Kinematic and Vorticity Analyses of the Western Idaho Shear Zone, USA

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THEMED ISSUE: EarthScope IDOR project (Deformation and Magmatic Modification of a Steep Continental Margin, Western Idaho–Eastern Oregon)

Kinematic and vorticity analyses of the western Idaho shear zone, USA

Scott Giorgis1,*, Zach Michels2, Laura Dair1, Nicole Braudy2, and Basil Tikoff2 1DEPARTMENT OF GEOLOGICAL SCIENCES, STATE UNIVERSITY OF NEW YORK AT GENESEO, 1 COLLEGE CIRCLE, GENESEO, NEW YORK 14454, USA 2DEPARTMENT OF GEOSCIENCE, UNIVERSITY OF WISCONSIN–MADISON, 1215 W DAYTON STREET, MADISON, WISCONSIN 53706, USA

ABSTRACT

The western Idaho shear zone (WISZ) is a Late Cretaceous, mid-crustal exposure of intense shear localized in the Cordillera of western North America. This shear zone is characterized by transpressional fabrics, i.e., downdip stretching lineations and vertical foliations. Folded and boudinaged late-stage dikes indicate a dextral sense of shear. The vorticity-normal section is identified by examining the three-dimensional shape preferred orientation of feldspar populations and the intragranular lattice rotation in quartz grains in deformed quartzites. The short axes of the shape preferred orientation ellipsoid gather on a plane perpendicular to the vorticity vector. In western Idaho this plane dips gently to the west, suggesting a vertical vorticity vector. Similarly, sample-scale crystallographic vorticity axis analysis of quartzite tectonites provides an independent assessment of vorticity and also indicates a subvertical vorticity vector. Constraints on the magnitude of vorticity are provided by field fabrics and porphyroclasts with strain shadows. Together these data indicate that the McCall segment of the WISZ dis- plays dextral transpression with a vertical vorticity vector and an angle of oblique convergence ≥60°. North and south of McCall, movement is coeval on the Owyhee segment of the WISZ and the Ahsahka shear zone. Together, the kinematics of these shear zones are consistent with northeast-southwest–directed convergence. Plate motion in this orientation acting on a curved plate boundary could have produced pure shear–dominated transpression in the Owyhee (a = 40°) and McCall (a = 60°) segments of the WISZ, while causing reverse-sense shearing (a = 90°) in the Ahsahka shear zone.

LITHOSPHERE; v. 9; no. 2; p. 223–234 | Published online 7 September 2016 doi: 10.1130/L518.1

INTRODUCTION The utility of a vorticity analysis is its ability to constrain the type of deformation undergone by a shear zone and make predictions of other Clear constraints on the vorticity of a high strain zone are fundamental quantities such as finite strain, infinitesimal strain, and the principal direc- to understanding the deformation history of that zone. Vorticity is a ten- tions of motion, known as flow apophyses. The flow apophyses define the sor quantity (e.g., Malvern, 1969) and therefore has both direction and relative motion of the shear zone walls with respect to each other. At the magnitude. In structural geology both the orientation and the magnitude plate boundary scale, flow apophyses characterize how the plate on one of vorticity provide constraints on the kinematics of deformation. First, side of a shear zone moves relative to the other, i.e., relative plate motion. the orientation of maximum spin defines a vorticity vector, i.e., the axis Strike-slip plate boundaries, such as the San Andreas fault, are dominated of maximum rotation (e.g., Tikoff and Fossen, 1995). Two-dimensional by simple shear. Obliquely convergent boundaries, such as the Alpine fault kinematic vorticity analysis typically assumes that the vorticity vector is zone in New Zealand, include components of pure shear (e.g., Little et al., oriented normal to the section in which data are gathered, that is normal 2005). Defining the relative plate motion, which is related to vorticity, is to the lineation-parallel, foliation-normal surface. Field fabrics, however, central to understanding the role of an ancient shear zone in the greater

may not be reliable indicators of vorticity orientation, as shown theoreti- tectonic history of the region. Mean vorticity (Wm) is a common measure cally by Lin et al. (1998) and others. Second, kinematic vorticity is a mea- of vorticity used in the structural geology community and it varies from sure of the relative amounts of the coaxial versus noncoaxial components zero (100% pure shear) to one (100% simple shear; e.g., Xypolias, 2010). acting in a shear zone (e.g., Means et al., 1980). This approach is used Transpression is a three-dimensional (3D) kinematic model often used because most shear zones lack fabric elements that provide quantitative to describe the flow of material at the plate tectonic scale (e.g., Sanderson information about the rates of rotation, but those same fabric elements and Marchini, 1984). Although there are many forms of transpressional can be used to distinguish between coaxial versus noncoaxial components deformation, for the purposes of this paper we refer to transpression as a of rotation (e.g., Passchier, 1987; Simpson and De Paor, 1993; Bailey et volume-constant, monoclinic system with shortening orthogonal to the al., 2004; Jessup et al., 2007; Xypolias, 2010; Iacopini et al., 2011; Li shear zone, vertical elongation, and no elongation parallel to the shear and Jiang, 2011). direction. For transpressional deformation, the angle of oblique convergence is used to quantify the relative plate motion (i.e., vorticity) of a system (e.g., Sanderson and Marchini, 1984; Fossen and Tikoff, 1993; *[email protected] Fig. 1). An angle of oblique convergence (a) of 0° describes a strike-slip

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up up 118°00′W 117°00′W 116°00′W115°00′W 114°00′W 47°00′N

Asahka Shear Zone

A Orofino north AN north T

vorticity IDAHO vorticity MON vector vector WASHINGTON 46°00′N 46°00′N (lineation α (lineation OREGON Grangeville α perdendicular) parallel) Columbia west west River Wrench dominated Pure shear dominated Basalt Riggins transpression transpression Fig. 3a Figure 1. Left: Block diagram showing wrench-dominated transpression. Accreted Right: Block diagram showing pure shear–dominated transpression. Both 45°00′N terranes 45°00′N result in vertical vorticity vectors, as well as straight and parallel flow lines McCall in the horizontal plane. Wrench-dominated transpression (at low finite strain values) results in the maximum stretching direction in the horizontal plane, and hence horizontal lineations. Pure shear–dominated transpres- Cascade sion always results in vertical maximum stretching and vertical lineations. Fig. 3b Sage Hen Reservoir 44°00′N 44°00′N plate boundary (W = 1), while an angle of 90° describes a purely con- m N Idaho

vergent system (Wm = 0). Transpressional and transtensional models of H A Boise batholith

