Abstract

The Syringa embayment (Idaho, USA) is in the Mesozoic accretionary margin of western North America, where the north-south–oriented lithospheric boundary bends abruptly to an east-west orientation near the 46th parallel. New geologic mapping, structural analysis, and laser ablation–inductively coupled plasma–mass spectrometry U-Pb zircon age data constrain the origin and prolonged evolution of this embayment. We agree with previous workers that the Syringa embayment may have initiated as an inherited Proterozoic rift boundary, producing a kink in the north-south–oriented continent margin. Structural analysis on the northwest-oriented Ahsahka shear zone that is adjacent to the accretionary boundary indicates dominantly reverse, southwest-vergent motion, as confirmed by crystallographic vorticity axis analysis on quartzites. New U-Pb zircon age dating brackets the age of deformation along the Ahsahka shear zone to between ca. 116 and 92 Ma. These dates indicate that deformation on the Ahsahka shear zone occurred simultaneously with deformation on the western Idaho shear zone to the south. Consequently, the Ahsahka and western Idaho shear zones are a continuous, albeit kinked, mid-Cretaceous shear zone system that maintained kinematic compatibility during oblique dextral convergence along the margin. The Syringa embayment has an additional younger structural history, specifically movement on a pair of northeast-trending dextrally transpressive structural zones, the Limekiln and Mount Idaho zones. The Mount Idaho deformation zone truncates the Ahsahka–western Idaho shear zone. Following truncation, continued orthogonal contraction was partitioned to the northeast of the Ahsahka shear zone in the northwest-trending Clearwater zone, which is bracketed by existing age data to ca. 73–54 Ma.

INTRODUCTION

The Mesozoic accretionary boundary in west-central Idaho (USA) has played an important role in tectonic reconstructions of the North American Cordillera (e.g., Burchfiel and Davis, 1972; Davis et al., 1978; Oldow et al., 1989; Jones et al., 1977; Brooks and Vallier, 1978; Dickinson, 1979). The mostly north-south–oriented boundary is unique along the North American accretionary margin in that accreted oceanic and fringing arcs are juxtaposed directly against continental North America along the nearly vertical Jurassic–Early Cretaceous Salmon River suture zone (Lund and Snee, 1988; Fig. 1). This structure was reactivated in the mid-Cretaceous by the western Idaho shear zone to accommodate dextral transpression in response to the accretion and translation of terranes along the western North American margin (McClelland et al., 2000; Tikoff et al., 2001; Giorgis et al., 2008).

At approximately the 46th parallel, the Salmon River suture zone abruptly bends to the west, forming a corner (Armstrong et al., 1977; Fleck and Criss, 2007; Mohl and Thiessen, 1995) referred to as the Syringa embayment (Fig. 1; Lund et al., 2008). Proposed origins for the bend in the boundary include Mesozoic rifting (Davis et al., 1978), oroclinal bending (Schmidt et al., 2003, 2009), northeast-directed subduction (Strayer et al., 1989), truncation of the north-south–striking accretionary boundary by sinistral strike-slip displacement on a west-northwest–striking structure (the Orofino shear zone; McClelland and Oldow, 2004, 2007; Snee et al., 2007), and a restraining bend at the northern end of the dextral transpressional western Idaho shear zone (Lund et al., 2008).

On the basis of new geologic mapping, structural analysis, and U-Pb age data, we test these hypotheses. Our salient conclusion is that the many structures that occur in the Syringa embayment can be separated in time and space, and that bending of the accretionary boundary in the embayment has progressively increased through its history. We agree that some aspects of previous models are correct, but other aspects do not fit with our new spatial, timing, and kinematic constraints for these structures. Thus, we agree with Lund et al. (2008) that Mesozoic initiation of the embayment during its early accretionary history was likely inherited from Precambrian–early Paleozoic tectonics, but argue that rather than functioning as a transpressional termination during later dextral transpression on the western Idaho shear zone, it was kinematically compatible with dextral transpression farther south. Similarly, we agree with McClelland and Oldow (2007) that the western Idaho shear zone is truncated, but argue that the individual structures that they include in the broad, northwest- to west-striking structural zone that they define as the Orofino shear zone were operating at distinctly different times and with different kinematics. We believe instead that the northeast-striking Mount Idaho zone was responsible for truncation of the western Idaho shear zone, and contributed to progressive bending of the embayment.

GEOLOGIC SETTING

The Salmon River suture zone was initially defined by the juxtaposition of disparate lithologic assemblages of oceanic versus continental origins (e.g., Vallier, 1977; Lund and Snee, 1988) and a dramatically sharp gradient in initial Sr isopleths that indicate a nearly vertical lithospheric boundary (e.g., Armstrong et al., 1977; Criss and Fleck, 1987). These features are continuous from the north-south–oriented Salmon River segment through the bend to the east-west–oriented Orofino segment of the suture zone. Three major tectonic assemblages occur across this lithospheric boundary (Fig. 1). From outboard (west) to inboard (east), these include the Blue Mountains oceanic arc assemblages, high-grade gneiss belt, and Laurentian metasedimentary assemblages intruded by plutons of the Idaho batholith.

In the outboard position well to the west and southwest of the lithospheric boundary, the Blue Mountains province (Fig. 1; Silberling et al., 1984) consists of Paleozoic and Mesozoic island arcs and basins that formed both in proximity to the Laurentian margin (Olds Ferry, Izee, and Baker terranes) and in a distal intraoceanic setting (Wallowa terrane; Dickinson, 1979; Hillhouse et al., 1982; Stanley and Whalen, 1989; Vallier, 1995; Schwartz et al., 2010; LaMaskin et al., 2008, 2011). Amalgamation of these terranes occurred in the Late Jurassic (Avé Lallemant, 1995; Schwartz et al., 2010) or earlier (LaMaskin et al., 2015), followed by intrusion of a distinct suite of diorite and quartz diorite plutonic rocks of Jurassic–Cretaceous age (Brooks and Vallier, 1978; Walker, 1986; Vallier, 1995). Most rocks in these terranes have undergone lower greenschist facies metamorphism. The Wallowa terrane is in the northern part of the Blue Mountains province and within the Syringa embayment. It comprises Permian plutonic basement overlain by Permian to Late Triassic arc-related volcanic and sedimentary rocks of the Seven Devils Group, Late Triassic to Early Jurassic carbonate platform to basin sedimentary rocks, and Late Jurassic clastic sedimentary rocks (Vallier, 1977, 1995; Walker, 1986; Morrison, 1963; Whalen, 1988; Follo, 1994; White and Vallier, 1994; Goldstrand, 1994).

Along the Laurentian margin east and northeast of the accretionary boundary, amphibolite facies metasedimentary rocks of the Mesoproterozoic Belt Supergroup and equivalents and Neoproterozoic Windermere Group equivalents are intruded by the Idaho batholith (Lund et al., 2003, 2008; Lewis et al., 2010). The Late Cretaceous to Paleocene Idaho batholith includes metaluminous and peraluminous phases of the Atlanta lobe and an early metaluminous phase of the Bitterroot lobe that range in age from 100 to 67 Ma and the main peraluminous phase of the Bitterroot lobe that ranges from 66 to 54 Ma (Fig. 1; Gaschnig et al., 2010). With the exception of early metaluminous phase intrusions, most plutons in the batholith are unfoliated, and biotite 40Ar/39Ar closure ages generally range from 80 to 50 Ma (Snee et al., 1995).

