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Journal of Structural Geology

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Quartz grainsize evolution during dynamic recrystallization across a naturalshear zone boundary

Haoran Xia∗, John P. PlattDepartment of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, USA

A R T I C L E I N F O

Keywords:Dynamic recrystallizationBulgeSubgrainGrainsize evolutionVincent fault

A B S T R A C T

Although it is widely accepted that grainsize reduction by dynamic recrystallization can lead to strain locali-zation, the details of the grainsize evolution during dynamic recrystallization remain unclear. We investigatedthe bulge size and grainsizes of quartz at approximately the initiation and the completion stages of bulgingrecrystallization across the upper boundary of a 500m thick mylonite zone above the Vincent fault in the SanGabriel Mountains, southern California. Within uncertainty, the average bulge size of quartz, 4.7 ± 1.5 μm, isthe same as the recrystallized grainsize, 4.5 ± 1.5 μm, at the incipient stage of dynamic recrystallization, andalso the same within uncertainties as the recrystallized grainsize when dynamic recrystallization is largelycomplete, 4.7 ± 1.3 μm. These observations indicate that the recrystallized grainsize is controlled by the nu-cleation process and does not change afterwards. It is also consistent with the experimental finding that thequartz recrystallized grainsize paleopiezometer is independent of temperature.

1. Introduction

During dynamic recrystallization, newly formed grains are oftensmaller than the original ones as shown in both experimentally andnaturally deformed rocks (e.g., Hirth and Tullis, 1992; Stipp et al.,2002). As a result, the average grainsize of a sample decreases afterdynamic recrystallization. Grainsize reduction resulting from dynamicrecrystallization may lead to changes in deformation mechanism (e.g.,Vissers et al., 1995; Platt and Behr, 2011a), strength of samples (e.g.,Poirier, 1980), and may cause strain localization (e.g., Tullis and Yund,1985; Rutter and Brodie, 1988; Jin et al., 1998; Drury, 2005). However,the grainsize evolution during dynamic recrystallization is still underdebate. Derby and Ashby (1987) proposed a theoretical model of dy-namic recrystallization, arguing that the recrystallized grainsize iscontrolled by two microscopic processes: nucleation of new grains, andgrain-boundary migration driven by strain energy. In this model, nu-cleation of new grains results in grainsize reduction while migration ofgrain boundaries leads to grain growth. The word “nucleation” used inthis paper refers to the process of forming new grains during dynamicrecrystallization by mechanisms including subgrain rotation and grain-boundary bulging. A nucleation-and-growth model was also proposedby Shimizu (1998, 2008, 2011), where nucleation takes place by sub-grain rotation while grain growth is driven by the distortional strainenergy associated with subgrain walls and free dislocations. De Bresseret al. (1998, 2001) argued that the recrystallized grains will grow after

nucleation until the strain rates contributed by diffusion creep anddislocation creep are the same. Austin and Evans (2007) suggested asomewhat different model, in which the steady-state recrystallizedgrainsize is a result of the balance between increasing surface energycaused by nucleation and decreasing surface energy resulting fromgrain growth. All of the above models predict that recrystallizedgrainsize should depend on temperature as well as stress.

An alternative approach was that of Twiss (1977), who equated thedislocation strain energy in the grain boundary to that in the enclosedvolume, and concluded that both grainsize and subgrain size are simplyfunctions of stress. The experimental recrystallized grainsize paleo-piezometers for minerals have employed the grainsize-stress relationand are temperature-independent within uncertainty (Van der Walet al., 1993; Post and Tullis, 1999; Stipp and Tullis, 2003; Stipp et al.,2006, 2010), though the recrystallized grainsize of some metals may betemperature dependent (e.g., Kodzhaspirov and Terentyev, 2012). Plattand Behr (2011b) argued that because strain-energy-driven grain-boundary migration increases grain-boundary curvature, whereas sur-face-energy-driven grain-boundary migration decreases it, the twoprocesses cannot normally operate at the same time in a recrystallizedaggregate. If dynamic recrystallization is driven by strain energy, thengrainsize should be set by the nucleation process. This implies that inthe dislocation creep regime, the grainsize evolution should not involvesurface-energy driven grain growth, and therefore may not be tem-perature dependent.

https://doi.org/10.1016/j.jsg.2018.01.007Received 25 June 2017; Received in revised form 16 January 2018; Accepted 23 January 2018

∗ Corresponding author. Now at: Chevron Energy Technology Company, Houston, TX 77002, USA.E-mail address: haoranxi@usc.edu (H. Xia).

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Available online 03 February 20180191-8141/ © 2018 Elsevier Ltd. All rights reserved.

