Lower Rhine Embayment Research Articles

Role of Charcoal Analysis for Interpreting Vegetation Change and Paleoclimate in the Miocene Rhine Embayment (Germany)

The potential of fossil wood and charcoal from browncoal deposits as sources of reliable paleoenvironmental information is explored with material from the Lower Rhine Embayment (Germany). The presence of charcoalified material demonstrates proof of natural wildfires in Tertiary mire environments, most probably during, or after periods of increased drainage and drying of surface vegetation and litter. The results presented suggest that sampling from charcoal layers may provide a more statistically reliable data set for study of such environments. Inclusion of taxa recovered from charcoal layers might compensate for the taphonomic and preservational bias of Tertiary lignitic floras based solely on the collection of lignitic wood. These data confirm the hypothesis that, during certain intervals of the Miocene, both monocotyledonous and dicotyledonous angiosperms might have dominated or represented significant portions of the peat-forming vegetation. The importance of palms and shrubby woody taxa, such as ‘Cyrilla’, is particularly striking from new evidence of charcoalified remains. In addition, certain wood anatomical features observed from well preserved lignitic wood and charcoal may be used as indicators of environmentally modulated growth: (1) clear growth rings testify to the existence of a seasonal climate; (2) wide variations in growth ring characters indicate variable environmental conditions; and (3) high incidence of dicotyledonous taxa, with abundant small vessels and scalariform perforation plates, is interpreted as evidence of a mesic environment.

Recent movement pattern of the Lower Rhine Embayment from tilt, gravity and GPS data

AbstractAs part of the activities of the Collaborative Research Centre ‘SFB 350’, measurements of geodetic and geodynamic changes in the area of the Lower Rhine Embayment and the Rhenish Shield are being performed at different scales in space and time. Continuous borehole tilt measurements and repeated microgravimetric surveys yield information on the local stability of the ground and changes in horizontal gravity gradients that are both dominated by seasonal fluctuations. Results of more than seven years of regular GPS campaigns are discussed in terms of vertical and horizontal point motions. The most prominent motions are man-induced effects occurring in or near the browncoal mining areas, where groundwater withdrawal produces subsidence of up to 2.2 cm/y in the area under investigation. Horizontal and vertical motions at other GPS points are smaller by one order of magnitude and in most cases are only marginally detectable. The eastward motion of two points in the Bergisches Land and the westward motion of two points in the Eifel near the Belgian border may be interpreted as a result of the ongoing extension of the Cenozoic rift system in the western part of the Eurasian plate.

Paleoseismological Investigations at the Rurrand Fault, Lower Rhine Embayment

AbstractFrom 1998 to 2000, we have studied the evidence for large paleoearthquakes at the Rurrand Fault. This fault represents the eastern border of the Roer Valley Graben, which is the tectonically most active region in the Lower Rhine Embayment. The purpose of our paleoseismological studies is to enlarge the seismicity data base for this region beyond instrumental records and historical reports using indications of surface-faulting events from stratigraphie conditions at active faults. Larger time spans considered in the earthquake catalogue will enable a more reliable statistical analysis which is required for seismic hazard assessment. Based on analyses of geological data and géomorphologie investigations, detailed geophysical surveying was carried out along the southern Rurrand Fault segment for the selection of a site appropriate to paleoseismological studies. Mapping of physical parameter contrasts with seismic reflection, VES, ERT, and GPR measurements along fault-crossing profiles inferred position and near-surface structure of the fault. At the site promising the best conditions, a trench was excavated across the fault near the city of Jiilich, Germany. Within a depth of about 4 m, the Rurrand Fault was exposed in an about 50 m-wide system of faults and fault zones, affecting the stratigraphie sequence with various displacement characteristics and amounts of throw. According to heavy mineral analyses, the deposition time of most the exposed sediment strata was assigned to Pliocene and Lower Pleistocene time. These geological units are covered by loess layers deposited through so-lifluction processes during the Weichselian glacial, i.e. some tens of ka B.P., or – with lower probability – during the Saalian glacial. Several faults which had also affected the loess reflect younger fault activity. However, clear paleoseismic features were not observed in the trench, thus an unambiguous proof of the occurrence of coseismic fault displacements could not be furnished. Recently, differential subsidence due to drainage takes place in the surroundings of the nearby opencast mining. An amount of some 0.35 m, concentrated in a very narrow lateral zone, has been observed during the last 40 a at about 1 km distance from the trench position. To date, the subsidence could not be clearly located in the trench exposure. Results from geodetic levelling campaigns will help to determine the offset residuals and to gain better insight into the ruling displacement processes at the Rurrand Fault.

