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_Journal of Structural Geology 32 (2010) 832–842_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Control of shear zone location and thickness by initial grain size variations in upper mantle peridotites Julie Newman a,*, Martyn R. Drury b a Department of Geology and Geophysics, Texas A&M University, College Station, TX 77843, USA b Department of Earth Sciences, Faculty of Geoscience, Utrecht University, 3508TA Utrecht, Netherlands Article history: Received 22 February 2010 Accepted 2 June 2010 Available online 10 June 2010 Keywords: Localization Peridotite Mylonite Reaction Grain size Abstract The Turon de T?cou?re peridotite, North Pyrenean fault zone, France, contains protomylonites grading to ultramylonites. Grain size reduction to 0.005–0.025 mm took place by reaction during deformation. Within the protomylonite domain, the size of the porphyroclasts suggests an initial grain size in this domain of 2–10 mm. In the ultramylonite domain, porphyroclasts are polycrystalline with an internal grain size of 0.1–0.2 mm, indicating that the ultramylonite domain had a finer initial grain size than the protomylonite domain. Lattice-preferred orientations and misorientations within a polycrystalline porphyroclast in the ultramylonite domain suggest an early episode of dislocation creep accommodated dynamic recrystallization, restricted predominantly to the present-day ultramylonite domain. Later grain size reduction by reaction, resulting in very fine-grained ultramylonites, was concentrated in the same domain. For reaction-involved deformation, initial grain size may be crucially important as it controls the surface area available for nucleation of new grains. As a result of the finer initial grain size within the present-day ultramylonites, a critical percentage of fine-grained matrix developed in the present-day ultramylonite domain earlier than in the present-day protomylonite domain, resulting in localization of further deformation in the ultramylonites. This study suggests that zones of locally smaller grain size can act as heterogeneities that control the location and width of shear zones formed by reaction-softening. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Strain localization has been widely researched and well documented in upper and middle crustal settings. While strain within the lower crust and upper mantle (within the “ductile regime”) is often assumed to be homogeneous, field studies have documented localized deformation that took place in these rocks under the high pressures and temperatures prevalent in these settings (e.g., Boullier and Gueguen, 1975; Rubie, 1983; Nicolas, 1986; Rutter and Brodie, 1988; Handy, 1989; Drury et al., 1991; Vissers et al., 1991; Jaroslow et al., 1996; Denghui et al., 1998; Jin et al., 1998; Newman et al., 1999; Dijkstra et al., 2002; Michibayashi and Mainprice, 2004; Warren and Hirth, 2006; Warren et al., 2008; Webber et al., 2008, 2010; Toy et al., 2010). The processes responsible for strain localization in ductilely deforming rocks have been studied through field research, as well as experimental and theoretical investigations. Field studies have identified a number of mechanisms responsible for, or associated with, localization in ductilely deforming rocks. * Corresponding author. E-mail address: newman@geo.tamu.edu (J. Newman). 0191-8141 $ e see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.06.001 including grain size reduction by either dynamic recrystallization (Rutter and Brodie, 1988; Warren and Hirth, 2006) or by reaction (Brodie and Rutter, 1987; Handy, 1989; Furusho and Kanagawa, 1999; Kruse and St?nitz, 1999; Newman et al., 1999; Handy and St?nitz, 2002; Dijkstra et al., 2002), by the introduction of fluids (Brodie, 1980; Vissers et al., 1995) or melt (Dijkstra et al., 2002), or as a result of compositional variations (Toy et al., 2010; Webber et al., 2010). Experimental and theoretical investigations have extended our understanding of localization processes in ductilely deforming materials through quantitative evaluation of the influence of variables such as relative viscosities in polyphase materials, relative abundance and spatial distribution of “hard” and “soft” phases, and changing rheologies during deformation (e.g., Tharp, 1983; Jordan, 1987; Rutter and Brodie, 1988; Handy, 1994; Bloom?eld and Covey-Crump, 1993; Govers and Wortel, 1995; Dell’Angelo and Tullis, 1996; de Bresser et al., 1998, 2001; Mont?si and Zuber, 2002; Holyoke and Tullis, 2006; Takeda and Griera, 2006; Jessell et al., 2009). While the mechanisms responsible for localization have been explored, less attention has been devoted to the controls on the location and scale of localization in ductilely deforming rocks. Field, experimental and theoretical studies indicate that the initiation of J. Newman, M.R. Drury Journal of Structural Geology 32 (2010) 833–842 localized deformation requires a geometric heterogeneity. Within the upper and middle crust, fractures may provide sufficient heterogeneity to result in localized deformation, often aided by the introduction of fluids (e.g., Mitra, 1978; Beach, 1980; O’Hara, 1988, 1990; Fitz Gerald and St?nitz, 1993; Newman and Mitra, 1993; St?nitz and Fitz Gerald, 1993). Within the lower crust, where high temperatures and pressures do not favor fracturing, heterogeneities may be provided by variations in rock type or by zones through which fluids have been introduced. However, the upper mantle is thought to be compositionally more homogeneous than the crust, and free fluids may not always be present in the upper mantle (e.g., Newman et al., 1999). We have attempted to determine the controls on location and scale of localization in mantle rocks, with respect to a shear zone developed within upper mantle peridotites and exposed as the Turon de T?cou?re peridotite body, France. Strain localization resulted, in part, from a reaction associated with decreasing temperatures and pressures. The deformation microstructures, deformation mechanisms and chemistry of these rocks are discussed in detail in Vissers et al. (1997) and Newman et al. (1999), and are reviewed below. This contribution focuses on polycrystalline