Quartz Grainsize Evolution During Dynamic Recrystallization Across a Natural Shear Zone Boundary

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Journal of Structural Geology 109 (2018) 120–126

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Journal of Structural Geology

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Quartz grainsize evolution during dynamic recrystallization across a natural T shear zone boundary

∗ Haoran Xia , John P. Platt

Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, USA

ARTICLE INFO ABSTRACT

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

1. Introduction nucleation until the strain rates contributed by diﬀusion creep and dislocation creep are the same. Austin and Evans (2007) suggested a During dynamic recrystallization, newly formed grains are often somewhat diﬀerent model, in which the steady-state recrystallized smaller than the original ones as shown in both experimentally and grainsize is a result of the balance between increasing surface energy naturally deformed rocks (e.g., Hirth and Tullis, 1992; Stipp et al., caused by nucleation and decreasing surface energy resulting from 2002). As a result, the average grainsize of a sample decreases after grain growth. All of the above models predict that recrystallized dynamic recrystallization. Grainsize reduction resulting from dynamic grainsize should depend on temperature as well as stress. recrystallization may lead to changes in deformation mechanism (e.g., An alternative approach was that of Twiss (1977), who equated the Vissers et al., 1995; Platt and Behr, 2011a), strength of samples (e.g., dislocation strain energy in the grain boundary to that in the enclosed Poirier, 1980), and may cause strain localization (e.g., Tullis and Yund, volume, and concluded that both grainsize and subgrain size are simply 1985; Rutter and Brodie, 1988; Jin et al., 1998; Drury, 2005). However, functions of stress. The experimental recrystallized grainsize paleo- the grainsize evolution during dynamic recrystallization is still under piezometers for minerals have employed the grainsize-stress relation debate. Derby and Ashby (1987) proposed a theoretical model of dy- and are temperature-independent within uncertainty (Van der Wal namic recrystallization, arguing that the recrystallized grainsize is et al., 1993; Post and Tullis, 1999; Stipp and Tullis, 2003; Stipp et al., controlled by two microscopic processes: nucleation of new grains, and 2006, 2010), though the recrystallized grainsize of some metals may be grain-boundary migration driven by strain energy. In this model, nu- temperature dependent (e.g., Kodzhaspirov and Terentyev, 2012). Platt cleation of new grains results in grainsize reduction while migration of and Behr (2011b) argued that because strain-energy-driven grain- grain boundaries leads to grain growth. The word “nucleation” used in boundary migration increases grain-boundary curvature, whereas sur- this paper refers to the process of forming new grains during dynamic face-energy-driven grain-boundary migration decreases it, the two recrystallization by mechanisms including subgrain rotation and grain- processes cannot normally operate at the same time in a recrystallized boundary bulging. A nucleation-and-growth model was also proposed aggregate. If dynamic recrystallization is driven by strain energy, then by Shimizu (1998, 2008, 2011), where nucleation takes place by sub- grainsize should be set by the nucleation process. This implies that in grain rotation while grain growth is driven by the distortional strain the dislocation creep regime, the grainsize evolution should not involve energy associated with subgrain walls and free dislocations. De Bresser surface-energy driven grain growth, and therefore may not be tem- et al. (1998, 2001) argued that the recrystallized grains will grow after perature dependent.

∗ Corresponding author. Now at: Chevron Energy Technology Company, Houston, TX 77002, USA. E-mail address: [email protected] (H. Xia). https://doi.org/10.1016/j.jsg.2018.01.007 Received 25 June 2017; Received in revised form 16 January 2018; Accepted 23 January 2018 Available online 03 February 2018 0191-8141/ © 2018 Elsevier Ltd. All rights reserved. H. Xia, J.P. Platt Journal of Structural Geology 109 (2018) 120–126

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 related schists (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. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

A key diﬀerence between the nucleation and growth models (Derby recrystallization, or in two samples deformed at the same conditions and Ashby, 1987; De Bresser et al., 1998, 2001, Shimizu, 1998, 2008; but recording diﬀerent stages of recrystallization. The latter approach Austin and Evans, 2007) and other models is whether the recrystallized was made possible by the mylonites above the Vincent fault in the San grains grow after nucleation. Stipp and Kunze (2008) reported that the Gabriel Mountains, southern California. This paper investigates the recrystallized grainsize produced by bulging recrystallization in natu- grainsize of recrystallized quartz in two samples from either side of the rally deformed quartz is about the same as grain boundary bulges and upper boundary of the mylonite zone, which are only 8 m apart but subgrains, but their results lacked quantitative measurements for sub- show approximately the initiation stage and the completion stage of grains and bulges. In order to investigate the grainsize evolution during dynamic recrystallization, respectively. With these two samples, we aim dynamic recrystallization, one could examine the size of recrystallized to examine the grainsize evolution during quartz dynamic re- grains, either in one sample at various temporal stages of crystallization.

