Simultaneous batholith emplacement, terrane/continent collision, and oroclinal bending in the Blue Mountains Province, North American Cordillera
2015; Wiley; Volume: 34; Issue: 6 Linguagem: Inglês
10.1002/2015tc003859
ISSN1944-9194
AutoresJiří Žák, Kryštof Verner, Filip Tomek, František V. Holub, Kenneth Johnson, Joshua Schwartz,
Tópico(s)earthquake and tectonic studies
ResumoTectonicsVolume 34, Issue 6 p. 1107-1128 Research ArticleFree Access Simultaneous batholith emplacement, terrane/continent collision, and oroclinal bending in the Blue Mountains Province, North American Cordillera Jiří Žák, Corresponding Author Jiří Žák Institute of Geology and Paleontology, Faculty of Science, Charles University, Prague, Czech Republic Correspondence to: J. Žák, jirizak@natur.cuni.czSearch for more papers by this authorKryštof Verner, Kryštof Verner Czech Geological Survey, Prague, Czech Republic Institute of Petrology and Structural Geology, Faculty of Science, Charles University, Prague, Czech RepublicSearch for more papers by this authorFilip Tomek, Filip Tomek Institute of Geology and Paleontology, Faculty of Science, Charles University, Prague, Czech Republic Institute of Geology, Czech Academy of Sciences, Prague, Czech RepublicSearch for more papers by this authorFrantišek V. Holub, František V. Holub Institute of Petrology and Structural Geology, Faculty of Science, Charles University, Prague, Czech RepublicSearch for more papers by this authorKenneth Johnson, Kenneth Johnson Department of Natural Sciences, University of Houston-Downtown, Houston, Texas, USASearch for more papers by this authorJoshua J. Schwartz, Joshua J. Schwartz Department of Geological Sciences, California State University, Northridge, California, USASearch for more papers by this author Jiří Žák, Corresponding Author Jiří Žák Institute of Geology and Paleontology, Faculty of Science, Charles University, Prague, Czech Republic Correspondence to: J. Žák, jirizak@natur.cuni.czSearch for more papers by this authorKryštof Verner, Kryštof Verner Czech Geological Survey, Prague, Czech Republic Institute of Petrology and Structural Geology, Faculty of Science, Charles University, Prague, Czech RepublicSearch for more papers by this authorFilip Tomek, Filip Tomek Institute of Geology and Paleontology, Faculty of Science, Charles University, Prague, Czech Republic Institute of Geology, Czech Academy of Sciences, Prague, Czech RepublicSearch for more papers by this authorFrantišek V. Holub, František V. Holub Institute of Petrology and Structural Geology, Faculty of Science, Charles University, Prague, Czech RepublicSearch for more papers by this authorKenneth Johnson, Kenneth Johnson Department of Natural Sciences, University of Houston-Downtown, Houston, Texas, USASearch for more papers by this authorJoshua J. Schwartz, Joshua J. Schwartz Department of Geological Sciences, California State University, Northridge, California, USASearch for more papers by this author First published: 08 May 2015 https://doi.org/10.1002/2015TC003859Citations: 12AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract The North American Cordillera is a classic example of accretionary orogen, consisting of multiple oceanic terranes attached to the western margin of Laurentia during the Mesozoic times. Although the Cordillera is linear for most parts, terrane boundaries are at a high angle to the overall structural grain in several segments of the orogen, which has been a matter of longstanding controversy as to how and when these orogenic curvatures formed. This paper discusses mechanisms, kinematics, and timing of initiation of one of these major curvatures, the Blue Mountains Province in northeastern Oregon. Here magmatic fabric patterns and anisotropy of magnetic susceptibility in the Wallowa batholith record three phases of progressive deformation of the host Wallowa terrane during Early Cretaceous. First is terrane-oblique ~NE-SW shortening, interpreted as recording attachment of the amalgamated oceanic and fringing terranes to the continental margin during dextral convergence at ~140 Ma. Deformation subsequently switched to pure shear-dominated ~NNE-SSW shortening associated with crustal thickening, caused by continued impingement of the