Structural control of geothermal reservoirs in extensional tectonic settings: An example from the Upper Rhine Graben

In extensional tectonic settings major structural elements such as graben boundary faults are typically oriented subparallel to the maximum horizontal stress component S_Hmax. They are often structurally accompanied by transfer zones that trend subparallel to the extension direction. In the Upper Rhine Graben, such transfer faults are typically characterized by strike-slip or oblique-slip kinematics. A major re-orientation of the regional stress field by up to 90° of the Upper Rhine Graben in the Early Miocene led to the present-day normal and strike-slip faulting regimes in the North and South of the Upper Rhine Graben, respectively, and a transition zone in-between. Consequently, conditions for fault frictional failure changed significantly. Moreover, it has been observed during tracer and stimulation experiments that such transfer faults may be of major importance for the hydraulic field of geothermal reservoirs under the present stress condition, especially, when located between production and injection well.In this context we have investigated slip and dilation tendencies (T_S and T_D) of major structural elements at reservoir scale for two representative geothermal sites, Bruchsal (Germany) and Riehen (Switzerland), located close to the Eastern Main Boundary Fault of the Upper Rhine Graben. We have evaluated the quality and uncertainty range of both tendencies with respect to potential variation in S_Hmax orientation. Despite significant differences in orientation of the structures and the stress regimes, the resulting variation of T_S and T_D reveal major similarities concerning the reactivation potential of both, the graben-parallel structures and the transfer faults. The conditions of criticality for tensile failure and non-criticality for shear failure suggest that transfer faults are most likely naturally permeable structures with low stimulation potential. This is in agreement with the absence of both immediate tracer recovery and seismicity in the studied geothermal sites.     

The south-western Black Forest and the Upper Rhine Graben Main Border Fault: thermal history and hydrothermal fluid flow

The thermal history of the south-westernmost Black Forest (Germany) and the adjacent Upper Rhine Graben were constrained by a combination of apatite and zircon fission-track (FT) and microstructural analyses. After intrusion of Palaeozoic granitic plutons in the Black Forest, the thermal regime of the studied area re-equilibrated during the Late Permian and the Mesozoic, interrupted by enhanced hydrothermal activity during the Jurassic. At the eastern flank of the Upper Rhine Graben along the Main Border Fault the analysed samples show microstructural characteristics related to repeated tectonic and hydrothermal activities. The integration of microstructural observations of the cataclastic fault gouge with the FT data identifies the existence of repeated tectonic-related fluid flow events characterised by different thermal conditions. The older took place during the Variscan and/or Mesozoic time at temperatures lower than 280°C, whereas the younger was probably contemporary with the Cenozoic rifting of the Upper Rhine Graben at temperatures not higher than 150°C.     

Neotectonics and intraplate continental topography of the northern Alpine Foreland

Research on neotectonics and related seismicity has hitherto been mostly focused on active plate boundaries that are characterized by generally high levels of earthquake activity. Current seismic hazard estimates for intraplate domains are mainly based on probabilistic analyses of historical and instrumental earthquake catalogues. The accuracy of such hazard estimates is limited by the fact that available catalogues are restricted to a few hundred years, which, on geological time scales, is insignificant and not suitable for the assessment of tectonic processes controlling the observed earthquake activity. More reliable hazard prediction requires access to high quality data sets covering a geologically significant time span in order to obtain a better understanding of processes controlling on-going intraplate deformation.The Alpine Orogen and the intraplate sedimentary basins and rifts in its northern foreland are associated with a much higher level of neotectonic activity than hitherto assumed. Seismicity and stress indicator data, combined with geodetic and geomorphologic observations, demonstrate that deformation of the Northern Alpine foreland is still on-going and will continue in the future. This has major implications for the assessment of natural hazards and the environmental degradation potential of this densely populated area. We examine relationships between deeper lithospheric processes, neotectonics and surface processes in the northern Alpine Foreland, and their implications for tectonically induced topography.For the Environmental Tectonics Project (ENTEC), the Upper and Lower Rhine Graben (URG and LRG) and the Vienna Basin (VB) were selected as natural laboratories. The Vienna Basin developed during the middle Miocene as a sinistral pull-apart structure on top of the East Alpine nappe stack, whereas the Upper and Lower Rhine grabens are typical intracontinental rifts. The Upper Rhine Graben opened during its Late Eocene and Oligocene initial rifting phase by nearly orthogonal crustal extension, whereas its Neogene evolution was controlled by oblique extension. Seismic tomography suggests that during extension the mantle-lithosphere was partially decoupled from the upper crust at the level of the lower crust. However, whole lithospheric folding controlled the mid-Miocene to Pliocene uplift of the Vosges–Black Forest Arch, whereas thermal thinning of the mantle–lithosphere above a mantle plume contributed substantially to the past and present uplift of the Rhenish Massif. By contrast, oblique crustal extension, controlling the late Oligocene initial subsidence stage of the Lower Rhine Graben, gave way to orthogonal extension at the transition to the Neogene.The ENTEC Project integrated geological, geophysical, geomorphologic, geodetic and seismological data and developed dynamic models to quantify the societal impact of neotectonics in areas hosting major urban and industrial activity concentrations. The response of Europe's intraplate lithosphere to Late Neogene compressional stresses depends largely on its thermo-mechanical structure, which, in turn, controls vertical motions, topography evolution and related surface processes.     

