# Temporal and spatial evolution of Northern Cascade Arc magmatism revealed by LA-ICP-MS U-Pb zircon dating

Emily Mullen, Jean-Louis Paquette, Jeffrey Tepper, I. Stewart Mccallum

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Emily Mullen, Jean-Louis Paquette, Jeffrey Tepper, I. Stewart Mccallum. Temporal and spatial evolution of Northern Cascade Arc magmatism revealed by LA-ICP-MS U-Pb zircon dating. Canadian journal of earth sciences, 2018, 55 (5), pp.443 - 462. 10.1139/cjes-2017-0167. hal-01789556

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Temporal and spatial evolution of Northern Cascade Arc magmatism revealed by LA-ICP-MS U-Pb zircon dating

<table><tr><td>Journal:</td><td>Canadian Journal of Earth Sciences</td></tr><tr><td>Manuscript ID</td><td>cjes-2017-0167.R1</td></tr><tr><td>Manuscript Type:</td><td>Article</td></tr><tr><td>Date Submitted by the Author:</td><td>12-Jan-2018</td></tr><tr><td>Complete List of Authors:</td><td>Mullen, Emily; Laboratoire Magmas et Volcans, Universite Clermont Auvergne<br/>Paquette, Jean-Louis; Laboratoire Magmas et Volcans, Universite Clermont Auvergne<br/>Tepper, Jeffrey; University of Puget Sound, Geology Department<br/>McCallum, I. Stewart; University of Washington, Earth and Space Sciences</td></tr><tr><td>Is the invited manuscript for consideration in a Special Issue?:</td><td>N/A</td></tr><tr><td>Keyword:</td><td>geochronology, U-Pb zircon dating, Cascade Arc, Chilliwack Batholith, Mt. Baker volcanic field</td></tr></table>
1 Temporal and spatial evolution of Northern Cascade Arc magmatism revealed by LA-ICP-

2 MS U-Pb zircon dating

3

4

Emily K. Mullen<fnref n="1" />*, Jean-Louis Paquette<fnref n="1" />, Jeffrey H. Tepper<fnref n="2" />, I. Stewart McCallum<fnref n="3" />

5

6

7

8

9

USA

10

11

<footnote n="1">¹ Laboratoire Magmas et Volcans, Université Clermont Auvergne, 63178 Aubière, France</footnote>

<footnote n="2">² University of Puget Sound, Geology Department, Tacoma, WA 98416-1048, USA</footnote>

<footnote n="3">³ University of Washington, Department of Earth and Space Sciences, Seattle, WA 98195-1310,</footnote>

<footnote>* mullen.emka@gmail.com</footnote>
12 **ABSTRACT**

We present thirty new LA-ICP-MS U-Pb zircon dates for intermediate to silicic plutons of the Northern Cascade Arc with emphasis on the Chilliwack batholith-Mount Baker magmatic focus, located in southwestern British Columbia and northern Washington. Chilliwack magmatism commenced at ~35 Ma in southern B.C. and the most voluminous plutons define a cluster at ~32-29 Ma, documenting an early flare-up. During the same interval, the Index, Squire Creek, and Cascade Pass intrusions were emplaced south of the Chilliwack batholith. North of the Chilliwack, maximum pluton ages become progressively younger northward, tracking the northerly migration of the edge of the Farallon-Juan de Fuca-Explorer plate system relative to North America. Chilliwack magmatism continued from ~29 Ma to 22 Ma at a slightly reduced flux, followed by a lull from 22 to 11 Ma during which magmatism shifted north to the Mount Barr batholith (18 Ma). Chilliwack magmatism resumed by 11 Ma but was intermittent and the intrusive flux was significantly lower. The temporal decrease in intrusive flux displayed by the Chilliwack batholith correlates with the declining convergence rate of the Juan de Fuca plate since arc inception. The 11 Ma-to-present magmatism extends a pattern of southwesterly migration of the magmatic focus previously identified from ~4 Ma (Hannegan caldera) to the modern Mt. Baker volcanic field. Crustal rotation accounts for the rate of the first ~7 Myr of migration. However, the migration rate more than doubled at ~4 Ma, coinciding with separation of the Explorer plate and initiation of Juan de Fuca plate rollback.

31

32 **Keywords:** geochronology, U-Pb zircon dating, Cascade Arc, Chilliwack Batholith, Mt. Baker volcanic field
34 RÉSUMÉ

Sont reportées trente nouvelles datations U-Pb zircons par LA-ICP-MS de plutons acides-intermédiaires du Nord de l'arc des Cascades, principalement l'ensemble batholithe de Chilliwack-Mont Baker (SW Colombie Britannique et nord Etat de Washington). Ce magmatisme a débuté il y a 35 Ma (Sud Colombie Britannique) et les plutons les plus volumineux représentent un pic d'activité précoce de 32 Ma à 29 Ma. Durant cette période, les intrusions d'Index, Squire Creek, et Cascade Pass se mettent en place au sud du Batholithe de Chilliwack. Dans la partie septentrionale, l'âge maximum des plutons devient progressivement plus jeune vers le nord en parallèle de la migration du système de plaques Farallon-Juan de Fuca-Explorer par rapport à l'Amérique du Nord. Un flux magmatique réduit s'est poursuivi de 29 Ma à 22 Ma, suivi par une période de repos de 22 Ma à 11 Ma au cours de laquelle le magmatisme s'est déplacé vers le nord (batholithe du Mont Barr 18 Ma). Un magmatisme intermittent reprend à partir de 11 Ma avec un flux réduit corrélé à la réduction du taux de convergence de la plaque Juan de Fuca. De 11 Ma à l'actuel, le magmatisme migre vers le SW depuis ~4 Ma (Hannegan caldera) vers le domaine volcanique actuel du Mont Baker. Une rotation de la croûte explique le taux de migration durant les 7 premiers Ma. Ce taux double à ~4 Ma, ce qui coïncide avec la séparation de la plaque Explorer et le début du rollback de la plaque Juan de Fuca.
## 51 INTRODUCTION

A growing number of studies in exhumed paleo-arc systems have documented the episodic nature of arc magmatism through space and time (e.g., Paterson and Ducea, 2015; Kirsch et al., 2016), with important implications for our understanding of the assembly and evolution of arcs and role of tectonic processes. A key element in these investigations has been the ability to determine high-quality crystallization ages on magmatic rocks. With exposures of some of the youngest arc plutons in the world, the Cascade Arc of western North America (Fig. 1) offers an opportunity to examine the temporal and spatial evolution of magmatism in a still-active arc.

Recent comprehensive, high-precision $^{40}$Ar/$^{39}$Ar and K-Ar dating studies have established chronologies for many of the active Cascade volcanic fields (e.g., Hildreth and Lanphere, 1994; Hildreth et al., 2003; Bacon and Lanphere, 2006; Clynne et al., 2008; Schmidt and Grunder, 2009; Muffler et al., 2011; Fleck et al., 2014; Hildreth and Fierstein, 2015), and several studies have sought to define the evolution of magma compositions and production rates in the Central and Southern Cascades (McBirney, 1978; Verplanck and Duncan, 1987; Priest, 1990; Sherrod and Smith, 2000; du Bray and John, 2011; du Bray et al., 2014). However, these studies did not include a major segment of the arc, the Northern Cascades (here defined as that part of the arc north of Mount Rainier) of Washington and British Columbia, which offers the best exposures of the plutonic roots of the arc due to rapid Pleistocene uplift and extensive glacial erosion. Most of the existing radiometric dates for the early products of the Cascade Arc were also measured by old and relatively imprecise K-Ar or zircon fission track dating, with uncertainties in the millions of years. As a consequence, the timing of arc initiation as a function of position along the arc axis, and the evolution of the distribution and volumes of magmatism through time, are incompletely known.
74

