Introduction
Brittle faults and ductile shear zones and their associated damage and high-strain zones have a localized yet influential impact on crustal mechanics and
fluid flow see reviews by. Physical properties
in and around the fault core and damage zone in the brittle regime generally
differ from the host rock by several orders of magnitude, asserting
tremendous influence on fluid flow, deformation, earthquake rupture, and the
development of economically exploitable resources. Less is known about the
nature of the physical properties of ductile shear zones, and their role in
crustal mechanics and fluid flow distribution once they are exhumed
e.g.,. Thus, understanding the nature of the
geometrical distribution and temporal evolution of the properties associated
with ductile shear zones and their transition into brittle faults and damage
zones is integral to assess crustal mechanics and fluid flow distribution.
The characterization of brittle faults and damage zones has received much
attention see review by. Previous studies from the
laboratory (cm) to field outcrop (km) scale have developed into generalized
models for the mechanical and hydraulic behavior of fault zones
e.g.,. These models
suggest that the fault zone consists of single or multiple high-strain cores
surrounded by a damage zone where the physical properties are a function of
the rock matrix, fracture density, and fault core. Brittle faults generally
increase in fracture density in the damage zone towards the fault core
, thereby increasing permeability and
reducing elastic and mechanical strength from the intact rock towards the
central fault core. show that the microfracture density
that enhances permeability around brittle faults scales with the displacement of
the fault. Laboratory experiments on Westerly granite indicate that
increasing permeability due to microfracturing occurs regardless of the
tectonic faulting regime .
In the ductile deformation regime, shear zones are understood to develop
anisotropic properties due to mineral alignment of anisotropic minerals in
preferred elongation directions (shape-preferred orientation or SPO) and/or
alignment of the crystallographic axis (crystallographic preferred
orientation or CPO) of minerals . The characteristics of
ductile shear zones have been studied in terms of their anisotropic velocity
structure to assess observations in middle to lower crustal seismic
reflectivity see review by. suggested
that the physical properties in ductile shear zones should also be considered
as transitional (i.e., the seismic velocities would grade into the ductile
shear zone core). The strength of ductile shear zones is typically studied in
terms of viscous rheology e.g.,. However, these shear
zones are often “frozen in” and preserve their textural features that when
exhumed behave with elastic and frictional failure criteria in the upper
crust. In preserved ductile shear zones, mechanical and fluid flow properties
have typically been studied separately. studied the porosity
and mineral structure across a mylonitic shear zone. Using empirical
relationships between porosity and pore throat diameter, these authors were
able to discern that the permeability decreases in the highest strained
sample. performed triaxial deformation experiments across
the brittle–ductile transition in Westerly granite and show that porosity
changes in the ductile regime is compactant, while the brittle regime is
marked by dilation.
The models for elasticity and permeability through brittle and ductile shear
zones have been mostly derived from outcrop examples and references
therein. To date, there have been limited systematic mechanical
and fluid flow studies on boreholes that directly penetrate fossil ductile
shear zones. Drilling into fractured crystalline rock for geothermal
exploitation has been ongoing since the 1970s . Recent
drilling through the Alpine Fault in New Zealand revealed how ductile
mylonites have been exhumed, altering the rocks to a typical brittle fault
damage zone in the vicinity to the fault . Although much
precaution is taken in outcrop studies, core material provides the
opportunity to sample systematically into a fault zone eliminating issues of
surface weathering and processes that may alter physical properties. However,
precaution should be taken when assessing the extent and timing of
hydrothermal alteration associated with faults at depth. Additionally, focus
on the relationship between elasticity, mechanical strength, and permeability
in the transition zone and core of faults has been inherently focused on
brittle structures. However, with the importance of meeting sustainable energy demands via geothermal energy and promoting safe geological waste disposal, the impact of preserved ductile structures in granitic rocks on mechanics and fluid flow are also if increasing significance..