A high-grade gneiss assemblage of probable Mesozoic age occurs along the isotopically defined lithospheric boundary (Fig. 1). It comprises hornblende schist and gneiss, amphibolite, marble, and quartzite that are outboard of the north-south–oriented boundary (Hamilton, 1963; Onasch, 1987; Selverstone et al., 1992; Manduca et al., 1993; Lund, 1995) and continues through the Syringa embayment to the north as the Orofino metamorphic assemblage, which straddles the isotopic boundary (Anderson, 1930; Hietanen, 1962; Snee et al., 2007; Kauffman et al., 2009). The rocks of the high-grade gneiss assemblage have sedimentary and possibly volcanic protoliths, and most agree that they may be parts of the Wallowa and other terranes. Along the north-south–oriented accretionary boundary south of the Syringa embayment, this assemblage occurs at the highest structural levels of a west-vergent thrust belt developed along the eastern margin of the Blue Mountains terranes (Hamilton, 1963; Lund and Snee, 1988; Onasch, 1987; Kauffman et al., 2014; Schmidt et al., 2016). In the Syringa embayment, the northeastern and northern margins of the Wallowa terrane are largely covered by rocks of the Miocene Columbia River Basalt Group, and it is unknown whether this thrust belt continues in the embayment. However, the Ahsahka thrust, a southwest-vergent fault (Strayer et al., 1989) that occurs within the high-grade gneiss assemblage, separates rocks of similar metamorphic grade, but differing metamorphic fabrics and 40Ar/39Ar cooling histories (Fig. 1; Davidson, 1990; Snee et al., 2007; Kauffman et al., 2009). As constrained along the Salmon River suture zone, oldest metamorphic ages for the high-grade gneiss assemblage range from 144 to 136 Ma (Getty et al., 1993). Peak metamorphic pressures were attained by 128 ± 3 Ma, followed by rapid exhumation and cooling, with the biotite 40Ar/39Ar system closing by ca. 100 Ma (Selverstone et al., 1992; Getty et al., 1993; Snee et al., 1995). Eastern and northeastern parts of the gneiss assemblage along both the north-south–oriented boundary and in the Syringa embayment are intruded by magmatic epidote-bearing quartz diorite, diorite, and tonalite-trondhjemite plutons emplaced at 8–11 kbar (Manduca et al., 1992; Selverstone et al., 1992; Getty et al., 1993) and dated by U-Pb zircon methods as ca. 120–110 Ma (Manduca et al., 1993; Lee, 2004; McClelland and Oldow, 2007; Unruh et al., 2008; Kauffman et al., 2009, 2014).

Along the north-south–oriented accretionary boundary, the Salmon River suture zone coincides with the isotopically defined boundary and is intruded by tabular mid-Cretaceous plutons that are elongate parallel to the suture zone (Lund and Snee, 1988; Manduca et al., 1993; Snee et al., 1995; Lund, 2004). These plutons intrude rocks of the high-grade gneiss assemblage to the west and Laurentian wall rocks to the east and obscure older fabrics associated with the initial accretion of outboard terranes. The age of this suturing is controversial, but is regarded to be constrained by the oldest garnet metamorphic ages in the high-grade gneiss assemblage to the west (144–136 Ma; Getty et al., 1993). A relatively narrow zone (∼5 km) of ductile deformation, the western Idaho shear zone (McClelland et al., 2000; Giorgis et al., 2005, 2008), is in the tabular suture zone plutons and minor country-rock screens. Fabrics in this zone consist of steeply east-southeast–dipping foliation and downdip-plunging lineation and include transpressional fabrics that are crystal plastic in ca. 105 Ma plutons and magmatic in ca. 90 Ma plutons (Giorgis et al., 2008). The western Idaho shear zone is interpreted as a spatially and temporally overprinting deformation zone that is relatively late in the protracted deformation that has occurred along the lithospheric boundary (McClelland et al., 2000; Giorgis et al., 2008), although not all agree on the degree to which this structure overprints previous structures (e.g., see Gray et al., 2012).

The nature of the suture zone and western Idaho shear zone as they continue north into the Syringa embayment is not well known due to extensive cover, and is debated. Hypotheses that address this question can be distilled down to two basic ideas. McClelland and Oldow (2004, 2007) and Snee et al. (2007) posited that the Salmon River suture zone and overprinting western Idaho shear zone originally continued north along the margin and were truncated and offset to the west in the Late Cretaceous by their proposed west-northwest–striking sinistral transpressional Orofino shear zone; they included the Ahsahka thrust as well as the Clearwater zone (Fig. 1; Strayer et al., 1989; Davidson, 1990; Snee et al., 2007) in this shear zone and speculated that the Orofino shear zone continues west from the northwest end of the Ahsahka thrust under Columbia River Basalt Group rocks (Fig. 1). Their main evidence for truncation of the accretionary margin by the Orofino shear zone was based on the discovery of deformation within the Clearwater zone that is considerably younger than deformation along the north-south–oriented boundary farther south. Yates (1968) originally recognized the pronounced northwest trend in the Clearwater zone, which he termed the trans-Idaho discontinuity, and Dickinson (2004) suggested that this structure reactivated a Neoproterozoic transform fault of the nascent Cordilleran margin. Sims et al. (2005) and Lund et al. (2008) attributed Late Cretaceous to Paleocene deformation in the Clearwater zone to east-west–oriented contraction in the Syringa embayment.

In contrast to the truncation mechanism proposed by McClelland and Oldow (2004, 2007), Lund et al. (2008) argued that the embayment was originally inherited from reactivation of structures associated with Neoproterozoic–early Paleozoic rifting. During deformation on the Salmon River suture zone and western Idaho shear zone, the embayment served as a transpressional termination zone and underwent focused contraction as a result of clockwise rotation of the Wallowa terrane against Laurentia during dextral oblique translation. In this model, ensuing orthogonal contraction in the embayment produced a crustal wedge of oceanic rocks that delaminated Laurentian crust and formed the Coolwater structural culmination in the Late Cretaceous (Fig. 1). The antiformal Coolwater culmination exposes paragneiss derived from multiple sources, including arc terranes and Precambrian Laurentia. A tectonic wedge of oceanic rocks is thought to have been inserted into the Laurentian margin between 98 Ma (protolith age of arc terrane paragneiss) and 73 Ma (youngest age of deformed plutons). Crustal thickening, melting, and intrusion within the wedge, and subsequent folding to form the Coolwater culmination, continued until ca. 61 Ma (zircon rim analyses; Lund et al., 2008).

We assess both of these models for the Syringa embayment by compiling a large body of recent geologic mapping in the Syringa embayment, apply structural and microstructural analysis to shear zones that are both previously known and newly discovered in the embayment, and constrain the ages of these shear zones using laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb zircon methods applied to deformed and undeformed plutons.

METHODS

Structural Analysis

Geologic Mapping and Outcrop-Scale Structural Analysis

We compiled recent mapping in the Syringa embayment (completed by Lewis and Schmidt and others) at 1:24,000 scale and published in a series of 100,000 scale maps by the Idaho Geological Survey (Fig. 2; Lewis et al., 2005, 2007a, 2007b; Kauffman et al., 2009, 2014). We have also selected important locations for which we have constructed more detailed maps with accompanying stereonet plots depicting structural elements (see Data Repository Figs. DR1a, DR1b, DR1c1). Our focus for macrostructural studies included characterizing and measuring fabrics in shear zones, describing kinematic indicators in outcrop and in oriented thin sections, and determining relative age relationships among structures.

Microstructural Studies

Because of their importance in the evolution of the Syringa embayment, we focused microstructural studies on two shear zones, the Ahsahka shear zone and Clearwater zone. In general, rocks of this region are deformed plutonic rocks, with minor metasedimentary rocks. However, we targeted known quartzite localities within these shear zones, which allowed us to conduct quantitative microkinematic analyses. Four sample sites were selected (Fig. 2), one in the Ahsahka shear zone (12-TSL-8) and three in the Clearwater zone (12-TSL-29, 13-TSL-70, and 12-TSL-7).

