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_Journal of Structural Geology 32 (2010) 1430e1443_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com locate jsg Asymmetrical quartz crystallographic fabrics formed during constrictional deformation W.A. Sullivan a,*, R.J. Beaneb a Department of Geology, Colby College, 5803 Mayflower Hill, Waterville, ME 04901, USA b Department of Geology, Bowdoin College, 6800 College Station, Brunswick, ME 04011, USA Article info Article history: Received 15 April 2010 Received in revised form 16 July 2010 Accepted 8 August 2010 Available online 17 August 2010 Keywords: Quartz crystallographic fabric Quartz c-axis fabric Quartz a-axis fabric Constrictional strain L tectonite Western Hayfork terrane Abstract Numerical simulations predict unique quartz crystallographic fabric patterns for plane strain, ?attening, and constriction. Multiple studies support the predictions for plane strain and ?attening. To test predictions for constriction, this paper analyzes five examples of quartz crystallographic fabrics from a 1km-wide domain of L tectonites in the Pigeon Point high-strain zone, Klamath Mountains, California, U.S.A. These samples were deformed under greenschist-to amphibolite-facies conditions. Quartz c-axis fabrics are similar to the predicted double-girdle fabrics except that amphibolite-facies samples exhibit c-axis maxima and are distinctly asymmetrical about the elongation lineations. Activation of different slip systems combined with small deviations from pure constriction account for the c-axis maxima, and noncoaxial flow accounts for the fabric asymmetry. The simple-shear component is randomly oriented in geographic coordinates throughout the domain of L tectonites. These data confirm that numerical simulations predict the quartz c-axis fabric geometry developed during constriction for some deformation conditions, and they confirm the quartz a-axis patterns predicted for constriction for the first time. These data also demonstrate that the relationship between quartz crystallographic fabrics and strain geometry is not straightforward, and they indicate that a-axis fabrics may be more useful indicators of strain geometry variations. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Crystallographic fabrics are a common feature of plastically deformed rocks from the crust and the mantle. Quartz and olivine are the two most widely studied and well-understood minerals in terms of the crystallographic fabrics that develop during plastic deformation. Of these two, quartz is by far the most common mineral in crustal rocks exposed in the continents. Moreover, it is often concentrated by sedimentary processes or as vein fill, so quartz-rich rocks are found in almost all continental metamorphic terranes. For these reasons, the development of quartz crystallographic fabrics during plastic deformation has been an active area of research for more than four decades. Indeed, it is widely accepted that quartz crystallographic fabric formation and the resulting fabric geometry are sensitive to variations in deformation temperature and strain rate (Lister, 1981; Wenk et al., 1989; Jessell and Lister, 1990; Okudaira et al., 1995; Kruhl, 1998; Stipp et al., 2002a,b; Heilbronner and Tullis, 2006), the noncoaxiality of flow (e.g. Tullis, 1977; Lister and Hobbs, 1980; Schmid and Casey, 1986; Dell’Angelo and Tullis, 1989; Law et al., 1990; Takeshita et al., 1999), and distortional strain geometry (Tullis et al., 1973; Majoribanks, 1976; Tullis, 1977; Lister and Hobbs, 1980; Price, 1985; Law, 1986; Schmid and Casey, 1986). This sensitivity to different deformation parameters, combined with the relative ease of measuring them, makes quartz crystallographic fabrics an important tool for analyzing natural high-strain zones, and they have been used to characterize deformation in exhumed metamorphic terranes from all over the world (e.g. Law et al., 1984, 2004; Lee et al., 1987; Wallis, 1995; Xypolias and Koukouvelas, 2001; Sullivan and Law, 2007; Toy et al., 2008; Barth et al., 2010). The initial link between distortional strain geometry and quartz c-axis fabric geometry was made by Lister and Hobbs (1980) using numerical simulations of plastic deformation based on the Taylor-Bishop-Hill model of slip system activation (Fig. 1). Schmid and Casey (1986) subsequently deduced the probable geometry of a-axis fabrics based on the observation that slip in the ha direction dominates most naturally deformed quartzites (Fig. 1). A variety of fabrics from naturally and experimentally deformed samples support the results of Lister and Hobbs’ (1980) numerical simulations for plane strain and ?attening deformations (Tullis et al., 1973; Majoribanks, 1976; Tullis, 1977; Compton, 1980; Law et al., 1984; Price, 1985; Schmid and Casey, 1986; Law, 1986). However, well-documented natural fabrics produced under apparent constrictional strain conditions are rare in the literature, and constrictional deformation of rocks has not been reproduced in experiments. Because of this, Lister and Hobbs’ (1980) model remains the only link between quartz crystallographic fabric geometry and constrictional strain 30 years after its publication. As far as we are aware, only a single sample from a single study has yielded both a measured prolate strain geometry and a double-girdle quartz c-axis fabric with girdles symmetrically arranged about the lineation (Burg and Teyssier, 1983 reproduced in Price, 1985) as predicted for pure constrictional deformation by Lister and Hobbs’ (1980) model. To fill this void, this paper presents five exceptionally well-documented examples of quartz crystallographic fabrics developed in a 1-km-wide domain of L tectonites in the Pigeon Point high-strain zone, Klamath Mountains, California, U.S.A. The quartz c-axis fabrics developed in these samples are similar to those predicted by Lister and Hobbs’ (1980) numerical simulations except that in four of the five samples exhibit c-axis maxima and the c-axis girdles are distinctly asymmetrical about the mineral elongation lineations. Phyllosilicate fabrics in these rocks define a weak, randomly oriented foliation not detectable by looking at the samples. Quartz crystallographic fabric geometry is unrelated to the weak phyllosilicate foliation, and the fabric asymmetry is randomly oriented in a geographic reference frame. These data show that Lister and Hobbs’ (1980) model does predict quartz c-axis fabric geometry during constrictional deformation for some deformation conditions. They also provide the first confirmation of the quartz a-axis patterns that Schmid and Casey (1986) predicted would form during constrictional deformation. At the same time, these data demonstrate that the relationship between crystallographic fabric geometry and finite strain geometry is not as straightforward as generally assumed, especially for non-plane strain, noncoaxial deformations, and they indicate that a-axis fabrics may be more useful indicators of finite strain geometry under a variety of deformation conditions. 