Paleomagnetism and rotation history of the Blue Mountains, Oregon, USA
Published:December 28, 2018
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Bernard A. Housen*, 2018. "Paleomagnetism and rotation history of the Blue Mountains, Oregon, USA", Tectonics, Sedimentary Basins, and Provenance: A Celebration of the Career of William R. Dickinson, Raymond V. Ingersoll, Timothy F. Lawton, Stephan A. Graham
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An important element in reconstructions of the Cordilleran margin of North America includes longstanding debate regarding the timing and amount of rotation of the Blue Mountains in eastern Oregon, and the origin of geometric features such as the Columbia Embayment, which was a subject of some of Bill Dickinson’s early research. Suppositions of significant clockwise rotation of the Blue Mountains derived from Dickinson’s work were confirmed in the 1980s by paleomagnetic results from Late Jurassic–Early Cretaceous plutonic rocks, and secondary directions from Permian–Triassic units of the Wallowa–Seven Devils arc that indicate ~60° clockwise rotation of the Blue Mountains.
This study reports new paleomagnetic data from additional locations of these Late Jurassic–Early Cretaceous plutonic rocks, as well as Jurassic sedimentary rocks of the Suplee-Izee area. Samples from three sites from the Bald Mountain Batholith, two sites from small intrusive bodies near Ritter, Oregon, and six sites from the Wallowa Batholith have well-defined magnetization components essentially identical to those found by previous workers. The combined mean direction of both sets of data from these Late Jurassic to Early Cretaceous intrusive rocks is D = 30, I = 63, α95 = 6°.
Samples from Jurassic sedimentary rocks in the Suplee-Izee area include four sites of the Lonesome Formation, three sites of andesitic volcanics in the Snowshoe Formation, and three sites from the Trowbridge Formation. The Lonesome and Trowbridge samples all had very well-defined, two component magnetizations. The in-situ mean of the combined Lonesome and Trowbridge Formations is D = 28, I = 63, α95 = 15°. Upon tilt-correction, the site means of these units scatter and fail the paleomagnetic fold test in spectacular fashion. The similarity between the directions obtained from the remagnetized Jurassic rocks, and from the Late Jurassic to Early Cretaceous plutonic rocks suggests that a widespread remagnetization accompanied emplacement of the intrusives. Similar overprints are found in Permian and Triassic rocks of the Blue Mountains. Directions from 64 sites of these rocks yields a mean of D = 33°, I = 64°, k= 26, α95 = 3.7°. Comparing the directions with North America reference poles, a clockwise rotation of 60° ± 9° with translation of 1000 ± 500 km is found. Together with data from Cretaceous and Eocene rocks, clockwise rotation of the Blue Mountains has occurred throughout the past ca. 130 Ma, with long-term rotation rates of 0.4 to 1 °/Ma. Approximately 1000 km of northward translation also occurred during some of this time.
This paper includes paleomagnetic results from a reconnaissance study of Jurassic sedimentary rocks of the Suplee-Izee area (Dickinson and Vigrass, 1965) of the Blue Mountains in east-central Oregon. The Suplee-Izee area was investigated by Bill Dickinson as part of his Ph.D. work, and the published results are one of the earliest examples of the many research contributions he made over the course of his long career. The objective of the project was to use the paleomagnetic signal recorded by those sediments to test paleogeographic models of the North American Cordillera and its accreted terranes (e.g., Dickinson, 2004). As will become clear, the paleomagnetic signal recorded by these rocks was acquired after they were tilted. This post-tilting remagnetization negates the use of the Suplee-Izee rock’s paleomagnetism to examine their Jurassic paleolatitude. Instead, these results are combined with new data from Late Jurassic–Early Cretaceous plutonic rocks that intrude the Blue Mountains, and focuses on another longstanding subject of study by Bill Dickinson—the origin and significance of the Columbia Embayment (Dickinson and Thayer, 1978; Dickinson, 2004), and the rotation history of the Blue Mountains and related terranes (Dickinson, 2002).
Columbia Embayment and the Blue Mountains
A 50°–60°CW (clockwise) rotation of the Blue Mountains (Fig. 1) is a common feature of many tectonic reconstructions of the North American Cordillera (e.g., Hamilton, 1969; Dickinson and Thayer, 1978; Dickinson, 2002, 2004; Riddihough et al., 1986; Wyld and Wright, 2001; Wyld et al., 2006). The age of this rotation is variably interpreted as either post–Late Jurassic to Early Cretaceous time (see, for example, Riddihough et al., 1986; Žák et al., 2015), or younger, i.e., from post-Cretaceous time. In particular, Dickinson (2002) proposed a model for Basin and Range extension that called for an Eocene age for the CW rotation of the Blue Mountains. The majority of these models also call for rotation of the Blue Mountains as a single, large block, with restoration of the CW rotation reorienting the Blue Mountains to form an elongate, north-south–striking arc system that connects to the Klamath Mountains terranes to the south.
