Pdf/12/3/900/4092675/900.Pdf 900 by Guest on 24 September 2021 Research Paper

Research Paper

GEOSPHERE Linking deep and shallow crustal processes during regional transtension in an exhumed continental arc, North Cascades, GEOSPHERE; v. 12, no. 3 northwestern Cordillera (USA) doi:10.1130/GES01262.1 Robert B. Miller1, Stacia M. Gordon2, Samuel Bowring3, Brigid Doran1, Noah McLean3*, Zachary Michels1*, Erin Shea1*, and Donna L. Whitney4 11 figures; 3 tables; 1 supplemental file 1Department of Geology, San Jose State University, One Washington Square, San Jose, California 95192-0102, USA 2Department of Geological Sciences and Engineering, University of Nevada-Reno, 1664 N. Virginia Street, Reno, Nevada 89557, USA 3Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue MIT Building 54, Cambridge, Massachusetts 02139, USA CORRESPONDENCE: robert.b.miller@sjsu.edu 4Department of Earth Sciences, University of Minnesota, 310 Pillsbury Drive SE, Minneapolis, Minnesota 55455, USA

CITATION: Miller, R.B., Gordon, S.M., Bowring, S., Doran, B., McLean, N., Michels, Z., Shea, E., and Whitney, D.L., 2016, Linking deep and shallow ABSTRACT basin, sediments were deposited in part at ca. 51 Ma, folded shortly afterward, crustal processes during regional transtension in an and then covered by ca. 49 Ma Teanaway basalts and intruded by associated exhumed continental arc, North Cascades, north- The North Cascades orogen (northwestern USA) provides an exceptional mafic dikes. Directly after dike intrusion, the fault-bounded Chumstick basin western Cordillera (USA): Geosphere, v. 12, no. 3, p. 900–924, doi:10.1130/GES01262.1. natural laboratory with which to evaluate potential temporal and kinematic subsided rapidly. Extension directions from these dikes and from Eocene dikes links between processes operating at a wide range of crustal levels during col- that intruded the Cascades core are dominantly oblique to the overall trend of

Received 6 September 2015 lapse of a continental arc, and particularly the compatibility of strain between the orogen (275°–310° versus ~320°, respectively) and to the northwest-south- Revision received 9 February 2016 the upper and lower crust. This magmatic arc reached a crustal thickness of east to north-south ductile flow direction in the Skagit and Swakane rocks. Accepted 27 April 2016 ≥55 km in the mid-Cretaceous. Eocene collapse of the arc during regional This discordance implies that coeval extensional strain was decoupled be- Published online 11 May 2016 transtension was marked by magmatism, migmatization, ductile flow, and tween the brittle and ductile crust. Strain orientations at all depths in the exhumation of deep crustal (8–12 kbar) rocks in the Cascades crystalline core Cascades core contrast with the approximately east-west extension driven by coeval with subsidence and rapid deposition in nonmarine basins adjacent orogenic collapse in coeval metamorphic core complexes ~200 km to the east. to the core, and intrusion of dike complexes. The Skagit Gneiss Complex is Arc-oblique to arc-parallel flow in the Cascades core probably resulted in part the larger of two regions of exhumed deep crust with Eocene cooling ages in from dextral shear along the plate margin and from along-strike gradients in the Cascades core, and it consists primarily of tonalitic orthogneiss emplaced crustal thickness and temperature. mainly in two episodes of ca. 73–59 Ma and 50–45 Ma. Metamorphism, melt crystallization, and ductile deformation of migmatitic metapelite overlap the orthogneiss emplacement, occurring (possibly intermittently) from ca. 71 to INTRODUCTION 53 Ma; the youngest orthogneisses overlap 40Ar/39Ar biotite dates, compatible with rapid cooling. Gently to moderately dipping foliation, subhorizontal Collisional orogens and some contractional continental magmatic arcs orogen-parallel (northwest-southeast) mineral lineation, sizable constrictional have gross similarities in their structure, including thickened crust (50–70 km) domains, and strong stretching parallel to lineation of hinges of mesoscopic and broad plateaus that have zones of partially molten lower to middle crust folds in the Skagit Gneiss Complex are compatible with transtension linked (e.g., Tibet and Altiplano-Puna; Nelson et al., 1996; Schilling et al., 2006; Ward to dextral-normal displacement of the Ross Lake fault zone, the northeastern et al., 2014). Studies of exhumed orogenic crust and models for lateral and boundary of the Cascades core. The other deeply exhumed domain, the 9–12 vertical crustal flow have most commonly been formulated based on colli- kbar Swakane Biotite Gneiss, has a broadly north-trending, gently plunging sional orogens, such as the Himalayas (e.g., Law et al., 2006, and references lineation and gently to moderately dipping foliation, which are associated therein). Much less is known about the deep levels of continental arcs, which with top-to-the-north noncoaxial shear. This gneiss is separated from overly- contain the rocks that record the thermal, rheological, and mechanical tran- ing metamorphic rocks by a folded detachment fault. The Eocene Swauk and sition of an orogen from contractional crustal thickening to extension and/or Chumstick basins flank the southern end of the Cascades core. In the Swauk transtension. For example, in the North American Cordillera, more research has concentrated on deep crustal flow and exhumation of the hinterland *Current addresses: McLean: Department of Geology, University of Kansas, Lindley Hall, 1475 metamorphic core complexes (e.g., MacCready et al., 1997; Vanderhaeghe Jayhawk Blvd. Room 120, Lawrence, Kansas 66045; Michels: Department of Geoscience, Uni- For permission to copy, contact Copyright versity of Wisconsin, 1215 W Dayton Street, Madison, Wisconsin 53706; Shea: Department of and Teyssier, 1997; Teyssier et al., 2005; Gervais and Brown, 2011) than on the Permissions, GSA, or [email protected]. Geological Sciences, University of Alaska, Anchorage, Alaska 99508. Mesozoic arc system to the west (Fig. 1). Similarly, lower to middle crustal flow

GEOSPHERE | Volume 12 | Number 3 Miller et al. | Linking crustal processes Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/3/900/4092675/900.pdf 900 by guest on 24 September 2021 Research Paper

124° reached thicknesses of ≥55 km (Miller and Paterson, 2001) in the Late Creta- 51° F r a ceous, and a wide range of Cretaceous crustal levels (0 to ≥40 km) is exposed. se Fr It is interpreted as the western margin of a proposed Late Cretaceous to Eo- au

l cene orogenic plateau, the eastern margin of which is marked by metamorphic t CBTS core complexes (Whitney et al., 2004). Eocene migmatization, plutonism, and 121° exhumation of some of the deepest (8–12 kbar) rocks of the Cascades crystal- 50° Pa line core were in part coeval with motion on dextral strike-slip and oblique-slip sa y faults, formation of nonmarine, transtensional basins, and intrusion of exten- te n Fault sive basaltic to rhyolitic dike swarms (e.g., Haugerud et al., 1991; Gordon et al., 2010a; Eddy et al., 2016). This orogen thus offers an exceptional opportunity to evaluate links between contemporaneous deep and shallow crustal processes during regional transtension. Furthermore, the record of how different crustal NWCS RL levels evolved provides a view of the construction and collapse of a thick con- F E tinental arc that may be analogous in some respects to the Cenozoic central CFB Andes (Western Cordillera and Altiplano) (e.g., Scheuber and Reutter, 1992; Beck and Zandt, 2002), where deep crust has not been exhumed. C

N h

48° e

l We integrate a large body of different types of data to arrive at a synthe- a

122° n

Bl sis of processes occurring from the middle crust to the surface during a time

En o c

ALASKA-ALEUTIAN t k iat interval largely centered on the Eocene (60–45 Ma). We address the potential RANGE SC 50 km Faul interplay of deformation, metamorphism, partial melting, and magmatism at F Wenatchee Block t different depths, including the temporal and dynamic relations of these pro- COAST cesses to exhumation of arc orogenic crust, formation of nonmarine basins, RANGE and intrusion of dike swarms. Particular emphasis is placed on how strain was IDAHO BATHOLITH partitioned at different depths in the crust during transtension and whether the upper crust was decoupled from the deeper, ductilely flowing lower crust 500km (e.g., Tibetan Plateau, Royden et al., 1997; Altiplano-Puna, Husson and Semere, SIERRANEVADA 2003; Gerbault et al., 2005; Ouimet and Cook, 2010).

Overview of North Cascades Geology PENINSULAR RANGES The Coast Plutonic Complex and its southeast extension, the crystalline core of the North Cascades (Cascades core), form a >1500-km-long plutonic Figure 1. Generalized map of Mesozoic and Paleogene western North American Cordi and metamorphic belt that represents an exhumed Cretaceous and Paleogene lleran arc plutons and metamorphic core complexes. Inset emphasizes distribution of magmatic arc (Figs. 1 and 2) (e.g., Armstrong, 1988; Tabor et al., 1989; Miller metamorphic rocks (purple) and plutons (orange) in the Cascades core and southern Coast belt. The Coast belt thrust system (CBTS), eastern Cascades fold belt (ECFB), north- et al., 2009a). In the southern Coast Mountains and North Cascades, arc plutons west Cascades thrust system (NWCS), and reverse shear zones in the Cascades core are intrude small late Paleozoic to Cretaceous oceanic and arc terranes (Journeay also shown. The dextral Straight Creek–Fraser fault (SCF) displaces the Cascades core and Friedman, 1993). The Cascades core includes the Chelan Mountains ter- from the main part of the Coast belt. The Entiat fault, Pasayten fault, and Ross Lake fault zone (RLF) are also major high-angle faults. The Cascades core is divided by the Entiat rane, Nason terrane, and Swakane terrane. The Chelan Mountains terrane con- fault into the Chelan and Wenatchee blocks, which have different thermal histories. sists of the Napeequa and Cascade River–Holden units, and metasupracrustal rocks in the Skagit Gneiss Complex are also commonly considered part of this terrane (Fig. 3) (Tabor et al., 1989). The Napeequa unit is dominated by amphibo- is poorly documented in other exhumed continental magmatic arcs, with a lite, siliceous schist, and quartzite (metachert), and contains widespread biotite few exceptions (e.g., Fiordland, New Zealand; Klepeis et al., 2004; Klepeis and schist and ultramafic rock, and minor marble and calc-silicate rock. It may cor- King, 2009; Betka and Klepeis, 2013). relate with low-grade Mississippian–Jurassic rocks northeast of the Cascades In this study we focus on the 96–45 Ma North Cascades magmatic arc of the core (e.g., Monger, 1986; Miller et al., 1993b). The Cascade River–Holden unit State of Washington (USA) and southwest British Columbia (Canada), which is made up of hornblende-biotite schist and gneiss, amphibolite, calc-silicate is the southern continuation of the Coast Mountains batholith. This arc likely rock, and metaconglomerate (e.g., Misch, 1966; Cater, 1982; Brown et al., 1994).

Figure 2. Geologic map emphasizing Eocene tectonic elements of the Cascades core and adjacent areas. Dextral zameen F strike-slip motion on the Straight Creek fault has been restored. Eocene plutons and units with young cooling ages (Skagit Gneiss Complex and Swakane Biotite Gneiss) in the Cascades core are emphasized. CM—Cooper Mountain batholith; DH—Duncan Hill pluton; GH—Golden Horn batholith; LFZ—Leavenworth fault zone; RC— Railroad Creek pluton; RLFZ—Ross Lake fault zone. Eocene fold traces are shown in Eocene basins and Cascades ault core. Sources are described in the text.

124º 122º P a G s H a S y k t 49º a e Vancouver g i n t R L F Z S G 60 F w n a e a i u k s l a s 50 t 50 n e G n e RC Fig. 3 60 is s

anut 70 50 CM Chuck Cowichan

Fold-Thrust Belt St D ? H r 50 a ? E i g 80 n h t i ? Leech River Fault t a t Fa ? 60 >60 u lt <50 Fig. 7 C 85 50 r Pacific Ocean e 48 º e k Swakane Gneiss Swauk LFZ

l t Roslyn Chumstick Teanaway

t Inferred boundary of Crescent n Cenozoic undifferentiated e basalt from EarthScope data c s Eocene nonmarine re C Eocene nonmarine volcanic S e a t t l e Eocene metamorphic 50 km Leech River schist Paleogene marine volcanic - Crescent Eocene Plutons 47º Cascades Core/ Coast Plutonic Complex Pre-Tertiary basement 80 Cooling Age Isochron Widely inferred eastern boundary of (Biotite) Crescent basalt in subsurface

121° 00′

Chilliwack RLF 34-2

Ho SC ND 48 zameen PO 47 68 FL 45 ER 65 DL Flt y 20 73 RM RC SCF 48 Hw 120° 30′ Figure 3. Map of the Skagit Gneiss Complex and adjacent units emphasizing U-Pb iso- Golden tope dilution–thermal ionization mass spec- RLFZ Horn 48 trometry zircon ages from orthogneisses. Numbers in plutons are inferred crystalliza- Meth 48° 30′ Eldorado tion ages. Abbreviations for orthogneisses Basi refer to our dated samples in Table 1 and in Skagit 68 ow Supplemental Table 1 (see text footnote 1). n NQ Gneiss Black Dates without abbreviations are from other 50 NCF sources (Mattinson, 1972; Miller et al., 1989; Complex Peak Miller and Bowring, 1990; Haugerud et al., GPTB NQ 1991). Map is mainly from Tabor et al. ST (1987a, 2003), Miller (1987), Hopson and •66 48 Mattinson (1994), and our mapping. Abbre Fo RF• 49• viations: ND—northern domain, CD—cen- CD PC g tral domain, and SD—southern domain 10 km gy De N of the Skagit Gneiss Complex. ER—Elijah 59 Oval Ridge; GPTB—Gabriel Peak tectonic belt; 89 Pk w NCF—North Creek fault; NQ—Napeequa unit; RC—Ruby Creek heterogeneous plu- Stepover in RLF 65 48 F tonic belt; RLF—Ross Lake fault; RM—Ruby • ault Post-Eocene Undi erentiated GG Mountain; RRC—Railroad Creek pluton; RRC Chumstick Fm. (Eocene) SD SC—Skymo Complex; SCF—Straight Creek 46 49 fault; Pk—peak; Mtn—mountain; Flt—fault. Eocene plutons SwakSSw SL SLs wwa 73 • • ak E • 49 Skagit Orthogneiss (Cretaceous-Eocene) ka ntia FP aneana • nen Skagit Banded Gneiss (Cretaceous-Eocene) e t 48 Fa Cooper Mtn Swakane Gneiss (Cretaceous) ul t Pre-Eocene plutons Duncan Hi 48° River - Holden Cascade River-Holden (Triassic) Cascade Napeequa (Mississippian - Jurassic?) ll Mesozoic Undi erentiated 46

This unit and approximately coeval orthogneisses represent a Late Triassic arc. taceous metapsammitic Swakane Biotite Gneiss (Fig. 2) (Matzel et al., 2004; Much of the southern part of the Cascades core consists of the dominantly Paterson et al., 2004; Gatewood and Stowell, 2012). metapelitic and metapsammitic Chiwaukum Schist of the Nason terrane (e.g., Much of the Cascades core was intruded, metamorphosed, and ductilely Plummer, 1980; Tabor et al., 1987b), and includes Early Cretaceous protoliths shortened in the mid-Cretaceous (ca. 100–85 Ma) synchronous with regional (Brown and Gehrels, 2007). The structurally deepest terrane is the Late Cre- thrusting and folding (Fig. 1) (e.g., Misch, 1966; Brown, 1987; McGroder, 1991).