Morphometric analysis of active normal faulting in slow-deformation areas : examples in the Lower Rhine Embayment

AbstractWe studied the applicability of classical scarp degradation modelling to active normal faults in the Lower Rhine Embayment. Our quantitative analysis was conducted on the frontal Bree fault scarp (Feldbiss fault) in Belgium and the Peel fault scarp near the city of Neer in the Netherlands. Vertical offset and diffusion age of these scarps have been modelled from elevation profiles across the studied faults using the diffusion equation. For that purpose, a computer-program (profil 2000) has been written, providing a sensitivity analysis of the determined parameters in function of the spatial repartition of the elevation measurements along the considered profiles. The results of this morphometric analysis have been validated by a comparison with the geologic record of the tectonic activity observed in the trenches excavated at the sites where the measurements have been conducted.We conclude that the modelling can only be applied to study tectonic activity since the Last Glacial Maximum (±14-19 kyr BP) because the surface expression of older paleoearthquakes in unconsolidated Late Pleistocene sediments has been erased by the strong erosive phase that occurred at the end of this glacial period. Even for Holocene scarps, morphologic dating seems very difficult because man-made perturbations destroyed surface evidence of the very recent fault activity in many sites. Nevertheless, we estimate that an appropriate value for the mass diffusivity constant for~ 1-m-high scarps in the investigated region is 0.002 to 0.010 m2/yr. On the other hand, vertical offsets can be determined with a good precision. These amount to respectively ~1 m and 1,3 m since the Last Glacial Maximum on the Feldbiss fault in Belgium and the Peel fault near Roermond in the Netherlands.

Local seasonal gravity changes in the lower rhine embayment, Germany

Repeated high resolution gravity measurements with LaCoste and Romberg gravity meters have been carried out for nearly two years in the Lower Rhine Embayment, Germany. In the long run, the observations aim at detecting long term gravity changes that are associated with surface subsidence due to groundwater withdrawal in a nearby browncoal mining area. Concurrently, short term gravity variations on local scales are also investigated to find out their influence on long term gravity observations. For that reason, two local networks with extensions of a few kilometers each have been set up, one of them across an active fault where presently large height changes are occurring (up to 2.2 cm/a), the other one in a stable intra-block site showing no changes of height. The gravity data, so far, do not reveal any significant long term changes. However, seasonal variations of up to 500 nms−2 are observed between stations on both sides of a steeply dipping tectonic fault. This is where significant height changes due to ground water withdrawal occur, while gravity differences between stations on the same side of that fault as well as the observations in the other network remain constant throughout the observation period. The physical reason for the gravity variations is not yet clear. We assume, that different types of vegetation and soil cause seasonal changes in soil moisture, and thus in subsurface density. This study emphasizes that stations for repeated high precision gravity observations (aiming e. g. to detect seismogenic precursory anomalies) may be largely influenced by local, effects of presumably hydrological origin and that networks even on the 100 m scale can considerably suffer from systematic distortions.

Point stability at shallow depths: experience from tilt measurements in the Lower Rhine Embayment, Germany, and implications for high-resolution GPS and gravity recordings

