Jacques Précigout
email: jacques.precigout@univ-orleans.fr

Greg Hirth

B-type Olivine Fabric induced by Grain Boundary Sliding

Keywords: Lattice Preferred Orientation, fabric strength, deformation mechanism, Ronda peridotite

)[001] slip in laboratory samples, and (4) our observations are supported by experiments that document B-type fabric in olivine aggregates where deformation involves a component of GBS. Our results have important implications for interpreting the rheological properties of shear zones and upper mantle structures via microstructural and seismic observations.

Introduction

Strain-induced olivine LPOs have a strong and predictable effect on seismic wave propagation, providing a powerful tool to interpret the kinematics and dynamics of flow within inaccessible regions of the mantle (e.g., [START_REF] Ismaïl | An olivine fabric database: an overview of upper mantle fabrics and seismic anisotropy[END_REF][START_REF] Silver | Mantle structural geology from seismic anisotropy[END_REF]. However, over the last 10 years or so, numerous experimental results and natural observations indicate that olivine LPO can vary significantly depending on the conditions of deformation. Of particular importance is the observation of B-type fabric, where the olivine [100] axes align in the flow plane, normal to the flow direction and [001] axes align parallel to the flow direction. This fabric geometry strongly affects the relationships between deformation kinematics and seismic anisotropy (e.g., [START_REF] Jung | Water-induced Fabric Transitions in Olivine[END_REF]. Although critical for the interpretation of upper mantle structure and dynamics, the deformation conditions where olivine B-type fabric develops are still actively debated (e.g., [START_REF] Karato | Geodynamic Significance of Seismic Anisotropy of the Upper Mantle: New Insights from Laboratory Studies[END_REF]Sundberg and Cooper, 2008;[START_REF] Kohlstedt | Shearing Melt Out of the Earth: An Experimentalist's Perspective on the Influence of Deformation on Melt Extraction[END_REF][START_REF] Jung | Deformation microstructures of olivine in peridotite from Spitsbergen, Svalbard and implications for seismic anisotropy[END_REF][START_REF] Ohuchi | High pressure and temperature fabric transitions in olivine and variations in upper mantle seismic anisotropy[END_REF].

During viscous deformation, olivine aggregates deform by either motion of intracrystalline defects along lattice-controlled slip systems (dislocation creep), or grain boundary sliding (GBS) accommodated by grain boundary diffusion (diffusion creep) or motion of dislocations (DisGBS). Traditionally, the development of LPO is attributed to viscous deformation by dislocation creep (e.g., [START_REF] Nicolas | Formation of anisotropy in upper mantle peridotitesa review[END_REF]. In contrast, the weakening or absence of LPO in fine-grained mylonitic rocks is attributed to a transition to diffusion creep (e.g., [START_REF] Fliervoet | Crystallographic preferred orientations and misorientations in some olivine rocks deformed by diffusion and dislocation creep[END_REF][START_REF] Warren | Grain size sensitive deformation mechanisms in naturally deformed péridotites[END_REF][START_REF] Drury | Crystal preferred orientation in peridotite ultramylonites deformed by grain size sensitive creep, Étang de Lers, Pyrenees, France[END_REF].

However, recent experimental observations demonstrate the formation of LPO during diffusion creep of Fe-free olivine (i.e. forsterite) [START_REF] Miyazaki | Olivine crystals align during diffusion creep of Earth's upper mantle[END_REF]. Furthermore, olivine B-type fabric has been observed in experimentally deformed harzburgite at conditions where diffusion creep is interpreted to be the dominant deformation mechanism (e.g., Sundberg and Cooper, 2008).

In the core of mantle shear zones, substantial grain size reduction may result in a transition from dislocation creep to grain-size-sensitive diffusion creep or DisGBS (e.g., [START_REF] Warren | Grain size sensitive deformation mechanisms in naturally deformed péridotites[END_REF][START_REF] Drury | Crystal preferred orientation in peridotite ultramylonites deformed by grain size sensitive creep, Étang de Lers, Pyrenees, France[END_REF][START_REF] Hansen | Grain boundary sliding in San Carlos olivine: Flow law parameters and crystallographic-preferred orientation[END_REF], promoting a decrease in effective viscosity and subsequent strain localization (e.g., [START_REF] Précigout | Mantle weakening and strain localization: implications for the longterm strength of the continental lithosphere[END_REF].