Crustal thickness in the core probably reached ≥55 km at ca. 90 Ma (Miller to be ca. 49–45 Ma (Tabor et al., 1984; Eddy et al., 2016). Dikes occur in granitic and Paterson, 2001). After the major shortening event, magmatism (79–45 Ma), swarms, some of which are probably related to shallow plutons in the Cas- metamorphism, and deformation were focused in the northeastern part of the cades core; the most voluminous are in a mafic swarm that intruded the Eo- Cascades core (Chelan block; Fig. 1). The Swakane Biotite Gneiss was buried to cene Swauk basin, and others are isolated bodies that intruded parts of the ~35–40 km depth by 68 Ma (Matzel et al., 2004; Paterson et al., 2004; Gatewood core. All magmatism, metamorphism, and ductile deformation ended by ca. and Stowell, 2012). High-angle Paleogene faults (Fig. 1) underwent variable 43 Ma, when the crystalline core rocks were exhumed to the near surface, amounts of dextral strike slip and dip slip, and a transition from components as recorded by 40Ar/39Ar and K-Ar hornblende and biotite dates (Engels et al., of reverse slip to normal slip on some of these faults likely coincided with a 1976; Wernicke and Getty, 1997; Tabor et al., 2003; Matzel, 2004). Subsequent regional change from transpression to transtension (Miller and Bowring, 1990; subduction of the Farallon plate resulted in the modern Cascades arc (e.g., Haugerud et al., 1991). Tepper, 1996) starting ca. 40 Ma (du Bray and John, 2011). The youngest 40Ar/39Ar and K-Ar ages (e.g., Wernicke and Getty, 1997; Paterson In the following we integrate new data from our multifaceted research with et al., 2004) and ductile deformation are found in the two domains of deep crust those of previous workers to provide a view of the Eocene crustal architecture that underwent substantial exhumation accompanied by Eocene near-isothermal and processes operating throughout the crustal section. These data include decompression and subsequent rapid cooling (Fig. 2). These domains include the material in theses by some of the authors (Michels, 2008; Shea, 2008; Doran, orthogneiss-dominated, partially migmatitic 8–10 kbar Skagit Gneiss Complex, 2009; McLean, 2012), publications arising from group research (Miller and and the largely nonmigmatitic 9–12 kbar Swakane Biotite Gneiss (Misch, 1968; Bowring, 1990; Gordon et al., 2010a, 2010b; Wintzer, 2012), and other publica- Haugerud et al., 1991; Whitney, 1992b; Valley et al., 2003; Gordon et al., 2010a). tions, notably Misch (1968), Haugerud et al. (1991), Paterson et al. (2004), and Ductile deformation, late magmatism, partial melting of Skagit rocks, and exhu- Eddy et al. (2016). The main new data sets presented herein include high-preci- mation of these domains overlapped with regional transtension, indicated in the sion U-Pb (chemical abrasion–isotope dilution–thermal ionization mass spec- shallow crust by coeval formation and rapid subsidence of Eocene nonmarine trometry, CA- ID-TIMS) zircon dating of 10 orthogneiss bodies in the Skagit basins bounded by oblique-slip (dextral and normal) faults (e.g., Johnson, 1985; Gneiss Complex (Fig. 3; Table 1; see Supplemental Table 11 and http://geochron Evans, 1994; Eddy et al., 2016), and intrusion of abundant Eocene dikes. These .org/dataset /html /geochron _dataset _2016 _01 _22 _kCnxS for details of dates); basins accumulated clastic rocks that in the central Washington Cascades may mapping and structural analysis of the central and southern parts of the gneiss have reached ≥12 km in thickness (Tabor et al., 1984; Evans, 1994). complex and the upper contact of the complex; structural fabrics in Eocene Several studies have argued that ridge subduction occurred at ca. 50 Ma plutons; and orientations and extension directions of Eocene dikes in the shal- at the latitude of northern Washington (Thorkelson and Taylor, 1989; Cowan, low crust. The data description begins with the deep crust, focusing on the 2003; Haeussler et al., 2003; Madsen et al., 2006) and created a slab window P-T-t-d (pressure-temperature-time-deformation) history of the Skagit Gneiss beneath the Cascades at that time. In the Cascades core, magmatism changed Complex and to a lesser extent the Swakane Biotite Gneiss, followed by analy Dates (Ma) Composition Isotopic Ratios Corr. Coeff. 206Pb/ 207Pb/ 206Pb/ 207Pb/ 206Pb/238U 238U ±2 207Pb/ ±2 206Pb±2 Th/ Pb* Pbc Pb*/ 206Pb/ 238U 207Pb/206Pb - Fraction aabs 235U babs aabs % disc cU d(pg) e(pg) fPbc g204Pb h i,a±2 % 235U i±2 % i,a±2 % 207Pb/235U Fraction SGC-63 zirconIGSN: SSR.NMM000011Stehekin orthogneiss. Youngest Th-corrected 206Pb/238U date: 65.953 ± 0.055 Ma z8 70.864 0.05171.34 0.35 87 11 22 0.22015.20.6423 15370.0110530.072 0.0727860.51 0.047780 0.470.470 z8 z25 71.236 0.06771.45 0.34 79 10 13 0.16016.80.5332 21140.0111120.095 0.0729060.49 0.047607 0.430.710 z25 from dominantly tonalite at 96–60 Ma to mainly granodiorite at 50–45 Ma sis of fabrics and regional strain in Eocene plutons, and then on high-angle zM13 65.953 0.05566.21 0.52 76 18 17 0.16115.91.1813 903 0.0102830.084 0.0673840.81 0.047545 0.760.645 zM13 zS17 70.989 0.04970.94 0.30 69 93 0.14729.91.0628 18790.0110730.069 0.0723640.43 0.047419 0.400.569 zS17 zS18 70.397 0.04370.55 0.25 76 8110.223 25.2 0.79 32 20920.0109800.062 0.0719530.37 0.047547 0.330.680 zS18 zS21 66.185 0.07666.88 0.74 92 26 31 0.1726.5 0.65 10 679 0.0103200.115 0.0680841.15 0.047870 1.080.581 zS21 zS22 70.713 0.05771.05 0.46 82 15 18 0.2038.6 0.49 18 11730.0110300.082 0.0724800.67 0.047682 0.630.551 zS22 zS30 77.193 0.08277.56 0.87 89 26 16 0.2327.2 0.75 10 633 0.0120460.107 0.0793821.17 0.047814 1.100.682 zS30 NC-197 zirconIGSN: SSR.NMM000010Diablo Lake orthogneiss.Wtd mean Th-corrected 206Pb/238U date: 44.857 ± 0.023 Ma (Misch, 1966; Miller et al., 2009a). Most of the widespread dikes are inferred faults, nonmarine basins, and dike swarms in the shallow crust. z1 44.819 0.05444.86 0.61 47 33 14 0.2757.4 0.64 12 762 0.0069770.121 0.0451671.39 0.046974 1.360.248 z1 z2 44.832 0.04044.67 0.34 36 18 -8 0.27316.00.7422 14180.0069790.090 0.0449750.78 0.046761 0.760.249 z2 z5 44.887 0.03344.83 0.15 42 85 0.30832.00.5757 36360.0069870.073 0.0451380.34 0.046873 0.320.422 z5

SGC-54 zirconIGSN: SSR.NMM000006Purple Creek orthogneiss.Wtd mean Th-corrected 206Pb/238U date: 49.377 ± 0.032 Ma L1b 51.537 0.19450.65 2.50 9120 -273 0.3152.3 0.73 3 217 0.0080270.377 0.0511525.05 0.046240 4.990.206 L1b s5z 49.359 0.06650.08 0.72 85 34 45 0.52610.20.9211 686 0.0076860.134 0.0505551.47 0.047725 1.440.244 s5z s10 49.417 0.05749.67 0.63 62 31 26 0.3488.7 0.69 13 820 0.0076950.116 0.0501291.30 0.047267 1.280.238 s10 s21 49.350 0.05949.57 0.63 60 30 24 0.3578.4 0.66 13 828 0.0076850.119 0.0500281.30 0.047236 1.260.344 s21 zS1 53.613 0.06753.41 0.65 44 29 -100.199 7.70.6113 852 0.0083510.125 0.0540041.25 0.046920 1.210.371 zS1 zs3 62.938 0.05863.68 0.53 92 20 34 0.24811.80.6319 12430.0098110.092 0.0647290.86 0.047871 0.840.302 zs3 zS11 49.377 0.08748.90 1.09 25 54 -620.180 4.80.697 478 0.0076890.176 0.0493342.29 0.046555 2.250.292 zS11

SGC-02 zirconIGSN: SSR.NMM000008Rainbow Falls orthogneiss.Wtd mean Th-corrected 206Pb/238U date: 48.158 ± 0.032 Ma s3z 48.174 0.04648.08 0.46 43 23 10.172 12.6 0.82 15 10420.0075010.096 0.0484920.98 0.046908 0.960.271 s3z s4z 52.211 0.13053.36 1.61 10572530.221 3.60.755 331 0.0081320.251 0.0539583.09 0.048144 3.050.234 s4z s9z 48.118 0.07448.10 0.87 47 44 80.256 6.20.768 549 0.0074920.154 0.0485101.86 0.046980 1.830.226 s9z s23z 56.816 0.10356.80 1.29 56 55 60.193 6.61.046 438 0.0088520.183 0.0575362.33 0.047159 2.300.225 s23z TABLE 1. SUMMARY OF U-Pb ZIRCON CRYSTALLIZATION AGES z22 48.113 0.08648.10 1.06 47 53 90.231 7.71.167 449 0.0074920.179 0.0485122.26 0.046986 2.230.226 z22 zs5 48.187 0.06848.05 0.82 41 41 -4 0.2565.0 0.51 10 652 0.0075030.141 0.0484581.74 0.046862 1.700.324 zs5

NMNC-581 zircon IGSN: SSR.NMM000009Protomylonitic tonalitic orthogneiss.Wtd mean Th-corr. 206Pb/238U date: 48.490 ± 0.027 Ma z1 48.451 0.04347.95 0.32 23 15 -750.440 9.10.3725 15180.0075440.090 0.0483550.69 0.046507 0.620.739 z1 z2 48.515 0.03448.47 0.25 46 12 50.308 10.9 0.44 25 15870.0075540.071 0.0488920.52 0.046962 0.480.562 z2 z3 49.099 0.16550.29 2.34 108106 56 0.3381.3 0.37 3 229 0.0076460.338 0.0507804.77 0.048192 4.500.812 z3

NC-95 zircon IGSN: SSR.NMM000013Sunrise Lake sheet. Youngest Th-corrected 206Pb/238U date: 49.198 ± 0.043 Ma zL1 49.567 0.05249.87 0.37 65 17 29 0.3059.0 0.55 16 10460.0077190.105 0.0503460.76 0.047327 0.700.576 zL1 Age zM1 49.466 0.03449.61 0.21 56.5 9.4190.656 18.4 0.61 30 17680.0077030.069 0.0500700.43 0.047165 0.390.564 zM1 zM2 49.360 0.03949.41 0.20 51.7 9.0130.366 9.60.3329 18370.0076860.078 0.0498630.41 0.047070 0.380.523 zM2 zM4 49.564 0.04049.82 0.31 62 14 26 0.42210.90.5819 11700.0077180.080 0.0502850.64 0.047273 0.600.494 zM4 zS5 49.434 0.11849.87 1.56 71 72 35 0.3182.3 0.64 4 249 0.0076980.239 0.0503423.22 0.047451 3.040.750 zS5 zS6 49.198 0.04349.32 0.31 55 14 18 0.3255.9 0.31 19 12080.0076610.087 0.0497700.63 0.047138 0.590.554 zS6 Abbreviation Sample (Ma) Unit zS7 49.627 0.10450.14 1.42 75 66 38 0.4112.3 0.57 4 262 0.0077280.211 0.0506212.90 0.047528 2.760.696 zS7 zS9 49.329 0.17148.54 2.54 10 122-2350.333 3.71.612 161 0.0076820.348 0.0489685.35 0.046255 5.090.787 zS9 zS12 49.272 0.05349.32 0.22 51.5 9.3120.423 8.70.2831 19240.0076730.108 0.0497680.45 0.047065 0.390.623 zS12

NC-92 zircon IGSN: SSR.NMM000012Sunrise Lake tonalite. Wtd mean Th-corrected 206Pb/238U date: 49.484 ± 0.026 Ma zM7 49.494 0.03349.71 0.26 60 12 24 0.40229.21.4021 13050.0077070.067 0.0501770.53 0.047238 0.490.547 zM7 zM9 66.092 0.05066.37 0.26 76.4 8.7180.333 9.00.2535 22340.0103050.077 0.0675490.41 0.047561 0.370.588 zM9 zM12 50.028 0.03450.11 0.14 53.8 5.6150.444 14.3 0.30 47 28690.0077910.069 0.0505840.28 0.047111 0.230.684 zM12 zS8 49.477 0.06649.59 0.52 55 24 17 0.3513.4 0.31 11 701 0.0077050.135 0.0500471.08 0.047133 1.020.439 zS8 zXL1b 64.137 0.05564.47 0.48 77 17 20 0.4297.1 0.46 16 968 0.0099990.086 0.0655480.77 0.047567 0.720.664 zXL1b DL NC-197 44.857 ± 0.023 Diablo Lake orthogneiss (see text) zXL3a 49.462 0.06748.68 0.56 10 26 -231 0.36512.51.1311 703 0.0077020.136 0.0491051.17 0.046260 1.090.656 zXL3a zXL3b 50.163 0.03050.24 0.16 53.9 7.2150.350 28.8 0.74 39 24380.0078120.060 0.0507220.33 0.047112 0.300.558 zXL3b zXL19 49.444 0.11148.70 1.50 12 72 -196 0.4726.8 1.79 4 248 0.0077000.225 0.0491273.15 0.046297 3.000.718 zXL19

H20FG zirconIGSN: SSR.NMM000007 "Flecked" gneiss. Wtd mean Th-corrected 206Pb/238U date: 47.197 ± 0.020 Ma z1 47.193 0.03847.3010.090 52.8 5.0190.178 31.0 0.40 78 50950.0073480.080 0.0476870.19 0.047092 0.210.034 z1 z2 47.184 0.03447.17 0.24 47 11 90.220 12.6 0.48 26 16950.0073460.072 0.0475560.51 0.046971 0.470.662 z2 FL H2OFG 47.197 ± 0.020 Flecked gneiss (Miller et al., 2009b, p. 402) z3 66.931 0.04166.80 0.13 61.9 4.3-20.124 45.9 0.41 111 74280.0104370.061 0.0679950.21 0.047272 0.180.571 z3 z6 47.213 0.03447.23 0.38 48 19 12 0.33018.20.9419 12290.0073510.072 0.0476180.83 0.047004 0.790.585 z6

ES-32 zirconIGSN: SSR.NMM000005Ferry Peak orthogneiss. Youngest Th-corrected 206Pb/238U date: 72.787 ± 0.046 Ma zM2 84.798 0.07584.92 0.77 88 22 70.365 3.70.2415 954 0.0132410.089 0.0872270.95 0.047799 0.920.354 zM2 zM4 76.462 0.06275.89 0.22 58.0 6.8-25 0.21447.20.6178 50460.0119320.081 0.0776080.29 0.047195 0.280.265 zM4 zM6 76.145 0.05976.26 0.21 79.9 5.68 0.23115.40.2367 43560.0118820.079 0.0779980.28 0.047630 0.240.667 zM6 FP ES-32 72.787 ± 0.046* Ferry Peak orthogneiss (Shea, 2008) zM7 72.787 0.04672.70 0.18 69.8 5.71 0.21825.30.4260 39250.0113550.063 0.0742240.26 0.047429 0.240.480 zM7 zM8 75.363 0.05475.47 0.24 78.8 6.38 0.18022.70.3565 42520.0117590.072 0.0771570.33 0.047609 0.260.943 zM8 zM9 77.558 0.05577.50 0.38 76 11 20.248 13.2 0.55 24 15680.0121040.072 0.0793140.51 0.047547 0.470.581 zM9 zS13a 76.905 0.09976.67 1.14 69 35 -6 0.2154.4 0.57 8 520 0.0120010.129 0.0784291.55 0.047418 1.470.639 zS13a

ES-02 zirconIGSN: SSR.NMM000004Tonalitic orthogneiss. Wtd mean Th-corrected 206Pb/238U date: 48.876 ± 0.094 Ma zM1 60.272 0.07160.01 0.58 50 22 -120.369 4.20.3114 857 0.0093940.118 0.0608851.00 0.047030 0.920.739 zM1 GG ES-02 47.876 ± 0.094 Tonalitic orthogneiss (Shea, 2008) zM7 83.576 0.06383.09 0.34 69.0 9.3-16 0.37011.40.3334 21450.0130490.076 0.0852680.43 0.047413 0.390.553 zM7 zS7 47.747 0.13046.91 1.85 492-4250.413 1.60.523 207 0.0074340.274 0.0472814.04 0.046147 3.840.756 zS7 zS10 48.019 0.13749.92 1.86 14285670.390 1.50.434 238 0.0074770.286 0.0503883.82 0.048900 3.610.739 zS10 zS15 77.131 0.10077.64 1.13 93 34 20 0.1907.3 0.96 8 517 0.0120370.131 0.0794611.51 0.047900 1.420.705 zS15

a Corrected for initial Th/U disequilibrium using radiogenic 208Pb and Th/U[magma] = 4 ± 1 (2s). b Isotopic dates calculated using 238 = 1.55125E-10 (Jaffey et al. 1971) and 235 = 9.8485E-10 (Jaffey et al. 1971). PC SGC-54 49.377 ± 0.023 Purple Creek orthogneiss (Michels, 2008) c % discordance = 100 - (100 * (206Pb/238U date) / (207Pb/206Pb date)) d Th contents calculated from radiogenic 208Pb and the 230Th-corrected 206Pb/238U date of the sample, assuming concordance between the U-Pb and Th-Pb systems. e Total mass of radiogenic Pb. f Total mass of common Pb. g Ratio of radiogenic Pb (including 208Pb) to common Pb. h Measured ratio corrected for fractionation and spike contribution only. i Measured ratios corrected for fractionation, tracer and blank. PO NC-581 48.490 ± 0.027 Protomylonitic orthogneiss (Miller et al., 2009b, p. 398) RF SGC-02 48.158 ± 0.032 Rainbow Falls orthogneiss (Michels, 2008) 1 Supplemental Table 1. U-Pb age data in Excel file SL NC-92 49.484 ± 0.026 Sunrise Lake tonalite (.xls). Please visit http://dx .doi.org/10 .1130/GES01262 .S1 or the full-text article on www.gsapubs.org to SLs NC-95 49.198 ± 0.043 Sunrise Lake sheet; cuts foliation in tonalite view the Supplemental Table. A data set containing ST SGC-63 65.866 ± 0.055 Stehekin orthogneiss (Michels, 2008) details of dates for high-precision U-Pb zircon dating Note: Abbreviations as in text Figure 3. Sample identiﬁcations and details as in Supplemental Table 1 (see text footnote 1); see http:// of 10 orthogneiss bodies in the Skagit Gneiss Com- geochron.org/dataset/html/geochron_dataset_2016_01_22_kCnxS. plex is located at http://geochron .org /dataset /html *Youngest grain; signiﬁcant inheritance. /geochron_dataset _2016 _01 _22 _kCnxS.

