In order to determine the spatial relationship of the physical properties in the shear zone a continuous set of samples was cored every 0.1 m in the transition zone from 19.6 m to the border of the first MC at 20.1 m. Abundant fractures in the damage zone between the two MCs prevented continuous coring. Two mutually perpendicular core samples, one parallel (x1) and one perpendicular (x3) to the Grimsel granodiorite foliation were taken to characterize the physical property and anisotropy changes as a gradient away from the mylonitic core. Sampling farther than 19.5 m was not possible due to previously made overcoring stress measurements (Fig. c). In order to optimize the number of samples, the x1 direction was taken ∼ 15∘ off axis from the lineation (Fig. d). Foliation perpendicular samples could not be taken at 19.5 and 20.1 m because of breaks in the core. The x1 and x3 samples were bored out of the core using a diamond drill bit (2.54 cm inner diameter) with water as the cooling fluid. The 2.49 to 5.56 cm long samples were ground and polished to craft parallel ends. To characterize the MC, parallel and perpendicular to foliation samples were taken at 20.2 and 23.6 m, respectively. There is a maximum length (∼ 2.49 cm) to diameter (∼ 2.53 cm) ratio of approximately 1:1 in the MC samples, due to the extremely fissile nature of these rocks. Additionally, these two samples come from separate but similar MC at the base of the borehole due to limited sample material. Since the seismic velocity measurements require longer samples due to signal noise and wave propagation issues, the MC samples are only long enough to perform only permeability measurements. Additionally, two sets of perpendicular samples were taken 5 and 7 m from the start of the borehole as a background Grimsel granodiorite reference.

Thin sections were prepared directly from the ends of the samples and observed under optical microscopy. Quantitative mineral analysis was obtained at the University of Geneva using QEMSCAN^® Quanta 650F, an automated scanning electron microscope with mineral identification based on a combination of back-scattered electron values, energy-dispersive X-ray spectra, and X-ray count rates. High-resolution mineralogical and petrographic maps were obtained with the QEMSCAN^® at a scanning resolution of 5 µm, which measures the mineral coverage in percent area.

Measurements of matrix volume and mass were performed after the samples were dried in an oven at 100∘C for 24 h for the granodiorite samples and 40 ∘C for the fragile MC samples. The matrix volume was measured using a helium pycnometer (AccuPyc 1330, Micromeritics^®). The dry mass was measured with a precision balance. The bulk rock density ρbulk was calculated as the dry mass divided by the matrix volume of the sample. The porosity (ϕ) of each cylindrical sample was calculated from the geometrical volume (Vtot) and the matrix volume (Vm) from the helium pycnometer ϕ=(Vtot-Vm)/Vtot.

A hydrostatic pressure vessel was used to measure the gas permeability of each sample (detailed description of the apparatus and measurement technique in ). The hydrostatic pressure vessel is equipped to measure samples of 2.5 cm in diameter and up to ∼ 5 cm in length at confining pressures up to 20 MPa. Hydraulic oil is used as the confining fluid, which is controlled with a screw-type displacement pump that regulates the confining pressure within ± 0.05 MPa. The sample assembly consists of the cylindrical rock specimen placed between two stainless steel disks fastened by a soft PVC tube to isolate the sample from the confining fluid. The two stainless steel disks have interconnected circular grooves to distribute the fluid across the cross-sectional area of the sample. The disks are connected via a plumbing system to the upstream and downstream reservoirs, which can be isolated and filled with the injected gas. The upstream and downstream reservoir, plus their associated plumbing network, have volumes of 50.8 cm3 and 21.2 cm3, respectively. The gas pressure in the two reservoirs is measured within 0.05  %.