deformation are ideal for plate-scale tectonics because they are the only I deformations characterized by straight and parallel flow lines in the hori- OREGON zontal plane (e.g., Fossen and Tikoff, 1998; Tikoff and Teyssier, 1998). Owyhee We present the results of a vorticity analysis of deformed rocks within 50 mi the western Idaho shear zone (WISZ; Fig. 2). We use this shear zone Mountains 43°00′N 43°00′N because (1) it has been modeled as a transpressional shear zone (e.g., 50 km Giorgis and Tikoff, 2004; Giorgis et al., 2005; Davis and Giorgis, 2014); (2) it has a primary bend in its orientations (Braudy et al., 2016; Schmidt et al., 2016), which should correspond to different kinematics (Benford KEY et al., 2010); and (3) it is locally well exposed. In this contribution, we Quaternary to Tertiary volcanic rocks and alluvium quantitatively constrain both the orientation of the vorticity vector and Eocene to Jurassic intrusive rocks the kinematic vorticity number for deformation in this shear zone. First, Jurassic to Permian accreted island-arc rocks we use 3D shape preferred orientation (SPO) of feldspar populations to estimate a vorticity vector and kinematic vorticity number. Second, we use Paleozoic and older North American continental rocks a statistically robust technique to determine the orientation of bulk vortic- Western Idaho shear zone and Ahsahka Shear zone ity vector by averaging the crystallographic rotation axes calculated from (& abrupt 87Sr/86Sr isotopic break) individual deformed grains (Michels et al., 2015, following Bestmann Figure 2. The western Idaho shear zone is located on the western edge of and Prior, 2003; Reddy and Buchan, 2005). The WISZ records a vertical the Idaho Batholith and connects with the Ahsahka shear zone (Schmidt vorticity vector and transpressional deformation (this study), while the et al., 2016) north of Grangeville, Idaho (modified from Tikoff et al., 2001; simultaneous and kinematically linked Ahsahka shear zone records thrust Benford et al., 2010). motion and a horizontal vorticity vector (Schmidt et al., 2016). Using the vorticity vector and the kinematic vorticity number studies together, we conclude that the majority of strain in the WISZ accumulated under (Fig. 2). South of the Sage Hen area on West Mountain (Fig. 3), the folia- transpressional conditions with an angle of oblique convergence of ~60° tion is oriented ~020, and dips steeply eastward. Lineation, also defined in the McCall region. by aligned amphiboles and stretched quartz, pitches steeply downdip to the east. Removal of the effects of tilting due to Miocene and Holocene MCCALL AND WEST MOUNTAIN FIELD AREAS Basin and Range extension yields a subvertical foliation and lineation (Tikoff et al., 2001). The WISZ extends from the Owyhee Mountains The WISZ near McCall, Idaho, deforms a series of granitic units on the south of Boise (Benford et al., 2010) to north of Riggins and is ideally western edge of the Idaho Batholith in the Late Cretaceous (Manduca et al., located to accommodate some of the Late Cretaceous northward transla- 1993; Giorgis et al., 2008). The Blue Mountain terrane is west of the shear tion of terranes in the Cordillera (Fig. 2; e.g., Wyld et al., 2006; Wright zone and on the basis of paleomagnetic data, is thought to have traveled and Wyld, 2007). hundreds of kilometers northward to its present location (Hillhouse et al., While there is general consensus that the WISZ is a transpressional 1982). Fabrics in the WISZ indicate dextral transpression (McClelland et structure (Lund and Snee, 1988; McClelland et al., 2000; Tikoff et al., al., 2000). Foliation, defined by aligned micas, amphiboles, and stretched 2001), there are few published constraints on the vorticity of this structure. quartz grains, strikes north-south and dips steeply to the east near McCall In Giorgis and Tikoff (2004) we used potassium feldspar SPO fabrics to

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A B

Figure 3. Geological maps and sample locations. PHD—porphyroclast hyperbolic distribution; SPO—shape preferred orientation. (A) From the McCall field area. (B) From the West Mountain field area. CVA—crystallographic vorticity analysis.

determine an angle of oblique convergence of 45°. More rigorous analysis, because the WISZ typically exhibits distributed, rather than localized, however, suggests that these fabrics are consistent with a wide variety of shearing. An exposure of the Sage Hen orthogneiss, located just south of vorticity values (Davis and Giorgis, 2014). Strain analysis of the strontium Cascade, exhibits clear dextral shearing during distributed deformation isotopic gradient suggests that this shear zone records a large amount of (Fig. 4). In this location, one mafic dike is folded and another is boudi- contraction (Giorgis et al., 2005). The Giorgis et al. (2005) study, however, naged on a subhorizontal, glacially polished surface. These dikes are not only provided information about the contractional component of deforma- sheared into parallelism with the main fabric, unlike most other features tion, not the strike-slip component, and therefore offers no insight into in the WISZ. Consequently, we interpret that the two mafic dikes intruded the vorticity of the system. late in the deformation history and only record a small proportion of the overall finite strain. The asymmetric boudin pods show right step-over SENSE OF SHEAR morphology and provide an unambiguous dextral shear sense. The orientations of the folded dike and boudinaged dike are also con- Shear sense indicators in the WISZ can be difficult to interpret. Sym- sistent with dextral transpression (Fig. 4). At this location, the strike of metric or domainal shear sense indicators are found on surfaces perpen- WISZ foliation is north-south, the boudinaged dike has a trend of ~340, dicular to foliation and parallel to lineation (e.g., Manduca et al., 1993). and the folded dike has a trend of ~060. Using the kinematic model of In contrast, asymmetric shear sense indicators are found on surfaces Tikoff and Fossen (1993) for dextral transpression along a north-south– perpendicular to both foliation and lineation (McClelland et al., 2000). oriented shear zone, shortening in the entire northeast-southwest quad- However, for an individual outcrop, the sense of shear can be ambigu- rants (000–090 and 180–270) is predicted and elongation is possible for ous. Furthermore, because the finite strain is so high in this zone, almost some orientations in the northwest-southeast quadrants (090–180 and all contacts have been rotated into parallelism to the overall trend of the 270–360). This prediction is consistent with the field observations. In shear zone, with the major contacts often reactivated as normal faults contrast, sinistral transpressional deformation will result in shortening (e.g., Tikoff et al., 2001). only in the northwest-southeast quadrants; thus, the finite strain field A few critical outcrops provide unambiguous evidence for dextral off- recorded by the dikes is incompatible with any sinistral component of set. For example, small outcrops of the Little Goose Creek complex show deformation. A contraction-only deformation (pure shear with vertical evidence for north-south–oriented localized structures with clear dextral elongation and horizontal shortening) will result in shortening only in the separation (McClelland et al., 2000). This outcrop is unusual, however, horizontal plane. Note that with more boudinaged and/or folded dikes it

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might be possible to more precisely constrain the vorticity of deformation A in addition to the sense of shear (e.g., Kuiper and Jiang, 2010). With a small set of observations, however, we come to the more limited conclu-

sion that this outcrop is consistent with dextral transpression (or dextral N simple shear) kinematics characterizing the deformation in the WISZ.

ORIENTATION OF VORTICITY VECTOR

Populations of Rigid Clasts

The orientation of the vorticity vector corresponds with the axis about which rotation occurs due to the simple shear component of deformation (e.g., Tikoff and Fossen, 1995). A plane oriented normal to this vector, i.e., the vorticity-normal section, contains the maximum amount of rota-

