Strain distribution within a km-scale, mid-crustal shear zone: The Kuckaus Mylonite Zone, Namibia

Accepted Manuscript Strain distribution within a km-scale, mid-crustal shear zone: the Kuckaus Mylonite Zone, Namibia S.F. Rennie, Å. Fagereng, J.F.A. Diener PII:

S0191-8141(13)00154-5

DOI:

10.1016/j.jsg.2013.09.001

Reference:

SG 2956

To appear in:

Journal of Structural Geology

Received Date: 5 March 2013 Revised Date:

30 August 2013

Accepted Date: 11 September 2013

Please cite this article as: Rennie, S.F., Fagereng, Å., Diener, J.F.A., Strain distribution within a kmscale, mid-crustal shear zone: the Kuckaus Mylonite Zone, Namibia, Journal of Structural Geology (2013), doi: 10.1016/j.jsg.2013.09.001. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

*Highlights (for review)

ACCEPTED MANUSCRIPT We map a two kilometre wide, Proterozoic mid-crustal strike-slip shear zone

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The shear zone developed in previously deformed gneisses

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Strain within the shear zone is heterogeneously distributed

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Strain distribution is related to composition, grain-size, and pre-existing foliation

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Strain localization increased with time as a function of decreasing grain size

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*Manuscript Click here to view linked References

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Strain distribution within a km-scale, mid-crustal shear

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zone: the Kuckaus Mylonite Zone, Namibia

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S. F. Rennie, ˚ A. Fagereng∗ & J. F. A. Diener

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Department of Geological Sciences, University of Cape Town, Private Bag X3, Rondebosch

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7701, South Africa

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corresponding author: [email protected]

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ABSTRACT

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The subvertical Kuckaus Mylonite Zone (KMZ) is a km-wide, crustal-scale, Proterozoic, dextral strike-slip shear zone in the Aus granulite terrain, SW Namibia. The KMZ was

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active under retrograde, amphibolite to greenschist facies conditions, and deformed felsic

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(and minor mafic) gneisses which had previously experienced granulite facies metamorphism

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during the Namaqua Orogeny. Lenses of pre- to syn-tectonic leucogranite bodies are also

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deformed in the shear zone. Pre-KMZ deformation (D1 ) is preserved as moderately dipping

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gneissic foliations and tightly folded migmatitic layering. Shear strain within the KMZ is

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heterogeneous, and the shear zone comprises anastomosing high strain ultramylonite zones

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wrapping around less deformed to nearly undeformed lozenges. Strain is localized along the

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edge of leucogranites and between gneissic lozenges preserving D1 migmatitic foliations.

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Strain localization appears controlled by pre-existing foliations, grain-size, and

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compositional anisotropy between leucogranite and granulite. The local presence of

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retrograde minerals indicate that fluid infiltration occurred in places, but most

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ultramylonite in the KMZ is free of retrograde minerals. In particular, rock composition

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and D1 fabric heterogeneity are highlighted as major contributors to the strain distribution

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in time and space, with deformation localization along planes of rheological contrast and

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along pre-existing foliations. Therefore, the spatial distribution of strain in crustal-scale

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ductile shear zones may be highly dependent on lithology and the orientation of pre-existing

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fabric elements. In addition, foliation development and grain size reduction in high strain

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zones further localizes strain during progressive shear, maintaining the anastomosing shear

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zone network established by the pre-existing heterogeneity.

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Key words: shear zones; strain localization; ductile deformation; rheology; high strain

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zones; mylonites

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Models of major crustal fault zones call for a rheological division with a shallow

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pressure-dependent frictional regime separated from a deeper temperature-dependent

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viscous regime by a frictional-viscous transition (e.g. Sibson, 1984; Scholz, 1988; Schmid and

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Handy, 1991; Fagereng and Toy, 2011). The presence of fault zones requires deformation

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localization; however, whereas the generalized model for the brittle crust involves localized

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slip on one or more discrete fault planes (e.g. Chester and Chester, 1998; Faulkner et al.,

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2003), the overall geometry of the deforming zone in the viscous regime is classically drawn

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as a downward-widening shear zone (e.g. Sibson, 1977; Scholz, 1988). Such widening with

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increasing depth and temperature is predicted because the yield strength of undeformed

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rock decreases with temperature in the dislocation creep regime, thus decreasing the degree

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of strain localization (e.g. Platt and Behr, 2011b). Generalized models do, however, tend to

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assume that deformation occurs in isotropic, homogeneous rock. The pattern of strain

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localization in natural mid- to lower crustal shear zones, where the strain distribution is

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affected by pre-existing structures and mixed rheological behaviour of rocks of

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heterogeneous composition, is therefore not known in detail.

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Introduction

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As identified by Carreras (2001), studies in structural geology tend to focus on outcrop scale (up to 1:100) or the scale of regional geological maps (greater than 1:10,000). There is

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therefore a lack in observational data at intermediate scale, calling for detailed study of

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structures that are sufficiently well exposed to be mapped in detail at the kilometre scale.

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Here we examine the Kuckaus Mylonite Zone (KMZ), a crustal-scale shear zone in

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southwestern Namibia. This particular shear zone offers unparalleled km-scale exposure and

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has never previously been the focus of a comprehensive study. This study is primarily

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concerned with the geometry, strain distribution and deformation history of the KMZ as

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exhibited on a 2 × 7 km map-scale. The geometrical data allow discussion of strain

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localization, and elucidate the factors that contribute to spatial and temporal deformation

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localization within the shear zone.

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The Kuckaus Mylonite Zone (KMZ) is located in the northwestern region of the Namaqua

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Metamorphic Complex (NMC), a Grenvillian-age metamorphic belt associated with the

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formation of Rodinia at 1100 - 1000 Ma (Rodgers and Santosh, 2003). The NMC extends

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across South Africa and southwest Namibia for about 1500 km and is bordered by the

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Kalahari Craton to the north and by the Pan-African Saldania belt to the south (Thomas

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et al., 1994). The NMC consists of middle to lower crustal high grade metamorphic rocks

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and related igneous rocks, which experienced peak metamorphism of high-temperature

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low-pressure amphibolite to granulite facies conditions (Jackson, 1976; Blignaut, 1977;

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Waters and Whales, 1984; Waters, 1986, 1988; Diener et al., 2013).