Such a mantle shear zone has been documented in the Ronda massif (southern Spain; Van der Wal and Vissers, 1993;1996;[START_REF] Platt | Exhumation of the Ronda peridotite and its crustal envelope: constraints from thermal modelling of a P-T-time array[END_REF], where strain localization has been interpreted to result from activation of DisGBS [START_REF] Précigout | Strain localization in the subcontinental mantlea ductile alternative to the brittle mantle[END_REF]. In this paper, we investigate the relationships between olivine LPO and deformation mechanisms in the kilometer-scale Ronda shear zone. We illustrate a transition from a flow-parallel [100]-axis olivine LPO (A-type fabric) to a flow-normal [100]-axis LPO (B-type fabric). We then discuss the processes that can induce this transition to B-type fabric in Ronda based on comparison of our results with experimental data and independent constraints on the natural deformation conditions.

The Ronda peridotite

The Ronda massif belongs to the Betic cordillera in southern Spain and encompasses several sheets of mantle peridotite that were emplaced onto the thinned crust of the internal Betics during the Early Miocene (Fig. 1; Van der Wal and Vissers, 1993). During exhumation, the Ronda peridotite experienced several stages of deformation and partial melting, evidence for which is well preserved in the western Ronda massif (Van der Wal and Vissers, 1996). In the northwestern rim of the exposed mantle section, a mylonitic domain overprints a tectonite, which together represent a kilometer-scale mantle shear zone that results from strain localization beneath the host crustal rocks (Fig. 1; Van der Wal and Vissers, 1996;[START_REF] Platt | Exhumation of the Ronda peridotite and its crustal envelope: constraints from thermal modelling of a P-T-time array[END_REF][START_REF] Précigout | Strain localization in the subcontinental mantlea ductile alternative to the brittle mantle[END_REF]2013); the mylonite and tectonite are classified as subcontinental lithospheric mantle (Van der Wal and Vissers, 1996). The tectonic setting of this mantle shear zone is still debated, but the large syn-tectonic decompression recorded in both the mylonite and overlying rocks suggests that deformation occurred during extension of continental lithosphere [START_REF] Platt | Exhumation of the Ronda peridotite and its crustal envelope: constraints from thermal modelling of a P-T-time array[END_REF][START_REF] Précigout | Strain localization in the subcontinental mantlea ductile alternative to the brittle mantle[END_REF][START_REF] Garrido | Garnet lherzolite and garnet-spinel mylonite in the Ronda peridotite: Vestiges of Oligocene backarc mantle lithospheric extension in the western Mediterranean[END_REF][START_REF] Précigout | Deformation and exhumation of the Ronda peridotite (Spain)[END_REF].

In the study area, the mylonite and tectonite are mainly composed of serpentinized spinel-bearing lherzolite and harzburgite with minor garnet-bearing peridotite in the uppermost sub-crustal mylonite (Fig. 2). These peridotites exhibit a steeply dipping foliation that becomes more pronounced towards the upper mylonite owing to increasing strain. We refer to the lower half of the mylonitic domain, which lies above the tectonite, as the lower mylonite and the upper half of the mylonitic layer, which lies just beneath the host crustal rocks, as the upper mylonite. The stretching lineation, defined by cm-scale elongated orthopyroxene (Fig. 2), is close to horizontal and shear criteria indicate sinistral kinematics with a minor component of coaxial shortening (Van der Wal and Vissers, 1996;[START_REF] Balanyá | Alternating contractional and extensional events in the Alpujarride nappes of the Alboran Domain[END_REF][START_REF] Précigout | Deformation and exhumation of the Ronda peridotite (Spain)[END_REF].

As shown in [START_REF] Précigout | Deformation and exhumation of the Ronda peridotite (Spain)[END_REF], the large-scale strain gradient across the mantle shear zone has been revealed by the deformation pattern of garnet-bearing pyroxenite layers in both the tectonite and mylonite. In the tectonite, pyroxenite layers show isoclinal folds and modest boudinage. In comparison, they become strongly transposed and highly boudinaged in the lower mylonite, and then highly stretched in the upper mylonite. These structures indicate progressive strain increasing from the tectonite to mylonite as pyroxenite layers rotate with respect to the principal strain axes, leading to a change in their deformation pattern from predominately shortening to stretching [START_REF] Ramsay | Shear zone geometry: a review[END_REF].

Petrological data indicate syn-kinematic temperatures from around 1100°C in the tectonite to 800-850°C in the upper mylonite (Van der Wal and Vissers, 1993;[START_REF] Garrido | Garnet lherzolite and garnet-spinel mylonite in the Ronda peridotite: Vestiges of Oligocene backarc mantle lithospheric extension in the western Mediterranean[END_REF]. The mean grain size of olivine and pyroxene decreases progressively from ~1 mm in the tectonite to 50-300 µm in the upper mylonite, consistent with the decrease in temperature recorded during deformation at high strain (Fig. 2; Van der Wal and Vissers, 1996;[START_REF] Précigout | Strain localization in the subcontinental mantlea ductile alternative to the brittle mantle[END_REF]. A syn-tectonic pressure of around 1.8-2.0 GPa has been documented in the mylonite [START_REF] Garrido | Garnet lherzolite and garnet-spinel mylonite in the Ronda peridotite: Vestiges of Oligocene backarc mantle lithospheric extension in the western Mediterranean[END_REF].