tion in the deformation. Data to constrain the 2D mean vorticity (Wm) of 0.5 m deformation in a monoclinic system should be collected in this plane (e.g., Xypolias, 2010). Numerical modeling of oblate spheroidal objects, such as feldspar porphyroclasts, suggests that the short axis of these objects B should gather in the vorticity-normal section for a contractional monoclinic deformation (Passchier, 1987). Although it is difficult to measure the N orientation of a single feldspar in the field or hand sample, it is possible calculate the SPO of a population of feldspars by gathering measurements on three orthogonal planes (e.g., Robin, 2002). The presence of large (1–3 cm) K-feldspar porphyroclasts is the defining characteristic of a portion of the Little Goose Creek Complex (Fig. 3; e.g., Manduca et al., 1993). In thin section, the mean grain size of the matrix surrounding these porphyroclasts is an order of magnitude smaller than the porphyroclasts (<1 mm) and they are rarely mantled with strain shadows (Giorgis and Tikoff, 2004). SPO measurements of the population of K-feldspar porphyroclasts, in three mutually perpendicular planes (~50 K-feldspar porphyroclasts per plane), were gathered at 23 stations across the WISZ (Giorgis and Tikoff, 2004). A 3D SPO ellipsoid was calculated 0.5 m for each station using the method of Robin (2002); particular attention was paid to the short axes. The short axes of these SPO ellipsoids collect in a shallow west-dipping plane (Fig. 5). After correction for the effects Map View C of post-Miocene rotation on normal faults (~15° of westward tilting; N Giorgis et al., 2006), the short axes of these SPO ellipsoids collect in a subhorizontal plane (Fig. 5). The pole to that plane is nearly vertical. We suggest that the pole to the plane defined by the short axis of the SPO ellipsoids represents the vorticity vector for the deformation. Numeri- cal models demonstrate that the short axes of the ellipsoid describing the SPO of a population of feldspars should collect in the vorticity-normal section (Giorgis and Tikoff, 2004). It is well documented that rigid objects will continue to rotate about the vorticity vector due to the simple shear WISZ F component of deformation (e.g., Jezek et al., 1994). Thus, the long axis of an individual rigid clast becomes attracted to the vertical direction because it is a fabric destination (Fossen et al., 1994) or attractor (Pass- abric chier, 1987). However, the rigid object keeps rotating around a vertical axis (in the horizontal plane). A population of rigid objects is therefore Shear Bands expected to have both short and intermediate SPO axes in the horizontal plane. In the case of the WISZ, the rigid K-feldspar porphyroclasts typically have a tabular geometry (i.e., the length of the maximum and intermediate axes are approximately equivalent), making the short axis of the SPO the most reliable indicator for vorticity analysis. If this interpretation is correct, the vorticity-normal section is horizon- Figure 4. (A, B) Photographs from critical locality in the Sage Hen ortho- tal for the WISZ, which implies that the vorticity vector is vertical and gneiss (West Mountain, Idaho; WISZ—western Idaho shear zone). Both subparallel to the stretching lineation (Fig. 5). Vertical foliation, vertical field photos are taken on a curviplanar, subhorizontal surface dipping ~8° to the southwest. Late mafic dikes show folding (A) and boudinage (B) stretching lineation, and a vertical vorticity vector are all consistent with patterns consistent with a distributed dextral component of wrench. Scales monoclinic transpression (e.g., Sanderson and Marchini, 1984; Fossen are approximate. (C) Schematic map relationship. and Tikoff, 1993).

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direction of lineation and map y-direction normal to the foliation plane). A. A B Data were acquired at the University of Wisconsin–Madison Department of Geoscience, using a Hitachi S3400N variable pressure scanning electron microscope (tungsten filament; 20 kV and 25 Pa conditions) equipped with an Oxford Instruments Nordlys II detector. Matrices of orientation maps were collected for each sample at step sizes ranging from 10 to 20 µm using a 3-step-overlap between maps. Map matrices were stitched together and processed to reduce noise (minimum nearest neighbor of 6) using the Channel5 (Oxford Instruments; www.oxford-instruments.com) software suite. On an average, the processed maps comprise 81% indexed solutions with a mean angular deviation <1°. C D Grain detection analyses were conducted using the free and open source software toolbox MTEX (4.0; http://mtex-toolbox.github.io/) (Hielscher, 2007; Schaeben et al., 2007; Hielscher and Schaeben, 2008; Bachmann et 3 al., 2010a) for MATLAB (MathWorks; http://www.mathworks.com). Grain 3 2 sets were constructed from electron backscatter diffraction orientation maps (e.g., Bachmann et al., 2010b) using a 10° misorientation thresh- old for grain boundary identification. Grains sets were filtered to identify 11 subsets in which each grain encompasses >3 orientation solutions. The 2 filtered grain sets comprise our selection of specimen-scale representative orientation data used for subsequent grain-scale CVA analysis and characterization of sample-scale quartz lattice preferred orientation. In the WISZ Figure 5. Lower hemisphere equal area plots of data from the McCall sec- samples, we calculate grain-scale crystallographic vorticity axes using the tion of the western Idaho shear zone. (A) Stereogram plots of poles to foliation corrected for Miocene to recent tilting. (B) Poles to lineation. All method described in Michels et al. (2015). We define the bulk vorticity axis contours are Kamb contours. (C) Rotation path of the short axes of three as the axis that matches the maximum orientation density of CVAs based different oblate spheroids deformed in monoclinic transpression (modified on nonparametric kernel density estimation using a 1° resolution and a de from Giorgis and Tikoff, 2004). Regardless of initial orientation, the short la Valée Poussin kernel with a 10° half-width (e.g., Michels et al., 2015). axes gather in the horizontal plane, i.e., in the plane perpendicular to the vorticity vector. (D) Stereogram plot of the tilt-corrected orientation of the Results short axis of the fabric ellipsoid (circles) measured from the shape preferred Quartzite samples in the WISZ were gathered from the Sage Hen orientation of populations of feldspar porphyroclasts in the Little Goose Creek complex. The best-fit plane to these axes (dashed line) is nearly metasedimentary screen on West Mountain (Fig. 3; Braudy et al., 2016). horizontal and the pole to that plane (diamond) is the vorticity vector. This screen contains pelite, marble, and quartzite, and is found in the Note that the vorticity vector is nearly parallel to the stretching lineation western portion of the Payette River tonalite. Both the metasedimentary (contours), consistent with pure shear–dominated transpression. screen and the surrounding tonalite are deformed along the eastern margin of the WISZ. Proterozoic detrital zircon ages from the quartzites clearly require a sedimentary source (Braudy et al., 2016). Crystallographic Vorticity Analysis Sample-scale representative populations of quartz grains were analyzed, such that CVA was calculated for each quartz grain with a grain Background boundary that encompasses three or more orientation solutions. For direct Crystallographic vorticity axis (CVA) analysis is a new quantitative comparison with regional data, we rotate the vorticity axes into a geo- method for identifying the position of grain-scale vorticity axes (one vor- graphic reference frame based on the orientation of foliation and lineation ticity axis per grain) in geologically deformed aggregates (Michels et al., measured in the field (Fig. 6). In the specimen reference frame, the bulk 2015). Specifically, CVA analysis uses rotational statistics to calculate the vorticity axes from the samples plots near the specimen x direction (i.e., position of a rotational axis that matches the intragranular dispersion of at the edge of the outer circle in a specimen equal area projection). In a crystallographic orientations in a single deformed grain. In Michels et al. geographic reference frame, the bulk vorticity axes plunge downdip in (2015) it was demonstrated that the preferred orientation from a specimen- the steeply oriented WISZ foliation. representative population of crystallographic vorticity axes can be used to The results of our CVA analyses indicate that the vorticity vector in track the position of the bulk vorticity vector in monoclinic shear zones each sample is close to parallel with the long axis of the stretching linea- (i.e., flow fields ranging from subsimple shear to pure shear–dominated tion. The subparallel relationship between vorticity and lineation defies transpression). The major advantage of this method is that it provides an the kinematic geometry necessary for simple shear–dominated deforma- objective quantitative determination of the vorticity vector orientation, tion (lineation is always perpendicular to the vorticity vector in simple independent of any presumptions about the relationship between vorticity shear). Alternatively, these vorticity results are entirely consistent with and fabric elements such as lineation, foliation, and crystallographic texture. pure shear–dominated transpression (parallel vertical stretching lineation and vertical vorticity vector). If post-Miocene fault movements are Methods restored, these CVAs have subvertical orientations. Crystallographic orientation maps were collected by means of electron backscatter diffraction analysis of three oriented quartzite samples from Quartz Crystallographic Preferred Orientation a metasedimentary screen in the WISZ near West Mountain. Analyzed sample surfaces were cut parallel to the macroscopic stretching lineation We present the following methods and data to (1) provide crystal- and perpendicular to the trace of foliation (i.e., map x-direction parallel to lographic texture context for the samples from which we calculate our