T

A key difference between the nucleation and growth models (Derbyand Ashby, 1987; De Bresser et al., 1998, 2001, Shimizu, 1998, 2008;Austin and Evans, 2007) and other models is whether the recrystallizedgrains grow after nucleation. Stipp and Kunze (2008) reported that therecrystallized grainsize produced by bulging recrystallization in natu-rally deformed quartz is about the same as grain boundary bulges andsubgrains, but their results lacked quantitative measurements for sub-grains and bulges. In order to investigate the grainsize evolution duringdynamic recrystallization, one could examine the size of recrystallizedgrains, either in one sample at various temporal stages of

recrystallization, or in two samples deformed at the same conditionsbut recording different stages of recrystallization. The latter approachwas made possible by the mylonites above the Vincent fault in the SanGabriel Mountains, southern California. This paper investigates thegrainsize of recrystallized quartz in two samples from either side of theupper boundary of the mylonite zone, which are only 8m apart butshow approximately the initiation stage and the completion stage ofdynamic recrystallization, respectively. With these two samples, we aimto examine the grainsize evolution during quartz dynamic re-crystallization.

Fig. 1. Geologic setting of the mylonite zone above the Vincent fault in the San Gabriel Mountains, southern California. (a) Major faults (red lines) and Laramide subduction relatedschists (grey areas) of southern California. Modified after Jennings (1977). (b) Geologic map of the East Fork area of the San Gabriel Mountains. Modified after Dibblee (2002a, 2002b,2002c, 2002d) and Xia and Platt (2017). (c) Transect along A-A′ in panel (b). Modified after Xia and Platt (2017). (d) Photo of the mylonitic front outcrop and sample locations. (Forinterpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

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2. The Vincent fault mylonites in the San Gabriel Mountains,southern California

The Vincent fault in the San Gabriel Mountains (Fig. 1) separates thelate Cretaceous-Paleocene subduction-associated Pelona schist in thelower plate from the Mesozoic magmatic arc consisting of a Meso-Proterozoic gneiss complex and Mesozoic granitoid rocks in the upperplate (Ehlig, 1981; Jacobson, 1997; Xia and Platt, 2017). The bottom100–1000m of the upper plate gneiss is mylonitized with a few weaklydeformed lenses, forming a mylonite zone (Ehlig, 1981; Xia and Platt, inreview). The mylonites were deformed during retrograde meta-morphism with a mineral assemblage of plagioclase + quartz + amphi-bole + epidote + chlorite + white mica + biotite + titanite + calcite.The pressure-temperature (P-T) conditions constrained by the phengitebarometer and the titanium-in-quartz thermobarometer for the struc-turally lower section of the mylonite zone, or the Lower Mylonite Zone,are 0.46 + 0.14/–0.13 GPa to 0.52 + 0.15/–0.14 GPa and362 ± 28 °C to 409 + 39/–42 °C and those for the structurally upper

section of the mylonite zone, or the Upper Mylonite Zone, are0.19 ± 0.12 GPa to 0.22 + 0.09/–0.10 GPa and 323 ± 27 °C to341 ± 26 °C (Xia and Platt, in review). The mylonites are argued tohave been formed during the eastward normal sense shear of the my-lonite zone based on deformation fabrics, kinematic indicators, de-formation conditions, and geochronological evidence from the Pelonaschist in the lower plate and the rocks in the upper plate (Xia and Platt,2017, in review). Amphibole and white mica Ar/Ar cooling ages anddetrital zircon fission track ages of both plates indicate that the mylo-nite zone was active between ∼58 Ma and ∼51.1 Ma (Jacobson, 1990;Grove and Lovera, 1996; Grove et al., 2003; Xia and Platt, 2017).

About 90% of the quartz grains in the upper-plate mylonites arerecrystallized, dominantly by subgrain rotation (SGR) in the LowerMylonite Zone (Fig. 2a) and bulging (BLG) in the Upper Mylonite Zone(Fig. 2b). In a sample from the base of the Upper Mylonite Zone, someof the quartz grains were recrystallized by SGR and others by BLG,indicating that the Upper Mylonite Zone overprinted the Lower Mylo-nite Zone. The recrystallized quartz grainsizes range between19.3 ± 3.2 μm and 46.4 ± 7.4 μm in four samples from the LowerMylonite Zone and between 4.7 ± 1.3 μm and 9.2 ± 1.6 μm in foursamples from the Upper Mylonite Zone (Fig. 3).