Due to the low porosity and permeability in the granodioirite and MC, the transient step technique was used to perform and analyze the flow experiments . Experiments were performed at room temperature and an effective pressure of 10 MPa, chosen to represent the effective stress conditions in the GTS. Three confining pressure and pore pressure configurations that preserved an effective pressure of 10 MPa were performed to assess the Klinkenberg gas slippage effect . For each sample a pressure difference of 0.5 MPa was imposed between the upstream (Pus) and downstream (Pds) reservoir and allowed to equilibrate at each of the pore pressure configurations (e.g., Pus and Pds = 1.0 and 0.5 MPa, 3.0 and 2.5 MPa, and 7.0 and 6.5 MPa, respectively). In some cases, the sample permeability was so low that reaching a full equilibrium between the up- and downstream reservoir was not possible within laboratory timescale. For these samples, only the beginning part of the partial pressure gradient equilibration has been assessed (correlating to a pressure drop of < 0.1 MPa).

developed a full analytical solution to the differential equation describing the gas pressure inside the sample as a function of the distance along the sample and time to estimate permeability. developed a simple analytical expression to estimate permeability from the measured pressure curves. The simple analytical solution k=βμϕL2sf(Vsa/Vus,Vsa/Vds) is a function of the compressibility, β, and viscosity, μ, porosity, ϕ, length of the sample, L, slope of the differential pressure vs. time, s, and a function of the ratio between the volume of the sample (Vsa) and the volume of the up- and downstream reservoirs (Vus and Vds, respectively). The solution is accurate within 0.3  % of the full expression if the pore volume is less than the reservoir volumes, which is true for our experiments. Since the pressure difference in the two reservoirs is small, we used an average pore pressure to determine the compressibility and viscosity of the argon gas using the NIST database .

A separate hydrostatic oil-medium pressure vessel, capable of reaching high confining pressures, was used to measure the P- and S-ultrasonic elastic wave velocities using the pulse transmission technique . The measurements were conducted on the mutually perpendicular samples up to 260 MPa and at room temperature conditions (detailed description of the measurements found in ). The mechanical impulse is directed into the sample by mounting the lead zirconate titanate piezoceramic transducer inside a “head” assembly that also contains a buffer rod, reducing the dispersion of energy. The setup is configured so that one transducer transforms the electrical impulse (1 MHz resonance frequency) and emits a mechanical wave at the coupling of the transducer with the sample. After passing through the sample, another transducer converts the mechanical wave back into an electrical signal. The electronic system consists of a Hewlett Packard^® 214B Pulse Generator that is connected to the transducers with coaxial cables and the output is recorded directly with a computer. To prevent oil seepage from the confining fluid into the sample, a thin polyolefin heat shrink tube is fitted over the ends of the transducers and the sample.

The velocity in the rock is given by Vp,s=Ltsamplewithtsample=ttotal-tsystem, where the P- and S-wave velocities, Vp,s, are a function of the travel time through the sample, tsample, and its length, L. The travel time through the sample is determined by subtracting the travel time of the cabling in the source–receiver system, tsystem, from the total time of flight of the impulses recorded, ttotal.

The waveforms are recorded at stepwise increases or decreases in pressure in the loading and unloading cycles performed for each P- and S-wave experiment. Measurements were recorded across the full pressure range of 30 to 260 MPa of the apparatus to investigate the properties closest to present-day low-pressure conditions at the GTS (minimum principal stress 8 to 12 MPa, maximum principal stress 13–17 MPa ) and to study the poro-elastic effect on seismic velocities after crack closure at high pressure . The measurements were made at room temperature and in dry, undrained conditions. Recordings of the waveform were measured within ±2 MPa and a travel time accuracy of ±0.01 µs.

Velocity anisotropy (AV) was estimated from the maximum, minimum, and mean velocities using AVp,s=Vp,smax⁡-Vp,smin⁡Vp,smean⋅100. Estimates of the dynamic elastic moduli were also calculated for each experiment. The P- and S-wave moduli are represented in the general form as cxx=ρVp,s2. The P-wave moduli for the vertical (x3) and maximum horizontal (x1) samples are represented by c33 and c11, respectively. Similarly, the S-wave moduli, also known as shear modulus (μ), for the vertical and maximum horizontal samples are represented by c44 and c66, respectively. The elastic moduli are estimated by applying the isotropic equations to the vertical and horizontal components separately in order to estimate the P- and S-wave moduli . show that the error in applying the isotropic equations to the vertical and horizontal components separately in the absence of having the 45∘-oriented sample is negligible. The dynamic Young's moduli are approximated for the parallel (E1) and perpendicular (E3) components using the following equations: E1=c66(3c11-4c66)c11-c66,E3=c44(3c33-4c44)c33-c44. The dynamic Poisson's ratio for the parallel (ν1) and perpendicular (ν3) sample is calculated using the isotropic equation ν=12(Vp/Vs)2-2[(Vp/Vs)2-1].