Mineral fabric and modal composition

Lattice preferred orientations (LPOs) and fabric strength

We measured LPOs of olivine and pyroxenes in 37 peridotite samples from the tectonite to the upper mylonite using Electron Backscatter Diffraction (EBSD). Olivine LPOs are displayed in Figure 3, the orthopyroxene and clinopyroxene LPO pole figures are provided in data repository. In Figure 4, we illustrate the general variation in olivine fabric across the Ronda shear zone using both the axis pole figures and plots of the orientation of maximum density. In the upper tectonite and lower mylonite, the olivine LPOs generally show a [100] maxima parallel to the lineation (X), a [010] maxima normal to the foliation (Z) and a [001] maxima normal to the lineation in the foliation plane. This flow-parallel [100]-axis olivine LPO is typical of A-type fabric, which is common in lithospheric peridotite and results from the dominance of the (010)[100] dislocation slip system (e.g., [START_REF] Zhang | Lattice preferred orientation of olivine aggregates deformed in simple shear[END_REF]. In contrast, many samples of the upper mylonite preserve LPOs with [100] and [001] axes normal and parallel to the lineation, respectively, while the [010] axis remains normal to the foliation. This flow-normal [100]-axis olivine is typical of B-type fabric [START_REF] Jung | Water-induced Fabric Transitions in Olivine[END_REF]). It appears locally within outcrops that display weak LPOs in the upper mylonite and progressively overprints the A-type fabric in the mylonitic domain over a distance of 300 meters. This overprinting gives rise to a transitional [010]-fiber fabricwhich we refer to as A/B-type fabric -characterized by [100] and [001] axes girdles within the foliation plane (Figure 4B and 4C). The overall transition from dominantly A-type to B-type fabric is well illustrated by the transition from generally lineation parallel [100] maximum to lineation perpendicular [100] maximum shown in Figure 4D. 4D and4E illustrates that the transition in olivine fabric type is also correlated with variations in fabric strength. For this analysis we used both the M index (Skemer et al., 2005) and J index [START_REF] Bunge | Texture analysis in Materials Science[END_REF]. Although the M index and J index provide very similar information [START_REF] Schaeben | Comment on -The misorientation index: Development of a new method for calculating the strength of lattice-preferred orientation‖ by Philip Skemer, Ikuo Katayama, Zhenting Jiang, Shun-Ichiro Karato[END_REF]Skemer and Karato, 2007), they rely on different parameters. Thus, we present both indices to document the olivine fabric in Ronda. In the upper tectonite, the M index and J index are nearly constant with a value of 0.1 ± 0.05 and 3.8 ± 0.3, respectively. Upon entering the mylonitic domain, both the M index and J index increase progressively up to 0.325 and 8.7 in the lower mylonite, and then decrease down to 0.05 ± 0.04 and 2.4 ± 0.4 in the upper mylonite, respectively. With respect to the fabric types, dominantly A-type fabrics are observed where the fabric strength remains constant then progressively increases. In contrast, the location where the fabric strength starts decreasing in the lower mylonite corresponds to where the transitional A/B-type LPO is observed. Finally both the M index and J index are lowest where B-type fabric is observed near the peridotite/crust contact. Our findings thus document that the transition from A-type to B-type fabric is spatially correlated with the transition to lower fabric strength in the mylonite.

Comparison of Figures

Misorientation axes and subgrain orientations

To investigate the origin of B-type fabric in Ronda, we analyzed misorientation axes across olivine subgrain boundaries to constrain the dominant dislocation slip system(s) activated during deformation of the Ronda peridotite (e.g., [START_REF] Kruse | Dynamic recrystallization processes in plagioclase porphyroclasts[END_REF][START_REF] Mehl | Arc-parallel flow within the mantle wedge: Evidence from the accreted Talkeetna arc, south central Alaska[END_REF]Soustelle et al., 2009). Subgrain boundaries were not particularly abundant in the samples, especially where B-type fabric occurs. Nonetheless, we collected the subgrain misorientation data in samples throughout the Ronda shear zone. In Figure 5a 010)[100] slip system, i.e., the A-type slip system [START_REF] Lloyd | Misorientation analysis and the formation and orientation of subgrain and grain boundaries[END_REF][START_REF] Mehl | Arc-parallel flow within the mantle wedge: Evidence from the accreted Talkeetna arc, south central Alaska[END_REF].