PALEOGENE MIDDLE TO DEEP CRUST overall are most similar to those in the Napeequa unit, except in the southwest, where hornblende-biotite schist resembles the Cascade River–Holden Skagit Gneiss Complex unit (Miller et al., 1994).

Map Pattern and Rock Units P-T Conditions and Migmatization

The Skagit Gneiss Complex extends for >100 km along strike, and is as Amphibolite facies assemblages occur throughout the Skagit Gneiss Com- much as 30 km wide (Misch, 1966, 1968; Haugerud et al., 1991) (Figs. 2 and 3). It plex (Misch, 1968). Sillimanite and, to a lesser extent, kyanite occur in local is bounded on the northeast by the Ross Lake fault zone and is intruded on the metapelite, mainly in the northern part of the complex (Misch, 1968). These southwest by Eocene plutons or is in contact with the Napeequa unit, Late Tri- metapelites record maximum conditions of 8–10 kbar at 650–725 °C (Whitney, assic arc rocks, or Cretaceous orthogneiss (Fig. 3). An intrusive contact with the 1992b; Gordon et al., 2010a) that were followed by a near-isothermal decom- ca. 48 Ma Cooper Mountain batholith marks the southern boundary and the pression path to P < 5 kbar (Whitney, 1992b). complex is truncated by the intersection of the Ross Lake and Straight Creek– The northern half of the complex also contains migmatites with low-K Fraser faults to the north (Figs. 2 and 3). The Skagit Gneiss Complex consists (trondhjemitic), layer-parallel to discordant leucosomes (Fig. 4A) that likely of orthogneiss, “banded gneiss,” which is strongly foliated and commonly formed under a combination of subsolidus processes (Misch, 1968; Yardley, migmatitic gneiss of uncertain protoliths, and metasupracrustal rocks (Misch, 1978; Babcock and Misch, 1989) and anatexis driven by water-saturated melt- 1968). Orthogneiss makes up ≥75% (Haugerud et al., 1991) of the complex, and ing and decompression (Whitney, 1992a; Whitney and Irving, 1994; Gordon on the basis of our mapping, probably >90% of its central and southern parts. et al., 2010a). Metamorphism and melt crystallization in the northern Skagit Much of the northern half is migmatitic (Misch, 1968). Gneiss Complex likely occurred (possibly intermittently) from 71 to 47 Ma, on The dominant protoliths of Skagit orthogneisses are biotite tonalite and the basis of U-Pb CA-ID-TIMS zircon and monazite dates from three leuco- leucotonalite; hornblende-bearing tonalite and biotite granodiorite are mod- somes, interpreted to have formed under anatectic conditions, biotite gneiss, erately abundant, and granite to quartz monzonite and diorite orthogneiss and metapelite in an ~7 km, across-strike transect in the Highway 20 corri- occur locally (Misch, 1968; Haugerud et al., 1991; Miller et al., 1994; our map- dor (Gordon et al., 2010a); these crystallization ages overlap with those of the ping). Crystallization ages of the orthogneiss protoliths range from ca. 89 to orthogneisses. 45 Ma, and the largest volume of orthogneiss probably intruded between ca. 73 and 59 Ma (Fig. 3; U-Pb CA-ID-TIMS zircon data in Table 1; other ID‑TIMS Structure of Skagit Gneiss Complex zircon dates from Mattinson, 1972; Miller et al., 1989; Miller and Bowring, 1990; Haugerud et al., 1991). Most of these orthogneisses are sheeted, or hetero The structure of the Skagit Gneiss Complex varies along strike and can geneous on the outcrop to 100 m scale, and are intimately intruded by 10-cm- be considered in terms of three domains (shown in Figs. 3 and 5) that are thick to 10-m-thick leucocratic, commonly pegmatitic sheets. An apparent distinguished by different foliation orientations. Foliation is generally defined intrusive hiatus occurred between the ca. 59 and 50–45 Ma orthogneisses by aligned biotite, quartz, and quartz-plagioclase aggregates, and in some (Fig. 3). Distinctive tonalitic orthogneiss sheets (most <20 m thick) that occur as rocks by hornblende. Most of the complex is foliated and lineated, but fabric late intrusive bodies scattered throughout the gneiss complex contain plagio intensity varies considerably. The weakest fabrics are in orthogneiss and par- clase phenocrysts that are commonly cored by igneous-appearing sphene. ticularly in the younger (ca. 50–48 Ma) orthogneiss in the central and southern One crystallized at 47.192 ± 0.002 Ma (FL, Fig. 3; Table 1). The youngest intru- domains. In addition, constrictional fabrics are common in some areas, and sions are ca. 45 Ma (44.857 ± 0.023 Ma; DL, Fig. 3; Table 1; also a less precise are recognized qualitatively by stronger lineation than foliation, and by strain age in Haugerud et al., 1991) bodies, ranging from <1 m to ~750 m across, of markers, such as in metamorphosed fragmental rocks and quartz-plagio biotite granodiorite to granite of the Diablo Lake orthogneiss. These bodies clase aggregates in orthogneisses (Misch, 1968; our observations). They are discordantly cut foliation and contacts in the gneiss complex and have a weak most notable in the young (ca. 45 Ma) Diablo Lake orthogneiss (Misch, 1968; or no foliation, but have a strong mineral lineation, which is parallel to that in Haugerud et al., 1991; Wintzer, 2012) and parts of the stepover in the Ross the older gneisses (Misch, 1968, 1977a; Haugerud et al., 1991; Wintzer, 2012). Lake fault zone. This stretching lineation is marked best by biotite and quartz aggregrates that In much of the gneiss complex, foliation strikes northwest (Fig. 5A), and have undergone grain-size reduction. on a regional scale, the complex forms a northwest-southeast–trending anti Metasedimentary and metavolcanic rocks commonly occur as inclusions clinorium (Tabor et al., 1989). Mineral lineation typically plunges gently to within, and/or rafts between, orthogneiss bodies (Misch, 1968). Most are the northwest or southeast (e.g., Brown and Talbot, 1989), but is domainal <25 m wide, but zones rich in metasupracrustal rocks reach ~3 km across in orientation (Fig. 5B). Outcrop-scale folds are common in well-layered (Tabor et al., 2003). Amphibolite and biotite schist dominate, and lithologies gneiss and migmatite in the northern domain. Earliest structures are tight to

A B E

C D

Figure 4. Field photographs of Skagit Gneiss. (A) Upright open fold of leuco some and foliation in a Skagit migmatite. Leucosomes in this outcrop have crystallization ages ranging from 69 to 63 Ma (Gordon et al., 2010a). (B) Strong stretching in face that is subparallel to fold hinge line and mineral lineation. (C) Plagioclase porphyroclasts in foliation-parallel shear zone in mylonitic paragneiss. (D) Discordant shear zone cutting foliation in ortho gneiss. Note the fine-grained, weakly deformed intrusive material in the shear zone. (E) Low-angle extensional fault in stepover in the Ross Lake fault. Cretaceous metaclastic and plutonic rocks structurally overlie metasedimentary and metavolcanic rocks of the Napeequa unit, which in turn is structurally above the Skagit orthogneiss (lowest exposure). Relief from saddle in foreground to summit of peak is ~400 m.

isoclinal, gently inclined to recumbent folds that typically have wavelengths the upright map-scale folds (Fig. 5C). Hinge lines mostly plunge southeast, of <25 cm (Misch, 1977a; Wintzer, 2012). Locally, the dominant foliation is are subparallel to mineral lineations, and are commonly stretched parallel to axial-planar to folded leucosomes and compositional layers, but it is gen- lineation (Fig. 4B). erally folded by the earliest structures. Hinge lines are variably oriented re- Map-scale folds of foliation and unit contacts are mainly upright to steeply flecting subsequent refolding, and the average orientation of axial surfaces inclined structures (Figs. 6A, 6B; e.g., Misch, 1966; Tabor and Haugerud, 1999; is approximately parallel to foliation (Wintzer, 2012; our observations). The Tabor et al., 2003; Miller et al., 2009b; Wintzer, 2012). They most commonly dominant mesoscopic folds are upright to inclined structures that range from plunge gently southeast or northwest, but in parts of the complex the axial gentle to tight, and have wavelengths of ~1 cm to 20 m (Fig. 4A) (Misch, 1968; traces are curved. In the south, symmetric gentle to open map-scale folds of Haugerud et al., 1991; Wintzer, 2012). These folds are probably parasitic to foliation have axial traces that trend west-northwest near the Cooper Mountain

A 121° 00

Chilliwack

ND zamee F n A′ RM ER A RC lt

SCF 20 120° 30′ y 20 2 Hw Golden Horn Figure 5 (on this and following two pages). RLFZ Structural maps of Skagit Gneiss Complex Meth constructed mostly from our averaged 48° 30′ Eldorado data with additional data in the north Basi ow from Tabor et al. (2003). Note the positions Skagit n of cross-sections A-A′ and B-B′. Abbrevia- NQ Black tions: ND—northern domain, CD—central Gneiss N Complex Peak CF domain, and SD—southern domain of the NQ Skagit Gneiss Complex; ER—Elijah Ridge; GPTB GPTB—Gabriel Peak tectonic belt; NQ— CD Napeequa unit; RC—Ruby Creek plutonic F oggy Dew belt; RLFZ—Ross Lake fault zone; RM— Ruby Mountain; SCF—Straight Creek 10 km fault; Pk—peak; Mtn—mountain; Flt— Oval fault. Note the stepover in the Ross Lake N fault zone (red dots). (A) Simplified map

Pk B ′ of foliation attitudes. Attitudes in Eocene Stepover in RLF SD Fa plutons are magmatic structures. u lt Post-Eocene Undi erentiated RRC Chumstick Fm. (Eocene) SwakS Eocene plutons w a k aneana Skagit Orthogneiss (Cretaceous-Eocene) nen B e En Skagit Banded Gneiss (Cretaceous-Eocene) t Cooper Mtn i a t Swakane Gneiss (Cretaceous) F a u Duncan Hill Pre-Eocene plutons 48° lt - Holden Cascade River Cascade River-Holden (Triassic) Napeequa (Mississippian - Jurassic?) Mesozoic Undi erentiated

batholith, and curve to a northwest trend to the north (Fig. 5C). Gently plung- main, foliation defines a broadly fan-like antiform; in the west, it strikes north- ing mineral lineation similarly swings from east-southeast near the batholith west and dips southwest, and in the east, it strikes north-northeast and dips to northwest farther to the north, and to northwest-southeast and northeast east. Lineation shows a similar swing (Adams, 1961; our data). In the northern in the northern part of the complex (Fig. 5B). Most map-scale folds die out domain, gently southeast-plunging map-scale folds have wavelengths of a northward from the Cooper Mountain batholith, whereas the longest (8 km) few kilometers (Fig. 6A, A-A′). The northwest-striking foliation of this corridor wavelength and westernmost antiform continues for ≥30 km. In the central do- changes in the east near the stepover in the Ross Lake fault zone where strikes

B 121° 00 Chilliwack

ND zamee F n A′

ER A RM RC lt

SCF 120° 30′ y 20 Hw Golden Horn RLFZ 48° 30′ Eldorado Meth Basi Skagit o n w NQ Gneiss Black NCF Complex Peak Figure 5 (continued). (B) Simplified map of NQ GPTB lineation trends (plunges <30°). Lineations CD in plutons are magmatic.

Fo g g 10 km y Dew N Oval Pk B Stepover in RLF ′ SD Fa Post-Eocene Undi erentiated ul t Chumstick Fm. (Eocene) RRC

Eocene plutons S waw ka a Skagit Orthogneiss (Cretaceous-Eocene) k aan nen Skagit Banded Gneiss (Cretaceous-Eocene) e B E Cooper Mtn Swakane Gneiss (Cretaceous) nt i a t Pre-Eocene plutons Fa Duncan Hil 48° u Holden Cascade River-Holden (Triassic) lt Cascade River - Napeequa (Mississippian - Jurassic?) l Mesozoic Undi erentiated

rotate to northeast and east-west (Fig. 5A), and dips are lower compared to to steeply. These open folds record approximately orogen-parallel shortening the south. Mineral lineation here mostly plunges gently to the south, or to that deformed foliation in rocks as young as the ca. 47.2 Ma sheet (FL, Fig. 3; the south-southwest and north-northeast, in contrast to the gentle southeast Table 1) in the northern domain. plunge of lineation away from the stepover. Noncoaxial flow is recognized in some areas at the outcrop scale to micro The latest folds are defined by the large swings in the regional trends of scale by asymmetric structures, but well-defined ductile shear zones are un- foliation, lineation, and hinge lines (Fig. 5), and probably plunge moderately common at the 10 m scale or larger. These asymmetric structures, which are

C 121° 00′

ChilliwackChillill wackk

H ND oz A′ ameen ER

RM Flt A RC

SCF 120° 30′ y 20 Hw Golden Horn RLFZ 48° 30′ Eldorado Meth GP Basi Skagit T ow B n NQ Gneiss Black NCF Complex Peak Figure 5 (continued). (C) Simplified map NQ showing axial traces of map-scale upright CD folds. F oggy Dew N 10 km Oval

Pk B ′ Stepover in RLF SD Fa ul Post-Eocene Undi erentiated RRC t Chumstick Fm. (Eocene) E SwakS n Eocene plutons w t ia a t k F Skagit Orthogneiss (Cretaceous-Eocene) ana e au nen e lt B Skagit Banded Gneiss (Cretaceous-Eocene) Cooper Mtn Swakane Gneiss (Cretaceous) Duncan Hill Pre-Eocene plutons 48° den scade River - Hol Cascade River-Holden (Triassic) Ca Napeequa (Mississippian - Jurassic?) Mesozoic Undi erentiated

used to determine shear sense, include S-C relations, shear bands (C′ surfaces), layering. Widespread, but less common, generally ≤15-cm-thick shear zones oblique quartz foliation, asymmetric plagioclase porphyroclast systems (Fig. cut foliation at moderate to high angles, are generally steeper than the folia- 4C), mica fish, asymmetric leucosomes and boudins, and foliation curvature. tion-parallel ones, and are highly variable in strike. Lineations in the late zones These features are concentrated in weaker micaceous layers, particularly in also show considerable scatter, but the majority plunges gently to moderately well-layered gneisses next to orthogneiss bodies and pegmatitic sheets. Shear to the east-southeast and southeast. Similar kinematic indicators occur in both zones are generally subparallel to the dominant foliation and compositional types of shear zones.

A A′ SW NE

2000 t

n Ruby Ck. i

k (48) a 67 1000 e 48 47 r 68 Chillwack 69–63 amphibolite 45 B s 65 r (34) te 0

Skagit Orthogneiss banded non-migmatitic Napeequa gneiss

Schist lt

F ew

B D

y B′

g g

SW 49 o 50 F NE 60 89 65 65

rs 2000

et 48 m x 1000 Skagit Orthogneiss Oval Peak Pluton Cooper Napeequa Mtn Schist C C′ S N

1500

mete 1000

500

Swauk Formation No Vertical Exaggeration

Figure 6. Cross sections through the Skagit Gneiss Complex (A-A′ and B-B′) and Swauk Formation (C-C′). Cross-section lines are in Figures 5 and 7. Numbers are crystallization ages (U-Pb zircon; isotope dilution–thermal ionization mass spectrometry). Dashes are traces of foliation (A-A′ and B-B′) and beds (C-C′). Ck—creek.

