Transcarpathian Depression: Study of Low-Velocity Zones in the Earth’s Crust Based on the Seismic Regional Profiles Data

Abstract

Transcarpathian depression (TD) is located in the junction zone of the eastern margin of ALCAPA terrain and the northern part of Tisza-Dacia one, buried under thick Neogene molasses. The Earth’s crust structure of the TD is not clearly understood. The purpose of this work is to refine the TD crust structure and identify low-velocity zones by interpreting wave images obtained from WARR PANCAKE profile data using the finite-difference reflection/refraction migration method. The crustal domain beneath the TD is interpreted to be limited from southwest by a fault gently southwest dipping, traced from Pieniny Klippen Belt on the surface. Within domain two suture zones being the Alpine Tethys ocean remnants and cemented the European plate and ALCAPA microplate, were distinguished: Piemont-Liguria and PKB ones, as well as a rootless fragment of ALCAPA terrain. The Outer Carpathians thrust belt of 13 km depth borders the TD by subvertical Transcarpathian fault. The crust structure of the Pannonian segment is interpreted to be a pile of thick- and thin-skinned basement nappes of the Tisza terrain and cover nappes with superimposed younger extensional structures. High-reflectivity and low-velocity zone at depths of 10­—20 km is identified. The zone follows the pattern of isotherms in the temperature range of 300—500°. On the deep seismic sounding (DSS) Chop—Velykiy Bychkiv profile, running along TD and crossing the PANCAKE line, two low-velocity zones were also distinguished. Published data on numerical and physical modeling, the deep well cores, as well as fault zones in natural outcrops suggest that the low-velocity zones have increased porosity, fracturing, and fluid saturation. Our results suggest a high hydrocarbon potential of the TD, associated with the low-velocity zones.