Introduction
Crustal-scale deformation is commonly localized into major faults, in the
upper crust and ductile shear zones in the lower crust
e.g.. This
observation requires the presence of weakening mechanisms that both initiate
and sustain strain localization into lithospheric high-strain zones
. Such mechanisms include (1) high fluid pressures, which
reduce the effective coefficient of friction and lead to localized and
transient embrittlement as well as enhance
microfracturing and mass transfer processes ; (2) grain-size
reduction and activation of diffusive mechanisms, particularly in the
presence of a reactive fluid phase ; (3) geometric
softening by alignment of easy slip planes and the shear plane, by
development of a crystal-preferred orientation ;
(4) reaction weakening by creation of grain-scale porosity
and a fine-grained reaction product ; and (5) introduction of a new, weaker, mineral phase through retrograde reactions,
for example the development of an interconnected phyllosilicate fabric
.
The mechanisms listed above are only effective at partitioning strain into a
major shear zone if such a localized zone already exists. Shear zone
initiation has been linked to the existence of brittle precursors, providing
tabular zones of fine-grained material deforming by diffusion creep at high-strain rates with low driving stresses .
However, shear zones can also reactivate existing ductile fabrics
or initiate through localized reaction softening
. In a prograde metamorphic setting, it is fairly
straightforward to imagine how an existing brittle discontinuity will
transform into a ductile shear zone during progressive burial and increased
temperature. Similarly, fluid release during prograde dehydration at
greenschist facies or above will provide a fluid phase, likely under
low-porosity, undrained conditions, which can lead to increased fluid pressure
associated with reaction weakening e.g.. On the other
hand, in a retrograde metamorphic setting, rocks are not dehydrating, but are
fluid absent and capable of rehydration e.g.;
therefore a fluid source required for fluid assisted weakening is less
obvious e.g.. Similarly, if a shear zone
develops within dry, previously migmatized crust under retrograde conditions,
there are unlikely to be brittle discontinuities on which a shear zone can
initiate , unless fine-grained pseudotachylytes have formed
through propagation of earthquake ruptures into the viscous regime
or local brittle structures formed through
fluid-absent, high-stress failure or acceleration within
crystal-plastic shear zones . Under retrograde conditions, it
is, therefore, not intuitive to envision how initial shear zone weakening
occurs, as an external fluid is required for reaction weakening, but requires
deformation to be introduced to the site of initiation. It is also not clear
what weakening mechanisms remain active through the life of a retrograde
shear zone, as elevated fluid pressures are hard to maintain in an open
system, but retrograde reactions may lead to the growth of fine-grained
products and the formation of new interconnected weak fabrics. We address
these questions here by investigating the retrograde metamorphic history of
an exhumed, crustal-scale strike-slip shear zone, the Kuckaus Mylonite Zone
in southern Namibia, to constrain its pressure–temperature–fluid history
and discuss the implications of our results for weakening mechanisms and
strain localization.
Geological setting of the study area. (a) Map of southern
Africa showing the extent of the Namaqua Metamorphic Complex (NMC).
(b) Location of the study area to the south-west of Aus in southern
Namibia. The Kuckaus Mylonite Zone (KMZ) forms part of the larger Marshall
Rocks–Pofadder shear zone (MRPSZ) and has a similar orientation, kinematics
and age as the southern Namaqua Front that separates the Richtersveld and
Gordonia subprovinces of the NMC and the Lord Hill–Excelsior shear zone
that separates the Gordonia and Konkiep subprovinces. After
and .
The problem of fluid flow and localized deformation in ductile shear zones is
not restricted to exhumed examples, such as those presented here, but is also a
current question of interest in active plate boundaries. For example, the
discovery of tectonic tremor and slow slip on the deep extensions of the
Alpine and San Andreas faults highlight
the presence of localized structures below the brittle–viscous transition.
The nature of these seismic signals also require the deep ductile roots of
these major faults to be significantly weaker than their surrounding rocks
. As opposed to similar features in
subduction zones, commonly associated with prograde metamorphism and high
fluid pressures, the tremors on the deep San Andreas and Alpine faults cannot
be easily explained by in situ production of fluids in low-porosity
fault zones. It is possible that San Andreas fault tremor is related to
retrograde weakening mechanisms associated with the introduction of an
external fluid . An additional question to address here
is, therefore, how fluid flow and shear zone weakening mechanisms on the
Kuckaus Mylonite Zone can serve as an analogue to those occurring on the deep
extension of active faults exhibiting tremor and slow slip under retrograde
P–T conditions.
Field photographs of mafic lenses within the KMZ.
(a) Detail of the unsheared core of a lens, showing coarse-grained
amphibolite with evidence of small-scale migmatization in the form of
leucosome stringers and ponds. (b) Boundary of mafic lens showing
the increase in strain over a distance of 35 cm. Hammer is 40 cm long.
Regional and outcrop geology
The Kuckaus Mylonite Zone KMZ; first described by forms
a segment of the larger Marshall Rocks–Pofadder shear zone system
MRPSZ, after that extends for more than 550 km from the
Atlantic coast north of the town of Lüderitz in Namibia to its southern
termination near the town of Pofadder in South Africa
Fig. ;. The MRPSZ strikes WNW–ESE
and exhibits a dextral sense of strike-slip displacement . The MRPSZ is localized in mid-crustal rocks of
the Namaqua Metamorphic Complex that experienced high-temperature
(T)–low-pressure (P) amphibolite- to granulite-facies peak metamorphic
conditions during the Mesoproterozoic at ca. 1200–1050 Ma . The development of the MPRSZ post-dates peak
metamorphism, with deformation having occurred under retrograde amphibolite-
to greenschist-facies conditions, at ca. 1005–960 Ma . Shearing along the MRPSZ resulted
in the development of proto- to ultramylonites, indicating that deformation
predominantly occurred by viscous flow of quartz and micas ; however, a discrete cataclastic
overprint indicative of lower-temperature brittle deformation is present in
the south-eastern parts of the MRPSZ .