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Geological Setting

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The KMZ is a kilometre-wide shear zone located between, and oriented subparallel to, two major crustal-scale shear zones, the Lord Hill - Excelsior shear zone to the north and

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the Tantalite Valley - Pofadder shear zone to the south (Fig. 1) (Jackson, 1976; Joubert,

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1986; Colliston and Schoch, 1998). These shear zones have, at least in places, been

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suggested to be terrane-bounding and separate the Gordonia subprovince from the

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Richtersveld and Bushmanland Subprovinces to the south and Sinclair Terrane to the north

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(Fig. 1)(Jackson, 1976; Hartnady et al., 1985; Joubert, 1986; Thomas et al., 1994; Becker

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et al., 2006). The KMZ is exposed along strike for ∼ 145 km, limited by overlying Nama

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Group sedimentary rocks to the SE and the Atlantic Ocean to the NW (Jackson, 1976).

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The KMZ exposure described here is a ∼ 7 km long by 1-2 km wide continuous outcrop surrounded by recent sand (Fig. 2), ∼ 35 km SW of the town of Aus (Fig. 1), within the

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Aus granulite terrain. The Aus terrain predominantly consists of a variety of pre- and

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syn-tectonic gneisses and minor gneisses of supracrustal origin, all of which have been

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metamorphosed and migmatised at peak conditions of 5.5 kbar and 825◦ C (Jackson, 1976;

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Diener et al., 2013). The KMZ localizes almost exclusively in the felsic gneisses, and

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contains only minor enclaves of mafic granulite. Given that the KMZ clearly crosscuts the

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migmatites and high-grade fabrics related to peak metamorphism, shearing on the KMZ

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post-dates peak metamorphism and occurred under retrograde conditions (Jackson, 1976).

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In the mapped area, the KMZ separates the Tsirub Gneiss from an undifferentiated

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biotite gneiss (Figs. 2 and 3)(Jackson, 1976). The age relationship between the two gneisses 4

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has not been established, and their correlation falls outside the scope of this study; however,

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we will use the terms Tsirub Gneiss and biotite gneiss to distinguish the country rocks on

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opposite sides of the shear zone. The Tsirub Gneiss is a garnet-bearing tonalitic to granodioritic orthogneiss that was

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emplaced shortly before the peak granulite facies metamorphic event (Jackson, 1976; Diener

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et al., 2013). On a regional scale the Tsirub Gneiss has been migmatised and contains

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abundant leucogranite segregations (Fig. 4a). Within the study area, this gneiss is

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medium-coarse grained and equigranular, and consists of two feldspars, quartz and biotite,

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with rare hornblende and is garnet-bearing in places. Mesoscale leucosome structures are

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generally absent or poorly developed. The biotite gneiss is typically medium-grained and

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equigranular, and consists of two feldspars, quartz and biotite. The biotite gneiss contains a

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well-developed and pervasive cm-scale stromatic migmatitic banding (Fig. 4b). This

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banding is the main textural feature used to distinguish the two rock units. Coarse-grained leucogranites comprising megacrystic K-feldspar and quartz with minor

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biotite and garnet are distributed throughout the KMZ (Fig. 2). The biotite does not form

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interconnected networks around the quartz and feldspar, but is in places aligned to form a

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weak fabric. Two types of leucogranite can be distinguished based on fabric development:

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‘Type 1’ has a very poorly developed foliation defined by biotite and quartz, with no

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observed lineations or shear-sense indicators, whereas ‘Type 2’ displays a strongly developed

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and pervasive shear fabric defined by quartz, feldspar and locally biotite (Fig. 4c). Euhedral

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feldspars have a shape preferred orientations and are thus interpreted as magmatically

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aligned. The two leucogranite types are interpreted as pre- to syn-tectonic, relative to KMZ

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development. Type 1 behaved as more competent than the surrounding gneisses, and are

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therefore either pre-tectonic, or represent syn-tectonic intrusions that crystallised fast

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relative to deformation rates. Type 2 are likely syn-tectonic because strain clearly

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partitioned preferentially into the granites which if crystalline would have been more

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competent than the surrounding gneiss.

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Mafic granulite lenses occur in places, but are a very minor component in the KMZ.

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They contain a hornblende- and plagioclase-rich assemblage with epidote, titanite and

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minor quartz and preserve a coarse grained granulite texture in low strain areas. The lenses

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occur as discrete units ranging in size from a few decimeters to 10 - 15 m long and up to 3 5

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m wide (Fig. 4d).

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Rocks in the KMZ show evidence for three deformation events, recorded in cross-cutting

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fabric elements. The first of these, D1 , is associated with peak metamorphism and predates

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shearing and formation of the KMZ; D2 is related to formation of the retrograding KMZ,

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and D3 is defined by discrete cataclastic faults that cross-cut and offset the KMZ fabrics.

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3.1

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D1 is associated with peak metamorphism, and reflected in an S1 gneissose foliation and

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migmatitic banding. In the Tsirub Gneiss, S1 is defined by the alignment of biotite, and

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generally moderately inclined to the NNE (Fig. 5a). The fabric orientation is very uniform

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on a regional scale, and Jackson (1976) and Diener et al. (2013) also report an east-west

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striking, moderately north dipping S1 in the Tsirub Gneiss to the north of the KMZ.

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Deformation Fabrics

In the biotite gneiss, S1 is generally defined by a migmatitic banding of alternating, centimetres-thick, layers of melanosome and leucosome (Fig. 4b). This banding is folded

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around NW-SE trending, subhorizontal to shallowly plunging fold hinges, and therefore dips

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at a range of angles to the N and S (Fig. 5b). The folds are generally upright, open to

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tight, and in places associated with thin axial planar leucosomes (as also described by

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Diener et al., 2013). We therefore consider these folds to have formed during D1 , as they

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primarily developed under high-temperature, melt-bearing conditions. However, they may

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have been locally modified or re-oriented during D2 , as described below.

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3.2

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D2 is related to post-D1 retrograde metamorphic conditions and shearing in the KMZ, and

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is represented by a mylonitic S>L-tectonite fabric which overprints S1 . The lineation, L2 , is

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defined primarily by rodded quartz and has NW and SE trends with sub-horizontal plunge

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(Fig 5c). However, as elongate minerals are relatively rare in the KMZ, the lineation is

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D2

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generally poorly developed. The S2 foliation is only present within the KMZ and is defined

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by phyllosilicates (predominantly biotite, with minor chlorite and muscovite in variable

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proportions) and/or quartz. This mineral assemblage and the presence of brittlely deformed

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feldspars indicate deformation under middle to upper greenschist facies conditions (Tullis

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and Yund, 1991). S2 strikes NW-SE on average, and is generally subvertical, although some

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moderate dips (30-65◦) occur on the outer edges of the shear zone. The dip direction is

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generally to the SW, but in places also to the NE (Fig. 5d-f). Lozenges of biotite gneiss

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within the KMZ contain S1 fabric defined by migmatitic banding, systematically folded

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with fold hinges subparallel to L2 . This folding of S1 probably occurred at melt-present

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conditions, pre-dating D2 and concurrent with development of the migmatites. However,

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the orientation of the fold hinges may have been passively rotated during D1 , from E-W to

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NW-SE.