porphyroclasts, observed only within the highest strain zone of the peridotite body that, when compared with less deformed rocks outside the high strain zone, suggest that a variation in grain size existed within these rocks prior to the reaction-enhanced deformation. For reaction-involved deformation, initial grain size is crucially important as it controls both the surface area available for nucleation of new grains and the length scale for diffusive mass transfer. We argue that the initial variation in grain size across the Turon de T?cou?re peridotite body that existed prior to the reaction-enhanced deformation served as the heterogeneity that determined the location and thickness of the high strain zone. 2. Background et al. (1999; Fig. 11 and Tables 1–3). This continuous reaction is associated with the transition from the medium-pressure spinel lherzolite metamorphic facies to the low-pressure plagioclase lherzolite facies at pressures around 0.5–1 GPa. Note that while plagioclase was not present in the initial spinel lherzolite assemblage, we have included plagioclase1 as a reactant, because the chemical analyses of plagioclase grains suggest that they continued to react after formation (Newman et al., 1999). Geothermometry based on compositions of matrix grains formed during the reaction yield temperatures between 850 and 700 x14C, while porphyroclasts yield higher temperatures (>1000 x14C) (Newman et al., 1999). Porphyroclasts in the protomylonites are approximately 0.2–1 cm in diameter, and the reaction resulted in the formation of fine new grains, 2–25 mm in diameter, observed within protomylonites, mylonites and ultramylonites. The fine-grained matrix within the mylonites and ultramylonites exhibits a weak lattice preferred orientation with a concentration of _100_ perpendicular to the lineation (Newman et al., 1999, see Fig. 8b), few dislocations, and alignment of grain boundaries (Newman et al., 1999). These observations are consistent with deformation by grain-size sensitive creep (e.g., Boullier and Gueguen, 1975; Schmid et al., 1977; Brodie and Rutter, 1987; Drury and Humphreys, 1988; Passchier and Trouw, 2005; Sundberg and Cooper, 2008). Recent experimental results by Sundberg and Cooper (2008) particularly support this interpretation. They deformed fine-grained (2–5 mm) olivine-orthopyroxene aggregates at low stresses (<20 MPa) and observed an olivine LPO similar to the one observed from the Turon de T?cou?re rocks, with olivine _100_ perpendicular to the lineation. Sundberg and Cooper (2008) suggest that their samples deformed by diffusion-accommodated grain boundary sliding. Weakening of the rocks, and strain localization, within the mylonite and ultramylonite domains resulted from a change in the dominant deformation mechanism from dislocation creep within porphyroclasts to diffusion creep within the fine-grained matrix. Turon de T?cou?re is exposed in the Northern Pyrenean Zone, north of the North Pyrenean Fault. Alpine thrusting and subsequent erosion has brought the Variscan basement of the Axial Zone and the North Pyrenean peridotite massifs to the surface (Fig. 1a; Fabri?s et al., 1991; Vissers et al., 1997). The Turon de T?cou?re peridotite is about 600 m across (Fig. 1b). The rocks exposed in the body are spineland plagioclase-bearing lherzolite mylonites. Protomylonites (>40% matrix) dominate the southwest portion of the body, with mylonites (>80% matrix) making up the northeast portion. A layer, about 25–40 m thick, of ultramylonites (>90% matrix) transects the mylonite domain at a low angle. The mylonites at Turon de T?cou?re formed by a reaction-dominated deformation event (Newman et al., 1999). The ?negra' _end_ Ключевые слова: clast, polycrystalline porphyroclasts, localization, ultramylonite domain, nucleation, mylonites, brodie, griera jessell, band, length scale, misorientation, grain, intermediate-size grain, shear localization, day, surface area, evidence, solid, deformation dominated, viscosity, journal structural geology, event, grain size, pyroxenite layer, development, grain polycrystalline, geophysics, earth, large grain, stnitz newman, newman, dislocation creep, upper, polygonal grain, diffusion creep, protomylonite domain, mantle, deformed, creep, intermediate-size, ?ne-grained matrix, mm, mylonite domain, higher percentage, phase, drury, deformation, letters, present-day, large, weak, hirth, relative, mylonite ultramylonite, angle boundary, rock, handy, ultramylonite, preferred, turon t?cou?re, intermediate-sized grain, hara, porphyroclasts, subgrains, theoretical study, white, variation, present-day mylonite, preferred orientation, dynamic, grain boundary, jordan, critical percentage, tullis takeda, mitra, mineralogy, matrix, misorientations, tectonophysics, covey-crump govers, polyphase band, ?ow, nicolas, shear zone, foliation, sensitive, beach, result, mylonite, spiers, hoogerduijn strating, structural, reaction increased, matrix grain, structural geology, fitz gerald, dynamic recrystallization, bresser, grained, higher, misorientation angle, ecors, location, mechanism responsible, accommodated, stnitz, boundary, initial grain, elsevier, american, kohlstedt, porphyroclast, polycrystalline, earlier deformation, ?ne-grained, trimby, reaction, tcoure, ne-grained matrix, subgrains arrow, central clast, tcoure rock, journal, peridotite, experimental, recrystallization, microstructures, geology, rutter, shear, tectonics, present-day ultramylonite, size reduction, long, subgrain boundary, metamorphic, angle, prior, petrology, griera, weak phase, orthopyroxene, ultramylonite zone, study, ultramylonites, schmid, journal structural, observed, size, static recrystallization, central grain, domain, fabri?s, olivine, deformation microstructures, upper mantle, science, orientation, high, geophysical letters, strength, geophysical, plagioclase, protomylonites, temperature, initial, boundary misorientations, strain localization, mineral chemistry, central, heterogeneity, tharp, t?cou?re, mechanism, turon, polycrystalline porphyroclast, zone, st?nitz, cm-scale porphyroclasts, dislocation, olivine grain, brink, strain, wa, percentage, jessell, protomylonite, minor orthopyroxene, journal geophysical, reduction