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 conver- gence 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 LITHOSPHERE© 2016 Geological | Volume Society 9 of| AmericaNumber 2| |For www.gsapubs.org permission to copy, contact [email protected] 223 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. 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 trans pression. Accreted Right: Block diagram showing pure shear–dominated trans pression. 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 O vergent system (Wm = 0). Transpressional and transtensional models of H A Boise batholith D 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).