Recent drilling at the Grimsel Test Site (GTS), an underground research
laboratory owned and operated by NAGRA (National Cooperative for the Disposal of Radioactive Waste) located
in central Switzerland, penetrated a ∼ 5 m thick ductile–brittle
fault damage zone relict in the Grimsel granodiorite host rock
. Alpine tectonism produced multiple stages of ductile and
subsequently brittle deformation in the Grimsel granodiorite, a member of the
Aar massif e.g.,. The
relict shear zone penetrated by the borehole is bounded by two foliated
ductile shear zones, which initially localized ductile deformation and
further reactivated brittlely between the two foliated shear zones. This
study concentrates on characterizing the elastic and fluid flow properties
from the surrounding granodiorite rock mass through the ductile transition
zone into the bounding foliated shear zone. Seismic P- and S-wave velocities
(Vp and Vs) and gas permeability (k) were
measured on core samples in the laboratory. This paper emphasizes that the
mineralogical changes entering the shear zone influence changes in physical
properties near the ductile–brittle damage zone. The results also provide
insight on the transient behavior of faults during the transition from
ductile to brittle regimes through exhumation processes and provide insight
on their effect on economic exploitation of such shear zones in terms of
geothermal energy or geological waste disposal.
Geologic setting and core details
(a) Geologic map of the Grimsel pass region
after; (b) borehole orientations
(this study – red borehole) with respect to the underground research
laboratory after; (c) the borehole in
this study depicting the location of the damage zone with coring and stress
measurement locations projected along the borehole. The shear zone at the
base is divided into three components: (1) transition zone (TZ),
(2) mylonitic core (MC), and (3) damage zone (DZ). (d) Photograph of
a saw-cut cross section through part of the transition zone and the mylonitic
core (black ellipses show subcored cylinders inclined to the cut surface).
Optical microscopy and
QEMSCAN® images of thin sections for each
x1 sample (S7x, S19.5x, S19.7x, S20.0x, and S23.6x). Panel
(a) shows the cross-polarized light image of the sample with the
corresponding QEMSCAN® area in (b).
Panel (c) depicts a particular microstructure of the sample. S7x –
typical host rock texture with twinned feldspar and quartz. S19.5x –
elongated feldspar with quartz in strain shadows between fine-grained mica.
S19.7x – fractured feldspar with quartz grains filling the fracture. S20.0x
– rounded sericitic feldspar clast with fine-grained quartz and mica
infilling the strain shadows between lenses of large mica grains and fine
recrystallized quartz/mica. S23.6x – very fine-grained foliated biotite and
quartz grains surrounding euhedral plagioclase. Sample locations depicted in
Fig. .
The GTS is located in the Aar Massif in central Switzerland
(Fig. a). The underground laboratory is situated in Grimsel
granodiorite in the Haslital. place the age of the Grimsel
granodiorite at 299 ± 2 Ma. From field relations and dating, the
Grimsel granodiorite has the same age as the Central Aar granite. These
intrusions postdate Variscan collision, and there are no identified
pre-Alpine deformation structures.
Detailed deformation histories of the Grimsel region are available
, which are
summarized here. From argon–argon and rubidium–strontium dating, as well as
field relations, beginning around 21 Ma and continuing until approximately
10 Ma, ductile deformation resulting from transpression created NNE–SSW,
E–W, and NW-SE striking shear zones with steep dips to the south
(Fig. a). Ductile deformation is believed to occur in two
stages: (1) NNW-vergent thrusting from 21 to 17 Ma and (2) transpression
causing dextral shearing of preferentially oriented oversteepened stage 1
structures from 14 to 10 Ma. Beginning around 9 Ma steady exhumation caused
retrograde ductile–brittle deformation in the form of discrete fractures,
and subsequent embrittlement of these shear zones, which has produced fault
breccias, cataclasites, and fault gouge.
The Aar granites experienced 300–450 ∘C and 150 to 250 MPa peak
conditions during Alpine metamorphism
. Our thin section observation
shows fracturing of feldspar and undulose extinction along with subgrain
boundaries in quartz, which are consistent with the inferred metamorphic
temperatures.
