The Cretaceous intrusive units of the Sahwave and Nightingale ranges in northwestern Nevada, USA, located between the Sierra Nevada and Idaho batholiths, represent a critical segment of Cretaceous arc magmatism. U-Pb zircon age dating shows that the older, 104 Ma Power Line intrusive complex is dominantly granodioritic in composition, while the younger 94–88 Ma Sahwave Range intrusive suite (the Juniper Pass, Bob Springs, and Sahwave plutons) is similar in composition (tonalite to granodiorite) and age to the plutons of the Tuolumne intrusive suite of the east-central Sierra Nevada batholith. We present new field measurements, microstructural observations, and anisotropy of magnetic susceptibility analyses of the Power Line intrusive complex and Sahwave Range intrusive suite. The Power Line intrusive complex is characterized by a vertical, N–S-striking, solid-state foliation and down-dip lineation. Evidence of dextral shearing is observed on subhorizontal planes that are perpendicular to the lineation, which is consistent with pure shear-dominated transpression. This fabric is similar in style and timing to both the western Idaho shear zone of the Idaho batholith and mid-Cretaceous shear zones of the central Sierra Nevada. The plutons of the Sahwave Range intrusive suite are not affected by the pure shear-dominated transpressional fabric observed in the Power Line intrusive complex, which indicates that this deformation ceased by ca. 94 Ma. Rather, the Juniper Pass pluton contains an E–W-striking magmatic foliation fabric that rotates to a steep NW–SE-striking, solid-state foliation in the younger Sahwave pluton. These fabrics are strikingly similar to fabrics in the Tuolumne intrusive suite, Sierra Nevada, California, USA. Recent work in the western Idaho shear zone also indicates that late-stage deformation occurred there until ca. 85 Ma. Therefore, the intrusions of northwestern Nevada provide a tectonic link between the Sierra Nevada and Idaho batholiths, which suggests that two distinct phases of mid-Cretaceous, transpressional deformation occurred in at least three magmatic arc segments of the western U.S. margin.
Steeply dipping Cretaceous–Paleogene shear zones (Armstrong, 1988; Bateman, 1992; Manduca and Silver, 1993; Tobisch et al., 1995; Tikoff and St. Blanquat, 1997; Andronicos et al., 1999; Ducea, 2001; Schmidt et al., 2002; Van Buer and Miller, 2010; Attia et al., 2022) span the North American continental margin from Mexico to Canada. Many of these shear zones occur within or adjacent to the major magmatic arcs and are oriented parallel to the axis of arc magmas (e.g., Contreras-Reyes et al., 2021). The kinematics (wrenching versus transpression versus reverse motion), magnitude of recorded strain, and timing of these shear zones vary. Furthermore, the relationship of these shear zones (strain localization and slip partitioning) to arc magmatism (transfer of heat and mass) and the overall regional tectonics is debated (Blanquat et al., 1998; Sieh and Natawidjaja, 2000; St. Blanquat et al., 2011; Attia et al., 2022). This paper focuses on the relationships between mid-Cretaceous shear zones hosted in intrusive suites across the western U.S. (e.g., Hildebrand and Whalen, 2021; Tikoff et al., 2023) and how the evolution of magmatism and deformation contributed to the formation and modification of the continental margin.
The two major batholiths located entirely in the western U.S. are the Sierra Nevada and Idaho batholiths, and both show evidence of mid-Cretaceous deformation. The western Idaho shear zone, located on the western margin of the Idaho batholith, records dextral transpression from 100 Ma to 85 Ma (e.g., Tikoff et al., 2001; Giorgis et al., 2008; Michels et al., 2015; Braudy et al., 2017). The Sierra Nevada batholith also records a 100 Ma phase of deformation in the Bench Canyon (McNulty, 1995), Courtright-Wishon (Torres-Andrade, 2022), and Sing Peak (Krueger and Yoshinobu, 2018) shear zones. The younger Sierra Crest shear zone system is a series of dextral shear zones active in the eastern Sierra Nevada batholith during the Cathedral Range intrusive event (Greene and Schweickert, 1995; Tikoff and St. Blanquat, 1997; Tikoff et al., 2005; Cao et al., 2015).
Our understanding of structural and temporal continuity between the Idaho batholith and the Sierra Nevada batholith is hindered by the effects of Basin and Range extension. There are, however, exhumed segments of Cretaceous arc plutons in this “gap” in northwestern Nevada, USA (e.g., Wyld, 2000; Van Buer and Miller, 2010). The largest and most continuous Cretaceous pluton segment in Nevada is the Sahwave Range intrusive suite. The plutons that comprise the suite are strikingly similar in composition, age, and emplacement pattern (Van Buer and Miller, 2010) to the plutons of the Tuolumne intrusive suite. Van Buer and Miller (2010) concluded that these plutons constitute a northward continuation of the Sierra Nevada batholith.
This contribution documents evidence of two periods of deformation between 100 Ma and 85 Ma in the plutons of the Sahwave Range and the Nightingale Range of northwest Nevada. We characterize the internal fabrics of the Early Cretaceous Power Line intrusive complex and the Late Cretaceous Sahwave Range intrusive suite (Juniper Pass, Bob Spring, and Sahwave plutons) and use microstructures and anisotropy of magnetic susceptibility (AMS) to characterize solid-state and submagmatic fabrics in these plutons. These new structural data from northwestern Nevada provide evidence for a link between shear zone-associated 100 Ma and 88 Ma deformational events in both the Idaho batholith and the Sierra Nevada batholith.
Evidence of deformation at ca. 100 Ma (mid-Cretaceous) is recognized throughout much of the U.S. Cordillera (Hildebrand and Whalen, 2021; Busby et al., 2023; Tikoff et al., 2023). This change correlates to a significant change in plate motion that is hypothesized to have occurred at 105–100 Ma in the Pacific basin (Matthews et al., 2012). Oldow (1983) proposed a margin-wide change in motion, from left-lateral to right-lateral, based on the orientation of fabrics in NW Nevada. Below, we review arc magmatism and the regional deformation that occurred between 100 Ma and 85 Ma in the Sierra Nevada batholith, the Idaho batholith, and NW Nevada (Fig. 1).
The Sierra Nevada Batholith
The Sierra Nevada batholith is part of the California triad that includes the Franciscan mélange, the Great Valley forearc, and the Sierra Nevada magmatic arc. The 125–85 Ma magmatism in the Sierra Nevada arc exhibits an eastward younging of pluton ages, an increase in K2O, and is commonly thought to have formed as a result of eastward subduction of oceanic plates from the Pacific basin beneath North America (e.g., Chen and Moore, 1982; Saleeby and Dunne, 2015; Surpless et al., 2019).
The 87Sr/86Sr = 0.706 isopleth (Sr 0.706 line; Armstrong et al., 1977; Saleeby, 1981; Kistler, 1990; Lackey et al., 2012) is generally used to define the boundary between the accreted oceanic terranes to the west and North American continental crust to the east. The boundary is thought to result from a combination of Precambrian rifting (e.g., Lund, 2008), and possibly left-lateral truncation of this margin during the Paleozoic/early Mesozoic (e.g., Davis et al., 1978; Burchfiel et al., 1992; Clemens-Knott and Gevedon, 2023). This interpretation is constrained by the wall rocks of these intrusions, which consist locally of metamorphosed Paleozoic sedimentary rocks of western North America (e.g., Greene and Schweickert, 1995; Stevens and Greene, 2000). We note that alternative models exist for the Sr 0.706 line (e.g., Levy and Christie-Blick, 1991; Attia et al., 2022).
The pre-100 Ma granites were generally emplaced west of the Sr 0.706 line (e.g., Bateman, 1992; Lackey et al., 2012), while the post-100 Ma plutons are emplaced east of the Sr 0.706 line and are typically granodioritic in composition. Hildebrand and Whalen (2021) use chemical discrimination diagrams to distinguish between pre-100 Ma “magmatic arc” granites and post-100 Ma “slab breakoff” granites.
