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Journal of Structural Geology 50 (2013) 161e175

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W.A. Sullivan

Department of Geology, Colby College, 5803 Mayﬂower Hill, Waterville, ME 04901, USA article info abstract

Article history: Rocks with a pure linear fabric, or L tectonites, often indicate nearly perfect constrictional deformation. Received 29 October 2011 This paper assimilates published data, models, and interpretations to understand the forcing mecha- Received in revised form nisms that can form L tectonites. Most noncoaxial kinematic geometries that can result in constrictional 6 January 2012 deformation involve vorticity-parallel shortening. Local variations in external boundary conditions that Accepted 23 January 2012 localize components of constriction include releasing and restraining bends in shear zones, linear Available online 31 January 2012 channels in shear zone boundaries, intersections between shear zones, and foliation triple points between ballooning diapirs. Internally, L tectonites are often localized in fold hinge zones, and rheologic Keywords: L tectonites variations partition constriction into discrete domains. Constriction The most common external kinematic framework that can form L tectonites involves simultaneous Constrictional strain transport-perpendicular shortening in two directions. Hence, large domains of L>S and L tectonites are Strain partitioning a common feature of orogen-parallel elongation. In every case, external variations in boundary condi- Lineation tions and/or internal variations in structural setting and rheology localize constriction to form L tectonites. External boundary conditions are important in density-driven vertical tectonics. Elsewhere, internal variations in structural setting and rheology are more important. The most common are the formation of L tectonites in fold hinge zones and in compositionally homogeneous rocks while heterogeneous rocks accommodate constriction by folding. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction geologic settings at all scales. These include but are not limited to the hinge zones of folds (e.g., Holst and Fossen, 1987; Sylvester and Deviation from plane strain is not only common in nature, it is Janecky, 1988; Poli and Oliver, 2001; Solar and Brown, 2001; probably the norm (Pfiffner and Ramsay, 1982, their Fig. A1). This is Sullivan, 2006); contractional, extensional, and transcurrent ductile especially true in the middle and lower crust where complex shear zones (e.g., Hossack, 1968; Chapman et al., 1979; Flinn 1992; ductile deformation zones can simultaneously accommodate many Fossen, 1993; Fletcher and Bartley, 1994; Lin and Jiang, 2001; different deformation components. Unfortunately, reliable finite Sullivan, 2009); areas of density-driven vertical tectonics (e.g., Balk, strain markers are exceedingly rare in nature. Therefore, geologists 1949; Hoy et al., 1962; Bouhallier et al., 1995); zones of tectonic in the field wishing to evaluate three-dimensional deformation are extrusion around rigid indenters (e.g., Dias and Ribeiro, 1994; often forced to make assumptions about finite strain geometry Piazolo et al., 2004); zones of subhorizontal lower-crustal flow in based on the shape fabrics visible in ductiley deformed rocks. Rocks large orogenic belts (Poli and Oliver, 2001; Duclaux et al., 2007; with strong foliations and no visible lineations, or S tectonites, Dumond et al., 2010); and ultra-high-pressure terranes (Zulauf, indicate flattening strain; rocks with both strong foliations and 1997; Kurz et al., 2004). Extensive domains of L>S and L tecton- mineral lineations, or L-S tectonites, indicate near-plane strain; and ites are also predicted by numerical simulations of transtension rocks exhibiting strong linear alignments of minerals with no zones (Dewey et al., 1998 ; Fossen and Tikoff, 1998; Jiang and visible foliations (Fig. 1), or L tectonites, indicate constrictional Williams, 1998) and may be a common feature of transtension in strain (Flinn, 1965). Of these cases, constrictional deformation is by the lower crust (Dewey, 2002). far the least common (Pfiffner and Ramsay, 1982, their Fig. A1). L tectonites are typically found in close spatial association with Nevertheless, constrictional or nearly constrictional deformation is more common L-S tectonites and even S tectonites (e.g., Hossack, an important feature at many active and ancient plate boundaries, 1968; Holst and Fossen, 1987; Piazolo et al., 2004; Sullivan, 2006, and significant domains of L tectonites occur in a wide range of 2009). These spatial variations between L, L-S, and S tectonites might provide significant information about the way strain is partitioned in the middle and lower crust, but this strain phenomenon E-mail address: [email protected]. has commonly been overlooked in the literature. Indeed, many

2. Recognizing L tectonites formed by constriction

2.1. Introduction

A variety of processes can lead to the formation of strong linear fabrics. Overprinting deformation phases can form prolate strain markers by superimposing deformations with different maximum shortening directions (Ramsay, 1967). Similarly, overprinting of nearly orthogonal planar fabrics can result in strong, penetrative intersection lineations commonly called pencil cleavage (Cloos, 1946; Crook, 1964). Therefore, it is often necessary to establish the existence of true constrictional deformation d even in rocks with suitable ﬁnite strain markers. Fortunately, true constrictional deformation normally produces unique microstructural features and distinct crystallographic fabric patterns that can be used to distinguish L tectonites formed by a single phase of constrictional deformation from other L tectonite fabrics.

