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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. [ABSTRACT FROM AUTHOR]

Fracture porosity is a crucial parameter in hydrocarbon and geothermal reservoir exploration and a major challenge in the absence of nearby exploration wells. The Sankt Gallen geothermal project targets a fault zone that affects Mesozoic sediments at a depth of about 4500 m. Spatial extension of these sediments, a major fault zone and indication for graben structures in the crystalline basement are observed in 3D seismic. Both the graben and the fault zone coincide with negative gravity anomalies acquired and analyzed during this study. Forward modeling of gravity anomalies based on a 3D seismic survey is used to estimate possible fracture porosity. After stripping gravity effects of geothermally irrelevant geological units from the residual anomaly, most likely only local structures related to the fault zone account for remaining anomalies. Synthetic case study on the effect of density variation and considerable gas content in the well support possible fracture porosity between about 4% and 8%. [ABSTRACT FROM AUTHOR]

Preferential fluid pathways along regional fault systems are crucial for the spatial localization of volcanic and geothermal manifestations. Differences in electric properties of fluids and hydrothermal alteration products against the unaltered matrix allow for visualization of such pathways. Unfavorable geometry for 2-D inversion resulting from the fluid pathways in regional faults often being aligned along the geo-electric strike can be overcome by using 3D inversion of magnetotelluric data. For a section in the 1400 km long Liquiñe-Ofqui Fault System (LOFS), we demonstrate the potential of 3-D inversion of magnetotelluric data for visualization of fluid pathways. Six out of eight electric resistivity anomalies at intermediate depth are connected to volcanic or geothermal surface manifestations. Deep and highly conductive anomalies are detected in the vicinity of the volcanic chain and where the LOFS is crosscut by a fault that belongs to the Andean transversal fault system. The anomaly related to the fault crossing reveals a vertical pathway extending to the surface. Phase tensor analyses indicate a structural origin of the latter anomaly that may be connected to a line of volcanic cones that occurs in the NE of the Villarrica Volcano. • 3-D inversion created the first complete visualization of preferential fluid pathways. • Structural control of preferential magmatic and geothermal fluid pathways • Connection at surface low resistivity bodies with the fault-related system • Results suggest that damage zones may provide optimal vertical pathways. [ABSTRACT FROM AUTHOR]

The active volcanic arc of the Andes is controlled by subduction of the Nazca and Antarctic Plates beneath the South American Plate. The Southern Volcanic Zone is the segment with the most active volcanoes in the Andes and the one where volcano-tectonic interactions are more evident. Magmatic fluids are there channeled through the brittle crust by faults and fractures, becoming an important region for understanding the mechanisms associated with melt migration and storage in subduction zones. The Liquiñe–Ofqui Fault System and Andean Transverse Faults, control the overall architecture of the volcanic arc and play an important role on magmatic transport and compositional partitioning. Evidence of this is the oblique chain composed of Villarrica, Quetrupillán and Lanín stratovolcanoes and a number of minor eruptive centers, where a wide range of magma compositions have been erupted with basalts and basaltic andesites dominating the suite. We used long–period and broad–band magnetotelluric stations deployed surrounding of Villarrica volcano to investigate the implications of crustal faulting and volcanism and their consequence on crustal reservoirs. Inversion of the data was used to generate three–dimensional electrical resistivity models. The resistivity distribution shows the upper crust as highly resistive, but below and east of Villarrica volcano, the model suggests the presence of a magmatic reservoir at shallow crustal levels (between 1.5 and 3 km b.s.l.), possibly corresponding to a transient magma storage. Meanwhile, the middle crust contains several intermediate to low conductive features interpreted as fluid pathways and/or melt storage regions, respectively, revealing the important role of fault systems. The lower crust also contains low resistivity zones indicating the presence of partial melt and/or fluids, associated with deep reservoirs (8–20 km), with a significant proportion of them likely non eruptible parts. This would suggest that melt is accumulated as highly crystallized mush or disconnected melt pockets and highlight the complex vertical extent of the structurally-controlled plumbing systems even in thin crust settings. • 3-D inversion of long–period and broad–band magnetotelluric stations. • Investigate the implications of crustal faulting and volcanism and their consequence on crustal reservoirs. • Low resistivity anomalies indicating the presence of partial melt and/or fluids, associated with shallow (1.5 km) and deep (8–20 km) reservoirs. • Shallow magmatic reservoir, below and east of Villarrica volcano, possibly correspond to a temporary magma storage. • Deep reservoirs may represent a deep source for the shallow reservoir. [ABSTRACT FROM AUTHOR]