Geology and Paleomagnetic Sampling
Rocks in the Blue Mountains of eastern Oregon and western Idaho (Fig. 2) have been divided into four terranes that trend generally east and northeast toward the Salmon River suture zone (Silberling et al., 1984). The Wallowa and Olds Ferry terranes are composed of Permian to Upper Triassic plutonic and volcanic rocks that formed in a magmatic arc and are overlain by Upper Triassic to Jurassic marine sedimentary rocks (Vallier, 1995). The Baker terrane is a mélange complex of deformed and metamorphosed Triassic argillite and chert, with olistostromal blocks of Permian limestone, that formed by marine sedimentation followed by intense deformation in an oceanic subduction zone (e.g., Schwartz et al., 2010). The Izee terrane consists of Upper Triassic to Upper Jurassic sedimentary rocks with a range of ages and depositional environments (dominantly shallow to deep marine). These strata depositionally overlie rocks of the Wallowa, Olds Ferry, and Baker terranes and were proposed as an overlap assemblage that linked the three other terranes to each other by Late–Middle Jurassic time (White et al., 1992; Dorsey and LaMaskin, 2007). The nature of this proposed overlap was questioned by LaMaskin et al. (2015), who determined that correlations between units such as the Coon Hollow Formation in the Wallowa terrane and the Suplee-Izee strata are questionable and thus do not establish a firm link between these terranes during Jurassic time. Late Jurassic to Early Cretaceous granitic plutons, such as the Bald Mountain and Wallowa Batholiths, crosscut these earlier units and constrain the age of amalgamation of the Blue Mountain terranes (LaMaskin et al., 2015). Isolated areas located mainly in the western portion of the Blue Mountains have exposures of Upper Cretaceous marine sedimentary rocks. The largest of these exposures are of the Hudspeth and Gable Creek Formations in the Mitchell Inlier (Oles and Enlows, 1971; Housen and Dorsey, 2005; Surpless and Gulliver, this volume). Finally, Eocene volcaniclastic sedimentary rocks, volcanics, and shallow-level intrusives of the Clarno Formation (Grommé et al., 1986) occur in the Mitchell and John Day area.
Previous paleomagnetic studies in Blue Mountains have investigated Late Permian and Middle Triassic volcanic rocks, the Late Jurassic to Early Cretaceous plutons, the Cretaceous sediments of the Mitchell Inlier, and the volcanics and volcaniclastic sediments of the Eocene Clarno Formation. Results from 29 sites of volcanic rocks and volcaniclastic sedimentary rocks from the Middle to Upper Triassic (Ladinian–Carnian) Wild Sheep Creek Formation of the Seven Devils Group (Wallowa terrane) and the Upper Triassic (Carnian–Norian) Huntington Formation (Olds Ferry terrane) were reported in Hillhouse et al. (1982). Using a combination of demagnetization techniques and fold tests, Hillhouse et al. (1982) identified three ancient components of magnetization. Two of these components (designated C1, carried only by Seven Devils rocks, and C2, carried mainly by Huntington rocks) were deemed to be primary (Middle to Late Triassic) magnetizations, because the directions clustered better following tilt correction. The other ancient component (C3) was apparently a postfolding secondary magnetization. Paleomagnetic results from the Upper Permian Hunsaker Formation in the Blue Mountains (Harbert et al., 1995) indicated magnetization in the Northern Hemisphere during the Late Permian Kiaman reversed superchron. This result resolved the hemisphere ambiguity of the results of Hillhouse et al. (1982), showing that all units were in the Northern Hemisphere for Middle to Late Triassic time. The pole positions for the C1 and C2 magnetizations are discordant both with each other and with Middle to Late Triassic poles from the North American craton. The discordance can be explained by presuming a 53° counterclockwise (CCW) vertical-axis rotation between the Seven Devils–Wallowa terrane (C1 pole) and primarily Huntington–Olds Ferry terrane (C2 pole) rocks and an additional 20°–40°CCW rotation with respect to North America (C2 pole compared to Middle–Late Triassic North American craton poles). The paleolatitudes obtained for the Seven Devils and Huntington rocks indicate magnetization at 18°N (+11°/–9°) latitude, which is within 4° of the Middle to Late Triassic paleolatitude for the Blue Mountains predicted from the North American apparent polar wander path (APWP; Beck and Housen, 2003; Kent et al., 2015).