In a few outcrops, asymmetric fabrics in foliation-parallel shear zones (such as in Fig. 4A) and restoring foliations to an approximately subhorizon- show a reversal in shear sense from sinistral to dextral across fold hinges. tal orientation. The best-documented cases have top-to-the-northwest shear in both limbs In the southern domain, after unfolding we found examples of the top-to- and in the hinge. This reversal and the shear direction (mineral lineation) the-southeast and top-to-the-northwest shear. In the central part of the gneiss do not result from flexural shear, as the lineations and hinge lines are sub complex, we rarely observed shear-sense indicators outside of the dextral Ross parallel (cf. Goscombe and Trouw, 1999). On the basis of these observations, Lake fault zone (see following). In the northern domain, shear-sense indicators and following Wintzer (2012), we reevaluated kinematic patterns for the foli- are widespread (Wintzer, 2012; our data), including in orthogneiss as young as ation-parallel shear zones after unfolding the upright, gently plunging folds ca. 48–47 Ma, but are not recognized in the ca. 45 Ma Diablo Lake orthogneiss

bodies. Both top-to-the-northwest (mostly dextral) and top-to-the-southeast fine the timing of deformation. In comparison, the 48.5–47.2 Ma orthogneisses (mostly sinistral) shear are recorded throughout the corridor. Noncoaxial shear (FG, PO, Fig. 3; Table 1) in the northern part of the complex record significant initiated at amphibolite facies conditions, as cummingtonite and sillimanite solid-state deformation. Moreover, ca. 45 Ma Diablo Lake orthogneiss bodies are in C-surfaces and plagioclase is recrystallized in foliation-subparallel zones crosscut foliation, although these orthogneisses have strong solid-state linea- (Gordon et al., 2010a), and some of the shear zones at moderate angles to tion (Misch, 1968; Haugerud et al., 1991; Wintzer, 2012). All ductile deformation foliation in the southern domain appear to have been filled with melt (Fig. 4D). ended before intrusion of the 34 Ma phase of the Chilliwack batholith (Fig. 3). Shear continued during cooling to greenschist facies conditions in both types Most K-Ar and 40Ar/39Ar biotite and hornblende dates from the Skagit of shear zones, as in some zones quartz shows basal slip, bulging recrys- Gneiss Complex range from ca. 50 to 44 Ma (Engels et al., 1976; Wernicke tallization, and considerable unrecovered strain, and plagioclase is microfrac- and Getty, 1997; Tabor et al., 2003; Gordon et al., 2010b). A sample from near tured and not recrystallized (cf. Gordon et al., 2010a, 2010b; Wintzer, 2012). (~2.5–5 km) dated migmatites gives 40Ar/39Ar hornblende and biotite dates of Overall patterns of noncoaxial flow are clearly complicated in the Skagit 47.1 ± 1.3 Ma and 45.2 ± 0.2 Ma, respectively (Wernicke and Getty, 1997), and Gneiss Complex. We thus further refined our analyses by looking at shear sense muscovite from two structurally higher pegmatite samples on Ruby Mountain in some of the youngest dated orthogneisses, including the 47.192 Ma ± 0.002 Ma, (Fig. 3) yield muscovite 40Ar/39Ar dates of 47.1 ± 0.3 Ma and 46.8 ± 0.3 Ma (Gor- plagioclase-phyric, sphene-cored, orthogneiss sheet and a 48.490 Ma ± 0.027 Ma don et al., 2010b). Overall, the temporal overlap of the youngest zircon (ca. (PO, Fig. 3; Table 1) protomylonitic orthogneiss body. There appears to be an 47 Ma) and monazite (ca. 49–46 Ma) dates from migmatites and orthogneisses association of the youngest greenschist facies microstructures and top-to-the- with the 40Ar/39Ar and K-Ar dates indicates rapid cooling at 47–45 Ma. northwest shear, but we also found examples with top-to-the-southeast shear. We estimated the minimum shortening of the gneiss complex recorded Boundaries of the Skagit Gneiss Complex by map-scale upright folds in two cross sections (Figs. 6A, 6B) by assuming flexural slip and using simple line-length balancing. Results are≥ 33% in the The relationships of the Skagit Gneiss Complex to adjacent, generally northern domain and ≥25% in the southern domain. The magnitude of short- lower grade rocks are important for models of crustal flow and partitioning of ening by foliation development and the homogeneous shortening indicated by deformation at different crustal levels. Lengthy segments of the western and the thickening of layers in fold hinges is unknown. southern boundaries are intrusive contacts with younger weakly deformed plutons. Where the gneiss complex is in contact with older rocks on the west Timing of Deformation and Cooling (Fig. 3), foliation and lineation are concordant across the contact (Cater and Wright, 1967; Miller, 1987; Tabor et al., 2003; our mapping). The northeast The Skagit Gneiss Complex underwent a protracted history of meta- boundary in the Ross Lake fault zone and the upper contact with the Napeequa morphism and deformation (Table 2). The overprinting by the extensive ca. unit are more complicated. 73–45 Ma magmatism, metamorphism, and deformation makes it difficult to The ≥10-km-wide Ross Lake fault system (Misch, 1966; Miller, 1994) is part correlate any structures within the complex to the mid-Cretaceous (ca. 100– of a 500-km-long zone of Paleogene, northwest-striking high-angle faults 80 Ma) regional shortening that is well documented in the Wenatchee block of (Fig. 1) (e.g., Monger, 1986). The northernmost strand of the Ross Lake fault is the Cascades core and in low-grade rocks adjacent to the core. Similarly, the a vertical mylonite zone in British Columbia that separates upper-amphibolite- high-P metamorphism that initiated by 75 Ma in the core (Miller et al., 1993a, facies rocks of the Skagit Gneiss Complex from sub-greenschist-facies rocks to 1993b; Brown et al., 1994) has been overprinted. the east (McTaggart and Thompson, 1967). Dextral shear and 6–12 km of north- Orthogneisses that yield inferred crystallization ages as young as 47.2 Ma east-side-down normal slip occurred from ca. 50(?) to after 45 Ma (Haugerud, (FL, Fig. 3; Table 1), ca. 69–51 Ma leucosomes, and metasupracrustal rocks with 1985) and predated Oligocene magmatic rocks to the south. In Washington, the monazite dates as young as 47 Ma (Gordon et al., 2010a) display the dominant fault is intruded by the ca. 48 Ma Ruby Creek plutonic belt, and steps westward foliation and lineation in the Skagit Gneiss Complex. In two of the dated migma- 7 km across a gently to moderately dipping stepover zone to the Gabriel Peak tite localities, a 65–64 Ma and a 53 Ma discordant leucosome cut the foliation of tectonic belt (Figs. 3 and 4E) (Miller et al., 1994; Gordon et al., 2010b). This the host metapelite and biotite gneiss, indicating that foliation initiated by 65 Ma mylonitic, moderately to steeply northeast-dipping belt records dextral shear in at least part of the gneiss complex. Both discordant leucosomes were later in the north and reverse shear farther south (Fig. 3). Transpressional defor- deformed under greenschist-facies conditions (Gordon et al., 2010a). mation in the belt occurred from 65 Ma (and earlier?) to ca. 58 Ma (Miller and Fabric development was in part time transgressive. Solid-state foliation Bowring, 1990; Miller et al., 1994). The tectonic belt merges to the south with and lineation in orthogneisses in the southern part of the complex, and folding the North Creek fault to form the Foggy Dew fault zone (Fig. 3), which sepa of the 49.484 ± 0.026 Ma Sunrise Lake tonalite (SL, Fig. 3; Table 1) predated rates sillimanite-bearing mylonites from greenschist facies rocks to the east, intrusion of the ca. 48 Ma Cooper Mountain batholith. A 49.198 ± 0.043 Ma and underwent oblique dextral-normal slip (down to the east) between ca. 50 felsic sheet (SL1, Fig. 3; Table 1) that cuts foliation in the tonalite may better de- and 48 Ma (Miller and Bowring, 1990).

TABLE 2. SUMMARY OF CHRONOLOGY OF EOCENE PROCESSES Upper Crust Mid Crust <40 Straight Creek Fault (48–34(?) Ma)-

Dextral/top-NE shear northern RLFZ (50(?)–45 Ma)- -Upper Chumstick–Roslyn (46–35(?) Ma) n Dextral/top-ESE normal shear, FDFZ (50(?)–48 Ma)- Entiat Fault: dextral normal (50–45(?) Ma)- -Chumstick (strike-slip basin) (49–46 Ma) 45 Skagit -Plutonism (48–45 Ma) -Rapid Cooling (50–45 Ma) -Plutonism (50–49 Ma) -Dike swarms (49–46 Ma)

-Skagit Orthogns (50–45 Ma) Fault motio n -Teanaway volcanism (49 Ma)

Swakane 4 m Magma

Magma -Rapid Cooling (51–46 Ma) 6 2 Cooling 50 3 Ma -Regional magmatic foliation & lineation (49–45 Ma) - mid & upper crust -Folding of Swauk (51–49 Ma)

Skagit Swakane -Swauk–Chuckanut deposition (<59–51 Ma) -Folding with steep axes (49–48 Ma) -Upright folding (>50–48 Ma) 55 -Upright folding (50–48 Ma) -Top-N shear (>50–48 Ma) -Foliation & lineation (>50–45 Ma) -Foliation & lineation (>51–48 Ma) -Metamorphism (>69–47 Ma) -Metamorphism (≥68–50 Ma) Basin deposition & deformatio 5 Metam & ductile def 1 >60 See the text and following other key sources for documentation. 1Metamorphism and ductile deformation: Misch (1968), Haugerud et al. (1991), Whitney (1992b), Matzel et al. (2004); Paterson et al. (2004), Gordon et al. (2010a, b), Gatewood and Stowell (2012), Wintzer (2012). 2Magmatism (mid-crust): Table 1 and Supplemental Table 1, Mattinson (1972), Miller et al. (1989), Miller and Bowring (1990), Haugerud et al. (1991). 3Cooling: Wernicke and Getty (1997), Matzel (2004), Paterson et al. (2004), Gordon et al. (2010b). 4Fault motion: Tabor et al. (1984, 1989), Miller and Bowring (1990); Gordon et al. (2010b). 5Basin deposition and deformation: Tabor et al. (1984), Johnson (1985), Evans (1994), Eddy et al. (2016). 6Magmatism (upper crust): Eddy et al. (2016). FDFZ = Foggy Dew fault zone, RLFZ = Ross Lake fault zone.

The gently to moderately northeast-dipping upper contact of the Skagit 40Ar/39Ar cooling ages (ca. 47 Ma) on both sides of the contact argue against Gneiss Complex is best exposed on Ruby Mountain (Fig. 3), where we ana- this interpretation (Gordon et al., 2010b). Thus, the Skagit-Napeeqa contact lyzed the structure. There, orthogneiss and local pelitic schist are structurally was overprinted by a high-temperature shear zone that was localized at this below amphibolite, quartzite, and metaperidotite of the Napeequa unit, and contact. Localization probably resulted from the rheological contrast between Skagit and Napeequa rocks are both intruded by trondhjemitic pegmatites. the Skagit orthogneiss and Napeequa rocks, possibly reactivating an older This contact is part of a shear zone marked by a strain gradient with pro- shear zone, and/or deformation may have transposed an intrusive contact. nounced intensification of lineation and the formation of constrictional (L >> S) fabrics. Protomylonitic gneiss near the contact has moderate- to high-tempera- EOCENE DUCTILE DEFORMATION IN ture microstructures, marked by recrystallization of hornblende, plagioclase, THE SWAKANE BIOTITE GNEISS and quartz, and is cut by widespread contact-parallel ductile shear zones. In addition, kinematic indicators give conflicting senses of noncoaxial shear from The second major domain of rocks with Eocene 40Ar/39Ar cooling ages and outcrop to outcrop. In one scenario, the intense fabrics and juxtaposition of ductile deformation is south of the Skagit Gneiss Complex within the Cretaceous different rock types represent a detachment zone; however, the inconsistent Swakane Biotite Gneiss. The Dinkelman décollement separates the main body sense of shear, similar P-T conditions of 8–10 kbar at 650 °C, and muscovite of Swakane Biotite Gneiss from the structurally overlying Napeequa unit and

91–72 Ma arc plutons (Fig. 2) (Matzel et al., 2004; Paterson et al., 2004). A nar- display weak solid-state deformation (our mapping), but overlap temporally row belt of Swakane Biotite Gneiss is also exposed southwest of the Entiat fault with late ductile deformation of the complex. Orientations (Figs. 5A, 5B) of (Fig. 2). In the following we briefly summarize the major features of the gneiss magmatic foliation and lineation, defined best by biotite and to a lesser extent that are most relevant to the Eocene evolution of the rocks, using previously plagioclase and hornblende in these plutons, may provide “snapshots” of the published data. regional strain field during emplacement (e.g., Paterson et al., 1998). Foliations The dominantly arkosic Cretaceous clastic protoliths of the Swakane Biotite in the intrusions are in places discordant to pluton contacts and subparallel to Gneiss were buried and metamorphosed at peak conditions of ~640–750 °C host-rock structures and the regional structural grain (Figs. 5A, 5B), and/or in- at 9–12 kbar (Valley et al., 2003; Gatewood and Stowell, 2012). Sm-Nd garnet tensify in strength independent of contacts. Thus, these fabrics are interpreted dates of 73.5 ± 1.2 to 66.8 ± 0.7 Ma from the gneiss (Gatewood and Stowell, to record tectonic strain and compliment results from the Skagit Gneiss Com- 2012) and a U-Pb zircon age of 68.36 ± 0.07 Ma from a syntectonic leucogranite plex and Swakane Biotite Gneiss. sheet (Matzel et al., 2004) are interpreted to date the amphibolite facies meta- On the southwest side of the Skagit Gneiss Complex are the markedly morphism. Hornblende 40Ar/39Ar and K-Ar dates of 57.9 ± 0.5 Ma and 50.8 ± elongate, ca. 46–45 Ma granodioritic Duncan Hill and Railroad Creek plutons 1.4 Ma, respectively, and biotite dates ranging from 49.5 ± 0.3 to 46.2 ± 1.5 Ma (Fig. 3) (Cater, 1982; Dellinger, 1996). The tilted Duncan Hill pluton (~2–13 km record the cooling of the Swakane Biotite Gneiss in the Chelan block (Tabor paleodepth) has a northwest-striking, vertical to steeply northeast-dipping, et al., 1987a: Matzel, 2004; Paterson et al., 2004). Some of the gneiss was at the moderate to strong magmatic to solid-state foliation and northwest-trending surface by ca. 48 Ma, as it is basement to part of the Eocene Chumstick basin lineation in its deeper northwest end (Figs. 5A, 5B) (Cater, 1982; Dellinger, 1996; (e.g., Gresens et al., 1981; Evans, 1994). our data) that has been interpreted to record regional strain by Haugerud et al. Structures associated with burial were probably largely obscured by per- (1991). In comparison, our structural mapping indicates that the Railroad Creek vasive ductile, noncoaxial shear that occurred at decreasing temperatures, pluton also has variable fabric intensity. In the south, the eastern margin of presumably during exhumation (Paterson et al., 2004). Gently dipping folia- the pluton has a very weak magmatic foliation. A moderately strong mag- tions and the Dinkelman décollement are folded into a ≥10 km wavelength, matic fabric that is variably overprinted by solid-state deformation is found gently northwest-plunging antiform. Lineation plunges gently to the north to the west, from the pluton interior to the margin. Foliation strikes north to to north-northeast (Paterson et al., 2004; Miller et al., 2006). Mesoscopic fold northwest and dips steeply, and lineation trends northwest or southeast, and hinge lines are more scattered, ranging from northwest to north-south, to plunges gently (Figs. 5A, 5B). Lineation is stronger than foliation near the west- south-southwest. Abundant kinematic indicators show that the gneiss records ern contact. The strain gradient and solid-state deformation in the interior of dextral shear on the southwest limb and sinistral shear on the northeast limb the pluton are compatible with an origin by regional tectonic strain. of the antiform (Paterson et al., 2004). Restoration of foliation to shallower The western part of the ca. 48 Ma Cooper Mountain batholith (Shea, 2008) dip by unfolding of the antiform results in uniform top-to-the-north to top-to- has a weak northwest-striking foliation where we have mapped it (Fig. 5A). the-north-northeast shear (Paterson et al., 2004). Latest major motion on the Anisotropy of magnetic susceptibility data from the batholith generally show Dinkelman detachment is also top-to-the-north, oblique to the strike of the oro- northwest-striking magnetic foliation and gently to moderately northwest- or gen (~315°–320°). Differences in 40Ar/39Ar and K-Ar cooling ages between the southeast-plunging magnetic lineation, which has been interpreted to record upper and lower plates of ~10–13 and 10–16 m.y. for hornblende and biotite, re- regional tectonic strain (Fawcett et al., 2003). spectively, suggest that late motion was extensional and resulted in excision of In summary, magmatic and mostly weak solid-state foliation in Eocene ≥8 km of crust (Matzel, 2004; Paterson et al., 2004). Undeformed dikes intrude plutons on average strike northwest, are steep, and probably record north- both the hanging wall and footwall of the décollement, although we have not east-southwest shortening. Lineations are dominantly subhorizontal and been able to trace any single dike across the structure. The two dated dikes in northwest or southeast trending, and likely indicate weak northwest-southeast the footwall close to the décollement are 48.4 ± 2.2 Ma (K-Ar hornblende) and tectonic stretch. 47.8 ± 1.9 Ma (K-Ar biotite) (Tabor et al., 1987a). Displacement on the décolle- ment probably occurred until ca. 48–47 Ma, as indicated by 40Ar/39Ar and K-Ar cooling ages. EOCENE FAULTS

