Faculté des sciences

Multidisciplinary approach to understand the localization of geothermal anomalies in the Upper Rhine Graben from regional to local scale

Baillieux, Paul ; Schill, Eva (Dir.)

Thèse de doctorat : Université de Neuchâtel, 2012.

This thesis is devoted to understanding the localization of geothermal anomalies in extensional tectonic settings away from active volcanic areas. In this context, the European Cenozoic Rift System (ECRIS) hosts some of the major geothermal anomalies in Europe. Its central segment, the Upper Rhine Graben (URG), stretching over 300km between Basel Switzerland) and Frankfurt (Germany), reveals... Plus

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    Summary
    This thesis is devoted to understanding the localization of geothermal anomalies in extensional tectonic settings away from active volcanic areas. In this context, the European Cenozoic Rift System (ECRIS) hosts some of the major geothermal anomalies in Europe. Its central segment, the Upper Rhine Graben (URG), stretching over 300km between Basel Switzerland) and Frankfurt (Germany), reveals several surface heat flow anomalies in the order of up to > 150 mW m-2 (compared to an average of 60 mW m-2 in Europe) and temperatures up to 120°C at 1km depth, when usually a mean temperature gradient of 30°C km-1 is observed.
    In the Upper Rhine Graben, geothermal activity has mainly been attributed to free hydrothermal convection at graben scale, as well as free convection along the major faults in the area of Landau and at the European Enhanced Geothermal System (EGS) test-site Soultz.
    These local phenomena emphasize that temperature is not homogeneously distributed in the URG and neither are geothermal anomalies. Different geological facts were solicited to explain the localization of geothermal anomalies in the URG. The geothermal anomaly at Soultz has been on the one hand attributed to recent compressive shear strain occurring parallel to the central segment of the graben, allowing hydrothermal circulation to occur along favorably oriented fracture zones in the basement and in the high porosity sandstone aquifer above it. On the other hand it has been correlated to the lithological nature of the basement and its inherited orientations.
    In this thesis, 3D geological modeling, boreholes temperatures distribution, gravity, magnetics, slip and dilation tendencies analysis, neotectonic activity and geodynamic modeling have been used to investigate geological patterns associated with the localization of geothermal anomalies at graben-wide scale (the URG) and pluri-kilometric scale (in the area of the EGS Soultz).
    The comparison of geophysical data with the temperature distribution at graben scale has shown that zones of temperature highs can be related to the occurrence of low density basement that can be attributed to crystalline ridges offering an optimal radiogenic heat production and heat conductivity but also to porosity changes associated with faulting and hydrothermal circulation. In particular, zones of magnetic highs and gravity lows are related to additional temperatures in the order 10-20°C at 2000m true-vertical depth (TVD) at a graben wide scale (temperature 100-110 °C compared to a mean 90°C with minimum value of 75°C).
    Additionally, the occurrence of compression shear and uplift regime, with low Quaternary sedimentation, in the central segment of the URG appears to be linked to the major geothermal anomalies (120 to > 140 °C at 2000 m TVD) in the area of Soultz, Landau and Speyer. This may be explained by a change in tectonic regime from the Early Miocene to up to Present, and is in favor of the interpretation of the central segment of the URG being a restraining band separating two-pull apart basin in a sinistral strike-slip regime. Surprisingly, a relative low seismicity is observed in this area, which to an uncertain extent, maybe linked to the occurrence of naturally circulating fluids.
    More locally in Soultz area, a new 3D geological model has been elaborated on the basis of a high density set of 2D seismic profiles and deep boreholes to understand the links between structural and lithological patterns and the temperature distribution. A mean temperature anomaly of 40 °C at top basement has been linked to a light and magnetic granodioritic pluton offering a rather high radiogenic heat production. Moreover, hydrothermal circulation has been found to occur along N-S directed major faults with a West dipping signature in the western side of horst structures, and is held responsible for temperature anomalies above 60 °C, and this correlates with magnetotelluric observations and fracture orientations in Soultz boreholes. The majority of faults are favorably oriented in the current stress field to be reactivated and undergo dilation at depth and thus allow hydrothermal convection, but no clear relationship between these phenomena has been observed in the slip and dilation tendency analyses, and this can possibly be explained by asymmetric deformation patterns in the area, or by the sealing of the other faults due to intense mineralization. Another possible explanation is the fluid circulation characterized by an upward flow of hot water with a meteoric signature as a result of fluid inclusions analyses, and this deep circulation is interpreted to be coming from the Vosges mountain basement to the West, and this correlates with gravity residuals analyses.
    Finally, the geodynamic modeling of the graben opening showed that simple models can explain the patterns of deformation observed along the deep seismic profiles perpendicular to the graben. In particular, the observed graben asymmetric geometry is reproduced and asymmetric localized faulting is observed on the side opposite to the master fault accommodating the majority of vertical deformation.