121 H. Xia, J.P. Platt Journal of Structural Geology 109 (2018) 120–126

Fig. 2. Photomicrographs (cross polarized light). (a) A typical Lower Mylonite Zone sample. Quartz grains (Qtz) are nearly completely recrystallized by subgrain rotation mechanism. The recrystallized 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.

section of the mylonite zone, or the Upper Mylonite Zone, are 0.19 ± 0.12 GPa to 0.22 + 0.09/–0.10 GPa and 323 ± 27 °C to 341 ± 26 °C (Xia and Platt, in review). The mylonites are argued to have been formed during the eastward normal sense shear of the mylonite zone based on deformation fabrics, kinematic indicators, deformation conditions, and geochronological evidence from the Pelona schist 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 and detrital zircon ﬁssion track ages of both plates indicate that the mylonite 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 are recrystallized, dominantly by subgrain rotation (SGR) in the Lower Mylonite Zone (Fig. 2a) and bulging (BLG) in the Upper Mylonite Zone (Fig. 2b). In a sample from the base of the Upper Mylonite Zone, some of 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 between 19.3 ± 3.2 μm and 46.4 ± 7.4 μm in four samples from the Lower Mylonite Zone and between 4.7 ± 1.3 μm and 9.2 ± 1.6 μm in four samples from the Upper Mylonite Zone (Fig. 3). The Upper Mylonite Zone is structurally overlain by unmylonitized Fig. 3. Dynamically recrystallized grainsize of quartz in Vincent fault mylonites as a gneiss, where the minerals are coarse grained and show little to no function of structural distance from the mylonitic front (after Xia and Platt, in review, and evidence of plastic deformation. The boundary between the top of the this paper). Upper Mylonite Zone and the bottom of the overlying unmylonitized gneiss is a zone a few meters thick, and we refer to this as the mylonitic 2. The Vincent fault mylonites in the San Gabriel Mountains, front. Quartz grains in gneiss right above the mylonitic front exhibit southern California undulose extinction with serrate grain boundaries. Newly recrystallized grains are sparsely distributed along quartz grain boundaries. Two The Vincent fault in the San Gabriel Mountains (Fig. 1) separates the samples, PS111 (34.279917 °N, 117.75125 °W) and PS114 (34.279859 late Cretaceous-Paleocene subduction-associated Pelona schist in the °N, 117.74741 °W), approximately 8 m apart, were collected below and lower plate from the Mesozoic magmatic arc consisting of a Meso- above the mylonitic front, respectively (Fig. 1d). About 2% of quartz in Proterozoic gneiss complex and Mesozoic granitoid rocks in the upper PS114 from the gneiss is recrystallized and represents the incipient plate (Ehlig, 1981; Jacobson, 1997; Xia and Platt, 2017). The bottom stage of recrystallization (Fig. 4a). Quartz grains in PS111 from the 100–1000 m of the upper plate gneiss is mylonitized with a few weakly Upper Mylonite Zone, on the other hand, are almost completely re- deformed lenses, forming a mylonite zone (Ehlig, 1981; Xia and Platt, in crystallized by BLG (Fig. 4b). Dynamic recrystallization of quartz in review). The mylonites were deformed during retrograde meta- PS114 is likely to be coeval with the mylonites, as there is no other morphism with a mineral assemblage of plagioclase + quartz + amphi- recognized deformation event producing similar microstructures in this bole + epidote + chlorite + white mica + biotite + titanite + calcite. area. We assume that the P-T conditions prevailing during re- The pressure-temperature (P-T) conditions constrained by the phengite crystallization at the mylonitic front were the same as those determined barometer and the titanium-in-quartz thermobarometer for the struc- in the Upper Mylonite Zone, and considering the short distance between turally lower section of the mylonite zone, or the Lower Mylonite Zone, our two samples bracketing the front, it is reasonable to assume that the are 0.46 + 0.14/–0.13 GPa to 0.52 + 0.15/–0.14 GPa and P-T and stress conditions of these two samples were the same when 362 ± 28 °C to 409 + 39/–42 °C and those for the structurally upper dynamic recrystallization occurred.

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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 vague recrystallized grain shape fabric, indicating a dextral (top-to-SE) sense of shear. Epidote (Ep) can be observed.