Crystallographic vorticity axis analysis. Crystallographic vorticity axis (CVA) analysis is a new method for investigating deformation kinematics at the microstructural scale (Michels et al., 2015). This method determines a rotation axis in the specimen reference frame that best characterizes crystal lattice dispersion within a single grain (intragranular crystallographic orientations). Specifically, the CVA analysis method utilizes a type of principal components analysis in the non-Euclidean space of rotations (principal geodesic analysis; Fletcher et al., 2004) to compute the best-fit grain-scale vorticity axis. A bulk sample-scale vorticity axis can be derived from the preferred orientation of all measured individual dispersion analyses using directional statistics (Michels et al., 2015).

Electron backscatter diffraction. Crystallographic orientation maps were collected by electron backscatter diffraction (EBSD) analysis from thin sections cut parallel to the macroscopic stretching lineation and perpendicular to the trace of foliation (i.e., map x-direction parallel to direction of elongation and/or lineation and map y-direction normal to the foliation plane). Data were acquired at University of Wisconsin–Madison (Geoscience Department), 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 25 µm using a 3-step overlap between maps and stitched together and processed to reduce noise (minimum nearest neighbor of 6) using the Channel 5 (Oxford Instruments; www.oxford-instruments.com) software suite. On an average, the processed maps comprise 76% indexed solutions with a mean angular deviation <1.

Grain detection analysis was conducted using the free and open source software toolbox MTEX (v4.0) (Bachmann et al., 2010a; Hielscher, 2007; Schaeben et al., 2007; Hielscher and Schaeben, 2008) for Matlab (Mathworks; www.mathworks.com). Grain sets were constructed from EBSD orientation maps (e.g., Bachmann et al., 2010b) using a 10° misorientation threshold for grain boundary identification. Grains were filtered to identify subsets in which each grain encompasses >3 orientation solutions. The subset grains comprise the data selected for subsequent grain-scale crystallographic vorticity analysis.

CVAs. CVA analysis was conducted on quartz grains in the four samples. We calculated populations of grain-scale vorticity axes from a specimen-scale selection of grains (233–446 grains per sample) and estimated a preferred specimen-scale vorticity axis in the specimen reference frame using nonparametric kernel density estimation (de la Vallée Poussin kernel with a 10° half-width). For direct comparison with field data and other results in this paper, we also present the vorticity axes relative to a geographic reference frame (i.e., Lower Hemisphere equal-area projection).

Quartz crystallographic fabric and slip system analysis. The crystallographic preferred orientation (CPO) of quartz was quantitatively characterized by calculating an orientation distribution function using 1° resolution and a symmetrized, antipodal kernel density estimation with a de la Vallée Poussin kernel and a 10° half-width. The function is normalized to multiples of uniform density and provides a measure by which to characterize and identify patterns of crystallographic orientations in the samples.

We use the distribution of low-angle (2°–10°) intragranular quartz misorientation axes to infer the dominant slip system during deformation (e.g., Nicolas and Poirier, 1976; Lloyd and Freeman, 1994; Lloyd et al., 1997; Neumann, 1996). Each crystallographic misorientation corresponds to a three-dimensional rotation between two selected crystal orientations and can be represented by an axis of rotation and a magnitude of rotation about that axis. When considered in the crystal reference frame, the preferred orientations of low-angle misorientation axes identify the dominant crystal directions associated with lattice rotation and propagation of dislocations resulting from intragranular deformation.

U-Pb Zircon LA-ICP-MS Analyses

Our strategy for determining lower and upper age constraints on shear zones in the Syringa embayment was to target both deformed and undeformed plutonic rocks within these shear zones for U-Pb zircon LA-ICP-MS analyses.

Rock samples were collected in quantities of 5–10 kg, and zircons were isolated using standard crushing, milling, and mineral separation procedures. Grains hand-picked under a binocular microscope were mounted in epoxy with standards, polished to expose centers, carbon coated, and imaged with scanning electron microscopy–cathodoluminescence (SEM-CL) at the University of Idaho (Moscow, Idaho). Analyses were conducted at the Washington State University GeoAnalytical Laboratory with a New Wave Nd:YAG UV 213 nm laser coupled to a Thermo Finnigan Element 2 single-collector, double-focusing, magnetic sector ICP-MS with a 30 μm laser spot diameter and 10 Hz repetition rate following the methods of Chang et al. (2006). Time-dependent fractionation was corrected for using the intercept method, and time-independent fractionation was corrected by normalizing unknown analyses to bracketing standards (Peixe, 564 Ma, Dickinson and Gehrels, 2003; FC1, 1099 Ma, Paces and Miller, 1993), with Peixe serving as the primary zircon standard. Common Pb corrections were considered unnecessary given the concordance of most results and comparatively small magnitude of adjustments (<0.2 m.y.) that would be made if a 207Pb-based common Pb correction was applied to 206Pb/238U ages of the few slightly discordant analyses.

The final ages are weighted means of pooled 206Pb/238U ages calculated using Isoplot (Ludwig, 2003), except where noted. Weighted mean ages incorporate all analyses in the main data clusters observed in Tera-Wasserburg diagrams to avoid biasing the results with potentially subjective data filtering. Weighted means (along with the mean square of weighted deviates and probabilities of fits) were calculated using internal uncertainties only (estimated by the standard errors of sample analyses). The standard deviations of the analyses of zircon standards that bracket the unknowns were then added quadratically to the weighted mean 2σ errors to calculate the total uncertainties cited later in the text.

RESULTS

Structural Relationships in the Syringa Embayment

Our geologic map compilation and structural analysis add considerably more detail to the geology of the Syringa embayment (Fig. 2). In general, the tectonostratigraphic assemblages and plutonic belts along the north-south segment of the Salmon River suture zone to the south continue through the Syringa embayment as far as pre-Cenozoic exposures can be traced before they disappear completely beneath Columbia River Basalt Group cover rocks to the west. The embayment is bordered to the north and northeast by a complex structural belt consisting of numerous high-strain zones of varying width. In this section we detail the spatial relationships and characterize the fabrics, kinematics, quartz crystallographic analysis results, and relative age constraints of these shear zones from oldest to youngest in apparent age. Note that assemblages that trend approximately north-south inboard and adjacent to the embayment, such as the Neoproterozoic Windermere rocks and their equivalent assemblages, are not truncated or offset by east-southeast–west-northwest–striking shear zones that border the embayment (Fig. 1). Also notable in the detailed geometry of the Syringa embayment shown in Figure 2 is the segmented nature of the border of the embayment, with the various segments differing significantly in trend. Two newly recognized northeast-striking structural zones in the Wallowa terrane, the Limekiln structural zone to the northwest and the Mount Idaho structural zone to the southeast, divide the border of the embayment into these segments of varying trend. South of the Mount Idaho structural zone the boundary is north-northeast trending. Within the Syringa embayment north of this structure the lithospheric boundary is northwest trending, and west of the Limekiln structural zone it is trending nearly due west (Fig. 2).

Woodrat Mountain Shear Zone

Along the northwest-striking segment of the Syringa embayment, rocks of the arc-related Orofino series and continental Neoproterozoic Syringa metamorphic sequence are juxtaposed along the northwest-striking Woodrat Mountain shear zone (Fig. 2). The Woodrat Mountain shear zone is a moderately to steeply east-northeast–dipping mylonite belt that may form one of the few relict segments of the Salmon River suture that has not been obscured by intrusion, deformation, or deposition of younger rocks (Fig. 2; Lewis et al., 2007a). The Woodrat Mountain shear zone is characterized by moderately to steeply northeast-dipping mylonite foliation and steeply pitching stretching lineation. Kinematic indicators such as mica fish, σ-type clasts, and S-C fabrics show consistent top-to-the-southwest sense of shear parallel to mineral stretching lineation (Lewis et al., 2014). East of the town of Orofino, an intruded segment of the shear zone is apparently folded by a regional-scale S-shaped fold (Fig. 2; Lewis et al., 2007a). Fabrics in wall rock to either side of the shear zone and in the pluton that intrudes the shear zone are not folded, but rather strike north-northwest and cut across the shear zone. We consider this overprinting fabric to have formed as part of the Clearwater zone, discussed in the following. These relationships suggest that the Woodrat Mountain shear zone is the oldest identifiable shear zone along this segment of the lithospheric boundary.