Photomicrographs. (a) Overview of the unsheared texture in
KMZ28. Long axis is 4.5 mm, plane-polarized light (ppl). (b) KMZ29
showing elongated hornblende grains alternating with chlorite–epidote foliae.
Long axis is 4.5 mm, ppl. (c) KMZ30 consisting of rounded
plagioclase and hornblende porphyroclasts in a fine-grained and mylonitized
matrix. Long axis is 4.5 mm, cross-polarized light (xpl). Panels (d) and
(e) illustrate the largely static breakdown of hornblende to
chlorite and epidote with no preferred alignment in KMZ28. Long axis is
2.2 mm and (d) is in ppl and (e) in xpl. Panels (f) and
(g) illustrate subgrain formation and disaggregation of hornblende
parallel to the foliation in KMZ29. Long axis is 2.2 mm and (f) is
in ppl and (g) in xpl.
In the study area, the KMZ occurs in granitic gneisses that form part of the
Aus granulite terrain . These rocks experienced
peak metamorphic conditions of 5.5 kbar and 825 ∘C, with the timing
of metamorphism constrained at ca. 1065–1045 Ma .
Metamorphism is inferred to have been dominated by heating and cooling, with
only minor attendant crustal thickening and burial . The
post-peak metamorphic retrograde path involved near-isobaric cooling,
indicating that the terrain remained at depth as it cooled to a stable
geotherm .
The shear zone core of the KMZ is about 1000 m in width and consists of
anastomosing high-strain ultramylonite zones that wrap around lower-strain
lozenges . Rock types within the KMZ are dominated by
granitic gneisses and mylonites, and only minor enclaves and lenses of
retrogressed mafic granulite are present. These mafic lenses occur as
discrete units, range from a few centimetres to 10–15 m long and are up
to 5 m in width . Larger mafic lenses have a core of
coarser-grained gneisses that are not pervasively mylonitized and in which
remnant migmatitic granulite-facies textures can be recognized
(Fig. a). The core is enveloped by more intensely sheared
and retrogressed mylonitic schists, with the increase in strain occurring
over a distance of 10–50 cm (Fig. b). The fabric in the
coarser-grained gneisses and enveloping mylonites has a similar orientation
to the penetrative subvertical foliation in the KMZ, and the weakly developed
amphibole lineation is parallel to the subhorizontal quartz rodding lineation
that is present in the granite gneisses and mylonites .
Whereas the volumetrically dominant felsic gneisses and mylonites do not
contain mineral assemblages that record distinctive P–T conditions, the
mafic enclaves provide a record of recrystallization conditions from a
preserved migmatitic core to a largely recrystallized mylonitic envelope.
Therefore, to constrain the P–T–fluid conditions of shear zone
deformation, three samples were chosen as a representative section from the
core of a low-strain lens, preserving peak fabrics and mineral assemblages,
into the well-developed retrograde mylonite zone, and these are described
further below.
Petrography and mineral chemistry
Petrography
Backscatter electron images. (a) Mineral elongation and
alignment in KMZ29. (b) σ-clast of epidote showing
preferential growth of chlorite in its pressure shadows. (c) Detail
of the pressure shadow outlined by the box in (b).
(d) Rounded plagioclase and hornblende clasts enveloped by a
fine-grained and foliated chlorite–epidote–plagioclase–quartz assemblage
in KMZ30. Note the preferred growth of chlorite in the pressure shadows of
plagioclase and hornblende. (d) Close-up of the area outlined
in (d), demonstrating chlorite–epidote growth in the pressure
shadow of plagioclase. (e) Close-up of the plagioclase–chlorite
grain boundary outlined in (e), showing the presence of pore spaces
at grain boundary irregularities (arrowheads).
The three samples are from the relatively low-strain core of a mafic lens
(sample KMZ28), the schistose collar (sample KMZ29) and the mylonitic
envelope (sample KMZ30; all collected from 26∘48′10′′ S
015∘57′50′′ E). All three samples are hornblende, plagioclase and
quartz-bearing amphibolites and contain variable proportions of additional chlorite,
epidote and sphene (Fig. ). Whereas the similar mineral
assemblages and mineral compositions suggest that all three samples were
derived from a common protolith, the texture, grain-size and fabric intensity
varies dramatically between the samples.
The low-strain sample KMZ28 is coarse-grained and equigranular, with typical
grain sizes on the order of 0.2–1 mm (Fig. a). The
sample is dominated by hornblende, plagioclase and quartz, with chlorite and
epidote only present as subordinate and fine-grained phases on the edges of
hornblende and plagioclase (Fig. d, e). Hornblende and
plagioclase are weakly aligned, giving the sample a poorly developed
gneissose fabric. Notably, fine-grained chlorite and epidote do not show a
preferred orientation (Fig. d, e).
Sample KMZ29 is a medium-grained schist, consisting of elongate hornblende
and chlorite–epidote foliae with typical grain sizes on the order of
0.1–0.5 mm (Fig. b). Some hornblende grains appear to be
larger, but are in fact aggregates made up of discrete subgrains
(Fig. f, g). Chlorite foliae form an interconnected
network, giving the sample a well-developed schistosity
(Fig. b, f, g). Hornblende and plagioclase aggregates are
elongate and aligned parallel to this fabric (Figs. f, g;
a, b). In places, chlorite can be seen to preferentially
occur in the pressure shadows of larger porphyroclasts
(Fig. b, c).