3. Methods host grain boundaries are serrated. Bulges are shown by the sutured grain boundaries. The root of a bulge, where the bulge meets its host The samples were sectioned parallel to lineation and perpendicular grain, may form a subgrain wall (Fig. 5a and c), as reported by Stipp to foliation. The thin sections were prepared using colloidal silica on a and Kunze (2008). The average size of the bulges is 4.7 ± 1.5 μm Buehler VibroMet 2 in order to remove the surface crystal lattice da- (Fig. 6a). The average aspect ratio (long axis/short axis) of the bulges is maged during mechanical polishing. Quartz crystallographic orienta- 1.33 ± 0.25. Scattered recrystallized quartz grains are developed tion was collected using a Hikari electron backscatter diffraction along the relict grain boundaries in this sample, taking up approxi- (EBSD) detector mounted on a JEOL-7001F scanning electron micro- mately 2% of the entire quartz (Figs. 3a and 4b). The average grainsize scope (SEM) at the Center for Electron Microscopy and Microanalysis of of recrystallized quartz is 4.5 ± 1.5 μm(Fig. 6b) and the average as- University of Southern California. The acceleration voltage was be- pect ratio is 1.90 ± 0.46. Because of the very low strain, and hence low tween 20 and 25 kV, the working distance between 15and 17 mm, and degree of recrystallization, the number of measurements of bulges the step size 0.15–0.40 μm. The data collection and processing were (n = 24) and recrystallized grains (n = 103) is small, but the low var- performed using EDAX OIM software. Grain boundaries are defined as iance in the measurements suggests that the mean values we have ob- having a misorientation of 10° or higher. Grainsize is treated as the tained are robust. diameter of a circle with the same area as the grain, while the average In the mylonite sample PS111, quartz is close to completely re- grainsize of a sample is the root mean square (RMS) of all the measured crystallized (Figs. 4b, 5e and 5f; also see the supplementary file), grains in that sample. Grainsize was measured using the OIM Analysis forming very fine-grained new crystals with an average grainsize of program. The relict grains were distinguished from the recrystallized 4.7 ± 1.3 μm(Fig. 6c), based on a statistically significant sample size ones following the procedure outlined by Behr and Platt (2011) and (n = 193). This grainsize corresponds to a shear stress of 97.8 + 20.9/ Cross et al. (2017). A value of 2.05° was used for the grain orientation –13.5 MPa using the EBSD-based calibrations of the Stipp and Tullis spread threshold (Cross et al., 2017), which is a quantitative mea- (2003) piezometer (Cross et al., 2017), or 71.4 + 15.3/–9.8 MPa with surement of the lattice distortion. The area of a bulge is defined by the the apparatus correction of Holyoke and Kronenberg (2010). The shear area of either the bulge-related subgrain when a subgrain boundary is stress in a plane stress setting is the differential stress divided by 3 shown at the root of the bulge, or the portion of a grain that intruded (Behr and Platt, 2013). The misorientation angle distribution (Fig. 5d) into an adjacent grain if there is no subgrain wall formed at the root of shows that grain boundaries < 15° are dominant, suggesting the op- the bulge. Bulge size is defined as the diameter of a circle with the same eration of subgrain rotation. The peak between 40° and 70° may reflect area as the bulge. Only elongate-shaped bulges were measured using the preservation of original high angle grain boundaries. On the or- μ ImageJ, because bulges smaller than Dmin defined by Platt and Behr ientation maps (Fig. 5e), relict grains, often over 10 m in size, exhibit (2011b) may shrink and disappear due to surface energy; the elongate variation in intragranular orientation, subgrain boundaries, and have shape of the bulges, however, indicates that they are above Dmin and are irregular grain boundaries. Recrystallized grains may contain grain able to eventually form new grains. The average bulge size is the RMS boundaries curved both toward and away from the grain center of all the measured bulges in the sample. (Fig. 5f). The crystallographic preferred orientation (CPO) of quartz Data were expressed as mean ± standard deviation. Student's un- grains in this sample shows asymmetric small-circle girdles around the paired t-test was performed to identify significant differences. F-test normal to foliation (Z axis), indicating a top-to-the-southeast sense of was performed to examine if the variances of two populations are equal. shear (Fig. 5g). P < 0.05 is considered significant. All data analyses were performed Student's t-test was used to compare the bulge size and the re- using Stata Statistical Software: Release 13. College Station, TX: crystallized quartz grainsize in PS114. As a premise of the t-test, F-test StataCorp LP. was performed first to examine the equality of variances between the bulge size and recrystallized grainsize, and no significant difference was found. There is no statistically significant difference between the 4. Results average bulge size and the average recrystallized grainsize (p = 0.46) assuming equal variance as validated by the F-test in the first step. That In the gneiss sample PS114, most quartz is coarse-grained, showing is, the average bulge size is the same as the average size of the re- some internal variation of crystallographic orientation and subgrain crystallized grains within uncertainty in PS114. boundaries (Fig. 4a; Fig. 5a and b; also see the supplementary file). The