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The intensity of D2 fabrics within the mylonites is variable, being these subdivided into protomylonite, mylonite, and ultramylonite as defined by Sibson (1977), based on volume

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percentage of recrystallised matrix. Thus, increasing matrix proportion and decreasing

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average grain size are correlated with increasing strain intensity. In the field, each rock type

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is differentiated primarily based on the inferred degree of dynamic recrystallisation of

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quartz and feldspar in hand specimen. It must be noted that the mylonites contain micas

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but, in all cases, the micas are recrystallised and form anastomosing networks around the

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more competent quartz and feldspar. Following these criteria the rock types are defined as

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follows: 1) Protomylonite has angular coarse-grained clasts of feldspar, the quartz crystals

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are mostly coarse but may exhibit rounding and a degree of rotation which is interpreted as

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partial recrystallisation. Shearing was primarily accommodated by relatively incompetent

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mica networks (Fig. 6a). 2) Mylonite has medium to coarse-grained, angular clasts of

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feldspar but in most cases the feldspar crystals appear rotated relative to the foliation

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which wraps around them. 90% - 100% of the quartz is fine grained and appears to have

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been smeared out and annealed (Fig. 6b). 3) Ultramylonites have 100% fine- to very

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fine-grained quartz crystals and 90% - 100% fine- to very fine-grained feldspar crystals.

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Where the feldspar is medium-grained, it is strongly rounded (Fig. 6c). Angular feldspar

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clasts commonly contain microfractures showing the same shear sense as the mylonites (Fig.

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6b,d). However, the mylonites consistently contain an interconnected network of quartz

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and/or biotite, such that the feldspars represent competent clasts within clastomylonites,

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whose bulk strength and deformation style are governed by the rheology of the weaker

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quartz and biotite (c.f. Handy, 1990; Handy et al., 1999).

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Along with the average grain-size decrease the mylonites also show a more intense S2 foliation: protomylonites have sparse, widely-spaced and poorly developed foliation,

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mylonites have relatively closely spaced, well developed foliation, and ultramylonites have

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very closely spaced, very well developed foliation. Microscopically, the protomylonites lack

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interconnected and aligned biotite clasts (Fig. 6b), whereas layers of interconnected,

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aligned, biotite crystals are present in mylonites (Fig. 6d), and layers of fine grained biotite

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are also seen in ultramylonites (Fig. 6f). The average foliation orientation does not vary

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significantly between the different mylonite types; however, the range in observed

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orientations decreases slightly with increasing degree of mylonitisation (Fig. 5d-f).

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The sub-vertical S2 and sub-horizontal L2 indicate that the KMZ has a strike-slip sense

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of shear with the horizontal map-view approaching the XZ plane of the finite strain

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ellipsoid. The shear sense of the KMZ can be inferred from the macro-scale map-view,

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where the strike of S2 forms an acute angle varying from < 5

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zone walls (Fig. 7). From an inferred initial angle of 45◦ to the shear zone walls (Ramsay,

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1980), the S2 foliations have therefore been rotated clockwise which indicates a right-lateral

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shear-sense (Fig. 7). A dextral strike-slip sense of shear is also indicated by numerous

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small-scale shear sense indicators such as σ-clasts, δ-clasts, and fractured feldspars in the

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protomylonite. Left-lateral shear sense indicators exist, but make up less than 5 % of the

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total number of indicators observed.

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Adjacent to the largest leucogranite lozenge, central in the mapped KMZ, there are a

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number of small dextral cataclastic fault zones, tens of metres in length (Fig. 2). These

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faults are subparallel to D2 foliation, with the same shear sense. The fault rocks have not

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been viscously deformed, and therefore post-date the viscous shear leading to mylonite

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development in the KMZ. However, the cataclasites record the same kinematics as the

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mylonites, and may therefore be a late feature of the same deformation episode, or related

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to local, transient embrittlement, caused by increased fluid pressure, high strain rate, or

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magmatic embrittlement if the leucogranite intrusion was indeed syn-tectonic.

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3.3

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The study area also contains N-S striking, subvertical, cataclastic faults, tens to hundreds of

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meters long. These faults cross-cut the mylonites at an oblique angle and are not associated

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with the main KMZ shearing event. These faults are therefore considered related to a D3

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event, likely associated with the break-up of Gondwana as inferred for similarly oriented

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structures throughout southern Namibia and western South Africa (Viola et al., 2012). D3

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structures do not fall under the scope of this study and will not be discussed further.

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Although the KMZ as a whole represents a zone of localized strain, deformation within the

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KMZ is variably partitioned into high and low strain zones, such that lozenges of relatively

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undeformed blocks also occur throughout the KMZ. In addition, the style of deformation

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and strain distribution between the two main rock units is distinct, with the Tsirub Gneiss

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characterized by distributed and relatively homogenous shearing, whereas strain in the

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biotite gneiss is highly variable and more localized.

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4.1

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Lozenges, here defined as relatively unstrained lenses surrounded by zones of significantly

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higher strain (Ponce et al., 2013), generally form either (1) from rheological heterogeneities,

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where more competent rocks are surrounded by more deformed, less competent rock; or (2)

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in rheologically homogeneous rock where interconnected networks of high strain zones

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surround relatively low strain lozenges. Ponce et al. (2013) have suggested that the second

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mechanism is generally controlled by the orientation of pre-existing fabric in the otherwise

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homogeneous rock. In the KMZ, lozenges occur both as competent lenses surrounded by

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incompetent matrix, and as low strain lenses within a single lithology.

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Deformation Distribution

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Lozenges

Compositionally-controlled lozenges are represented by leucogranite lenses within the

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gneisses (Fig. 2). Mineralogically and texturally the leucogranites differ from the

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surrounding country rock in that they are very coarse-grained and contain little to no 9

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phyllosilicate. The Type 1, relatively competent, leucogranites include two major lozenges

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in the central NW (Fig. 2), with long axes of 500 m and 2500 m respectively and short axes

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between 125 and 200 m. A number of smaller leucogranite lozenges within the Tsirub

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Gneiss have long axes ranging from 20 to 250 m and short axes ranging from 1 to 20 m.

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These leucogranites strike sub-parallel to the shear zone walls and are lensoidal to sigmoidal

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in shape. The two largest leucogranites have a protomylonite fabric in an ∼ 1 m thick zone

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around the edge, but otherwise their igneous texture is unaffected by deformation. The

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smaller Type 2, less competent, leucogranites form lozenges that are more deformed than

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the surrounding gneisses, with a strong planar and linear D2 fabric.