Most major map-scale Eocene faults cutting and bounding the Cascades STRAIN PATTERNS FROM EPIZONAL EOCENE PLUTONS core and Eocene basins strike north to northwest and are steep. The most prominent is the right-lateral Straight Creek–Fraser fault, which forms the Shallow to middle crustal Eocene (ca. 48–45 Ma) plutons intrude, or are western boundary of the Cascades core (Figs. 1 and 2) and has displaced core near, the Skagit Gneiss Complex, and include the Cooper Mountain, Duncan rocks ~110–160 km southward from equivalent rocks in southern British Co- Hill, and Railroad Creek plutons (Figs. 2 and 3). These intrusions generally lumbia (e.g., Misch, 1977b; Vance and Miller, 1981; Tabor et al., 1984; Monger,

1986). Fault motion initiated by 48 Ma and ended by 34 Ma (Tabor et al., 1984; EOCENE NONMARINE BASINS AND THEIR DEFORMATION Umhoefer and Miller, 1996). The northwest-striking Entiat fault is the eastern boundary of the Eocene The Eocene nonmarine basins (Figs. 2 and 7) that flank the Cascades core Chumstick basin, separating the basin from the Swakane Biotite Gneiss consist of very thick sequences of sandstone and mudstone, with variable (Fig. 7). To the northwest, the clastic rocks pinch out and the fault separates the amounts of conglomerate and local tuff (e.g., Johnson, 1984). Most important Wenatchee block on the southwest, which contains mid-Cretaceous plutons for comparison to the processes occurring in the deeper crust are the Chucka and has largely Late Cretaceous 40Ar/39Ar and K-Ar biotite cooling ages, from nut, Swauk, and Chumstick sequences (Fig. 2), which overlap in age with the the Chelan block on the northeast, which is intruded by abundant 79–45 Ma Eocene crystalline rocks. plutonic rocks and generally has Eocene biotite cooling ages (Fig. 1) (e.g., The Chuckanut Formation is west of the Skagit Gneiss Complex, but before Tabor et al., 1989). The southeast segment of the Entiat fault has down-to-the- dextral slip on the Straight Creek fault the formation was farther south, west of southwest normal separation and was active during Eocene deposition in the the Swauk Formation (e.g., Vance and Miller, 1981). A tuff deep in the Chucka Chumstick basin (Gresens et al., 1981; Evans, 1994). The fault probably has nut section is 56.835 ± 0.087 Ma, and an intercalated felsic tuff (Silver Pass a major strike-slip component given its straightness in map view, and Tabor Volcanic Member) in the Swauk Formation is 51.364 ± 0.067 Ma (U-Pb zircon, et al. (1987b) suggested 30–40 km of dextral strike slip, using a tenuous pierc- CA-ID-TIMS; Eddy et al., 2016). ing point, the hinge line of the folded contact between the Swakane Biotite The Swauk and Chuckanut strata were openly to tightly folded. Folds in Gneiss and Napeequa unit (Fig. 2). the Swauk Formation mostly have wavelengths of 100 m to 2 km and steep The steep north-northwest–striking Leavenworth fault forms the western axial planes (Fig. 6C) (Tabor et al., 1982, 2000; Doran, 2009). Axial traces of boundary of the Chumstick basin and separates it from the Eocene Swauk For- these folds change from northwest in the west, to mainly east-southeast or mation and the Cascades core (Figs. 2 and 7). The fault consists of numerous west-northwest in the central region, to largely northwest in the easternmost segments that range in strike from ~315° to 000° (Tabor et al., 1982, 1987a). part of the Swauk Formation (Tabor et al., 1982; our mapping; Fig. 7). It has been interpreted as a dextral strike-slip fault largely on the basis of its The Swauk Formation is overlain via an angular unconformity by the dom- length, stratigraphic relations, and rapid depositional rates of the Chumstick inantly basaltic Teanaway Formation and is intruded by the voluminous mafic basin (e.g., Johnson, 1985; Evans, 1994). Cheney and Hayman (2009) sug- Teanaway dikes (Foster, 1958; Tabor et al., 1984), both of which help bracket gested that steep reverse faults are localized in and near the fault zone. the timing of Swauk folding. An intercalated rhyolite flow near the base of

121° 120° 30′

LeavenworthLe EntiatEEn Fault Cenozoic (Undifferentiated) ntn ava tia ve iaata en t Roslyn Fm. (Eocene) FFa w EagleEaE Creekaua ag Chumstick Fm. (Eocene) oro ulu t gle ltlt Straight le tht e Teanaway Fm. (Eocene) C Figure 7. Simplified map emphasizing re 47° 30′ eek Swauk Fm. (Eocene) Eocene sedimentary and volcanic rocks, ChumsticCChhuummsstick k adjacent units, and Eocene faults, dikes, F Swakane Gneiss (Cretaceous) Creek (1) aault and folds in the Swauk basin of the cen- ult C FaulF t Mesozoic (Undifferentiated) (3) au t tral Washington Cascades. Fm.—forma- ltlt tion. C-C′ is the line of the cross section in ZoneZoZ Mean Dike Trends Fault o Figure 6C. Lines 1–4 are transects where (2) ne Fold Axes numerous dike orientations were mea- (4) C′ sured and extensions were calculated (see

Zone Swauk Table 3). Dikes and some folds are from our Teanaway mapping, and other folds and contacts are 47° 15′ modified mainly from Tabor et al. (1982, Roslyn 2000). N

Columbia River 0 10 Basalt km

the Teanaway Formation gives a U-Pb zircon age of 49.341 ± 0.070 Ma (Eddy et al., 2016). Conformably overlying the Teanaway lavas are clastic rocks of the A mid(?) to late Eocene Roslyn Formation. The Swauk basin is juxtaposed on the east against the Chumstick basin by the Leavenworth fault, and the Entiat fault forms the northeast boundary of the Chumstick basin (Fig. 7). These two faults have components of dextral slip (Tabor et al., 1984). The Chumstick Formation contains numerous tuffs (McClincy, 1986); one from near the base of the section is 49.147 ± 0.073 Ma and another ~6000 m higher in the lower-middle part of the section is 47.981 ± 0.059 Ma (U-Pb zircon, CA-ID-TIMS; Eddy et al., 2016), indicating very high sedimenta tion rates of ~6–7 m/k.y. during the interval between eruption of these tuffs. This depositional rate, the large thickness of the lower half of the formation, migrating depocenters, and monolithologic breccias containing large (averaging >1 m) clasts, some of which are >25 km from their source, provide strong evidence that the Chumstick Formation accumulated in a transtensional basin (Evans, 1994). Eddy et al. (2016) showed that the onset of rapid sedimentation in the basin corresponds to a region-wide reorganization of sedimentation patterns and the onset or acceleration of movement on the regional strike-slip faults. B EOCENE DIKES

Dike swarms give insights into the regional strain field in the shallow crust. Felsic to mafic Eocene dikes widely intrude Cascades core rocks and the Eo- cene Swauk Formation. The best studied, most abundant, and southernmost dikes are in the ca. 49.3 Ma (U-Pb date for Teanaway lavas) mafic Teanaway dike swarm, which primarily intrudes the Swauk Formation (Fig. 8; Foster, 1958; Tabor et al., 1984; Doran, 2009). This swarm extends ~75 km from east to west and a maximum of 18 km from north to south (Fig. 7). We measured orientations and thicknesses of 392 Teanaway dikes throughout the basin. In 4 detailed transects in the western and central parts of the swarm (Fig. 7), average strikes range from 036° to 017° (Table 3), dips range from 72° to 62°, and thicknesses are from 20 to 12 m. By comparison, 145 dikes intruding the eastern part of the basin have a mean strike of 040°, dip of 78°, and thickness of 14 m (Mendoza, 2008). Average subhorizontal extension directions inferred for the dikes range from 306°–126° to 287°–107° in the western and central regions, to 310°–130° in the east. Minimum extensions of ~16% and 43% were determined from 2 of the Figure 8. Photographs of Eocene Teanaway dikes. (A) Example of a very thick Teanaway dike strike-normal transects across the western and central Swauk basin (Table 3). (note person for scale). (B) Two Teanaway dikes cutting bedded Swauk Formation. These extension values were calculated from the number and thickness of dikes over the length of a transect. have also been reported from southeast of the pluton (Waters, 1926; Tabor Orientations of Eocene dikes (Fig. 9) that intrude the Cascades core are less et al., 1987a; Sylva and Miller, 2014). Granitic dikes next to the southern margin well documented. The few dated dikes that intrude the Chelan block are ca. of the ca. 48 Ma Cooper Mountain batholith on average strike 005° (Raviola, 48–46 Ma (K-Ar; Tabor et al., 1987a). Granite and rhyolite dikes next to the shal- 1988). Our study of dikes scattered throughout the Skagit Gneiss Complex in- low part of the ca. 46 Ma Duncan Hill pluton commonly strike northeast (Tabor dicates that they have a broad range of modal compositions and textures, are et al., 1987a; Dellinger, 1996; Bryant and Miller, 2014). Northwest-striking dikes typically 10 cm to 2 m thick, and some are younger than ca. 48.2 Ma ortho

TABLE 3. TEANAWAY DIKE ORIENTATIONS AND EXTENSION DIRECTIONS shallow crust (Fig. 10; Table 2). In contrast to some orogens (e.g., Tauern win- Transect Total dike dow; Selverstone, 1988; Axen et al., 1998), where the upper rigid crust was Number Mean trend length thickness Extension not strained while the middle to lower crust underwent ductile flow, different Transect of dikes (°) (km) (km) (%) crustal levels of the North Cascades underwent coeval deformation, suggest- 133 036 ing a degree of coupling between the crustal layers. 236 028 5.7 0.77 15.5 386 021 7.8 2.08 36.3 438 017 Compatibility of Strain in the Ductile and Brittle Crust

Combined 193 024 The presence of both Eocene brittle and ductile structures allows further Note: Excludes dikes dipping <50°; does not include data in eastern part of the dike evaluation of structural patterns and the degree of strain coupling between dif- swarm. Transect locations shown in text Figure 7. ferent crustal levels during regional transtension (Fig. 10). In particular, our discussion focuses on deformation styles, extension directions, and noncoaxial shear at different depths, and the Eocene folding that involved the Swauk basin gneisses (RF, Fig. 3; Table 1) that they cut, but ages are poorly known. Those and the Cascades crystalline core. dikes that intruded the southern part of the gneiss complex have an average Mesoscopic structures in some of the Skagit rocks are likely composite, as strike of 032° (Shea, 2008), whereas dikes in the central part of the complex on foliation in some places predates 65 Ma leucosomes and elsewhere deforms average strike 092° (Michels, 2008). Ductile stretching recorded by lineation widespread 49–45 Ma orthogneisses (see preceding discussion). The concor- trends in the latter area are also anomalous, averaging 010°, and are compati- dance of fabrics in Eocene orthogneisses with those in older orthogneisses ble with the extension direction from the dikes there (Fig. 9). implies that Eocene fabrics are widespread. Fabrics reflecting ductile transten- On a regional scale, dike extension directions, from those intruding the sion include the combination of gently to moderately dipping foliations, gently Swauk Formation in the south to the Skagit Gneiss Complex in the north, range plunging lineations, L > S fabrics in sizable (>10 km2) domains, and strong from 275°–095° to 002°–182°, to northeast-southwest south of the Duncan Hill stretching of fold hinges parallel to lineation (Fig. 4B) (e.g., Robin and Cruden, pluton (Fig. 9). Extension was thus highly variable, but a significant majority 1994; Tikoff and Teyssier, 1994; Dewey, 2002; Fossen et al., 2013), most promi of the dikes, including the voluminous Teanaway swarm, indicate an extension nently in the ca. 45 Ma Diablo Lake orthogneiss bodies. The extension direc- direction that is considerably more consistent (275°–095° to 310°–130°) (Fig. 9). tion in the Skagit Gneiss Complex is largely northwest-southeast (~330°–150°), On average, extension was broadly arc parallel to arc oblique, rather than arc except near segments of the Ross Lake fault zone. normal (Figs. 9 and 10). The wide range in extension directions may be ex- Noncoaxial ductile shear in the Skagit Gneiss Complex is localized in weaker plained by complex spatial variations of the strain field, a rapidly changing metasedimentary rocks, as described above. Both top-to-the-northwest and strain field during emplacement of dikes of different ages, and/or later vertical top-to-the-southeast shear occur in the northern domain with top-to-the-north- axis rotations. There are insufficient age data for the dikes with different orien- west shear more common, particularly in dated Eocene orthogneisses. This tations to choose among these three interpretations. shear likely predated upright folding, and thus originally probably occurred on gently dipping surfaces. Overall, we infer that on the map scale, deforma tion was general shear marked by a component of top-to-the-northwest non DISCUSSION coaxial shear. The ductile ca. 50–48 Ma strain of the Skagit Gneiss Complex was broadly The North Cascades expose a >40 km crustal section, allowing evaluation coeval with ductile dextral-normal shear in the Ross Lake fault zone, which was of processes occurring in the deep to shallow crust. We use our new results top-to-the-northeast in the north and top-to-the-east-southeast in the south and previously published data to summarize these processes, and their tem- (Table 2) (Miller and Bowring, 1990; Haugerud et al., 1991), and the approxi- poral, kinematic, and other links of tectonic processes at different crustal levels mately north-south stretching in the dilational stepover of the Ross Lake fault. in the Eocene. Eocene deformation may have overlapped earliest movement on the Straight Creek fault (Table 2). Interaction of Shallow and Deeper Crustal Levels The Swakane Biotite Gneiss is characterized by pervasive, top-to-the-north to north-northeast noncoaxial shear and north-south to north-northeast– In the North Cascades, a relatively short time interval (ca. 53–45 Ma) was south-southwest subhorizontal to moderately plunging stretching (Figs. 9 and marked by (1) plutonism, metamorphism, partial melting, ductile flow, and 10). The fabrics thus indicate largely subvertical shortening and top-to-the- rapid cooling of metamorphic rocks; (2) rapid subsidence and deposition in north subhorizontal flow prior to the late upright folding; some of the flow nonmarine basins; (3) brittle faulting; and (4) extensive dike intrusion in the overlapped with the Eocene deformation in the Skagit rocks (Table 2).

121° 00′

Displacement direction of HW in shear zone Stretching direction- lineations Dike trend

RLF

48 48° 30′

Skagit Gneiss Complex

Figure 9. Summary map of Eocene structures and extension directions in the North and Central Cascades, including dikes, stretching lineations, magmatic 45 lineations in plutons, and faults. Eocene plutons (orange with names), dikes, and basins (yellow) are highlighted, as are the Skagit Gneiss Complex and Swakane Biotite Gneiss (purple). NQ—Napeequa unit; RLF—Ross Lake fault; RRC—Railroad 48° Creek pluton; HW—hanging wall; Mtn— mountain.