The dynamic bulk modulus for the parallel (K1) and perpendicular (K3) sample is calculated using the isotropic equation K=ρ(Vp2-43Vs2).

In general there is a decrease in grain size in the TZ toward the MC (Fig. d; data available in Wenning et al., 2018). Additionally, millimeter-thick shear bands become more frequent nearer to the MC until reaching the sharp boundary with the MC. The MC itself is heterogeneously layered and folded. The compositional and microstructural transition from the “host” granodiorite, through the transition zone (TZ), and the mylonitic core (MC) are depicted in Fig.  and the rock composition is summarized in Table . The density of granodiorite samples irrespective of their proximity to the MC varies between 2.72 and 2.78 gcm-3 with porosity varying between 0.4 to 1  % (Table ). In general, the samples do not have visible open microcracks; thus, the porosity occurs between grain contacts (i.e., intergranular micropores). The density of the MC from both sampling locations is 2.80 and 2.84 gcm-3 and porosity estimates are 0 and 1 %, respectively.

The samples (S5 to S20.1) from the granodiorite are made up of various amounts of plagioclase (albite), quartz, K-feldspar, biotite/phlogopite, muscovite, and epidote. The amount of each mineral phase and microstructure depends on the vicinity to the mylonitic core. In the samples taken from the host granodiorite (S5 and S7) as well as samples farthest from the mylonitic core (S19.5 and S19.6) the microstructure and composition is similar. Plagioclase is the most abundant mineral phase (∼ 40 %). The sub-millimeter to > 10 mm big plagioclase grains are rounded to subangular. The grain size of plagioclase varies from sub-millimeter to > 10 mm. Needle-like sericite inclusions (< 0.1 mm) form within the plagioclase cleavage planes, indicating that hydrothermal alteration occurred. Quartz subgrains also develop along the boundaries and within large plagioclase grains. In larger plagioclase grains brittle fractures are filled with biotite and quartz. Quartz is the second-most abundant mineral phase (∼ 17 to 25 %). Quartz grains of variable size (< 1–2 mm) typically occur as many rounded to subhedral individual subgrains that form lenses or develop in the strain shadows of plagioclase clasts (Fig. ). The main difference between the host granodiorite (S5 and S7) and the beginning of the transition zone (S19.5 and S19.6) is the K-feldspar concentration, which is ∼ 15 to 17 % and ∼ 5 %, respectively. Phyllosilicates in the form of biotite and muscovite form anastamosing lenses of mixed muscovite and biotite with variable thickness across the thin section, which comprise about 15 to 18 % of the total mineralogy. Biotite forms < 0.1 to 1 mm grains, of which the individual grains are randomly oriented in the anastamosing lenses.

A progressive change in the overall microstructure, state of the individual minerals, and the mineral composition is observed in the transition zone between samples S19.7 and S20.1. In Sample S19.7 the foliation becomes more continuous across the thin section when compared to the host granodiorite samples. Plagioclase deforms brittlely in the form of fracturing (Fig. ), while grain boundary migration, undulose extinction, and subgrain rotation is observed in the quartz grains indicates ductility. Plagioclase and quartz grain size are similar to host granodiorite. Lenses of biotite and muscovite extend across the thin section more continuously; however, the lenses form variable thicknesses that wrap around the intermixed plagioclase and quartz. The anastomosing lenses are still present. In the samples nearest the MC (S20.0 and S20.1), the continuity and thickness of the mica-rich layers across the sample are the most developed. The foliation planes are oriented towards the parallel alignment of the individual grains. Biotite and muscovite grains are especially larger in samples S20.0 and S20.1, where individual grains can be >5 mm long. While lenses of fine-grained phyllosilicates occur, the overall grain size, continuity, layer thickness, and orientation of the individual grains is greater and more continuous. The total phyllosilicate amount increases from ∼ 15 to 18 % in samples S5 to S19.6 to ∼ 30 % in samples S20.0 and S20.1, with the other tectosilicates (plagioclase, K-feldspar, and quartz) reducing as a result. Biotite and quartz appear in the strain shadows of the plagioclase clasts (Fig. ).