Sample KMZ30 is a mylonite, consisting of approximately 30 %
0.1–0.8 mm-sized rounded plagioclase and hornblende clasts in a matrix of
very fine-grained (5–25 µm) mylonitized plagioclase, chlorite, epidote
and quartz (Figs. c, d). Chlorite and
prismatic epidote are the main fabric-defining minerals whereas quartz,
plagioclase and hornblende grains are elongated parallel to the fabric
(Fig. d, e). Chlorite and epidote preferentially occur in the
pressure shadows of plagioclase and hornblende clasts
(Fig. d, e), whereas small cavities are present at grain
boundary irregularities (e.g. Fig. f).
Representative mineral compositions. b.d.: below detection limit; n.d.: not determined.
KMZ28
KMZ29
KMZ30
hb
pl core
pl rim
chl
ep
hb
pl core
pl rim
chl
ep
hb
pl core
pl rim
chl
ep
SiO2
45.83
51.15
63.37
26.39
37.64
47.58
53.45
61.67
29.17
38.9
44.41
55.96
60.11
26.29
37.53
TiO2
1.00
0.01
0.01
0.1
0.03
0.83
b.d.
0.05
0.03
0.09
0.42
b.d.
0.01
0.05
b.d.
Al2O3
10.61
31.97
23.49
22.33
27.04
8.51
30.86
22.85
19.1
27.76
11.5
28.67
24.8
22.37
23.52
Cr2O3
0.22
n.d.
n.d.
0.08
b.d.
0.14
n.d.
n.d.
0.24
b.d.
0.03
n.d.
n.d.
b.d.
0.03
FeO
15.5
0.05
0.28
23.36
9.45
13.18
0.09
1.26
20.11
8.1
18.52
0.17
0.14
26.58
14.21
MnO
0.3
0.02
0.05
0.23
b.d.
0.34
0.05
0.04
0.27
0.19
0.58
0.03
b.d.
0.48
0.27
MgO
12.36
b.d.
b.d.
15.8
b.d.
14.76
b.d.
0.8
19.56
0.05
10.14
0.02
b.d.
13.44
0.01
CaO
11.39
13.3
3.75
0.12
23.23
11.77
11.75
3.18
0.08
22.93
11.84
9.6
4.88
0.09
22.59
Na2O
0.73
3.38
8.49
0.04
0.08
0.74
4.46
7.01
b.d.
0.03
0.95
5.96
8.13
0.05
0.01
K2O
0.36
0.06
0.1
0.46
0.14
0.24
0.07
2.36
0.03
0.02
0.61
0.09
0.93
0.34
0.01
Total
98.28
99.94
99.55
88.9
97.61
98.17
100.72
99.71
88.58
98.08
98.99
100.5
99
89.69
98.18
Oxygens
23
8
8
14
25
23
8
8
14
25
23
8
8
14
25
Si
6.72
2.32
2.8
2.71
6.03
6.91
2.39
2.77
2.94
6.13
6.59
2.5
2.7
2.72
6.13
Ti
0.11
0
0
0.01
0
0.09
–
–
0
0.01
0.05
0
0
0
0
Al
1.83
1.71
1.22
2.71
5.11
1.46
1.63
1.21
2.27
5.16
2.01
1.51
1.31
2.73
4.53
Cr
0.03
–
–
0.01
0
0.02
0
0
0.02
0
0
–
–
0
0
Fe*
1.9
0
0.01
2.01
1.27
1.60
0
0.05
1.7
1.07
2.3
0.01
0.01
2.3
1.94
Mn
0.04
0
0
0.02
0
0.04
0
0
0.02
0.02
0.07
0
0
0.04
0.04
Mg
2.7
0
0
2.42
0
3.19
0
0.05
2.94
0.01
2.24
0
0
2.07
0
Ca
1.79
0.65
0.18
0.01
3.99
1.83
0.56
0.15
0.01
3.87
1.88
0.46
0.24
0.01
3.95
Na
0.21
0.3
0.73
0.01
0.02
0.21
0.39
0.61
0
0.01
0.27
0.52
0.71
0.01
0
K
0.07
0
0.01
0.06
0.03
0.04
0
0.14
0
0
0.12
0.01
0.05
0.04
0
Total
15.38
4.98
4.95
9.96
16.44
15.4
4.99
4.99
9.91
16.29
15.55
5
5.02
9.94
16.61
Xan
0.68
0.20
0.59
0.20
0.47
0.25
XFe
0.56
0.6
0.47
0.51
0.65
0.66
Xps
0.20
0.17
0.30
* All Fe is recalculated as Fe2+, except for epidote,
where all Fe is assumed to be Fe3+.
Mineral chemistry
Mineral compositions were determined using a JEOL JXA-8100 electron
microprobe housed at the University of Cape Town. Analyses were carried out
using a 15 kV acceleration voltage, 20 nA probe current and 2 to 3 µm
spot size. Counting times were 5 s for both backgrounds and 10 s
for peaks on all elements. Compositions were quantified using
natural mineral standards. Representative mineral compositions for the three
samples are presented in Table .
