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Earth and Planetary Science Letters 413 (2015) 2536
Contents lists available at ScienceDirect
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by(Ealsem
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ht00epartment of Geology, University of Otago, Dunedin 9054, New
Zealandock Mechanics Laboratory, Department of Earth Sciences,
Durham University, Durham, England, United Kingdomstituto Nazionale
di Geosica e Vulcanologia (INGV), Rome 00143, Italyipartimento di
Geoscienze, Universit degli Studi di Padova, Via G. Gradenigo 6,
35131 Padova, Italy
r t i c l e i n f o a b s t r a c t
ticle history:ceived 14 October 2014ceived in revised form 13
December 2014cepted 21 December 2014ailable online 14 January
2015itor: P. Shearer
ywords:calizationlciteugenamic weakeningrthquakesperiments
To determine the role of strain localization during dynamic
weakening of calcite gouge at seismic slip rates, single-slide and
slideholdslide experiments were conducted on 23-mm thick layers of
calcite gouge at normal stresses up to 26 MPa and slip rates up to
1 ms1. Microstructures were analyzed from short displacement (
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26 S.A.F. Smith et al. / Earth and Planetary Science Letters 413
(2015) 2536
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immitexdiic strains must occur at fault irregularities such as
bends and epovers (Sibson, 1986; Pavlis et al., 1993).Laboratory
experiments have demonstrated that bare rock sur-
ces and gouge layers experience dynamic weakening when the ip
velocity and sliding displacement approach values character-tic of
earthquakes (Di Toro et al., 2011). A variety of physical echanisms
have been proposed to explain the dynamic weak-ing behavior
observed in the laboratory and postulated to oc-r in natural
faults. In particular, mechanical and microstructural ta collected
from experiments performed on solid rocks (bare rfaces) are
consistent with the activity of ash heating and eakening at
asperity contacts (Rice, 2006; Beeler et al., 2008;oldsby and
Tullis, 2011; Kohli et al., 2011), silica gel lubrication oldsby
and Tullis, 2002; Di Toro et al., 2004) and frictional melt-g (Di
Toro et al., 2006; Nielsen et al., 2008).Gouge layers deformed at
high velocities typically show a rrow (99 wt% calcite, with
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S.A.F. Smith et al. / Earth and Planetary Science Letters 413
(2015) 2536 27
Fipaanbeinthampogrre
nogrmfu7wIncotounthjethonan
icrcacag. 1. Experimental set-up and sample assembly for gouge
experiments. (a) SHIVA apparatus with main components labeled. (b)
Scale diagram of gouge holder with main rts labeled. Details of
calibration tests can be found in the Supplementary Information of
Smith et al. (2013). The gouge layer (yellow) is contained between
the outer d inner rings that slide over a base disc located in the
stationary side (sliding contacts in red). Normal stress (n) is
applied to the gouge layer by the loading assembly hind the axial
column (Di Toro et al., 2010). Normal stress on the sliding rings
is modulated by inner and outer springs. (c) Photograph of calcite
gouge layer (35/55 mm t./ext. diameters) prior to an experiment. A
thin layer of high-temperature grease is applied to the sliding
surfaces of the rings to reduce friction. (d) Scale diagram showing
e geometry of the annular gouge layer. Where in contact with the
gouge layer, the rotary and stationary pieces have surface
roughness with wavelength of 400 m and plitude of 200 m. The dashed
line indicates the region where localization typically occurs in
the gouge layers (see text for details). (e) Optical
photomicrograph in plane larized light (main image) and
backscattered scanning electron microscope image (inset) of calcite
gouge compacted to 15 MPa without shear. The starting material has
a ain size of
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28 S.A.F. Smith et al. / Earth and Planetary Science Letters 413
(2015) 2536
FiSlithgoofanin0.0
foCath(s20
3.
3.
itatin(ipoeagoneaxg. 2. Evolution of shear stress, slip velocity
and axial shortening during slideholdslide experiments on calcite
gouge (s753) and solid cylinders of calcite marble (s758). p is
reported on a log scale to highlight the mechanical behavior in the
early stages of slip, but similar plots in Figs. 47 show slip
values on a linear scale. (a) Slide 1: e onset of weakening in
solid marble (grey arrow) occurred after c. 0.002 m of slip at a
slip velocity of c. 0.1 ms1 (grey star on slip velocity curve).