47° 30′

9 120° 30’

Ross La En ke tia t F au lt

Upper Crust x St . (Brittle) r N aight Cr Figure 10. Illustration of features at different structural levels in the Eocene crust of the North Cascades. Red lines are dikes ee x . and arrows show associated extension di- k Fault rections. Bold converging black arrows are Zo shortening directions during transtension ne Mid-Crust and double-headed arrows show orienta- GIT . SKA S (Ductile) tions of stretching lineations (extension NEIS . x G directions). Basin fill is indicated by dots (sandstone and mudstone) and circles (conglomerates). Note the discordance between the extension direction in the upper crust and mid crust. ANE WAK F S EISS ault GN Migmatite

Zo n atio ne Foli

The two deep-crustal domains in the Chelan block were exhumed in similar ented ~45° from the strike of the fault zone, at ~095°–275°, whereas in parti- manners. The rapid cooling and, presumably, exhumation of the Skagit Gneiss tioned transtension it should be broadly perpendicular, ~050°–230° (e.g., Tikoff Complex are compatible with vertical thinning associated with subhorizontal and Teyssier, 1994). Near a major strike-slip zone, high strains may rotate fab- stretching and constrictional strain, and a component of normal slip on the rics into subparallelism with the zone, but this seems unlikely to affect patterns dextral Ross Lake fault system. Similarly, exhumation of the Swakane Biotite at distances of kilometers from the zone. The stretching direction must be in- Gneiss was broadly synchronous with vertical ductile shortening, and proba- fluenced by other poorly understood factors, such as along-strike changes in bly with dip slip on the Entiat fault. crustal thickness (see following). The overall pattern of ductile Eocene deformation in the Cascades core Transtension in the brittle crust was manifested by dextral strike-slip thus includes subvertical shortening and subhorizontal northwest-southeast faults, widespread dike swarms, and subsidence and thick accumulation of to north-south stretching in the Skagit and Swakane rocks; top-to-the-the- clastic sediments in nonmarine basins (Figs. 9 and 10). Orientations of struc- north to north-northeast noncoaxial shear in the Swakane Biotite Gneiss, and tures indicate complex partitioning of deformation. Dextral strike slip on the less prominent top-to-the-northwest shear in the Skagit Gneiss Complex; and northwest-striking Ross Lake fault zone (Foggy Dew, Ross Lake faults) prob- dextral-normal shear in the Ross Lake fault zone (Figs. 9 and 10). The dis- ably overlapped with motion of the similarly oriented Entiat fault and the placement direction in the fault zone is generally oblique to stretching in the north-south–striking Straight Creek–Fraser fault (Fig. 1; Table 1). The north- gneisses. west-striking faults have components of down-to-the-southwest (Entiat fault) The orientation of the stretching lineation relative to the Ross Lake fault and down-to-the-northeast to east (Ross Lake and Foggy Dew faults) displace- zone and the overall trend of the orogen suggest that a simple transtensional ment. Orientations of the Eocene dikes indicate that extension was dominantly model cannot be applied. In a dextral wrench mode, stretching should be ori- west-northwest–east-southeast to northwest-southeast, but in detail markedly

variable (Fig. 9). The average strike of the Teanaway dike swarm is ~30° to that Oregon and Vancouver Island (Fig. 2), where the terrane boundary is marked of the Straight Creek fault, which is an appropriate orientation for the strain by thrusting and folding (Johnston and Acton, 2003; Wells et al., 2014). Colli- field associated with a transtensional dextral wrench fault (e.g., Sanderson and sion in southwest Oregon occurred between ca. 51 and 47 Ma. Marchini, 1984) (Fig. 7). The magmatic fabrics in Eocene plutons that are interpreted to record re- What Drove Orogen-Parallel Extension? gional strain formed at depths intermediate between those of the metamorphic rocks and the dikes. Magmatic lineations trend northwest or southeast In the Paleogene, ca. 55 Ma, there was a fundamental change from regional and are gently to moderately plunging. These orientations are broadly similar transpression to transtension in the western Cordillera at the latitude of the to those in the ductile crust. Coast Mountains and North Cascades (e.g., Parrish et al., 1988; Miller and Bow- Extension directions from ductile and brittle structures and the sense of ring, 1990). This swing in deformation regimes is interpreted as a response to noncoaxial shear in different areas define a complex Eocene strain field (Figs. a change in the motion between the North American and Farallon/Kula plates 9 and 10). Overall, the ductilely deformed rocks underwent largely orogen- (e.g., Engebretson et al., 1985). In addition, a slab window may have formed parallel to orogen-oblique, subhorizontal flow. The shallow crustal extension during postulated subduction of the Farallon-Kula ridge or Farallon-Resurrec- direction inferred from dikes is also oblique to the strike of the orogen and to tion ridge beneath the North American plate ca. 50 Ma (e.g., Breitsprecher the flow direction in deeper crustal rocks. Differences in extension between et al., 2003; Madsen et al., 2006). the dikes and Skagit and Swakane rocks average ~20°–25°and 65°–70°, respec- The orogen-parallel to orogen-oblique extension in the brittle and ductile tively (Fig. 9). levels of the Washington Cascades indicates that plate motion was not sim- ply partitioned into dextral strike slip parallel to the northwest-trending plate Compatibility of Folding of the Swauk Formation boundary and extension normal to the boundary. Furthermore, orogenic col- and Skagit Gneiss Complex lapse did not result in extension normal (northeast-southwest) to the strike of the arc, as in the metamorphic core complexes to the east. A distinctive component of the deformation history is the analogous ge- We postulate that orogen-parallel to orogen-oblique ductile stretching was ometry and timing of Eocene folds of the Swauk Formation and Skagit Gneiss driven by a combination of dextral shear along the plate margin and along- Complex that may also be in part synchronous with upright folding of folia- strike differences in crustal thickness and temperature (cf. Paterson et al., tion in the Swakane Biotite Gneiss and the Dinkelman décollement. Folding is 2004). Crustal thickness of the Cascades core may have decreased from north bracketed between ca. 51 and 49 Ma in the Swauk Formation, and the latest to south as a result of differences in the magnitudes of mid-Cretaceous short- folding occurred ca. 49–48 Ma in the southern part of the gneiss complex and ening. Shortening was driven by eastward underthrusting of the rigid Insular continued until at least 48 Ma (ending by 45 Ma) in the north. Fold geometries superterrane (McGroder, 1991). In addition, the contractional belt probably ter- are broadly analogous in the Swauk and Skagit rocks (Fig. 6). In both settings, minates at the southern end of the Coast Mountains–North Cascades arc (cf. axial traces of the larger folds curve considerably (Figs. 5C and 7). In the Swauk Monger et al., 1982, 1994), leading to less thickening at this end of the orogen. rocks, the trend may result in part from the orientation of boundaries of the Much of the southern part of the Cascades core, represented by the Nason basin, and in the west, from rotation due to strike slip on the Straight Creek terrane, had cooled below biotite K-Ar and 40Ar/39Ar closure temperatures by fault. In the Skagit rocks, the late moderately to steeply plunging folds of folia- 80 Ma (e.g., Tabor et al., 1982, 1987a; Matzel, 2004), in marked contrast to the tion described here are difficult to explain. Such large folds with steep axes are Skagit Gneiss Complex, where upwelling of hot asthenosphere in the inferred commonly thought to not extend into the deep crust (cf. Johnston and Acton, slab window may have helped localize late metamorphism, magmatism, and 2003) and may be truncated by a detachment. partial melting. The top-to-the-north shear in the Swakane Biotite Gneiss and The origin of this short-lived folding is uncertain. Folding may occur during top-to-the-northwest flow in some of the Skagit Gneiss Complex suggests that transtension, but the associated shortening is limited (Fletcher and Bartley, during at least part of this deformation, the hot and weak middle to deep crust 1994; Fossen et al., 2013). If the folds are related to transtension, the low angle flowed southward from the region of 79–45 Ma magmatism, metamorphism, (<20°) of many of the hinge lines to the northwest-trending, dextral strike-slip and partial melting in the Skagit Gneiss Complex toward the region of thinner faults (Fig. 5C) that involve the Cascades core suggests that the divergence and cooler crust in the south. We speculate that this crustal flow is broadly angle is low (cf. Fossen et al., 2013). The folding more likely reflects a short- similar to the central Andes where orogen-parallel lower crustal flow is inter- lived episode of regional shortening that interrupted the overall transtensional preted to have occurred in response to differences in upper crustal shortening regime. In one interpretation, folding results from collision and accretion of the in the Altiplano and Puna Plateau (e.g., Hindle et al., 2005; Ouimet and Cook, Siletz-Crescent terrane (Siletzia) (Miller and Umhoefer, 2013; Eddy et al., 2016), 2010). Similarly, deep crustal flow discordant to shallower level extension is which is a large igneous province west of the North Cascades (Fig. 2). This also thought to have been driven by differences in crustal thickness in the Ruby collision has been inferred to account for Eocene deformation in southwest Mountains metamorphic core complex of Nevada (MacCready et al., 1997).

Relationships to Metamorphic Core Complexes to the East 120°

a bi

The North Cascades and Omineca metamorphic core complexes to the rt C east were sites of thick (>55 km) crust in the Late Cretaceous–Paleogene, and O olum A underwent extension, cooling, and exhumation in the Eocene (e.g., Parrish S RO T I C et al., 1988; Miller and Paterson, 2001; Paterson et al., 2004). Both regions also N KY P T Albe M 54° British C O L E U contain migmatites with similar Eocene melt-crystallization ages (e.g., Vander U R N M O TA T IN haeghe et al., 1999; Crowley et al., 2001; Hinchey et al., 2006; Kruckenberg O M O FO N I L et al., 2008; Gordon et al., 2008, 2010a). The two areas are currently connected N N D I T E A C A C N by a regionally flat Moho, with crustal thicknesses of ~30 km (Cook et al., N A D - T E H 1992). Based on these characteristics, Whitney et al. (2004) postulated that the R U B S Omineca and North Cascades metamorphic and plutonic belts were dynami E T N L T B cally coupled and represent the exhumed eastern and western margins, re- O E L R T spectively, of an orogenic plateau. TH C CA R Pervasive Eocene flow and kinematics recorded in gneisses and mylonites S Y C S associated with the major detachment of the Shuswap metamorphic core A T D A complex of the Omineca belt (Fig. 11) indicate largely east-west extension L E L S mplex I (e.g., Read and Brown, 1981; Brown and Journeay, 1987; Carr et al., 1987; Par- N CANADA Co E rish et al., 1988; Teyssier et al., 2005). In the southwestern part of this com- Shuswap

B plex, however, the Okanogan dome records west-northwest–east-southeast E USA B 49° L E T to northwest-southeast extension (Kruckenberg et al., 2008). In comparison, L T ductile flow was more northwest-southeast to north-south in the Cascades core. We infer that these differences in part reflect the position of the thick crust relative to the plate margin. In much of the Shuswap complex, extension was probably driven by orogenic collapse in a largely extensional regime (e.g., Teyssier et al., 2005), whereas in the western part of the Okanogan dome and aho

shington Id

North Cascades, dextral shear associated with the obliquity of the plate bound- W ary modified the strain field and resulted in transtensional flow. Any linked deep crustal flow beneath the hypothesized orogenic plateau must thus be marked by a major swing in the direction of flow. In the Omineca metamorphic Figure 11. Eocene ductile extension directions in metamorphic core complexes in part of the core complexes, a broad zone of ductilely deformed rocks has been kinemati- northwest Pacific Cordillera, and middle to deep arc crust in the Coast Plutonic Complex and North Cascades. Sources include Andronicos et al. (2003), Coleman and Parrish (1991), Foster cally and thermally linked for at least part, if not all, of the deformation history et al. (2007), Friedman and Armstrong (1988), Kruckenberg et al. (2008), and our data. to the structurally higher detachment faults during exhumation of the footwall. This relationship may also apply to the Swakane Biotite Gneiss, but it is not apparent for the Skagit Gneiss Complex. Instead, exhumation and ductile shear dextral-normal ductile and brittle shear in the bounding Ross Lake fault sys- in the gneiss complex were in part coeval with the dextral-normal shear in the tem. The other deep-seated domain, the Swakane Biotite Gneiss, records Ross Lake fault system. pervasive Eocene top-to-the-north to north-northeast noncoaxial shear. Strain patterns in the Swakane and Skagit rocks are compatible with transtension and orogen-parallel to orogen-oblique subhorizontal stretching. CONCLUSIONS The shallow crust in the Eocene was marked by dextral and normal slip on regional northwest- and north-south–striking high-angle faults, basin sub The North Cascades orogen exposes a wide range of crustal levels that sidence, rapid deposition, and widespread intrusion of basaltic to rhyolitic recorded Eocene transtension differently. The largest domain of deep crust dikes. On a regional scale, extension directions from dikes are highly variable, exhumed in the Eocene, the Skagit Gneiss Complex, reveals a long record of but are mostly oblique to the trend of the orogen. magmatism, metamorphism, and partial melting. The complex has gently to The brittle and ductile levels of the North Cascades crust were deformed co- moderately dipping foliation, gently northwest- and southeast-plunging linea- evally, but record different strain geometries and extension directions. These tion, and sizable constrictional domains, all of which formed in part during differences illustrate the problems with extrapolating strain patterns from one