amalgamated Blue Mountains superterrane into a presumed westward concave reentrant in the continental margin at ~135–128 Ma. Upon impingement (at ~126 Ma), the northern portion of the superterrane became "locked," leading to reorientation of the principal shortening direction to ~NNW-SSE while its still deformable southern portion rotated clockwise about a vertical axis. We thus propose oblique bending as the main mechanism of the orocline formation whereby horizontal compressive forces resulting from plate convergence acted at an angle to the terrane boundaries. Key Points Wallowa batholith was emplaced during protracted terrane/continent collision Blue Mountains superterrane impinged into a continental reentrant Pluton fabrics record Early Cretaceous onset of oroclinal bending 1 Introduction The North American Cordillera developed as an accretionary orogen along the western margin of Laurentia during the Mesozoic to Cenozoic times (Figure 1) [e.g., Coney et al., 1980; Monger, 1997; Dickinson, 2004; Piercey and Colpron, 2009; Hildebrand, 2013]. Although the polarity and significance of the past subductions have been a matter of debate [e.g., Johnston, 2008; Hildebrand, 2009, 2013; Sigloch and Mihalynuk, 2013], the prevailing view is that the overall eastward motion of the paleo-Pacific basin [e.g., Hamilton, 1969; Atwater, 1970; Engebretson et al., 1985], itself composed of several oceanic plates, brought a number of Late Paleozoic to Early Mesozoic intraoceanic-arc systems and associated sedimentary basins (in some reconstructions inferred to compose a large ribbon continent) [e.g., Johnston, 2001, 2008; Hildebrand, 2009, 2013] to the proximity of the Laurentian margin. These oceanic units were then successively attached to and variously displaced along this continental margin as tectonostratigraphic terranes (Figure 1). Much of the western North American Cordillera is linear and roughly parallels the leading continental edge, however, some portions exhibit a significant curvature and are at a high angle to the overall ~NNW-SSE tectonic grain (Figure 1; present-day coordinates are used throughout this paper). From north to south, the major Cordilleran orogenic curvatures include the Bering Strait region northwest of Alaska [e.g., Patton and Tailleur, 1977; Amato et al., 2004] (not shown in Figure 1), several oroclines in central and southern Alaska [e.g., Johnston, 2001; Glen, 2004], the Southern Vancouver Island orocline [Johnston and Acton, 2003], the Blue Mountains Province (Figure 1), forming what was variously referred to as the Columbia or Idaho orocline [e.g., Carey, 1958; Hamilton and Myers, 1966; Taubeneck, 1966; Greenwood and Reid, 1969] around an area covered by Tertiary volcanic and sedimentary rocks (Figure 1), and an orocline at the southern termination of the Sierra Nevada batholith [McWilliams and Li, 1985]. Figure 1Open in figure viewerPowerPoint Schematic geologic map showing the principal tectonostratigraphic terranes, their inferred afinities, and major oroclines in the North American Cordillera. Base map modified from Piercey and Colpron [2009]. As exemplified by the Blue Mountains Province, large uncertainty persists regarding the driving forces, kinematics, exact amount of rotation, and timing of formation of these orogenic curvatures. In this paper, we examine in detail a portion of the Early Cretaceous Wallowa batholith that, on a regional scale, intrudes the "hinge" of the Blue Mountains orocline (Figures 2 and 3). Furthermore, the existing structural and geochronologic data establish that batholith emplacement was broadly coeval with collision of the previously amalgamated oceanic terranes ("the Blue Mountains superterrane"; Figure 2) with the North American craton. The Wallowa batholith is thus one of the key areas to explore the nature of crustal deformation, kinematics, and temporal relations of plutonism to terrane/continent collision and to vertical-axis lithospheric rotations in the Cordilleran orogen. Figure 2Open in figure viewerPowerPoint Simplified geologic map showing terranes, their boundaries, other principal tectonic features, and main plutonic units in the Blue Mountains Province accreted to the North American craton. The Wallowa batholith