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In addition to leucogranite lozenges, both the Tsirub Gneiss and the biotite gneiss contain lozenges that are weakly affected by D2 , but otherwise are mineralogically near

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identical to the surrounding, mylonitised gneiss. These lenses contain a well-developed D1

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fabric, which was locally folded but not mylonitised during D2 . We refer to the lenses as

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gneissic lozenges, and they represent lenses formed in D2 , that internally preserve a D1

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fabric. S1 orientations in Tsirub Gneiss, preserved within the gneissic lozenges and observed

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outside the KMZ, are generally 60 - 70◦ from S2 in 3-D (Fig. 5) but the foliation traces of

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S1 and S2 are < 45◦ apart in the XZ plane approximated in map view. The Tsirub Gneiss

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only contains four gneissic lozenges in the mapped area, whereas the biotite gneiss has

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numerous gneissic lozenges, particularly in the SE quadrant of the mapped area (Fig. 2). In

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the biotite gneiss the gneissic lozenges stand out as containing a folded migmatitic banding

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which is not recognizable in surrounding mylonites. The lozenges have long axes ranging

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from 25 to 800 m and short axes ranging from 1 to 30 m. The lozenges have sigmoidal

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shapes with long axes oriented at an acute anti-clockwise angle of between 10◦ and 30◦ from

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the average shear zone walls (Fig. 2).

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4.2

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The KMZ contains a number of intensely foliated mylonites and ultramylonites which wrap

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around the leucogranites and gneissic lozenges, forming an anastomosing network similar to

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several ductile to brittle-ductile anastomosing shear zone networks described in a range of

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other locations (e.g. Ramsay and Graham, 1970; Choukroune and Gapais, 1983; Corsini

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Strain partitioning throughout the KMZ

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et al., 1996; Hudleston, 1999; Stewart et al., 2000; Bhattacharyya and Hudleston, 2001;

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Carreras, 2001; Bhattacharyya and Czech, 2008; Carreras et al., 2010). Figure 7 shows how

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the foliation anastomoses around the lozenges in map view, and Fig. 3 shows the foliation in

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cross-sectional view. There is a variation in the strike of the foliations, which are

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sub-parallel to the lozenge edges, as they wrap around the lozenges or where lozenges

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narrow and pinch out along strike (Fig. 7).

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The Tsirub Gneiss is predominantly deformed to form protomylonite, with strain

relatively evenly distributed throughout this rock unit, as exhibited by the general lack of

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lozenges and high strain zones (Fig. 2). The only zones of mylonite and ultramylonite

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within the Tsirub Gneiss occur around the Type 1 leucogranite lozenges near the centre of

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the shear zone, so that there is a general increase in strain intensity from the shear zone

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boundary toward the centre. The smaller Type 1 leucogranite lozenges within the Tsirub

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Gneiss are surrounded by a narrow 1 to 3 m wide rim of mylonite. Along the NW contact

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between the major leucogranite lozenge and the Tsirub Gneiss, the ultramylonite rim is

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∼ 6 m thick followed by a 10 - 15 m thick mylonite zone which gradually grades into

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protomylonite away from the leucogranite lozenge.

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In the biotite gneiss, the deformation is more heterogeneously distributed, and is

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characterised by zones of mylonite and ultramylonite anastomosing around lozenges that

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internally show folded migmatitic banding (Figs. 2, 3 and 7). Overall, the deformation

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intensity in the biotite gneiss increases away from the southern shear zone wall, toward the

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contact with the Tsirub Gneiss. With the exception of the gneissic lozenges and their

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peripheries, the northern half of the biotite gneiss is mylonite or ultramylonite (Fig. 2).

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Like the Tsirub Gneiss, the most intense deformation in the biotite gneiss is along the

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contact with the major Type 1 leucogranite lens in the central NW (Figs. 2, 3 and 7). Here,

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the biotite gneiss - leucogranite contact is a 5 to 8 m thick zone of ultramylonite which

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grades into a 50 to 200 m wide zone of mylonite, which in turn grades into protomylonite

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and finally into the country rock with only a D1 fabric (Fig. 2).

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The proportions of both relatively high and low strain rocks is much greater in the

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biotite gneiss than in the Tsirub Gneiss. Within the KMZ area, only 4% of the Tsirub

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Gneiss is mylonite or ultramylonite, whereas 19% of the biotite gneiss is mylonite or

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ultramylonite. Similarly, the biotite gneiss also has a higher percentage of unsheared rocks, 11

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292

with about 20 % of this unit consisting of low strain lozenges, compared to less than 1 % in

293

the Tsirub Gneiss. About 95% of Tsirub Gneiss exposures are protomylonite, whereas only

294

60% of biotite gneiss is protomylonite. The strain distribution in the KMZ is illustrated qualitatively along three sections

295

perpendicular to the shear zone margins in Fig. 8. These sections show that deformation

297

localized predominantly in two places: (1) along the edge of leucogranite lozenges and (2) in

298

the zones between D1 lozenges in the biotite gneiss (Fig. 8), emphasizing the observation

299

that the KMZ comprises low strain lenses separated by high strain zones, and that strain is

300

highly heterogeneous across the shear zone.

301

5

302

On initiation of a shear zone a flattening fabric may form perpendicular to the greatest

303

compressional stress (Ramsay, 1980). Following initiation, in simple shear the foliations

304

generally track finite strain and therefore rotate towards the shear zone walls. A pure shear

305

component of shear zone flattening approximately perpendicular to the shear zone walls

306

may lead to additional rotation of the foliation toward the shear zone margins. It is inferred

307

that a well-developed, closely spaced fabric in a fine-grained mylonite or ultramylonite is

308

indication of intense deformation and high strain (Sibson, 1977; Ramsay, 1980).

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Discussion

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Strain may localize for a number of reasons. Strength anisotropy caused by pre-existing foliations (Williams and Price, 1990; Carreras, 2001; Gomez-Rivas and Griera, 2012; Ponce

311

et al., 2013; Montesi, 2013), compositional anisotropy (Cobbold, 1983; Handy, 1990;

312

Goodwin and Tikoff, 2002; Fagereng and Sibson, 2010), and pre-existing brittle

313

discontinuities (Segall and Simpson, 1986; Mancktelow and Pennacchioni, 2005), lead to

314

initial localization determined by pre-existing geometry. On the other hand, grain size

315

variations (Rutter and Brodie, 1988), crystal preferred orientation (Poirier, 1980), and

316

addition of a hydrous phase allowing retrograde reactions (Imber et al., 1997; Jefferies et al.,

317

2006; Fagereng and Diener, 2011) may all result from progressive viscous shearing and lead

318

to increasing localization with time. The different reasons and suggested effects on

319

deformation localization in the KMZ are discussed below.