The Cathedral Range intrusive event (94–83 Ma; Evernden and Kistler, 1970; Kistler et al., 1986; Bateman, 1992; Coleman et al., 2004) consists of several compositionally zoned intrusive suites, which are elongated, with their long axes oriented NNW. From south to north, these are: the Mount Whitney intrusive suite (e.g., Hirt, 2007); the John Muir intrusive suite (e.g., Coleman et al., 2004); the Tuolumne intrusive suite (e.g., Bateman and Chappell, 1979; Kistler et al., 1986); and the Sonoran intrusive suite (Leopold, 2016). Each intrusive suite displays an inward younging that is generally mirrored by geochemical trends ranging from more mafic to more felsic (e.g., Stern et al., 1981; Chen and Moore, 1982; Kistler et al., 1986; Coleman and Glazner, 1997; Gray et al., 2008). The younger megacrystic K-feldspar granites are often asymmetrically nested within (Tuolumne and Mount Whitney intrusive suites) or aligned en echelon (Mono Pass intrusive suite) with older, more mafic equigranular granodiorites (Fig 2; John and Robinson, 1982; Bateman, 1992; Hirt, 2007; Saleeby et al., 2008).
Ca. 100 Ma Initiation of Transpressional Deformation
Deformation in the Sierra Nevada at ca. 100 Ma is recorded in the Courtwright-Wishon and Sing Lake shear zones (Krueger and Yoshinobu, 2018; Torres-Andrade, 2022). The Courtwright-Wishon shear zone is a high-temperature, solid-state shear zone (Tobisch et al., 1995) and is characterized by NW-striking, steeply dipping mylonitic and ultramylonitic fabrics superimposed on a weak foliation. The shear zone is truncated by younger intrusions, including the Mount Givens pluton (90 Ma, as determined by U-Pb on zircon dating). Recent works suggest that the Courtwright-Wishon shear zone records dextral transpressional shearing with a component of contraction, as it preserves dextral shear-sense indicators perpendicular to a subvertical lineation (Torres-Andrade, 2022).
The slightly younger 98 Ma Sing Lake shear zone deforms the Jackass Lakes granodiorite and records continuous deformation, as evidenced by a near-solidus to subsolidus fabric gradient (Krueger and Yoshinobu, 2018). The Sing Lake shear zone records a steep, NW–SE foliation, a moderate north-plunging mineral lineation, and evidence of both dextral shear and west-directed shortening. These fabrics indicate transpressional strain. The similar deformational history and orientation of these mid-Cretaceous shear zones suggest that they may have been part of a larger shear zone system prior to their truncation by younger intrusions (McNulty et al., 2000; Torres-Andrade, 2022).
Ca. 92–85 Ma Transpressional Deformation
The Sierra Crest shear zone system is a series of dextral transpressional shear zones exposed along the eastern margin of the batholith (Tikoff and St. Blanquat, 1997; Nadin et al., 2016). In the center, from north to south of the Sierra Nevada batholith, they are the Cascade Lake shear zone (Tikoff et al., 2005; Cao et al., 2015), the Gem Lake shear zone (Greene and Schweickert, 1995), the Rosy Finch shear zone (Tikoff and Teyssier, 1992; Tikoff and St. Blanquat, 1997), and the proto-Kern Canyon fault (Busby-Spera and Saleeby, 1990; Wood and Saleeby, 1998; Nadin and Saleeby, 2008; Fig. 2). Together, these shear zones record dextral deformation for over 300 km along the axis of Sierra Nevada magmatism.
Tikoff and Teyssier (1992) proposed a p-shear model for emplacement of these large intrusive suites, in which plutons intruded into dilatational jogs/stepovers in the right-lateral shear zone system. The Sierra Crest shear zone system records synmagmatic shearing during the incremental emplacement of the nested intrusive suites (Tuolumne, Mono Pass, and Whitney) of the Cathedral Range intrusive suites. Fabrics within the Rosy Finch and Cascade Lake shear zones occur in plutonic units and display a range of microstructural textures that record submagmatic to subsolidus deformation indicative of high-temperature shearing. Dextral transpressional deformation is broadly constrained to coincide with emplacement of the Cathedral Peak and Mono Creek plutons at 87 Ma (Tikoff et al., 2005; Cao et al., 2015). Bartley et al. (2005) suggested that dextral shearing must have been active at 92 Ma, during emplacement of the oldest member of the Mono Pass intrusive suite (Lamarck granodiorite). Sharp et al. (2000) dated the timing of deformation in the Gem Lake shear zone at 82–80 Ma (Sharp et al., 2000). We infer from this data that dextral deformation occurred during the emplacement of the intrusive suites from the Cathedral Range intrusive epoch (e.g., Attia et al., 2022).
The ca. 120–100 Ma Magmatic Arc and the Western Idaho Shear Zone
A ca. 120–100 Ma magmatic arc (e.g., Giorgis et al., 2005; Gaschnig et al., 2017) is preserved along the western margin of the Idaho batholith. These igneous rocks were highly telescoped into a gneissic belt (e.g., Taubeneck, 1971) during deformation of the 100–85 Ma western Idaho shear zone and are not considered to be part of the Idaho batholith proper (Fig. 3). Younger intrusions that were emplaced adjacent to and within the western Idaho shear zone after ca. 92 Ma are considered part of the Idaho batholith (the “border zone” suite of Gaschnig et al., 2010).
Undeformed parts of the Early Cretaceous magmatic arc occur locally west of the western Idaho shear zone near McCall, Idaho, USA, in the Hazard Creek complex. The westernmost Hazard Creek complex (118 ± 5 Ma in age, as determined by U-Pb on zircon data; Manduca and Silver, 1993) is composed of variably deformed tonalites, trondhjemites, granodiorite, and granites that commonly contain magmatic epidote (Taubeneck, 1971; Manduca and Silver, 1993), which is indicative of emplacement pressures of ≥7 kbar (Zen and Hammarstrom, 1984). More recent studies indicate a variety of Early Cretaceous and Late Jurassic ages in this unit (Patzke, 2017). Other isolated Early Cretaceous (e.g., Gaschnig et al., 2017) plutons occur to the west in the Blue Mountain terranes. Other parts of the Early–middle Cretaceous magmatic arc were deformed within the western Idaho shear zone. The Little Goose Creek complex—located east and inboard of the Hazard Creek complex—contains tonalites, granodiorite, and granites (110 ± 5 Ma, 105.2 ± 1.5 Ma, as determined by U-Pb on zircon dating; Manduca et al., 1992; Giorgis et al., 2008) and is completely deformed by the western Idaho shear zone.
The Sr 0.706 line was located within the Cretaceous magmatic arc in Idaho, similar to the Sierra Nevada batholith. In Idaho, however, western Idaho shear zone deformation obscures our interpretation of the magmatic history. The western Idaho shear zone currently contains the Sr 0.706 line, which is oriented approximately north–south near McCall, Idaho, and has a very steep gradient of Sr and O values (Armstrong et al., 1977; Fleck and Criss, 1985; Manduca et al., 1992; King et al., 2007). The position of the abrupt juxtaposition is consistent with a rapid transition of wall rocks derived from the accreted island arc terranes versus those derived from the North American craton, which occur within an ~10-km-thick zone. The steep isotopic gradient and sharp juxtaposition of wall rocks are caused by the western Idaho shear zone (Giorgis et al., 2005).
Deformation in the Idaho Batholith at 100–85 Ma: Western Idaho Shear Zone
The dextral, pure shear-dominated transpressional structures have been very well characterized (Giorgis and Tikoff, 2004; Giorgis et al., 2008, 2017; Michels et al., 2015; Braudy et al., 2017). Foliations dip steeply (60°–70°) eastward, and lineations are down dip. The Cretaceous fabrics restore to subvertical orientations when the tilt of overlying Miocene basalt flows is removed (Tikoff et al., 2001). Near McCall, the fabric strikes N–S. Dextral shear indicators are found on subhorizontal planes, perpendicular to the stretching lineation. Shear sense in the western Idaho shear zone is characterized by a vertical vorticity vector (the axis about which rotational strain occurred) nearly parallel to the vertical field lineation, which is consistent with transpressional kinematics (Giorgis et al., 2017). Populations of rigidly rotated feldspar porphyroclasts also reveal a subvertical vorticity vector (Giorgis and Tikoff, 2004). A vertical vorticity vector was also inferred from quartzites in wall-rock screens using the crystallographic vorticity axis (CVA) method (Michels et al., 2015). Field fabrics, including boudinaged and folded dikes, indicate a right-lateral wrench component of deformation (e.g., Braudy et al., 2017).