intruded into the hinge of the orogenic curvature. Redrafted from Schwartz et al. [2011a]. Figure 3Open in figure viewerPowerPoint Simplified geologic map of the Wallowa batholith, which is largely concealed beneath the Tertiary Columbia River Basalt Group. The batholith is composite and consists of four component intrusions emplaced over a time span of about 15 Myr during Early Cretaceous. Geology compiled from Walker [1979] and Taubeneck [1987] and radiometric ages from Johnson et al. [2011]. Below, we first briefly review the principal tectonic elements of the Blue Mountains Province and geology of the Wallowa batholith and then concentrate on the host rock deformation structures and multiple magmatic to solid-state fabrics recorded in the batholith. The field observations are complemented by a magnetic fabric analysis in three main component plutons of the batholith to obtain an independent quantitative information on their internal structure and to infer possible strain patterns recorded by these plutons. Finally, we use these data sets as a background for discussion on kinematics and potential geodynamic causes of the vertical-axis lithospheric rotations in the North American Cordillera. 2 Principal Lithotectonic Elements of the Blue Mountains Province 2.1 Overview The Blue Mountains Province is an erosional inlier that exposes the uplifted Late Paleozoic to Mesozoic variably metamorphosed basement rocks from beneath Tertiary and Quaternary deposits and Columbia River flood basalts. The basement comprises the following principal lithotectonic units [e.g., Dorsey and LaMaskin, 2007, 2008; LaMaskin et al., 2009, 2011; Schwartz et al., 2010, 2011a]: The outboard Wallowa terrane (Figure 2) is an oceanic island arc assemblage consisting of Permian and Triassic volcanic and volcaniclastic rocks. The island arc volcanic complex is overlain unconformably by Triassic to Lower Jurassic siliciclastic and limestone successions of the Hurwal and Martin Bridge Formations, respectively [e.g., Stanley et al., 2008], and these rocks are in turn unconformably overlain by a Middle to Upper Jurassic flysch-like succession [LaMaskin et al., 2008]. The Wallowa terrane was correlated with the Wrangellia or Stikinia terranes farther north in the Canadian Cordillera [e.g., Sarewitz, 1983; Mortimer, 1986; Wernicke and Klepacki, 1988; Schwartz et al., 2011a]. The less extensively exposed, inboard Olds Ferry terrane (Figure 2) is also an island arc complex that consists of Middle Triassic to Lower Jurassic weakly metamorphosed volcanic and volcaniclastic rocks of chiefly andesitic composition [Brooks and Vallier, 1978; Vallier, 1995]. This terrane was interpreted as a fringing arc complex that developed along the North American passive margin [e.g., Ferns and Brooks, 1995]. The Baker terrane (Figure 2) is a subduction–accretionary wedge–fore-arc complex located between the two island arc assemblages and was thrust over the Wallowa terrane during Middle to Late Jurassic terrane convergence [Ferns and Brooks, 1995; Schwartz et al., 2010; Žák et al., 2012a]. The terrane contains fault-bounded island arc igneous and sedimentary rocks ranging in age from Middle Devonian to Early Jurassic and extensively disrupted fragments of ocean floor, including mélanges with blocks of moderate-pressure metamorphic rocks [Schwartz et al., 2011a]. Part of the Baker terrane is overlain unconformably by a Permian to Triassic siliciclastic successions [Ferns and Brooks, 1995; Schwartz et al., 2011a]. The Izee unit (Figure 2) comprises Triassic and Jurassic sedimentary successions that can be divided into two marine siliciclastic Upper Triassic to Early to early Late Lower Jurassic megasequences separated by an angular unconformity [Dorsey and LaMaskin, 2007]. The Izee basin was earlier interpreted as a separate fore-arc basin terrane [Dickinson, 1979] but recently was reinterpreted as a regional overlap succession that rests unconformably upon the Baker and Olds Ferry terranes [Dorsey and LaMaskin, 2007]. To the east, the Blue Mountains oceanic terranes are juxtaposed against the western margin of the North American craton (Laurentia) along the Salmon River suture zone (Figure 2). This zone comprises several west dipping