SUMMARY From 1996 to 1999, we have studied ground tilts at depths of between 2 m and 5 m at three sites in the Lower Rhine Embayment (LRE), western Germany. The LRE is a tectonically active extensional sedimentary basin roughly 50 kmr100 km. The purpose of the tilt measurements was (a) to provide insight into the magnitude, nature and variability of background tilts and (b) to assess possible limitations of high-resolution GPS campaigns and microgravity surveys due to natural ground deformation. The tilt readings, sensed by biaxial borehole tiltmeters of baselength 0.85 m, cover a frequency range from 10 x8 Hz to 10 x2 Hz (periods from minutes to years). Assuming that the tilt signals represent ground displacements on a scale typically not larger than several times the tiltmeters’ baselength, and that tilt signals at shallow depth could in a simple geometric way be related to changes in surface elevation and gravity, we try to estimate the magnitude level of point movements and corresponding Bouguer gravity effects that is generally not surpassed. The largest tilt signals observed were some t50 mrad yr x1 .I f they were observable over a ground section of extension, e.g. 10 m, the converted rates may correspond to about t0.5 mm per 10 m yr x1 in vertical ground displacement, and t0.1 mgal yr x1 in Bouguer gravity effect, respectively. Large signals are mostly related to seasonal effects, probably linked to thermomechanical strain. Other causes of ground deformation identified include seepage effects after rainfalls (order of t10 mrad) and diurnal strains due to thermal heating and/or fluctuations in the water consumption of nearby trees (order of t1 mrad). Episodic step-like tilt anomalies with amplitudes up to 22 mrad at one of the observation sites might reflect creep events associated to a nearby active fault. Except for short-term ground deformation caused by the passage of seismic waves from distant earthquakes, amplitudes of non-identified tilt signals in the studied frequency range seem not to exceed t2 mrad. As the larger tilt signals are close to the precision achieved with modern GPS systems and superconducting gravimeters when converted into height and gravity changes, further enhancement in resolution of these techniques may require simultaneous recording of local ground deformation at the observation sites.

SENSITIVITY OF PARAMETERS FOR PROBABILISTIC SEISMIC HAZARD ANALYSIS USING A LOGIC TREE APPROACH

The sensitivity of different parameters used in probabilistic seismic hazard calculation is investigated by different logic tree runs with alternative magnitude sets, source zone models and attenuation relations, and with different sets of values for the seismicity parameters and the <r-value. Also the influence from the different parameters on the hazard uncertainty, represented by fractiles, is investigated. The calculations are made for peak ground acceleration at a site near Aachen in the Lower Rhine Embayment. The model where the site is located in a larger source zone gives lower hazard values. This is typical for the case where the seismicity near the site is high relative to its surrounding. The hazard curves for the different attenuation functions are similar, an effect of the similarity of the functions themselves. However, a large sensitivity of this parameter is indicated for small mean return periods. An increased α-value implies a moderate increase of the hazard at long mean return periods. The hazard is increasing for decreased focal depth, decreased β-value and increased maximum expected magnitude, respectively. However, the effects are noteworthy only at low hazard levels for variations in the focal depth and to some extent in the maximum expected magnitude. Finally, decreasing the minimum magnitude thought to be of engineering relevance causes a drastic increase of the hazard at small mean return periods.

Clariant reports strong sales growth in 2000

We present a comprehensive statistical analysis of petrophysical properties of rocks of the northeastern Rhenish Massif and the Lower Rhine Embayment in Germany. Properties measured comprise thermal conductivity, specific heat capacity density, porosity, hydraulic permeability, and compressional wave velocity. This robust and statistically reliable data is generally useful for numerical modeling of heat transport processes and helps, in particular, reducing the risk of failure in projects of geothermal energy use. We measured the thermophysical properties of rocks from two geological settings: (1) predominantly little consolidated Tertiary rocks forming the young sedimentary cover of the Lower Rhine Embayment in the condition they arrived in the laboratory; (2) well consolidated Paleozoic rocks from the northeastern Rhenish Massif in both dry and saturated condition. We tested a total of 476 samples from different lithologies in both settings in a comprehensive laboratory program consisting of mineralogical analyses and various petro- and thermophysical measurements at ambient and elevated p-T-conditions. This yields relations between composition and thermophysical properties of different sedimentary rock types and allows distinguishing between effects due to rock matrix and structure. The results are used to prove petrophysical rock models and allow predicting thermal properties of distinct rock types for greater depth. The results show that the thermophysical properties of Paleozoic rocks are mainly controlled by their mineralogical compositions, while thermophysical characteristics of Tertiary rocks are the result of a superposition of properties of their mineral content and the water-filled pore volume.