Wilson and Cox (1980) reported paleomagnetic results from Late Jurassic to Early Cretaceous plutonic rocks in the Wallowa batholith, the Cornucopia Stock, and a small, unnamed pluton near Ritter, Oregon, in the Baker terrane. They found well-defined magnetizations with both normal and reverse polarity, which they interpreted to represent primary magnetizations. They argued that postmagnetization tilting of these plutons was insignificant because of the overall consistency of the directions from the large geographic region sampled. The pole obtained from magnetization of the plutons is discordant to poles from Late Jurassic to Early Cretaceous rocks of North America. Assuming a Late Jurassic magnetization age, the discordance can be explained by a 60°CW rotation of the Blue Mountains relative to North America (Wilson and Cox, 1980; Hillhouse et al., 1982). The paleolatitude derived from magnetization of the plutons (43°N ± 7°) is concordant with a Late Jurassic expected paleolatitude of the Blue Mountains predicted by North American APWP. The overprint found in the Triassic Seven Devils and Huntington rocks (C3 pole of Hillhouse et al., 1982) is similar to the mean direction of the plutons reported by Wilson and Cox (1980), suggesting that remagnetization of some of the Permian–Triassic rocks may have been coeval with late-stage plutonism in the Blue Mountains. Updated ages of Blue Mountains plutonic rocks may call for reinterpretation of some of the magnetization ages of Wilson and Cox (1980), who reported ages from 150 to 130 Ma for these rocks. Johnson et al. (1997) reported a 118 Ma 40Ar/39Ar age for the Cornucopia Stock, and an age of 130 Ma was reported by Schwartz et al. (2011) for the Hurricane Divide pluton of the Wallowa Batholith, the unit sampled by Wilson and Cox (1980). These ages indicate that the plutons and their magnetization are Early Cretaceous, rather than Late Jurassic, in age.
The paleomagnetism of Late Cretaceous (Albian–Cenomanian) marine strata of the Hudspeth and Gable Formations found in the Mitchell Inlier, in the western portion of the Blue Mountains, was reported by Housen and Dorsey (2005). They found that the magnetization of the majority of these rocks passed paleomagnetic fold, conglomerate, and baked contact tests and is therefore primary. The mean direction and resulting paleopole indicated both significant CW rotation (37°) and latitudinal displacement (1200–1700 km) since the Late Cretaceous. Analysis of magnetic anisotropy and site-level directional dispersion suggested that the detrital paleomagnetic inclination error was minor (<5°). A more robust follow-up study (Callebert et al., 2017) resampled these units, collecting ~850 specimens and reporting >400 well-defined paleomagnetic directions from the Gable Creek and Hudspeth Formations. This work yielded mean directions similar to those reported by Housen and Dorsey (2005) and more robust estimates of paleomagnetic inclination error using both anisotropy of partial anhysteretic remanent magnetization (ApARM) fabrics and elongation-distribution analysis (Tauxe et al., 2008). While further refinement of this updated study is pending, the fabric and elongation methods suggest a detrital inclination error of 5°–10°. This result reduces the estimate of latitudinal displacement to ~500 km and increases the inferred CW rotation of these rocks to 48°.
The paleomagnetic study reported in Grommé et al. (1986) included results from 46 sites collected from extrusive volcanics with well-defined flow bedding in the Eocene Clarno volcanics, which crop out in the western and central portions of the Blue Mountains. The paleomagnetic results included both normal and reverse polarity directions that were well clustered and antipodal to each other. Updated geochronology (Bestland et al., 1999) indicates these rocks are 54–43 Ma (early to middle Eocene) in age. The mean direction and resulting paleopole from the Clarno volcanics indicate that 16°CW rotation, with no latitudinal translation, occurred since middle Eocene time. As discussed in Housen and Dorsey (2005), these results collectively indicate a protracted history of CW rotation of the Blue Mountains during the past 130 m.y.