Ahsahka Shear Zone

Along the northwest segment of the Syringa embayment, Davidson (1990) and Snee et al. (2007) mapped the Ahsahka thrust as the southwest limit of well-developed mylonitic fabrics in the high-grade gneiss assemblage (Fig. 2). We mapped this structural zone as the 4–7-km-wide mylonitic Ahsahka shear zone, which deforms Early Cretaceous orthogneiss and amphibolite facies calcareous hornblende gneiss and schist, calc-silicate gneiss and schist, and local quartzite and marble layers, with the Ahsahka thrust delineating the southwest margin of the shear zone as originally defined by Davidson (1990) (Data Repository Fig. DR1a; Kauffman et al., 2009). The northeast margin of the shear zone is defined by a gradual decrease in fabric intensity and is less well constrained than the southwest margin. The isotopic boundary along this part of the margin occurs within the Ahsahka shear zone. Extensive younger rock cover of the northwest end of the shear zone prevents determining whether it bends to the west to follow the east-west–oriented segment of the boundary or joins the Woodrat Mountain shear zone. This structure was ascribed to part of the Orofino shear zone by McClelland and Oldow (2007).

Fabrics in the Ahsahka shear zone consist of mylonite and mylonite gneiss. Foliation defined by mineral bands and platy mineral alignment strikes northwest and dips ∼40°–70° northeast. Lineation is defined by elongate minerals such as hornblende, muscovite, and biotite that typically plunge downdip (Fig. 3A; Data Repository Fig. DR1a; Kauffman et al., 2009; Stetson-Lee, 2015). Quartz microstructures in mylonites include domains of undulose extinction and uncommon deformation lamellae (Fig. 3B). Grain boundaries are dominantly interlobate and occasionally amoeboid, and commonly show evidence of bulging grain boundary migration. Feldspar grains are generally fractured, and typically are surrounded by anastomosing biotite when both minerals are present. These microstructures collectively indicate that the rocks were subjected to medium-grade metamorphism that caused crystallographic reorientation of quartz and brittle deformation of feldspar. Orofino series rocks southwest of the shear zone lack pervasive gneissic foliation, but are cut by numerous discrete northeast-dipping ductile shear zones.

Shear-sense indicators on foliation-perpendicular and lineation-parallel surfaces in the Ahsahka shear zone display consistent reverse (top to southwest) sense of shear (e.g., Fig. 3C). Indicators include σ-type grains on feldspar porphyroclasts, imbricated porphyroclasts, sigmoidal foliation between adjacent shear bands, and small-scale offset of veins along shear bands. Thin sections cut parallel to this orientation show consistent reverse shear sense, including σ-type pressure shadows, mica fish, and weakly developed S-C fabric. On lineation-normal surfaces, shear-sense indicators were much less prevalent and inconsistent in orientation. Lineation-normal thin sections generally display clearly defined foliation, but rarely contain consistent shear-sense indicators. These data support southwest-directed, reverse shearing in the Ahsahka shear zone, consistent with the findings of Strayer et al. (1989), Davidson (1990), and Snee et al. (2007).

Results from quartz crystallographic vorticity vector analysis of quartzite sample 12-TSL-8 from the northwestern end of the Ahsahka shear zone exhibit subhorizontal vorticity vectors that trend northwest-southeast (Fig. 4A). Thus, vorticity axes are within the plane of foliation and perpendicular to the lineation in the Ahsahka shear zone, compatible with interpretations of Cretaceous southwest-directed (northeast over southwest) reverse motion for the Ahsahka shear zone (Strayer et al., 1989; Davidson, 1990; Snee et al., 2007).

Quartz crystallographic data from the sample 12-TSL-8 are shown in Figure 4 relative to both a specimen (Fig. 4B; approximately vorticity-normal) and geographic (Fig. 4C) reference frame. Intragranular misorientation axes are also shown in Figure 4D and are plotted relative to the crystallographic reference frame in order to visualize the coincidence between misorientation and crystallographic axes. In this Ahsahka shear zone sample, quartz <c> axes form a point maximum subparallel to the inferred vorticity axis (normal to specimen reference surface) and <a> axes form point concentrations in a girdle orthogonal to it (subparallel to specimen reference surface). Low-angle misorientation axes show a strong association with the <c> axis direction and indicate that intragranular slip was predominantly accommodated by lattice rotation about quartz <c> axes. These data are consistent with CPO and misorientation axis patterns developed during prism <a> slip (dislocation propagation along {m} planes in the direction of the <a> axis). The inference that prism <a> slip was dominant in quartz constrains deformation conditions in the Ahsahka shear zone to temperatures of ∼450–650 °C (Bouchez, 1977; Lister and Hobbs, 1980; Behrmann and Platt, 1982; Lister and Dornsiepen, 1982; Mainprice et al., 1986). These results are consistent with interpretations of amphibolite metamorphic conditions from previous studies (Hietanen, 1962; Strayer et al., 1989; Davidson, 1990; Lund et al., 2003; Snee et al., 2007). All of the quartz microstructural data (CPO, intragranular misorientation axes, CVA analyses) are consistent with dominantly simple shear deformation (Lister and Hobbs, 1980; Law et al., 1984; Law, 2010), and when rotated into geographical space indicate reverse, top-to-the-southwest shearing on the Ahsahka shear zone.

In the region east of the town of Orofino, a series of kilometer-scale Z-shaped folds deform metamorphic layering and mylonite fabrics within the Ahsahka shear zone (Data Repository Fig. DR1a; Kauffman et al., 2009). The regional S-shaped fold that folds the Woodrat Mountain shear zone farther east does not appear to fold fabrics and metamorphic layering within the Ahsahka shear zone. Thus, relative fabric and structural relationships suggest that deformation initiated on the Woodrat Mountain shear zone early in the structural history of the embayment followed by folding on the S-shaped fold. The Ahsahka shear zone came later, either accompanied by or followed by folding on Z-shaped folds.

Northeast-Striking Structural Zones

The pair of northeast-striking structural zones, Limekiln zone to the north and the Mount Idaho zone to the south, cross the Syringa embayment (Fig. 2). Evidence for these structures includes faulting and folding of the Mesozoic and older rock assemblages of the Wallowa terrane. Both zones record an extended history of deformation, including an episode of Late Cretaceous contraction with dextral strike-slip displacement. Miocene cover sequences of the Columbia River Basalt Group are deformed along parts of these structural zones, interpreted to reflect later structural reactivation.

The Limekiln structural zone forms the southeast margin of the Neogene Lewiston basin and includes a series of northeast-striking faults and shallowly northeast-plunging folds and monoclines in rocks of the Miocene Columbia River Basalt Group along the Waha escarpment (Kauffman et al., 2009). There is evidence, however, for an older history along the Limekiln structural zone in that Mesozoic volcanic and sedimentary rocks of the Wallowa terrane show reverse-dextral offset. Where the structural zone crosses the Snake River, the Limekiln fault is nearly completely covered, but is constrained to be a northeast-striking, steeply northwest-dipping fault zone that juxtaposes Middle to Late Triassic rocks in the in hanging wall with latest Triassic–earliest Jurassic rocks in the footwall. We estimate 10–15 km of dextral oblique displacement, constrained by restoration of the hanging wall rocks back to their location in the regional homoclinal northwest-dipping section. Vertical offset on the fault is limited to a few kilometers, due to the similarity in metamorphic grade across the shear zone (Kauffman et al., 2009). The projected Limekiln structural zone does not appear to crosscut the Ahsahka shear zone in the region around Orofino. However, it does coincide with the northwest to west bend in tectonic assemblages and structures north of the town of Orofino (Fig. 2).

