Introduction
Crustal-scale deformation is commonly localized into major faults, in the
upper crust and ductile shear zones in the lower crust
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
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 .
[Figure omitted. See PDF]
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 – 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.
[Figure omitted. See PDF]
Regional and outcrop geology
The Kuckaus Mylonite Zone
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.
[Figure omitted. See PDF]
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 – 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 ––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) -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).
[Figure omitted. See PDF]
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 264810 S 0155750 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 m) 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 | |||
SiO | 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 | ||
TiO | 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. | ||
AlO | 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 | ||
CrO | 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 | ||
NaO | 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 | ||
KO | 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 | ||
0.68 | 0.20 | 0.59 | 0.20 | 0.47 | 0.25 | ||||||||||||
0.56 | 0.6 | 0.47 | 0.51 | 0.65 | 0.66 | ||||||||||||
0.20 | 0.17 | 0.30 |
All Fe is recalculated as Fe, except for epidote, where all Fe is assumed to be Fe.
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 m 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 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 0.68, 0.59 and 0.47 are observed in the different samples, whereas plagioclase rims are consistently oligoclase with 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 of 0.5–0.66, mimicking that of hornblende and indicating that the variation in between samples is likely controlled by the bulk composition (see also Table ). Epidote has a pistacite (FeAl + Fe) content of 0.17–0.3.
Bulk compositions (in mole %) used to construct the pseudosections.
SiO | TiO | AlO | FeO | MgO | CaO | NaO | O | HO | |
---|---|---|---|---|---|---|---|---|---|
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 ( | 43.64 | 0.83 | 8.88 | 8.23 | 11.69 | 9.27 | 1.87 | 0.60 | 15 |
Fig. a ( | 49.80 | 0.95 | 10.13 | 9.39 | 13.35 | 10.57 | 2.13 | 0.68 | 3 |
Fig. b ( | 45.35 | 0.86 | 7.83 | 7.51 | 13.21 | 7.70 | 0.93 | 0.60 | 16 |
Fig. b ( | 52.91 | 1.00 | 9.14 | 8.76 | 15.42 | 8.98 | 1.09 | 0.71 | 2 |
Fig. c ( | 54.15 | 0.84 | 9.03 | 6.99 | 6.31 | 7.16 | 2.89 | 0.62 | 12 |
Fig. c ( | 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 , 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
– 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 in plagioclase.
[Figure omitted. See PDF]
Mineral equilibria modelling
Mineral equilibria calculations were performed with the THERMOCALC
programme by using the new and expanded internally consistent
thermodynamic dataset by
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 KO, MnO, CrO ( 0.03 wt %) and PO ( 0.2 wt %) and assuming approximately 15 % of
total Fe to be present as Fe, in line with typical values for
metamafic rocks
– pseudosections
Fluid-saturated – 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 below 450 C and contain the typical amphibolite-facies assemblage of hornblende–plagioclase–ilmenite–quartz at above 550–600 C (Fig. ). In the 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 zone, the inferred equilibrium assemblage of hornblende–plagioclase–chlorite–epidote–sphene occurs in a narrow, -sensitive field at around 450 C and between 2 and 4 kbar in KMZ28 and KMZ29 (Fig. a, b), but spans the entire range of interest in KMZ30 (Fig. c). This field is bound by the removal of plagioclase to lower and the loss of epidote to higher . Contours of calculated for KMZ29 indicate that the composition of plagioclase varies substantially, from to , at the stability conditions of the inferred equilibrium assemblage (Fig. b).
– pseudosections
Calculated – 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 of 4 kbar in order to bracket the peak-to-retrograde evolution, with HO content chosen to vary such that the samples are fluid saturated and undersaturated over the range of interest. THERMOCALC outputs HO content as a mol fraction, but this approximates volume percent when normalized to 1 oxide sum total.
– pseudosections for mafic schists from the KMZ. HO-saturated assemblages are separated from HO-undersaturated assemblages by the thick dashed line. The observed assemblages are outlined by red boxes.
[Figure omitted. See PDF]
The pseudosections all show a similar topology and exhibit the same features, notably that all samples require high HO content to be fluid saturated at low- greenschist-facies conditions ( mol %), but that only about a third of this fluid is required for fluid saturation under amphibolite- to granulite-facies conditions ( mol %; Fig. ). Consequently, all samples undergo large changes to their fluid content in the range between 450 and 550–600 C, with vol. % HO 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 HO that these rocks could have retained from peak
metamorphic conditions
Discussion
– 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 – 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 – conditions experienced by these rocks at 2.7–4.2 kbar and 450–480 C. Whereas is tightly constrained, the 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 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
Whereas the KMZ lacks an obvious lower- history, it is localized in
lithologies that experienced earlier granulite-facies metamorphism, such that
it could potentially have an inherited higher- 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
Summary of estimated – conditions for the KMZ, constrained from the – overlap of the equilibrium assemblages that occur in the various mafic schists.
[Figure omitted. See PDF]
The tectonic and – 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. 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
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 – conditions of shearing
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. % HO (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 10 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 10 can be achieved through episodic seismic pumping of a meteoric fluid and local, transiently enhanced permeability of 10 to 10 m. 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 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 – 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 – 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
The San Andreas fault is inferred to be weak and wet
Conclusions
The KMZ was localized in dry, high-grade mid-crustal gneisses. Once shearing was active, it allowed externally derived fluids to infiltrate the KMZ and activate a number of positive feedback processes that allowed weakening and continued strain localization. The observed mineral assemblages lead us to conclude that the KMZ was fluid bearing during deformation, but the general absence of hydrofractures and hydrothermal precipitates indicate that fluid pressures and the fluid/rock ratio remained low. In this regard the KMZ differs from most other exhumed and active continental retrograde shear zones for which high fluid/rock ratios have been suggested or for which high fluid pressures are inferred . It consequently appears that retrograde shearing can be sustained under a variety of fluid regimes, from dry and entirely fluid absent , through low-volume fluid presence such as described here, to examples where shear zones are inundated and dominated by fluid. There is also a range in cases from where high fluid pressures are sustained by a combination of high fluid volumes and low permeability, to shear zones where either low fluid volumes or high permeability prevent the build-up of high fluid pressures. Therefore we conclude that the dominant contribution of fluids to sustaining localized deformation under retrograde conditions can be through reaction weakening, by producing weaker mineral phases and facilitating grain-size reduction. We also conclude that weak lower crustal, fluid-bearing shear zones do not need to imply high fluid pressures, but can also be significantly weaker than surrounding wall rocks from reaction weakening at hydrostatic fluid pressure conditions.
Acknowledgements
Luca Menegon and Florian Fusseis are thanked for helpful reviews that improved and clarified the manuscript. This work is based on research supported by research development grants from the University of Cape Town. We thank Koos and Anna Bosman for access to their land and their hospitality during field work. Edited by: F. Rossetti Reviewed by: L. Menegon and F. Fusseis
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Abstract
The Kuckaus Mylonite Zone (KMZ) forms part of the larger Marshall Rocks–Pofadder shear zone system, a 550 km-long, crustal-scale strike-slip shear zone system that is localized in high-grade granitoid gneisses and migmatites of the Namaqua Metamorphic Complex. Shearing along the KMZ occurred ca. 40 Ma after peak granulite-facies metamorphism during a discrete tectonic event and affected the granulites that had remained at depth since peak metamorphism. Isolated lenses of metamafic rocks within the shear zone allow the
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1 Department of Geological Sciences, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
2 Department of Geological Sciences, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa; present address: School of Earth & Ocean Sciences, Cardiff University, Cardiff CF10 3AT, UK