The timing of western Idaho shear zone deformation is well constrained. Braudy et al. (2017) concluded that the western Idaho shear zone initiated between ca. 105 Ma and 103 Ma (105.3 ± 3 Ma, 103.9 ± 2.7 Ma; U-Pb zircon dates of Crevice pluton; Gray et al., 2012). This interpretation is supported by U-Pb titanite dating that constrains western Idaho shear zone initiation to between 98 Ma and 96 Ma (Harrigan, 2022). Cessation of deformation was constrained by a cross-cutting pegmatite dike dated at 90 Ma (Giorgis et al., 2008). The youngest pluton (88.2 ± 3 Ma; U-Pb on zircon), at West Mountain, south of McCall, is the undeformed, which is consistent with cessation of western Idaho shear zone deformation at ca. 90 Ma. However, titanite U-Pb dating from Harrigan (2022) suggests that the western Idaho shear zone was locally active until ca. 85 Ma.
The western Idaho shear zone continues south into the Owyhee Mountains in southwestern Idaho. Relative to the McCall segment, the shear zone in the northern Owyhee segment is wider, less well developed, and contains an 020-oriented foliation (Benford et al., 2010). This fabric orientation is consistent with mapping in West Mountain, located between McCall and the Owyhee Mountains (Braudy et al., 2017). Along-strike variations in deformation and geochemical characteristics are attributed to the irregularity of the original continental margin (Tikoff et al., 2023). Dextral transpressional deformation is also recorded in the strongly deformed metasedimentary and intrusive rocks of the Cretaceous Sawtooth metamorphic complex exposed east of the central Idaho batholith (Ma et al., 2017). Intrusions from the Sawtooth metamorphic complex exhibit a N–S-oriented fabric. Zircon U-Pb dates for the deformed intrusions from this area indicate that transpressional deformation likely initiated between 95 Ma and 92 Ma and ca. 84 Ma and ended by 77 Ma. The transpressional deformation in the Sawtooth metamorphic complex and the western Idaho shear zone was kinematically compatible, which suggests that these shear zones may represent a regional transpression system (Ma et al., 2017).
Northwest Nevada Plutons
The fragmented record of magmatism between the Sierra Nevada batholith and the Idaho batholith represents a significant unknown in our understanding of the Cretaceous margin. Van Buer and Miller (2010) suggest that the Sierra Nevada batholith continues northward into northwestern Nevada. The Late Cretaceous Sahwave Range intrusive suite (Sahwave intrusive complex of Van Buer and Miller, 2010) is ~100 km NE of the main axis of the Sierra Nevada batholith. It is a nested intrusive suite comprising three NNE-elongated plutons emplaced between 94 Ma and 88 Ma that range in composition from older granodioritic plutons to younger granitic plutons (Fig. 4). From oldest to youngest, they are the granodiorite of Juniper Pass (92.7 ± 1.4 Ma), the granodiorite of Bob Springs (92.8 ± 1.7 Ma), and the Sahwave granodiorite/granite (88.5 ± 2 Ma; U-Pb zircon ages; Van Buer and Miller, 2010). In terms of similarities between the plutons of the Sahwave Range and Tuolumne intrusive suites: (1) the Juniper Pass corresponds to the Kuna Crest/Glen Aulin, (2) the Bob Springs corresponds to the Half Dome, and (3) the Sahwave corresponds to the Cathedral Peak.
In addition to the Sahwave Range intrusive suite, there are a variety of igneous rocks that were intruded in the Early Cretaceous in northwestern Nevada (Wyld, 1996, 2000; Wyld and Wright, 2001; Trevino et al., 2021); one particular example is the Power Line intrusive complex (104.9 ± 0.8 Ma) in the Nightingale Range (Van Buer and Miller, 2010). The main unit of the Power Line intrusive complex is a medium-grained biotite hornblende granodiorite, and it is intruded by the granodiorite of Juniper Pass and the School Bus granodiorite. It is distinctly older than the plutons of the Sahwave Range intrusive suite and records an older deformational event.
Despite being compositionally similar, the Sahwave Range intrusive suite rocks are isotopically more primitive—87Sr/86Sr of ~0.7046, εNd of ~−0.2—than those of the Tuolumne intrusive suite. The plutons of the Sahwave Range intrusive suite, although located eastward currently, are located westward of the Sr 0.706 line. Specifically, the Sahwave Range intrusive suite intruded the back-arc marine sedimentary strata of the Middle Jurassic to Upper Triassic Jungo basinal terrane (Silberling and Roberts, 1962; Quinn et al., 1997; Wyld, 2000), which likely correlates to the Quesnellia rocks. This geometry requires that the mid-Cretaceous magmatic arc did not radically change in composition along strike, despite a transition in the lithospheric type into which magmas were emplaced.
Dextral strike-slip shearing has been documented in this region as the western Nevada shear zone (Wyld and Wright, 2001). The western Nevada shear zone trends NNE from northwest Nevada to southeast Oregon, USA, which separates the Jungo and Black Rock terranes to the east from the allochthonous western assemblages (Fig. 5). The western Nevada shear zone is not exposed; it is covered by Cenozoic strata and intruded by Late Cretaceous plutons.
Field Observations and Sampling
Field observations were made in: (1) the Power Line intrusive complex in the Nightingale Range and (2) the plutons of the Sahwave Range intrusive suite (Sahwave granodiorite, granodiorite of Bob Spring, and granodiorite of Juniper Pass); this group of plutons was named the Sahwave batholith by Van Buer and Miller (2010). Field observations included measurements of foliation, lineation, and magnetic susceptibility. Oriented samples were collected at 122 locations, including 88 sites sampled with a coring drill. Optically visible microstructures were examined in thin sections cut parallel to the XZ kinematic plane, and AMS was measured on cores.
Optical Microscopy and Microstructure
Microstructures were observed at 88 localities within the Sahwave Range intrusive suite and the Power Line intrusive complex. Samples selected for micro-structural analysis are a representative subset of AMS stations. In outcrop, samples were collected from homogeneous host rock (Fig. 6). For a subset of samples, thin sections were prepared in the standard XZ fabric orientation. Due to sampling limitations, some thin sections were cut from 1-inch core rounds used in the AMS analysis.
Additional analyses of rock magnetics are required to characterize the composition and domain state of the magnetic components so the results of AMS analyses can be interpreted. Magnetic properties were measured on samples from six sites, two from each of the three rock units. All rock magnetic experiments were carried out at the University of Texas at Dallas Paleomagnetic Laboratory, Dallas, Texas, USA. Magnetic composition was evaluated using a multifunction Kappabridge with a CS4 attachment for thermal experiments and domain state of the ferrimagnetic phase. The paramagnetic versus ferrimagnetic grain contributions per specimen were evaluated using a vibrating sample magnetometer for hysteresis and isothermal remnant magnetization curves.
Anisotropy of Magnetic Susceptibility (AMS)
Magmatic fabrics preserved in granitic rocks are often weak and difficult to observe, and an alternative method is required to evaluate the strength and orientation of these fabrics. AMS is a rapid, non-destructive technique that quantifies the fabric ellipsoid in a rock sample using the magnetic susceptibility of paramagnetic and ferromagnetic grains. Numerous studies have successfully confirmed that AMS fabrics measured in granites accurately reproduce field measurements of both magmatic and solid-state deformational fabrics (Hrouda, 1982; Tarling and Hrouda, 1993; Bouchez, 1997). Arc granites are ideal candidates for AMS analyses for two main reasons: (1) they contain titanomagnetite and other Fe-rich phases (e.g., I-type granites), and (2) they often contain a medium- to coarse-grained crystalline matrix. These conditions favor the growth of multidomain to pseudo-single domain ferrimagnetic minerals (e.g., titanomagnetite) and mafic paramagnetic minerals (e.g., biotite and hornblende). Both the aforementioned mineral assemblages faithfully record magnetic fabrics due to the strength of their bulk susceptibility (5 × 10−5 × 10−3 SI) and the preferred orientation of their shapes.