The MC sample (S23.6) is constituted of very fine-grained ultramylonitic (more than 90 % grain size reduction) plagioclase (31 %), biotite (27 %), quartz (22 %), and epidote (12 %) making up the main mineral constituents. The foliation is defined by the biotite-quartz/plagioclase preferred shape orientation (layers typically < 0.1 mm). Recrystallized plagioclase and quartz form rotated clasts within the biotite-quartz foliation. The shear zones in the region are often interpreted as former mafic dykes e.g.,. The MC has a gradient of deformation between the Grimsel granodiorite in the TZ and the MC, and most notably there is a heterogeneous layering within the ultramylonite with larger grain lenses that are compositionally similar to the granodiorite (e.g., Fig. d). We interpret this structure more broadly as a mylonitic shear zone with a strain gradient of decreasing deformation away from the mylonitic cores.

Seismic velocities (Fig.  and Table ; data available in Wenning et al., 2018) are reported for the 30 MPa confining pressure measurement (i.e., the closest measurement to the stress magnitudes in the GTS). The measured velocities parallel to foliation at 30 MPa, (Fig.  and Table ) show an increase across the ∼ 0.5 m transition zone. In the two samples taken from the host granodiorite (S5 and S7) P-wave velocity parallel to the foliation (Vpx1) are ∼ 5.5 kms-1. Samples S19.5 to S19.7 taken from the beginning of the transition zone have comparable velocities to the host granodiorite samples (5.55 to 5.61 kms-1). Transitioning towards the mylonitic core the Vpx1 increases steadily and reaches a maximum in sample S20.1 (6.14 kms-1) directly adjacent to the MC at 20.2 m. The S-wave velocity follows a similar trend where the host samples and the samples farthest from the mylonitic core have a Vsx1 of ∼ 3.42 to 3.54 kms-1 and the velocities increase steadily and reach a maximum nearest the MC (S20.1: Vsx1 = 3.83 kms-1). The velocities measured perpendicular to the foliation, Vpx3 and Vsx3, fluctuate without a consistent trend between 4.98 and 5.21 kms-1 and 3.20 and 3.35 kms-1, respectively. The P- and S-wave anisotropy is generally lower away from and higher near the MC. However, there are outliers (e.g., S19.6), which can be attributed to bias from either a slower x3 velocity or a faster x1 velocity. The seismic velocities are in general agreement with previous measurements on Grimsel granodiorite and other granodiorite samples .

The dynamic elastic moduli behave like the velocities because the density remains consistent for each sample. Therefore, the velocities exert greater influence on the x1 and x3 moduli. Approaching the MC each respective dynamic elastic moduli increases by 10 to 20 GPa for the x1 sample, while the x3 remains almost constant, which corroborates previous measurements on Grimsel granodiorite . The dynamic Poisson's ratio remains relatively uniform throughout.

Additionally, seismic velocities were measured up to confining pressures of 260 MPa in order to determine the intrinsic crack-free velocities of the rocks (Fig. ). Both Vp and Vs velocity contours show that for a given confining pressure, the velocities parallel to foliation tend to increase to maximum values closest to the mylonitic core and that there is minimal sporadic variation in the perpendicular-to-foliation velocity measurements.