The Upper Mylonite Zone is structurally overlain by unmylonitizedgneiss, where the minerals are coarse grained and show little to noevidence of plastic deformation. The boundary between the top of theUpper Mylonite Zone and the bottom of the overlying unmylonitizedgneiss is a zone a few meters thick, and we refer to this as the myloniticfront. Quartz grains in gneiss right above the mylonitic front exhibitundulose extinction with serrate grain boundaries. Newly recrystallizedgrains are sparsely distributed along quartz grain boundaries. Twosamples, PS111 (34.279917 °N, 117.75125 °W) and PS114 (34.279859°N, 117.74741 °W), approximately 8m apart, were collected below andabove the mylonitic front, respectively (Fig. 1d). About 2% of quartz inPS114 from the gneiss is recrystallized and represents the incipientstage of recrystallization (Fig. 4a). Quartz grains in PS111 from theUpper Mylonite Zone, on the other hand, are almost completely re-crystallized by BLG (Fig. 4b). Dynamic recrystallization of quartz inPS114 is likely to be coeval with the mylonites, as there is no otherrecognized deformation event producing similar microstructures in thisarea. We assume that the P-T conditions prevailing during re-crystallization at the mylonitic front were the same as those determinedin the Upper Mylonite Zone, and considering the short distance betweenour two samples bracketing the front, it is reasonable to assume that theP-T and stress conditions of these two samples were the same whendynamic recrystallization occurred.

Fig. 2. Photomicrographs (cross polarized light). (a) A typical Lower Mylonite Zone sample. Quartz grains (Qtz) are nearly completely recrystallized by subgrain rotation mechanism. Therecrystallized grains show new grain shape fabric, indicating a dextral (top-to-SE) sense of shear. Calcite (Cal) shows the foliation. (b) A typical Upper Mylonite Zone sample. Quartz (Qtz)is almost completely recrystallized by bulging mechanism. Plagioclase (Plg) grains occur as porphyroclasts and white mica (Wm) defines the foliation.

Fig. 3. Dynamically recrystallized grainsize of quartz in Vincent fault mylonites as afunction of structural distance from the mylonitic front (after Xia and Platt, in review, andthis paper).

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3. Methods

The samples were sectioned parallel to lineation and perpendicularto foliation. The thin sections were prepared using colloidal silica on aBuehler VibroMet 2 in order to remove the surface crystal lattice da-maged during mechanical polishing. Quartz crystallographic orienta-tion was collected using a Hikari electron backscatter diffraction(EBSD) detector mounted on a JEOL-7001F scanning electron micro-scope (SEM) at the Center for Electron Microscopy and Microanalysis ofUniversity of Southern California. The acceleration voltage was be-tween 20 and 25 kV, the working distance between 15and 17mm, andthe step size 0.15–0.40 μm. The data collection and processing wereperformed using EDAX OIM software. Grain boundaries are defined ashaving a misorientation of 10° or higher. Grainsize is treated as thediameter of a circle with the same area as the grain, while the averagegrainsize of a sample is the root mean square (RMS) of all the measuredgrains in that sample. Grainsize was measured using the OIM Analysisprogram. The relict grains were distinguished from the recrystallizedones following the procedure outlined by Behr and Platt (2011) andCross et al. (2017). A value of 2.05° was used for the grain orientationspread threshold (Cross et al., 2017), which is a quantitative mea-surement of the lattice distortion. The area of a bulge is defined by thearea of either the bulge-related subgrain when a subgrain boundary isshown at the root of the bulge, or the portion of a grain that intrudedinto an adjacent grain if there is no subgrain wall formed at the root ofthe bulge. Bulge size is defined as the diameter of a circle with the samearea as the bulge. Only elongate-shaped bulges were measured usingImageJ, because bulges smaller than Dmin defined by Platt and Behr(2011b) may shrink and disappear due to surface energy; the elongateshape of the bulges, however, indicates that they are above Dmin and areable to eventually form new grains. The average bulge size is the RMSof all the measured bulges in the sample.

Data were expressed as mean ± standard deviation. Student's un-paired t-test was performed to identify significant differences. F-testwas performed to examine if the variances of two populations are equal.P < 0.05 is considered significant. All data analyses were performedusing Stata Statistical Software: Release 13. College Station, TX:StataCorp LP.