The AMS ellipsoid is approximated by a symmetrical second-order tensor, with eigenvalues that define the principal susceptibility magnitudes (K1 ≥ K2 ≥ K3). The bulk magnetic susceptibility (Km) is the sum of the paramagnetic, ferromagnetic, and diamagnetic mineral contributions to the sample and is the arithmetic mean of the principal susceptibilities measured in low field. The orientation of the AMS ellipsoid is assumed to reflect the alignment of the bulk fabric in most cases. K1 represents the magnetic lineation, and K3 is the pole to the magnetic foliation. Pj, the corrected degree of anisotropy, is the deviation of the AMS ellipsoid from a sphere; the Pj value increases from 1 (sphere) to infinity with increasing anisotropy. Ellipsoidal shape, T, is used to approximate the geometry of finite strain for the last major deformational event that occurred within the intrusive suite. T = 0 indicates plane strain, −1 < T < 0 indicates prolate strain, and 0 < T < 1 indicates oblate strain.
On average, 15 cylindrical subspecimens were collected and cut from eight to 10 drill cores per site. AMS was measured on a KLY-3S Kappabridge operating at 300 A/m @ 875 Hz at the University of Wisconsin–Madison, in Madison, Wisconsin, USA. Of the 88 sites, none were rejected. Data from the sites were averaged using the Anisoft 4.2 software package and further reduced using scripts written for AMS in the R programming language (Roberts et al., 2019).
Power Line Intrusive Complex
The Power Line intrusive complex is characterized by large and relatively equant matrix grains of potassium feldspar porphyroclasts, plagioclase feldspar, quartz, biotite, and sparse oxides. The Power Line complex contains a solid-state foliation that strikes N and dips steeply (Van Buer and Miller, 2010). The solid-state foliation is defined by mafic folia, predominately recrystallized biotite, ribboned quartz, and potassium feldspar porphyroclasts. Lineation is subvertical and defined by biotite, ribboned quartz, and feldspar porphyroclasts. Sigma-type porphyroclasts of potassium feldspar record dextral shear sense in the YZ (horizontal) plane. In contrast, no consistent shear sense is observed in the XZ (vertical) plane.
Microstructural observations from the western margin of the Power Line intrusive complex show medium- to low-temperature solid-state deformation and metamorphic recrystallization textures (Fig. 6). Significant recrystallization of biotite and myrmekitic textures is associated with potassium feldspar porphyroclasts. Large feldspar grains display flame perthite nucleating along the margins of both large and small potassium feldspar grains, which could be indicative of high-differential stress (Pryer and Robin, 1995, 1996). In addition, cracks filled with quartz are present within plagioclase and potassium feldspar grains, which indicates that deformation occurred while melt was still present (Bouchez et al., 1992). Observations from the center of the unit show quartz and feldspar microstructures similar to those observed at the western margin of the Power Line intrusive complex.
Grain size reduction is evidenced by fine, recrystallized quartz and feldspar domains with core and mantle structures and ribbons of recrystallized grains. Large quartz grains show significant subgrain devolvement with abundant checkerboard extinction indicative of high-temperature dynamic recrystallization via grain boundary migration, while small (<200 um) subequant quartz is more indicative of subgrain rotation. Smaller, recrystallized quartz grains have a polygonal texture and are intermixed with equant feldspar.
The biotite from the Power Line unit is the most striking feature in thin section and varies significantly between the margin and the interior of the intrusive complex. The biotite texture of the margin is defined by fully recrystallized biotite mica fish in the horizontal plane that serve as an additional dextral shear-sense indicator. The biotite grain size is small compared to the interior, birefringence is high, and individual lathes are devoid of inclusions. The grains do not exhibit much internal deformation despite being realigned parallel to the fabric. In contrast, biotite from the interior sites is larger, rich in inclusions, and has undergone resorption, as grain margins are ratty and smeared. The presence of kinking and wavy extinction suggests that the internal biotite grains preserve some internal deformation and did not fully recrystallize during fabric development.
We interpret these features to represent a combination of high-temperature and mid–low-temperature deformation in the interior of the unit.
The Power Line intrusive complex yields the lowest magnetic susceptibility values of all units, 0.01–0.09 × 10−5. These low values indicate that the AMS fabric is carried by paramagnetic minerals. Rock magnetic experiments of the Power Line intrusive complex yield a negative susceptibility trend in temperature versus susceptibility, as well as flat hysteresis curves, both of which indicate a predominantly paramagnetic and diamagnetic mineral assemblage and a general lack of ferrimagnetic phases (see Supplemental Material1).
The magnetic foliation plane in the Power Line granodiorite strikes NW–SE to N–S and dips steeply. The magnetic fabrics are consistent with field measurements. The western margin of the Power Line intrusive complex preserves a more NW–SE-trending magnetic foliation but becomes progressively more N–S oriented on the eastern margin. The magnetic lineation of the Power Line granodiorite is generally subvertical. A subvertical N–S magnetic lineation is observed along the western marginal outcrops, whereas the easternmost outcrops yield a more vertical NW–SE magnetic lineation (Fig. 7).
The highest degrees of anisotropy are found along the eastern contact in the southeastern portion of the Nightingale Range (Fig. 8). Distinct clusters of strongly oblate- to triaxial-shaped ellipsoids and triaxial- to prolate-shaped ellipsoids are localized in certain regions of the pluton. The more oblate-shaped clusters of ellipsoids are found in the western and southeastern margins of the Nightingale Range, whereas the triaxial- to prolate-shaped clusters of ellipsoids are found in northern outcrops of the Power Line intrusive complex in the Nightingale Range.
The corrected degree of anisotropy also varies across the Power Line intrusive complex from 1.05 to 1.8. There appears to be no relationship between ellipsoidal shape and bulk magnetic susceptibility, nor does there appear to be a relationship between the corrected degree of anisotropy with bulk magnetic susceptibility. High susceptibilities do not correspond with higher degrees of anisotropy within this unit.
Sahwave Granodiorite (Sahwave Range Intrusive Suite)
We address the units of the Sahwave Range intrusive suite from youngest to oldest.
The Sahwave granodiorite exhibits a weak foliation that is defined by the preferred orientation of K-feldspar megacrysts and alignment of biotite laths. Field measurements of the weak mineral foliation trend N–S throughout the unit; lineation is not visible in the field. The eastern margin of the pluton hosts a 2-m-thick, N–S-striking, solid-state shear zone of mylonitized granodiorite.
The microstructures in the Sahwave granodiorite vary from the margin to the interior of the pluton. Microstructures from the margins of the pluton include coarse quartz grains with deeply sutured and cuspate-lobate grain boundaries that display weak checkerboard extinction (Fig. 6). Mafic phases—predominately biotite—are aligned parallel to a weak foliation defined by quartz elongation and the long axis of feldspar clasts. Plagioclase grains vary from <1 mm and subequant with slightly wavy boundaries to >2 cm with cuspate-lobate boundaries. Potassium feldspar is often poikilitic and contains smaller, subequant feldspar grains. Poikilitic potassium feldspar domains display patchy microcline tartan twinning and myrmekite around the margins.
Sites from the center of the unit show a bimodal feldspar population of large and small subequant grains. Larger grains of plagioclase feldspar show cuspate-lobate margins and sericitization within the core of the grain, with relatively unaltered grain margins. In this sample, we observed both patchy tartan twinning and flame perthite nucleating along the margins of both large and small potassium feldspar grains. The latter is indicative of high-differential stress (Pryer and Robin, 1995, 1996).