All samples show the same trends in mineral compositions, with the amphibole
in all samples being hornblende (sensu lato), with appreciable Al and
Na content (1.5–2 and 0.2–0.25 cations per formula unit respectively) and
XFe of 0.47–0.65 (Table ). Large plagioclase
grains in all samples exhibit moderate compositional zonation, with
anorthite-rich cores grading to more albite-rich rims. Core compositions of
Xan= 0.68, 0.59 and 0.47 are observed in the different samples,
whereas plagioclase rims are consistently oligoclase with Xan= 0.2–0.25 (Table ). The composition of strongly
recrystallized plagioclase, such as in the foliated matrix of KMZ29 and KMZ30
is the same as that of the oligoclase rims found on large grains. Chlorite
has XFe of 0.5–0.66, mimicking that of hornblende and indicating
that the variation in XFe between samples is likely controlled by
the bulk composition (see also Table ). Epidote has a
pistacite (Fe3+/Al + Fe3+) content of 0.17–0.3.
Bulk compositions (in mole %) used to construct the pseudosections.
SiO2
TiO2
Al2O3
FeO
MgO
CaO
Na2O
O
H2O
Fig. a
51.33
0.98
10.45
9.68
13.76
10.90
2.20
0.70
excess
Fig. b
53.98
1.02
9.32
8.94
15.73
9.17
1.11
0.72
excess
Fig. c
61.53
0.95
10.26
7.95
7.17
8.14
3.28
0.71
excess
Fig. a (MH2O=15)
43.64
0.83
8.88
8.23
11.69
9.27
1.87
0.60
15
Fig. a (MH2O=3)
49.80
0.95
10.13
9.39
13.35
10.57
2.13
0.68
3
Fig. b (MH2O=16)
45.35
0.86
7.83
7.51
13.21
7.70
0.93
0.60
16
Fig. b (MH2O=2)
52.91
1.00
9.14
8.76
15.42
8.98
1.09
0.71
2
Fig. c (MH2O=12)
54.15
0.84
9.03
6.99
6.31
7.16
2.89
0.62
12
Fig. c (MH2O=2)
60.30
0.94
10.06
7.79
7.03
7.98
3.22
0.70
2
Inferred equilibrium mineral assemblages
All samples contain mineral assemblages and mineral compositions indicative
of having equilibrated under amphibolite-facies metamorphic conditions. Some
remnants of the preceding granulite-facies history of these rocks is
preserved as relict textures in outcrop (Fig. a), and
possibly in the composition of anorthite-rich plagioclase cores
(Table ), but overall these rocks have been pervasively
re-equilibrated (and likely rehydrated) during retrogression. The current
equilibrium assemblage in all samples is interpreted to consist of
hornblende, plagioclase with Xan∼0.25, chlorite, epidote,
sphene and quartz. The petrography and microstructures indicate that much of
this assemblage crystallized synkinematically, as illustrated by the
preferential occurrence of chlorite and epidote in the pressure shadows of
larger grains (Fig. b–e). Similarly, pre-existing
hornblende and plagioclase were pervasively recrystallized and their
compositions re-equilibrated during KMZ-related shearing.
Given the close proximity of the three samples, coupled to the similarities
in their mineral assemblages and compositions and the apparent mineral
reactions that occurred within each, we infer that they are derived from the
same protolith, but experienced different degrees of KMZ-related shearing and
perhaps fluid flow, retrogression and metasomatism. The three samples fall on
a broad compositional trend, manifested by increases in Si and Na coupled to
decreases in Fe, Mg and Ca, indicating a degree of open-system behaviour
during their KMZ-related metamorphic history e.g.. Sample KMZ28 is largely unaffected by shearing and is also
the least altered, whereas KMZ30 is pervasively recrystallized, mylonitised
and also the most strongly metasomatized. It is conceivable that these
compositional differences were established prior to KMZ shearing, but it is
highly unlikely that they would have led to rheological changes that can
explain the strain gradient between the samples. Hence, we interpret the
three samples as variably deformed versions of the same protolith. This,
coupled to the synkinematic growth of the assemblage, indicates that the
P–T conditions under which the assemblages were equilibrated also
brackets the conditions of shearing in the KMZ.
P–T pseudosections calculated for mafic schists from the KMZ.
The inferred equilibrium mineral assemblages are outlined by red boxes and
indicated in bold type. Contours in (b) are for Xan in
plagioclase.
Mineral equilibria modelling
Mineral equilibria calculations were performed with the THERMOCALC
programme by using the new and expanded internally consistent
thermodynamic dataset by dataset 6.2 created 6 February
2012. Calculations were performed in the
Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3
(NCFMASHTO) model system, disregarding the minor components MnO (less than
0.2 wt %) and K2O (less than 1.5 wt %). The activity-composition models
used are those by for chlorite, plagioclase and ilmenite,
by for amphibole and by for epidote. Albite,
sphene, quartz and H2O are pure end-member phases. Although a melt model
appropriate for mafic rocks is included in , melt was not
considered in the calculations as the conditions of interest are far below
the solidus. However, the calculated equilibria above ∼ 650 ∘C are likely to be metastable to melt.
The bulk compositions used in the pseudosection calculations were determined
by XRF analysis at the University of Cape Town. Analyses were converted to
the NCFMASHTO model system by disregarding K2O, MnO, Cr2O3 (< 0.03 wt %) and P2O5 (< 0.2 wt %) and assuming approximately 15 % of
total Fe to be present as Fe3+, in line with typical values for
metamafic rocks cf. the compilation of. During initial
modelling, the samples were assumed to be fully hydrated, such that water
(H2O) was taken to be present in excess. The possibility of variable
rehydration during retrogression is explicitly considered later. The bulk
compositions used to construct the pseudosections are presented in
Table .