thrust sheets derived from the Wallowa terrane, pinches out the Baker and Olds Ferry terranes, and is separated from the easterly Cretaceous-Tertiary plutons of the Idaho batholith along the Western Idaho shear zone. The Salmon River suture zone also coincides with the 0.706Sr line which marks an abrupt increase in contamination of plutonic rocks by radiogenic Precambrian continental crust [e.g., Armstrong et al., 1977; Manduca et al., 1992, 1993; Giorgis et al., 2005]. Although the Salmon River suture zone records a protracted kinematic history [e.g., Manduca et al., 1993; McClelland et al., 2000; Gray and Oldow, 2005; Giorgis et al., 2008], most relevant for this study are the Early Cretaceous events coeval with emplacement of the Wallowa batholith. Recent petrologic data and Sm-Nd garnet ages suggest prograde amphibolite-facies metamorphism and garnet growth at ~141–124 Ma and were interpreted as recording crustal thickening via stacking of thrust sheets and dating the collision of the previously amalgamated terranes with the western North American margin [Selverstone et al., 1992; Getty et al., 1993; McKay, 2011; Stowell et al., 2014]. (6) To the northeast, the Wallowa terrane, Salmon River belt, and Western Idaho shear zone abut against the North American craton margin. In the map (Figure 2), all these units as well as the Sri isopleth take an abrupt, approximately 90° bend from ~NNE-SSW in the east to ~E-W in the north; the bend has been referred to as the Syringa embayment. The origin of the bend has been a matter of controversy (see Lund et al. [2008] for discussion). In one view, the embayment is an inherited preaccretion feature of the rifted North American craton margin [Schmidt et al., 2003, 2009] with the ~E-W segment representing a transform fault and the ~NNE-SSW segment representing part of the now shortened rift structure [Tikoff et al., 2014]. Recent paleomagnetic data suggest that the original orientation of the two segments may have been 60° and 330°, respectively [Tikoff et al., 2014]. In an opposite view, formation of the bend is a result of sinistral transpressional shearing where the NNE-SSW trending assemblages have been truncated along the Late Cretaceous ~NW-SE to ~E-W trending Orofino shear zone (Figure 2; active from ~90 to ~70 Ma) [McClelland and Oldow, 2007]. 2.2 Geometry of the Terrane Boundaries and Vertical-Axis Block Rotations In a regional map view, the terrane boundaries in the Blue Mountains Province generally trend ~E-W in the west and then continuously reorient to the ~NNE-SSW trend in the east (Figure 2). The ~E-W trend of the Blue Mountains terranes is at a high angle to the ~NNW-SSE trend of the Sierra Nevada and Canadian Cordillera terranes (Figure 1). The curved terrane boundaries in the Blue Mountains Province are thus suggestive of significant clockwise vertical-axis rotation (Figure 2), a notion that is supported by several paleomagnetic studies (see Housen and Dorsey [2005] for discussion). Wilson and Cox [1980] inferred rotation by 60° ± 29° on the basis of samples from plutonic and contact metamorphic rocks of the Wallowa batholith and Baker terrane. Similarly, Hillhouse et al. [1982] estimated rotation by 66° ± 21° from Upper Triassic volcanogenic rocks of the Wallowa terrane. Cretaceous sedimentary rocks of the Mitchell Inlier (Figure 2), recently interpreted by Schwartz and Johnson [2014] as separated from the central Blue Mountains by a large-magnitude shear zone, indicate a lesser amount of rotation of 37° ± 7°. The rotation was resolved into 21° from the mid-Cretaceous to early/middle Eocene and an additional 16° after Eocene [Housen and Dorsey, 2005; see also Grommé et al., 1986]. It follows from the above that the paleomagnetic data from igneous and metamorphic rocks lack paleohorizontal control and involve an unknown amount of tilt of the sampled units and that large rotations are consistently derived from older rocks in the Blue Mountains Province; thus, all of the above estimates may be correct. 