Quartz grains also show a bimodal size distribution. The larger grain-size population exhibits amoeboidal shapes, and cuspate lobate boundaries and chessboard extinction provide evidence of grain-boundary migration. The smaller grain size occurs in domains of ribboned quartz, in which relatively equant small quartz grains exist with abundant triple junctions. Some of the smaller recrystallized quartz grains show minor bulging recrystallization, as evidenced by bulges and small recrystallized grains along grain boundaries. Subhedral potassium feldspar grains also display a bimodal size distribution, with tartan twinning and flame perthite along the grain margins, cuspate-lobate grain boundaries, and the development of myrmekite. Large biotite grains show weak kinking and alignment. Iron oxides and euhedral titanite are more abundant in the center of the pluton.
The Sahwave granodiorite’s bulk magnetic susceptibility ranges from 2.5 × 10−3 in the center of the pluton to 0.75 × 10−3 at the margins. Temperature-versus-susceptibility analyses of all units of the Sahwave intrusive suite yield Curie point temperatures ranging from 560 °C to 580 °C, which are indicative of predominately titanomagnetite and titanomaghematite ferrimagnetic carriers. Hysteresis curves and isothermal remanent magnetization (IRM) analyses indicate that the ferrimagnetic phase is multidomain to pseudo-single domain for the Sahwave Range intrusive suite (Fig. 9). Thus, we interpret that titanomagnetite is the dominant carrier of the AMS signal, which typically indicates a straightforward correlation between field-based and magnetic fabrics.
The most detailed sampling was conducted within the Sahwave granodiorite with two transects (N–S and E–W) across the unit (Fig. 10). Oriented drill core sample sites from the Sahwave granodiorite were taken every 500 m in two transects (N–S and E–W) across the unit. The AMS fabric orientation vary from west to east across the Sahwave granodiorite. Magnetic foliations strike E–W in the west and NNW–SSE within the interior of the unit, with a clear gradation between these two orientations. The magnetic lineation (K1) orientation also varies from the interior of the pluton toward either margin. A steep to subvertical magnetic lineation is observed from the center of the pluton, which gradually shallows toward the western margin.
AMS ellipsoidal data exhibit prolate shape ellipsoids along the eastern margin and predominately oblate ellipsoids in the interior of the unit; triaxial ellipsoids are found between the two (Fig. 8). This pattern is mirrored along the western side, with prolate AMS ellipsoids near the contact with the granodiorite of Bob Springs.
Pj values are weaker along the margin (1.13) and increase toward the interior of the unit (1.2). A Pj value of 1.20 and above appears to be a reliable indicator of the presence of solid-state deformation in the Sahwave granodiorite, with lower Pj values correlating with the magmatic textures.
The Granodiorite of Bob Spring (Sahwave Range Intrusive Suite)
The granodiorite of Bob Spring is characterized as a medium-grained biotite granodiorite or granite with poikilitic K-feldspar phenocrysts of up to ~2 cm long. A weak foliation defined by mafic mineral alignment was occasionally observed in this unit but was difficult to measure in the field, and trends were variable. Part of the western margin of the Bob Spring pluton bulges into the Nightingale Range, and the fabric pattern observed in the unit approximately parallels the inferred intrusive contact. Lineation was not observed in the field.
Microstructural observations from thin sections show quartz-filled fractures in plagioclase, partially recrystallized quartz, and thin films of alkali feldspar lining many plagioclase laths. All sections reveal recrystallized subordinate plagioclase at the boundaries of large plagioclase porphyroclasts. We also observe deformed polysynthetic albite twins in plagioclase, and some examples of kinked crystals are present. Potassium feldspar crystals also show examples of deformational twins, and kinking and is commonly observed with myrmekite along the margins. Quartz crystals show cuspate-lobate boundaries, which indicate that some higher temperature deformation possibly occurred during development of the weak N–S fabric.
We interpret the microstructures in the granodiorite of Bob Spring as evidence of a predominately submagmatic fabric (Fig. 6).
The bulk magnetic susceptibility values of the granodiorite of Bob Spring range from 0.53 × 10−3 to 8.65 × 10−3. Lower magnetic susceptibilities were observed in the Nightingale Range (0.53–3.74 × 10−3), and higher susceptibilities were more prevalent in the Sahwave Range (4.18–8.65 × 10−3).
The AMS data from 23 granodiorites of the Bob Spring sites reveal a steep, N–S-striking magnetic foliation. The western margin of the pluton in the Nightingale Range yields a more NE–SW-striking foliation that gradually becomes N–S in the outcrops of the Sahwave Range. The magnetic lineation pattern varies (Fig. 10).
Oblate-, triaxial-, and prolate-shaped ellipsoids were identified in the unit, with oblate-shaped ellipsoids being the most common. Oblate-shaped ellipsoids are found in the Sahwave Range outcrops. Prolate- and triaxial-shaped ellipsoids are found in the Nightingale Range outcrops along the presumed western margin of the unit. The Pj values do not vary significantly across the granodiorite of Bob Spring (Fig. 8).
The Granodiorite of Juniper Pass (Sahwave Range Intrusive Suite)
The granodiorite of Juniper is exposed predominately in the Sahwave Range and is the most compositionally heterogeneous, varying from a granodiorite to a tonalite or a quartz diorite. Though compositionally variably, the Juniper Pass granodiorite has the most discernible and consistent field fabric of the intrusive suite plutons of the Sahwave Range. The strong E–W foliation is defined by the alignment of mafic minerals (biotite and hornblende) and euhedral plagioclase. Mafic schlieren and enclaves are parallel to the mineral fabric, with many of these enclaves also displaying elongation parallel to the mineral fabric.
Quartz grains display weak checkerboard undulatory extinction, and large grains (>1 mm) are often highly fractured (Fig. 6). Smaller quartz grains also show this texture, with some evidence for grain-size reduction by dynamic recrystallization methods (bulging recrystallization). Small biotite grains are unaltered and euhedral, whereas large grains display alteration. Hornblende shows significant alteration to biotite. Small plagioclase feldspar grains (<500 µm) are rounded, with cuspate/lobate boundaries. Poikilitic potassium feldspar displays patchy microcline tartan twinning and cracks infilled by quartz and/or plagioclase, which indicates brittle deformation in the presence of residual melt or fluids.
Microstructural observations from the granodiorite of Juniper Pass are interpreted to show evidence of both high-temperature solid-state and submagmatic fabrics, as textures from the interior of the unit are more deformed than what we observed at the margin.
The granodiorite of Juniper Pass yields a wide range of bulk magnetic susceptibility values from 0.01 × 10−3 to 8.02 × 10−3, although average values range between 2.24 × 10−3 and 3.69 × 10−3.
The AMS data from the granodiorite of Juniper Pass reveal a steep, E–W-striking magnetic foliation that is consistent with field measurements in the Sahwave Range. In contrast, the western margin of the granodiorite of Juniper Pass in the Nightingale Range yields a more NW–SE-trending foliation. The orientation of the magnetic lineation fabric is inconsistent across the unit and varies from site to site (Fig. 10).
Oblate-, prolate-, and triaxial-shaped ellipsoids are clustered in certain regions of the pluton (Fig. 8). Oblate-shaped ellipsoids (T values of 0.5–1) are found in the north, triaxial-shaped ellipsoids are found in the south (T values of −0.5 to 0.5), and prolate-shaped ellipsoids (T values of −1 to −0.5) are found in the western outcrops of the Nightingale Range.
The Pj values also vary significantly across the granodiorite of Juniper Pass, from 1.05 to 1.40. The mean Pj value of the sites is 1.16, with the sites exhibiting submagmatic fabrics with Pj values of 1.0–1.13. These submagmatic fabrics are observed predominately in the northern outcrops of the granodiorite of Juniper Pass in the Sahwave Range.
Field and Magnetic Fabrics of the Sahwave Range Intrusive Suite
The E–W field foliation observed across the granodiorite of Juniper Pass is consistent with AMS orientation data that define an E–W-striking, oblate fabric. We interpret the oblate magnetic ellipsoids, consistent foliation, and lack of a consistent lineation to result from flattening strains. These orientations, combined with the microstructures observed in granodiorite of Juniper Pass that documents submagmatic to high-temperature solid-state textures, suggest that flattening occurred during and after the emplacement of magma. Furthermore, when we compare the microstructures with the Pj values, we observe that higher degrees of anisotropy (1.20) correspond to sites with subsolidus microstructures. Thus, we interpret these higher Pj values to be a reliable indicator of high-temperature, solid-state deformation within the unit.