Methane exchange between coal-bearing basins and the atmosphere: the Ruhr Basin and the Lower Rhine Embayment, Germany

A precise knowledge of methane exchange processes is required to fully understand the recent rise of atmospheric methane concentration. Three of these processes take place at the lithosphere/atmosphere boundary: bacterial consumption of methane and emission of bacterial or thermogenic methane. This study was initiated to quantify these processes on a regional scale in the Ruhr Basin and the Lower Rhine Embayment. Since these areas are subject to bituminous coal and lignite mining, natural and anthropogenically-induced methane exchange processes could be studied. The methane emission and consumption rates and their carbon isotope signal were measured at the lithosphere/atmosphere boundary using flux chambers. On most of the soils studied, methane consumption by bacteria was identified. Thermogenic methane was released only at some of the natural faults examined. In active and abandoned bituminous coal mining areas methane emissions were restricted to small areas, where high emission rates were measured. The carbon isotope composition of methane at natural faults and in mining subsidence troughs was typical of thermogenic methane (−45 to −32 ‰ δ13C). Methane exchange balancing revealed that natural methane emissions from these two basins represent no source of atmospheric importance. However, methane release by upcast mining shafts dominates the methane exchange processes and is by about two orders of magnitude greater than methane consumption by bacterial oxidation in the soils.

Long-term seismicity in regions of present day low seismic activity: the example of western Europe

In western Europe, the knowledge of long-term seismicity is based on reliable historical seismicity and covers a time period of less than 700 years. Despite the fact that the seismic activity is considered as low in the region extending from the Lower Rhine Embayment to England, historical information collected recently suggests the occurrence of three earthquakes with magnitude around 6.0 or greater. These events are a source of information for the engineer or the scientist involved in mitigation against large earthquakes. We provide information relevant to this aspect for the Belgian earthquake of September 18, 1692. The severity of the damage described in original sources indicates that its epicentral intensity could be IX (EMS-98 scale) and that the area with intensity VII and greater than VII has at least a mean radius of 45 km. Following relationships between average macroseismic radii and magnitude for earthquakes in stable continental regions, its magnitude M s is estimated as between 6.0 and 6.5. To extend in time our knowledge of the seismic activity, we conducted paleoseismic investigations in the Roer Graben to address the question of the possible occurrence of large earthquakes with coseismic surface ruptures. Our study along the Feldbiss fault (the western border of the graben) demonstrates its recent activity and provides numerous lines of evidence of Holocene and Late Pleistocene large earthquakes. It suggests that along the 10 km long Bree fault scarp, the return period for earthquakes with magnitude from 6.2 to 6.7 ranges from 10,000 to 20,000 years during the last 50,000 years. Considering as possible the occurrence of similar earthquakes along all the Quaternary faults in the Lower Rhine Embayment, a large earthquake could occur there each 500–1000 years. These results are important in two ways. (i) The evidence that large earthquakes occur in western Europe in the very recent past which is not only attested by historical sources, but also suggested by paleoseismic investigations in the Roer Graben. (ii) The existence of a scientific basis to better evaluate the long-term seismicity in this part of Europe (maximal magnitude and return period) in the framework of seismic hazard assessment.

A review of charcoal analysis as a tool for assessing Quaternary and Tertiary environments: achievements and limits

Identification of charcoal fragments (≥2 mm) using reflected light microscopy is routinely carried out to study former natural ecosystems, assess changes in vegetation communities over time and identify the influence and patterns of behaviour of human populations on vegetation. Material studied comes either from archaeological horizons or soil profiles. Despite undeniable limits, informative results have been obtained in the last 25 years, mostly in southwestern Europe, where charcoal analysis has been widely applied in Quaternary studies. Examples of these achievements are provided, focusing on the major vegetation trends since the Upper Palaeolithic to the Middle Ages. The techniques used for charcoal analysis in the reconstruction of Quaternary vegetation are now being used to study Tertiary charcoal and lignites. The study of fossil charcoal can be a highly informative method for interpreting ancient ecosystems as species easily degraded may be preserved by charcoalification. Some results from the Lower Rhine Embayment (Germany) are presented here.