3 Geology of the Wallowa Batholith The Wallowa batholith is the largest (~620 km2) of multiple compositionally diverse plutonic bodies that intruded the Blue Mountains Province (Figure 2) [Krauskopf, 1943]. The batholith is composite and from north to south consists of four main plutons (Figures 3 and 4) [Taubeneck, 1987; Johnson et al., 2011]: Pole Bridge (140.2 ± 1.4 Ma), Hurricane Divide (130.2 ± 1.0 Ma), Craig Mountain (125.6 ± 0.6 Ma), and Needle Point (130.8 ± 1.5 Ma; not examined in this study). Overlapping ages and identical compositions of the ~130 Ma Hurricane Divide and Needle Point plutons (K. Johnson, unpublished data) suggest that they are virtually the same intrusion bifurcated by the younger Craig Mountain pluton (Figure 3). From north to south, the interpluton contacts change orientation from ~NE-SW to ~E-W and define a fan-like pattern, with the youngest Craig Mountain pluton in the middle tapered westward. In addition, the Sawtooth stock dated at 129.9 ± 2.1 Ma (all ages are based on U-Pb sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) measurements of igneous zircons) [Johnson et al., 2011] intruded the easterly batholith host rock and is correlative both in terms of composition and age with the Hurricane Divide pluton (Figures 3 and 4). Figure 4Open in figure viewerPowerPoint Structural map of the northeastern portion of the Wallowa batholith and its host rocks. Note different magmatic to solid-state fabric patterns in each pluton. Rocks from the Pole Bridge, Hurricane Divide, and Sawtooth plutons are broadly of the same lithology, ranging from low-silica tonalite along pluton margins to amphibole-biotite granodiorite in pluton cores. Minor hornblende gabbro bodies intrude all of the tonalite-granodiorite plutons. These plutons are interpreted to be derived from a depleted mantle source during Late Jurassic-Early Cretaceous crustal thickening broadly coeval with thrust loading in the Salmon River belt [Johnson et al., 2011]. On the other hand, the Craig Mountain pluton is more felsic and primarily granodiorite. Rocks of the Craig Mountain pluton are consistent with partial melting of thickened crust [Johnson et al., 2011] following attachment of the Blue Mountains superterrane to the continental margin at ~141–124 Ma. The last stage of Wallowa magmatism is represented by small satellite bodies of cordierite-bearing trondhjemite to the south of the main batholith [e.g., Taubeneck, 1964; Johnson et al., 1997, 2002]. In summary, existing geochemical data indicate that the Wallowa batholith formed in a suprasubduction setting associated with amalgamation of the Blue Mountains superterrane to the North American craton [Johnson et al., 2011]. However, it remains unclear, and beyond the scope of this paper, whether Wallowa plutonism formed above an oceanic plate subducting eastward (Farallon plate) or westward (oceanic basin once underlying the Salmon River belt) beneath the Blue Mountains superterrane. Contrasting views regarding subduction zone polarity are presented in Selverstone et al. [1992], McClelland et al. [2000], Dorsey and LaMaskin [2007, 2008], and Schwartz et al. [2011b, 2014, and references therein]. The remainder of the Wallowa batholith is largely concealed by basalt flows of the Miocene Columbia River Group; intrusive contacts against the host rocks of the Wallowa oceanic arc terrane are exposed only along its northeastern and southeastern margins (Figures 3 and 4) [Krauskopf, 1943; Weis et al., 1976]. In a stratigraphic order, these variably metamorphosed volcanic and sedimentary arc-related host rocks comprise (1) a Triassic succession, the base of which is not exposed, of altered basic to intermediate volcanic rocks and marine mudstones, sandstones, and conglomerates, overlain conformably by (2) an ~300–460 m thick succession of Upper Triassic (Carnian-Norian) limestone and calcareous shales of the Martin Bridge Formation, and by (3) Upper Triassic to Lower Jurassic (Norian-Sinemurian) bedded siliciclastic sedimentary rocks of the Hurwal Formation comprising shales, siltstones, and quartzites of unknown total thickness [Weis et al., 1976; Stanley et al., 2008]. Both successions have been heterogeneously overprinted by contact metamorphism which produced a variety of hornfels from siliciclastic protoliths and was most intense in the carbonate lithologies. The latter are commonly recrystallized to marbles and calc-silicate rocks with tremolite, garnet, epidote, diopside, and wollastonite [Krauskopf, 1943]. Away from the batholith, the degree of regional metamorphism is low in general and does not exceed greenschist-facies conditions, consistent with Al in hornblende barometry indicating a shallow emplacement depth of less than 7 km [Žák et al., 2012b]. 