Backward modeling of the rifting kinematics in the Upper Rhine Graben: insights from an elastic-perfect contact law on the restoration of a multi-bloc domain

This paper addresses the problem of volume restoration for 3-D sedimentary basin kinematic deformation. The primary purpose is methodological and concerns the use of contact mechanics with the finite element method, in order to deform a geological multi-bloc domain. This approach is applied to backward model the later stage of rifting of a segment of the southern Upper Rhine Graben (France–Germany border). Preliminary results from our modeling demonstrate the ability of the method not only to handle complex geometries, but also to successfully perform retro-deformation of a complex geological domain. In addition, they provide or confirm crucial information on the rifting evolution and tectonic features of this segment of the Upper Rhine Graben, such as the distribution of deformation, the asymmetry of the graben and a significant left-lateral strike-slip component of displacement.     

Major palaeohydrographic changes in Alpine foreland during the Pliocene - Pleistocene

The changing palaeogeographical pattern of Alpine deposits across the European forelands can be traced by identifying mineral assemblages and establishing the chronology of Pliocene-Pleistocene deposits in Alpine foreland. In the late Miocene, the upper courses of the Rhine and the Aar flowed east from the Swiss molasse plain towards the Danube. In the early Pliocene (Brunssumian, 5-3.2 Ma), these same rivers headed north wards towards the Rhine Graben of Alsace. In the early Reuverian, these streams were captured south of the Rhine Graben by the Doubs. They ceased their northward flow and headed west to feed the Bresse Graben. This phase is dated to the Lower and Middle Reuverian (3.2-2.6 Ma). From the Upper Reuverian (2.6 Ma) to the present day, the Rhine has adapted approximately its present course towards the North Sea, south to north along the Rhine Graben and across the Rhine Schist Massif to feed the Dutch Grabens. This changing pattern of capture and alteration of the hydrographic system of the upper reaches of the Rhine and the Aar can be explained by local tectonic movements.     

The northern Upper Rhine Graben: basin geometry and early syn-rift tectono-sedimentary evolution

A large-scale transfer zone subdivides the northern parts of the Upper Rhine Graben into a northern and a southern sub-basin. These sub-basins display the geometry of asymmetric half-grabens with opposing tilt directions. The transfer zone connects the western master fault of the northern half-graben with the eastern master fault of the southern half-graben. In the northern Upper Rhine Graben early syn-rift sedimentation (Late Priabonian to Late Rupelian) was controlled by the tectonically induced subsidence of these half-grabens (autogenetic), as well as by regional third-order sea level variations (allogenetic). Within the graben, lateral changes in subsidence rates (in dip and strike direction of fault blocks) controlled the development of accommodation space and thus, sediment thickness and facies. Furthermore, a low-displacement segment along the western border fault acted as a sediment entry point. Tectonics controlled the distribution of early syn-rift deposits and the palaeogeography of the northern Upper Rhine Graben.     