P–T pseudosections
Fluid-saturated P–T pseudosections for samples KMZ28, KMZ29 and KMZ30
are presented in Fig. . The phase relations in all three
samples have a similar topology and consist of the typical
greenschist-facies assemblage
actinolite–chlorite–epidote–albite–sphene–quartz at T below
450 ∘C and contain the typical amphibolite-facies assemblage of
hornblende–plagioclase–ilmenite–quartz at T above 550–600 ∘C
(Fig. ). In the T range between 450 and
550–600 ∘C, these rocks undergo a series of phase changes, notably
(1) the introduction of hornblende at the expense of actinolite, (2) the
introduction of plagioclase and the demise of albite and epidote, (3) the
replacement of sphene by ilmenite and finally (4) the demise of chlorite
(Fig. ). Within this T zone, the inferred equilibrium
assemblage of hornblende–plagioclase–chlorite–epidote–sphene occurs in a
narrow, T-sensitive field at around 450 ∘C and between 2 and 4 kbar in KMZ28 and KMZ29 (Fig. a, b), but spans the entire
P range of interest in KMZ30 (Fig. c). This field is
bound by the removal of plagioclase to lower T and the loss of epidote to
higher T. Contours of Xan calculated for KMZ29 indicate that the
composition of plagioclase varies substantially, from Xan=0.32 to
Xan=0.82, at the stability conditions of the inferred equilibrium
assemblage (Fig. b).
T–MH2O pseudosections
Calculated T–MH2O pseudosections allow the degree of
fluid saturation in these rocks to be quantitatively evaluated and are
presented for the three samples in Fig. . The diagrams are
calculated at constant P of 4 kbar in order to bracket the
peak-to-retrograde evolution, with H2O content chosen to vary such that
the samples are fluid saturated and undersaturated over the T range of
interest. THERMOCALC outputs H2O content as a mol fraction, but this
approximates volume percent when normalized to 1 oxide sum total.
T–MH2O pseudosections for mafic schists
from the KMZ. H2O-saturated assemblages are separated from
H2O-undersaturated assemblages by the thick dashed line. The observed
assemblages are outlined by red boxes.
The pseudosections all show a similar topology and exhibit the same features,
notably that all samples require high H2O content to be fluid saturated at
low-T greenschist-facies conditions (MH2O=11-15 mol %), but
that only about a third of this fluid is required for fluid saturation under
amphibolite- to granulite-facies conditions (MH2O=3-5 mol %;
Fig. ). Consequently, all samples undergo large changes to
their fluid content in the range between 450 and 550–600 ∘C, with
∼7-10 vol. % H2O being produced if the rocks were heating up;
conversely the same amount of rehydration from an external source is required to maintain
fluid saturation during cooling (Fig. ).
The maximum amount of H2O that these rocks could have retained from peak
metamorphic conditions T>750 ∘C; is 3–5 mol %, depending on the sample (Fig. ). However, the mineral
assemblages observed in the three samples all straddle the H2O saturation
line at ∼ 480 ∘C, indicating that they can occur at
fluid-saturated or slightly fluid-undersaturated conditions, but require a
higher H2O content of at least 5.5–11.5 mol %, depending on the sample
(Fig. , red boxes). The observed assemblages are bound by the
loss of epidote and plagioclase to lower H2O content. As H2O content is
decreased further at 480 ∘C, ilmenite is introduced, prior to the
loss of sphene and chlorite, and finally the introduction of orthopyroxene
and garnet at the very low H2O content that characterized peak metamorphic
conditions (MH2O< 3–5 mol %; Fig. ).
Discussion
P–T conditions of shearing
The three samples described above were affected by KMZ-related retrograde
deformation to different degrees and can, therefore, be used to effectively
constrain the P–T conditions under which quasi-plastic shearing in the
KMZ occurred. The stability of the observed assemblages are summarized in
Fig. , and the overlap between the samples constrains the
most likely P–T conditions experienced by these rocks at 2.7–4.2 kbar
and 450–480 ∘C. Whereas T is tightly constrained, the P estimate
is less precise and straddles the kyanite–andalusite phase boundary.
However, no aluminosilicates are present in the KMZ, nor have they been
reported for other parts of the MRPSZ; consequently the presence of
kyanite or andalusite cannot be used to refine the P estimate further.
The estimates indicate that the bulk of shearing in the KMZ occurred at 450–480 ∘C, at a depth of 10–16 km,
assuming overlying granitic crust. These conditions are significantly warmer than the brittle–viscous transition in
quartz , and the unstable–stable frictional transition in granitic rocks , and roughly
coincides with the onset of crystal-plastic deformation of feldspar at geological strain rates .
These inferred conditions are consistent with frictional–viscous flow sensu in felsic KMZ
mylonites, where bulk deformation occurred by viscous flow of interconnected quartz–phyllosilicate networks surrounding
locally brittle feldspar clasts . The lack of a greenschist-facies overprint on the mineralogy of the
samples, as well as lack of a pervasive brittle overprint to the deformation, strongly indicates that shearing in the
KMZ ceased at these T and that deformation along the shear zone did not lead to progressive exhumation and attendant
cooling as the KMZ developed cf..
Whereas the KMZ lacks an obvious lower-T history, it is localized in
lithologies that experienced earlier granulite-facies metamorphism, such that
it could potentially have an inherited higher-T history. The available age
data indicate that the MRPSZ was active at ca. 1005–960 Ma , such that the KMZ could post-date peak metamorphism
1065–1045 Ma; by as little as 40 Ma. However, apart from
local textural indications preserved in low-strain areas (e.g.
