The shape of the Variscan Belt in Central Europe: Strike-slip tectonics versus oroclinal bending (original) (raw)
Related papers
International Journal of Earth Sciences
Geophysical and geological data from the eastern sector of the Central European Variscan belt are presented and reviewed in the regional tectonic context. Matched filtering of isostatic gravity, guided by results of spectral analysis, along with other derivatives of gravity and magnetic fields reveal a dominant WNW-ESE-trending pre-Permian structural grain in the external zones of the Variscan belt in Poland. This trend is confirmed by regional distribution of dips in Carboniferous and Devonian strata that were penetrated by boreholes beneath Permian-Mesozoic sediments. Based on these data, two alternative concepts explaining the connection of the Variscan belt and its NE foreland, those of strike-slip tectonics versus oroclinal bending, are discussed. The WNW-ESE structural trend in the Variscan foreland is parallel to a set of major strike-slip fault zones in the area, including those of Upper Elbe, Intra-Sudetic, Odra, Dolsk and Kraków-Lubliniec. These faults are considered to convey a significant dextral displacement between Laurussia and Gondwana. The revised position of the Variscan deformation front shows a similar, uninterrupted, generally WNW-ESE trend, up to the SE border of Poland, which indicates an initial continuation of the Variscan belt into the area of the present-day Western Carpathians. The geometry of the Variscan deformation front along with the pattern of the Variscan structural grain are inconsistent with the idea of an oroclinal loop affecting the external, non-metamorphic Variscan belt. However, the data presented do not entirely rule out an oroclinal loop within the Variscan internides. The still possible options are (1) a semi-oroclinal model postulating ~ 90° bending of the Variscan tectonostratigraphic zones into parallelism with the WNW-ESE strike-slip faults or (2) an orocline limited only to the belt linking the Wolsztyn High and Moravo-Silesian non-to weakly-metamorphic fold-and-thrust belt. Regardless of the kinematic model preferred, our data indicate that structural evolution of the Polish Variscides was concluded with the end-Carboniferous NNE-SSW shortening that resulted in the present-day extent of the Variscan deformation front. magnetic field attenuates faster than the gravity field with increasing distance from a source, magnetic anomalies are more accurate at revealing shallow features located within the upper and middle crust, such as e.g., the top of magnetic basement, whereas the gravity field has a better potential to represent deeper crustal structures . Magnetic susceptibility of sedimentary rocks is one order of magnitude weaker than that of crystalline rocks, and, thus, sedimentary cover is practically transparent for the magnetic field, despite of possible local occurrences of intra-sedimentary volcanic rocks.
Geological Quarterly, 2006
The structure and evolution of the Polish part of the Variscan Orogenic Belt is reviewed, based on published data and interpretations. The Sudetic segment of the Variscides, together with adjacent areas, experienced multi-stage accretion during successive collisional events that followed the closure of different segments of the Rheic Ocean. In SW Poland, Variscan tectono-stratigraphic units are tectonically juxtaposed and often bear record of contrasting exhumation/cooling paths, constrained by palaeontological and geochronological data. This points to the collage-type tectonics of this area. A three-partite subdivision of the Sudetes is proposed that reflects timing differences in deformation and exhumation of the respective segments. The Central, West and East Sudetes were deformed and amalgamated during the Middle/Late Devonian, at the turn from the Devonian to Carboniferous and during Early Carboniferous times, respectively. Problems in extending the classical tectono-stratigrap...
Tectonophysics, 2002
The tectonic evolution of the region adjacent to the SW border of the East European craton is crucial for understanding the Palaeozoic amalgamation of Europe and its later evolution. We present a structural analysis of the brittle deformation (fault-slip data inversion) and folding carried out in the Cambrian to Cenozoic rocks of the European platform in SE Poland to characterize the Late Palaeozoic, Maastrichtian-Palaeocene and Miocene tectonics of this region. The Variscan (Late Carboniferous) palaeo-stress pattern was characterized by N-S to NNE-SSW-oriented compression, as inferred from the folding of the Palaeozoic (mainly Devonian) rocks. The compression was associated with the northwesterly (present-day coordinates) motion of Gondwana towards the northerly situated Laurussia. Subsequently, the Mid-Polish Trough (MPT) developed in an extensional regime. The stress regime changed to compressional at the Maastrichtian-Palaeocene transition in response to the Africa-Europe plate kinematics. The compressional regime induced the tectonic inversion of the Mid-Polish Trough. The stress pattern was characterized by NE-SW-oriented compression in the Holy Cross Mts. (HCM) area and in the Radomsko High. Folds and faults in the Mesozoic cover revealed a sinistral reactivation of the Holy Cross Fault (HCF) and of NW-SE-trending faults. The Alpine stress patterns were related to the subduction of the European platform beneath the Carpathian orogen. The flexural bending of the subducted lithosphere induced a homogeneous Middle Miocene N-S extension in the Carpathian foreland. Subsequently, the northward propagation of the Carpathian front resulted in late Middle Miocene N-S compression, restricted to the proximal area of the frontal thrusts, implied by a strain partitioning at a low friction contact between the Carpathian nappes and the European platform. Since the Palaeozoic, the Teisseyre-Tornquist Zone (TTZ) behaved as lithospheric weak zone, which has been reactivated during the Maastrichtian-Palaeocene change in stress regime. It resulted in the tectonic inversion of the Polish Trough. Inherited faults conditioned superficial stress perturbations and focused the deformation.