For this study, paleomagnetic samples were collected using either a gas-powered drill, or as oriented block samples. In the Suplee-Izee area, samples were collected from sedimentary rocks at four sites of the Lonesome Formation, three sites of andesitic volcanics of the Snowshoe Formation, and three sites from sedimentary rocks of the Trowbridge Formation. In the central portion of the Blue Mountains, three sites from the Bald Mountain Batholith, two sites from small intrusive bodies near Ritter, Oregon, and seven sites from the Hurricane Divide pluton of the Wallowa Batholith were collected. The site locations and their site-mean paleomagnetic directions (described below) are given in Table 1. The samples were cut into standard (2.2-cm-long) specimens that were demagnetized using either thermal or alternating field methods with remanence measured in a WSGI 755 DC-SQUID magnetometer, housed in a magnetically shielded space in the Pacific Northwest Paleomagnetism Laboratory at Western Washington University.
The majority of the samples had well-defined magnetizations (Fig. 3) with two components indicated by examination of orthogonal vector plots of the demagnetization results. Most of the Jurassic sedimentary rocks had similar magnetization behavior, with a well-defined second removed component defined by demagnetization steps higher than either 25 mT (alternating field) or 250 °C (thermal; Fig. 3). The in situ mean direction of this component from the Lonesome and Trowbridge Formations was D = 28°, I = 63°, k = 27, α95 = 15°, N = 5. Correcting these directions for bedding tilts measured at each site significantly increased their scatter (Fig. 4), and so I infer that they are the result of a postdeformation remagnetization event. The andesites of the Jurassic Snowshoe Formation were poorly resolved and did not yield useful directions.
Samples from the plutonic rocks had well-defined directions (Fig. 3), with site mean directions that were very similar to those reported by Wilson and Cox (1980). This work extends the previous work to include well-defined directions from the Bald Mountain Batholith, and additional sites from the Ritter, Oregon, area and the Hurricane Divide pluton of the Wallowa Batholith. The combined mean direction of all of the plutonic rocks (this study plus data from Wilson and Cox, 1980) is D = 29°, I = 66°, k = 56, α95 = 4°, N = 22 (Fig. 5). A combined mean direction from a total of 64 sites from the remagnetized strata reported by Hillhouse et al. (1982) and Harbert et al. (1995), and from the Suplee-Izee area in this study, and from the plutonic rocks is D = 33°, I = 64°, k = 26, α95 = 3.7°, N = 64.
INTERPRETATION AND DISCUSSION
The postfolding age of magnetization of the Jurassic strata in the Suplee-Izee area, and the similarity between the in situ mean direction of these rocks and the combined mean direction of the plutonic rocks strongly suggest that a remagnetization event has affected the Suplee-Izee area (Fig. 6). These strata are folded, though not as severely as underlying units (Dickinson and Vigrass, 1965; Dorsey and LaMaskin, 2007), and they are not metamorphosed. These observations suggest that the remagnetization involved some combination of orogenic fluids (Van der Voo and Torsvik, 2012; Izquierdo-Llavall et al., 2015) and localized pressure solution associated with incipient deformation (e.g., Housen et al., 1993). Both Hillhouse et al. (1982) and Harbert et al. (1995) recognized that many Permian and Triassic rocks in the eastern portion of the Blue Mountains have been remagnetized, and that the similarity between the paleomagnetic directions obtained from these remagnetized strata and the directions obtained from the plutonic rocks reported by Wilson and Cox (1980) suggests that the remagnetization event recorded in the Permian–Triassic rocks was likely coeval, or close in age, with the intrusion of the plutonic rocks (see Fig. 6).
The ages of magnetization of the plutonic rocks reported here, and in Wilson and Cox (1980), range from 140 Ma for the Bald Mountain Batholith (Schwartz et al., 2014), to 135 Ma for the Hurricane Divide pluton of the Wallowa Batholith (Schwartz et al., 2011), to 115 Ma for the Cornucopia Stock (Johnson et al., 1997). There are no known published ages for the plutonic rocks near Ritter, Oregon, but they are presumed to be coeval with the other plutons. The magnetic polarity of these plutons can provide some additional constraints on their magnetization ages via a comparison with the geomagnetic polarity time scale (Gradstein et al., 2004). The normal polarity of the Cornucopia Stock is consistent with a magnetization age during the Cretaceous long-normal superchron (125–83 Ma). In addition, all of the in situ directions from remagnetized strata of the Blue Mountains have normal magnetic polarity; this may suggest that the remagnetization in the Blue Mountains occurred during the long-normal superchron, following the intrusion of most of the Blue Mountains plutons.