This study presents new U-Pb zircon ages for ancestral magmas of the Northern Cascade Arc,

measured by laser ablation (LA) ICP-MS, to refine the spatial and temporal evolution of this

sector of the arc. LA-ICP-MS is an ideal method for dating such relatively young samples

because analytical uncertainties of less than 1% can be achieved at the 2σ level, which translates

to tens or a few hundreds of thousands of years, shorter than the timescales of the magmatic

processes of interest. This technique also affords the high spatial resolution needed to identify

inherited zircons and multiple age domains within individual grains as well as the ability to

rapidly produce large datasets. We primarily examine the ~960 km² composite Chilliwack

batholith of northern Washington and southern British Columbia and its successor, the currently

active Mount Baker volcanic field, which together preserve one of the longest and most

continuously exposed magmatic records in the Cascades (Figs. 1, 2). In addition to 19 new dates

for the area comprising the Chilliwack-Mt. Baker magmatic focus (as defined in Figs. 1 and 2),

we have determined ages for 6 arc plutons north of the Chilliwack batholith (Pemberton Belt;

Fig. 1) and 5 samples from intrusions south of the batholith. Our goals are to improve the

knowledge of Northern Cascades magmatic chronologies, determine how magma fluxes and

distributions have changed spatially and temporally, and to evaluate the contributing roles of

forcing factors both internal and external to the arc, such as tectonic processes and the structure

and characteristics of the mantle and crust of the arc system (Paterson and Ducea, 2015).

93

## GEOLOGICAL CONTEXT

### 1. The Cascade Arc system
The Cascade Arc currently extends from northern California to southwestern British Columbia and is related to the northeasterly subduction of the Juan de Fuca oceanic plate (Fig. 1), a remnant of the once considerably larger Farallon plate (e.g., Schellart et al., 2010). Cascadia subduction was initiated after a westward trench jump following the accretion of the Siletz terrane, a process that was essentially complete by 45 Ma, although related basaltic volcanism of the Coast Range persisted through the earliest Cascade Arc magmatism (Wells et al., 2014). The oldest radiometrically measured dates for magmatic products of the Cascade Arc cluster at ~35-40 Ma with a few outlying points of up to ~42 Ma (Lux, 1981; Sherrod and Smith, 2000; du Bray and John, 2011). Over time, the Juan de Fuca plate has become progressively smaller, younger, and hotter at the trench, resulting in increased buoyancy and resistance to subduction (Riddihough, 1984). Consequently, the orthogonal convergence rate between the Juan de Fuca and North American plates has decreased by a factor of five since the initiation of subduction (Verplanck and Duncan, 1987). At ~4 Ma, the youngest and most northerly portion of the Juan de Fuca plate detached to create the Nootka fault zone and establish the independent Explorer microplate (Fig. 1), which has ceased to subduct and is in the process of breaking up and becoming captured by North America (Riddihough, 1984; Audet et al., 2008; Rohr and Tryon, 2010; Rohr, 2015).

A bend in the arc axis in Washington subdivides the arc into two major sectors, the Garibaldi Belt (originally defined as extending from Glacier Peak to Bridge River Cones) and the High Cascades (Mt. Rainier to Lassen Peak) (Fig. 1). On the basis of geochemical data, Mullen et al. (2017) recommend including Glacier Peak in the High Cascades suite rather than the Garibaldi Belt. Following du Bray and John (2011), we use the term “ancestral” to refer to magmatism
that preceded the “modern” Cascades of approximately 4 Ma to recent. In the High Cascades, ancestral magmatism is preserved in a belt of mainly volcanic rocks known as the Western Cascades located trenchward of the modern arc (Fig. 1). The opposite relationship is observed in the Garibaldi Belt, where ancestral magmas are mainly intrusive and located to the east of the modern arc in a swath known as the Pemberton Belt (Souther, 1977; Woodsworth et al., 1991) (Fig. 1). The Garibaldi and Pemberton Belts merge at Mt. Meager (Souther and Yorath, 1991). The dearth of preserved volcanic rocks in the Pemberton Belt is attributed, in part, to the rapid uplift and erosion that have characterized northern Washington and British Columbia since the late Miocene (Reiners et al., 2002). An along-strike change in the crustal stress regime, from extensional in the High Cascades to compressional in the Garibaldi Belt, is also likely to be a factor in reduced eruptive volumes in the Northern Cascades (Hildreth, 2007).

The Northern Cascade Arc was not included in du Bray and John's (2011) assessment of arc evolution due to an apparently uncertain affiliation of these magmas with subduction. However, geochemical data clearly demonstrate that mafic through silicic magmas in the Northern Cascades segment are calc-alkaline with a trace element "arc signature" at least as far north as Mt. Meager, British Columbia (e.g., Tabor and Crowder, 1969; Berman and Armstrong, 1980; Coish and Journeay, 1992; Tepper, 1991, 1996; Tepper et al., 1993; Mullen and McCallum, 2014; Mullen and Weis, 2013; 2015; Mullen et al., 2017; authors' unpublished data). On the northern periphery of Mt. Meager, recently-erupted alkalic mafic lavas at Salal Glacier and Bridge River are related to asthenosphere upwelling through a slab gap along the subducted trace of the Nootka fault (Mullen and Weis, 2013). Farther north, two isolated volcanic fields,
141 Franklin Glacier and Silverthrone, lie inboard of the Explorer plate and their relationship to the
142 arc remains uncertain (Mullen et al., 2017).

143
144 The most extensive area of plutonic rocks in the Northern Cascades is the Chilliwack batholith,
145 which straddles the Washington-British Columbia border (Figs. 1, 2). The batholith includes at
146 least 55 plutons ranging in area from <1 to >100 km² that were emplaced at depths of <10 km,
147 plus subordinate erosional volcanic remnants (Tepper, 1991, 1996; Tepper et al., 1993; Tabor et
148 al., 2003). Petrologic and geochemical studies have established that Chilliwack magmas are I-
149 type, medium-K to (less commonly) low-K, and range in composition from gabbro to alaskite
150 with granodiorite and tonalite as the predominant lithologies (Richards and McTaggart, 1976;
151 Tepper, 1991, 1996; Tepper et al., 1993; Tabor et al., 2003). Radiometric dates previously
152 measured for the Washington sector of the Chilliwack batholith were compiled by Engels et al.
153 (1976) and more recently by Tabor et al. (2003); most were obtained by K-Ar, zircon fission
154 track, multi-grain U-Pb zircon, or Rb-Sr isochron methods. K-Ar dates have been reported for
155 the British Columbia portion of the batholith by Baadsgaard et al. (1961), Richards and
156 McTaggart (1976), and Mathews et al. (1981). Overall, these previously measured dates range
157 from ~2 to 35 Ma.

158
159 At the western margin of the Chilliwack batholith, plutonic rocks of the Lake Ann stock are
160 truncated by the eastern margin of the Kulshan caldera (Fig. 2), which represents the easternmost
161 expression of the ~220 km² Mt. Baker volcanic field (Hildreth, 2007). Hildreth et al. (2003)
162 carried out a comprehensive ⁴⁰Ar/³⁹Ar dating study showing that the Mt. Baker volcanic field has
163 been active since ~1.3 Ma, culminating in the ~40 ka to recent andesitic Mt. Baker stratocone.
164 Additional <sup>40</sup>Ar/<sup>39</sup>Ar, K-Ar, and ID-TIMS U-Pb zircon ages (Hildreth et al., 2003; Tucker et al., 2007) extend the magmatic record intermittently back to the 3.722 Ma Hannegan caldera, which is embedded within older Chilliwack plutons and intruded by younger ones (Fig. 2).