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Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/2/223/1001539/223.pdf by guest on 28 September 2021 GIORGIS ET AL. m m m es c-slip a a a es n = 33,155 n = 45,968 n = 35,289 tz slip systems m m m isorientation ax M misorientation ax c c c prism slip related to quar ) Max = 2.2 Max = 9.3 Max = 12.2 rence frame fe prism slip medium T (>500 ºC a

ticity-normal re e to n = 2582 grains n = 797 grains n = 814 grains CPO: vor DF rence frame fe stems relativ Max = 2.2 Max = 9.3 Max = 12.2 tz slip sy nematic re ki Quar rence frame c fe ) slip high T (> 650 ºC CPO: specimen re n = 2582 grains n = 797 grains n = 814 grains EG E

ral s,

xt icity axi

rt N

oliation f nematics ki n = 2582 n = 79 7 n = 81 4 tical vo transpressional r geographic es Ve compatible with de n ticity ax

vor -shear liation icity parallel to fo rt specimen with pure Vo lineation, compatible Max = 1.97 Max = 2.19 Max = 2.96 dominated transpressio B Figure 6. Data associated with deformed quartzite from West Mountain in the western Idaho shear zone. (A) Equal area projections of intragranular vorticity axes based on axes vorticity of intragranular projections (A) Equal area Idaho shear zone. Mountain in the western West from quartzite with deformed associated Data 6. Figure and geographic y direction) fabric positive view looking toward (left, frame in both specimen reference shown plots are sample, each For axis analysis. vorticity crystallographic triangles black In all plots, density. multiples of uniform to units and shading correspond contour foliation; of macroscopic the edge trace arcs Dashed (right). frame reference of the lineation. the orientation indicate circles and white the sample, axis for as the bulk vorticity density that is interpreted mean uniform the position of greatest show (C) Crystallographic (right). frame reference and geographic (left) frame appear in specimen reference would transpression shear–dominated pure (B) How Max—maximum. frame. reference vorticity-normal for (D) CPO pattern frame. a specimen reference for diffraction, backscatter electron by samples analyzed for (CPO) pattern orientation preferred (schematic occurs slip or prism slip [c] whether on depending patterns, CPO of interpretations dependent (T) Temperature (E) shown. are quartz of axes and [c] Both slip system. of active in terms axes of misorientation (G) Interpretation the [c] axes. around clusters data show (F) Misorientation 1986). and Casey, CPOs based on Schmid the CPO. interpret to way (D) is the correct frame reference and hence the vorticity-normal active, slip was that prism The data indicate AC

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CVA analyses; and (2) set the foundation for a discussion about quartz frame in order to assess the predominant crystal slip mechanisms. In this crystallographic preferred orientation (CPO) interpretation in pure shear– manner, the relationship between the preferred low-angle misorientation versus simple shear–dominated monoclinic transpressional deformation. axes and crystallographic axes can be used to determine the active quartz slip system (e.g., Nicolas and Poirier, 1976; Lloyd and Freeman, 1994; Methods Lloyd et al., 1997; Neumann, 1996; Fig. 6), independent from interpre- To quantitatively characterize the quartz crystallographic orientation tations made in the specimen, kinematic, or geographic reference frame. texture in the WISZ samples, we compute an orientation density function using the mean crystallographic orientations of quartz grains (one Results orientation per grain) in each sample. The orientation density function Intragranular misorientation axes in all three samples exhibit a pre- is calculated using 1° resolution and a symmetrized, antipodal kernel ferred association with quartz c-axis orientations (i.e., low-angle mis- density estimation with a de la Valée Poussin kernel and a 10° half-width. orientation axes form a cluster centered about the quartz c-axis) (Fig. 6). The density function is normalized to multiples of uniform density and This result indicates that the quartz c-axis is the main rotational axis about provides a measure by which we characterize and identify the patterns which lattice deformation occurs to accommodate intracrystalline slip. of quartz CPOs in the samples (Fig. 6). This crystallographic relationship with misorientation axes is characteristic of the prism slip ({m} slip) system in quartz (Fig. 6). However, Results the distribution of quartz c-axes in the specimen reference frame forms a We present quartz crystallographic texture data (Fig. 6) in both the cluster that is subparallel to the stretching lineation (i.e., fabric attractor; specimen (i.e., acquisition) reference frame and the kinematic reference Passchier, 1987) with axes normal to it (Fig. 6). This latter association frame (i.e., vorticity-normal section) in order to discuss the potential between quartz c-axes and stretching lineation is characteristic of prism pitfalls of interpreting deformation conditions from quartz CPO patterns [c] slip ({m}[c] slip) in quartz for simple shear–dominated deformation. that developed as a result of pure shear–dominated transpression (see The results of quartz CPO pattern interpretation for the WISZ samples Discussion). In all the samples, quartz c-axes form a clustered distribu- vary depending on the reference frame in which they are considered. Spe- tion centered about a direction that is subparallel to the bulk stretching cifically, when viewed in the typical X-Z fabric reference frame, the CPO lineation in the fabric and the calculated bulk CVA orientation, and quartz matches patterns developed during prism [c] slip at temperatures >650 °C; a-axes form a symmetric girdle of clusters normal to it. In a vorticity- whereas, when viewed in a vorticity-normal projection, the CPO pattern normal reference frame (i.e., a projection centered about the preferred matches those developed by dominantly prism slip at lower tem- CVA orientation), the concentration of quartz c-axes plots at the center peratures (> 500 °C). However, intragranular misorientation axes from of an equal area projection (Fig. 6), with a girdle of a-axes clusters that the WISZ samples indicate that prism slip dominated during crystal- traces a great circle arc along the outer edge of the plot. plastic deformation. This result suggests that plotting CPO in fabric ref- In simple shear and wrench-dominated transpression, the X-Z fabric erence frame is not always appropriate. The CPO plotted in the fabric ellipsoid reference frame is identical to the vorticity-normal (i.e., kinematic) reference plane would (erroneously) suggest that prism [c] slip was active reference frame, and serves as the most appropriate framework in which to in the WISZ. However, prism slip is the dominant slip system. The interpret kinematics and CPO patterns. For this reason, when the specific main reason is that for pure shear–dominated transpressional deformations, deformation geometry is unknown or assumed, the fabric X-Z plane is the X-Z (finite strain) plane is not coincident with the vorticity-normal commonly selected for microstructural investigation and analysis. How- plane (the Y-Z finite strain) plane. Our data suggest that a vorticity-normal ever, in pure shear–dominated transpression, the vorticity-normal surface kinematic perspective may be a more useful way to interpret CPO data. is, instead, coincident with the Y-Z plane of the fabric ellipsoid reference frame, with the vorticity axis parallel to the X-direction of the fabric ellip- MAGNITUDE OF VORTICITY soid. This orientation raises the question of which reference frame is most suitable interpreting the kinematic significance of quartz texture data for Fabric Orientation transpressional deformation. In the following n, we present an independent assessment of quartz slip systems based on a misorientation axis analysis in In transpression the orientation of the maximum stretching direction is a the crystallographic reference frame. In the Discussion, we explore some function of the vorticity of deformation (e.g., Fossen and Tikoff, 1993). In ramifications of reference frame selection for data analysis and potential pure shear–dominated transpression, i.e., transpression with a high degree of pitfalls of quartz texture interpretation in transpressional systems. contraction, the maximum stretching direction is always vertical. In wrench- dominated transpression, i.e., transpression with a high degree of strike-slip Misorientation and Slip-System Analysis of Quartz motion, the maximum stretching direction starts out horizontal and will switch to vertical at very high strain. The boundary between pure shear–