Previous comparisons between the paleomagnetic results from the Blue Mountains plutons (Wilson and Cox, 1980), and the results obtained from remagnetized strata (Hillhouse et al., 1982; Harbert et al., 1995) have largely presumed a Late Jurassic magnetization age for comparisons with paleomagnetic reference poles from North America. As discussed above, it is clear that the magnetization ages for both the plutonic rocks and the remagnetized strata of the Blue Mountains are Early Cretaceous. While there are some significant differences in the Jurassic portions of the North American APWP (Beck and Housen, 2003; Torsvik et al., 2008; Kent and Irving, 2010; Kent et al., 2015), there is close agreement between different APWP compilations for Cretaceous time. The main feature of the North American APWP for this time is the clustering of paleomagnetic poles, indicating very little motion of the North American craton with respect to the spin axis between 135 Ma and 85 Ma. Using the APWP compilations of Beck and Housen (2003) and Kent et al. (2015), an expected paleolatitude versus time for a location in the center of the Blue Mountains can be calculated (Fig. 7). Both compilations indicate rapid northward motion of the North American margin throughout Jurassic time, with very rapid motion, to very high paleolatitudes, indicated by the Kent et al. (2015) compilation.
Paleomagnetic directions from both the plutons of the Blue Mountains and those obtained from remagnetized strata of Permian–Jurassic rocks lack robust constraints on paleohorizontal for the time of their magnetization. In many evaluations of discordant paleomagnetic data from the North Cascades–Coast plutonic complex (e.g., Butler et al., 1991), or coastal and Baja California (Dickinson and Butler, 1998), Bill Dickinson and colleagues consistently ascribed the discordance to large-scale crustal tilting rather than a combination of latitudinal translation and rotation (e.g., Beck, 1976; Irving et al., 1996; Cowan et al., 1997). Interpretation of the paleomagnetic results obtained by Wilson and Cox (1980), and those of the plutons and remagnetized strata presented here are also made uncertain by a lack of paleohorizontal control. While there is some evidence for deformation of the Blue Mountains plutons (see, for example, Žák et al., 2015), there are no systematic studies of barometry or differential uplift that can be used to evaluate or correct for possible postmagnetization tilt of these rock units. The paleomagnetic directions are not significantly streaked or elongate, which does rule out tilt, deformation, or rotation during cooling and acquisition of magnetization (Beck, 1992), but postcooling tilt cannot be ruled out using those considerations.
A compilation of paleomagnetic data from Early to Late Cretaceous rocks from cratonic North America in Housen et al. (2003) has been updated with additional results (Housen, 2017); the resulting mean pole (71.8°N, 192.7°E, A95 = 2.4°, N = 27, derived from units 130–85 Ma in age) provides a robust reference for the paleogeographic position of North America. The expected direction for the Blue Mountains calculated from this reference pole (Ix = 71°, Dx = 333°, expected paleolatitude = 55°N) differs significantly from the mean direction obtained from the plutonic rocks of the Blue Mountains (Fig. 6). One end-member interpretation would attribute the entire difference to a tilt of 25° (top-up-to-the-west) about a horizontal axis with an azimuth of 350° (Fig. 6). The other end member would attribute the entire difference to a 60° ± 9° vertical-axis rotation and 1000 (±500) km of latitudinal translation (for examples, see Beck and Nosen, 1972; Beck, 1980). While it is not possible to rule out either of these end-member interpretations, substantial postmagnetization tilting of the Blue Mountains is unlikely for the following reasons. The area in question is ~200 km wide (Fig. 2), so that uniform application of a large (25°) tilt of the Blue Mountains as a rigid block implies exposure of unrealistically deep (>100 km) rock on the eastern side. Tilt of smaller, discrete crustal blocks or panels (as in Butler et al., 1991) would be possible, but it would be implausibly uniform to produce the consistent pattern of directions in plutons and remagnetized sedimentary rocks over a 200 km × 300 km area as observed here. The preferred interpretation is that postmagnetization tilt of the plutonic and remagnetized rocks is negligible.
