EGS are planned in areas with high temperatures at depths of 3-6 km. Until now areas have been selected largely on the basis of observations of high temperature gradients near surface (e.g. volcanic areas such as Iceland) and/or relative high temperatures assessed in deep boreholes drilled mainly for Hydrocarbon exploration and production (e.g. Gross Schönebeck).
For assessing the exploration potential of continental regions for geothermal energy we need to look beyond depths of temperatures known from shallow wells and need capability to predict temperatures at depth in areas where no well control is available. For this reason knowledge the thermo-mechanical signature of the lithosphere and crust is important to obtain critical constraints for the crustal stress field and basement temperatures.
Thermal characteristics of lithosphere beyond well control
Predicting the temperature of the earth at depths beyond existing well control and in areas where no well data is available has been a prime topic in earth science for centuries. Tectonic models integrating geological and geophysical databases in a process-oriented approach provides a continent-wide approach in first order temperature prediction of the lithosphere. In these models the lithosphere constitutes a mechanically strong topmost layer of the earth, which is c.a. 100-200 km thick and which is floating on the astenosphere, which behaves –on geological times- as fluid. The base of the lithosphere is marked by a relatively constant temperature of around 1330 °C.
The actual temperatures extrapolated at depth are sensitive to surface heat flow, and thermal parameters of the rocks (thermal conductivity and heat production essentially). Knowledge of rock conductivity is based on measurements in boreholes and values extrapolated from laboratory experiments, taking into account mineralogical, temperature and porosity effects.
Continental scale transient effects
Due to plate tectonics in the earth, the continents deform. This deformation is most pronounced at the boundaries of plates, e.g. spreading ridges or subduction zones. However plate boundaries forces are transmitted deeply into to the continental interior resulting in intraplate deformation. This deformation is reflected by extension or compression of the interiors of the lithosphere (Ziegler et al., 1998),which can have a significant effect on predicted heat flow and temperature.
The McKenzie passive stretching model has been the first tectonic model delivering a quantitative assessment of kinematic extension of the lithosphere in relation to the sedimentary infilling history of basins. Tectonic models show that extension can be accompanied or preceded by a period of deep lithospheric thermal upwelling, related to mantle plumes. This upwelling, and associated magmatic activity results in a significant increase of heat flow. Examples in Europe include portions of the Rhine Graben, the Eifel area and the Massif Central. In addition back-arc extensional settings can be marked by a mantle upwelling effect as witnessed in the Panonian Basin.
Lithosphere compression typically results in crustal thickening and mountain building. The lithospheric thickening results in relatively low heat flows. However elevated mountains can be actively eroded in fault bounded zones, resulting in elevated heat flows close to the surface. Exhumation with elevated heat flows can also occur in lithosphere extension in particular on rift flanks of extensional basins.
The earth is marked by a multistage deformation history in which extension can occur over areas that have been previously marked by a compressional orogeny with significant mountain building. Extension over such areas can result in thinning and thermal attenuation of the crust in absence of significant sediment infill and loss of heat production in the crust. Consequently the heat flow can be strongly elevated. Areas in Europe include the Western Mediterranean (western Italy, Larderello) and the Aegean (e.g. Milos Island). In these areas, active deep mantle processes may partly enhance heat flows.
The temperatures in crust and mantle lithosphere can be used to calculate the rheology of the lithosphere. Doing so, the development of innovative combinations of numerical and analogue modeling techniques is key to thoroughly understand the spatial and temporal variations in crustal stress and temperature. From these models we derive that the strength of continental lithosphere is controlled by its depth-dependent rheological structure in which the thickness and composition of the crust, the thickness of the mantle–lithosphere, the potential temperature of the asthenosphere, the presence or absence of fluids and strain rates play a dominant role.The strength of the European lithosphere typically shows a stratification into a brittle upper crustal, ductile lower crust and strong ductile upper mantle.
The layered rheology is clearly reflected in the way stress is transmitted from the plate boundaries into the European continent resulting in upper crustal extensional and strike faulting and complex stress and strain interactions in the Mediterranean domain.
Field studies of kinematic indicators and numerical modeling of present-day and paleo-stress fields in selected areas have yielded new constraints on the causes and the expression of intraplate stress fields in the lithosphere, driving basin (de)formation. The actual basin response to intraplate stress is strongly affected by the rheological structure of the underlying lithosphere, the basin geometry, fault dynamics and interplay with surface processes. Integrated basin studies show that rheological layering and strength of the lithosphere plays an important role in the spatial and temporal distribution of stress-induced vertical motions, varying from subtle faulting to basin reactivation and large wavelength patterns of lithospheric folding, demonstrating that sedimentary basins are sensitive recorders to the intraplate stress field. The long lasting memory of the lithosphere, in terms of lithospheric scale weak zones, appears to play a far more important role in basin formation and reactivation than hitherto assumed. A better understanding of the 3-D linkage between basin formation and basin reactivation is, therefore, an essential step in research that aims at linking lithospheric forcing and upper mantle dynamics to crustal vertical motions and stress, and their effect on sedimentary systems and heat flow. Vertical motions in basins can become strongly enhanced, through coupled processes of surface erosion/sedimentation and lower crustal flow. Furthermore patterns of active thermal attenuation by mantle plumes can cause a significant spatial and modal redistribution of intraplate deformation and stress, as a result of changing patterns in lithospheric strength and rheological layering. Novel insights from numerical and analogue modeling aid in quantitative assessment of basin and basement histories and shed new light on tectonic interpretation, providing helpful constraints for geothermal exploration and production, including understanding and predicting crustal stress and basin and basement heat flow.
CLOETINGH, S., ZIEGLER, P.A, BEEKMAN, F., ANDRIESSEN, P.A.M., MATENCO, L., BADA, G., GARCIA-CASTELLANOS, D., HARDEBO,L.N., DEZES, P., SOKOUTIS, D. (2005) Lithospheric memory, state of stress and rheology: neotectonic controls on Europe’s intraplate continental topography. Quaternary Science Reviews, 24, 241–304
GOES, S., GOVERS, R., VACHER, P. (2000) Shallow upper mantle temperatures under Europe from P and S wave tomography. Journal of Geophysical Research 105, 11153-11169.