Methods dominated and wrench-dominated transpression occurs at Wm = ~0.8 or an Misorientation analysis provides a complimentary type of texture angle of oblique convergence (a) of ~20° (e.g., Fossen and Tikoff, 1993). characterization to the CPO analysis described here, and intragranular Assuming that the vertical stretching lineation observed in the field is misorientations can be used to constrain dominant slip systems during a good proxy for the maximum stretching direction, the WISZ is char- deformation independent of information about the orientations of folia- acterized by either very high strain wrench dominated transpression, or tion and lineation. A crystallographic misorientation is the 3D rotation pure shear–dominated transpression. Although porphyroclast-based strain between any two selected crystal orientations. Each misorientation rota- analyses in the Little Goose Creek Complex do not provide very precise tion can be described by an axis of rotation and an angular magnitude of results (Davis and Giorgis, 2014), they indicate that strains are not high rotation about that axis. For each quartz grain in the WISZ samples, we enough to produce a vertical lineation in a wrench-dominated transpres- calculate the misorientation between every measured lattice orientation sional deformation. This result suggests that the kinematics recorded by within the grain and the mean intragranular orientation for that same grain. the finite strain fabric of the WISZ are pure shear–dominated transpression

We consider the distribution of misorientation axes in the crystal reference (Wm <0.8 or a >20°).

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Porphyroclast Hyperbolic Distribution Method A up The porphyroclast hyperbolic distribution (PHD) method seeks to estimate the vorticity of a shear zone by defining the zone of back rota- north tion (Simpson and De Paor, 1993). This is a 2D approach that assumes that porphyroclasts are elliptical in cross section with no initial preferred orientation to their long axes. In simple shear all porphyroclasts rotate α in the same direction, i.e., clockwise for dextral simple shear zone, and there is no zone of back rotation. In pure shear, grains will rotate either west clockwise or counterclockwise depending on the orientation of their long axis and the aspect ratio of the grain. This approach is typically used in plane strain shear zones that exhibit some combination of pure and simple shear (e.g., Simpson and De Paor, 1993; Bailey et al., 2004). Transpression is also a mixture of pure and simple shearing. However, the elongation component of pure shear is located orthogonal to the maximum vorticity plane (e.g., vertical). The 2D numerical modeling based on Ghosh and Ramberg (1976) shows that the shortening component of the pure shear in transpression creates domains of clockwise versus counterclockwise rotation (Giorgis et al., 2004). Therefore in transpression, the relative size of the clockwise versus counterclockwise rotational domains is a function of the relative percentage of pure shear to simple shear. This ratio can be characterized as either the angle of oblique convergence (i.e., B oblique flow apophysis) or the kinematic vorticity number. However, the relationship between the angle of oblique convergence and the kinematic vorticity number is different from that of 2D general shear (Tikoff and Fossen, 1995). In our simplified analysis, we assume that the angle of the oblique flow apophyses separates the clockwise versus counterclockwise rotational domains.

5 mm 5 mm Methods The PHD analysis is based on measurements of the aspect ratio, orientation of the long axis, and sense of rotation of s-type porphyroclasts. Typically, these data are collected in thin section. The feldspars of the Little Goose Creek Complex, however, are too large for thin section analysis. In addition, most do not have strain shadows to provide information about the sense of rotation of that grain. The data from this study were measured directly from the outcrop surfaces oriented perpendicular

to foliation and lineation gathered over a 2-km-wide traverse across the 5 mm 5 mm shear zone (Figs. 3 and 7). By applying this method at the map scale rather than the thin section scale, we assume that deformation in the WISZ is homogeneous. Regional scale shear zones may subdivide into anastomosing networks of localized shear zones at the outcrop scale (e.g., Huddleston, 1999). Field observations in the McCall area of the C α = 60-90° clockwise rotated clast WISZ, however, show no evidence for a penetrative system of localized, counter clockwise rotated clast anastomosing shear zones at the outcrop scale, which suggests that our assumption of a homogeneous system is tenable.

Figure 7. Porphyroclastic hyperbolic distribution (PHD) vorticity analysis. 0° 180° (A) Block diagram of transpression (e.g., Sanderson and Marchini, 1984) D showing the straight and parallel flow lines in the horizontal plane. Por- 160 dextral sinistral phyroclasts with sufficiently large aspect ratios will either back rotate 80% (i.e., antithetic to the sense of shear) or forward rotate (synthetic to the 140 sense of shear) depending on their initial orientation. (B) Field photos of 70% rare porphyroclasts with strain shadows showing both a clockwise and 120 a counterclockwise sense of rotation. (C) Polar plot of 179 porphyroclasts 60% 100 with strain shadows. The orientation of the long axis of the porphyroclast 50% relative to local foliation (azimuth) is plotted against the aspect ratio of 80 the porphyroclast (radius). (D) Graph illustrating how well a given orienta- Matching Clasts (N ) tion of the convergent flow apophysis divides the porphyroclasts with a 020406080 100 120 140 160 180 clockwise versus counterclockwise sense of rotation. Orientation of Convergent Flow Apophysis (°)