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5.1

321

The presence of an interconnected, relatively weak, mineral phase leads to significant bulk

322

rock weakening compared to the strength of a rock where the load bearing framework is a

323

stronger phase (Handy, 1990; Handy et al., 1999). It follows that strain localization can

324

occur very efficiently in rocks with a foliation defined by a weak mineral, or a mineral with

325

a highly non-linear flow law (Montesi, 2013). Holyoke and Tullis (2006) have shown

326

experimentally that strain localization, to strain rates more than an order of magnitude

327

greater than in surrounding rock, can occur along interconnected layers of biotite developed

328

during progressive strain. In these experiments, continued strain weakening occurred along

329

biotite foliations by grain size reduction and reaction weakening, such that the foliated rock

330

exhibits strain softening behaviour. Similarly, Johnson et al. (2004) used a field example

331

and numerical modeling of an initially homogeneous tonalite to show that the development

332

of a biotite foliation allows for ductile shear at strain rates several orders of magnitude

333

above bulk strain rates. The presence of a pre-existing phyllosilicate foliation is therefore a

334

possible means of localizing strain within a shear zone. Such reactivation of a pre-existing

335

foliation has, for example, been invoked to explain progressive strain localization and the

336

development of the orogen-scale Wenchuan-Maowen Shear Zone in the Longmen Shan,

337

China (Worley and Wilson, 1996). In our current example, the presence of S1 may therefore

338

also effect strain localization and fabric development during D2 . Considering the

339

microstructural differences between protomylonite, mylonite, and ultramylonite in the KMZ

340

(Fig. 6), it is clear that increased strain localization is coupled to increased presence of

341

through-going, interconnected biotite layers.

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Pre-existing viscous fabric

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The strength of biotite is highly anisotropic, and the weakness of biotite indicated in

343

previous studies (Johnson et al., 2004; Holyoke and Tullis, 2006) involves slip along (001)

344

crystallographic planes (Kronenberg et al., 1990). Therefore, for S1 to become reactivated

345

as a shear surface during D2 development of the KMZ, S1 must be oriented at a favourable

346

angle. In the KMZ, the shear zone boundaries, likely parallel to the ideal slip plane (e.g.

347

Ramsay, 1980), are subvertical and strike NW-SE. In 3-D, the average acute angle between

348

S1 in the Tsirub Gneiss (Fig. 5a) and S2 mylonitic foliations (Fig. 5d-f) is 65 - 70◦ . Because

349

of this high angle between pre-existing biotite foliation and the shear plane, we suggest that

350

a new foliation initiated and developed in response to shearing, rather than slip occurring 13

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along the pre-existing biotite foliation. Because the pre-existing foliation was uniformly

352

mis-oriented (Fig. 5a)(Diener et al., 2013), the shear zone foliation probably developed

353

relatively homogeneously, with the exception of strain localization and enhanced fabric

354

development near the Type 1 leucogranites (further discussed below). This could explain

355

the general lack of gneissic lozenges in the Tsirub Gneiss. When formed, however, the

356

biotite foliation would likely facilitate progressive strain weakening within the sheared

357

Tsirub Gneiss (Holyoke and Tullis, 2006).

358

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In the biotite gneiss, S1 migmatitic banding has a highly variable orientation resulting from folding around shallowly plunging NW-SE trending fold hinges, and therefore includes

360

both steeply dipping planes at a low angle to the shear zone boundaries, and planes highly

361

oblique to the KMZ slip plane. We suggest that the steeply dipping S1 planes were

362

favourably oriented to be easily reactivated and undergo further slip during D2 . On the

363

other hand, fold hinges where S1 had a shallow orientation were not suitably oriented for

364

slip and developed into gneissic lozenges. The end result is therefore a heterogeneously

365

strained mylonitised migmatite, that within lozenges preserves migmatitic fabrics, at high

366

angles to the shear plane. The current orientation of D1 fold hinges is near-parallel to L2 ,

367

and they are also tighter than D1 folds away from the KMZ (c.f. Diener et al., 2013). This

368

difference in fold orientation and geometry may arise from the passive rotation of fold

369

hinges during shearing, coupled to either a transpressional component of strain that has

370

tightened the pre-existing migmatitic folds, or shortening of lozenge short axes during

371

progressive simple shear.

372

5.2

373

In the KMZ there are compositional differences both within and between lithologies,

374

although the most significant compositional contrast is between leucogranite and gneiss.

375

The main differences between these lithologies is that the Type 1 leucogranites are

376

coarse-grained and have a high K-feldspar content coupled to a low phyllosilicate content

377

and no pre-KMZ foliation. There is also some minor heterogeneity within the gneisses,

378

where differing proportions of quartz, feldspar and phyllosilicates, or heterogeneous

379

amounts of retrogressive minerals occur within zones of both Tsirub Gneiss and biotite

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Compositional Anisotropy

14

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gneiss. The Type 2 leucogranites, on the other hand, appear to have accommodated more

381

strain than the surrounding gneiss, implying that these leucogranites were syn-tectonic and

382

preferentially deformed while in a weak, liquid state (c.f. Passchier et al., 2005). Aligned,

383

angular K-feldspar crystals attest to a preferred orientation of prismatic minerals likely

384

attained in the liquid state.

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Generally, the Type 1 leucogranites are more competent than the surrounding country

386

rock because of the lower phyllosilicate content and the coarse grain-size, leading to strain

387

accumulating preferentially around the leucogranites (Handy, 1990; Goodwin and Tikoff,

388

2002; Fagereng, 2012). The boundary between the Type 1 competent leucogranites and the

389

country rock became a major plane of anisotropy which led to localization of strain on the

390

edges of the leucogranite blocks as the less competent country rock accommodated the

391

additional strain that the leucogranites did not. As strain in the KMZ is primarily

392

accommodated by viscous flow of quartz and biotite, the presence of only trace quantities of

393

biotite in the leucogranite causes strain partitioning into the surrounding country rock.

394

Goodwin and Tikoff (2002) came to a similar conclusion of localized strain, at much smaller

395

scale, around competent porphyroclasts in the Rosy Finch shear zone, Sierra Nevada, where

396

localized strain occurred at the edges of relatively undeformed feldspar crystals.

397

5.3

398

Brittle deformation of feldspars, at the microscale, indicate frictional-viscous flow in the

399

mylonites, but macroscopic ductile deformation and an interconnected network of viscously

400

deforming, weak mineral phases (quartz, biotite, minor chlorite and muscovite) show that

401

the majority of the strain was accommodated by viscous flow. Deformation therefore likely

402

occurred in a temperature range between 350◦ C and 450◦C, where feldspar is brittle, but

403

deformation is accommodated by shearing flow along interconnected networks of quartz and

404

biotite, which are viscous in this temperature range (Handy et al., 1999). In this case of

405

strain localization along planes of weak phyllosilicates, retrograde metamorphism may aid

406

deformation by leading to formation of additional micaceous minerals (e.g. Imber et al.,

407

1997).