Instead, the calcite uges showed a prolonged phase of strengthening
prior to peak shear stress and dynamically weakened (red arrow)
after 0.2 m of slip at a much higher slip velocity 1 ms1 (red star
on slip velocity curve). Slide 2: the onset of weakening in both
solid marble and calcite gouge occurred after 0.002 m of slip at a
slip velocity of 0.1 ms1. (b) During slide 1, compaction was
negligible in the solid marble, but a phase of dilation occurred in
the calcite gouge layer prior to peak stress (between 0.1d 0.2 m
slip). Dilation ended in the gouge layer at peak stress and was
followed by compaction of 150 m. During slide 2, 50 m of further
compaction took place the calcite gouge layer after 0.05 m of slip.
In the solid marble, a short phase of dilation lasting 1 mm was
recorded just prior to peak stress (between 0.001 and 02 m). (For
interpretation of the references to color in this gure legend, the
reader is referred to the web version of this article.)
re each experiment. The experimental procedures for the solid
rrara cylinders were the same as those detailed above, except at a
different sample holder was used to grip the solid cylinders ample
procedures for solid cylinders described in Di Toro et al., 10;
Niemeijer et al., 2011).
Results
1. Dynamic weakening in calcite gouges and solid cylinders
Fig. 2a shows the evolution of shear stress and slip veloc-y for
two representative slideholdslide experiments performed 8.5 MPa
normal stress and 2.25 ms1 max. slip velocity us-g (i) a 2 mm-thick
layer of calcite gouge (red data, s753) and i) solid cylinders of
calcite marble (grey data, s758). Slip is re-rted on a log scale to
highlight the mechanical behavior in the rly stages of slip. For
clarity, only the slip velocity data for the uge experiment are
shown, but the slip velocity evolution was arly identical in both
experiments. Fig. 2b shows variations in ial displacement during
the two experiments, where positive
changes indicate compaction and negative changes indicate
dila-tion.
These two experiments illustrate important differences in the
mechanical behavior of calcite gouge layers and solid cylinders of
calcite marble at seismic slip rates. During slide 1, the solid
marble initially strengthened reaching peak shear stress (5.5 MPa)
after c. 0.002 m of slip (approximated by grey arrow in Fig. 2a).
The solid marble then dynamically weakened to a much lower shear
stress of 1 MPa after c. 1 m of slip. Dynamic weakening in the
solid marble initiated at a slip velocity of c. 0.1 ms1 (see grey
star on slip velocity curve in Fig. 2a).
Compaction was negligible during slide 1 in the solid marble
cylinders (Fig. 2b). Regularly spaced oscillations were observed in
the axial displacement (and shear stress) data with wavelengths of
c. 0.125 m in the solid marble experiments and c. 0.15 m in the
gouge experiments, equivalent in both cases to one full rotation of
the annular samples (which have slightly different internal and
external diameters as described above). These regular oscillations
reect small misalignments of the gouge holder or the rotary and
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S.A.F. Smith et al. / Earth and Planetary Science Letters 413
(2015) 2536 29
Fislicafacwenelayis
axof
prD(ac.imw(Ftw(wOdu
thdyarstimboshth
cotephtopa
wFineacslbl
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m0.oftomwacofma prdatotain
3.
imdithstw
toimcoanthFisltoMth(sanroth10th
3.
17mofg. 3. Distance to onset of dynamic weakening vs. normal
stress for 31 single-de and slideholdslide experiments performed on
2- or 3-mm thick layers of lcite gouge, as well as 13 single-slide
experiments on solid cylinders (bare sur-es) of calcite marble. In
all experiments the acceleration and deceleration rates re 7 ms2.