167 Advancement of the magmatic focus from the Hannegan caldera to the modern Mt. Baker entailed ~25 km of progressive southwesterly migration of the magmatic focus (Hildreth et al., 2003). On the basis of previously determined dates for the Chilliwack batholith, Tepper and Hildreth (2004) suggested that this migration pattern may have initiated as early as ~12 Ma.

171 Magmatic products of the Mt. Baker volcanic field are basaltic through rhyodacitic and display a complete compositional overlap with the Chilliwack batholith, demonstrating that the two suites are products of a single causal magmatic phenomenon (Hildreth et al., 2003). As such, the oldest Mt. Baker rocks and the youngest Chilliwack rocks could be considered members of both suites.

175

## 2. Basement rocks

177 The western margin of North America has long been the site of active subduction. The most recent Mesozoic and Cenozoic events have led to the docking of numerous allochthonous terranes, predominantly late Paleozoic to Tertiary in age, that constitute most of the ~35-40 km thick crust upon which the Cascade Arc has been built (Monger et al., 1982; Dickinson, 2004; Levander and Miller, 2012). In the Northern Cascades, this accreted crust was also intruded by plutons formed in Mesozoic to early Tertiary subduction zones preceding the formation of the Cascade Arc (Monger and Price, 2000; Dickinson, 2004; Brown and Gehrels, 2007).

184

185 The Chilliwack-Mt. Baker area is underlain by three distinct crustal blocks that are juxtaposed by major Tertiary faults. From west to east, these domains are known as the Northwest Cascades

186
187 System (NWCS), Cascades Core, and Methow-Pasayten belt (Fig. 2), which are separated by the
188 Straight Creek and Ross Lake faults, respectively. The Mt. Baker volcanic field lies entirely
189 within the NWCS, which consists of lithologically diverse and variably metamorphosed
190 Paleozoic to early Cretaceous terranes that are assembled into four major thrust sheets (Tabor et
191 al., 2003; Brown, 2012).

192
193 The Chilliwack batholith was emplaced mainly into the NWCS and Cascades Core (Fig. 2),
194 although the easternmost Chilliwack intrusions extend into the Methow-Pasayten belt. The
195 Cascades Core is a highly metamorphosed assemblage of Paleozoic to Tertiary accreted terranes
196 that are intruded by mid-Cretaceous to Eocene plutons (e.g., Miller et al., 2009). The terranes of
197 the Methow-Pasayten belt record little to no metamorphism and intrusive rocks are sparse. The
198 Chilliwack batholith is centered upon the north-trending strike-slip Straight Creek fault, and in
199 map view the batholith displays a north-south elongation parallel to the fault (Fig. 2), implying
200 that pluton emplacement was partially controlled by this structure (Woodsworth et al., 1991).
201 However, none of the plutons shows evidence for displacement on the fault (Tepper, 1991).

202
203 SAMPLES
204 Age determinations were carried out on 19 samples from the Chilliwack-Mt. Baker magmatic
205 focus and 13 samples from neighboring arc plutons to the north and south of this area. Some of
206 the same units were dated by previous studies and the most recently published ages are listed in
207 Table 1. Except for the Kulshan caldera rhyodacite tephra, all of the samples are biotite- and/or
208 hornblende-bearing, fine- to medium-grained granitoids; additional petrographic details are
209 provided in Table 1.
210
211 North of the Chilliwack batholith, new dates were obtained on the Fall Creek stock at Mt.
212 Meager, the Hicks Lake pluton, the Mount Barr batholith, and the Silver Creek stock. These new
213 dates complement existing dates for the Rogers Creek intrusive complex and the Harrison Lake,
214 Doctors Point, and Salal Creek plutons (Fig. 1b). The Fall Creek sample is a rounded lithic clast
215 ejected from the conduit wall during the 2360 B.P. subplinian eruption of Mt. Meager (Campbell
216 et al., 2013). The Mount Barr batholith has been subdivided into four phases (Richards and
217 McTaggart, 1976) and the dated sample represents the largest (Conway) phase. South of the
218 Chilliwack batholith, new ages were determined on the Cascade Pass dike, the Index, Grotto, and
219 northern Snoqualmie batholiths, and the Squire Creek stock (Tabor et al., 1993, 2002, 2003)
220 (Fig. 1b). Sample localities are shown in Figs. 1b and 2. Due to disturbance by advances of the
221 Cordilleran ice sheet, the plinian fallout from the Kulshan caldera-forming eruption is preserved
222 only far south of the caldera (Hildreth et al., 2004). The sample studied here was collected ~200
223 km south of Mount Baker at Sumner, Washington, where the fallout is known as the Lake Tapps
224 tephra (Westgate et al., 1987) (Fig. 1b).

225
226 ANALYTICAL METHODS
227 1. U-Th-Pb analyses by LA-ICP-MS
228 Zircons were separated at the Laboratoire Magmas et Volcans (LMV) using standard techniques
229 of crushing, sieving, and isodynamic magnetic separation, followed by Wilfley table or heavy
230 liquids (bromoform and methylene iodide). Approximately 50 zircons per sample were mounted
231 in epoxy disks and ground and polished to expose crystal interiors. Cathodoluminescence (CL)
232 imaging was conducted at LMV on selected samples using a JEOL JSM-5910LV SEM. U-Th-
Pb isotope data were measured by laser ablation inductively coupled mass spectrometry (LA-ICP-MS) at LMV. Zircons were ablated using a Resonetics Resolution M-50 equipped with a 193 nm ultra-short-pulse ATL Atlex Excimer laser system (Müller et al., 2009) coupled to a Thermo Element XR high resolution ICP-MS. Helium carrier gas was supplemented with N₂ prior to mixing with Ar for sensitivity enhancement (Paquette et al., 2014). The laser was operated with a spot diameter ranging from 33 to 60 µm, a repetition rate of 3 Hz, and a fluence of 2.5 to 5.5 J/cm². Other instrumental operating conditions and data acquisition parameters are detailed in Table S1.

The Element ICP-MS was tuned to maximize the ²³⁸U intensity and minimize ThO⁺/Th⁺ (<1.5%) using the NIST SRM 612 glass. The signals of ²⁰⁴Pb (+Hg), ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb, ²³²Th, and ²³⁸U were acquired during each analysis. After pre-ablating each spot, background levels were measured on-peak with the laser off for ~30 seconds, followed by ~60 seconds of measurement with the laser firing and then ~30 seconds of washout time. Reduction of raw data was carried out using the GLITTER® software package of Macquarie Research Ltd (van Achterbergh et al., 2001). Isotope ratios (with ²³⁵U calculated as ²³⁸U/137.88) were corrected for laser-induced and instrumental mass fractionation via sample-standard bracketing using the GJ-1 zircon (²⁰⁶Pb/²³⁸U age of 601 Ma; Jackson et al., 2004) which was analyzed four times at the beginning and end of each analytical session, and twice after each set of 8 samples. Concentrations of U, Th, and Pb were calculated by normalization to the certified composition of GJ-1 (Jackson et al., 2004). ²⁰⁶Pb/²³⁸U ratios were corrected for ²³⁰Th disequilibrium following Schärer (1984) by using the Th/U ratio of the whole rock (where data are available) as that of the magma, or by assuming a value of 3.5 ± 0.5 which represents the range defined by Chilliwack granitoids (Tepper, 1991,
256 1996; Tepper et al., 1993; Mullen, 2011; authors' unpublished data). Data are not corrected for common Pb. Tera-Wasserburg $^{207}$Pb/$^{206}$Pb vs. $^{238}$U/$^{206}$Pb diagrams were generated for each sample using the Isoplot/Ex v. 2.49 software of Ludwig (2001) (Figs. 3-5). Error ellipses for each point are shown at the 2σ level and incorporate both internal and external uncertainties.
257
258
259
260 Data points were pooled to calculate a date and associated 2σ error for each sample (reported in Fig. 3 and Table 1) using Isoplot. The Harward 91500 zircon (1065 Ma; Wiedenbeck et al., 1995) was analyzed along with the samples, twice at the beginning and end of each session, to independently monitor the external precision and accuracy of the measurements. The Concordia age for 132 analyses of 91500 conducted over the course of the study was 1063.9 ± 2.4 Ma (2σ including decay constant errors; MSWD = 2.1 including concordance + equivalence) as shown in Table S2.
261
262
263
264
265
266

