Mid-crustal deformation is classically characterized by the transition from ductile to brittle deformation defining the frictional-to-viscous transition (FVT). Here we investigate an exhumed continental midcrustal basement section in order to envisage the relationship between ductile and brittle deformation at the FVT. Our detailed study from km-to micro-scale shows that, under greenschist metamorphic conditions, deformation is accommodated by a dense network of highly-localized ductile shear zones. In the investigated case it is not quartz which defines the overall ductile deformation behavior but the viscous granular deformation in shear zones with an ultrafine-grained polymineralic matrix consisting of quartz, feldspar, sheet silicates and epidote. During viscous granular flow mass transfer processes under the presence of fluids promote a chemo-mechanical mixing, resulting in grain size reduction and reaction softening. Coeval with this ductile deformation, fluid-assisted embrittlement occurs, as indicated by biotite-coated fractures, cataclasites and injection of non-cohesive polymineralic gouge material into secondary fractures inside the host rock. The embrittlement during predominant ductile deformation occurs in cycles, i.e. prolonged periods of slow viscous granular flow are interrupted by rapid brittle deformation. We interpret this fluid-assisted cyclic embrittlement evidenced by injection of the fluidized material into off-fault fractures as an alternative equivalent to pseudotachylites and as a microstructural indicator for paleo-seismic activity. With exhumation and associated cooling, localized deformation persists in the ultrafine-grained polymineralic shear zones but progressively transitions to cataclastic flow and finally to pressure-dependent frictional flow; always showing cycles of slow interseismic flow and fast seismic injection events. Overall, in the granitic crust of the Aar-massif, brittle and ductile deformation coexist up to deformation temperatures of minimum 450°C, indicating that the FVT has to be placed in a rather wide range from 8 km up to > 18-20 km in the granitoid crust.

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As estimated by the International Energy Agency, geothermal power can contribute to 3.5 % of worldwide power and 3.9 % to heat production by 2050. This includes the development of enhanced geothermal systems (EGSs) in low‐enthalpy systems. EGS technology is still in an early stage of development. Pushing EGS technologies towards market maturity requires a long‐term strategic approach and massive investments in research and development. Comprehensive multidisciplinary research programs that combine fundamental and applied concepts to tackle technological, economic, ecological, and safety challenges along the EGS process chain are needed. The Karlsruhe Institute of Technology (KIT) has defined a broad research program on EGS technology development following the necessity of a transdisciplinary approach. The research concept is embedded in the national research program of the Helmholtz Association and is structured in four clusters: reservoir characterization and engineering, thermal water circuit, materials and geoprocesses, and power plant operation. The proximity to industry, closely interlinked with fundamental research, forms the basis of a target‐orientated concept. The present paper aims to give an overview of geothermal research at KIT and emphasizes the need for concerted research efforts at the international level to accelerate technological breakthrough of EGS as an essential part of a future sustainable energy system.

We present a theory for the onset of localization in layered rate‐ and temperature‐sensitive rocks, in which energy‐related mechanical bifurcations lead to localized dissipation patterns in the transient deformation regime. The implementation of the coupled thermomechanical 2‐D finite element model comprises an elastic and rate‐dependent von Mises plastic rheology. The underlying system of equations is solved in a three‐layer pure shear box, for constant velocity and isothermal boundary conditions. To examine the transition from stable to localized creep, we study how material instabilities are related to energy bifurcations, which arise independently of the sign of the stress conditions imposed on opposite boundaries, whether in compression or extension. The onset of localization is controlled by a critical amount of dissipation, termed Gruntfest number, when dissipative work by temperature‐sensitive creep translated into heat overcomes the diffusive capacity of the layer. Through an additional mathematical bifurcation analysis using constant stress boundary conditions, we verify that boudinage and folding develop at the same critical Gruntfest number. Since the critical material parameters and boundary conditions for both structures to develop are found to coincide, the initiation of localized deformation in strong layered media within a weaker matrix can be captured by a unified theory for localization in ductile materials. In this energy framework, neither intrinsic nor extrinsic material weaknesses are required, because the nucleation process of strain localization arises out of steady state conditions. This finding allows us to describe boudinage and folding structures as the same energy attractor of ductile deformation.