In Figure 5b, we illustrate more examples of misorientation data across subgrain boundaries for samples from the tectonite, lower mylonite and upper mylonite that developed A-type, A/B-type and B-type fabric, respectively, as well as one sample that did not exhibit a well-defined fabric type. In each case, we find that the [100] axes are oriented approximately normal to the subgrain boundary. Furthermore, with the exception of sample 33, all of the samples that show a clear rotation signature exhibit smaller rotations of [001] or [010] relative to the other axes. These features suggest subgrain boundaries consistent with tilt boundaries comprised of (010)[100] or (001)[100] edge dislocations, similar to observations on high temperature peridotites that exhibit A-type fabric (e.g., [START_REF] Falus | The effect of dynamic recrystallization on olivine crystal preferred orientations in mantle xenoliths deformed under varied stress conditions[END_REF]. Sample 33 shows a more complex orientation, however, the fact that the subgrain is normal to the [100] axis is inconsistent with either a twist or tilt boundary from the (010)[001] -B-type‖ slip system. We did not find any subgrains with misorientation axes close to the [100] axis, and hence consistent with the (010)[001] -B-type‖ slip system, even in samples from the upper mylonite that exhibit the olivine B-type fabric (Fig. 5). Thus irrespective of the olivine fabric type, our misorientation dataset shows no evidence for a change in the character of the subgrains. We therefore conclude that B-type fabric did not result in intra-granular textures indicative of (010)[001] slip in Ronda. Furthermore, the observation of sub-structures indicative of (010)[100] slip in the mylonite indicates that these textures either formed synchronously with the formation of the B-type LPOor that they were not overprinted by the later deformation event.

Modal composition

The relationship between modal composition of the peridotite and fabric strength is illustrated in Figure 6. Overall, the Ronda samples do not show a strong correlation between either the fabric type, or fabric strength and modal composition. We only discern a weak relationship showing that samples with the strongest B-type fabrics have low olivine content.

However, for the discussion below, we emphasize that these samples are characterized by having generally high CPx content, rather than high OPx content.

Discussion

B-type fabric in the laboratory and nature

Differences in the geometry of olivine fabrics are generally related to transitions in the dominant slip system associated with changes in stress, pressure, temperature and water content (e.g., [START_REF] Katayama | New type of olivine fabric from deformation experiments at modest water content and low stress[END_REF]. [START_REF] Jung | Water-induced Fabric Transitions in Olivine[END_REF] (3) [START_REF] Jung | Deformation microstructures of olivine in peridotite from Spitsbergen, Svalbard and implications for seismic anisotropy[END_REF] reported a change from A-type fabric to B-type fabric in dry dunite (with 3% OPX) with increasing confining pressure from 2.5 to 3.1 GPa at relatively low stress (~150 MPa) and 1300°C. (4) [START_REF] Ohuchi | High pressure and temperature fabric transitions in olivine and variations in upper mantle seismic anisotropy[END_REF] documented the change from A-type to Btype fabric in dry dunite (without OPX) with increasing pressure from 5 to 7 GPa at a temperature of 1200-1400°C (see also [START_REF] Couvy | Shear deformation experiments of forsterite at 11 GPa -1400°C in the multianvil apparatus[END_REF]. The authors of the latter two studies concluded that B-type fabric arises from increasing confining pressure.

Olivine with B-type fabric has also been observed in naturally deformed peridotite. [START_REF] Mizukami | Natural examples of olivine lattice preferred orientation patterns with a flow-normal a-axis maximum[END_REF] showed B-type fabric in olivine neoblasts of a dunite from the Higashi-Akaishi peridotite (North Japan), which deformed at 700-800°C in the presence of water at a pressure between 1.5 and 3.0 GPa. Similar B-type fabric has been also described in southwestern Japan within the Imono peridotite body (Tasaka et al., 2008). Skemer et al.

Origin of B-type fabric in the Ronda shear zone

In Ronda, the well constrained deformation conditions and well preserved spatial transition from A-type to B-type fabric provide an excellent opportunity to evaluate the processes responsible for B-type fabric under geologic conditions. For the Ronda mylonite, several of the proposed mechanisms are relatively easy to rule out. First, a pressure-induced transition to B-type fabric can be ruled out based on petrologic data, as the A-type to B-type transition is observed in rocks that deformed during exhumation with pressure decreasing below 2.5 GPa [START_REF] Garrido | Garnet lherzolite and garnet-spinel mylonite in the Ronda peridotite: Vestiges of Oligocene backarc mantle lithospheric extension in the western Mediterranean[END_REF]. Second, the low-temperature conditions of deformation exclude the possibility of B-type fabric produced in the presence of melt.