Die erste Wirbeltierfauna aus der miozänen Braunkohle der Niederrheinischen Bucht (Ville-Schichten, Tagebau Hambach)

This paper describes the first Miocene vertebrate fauna from the Ville Series of the Lower Rhine Embayment in western Germany. The fossil material was discovered in the vast Hambach open cast lignite mine ofRheinbraun. The numerous remains, mostly isolated teeth and disarticulated bones, were accumulated in a channel-fill within the main lignite seam. The fauna consists of chondrichthyans, teleosteans, amphibians, reptiles, and mammals which inhabited brackish-estuarine, lacustrine-fluviatile, and terrestrial environments. This is the first fauna to provide an insight into the diversity of vertebrates which lived in the forested peat swamps of the Rhenish Tertiary. Based on the rich association of more than 70 mammalian taxa, including about 30 rodent species, this late Orleanian fauna can be correlated with the upper part of the Mammalian Neogene biozone MN 5. Therefore the middle part of the main lignite seam (horizon 6C of the local lithostratigraphy) can be placed in the early middle Miocene. This means an age of 15.2-16.0 Ma (Langhian or Reinbekian of the stratigraphy of the NW German Tertiary). For some mammalian taxa (Lanthanotherium, Plesiosorex,Miopetaurista, Myoglis, Fahlbuschia, ‘Lartetomys’, Anchitheriomys, Dorcatherium), Hambach 6C is one of the richest European localities. In addition this assemblage contains some very rare mammals (‘Lartetomys’, Anchitheriomys, Orygotherium, Pliopithecus). The Hambach vertebrates are of great palaeobiogeographic importance as they represent the north westernmost outpost of terrestrial Miocene faunas in Europe. Die hier vorgestellte Vertebratenfauna aus den miozanen Ville-Schichten der Niederrheinischen Bucht gibt erstmals einen Einblick in den Artenreichtum der Tierwelt in den Braunkohle-Sumpfwaldern des rheinischen Tertiars. Die zahlreichen zusammengeschwemmten, disartikulierten Reste, vor allem Zahne und Knochen, stammen aus einer in das Rheinische Hauptfloz eingeschnittenen Flusrinne im Tagebau Hambach derRheinbraun AG. Die Fauna umfast Fische, Amphibien, Reptilien und Saugetiere aus brackisch-astuarinen, limnisch-fluviatilen und terrestrischen Habitaten. Die mit uber 70 Spezies, darunter ca. 30 Nagetierarten, sehr reiche Mammalierassoziation reprasentiert eine spatorleanische Fauna, die in den oberen Teil der Saugerzone MN 5 eingestuft wird. Damit kann fur den mittleren Abschnitt des Hauptflozes (Schicht 6C) erstmals ein biostratigraphisch begrundetes, fruhmittelmiozanes Bildungsalter von ca. 15.2-16.0 Ma (Langh bzw. Reinbek nach der NW-deutschen Tertiar-Gliederung) angegeben werden. Fur einige Mammalier (Lanthanotherium, Plesiosorex,Miopetaurista, Myoglis, Fahlbuschia, ‘Lartetomys’, Anchitheriomys, Dorcatherium) ist Hambach 6C eines der reichsten Vorkommen. Der Nachweis von sonst z.T. extrem seltenen Saugertaxa (‘Lartetomys’, Anchitheriomys, Orygotherium, Pliopithecus) ist auch palaobiogeographisch von Bedeutung, da es sich um die nordwestlichste Fundstelle im terrestrischen Miozan Europas handelt.

Charophyten aus dem oberpliozänen Reuverton des Braunkohlen-Tagebaus Hambach (Niederrheinische Bucht, Deutschland)

Charophyten aus dem oberpliozänen Reuverton des Braunkohlen-Tagebaus Hambach (Niederrheinische Bucht, Deutschland)

Environmental history of the German Lower Rhine Embayment during the Middle Miocene as reflected by carbon isotopes in brown coal

Stable carbon isotope investigations have been carried out on Miocene brown coal from the Garzweiler Seam of the German Lower Rhine Embayment. Material studied included fossil wood from seven different taxa, and brown coal matrix. Isotope results from macrofossil analysis show variations of more than 6‰ within individual samples and reveal a general isotopic difference between angiosperm and gymnosperm wood specimens. According to mean carbon isotope values found for gymnosperms, angiosperms and brown coal matrix (−23.3‰, −26.0‰ and −25.8‰), the peat-forming vegetation of the Garzweiler Seam was dominated by angiosperm taxa. Results from brown coal matrix establish a continuous high-resolution depth profile of carbon isotope variations during the late Middle Miocene. They show a significant and characteristic isotope pattern with distinct medium- and short-term cycles (high-frequency variations) in the two main units of Garzweiler Seam (locally split into three units). The medium-term δ 13C variations are most likely caused by varying proportions of gymnosperms within the peat-forming vegetation while high-frequency oscillations seem to be a direct signal of environmental changes. A long-term decline of carbon isotope values observed within the complete Garzweiler Seam from base to top is presumably due to a cooling trend in the Miocene.