Fig. a), the mineralogical features have been pervasively
overprinted at the P–T conditions constrained above. One mineralogical
aspect that could potentially be inherited from the earlier high-T history
is the anorthite-rich composition preserved in plagioclase cores. However,
calculated isopleths of anorthite content in plagioclase (only shown for
KMZ29 in Fig. b) indicate that the plagioclase composition
varies widely at or near the P–T constrained for the KMZ. This does
not preclude the plagioclase cores being inherited from earlier higher-T
conditions, but shows that the core compositions are not far from
equilibrium, with the remaining assemblage at 450–480 ∘C. Consequently, it
appears plausible that shearing along the KMZ occurred at near-constant T
conditions of ∼ 450 ∘C at a depth within the range of 10 to
16 km and that the KMZ did not undergo progressive exhumation and cooling
during its development. The KMZ is, therefore, related to a discrete tectonic
event and does not form a continuum of earlier granulite-facies
metamorphism, which is supported by its large-scale structural discordance
with the granulite-facies structures and fabrics . It also follows that the KMZ likely acted as a
transcurrent shear zone, with little-to-no associated transpression or
transtension.
Summary of estimated P–T conditions for the KMZ, constrained
from the P–T overlap of the equilibrium assemblages that occur in the
various mafic schists.
The tectonic and P–T history of the KMZ is similar to that of modern
examples of major continental strike-slip shear zones that are localized in
isostatic/non-orogenic crust and do not involve large amounts of crustal
thickening or thinning. Such modern examples include the San Andreas fault
away from the Mendocino Triple Junction and transpressional/transtensional
bends and the North Anatolian fault east of the
Aegean transtensional zone . The thermal profile of these
shear zones follow the continental geotherm that is stable at the time, which
in the case of the KMZ is estimated at 30–45 ∘C km-1. The
uncertainty in this estimate is caused by uncertainty in the depth of the
KMZ, but the entire range is significantly warmer than for stable continental
lithosphere ∼ 20 ∘C km-1; e.g.and references
therein. However, given that the Aus granulite terrain experienced
granulite-facies metamorphism associated with lithospheric thinning only
40 Ma earlier, it is to be expected that the terrain was characterized by
higher-than-average heat flow at the time of shear zone formation
e.g.. In fact, the estimated geotherm is
comparable with those derived from crust underlain by young and thin
lithosphere, such as the northern San Andreas fault .
Fluid regime, fluid source and infiltration mechanisms
The rocks in which the KMZ localized were dehydrated and experienced partial
melting and melt loss during preceding metamorphism . In the
absence of rehydration, the rocks would have retained their (low) fluid
content and granulite mineralogy from peak metamorphic conditions and
experienced shearing and reworking under fluid-absent conditions
. If this were the case, the
samples would have consisted of orthopyroxene-bearing assemblages and would
have grown garnet at the P–T conditions of shearing
450–480 ∘C and MH2O<3-5 mol %;
Fig. ; cf.. For the rocks to have replaced their
high-T assemblages with the minerals observed in the samples described
here, they must have experienced an addition of H2O (Fig. ).
The observed assemblages in all three samples straddle the H2O saturation
line, indicating that they can occur at fluid-saturated or slightly
fluid-undersaturated conditions. However, given that the absolute fluid
content required by each assemblage differs widely between the samples, it
appears unlikely that the samples were all rehydrated to be just
fluid undersaturated, and it is more plausible to conclude that they were
fully rehydrated and fluid saturated during shearing along the KMZ.
The mineral assemblages described here, therefore, demand at least some
syntectonic retrograde rehydration during strike-slip displacement in a
dominantly viscous shear zone. Calculations show that rehydration requires
the addition of 4–8 vol. % H2O (Fig. ), such that a
fluid/rock ratio of at least 0.05–0.1 is necessary to ensure rehydration
and fluid saturation of the KMZ if fluid uptake is assumed to be efficient.
Other than the presence of hydrous mineral phases, formed syntectonically
from a relatively dry protolith, there are, however, few observed signs of
extensive fluid flow. Whereas the change in bulk composition between the
samples indicates a degree of metasomatic open-system behaviour, the shear
zone is largely barren of hydrothermal precipitates and quartz veins, other
than a few local (up to tens of metres along-strike) examples of
foliation-parallel veins with mylonitic lineation. A few very late,
subvertical, tens of centimetres thick, north-striking veins cross-cut the
KMZ mylonitic fabrics. Thus, we envisage that fluid flow during shearing was
sufficient to completely rehydrate the mineral assemblages and allow the
presence of a free fluid phase, but that it was not extensive enough to allow
widespread hydrothermal precipitation. Similarly, we envisage that fluid flow
occurred either along grain boundaries or through small-length-scale
fracture systems (which were subsequently healed, thus not preserved) rather
than through long-lived, channelized conduits.
Kilometre-scale, open-system diffusion of water through crustal shear zones
is not a unique phenomenon and was proposed by for major
retrogression in ductile shear zones cross-cutting the Archaean Lewisian
basement complex of northern Scotland, in similar settings and metamorphic
conditions to the current study. Similarly, significant retrograde fluid
influx has been inferred for shear zones at Broken Hill, Australia
, the French Pyrenees and the
Yellowknife gold district, Canada ; in the latter location,
fluid flux is associated with hydrothermal vein mineralization as well as
retrograde reactions. Based on these and other examples, it has been
estimated that typical fluid/rock ratios in retrogressed shear zones can
exceed 102 and can locally be much greater
. This is 3 orders of magnitude higher than the
minimum fluid/rock ratio required for the KMZ, which again indicates that,
although the KMZ was fluid saturated, it was likely not inundated with fluid
to the same extent as many other examples of retrograde shear zones.
Achieving retrograde rehydration requires a significant source of fluids.