2.2. Mineral shape fabrics

A suite of unique microstructural features commonly develop during constrictional or nearly constrictional deformation. The deformation fabric in constrictional L tectonites viewed on lineation-parallel faces closely resembles that seen in foliation- perpendicular faces of L-S and even S tectonites (Fletcher and Bartley, 1994; Sullivan, 2006, 2009; Sullivan and Beane, 2010). Long axes of mineral grains with anisotropic shapes are aligned, porphyroblasts and porphyroclasts commonly exhibit preferred alignments, dynamically recrystallized mineral aggregates exhibit shape-preferred orientations, and compositional banding and dissolution/precipitation features are parallel with grain-shape Fig. 1. Photographs of L tectonites: (a) L tectonite of the Boy Scout Camp Granodiorite alignments. However, lineation-normal faces of constrictional L from the Laramie Mountains, Wyoming, reproduced from Sullivan (2006). Pencil is tectonites exhibit a unique set of microstructural features that can be parallel with the penetrative lineation. (b) Prolate clast in a deformed tuff-breccia from used to infer true constrictional deformation. The long axes of por- the Pigeon Point high-strain zone, southern Klamath Mountains, California (repro- phyroblasts and porphyroclasts will show no shape-preferred duced from Sullivan, 2009). The average dimensions of the long, intermediate, and short axes of clasts measured in this outcrop are 34.2 cm, 4.5 cm, and 3.6 cm, orientation (Fletcher and Bartley, 1994; Solar and Brown, 2001; respectively. Pocket knife for scale is 9.8 cm long. Sullivan, 2009). Individual minerals with strongly anisotropic shapes such as amphiboles or pyroxenes will be oriented such that basal sections are commonly presented on lineation-normal faces, but the studies of areas with large domains of L tectonites fail to even intermediate axes of these minerals will show little or no preferred mention them. This paucity of knowledge concerning the nature orientation (Sullivan, 2006, 2009). Basal cleavage traces of micas will and significance of L tectonites exists, in part, because under- be randomly oriented when viewed in lineation-normal sections, standing the formation of L tectonites is a difficult problem but the h001i directions will consistently be subperpendicular to the requiring detailed mapping and three-dimensional structural lineations (Ramsay and Huber, 1983; Fletcher and Bartley, 1994; analyses of areas that have often undergone complicated, poly- Sullivan, 2006; Sullivan and Beane, 2010). In L tectonites that display phase deformational histories. Finally, the existing framework of compositional segregation, quartzo-feldspathic domains may be knowledge concerning the formation of L tectonites is very limited. encircled and separated by micaceous domains (Sullivan, 2009; Therefore, this article aims to accomplish three things. First, it will Sullivan and Beane, 2010). Similarly, in L tectonites where fluid- outline some criteria for distinguishing L tectonites formed during assisted diffusive mass transfer was an important deformation a single phase of true constrictional deformation from those mechanism, porphyroclasts viewed on lineation-normal faces will formed by overprinting fabric elements or other processes. Second, commonly be completely rimmed by seams of insoluble minerals the bulk of the paper combines case studies of ductile deformation (Sullivan, 2009). If present, shear bands or micro-shears will be zones that contain L tectonites with observations of numerical, anastomosing and randomly oriented about the lineations (Fletcher theoretical, and analogue models that predict constriction to and Bartley,1994). Finally, dynamically recrystallized aggregates in L highlight some of the mechanisms that can form L tectonites during tectonites will generally not exhibit grain-shape-preferred orienta- a single progressive deformation event. Finally, a series of case tions on lineation-normal faces despite displaying abundant studies are used to assess the relative importance of different evidence for lattice deformation and dynamic recrystallization forcing mechanisms that can lead to the formation of L tectonites. including undulose extinction, subgrain development, sutured grain The assembled results and conclusions presented in this article boundaries, and the nucleation of neoblasts (Fletcher and Bartley, provide geologists with a starting point for recognizing and inter- 1994; Sullivan, 2006, 2009; Sullivan and Beane, 2010). preting large domains of constrictional strain in ductiley deformed rocks. Additionally, assessing the mechanisms that localize 2.3. Crystallographic fabrics constriction provides a better understanding of the relative importance of the different forcing mechanisms that drive strain Quartz is one of the most well understood minerals in terms partitioning in all kinds of ductile deformation zones. of the crystallographic fabrics that develop during ductile W.A. Sullivan / Journal of Structural Geology 50 (2013) 161e175 163 deformation, and it is widely accepted that quartz c- and a-axis Minerals that commonly deform by diffusion-assisted grain fabric geometries are systematically related to finite strain geometry boundary sliding such as micas, amphiboles, and pyroxenes may (e.g., Tullis et al., 1973; Tullis 1977; Lister and Hobbs, 1980; Price, also exhibit unique crystallographic fabric patterns when they 1985; Law, 1986; Schmid and Casey, 1986; Barth et al., 2010; experience constrictional deformation. Micas should exhibit Sullivan and Beane, 2010). This relationship is relatively well a girdle of poles to the basal planes approximated by c axes centered established for coaxial constriction under greenschist-facies condi- about the lineation (Fig. 3a) (Ramsay and Huber, 1983; Sullivan and tions where quartz c-axis fabrics form girdles about the lineations Beane, 2010), whereas pyroxenes (Kurz et al., 2004) and amphiboles with opening angles of 130e160 and a-axis fabrics form girdles should exhibit lineation-parallel c-axis maxima (Fig. 3b) and girdles about the lineations with opening angles of 60e70 (Fig. 2a) (Lister of a and b axes centered about the lineation (Figs. 3c and d). In rocks and Hobbs,1980; Schmid and Casey,1986; Barth et al., 2010; Sullivan that experienced plane strain deformation, poles to the basal planes and Beane, 2010). With increasing temperature, quartz c-axis girdles of mica grains and pyroxene and amphibole a axes should cluster widen, and single-girdle c-axis fabrics may form during coaxial about the pole to the mesoscopic foliation (Compton, 1980; Ramsay constriction under amphibolite-facies conditions as prism hai slip and Huber, 1983; Kurz et al., 2004; Díaz-Azpiroz et al., 2007), and becomes dominant (Fig. 2b) (Barth et al., 2010; Sullivan and Beane, pyroxene and amphibole c axes should form more diffuse maxima 2010). However, quartz a-axis fabric geometry is much less sensi- about the lineation, possibly showing signs of a transition to a girdle tive to temperature variations because slip in the crystallographic parallel with the trace of the mesoscopic foliation (Fig. 3bed) (Kurz hai direction dominates crystaleplastic deformation of quartz up to et al., 2004; Díaz-Azpiroz et al., 2007). middle-amphibolite-facies conditions (Schmid and Casey, 1986; Stipp et al., 2002). Quartz a-axis fabrics are also better indicators of 3. Mechanisms of L tectonite formation finite strain geometry for samples that experienced noncoaxial deformation, and the characteristic double-girdle quartz a-axis 3.1. Hierarchy of forcing mechanisms fabrics indicative of constriction are recognizable under a much wider range of deformation conditions (Fig. 2b and c) (Sullivan and The mechanisms that determine strain geometry and strain path Beane, 2010). and that drive strain partitioning in ductile deformation zones can

