the shot points were also recorded along the Polish part of the S01 profile to produce dense recordings of wide-angle reflections. Processed near-vertical incidence (NVI) data recorded along the GRUNDY 2003 G01 line revealed highly reflective, laminated lower crust starting at ca. 7 s TWT (20–23 km depth). Moho was inferred as the base of the crustal reflectivity at ca. 11.5–12 s (33–37 km depth). Wide-angle data recorded along the S01 profile, processed and stacked using CDP method with up to 41-fold, revealed reflectivity associated with the lower crust and the Moho in two areas: east of the Odra Fault zone and close to the Dolsk Fault. Moho boundary deduced from both the NVI and wide-angle stacks fits well to the “refraction” Moho constrained by the ray-tracing modelling and joint refraction/reflection tomography of P g and P mP arrivals performed along the S01 profile. Part of the Moho boundary imaged along the S01 profile (close to the Dolsk Fault zone) is proposed to represent a metamorphic front of lower crustal rocks partly undergone eclogite transformation. This observation is consistent with the proposed southward underthrusting of Baltica lower crust below the Avalonian acretionary prism. The postulated thin (up to 10 km) high-velocity eclogite layer overlying “normal” velocity mantle would also help to reconcile the apparent contradiction of a high heat-flow observed over the area with the high P n velocities.