crustal level to another level and using these patterns to interpret ancient plate Bryant, K., and Miller, R.B., 2014, Structural analysis of Eocene dike swarms in and near the motions. These differences may reflect the complex regional and local bound- Duncan Hill pluton, North Cascades, Washington: Geological Society of America Abstracts with Programs, v. 46, no. 5, p. 9. ary conditions. Ductile flow of the hot and weak Skagit Gneiss Complex and Carr, S.D., Parrish, R.R., and Brown, R.L., 1987, Eocene structural development of the Val- Swakane Biotite Gneiss was probably in response to regional strike slip and halla complex, southeastern British Columbia: Tectonics, v. 6, p. 175–196, doi:10 .1029 extension, vertical shortening and collapse, and along-strike differences in /TC006i002p00175. Cater, F.W., 1982, The intrusive rocks of the Holden and Lucerne quadrangles, Washington; the crustal thicknesses in the Cascades core. In particular, the variation in crustal relations of depth zones, composition, textures, and emplacement of plutons: U.S. Geologi- thickness and the presence of thick, hot and weak lower crust in the northern cal Survey Professional Paper 1220, 108 p. part of the core may have led to the differences in extension directions. Cater, F.W., and Wright, T.L., 1967, Geologic map of the Lucerne quadrangle, Chelan County, Washington: U.S. Geological Survey Geological Quadrangle Map GQ-647, scale 1:62,500. Cheney, E.S., and Hayman, N.W., 2009, The Chiwaukum structural low: Cenozoic shortening of the central Cascade Range, Washington State, USA: Geological Society of America Bulletin, ACKNOWLEDGMENTS v. 121, p. 1135–1153, doi:10.1130/B26446.1. We thank Scott Paterson, Christian Teyssier, Mike Eddy, Niki Wintzer, Paul Umhoefer, Jamie Clay Coleman, M.E., and Parrish, R., 1991, Eocene dextral strike-slip and extensional faulting in the Johnson, and Jahan Ramezani for thoughtful discussions. Miller acknowledges Art Snoke for his Bridge River terrane, southwest British Columbia: Tectonics, v. 10, p. 1222–1238, doi:10.1029 major contributions to our understanding of crustal sections and Cordilleran tectonics, and his ex- /91TC01078. emplary service to the geologic community. Reviews by Margi Rusmore, an anonymous reviewer, Cook, F.A., Varsek, J.L., Clowes, R.M., Kanasewich, E.R., Spencer, C.S., Parrish, R.R., Brown, R.L., and editors Shan de Silva and Josh Schwartz were very helpful. The U.S. National Park Service Carr, S.D., Johnson, B.J., and Price, R.A., 1992, Lithoprobe crustal reflection cross section generously helped with logistics. This research was supported by National Science Foundation of the southern Canadian Cordillera, 1, Foreland thrust and fold belt to Fraser River fault: grants EAR-0511062, EAR-1119358, and EAR-1419787 to Miller, EAR-0510591 to Bowring, and EAR- Tectonics, v. 11, p. 12–35, doi:10 .1029 /91TC02332 . 0510326 to Whitney, and U.S. Geological Survey EDMAP grants 07HQAG00125 and 08HQAG0044 Cowan, D.S., 2003, Revisiting the Baranof–Leech River hypothesis for early Tertiary coastwise to Miller. transport of the Chugach-Prince William terrane: Earth and Planetary Science Letters, v. 213, p. 463–475, doi:10.1016/S0012-821X(03)00300-5. Crowley, J.L., Brown, R.L., and Parrish, R.R., 2001, Diachronous deformation and a strain gradient beneath the Selkirk allochthon, northern Monashee complex, southeastern Canadian Cordi REFERENCES CITED llera: Journal of Structural Geology, v. 23, p. 1103–1121, doi:10.1016 /S0191 -8141(00)00179 -6. Adams, J.B., 1961, Petrology and structure of the Stehekin-Twisp Pass area, North Cascades, Dellinger, D.A., 1996, The geology, petrology, geochemistry, mineralogy, and diapiric emplace- Washington [Ph.D. thesis]: Seattle, University of Washington, 171 p. ment of the Duncan Hill pluton, North Cascades, Washington [Ph.D. thesis]: Santa Barbara, Andronicos, C.L., Chardon, D.H., and Hollister, L.S., 2003, Strain partitioning in an obliquely con- University of California, 539 p. vergent orogen, plutonism, and syn-orogenic collapse: Coast Mountains batholith, British Dewey, J.F., 2002, Transtension in arcs and orogens: International Geology Review, v. 44, p. 402– Columbia: Tectonics, v. 22, 1012, doi:10 .1029 /2001TC001312 . 439, doi:10.2747/0020-6814.44.5.402. Armstrong, R.L., 1988, Mesozoic and early Cenozoic magmatic evolution of the Canadian Cordi Doran, B.A., 2009, Structure of the Swauk Formation and Teanaway dike swarm, Washington llera, in Clark, S.P., Jr., et al., eds., Processes in continental lithospheric deformation: Geo Cascades [M.S. thesis]: San Jose, California, San Jose State University, 97 p. logical Society of America Special Paper 218, p. 55–92, doi:10 .1130 /SPE218 -p55 . du Bray, E.A., and John, D.A., 2011, Petrologic, tectonic, and metallogenic evolution of the Ances- Axen, G.J., Selverstone, J., Byrne, T., and Fletcher, J.M., 1998, If the strong crust leads, will the tral Cascades magmatic arc, Washington, Oregon, and northern California: Geosphere, v. 7, weak crust flow?: GSA Today, v. 8, no. 12, p. 1–8. p. 1102–1133; doi:10.1130/GES00669.1. Babcock, R.S., and Misch, P., 1989, Origin of the Skagit migmatites, North Cascades Range, Eddy, M.P., Bowring, S.A., Umhoefer, P.J., Miller, R.B., McLean, N.M., and Donaghy, E.E., 2016, Washington State: Contributions to Mineralogy and Petrology, v. 101, p. 485–495, doi:10 High-resolution temporal and stratigraphic record of Siletzia’s accretion and triple junction .1007/BF00372221. migration from nonmarine sedimentary basins in central and western Washington: Geologi Beck, S.L., and Zandt, G., 2002, The nature of orogenic crust in the central Andes: Journal of cal Society of America Bulletin, v. 128, p. 425–441, doi:10 .1130 /B31335 .1 . Geophysical Research, v. 107, 2230, doi:10.1029 /2000JB000124 . Engebretson, D.C., Cox, A., and Gordon, R.G., 1985, Relative motions between oceanic and con- Betka, P.M., and Klepeis, K., 2013, Three-stage evolution of lower crustal gneiss domes at Break- tinental plates in the Pacific Basin: Geological Society of America Special Paper 206, 60 p., sea Entrance, Fiordland, New Zealand: Tectonics, v. 32, p. 1084–1106, doi:10.1002 /tect.20068. doi:10.1130/SPE206-p1. Breitsprecher, K., Thorkelson, D.J., Groome, W.G., and Dostal, J., 2003, Geochemical confirma- Engels, J.C., Tabor, R.W., Miller, F.K., and Obradovich, J.D., 1976, Summary of K-Ar, Rb-Sr, U-Pb, tion of the Kula-Farallon slab window beneath the Pacific Northwest in Eocene time: Geol- and fission track ages of rocks from Washington State prior to 1975 (exclusive of Columbia ogy, v. 31, p. 351–354, doi:10.1130/0091-7613(2003)031<0351:GCOTKF>2.0.CO;2. Plateau Basalts): U.S. Geological Survey Miscellaneous Field Studies Map MF-710. Brown, E.H., 1987, Structural geology and accretionary history of the of the Northwest Cascades Evans, J.E., 1994, Depositional history of the Eocene Chumstick Formation: Implications of tec- system, Washington and British Columbia: Geological Society of America Bulletin, v. 99, tonic partitioning for the history of the Leavenworth and Entiat–Eagle Creek fault systems: p. 201–214, doi:10.1130/0016-7606(1987)99<201:SGAAHO>2.0.CO;2. Tectonics, v. 13, p. 1425–1444, doi:10 .1029 /94TC01321 . Brown, E.H., and Gehrels, G.E., 2007, Detrital zircon constraints on terrane ages and affinities Fawcett, T.C., Burmester, R.F., Housen, B.A., and Alexander, I., 2003, Tectonic implications of and timing of orogenic events in the San Juan Islands and North Cascades, Washington: magnetic fabrics and remanence in the Cooper Mountain pluton, North Cascade Mountains, Canadian Journal of Earth Sciences, v. 44, p. 1375–1396, doi:10.1139 /E07 -040. Washington: Canadian Journal of Earth Sciences, v. 40, p. 1335–1356, doi:10.1139 /e03 -055. Brown, E.H., and Talbot, J.L., 1989, Orogen-parallel extension in the North Cascades crystalline Fletcher, J.M., and Bartley, J.M., 1994, Constrictional strain in a non-coaxial shear zone: Implica- core, Washington: Tectonics, v. 8, p. 1105–1114, doi:10.1029 /TC008i006p01105 . tions for fold and rock fabric development, central Mojave metamorphic core complex, Cali Brown, E.H., Carey, J.A., Dougan, B.E., Dragovich, J.D., Fluke, S.M., and McShane, D.P., 1994, fornia: Journal of Structural Geology, v. 16, p. 555–570, doi:10 .1016 /0191 -8141(94)90097 -3. Tectonic evolution of the Cascades crystalline core in the Cascade River area, Washington: Fossen, H., Teyssier, C., and Whitney, D.L., 2013, Transtensional folding: Journal of Structural Washington Division of Geology and Earth Resources Bulletin, v. 80, p. 93–113. Geology, v. 56, p. 89–102, doi:10.1016/j.jsg.2013.09.004. Brown, R.L., and Journeay, J.M., 1987, Tectonic denudation of the Shuswap metamorphic terrane Foster, D.A., Doughty, P.T., Kalakay, T.J., Fanning, C.M., Coyner, S., Grice, W.C., and Vogl, J., 2007, of southeastern British Columbia (Canada): Geology, v. 15, p. 142–146, doi:10 .1130 /0091 -7613 Kinematics and timing of exhumation of metamorphic core complexes along the Lewis and (1987)15<142:TDOTSM>2.0.CO;2. Clark fault zone, northern Rocky Mountains, USA, in Roeske, S.M., et al., eds., Exhumation

associated with continental strike-slip fault systems: Geological Society of America Special Johnston, S.T., and Acton, S., 2003, The Eocene Vancouver Island orocline—A response to sea- Paper 434, p. 207–232, doi:10.1130/2007.2434(10). mount accretion and the cause of fold-and-thrust belt and extensional basin formation: Tec- Foster, R.J., 1958, The Teanaway dike swarm of central Washington: American Journal of Sci- tonophysics, v. 365, p. 165–183, doi:10.1016/S0040-1951(03)00021-0. ence, v. 256, p. 644–653, doi:10.2475/ajs.256.9.644. Journeay, J.M., and Friedman, R.M., 1993, The Coast Belt thrust system: Evidence of Late Cre- Friedman, R.M., and Armstrong, R.L., 1988, Tatla Lake Metamorphic Complex: An Eocene meta- taceous shortening in southwest British Columbia: Tectonics, v. 12, p. 756–775, doi:10 .1029 morphic core complex on the southwestern edge of the Intermontane Belt of British Colum- /92TC02773. bia: Tectonics, v. 7, p. 1141–1166, doi:10.1029 /TC007i006p01141 . Klepeis, K.A., and King, D.S., 2009, Evolution of the middle and lower continental crust during Gatewood, M.P., and Stowell, H.H., 2012, Linking zircon U-Pb and garnet Sm-Nd ages to date the transition from contraction to extension in Fiordland, New Zealand, in Miller, R.B., and loading and metamorphism in the lower crust of a Cretaceous magmatic arc, Swakane Snoke, A.W., eds., Crustal cross-sections from the western North American Cordillera and Gneiss, WA, USA: Lithos, v. 146, p. 128–142, doi:10.1016/j.lithos.2012.04.030. elsewhere: Implications for tectonic and petrologic processes: Geological Society of Amer- Gerbault, M., Martinod, J., and Herail, G., 2005, Possible orogen-parallel lower crustal flow and ica Special Paper 456, p. 243–265, doi:10.1130/2009.2456(09). thickening in the Central Andes: Tectonophysics, v. 399, p. 59–72, doi:10 .1016 /j .tecto.2004 Klepeis, K.A., Clarke, G.L., Gehrels, G., and Vervoort, J., 2004, Processes controlling vertical cou- .12.015. pling and decoupling between the upper and lower crust of orogens: Results from Fiordland, Gervais, F., and Brown, R.L., 2011, Testing models of exhumation in collisional orogens: Syn New Zealand: Journal of Structural Geology, v. 26, p. 765–791, doi:10.1016 /j .jsg.2003.08.012. convergent channel flow in the southeastern Canadian Cordillera: Lithosphere, v. 3, p. 55– Kruckenberg, S.C., Whitney, D.L., Teyssier, C., Fanning, C.M., and Dunlap, W.J., 2008, Paleocene– 75, doi:10.1130/L98.1. Eocene migmatite crystallization, extension, and exhumation in the hinterland of the north- Gordon, S.M., Whitney, D.L., Teyssier, C., Grove, M., and Dunlap, W.J., 2008, Timescales of ern Cordillera: Okanogan dome, Washington, USA: Geological Society of America Bulletin, migmatization, melt crystallization, and cooling in a Cordilleran gneiss dome, the Valhalla v. 120, p. 912–929, doi:10.1130/B26153.1. complex, southeastern British Columbia: Tectonics, v. 27, TC4010, doi:1029 /2007TC002103 . Law, R.D., Searle, M.P., and Godin, L., eds., 2006, Channel flow, ductile extrusion and exhuma- Gordon, S.M., Bowring, S.A., Whitney, D.L., Miller, R.B., and McLean, N., 2010a, Time scales of tion in continental collision zones: Geological Society of London Special Publication 268, metamorphism, deformation, and crustal melting in a continental arc, North Cascades USA: 620 p., doi:10.1144/GSL.SP.2006.268.01.28. Geological Society of America Bulletin, v. 122, p. 1308–1330, doi:10.1130 /B30060 .1. MacCready, T., Snoke, A.W., Wright, J.E., and Howard, K.A., 1997, Mid-crustal flow during Tertiary Gordon, S.M., Whitney, D.L., Miller, R.B., McLean, N., and Seaton, N.C.A., 2010b, Metamorphism extension in the Ruby Mountains core complex, Nevada: Geological Society of America Bul- and deformation at different structural levels in a strike-slip fault zone, Ross Lake fault, North letin, v. 109, p. 1576–1594, doi:10.1130/0016-7606(1997)109<1576:MCFDTE>2.3.CO;2. Cascades, USA: Journal of Metamorphic Geology, v. 28, p. 117–136, doi:10.1111 /j.1525-1314 Madsen, J.K., Thorkelson, D.J., Friedman, R.M., and Marshall, D.D., 2006, Cenozoic to recent .2009.00860.x. plate configurations in the Pacific Basin: Ridge subduction and slab window magmatism in Goscombe, B., and Trouw, R., 1999, The geometry of folded tectonic shear sense indicators: western North America: Geosphere, v. 2, p. 11–34, doi:10 .1130 /GES00020 .1. Journal of Structural Geology, v. 21, p. 123–127, doi:10.1016 /S0191 -8141(98)00092 -3. Mattinson, J.M., 1972, Ages of zircons from the Northern Cascade Mountains, Washington: Geo- Gresens, R.L., Naeser, C.W., and Whetten, J.T., 1981, Stratigraphy and age of the Chumstick and logical Society of America Bulletin, v. 83, p. 3769–3784, doi:10 .1130 /0016 -7606(1972)83 [3769: Wenatchee Formations—Tertiary fluvial and lacustrine rocks, Chiwaukum graben, Washing- AOZFTN]2.0.CO;2. ton: Geological Society of America Bulletin, v. 92, p. 841–876, doi:10 .1130 /GSAB -P2 -92-841. Matzel, J.P., 2004, Rates of tectonic and magmatic processes in the North Cascades continental Haeussler, P.J., Bradley, D.C., Wells, R.E., and Miller, M.L., 2003, Life and death of the Resurrec- magmatic arc [Ph.D. thesis]: Cambridge, Massachusetts Institute of Technology, 249 p. tion plate: Evidence for its existence and subduction in the northeastern Pacific in Paleo- Matzel, J.P., Bowring, S.A., and Miller, R.B., 2004, Protolith age of the Swakane Gneiss, North cene–Eocene time: Geological Society of America Bulletin, v. 115, p. 867–880, doi:10 .1130 Cascades, Washington: Evidence of rapid underthrusting of sediments beneath an arc: Tec- /0016-7606(2003)115<0867:LADOTR>2.0.CO;2. tonics, v. 23, TC6009, doi:10.1029/2003TC001577. Haugerud, R.A., 1985, Geology of the Hozameen Group and the Ross Lake shear zone, Masel- McClincy, M., 1986, Tephrostratigraphy of the Chumstick Formation [M.S. thesis]: Portland, Ore- panik area, North Cascades, southwest British Columbia [Ph.D. thesis]: Seattle, University gon, Portland State University, 127 p. of Washington, 263 p. McGroder, M.F., 1991, Reconciliation of two-sided thrusting, burial metamorphism, and diachro- Haugerud, R.A., van der Heyden, P., Tabor, R.W., Stacey, J.S., and Zartman, R.E., 1991, Late Cre- nous uplift in the Cascades of Washington and British Columbia: Geological Society of Amer- taceous and early Tertiary plutonism and deformation in the Skagit Gneiss Complex, North ica Bulletin, v. 103, p. 189–209, doi:10 .1130 /0016 -7606 (1991)103 <0189: ROTSTB>2 .3 .CO;2 . Cascade Range, Washington and British Columbia: Geological Society of America Bulletin, McLean, N.M., 2012, Statistical considerations in high precision U-Pb geochronology, with an v. 103, p. 1297–1307, doi:10.1130/0016-7606(1991)103<1297:LCAETP>2.3.CO;2. application to the tectonic evolution of the North Cascades, Washington [Ph.D. thesis]: Cam- Hinchey, A.M., Carr, S.D., McNeill, P.D., and Rayner, N., 2006, Paleocene–Eocene high-grade bridge, Massachusetts Institute of Technology, 186 p. metamorphism, anatexis, and deformation in the Thor-Odin dome, Monashee complex, McTaggart, K.C., and Thompson, R.M., 1967, Geology of part of the northern Cascades in southern southeastern British Columbia: Canadian Journal of Earth Sciences, v. 43, p. 1341–1365, doi: British Columbia: Canadian Journal of Earth Sciences, v. 4, p. 1199–1228, doi:10 .1139 /e67 -081. 10.1139/e06-028. Mendoza, M.K., 2008, Tectonic implications of the Eocene Teanaway dike swarm in the eastern Hindle, D., Kley, J., Oncken, O., and Sobolev, S., 2005, Crustal balance and crustal flux from Swauk basin, central Washington: Geological Society of America Abstracts with Programs, shortening estimates in the Central Andes: Earth and Planetary Science Letters, v. 230, v. 40, no. 1, p. 66. p. 113–124, doi:10.1016/j.epsl.2004.11.004. Michels, Z., 2008, Structure of the central Skagit Gneiss Complex, North Cascades, Washington Hopson, C.A., and Mattinson, J.M., 1994, Chelan Migmatite Complex: Field evidence for mafic [M.S. thesis]: San Jose, California, San Jose State University, 95 p. magmatism, crustal anatexis, mixing and protodiapiric emplacement, in Swanson, D.A., and Miller, R.B., 1987, Geology of the Twisp River–Chelan Divide region, North Cascades, Washing- Haugerud, R.A., eds., Geologic field trips in the Pacific Northwest: Seattle, University of ton: Washington Division of Geology and Earth Resources Open-File Report 87-17, 12 p. Washington, p. 2K1–2K21. Miller, R.B., 1994, A mid-crustal contractional stepover zone in a major strike-slip system, North Husson, L., and Semere, T., 2003, Thickening the Altiplano crust by gravity-driven crustal chan- Cascades, Washington: Journal of Structural Geology, v. 16, p. 47–60, doi:10.1016 /0191 -8141 nel flow: Geophysical Research Letters, v. 30, 1243, doi:10.1029 /2002GL016877 . (94)90017-5. Johnson, S.Y., 1984, Stratigraphy, age, and paleogeography of the Eocene Chuckanut Formation, Miller, R.B., and Bowring, S.A., 1990, Structure and chronology of the Oval Peak batholith and northwest Washington: Canadian Journal of Earth Sciences, v. 21, p. 92–106, doi:10.1139 adjacent rocks: Implications for the Ross Lake fault zone, North Cascades, Washington: /e84-010. Geological Society of America Bulletin, v. 102, p. 1361–1377, doi:10.1130 /0016 -7606(1990)102 Johnson, S.Y., 1985, Eocene strike-slip faulting and nonmarine basin formation in Washington, <1361:SACOTO>2.3.CO;2. in Biddle, K.T., and Christie-Blick, N., eds., Strike-slip deformation, basin formation, and sedi Miller, R.B., and Paterson, S.R., 2001, Influence of lithological heterogeneity, mechanical anisot- mentation: Society of Economic Paleontologists and Mineralogists Special Publication 37, ropy, and magmatism on the rheology of an arc, North Cascades, Washington: Tectono p. 283–302, doi:10.2110/pec.85.37.0283. physics, v. 342, p. 351–370, doi:10.1016/S0040-1951(01)00170-6.