Intraplate deformation and 3D rheological structure of the Rhine Rift System and adjacent areas of the northern Alpine foreland

The lithosphere of the Northern Alpine foreland has undergone a polyphase evolution during which interacting stress-induced intraplate deformation and upper mantle thermal perturbations controlled folding of the thermally weakened lithosphere. In this paper we address relationships among deeper lithospheric processes, neotectonics and surface processes in the Northern Alpine foreland with special emphasis on tectonically induced topography. We focus on lithosphere memory and neotectonics, paying special attention to the thermo-mechanical structure of the Rhine Graben System and adjacent areas of the northern Alpine foreland lithosphere. We discuss implications for mechanisms of large-scale intraplate deformation and links with surface processes and topography evolution.     

Tertiary tectonics of the Dannemarie Basin, upper Rhine graben, and regional implications

Recently released seismic reflection data, together with previous seismic and well data, are used to describe the development of the Dannemarie basin, in the SW end of the Upper Rhine Graben. The Dannemarie Basin was formed during the main rifting phase of the Upper Rhine Graben as an asymmetrical graben trending NE–SW. Post-rift tectonism shifted the depocenter southward and changed the overall shape of the basin. Miocene Jura compression did not result in the formation of folds, as in the adjacent Mulhouse Horst. Strike slip faulting was dominant in the post-rift period and new faults were created, most notably the north trending and transpressional Belfort Fault. The boundary of the Dannemarie Basin with the Vosges Mountains is part of a restraining bend, which may account for the uplift of the southernmost part of the Vosges Mountains.     

The southeastern border of the Upper Rhine Graben: a 3D geological model and its importance for tectonics and groundwater flow

A 3D geological model of the area east of Basel on the southeastern border of the Upper Rhine Graben, consisting of 47 faults and six stratigraphic horizons relevant for groundwater flow, was developed using borehole data, geological maps, geological cross sections, and outcrop data. This model provides new insight into the discussions about the kinematics of the area between the southeastern border of the Upper Rhine Graben and the Tabular Jura east of Basel. A 3D analysis showed that both thin-skinned and thick-skinned tectonic elements occur in the modeled area and that the Anticline and a series of narrow graben structures developed simultaneously during an extensional stress-field varying from E–W to SSE–NNW, which lasted from the Middle Eocene to Late Oligocene. In a new approach the faults and horizons of the 3D geological model were transferred into discrete elements with distributed hydrogeological properties in order to simulate the 3D groundwater flow regime within the modeled aquifers. A three-layer approach with a horizontal regularly spaced grid combined with an irregular property distribution of transmissivity in depth permitted the piezometric head of the steady-state model to be automatically calibrated to corresponding measurements using more than 200 piezometers. Groundwater modeling results demonstrated that large-scale industrial pumping affected the groundwater flow field in the Upper Muschelkalk aquifer at distances of up to 2 km to the south. The results of this research will act as the basis for further model developments, including salt dissolution and solute transport in the area, and may ultimately help to provide predictions for widespread land subsidence risks.     

An overview on source rocks and the petroleum system of the central Upper Rhine Graben

International Journal of Earth Sciences - The petroleum system of the Upper Rhine Graben (URG) comprises multiple reservoir rocks and four major oil families, which are represented by four distinct...     

Rhine flood deposits recorded in the Gallo-Roman site of Oedenburg (Haut-Rhin, France)

From the first to the fourth century AD, the Gallo-Roman town of Oedenburg developed in the alluvial landscape of the southern Upper Rhine Graben. Throughout this period, the landscape mosaic, composed of palaeochannels, stable palaeoislands and river terraces, continued to evolve. A district of this town, situated on a lateral Rhine channel system, was archaeologically excavated. Large-scale excavation and cross-section analysis provide evidence of changing fluvial conditions during the period under study. At about AD 20 or earlier, this lateral part of the floodplain, affected by very fine sedimentation, was occupied by moribund marshy palaeochannels. When the first Gallo-Roman settlers occupied the site, they filled parts of the channels with woven brushwood in order to create an efficient circulation surface. The sedimentary infill of this palaeochannel records four different flood deposits interlayered with dated anthropogenic units (pavements, road embankments, and other structures). Archaeological analysis and dendrochronological dating indicate that these four flooding events occurred during a short time period between AD 20 and AD 145/146. These geoarchaeological observations focus on floods that do not seem to have significantly affected human occupation in this part of the Rhine floodplain. These results are set in the broader context of the Rhine catchment and the Alps.     

