Ulf Bayer

The Elbe Fault System in North Central Europe—a basement controlled zone of crustal weakness

Abstract The Elbe Fault System (EFS) is a WNW-striking zone extending from the southeastern North Sea to southwestern Poland along the present southern margin of the North German Basin and the northern margin of the Sudetes Mountains. Although details are still under debate, geological and geophysical data reveal that upper crustal deformation along the Elbe Fault System has taken place repeatedly since Late Carboniferous times with changing kinematic activity in response to variation in the stress regime. In Late Carboniferous to early Permian times, the Elbe Fault System was part of a post-Variscan wrench fault system and acted as the southern boundary fault during the formation of the Permian Basins along the Trans-European Suture Zone (sensu [Geol. Mag. 134 (5) (1997) 585]). The Teisseyre–Tornquist Zone (TTZ) most probably provided the northern counterpart in a pull-apart scenario at that time. Further strain localisation took place during late Mesozoic transtension, when local shear within the Elbe Fault System caused subsidence and basin formation along and parallel to the fault system. The most intense deformation took place along the system during late Cretaceous–early Cenozoic time, when the Elbe Fault System responded to regional compression with up to 4 km of uplift and formation of internal flexural highs. Compressional deformation continued during early Cenozoic time and actually may be ongoing. The upper crust of the Elbe Fault System, which itself reacted in a more or less ductile fashion, is underlain by a lower crust characterised by low P-wave velocities, low densities and a weak rheology. Structural, seismic and gravimetric data as well as rheology models support the assumption that a weak, stress-sensitive zone in the lower crust is the reason for the high mobility of the area and repeated strain localisation along the Elbe Fault System.

Crustal-scale pop-up structure in cratonic lithosphere: DOBRE deep seismic reflection study of the Donbas fold belt, Ukraine

The DOBRE project investigated the interplay of geologic and geodynamic processes that controlled the evolution of the Donbas fold belt, Ukraine, as an example of an inverted intracratonic rift basin. A deep seismic reflection profile provides an excellent image of the structure of the Donbas fold belt, which is the uplifted and compressionally deformed part of the late Paleozoic Pripyat-Dniepr-Donets basin. Both the effects of rifting and those of later structural inversion are recognized in the seismic and geologic data. The interpretation of the reflection data shows that the inversion of the Donbas fold belt occurred at the crustal scale as a mega-pop-up, which involved a major detachment fault through the entire crust and an associated back thrust. The DOBREflection image provides a simple concept of intracratortic basin inversion, the crustal pop-up being uplifted and internally deformed. The association of such a structure with inverted intracratonic basins such as the Donbas fold belt implies brittle deformation of relatively cold crust.

Salt movements in the Northeast German Basin and its relation to major post-Permian tectonic phases—results from 3D structural modelling, backstripping and reflection seismic data

Abstract The NW–SE-striking Northeast German Basin (NEGB) forms part of the Southern Permian Basin and contains up to 8 km of Permian to Cenozoic deposits. During its polyphase evolution, mobilization of the Zechstein salt layer resulted in a complex structural configuration with thin-skinned deformation in the basin and thick-skinned deformation at the basin margins. We investigated the role of salt as a decoupling horizon between its substratum and its cover during the Mesozoic deformation by integration of 3D structural modelling, backstripping and seismic interpretation. Our results suggest that periods of Mesozoic salt movement correlate temporally with changes of the regional stress field structures. Post-depositional salt mobilisation was weakest in the area of highest initial salt thickness and thickest overburden. This also indicates that regional tectonics is responsible for the initiation of salt movements rather than stratigraphic density inversion. Salt movement mainly took place in post-Muschelkalk times. The onset of salt diapirism with the formation of N–S-oriented rim synclines in Late Triassic was synchronous with the development of the NNE–SSW-striking Rheinsberg Trough due to regional E–W extension. In the Middle and Late Jurassic, uplift affected the northern part of the basin and may have induced south-directed gravity gliding in the salt layer. In the southern part, deposition continued in the Early Cretaceous. However, rotation of salt rim synclines axes to NW–SE as well as accelerated rim syncline subsidence near the NW–SE-striking Gardelegen Fault at the southern basin margin indicates a change from E–W extension to a tectonic regime favoring the activation of NW–SE-oriented structural elements. During the Late Cretaceous–Earliest Cenozoic, diapirism was associated with regional N–S compression and progressed further north and west. The Mesozoic interval was folded with the formation of WNW-trending salt-cored anticlines parallel to inversion structures and to differentially uplifted blocks. Late Cretaceous–Early Cenozoic compression caused partial inversion of older rim synclines and reverse reactivation of some Late Triassic to Jurassic normal faults in the salt cover. Subsequent uplift and erosion affected the pre-Cenozoic layers in the entire basin. In the Cenozoic, a last phase of salt tectonic deformation was associated with regional subsidence of the basin. Diapirism of the maturest pre-Cenozoic salt structures continued with some Cenozoic rim synclines overstepping older structures. The difference between the structural wavelength of the tighter folded Mesozoic interval and the wider Cenozoic structures indicates different tectonic regimes in Late Cretaceous and Cenozoic. We suggest that horizontal strain propagation in the brittle salt cover was accommodated by viscous flow in the decoupling salt layer and thus salt motion passively balanced Late Triassic extension as well as parts of Late Cretaceous–Early Tertiary compression.