The Mount Idaho structural zone to the south is well exposed east and south of the town of Grangeville (Fig. 2; Kauffman et al., 2014), where the ∼2-km-wide mylonite shear zone contains moderately southeast-dipping foliation and moderately east-northeast–plunging mylonite lineation (Data Repository Fig. DR1b). Well-developed kinematic indicators include sigma-shaped clasts, S-C fabrics, and extensional crenulations that consistently show reverse-dextral, top-to-the-west-southwest shear sense. The Mount Idaho shear zone continues to the northeast, where it truncates the Salmon River suture zone and overprinting western Idaho shear zone, with dextral strike separation estimated to be 10–20 km (Kauffman et al., 2014). The Mount Idaho structural zone delineates the southeastern extent of the Syringa embayment. Where the structural zone crosscuts the suture zone, the accretionary boundary changes orientation from north-northeast–striking to the south to northwest-striking to the north. Farther to the northeast, the Mount Idaho structural zone is disrupted by ca. 73–61 Ma faults and shear zones in the Clearwater zone and intruded by plutons of the Paleogene Bitterroot lobe of the Idaho batholith that are as young as 54 Ma (Lewis et al., 2007a; Gaschnig et al., 2010).

The southwest continuation of the Mount Idaho structural zone appears to have reactivated the Hammer Creek thrust and Klopton Creek thrust. These faults carry Permian–Triassic plutonic basement rocks northwest over Triassic–Jurassic volcano-sedimentary rocks within the Wallowa terrane early in their histories (Kauffman et al., 2014). We think that later deformation on these structures likely included dextral oblique southeast-over-northwest sense of shear. Kurz (2001) determined a dextral oblique component to the thrust slip vector in a fracture pattern analysis study along the Klopton Creek fault segment (calculated slip vector rakes ∼43° in the mean fault plane).

Clearwater Zone

Inboard and northeast of the Woodrat Mountain fault along the northwest-trending segment of the Syringa embayment are several younger northwest-striking faults and shear zones that occur in the Clearwater zone (Fig. 2; Sims et al., 2005; Lund et al., 2008). These structures deform Cretaceous orthogneiss (Fig. 3D) and amphibolite facies quartzite, schist, and calc-silicate gneiss of the Neoproterozoic Syringa metamorphic series. In most of the individual shear zones that compose the Clearwater zone foliation and lineation fabrics are defined by quartz subgrain boundaries and mica orientations (Fig. 3E). Quartz grains exhibit undulose extinction and dominantly interlobate to amoeboid grain boundaries that are consistent with medium- to high-grade metamorphism. Shear-sense indicators on foliation-perpendicular and lineation-parallel surfaces include S-C fabrics and σ-type grains, and display consistent reverse, top-to-the-southwest shear sense (Fig. 3F). Shear-sense indicators were much less prevalent and inconsistent in orientation on lineation-normal surfaces.

One of the larger shear zones, the Browns Creek Ridge shear zone, comprises a several kilometers wide belt of mylonite with steeply northeast-dipping foliation and steeply pitching lineation (Data Repository Fig. DR1c). Kinematic indicators consistently show northeast-side-up kinematics parallel to the mylonitic lineation (Lewis et al., 2007a). McClelland and Oldow (2007) reported ages of 73.4 ± 1.2 Ma and 72.9 ± 1.7 Ma (U-Pb zircon SHRIMP, sensitive high-resolution ion microprobe) for hornblende quartz diorite that contains well-developed mylonite fabrics in the Browns Creek Ridge shear zone. A second major structure, the Glade Creek fault (Morrison, 1968; Lewis et al., 1992, 1998, 2007a), is characterized by northwest-striking and vertical to steeply northeast-dipping foliation and steep downdip lineation (Fig. 3D; Data Repository Fig. DR1c). Mylonite fabrics in this shear zone also display northeast-side-up kinematics parallel to the mylonitic lineation.

Three quartzite samples (12-TSL-29, 13-TSL-70, 12-TSL-7; Fig. 2) collected in the Clearwater zone were used for detailed microstructural analysis, and were described in detail in Stetson-Lee (2015). Although we are uncertain about the full extent of the Clearwater zone, we collected samples that are very similar in mineralogy and deformational microstructure, and were deformed in similar medium- to high-grade metamorphic environments from an ∼60 km length of the structural zone. Sample 13-TSL-70 is from rocks that contain northwest-striking fabrics that overprint the older S-shaped fold in the Woodrat Mountain shear zone near Pierce (Fig. 2). Sample 12-TSL-29 was collected from mylonite rocks with northwest-striking fabrics north of Orofino, on the northwestern end of the Clearwater zone. Sample 12-TSL-7 is from an antiformally folded shear zone in the Coolwater structural culmination, which we include in the Clearwater zone on the basis of similar kinematics that show top-to-the-southwest shear sense and published age constraints by Lund et al. (2008) that mostly postdate the Ahsahka shear zone and predate intrusion of the 66–54 Ma main body of the Bitterroot lobe of the Idaho batholith. Deformed rocks in this structural culmination include amphibolite facies calcareous hornblende gneiss and schist, calc-silicate gneiss and schist, and local quartzite and marble layers (Lund et al., 2008).

Results from quartz crystallographic vorticity vector analysis of the three quartzite samples indicate subhorizontal vorticity vectors that trend northwest-southeast (Fig. 4A) within the plane of foliation and perpendicular to the lineation defined for individual structures in the Clearwater zone. These results are compatible with our interpretation of Late Cretaceous southwest-directed (northeast over southwest) reverse motion based on fabrics and shear-sense indicators for structures in the Clearwater zone.

Results from quartz crystallographic data from the three Clearwater zone quartzite samples are very similar to those from the Ahsahka shear zone described here (Fig. 4). Quartz <c> axes in the Clearwater zone samples form a point maximum subparallel to the inferred vorticity axis and <a> axes form point concentrations in a girdle orthogonal to it. Low-angle misorientation axes show a strong association with the <c> axis direction, consistent with intragranular slip by lattice rotation about the quartz <c> axes during prism <a> slip. Our collective microstructural results for the Clearwater zone are consistent with dominantly simple shear deformation and reverse, top-to-the-southwest shear at amphibolite metamorphic conditions.

U-Pb Zircon Age Relationships

Data from U-Pb zircon LA-ICP-MS analysis are summarized in Table 1 and results are displayed on Tera-Wasserburg plots in Figure 5. CL images of representative zircons analyzed from each sample are displayed in Figure 5; all CL images used for analysis are located in Data Repository Figure DR2. Five samples yielded acceptable results, which are discussed in the following. One sample produced highly discordant results.

Undeformed magmatic epidote–bearing biotite hornblende quartz diorite (sample 06RL400) was collected from a roadcut along U.S. Highway 12, north of Peck (Fig. 2). The outcrop is located ∼2 km south of the Sri 0.706 isopleth (Fleck and Criss, 2004) and provides broad age limits on intrusion and deformation in the northeastern part of the Wallowa terrane. The pluton intrudes hornblende gneiss and amphibolite interpreted as metavolcanic and metasedimentary rocks of the Orofino series in the high-grade gneiss assemblage (Kauffman et al., 2009). Zircons in this sample are mostly equant and show faint oscillatory zoning, and most grains lack distinct cores visible in CL (Fig. 5A; Data Repository Fig. DR2). All 16 analyses are clustered within 10 m.y. and yield a weighted mean Early Cretaceous age of 125.2 ± 1.1 Ma, which is interpreted as the crystallization age of the quartz diorite. This age is similar to Early Cretaceous ages from other nearby quartz diorite plutons (Fig. 2; Unruh et al., 2008) that are intruded by the Sixmile plutonic complex and, depending on location, deformed by the Ahsahka shear zone.