Gravity interpretation along seismic reflection profile DEKORP 9-N (northern Rhine Graben)

A 2-D gravity model, incorporating geophysical and geological data, is presented for a 110 km long transect across the northern Rhine Graben, coinciding with the 92 km long DEKORP 9-N seismic reflection profile. The Upper Rhine Graben is marked by a prominent NNE-striking negative anomaly of 30–40 mgal on Bouguer gravity maps of SW Germany. Surface geological contacts, borehole data and the seismic reflection profile provide boundary constraints during forward modelling.
Short-wavelength (5–10 km) gravity features can be correlated with geologic structures in the upper few km. At deeper levels, the model reflects the asymmetry visible in the seismic profile; a thicker, mostly transparent lower crust in the west and a thinner, reflective lower crust in the east. From west to east Moho depth changes from 31 to 26–28 km. The entire 40 mgal minimum can be accounted for by the 2–3 km of light sedimentary fdl in the graben, which masks the gravitational effects of the elevated Moho. The thickened lower crust in the west partly compensates for the mass deficit from the depressed Moho. A further compensating feature is a relatively low density contrast at the crust-mantle boundary of 0.25 g cm^-3. The Variscan must displays heterogeneity along the profile which cuts at an angle across the strike of Variscan structures. The asymmetry of the integrated crustal model, both at the surface and at depth suggests an asymmetric mechanism of rift development.     

Loess and palaeosols on the High Terrace at Sierentz (France), and implications for the chronology of terrace formation in the Upper Rhine Graben

Pedological investigations in combination with luminescence dating have been used to reconstruct the genesis and chronology of a sediment succession at Sierentz, France. The sequence comprises loess and palaeosols on top of gravel attributed to the High Terrace of River Rhine. According to the dating results, three phases of soil development occurred during different warm phases of Marine Isotope Stage (MIS) 7 (245–190 ka). Soil development on top of the gravel occurred either during early MIS 7 or during an earlier warm phase, possibly MIS 9. The results imply a minimum age of 250 ka for the formation of the High Terrace in this part of the Upper Rhine Graben, contrary to previous assumptions that correlated gravel sheet deposition with MIS 6 (ca. 150 ka). These results and recent findings at other sites suggest that the chronological setting of terrace formation north of the Alps is much more complex than previously assumed.     

Tracing sediment pathways by zircon fission track analysis: Oligocene marine connections in Central Europe

In this study, we use contrasting zircon fission track age signatures of Alpine detritus and detritus derived from the Variscan realm to trace sediment pathways in Central Europe. Our data show that the Molasse Basin was connected with the Rhine Graben Sea during the Mid-Oligocene, thus joining the North Sea to the Paratethys. Within the Rhine Graben Sea, fairly strong south–north directed currents existed, transporting sand-sized Alpine detritus nearly 300 km towards the north. A connection between the Rhône-Bresse Graben and the Rhine Graben and/or the French Molasse Basin and the Swiss Molasse Basin, by contrast, is not supported by the fission track data. This may be explained by the existence of submarine rises that hampered the transport of sand-sized sediment towards the north/northeast.     

Late Pleistocene fluvial dynamics in the Hochrhein Valley and in the Upper Rhine Graben: chronological frame