Fig. 2. (a) Quartz crystallographic fabric geometries predicted for coaxial deformation under greenschist-facies conditions (adapted from Schmid and Casey, 1986). X, Y, and Z are the maximum, intermediate, and minimum axes of the ﬁnite strain ellipsoid respectively. (b) Cartoons and schematic stereonets illustrating the effect of increasing deformation temperature on quartz crystallographic fabrics formed during coaxial constriction (adapted from Sullivan and Beane, 2010). The a-axis fabric geometry remains constant with two symmetrical cones of a axes appearing as two small-circle girdles on the stereonets. The opening angles of the c-axis fabric girdles increase with increasing deformation temperature as rhomb hai and basal hai slip become more important. (c) Geometry of crystallographic fabrics formed during noncoaxial, nearly constrictional deformation under lower-amphibolite-facies conditions (adapted from Sullivan and Beane, 2010). The shear plane lies in the plane deﬁned by the a-axis maxima, near the eastewest line of the diagrams. Note that the small-circle a-axis girdles are more readily recognized than the c-axis girdles. 164 W.A. Sullivan / Journal of Structural Geology 50 (2013) 161e175

a Mica c axes c Amphibole or pyroxene a axes Z Z

XY XY X/Y X/Y

K>1 K>1 K=1 K=1 K<1 K<1

Y/Z Y/Z

b Amphibole or d Amphibole or pyroxene c axes pyroxene b axes Z Z Y XY X X/Y X/Y

K>1 K>1

K=1 K=1 K<1 K<1

Y/Z Y/Z

Fig. 3. Possible crystallographic fabric patterns of anisotropic minerals deformed by diffusion-assisted grain boundary sliding. X, Y, and Z are the maximum, intermediate, and minimum axes of the finite strain ellipsoid respectively. Inferred from data presented by Compton (1980), Kurz et al. (2004), and Díaz-Azpiroz et al. (2007), Sullivan and Beane (2010); microstructural observations; and theory presented in Ramsay and Huber (1983). (a) Pattern of poles to basal planes of mica grains for different strain geometries. (b) Possible amphibole and pyroxene c-axis fabric patterns for different strain geometries. (c) Possible amphibole and pyroxene a-axis fabric patterns for different strain geometries. (d) Possible amphibole and pyroxene b-axis fabric patterns for different strain geometries. be thought of in a hierarchical manner wherein lower-order coaxial deformation (Flinn, 1962). However, numerical simulations mechanisms operate within the parameters dictated by higher- and theoretical studies of homogeneous deformation show us that order mechanisms. The first-order mechanism that determines six end-member monoclinic noncoaxial kinematic geometries can strain geometry is the external kinematic geometry of the defor- form strongly constrictional strains and therefore L>S and L tec- mation zone dictated by the regional structural or plate tectonic tonites (Fig. 4a). Moreover, many of these geometries include large setting. Examples of external kinematic geometries that produce or even dominant simple shear components (Dewey et al., 1998; different bulk strain paths include monoclinic transpression with Fossen and Tikoff, 1998; Jiang and Williams, 1998; Lin et al., 1998; vertical elongation, dip-slip shear zones with transport-parallel Tikoff and Fossen, 1999). An interesting feature of these models is elongation, and channel flow. Second-order forcing mechanisms that the shape of the finite strain ellipsoid changes throughout the include local external variations in the boundary conditions deformation history because strain from the coaxial component of imposed on deformation zones such as restraining bends in deformation accumulates more efficiently than strain from simple transpressional shear zones, ramps in dip-slip shear zones, and shear (Figs. 4bed) (Means et al., 1980; Tikoff and Fossen, 1995). For narrowing of a channel in channel flow. Third-order forcing any given kinematic geometry the exact strain path is a function of mechanisms include internal variations in structural setting and the relative significance of the incremental components of coaxial rheology. Second- and third-order forcing mechanisms create local deformation and simple shear, or the kinematic vorticity number domains with widely different kinematic geometries or partition (Dewey et al., 1998; Fossen and Tikoff, 1998; Tikoff and Fossen, different components of the bulk external deformation into 1999). Perhaps the most commonly cited of these kinematic different domains. Therefore, these are the mechanisms that drive geometries is thickening/narrowing shear in which coaxial strain partitioning and create spatial variations in strain geometry contraction parallel with the vorticity vector is coupled with elon- and strain magnitude in ductile deformation zones. gation of the shear zone normal to its boundaries (Fig. 4a) (Dewey et al., 1998; Fossen and Tikoff, 1998; Dewey, 2002). Strongly con- 3.2. First-order forcing mechanisms strictional strains are predicted for simple shear-dominated thickening/narrowing shear deformations, and perfectly prolate finite The most obvious kinematic framework that will form L tec- strain ellipsoids will only exist at one point along the strain paths tonites during homogeneous deformation is perfectly constrictional (Fig. 4b) (Tikoff and Fossen, 1995; Dewey et al., 1998). Practically W.A. Sullivan / Journal of Structural Geology 50 (2013) 161e175 165

Fig. 4. (a) End-member, monoclinic kinematic geometries that can result in constrictional deformation (modified from Tikoff and Fossen, 1999). Note, no geographic coordinates are implied, and the different deformation components may have any orientation. (b) Strain paths for thickening/narrowing shear with different simple shear components (modified from Dewey et al., 1998). Perfect constrictional strain occurs where the paths meet the X/Y axis. (c) Strain paths for lengthening shear with different simple shear components (Modified from Fossen and Tikoff, 1998). (d) Strain paths for widening shear with different simple shear components (Modified from Fossen and Tikoff, 1998). X, Y, and Z are the maximum, intermediate, and minimum axes of the finite strain ellipsoid respectively.