The NW–SE-striking, prolate fabrics of the granodiorite of the Juniper Pass sites in the Nightingale Range differ from the strong E–W fabric observed in the Sahwave Range. These prolate fabrics are similar in orientation and microstructure to those of the Power Line intrusive complex. This pattern suggests that the oldest component of the granodiorite of Juniper Pass experienced more deformation during the early stages of magmatic emplacement.
The granodiorite of Bob Spring was difficult to interpret due to sparser sampling; therefore, the projected fabric patterns are speculative. A consistent field fabric was difficult to observe, but AMS fabric data document a roughly N–S planar fabric. The microstructures indicate that submagmatic textures and Pj values are low despite having the highest mean susceptibilities.
The more constrictional strain pattern in the granodiorite of Bob Spring differs from the flattening strain patterns observed in the other units. From map patterns, the granodiorite of Bob Spring is seen “bulging” into the granodiorite of Juniper Pass in the Nightingale Range. We suggest that this pattern is likely a result of igneous intrusive processes rather than regional tectonics, consistent with similar patterns observed in the Sierra Nevada batholith during this time (e.g., Tikoff et al., 1999). The AMS data support this idea, as the internal fabric pattern of the granodiorite of Bob Spring roughly parallels the proposed intrusive contact. Another intrusive bulge occurs to the south, where the younger Sahwave granodiorite laterally intrudes the older granodiorite of Bob Spring. In this location, the fabric in the granodiorite of Bob Spring does not follow the contact, but instead yields a steep N–S-striking magnetic planar fabric. These lobate-shaped intrusive relationships may explain why there is no consistent lineation in the granodiorite of Bob Spring, as it is deformed by the Sahwave granodiorite in the submagmatic state.
The Sahwave granodiorite was sampled densely and yields the most compelling and complete fabric pattern. Bulk magnetic susceptibility in the Sahwave granodiorite varies slightly, with lower susceptibilities near the margin and higher susceptibilities toward the interior of the pluton. The field foliation of the Sahwave granodiorite is predominately NW–SE and steep, but the AMS data indicate that the magnetic foliation pattern strikes E–W in the west, rotates to nearly NNW–SSE within the interior of the unit, and maintains this orientation throughout the eastern margin. The magnetic lineation trends NW–SE and plunges subhorizontal to both the NW and SE across the unit. Pj values are weaker along the margin (1.13) and increase toward the interior of the unit (1.2). A value of Pj of 1.20 and above appears to be a reliable indicator of the presence of solid-state deformation in the Sahwave granodiorite, with lower Pj values correlating with the submagmatic microstructures.
Synthesis of Solid-State Fabrics in the Power Line Intrusive Complex
The field foliation of the Power Line intrusive complex is consistently NW–SE, and is corroborated by a steep NW–SE planar magnetic AMS fabric. The AMS data also reveal a steeply plunging linear magnetic fabric that varies slightly across the complex. A steep, south-plunging magnetic lineation occurs along the eastern margin, while a SE-oriented, moderately plunging magnetic lineation occurs on the western margin. These orientations loosely define a sigmoidal trajectory within the Power Line intrusive complex, which is consistent with dextral shear in the syn- to late-magmatic state.
Oblate-shaped ellipsoids dominate in the Power Line intrusive complex sites along the eastern margin and correspond with higher Pj values (1.2–14). We interpret the oblate-shaped ellipsoids from the eastern margin as indicators of flattening strains, which is consistent with transpressional kinematics. Triaxial- to prolate-shaped ellipsoids dominate along the western margin and yield lower Pj values (1.2).
Comparison of ca. 100 Ma Deformation along the Continental Margin
The deformation preserved in the Power Line intrusive complex is similar in geometry, composition, age, and age progression to that of the McCall and Owyhee segments of the western Idaho shear zone. Dates determined by U-Pb zircon laser ablation–inductively coupled plasma–mass spectrometry suggest a crystallization age of ca. 104 Ma for the Power Line intrusive complex (Trevino et al., 2021). The solid-state fabrics—particularly the vertical lineation—are not present in the units of the 94–88 Ma Sahwave Range intrusive suite. Thus, it appears that pure shear-dominated transpression did not occur during emplacement of the younger intrusive suite and is temporally constrained between 104 Ma and 94 Ma.
Therefore, all of the shear zone segments record pure shear-dominated transpressional fabrics at ca. 100 Ma. The western Idaho shear zone exhibits solid-state flattening fabrics that strike N–S to NE–SW. However, Tikoff et al. (2023b) interpret that the western Idaho shear zone fabrics have been rotated ~30° clockwise since their formation. Hence, the foliation would be restored to a NNW-striking orientation. In this orientation, they are parallel to the foliations in the Power Line intrusive suite. Both orientations are parallel to the inferred trend of the magmatic arc. The degree of fabric development in the Power Line intrusive suite is also consistent with that of the western Idaho shear zone, particularly in the Owyhee Mountains of SW Idaho (Benford et al., 2010). The Power Line intrusive complex, located 300 km south of the Owyhee segment of the western Idaho shear zone, is comparable in width to the McCall segment and in fabric development to the Owyhee segment (Fig. 11).
The pure shear transpressional fabrics of the Power Line intrusive complex are compatible with those documented at 100 Ma in the Sierra Nevada batholith (Table 1). The Courtwright shear zone (102–94 Ma; Torres-Andrade, 2022) and the Sing Lake shear zone (ca. 98 Ma; Krueger and Yoshinobu, 2018) are two such examples, consistent in both deformational style and age, to fabrics observed in the Power Line intrusive complex (104–94 Ma). Other dextral transpressional shear zones also occur in the Sierra Nevada batholith at this time (Table 1). We note that the lineation orientation plunges more shallowly in the Sing Lake shear zone relative to the deformation within the Power Line intrusive complex.
We hypothesize that the dextral transpressional deformation is continuous from the western Idaho shear zone through NW Nevada and into the Sierra Nevada batholith. The correspondence of the timing, kinematics, and along-strike location suggests that regional deformation is localized within the axis of active arc magmatism. Giorgis et al. (2008) and Tikoff et al. (2023a) attribute the formation of the western Idaho shear zone to the collision of the Insular terrane. If that is correct, the deformation in the Power Line intrusive complex could be attributed to the same causal mechanism. Fault-based reconstructions of the terranes of the Canadian Cordillera suggest that the southern end of the Insular terrane was located in northernmost California (Wyld et al., 2006) at 100 Ma, marginward of the Power Line intrusive complex. We note that the fault reconstructions of Wyld et al. (2006) are minimum offset reconstructions; paleomagnetically based reconstructions indicate significantly more offset but are still consistent with Insular terrane collision at NW Nevada latitudes at ca. 100 Ma (e.g., Tikoff et al., 2023a).
If the Insular terrane collision is the cause of the initiation of transpressional deformation along the axis of arc magmatism, it suggests that the Insular terrane collided outboard of the Sierra Nevada (Tikoff et al., 2023a). The paleomagnetic data (Enkin, 2006; Tikoff et al., 2023a) consistently indicate that the Insular terrane is located offshore California, as do recent models that involve tectonic reconstructions based on tomographic images of subducted slabs (tomotectonics; e.g., Sigloch and Mihalynuk, 2017; Clennett et al., 2020). As a result, a ca. 100 Ma dextral transpressional shear system would have been active from the Sierra Nevada through the Idaho batholith (Fig. 12).
Regardless of polarity, the collision would explain the long-standing problem of the cessation of arc magmatism in the east-central California, northwest Nevada, and Idaho magmatic arcs at 85 Ma (e.g., Bateman, 1992; Gaschnig et al., 2010). Since the Insular terrane contains a Late Cretaceous–Paleogene magmatic arc, a collision would result in the cessation of magmatism on North America (e.g., Maxson and Tikoff, 1996; Tikoff et al., 2023a). The hiatus between 100 Ma and 85 Ma is interpreted to record the time necessary to complete subduction of the descending plate. While these models are speculative, they emphasize the orogen-scale patterns of arc deformation at ca. 100 Ma (see Tikoff et al., 2023a). The plutons of northwestern Nevada clearly show this hiatus between 100 Ma and 85 Ma, and furthermore, the fabrics mirror this pattern of arc deformation at 100 Ma.