The role of water on the fabric transition is more problematic to assess, partly due to the subsequent alteration (serpentinization) of the rocks, but several observations suggest that the water concentration in olivine was relatively low. First, extrapolation of the Arrhenius relation for H concentration in olivine [START_REF] Zhao | Solubility of hydrogen in olivine: dependence on temperature and iron content[END_REF] indicates a solubility of ~460 H/10 6 Si at ~800 °C and 2 GPa; this value includes the multiplication factor of 3.5 suggested by FTIR calibration measurements of water in olivine [START_REF] Bell | Hydroxide in olivine: A quantitative determination of the absolute amount and calibration of the IR Spectrum[END_REF]. For comparison with the fabric diagram of [START_REF] Katayama | New type of olivine fabric from deformation experiments at modest water content and low stress[END_REF], which does not include the factor of 3.5, the water solubility is ~130 H/10 6 Si, falling within the A-type fabric regime. Second, no chlorite or amphibole is observed in the Ronda mylonite; the thermal stability of chlorite is ~800 °C at 2 GPa [START_REF] Hacker | Subduction factory 1. Theoretical mineralogy, densities, seismic wave speeds, and H 2 0 contents[END_REF]. Thus even though some water can remain dissolved in nominally anhydrous minerals at water activities that are too low to stabilize hydrous phases, the value for H solubility in olivine should be considered a maximum. Third, neither of the other -water-induced‖ olivine fabrics (i.e., Type-E or Type-C; [START_REF] Katayama | New type of olivine fabric from deformation experiments at modest water content and low stress[END_REF] are observed in the transitional region of the Ronda mylonite where A/B-type fabric occurs. While uncertainties remain on the temperature and stress dependence of the fabric transitions at low water contents (c.f. [START_REF] Karato | Geodynamic Significance of Seismic Anisotropy of the Upper Mantle: New Insights from Laboratory Studies[END_REF], these arguments suggest that the B-type fabrics in Ronda are not produced by diffusion/advection of water from the overlying crust into the mylonite.

By process of elimination, consideration of experimental results suggests that the transition to B-type fabric in Ronda occurs owing to an enhancement of grain size sensitive deformation mechanism. This hypothesis is further supported by two observations: [START_REF] Précigout | Strain localization in the subcontinental mantlea ductile alternative to the brittle mantle[END_REF] analysis of misorientation axes across subgrains indicate a dominance of the A-type slip system in samples that exhibit both A-type and B-type fabrics (Fig. 5), suggesting that the fabric transition did not occur due to a change in the dominant slip system. (2) The overprinting of A-type fabric by B-type fabric is spatially correlated with a decrease of both M index and J index , i.e., fabric strength, in the highest strain regions of the shear zone (Fig. 4).

The increase of fabric strength upon entering the shear zone suggests that dislocation creep dominated in the A-type fabric domain. However, if dislocation creep remained the dominant deformation mechanism in the core of the Ronda shear zone (upper mylonite), we would expect increasing, or constant fabric strength with increasing finite strain (e.g., [START_REF] Silver | Mantle structural geology from seismic anisotropy[END_REF][START_REF] Bystricky | High Shear Strain of Olivine Aggregates: Rheological and Seismic Consequences[END_REF][START_REF] Warren | Evolution of olivine lattice preferred orientation during simple shear in the mantle[END_REF]. In contrast, deformation within the inner shear zone is characterized by decreasing M index (and J index ) with increasing strain, suggesting a deformation mechanism dominated by grain size sensitive processes [START_REF] Warren | Grain size sensitive deformation mechanisms in naturally deformed péridotites[END_REF][START_REF] Précigout | Strain localization in the subcontinental mantlea ductile alternative to the brittle mantle[END_REF].

By integrating results from both laboratory and geological studies, we propose that the B-type fabric in the Ronda mylonite forms as a result of GBS. The interface-controlled diffusion mechanism proposed by Sundberg and Cooper (2008) which is contingent on the presence of OPx -is possible. However, our analysis of relationships between the modal composition and fabric strength indicate that the samples with the strongest B-type fabric contain abundant CPx, rather than OPx (Fig. 6); this observation is inconsistent with the Sundberg and Cooper mechanism owing to the additional compositional components involved with diffusion creep in the presence of CPx (i.e., Al, Ca). Furthermore, the results of [START_REF] Kohlstedt | Shearing Melt Out of the Earth: An Experimentalist's Perspective on the Influence of Deformation on Melt Extraction[END_REF] on partially molten dunite indicate that OPx is not required to produce B-type fabric during grain size sensitive creep. Based on these observations, we speculate that the GBS sliding process itself is responsible for the evolution of LPO in these samples.