In 2015, a series of boreholes were drilled in the Grimsel granodiorite
(Fig. b) for stress measurements, petrophysical property
characterization, and hydraulic stimulation of the shear zones
. The core material used in this study comes from the borehole
drilled from an offset of the main tunnel in the GTS that penetrates two
parallel shear zones. The well was drilled from 480 m below the ground
surface in a subhorizontal trajectory with an azimuth of 319∘. The
well penetrates ∼ 20 m of mostly non-fractured granodiorite
(Fig. c). The granodiorite is foliated and at 20.2 m intersects
a 20 cm thick foliated mylonitic shear zone, also defined as a foliated
mylonitic core (MC). The foliation intensity in the granodiorite decreases
towards the host rock ∼ 0.5 m from the MC through the transition
zone (TZ) and is concordant with the foliation in the steeply dipping
E–W-oriented MC . The TZ has a gradual decrease in grain
size of both matrix grains and the felsic clasts with more frequent mylonitic
shear bands towards the MC (Figs. d and ). The
MC itself is heterogeneously banded with mylonite and ultramylonite layers. A
brittle damage zone (DZ) mixed with small < 5 cm thick mylonitic
shear zones is bounded between the MC at 20.2 m and another 20 cm thick MC
at the end of the borehole (Fig. ). Less than 1 mm aperture
quartz-filled fractures intersect the MCs originating from within the damage
zone. However, these do not appear to penetrate entirely through the MCs.
Methods
Sample selection, preparation, and characterization
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.
Density, porosity, and permeability measurements
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 .
Elastic wave velocity measurements and calculations
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,sminVp,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).
Results
Characterization
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. ).
Sample composition, seismic velocity, and
permeability across the transition zone into the mylonitic core. Depth
corresponds to sample name in Table .
(a) Tectosilicates – quartz, plagioclase, and K-feldspar (red);
phyllosilicates – biotite and muscovite (green); and epidote (grey).
(b, c) P- and S-wave velocity parallel to foliation (orange) and
perpendicular to foliation (blue). Measured at 30 MPa hydrostatic confining
pressure. (d) Permeability parallel to foliation (orange) and
perpendicular (blue). Permeability is reported as the median value measured
for each sample, and the bars show the range of values in terms of minimum
and maximum. Measured at 10 MPa effective pressure. For all panels the
shaded region depicts the range of values from the host
granodiorite samples (S5 and S7), the circular markers show the values
measured through the transition zone (TZ), and the triangular markers show
the values measured in the mylonitic core (MC). Error bars are depicted where
the error is larger than the marker size.
Summary of sample composition: sample name refers to
depth in the borehole; rock type refers to either the host granodiorite,
transition zone (TZ), or mylonitic core (MC); dry bulk density and porosity
are reported as an average of individual measurements for each sample
(x1 and x3); and mineral composition is derived from the
QEMSCAN analysis of the x1-thin section in % area. Mineral
abbreviations: Bt – biotite; Phl – phlogopite; Ms – muscovite; Ep –
epidote; Ab – albite; Kfs – K-feldspar; and Qz – quartz.
Rock
Density
Porosity
Bt+Phl
Ms
Ep
Ab
Kfs
Qz
Other
Sample
type
(gcm-2)
(%)
(%)
(%)
(%)
(%)
(%)
(%)
(%)
S5
Host
2.73
< 1
9
6
6
43
17
17
2
S7
Host
2.73
< 1
10
4
5
39
15
26
2
S19.5
TZ
2.74
< 1
9
9
3
46
5
25
2
S19.6
TZ
2.75
< 1
6
8
5
45
5
28
2
S19.7
TZ
2.76
< 1
10
7
7
50
3
21
2
S19.8
TZ
2.75
< 1
9
7
4
45
3
30
2
S19.9
TZ
2.73
< 1
12
10
1
56
3
16
2
S20.0
TZ
2.77
< 1
15
13
1
42
4
22
2
S20.1
TZ
2.73
< 1
13
16
0
42
3
25
2
S23.6
MC
2.82
< 1
27
0
12
31
5
22
2
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.
(a) and (b) display the results for foliation parallel velocities (x1) separated
into Vp (a) and Vs (b).
(c) and (d) display the results for foliation perpendicular velocities (x3) separated into Vp (c)
and Vs (d).
For Vp and Vs red colors indicate faster velocities and blue colors depict slower velocities
(magnitude defined on the color bar).
Velocity measurements and elastic moduli calculation
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.
Summary of density, porosity, elastic properties and anisotropy, and
permeability obtained from laboratory measurements. Laboratory velocities
measured at 30 MPa confining pressure and permeability measured at 10 MPa
effective pressure.