Both deformed and undeformed samples were obtained from a roadcut along Idaho Highway 7 northwest of Orofino to constrain the age of the Ahsahka shear zone (Fig. 2). This outcrop is located ∼1 km south of the Sri 0.706 isopleth (Fleck and Criss, 2004). Mylonitized biotite hornblende quartz diorite contains equant to slightly prismatic zircons that show variably developed oscillatory zoning in CL (sample 06RL401; Figure 5B; Data Repository Fig. DR2). Analyses (n = 17) give a weighted mean late Early Cretaceous age of 115.8 ± 1.3 Ma. This age is interpreted as the crystallization age of the quartz diorite, which predates the strong mylonite fabric that is developed in the Ahsahka shear zone and constrains the age of mylonite fabrics in this intrusion to younger than 116 Ma. The undeformed pegmatite that cuts the mylonite fabric in the quartz diorite (sample 06RL402) yielded zircons that are slightly prismatic and contain coarse concentric and parallel oscillatory zoning (Fig. 5C; Data Repository Fig. DR2). Analyses (n = 11) resulted in a weighted mean age of 93.8 ± 1.3 Ma. We interpret these results as the crystallization age of the pegmatite, which constrains the youngest possible age of this part of the Ahsahka shear zone to ca. 94 Ma.

A weakly deformed biotite tonalite sill that cuts the mylonitized quartz diorite (sample 06RL403) was also sampled from the same outcrop northwest of Orofino that yielded samples 06RL401 and 06RL402. Elongate zircons display crudely developed parallel oscillatory zoning and no cores (Fig. 5D; Data Repository Fig. DR2). Six analyses give a weighted mean age of 92 ± 3 Ma, which is interpreted as the age of crystallization of the sill. This age is slightly younger than the age of the undeformed pegmatite, but is within error of both ages. We speculate that the tonalite sill is likely slightly older than the pegmatite, and that it was part of the final stages of deformation on the Ahsahka shear zone before the pegmatite dike intruded. Alternatively, the finer grain size or presence of biotite in this sample compared to the pegmatite (sample 06RL402) may have made this sample more susceptible to deformation during the waning stages of activity on the Ahsahka shear zone.

Undeformed biotite hornblende quartz diorite was sampled (sample 06RL404) from a roadcut along Idaho Highway 11, north of the 119–116 Ma Sixmile pluton to help constrain the age of the Ahsahka shear zone. Zircons in this sample are euhedral and prismatic and display concentric and parallel oscillatory zoning (Fig. 5E; Data Repository Fig. DR2. Cores are poorly visible in CL. The 17 analyses yielded a weighted mean age of 88.8 ± 1.2 Ma; we interpret this as the crystallization age for a relatively small stock that intruded subsequent to intrusion of the Sixmile pluton and deformation on the Ahsahka shear zone.

Weakly deformed biotite tonalite was sampled at Suttler Creek from the transitional Sri zone just west of the Woodrat Mountain thrust to constrain the age of transitional zone magmatism and the northeastern extent of the Ahsahka shear zone (sample 06RL405). Zircons in this sample are subhedral to euhedral and slightly prismatic (Fig. 5F; Data Repository Fig. DR2). Fine concentric oscillatory zoning is apparent in CL and some grains contain very visible cores. We performed 15 analyses on 3 grains. Core analyses gave results ranging from 257 to 194 Ma and one core analysis gave a 207Pb/206Pb age of 1678 Ma. Rim analyses range from 100 to 116 Ma and produce a weighted mean age of 109.2 ± 10.5 Ma. Although errors are unacceptably high for this sample, we interpret these results as a crystallization age of ca. 110 Ma for a pluton that contains xenolithic cores derived from both Cretaceous through Permian Wallowa terrane rocks and Proterozoic North American rocks.

DISCUSSION

Our geologic mapping, structural analysis, and geochronology results presented here suggest that deformation occurred in distinct belts in space and time in the Syringa embayment. In this section we integrate the ages and kinematics of the geological structures that occur across the lithospheric boundary in the Syringa embayment and use these results to test hypotheses that have been proposed for the evolution of the embayment. We also compare the structural history we develop for the Syringa embayment with the north-south–trending lithospheric boundary to the south, emphasizing deformation on the Ahsahka shear zone and its continuity with the western Idaho shear zone, as well as the Clearwater zone.

Ahsahka Shear Zone and Its Continuity with the Western Idaho Shear Zone

Results from our geochronology and structural analyses significantly improve our understanding of the age of activity and kinematic relationships for the Ahsahka shear zone. The mylonitized quartz diorite in the Ahsahka shear zone from northwest of Orofino yielded an age of 115.8 ± 1.3 Ma. This age is identical within error to an age of 116.7 ± 0.7 (U-Pb zircon SHRIMP; Snee et al., 2007; Unruh et al., 2008) obtained from a mylonite quartz diorite that is probably part of the same pluton located near Dworshak Dam. The mylonite zone is crosscut by undeformed pegmatite that yielded an age of 93.8 ± 1.3 Ma and a weakly deformed biotite tonalite sill that bore an age of 92 ± 3 Ma. McClelland and Oldow (2007) sampled a leucocratic pegmatite dike, located ∼4 km along strike farther northwest from our sample location, which displayed weakly developed mylonite foliation interpreted as a late syn-deformational dike. Their sample yielded a U-Pb zircon (SHRIMP) age of 86.5 ± 2.0 Ma. As constrained by 40Ar/39Ar hornblende results (Davidson, 1990; Snee et al., 2007), rocks in the Ahsahka shear zone had cooled below ∼550 °C by ca. 80 Ma. Our quartz petrofabric results, which display prism <a> slip and indicate deformation at amphibolite-grade temperatures, are in agreement with shear zone conditions at relatively high metamorphic temperatures. We interpret these results to indicate that the Ahsahka shear zone accumulated significant strain between ca. 116 and 92 Ma, accumulated very minor localized strain after ca. 87 Ma, and had ceased by ca. 80 Ma. This shearing occurred subsequent to collision of the Wallowa terrane and ensuing crustal thickening that is believed to have generated the 120–110 Ma tonalite-trondhjemite plutonic suite that resulted in the Sixmile plutonic complex and related intrusions that are in the footwall of the shear zone (Lee, 2004).

Our interpretation of timing relationships differs from that of McClelland and Oldow (2007); they considered deformation on the Ahsahka shear zone, with ages as young as ca. 87 Ma, to be part of their broader (in both space and time) Orofino shear zone, which includes mylonitic deformation on the Browns Creek Ridge shear zone that occurred after 73.4 ± 1.2 Ma. Our data, combined with the larger geochronology data set that has amassed since the work of McClelland and Oldow (2007), indicate that the Ahsahka shear zone and Browns Creek Ridge shear zone (part of the Clearwater zone) are separate structures in space and time. The 40Ar/39Ar data set strongly supports this conclusion with results of ca. 80 Ma for hornblende and 76–70 Ma for biotite from this part of the Ahsahka shear zone (Davidson, 1990; Snee et al., 2007). Clearly, this part of the Ahsahka shear zone is older than shear zones in the Clearwater zone; however, our results permit overprinting of the northeastern side of the Ahsahka shear zone by structures in the Clearwater zone, particularly along the poorly understood northwestern end of the Ahsahka shear zone.

We have constrained the Ahsahka shear zone to activity by at least 116 Ma and until 92 Ma with significant cooling by ca. 80 Ma. This age range overlaps with age constraints of 111–91 Ma for deformation on the western Idaho shear zone along the north-south–oriented lithospheric boundary where it has been defined near McCall, Idaho, where rocks also indicate 40Ar/39Ar hornblende closure ages of ca. 80 Ma (Giorgis et al., 2008; Braudy et al., 2016). These overlapping ages of deformation and exhumation of two shear zones along the lithospheric boundary provide clear evidence linking the Ahsahka shear zone with the western Idaho shear zone and indicate that deformation in these areas occurred simultaneously as a result of northeast-directed convergence.