4 Structure of the Batholith Host Rock On a regional map scale, lithologic contacts between the Late Triassic (meta) volcanic rocks and the Martin Bridge and Hurwal Formations define an arc with an ~NNW-SSE trending limb roughly parallel to the northeastern batholith margin, hinge zone located within the northeastern off-shoot of the Craig Mountain pluton, and an ~NNE-SSW trending limb parallel to the southeastern batholith margin (Figures 3 and 4). At this scale, the outer batholith margin is largely discordant to bedding and foliation in the host rock, and the interpluton contacts are at a high angle to the regional curvature of host rock markers (Figures 3 and 4). This tens of kilometers scale structure contains folds on a scale of kilometers to hundreds meters with the fold geometry and fabric orientation varying along strike; in some places, fold limbs have been overturned (Figures 4–7a and 7b). Below, the fabrics and folds are described in four domains from the northwest to the southeast along the batholith margin (Domains 1–4; Figure 4). In Domain 1, bedding is well preserved, chiefly planar, and oriented uniformly over large regions in the siliciclastic rocks of the Hurwal Formation (Figures 4-6c). Near the Pole Bridge pluton, the bedding strikes parallel to the pluton margin and dips at moderate angle to the ~ENE to ~E, slickenside-type striations on the bedding planes plunge at moderate angle to the ~NNE to ~NE (Figures 4 and 5). This easterly dipping strata are part of a major ~NNW-SSE trending Sawtooth syncline [Nolf, 1966; not mapped in this study] (Figures 4 and 5). Figure 5Open in figure viewerPowerPoint Schematic structural cross sections across the Pole Bridge, Hurricane Divide, and Craig Mountain plutons and their host rocks. See Figure 4 for location. Figure 6Open in figure viewerPowerPoint Host rock structures along the northeastern margin of the Wallowa batholith. (a) Distant view to the SE on top of the Wallowa batholith with a flat roof pendant made up of the siliciclastic rocks of the Hurwal Formation and east dipping marbles of the Martin Bridge Formation in the far left background; view from a ridge in the Hurricane Divide. (b) Overturned NW-SE trending isoclinal anticline cored by marbles of the Martin Bridge Formation, looking SE; ridge 2.2 km SSE of Frances Lake. (c) East dipping bedded siliciclastic succession of the Hurwal Formation; hammer for scale. World Geodetic System (WGS84) coordinates: N45.340177°, W117.410447°. (d) Minor isoclinal folds with their axial planes parallel to pervasive metamorphic foliation in marble of the Martin Bridge Formation; hammer for scale. WGS coordinates: N45.27841455°, W117.35172400°. In Domain 2, significant vertical relief establishes that the northeastern margin of the Hurricane Divide pluton is a flat-lying batholith roof (Figure 6a) rolling over into a steep wall (see Žák et al. [2012b] for details). The overall synclinal architecture described above is here superposed by smaller-scale folds with their axial planes at a high angle to the ~NNW-SSE structural grain (Figures 4 and 6b). For instance, we mapped a steeply inclined, tight to isoclinal, ~NNW-SSE to ~NW-SE trending anticline facing toward the pluton, where the marbles and calc-silicate rocks of the Martin Bridge Formation occupy the core and the overlying siliciclastic rocks of the Hurwal Formation are exposed in the limbs (Figures 4-6b). Furthermore, another anticlines and synclines occur in the nearby Hurwal Formation (Figures 4 and 5). The exception to this generally simple fold style is seen in the marbles where boudins of calc-silicate rocks have been folded more complexly. On outcrops, both formations exhibit remarkably contrasting patterns of deformation (Figures 6c and 6d). The marbles exhibit pervasive metamorphic foliation and compositional banding (Figure 6d) and have well-developed mineral and stretching lineations defined by elongated or fibrous grains and