We envision that GBS may produce B-type fabric via rigid rotation of grains -guided‖ by the orthorhombic olivine habit, resulting in alignment of the long [001] axes in the flow direction and alignment of the short [010] axes normal to the shear plane. Indeed, coaxial deformation experiments on olivine + clinopyroxene aggregates (Miyakazi et al., 2013) conducted at high temperature and room pressure indicate that GBS on specific boundaries (in conjunction with a shape preferred orientation) may produce LPO (mostly A-type fabric).

Although a shape preferred orientation is observed in the mylonite (e.g., Figure 2), posttectonic serpentinization of our samples makes statistical assessment of the details of shapepreferred orientation and grain boundary microstructures problematic. Furthermore, the conditions of Miyazaki et al. experiments (1150Miyazaki et al. experiments ( -1385 °C, room pressure, coaxial shear) °C, room pressure, coaxial shear) are quite different from those in the Ronda mylonite (850°C, 2 GPa, non-coaxial shear); hence direct comparisons are not straightforward. Finally, whereas [START_REF] Miyazaki | Olivine crystals align during diffusion creep of Earth's upper mantle[END_REF] documented GBS accommodated by diffusional creep, the subgrain development and grain size reduction by dynamic recrystallization shown in the Ronda samples provide evidence for dislocation mobility, suggesting the dominance of DisGBS in the mylonite [START_REF] Précigout | Strain localization in the subcontinental mantlea ductile alternative to the brittle mantle[END_REF]. Thus, whereas the Miyazaki et al. results demonstrate LPO formation during diffusion accommodated GBS (i.e. diffusion creep), we infer that the B-type fabric formed during DisGBS in Ronda.

Deformation mechanism(s) in the Ronda shear zone

To further assess the dominant deformation mechanisms under conditions where B-type fabric formed in the Ronda shear zone, we use deformation mechanism maps (DMM) constructed using olivine flow laws. As shown in Figure 7, at deformation conditions of 850°C and 2 GPa constrained for the upper mylonite [START_REF] Garrido | Garnet lherzolite and garnet-spinel mylonite in the Ronda peridotite: Vestiges of Oligocene backarc mantle lithospheric extension in the western Mediterranean[END_REF], and a recrystallized grain size between 50 and 300 m [START_REF] Précigout | Strain localization in the subcontinental mantlea ductile alternative to the brittle mantle[END_REF], extrapolation of laboratory flow laws indicate deformation conditions either near the boundary between the DisGBS and diffusion creep regimes (Fig. 7A: based on flow laws from [START_REF] Hirth | Rheology of the Upper Mantle and the Mantle Wedge: A view from the Experimentalists[END_REF] or within the DisGBS regime (Fig. 7B: based on flow laws from [START_REF] Hansen | Grain boundary sliding in San Carlos olivine: Flow law parameters and crystallographic-preferred orientation[END_REF].

In both cases, the DMMs predict DisGBS limited by GBSinstead of DisGBS limited by dislocation motion on the easy-slip system (e.g., [START_REF] Warren | Grain size sensitive deformation mechanisms in naturally deformed péridotites[END_REF]. Our microstructural evidence for dislocation mobility supports the interpretation that the mylonite deformed by DisGBS instead of diffusion creep. The spatial correlations between the fabric strength and fabric type also suggest that strain localization in the mylonite resulted from a transition from dislocation creep (preserved in the tectonite) to DisGBS, such as proposed in [START_REF] Précigout | Strain localization in the subcontinental mantlea ductile alternative to the brittle mantle[END_REF].