123 H. Xia, J.P. Platt Journal of Structural Geology 109 (2018) 120–126

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 PS114 showing 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 angles for 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 portion of 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 pole figure (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.). The orientation 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 adjacent grains or subgrains. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

5. Discussion average size of recrystallized grains in PS114. This implies that the size of recrystallized grains is controlled by the bulges, and hence by the The EBSD analysis confirms that quartz in both PS114 and PS111 nucleation process, at least during incipient recrystallization. The as- was recrystallized by the bulging mechanism. As shown by the data pect ratio of the recrystallized grains is larger than that of the bulges in analysis, the average bulge size is not significantly different from the PS114, probably because of the continued deformation of the

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Fig. 6. Bulge size and recrystallized grainsize distributions. Frequency calculated as number fractions of bulges/grainsizes. (a) Quartz bulge size distribution of PS114. (b) Grainsize distribution of recrystallized quartz in PS114. (c) Grainsize distribution of recrystallized quartz in PS111. recrystallized grains. The average grainsize of recrystallized quartz in which appears to invalidate the nucleation-and-growth model, and may the mylonite sample PS111 is the same as that in PS114 within un- rule out the possible roles of strain rate and surface energy in grainsize certainty, and the recrystallized grainsize distributions of both samples evolution after nucleation. Therefore, temperature is unlikely to be are similar (Fig. 6b and c). These similarities imply that there is no incorporated in the recrystallized grainsize paleopiezometer for quartz, significant change in grainsize during dynamic recrystallization. This at least in the BLG regime. conclusion is supported by other evidence from both naturally and experimentally deformed rocks where the recrystallized grain sizes are 6. Conclusions independent of either the amount of recrystallization or the amount of strain (Johanesen and Platt, 2015; Valcke et al., 2015; Heilbronner and Two samples associated with the Vincent fault in the San Gabriel Kilian, 2017; Platt and De Bresser, 2017). Mountains, southern California allowed us to investigate quartz grain- Several models propose that grain boundary migration driven by size evolution during dynamic recrystallization in the BLG regime. We either surface energy or strain energy can result in grain growth during conclude that: dynamic recrystallization after nucleation (Derby and Ashby, 1987; Derby, 1991, 1992, De Bresser et al., 1998, 2001, Shimizu, 1998, 2008). 1. At the initiation of dynamic recrystallization by the BLG mechanism, However, this is not consistent with our observation that the grainsizes the average size of the bulges (4.7 ± 1.5 μm) is the same as the of recrystallized quartz in both samples are the same within un- recrystallized quartz grains (4.5 ± 1.5 μm) within uncertainty. certainty, and the lack of evidence supporting grain growth after nu- When dynamic recrystallization is largely complete, the average cleation. In addition, the recrystallized quartz grains do not show 120° grainsize of recrystallized grains (4.7 ± 1.3 μm) remains the same triple junctions with straight grain boundaries, indicating that surface- as that at the initiation stage within uncertainty. energy-driven grain boundary migration (γ-GBM) was not dominant. 2. The recrystallized grainsize of quartz is controlled by the nucleation Instead, the fact that inward and outward curvature of grain boundaries process and does not change after nucleation. occurs in the same recrystallized quartz grains (Fig. 5f) imply that 3. The recrystallized grainsize paleopiezometer for quartz is unlikely to dislocation-density-driven grain boundary migration (ρ-GBM) was be temperature-dependent, confirming earlier experimental find- dominant. ings. Our observations are consistent with the suggestion that dynamic recrystallization by the BLG mechanism occurs in a region of stress–- Acknowledgement grainsize space where grain-boundary migration is primarily driven by lattice strain energy (i.e., ρ-GBM) (Bailey and Hirsch, 1962; Drury et al., This research was funded in part by NSF grant EAR-1250128 to 1985; Hirth and Tullis, 1992; Stipp and Kunze, 2008; Platt and Behr, John P. Platt. We would like to thank Toru Takeshita for handling the 2011b). In this region, surface energy does not contribute to grainsize manuscript and for his valuable comments. The constructive reviews by evolution, and hence does not lead to grain growth (Platt and Behr, Michael Stipp and Takamoto Okudaira have improved this manuscript. 2011b). No experimental or theoretical grain-growth law has been We are also grateful to Haiming Tang for his help on statistical analysis derived for ρ-GBM, and this process is unlikely to lead to a significant and to Tarryn Cawood for her help in the field. change in grainsize after nucleation. The concept of a temperature-independent recrystallized grainsize Appendix A. 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