Regional-scale palaeoclimate modelling on soft proxy-data basis — an example from the Upper Miocene of the Lower Rhine Embayment

A new method in palaeoclimatology, initially proposed by Mosbrugger and Schilling (1992), which makes it possible to extract a maximum of information from the available proxy-data is tested. It possesses a capacity to produce a detailed palaeoclimate reconstruction, which is consistent with both proxy-data and the physics of atmospheric models. Since proxy-data information is particularly poor on a global scale, our method focuses on smaller-scale regions of better proxy-data coverage. It consists of an iterative coupling of inverse mesoscale atmospheric modelling with proxy-based palaeoclimate reconstruction. Using this method, the palaeoclimate of the Tortonian (Upper Miocene) of the Lower Rhine Embayment (northwest Germany) is reconstructed with a high spatial resolution. The results include a physically consistent inter- and extrapolation of proxy-derived palaeoclimate data in space and onto variables not accessible from geological–palaeontological proxies alone. Moreover, the method delivers statistics of the large-scale atmospheric flows as information on the coupling of the regional climate to global fields.

Microtremor measurements used to map thickness of soft sediments

AbstractThe observations about the behavior of microtremor spectra presented here show that noise measurements can be used as a powerful tool to determine the thickness of soft cover layers. The most suitable method for this determination is Nakamura's technique, which is the ratio of the horizontal-component noise spectrum and that of the vertical component (H/V spectrum). The frequency of the main peak in these spectral ratios correlates well with the sediment thickness at the site. Using an extensive database of microtremor measurements carried out in the western Lower Rhine Embayment (Germany), it was possible to show that this correlation is clearly valid for a wide range of thickness, namely, from tens of meters to more than 1000 m. A simple formula was derived that, for the sediments to be found in the area investigated, directly calculates the cover thickness from the frequency of the main peak in the H/V spectrum. A comparison with calculated resonant frequencies suggests the relation derived from the noise measurements depending on the velocity depth function of the shear wave. Classical spectral ratios are shown to be strongly influenced by the noise level and are therefore less reliable in determining the resonant frequency of the subsoil. The practical relevance of the investigation is illustrated by means of cross sections, constructed from results of the microtremor analyses, which provide a convincing image of the surficial structure of the areas investigated.

Seismotektonisch induzierte Brunnenspiegelschwankungen in der Niederrheinischen Bucht

Seismotektonisch induzierte Brunnenspiegelschwankungen in der Niederrheinischen Bucht

The stress field in the Rhine Graben area inferred from earthquake focal mechanisms and estimation of frictional parameters