The data for slide 1 in gouges are approximated using best-t
expo-ntial decay functions (R2 = 0.81 for 3 mm thick layers, R2 =
0.99 for 2 mm thick ers). (For interpretation of the references to
color in this gure legend, the reader referred to the web version
of this article.)
ial columns, or slightly non-parallel sliding surfaces in the
case the solid marble samples.In comparison to the solid marble,
the calcite gouge showed a olonged phase of strengthening at the
start of slide 1 (Fig. 2a). ynamic weakening in the gouge initiated
after c. 0.2 m of slip pproximated by red arrow in Fig. 2a) at a
slip velocity of 1 ms1 (see red star on slip velocity curve in Fig.
2a). The min-um shear stress obtained by the gouge layer following
dynamic eakening was slightly higher than in the solid marble
samples ig. 2a). The gouge layer initially compacted by 50 m, then
be-een 0.08 and 0.2 m a transient phase of dilation was recorded
100 m dilation; Fig. 2b). Dilation ended once peak shear stress as
reached in the gouge layer, followed by renewed compaction. verall
compaction of c. 150 m was recorded in the gouge layer ring slide 1
(Fig. 2b).During slide 2, the evolution of shear stress was similar
in both e solid marble and calcite gouge (Fig. 2a). In both
experiments, namic weakening initiated after c. 0.002 m of slip
(grey and red rows in Fig. 2a) at a slip velocity of c. 0.1 ms1
(grey and red ars on slip velocity curve in Fig. 2a). The decay
from peak to min-um shear stress occurred over roughly the same
slip distance in th experiments, although as observed in slide 1
the minimum ear stress obtained by the gouge layer was slightly
higher than e solid marble cylinders (Fig. 2a).Compaction during
slide 2 in the gouges was relatively minor mpared to slide 1,
although 50 m of compaction occurred af-r c. 0.03 m of slip (Fig.
2b). In the solid marble, a short-lived ase of dilation occurred
between c. 0.001 and 0.002 m, just prior peak shear stress (Fig.
2b). Following peak shear stress, com-ction of 100 m was
observed.Fig. 3 summarizes the slip distance required to initiate
dynamic
eakening in calcite gouge layers (e.g. grey and red arrows in g.
2) and its dependence on normal stress and gouge layer thick-ss for
31 single-slide and slideholdslide experiments with an celeration
rate of 7 ms2. Also shown are data from 13 single-ide experiments
on solid cylinders (bare surfaces) of calcite mar-e. The main
results can be summarized as follows (Fig. 3):i) During slide 1 in
calcite gouges (red and green lled symbols) e initial strengthening
phase lasts between c. 3 and 30 cm. The ngth of the strengthening
phase decreases with increasing nor-al stress, and it also
decreases in thinner gouge layers. Above a rmal stress of 1520 MPa,
the length of the strengthening phase ay remain constant with
increasing normal stress, although more ta are required to conrm
this.ii) During slide 2 in gouges (red and green open symbols),
the
rengthening phase is much shorter, lasting a few millimeters or
ss. The strengthening phase in slide 2 is independent of both rmal
stress and layer thickness.iii) The length of the strengthening
phase during slide 2 in uges is comparable to that observed in
solid cylinders of cal-te marble (grey symbols) over the range of
investigated normal resses.
2. Microstructural evolution of calcite gouge layers
A series of experiments was performed at 8.5 MPa nor-al stress
with increasing total displacements in the range of 010.35 m to
provide insights in to the microstructural evolution the calcite
gouge layers during the transition from strengthening dynamic
weakening. Observations from three of these experi-ents are
summarized below (Figs. 4, 6, 7). All three experiments ere
performed with 3 mm-thick gouge layers, imposing a target
celeration rate of 7 ms2 and a target maximum slip velocity 1.1
ms1. An additional small displacement (0.028 m) experi-ent was
performed at a higher normal stress of 17.3 MPa with dolomite
strain marker constructed in the calcite gouge layer ior to
shearing (Fig. 5). In part a of each of Figs. 47, the grey ta
curves show the shear stress evolution of experiments taken larger
displacements, which serve to illustrate the typical dis-nce
required for the onset of dynamic weakening (also see data Fig.
3).
2.1. 0.01 m slip (s784)Due to the small displacement in this
experiment, the max-um slip velocity obtained was 0.27 ms1 (Fig.
4a). The total splacement was approximately an order of magnitude
lower than e c. 0.1 m required to initiate dynamic weakening at
this normal ress (grey curve in Fig. 4a; data in Fig. 3).