267

268 RESULTS

269 1. Zircon morphology, petrography, and cathodoluminescence imaging

270 The analyzed zircons are pale pink to pale yellow or colorless, and most grains are transparent
271 and euhedral with habits ranging from prismatic to acicular. Zircon was detected in thin sections
272 of several samples but is most abundant in the Lake Ann samples. Cathodoluminescence (CL)
273 images (Fig. 6) display fine oscillatory, patchy, and complex zoning patterns that are consistent
274 with magmatic growth (Corfu et al., 2003) and lack obvious signs of xenocrystic cores or
275 overgrowths.
276

276

277 2. U-Th-Pb data
The complete dataset of measured U-Th-Pb isotope ratios and abundances is provided in Table S2. Concordia ages are reported for six samples (Table 1, Figs. 3-5). For the other samples, the data define linear correlations on $^{207}$Pb/$^{206}$Pb vs. $^{238}$U/$^{206}$Pb diagrams (Figs. 3-5) that extend between a lower Concordia intercept age (reported in Table 1) and a common Pb composition at the upper intercept (Tera and Wasserburg, 1972). We interpret the Concordia and lower intercept ages as those of magma crystallization. Multiple spots were analyzed on some individual zircon grains, and the resulting ages were within error. Overall, Th/U ratios for most of the zircons are from 0.16 to 2.0, with several outliers as low as 0.04. U and Th contents are typically ~30 to 1300 ppm and ~10 to 1900 ppm, respectively, although a few analyses extend up to ~2800 ppm U and ~3400 ppm Th. These values are typical of igneous rocks (Lopez-Sanchez et al., 2016). Except for the Cascade Pass dike sample, where zircons define a bimodal age distribution, inherited zircons older than magma crystallization ages are rare and occur as only one or two grains in a few samples.

## 2.1. Chilliwack-Mt. Baker magmatic focus

The new dates are presented in order of increasing age. The rhyodacite tephra of Lake Tapps yielded a lower intercept age of 1.165 ± 0.013 Ma (Table 1, Fig. 3a), in excellent agreement with Hildreth et al.'s (2003) date of 1.149 ± 0.010 Ma ($^{40}$Ar/$^{39}$Ar plagioclase) from a Kulshan caldera rhyodacite ignimbrite, and confirming the work of Westgate et al. (1987) indicating that the Kulshan caldera was the most probable source of the Lake Tapps tephra. Three older inherited zircons were also present in the sample (red points in Figs. 3a, 3b); two grains at ~3 Ma (three spots analyzed) and one grain at ~200 Ma (three spots analyzed). Th/U ratios for the main zircon population are 0.3 to 1.2, and the inherited zircons are indistinguishable with Th/U = 0.3-0.4 for
301 the older grain and 0.6-0.7 for the younger grains. Dates for two granodiorites from different
302 parts of the Lake Ann stock, which was cut by the Kulshan caldera eruption, are outside of
303 respective 2σ errors at 2.344 ± 0.021 and 2.430 ± 0.016 Ma (Table 1, Fig. 3c-d). Both new ages
304 are younger than the 2.75 ± 0.13 Ma $^{40}$Ar/$^{39}$Ar biotite age of Hildreth et al. (2003). The sample
305 collected from the upper part of the intrusion (06-MB-68) crystallized between 49 and 123 kyr
306 before the sample from the center (06-MB-70), evidence for incremental growth of this magma
307 body. Each Lake Ann sample also yielded a single inherited zircon grain at ~4.3 and 4.5 Ma (red
308 points in Fig. 3c-d). The inherited grains display Th/U ratios of ~0.5, overlapping with those of
309 the main zircon populations (~0.4 to 0.9).

310
311 A zircon separate from a granite of the Mineral Mountain pluton (#RWT269-87) gave a lower
312 intercept age of 7.83 ± 0.06 (Table 1, Fig. 3e) that is consistent with dates of ~7.3 to 8.1 Ma
313 (multi-grain U-Pb TIMS) and 6.5 ± 2.6 Ma (fission track) previously measured on the same
314 separate (Tabor et al., 2003 and references therein). However, we also identified a single
315 inherited grain of ~14-15 Ma (red point in Fig. 3e) with a Th/U ratio of 0.5 that overlaps with the
316 main zircon population (~0.4 to 1.6). Farther to the northeast, the previously undated Whatcom
317 Camp granite (Tepper, 1991) returned a lower intercept age of 9.538 ± 0.065 Ma (Table 1, Fig.
318 3f). A quartz diorite from the Mount Sefrit pluton gave a lower intercept age of 10.27 ± 0.06 Ma
319 (Table 1, Fig. 3g), in agreement with our recalculated Rb-Sr isochron age of 10.08 ± 0.29 Ma for
320 the bio-plag-cpx-whole rock data of Tepper (1991) for an associated gabbro. In the eastern
321 sector of the batholith, the Indian Creek pluton returned a 11.33 ± 0.08 Ma lower intercept age
322 (Table 1, Fig. 3h) for a quartz monzonite, within the respective 2σ errors of previous dates of
323 12.1 ± 0.8 (K-Ar biotite, Mathews et al., 1981) and 10.8 ± 1.8 (K-Ar hornblende, Engels et al.,
324 1976). The previously undated granodiorite of Whatcom Arm (Tepper, 1991) gave a lower intercept age of 22.75 ± 0.17 Ma (Table 1, Fig. 3i).

326
327 In British Columbia, a lower intercept age of 27.38 ± 0.12 Ma for a granodiorite from the Radium Peak stock (Table 1, Fig. 3j) is consistent with the 27 ± 2 Ma K-Ar biotite age reported by Richards and McTaggart (1976). Samples from the previously undated Paleface stock and Western tonalite (Richards and McTaggart, 1976) were collected ~10 km apart, yet returned ages of 28.28 ± 0.15 Ma (lower intercept) and 28.76 ± 0.14 Ma (Concordia), respectively (Table 1, Figs. 3k-l), nearly within error. For the Eastern tonalite, two of the three dated samples, collected ~10 km apart, are within error at 29.41 ± 0.13 and 29.43 ± 0.16 Ma (Table 1, Figs. 3m-n) and both fall within the 2σ uncertainties of K-Ar biotite determinations (30 ± 2 and 29 ± 2 Ma; Richards and McTaggart, 1976). However, the third Eastern tonalite sample (#15-PB-15), which was collected farther northeast (Fig. 2), is at least 2.23 Myr older with a Concordia age of 31.93 ± 0.16 Ma (Table 1, Fig. 3o). In recognition of this difference, we refer to the older portion of the Eastern tonalite as a separate “Maselpanik lobe” (Fig. 2). Th/U ratios for all three Eastern tonalite samples are 0.2 to 0.8, except for a few analyses from one sample (#CB89-1) that are as low as 0.04. The low Th/U reflects elevated U contents (up to ~1270 ppm) relative to other analyses from the same sample (~60 to 580 ppm U). Nevertheless, the low-Th/U points are not associated with any age discrepancy outside of analytical error.