Permeability decreases (Fig.  and Table ) from the host granodiorite and farthest samples in the transition zone (0.99 to 8.38 × 10-19 m2) towards the samples nearest the mylonitic core (0.03 to 1.89 × 10-19 m2) along the x1 direction (data available in Wenning et al., 2018). The permeability perpendicular to the foliation x3 fluctuates from 0.52 to 4.14 × 10-19 m2. Permeabilities of similar host Grimsel granodiorite at 5 MPa are 10-18 to 10-20 m2 and measurements on Kola granodiorite samples range from approximately 10-18 to 10-20 m2 at effective pressures of 10 to 50 MPa . Directional permeability of the mylonitic core was measured on two samples from separate foliated shear zones due to difficulties in sample preparation (i.e., x3 is from 20.3 m depth and x1 is from 23.6 m depth). The permeability is 0.03 × 10-19 m2 parallel to foliation and 0.57 × 10-19 m2 perpendicular to the foliation. Flow along the boundary of a quartz-filled vein that crosscuts the perpendicular sample is believed to cause the increase in permeability perpendicular to foliation in the mylonitic core.

Many studies on the transition of elastic and fluid flow properties in and around fault cores and damage zones have been concentrated on outcrop material of brittle faults . present models for fault core geometries, with fault cores composed of fault gouge or cataclasite. expand this model to include both single-fault core damage zones and damage zones made up of several anastamosing faults. Laboratory measurements on samples from natural fault systems have led to the development of brittle fault permeability and elastic or mechanical properties that are microfracture dependent. In such systems, damage is concentrated in the fault core, which produces fault gouge or cataclasite that can either be higher or lower in permeability than the surrounding host rock. In the host rock directly contacting the fault zone, microfracturing due to strain displacement around the fault leads to increased permeability and decreased elastic or mechanical strength . Permeability decreases and elastic or mechanical strength increases moving away from the damage zone core, as the microfracture intensity decreases away from the shear zone e.g.,.

In ductile shear zones the alignment of anisotropic minerals in CPO or SPO due to strain accumulation has been a central focus for crustal reflectivity e.g.,. Additionally, temperature and fluid content can modify measured elastic wave velocities e.g.,. discuss the existence of transition zones in which the strain progressively increases towards the ductile shear zone core, causing gradual changes in the physical properties around such faults. The permeability across a mylonitic ductile shear zone was estimated from relationships between porosity and pore throat radius . They show that permeability is reduced in the central shear zone core but is higher in the surrounding strain gradient, which is higher than the host rock. performed triaxial deformation experiments in Westerly granite across the brittle–ductile transition with simultaneous measurements of porosity. The authors found that the deformation in the ductile regime is associated with compaction, while the brittle regime is primarily dilatant. For shear zones that have undergone the transition from ductile to brittle deformation, a competing process between microfracture and mineral-orientation-controlled physical properties can be envisaged.

The shear zone selected for measurements of seismic velocities and permeability in this study preserves both ductile and overprinting brittle structures. The shear zone penetrated by the borehole at GTS is characterized by foliation aligned with the mylonitic core that developed under viscous-flow deformation conditions . The foliation intensity is highest nearest the mylonitic core and decreases into the host granodiorite. The brittle fractures, which are bounded between the mylonitic cores, formed during a later brittle overprint. Figure  shows that the elastic and permeability transition into the mylonitic core is dissimilar to models derived from brittle fault zones e.g.,, even though brittle deformation is evident in the damage zone. The measurements from GTS show a trend of increasing velocities and stable to decreasing permeability in the plane parallel to foliation in the transition zone. In the direction perpendicular to foliation, the velocities and permeability have minor fluctuation in the vicinity of the mylonitic core. The ductile strain gradient in the transition zone does not appear to be influenced by the later stages of brittle deformation, as indicated by the increased seismic velocities and slightly decreasing permeability parallel to foliation towards the core in the transition zone. Should there be a brittle overprint, velocities would be expected to decrease due to microfracturing; however, this is not the case .