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Results vorticity axis orientations derived from our analysis of fabric ellipsoids. The data are presented on a polar plot showing aspect ratio, orienta- Specifically, the crystallographic approach records the same kinematics tion, and sense of rotation (Fig. 7B). Ideally, the s-porphyroclasts with as the field SPO data, i.e., a nearly vertical vorticity vector parallel to the a counterclockwise sense of rotation will form a domain that is clearly stretching lineation. Moreover, both approaches suggest a nearly, but not separated from clockwise domain. Instead, we observe two overlapping quite, vertical vorticity vector. In both cases the vorticity vector consis- populations (Fig. 7B). Therefore we evaluate these data by investigating tently plunges steeply to the north and is not exactly parallel to the mean every possible orientation of the convergent flow apophysis and tabulate stretching lineation. This could imply some triclinic component to the how many observations are consistent with that orientation (Fig. 7C). deformation, suggesting that the monoclinic approach taken here may be For example, a convergent flow apophysis oriented at 5° to the shear oversimplifying the kinematics of the WISZ. zone boundary would suggest that all of the clasts measured in this study should have a clockwise sense of rotation. There are 79 clasts that agree Relationship Between Vorticity and CPO of Quartz with this interpretation, but the remaining 100 clasts, i.e., the clasts with Prior work has attempted to establish the relationships between defor- a counterclockwise sense of rotation, disagree (Fig. 7C). This analysis mation and quartz crystallographic textures in naturally deformed rocks. suggests that a convergent flow apophysis oriented in the range of 60°–120° In many cases, crystallographic patterns in quartzitic tectonites can be is consistent with the greatest number of observations. Orientations >90° compared with results from experimental deformation studies (e.g., Green indicate sinistral transpressional kinematics. Given the dextral nature of et al., 1970; Tullis et al., 1973) and numerical models (e.g., Etcheco- the WISZ, we therefore interpret these data to indicate an angle of oblique par, 1977; Lister et al., 1978) to adequately establish the relationship convergence in the range of 60°–90° (Fig. 8). between crystallographic texture, strain symmetry, and finite strain axes (e.g., reviews by Schmid and Casey, 1986; Law 1990). The vorticity- DISCUSSION normal section is regarded as the critical reference frame in which to interpret crystallographic patterns. For simple shear–dominated systems, Orientation of Vorticity Vector the vorticity-normal reference frame is always perpendicular to the long axis of the finite strain ellipsoid (i.e., the fabric attractor and direction of In this study we derive vorticity vectors in two different ways. The the macroscopic stretching lineation). In addition, when the kinematic first is based on the orientation of short axes of fabric ellipsoids calcu- reference frame is unknown, workers often assume a simple shear–con- lated from populations of oblate rigid clasts. These short axes fall on trolled relationship between vorticity and lineation; the vorticity vector is a great circle, the pole to which we interpret as a vorticity vector. The assumed to lie in the foliation plane, perpendicular to lineation. However, nearly vertical vorticity vector is subparallel to the stretching lineations. in the case of pure shear–dominated transpression, lineation and vorticity Field-based SPO analysis requires exposures with fracture and/or folia- are parallel, and the vorticity-normal reference frame is centered about tion surfaces that are nearly orthogonal. The rugged, glaciated region the vorticity vector and the fabric attractor. Therefore, without proper just north of McCall provides these types of outcrops in the Little Goose identification of the vorticity vector orientation (parallel to lineation in Creek Complex. Farther south, suitable outcrops are not available, and pure shear–dominated transpression), interpretations of quartz CPO that we are unable to apply this same technique. Rather, we apply the CVA are based on a presumed simple shear–type relationship between linea- approach to quartzites in the WISZ on West Mountain. CVA analysis has tion and quartz c-axes CPO may lead to incorrect conclusions about the not yet been experimentally validated, and only a few consistency argu- dominant active slip system and the kinematics of the deformation history. ments have been demonstrated in support of the method thus far (Michels Viewed in the fabric reference frame, the quartz CPO patterns in the et al., 2015; Schmidt et al., 2016). We note that our results provide a new WISZ samples closely match the predicted pattern for prism [c] slip in type of consistency argument in that the results of CVA analysis match simple shear (Fig. 6). That is, the parallel relationship between quartz c-axes maxima and the fabric attractor (lineation) in the WISZ samples is consistent with CPO patterns formed by activation of the prism [c] 0 90 Angle of Oblique Convergence slip system during high-temperature (>650 °C) simple shear deforma- 0.1 80 tion. In contrast, when the quartz CPO data from the WISZ samples are viewed in a kinematic reference frame (i.e., normal to the bulk vorticity 0.2 70 axis determined by CVA analysis), the quartz CPO pattern is effectively )

k 60 0.3 flipped ~90° about the pole to foliation (Fig. 6). In this perspective, the 0.4 50 pattern of quartz [c] and axes closely match patterns produced by 0.5 40 dominantly prism slip during simple to subsimple shear deformation. rticity ( W 0.6 Our misorientation analysis results resolve this issue: the WISZ sam- Vo 30 0.7 ples deformed by dominantly prism slip. The interpretation of slip 0.8 20 0.9 systems from misorientation axes patterns is conducted in the crystallo-

10 ( α ) graphic reference frame and is thus independent of assumptions about the Ahsahka Shear Zone McCall (PHD) Owyhee Mountains McCall (fabrics) 1.0 0 geometric relationship to bulk vorticity and lineation. Consequently, the Figure 8. Summary of vorticity constraints on the McCall seg- vorticity-normal section appears as the better reference frame in which ment of the western Idaho shear zone. Local constraints on the to interpret transpressional fabrics. Furthermore, the computational McCall segment suggest a strong component of pure shear in independence of CVA analysis relative to misorientation index and CPO transpression (i.e., α ≥60°). Fabrics from the Owyhee segment data provides an objective manner with which to interpret quartz CPO agree with these constraints, while crystallographic vorticity patterns in a transpressional shear zone. analysis from the Ahsahka shear zone suggests an angle of oblique convergence in McCall of ~60° ± 10°. Vorticity analyses The critical aspect of our analysis is that we cannot expect to ade- from all three segments of this plate boundary overlap and are quately interpret quartz CPO patterns if we presume simple shear–domi- consistent with each other. nated kinematics and fabric geometry in lieu of independent determination.

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The results of intragranular misorientation analysis should broadly agree the Owyhee segment indicates that local angle of oblique convergence in with any slip system inferences made by interpreting CPO patterns, and that section must also be >20°. Both the Owyhee and McCall segments of therefore represent a useful double-check for the relevance of a chosen the WISZ are active at the same time (Giorgis et al., 2008; Benford et al., reference frame. Moreover, when there is reason to suspect that defor- 2010), and therefore it is reasonable to assume that these two segments had mation may deviate significantly from simple shear, an independent vor- different orientations during deformation. This inference is confirmed by ticity determination can be critically useful for identifying an appropri- the preservation of a primary bend in the shear zone in the Sage Hen area ate kinematic reference frame. An advantage of using CVA analysis for of West Mountain (Fig. 3; Braudy et al., 2016). Consequently, as assumed independent vorticity determination is that it can be conducted using the by Benford et al. (2010), the deformation may result from a regionally same crystallographic data as those used for CPO and misorientation consistent plate motion. If that is the case, then the McCall segment must analyses. The agreement between CPO patterns viewed in a vorticity- record an angle of oblique convergence of >40° for the Owyhee segment normal perspective and the misorientation analyses supports the overall to have an angle >20° (Figs. 8 and 9) (Benford et al., 2010). Results from theme of this paper: the WISZ is a pure shear–dominated transpression the PHD analysis support the conclusion that the WISZ is characterized zone in which vorticity and lineation are subparallel. by a strong contractional component of deformation (a = 60°–90°). Although these results are consistent from a regional tectonics perspec- Regional Implications tive, there are potential problems with using 2D data from the vorticity- normal section to characterize a 3D deformation such as transpression. Kinematic Vorticity Estimates Li and Jiang (2011) highlighted potential problems that could arise with The magnitude of vorticity provides information about the relative this method by failing to take into account the 3D shape of the rigid clasts contributions of coaxial versus noncoaxial deformation. For deformations used in a vorticity analysis. Specifically, the apparent 2D cross section such as transpression, the degree of coaxiality of deformation also pro- of an ellipsoid in the vorticity-normal section will change throughout a vides information about the orientation of relative plate motion recorded progressive deformation as that ellipsoid rotates in three dimensions. That by the shear zone (i.e., the angle of oblique convergence). The vertical is, the PHD method assumes that the ellipsoid rotates about an axis that stretching lineation and vertical foliation observed in the WISZ further is perpendicular to the plane of view. The 3D nature of ellipsoid rotation, south in the Owyhee segment provide further constraints (Benford et al., however, can also affect the PHD method. This method relies on breaking 2010). This segment of the WISZ is oriented at 020 as compared to 000 porphyroclasts into clockwise versus counterclockwise cohorts. Li and in the McCall segment. The presence of a vertical stretching lineation in Jiang (2011), however, found that triaxial clasts may rotate clockwise in

vorticity axes from CVA analysis

Figure 9. Schematic map of the western Idaho shear zone system, showing the Owyhee segment (oriented ~020), the McCall seg- Ahsahka ment (oriented ~000), and the Ahsahka segment (oriented ~330). Shear Zone CVA—crystallographic vorticity axis. A rigid plate converging with IDAHO α = 90° MONTANA WASHINGTON vorticity axis the North American craton at an OREGON N azimuth 060, in present-day coor- dinates, could produce the varying kinematics along a curved plate boundary; a = angle of oblique McCall vorticity axes convergence in transpression. The Segment from CVA analysis Owhyee and McCall segments are characterized by transpressional α = 60˚ deformation and nearly vertical vorticity vectors, while the Ahsahka segment is characterized by simple shear (?) deformation and horizontal vorticity vectors. Traces of shear zones are from Tikoff et al. (2001), α = 40° Owyhee Benford et al. (2010), and Schmidt et al. (2016). Segment vorticity