343
344 A zircon fission track age of 8.7 Ma was previously reported for the Hannegan Trail porphyritic tonalite (Tepper, 1991), but our sample from this unit gave a significantly older age (lower intercept) of 31.95 ± 0.24 Ma (Table 1, Fig. 3p). The largest and southernmost pluton in the
Chilliwack batholith, Thornton Creek, was previously dated by a variety of methods and most results were in the ~29 to 35 Ma range (Tabor et al., 2003 and references therein). Our new lower intercept age of 32.02 ± 0.16 Ma for a quartz diorite (Table 1, Fig. 3q) lies within the same range. Th/U ratios from this sample are 0.3 to 0.7, with the exception of a single low value of 0.04 due to high U (~480 ppm) compared to other grains (maximum ~320 ppm). For the Copper Lake pluton, a 33.96 ± 0.18 Ma lower intercept age for a hornblende quartz diorite (Table 1, Fig. 3r) is within error of Tepper's (1991) Rb-Sr isochron date of 34.03 ± 0.91 Ma (bio-hbl-plag-whole rock) for a diorite sample.

Identical K-Ar biotite ages of 27 ± 2 Ma have been reported for the Post Creek and Mt. Rexford stocks of British Columbia (Richards and McTaggart, 1976). However, both of our new ages (lower intercept) are significantly older, 32.34 ± 0.17 Ma for Mt. Rexford and 34.74 ± 0.24 for Post Creek (Table 1, Fig. 3s-t). Our date for the Post Creek stock is the oldest measured on Chilliwack batholith samples.

A new date of 86.91 ± 0.43 Ma for Tepper's (1991) tonalite of Copper Ridge Lookout (Table 1, Fig. 3u) indicates that this unit is not part of the Chilliwack batholith, but this date does fall within the ~71-96 Ma range reported for Cascade Core plutonic basement rocks, which are also predominantly tonalitic (Miller et al., 2009). Th/U ratios of 0.2 to 0.6 are also consistent with an igneous origin.

The new dating results were used to color-code the individual phases of the Chilliwack batholith by age (Fig. 2). For plutons not dated in this study, we used ages compiled and recommended by
370 Tabor et al. (2003) (Table S3). Diagrams displaying the new dates on east-west and north-south profiles (Fig. 7) highlight spatial shifts in magmatism through time.
371

372
373 The exposed areas of individual plutons were calculated digitally from geologic maps and are plotted as a function of time (Fig. 8) as a first-order proxy for the “apparent intrusive flux” (de Silva et al., 2015). Since the third dimensions of the plutons are unknown, the calculated fluxes should be viewed as estimates. Except for caldera fills and the area covered by the Mt. Baker volcanic field (shown for reference), the few small volcanic remnants in the Chilliwack are omitted from Fig. 8 because of uncertainties in intrusive to extrusive ratios (e.g., Paterson and Ducea, 2015) and unknown pre-erosive dimensions.
376
377
378
379

380
381
**2.2. North of the Chilliwack batholith**
382
The northernmost sample dated for this study, a quartz monzonite from the Fall Creek stock
383
underlying Mt. Meager, gave a lower intercept age of 6.646 ± 0.046 Ma (Table 1, Fig. 4a) that is
384
significantly younger than the previous 10.1 ± 1.2 (K-Ar biotite) determination of Stevens et al.
385
(1982). A new 18.13 ± 0.10 Ma lower intercept age (Table 1, Fig. 4b) for a quartz monzonite
386
from the Conway phase of the Mount Barr batholith, the largest of four mapped phases, overlaps
387
with Baadsgaard et al. (1961)'s K-Ar biotite date of 18.5 ± 1.8 Ma. Our result is also in the
388
range of K-Ar biotite dates reported for two of the three other phases of the batholith, Wahleach
389
Lake and Mt. Barr (~16-22 Ma) (Richards and McTaggart, 1976). The Silver Creek stock,
390
located just north of the Chilliwack batholith, was once thought to be the oldest in the Pemberton
391
Belt based on a K-Ar hornblende date of 36 ± 4 Ma (Richards and McTaggart, 1976). However,
392
the new date for Silver Creek, measured on a quartz diorite, is considerably younger at 23.77 ±
393 0.10 Ma (Table 1, Fig. 4c). A granite from the Hicks Lake pluton yielded a Concordia age of
394 26.23 ± 0.16 Ma (Table 1, Fig. 4d), comparable to the 25 ± 1.4 Ma K-Ar biotite date of Richards
395 and McTaggart (1976). One inherited zircon grain of ~174 Ma was also identified (red ellipse,
396 Fig. 4d). The Th/U ratio of the inherited grain (~0.4) overlaps with the 0.2 to 0.5 range defined
397 by the main zircon population.

398
399 **2.3. South of the Chilliwack batholith**
400
Multiple K-Ar and zircon fission track dates obtained for the Snoqualmie batholith indicate that
401 it generally decreases in age from ~25 Ma in the north to ~18 Ma in the south (Tabor et al., 1993
402 and references therein). Our new lower intercept age of 23.87 ± 0.15 Ma for a tonalite from the
403 northern Snoqualmie batholith (Fig. 5a) agrees with a K-Ar hornblende date of 23.0 ± 1.2 Ma for
404 a nearby sample locality (Tabor et al., 1993; sample #5). The Grotto batholith is located on the
405 northern boundary of the Snoqualmie batholith, and it has long been hypothesized that the two
406 intrusions represent a single body based upon similar ranges of previously measured ages (Tabor
407 et al., 1993, and references therein). An intensive search for zircon in our tonalite from the
408 Grotto batholith produced only nine grains, the fewest in our study. Nevertheless, these grains
409 yielded a lower intercept age of 24.14 ± 0.45 Ma (Fig. 5b), identical (within error) to our new
410 age for the northern Snoqualmie batholith and confirming that the two intrusions are most likely
411 continuous at depth.

412
413 The quartz diorite sample from the Cascade Pass dike is unique in this study in that the zircon
414 data delineate two distinct age populations, both of which display well-defined lower intercepts.
415 About 40% of the data define an age of 32.78 ± 0.28 Ma and the other ~60% define an age of
416 95.36 ± 0.57 Ma (Fig. 5c). The younger zircon grains are stubby, prismatic, yellow to pink, and
tend to be smaller than the older zircons, which are pink and more elongate. Th/U ratios of the
417
younger population (0.4 to 0.6) overlap with those of the older population (0.3 to 0.5), both of
418
which are typical of igneous rocks. We report the younger age (32.78 Ma) as that of magma
419
crystallization and the older age (95.36 Ma) as that of an inherited component. The new
420
solidification age for the dike is significantly older than the previously accepted age of ~18 Ma
421
for the Cascade Pass dike (Tabor et al., 2002).
422

423
424 New dates for two samples from different parts of the Index batholith, a granodiorite and
425 tonalite, are within analytical error at 33.26 ± 0.19 and 33.53 ± 0.15 Ma (Table 1, Fig. 5d-e) and
426 are consistent with the ~34 Ma best age estimate for the batholith based on multiple K-Ar, multi-
427 grain U-Pb zircon, Rb-Sr, and zircon fission track determinations that lie in the ~33-36 Ma range
428 (Tabor et al., 1993 and references therein). Th/U ratios are 0.2 to 0.9, except for one analysis
429 with a ratio of 0.04 that reflects a slightly elevated U content (~740 ppm). Our oldest date for
430 Northern Cascade arc magmas was measured on a tonalite from the Squire Creek stock, which
431 gave a lower intercept age of 35.84 ± 0.16 Ma (Fig. 5f) that overlaps with a number of K-Ar,
432 multi-grain U-Pb zircon, and zircon fission track dates of ~33 to 36 Ma (Tabor et al., 2002 and
433 references therein).
434

434
435 All of the new dates from this study are plotted versus latitude in Fig. 9, along with previous
436 radiometric dates for Northern Cascades volcanic and plutonic rocks from Mt. St. Helens to
437 Silverthrone. The areas defined by Northern Cascades intrusive rocks, calculated digitally from
438 geologic maps (Richards and McTaggart, 1976; Woodsworth, 1977; Ray and Coombes, 1985;
439 Evarts et al., 1987; Schasse, 1987; Monger, 1989; Journeay and Friedman, 1993; Tabor et al., 1993, 2000, 2002; du Bray et al., 2011), are plotted versus age in Fig. 8 to approximate the intrusive flux for this segment of the arc. Areas are tabulated in Table S3.