Salt redistribution during extension and inversion inferred from 3D backstripping

Abstract A 3D backstripping approach considering salt flow as a consequence of spatially changing overburden load distribution, isostatic rebound and sedimentary compaction for each backstripping step is used to reconstruct the subsidence history in the Northeast German Basin. The method allows to determine basin subsidence and the salt-related deformation during Late Cretaceous–Early Cenozoic inversion and during Late Triassic–Jurassic extension. In the Northeast German Basin, the deformation is thin-skinned in the basinal part, but thick-skinned at the basin margins. The salt cover is deformed due to Late Triassic–Jurassic extension and Late Cretaceous–Early Cenozoic inversion whereas the salt basement remained largely stable in the basin area. In contrast, the basin margins suffered strong deformation especially during Late Cretaceous–Early Cenozoic inversion. As a main question, we address the role of salt during the thin-skinned extension and inversion of the basin. In our modelling approach, we assume that the salt behaves like a viscous fluid on the geological time-scale, that salt and overburden are in hydrostatical near-equilibrium at all times, and that the volume of salt is constant. Because the basement of the salt is not deformed due to decoupling in the basin area, we consider the base of the salt as a reference surface, where the load pressure must be equilibrated. Our results indicate that major salt movements took place during Late Triassic to Jurassic E–W directed extension and during Late Cretaceous–Early Cenozoic NNE–SSW directed compression. Moreover, the study outcome suggests that horizontal strain propagation in the salt cover could have triggered passive salt movements which balanced the cover deformation by viscous flow. In the Late Triassic, strain transfer from the large graben systems in West Central Europe to the east could have caused the subsidence of the Rheinsberg Trough above the salt layer. In this context, the effective regional stress did not exceed the yield strength of the basement below the Rheinsberg Trough, but was high enough to provoke deformation of the viscous salt layer and its cover. During the Late Cretaceous–Early Cenozoic phase of inversion, horizontal strain propagation from the southern basin margin into the basin can explain the intensive thin-skinned compressive deformation of the salt cover in the basin. The thick-skinned compressive deformation along the southern basin margin may have propagated into the salt cover of the basin where the resulting folding again was balanced by viscous salt flow into the anticlines of folds. The huge vertical offset of the pre-Zechstein basement along the southern basin margin and the amount of shortening in the folded salt cover of the basin indicate that the tectonic forces responsible for this inversion event have been of a considerable magnitude.

Origin of the regional stress in the North German basin: results from numerical modelling

Abstract We use a thin sheet approach to investigate the effects induced by the Alpine collision on the deformation and regional stress in northern Europe, with special emphasis on the NE German Basin. Here new seismic crustal studies indicate a flexural-type basin, which may have been induced by compressive forces transmitted from the south, due to the Alpine orogeny. Finite-element techniques are used to solve the equations for the deformation of a continuum described by a linear creep rheology and a spatial resolution of about 0.5°. The model has been constrained by stress and seismic data. We show that a relatively strong lithosphere below the northern margin of the German Basin, at the transition with the Baltic Shield, may explain the characteristic regional stress field, in particular the fan-like pattern which is observed within the region. Furthermore, the predicted strain rate pattern resembles the seismically recognizable undeformed area of the North German Basin.

The Central European Basin System – an Overview

The geological knowledge of the Central European Basin system (CEBS, Fig. 2.1) is founded on more than 1,000 years of industrially exploring and exploiting mineral resources therein. At its southern rim within the Harz Mountains mining of Palaeozoic rocks for silver and other metals is well reported for at least one millennium, terminating the prospering Viking silver trade from Southeast Europe which is seen as one of the causes for the decline of the Viking power.