Such a source is not obvious in a depleted, dry, granulite terrain such as
the Aus granulites hosting the KMZ. suggest hydrothermal
circulation of prograde fluids, driven by a mantle heat source, as typical of
regional metamorphism at depths > 10 km, but no prograde fluid source is
available during strike-slip deformation of the KMZ. For retrograde
metamorphism and rehydration, imply that tectonic
juxtaposition of hot rocks and cool, fluid-saturated mineral assemblages can
drive dehydration and provide local fluids. Again, this is not feasible in
the KMZ, as the strike-slip motion provides neither a heat source nor a
fluid source. In a review of fluids in deep fault zones,
proposes that retrograde deformation with high fluid/rock ratios requires an
external, typically meteoric, fluid source. Such a meteoric fluid source is
evident in current deformation along and around the Alpine fault, New
Zealand, where retrograde deformation is associated with transpression and
topographically driven deep circulation of meteoric waters in the southern
Alps . However, this model requires a component on
crustal shortening and associated mountain building to create the hydraulic
head to drive surface fluids down to below the brittle–viscous transition.
suggest that high fluid/rock ratios of up to 103 can be
achieved through episodic seismic pumping of a meteoric fluid and local,
transiently enhanced permeability of 10-17 to 10-15 m2. In their
model, the meteoric fluids move down a gently dipping décollement and then
up through steeply dipping shear zones. No such gently dipping décollement
is know to exist below the KMZ, but one could potentially envisage
seismically driven meteoric fluid flow down to the KMZ, from the brittle
crust above , but this mechanism still requires high
permeability into the ductile lower crust. Such enhanced permeability may
come about through aseismic processes, considering that retrograde reactions
lead to volume change. The study by implies volume loss during retrograde
breakdown of feldspar coupled to fluid influx as the origin of
phyllonitisation, reaction weakening and mylonitisation in an overthrust
setting in the Appalachians. , on the other hand, suggest
extensive microfracturing associated with retrograde hydration and increase
in solid volume as another mechanism to get fluids into otherwise low-permeability, high-grade crystalline rocks. Even without associated
retrograde reaction, creep has been shown to enhance permeability through
generation of grain boundary microcracks , grain
boundary dissolution porosity and through a dynamic process
of creep cavitation . Porosity seen along grain
boundaries (Fig. f) indicates the possibility of grain
boundary permeability, allowing fluid flow and enhancing grain boundary
sliding . Thus, the presence of a
retrograde shear zone, once actively deforming, is a source of locally
elevated permeability – either from solid volume increase and associated
microfracturing, or solid volume decrease and associated grain boundary
dilatancy. This permeability may be transiently enhanced by earthquake
rupture and associated fault zone damage in the brittle crust overlying the
shear zone and potentially through downward propagation of such rupture
fronts into the ductile regime .
The above examples all include an element of dip-slip displacement. In
another example of fluids in a subvertical strike-slip shear zone, the deep
San Andreas fault is suspected to be fluid rich, based on high Vp/Vs in the
lower crust . A likely source for this fluid is a
serpentinized mantle wedge, where serpentinization dates to past subduction
and the absence of a cool, insulating slab now leads to heating and
dehydration . This San Andreas fault case is,
however, somewhat special in requiring a transition from subduction to
strike-slip tectonics, and we do not see evidence for the presence of a
similar serpentinized mantle wedge as a source of fluids for KMZ rehydration,
neither does the tectonic history of the Namaqua-Natal Belt call for an
initial subduction origin for the MRPSZ. Thus, while we do not preclude other
alternatives, our best hypothesis is that the external fluid source for KMZ
rehydration was meteoric, and fluid flow was allowed by local dilation and/or
increased fracture permeability within the shear zone, thus accounting for
retrograde metamorphic reactions within the mylonites and absence of such
reactions outside the KMZ.
Initiation and feedback mechanisms in a retrograde shear zone
In the previous section, we argue for an external fluid source to allow
retrograde rehydration reactions to occur in the KMZ. Although we suggest a
meteoric origin as most likely, another origin for the external fluid does
not change the arguments and conclusions that will next be made regarding the
effects of fluid infiltration and retrograde reactions on fault zone
rheology. Retrograde minerals within the mylonites define syntectonic fabric
elements (Fig. b–e), thus indicate syntectonic
retrogressive reaction and hydration. The implication that hydration,
metamorphism and deformation occurred concurrently and were mutually
enhancing raises a chicken-and-egg question regarding the onset of
retrograde metamorphism and shear zone deformation. However, if the
interpretation that the fluids source was external is correct, and we know
retrograde mineral assemblages are localized in the KMZ, it is implicit that
the shear zone must have been a region of enhanced permeability before
retrograde reactions could initiate. Moreover, if the fluid source was
near-surface and fluids came into the KMZ through the overlying brittle
crust, it is required that (1) a fault system in the brittle crust was
present and linked to the KMZ and (2) the KMZ was already there to
provide a low-permeability zone for such fluids to localize into.
concluded that in early stages of ductile deformation, fluids
preferentially flow into the deforming zone, and the consequent initiation of
retrograde metamorphic reactions initiate reaction softening and further
strain localization. Thus, as soon as a shear zone is active at retrograde
conditions and connected to an external fluid reservoir, we envisage a
positive feedback loop where reaction softening and ongoing metamorphic
reactions lead to progressive strain accumulation, weakening and
permeability enhancement. The feedback mechanisms involved include (1) grain-size reduction through growth of new minerals, enhancing diffusion rates
; (2) low cohesion along new grain boundaries,
enhancing grain boundary sliding ; (3) second phase pinning
that restricts grain size, maintaining a fine grain size that enhances grain-size sensitive creep as the main deformation mechanism
and (4) replacement of
strong mineral phases with weaker phyllosilicates .