442

443 DISCUSSION

444 1. Discrepancy between new and previous dates

445 About half of the new dates differ well outside of 2σ errors from previous age determinations on the same unit. For a given analytical method, however, the previous dates are systematically neither older nor younger. Argon loss and/or the lower closure temperature of the K-Ar system could explain why several of the new U-Pb zircon dates are older than previous K-Ar dates (e.g., Cascade Pass dike, Rexford, Post Creek, and Eastern tonalite), but these processes cannot account for why both new dates for the Lake Ann stock are younger than the $ ^{40}\text{Ar}/^{39}\text{Ar} $ biotite date of Hildreth et al. (2003). In contrast to those samples, a number of the new U-Pb zircon dates agree well with previous K-Ar ages (e.g., Index, Snoqualmie, Squire Creek, Radium Peak, Hicks Lake, Mount Barr, and Indian Creek). The significant difference between the new U-Pb date (31.95 Ma) and the zircon fission track date (8.7 Ma, Tepper 1991) for the Hannegan Trail unit may be the result of resetting by the adjacent Lower Ruth Creek unit, which was also dated at 8.7 Ma by Rb-Sr isochron (Tepper, 1991).

457

458 2. Provenance of inherited zircons

459 The inherited zircons from the Lake Ann and Mineral Mountain intrusions (Fig. 3c-e), as well as the younger (~3 Ma) inherited zircon population from the Kulshan caldera (Fig. 3a), are several millions of years older than magma crystallization ages, yet within the overall range defined by

460

461
the Chilliwack batholith. The Th/U ratios of the inherited grains are consistent with a magmatic growth (Lopez-Sanchez et al., 2016) and are indistinguishable from the Th/U ratios of other zircons in the same samples. We consequently interpret these inherited zircons as xenocrysts derived from earlier arc magmatic pulses.

With a significantly older age of ~200 Ma, the second inherited zircon in the Kulshan caldera sample (Fig. 3b) is consistent with a xenocryst derived from basement rock. However, the age of this xenocryst does not match the age ranges of the three units truncated by the caldera eruption (Tabor et al., 2003), i.e., the Middle Jurassic to Early Cretaceous Nooksack Formation (~180-100 Ma, Brown and Gehrels, 2007), the Paleozoic Chilliwack Group (~370-410 Ma, Brown et al., 2010), and the Lake Ann stock (~2.3-2.4 Ma, this study). Furthermore, none of the other NWCS units displays a ~200 Ma detrital zircon peak (Brown and Gehrels, 2007). However, the Nooksack Formation is correlative with the Harrison terrane in British Columbia (Monger and Journeay, 1994; Tabor et al., 2003), which extends back to middle Triassic (Mahoney et al., 1995) and does encompass the age of the xenocryst. Alternatively, structural reconstructions and seismic reflection-refraction data indicate that the NWCS occupies only the upper ~10 km of the crust and is underlain by a middle to lower crust composed of the Wrangellia terrane onto which the NWCS was obducted (Zelt et al., 1993; Monger and Journeay, 1994; Monger and Price, 2000; Miller et al., 1997; Brown, 2012). Wrangellia is presently exposed to the west on Vancouver Island, and its presence beneath the Chilliwack-Mt. Baker area is further supported by the emplacement of Mt. Baker within a structural window in which the NWCS thrust sheets were removed to reveal underlying rocks of the Nooksack Formation and Wells Creek volcanics that formed in close association with Wrangellia (Monger and Journeay, 1994; Tabor et al.,
485 2003). Major constituents of Wrangellia are the 203 to 164 Ma igneous rocks of the Bonanza Arc (Nixon and Orr, 2007), overlapping with the age of the inherited zircon in the Kulshan caldera sample. Th/U ratios of the inherited zircon are also consistent with a magmatic origin.

488
489 The Hicks Lake pluton is underlain by rocks of the Middle Triassic to Lower Cretaceous
490 Harrison terrane and its metamorphosed equivalent, the Slollicum schist (Monger, 1989;
491 Mahoney et al., 1995). The single Jurassic (~174 Ma) inherited zircon identified in our Hicks
492 Lake sample (Fig. 4d) overlaps with the age range of these basement rocks. A Th/U ratio of 0.4
493 for the inherited grain is typical of igneous rocks and consistent with the predominantly volcanic
494 and volcaniclastic origin of the Harrison terrane (Mahoney et al., 1995).

495
496 The Cascade Pass dike sample contains by far the largest proportion (~60%) of xenocrystic
497 zircons, which define a lower intercept age of 95.36 ± 0.57 Ma (Fig. 5c). Of several units in the
498 Cascade Core crustal block that were intruded by the dike, only the Eldorado orthogneiss, which
499 has a reported age of ~88-92 Ma based on zircon multi-grain fraction U-Pb TIMS dating (Tabor
500 et al., 2003 and references therein), is a close match to the age of the xenocrysts. Th/U ratios for
501 the Cretaceous zircon population are typical of magmatic rocks, consistent with derivation from
502 an orthogneiss. The other intruded units are the 60-70 Ma Skagit Gneiss Complex and the
503 Triassic Cascade River schist and Marblemount pluton (Tabor et al., 2003).

504
505 3. Temporal evolution of Chilliwack-Mt. Baker magmatism
506 The twenty new dates for the Chilliwack batholith delineate an episodic magmatic history that
507 was punctuated by an early flare-up and later interrupted by a prolonged lull. We subdivide
508 magmatism into three main phases: Phase 1 (~35 to 30 Ma) defines the highest intrusive flux in
509 the lifetime of the Chilliwack-Mt. Baker magmatic focus (Fig. 8). The earliest magmatism is
510 recorded by the Post Creek stock, the northernmost pluton in the batholith, at 34.74 Ma (Fig. 2).

511 Within less than 3 Myr, activity spread to the center (Copper Lake, 34.03 Ma) and the
512 southernmost reaches of the batholith to form largest intrusive body, Thornton Creek (118 km²,
513 Tabor et al., 2003), at 32.02 Ma (Fig. 2, 7b). Phase 1 magmatism was predominantly
514 concentrated in a north-trending belt in the western sector of the batholith, roughly coinciding
515 with the Straight Creek fault (Figs. 2, 7a). Farther east, the Maselpanik lobe of the Eastern
516 tonalite is centered on the Ross Lake fault zone (Fig. 2).

517
518 Over the following ~8 million years (Phase 2, ~30-22 Ma), the magmatically active zone
519 gradually broadened to the east (Fig. 7b). The intrusive flux (Fig. 8) remained steady but was
520 slightly lower than during the flare-up of Phase 1. Whatcom Arm (22.75 Ma) was the final
521 intrusion emplaced before a lull lasting ~11 Myr.