408

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Retrogression

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Some of the high strain zones in the KMZ appear to contain a greater proportion of 15

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409

retrograde chlorite and muscovite relative to the low strain zones. Similar observations

410

made by Imber et al. (1997) on the Outer Hebrides fault zone, Scotland, led to the

411

conclusion that the addition of a hydrous phase in a shear zone caused mechanical

412

weakening along the fluid conduit and associated increased strain localization. The presence of retrograde chlorite requires hydrous fluid addition, potentially aiding

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413

the localization of strain in these regions. However, the presence of chlorite is not

415

ubiquitous, and not all ultramylonites contain chlorite. Therefore, although some high

416

strain zones appear to be more chlorite-rich, this is not pervasive, indicating that strain

417

localization could also occur where the influence of a fluid phase was minor.

418

SC

414

The KMZ cross-cuts migmatites related to peak metamorphic conditions (Jackson, 1976), but D2 shearing did not initiate under granulite facies conditions. The KMZ was

420

active under post-D1 retrograde conditions, overprinting the peak metamorphic fabrics

421

along a near-isobaric cooling metamorphic path as described by Diener et al. (2013). This is

422

consistent with the strike-slip kinematics of the KMZ, which implies that it was not related

423

to significant crustal thickening or thinning. It follows that the KMZ was likely active under

424

constant or decreasing temperatures. Under such retrograde conditions, any active

425

metamorphic reactions would consume rather than produce fluids (Guiraud et al., 2001;

426

Tenczer et al., 2006; Diener et al., 2008). The presence of some retrograde minerals

427

indicates that partial rehydration of the KMZ did occur, but the relative scarcity of such

428

minerals implies that localized high strain deformation in the KMZ did not require fluids.

429

We interpret the patchy retrogression to reflect that fluid infiltration was a relatively

430

passive consequence of high strain deformation, rather than a driver for it.

431

5.4

432

Work by White (2004) and Platt and Behr (2011a) have concluded that grain-size reduction

433

caused by dislocation creep acts to weaken rock and localize strain, particularly because of

434

a transition from dislocation creep to grain-size sensitive diffusion creep below a particular

435

grain size. Similarly, Knipe (1989) has emphasised that progressive microstructural

436

development significantly affects the dominant deformation mechanisms in naturally

437

deforming rocks. Rutter and Brodie (1988) established an association between localized

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Grain size

16

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shear and grain size reduction. Similarly, in the KMZ the zones of smallest grain-size are

439

the highly sheared contacts of the major Type 1 leucogranite lozenges (Figs. 2 and 3).

440

There is therefore a clear correlation between grain size and strain, where the highest strain

441

zones indicated by foliation development are also the zones of finest grain size (Figs. 7 and

442

8), whereas the grain size is progressively larger through mylonites and protomylonites (Fig.

443

6) and into the undeformed wall rock.

444

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Grain size reduction associated with ductile shear zones may either be caused by

cataclastic grain size reduction or dynamic recrystallization (Handy et al., 2007). Although

446

increased strain is inferred to have decreased average grain size, based on field observations

447

of grain size and inferred strain intensity, this does not unequivocally show that decreasing

448

grain size also led to increased localization of strain. However, finer grain size would have

449

allowed a transition from dislocation to diffusion creep, as suggested in other locations

450

(Brodie and Rutter, 1987; White, 2004; Platt and Behr, 2011a), which would allow

451

significantly higher strain rate with decreasing grain size (Rutter, 1983). The increasing

452

degree of mylonitization would, however, also lead to the development of a crystal preferred

453

orientation, in which glide planes may be aligned with the shear plane, also causing

454

weakening and strain localization (Poirier, 1980; Schmid et al., 1987). However, kinking of

455

large biotite grains may lead to strain hardening, despite development of a crystal preferred

456

orientation aligned with the shear plane, and experiments by Holyoke and Tullis (2006)

457

have indicated that reaction softening and dynamic grain size reduction are the most likely

458

processes to maintain strain localization after the formation of an interconnected biotite

459

fabric.

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The observations in the KMZ therefore provide an additional example of a shear zone

461

where high strain zones with high foliation intensity and very fine grain size surround

462

relatively unsheared lenses. The lozenges have larger grain size and preserve pre-shear

463

fabrics, evidence that these lenses were not recrystallized, and that grain size reduction is a

464

function of strain intensity and dynamic recrystallization. Although strain weakening may

465

be aided by foliation development, we interpret the association between grain size and

466

strain intensity as a result of increasing strain rate with decreasing grain size; this increase

467

in strain rate could be caused by a transition to diffusion creep or a change in the flow law

468

for dislocation creep at smaller grain size. 17

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5.5

470

In the discussion above, strain localization appears to be controlled by three main factors:

471

the orientation of pre-existing fabrics, rheological contrasts caused by compositional

472

differences between gneiss and leucogranites, and localization by weakening related to grain

473

size reduction plus/minus fabric development. Whereas pre-existing fabrics and

474

compositional boundaries would impose initial, narrow zones of weaker rock, weakening

475

caused by grain size reduction and fabric development would lead to increasing localization

476

with time. Therefore, there are two end-member options for the time-evolution of the KMZ:

477

(1) the shear zone initiated along a pre-existing narrow zone, and widened to its current

478

observed width; or (2) the shear zone initiated as a wide zone and narrowed with time, as

479

strain became increasingly localized in what is currently observed as higher strain zones.

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Evolution of the KMZ

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If the KMZ initiated as a wide zone, strain would be required to, with time, localize between lozenges in the biotite gneiss, and against the leucogranites. The leucogranites

482

represent a major anisotropic boundary, along which deformation can be expected to

483

localize, leading to decreased grain size and further localization with time. Within the

484

biotite gneiss, shearing occurred along the biotite foliation where such interconnected

485

biotite layers were present and favourably oriented. Lozenges containing a folded fabric at

486

high angles to S2 foliation are preserved between zones of localized shear. Fabric

487

development within the Tsirub Gneiss likely occurred during the initial stages of shear

488

strain in the KMZ, prior to any strain localization. After some time, strain was localized

489

along and between the major pre-tectonic leucogranites and in the mylonite north of the

490

gneissic lozenges in the SE biotite gneiss (Fig. 8). This strain distribution highlights the

491

very heterogeneous strain that may occur in strike-slip shear zones, even where there is

492

little mineralogical variation in the deforming rock assemblage.