During the Pleistocene, the Rhine glacier system acted as a major south–north erosion and transport medium from the Swiss Alps into the Upper Rhine Graben, which has been the main sediment sink forming low angle debris fans. Only some aggradation resulted in the formation of terraces. Optically stimulated luminescence (OSL) and radiocarbon dating have been applied to set up a more reliable chronological frame of Late Pleistocene and Holocene fluvial activity in the western Hochrhein Valley and in the southern part of the Upper Rhine Graben. The stratigraphically oldest deposits exposed, a braided-river facies, yielded OSL age estimates ranging from 59.6 ± 6.2 to 33.1 ± 3.0 ka. The data set does not enable to distinguish between a linear age increase triggered by a continuous autocyclical aggradation or two (or more) age clusters, for example around 35 ka and around 55 ka, triggered by climate change, including stadial and interstadial periods (sensu Dansgaard–Oeschger cycles). The braided river facies is discontinuously (hiatus) covered by coarse-grained gravel-rich sediments deposited most likely during a single event or short-time period of major melt water discharge postdating the Last Glacial Maximum. OSL age estimates of fluvial and aeolian sediments from the above coarse-grained sediment layer are between 16.4 ± 0.8 and 10.6 ± 0.5 ka, and make a correlation with the Late Glacial period very likely. The youngest fluvial aggradation period correlates to the beginning of the Little Ice Age, as confirmed by OSL and radiocarbon ages.     

The position of the Rupelian/Chattian boundary in the southern Upper Rhine Graben based on new records of microfossils

This study analyses the regressive phase of the marine Froidefontaine Subgroup and the subsequent fluvio-lacustrine Niederroedern formation in the southern Upper Rhine Graben during the Late Rupelian and the position of the Rupelian/Chattian boundary. The study is based on the sedimentary record and several microfossil groups from two boreholes, with a focus on new records of fish otoliths, Bolboforma and Charophyta. The biostratigraphic evaluation of these groups provides evidence for a position of the Rupelian/Chattian boundary within the upper Niederroedern Formation. This is contrary to the results from a previous sedimentological approach, which places the boundary at the base of the Niederroedern Formation. The fish fauna indicates a biogeographic relation to southern France, the Molasse Basin of Switzerland and southern Germany.     

Eocene-Pliocene time scale and stratigraphy of the Upper Rhine Graben (URG) and the Swiss Molasse Basin (SMB)

We present a general stratigraphic synthesis for the Upper Rhine Graben (URG) and the Swiss Molasse Basin (SMB) from Eocene to Pliocene times. The stratigraphic data were compiled both from literature and from research carried out by the authors during the past 6 years ; an index of the stratigraphically most important localitites is provided. We distinguish 14 geographical areas from the Helvetic domain in the South to the Hanau Basin in the North. For each geographical area, we give a synthesis of the biostratigraphy, lithofacies, and chronostratigraphic ranges. The relationships between this stratigraphic record and the global sea-level changes are generally disturbed by the geodynamic (e.g., subsidence) evolution of the basins. However, global sea-level changes probably affected the dynamic of transgression–regression in the URG (e.g., Middle Pechelbronn Beds and Serie Grise corresponding with sea-level rise between Ru1/Ru2 and Ru2/Ru3 sequences, respectively) as well as in the Molasse basin (regression of the UMM corresponding with the sea-level drop at the Ch1 sequence). The URGENT-project (Upper Rhine Graben evolution and neotectonics) provided an unique opportunity to carry out and present this synthesis. Discussions with scientists addressing sedimentology, tectonics, geophysics and geochemistry permitted the comparison of the sedimentary history and stratigraphy of the basin with processes controlling its geodynamic evolution. Data presented here back up the palaeogeographic reconstructions presented in a companion paper by the same authors (see Berger et al. in Int J Earth Sci 2005).     

Recent vertical movements from precise levelling in the vicinity of the city of Basel, Switzerland

The southern end of the Upper Rhine Graben is one of the zones in Switzerland where recent crustal movements can be expected because of ongoing seismotectonic processes as witnessed by seismicity clusters occurring in this region. Therefore, in 1973 a control network with levelling profiles across the eastern Rhine Graben fault was installed and measured in the vicinity of the city of Basel in order to measure relative vertical movements and investigate their relationship with seismic events. As a contribution to EUCOR-URGENT, the profiles were observed a third time in the years 2002 and 2003 and connected to the Swiss national levelling network. The results of these local measurements are discussed in terms of accuracy and significance. Furthermore, they are combined and interpreted together with the extensive data set of recent vertical movements in Switzerland (Jura Mountains, Central Plateau and the Alps). In order to be able to prove height changes with precise levelling, their values should amount to at least 3–4 mm (1). The present investigations, however, have not shown any significant vertical movements over the past 30 years.