Late Cretaceous–Cenozoic evolution of the North German Basin—results from 3-D geodynamic modelling

Abstract The Late Cretaceous–Cenozoic evolution of the North German Basin has been investigated by 3-D thermomechanical finite element modelling. The model solves the equations of motion of an elasto-visco-plastic continuum representing the continental lithosphere. It includes the variations of stress in time and space, the thermal evolution, surface processes and variations in global sea level. The North German Basin became inverted in the Late Cretaceous–Early Cenozoic. The inversion was most intense in the southern part of the basin, i.e. in the Lower Saxony Basin, the Flechtingen High and the Harz. The lower crustal properties vary across the North German Basin. North of the Elbe Line, the lower crust is dense and has high seismic velocity compared to the lower crust south of the Elbe Line. The lower crust with high density and high velocity is assumed to be strong. Lateral variations in lithospheric strength also arise from lateral variations in Moho depth. In areas where the Moho is deep, the upper mantle is warm and the lithosphere is thereby relatively weak. Compression of the lithosphere causes shortening, thickening and surface uplift of relatively weak areas. Tectonic inversion occurs as zones of preexisting weakness are shortened and thickened in compression. Contemporaneously, the margins of the weak zone subside. Cenozoic subsidence of the northern part of the North German Basin is explained as a combination of thermal subsidence and a small amount of deformation and surface uplift during compression of the stronger crust in the north. The modelled deformation patterns and resulting sediment isopachs correlate with observations from the area. This verifies the usefulness and importance of thermomechanical models in the investigation of intraplate sedimentary basin formation.

Tertiary and Cretaceous paleoenvironments in the southwest Atlantic Ocean: Preliminary results of Deep Sea Drilling Project Leg 71

DSDP Leg 71 in the South Atlantic confirmed that the early opening (Neocomian-Aptian) of the South Atlantic was marked with restrictive circulation in which shale units high in organic carbon (1.7% to 4.1%) were deposited. Ratios of gaseous hydrocarbons and pyrolysis-fluorescence analyses suggest a fairly high degree of maturity of the black shale. A comparatively complete Cretaceous section provides biostratigraphic reference for the South Atlantic. Major erosion occurred at or near the Tertiary-Cretaceous boundary prior to the opening of the Drake Passage (Oligocene–middle Miocene). The early Tertiary was marked by mild climatic conditions and periods of exceptionally rapid sediment accumulation (as much as 44 m/m.y.) separated by hiatuses or condensed intervals. Paleomagnetic measurements recognize the Brunhes and Matuyama (with Jaramillo and Olduvai events), Gauss (with Kaena and Mammoth events), and Gilbert (with Cochiti event) Epochs. Correlation of this paleomagnetic scale with siliceous microfossil zonations was accomplished. Siliceous and calcareous microfossils reveal pronounced fluctuations of the Polar Front in the Pliocene and Pleistocene. Warm intervals occurred in the late Gilbert and middle Guass Epochs; cooler conditions were prevalent in the late Gilbert-early Gauss. The late Pliocene was marked by climatic deterioration with brief warmings in the uppermost Matuyama and upper Brunhes. Sedimentation rates dropped markedly from 180 m/m.y. in the early Pliocene (Gilbert Epoch) to 2.3 m/m.y. in the Pleistocene (early Brunhes).

Regional-scale structural role of Permian salt within the Central European Basin System

Abstract The Central European Basin System (CEBS) includes the former Northern and Southern Permian Basins together with superimposed Meso-Cenozoic sub-basins and contains a thick layer of Upper Permian (Zechstein) salt. This salt was mobilized in response to several post-Permian tectonic events. In order to analyse the regional relationship between the structural pattern of the Meso-Cenozoic sedimentary cover and the distribution of the Upper Permian salt, a 3D structural model of the CEBS has been constructed. In this model, the Permian salt is resolved as an extra layer for the entire basin system. According to the 3D structural model, the salt layer is strongly deformed as a result of halokinetic activity. The thickest salt is localized within salt walls and diapirs, reaching up to 9 km of thickness. A regional structural 3D analysis of the overburden in relation to underlying ductile salt demonstrates that the geometry of the sedimentary cover is strongly complicated by a variety of salt structures. The withdrawal of the Permian salt appears to have played a key role in both deposition and deformation of Meso-Cenozoic deposits in addition to tectonically forced regional subsidence.

Characteristics of Complex Intracontinental Sedimentary Basins

The term “basin” is used in different sciences such as physics, chemistry, mathematics, and the geosciences. It can be generally defined as a low, attracting some process coming to rest at the deepest point or the point of minimal energy. A complicated or complex basin then is a potential surface consisting of several minima or basins separated by saddles. A challenging problem is to find out, which minima will be approached (Despa and Berry 2001; Reddy and Pratap 2003; Van Siclen 2007).