We do not exclude a possibility that this early stage of ductile deformation
initiated on a pre-existing brittle fracture , or in rock
locally more hydrous ; however, we do not record evidence
of existing fractures, and local fluid infiltration would involve late
granulite-facies magmatic fluids related to granites not observed in direct
contact with the shear zone rocks we discuss here. Therefore it seems equally
or more likely that a new shear zone within a dry granulite terrane should
initiate either along an existing well-oriented fabric
, or around a stress riser such as a
pretectonic granitic intrusion .
describe a similar example of deformation localization
into a metre-scale fine-grained ultramylonite with increased metasomatic Si
content and suggest that fluids were required to initiate metasomatism
and localized deformation, but were introduced through brittle precursor
fracture systems. We do not see such definite evidence for a brittle
precursor, but also prefer a conclusion where deformation localization
allowed localized fluid flow, either through a brittle precursor or
activation of existing fabrics, locally or further afield. Note that shear
zone propagation may be more pertinent here than nucleation, given that the
KMZ may have initiated outside the study area in a different rock assemblage.
Independently of the exact initiation mechanism, we stress that as soon as a
shear zone has initiated in strong, dry rocks at retrograde conditions, the
interplay between reaction and deformation will cause further weakening,
enhancing strain localization, thus significant local weakening of the
lower crust will occur.
It has long been envisaged that as a viscous shear zone accumulates strain,
grain-size reduction caused by metamorphic growth of new grains and
recrystallization through dislocation creep competes with grain growth
through diffusion . Thus, retrograde shear zones are commonly
predicted to initiate with fine grain sizes and dominantly deform by
diffusion creep, but as grains grow and the mineral assemblage equilibrates
at the retrograde P–T conditions, grain size should increase and the
deformation mechanism may change to dominantly dislocation creep
. We note, however, that in our samples (Fig. ), as in microstructures reported by from
the KMZ, the highest strain rocks are characterized by a very fine-grained,
intensely foliated, retrograde mineral assemblage. This fine grain size may
be a result of second phase pinning preventing grain growth, thus
enhancing grain-size sensitive creep .
The general lack of veins may further imply that fluid pressures were not
sufficient to allow hydrofracturing and mineral precipitation; however,
precipitation may also have been hindered by low solubility and low dissolved
mineral content in cool fluids derived from above the shear zone. It is,
however, implied by a meteoric fluid source that connectivity with the
surface existed, at least temporarily, under which fluid pressure cannot have
been greater than hydrostatic. Thus, overall, we do not see high fluid
pressures as a necessary nor likely weakening mechanism in retrograde shear
zones that are connected to an external fluid source, particularly if this
fluid source is related to a surface-connected fracture system.
Implications for strength and deformation mechanisms in active retrograde viscous shear zones
We have inferred from the P–T path derived from low- and high-strain
rocks that the KMZ represents a retrograde shear zone deformed at relatively
constant temperature, just warmer than the brittle–viscous transition in
granitic rocks. Further, the mylonites record simultaneous retrograde grain
growth and strike-slip deformation, explained by localized fluid flow through
an active shear zone. Because the shear zone was hosted in dry, melt-depleted
and strong granulite-facies rocks e.g. that retain migmatitic and gneissic fabrics, the bulk of the
fluid must have been derived externally. The shear zone does not preserve
pervasive hydrofractures, but does preserve very fine-grained mineral
assemblages likely associated with diffusive mass transfer, as evidenced by a
strain localization in a fine-grained polyphase aggregate with preferential
growth of reaction products in dilatant sites. In addition, synkinematic
fine-grained plagioclase have a different composition than prekinematic
plagioclase cores (Table ), a possible sign of nucleation
from solution during deformation, after which fine-grained precipitates would
have deformed by grain-size sensitive creep . Coupled to the
inference of an external fluid source, the scarcity of veins and prevalence
of a fine-grained assemblage lead to the inference that the shear zone was
active at low fluid pressure conditions, and weakening relative to
surrounding wall rocks was caused by reaction weakening, involving grain-size
reduction and growth of relatively weak minerals (e.g. chlorite). These
inferences imply that as soon as a retrograde shear zone has formed, and as
long as it retains connection to a reservoir of fluids, such a shear zone is
a long-lived, strain-weakening feature that controls the strength of the
lower crust over its along-strike extent.
The San Andreas fault is inferred to be weak and wet
e.g., commonly explained by high fluid pressures
. The Alpine fault, New Zealand, has on the other hand been
interpreted as frictionally strong and potentially
weakened at depths by fluid presence . Both of these
faults are currently active under retrograde metamorphic conditions and
exhibit tectonic tremor, a persistent low-frequency seismic signal
characterized by lack of impulsive body wave arrivals, emanating from below
the brittle–viscous transition . These
tremor signals have, in both places, been interpreted as fluid-enabled slip
on the deep extension of the plate boundary faults, below the base of the
seismogenic zone . The KMZ is exhumed from the same
thermal and metamorphic regime where the San Andreas and Alpine fault tremor
occurs. The KMZ also deformed under fluid-present conditions and was
significantly weak compared to surrounding rock. However, our observations
and inferences along the KMZ require fluid presence, but do not require
elevated fluid pressure to explain this weakening. As suggested by
for the San Andreas fault, we also suggest that
deformation localization below the brittle–viscous transition at retrograde
conditions may be a function of reaction weakening and metamorphic
rehydration. At these conditions, localized slip may occur along weak planes
characterized by aligned phyllosilicates , possibly
associated with transient aseismic creep accommodated in fine-grained
mylonites deforming by diffusion creep. If this interpretation is correct,
high fluid pressures are not required for tremor and slow slip in the viscous
regime in retrograde shear zones. Instead, weakness of fluid-saturated,
retrograde faults and shear zones may be explained by reaction weakening
under hydrostatic fluid pressure conditions.