Nonetheless, guided by insights from recent rheological experiments [START_REF] Hansen | Grain boundary sliding in San Carlos olivine: Flow law parameters and crystallographic-preferred orientation[END_REF][START_REF] Hansen | The influence of microstructure on the deformation of olivine in the grain-boundary sliding regime[END_REF], we highlight where our interpretations remain ambiguous owing to a lack ofor conflictingexperimental data. First, the interpretation that the Ronda tectonite deformed by dislocation creep is inconsistent with the DMM constructed using the DisGBS flow law from Hansen et al. (Fig. 7D). In addition, for a temperature of 850 o C, the Hansen et al.'s flow law predicts only a modest difference (approximately a factor of three) between the strain rate of the mylonite with a grain size of ~100 m and that for the tectonite with a grain size of ~1 mm (Fig. 7B); this is inconsistent with the observation of significant strain localization into the mylonite (e.g. [START_REF] Vissers | Mylonite deformation in the upper mantle peridotites of the North Pyrenean Zone (France): implications for strength and strain localization in the lithosphere[END_REF]. Second, while our interpretation is generally consistent with the DMM constructed using the flow laws of Hirth and Kohlstedt (Fig. 7C), which predict a larger difference in strain rate for the mylonite and tectonite (Fig. 7A), the predicted magnitude of strain rate (~5.10 -15 s -1 ) is low for a shear zone (e.g. [START_REF] Vissers | Mylonite deformation in the upper mantle peridotites of the North Pyrenean Zone (France): implications for strength and strain localization in the lithosphere[END_REF]. Third, high temperature deformation experiments on olivine aggregates in the DisGBS regime do not produce B-type fabric; rather A-type fabric is observed [START_REF] Hansen | Grain boundary sliding in San Carlos olivine: Flow law parameters and crystallographic-preferred orientation[END_REF][START_REF] Hansen | The influence of microstructure on the deformation of olivine in the grain-boundary sliding regime[END_REF]. Fourth, a possible resolution of these apparent discrepancies is that either the diffusion creep or DisGBS process is enhanced by the presence of pyroxenes. Currently there are conflicting results on this topic. For example, the diffusion creep flow law for nominally melt free lherzolite [START_REF] Zimmerman | Rheological Properties of Partially Molten Lherzolite[END_REF] is the same within error as that determined for olivine [START_REF] Hirth | Rheology of the Upper Mantle and the Mantle Wedge: A view from the Experimentalists[END_REF]. In contrast, the results of Sundberg and Cooper (2008;2010) suggest an enhancement of diffusion creep in the presence of OPx. Thus, although our findings and recent experiments highlight GBS as an effective mechanism to generate a mineral fabric (B-type fabric in this study), our results illustrate that additional experimental and theoretical studies on the relationships between GBS and olivine fabric (in both the DisGBS and diffusion creep regimes) are needed to constrain the more general applicability of our results for the interpretation of strain localization and seismic anisotropy in the lithosphere, as well as how (or if) these processes are related to B-type fabric observed in high-pressure experiments (> 3 GPa) and/or hydrous peridotite.

Conclusion

Based on documentation of olivine LPO, fabric strength and subgrain misorientation axes, we interpret that the olivine B-type fabric in Ronda results from GBS, not from a transition in the dominant slip system, as traditionally proposed. Extrapolation of experimental flow laws indicates that GBS may contribute to the deformation over a wide range of conditions in the upper mantle. Thus, our results may have importance for the interpretation of upper mantle deformation processes through observation of seismic anisotropy. In any case, our observations highlight the need to further resolve the competing processes responsible for fabric transitions using a combination of analyses on both laboratory and naturally deformed samples. (Skemer et al., 2006) and J index calculations [START_REF] Bunge | Texture analysis in Materials Science[END_REF], and using standard parameters from [START_REF] Ismaïl | An olivine fabric database: an overview of upper mantle fabrics and seismic anisotropy[END_REF].

Figure captions

To compare the A-type/B-type fabric transition with the trend of fabric strength, we denote samples that show A-, A/B-and B-type fabrics with black circles, white squares in black circles and white squares, respectively. White dots indicate samples that do not show a clearcut fabric. The A-type/B-type transition is spatially correlated with a drop in fabric strength. X = lineation; Z = Foliation pole. temperatures of 850°C and 1100°C, respectively. The DMM displays the competing deformation mechanisms that control the rheology of olivine aggregates as a function of stress, grain size, strain rate, temperature and pressure. Here we calculate DMMs using four deformation mechanisms, as described in [START_REF] Warren | Grain size sensitive deformation mechanisms in naturally deformed péridotites[END_REF]: dislocation creep (disl.); DisGBS that includes both the easy-slip and GBS regimes [START_REF] Warren | Grain size sensitive deformation mechanisms in naturally deformed péridotites[END_REF]; diffusion creep (Diff.) and low-temperature plasticity. In A and C we use the DisGBS flow law from [START_REF] Hirth | Rheology of the Upper Mantle and the Mantle Wedge: A view from the Experimentalists[END_REF], and in B and D we use the DisGBS flow law from [START_REF] Hansen | Grain boundary sliding in San Carlos olivine: Flow law parameters and crystallographic-preferred orientation[END_REF]. The temperature and pressure are chosen based on the temperature and pressure records in the Ronda mylonite [START_REF] Garrido | Garnet lherzolite and garnet-spinel mylonite in the Ronda peridotite: Vestiges of Oligocene backarc mantle lithospheric extension in the western Mediterranean[END_REF]. Using the recrystallization

  figure shows the variation in the orientation of axes along a transect shown in the micrograph.

