Variscides in the Sudetes and the reworked Cadomian orogen: evidence from the GB-2A seismic reflection profiling in southwestern Poland

Abstract

The GB-2A profile shot perpendicular to major fault zones in SW Poland gave first seismic reflection insight, integrated with gravimetric and magnetic ones, into the crustal structure of the NE Bohemian Massif, eastern part of the Variscan belt. Under the West Sudetes there is a domal stack of well reflective, relatively dense, lower crustal rocks, with the Moho easily identifiable at the base of the laminated lower crust. Much poorer reflectors occur in the crust under the Fore-Sudetic Block (FSB)and Fore-Sudetic Monocline (FSM) further NE, with the Moho located in a c. 10 km thick transitional zone between crust and the upper mantle. The wedge-layered internal structure may imply crustal subduction or delamination with the northern block (terrane) pushed over and under the southern one, probably as early as during the Cadomian orogeny which exposed c. 680-540 Ma granodiorites at the surface. The main crustal suture of the A-subduction type is located beneath the Gory Kaczawskie Mts. This domal structure, with particularly well defined NE slope, is a real feature as confirmed also by gravimetric and magnetic modelling. The entire feature probably represents a Cadomian compressional event, then repeated during Variscan times, after Early Palaeozoic crustal extension. Alternatively, the crustal bulge under the Sudetes may represent a suture of two Cadomian terranes. The northern one would be compatible with rifted-off segments of proto-Baltica continental plate. The upper crust is composed of several wedge-like crustal blocks bounded by listric faults dipping generally to the north or north-east. Most important are two zones of this type: a southern zone, coinciding with the Main Intra-Sudetic Fault (MIF) and northern zone, the most important one, corresponding to the Sudetic Marginal Fault (SMF). Their geological history consists of repeated extensional-compressional deformations of a continental crust, accomplished in a dip-slip to strike-slip regime. From Carboniferous times extensional deformation seems to dominate. The Odra Fault Zone (OFZ) is identifiable only by magnetic modelling and cannot be taken as an eastern continuation of the Mid-German Crystalline Rise (MGCR).