Vp
Mean Vp
AVp
Vs
Mean Vs
AVs
E
μ
K
ν
k (m2)
Sample
Direction
(km s-1)
(km s-1)
(%)
(km s-1)
(km s-1)
(%)
(GPa)
(GPa)
(GPa)
×10-19
SBH5
x1
5.52
5.34
6.75
3.43
3.35
4.52
76
32
40
0.19
8.38
x3
5.16
3.28
68
29
33
0.16
4.14
SBH7
x1
5.50
5.30
7.55
3.42
3.32
6.30
76
32
40
0.18
5.95
x3
5.10
3.21
66
28
33
0.17
1.70
SBH19.5
x1
5.59
–
–
3.49
–
–
79
33
41
0.18
0.99
x3
–
–
–
–
–
–
–
SBH19.6
x1
5.61
5.30
11.55
3.54
3.37
10.01
81
35
41
0.17
2.11
x3
4.99
3.20
65
28
31
0.15
2.72
SBH19.7
x1
5.55
5.26
10.79
3.51
3.43
4.75
79
34
39
0.16
5.29
x3
4.98
3.35
67
31
27
0.09
1.36
SBH19.8
x1
5.70
5.37
12.14
3.55
3.43
7.15
82
35
43
0.18
5.60
x3
5.05
3.31
68
30
30
0.12
1.05
SBH19.9
x1
5.76
5.38
14.43
3.63
3.48
8.28
84
36
43
0.17
1.89
x3
4.99
3.34
67
30
27
0.09
1.37
SBH20.0
x1
5.99
5.60
13.85
3.68
3.48
11.55
90
38
49
0.20
1.55
x3
5.21
3.28
70
30
35
0.17
0.52
SBH20.1
x1
6.14
–
–
3.83
–
–
95
40
50
0.18
0.03
x3
–
–
–
–
–
-
-
SBH20.3
x3
–
–
–
–
–
–
–
–
–
–
0.57
SBH23.6
x1
–
–
–
–
–
–
0.03
Permeability 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.
Discussion
Shear zone characterization
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.
Conceptual model for the characteristics
of faults in crystalline rock and their associated petrophysical properties
during transition from ductile to brittle deformation conditions.
(a) Formation of ductile shear zones with increasing foliation in the transition
zone (TZ) nearest the mylonitic core (MC), (b) immature brittle fault with
ductile transition zone preserved outside the MCs and the damage zone (DZ)
between, and (c) mature brittle fault and DZ, where the brittle features
dominate the fluid flow and elastic properties near the brittle fault cores
dominated by gouge (FC-G) and breccia/cataclasite (FC-B/C)
after.
Ductile–brittle transition in the fault zone
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.
Implication for geothermal energy production and waste disposal
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.
Conclusions
The shear zone at the GTS displays contrasting behavior in a single shear
zone due to the fault evolution from ductile to brittle deformation. The
ductile history is frozen in outside the mylonitic cores and is characterized
by a transition zone of increasing foliation intensity from the host
granodiorite towards the mylonitic cores. In the transition zone, the seismic
velocity of the foliation-parallel samples increases towards the mylonitic
core, while the velocity perpendicular to the foliation remains fairly
constant. The permeability is also anisotropic and is lower in the samples
nearest to and within the mylonitic core, suggesting that both permeability
and seismic velocities in the transition zone are greatly influenced by the
amount and texture of phyllosilicates in the rock mass. Recent brittle
deformation is bounded between the foliated mylonitic cores and constitutes
macroscopic fractures and associated microfractures that rarely penetrate
through the mylonitic cores.
The evolution of the system from the formation of the localized shear zones
in the earliest observed ductile regime (ca. 21 Ma) and the current brittle
regime follows three steps: (1) the localization of ductile deformation,
(2) shearing along the rheological discontinuity causing higher foliation
intensity in the granodiorite nearest to and mylonitization of the mylonitic
core, and (3) subsequent brittle deformation along the foliated mylonitic
cores. We hypothesize that the properties of this shear zone suggest that
brittle deformation is immature in the sense that the overprint has not
effected the ductile transition zone.
Encountering such structures in geothermal reservoirs or waste disposal sites
would prove to be challenging. The elastic, mechanical, and fluid flow
heterogeneity caused by the mylonitic cores and their juxtaposition to a
brittle damage zone would need to be considered for the optimal engineering
design of any reservoir usage system.