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481

Alternatively it can be argued that the KMZ widened progressively with time.

494

Mechanisms for localized initiation are suggested by Mancktelow and Pennacchioni (2005)

495

and Fusseis et al. (2006) where a shear zone may initiate along a plane of anisotropy such as

496

the major pre-tectonic leucogranites or in pre-existing brittle fractures and then widen into

497

the surrounding rock. It has been suggested by Mancktelow and Pennacchioni (2005) that

498

shear zone widening is promoted by fluid diffusion into the rock surrounding the initiating

18

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499

plane of anisotropy. Fusseis et al. (2006) suggest that no fluid is needed to widen shear zones

500

but that a number of initiating fractures may connect into a continuous ductile network.

501

Although a case for shear zones widening with time has been successfully argued for in other locations, a narrowing of the shear zone with time is preferred for the KMZ. Cases for

503

shear zone delocalization with progressive strain are generally based on shear zones

504

initiating on pre-existing brittle fractures (e.g. Mancktelow and Pennacchioni, 2005; Fusseis

505

et al., 2006). There is no evidence for through-going brittle fractures leading to strain

506

localization along leucogranites or between low strain lozenges in the gneisses. Because the

507

rocks containing the shear zone were cooling from temperatures far warmer than the

508

frictional-viscous transition, prior to shear zone initiation, and because brittle deformation

509

is only observed locally and generally contained inside feldspar porphyroclasts, this

510

retrograde shear zone could not have initiated on a fracture system developed at depths

511

above the frictional-viscous transition. Therefore, we infer that it initiated in the viscous

512

mid- to lower crust in the absence of pre-existing brittle fractures. We envisage a shear zone

513

evolution where strain initially may have been distributed, but with progressive strain,

514

shearing occurred primarily on interconnected networks of biotite, either pre-existing or

515

developed as progressive shear led to development of a stronger biotite fabric (see

516

progression from protomylonite to ultramylonite in Fig. 6). Reaction softening and grain

517

size reduction associated with progressive, localized strain along biotite foliations then led

518

to strain softening (Holyoke and Tullis, 2006), causing further localization onto these

519

planes. In addition, competent leucogranite lenses led to increased stress and associated

520

increased strain rate in the gneisses close to leucogranite bodies. Overall, this evolution

521

would lead to heterogeneous strain, which differes significantly from the classical model of a

522

wide, distributed, homogeneous shear zone in the mid- to -lower crust. This model of strain

523

softening and increasing localization with time agrees with the recent suggestion by

524

Boutonnet et al. (2013) that continental strike-slip zones represent strong, long-lived strain

525

localization, and the conclusion of Montesi (2013) that foliation development is particularly

526

efficient for localizing strain.

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6

528

The KMZ is a NW-SE striking, 1 - 2 kilometer wide, sub-vertical, dextral strike-slip shear

529

zone developed under amphibolite to upper greenschist facies, retrograde metamorphic

530

conditions, and separates two granulite gneisses with different pre-existing fabrics. Variable

531

development of a D2 S > L shear fabric indicates that the shear zone comprises

532

anastomosing high strain zones separating leucogranite and gneissic lozenges that are

533

relatively unaffected by shear zone deformation. Based on observations of shear zone

534

fabrics, the following conclusions are made in relation to strain distribution, and the

535

evolution of a crustal-scale shear zone in the mid-crust:

SC

(1) Strain is heterogeneous and primarily localized in two places: a) at the contact

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536

Conclusions

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527

537

between relatively undeformed leucogranites and the surrounding gneisses; and b) in

538

between lozenges of biotite gneiss. The highest strain is recorded in a narrow ∼ 4 - 6 meter

539

zone between two relatively undeformed leucogranites.

540

(2) We suggest that strain localization occurred because of a major competency contrast between the leucogranites and the gneisses, such that strain was preferentially

542

partitioned into the gneisses near the leucogranites. Within the biotite gneiss, strain may

543

have been localized into anastomosing high strain zones controlled by variable orientation of

544

weak biotite foliation and progressively localized by reduction in grain-size during

545

deformation. It is unlikely that the anastomosing shear zone network within the biotite

546

gneiss developed from a pre-existing network of fractures, although it may have been

547

affected by variable orientation of folded biotite layers. In the Tsirub Gneiss the strain is

548

more distributed, without anastomosing high strain zones, probably because of the uniform

549

(mis)orientation of pre-existing weak biotite layers.

EP

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541

(3) Strain intensity is dependent on rheology which in turn depends on compositional

551

anisotropy. Grain-size reduction, particularly near the boundary of Type 1 competent

552

leucogranites, was likely a major contributor to the progressive development of high strain

553

intensity zones. The influx of a retrogressive hydrous phase appears to locally affect strain

554

intensity, but because secondary phases are not abundant, this is considered a minor effect

555

relative to composition, mechanical anisotropy, and grain size.

20

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556

(4) In response to progressive fabric development, the KMZ became narrower with time as localization continued, similar to experiments by Holyoke and Tullis (2006) and

558

numerical models by Johnson et al. (2004). Final shear localization occurs in the form of

559

shear zone sub-parallel brittle faults on the southern contact of the major pre-tectonic

560

leucogranite and the biotite gneiss.

561

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Overall, the KMZ is an example of a major viscous shear zone, representing a zone of strain localization, but is itself heterogeneous with high strain zones anastomosing around

563

lower strain lozenges. Strain distribution within the KMZ is controlled by rheological

564

heterogeneity, arising from variability in composition, orientation and continuity of

565

pre-existing foliations, and grain size. Because additional rheological contrast must develop

566

with time, as dynamic recrystallization leads to grain size reduction and foliation

567

development, strain likely became more localized with time in this particular shear zone,

568

and likely in other shear zones developed in comparable rocks under similar conditions.

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569

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562

The KMZ provides an example of a shear zone where an anastomosing network of high strain zones developed at the kilometre scale, and is therefore a tabular, km-wide, feature

571

where shearing strain accumulated progressively but heterogeneously in different rock units.

572

For shear zones in general this implies that the structures generally observed at outcrop

573

scale, such as development of lozenges, variable fabric development, and inferred progressive

574

changes in deformation mechanisms, also apply to crustal scale shear zones. We also

575

emphasise that a single, monolithological flow law is unlikely to be applicable for natural

576

shear zones, as the KMZ is an example of a shear zone in relatively homogeneous rocks, but

577

has still developed a heterogeneous internal geometry in response to pre-exising anisotropy

578

and variable progressive fabric development.

579

Acknowledgments

580

Fagereng and Diener acknowledge financial support from separate UCT Research

581

Development Grants. We sincerely thank Koos and Anna Bosman for their hospitality and

582

access to the field area. The manuscript benfitted significantly from critical reviews by E.