Compaction of c. 50 mas measured after 0.01 m of slip (Fig. 4a).The
calcite gouge layer contains a well-dened shear band up
20 m thick (between white arrows in Fig. 4b), dened in SEM ages
by a much ner grain size and more compact appearance mpared to the
surrounding gouge layer (Fig. 4b, c). The grain size d overall
appearance of the bulk of the gouge layer (i.e. outside e shear
band) are similar to the gouge starting material (compare gs. 1e
and 4b). As observed in all gouge experiments that reached ip
velocities >0.1 ms1, the shear band developed sub-parallel gouge
layer boundaries (a Y-shear after e.g. Logan et al., 1979;arone and
Scholz, 1989) and at a distance of c. 100 m from e surface
roughness on the stationary side of the gouge holder ee position of
dashed lines in Fig. 1d). The shear band consists of gular to
sub-angular calcite grains
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30 S.A.F. Smith et al. / Earth and Planetary Science Letters 413
(2015) 2536
Fibothgonarto
veco
ncogoDFigrsts(F
c.lacooflitrobydunototh
cog. 4. Mechanical data and microstructures of experiment s784,
stopped after 0.01 m of slip. (a) Plot of shear stress, slip
velocity and axial displacement vs. slip. The inset x shows a
detail of the rst 0.012 m of slip. (b) SEM image of gouge layer. A
narrow shear band of ne grain size (outlined by the white arrows)
is developed close to e stationary side of the gouge holder (see
approximate position of localization in Fig. 1d). The inset shows a
schematic representation of the location of the preserved uge layer
with respect to the original gouge layer boundaries. (c) Detail of
shear band in part b showing angular calcite clasts
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S.A.F. Smith et al. / Earth and Planetary Science Letters 413
(2015) 2536 31
Fiaxof(cawagelowgore
3.
inCodiwg. 5. Experiment s781 (17.3 MPa normal stress) performed
with a dolomite strain marker and stopped after 0.028 m of slip.
(a) Evolution of shear stress, slip velocity and ial displacement
in experiment s781. (b) SEM and EDS images of the calcite gouge
layer and dolomite strain marker. The small greyscale image shows
an SEM mosaic the entire preserved gouge layer and a representation
of the total experimental displacement. The colored image shows an
EDS chemical map of Mg (dolomite) and Ca lcite) distribution in the
gouge layer that was used to identify the distribution of dolomite
and reconstruct the strain distribution after shearing. The
dolomite marker s initially sub-perpendicular to gouge layer
boundaries. (c) Tracing of the dolomite strain marker and
interpretation of the three strain domains distinguished from the
ometry of the marker; low-strain domain, intermediate-strain
domain, and high-strain shear band. A series of R1-Riedel shears
offset the edges of the strain marker in the -strain domain. (d)
SEM image (location shown in parts b, c) of the high-strain shear
band. The shear band is 50100 m wide and much ner grained than the
adjacent
uge. The white arrow shows where one edge of the shear band is
dened by a discrete surface. (For interpretation of the references
to color in this gure legend, the ader is referred to the web
version of this article.)
2.3. 0.2 m slip (s631)Experiment s631 was stopped after 0.2 m of
slip (Fig. 6a). Dur-
g deceleration the shear stress recovered to nearly its peak
value. mparison to other experiments performed under the same
con-tions (grey curve in Fig. 6a; data in Fig. 3) indicates that
s631 as stopped approximately mid-way through the dynamic weak-
ening phase. A total of c. 200 m of compaction was measured,
although this included a transient phase of dilation between c.
0.05and 0.12 m (Fig. 6a). Dilation ended once peak shear stress was
reached and dynamic weakening initiated.
Compared to experiments stopped before the onset of dynamic
weakening (e.g. Figs. 5, 6), the bulk of the gouge layer has a
much
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32 S.A.F. Smith et al. / Earth and Planetary Science Letters 413
(2015) 2536
Fig. 6. Mechanical data and microstructures of experiment s631,
stopped after 0.2 m of slip. (a) Plot of shear stress, slip
velocity and axial displacement. (b) Optical pho-toscsliofinre
ntishtoobTh