Instead, within the transition zone the both elastic and fluid flow properties are controlled by mineralogical changes in the rocks. Microfractures in thin section are scarce; thus, most of the < 1  % porosity are intergranular micropores. It is important to note that these changes are localized within ∼ 1 m of the ductile mylonitic core. Since the material is bored from an underground research lab, alteration processes and weathering should be suppressed in such samples. The mineralogy of the samples shows a gradual change in composition, losing tectosilicates (Pl, Fsp, and Qz) and gaining phyllosilicates (Bt and Ms), through the transition zone (Fig. ). There is an increase in foliation intensity towards the mylonitic core. The faster foliation-parallel velocities are controlled by the alignment of the platy phyllosilicate minerals . Higher foliation-parallel permeability compared to flow perpendicular, as measured in the GTS samples, has been measured in previous studies e.g.,. In low grade to ductile deformation, changes in the mineralogy and foliation structure alter the connection of intergranular micropores of the platy phyllosilicate and tectosilicate minerals e.g.,. In this study the changes in mineralogy, most notably the phyllosilicate-to-tectosilicate ratio, is a driver in both the velocity and permeability anisotropy, where microcracks do not have a driving role due to their scarcity.

Outside the MCs, fractures along the borehole wall are uncommon, as indicated by optical televiewer images . Between the MCs the density of fractures is high enough to have been termed damage zone. Although some fractures penetrate the MCs, they are typically quartz-filled and generally do not connect the granodiorite on either side of the mylonitic core. In the damage zone itself, fluid flow properties and elasticity are governed by the micro and macroscopic fractures. In the damage zone the velocities are decreased and the permeability increases, indicated by logging and pump tests in the borehole . Since the microfracturing does not appear to have influence outside the MCs, the displacement of these brittle features is likely small . Similar mapped faults in the region have cataclastic gouge or fault breccia , indicating that the fault at the GTS in not mature and has not accommodated much of the brittle displacement since the ductile structure is still preserved.

The measurements at the GTS lead us to hypothesize how fault properties might vary not only in geometry but also in the transient evolution of the fault itself. For the transition from a ductile to brittle fault system in crystalline rock, two end-member behaviors can be envisaged: ductile and brittle. While the rock is undergoing ductile deformation and localizing along the mylonitic core shear zones, the deformation processes would be accommodated by crystal–plastic flow. The highly strained and extreme recrystallization in the ultramylonite in the mylonitic core, along with the seritization of the plagioclase in the transition zone, indicate that fluids were present and likely localized in the mylonitic core during deformation. However, once the deformation and ductile structures were frozen in, the ductile transition zone behaves in a manner where elasticity parallel to foliation increases, transitioning from the host rock to the ductile core parallel to the foliation, and permeability decreases in the core. On the other hand, in the brittle damage zone model, microfracturing induces permeability enhancement and weak elasticity nearest the fault core e.g.,. This case study from the GTS is a hybrid between the two end-member systems.

During the two ductile deformation phases, slip was accommodated along localized foliated shear zones that are mylonitic and ultramylonitic . The transition from highly foliated and extremely recrystallized mylonitic cores towards the host granodiorite represents a strain gradient, which is ∼ 0.5 m thick in this study (Fig. a). The highest foliation intensity in the granodiorite nearest the mylonitic core also creates a change in bulk mineralogy (i.e., more phyllosilicates and less tectosilicates; Fig. ) and microstructure (i.e., more laminated; Figs. d and ), which alter the petrophysical properties that, once frozen in, behave as those measured in the transition zone and mylonitic core in this study (i.e., higher seismic velocity parallel to foliation and lower permeability nearest the mylonitic core). Fluid flow channelization in ductile shear zones have been argued based on mobile elements (Ca, Mg, Na, and K) concentration, stabile isotopes (δ18O), and fluid phase observations . However, in this study and the study by the lowest-permeability measurements come from the mylonitic core. While the current measurements come from the frozen-in ductile microstructure, dynamic porosity changes might be occurring during deformation e.g.,, which could enhance the permeability in the mylonitic core. It is possible that the ductile shear zone would behave in such a way that the long-term permeability of the shear zone is low, creating a pressure seal. Then, during rupture, the seal releases pore pressure and causes short-term permeability enhancement in the form of microfractures and micropores around grains. This is corroborated by fractures in the feldspar grains while at conditions with quartz recrystalization.