Geological Quarterly, 2013
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 mantIe. 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 northeast. Most important are two zoncs of this type: a southcrn 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).
Tectonophysics, 2017
Recently acquired seismic reflection data provide better insight in the structural style of extensive sedimentary series overlying the SW slope of the East European Craton (EEC) in Poland. The two main seismic datasets-the POLCRUST-01 profile and PolandSPAN surveyyielded contrasting thick-and thinskinned structural models for the same structural units in SE Poland. We reattempt an interpretation of the POLCRUST-01 profile using techniques of cross-section balancing and restoration aided by 2D forward seismic modelling. An outcome is the thinskinned structural model is. This solution relies on a continuous top of the EEC crystalline basement well represented in the seismic data as well as on fragmentary, yet conclusive seismic geometries in shallow depth intervals proving the Ediacaran-Palaeozoic series to be thrust and folded. A Variscan (late Carboniferous) compressional regime is consequently invoked to explain thinskinned structuring of the pre-Permian sedimentary pile and >20 km of calculated shortening. We demonstrate an ambiguous nature of the top
Geological Magazine, 1997
The still highly disputable terrane boundaries in the Sudetic segment of the Variscan belt mostly seem to follow major strike-slip faults and shear zones. Their kinematics, expected to place important constraints on the regional structural models, is discussed in some detail. The most conspicuous is the WNW-ESE Intra-Sudetic Fault Zone, separating several different structural units of the West Sudetes. It showed ductile dextral activity and, probably, displacement magnitude of the order of tens to hundreds kilometres, during late Devonian(?) to early Carboniferous times. In the late Carboniferous (to early Permian?), the sense of motion on the Intra-Sudetic Fault was reversed in a semi-brittle to brittle regime, with the leftlateral offset on the fault amounting to single kilometres. The north-south trending Niemcza and northeast-southwest Skrzynka shear zones are left-lateral, ductile features in the eastern part of the West Sudetes. Similarly oriented (northeast-southwest to NNE-SSW) regional size shear zones of as yet undetermined kinematics were discovered in boreholes under Cenozoic cover in the eastern part of the Sudetic foreland (the Niedźwiedź and Nysa-Brzeg shear zones)
Journal of Structural Geology, 2020
We provide a detailed description of a late Carboniferous fold-and-thrust belt in the Lublin Basin based on seismic lines, including new, high-resolution acquisitions, and legacy borehole and gravimetric data. A series of regional cross-sections integrate results of the joint seismic-borehole-gravimetric interpretation. Cross-section restoration provides estimates of the minimum incremental and total shortening and reveals its along-strike variations. The Lublin Basin straddles a leading edge of the Variscan orogen and its undeformed foreland; therefore, it experienced only a minor deformation. It preserves a fossilized record of early phases of shortening: seismic scale, divergent intra-formational thrusts and folds in horizontal strata that were later passively rotated by subsequent folds developed above a regional decollement. Lateral variations of deformation styles-from displacement of the entire basin with little internal deformation to dispersion of shortening by numerous contractional structures-is attributed to along-strike changes in lithology and thickness of the Silurian detachment horizon. Lateral change of detachment strength is quantified using the critical wedge theory. Our observations deliver a new perspective on factors controlling the localization of incipient structures, their differentiation as a function of detachment strength and interplay between folding and thrusting along a leading edge of an organic system.
Tectonophysics, 2016
The Late Paleozoic Variscan orogen in Europe is the result of convergence and collision between Laurussia and Gondwana during closure of the Rheic Ocean. The orogen is divided into tectonostratigraphic zones that have a distinct curvature (Ibero-Armorican Arc, IAA) and record the Late Cambrian-Early Ordovician opening of the Rheic Ocean, the migration of terranes from the Gondwanan margin towards Laurussia, as well as the closure of that ocean and development of the IAA. Three models have emerged to explain the distribution of tectonostratigraphic zones: (1) indentation tectonics due to collision with Laurussia by a (Ibero-Aquitanian) promontory of Gondwana during the Devonian; (2) development of the Cantabrian orocline at ca. 295 Ma in the inner core of the IAA, an interpretation supported by abundant paleomagnetic, structural and geochronological data; and (3) orogen-scale modification due to large-scale strike-slip shear zones. The indentor and orocline models (1 and 2) have been viewed as incompatible because the former requires curvature along the Gondwanan margin prior to Devonian collision with Laurussia, whereas the latter requires that the tectonostratigraphic zones were much more linear prior to bending at ca. 295 Ma. Recent data have rekindled the hypothesis that the Cantabrian Orocline is connected to a second orocline to the south (Central Iberian Orocline), highlighting the possibility that inner (Gondwanan) and outer (Laurussian) zones of the Ibero-Armorican arc may be structurally discordant with respect to each other, implying that the geographic limits of orocline formation are presently unclear. The two models can be reconciled if the geography of Gondwana's leading edge was irregular, but the tectonostratigraphic zones remained approximately linear within the inner zones of the putative Ibero-Aquitanian indentor and (ii) deformation associated with initial collision was largely accommodated by sinistral (SW Iberia) and dextral (Armorican Massif) motion along shear zones on either side of the promontory.