Kinematics of the Ahsahka shear zone consistently indicate dominantly reverse simple shear. Lineation-parallel and foliation-perpendicular surfaces in outcrop and thin section are populated with folded veins, σ-type porphyroclasts, mica fish, and weakly developed S-C fabric, all of which indicate top-to-the-southwest sense of shear. Vorticity vectors and CPO patterns on the quartzite layers are both consistent with simple shear with top-to-the-southwest sense of shear: (1) CVA axes occur orthogonal to the field lineation and lie in the plane of the field foliation, consistent with dominantly simple shear deformation; and (2) CPO patterns are consistent with simple shear accommodated by prism <a> slip, corroborated by misorientation indices. Note that the quartzites deformed in the western Idaho shear zone show significantly different patterns, consistent with dextral transpression (Braudy et al., 2016; Giorgis et al., 2016). Thus, these kinematics along the arc-continent boundary vary along strike from dextral transpression in the south (Giorgis et al., 2008; Benford et al., 2010) to reverse-sense simple shear in the north. The regional implications are more fully discussed in Giorgis et al. (2016).

Despite kinematic compatibility, a significant difference between the western Idaho shear zone and the Ahsahka shear zone is that the western Idaho shear zone overprints the Salmon River suture zone and coinciding Sri 0.704/0.706 line along the north-south segment of the lithospheric boundary. Accordingly, the high-grade gneiss and Laurentian rock assemblages are juxtaposed across the western Idaho shear zone. In contrast, the Ahsahka shear zone and Sri 0.704/0.706 line occur southwest of the Woodrat Mountain shear zone and within the high-grade gneiss assemblage. Kinematic considerations suggest that orthogonal convergence on the Ahsahka shear zone requires more contraction on this segment than for the transpressional western Idaho segment farther south. A wider shear zone that accommodated less focused contraction than for the western Idaho shear zone or crustal wedging, as suggested by Lund et al. (2008) for the Coolwater culmination, are mechanisms that may have led to a broader zone in the Syringa embayment.

Northeast-Striking Structural Zones and Continued Evolution of the Embayment

The northeast-striking Limekiln and Mount Idaho structural zones are important newly recognized structures in the tectonic development of the Syringa embayment. They show significant displacement of Wallowa terrane assemblages and accommodated dextral oblique contraction. The northeast projection of the Limekiln structural zone coincides with the northwest to west bend along the border of the embayment near the town of Orofino. The Mount Idaho structural zone truncates the accretionary boundary, displacing the entire embayment inboard and to the northeast with respect to the north-south accretionary boundary that continues to the south. These structural zones were active following ca. 111–91 Ma deformation on the western Idaho shear zone, and the Mount Idaho zone is crosscut by ca. 73–61 Ma structures in the Clearwater zone. Portions of the northeast-striking structures were reactivated in the Miocene.

We speculate that a change in plate convergence directions along the Laurentian margin led to loss of kinematic compatibility between the Ahsahka shear zone and western Idaho shear zone and initiation of the northeast structural zones in the embayment. We envision that displacement on the northeast-striking structures was accompanied by clockwise rotation of the originally northeast-southwest–oriented reactivated transform boundary to its present east-west orientation during clockwise rotation that likely occurred in the embayment during dextral shear on the northeast-striking structural zones. This rotation was probably accommodated by sinistral transpression along the east-west–oriented segment of the embayment, a conclusion that is tentatively supported by our current knowledge of structural relationships for this time period in this poorly exposed part of the embayment. At Green Knob (Fig. 1), Anderson (1991) determined sinistral kinematics for mylonite fabrics in Neoproterozoic to early Paleozoic quartzite that include shallow east-plunging lineation. The fabrics predate intrusion of a 74 ± 1 Ma unfoliated syenite pluton (U-Pb zircon thermal ionization mass spectrometry; Bush et al., 2001). At Granite Point, Washington (Fig. 1), Murphy (2007) noted northeast-striking sinistral ductile shear zones that deformed 83.6 ± 0.7 Ma pegmatite dikes (U-Pb zircon SHRIMP age). These studies suggest sinistral shear during deformation within the time period ca. 84–74 Ma that likely overlapped with deformation on the northeast-striking structural zones in the embayment.

Timing and Kinematic Role of the Clearwater Structural Zone

The series of northwest-striking high-strain zones that compose the Clearwater zone show dominantly reverse kinematics with top-to-the-southwest shear sense. Results of microstructural analysis for shear zones in this structural zone are very similar to those from the Ahsahka shear zone. They suggest amphibolite-grade conditions, reverse kinematics, and simple shear–dominated vorticity. Published ages for shear zones in the Clearwater zone constrain its activity to largely post–73 Ma deformation in the Coolwater culmination (Lund et al., 2008) and in the Browns Creek Ridge shear zone (McClelland and Oldow, 2007) and pre–66 Ma to 54 Ma intrusion of the peraluminous main phase of the Bitterroot lobe of the Idaho batholith (Lewis et al., 2007a; Gaschnig et al., 2010). The complex zone of Late Cretaceous to Paleocene deformation that comprises the Clearwater zone was defined by McClelland and Oldow (2007) as part of the ∼6-km-wide Orofino shear zone that has been suggested to link kinematically with the Ahsahka shear zone to the west. Our new mapping and age data of 116–92 Ma deformation on the Ahsahka shear zone now calls into question this purported link. Rather than linking with the Ahsahka shear zone, it appears that the Clearwater zone is a spatially and temporally separate structural zone from the Syringa embayment and is unrelated to the Ahsahka shear zone.

Our results for Clearwater zone deformation also do not fit the model proposed by Lund et al. (2008) for a transpressional termination in the Syringa embayment. Our age constraints on the Ahsahka shear zone are in agreement with constraints for deformation on the western Idaho shear zone, both of which predate most of the deformation that has been constrained for the Clearwater zone, which we consider to include the Coolwater structural culmination. Furthermore, our kinematic and quartz microfabric analyses suggest deformation characterized by dominantly simple shear, reverse shear sense, and horizontal vorticity axes oriented parallel to strike of shear zones. These results are consistent with orthogonal contraction across the northwest-striking Clearwater zone rather than sinistral transpression as suggested by Lund et al. (2008). We envision continued dextral oblique convergence along the North American Cordilleran margin following truncation of the Ahsahka–western Idaho shear zone system by the northeast-striking structural zones. The locus of deformation simply shifted inboard from the accretionary boundary once the geometry of the long-lived Ahsahka–western Idaho shear zone system had become unfavorable due to constriction in the embayment.

Tectonic Evolution of the Syringa Embayment

Our new age and structural results are pertinent to the nature and timing of formation of the bend in the Syringa embayment. In the following discussion we address the origin and evolution of structures in the embayment and outline a likely history of its evolution (Fig. 6). We refer to structures in their present orientations, which permits their description relative to each other. However, we acknowledge that they likely were not in their present orientations when the Syringa embayment was evolving (Tikoff et al., 2014).