The paleomagnetic results summarized here can be used to constrain the rotation history of the Blue Mountains as follows. Very robust estimates of rotation are provided by the units that have good paleohorizontal control. The Clarno volcanics (Grommé et al., 1986) have directions that indicate 16° ± 10°CW rotation after 45 Ma, and the Gable Creek and Hudspeth Formations of the Mitchell Inlier (Housen and Dorsey, 2005) have directions that indicate 37° ± 7°CW rotation after 90 Ma. Results of a larger study of the Cretaceous sedimentary rocks of the Mitchell Inlier (Callebert et al., 2017) have more easterly declinations than reported by Housen and Dorsey (2005) and may indicate larger (48°) CW rotation for these rocks. The rotation estimate from the combined plutonic rock/remagnetized strata is 60° ± 9°CW (assuming tilt is negligible). Given the ages of the youngest plutonic results and the normal polarity of the remagnetized directions, the rotation should be considered to have occurred after 110–120 Ma. Taken together, these results indicate that the Blue Mountains have experienced a consistent pattern of CW rotation throughout most of the past 120 m.y. (Fig. 8). The majority of this rotation (~45°) has taken place between either 120 Ma and 45 Ma (if tilt is neglected) or between 90 Ma and 45 Ma (if only results with paleohorizontal are used). The corresponding rotation rates range between 0.6°/m.y. and 1.0°/m.y. If the age of rotation of the Blue Mountains is presumed to be younger, as proposed by Dickinson (2002), who linked CW rotation of the Blue Mountains to rotation observed in the Paleocene rocks of the Oregon and Washington Coast Ranges, then the rates of rotation would be much higher (2–3°/m.y.).
The paleomagnetic results discussed here also suggest that the Blue Mountains have experienced moderate-scale latitudinal displacement since Early Cretaceous time (Fig. 7). Assuming that tilt of the plutonic and remagnetized rocks is negligible, then the Early Cretaceous magnetization age (and comparable North American reference pole) indicate that the Blue Mountains mean inclination from these rocks provides a paleolatitude of 46°N (±5°). Using the Early Cretaceous reference pole discussed previously, the expected paleolatitude of the Blue Mountains is 55°N, implying a statistically significant poleward displacement of 1000 ± 500 km since ca. 120 Ma. The Blue Mountains would have been at the latitude of central Nevada and the northern portion of the Sierra Nevada. This moderate displacement is compatible with the low-end (1200 km) displacement estimated for the sedimentary rocks of the Mitchell Inlier by Housen and Dorsey (2005), but it is somewhat higher than the more robust estimate (~500 km) emerging from the restudy by Callebert et al. (2017). The displacement estimates for the Blue Mountains are very similar to those obtained from the Spences Bridge volcanics and related rocks of the Intermontane superterrane (Irving et al., 1995; Haskin et al., 2003), and from untilted plutonic rocks of Knight Inlet (Rusmore et al., 2013). Collectively the data support linkages between the Intermontane superterrane and the Blue Mountains. The inferred displacement does not agree with the fault-based reconstructions of Wyld et al. (2006), who proposed essentially zero post–90 Ma displacement of the Blue Mountains, and it is in excess of the <500 km displacement inferred from recent provenance studies (Surpless and Gulliver, this volume). However, the displacement proposed here is in agreement with several provenance-based constraints such as those proposed by Wyld and Wright (2001), Dorsey and LaMaskin (2007), and LaMaskin et al. (2011). As discussed in Housen and Dorsey (2005), the occurrence of a consistent and long-lived pattern of vertical-axis rotations of the Blue Mountains (Fig. 8) is consistent with both long-lived dextral shear of this portion of the Cordilleran margin and translation of outboard terranes as proposed by Beck (1976).
Alternatively, the anomalous inclination of the plutonic and remagnetized strata could be ascribed to postmagnetization tilting or (in the unremagnetized sedimentary rocks) unrecognized detrital inclination error. Invoking tilt would necessarily preclude use of the paleomagnetic directions from the plutons to constrain the rotation of the Blue Mountains, in which case, the only remaining evidence for CW rotation of the Blue Mountains would be from units younger than 90 Ma.
Many helpful edits and valuable suggestions for revisions of this manuscript were provided by reviewers Scott Bogue and Todd LaMaskin and by the editors of this volume. I would also thank Becky Dorsey, Kathy Surpless, Paul Umhoefer, Margi Rusmore, and Basil Tikoff for interesting conversations and collaboration on work related to this study. Russ Burmester and Myrl Beck provided numerous helpful comments and advice, as well as some interesting perspectives on Bill Dickinson over the years. Support from the National Science Foundation (grant EAR-141035), and the Western Washington University Office of Research and Sponsored Programs that enabled the field work, laboratory work, and production of this manuscript is gratefully acknowledged. Finally, this paper is dedicated to the memory of Chris Suczek, a long-time member of the Western Washington University Geology Department’s faculty, and an early Dickinson Ph.D. student.