522
523 The end of the lull, and the beginning of Phase 3, was marked by the emplacement of the Indian
524 Creek pluton (11.33 Ma) in the eastern sector of the batholith (Fig. 2). Indian Creek also
525 initiated a pattern of southwesterly migration of the magmatic focus that persisted over a distance
526 of ~45 km to culminate at the modern Mount Baker stratocone (Fig. 7a). Compared to pre-lull
527 Chilliwack activity, post-lull magmatism was more sporadic (Fig. 8) and the intrusive flux was
528 ~5 times lower (14 vs. 63 km²/Myr; calculations in Table S3). Following emplacement of the
529 Indian Creek pluton, the magmatic focus shifted to Whatcom Camp (9.538 Ma), followed by
530 Mineral Mountain (7.83 Ma) and then the Hannegan caldera at 3.72 Ma (Fig. 2). During the
531 ~7.6 Myr that elapsed between Indian Creek and the Hannegan caldera, the migration distance
532 was ~20 km, corresponding to an average migration rate of ~2.6 mm/yr. The Mount Sefrit
533 pluton also falls within Phase 3 at 10.27 Ma, but it is an exception to the migration pattern as it is
534 located farther west than predicted for its age (Fig. 2, 7a). However, as one of the rare mafic
535 plutons in the Chilliwack batholith (Tepper, 1996), the peripheral location of Mt. Sefrit is
536 consistent with the observation that mafic magmas in the Cascades tend to occur on the fringes
537 of the main magmatic focus (Hildreth, 2007). From ~3.7 Ma to the present, migration persisted
538 an additional ~25 km from Hannegan caldera to the modern Mount Baker (Hildreth et al., 2003),
539 following an azimuth similar to that defined by the first ~7.6 Myr of migration, but at an
540 accelerated rate of ~6.7 mm/yr.

541
542 The ~3.7 Myr of migration in the Hannegan-Mount Baker sequence was attributed by Wells and
543 McCaffrey (2013) to clockwise rotation of the arc crust, augmented by slab rollback. GPS
544 vectors indicate that the arc is rotating clockwise with respect to stable North America about a
545 pole in eastern Oregon, and paleomagnetic data confirm that this rotation has been occurring at a
546 similar rate for at least the past ~15 Myr (McCaffrey et al., 2007, 2013; Wells and McCaffrey,
547 2013). This crustal rotation is driven by a combination of Basin and Range extension and dextral
548 shear due to the oblique subduction of the Farallon plate (Wells and Heller, 1988). Rotation
549 around the eastern Oregon pole would have displaced the ancestral Southern and Central
550 Cascades (the Western Cascades) to the west of modern High Cascades and the Pemberton Belt
551 to the east of the modern Garibaldi Belt, both of which are observed (Fig. 1).

552
In the Chilliwack-Mt. Baker area, the modern vector for arc rotation, ~3.5 mm/yr along a northeasterly azimuth (Wells and McCaffrey, 2013), is comparable to both the rate (2.6 mm/yr) and direction of the first ~8 Myr of migration, from Indian Creek to the Hannegan caldera. The accelerated migration of the past 3.7 Myr requires an added velocity component which Wells and McCaffrey (2013) ascribed to slab rollback, as is occurring in the modern arc (Schellart et al., 2010). The onset of slab rollback may be related to the separation of the buoyant Explorer plate from the Juan de Fuca plate at ~4 Ma (Riddihough, 1984), which would have had the effect of removing a kinematic constraint on the Juan de Fuca plate, causing an increase in the dip of the slab and enhancing rollback.

The temporal decrease in magma productivity in the Chilliwack batholith, as indicated by the general reduction in intrusive flux through time (Fig. 8), correlates with the ~5-fold decrease in the normal component of the convergence rate from 35 Ma to the present (Verplanck and Duncan, 1987). Trace element data for Chilliwack-Mt. Baker magmas support this explanation, as magmas emplaced after the lull have a more muted “subduction signature” such as lower Ba/Nb, lower B/Be, and higher incompatible element concentrations (e.g., Nb, Th) (Tepper, 2002). These differences point to a declining fluid flux from the slab and decreased mantle melt fractions as subduction slowed and the slab became younger and warmer. Because the Farallon plate was larger, older, and cooler earlier in the history of the arc (Schellart et al., 2010), the slab would have dipped at a steeper angle, which could account for the more westerly location of the earliest-formed Chilliwack plutons (Fig. 2, Fig. 7a). The decrease in convergence rate does not appear to be related to the ~11 Myr lull in Chilliwack magmatism because, as discussed in the next section, this lull appears to be restricted to the Chilliwack-Mt. Baker magmatic focus.
576

## 4. Regional Patterns

South of the Chilliwack-Mt. Baker magmatic focus, few intrusive rocks have yielded dates within the ~25-32 Ma age range (Fig. 8c), in apparent contrast to the Chilliwack batholith.

However, a number of volcanic and volcaniclastic rock units date within this interval, most notably the areally extensive (>400 km²) Ohanapecosh Formation near Mt. Rainier (Schasse, 1987; Tabor et al., 2000; Jutzeler et al., 2014) (Fig. 9). Farther to the south, magma output rates calculated from volcanic rock exposures in the Central and Southern Cascade Arc display a temporal decrease that has also been attributed to the convergence rate decrease (Verplanck and Duncan, 1987; Duncan and Kulm, 1989; Priest, 1990; Sherrod and Smith, 2000; du Bray and John, 2011). However, peak output rates occurred at ~25 to 17 Ma (du Bray and John, 2011), slightly younger than in the Chilliwack batholith (Fig. 8).

The magmatic lull in the Chilliwack-Baker area from ~22 to 11 Ma does not appear to be an arc-scale phenomenon, as plutons were emplaced both to the immediate north and the south of the Chilliwack during this interval (Fig. 9). Our new date of 18.13 ± 0.1 Ma for the Conway phase of Mount Barr batholith, which lies adjacent to the northern margin of the Chilliwack batholith, falls within the ~16-22 Ma range determined by Richards and McTaggart (1976) on the basis of low-precision K-Ar dating. Farther to the north, the Doctors Point pluton was emplaced as recently as 19.3 ± 1.6 Ma (Ray, 1986) and the Rogers Creek intrusive complex has been dated at 16.7 ± 2.7 Ma (Fig. 9, 10; Wanless et al., 1978). Immediately south of the Chilliwack batholith, K-Ar hornblende ages for the Mt. Buckindy pluton range from 14.0 ± 1.8 to 16 ± 0.4 Ma (Tabor et al., 2002 and references therein), and four phases of the Tatoosh intrusive suite in the Mount
599 Rainier area were recently dated at 17.5 to 19.2 Ma by SIMS U-Pb zircon (du Bray et al., 2011).

600 However, with very few volcanic or plutonic rocks falling in the 15 to 5 Ma age range (Fig. 9),
601 the area to the south of the Chilliwack appears to have entered a period of quiescence at about
602 the same time as the re-establishment of Chilliwack magmatism (by ~11 Ma).

603
604 Compared to the Northern Cascades, where the oldest magmas are ~35-36 Ma, a number of K-
605 Ar and $^{40}$Ar/$^{39}$Ar ages of up to ~40 Ma have been reported for volcanic rocks in the central
606 Oregon and Mt. Rainier-Mt. St. Helens regions of the Cascades (Fig. 9; compilations of Sherrod
607 and Smith, 2000; du Bray et al., 2006; du Bray and John, 2011) that place arc initiation at up to
608 ~6 Myr earlier than in the north. However, most of the >35 Ma ages for the Southern and
609 Central Cascades are associated with large uncertainties and many were measured on samples for
610 which existing geochemical evidence is insufficient to establish an arc affinity (e.g., the Goble
611 volcanics; Beck and Burr, 1979; Madsen et al., 2006). New dating and geochemical studies will
612 be required to determine exactly when and where arc magmatism first initiated. Nevertheless,
613 within the resolution provided by currently available data for the Southern and Central Cascades,
614 arc magmatism appears to have initiated at around the same time all along the arc, as far north as
615 the Chilliwack batholith (Fig. 9).