aggregates of calcite, tremolite, and epidote. Foliation commonly encloses boudins of competent calc-silicate rocks and is axial planar to minor tight to isoclinal folds defined by folded calcite veins or quartzite and calc-silicate intercalations (Figure 6d). Farther south, bedding in the siliciclastics and metamorphic foliation and banding in the marbles strike ~E-W to ~NW-SE and dip moderately to steeply to the ~N to ~NE (Figures 4, 5, and 7a). Mineral lineation in the marbles tends to plunge moderately to the ~NE (Figures 4 and 7a). Figure 7Open in figure viewerPowerPoint Stereonets (equal area projection, lower hemisphere) showing orientation of fabric elements in the Wallowa terrane and in component plutons of the Wallowa batholith. In Domain 3 along the northwestern margin of the Craig Mountain pluton, lithologic contacts between the Martin Bridge and Hurwal Formations, as well as steep metamorphic foliation and bedding, dip steeply to the north and, again, appear deflected from the ~NNW-SSE to the ~E-W strike, i.e., toward parallelism with the nearby pluton margin (Figures 4 and 5). Similarly to Domain 2, the units here define an overturned steep isoclinal anticline cored by the marbles and facing toward the pluton (Figures 4 and 5). In the southeasterly Domain 4, a limb-parallel, ~NNE-SSW trending reverse to thrust fault was mapped that separates siliciclastic rocks of the Hurwal Formation from the Martin Bridge marbles (Figures 4 and 5). Bedding and foliation dip moderately to steeply to the ~ESE to ~SE (Figures 4, 5, and 7b), and mineral and stretching lineations are rare. In contrast to Domain 3, the intrusive pluton/host rock contact is discordant and truncates both the thrust fault and mesoscopic fabrics in the host rock. 5 Multiple Magmatic to Solid State Fabrics in the Wallowa Batholith Mesoscopic magmatic foliation in the Pole Bridge, Hurricane Divide, and Craig Mountain plutons is commonly defined by planar shape-preferred orientation of biotite, hornblende, and feldspar grains or aggregates (Figure 8a) and by the alignment of flattened microgranular enclaves and elongated host rock xenoliths (Figures 8a–8d). Lineation is defined by linear shape-preferred orientation of euhedral to subhedral hornblende crystals or elongated biotite aggregates in the foliation plane. On many outcrops, however, lineation is difficult to measure due to the lack of suitably oriented, foliation-parallel surfaces. Figure 8Open in figure viewerPowerPoint Field examples of magmatic fabrics in the Wallowa batholith and their geometric and temporal relations to other magmatic structures. (a) Close-up of subvertical mineral foliation in the Hurricane Divide pluton, defined mainly by amphibole and biotite. Stretched microgranular enclave is aligned parallel to the mineral fabric in the host granodiorite. Swiss Army penknife for scale is 9 cm long. WGS84 coordinates: N45.24463205°, W117.34662060°. (b) Steeply dipping swarm of flattened microgranular enclaves aligned parallel to mineral foliation in the host granodiorite of the Hurricane Divide pluton. WGS84 coordinates: N45.25134667°, W117.4067383°. (c) Microgranular enclave near larger host rock xenolith, both with their long axes parallel to mineral foliation in the host granodiorite of the Hurricane Divide pluton. SOG 2.0 Bowie knife for scale is 28 cm long. WGS84 coordinates: N45.24509900°, W117.34090380°. (d) Close-up of mineral fabric overprinting at a high angle a composite aplitic-mafic cumulate dike. Note microgranular enclave parallel to the host mineral fabric; the Hurricane Divide pluton. Pencil for scale is 14 cm long. WGS84 coordinates: N45.24362002°, W117.34236385°. (e) Mineral fabric in the host granodiorite overprinting a folded aplitic dike. Swiss Army penknife for scale is 9 cm long. WGS84 coordinates: N45.21056347°, W117.3931542°. (f) Aplitic dike folded into an open magmatic fold, magmatic foliation in the host granodiorite is axial planar to this fold; the Craig Mountain pluton. WGS coordinates: N45.201704°, W117.208478°. A similar feature of all plutons in the batholith is a moderately to steeply plunging lineation (~60–90° plunge) and a steep foliation (~70–90° dip), the s
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