583

Druguet and an anonymous reviewer.

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584

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Figure Captions Figure 1: 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 southwest of

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Aus in southwest Namibia. The Kuckaus Mylonite Zone (KMZ) lies within the Gordonia Subprovince of the NMC, separated from the Sinclair Subprovince to the northeast by the larger Lord Hill - Excelsior shear zone, and from the Richterveld Subprovince to the southwest

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by the Pofadder - Tantalite Valley shear zone (after Becker et al., 2006).

Figure 2: Geological map of the Kuckaus Mylonite Zone, in a two by seven kilometer area of

tary material for a more detailed map.

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near-continuous exposure. Cross-sections are presented in Fig. 3. See the online supplemen-

Figure 3: Vertical cross-sections through the KMZ, oriented approximately perpendicular to stretching lineation (see Fig. 2 for legend, locations and orientations).

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Figure 4: Field photographs of the main rock units exposed in the KMZ. a) Tsirub Gneiss with a planar, shallow to moderately dipping S1 defined by biotite; b) Biotite gneiss with a tightly folded D1 fabric defined by alternating leucosome-melanosome layers; c) Leucogranite

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with coarse grained, equigranular feldspar and quartz; d) Mafic granulite with hornblende and plagioclase-rich layers.

Figure 5: Lower hemisphere, equal area stereoplots showing: a) poles to S1 in Tsirub Gneiss

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where unaffected by the KMZ, great circle shows average S1 ; b) S1 in biotite gneiss low strain lozenges containing a folded migmatitic banding; c) stretching lineations in the KMZ; d) poles to S2 in ultramylonite, great circle shows average S2 ; e) poles to S2 in mylonite, great circle shows average S2 ; f) poles to S2 in protomylonite, great circle shows average S2 .

29

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Figure 6: Three subdivisions of mylonite in outcrop (left column) and photomicrographs in cross-polarized light (right column). The thin sections were cut parallel to lineation and perpendicular to foliation, outcrop photographs are taken of approximately horizontal surfaces

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approximating the XZ plane of the finite strain ellipsoid. a) and b) show protomylonite with weakly developed foliation and lacking interconnected networks of phyllosilicates. Note fractured feldspars and relatively large grain size of all components. c) and d) show mylonite, where an interconnected network of biotite and chlorite has developed. The phyllosilicates

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show a shape preferred orientation subparallel to the shear plane. Note smaller grain size of all components, and brittle deformation of isolated feldspar clasts. e) and f) show ultramylonite with strong foliation defined by mineralogical segregation in very fine grained phyllosilicates

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(mainly biotite) and quartzofeldspathic bands. The shape preferred orientation seen in mylonites is no longer apparent, potentially indicating a change in deformation mechanism from dislocation to diffusion creep (c.f. Holyoke and Tullis, 2006). Notice the overall trend of decreasing grain size and increasingly well developed foliation from protomylonite to ultra-

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mylonite. Dextral shear sense inferred for all photographs and photomicrographs shown.

Figure 7: S2 foliation traces (dashed lines) showing the variation in strike around the lozenges. This map shows clearly the acute angle that the foliation makes with the shear zone walls indicating a right lateral shear sense. The intensity of foliation is indicated by line density,

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and highlights high strain zones adjacent to leucogranites and between gneissic lozenges. S1

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foliation traces are shown where preserved inside lozenges (dotted lines).

Figure 8: Schematic representation of strain intensity across the shear zone, parallel to cross sections in Fig. 3. The y-axis shows strain as indicated by level of mylonitization, and is thus a qualitative, non-linear measure of strain. UM = ultramylonite, M = mylonite, PM = protomylonite, D1 = preserved pre-KMZ fabric. A-A’ shows a wide zone of high strain in the biotite gneiss increasing towards the major leucogranites and a narrower zone in the Tsirub Gneiss. Peak strain is between the two major leucogranites. B-B’ illustrates a wide zone of high strain in the biotite gneiss and a narrower zone in the Tsirub Gneiss. C-C’ shows how strain in the biotite gneiss is localized between D1 lozenges and is negligible near and within each lozenge. 30

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(a)

Namibia

Botswana Kaapvaal Craton

b

NMC

16ºE

(b)

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South Africa

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Figure 1

rd

Aus

Fig. 2

KM

Z

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28ºS

Keetmanshoop

lsio

rS

he

Oranjemund

ar

Zo

ne

NAMIBIA

Gordonia Subprovince Grunau

Ta n

tal

ite

Va lle

y–

Po

fad

ATLANTIC OCEAN

19ºE

Sinclair Subprovince

Ex

ce

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27ºS

Hil

l–

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Lüderitz

18ºE

17ºE

Lo

Richtersveld Subprovince

de

rS

he

ar

Zo

ne

Orange River

100 km

SOUTH AFRICA

Figure 2

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A’

Quaternary Sands Late Cataclastic Fault Zones

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B’

Type 2 Leucogranite Type 1 Leucogranite Mafic Lenses

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Biotite Gneiss Ultramylonite

Ultramylonite

Mylonite

Mylonite

Protomylonite

Protomylonite

D1 biotite gneiss & lozenges

D1 Tsirub Gneiss & lozenges 1000 m

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A

Tsirub Gneiss

AC C

EP

C’

B

C

N

Figure 3

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1250 m

1000 m

A

500 m

A’

1000 m

M AN U

SC

1250 m

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1125 m

1125 m

500 m

1500 m

2000 m

AC C

1250 m

1000 m

EP

B

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1000 m

1125 m 1000 m

C

500 m

1000 m

C’

2500 m

B’

a Figure 4

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b

M AN U

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S1

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c

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25 cm

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d

1.5 cm

Figure 5 a

b

c

M AN U

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n = 24

f

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e

n = 68

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d

n = 29

n = 38

n = 151

n = 80

Figure 6 a

1 cm

c

500 µm

1 cm

500 µm

f

AC C

e

EP

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M AN U

d

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b ACCEPTED MANUSCRIPT

1 cm

500 µm

Figure 7

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S2 foliation traces S1 foliation traces Leucogranite Gneissic Lozenges

1000 m

N

Figure 8

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UM PM

leucogranite leucogranite

100

750

UM PM

A’

EP

leucogranite

1500

1650

Tsirub Gneiss 2250

B’

Distance (m)

UM M

Strain

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D1

biotite gneiss

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M

Strain

750

1150

M AN U

Distance (m)

Tsirub Gneiss

SC

D1

B

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M

Strain

A

biotite gneiss

PM D1

C

300

biotite gneiss 750

Distance (m)

1250

C’