speaking, L tectonites without recognizable foliations in outcrop kinematic geometries include coaxial vorticity-vector-parallel should exist along the segments of the strain paths that lie near the shortening (YI-type shear zones of Passchier, 1998). The sole constrictional axis of the Flinn diagram (Fig. 4b). The other five end- outlier, widening shear, can also produce bulk flattening strains if member monoclinic kinematic geometries in Fig. 4a can also the kinematic vorticity number is greater than 0.5 (Fig. 4d) (Fossen produce strain in the constrictional field, but only thickening/nar- and Tikoff, 1998). Therefore, regional vorticity-vector-parallel rowing shear and lengthening/narrowing shear can produce perfect shortening is one of the surest ways to generate large domains of constriction with a non-zero simple shear component (Fig. 4c and d) constrictional strain. (Fossen and Tikoff, 1998). However, kinematic geometries inter- To date, no triclinic models have been published that predict mediate between these end members can also produce perfect perfect constrictional strains and hence L tectonites, but numerical constriction. For instance, perfect constriction can develop under simulations of triclinic transtension predict finite strains that lie a kinematic geometry similar to lengthening shear if the vorticity- well into the constrictional field (Lin et al., 1998; Jiang and vector-parallel shortening is greater than the vorticity-vector- Williams, 1998). Triclinic transtension models predict progressive normal shortening component. All but one of these end-member rotation of the long axes of the finite strain ellipsoids towards 166 W.A. Sullivan / Journal of Structural Geology 50 (2013) 161e175 parallelism with the elongation direction of the coaxial component networks. Each of these deviations will lead to strain heterogene- of deformation and progressive changes in finite strain geometry ities and local non-plane strain deformation, and most of these that are not as extreme as the shape changes predicted by mono- geometries can develop local domains of L tectonites under the clinic numerical simulations. Making any of the kinematic geom- right conditions. etries depicted in Fig. 4a triclinic can also result in bulk deformation in the constrictional field (Lin et al., 1998). 3.3.2. Variations in shear zone boundaries All of these numerical simulations and simple conceptual Flinn (1961) investigated the effects of zone-normal contraction models make the basic assumption of steady state homogeneous coupled with extrusion of material out of the deformation zone deformation. Because of this, they do not explain all of the finite within high-strain zones that contain rectangular asperities in their strain patterns observed in natural deformation zones which are walls using an unscaled analogue model. This model demonstrates subject to various second- and third-order forcing mechanisms and that linear asperities in high-strain-zone boundaries can drive local rarely experience steady state deformation. However, these models convergent flow resulting in constrictional deformation during do demonstrate that multiple noncoaxial deformation geometries zone-normal contraction coupled with elongation parallel with the are likely to develop large domains of strain in the constrictional channel. Using this geometry, the width of the zone must be less field, and they provide an important conceptual tool for analyzing than the depth of the asperity to initiate convergent flow into the natural deformation zones in terms of the potential kinematic groove (Flinn, 1961). These results are for bulk flattening defor- geometries that the deformation fabrics may record. mations, and bulk plane-strain or constrictional deformations will likely favor convergent flow into linear asperities in high-strain- 3.3. Second-order forcing mechanisms zone boundaries and the development of L tectonites (Fig. 5). Such an asperity in the upper boundary of the Pigeon Point high- 3.3.1. Introduction strain zone helped form a one-km-wide domain of L tectonites by In three dimensions, shear zones commonly deviate from the preferentially partitioning a relatively small component of zone- classic straight-sided model (e.g., Rykkelid and Fossen, 1992; Lin parallel, transport-perpendicular contraction coupled with and Jiang, 2001; Strine and Wojtal, 2004; Resor and Snoke, 2005; transport-parallel elongation into a narrow corridor within the Sullivan, 2009; Carreras et al., 2010). Such deviations include linear high-strain zone (Sullivan, 2009). asperities in shear zone boundaries; releasing and restraining Bends in shear zones can also help form local domains of L bends in strike-slip, transpressional, and transtensional shear tectonites. For example, in restraining bends in transpressional systems; frontal, lateral, and oblique ramps in dominantly dip-slip shear zones, zone-normal contraction coupled with vertical elon- shear systems; transfer zones between segments in ductile shear gation is combined with local transport-parallel contraction systems; and intersections between shear zones in complex (Fig. 6a) (Lin and Jiang, 2001). This geometry will create subvertical,

Fig. 5. Block diagram cartoon illustrating the localization of constriction in a linear asperity in a high-strain zone undergoing transport-perpendicular shortening coupled with transport-parallel elongation. The linear channel concentrates a relatively small component of subhorizontal, transport-perpendicular shortening, and the bulk deformation geometry is similar to the lengthening shear framework of Fig. 4a. Also note the folding of different rheologic units in the L tectonite domain with fold axes parallel with the maximum elongation direction as observed in the Pigeon Point high-strain zone (Sullivan, 2009). W.A. Sullivan / Journal of Structural Geology 50 (2013) 161e175 167

Fig. 6. (a) Block diagram cartoon illustrating a restraining bend in a dextral, transpressional shear zone. In the bend, zone-normal shortening is combined with local transport- parallel shortening to produce a constrictional deformation geometry similar to the widening shear geometry in Fig. 4a. (b) Block diagram cartoon illustrating a releasing bend in a dextral, transcurrent shear zone. In the bend subvertical shortening coupled with transport-parallel elongation is combined with simple shear to produce a constrictional deformation geometry similar to the lengthening/narrowing shear geometry of Fig. 4a. transport-perpendicular L tectonite fabrics as material is extruded transport-parallel elongation coupled with vertical shortening out of the restraining bends. Similar geometries should create (Fig. 6b). This geometry could help form local subhorizontal L tec- transport-perpendicular L tectonite fabrics in ramps or bends in tonite fabrics subparallel with the overall transport direction of the any shear zones that include zone-normal contraction coupled shear system. A similar geometry should form at frontal ramps in with transport-perpendicular elongation. At releasing bends in normal-sense, dip-slip shear zones, leading to transport parallel L strike-slip shear zones, simple shear may be locally combined with tectonites. 168 W.A. Sullivan / Journal of Structural Geology 50 (2013) 161e175