Synthesis of Microstructural and Magnetic Fabrics in the Sahwave Range Intrusive Suite
The Sahwave Range intrusive suite records a younger stage of both magmatism and deformation along the magmatic arc of the western U.S. We first summarize these fabrics in Figure 13 before comparing them to those of the Sierra Nevada batholith (Fig. 14).
The E–W planar field fabric observed across the granodiorite of Juniper Pass is consistent with AMS orientation data that define an E–W-oriented, oblate planar fabric. The granodiorite of Juniper Pass, however, yields neither an observable field lineation nor do AMS data reveal a consistent magnetic lineation orientation. We interpret these data to indicate flattening strains. These fabric orientations, combined with the microstructures that document submagmatic to high-temperature, solid-state microstructures, indicate that flattening occurred during and post-incremental magmatic emplacement. Submagmatic fabrics are observed predominately at sites of the northern outcrops of the granodiorite of Juniper Pass in the Sahwave Range. In contrast, the granodiorite of Juniper Pass sites in the Nightingale Range shows significant variability from the strong E–W fabric observed in the Sahwave Range. Most of these sites have higher degrees of anisotropy (>1.20) and exhibit subsolidus microstructures. Prolate fabrics in the Nightingale Range are NW–SE trending and are more similar in orientation and texture to those of the Power Line intrusive complex (Fig. 10). This pattern could suggest that the oldest component of the granodiorite of Juniper Pass may have experienced some deformation from an event during the early stages of magmatic emplacement.
The granodiorite of Bob Spring was difficult to interpret due to sparser sampling; therefore, the projected fabric patterns are speculative. The pluton records a roughly N–S planar fabric with dominantly submagmatic microstructures. The granodiorite of Bob Spring also yields predominantly prolate- to triaxial-shaped ellipsoids that are indicative of constriction to plane strains. The more constrictional strain pattern in the granodiorite of Bob Spring differs from the flattening strains observed in all other units.
As seen in map patterns, a lobe of the granodiorite of Bob Spring forcefully intrudes into the granodiorite of Juniper Pass in the Nightingale Range. We interpret that this pattern is likely a result of igneous intrusive processes rather than regional tectonics. It is worth noting that similar lobes—often interpreted as intrusive bulges—occur in the time-equivalent intrusive suites of the Sierra Nevada batholith (e.g., Tikoff et al., 1999; Memeti et al., 2010). The AMS data support this bulging lobe hypothesis, as the internal fabric pattern of the granodiorite of Bob Spring roughly parallels the proposed intrusive contact. Another intrusive bulge occurs to the south, where the younger Sahwave granodiorite laterally intrudes the older granodiorite of Bob Spring. In this location, the fabric in the granodiorite of Bob Spring does not follow the contact but yields a steep N–S-striking magnetic foliation. These lobate-shaped intrusive relationships may explain the lack of consistent lineation in the granodiorite of Bob Spring as it is deformed by the Sahwave granodiorite in the submagmatic state.
The Sahwave granodiorite yields the most compelling and complete fabric pattern, as a result of being: (1) well-exposed, (2) densely sampled, and (3) not overprinted by younger magmatism. The field foliation of the Sahwave granodiorite was predominately NW–SE and steep, as corroborated by the AMS data. The AMS data reveal a magnetic foliation pattern that strikes more E–W in the west, rotates to nearly NNW–SSE within the interior of the unit, and maintains this orientation through the eastern margin. The magnetic lineation trends NW–SE and plunges shallowly to both the NW and SE across the unit. A Pj value of ≥1.20 is a reliable indicator of the presence of solid-state deformation in the Sahwave granodiorite, with lower Pj values correlating with the submagmatic textures. Pj values are weaker along the margin (1.13) and increase toward the interior of the unit (1.2).
We interpret the spatial patterns of Pj and T as evidence of a strain gradient across the Sahwave granodiorite. The highest Pj values are observed in the interior of the pluton and are associated with a flattening strain (oblate ellipsoid). The lower Pj values associated with constrictional strain (prolate-shaped ellipsoids) are observed along the eastern margin. Plane strains (triaxial) and predominately flattening strains (oblate) occur in a gradient that increases in Pj toward the interior of the unit. This pattern is mirrored along the western side, with prolate-shaped ellipsoids near the contact with the granodiorite of Bob Springs.
Comparison of Fabric Development in Sahwave Range Intrusive Suite with Coeval Intrusive Suites of the Sierra Nevada Batholith
Van Buer and Miller (2010) recognized the striking similarity in composition of the Sahwave Range intrusive suites and the time-equivalent intrusive suites of the Sierra Nevada batholith, specifically the Tuolumne intrusive suite. The plutons of the Tuolumne intrusive suite show lobes of younger plutons intruded into older plutons (e.g., Memeti et al., 2010) and gradational contacts (e.g., Bateman, 1983). We observe the same features in the Sahwave Range intrusive suite.
We also observe that the fabric orientations between these intrusive suites are also similar. AMS magnetic lineations from the Sahwave granodiorite plunge moderately to the SSE and NNW. Additionally, the southeast-plunging lineations are combined with dextral shear-sense indicators observed near the eastern contact (Fig. 12). This pattern is consistent with both upward ascent and emplacement of the pluton as well as northward horizontal displacement. Similar AMS, microstructural, and field patterns are observed in the Tuolumne intrusive suite. Tikoff et al. (2005) documented the presence of shallow–moderately southeast-plunging lineations along the northeastern margin of the Tuolumne intrusive suite (also see Tikoff and Greene, 1997; Cao et al., 2015). Furthermore, Tikoff et al. (2005) demonstrated a change from magnetic foliations from NW-oriented near the contact to E–W-oriented in the center of the Cathedral Peak granodiorite of the Tuolumne intrusive suite. This change in foliation orientation is also observed in the field (Bateman, 1983). Another similarity is the presence of late stage, low-temperature localized shear zones. The movement is dextral in cases where the shear zones are parallel to contacts, and sinistral in cases where they form at a high angle to contacts (e.g., Martel et al., 1988). The late-stage dextral shear zone parallel to the contact in the Sahwave granodiorite is consistent with this trend.
The fabrics and internal structures of the Sahwave Range and Tuolumne intrusive suites are not identical (Table 2). There are three problems with a direct comparison of these units: (1) a systematic AMS study of the Half Dome granodiorite has not been conducted; (2) the granodiorite of Bob Spring is so poorly exposed that larger-scale trends are inherently interpretational; and (3) the granodiorite of Juniper Pass has different fabric patterns relative to the Kuna Crest granodiorite, although their composition and timing are equivalent. The E–W foliation in the granodiorite of Juniper Pass is only locally observed in the Kuna Crest granodiorite, in which the foliation mostly trends NNW. We attribute these minor variations to differences in igneous processes associated with emplacement of the pluton.
How to interpret magmatic fabrics in plutons that were incrementally emplaced remains an unanswered question. The range of U-Pb dates of zircon from the Tuolumne intrusive suite likely requires that incremental emplacement occurred in the Sierra Nevada batholith (Coleman et al., 2004). The same incremental emplacement is inferred for the plutons of the Sahwave Range intrusive suite, based on the similarity of size, composition, and timing of the plutons. A typical way to interpret magmatic fabrics is that they reflect the flow of a viscous magma—in response to both regional and internal forces—prior to full crystallization. But this interpretation requires that large portions of the pluton remained as magmas for long periods of time, >1 m.y., to produce a coherent pattern across the entire pluton. Alternatively, significant post-magmatic textural and mineralogical modification may have occurred as a result of multiple pulses of magmatic injection. In this case, late-stage recrystallization—induced by the intrusion of magmatic batches and/or hydrothermal fluids—would tend to eradicate the microstructural record of any earlier solid-state processes (e.g., Bartley et al., 2019). We acknowledge the ambiguity in how to interpret the magmatic fabrics in both of these plutonic systems, although we make the standard assumption that some silicate melt was present when the magmatic fabric formed.