Altogether 98 focal mechanisms of micro to moderate earthquakes (ML = 0.5−5.9) have been used to analyze the state of stress in the Rhine Graben and adjacent areas. Due to dense networks in the investigated area fault plane solutions determined by the first motion method can be fixed with an accuracy better than 10°. The majority of the focal mechanisms is characterized by strike-slip and normal faulting movements; only a few events are of reverse faulting type.To calculate the orientations of the principal stress axes and the shape of the stress tensor we applied the inversion method of Gephart and Forsyth (1984) to three subsets of the data, each of which has been chosen taking into account the local tectonic units (northern Alpine foreland, southern Rhine Graben, northern Rhine Graben including Rhenish Massif and lower Rhine Embayment). For all of the data subsets, the inversions revealed a stable axis of minimum principal stress σ3, oriented horizontally to subhorizontally and striking in a WSW-ENE to SW-NE direction. The azimuth of σ1, the maximum principal stress, is about 150°. However, the confidence regions show that horizontal and vertical orientations of σ1, as well as orientations in between, are of the same probability. Therefore, an unequivocal distinction between strike-slip and extensional regimes is not possible. When comparing the results from the data subsets, a slight counterclockwise rotation from south to north of about 30° of the σ1−σ2 plane plane as well as of the direction of σ3, can be observed. The relative stress magnitude R (R = (σ2 − σ1)/(σ3 − σ1)) has a value of about 0.5 for all subsets, indicating that for the deviatoric part of the stress tensor, σ1 and σ3 are of the same magnitude but of opposite sign. The directions of the horizontal stresses, SHmax and Shmin, inferred from the analysis of focal mechanisms are in good agreement with the general features for western Europe in the European Stress Map.More detailed investigations have been performed for the southern Rhine Graben area. A division into an upper crustal (<15 km) and a lower crustal (>15 km) dataset makes possible the resolution of an individual stress regime for each subset: a strike-slip regime for the upper and an extensional regime for the lower crust, with the same azimuths of the principal stress axes for both depth ranges. Such a change of the stress tensor with depth fits into the tectonic model of detachment zones in the lower crust in the northern front of the Alps, a detachment caused by south-plunging subduction of the European plate beneath the Alps and the Adriatic plate.On the basis of the resolved orientations of the principal stress axes and the shape of the stress tensor, we furthermore estimated absolute stress magnitudes and the ranges of frictional parameters acting on the individual faults. Assuming hydrostatic pore pressure on the faults, frictional coefficients between 0.10 and 0.64 are required to enable slip. On the other hand, when the frictional coefficient has been fixed to 0.65, the required pore pressure values range from 0.38 to 0.62 of lithostatic with an average of 0.48. The absolute stress magnitudes inferred from the analysis of the focal mechanisms compare favourably with the in-situ measurements of absolute stresses from the KTB borehole at Windischeschenbach (German continental deep drilling program).

Mining-induced crustal deformation in northwest Germany: modelling the rheological structure of the lithosphere

Large-scale coal mining in Germany causes deloading of the crust by the removal of weight. The crust responds by regional uplift while locally, in the mining areas proper, the surface subsides by sagging above extracted coal seams and removed groundwater. In this study only the regional uplift is addressed. The deloading consists of the total annual mass of coal and ground water removal since the 19th century for all significant mines. The uplift data are from repeated levellings (between about 1950 and 1980), reduced to mean annual rates of elevation change. These observations are interpreted as the visco-elastic response of the crust and uppermost mantle but alone they cannot discriminate layering from a homogeneous viscous half-space or an elastic layer above such a half-space. If the seismically determined layered structure of upper and lower crust, lithosphere mantle and asthenosphere is assumed to constrain the rheological structure, a plausible model can be found by inversion (only the lower crustal and asthenospheric effective Maxwell viscosities are subjected to inversion). A model, consistent with the observations, has the following properties: 15 km of elastic upper crust; 15–20 km thick lower crust of surprisingly low viscosity of 6 · 10 17±0.3 Pas; 30–50 km visco-elastic mantle lithosphere with an assumed average viscosity of about 10 22 Pas; a visco-elastic asthenosphere (assumed bottomless) with 5 · 10 19±0.6 Pas. The low viscosity values are consistent with active rifting and above-average heat flow in the region. The very low effective viscosity of the lower crust in this model (it could be even lower if the thickness is smaller) may be explained by elevated stresses, quartz rheology and water content as suggested by xenoliths from the lower crust below the Eifel region. As a by-product, the tectonic subsidence in the Lower Rhine Embayment (filled with Tertiary and Quaternary sediments) can be better defined by taking the anthropogenic effect into account.

13C/12C ratio double cyclicity in a Miocene browncoal: isotopic signals and orbital forcing

A detailed stable carbon isotopic profile of a late Miocene browncoal seam from the Lower Rhine Embayment, Germany, reveals two clear separate cycles of different frequency: this is the first instance of this type of isotopic signature being recognized within coals. The ratio of the two frequencies suggests this isotopic signal possibly resulted from climatic or vegetational responses to orbital forcing. The cyclicity has been enhanced by the application of conventional filtering methods on the data set. An analysis of the seam’s palynology indicates a correlation between heavier isotopic compositions and the presence of Sequoiapollenites polyformosus, whose parent plant is believed to have favoured moist climates and higher groundwater tables.