3.3.3. Intersecting shear zones The preservation of constrictional fabrics within diapirs is Complex networks of anastomosing shear zones occur at largely a function of the duration of the strain clock of the rocks in a variety of scales, and they can lead to significant local variations in question. As material circulates within the rising diapiric body, the strain path and strain geometry (e.g., Corsini et al., 1996; initial constrictional fabrics in the center are rotated to the outside Bhattacharyya and Hudleston, 2001; Carreras et al., 2010). Although of the body and overprinted by flattening strains (Talbot and natural examples of L tectonites forming at shear zone triple points Jackson, 1987; Cruden, 1990). In systems where the rate of circu- do exist, I have not found them explicitly described in the literature. lation is great enough, material may rotate through the diapiric However, one possible natural example of shear zone intersections body several times during ascent, giving rise to a complex strain creating domains of L tectonites occurs in the Archean Dharwar history with multiple overprinting periods of constriction and craton of India where subvertical high-strain zones in greenstone flattening (Cruden, 1990). belts form triple points that are dominated by L and L>S tectonites The wall rocks around rising diapirs may also experience con- (Bouhallier et al., 1995). This area is interpreted as an example of strictional deformation. For instance, L and L>S tectonites are found vertical tectonics and sagduction (Bouhallier et al., 1995). If this in high-strain zone triple points between ballooning diapiric bodies interpretation is correct, the individual high-strain zones have (Brun et al., 1981; Bouhallier et al., 1995). Also, numerical simula- a bulk channel-flow deformation geometry. Local domains of tions and natural examples indicate that L>S and L tectonites may constriction and flattening can also be produced in a network of form at foliation triple points in the country rocks near the ends of shear zones undergoing bulk simple shear by translating the isolated ballooning plutons where the strain field created by the intervening rheologically stronger domains to create local zones of plutons interacts with a regional strain field (Guglielmo, 1993, transtension and transpression (Hudleston, 1999). 1994; Bouhallier et al., 1995). In this case deformation is also non- steady state even though the regional strain field is constant 3.3.4. Density-driven vertical tectonics because the pluton is expanding. Hence L tectonites will only exist Diapirism has been invoked as a mechanism for the emplace- at any one location for a segment of the deformation history ment of a variety of crustal features including salt domes (e.g., (Guglielmo, 1993, 1994). Jackson and Talbot, 1986; Talbot and Jackson, 1987), plutons (e.g., Sylvester et al., 1978; Ramsay, 1989), and gneiss domes (e.g., Eskola, 3.4. Third-order forcing mechanisms: structural setting 1949; Ramberg, 1980; Whitney et al., 2004). Both conceptual models and analogue simulations of rising diapirs indicate that A number of authors have reported L and L>>S tectonites in the these structures are characterized by upward flow through the hinge zones of natural folds (e.g., Holst and Fossen, 1987; Sylvester middle of the dome and then downward flow along the external and Janecky, 1988; Snoke et al., 2001; Solar and Brown, 2001; margins (Talbot and Jackson, 1987; Cruden, 1988, 1990). This flow Sullivan, 2006). Deformation in folded layers is non-steady state geometry leads to flattening strains around the outside of the because they are constantly rotating with respect to the external diapiric dome and constrictional strains in the center of the dome kinematic framework and deformation zone boundaries. This (Fig. 7). Additionally, downward return flow surrounding the rising results in progressive overprinting of earlier deformation features, diapir results in convergent flow in the trailing tail or stalk beneath and this overprinting can create L tectonites in bulk plane strain or the dome, and this area may also form large domains of L tectonites even flattening strain environments during a single progressive (Fig. 7)(Cruden, 1990). The lower structural levels of plutons and deformation event. Fig. 8 illustrates one possible overprinting gneiss domes inferred to have been emplaced by diapiric rise are scenario that can lead to the formation of L tectonites in fold hinge only rarely observed. However, a number of workers have reported zones during simple shear-dominated deformation. In this case, the halite and anhydrite L tectonites in the centers of salt diapirs (e.g., folded layer is subjected to simultaneous simple shear and a small Balk, 1949, 1953; Hoy et al., 1962). component of coaxial fold hinge-parallel elongation. The early increments of deformation create an S>L-tectonite fabric (Fig. 8a). As folding progresses, the strain markers are rotated so that one of the long axes of the strain ellipsoid becomes parallel with the maximum shortening direction in the fold hinge zone (Fig. 8b) forming L and L>S tectonites. L and L>S tectonites may also form by progressive rotation of layers in the hinge zones and short limbs of F l a t t e n i n g asymmetrical folds that form in layers initially oriented nearly parallel with the maximum shortening and maximum elongation directions during nearly coaxial deformation. In such case the foli- Diapiric Cascading ations outside of the fold hinge zones will not be axial planar. dome folds: Potential Hudleston (1977) and Holst and Fossen (1987) invoked the L tectonites in hinge zones progressive rotation mechanism depicted in Fig. 8 to explain constrictional strains in fold hinge zones formed under bulk plane strain C o n s t r i c t i o n and flattening strain conditions, but they did not include a coaxial component of deformation in the model. Sylvester and Janecky (1988) also invoked the progressive rotation model to explain the formation of L tectonites in fold hinge zones, but their model also included a change in external boundary conditions to initiate folding with fold axes parallel with the regional transport direction. Regardless of the mechanism, the formation of L tectonites by progressive rotation of folded layers is dependent on the relative rates of shortening, fold-axis-parallel elongation, and rotation of Fig. 7. Block diagram cartoon of a rising diapiric body. Constrictional strain develops in the folded layers. Additionally, at any one point in the folded layer L the center of the body and the trailing tail. L tectonites may also form in the hinge zones of the folds developed by return flow around the rising diapir (modified from tectonites will only exist for an increment of the deformation Whitney et al., 2004). history before they are overprinted by continued layer-parallel W.A. Sullivan / Journal of Structural Geology 50 (2013) 161e175 169

In a km-scale synform exposed in the Laramie Mountains of eastern Wyoming, L tectonites were preferentially developed in compositionally homogenous units whereas compositionally heterogeneous units deformed by progressive folding with fold axes parallel with the maximum elongation direction (Fig. 9) (Sullivan, 2006). The formation of L tectonites in this case is best explained by relative thickening of fold hinge zones during the formation of class 2 and 3 folds (Ramsay, 1967) coupled with a large component of fold hinge-parallel elongation during bulk near- coaxial deformation in the constrictional ﬁeld. Relative hinge zone thickening may also form L tectonites under any of the noncoaxial kinematic geometries depicted in Fig. 4a as long as the hinge zones are parallel with the maximum elongation direction, and the formation of fold axes subparallel with the maximum ﬁnite elongation direction seems to be the only strict requirement for forming L tectonites in fold hinge zones.

3.5. Third-order forcing mechanisms: internal rheological variations

Variations in rheology are often considered the most important internal control of strain partitioning (Lister and Williams, 1983; Goodwin and Tikoff, 2002), and three-dimensional rheological heterogeneities can inﬂuence the development of L tectonite fabrics in a number of ways. Theoretically, during general-shear deformation, simple shear will be preferentially partitioned into rheologically weak units (Lister and Williams, 1983; Jiang, 1994a, b). In such