2. Geologic setting The samples documented in this study are from a 1-km-wide domain of well-developed L tectonites in the middle Jurassic Pigeon Point high-strain zone, Klamath Mountains, California, U.S.A. (Wright and Fahan, 1988; Sullivan, 2009) (Fig. 2). The Pigeon Point high-strain zone is a gently SE-dipping zone of intense plastic deformation that cuts Jurassic metavolcanic and metasedimentary rocks of the western Hayfork terrane. Foliation surfaces in LeS- and L > S-tectonites in the Pigeon Point high-strain zone are shallowly to moderately dipping and poles to foliations define a partial great circle distribution roughly centered about the mineral elongation lineations (Fig. 2c). Elongation lineations, including the domain of L tectonites, plunge gently to the ESE (Fig. 2c). Overall, this pure-shear-dominated high-strain zone accommodated a subordinate component of top-to-the-WNW-directed, reverse-sense displacement coupled with zone-normal contraction and transport-parallel elongation (Sullivan, 2009). Sullivan (2009) concluded that the domain of L tectonites in the Pigeon Point high-strain zone accommodated a component of constrictional deformation that was concentrated in a convex-upwards groove in the upper boundary of the high-strain zone. The geometry of the high-strain-zone boundary and resulting strain localization was likely related to magmatic heating that catalyzed intense plastic deformation in the first place (Sullivan, 2009). Rock units cut by the Pigeon Point high-strain zone include a structurally lower mafic metavolcaniclastic unit that contains metamorphosed mafic tuff and tuff breccia and an upper metasedimentary unit primarily composed of siliceous meta-argillite and subordinate metachert (Wright and Fahan, 1988). Two 20e60-m-wide syntectonic hornblende-gabbro pyroxenite composite dikes are exposed in the center of the domain of L tectonites (Fig. 2b) (Wright and Fahan, 1988; Sullivan, 2009). Throughout most of the Pigeon Point high-strain zone, mafic metavolcanic rocks contain the metamorphic mineral assemblage blue-green actinolite ? green actinolite ? epidote ? brown biotite ? quartz ? chlorite ? calcite dolomite and meta-argillites contain the assemblage quartz ? chlorite ? white mica._ Ключевые слова: dark gray, wenk, tullis stipp, wallis, minimum, journal structural, deformation condition, geological society, preferred, geology, garnet amphibolite, crystallographic fabric, a-axis girdle, shear plane, lattice parameter, quartz, collected, hobbs, preferred orientation, point, eds, law, tullis, pigeon point, introduction, fabric, structural, dominant slip, casey law, plagioclase, opening, strain geometry, grain-boundary intersection, fabric development, plot, elongation, simulation, foliation, constrictional deformation, plane, high-strain, grain-boundary sliding, fabric asymmetry, garnet grain, phyllosilicate grain, takeshita, sullivan, de?ned, kinematic geometry, exhibit, ?ow, london, geophysical, viewed, hai, amphibolite-facies metamorphism, small deviation, data, amphibolite, quartz grain, rst conrmation, fahan sullivan, geological, shape, colby, lie close, vertical plane, quartzite journal, price schmid, development, constrictional strain, point high-strain, xey plane, kruhl, phyllosilicate-rich domain, plot center, ?nite strain, outcrop scale, geometry, plot producing, hand sample, high-temperature sample, foliation dened, heilbronner, center, structural geology, girdle, deductive conclusion, reference, brady perkins, sullivan beane, direction, schmid casey, cartoon depicting, deformation temperature, lineation-parallel, deformation recorded, lineation lie, feldspar, slip, quartz crystallographic, zone, strain, high, deformation, condition, high-strain-zone boundary, distinctly asymmetrical, american, tectonite, sample collected, symmetrical cone, pattern, journal structural geology, numerical simulation, c-axis girdle, primitive, journal, bulletin, slip direction, small-circle, amphibole, hornblende, brady, quartz c-axis, tectonite sample, lineation, opening angle, distinct maximum, reference frame, table note, maximum, universal stage, domain, lineation-normal face, crystal, fabric geometry, perkins, metavolcaniclastic unit, plastic deformation, simple-shear component, boundary, randomly oriented, additional evidence, mineralogist, quartz crystallographic fabric, prior, a-axis fabric, weak, quartz-rich domain, quartz fabric, type, maximum intermediate, deformation parameter, shear, elsevier, frame, amphibolite-facies domain, angle, schmid, quartzite, long-axis orientation, lie, ax, lister, tectonophysics, hirth, area, microstructures, constrictional, component, c-axis maximum, tullis law, a-axis, wa, powell, table, amphibolite-facies, data collected, samples, orientation, beane, price, plane strain, recrystallization, tectonites, c-axis, deformed, casey, largest petal, metamorphic, majoribanks, temperature, axis, noncoaxial, a-axis pattern, metamorphism, grain, small-circle girdle, rock, geographic horizontal, phyllosilicate, weak foliation, mineral, caseys prediction, sample, electron, hobbs model, dominant role, c-axis fabric, pigeon, ?nite, lister hobbs, dynamic recrystallization, high-strain zone, analysis, mesoscopic lineation, compton, yund hirth, society, teyssier, garnet, high angle, crystallographic