The accretionary boundary initiated with Jurassic or Early Cretaceous collision of Blue Mountains terranes with the North American margin. The initial juxtaposition of the Wallowa terrane that currently occupies the Syringa embayment with North America may have occurred farther south along the margin, with subsequent northward displacement on the western Idaho shear zone. The original geometry of this collision zone in the Syringa embayment is poorly constrained, but it is likely that an early less kinked form of the embayment was initially present, that included the northwest-striking Woodrat Mountain shear zone (Fig. 6A). We speculate that this northwest-trending segment reactivated structures in the Neoproterozoic–early Paleozoic rifted Laurentian margin during the Mesozoic as proposed by Lund (2008) and Lund et al. (2010). Contraction continued across the collisional belt following initial terrane accretion, likely forming a west-vergent fold and thrust belt that is currently covered by Neogene rocks (Fig. 2). We speculate that the thrust belt continues under Columbia River Basalt Group cover sequences into the Syringa embayment from the south. This episode of contractional deformation would have resulted in crustal thickening, deep burial of Riggins Group and Orofino assemblage rocks that recorded peak metamorphic garnet Lu-Hf ages ranging from 121 to 113 Ma (Fig. 6A; Wilford, 2012), and subsequent deep emplacement of the high Sr/Y plutons that include the 119–116 Ma Sixmile pluton (Fig. 6B; Lee, 2004; Kauffman et al., 2009). Some of these plutons were later deformed in the Ahsahka shear zone.

The origin of the east-west segment in the embayment for this early time is enigmatic. Figure 6A shows three scenarios for its origin as (1) a northeast-southwest–striking tear structure that reactivated an older transform fault; (2) a northwest-striking continuation of the Woodrat Mountain shear zone; or (3) an east-west–striking tear structure. Both (1) and (2) require subsequent rotation of the segment to its current east-west orientation. We prefer the scenario in which the segment originated as a northeast-southwest–trending transform in the continental margin rift system. Its anomalous present orientation requires 30°–40° of subsequent clockwise rotation with respect to the orientation of the Woodrat Mountain shear zone as described here.

The margin underwent a distinct episode of dextral transpression along the superposed western Idaho shear zone in the mid-Cretaceous. This deformation was centered along the Salmon River suture zone on the north-south–oriented segment of the boundary and involved mostly contractional deformation with a smaller component of dextral shear during the interval 111–91 Ma (e.g., Giorgis et al., 2008). Deformation that was coeval and kinematically compatible with the western Idaho shear zone occurred on the Ahsahka shear zone farther north, in what is now the Syringa embayment, during the interval 116–92 Ma (Fig. 6B). Kinematic compatibility between these two segments requires more shortening along the purely contractional Ahsahka shear zone than along the transpressional western Idaho shear zone to the south (see Giorgis et al., 2016). We speculate that this additional shortening was accommodated by progressive widening of the Ahsahka shear zone in a southwestward direction as it evolved and by lower crustal wedging that shifted the Sri boundary southwest from its likely original position superposed on the Woodrat Mountain suture. We envision that the east-west segment of the Syringa embayment accommodated kinematically compatible sinistral displacement while the Ahsahka shear zone was active. However, other scenarios are possible.

Post–90 Ma, the Ahsahka–western Idaho shear zone system was clearly truncated by the southernmost of the two northeast-trending dextral transpressional structural zones that cut across the Wallowa terrane (Fig. 6C). These zones, the Limekiln and Mount Idaho structural zones, are oriented orthogonal to the trace of the Ahsahka shear zone. Although these structural zones are generally compatible with dextral transpression along the North American margin after 90 Ma they are not kinematically compatible with the transpressional strain regime defined for the Ahsahka–western Idaho shear zone system. The two northeast-trending structural zones accommodated displacement of the northeasternmost corner of the Wallowa terrane deeper into the continental margin, kinking and truncating the long-lived Ahsahka–western Idaho shear zone system. We speculate that the east-west segment of the embayment rotated clockwise to its present orientation accompanied by sinistral shear after 90 Ma based on published evidence form Green Knob, Idaho, and Granite Point, Washington, as described herein. Early phases of the Idaho batholith intruded toward the end of this episode of deformation.

The evolution of a kinematic bind in the margin in the Syringa embayment implies that deformation along the margin was focused elsewhere after cessation of the Ahsahka–western Idaho shear zone system. We hypothesize that this occurred on the northwest-striking Clearwater zone soon after 90 Ma (Fig. 6D). This episode of deformation was active after intrusion of ca. 73 Ma quartz diorite at Browns Creek Ridge that has been mylonitized (McClelland and Oldow, 2007), but also after 69 Ma due to the presence of mylonitized intrusive rocks in the Clearwater structural culmination (Lund et al., 2008). However, we are unaware of any constraints on the actual initiation of this episode of deformation in the Clearwater zone; it is possible that deformation initiated prior to 73 Ma. The orientation of the Clearwater zone as well as the kinematics and vorticity of individual shear zones within it are compatible with the overall convergence direction recorded in the Ahsahka–western Idaho shear zone system. This implies that the Clearwater zone accommodated continued northeast-directed convergence after cessation of the Ahsahka–western Idaho shear zone system. Intrusion of the main phase of the Bitterroot lobe of the Idaho batholith ensued during the period ca. 66–54 Ma, following deformation on the Clearwater zone (Fig. 6E) (Gaschnig et al., 2010).

CONCLUSIONS

Our new geologic mapping, structural analysis, and U-Pb zircon ages permit a refined tectonic model for the evolution for the Syringa embayment. Our salient conclusions include the following.

  1. The Syringa embayment appears to be a long-lived feature in the Cordilleran accretionary margin that progressively evolved to its present geometry through a series of distinct deformation events. It is likely that the embayment initiated as a less kinked bend in the Mesozoic Laurentian continental margin. The Woodrat Mountain shear zone, a probable expression of the Salmon River suture zone, may have reactivated structures associated with Neoproterozoic–early Mesozoic rifting of the margin, causing the mostly north-south–trending accretionary margin to deviate locally to a northwest trend in the embayment.

  2. We have constrained the age range for the northwest-striking Ahsahka shear zone to between ca. 116 and 92 Ma and determined consistent top-to-the-southwest thrust kinematics and horizontal northwest-trending vorticity vector for the ductile fabrics within the shear zone. These results indicate that the Ahsahka shear zone is the northward continuation of the dextral transpressive western Idaho shear zone to the south. The Ahsahka shear zone formed along the northwest-trending inherited bend in the margin, accommodating reverse-sense displacement to maintain kinematic compatibility with dextral transpression on the north-south–oriented western Idaho shear zone. Our new age constraints indicate that the Ahsahka shear zone is not part of the Orofino shear zone proposed by McClelland and Oldow (2004, 2007), which they suggested to have truncated the western Idaho shear zone. Our kinematic constraints for the Ahsahka shear zone imply that the Syringa embayment was kinematically compatible with dextral transpression on the western Idaho shear zone, rather than serving as a transpressional termination as suggested by Lund et al. (2008).

  3. Two northwest-trending structural zones that accommodated both contractional and dextral strike-slip shear, the Limekiln zone to the north and Mount Idaho zone to the south, cut across the Syringa embayment. The Mount Idaho zone severs the junction between the western Idaho shear zone and the Ahsahka shear zone and displaces it 10–20 km in a dextral sense. The Mount Idaho structural zone is cut in turn by younger structures to the east in the Clearwater structural zone. We speculate that this pair of northeast-trending structural zones formed ca. 90 Ma in response to changing plate kinematics along the Cordilleran margin. The northeast-trending zones effectively kinked and truncated the western Idaho–Ahsahka shear zone, constraining the Syringa embayment to its present geometry. Later deformation along the margin was then partitioned into the Clearwater zone, farther within the North American continent, which also displays consistent top-to-the-southwest reverse kinematics and horizontal northwest-trending vorticity vector in its shear zones.

We thank Maureen Kahn for helpful discussions. Loudon Stanford helped to compile map and structural data for our structural analysis. Reviews by two anonymous reviewers significantly improved our original manuscript and we thank them for their constructive suggestions. Work by Michels, Stetson-Lee, and Tikoff was supported by EAR-0844260 and EAR-1251877 to B. Tikoff (University of Wisconsin-Madison).

1GSA Data Repository Item 2016276, which includes detailed structural maps and cathodoluminescence images of zircons used for U-Pb analysis, is available at www.geosociety.org/pubs/ft2016.htm, or on request from editing@geosociety.org.