IDAHO OREGON axis

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Fleck, R.J., and Criss, R.E., 1985, Strontium and oxygen isotopic variations in Mesozoic and Ter- Lund, K., 2004, Geologic setting of the Payette National Forest, west-central Idaho: U.S. Geo- tiary plutons of central Idaho: Contributions to Mineralogy and Petrology, v. 90, p. 219–308. logical Survey Professional Paper 1666, p. 1–44. Fossen, H., and Tikoff, B., 1993, The deformation matrix for simultaneous simple shearing, Lund, K., and Snee, L.W., 1988, Metamorphism, structural development, and age of the con- pure shearing and volume change, and its application to transpression-transtension tec- tinent–island arc juncture in west-central Idaho, in Ernst, W.G., ed., Metamorphism and tonics: Journal of Structural Geology, v. 15, p. 413–422, doi:10.1016 /0191 -8141 (93)90137-Y. crustal evolution of the western United States: Rubey Volume VII: Englewood Cliffs, New Fossen, H., and Tikoff, B., 1998, Extended models of transpression and transtension, and Jersey, Prentice-Hall, p. 296–331. application to tectonic settings, in Holdsworth, R.E., et al., eds., Continental transpres- Malvern, L.E., 1969, Introduction to the mechanics of a continuous medium: Englewood Cliffs, sional and transtensional tectonics: Geological Society, London, Special Publication 135, New Jersey, Prentice-Hall, 713 p. p. 15–33, doi:10.1144/GSL.SP.1998.135.01.02. Manduca, C.A., Kuntz, M.A., and Silver, L.T., 1993, Emplacement and deformation history of Fossen, H., Tikoff, B., and Teyssier, C.T., 1994, Strain modeling of transpression and transten- the western margin of the Idaho batholith near McCall, Idaho: Influence of a major ter- sional deformation: Norsk Geologisk Tidsskrift, v. 74, p. 134–145. rane boundary: Geological Society of America Bulletin, v. 105, p. 749–765, doi:10.1130 Gaschnig, R.M., Vervoort, J.D., Lewis, R.S., and Tikoff, B., 2013, Probing for Proterozoic and /0016-7606(1993)105<0749:EADHOT>2.3.CO;2. Archean crust in the northern US Cordillera with inherited zircon from the Idaho batho- McClelland, W.C., Tikoff, B., and Manduca, C.A., 2000, Two-phase evolution of accretionary lith: GSA Bulletin, v. 125, p. 73–88. margins: Examples from the North American Cordillera: Tectonophysics, v. 326, p. 37–55, Ghosh, S.K., and Ramberg, H., 1976, Reorientations of inclusions by combinations of pure doi:10.1016/S0040-1951(00)00145-1. shear and simple shear: Tectonophysics, v. 17, p. 133–175. Means, W.D., Hobbs, B.E., Lister, G.S., and Williams, P.F., 1980, Vorticity and non-coaxiality Giorgis, S., and Tikoff, B., 2004, Constraints on kinematics and strain from feldspar por- in progressive deformation: Journal of Structural Geology, v. 2, p. 371–378, doi:10.1016 phyroclast populations, in Alsop, I., et al., eds., Transport and flow processes in shear /0191-8141(80)90024-3. zones: Geological Society, London, Special Publication 224, p. 265–285, doi:10.1144 /GSL Michels, Z.D., Kruckenberg, S.C., Davis, J.R., and Tikoff, B., 2015, Determining vorticity axes .SP.2004.224.01.17. from grain-scale dispersion of crystallographic orientations: Geology, v. 43, p. 803–806, Giorgis, S., Markley, M., and Tikoff, B., 2004, Vertical-axis rotation of rigid crustal blocks driven doi:10.1130/G36868.1. by mantle flow, in Grocott, J., et al., eds., Vertical coupling and decoupling in the litho- Neumann, B., 1996, Texturbildende Prozesse in rekristallisierten Quarz- polykristallen-Einzel- sphere: Geological Society, London, Special Publication 227, p. 83–100, doi:10.1144 /GSL korn-und Gesamttexturanalysen: Geotektonische Forschungen 87, 154 p. .SP.2004.227.01.05 Nicolas, A., and Poirier, J.P., 1976, Crystalline plasticity and solid state flow in metamorphic Giorgis, S., Tikoff, B., and McClelland, W.C., 2005, Missing Idaho arc: Transpressional modi- rocks: London, Wiley, 462 p. fication of the87 Sr/86Sr transition on the western edge of the Idaho batholith: Geology, Passchier, C.W., 1987, Stable positions of rigid objects in non-coaxial flow: A study in vorticity v. 33, p. 469–472, doi:10.1130/G20911.1. analysis: Journal of Structural Geology, v. 9, p. 679–690, doi:10 .1016 /0191 -8141 (87)90152 -0. Giorgis, S., Tikoff, B., Kelso, P., and Markley, 2006, The role of material anisotropy in the Reddy, S.M., and Buchan, C., 2005, Constraining kinematic rotation axes in high-strain zones: neotectonic extension of the western Idaho shear zone, McCall, Idaho: GSA Bulletin, v. A potential microstructural method?, in Gapais, D., et al., eds., Deformation mechanisms, 118, p. 259–273. rheology and tectonics: From minerals to the lithosphere: Geological Society, London, Giorgis, S., McClelland, W., Fayon, A., Singer, B., and Tikoff, B., 2008, Timing of deformation Special Publication 243, p. 1–10, doi:10.1144/GSL.SP.2005.243.01.02. and exhumation in the western Idaho shear zone, McCall, Idaho: Geological Society of Robin, P.F., 2002, Determination of fabric and strain ellipsoids from measured sectional —The- America Bulletin, v. 120, p. 1119–1133, doi:10 .1130 /B26291 .1. ory: Journal of Structural Geology, v. 24, p. 531–544, doi:10.1016 /S0191 -8141(01)00081 -5. Green, H.W., II, Griggs, D.T., and Christie, J.M., 1970, Syntectonic and annealing recrystal- Sanderson, D.J., and Marchini, W.R.D., 1984, Transpression: Journal of Structural Geology, lization of fine-grained quartz aggregates, in Paulitsch, P., ed., Experimental and natural v. 6, p. 449–458, doi:10.1016/0191-8141(84)90058-0. rock deformation: Berlin, Springer, p. 272–335. Schaeben, H., Hielscher, R., Fundenberger, J.J., Potts, D., and Prestin, J.U.R., 2007, Orienta- Hielscher, R., 2007, The radon transform on the rotation group—Inversion and application tion density function-controlled pole probability density function measurements: Auto- to texture analysis [Ph.D. thesis]: Freiberg, Germany, Technische Universität Bergakad- mated adaptive control of texture goniometers: Journal of Applied Crystallography, v. 40, emie Freiberg. p. 570–579, doi:10.1107/S0021889807019711. Hielscher, R., and Schaeben, H., 2008, A novel pole figure inversion method: Specification Schmid, S.M., and Casey, M., 1986, Complete fabric analysis of some commonly observed of the MTEX algorithm: Journal of Applied Crystallography, v. 41, p. 1024–1037, doi:10 quartz c-axis patterns, in Hobbs, B.E., and Heard, H.C., eds., Mineral and rock deforma- .1107/S0021889808030112. tion: Laboratory studies: The Paterson volume: American Geophysical Union Geophysical Hillhouse, J.W., Gromme, C.S., and Vallier, T.L., 1982, Paleomagnetism and Mesozoic tectonics Monograph 36, p. 263–286, doi:10.1029 /GM036p0263. of the Sevin Devils volcanic arc in northeastern Oregon: Journal of Geophysical Research, Schmidt, K.L., Lewis, R.S., Vervoort, J., Stetson-Lee, T.A., Michels, Z.D., and Tikoff, B., 2016, v. 87, p. 3777–3794, doi:10 .1029 /JB087iB05p03777. Tectonic evolution of the Syringa Embayment in the central North American Cordilleran Hudleston, P.J., 1999, Strain compatibility and shear zones: Is there a problem?: Journal of accretionary boundary: Lithosphere, doi:10.1130/L545.1, (in press). Structural Geology, v. 21, p. 923–932. Simpson, C., and De Paor, D.G., 1993, Strain and kinematic analysis in general shear zones: Iacopini, D., Frassi, C., Carosi, R., and Montomoli, C., 2011, Biases in three-dimensional vor- Journal of Structural Geology, v. 15, p. 1–20. ticity analysis using porphyroclast systems: Limits and application to natural examples, Tikoff, B., and Fossen, H., 1993, Simultaneous pure and simple shear: The unifying deformation in Prior, D.J., et al., eds., Deformation mechanisms, rheology, and tectonics: Microstruc- matrix: Tectonophysics, v. 217, p. 267–283, doi:10.1016/0040-1951(93)90010-H. tures, mechanics and anisotropy: Geological Society, London, Special Publication 360, Tikoff, B., and Fossen, H., 1995, The limitations of three-dimensional kinematic vorticity analy- p. 301–318, doi:10.1144/SP360.17. sis: Journal of Structural Geology, v. 17, p. 1771–1784, doi:10.1016 /0191 -8141(95)00069 -P. Jessup, M.J., Law, R.D., and Frassi, C., 2007, The rigid grain net (RGN): An alternative method Tikoff, B., and Teyssier, C., 1998, Strike-slip partitioned transpression of the San Andreas