In general, dextral transpressional deformation in the Sierra Nevada batholith appears to have occurred throughout the entire emplacement period of the Late Cretaceous intrusive suites (Tikoff and St. Blanquat, 1997). Dextral shearing certainly occurred on the NE margin of the Tuolumne intrusive suite during emplacement (e.g., Cao et al., 2015), but the presence of shearing during the earlier (92–88 Ma) phases of emplacement remains controversial. Bartley et al. (2005) documented dextral motion during emplacement of the ca. 92 Ma Mt. Lamarck granodiorite in the Mono Pass intrusive suite; the en echelon nature of the Mono Pass intrusive suite allows better resolution of the earlier phases of deformation.
In summary, we interpret the fabrics across all units of the Sahwave Range intrusive suite to be a distributed zone of non-coaxial, dextral deformation that was active during magmatic emplacement from 94 Ma to 88 Ma. Furthermore, this dextral shearing facilitated magmatic emplacement and is part of a larger shear zone system along the trend of the Late Cretaceous arc in western Nevada and eastern California.
A Tectonic-Assisted, Pull-Apart Emplacement Model for the Sahwave Range Intrusive Suite
The striking similarity of these two systems allows us to utilize the better-exposed and better-constrained intrusive suites in the Sierra Nevada batholith to interpret the emplacement of the Sahwave Range intrusive suite. Tikoff and St. Blanquat (1997) proposed that the large plutons in the eastern Sierra Nevada were emplaced into a shear zone system that created en echelon pull-apart zones (P-shear model; Tikoff and Teyssier, 1992). In this model, pluton emplacement occurs in dilatational jogs created by these shear zones, which allows the accumulation of such large plutons. In cases where magmatic supply outpaces shear zone movement, lobes or bulges are formed. However, the magmatic emplacement also results in softening of the crust, which facilitates greater deformation in these systems (Tommasi et al., 1994).
We interpret that the Sahwave Range intrusive suite was emplaced into a pull-apart zone during dextral shearing as a result of oblique terrane collision. The granodiorite of Juniper Pass intruded the Jungo terrane and Power Line intrusive complex at 94 Ma during the initial stages of strike-slip movement and extension. Incremental emplacement continued for millions of years, as the granodiorite of Bob Springs intruded the granodiorite of Juniper Pass, causing a net increase of heat and mass (and fluids) to the system. Incremental emplacement and evolving deformational conditions localized simple shear into the weaker, hotter, younger rocks of the granodiorite of Bob Spring. Magnetic fabrics indicate both upward and horizontal flow from 94 Ma to 88 Ma within the granodiorite of Bob Spring, which is consistent with dextral shearing during emplacement. Simple shear deformation persisted during the emplacement of the younger Sahwave granodiorite as deformation further localized during and after emplacement. We interpret the gradational clockwise rotation of fabrics across all units of the Sahwave batholith (E–W in the granodiorite of Juniper Pass to NW–SE in the Sahwave granodiorite) to result from non-coaxial, dextral shearing during the incremental emplacement of magma. Moreover, the fabric rotation is both gradual (at the batholith scale) and discrete (at the individual pluton scale), and presumably reflects the interplay between magmatism and deformation during the incremental emplacement of the Sahwave Range intrusive suite. Finally, this emplacement model is supported by the presence of late-stage shearing on the eastern side of the Sahwave granodiorite, and the similarity of these fabric patterns to those of the eastern side of the Tuolumne intrusive suite.
Because of subsequent Basin and Range extension, the inferred large-scale shear zones are admittedly cryptic for the Sahwave Range intrusive suite. Major normal faults occur on the sides of the intrusive suite, where the suspected shear zone would occur. It is likely, in fact, that vertical Cretaceous shear zones would be preferentially reactivated by Miocene-era normal faults, as observed in western Idaho (e.g., Tikoff et al., 2001; Giorgis et al., 2006).
We conducted a first-order restoration of the northwestern portion of the Basin and Range, to place the plutonic rocks of northwestern Nevada into their correct Cretaceous position between the Sierra Nevada and Idaho batholiths. We retro-deformed the Basin and Range extension following restorations by Wernicke and Snow (1998) and McQuarrie and Wernicke (2005). The Sierra Nevada batholith is restored southward by ~300 km, based on estimates of movement for the eastern California shear zone system and Walker Lane shear zone (e.g., Atwater and Stock, 1998).
For the 100 Ma restoration, the Blue Mountain terranes of western Idaho and eastern Oregon also restore southward by ~400 km. This reconstruction would create continuity between the Blue Mountain terranes and the Late Triassic Jungo terrane, as well as alignment among the Power Line, Owyhee, and McCall segments of pure shear-dominated transpression.
In addition, for the 85 Ma restoration, ~400 km of dextral offset along the trace of the Sierra Nevada batholith was determined by the offset of the Sr 0.706 line associated with the Mina deflection (Kistler, 1990). This restoration would move terranes in the western part of the Sierra Nevada Mountains southward relative to the eastern Sierra Nevada Mountains (e.g., Tikoff et al., 2023). The net effect of this restoration is the creation of a relatively continuous magmatic arc from California to Idaho in the Early Cretaceous. Other authors have drawn similar conclusions (Dickinson, 1970; Van Buer and Miller, 2010). Our structural comparison further validates this conclusion, as the diffuse shearing recorded by the fabrics of the Sahwave Range intrusive suite provides structural evidence of an additional northern segment of the Sierra Crest shear zone system in Nevada.
We documented fabrics associated with two distinct shearing events in the plutonic suites of the Sahwave and Nightingale Ranges of northwestern Nevada, which occupy an intermediate position between the Sierra Nevada and Idaho batholiths. We concluded that the two periods of dextral deformation were continuous along the axis of magmatism parallel to the continental margin of the western U.S.
The Power Line intrusive complex records ca. 100 Ma solid-state deformation. Field fabrics, AMS, and microstructures all support pure shear-dominated dextral transpression kinematics. Deformation in the Power Line intrusive complex must have ceased by 94 Ma, because similar fabrics are not observed in the 94 Ma granodiorite of Juniper Pass. This deformation is consistent with pure shear-dominated transpressional deformation in both the western Idaho shear zone and the central part of the Sierra Nevada batholith at this time. These data suggest that a regional transpression event initiated at ca. 100 Ma that was focused along the axis of the magmatic arcs in the western U.S.
The fabrics in the Sahwave Range intrusive suite are also similar to those of the Tuolumne intrusive suite. The younger Bob Spring and Sahwave plutons record N–S-trending, magnetic foliations, although lineations are more shallowly plunging in the latter pluton. In contrast, the Juniper Pass pluton contains E–W-trending foliation and vertical lineations. We interpret these fabrics as records of syn-emplacement dextral shearing associated with the same wrench-dominated transpression recorded by the Sierra Crest shear zone system.
Dextral transpressional fabrics formed at 100–85 Ma in the plutons of northwestern Nevada and extend northward into the western Idaho shear zone and southward into the Sierra Nevada batholith. This continuity suggests a through-going dextral shear zone along the trend of active magmatism in the central part of the western U.S. Given that the Insular terrane was located offshore of northwestern Nevada and Idaho at 100 Ma (e.g., Wyld et al., 2006)—and paleomagnetic data indicate that it extended much farther southward (e.g., Enkin, 2006; Tikoff et al., 2023a)—we ascribe this deformation to the collision of the Insular superterrane.
This project was supported by two Geological Society of America Graduate Student Research Grants. We gratefully acknowledge John Geissman of the University of Texas at Dallas for use of the rock magnetism lab, Claire Ruggles and Amanda Feltz for core preparation, and lastly the Structure Group at the University of Wisconsin–Madison for many useful discussions that aided in the preparation of this paper. We also thank John Bartley and Sean Regan, whose reviews greatly improved the final version of this manuscript.