Fig. 8. Cartoons showing the progressive rotation of strain markers in an evolving fold case, a component of nearly coaxial, constrictional strain may be hinge zone. (a) Time 1 d the early increments of deformation create an S>L-tectonite preferentially localized in rheologically strong domains leading to fabric. (b) Time 2 d the strain markers are rotated so that one of the long axes of the the formation of L and L>S tectonites. Such strain path partitioning strain ellipsoid becomes parallel with the maximum shortening direction in the fold led to the development of a local domain of L>>S tectonites in the hinge zone forming L and L>S tectonites. Raft River shear zone where coaxial constriction was localized in quartzecobbleeconglomerate beds and simple shear was absorbed by rheologically weaker phyllonites (Sullivan, 2008). Conversely, in shortening or rotated out of a migrating fold hinge zone. However, certain deformation geometries where simple shear-dominated if the rates of rotation and layer-parallel shortening reach a near- deformation favors the development of constrictional strains such as steady state, it is possible for L tectonites to exist in fold hinge lengthening/narrowing shear (Fig. 4), the localization of simple zones for a signiﬁcant portion of the deformation history. shear may cause L tectonites to form in discrete domains of rheo- Quartz crystallographic fabrics collected from progressively logically weak rocks. Internal rheological variations do not always rotated fold hinge zones often do not show a direct relationship affect strain geometry. For example, in the Pigeon Point high-strain between the c-axis pattern and the bulk kinematic framework zone a large domain of L tectonites cuts multiple rock types in both because of the continual rotation of the rocks during deformation amphibolite- and greenschist-facies domains that are inferred to (Law et al., 1986; Stünitz, 1991; Hongn and Hippertt, 2001). There- have very different rheologies (Sullivan, 2009). fore, L tectonites formed by progressive rotation of strain markers in In the Laramie Mountains fold locality mentioned above L tec- fold hinge zones may not exhibit the quartz crystallographic fabrics tonites preferentially formed in compositionally homogeneous expected to develop during perfect constrictional deformation rocks including a w1-km-wide granodiorite pluton in the hinge of unless the bulk external kinematic framework under which folding a km-scale synform and local pods of homogeneous granite in occurred is constrictional. Similarly, they might also exhibit micro- compositionally heterogeneous banded orthogneiss (Fig. 9) structures indicative of two successive overprinting events. (Sullivan, 2006). At the same time, compositionally heterogeneous Constrictional strains in fold hinge zones are predicted by domains in this deformation zone accommodated constriction by various three-dimensional analogue-modeling studies of folding folding with axes parallel with the bulk elongation direction, and (Grujic and Mancktelow, 1995; Kobberger and Zulauf, 1995; Zulauf these domains tend to contain L>S and L-S tectonites (Sullivan, and Zulauf, 2005). These models indicate that L tectonites may also 2006). Very similar variations in the style of deformation form near fold hinge zones without rotation of initially shortened between penetrative L tectonite development in homogeneous layers. For instance, strong L and L>S fabrics can develop in weak granitic units and folding of compositionally heterogeneous rocks layers around fold hinge zones wherein the maximum ﬁnite elon- occur in the footwall of the Mojave metamorphic core complex gation direction is parallel with the layering without rotation of (Fletcher and Bartley, 1994). These parallel observations from initial fabrics if there is an extremely large viscosity contrast different plate tectonic and structural settings indicate that such (>400:1) between the layers (Grujic and Mancktelow, 1995). L rheologically driven changes in the way constrictional deformation tectonites may also form parallel with fold axes throughout folded is accommodated may be common in nature. A number of layers that experience coaxial constriction. In such case, viscosity processes may give rise to this style of strain partitioning. For contrasts large enough to initiate folding also result in boudinage of instance, rheologically driven strain path partitioning between the competent layer with boudin necks forming perpendicular to rheologically strong homogeneous units and rheologically weak the maximum elongation direction and the linear fabric (Kobberger heterogeneous units may concentrate coaxial constriction in the and Zulauf, 1995; Zulauf and Zulauf, 2005). homogeneous units. Similarly, rheologically driven strain path 170 W.A. Sullivan / Journal of Structural Geology 50 (2013) 161e175

Fig. 9. Block diagram cartoon illustrating the deformation geometry and strain pattern in the Laramie Mountains L tectonite locality (Sullivan, 2006). L tectonites are localized in compositionally homogeneous rocks centered in the fold hinge zone while the limbs of the fold are L>S and L-S tectonites. Compositionally heterogeneous rocks accommodate constriction by folding with axes parallel with the maximum ﬁnite elongation direction. Metamorphosed diabase dikes outline the extent of the deformation zone.

partitioning between the different rheologic layers in composi- 4. Relative importance of different forcing mechanisms tionally heterogeneous units may prevent constrictional strain geometries from developing in these units at the present state of 4.1. Importance of ﬁrst-order forcing mechanisms strain. Alternatively, active folding in rheologically heterogeneous units may lead to strain variations between different domains Few detailed case studies completely explain the formation of L within folds. These different domains would be L>S, L-S, or S>L tectonites, but at least twenty-one case studies of naturally tectonites that share a common maximum elongation direction but deformed rocks explicitly note the presence of L or L>>S tectonites have opposed intermediate and minimum elongation directions and provided sufﬁcient data to draw conclusions about at least one that combine to accommodate constrictional strain. This is the forcing mechanism leading to their formation (Table 1). Many of preferred model in the Laramie Mountains locality because folia- these case studies are of areas that record complicated, polyphase tions in compositionally heterogeneous rocks that are coeval with deformation histories, but the authors of these studies concluded the L tectonite fabric form a girdle about the maximum elongation that the formation of L tectonites was the result of a single, direction (Sullivan, 2006). progressive deformation phase rather than clear overprinting of W.A. Sullivan / Journal of Structural Geology 50 (2013) 161e175 171

Table 1 Case studies that explain L tectonite formation in crustal deformation zones.

Reference Degree of First-order, external Second-order, external Third-order, internal Third-order, internal depth kinematic framework boundary conditions rheologic variations structural setting Balk, 1949 1 Rising salt diapirs Center of diapir ———————————— ————————————

Balk, 1953 1 Rising salt diapirs Center of diapir ———————————— ————————————

Bouhallier et al., 2 Diapirs or sagduction Triple junctions between ———————————— ———————————— 1995 a ballooning diapirs

Bouhallier et al., 2 Diapiric ridges undergoing Centers of diapirc ridges and ———————————— ———————————— 1995 a horiz. shortening mantling rocks at ends of ridges

Braathen et al., 1 Lengthening shear ———————————— ———————————— ———————————— 2000

Brun et al., 1981 2 Diapirs or sagduction Triple junctions between ———————————— ———————————— ballooning diapirs

Dias and Ribeiro, 1 Vorticty-vector-parallel ———————————— ———————————— ———————————— 1994 shortening

Dumond et al., 2010 1 Lengthening shear ———————————— ———————————— ————————————

Fletcher and 2 Lengthening shear Not important L tectonites in Not important Bartley, 1994 homogeneous rocks

Fossen, 1993 1 Vorticty-vector-parallel ———————————— ———————————— ———————————— shortening

Holst and Fossen, 2 Noncoaxial ﬂattening Not signiﬁcant Not important L tectonites in fold 1987 hinge zones

Hoy et al., 1962 1 Rising salt diapir Center of diapir ———————————— ————————————

Lin and Jiang, 1 Transpression with Bend in shear zone ———————————— ———————————— 2001 vertical elongation

Mancktelow and 1 Lengthening shear ———————————— ———————————— ———————————— Pavlis, 1994

Passchier et al., 1997 2 Lengthening shear ———————————— ———————————— ————————————