for estimating mean kinematic vorticity number (Wm): Journal of Structural Geology, v. 29, fault system: A lithospheric-scale approach, in Holdsworth, R.E., et al., eds., Continental p. 411–421, doi:10.1016/j.jsg.2006.11.003. transpression and transtensional tectonics: Geological Society, London, Special Publica- Jezek, J., Melka, R., Schulmann, K., and Venera, Z., 1994, The behavior of rigid triaxial ellip- tion 135, p. 143–158, doi:10.1144/GSL.SP.1998.135.01.10. soidal particles in viscous flows—Modeling of fabric evolution in a multiparticle system: Tikoff, B., Kelso, P., Manduca, C., Markley, M.J., and Gillaspy, J., 2001, Lithospheric and crustal Tectonophysics, v. 229, p. 165–180, doi:10.1016/0040-1951(94)90027-2. reactivation of an ancient plate boundary: The assembly and disassembly of the Salmon Kuiper, Y.D., and Jiang, D., 2010, Kinematics of deformation constructed from deformed planar River suture zone, Idaho, USA, in Holdsworth, R., et al., eds., The nature and tectonic and linear elements: The method and its application: Tectonophysics, v. 492, p. 175–191, significance of fault zone weakening: Geological Society, London, Special Publication doi:10.1016/j.tecto.2010.06.009. 186, p. 213–231, doi:10.1144/GSL.SP.2001.186.01.13. Law, R.D., 1990, Crystallographic fabrics: A selective review of their applications to research Tullis, J., Christie, M., and Griggs, D.T., 1973, Microstructures and preferred orientations in structural geology, in Knipe, R.J., and Rutter, E.H., eds., Deformation Mechanisms, Rhe- of experimentally deformed quartzites: Geological Society of America Bulletin, v. 84, ology and Tectonics: Geological Society, London, Society Publications, v. 54, p. 335–352. p. 297–314, doi:10.1130/0016-7606(1973)84<297:MAPOOE>2.0.CO;2. Li, C., and Jiang, D., 2011, A critique of vorticity analysis using rigid clasts: Journal of Struc- Wright, J.W., and Wyld, S.J., 2007, Alternative tectonic model for Late Jurassic through Early tural Geology, v. 33, p. 203–219, doi:10.1016/j.jsg.2010.09.001. Cretaceous evolution of the Great Valley Group, California, in Cloos, M., et al., eds., Con- Lin, S., Jiang, D., and Williams, P.F., 1998, Transpression (or transtension) zones of triclinic vergent margin terranes and associated regions: A tribute to W.G. Ernst: Geological So- symmetry: Natural example and theoretical modeling, in Holdsworth, R.E., et al., eds., ciety of America Special Paper 419, p. 81–95, doi:10 .1130 /2007 .2419(04). Continental transpressional and transtensional tectonics: Geological Society, London, Wyld, S.J., Umhoefer, P., and Wright, J.E., 2006, Reconstructing northern Cordilleran terranes Special Publication 135, p. 41–57, doi:10.1144/GSL.SP.1998.135.01.04. along known Cretaceous and Cenozoic strike-slip faults: Implications for the Baja B.C. Lister, G.S., Paterson, M.S., and Hobbs, B.E., 1978, The simulation of fabric development in hypothesis and other models, in Haggart, J.W., et al., eds., Paleogeography of the North plastic deformation and its application to quartzite: the model: Tectonophysics, v. 45, p. American Cordillera: Evidence for and against large-scale displacement: Geological As- 107–158, doi:10.1016/0040-1951(78)90004-5. sociation of Canada Special Paper 46, p. 277–298. Little, T.A., Cox, S., Vry, J.K., and Batt, G., 2005, Variations in exhumation level and uplift rate Xypolias, P., 2010, Vorticity analysis in shear zones: A review of methods and applications: along the oblique-slip Alpine fault, central Southern Alps, New Zealand: Geological So- Journal of Structural Geology, v. 32, p. 2072–2092, doi:10 .1016 /j .jsg.2010.08 .009. ciety of America Bulletin, v. 117, p. 707–723, doi:10.1130 /B25500 .1. Lloyd, G.E., and Freeman, B., 1994, Dynamic recrystallization of quartz under greenschist con- MANUSCRIPT RECEIVED 27 JANUARY 2016 ditions: Journal of Structural Geology, v. 16, p. 867–881, doi:10 .1016 /0191 -8141 (94)90151 -1. REVISED MANUSCRIPT RECEIVED 27 MAY 2016 Lloyd, G.E., Farmer, A.B., and Mainprice, D., 1997, Misorientation analysis and the formation MANUSCRIPT ACCEPTED 3 AUGUST 2016 and orientation of subgrain and grain boundaries: Tectonophysics, v. 279, p. 55–78, doi: 10.1016/S0040-1951(97)00115-7 Printed in the USA

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