Miller, R.B., and Umhoefer, P.J., 2013, Evidence for collision of Siletzia in the central and North Plummer, C.C., 1980, Dynamothermal contact metamorphism superposed on regional meta- Cascades: Geological Society of America Abstracts with Programs, v. 45, no. 7, p. 166. morphism in the pelitic rocks of the Chiwaukum Mountains area, Washington Cascades: Miller, R.B., Bowring, S.A., and Hoppe, W.J., 1989, Paleocene plutonism and its tectonic im- Geological Society of America Bulletin, v. 91, p. 1627–1668, doi:10 .1130 /GSAB -P2 -91-1627. plications, North Cascades, Washington: Geology, v. 17, p. 846–849, doi:10.1130 /0091 -7613 Raviola, F.P., 1988, Metamorphism, plutonism and deformation in the Pateros–Alta Lake region, (1989)017<0846:PPAITI>2.3.CO;2. north-central Washington [M.S. thesis]: San Jose, California, San Jose State University, Miller, R.B., Brown, E.H., McShane, D.P., and Whitney, D.L., 1993a, Intra-arc crustal loading and 181 p. its tectonic implications, North Cascades crystalline core, Washington and British Columbia: Read, P.B., and Brown, R.L., 1981, Columbia River fault zone: Southeastern margin of the Geology, v. 21, p. 255–258, doi:10.1130/0091-7613(1993)021<0255:IACLAI>2.3.CO;2. Shuswap and Monashee complexes, southern British Columbia: Canadian Journal of Earth Miller, R.B., Whitney, D.L., and Geary, E.D., 1993b, Tectono-stratigraphic terranes and the meta- Sciences, v. 18, p. 1127–1145, doi:10.1139/e81-108. morphic history of the northeastern part of the crystalline core of the North Cascades: Evi- Robin, P.-Y.F., and Cruden, A.R., 1994, Strain and vorticity patterns in ideally ductile transpression dence from the Twisp Valley Schist: Canadian Journal of Earth Sciences, v. 30, p. 1306–1323, zones: Journal of Structural Geology, v. 16, p. 447–466, doi:10.1016 /0191 -8141(94)90090 -6. doi:10.1139/e93-112. Royden, L.H., Burchfiel, B.C., King, R.W., Wang, E., Chen, Z., Shen, F., and Lin, Y., 1997, Surface Miller, R.B., Haugerud, R.H., Murphy, F., and Nicholson, L.S., 1994, Tectonostratigraphic frame- deformation and lower crustal flow in eastern Tibet: Science, v. 276, p. 788–790, doi:10 .1126 work of the northeastern Cascades: Washington Division of Geology and Earth Resources /science.276.5313.788. Bulletin, v. 80, p. 73–92. Sanderson, D., and Marchini, W.R.D., 1984, Transpression: Journal of Structural Geology, v. 6, Miller, R.B., Paterson, S.R., Lebit, H., Alsleben, H., and Lüneburg, C., 2006, Significance of p. 449–458, doi:10.1016/0191-8141(84)90058-0. composite lineations in the mid- to deep crust: A case study from the North Cascades, Scheuber, E., and Reutter, K.-J., 1992, Magmatic arc tectonics in the central Andes between 21° Washington: Journal of Structural Geology, v. 28, p. 302–322, doi:10 .1016 /j .jsg.2005.11 and 25°S: Tectonophysics, v. 205, p. 127–140, doi:10.1016/0040-1951(92)90422-3. .003. Schilling, F.R., et al., 2006, Partial melting in the central Andean Crust: A review of geophysical, Miller, R.B., Paterson, S.R., and Matzel, J.P., 2009a, Plutonism at different crustal levels: Insights petrophysical, and petrologic evidence, in Oncken, O., et al., eds., The Andes: Active subduc- from the ~5–40 km (paleodepth) North Cascades crustal section, Washington, in Miller, R.B., tion orogeny: Berlin, Springer, p. 459–474, doi:10.1007/978-3-540-48684-8_22. and Snoke, A.W, eds., Crustal cross sections from the western North American Cordillera Selverstone, J., 1988, Evidence for east-west crustal extension in the Eastern Alps: Implica- and elsewhere: Implications for tectonic and petrologic processes: Geological Society of tions for the unroofing history of the Tauern window: Tectonics, v. 7, p. 87–105, doi:10.1029 America Special Paper 456, p. 125–149, doi:10.1130/2009.2456(05). /TC007i001p00087. Miller, R.B., Gordon, S.M., Bowring, S., Doran, B., Mendoza, M., McLean, N., Michels, Z., Shea, Shea, E.K.M., 2008, Structure of the southern Skagit Gneiss Complex, North Cascades, WA [M.S. E., Whitney, D.L., and Wintzer, N., 2009b, Linking deep and shallow crustal processes in an thesis]: San Jose, California, San Jose State University, 90 p. exhumed continental arc, in O’Connor, J.E., et al., eds., Volcanoes to vineyards: Geological Sylva, N.L., and Miller, R.B., 2014, Structure of Eocene dikes south of the Duncan Hill pluton, field trips through the dynamic landscape of the Pacific Northwest: Geological Society of North Cascades, Washington—Implications for regional strain during postulated ridge sub- America Field Guide 15, p. 373–406, doi:10.1130/2009.fld015(19). duction: Geological Society of America Abstracts with Programs, v. 46, no. 5, p. 9. Misch, P., 1966, Tectonic evolution of the northern Cascades of Washington State—A west Cordi Tabor, R., and Haugerud, R., 1999, Geology of the North Cascades: Seattle, Washington, The lleran case history: Canadian Institute of Mining and Metallurgy, v. 8, p. 101–148. Mountaineers Books, 143 p. Misch, P., 1968, Plagioclase compositions and non-anatectic origin of migmatitic gneisses in the Tabor, R.W., Waitt, R.B., Jr., Frizzell, V.A., Jr., Swanson, D.A., Byerly, G.R., and Bentley, R.D., 1982, Northern Cascade Mountains of Washington State: Contributions to Mineralogy and Petrol- Geologic map of the Wenatchee 1:100,000 quadrangle, central Washington: U.S. Geological ogy, v. 17, p. 1–70, doi:10 .1007 /BF00371809. Survey Miscellaneous Investigations Series Map I-1311, scale 1:100,000. Misch, P., 1977a, Bedrock geology of the North Cascades, in Brown, E.H., and Ellis, R.C., eds., Tabor, R.W., Frizzell, V.A., Jr., Vance, J.A., and Naeser, C.W., 1984, Ages and stratigraphy of lower Geological excursions in the Pacific Northwest: Bellingham, Western Washington Univer- and middle Tertiary sedimentary and volcanic rocks of the central Cascades, Washington: sity, p. 1–62. Application to the tectonic history of the Straight Creek fault: Geological Society of America Misch, P., 1977b, Dextral displacements at some major strike faults in the North Cascades [abs.]: Bulletin, v. 95, p. 26–44, doi:10.1130/0016-7606(1984)95<26:AASOLA>2.0.CO;2. Geological Association of Canada Programme with Abstracts, v. 2, p. 37. Tabor, R.W., Frizzell, V.A., Jr., Whetten, J.T., Waitt, R.B., Swanson, D.A., Byerly, G.R., Booth, D.B., Monger, J.W.H., 1986, Geology between Harrison Lake and Fraser River, Hope Map area, south- Hetherington, M.J., and Zartman, R.E., 1987a, Geologic map of the Chelan 30-minute by western British Columbia, in Current research, Part B: Geological Survey of Canada Paper 60-minute Quadrangle, Washington: U.S. Geological Survey Investigations Series I-1661, 86-1B, p. 699–706. scale 1:100,000, 56 p. Monger, J.W.H., Price, R.A., and Tempelman-Kluit, D.J., 1982, Tectonic accretion and the origin Tabor, R.W., Zartman, R.E., and Frizzell, V.A., Jr., 1987b, Possible tectonostratigraphic terranes of the two major metamorphic and plutonic welts in the Canadian Cordillera: Geology, v. 10, in the North Cascades crystalline core, Washington, in Schuster, J.E., ed., Selected papers p. 70–75, doi:10.1130/0091-7613(1982)10<70:TAATOO>2.0.CO;2. on the geology of Washington: Washington Division of Mines and Geology Bulletin 77, Monger, J.W.H., van der Heyden, P., Journeay, J.M., Evenchick, C.A., and Mahoney, J.B., 1994, p. 107–127. Jurassic–Cretaceous basins along the Canadian Coast belt; their bearing on pre-mid-Creta- Tabor, R.W., Haugerud, R.A., and Miller, R.B., 1989, Overview of the geology of the North Cas- ceous sinistral displacements: Geology, v. 22, p. 175–178, doi:10 .1130 /0091 -7613(1994)022 cades: International Geological Congress Trip T307: American Geophysical Union, 62 p. <0175:JCBATC>2.3.CO;2. Tabor, R.W., Frizzell, V.A., Jr., Booth, D.B., and Waitt, R.B., Jr., 2000, Geologic map of the Snoqualmie Nelson, K.D., et al., 1996, Partially molten middle crust beneath southern Tibet: Synthesis of Pass 30 minute by 60 minute quadrangle, Washington: U.S. Geological Survey Geologic Project INDEPTH results: Science, v. 274, p. 1684–1688, doi:10.1126 /science .274.5293 .1684. Investigations Series I-2538, scale 1:100,000. Ouimet, W.B., and Cook, K.L., 2010, Building the central Andes through axial lower crustal flow: Tabor, R.W., Haugerud, R.A., Hildreth, W., and Brown, E.H., 2003, Geologic map of the Mount Tectonics, v. 29, TC3010, doi:10.1029/2009TC002460. Baker 30- by 60-minute quadrangle, Washington: U.S. Geological Survey Geologic Investi- Parrish, R.R., Carr, S.D., and Parkinson, D.L., 1988, Eocene extensional tectonics and geochro- gations Series I-2660, scale 1:100,000. nology of the southern Omineca belt: British Colombia and Washington: Tectonics, v. 7, Tepper, J.H., 1996, Petrology of mafic plutons associated with calc-alkaline granitoids, Chilliwack p. 181–212, doi:10.1029/TC007i002p00181. batholith, North Cascades, Washington: Journal of Petrology, v. 37, p. 1409–1436, doi:10.1093 Paterson, S.R., Fowler, T.K., Jr., Schmidt, K.L., Yoshinobu, A.S., Yuan, E.S., and Miller, R.B., 1998, /petrology/37.6.1409. Interpreting magmatic fabrics in plutons: Lithos, v. 44, p. 53–82, doi:10.1016 /S0024 -4937 Teyssier, C., Ferré, E.C., Whitney, D.L., Norlander, B., Vanderhaeghe, O., and Parkinson, D., 2005, (98)00022-X. Flow of partially molten crust and origin of detachments during collapse of the Cordilleran Paterson, S.R., Miller, R.B., Alsleben, H., Whitney, D.L., Valley, P.M., and Hurlow, H., 2004, Driving orogen, in Bruhn, D., and Burlini, L., eds., High-strain zones: Structure and physical prop- mechanisms for >40 km of exhumation during contraction and extension in a continental erties: Geological Society of London Special Publication 245, p. 39–64, doi:10 .1144 /GSL .SP arc, Cascades core, Washington: Tectonics, v. 23, TC3005, doi:10.1029 /2002TC001440 . .2005.245.01.03.

Thorkelson, D.J., and Taylor, R.P., 1989, Cordilleran slab windows: Geology, v. 17, p. 833–836, doi: Wells, R., Bukry, D., Friedman, R., Pyle, D., Duncan, R., Haeussler, P., and Wooden, J., 2014, 10.1130/0091-7613(1989)017<0833:CSW>2.3.CO;2. Geologic history of Siletzia, a large igneous province in the Oregon and Washington Coast Tikoff, B., and Teyssier, C., 1994, Strain modeling of displacement-field partitioning in transpres- Range: Correlation to the geomagnetic polarity time scale and implications for a long-lived sional orogens: Journal of Structural Geology, v. 16, p. 1575–1588, doi:10 .1016 /0191 -8141 Yellowstone hotspot: Geosphere, v. 10, p. 692–719, doi:10 .1130 /GES01018 .1 . (94)90034-5. Wernicke, B., and Getty, S.R., 1997, Intracrustal subduction and gravity currents in the deep crust: Umhoefer, P.J., and Miller, R.B., 1996, Mid-Cretaceous thrusting in the southern Coast Sm-Nd, Ar-Ar, and thermobarometric constraints from the Skagit Gneiss Complex, Wash- belt, British Columbia and Washington, after strike-slip fault reconstruction: Tectonics, v. 15, ington: Geological Society of America Bulletin, v. 109, p. 1149–1166, doi:10.1130 /0016 -7606 p. 545–565, doi:10.1029/95TC03498. (1997)109<1149:ISAGCI>2.3.CO;2. Valley, P.M., Whitney, D.L., Paterson, S.R., Miller, R.B., and Alsleben, H., 2003, Metamorphism of Whitney, D.L., 1992a, Origin of CO2-rich fluid inclusions in leucosomes from the Skagit migma- deepest exposed arc rocks in the Cretaceous to Paleogene Cascades belt, Washington: Evi- tites, North Cascades, Washington: Journal of Metamorphic Geology, v. 10, p. 715–725, doi: dence for large-scale vertical motion in a continental arc: Journal of Metamorphic Geology, 10.1111/j.1525-1314.1992.tb00118.x. v. 21, p. 203–220, doi:10.1046/j.1525-1314.2003.00437.x. Whitney, D.L., 1992b, High pressure metamorphism in the western Cordillera of North America: Vance, J.A., and Miller, R.B., 1981, The movement history of the Straight Creek fault in Washing- An example from the Skagit Gneiss, North Cascades, Washington: Journal of Metamorphic ton State: The last 100 million years (mid Cretaceous to Holocene): Geological Association Geology, v. 10, p. 71–85, doi:10.1111/j.1525-1314.1992.tb00072.x. of Canada Programme with Abstracts, p. 39–41. Whitney, D.L., and Irving, A.J., 1994, Origin of K-poor leucosomes in a metasedimentary migma- Vanderhaeghe, O., and Teyssier, C., 1997, Formation of the Shuswap metamorphic core complex tite complex by ultrametamorphism, syn-metamorphic magmatism, and subsolidus pro- during late-orogenic collapse of the Canadian Cordillera: Role of ductile thinning and partial cesses: Lithos, v. 32, p. 173–192, doi:10.1016/0024-4937(94)90038-8. melting of the mid-to lower crust: Geodinamica Acta, v. 10, p. 41–58, doi:10 .1080 /09853111 .1997.11105292. Whitney, D.L., Paterson, S.R., Schmidt, K.L., Glazner, A.F., and Kopf, C., 2004, Growth and demise Vanderhaeghe, O., Teyssier, C., and Wysoczanski, R., 1999, Structural and geochronological con- of continental arcs and orogenic plateaux in the North American Cordillera: From Baja to straints on the role of partial melting during the formation of the Shuswap metamorphic British Columbia, in Grocott, J., et al., eds., Vertical coupling and decoupling in the litho- core complex at the latitude of the Thor-Odin Dome, British Columbia: Canadian Journal of sphere: Geological Society of London Special Publication 227, p. 167–176, doi:10 .1144 /GSL Earth Sciences, v. 36, p. 917–943, doi:10.1139 /e99 -023. .SP.2004.227.01.09. Ward, K.M., Zandt, G., Beck, S.L., Christensen, D.H., and McFarlin, H., 2014, Seismic imaging of Wintzer, N.E., 2012, Deformational episodes recorded in the Skagit Gneiss Complex, North Cas- the magmatic underpinnings beneath the Altiplano-Puna volcanic complex from the joint cades, Washington, USA: Journal of Structural Geology, v. 42, p. 127–139, doi:10 .1016 /j .jsg inversion of surface wave dispersion and receiver functions: Earth and Planetary Science .2012.06.004. Letters, v. 404, p. 43–53, doi:10.1016/j.epsl.2014.07.022. Yardley, B.W.D., 1978, Genesis of the Skagit Gneiss migmatites, Washington, and the distinction Waters, A., 1926, Corbaley Canyon dike swarm, Washington [B.S. thesis]: Seattle, University of between possible mechanisms of migmatization: Geological Society of America Bulletin, Washington, 58 p. v. 89, p. 941–951, doi:10.1130/0016-7606(1978)89<941:GOTSGM>2.0.CO;2.