  first documented B-type fabric in experimentally deformed olivine aggregates, and hypothesized that it formed due to the dominance of the (010)[001] slip system under high stresses with a combination of high water content and high confining pressure. However, over the last 10 years, at least five other experimental studies have shown that B-type fabric also forms under dry conditions over a wide range of pressures. (1) Holtzman et al. (2003), Kohlstedt and Holtzman (2009) and Qi et al. (2013) produced B-type fabric during general shear and torsional shear of fine-grained, partially molten olivine aggregates at low differential stress (below 150 MPa) and a confining pressure of 300 MPa. These authors concluded that the B-type fabric formed owing to the grain-scale effect of aligned melt pockets on the GBS component of deformation. (2) Sundberg and Cooper (2008) observed B-type fabric in fine-grained anhydrous harzburgite (35-50% OPX) at low stress (< 200 MPa), moderate confining pressure (1.5 GPa) and 1200°C. Based on the grain size and rheological data, they concluded that B-type fabric formed during diffusion creep, although the stress exponents determined for the pyroxene-rich samples were statistically higher (~1.8) than 1 (the theoretical value of n for diffusion creep).

(

  2006) documented B-type fabric in a peridotite mylonite from the Cima di Gagnone massif (Swiss Alps), which recorded deformation conditions of 800 ± 175°C at undefined pressure; the presence of chlorite and tremolite indicated also the presence of water during deformation in the Cima di Gagnione massif. In constrast, Drury et al. (2011) identified B-type fabric in water-poor low-temperature (< 750°C) peridotite ultramylonite in the French Pyrenees (Etang de Lerz). Based on the results of Sundberg and Coopers's (2008) experimental study, Drury et al. concluded that B-type fabric formed during diffusion-accommodated GBS (i.e., diffusion creep) in the extremely fine-grained rocks.

Figure 1 -

 1 Figure 1 -Location, structural map and cross-section of the study area in the Ronda

Figure 2 -

 2 Figure 2 -Microstructures in thin section (in polarized light) across the Ronda shear zone.

Figure 3 -

 3 Figure 3 -Lattice Preferred Orientation (LPO) of olivine across the Ronda shear zone. The

Figure 4 -

 4 Figure 4 -Geometry of olivine LPO and fabric strength across the Ronda shear zone. A)

Figure 5 -

 5 Figure 5 -Misorientation axis data across subgrain boundaries. A) Example of subgrain

Figure 6 -

 6 Figure 6 -M index of olivine LPO vs. modal composition of peridotite (in weight %) across the

Figure 7 -

 7 Figure 7 -Olivine Deformation Mechanism Maps (DMM) for deformation conditions of the

Acknowledgements

We thank Dr. Karsten Kunze (ETH-Zürich, Switzerland) and Dr. Bjarne Almqvist (ETH-Zürich) for their help to get the LPO data and their fruitful discussions. We also thank Dr. Carlos J. Garrido (CSIC, Granada, Spain) for his help in getting samples from the Ronda peridotite. We are moreover grateful for helpful reviews of the manuscript from Dr. Lars Hansen and anonymous reviewers. This work was funded by NSF award EAR-0738880 and the ETH-Zürich (ETH Fellowship application FEL-01 09-3).

Skemer, P., Katayama, I., Jiang, Z., Karato, S.-I., 2005. The 7A and7B shows the predicted strain rate at the stress predicted by the mylonite grain size, but considering grain size of the protolith, i.e., the tectonite (e.g., [START_REF] Platt | Lithospheric shear zones as constant stress experiments[END_REF]. The arrow illustrates the grain size reduction related to strain localization in the mylonite.

Supplementary material

Supplementary Table 1 -Whole-rock major elements, modal compositions (in weight %) and fabric strength of the studied samples. Chemical analyzes have been performed at the Franklin & Marshall College (Lancaster, PA., USA). The modes have been calculated from the CIPW method. LOI = loss of ignition; Ol = olivine; Opx = orthopyroxene; Cpx = clinopyroxene; Spl = Spinel; M = M index ; J = J index .

Supplementary Figure 1 -Set of lattice preferred orientation data for orthopyroxene and clinopyroxene within our 37 samples across the Ronda shear zone. Orientations of the [100],

[010] and [001] axes are plotted in lower-hemisphere pole figures with respect to X (lineation) and Z (foliation pole) deformation axes. We took one measurement per grain.