Piazolo et al., 2004 1 ———————————— ———————————— Not important ————————————

Poli and Oliver, 3 Regional convergent Not important Not important L tectonites in fold 2001 pipe ﬂow hinge zones

Solar and Brown, 2 ———————————— ———————————— ———————————— L tectonites in fold 2001 hinge zones

Sullivan, 2006 3 Nearly coaxial Not important L tectonites in L tectonites in fold constriction homogeneous rocks hinge zones

Sullivan, 2008 2 Lengthening shear Not important rheologically driven strain Constriction in channel-form path partitioning quartzite beds

Sullivan, 2009 3 Lengthening shear Linear channel in zone Not important Not important boundary

Degree of depth: 1 e strain variations are side topic; 2 e strain variations are primary topic; 3 e L tectonites are primary topic ———————————— indicates that the data are insufﬁcient to draw conclusions; “Not important” indicates that the mechanism did not play an important role in creating L tectonites. a Bouhallier et al. (1995) described L tectonite formation in two different environments, and it is treated as two case studies.

different deformation phases. However, as with all case studies of separately. The remaining thirteen examples record approximately naturally deformed rocks, the complete deformation paths leading monoclinic external kinematic frameworks, and eleven of these to L tectonite formation cannot be totally constrained. Nineteen of record bulk deformation in the constrictional ﬁeld (Table 1). Two of the twenty-one case studies in Table 1 provide conclusive infor- these areas of bulk monoclinic constriction record coaxial mation about external kinematic frameworks. Six of these nineteen constriction or nearly coaxial pipe-style ﬂow. The other nine record case studies involve complex kinematic frameworks related to vorticity-parallel shortening, and seven of these underwent bulk density-driven vertical tectonics, and they will be considered lengthening shear style deformation (Table 1). The localities 172 W.A. Sullivan / Journal of Structural Geology 50 (2013) 161e175

Fig. 10. Block diagram cartoon illustrating the localization of constriction in an area of orogen-parallel extensional detachment faulting. Note that compositionally heterogeneous rocks accommodate subhorizontal transport-perpendicular shortening by folding with axes parallel with the maximum elongation direction while homogeneous rocks form L tectonite fabrics. The overlying detachment surface is also folded. The bulk deformation geometry is similar to the lengthening shear framework of Fig. 4a. Inspired by Mancktelow and Pavlis (1994) and Braathen et al. (2000).