4. Results

In the gneiss sample PS114, most quartz is coarse-grained, showingsome internal variation of crystallographic orientation and subgrainboundaries (Fig. 4a; Fig. 5a and b; also see the supplementary file). The

host grain boundaries are serrated. Bulges are shown by the suturedgrain boundaries. The root of a bulge, where the bulge meets its hostgrain, may form a subgrain wall (Fig. 5a and c), as reported by Stippand Kunze (2008). The average size of the bulges is 4.7 ± 1.5 μm(Fig. 6a). The average aspect ratio (long axis/short axis) of the bulges is1.33 ± 0.25. Scattered recrystallized quartz grains are developedalong the relict grain boundaries in this sample, taking up approxi-mately 2% of the entire quartz (Figs. 3a and 4b). The average grainsizeof recrystallized quartz is 4.5 ± 1.5 μm (Fig. 6b) and the average as-pect ratio is 1.90 ± 0.46. Because of the very low strain, and hence lowdegree of recrystallization, the number of measurements of bulges(n= 24) and recrystallized grains (n= 103) is small, but the low var-iance in the measurements suggests that the mean values we have ob-tained are robust.

In the mylonite sample PS111, quartz is close to completely re-crystallized (Figs. 4b, 5e and 5f; also see the supplementary file),forming very fine-grained new crystals with an average grainsize of4.7 ± 1.3 μm (Fig. 6c), based on a statistically significant sample size(n = 193). This grainsize corresponds to a shear stress of 97.8 + 20.9/–13.5 MPa using the EBSD-based calibrations of the Stipp and Tullis(2003) piezometer (Cross et al., 2017), or 71.4 + 15.3/–9.8 MPa withthe apparatus correction of Holyoke and Kronenberg (2010). The shearstress in a plane stress setting is the differential stress divided by 3(Behr and Platt, 2013). The misorientation angle distribution (Fig. 5d)shows that grain boundaries< 15° are dominant, suggesting the op-eration of subgrain rotation. The peak between 40° and 70° may reflectthe preservation of original high angle grain boundaries. On the or-ientation maps (Fig. 5e), relict grains, often over 10 μm in size, exhibitvariation in intragranular orientation, subgrain boundaries, and haveirregular grain boundaries. Recrystallized grains may contain grainboundaries curved both toward and away from the grain center(Fig. 5f). The crystallographic preferred orientation (CPO) of quartzgrains in this sample shows asymmetric small-circle girdles around thenormal to foliation (Z axis), indicating a top-to-the-southeast sense ofshear (Fig. 5g).

Student's t-test was used to compare the bulge size and the re-crystallized quartz grainsize in PS114. As a premise of the t-test, F-testwas performed first to examine the equality of variances between thebulge size and recrystallized grainsize, and no significant difference wasfound. There is no statistically significant difference between theaverage bulge size and the average recrystallized grainsize (p= 0.46)assuming equal variance as validated by the F-test in the first step. Thatis, the average bulge size is the same as the average size of the re-crystallized grains within uncertainty in PS114.

Fig. 4. Photomicrographs (cross polarized light). (a) Sample PS114. Coarse quartz (Qtz) grains show patchy and undulose extinction. Along their serrated grain boundaries, bulges(shown by arrows) and recrystallized grains occur. (b) Sample PS111. Plagioclase (Plg) and quartz (Qtz) mostly recrystallized, forming fine-grained crystals. Quartz grains show a vaguerecrystallized grain shape fabric, indicating a dextral (top-to-SE) sense of shear. Epidote (Ep) can be observed.

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5. Discussion

The EBSD analysis confirms that quartz in both PS114 and PS111was recrystallized by the bulging mechanism. As shown by the dataanalysis, the average bulge size is not significantly different from the

average size of recrystallized grains in PS114. This implies that the sizeof recrystallized grains is controlled by the bulges, and hence by thenucleation process, at least during incipient recrystallization. The as-pect ratio of the recrystallized grains is larger than that of the bulges inPS114, probably because of the continued deformation of the

Fig. 5. (a) Orientation map along part of a grain boundary in PS114 showing a bulge and subgrain boundary at its root. (b) Orientation map along part of a grain boundary in PS114showing a newly recrystallized grain which preserves the bulge shape. (c) Misorientation along Line A-A′ crossing a bulge in panel (a). (d) Frequency distribution of misorientation anglesfor PS111. (e) Orientation map and grainsize distributions (inset, frequency computed as number fraction of grains) of PS111. (f) Enlarged part of panel (e). Black arrows show the portionof grain boundaries curved toward the center of the pink grain while grey arrows show the outward curvature of grain boundaries with respect to the pink grain. (g) Quartz c-axis polefigure (lower-hemisphere and equal-area projection) of panel (e). Contour intervals are as shown in the legend where numerals indicate multiples of random distribution (m.r.d.). Theorientation maps are color coded in (a), (b), (e) and (f) as shown in the inverse pole figures (legend), where the crystallographic orientations are shown with respect to the sample Y-axis(parallel to foliation and normal to lineation). Also, the grain and subgrain boundaries are as shown in the legend, where numerals indicate the misorientation angles between adjacentgrains or subgrains. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