The later stages of brittle deformation formed along the suitably oriented ductile shear zones resulting in the current fault zone geometry at the GTS shear zone (Fig. b). The brittle deformation is bounded by the mylonitic cores. Due to the lack of damage outside these mylonitic cores, this system is believed to be an “immature” fault, with minimal brittle slip. Outside the mylonitic cores the properties are governed by the frozen-in ductile structures. Inside the mylonitic cores the properties are heterogeneously dispersed due to the macrofractures and their associated small-scale microfractures, which reduce the seismic velocity. Recent borehole measurements from pump tests in the damage zone indicate that the transmissivity in the damage zone is ∼ 10-8 to 10-7 m2 s-1, while the host granodiorite has a transmissivity of ∼ 10-13 to 10-12 m2 s-1 .

Finally, in the Grimsel region there are more “mature” brittle faults with a more pronounced damage zone and altered fault core composition . These mature brittle fault cores consist of gouges, cataclasites, and fault breccias in the middle of a fractured damage zone (Fig. c). The properties are expected to behave similar to the fault zone model of , where there is an inverse relationship between low seismic velocity (i.e., elasticity) and high permeability around the fault core, arising due to the extensive microfracturing in the brittle damage zone. The fault core in such a brittle fault typically has lower permeability than the surroundings due to the clay minerals in the gouge, cataclasite, or fault breccia e.g.,.

As rocks are exhumed and cooled, this system would transition from the ductile shear zone to a brittle damage zone. Thus, their mechanical properties and how fluids percolate through the entire shear zone would be highly dependent on the transient condition (depth or fault maturity) in which the fault occurs.

In crystalline rocks the elastic, mechanical, and fluid flow properties are important characteristics for the successful exploitation of natural resources. Mechanically, bulk strain can localize in the fault zone and the mechanical properties can govern earthquake rupture and fracture propagation. In terms of fluid flow, fault zones can act as both fluid conduits and barriers e.g.,. These can be significant in terms of building or releasing pore fluid pressures closely coupled to earthquake rupture . The elastic, mechanical, and fluid flow properties of fault zones are also directly linked to geothermal projects , as well as the security of long-term waste storage . With the technological advance of horizontal drilling and hydraulic fracturing the influence and interplay of mechanical and fluid flow anisotropy and heterogeneity are important when addressing stimulation in structurally complex environments e.g.,.

The case at GTS emphasizes the interplay between properties controlled by matrix mineral and fracture-controlled properties. The shear zone at GTS serves as a proxy structure expected in a geothermal reservoir. Understanding the orientation of subsurface foliation and proximity to shear zones can assist the efficiency of energy production. The interplay of matrix and fracture flow in such systems should be considered as an additional complexity. When considering the circulation of fluids, well placement in a geothermal injection/production system would need to address the geometry of subsurface heterogeneities. Present-day hydrothermal fluids in the Grimsel region flow in “pipe”-like channels (i.e., they are not uniform across the shear zone) . Mapping such structures in a crystalline basement will prove to be a challenge for the successful development of geothermal energy as a resource. Hydraulic stimulation is almost certainly required to enhance fluid flow in such crystalline systems. Mechanical anisotropy and heterogeneous pore geometries have been shown to have considerable influence on damage evolution and failure mode e.g.,. The importance of understanding the effect of mechanical discontinuities on hydraulic stimulation is shown with numerical models of damage propagation across mechanical layers e.g.,. The heterogeneous and anisotropic elastic and fluid flow properties at GTS show that mechanical/elastic foliation heterogeneity must be determined, along with stress magnitude and orientation when planning the optimal borehole placement, trajectory, and stimulation design. The low permeability in the ductile mylonitic cores measured in this study suggest that there might be significant compartmentalization around such structures.