described by Dias and Ribeiro (1994) and Fossen (1993) may also crust. Upper crustal rocks record simultaneous contractional record bulk lengthening shear style deformation, but these datasets faulting and extensional faulting. L tectonites occur in the ductile do not completely constrain the external kinematic frameworks high-strain zone beneath the detachment fault. This zone experi- beyond general vorticity-parallel shortening. Therefore, it seems enced bulk lengthening shear style deformation that combines that L tectonites are most likely to form under external kinematic horizontal shortening, minor vertical shortening, and simple shear frameworks that produce deformation in the constrictional field, parallel with the coaxial elongation direction. Both the detachment and that lengthening shear is the most common external kinematic surface and the underlying compositionally heterogeneous rocks framework leading to L tectonite formation. This has significant are folded while compositionally homogeneous rocks in the high- implications for interpreting very large domains of L>S and L tec- strain zone beneath the detachment fault form L tectonites. This tonites because lengthening shear kinematic geometries are most cartoon is also a good approximation of the style of deformation in likely to develop in extensional environments. Moreover, four of the purely extensional central Mojave metamorphic core complex the seven lengthening shear examples and one of the uncon- in the basin and range province of North America (Fletcher and strained vorticity-parallel shortening examples in Table 1 are Bartley, 1994). interpreted to have formed during orogen-parallel elongation at Six of the case studies in Table 1 are from areas of density- convergent plate boundaries. Hence, it seems that orogen-parallel driven vertical tectonics. Three of these are salt diapirs with L elongation is one of the most common regional settings for tectonites in the centers of the diapiric bodies (Balk, 1949, 1953; developing large domains of constrictional strain. Interestingly, L Hoy et al., 1962). Bouhallier et al. (1995) described deformation tectonites appear to be relatively rare in the gneiss domes and patterns in and around large-scale dome-and-basin-style struc- metamorphic core complexes related to orogen-parallel elongation tures in the Archean Dharwar craton that they interpreted as in the Himalayan orogen (M. Jessup personal communication, examples of density-driven vertical tectonics. In this area two 2011). Fig. 10 presents a model of L tectonite formation in a region distinct L tectonite fabrics are found in three environments: (1) of orogen-parallel extensional detachment faulting developed from a subvertical L tectonite fabric is found at triple points of steeply Mancktelow and Pavlis (1994) and Braathen et al. (2000). In this dipping high-strain zones between granitic domes (2) a sub- model there is a regional detachment between the upper and lower horizontal to shallowly plunging L tectonite fabric is found in the W.A. Sullivan / Journal of Structural Geology 50 (2013) 161e175 173 centers of laterally shortened diapiric ridges, and (3) a sub- forming domains of L tectonites outside of areas of density-driven horizontal to shallowly plunging L tectonite fabric is found in vertical tectonics. shallowly dipping high-strain zone triple points around these diapiric ridges. If the interpretation of vertical tectonics is correct, 4.2.3. Third-order mechanisms then these L tectonite domains did not form under a homogeneous Six case studies provide constraints on both categories of third- regional kinematic framework matching any simple monoclinic or order forcing mechanism d internal variations in rheology and triclinic model deformation. The steeply plunging L tectonites internal variations in structural setting (Table 1). None of these are record complex external kinematic frameworks partially driven by from areas of density-driven vertical tectonics. In five of these cases diapirism or sagduction, and bulk deformation between the dia- one or both of these third-order mechanisms were directly piric bodies was probably channel- or pipe-flow with nearly responsible for forming domains of L and L>>S tectonites without coaxial flow in the centers of the zones. Similar foliation triple the assistance of any second-order mechanism (Holst and Fossen, points and local L>>S tectonites are also found between gneiss 1987; Fletcher and Bartley, 1994; Poli and Oliver, 2001; Sullivan, domes in Finland (Brun et al., 1981). The shallowly plunging L 2006, 2008). This strengthens the conclusion that third-order tectonites described by Bouhallier et al. (1995) record regional mechanisms are more important than external variations in shortening and relative diapiric rise that also combine to create an boundary conditions in localizing constrictional deformation overall channel- or pipe-flow deformation geometry in and around outside of areas of density-driven vertical tectonics. the diapiric ridges. However, in the absence of detailed kinematic Interestingly, internal rheological variations played little or no analyses, these zones may be interpreted as triple points between role in forming L tectonites in the majority of cases where this can be transpressional shear zones. In such case, these structures would constrained (Table 1). The most common rheological mechanism be classified as examples of second-order variations in external leading to the formation of L tectonites appears to be the localization boundary conditions. of L tectonite fabrics in compositionally homogeneous units while Interestingly, a number of model kinematic geometries in Fig. 4a compositionally heterogeneous units accommodated constriction are not represented in Table 1. The most notable of these is thick- by folding (Fletcher and Bartley, 1994; Sullivan, 2006), but this is ening/narrowing shear. The other two thickening geometries are certainly not a statistically significant dataset. There is only one also absent. There are three potential explanations for this. First, unambiguous example of rheologically driven strain path parti- because gravitationally driven vertical shortening is such an tioning (Sullivan, 2008), but it could be argued that rheologically important deformation component, thickening shear zones tend driven strain path partitioning helps localize L tectonites in not to develop in purely contractional or extensional settings where compositionally homogeneous rocks. Fold hinge zones are by far the dip-slip fault systems dominate. Second, lower-crustal exposures in most common internal structural setting for L tectonite formation transtensional settings where thickening/narrowing shear is ex- (Holst and Fossen, 1987; Poli and Oliver, 2001; Solar and Brown, pected to dominate (Dewey, 2002) are almost entirely confined to 2001; Sullivan, 2006). In the Raft River shear zone, local domains metamorphic core complexes that are dominated by dip-slip of L>>S tectonites formed in channel-form metaconglomerate motion. Finally, it may be that spatial partitioning of strike-slip bodies (Sullivan, 2008), and this is the only other example of an and extensional components is the norm at transtensional internal structural setting helping to localize constriction. boundaries. 5. Where to find L tectonites 4.2. Importance of second- and third-order forcing mechanisms Finding L tectonite localities in the literature involves a good 4.2.1. Overview deal of luck, and a comprehensive survey of L tectonite occurrences The case studies in Table 1 provide significantly less information is simply not feasible. However, explicit references to L tectonites about the relative importance of second- and third-order forcing appear to be more common in certain areas or certain geologic mechanisms. Nevertheless, it is important to note that in every case environments. For example, explicit descriptions of L>S and L where second- or third-order mechanisms can be evaluated, one or tectonites are more common in the literature dealing with the more of these mechanisms concentrated a component of con- Norwegian Caledonides than other Phanerozoic orogenic belts (e.g., strictional deformation into discrete zones to form localized Flinn, 1961; Hossack, 1968; Chapman et al., 1979; Holst and Fossen, domains of L tectonites. Therefore, in most cases local variations in 1987; Sylvester and Janecky, 1988; Fossen, 1993; Braathen et al., boundary conditions, rheology, and/or structural setting are 2000). This could be related to the nature of this orogenic belt, or required to localize constriction and form L tectonites. it could be because some of the first studies of spatial variations in strain geometry come from this region and geologists working in 4.2.2. Second-order mechanisms the area are simply more likely to emphasize the presence of L Eleven case studies in Table 1 provide conclusive information tectonites. Extensive domains of L and L>S tectonites also appear to about the relative importance of second-order forcing mechanisms, be more common in Archean and Paleoproterozoic orogenic belts and local variations in external boundary conditions played an world wide (e.g., Bouhallier et al., 1995; Passchier et al., 1997; Lin important role in seven of these cases including all six examples of and Jiang, 2001; Poli and Oliver, 2001; Piazolo et al., 2004; density-driven vertical tectonics. However, there are only two Sullivan, 2006; Duclaux et al., 2007; Dumond et al., 2010). Given the examples of external boundary conditions playing an important global distribution of these localities and the diverse group of role in concentrating constrictional deformation to form L tecton- geologists studying them, it is unlikely that there is a systematic ites in shear zones. These are: (1) a bend in a transpressional shear bias in reporting L tectonites from these terranes. One possible zone (Lin and Jiang, 2001) and (2) a linear asperity in a shear zone explanation for the greater occurrence of L tectonites in very old boundary (Sullivan, 2009). Second-order forcing mechanisms orogenic belts is that the higher thermal gradients of these orogens played little or no role in forming domains of L tectonites in the two favored both orogen-parallel elongation and diapirism and the other shear zones where these mechanisms can be constrained formation of gneiss domes. Alternatively, it could be that con- (Table 1)(Fletcher and Bartley, 1994; Sullivan, 2008). Therefore, it strictional flow is more common in the lower half of the crust than seems reasonable to conclude that variations in external boundary in the upper half, and deep crustal exposures are more common in conditions are less important than third-order mechanisms in very old and deeply eroded orogenic belts. 174 W.A. Sullivan / Journal of Structural Geology 50 (2013) 161e175

6. Summary and conclusions from the J. David Love Foundation, the Gregg Ranch Foundation, the Geological Society of America, and the Mellon Foundation. The final A variety of mechanisms can form strong linear fabrics including version of this manuscript was improved by reviews from C. Fer- superimposed plane strain or even flattening strain deformations, nández and H. Fossen, but any remaining errors or oversights are but true constrictional deformation is distinguished by a unique set my own. of microstructural features and crystallographic fabric patterns. In lineation-normal thin sections basal planes of phyllosilicates and amphibole cleavage traces are randomly oriented and porphyr- References oclasts and dynamically recrystallized aggregates show no shape- Balk, R., 1949. Structure of Grand Saline salt dome, Can Zandt County, Texas. preferred orientation. The relationship between quartz crystallo- American Association of Petroleum Geologists Bulletin 33, 1791e1829. graphic fabric geometry and strain geometry is well established, Balk, R., 1953. Salt structure of Jefferson Island salt dome, Iberia Vermilion Perishes, Louisiana. American Association of Petroleum Geologists Bulletin 37, and quartz a-axis fabrics are less sensitive to variations in defor- e fl 2455 2474. mation temperature and the noncoaxiality of ow making them Barth, N.C., Hacker, B.R., Seward, G.E., Walsh, E.O., Young, D., Johnston, S., 2010. excellent indicators of constrictional deformation. Mica, amphibole Strain within the ultrahigh-pressure Western Gneiss region of Norway recorded and pyroxene crystallographic preferred orientations formed by Quartz CPOs. In: Law, R.D., Butler, R.W.H., Holdsworth, R.E., Krabbendam, M., Strachan, R.A. 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