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recrystallized grains. The average grainsize of recrystallized quartz inthe mylonite sample PS111 is the same as that in PS114 within un-certainty, and the recrystallized grainsize distributions of both samplesare similar (Fig. 6b and c). These similarities imply that there is nosignificant change in grainsize during dynamic recrystallization. Thisconclusion is supported by other evidence from both naturally andexperimentally deformed rocks where the recrystallized grain sizes areindependent of either the amount of recrystallization or the amount ofstrain (Johanesen and Platt, 2015; Valcke et al., 2015; Heilbronner andKilian, 2017; Platt and De Bresser, 2017).

Several models propose that grain boundary migration driven byeither surface energy or strain energy can result in grain growth duringdynamic recrystallization after nucleation (Derby and Ashby, 1987;Derby, 1991, 1992, De Bresser et al., 1998, 2001, Shimizu, 1998, 2008).However, this is not consistent with our observation that the grainsizesof recrystallized quartz in both samples are the same within un-certainty, and the lack of evidence supporting grain growth after nu-cleation. In addition, the recrystallized quartz grains do not show 120°triple junctions with straight grain boundaries, indicating that surface-energy-driven grain boundary migration (γ-GBM) was not dominant.Instead, the fact that inward and outward curvature of grain boundariesoccurs in the same recrystallized quartz grains (Fig. 5f) imply thatdislocation-density-driven grain boundary migration (ρ-GBM) wasdominant.

Our observations are consistent with the suggestion that dynamicrecrystallization by the BLG mechanism occurs in a region of stress–-grainsize space where grain-boundary migration is primarily driven bylattice strain energy (i.e., ρ-GBM) (Bailey and Hirsch, 1962; Drury et al.,1985; Hirth and Tullis, 1992; Stipp and Kunze, 2008; Platt and Behr,2011b). In this region, surface energy does not contribute to grainsizeevolution, and hence does not lead to grain growth (Platt and Behr,2011b). No experimental or theoretical grain-growth law has beenderived for ρ-GBM, and this process is unlikely to lead to a significantchange in grainsize after nucleation.

The concept of a temperature-independent recrystallized grainsizepaleopiezometer was challenged by theories that relate dynamicallyrecrystallized grainsize to the rate of energy dissipation (e.g., Austinand Evans, 2007), to the field boundary between grainsize sensitive andgrainsize insensitive creep (e.g., De Bresser et al., 1998, 2001), or to abalance between grainsize reduction and grain growth (e.g., Derby andAshby, 1987; Shimizu, 1998), because all these processes may involve atemperature dependence related to the activation energies for creep andgrain-boundary migration (or in the case of the field boundary hy-pothesis, the difference in the activation energies of the creep me-chanisms involved). The existing experimental recrystallized grainsizepiezometers are independent of temperature within the uncertainties,however (Van der Wal et al., 1993; Post and Tullis, 1999; Stipp andTullis, 2003; Stipp et al., 2006, 2010). Also as shown by this study, thegrainsize of recrystallized quartz does not change after nucleation,

which appears to invalidate the nucleation-and-growth model, and mayrule out the possible roles of strain rate and surface energy in grainsizeevolution after nucleation. Therefore, temperature is unlikely to beincorporated in the recrystallized grainsize paleopiezometer for quartz,at least in the BLG regime.

6. Conclusions

Two samples associated with the Vincent fault in the San GabrielMountains, southern California allowed us to investigate quartz grain-size evolution during dynamic recrystallization in the BLG regime. Weconclude that:

1. At the initiation of dynamic recrystallization by the BLG mechanism,the average size of the bulges (4.7 ± 1.5 μm) is the same as therecrystallized quartz grains (4.5 ± 1.5 μm) within uncertainty.When dynamic recrystallization is largely complete, the averagegrainsize of recrystallized grains (4.7 ± 1.3 μm) remains the sameas that at the initiation stage within uncertainty.

2. The recrystallized grainsize of quartz is controlled by the nucleationprocess and does not change after nucleation.

3. The recrystallized grainsize paleopiezometer for quartz is unlikely tobe temperature-dependent, confirming earlier experimental find-ings.

Acknowledgement

This research was funded in part by NSF grant EAR-1250128 toJohn P. Platt. We would like to thank Toru Takeshita for handling themanuscript and for his valuable comments. The constructive reviews byMichael Stipp and Takamoto Okudaira have improved this manuscript.We are also grateful to Haiming Tang for his help on statistical analysisand to Tarryn Cawood for her help in the field.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.jsg.2018.01.007.

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