616
617 To the north of the Chilliwack, maximum pluton ages rapidly diminish northwards, indicating
618 progressively more recent onset of arc magmatism (Fig. 9). We propose that this pattern reflects
619 the northerly migration of the Juan de Fuca/Explorer plate system (relative to North
620 America) to its current position (Fig. 10). Although the northern edge of the Explorer plate
621 offshore is conventionally placed at the termination of the Queen Charlotte transform fault (as
shown in Fig. 1), recent detailed geophysical surveys (Rohr and Tryon, 2010; Rohr, 2015) have shown that the Revere-Dellwood fracture represents an offset continuation of the Queen Charlotte transform and that the Winona basin is an isolated fragment of oceanic crust that is not being subducted. In this model, the magmatic centers of the Tuzo Wilson and Dellwood Knolls are not true spreading centers but the result of localized extensional regimes in the zone between the two strands of the transform (Fig. 10). Consequently, we place the northern edge of the offshore plate system at the southern end of the Winona basin in the vicinity of the Brooks Peninsula, a conclusion that is also supported by seismic imaging (Cassidy et al., 1998; Audet et al., 2008). Magnetic anomalies indicate that the northern edge of the plate system has remained stable in its current position for the past ~6.5 Ma (Riddihough, 1984). However, the magnetic anomalies needed to reconstruct the position of the triple junction prior to that time have been subducted (e.g., Atwater and Molnar, 1973; Wilson, 1988; Wright et al., 2016). The only constraints provided by extant anomalies are that the Juan de Fuca-Pacific spreading ridge extended at least as far north as the latitude of central Oregon at ~42 Ma (Wells et al., 2014), the Washington-Oregon boundary at ~38 Ma, and southern Vancouver Island by ~21 Ma (Atwater and Molnar, 1973). Several reconstructions of the triple junction have been proposed based on occurrences of magmas possibly related to slab windows (Breitsprecher et al., 2003; Madsen et al., 2006; Wright et al., 2016) and models for the formation and accretion of the Siletz terrane (McCrory and Wilson, 2013; Wells et al., 2014). However, these reconstructions differ significantly in their placement of the triple junction through time relative to the coastline. We argue that the best temporal record of the position of the subducting plate relative to the North America plate is given by the ages of Northern Cascade Arc magmas. The northerly younging of arc plutons is consistent with the model in Fig. 10, which shows the Farallon plate present
beneath all of the Cascades as far north as the Chilliwack batholith by ~35 Ma. Farther north at
Mt. Meager, the new 6.46 Ma date for the Fall Creek stock indicates that the subducting plate
only arrived beneath the northern Pemberton Belt nearly 30 Myr later. The motion of the Juan
de Fuca plate relative to the North American plate based upon the slab migration contours shown
in Fig. 10 is ~1 cm/year, consistent with the detailed plate model of Madsen et al. (2006).

We present a refined magmatic chronology for silicic plutons of the Northern Cascade Arc from
Snoqualmie batholith in central Washington to Fall Creek in southern B.C., with emphasis on
samples from the Chilliwack batholith-Mount Baker magmatic focus. The new LA-ICP-MS U-
Pb zircon dates are associated with significantly smaller uncertainties than previous age
determinations, and many differ outside of analytical error from previously published ages. The
new dates provide evidence for major variations in magmatic flux over the lifetime of the
Chilliwack-Mt. Baker magmatic focus and demonstrate that the southwesterly migration pattern
first identified in the ~3.7 Ma to present interval between the emplacement of the Hannegan
caldera and the modern Mt. Baker volcanic field began ~7 Myr earlier. New dates for other
plutons of the Pemberton Belt constrain the location of the northern edge of the Juan de
Fuca/Explorer plate system (relative to North America) over the last 35 million years. Our main
conclusions are as follows:

(1) The highest intrusive flux is associated with the magmatic flare-up of Phase 1 (30-35 Ma).
Phase 1 intrusions are largely located in the western part of the arc and extend from Post Creek
stock, the northernmost pluton in the Chilliwack batholith, to Thornton Creek, the largest and
668 southernmost pluton. Most of the intrusions are concentrated in a north-south trending belt
669 straddling the Straight Creek fault. South of the Chilliwack batholith, Phase 1 units include the
670 Index batholith, Cascade Pass dike, and Squire Creek stock.

671
672 (2) Slightly lower-flux magmatism persisted uninterrupted throughout Phase 2 (30-22 Ma),
673 during which the magmatically active zone in the Chilliwack batholith broadened to the east.
674 Phase 2 includes the Snoqualmie and Grotto batholiths to the south and the Silver Creek and
675 Hicks Lake plutons to the north of the Chilliwack batholith.

676
677 (3) The ~13 Myr period of essentially continuous activity of Phases 1 and 2 was followed by a
678 nearly 11 Myr period of dormancy (lull) in the Chilliwack-Mt. Baker magmatic focus, during
679 which magmatism was active only to the north and south of the Chilliwack batholith. New dates
680 confirm that the Mt. Barr batholith, located immediately north of the Chilliwack, and the Tatoosh
681 magmatic suite south of the Chilliwack, formed during the lull.

682
683 (4) Post-lull magmatism (Phase 3) resumed in the east-center of the Chilliwack batholith with the
684 Indian Creek pluton at 11 Ma. However, Phase 3 magmatism was intermittent and the intrusive
685 flux was reduced by a factor of five. Phase 3 magmas define a striking pattern of ~45 km of
686 southwesterly migration over ~11 Myr, extending the pattern recorded within the 3.7 Ma to
687 present Mt. Baker-Lake Ann stock-Hannegan caldera suite. Arc rotation since the Miocene can
688 account for the rate and direction associated with the first ~7 Myr of migration. Doubling of the
689 migration rate at ~4 Ma coincided with separation of the Explorer plate and the onset of Juan de
690 Fuca plate rollback.
691
692 (5) The oldest dates for Cascade Arc magmatism rapidly decrease to the north of the Chilliwack batholith. This pattern is consistent with reflecting the northerly migration of the northern edge of the Farallon-Juan de Fuca-Explorer plate system relative to the North American plate. From the Chilliwack batholith to the south, the earliest magmatism is consistently in the ~35-40 Ma range, indicating that Farallon plate subduction was well-established along the entire arc by this time.
693
694
695
696
697

698
699 (6) A general decrease in intrusive flux through time mirrors the pattern defined by volcanic rocks in the Central and Southern Cascades. These observations are consistent with the progressive reduction in the convergence rate of the subducting plate, which has become younger and hotter at the trench, and therefore more resistant to subduction, through time.
700
701
702
703

704 ACKNOWLEDGEMENTS

705 We wish to thank C. Fonquernie (LMV) for assistance with zircon separations and J.-M. Hénot
706 (LMV) for cathodoluminescence imaging. We appreciate the thorough reviews of Associate
707 Editor C. McFarlane and an anonymous reviewer, and thank A. Polat for editorial handling. We
708 gratefully acknowledge financial support from LabEx ClerVolc and a Région Auvergne
709 fellowship to E.K.M. This is LabEx ClerVolc contribution 277.
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1099 Westgate, J. A., Easterbrook, D. J., Naeser, N. D. and Carson, R. J. (1987) Lake Tapps tephra:
1100 An early Pleistocene stratigraphic marker in the Puget Lowland, Washington. Quaternary
1101 Research 28, 340-355.
1102
1103 Wiedenbeck, M., Allé, P., Corfu, F., Griffin, W.L., Meier, M., Oberli, F., von Quadt, A.,
1104 Roddick, J.C. and Spiegel, W. (1995) Three natural zircon standards for U-Th-Pb, Lu-Hf, trace
1105 element and REE analyses. Geostand. Newsl. 19(1), 1-23.
1106
1107 Wilson, D.S. (1988) Tectonic history of the Juan de Fuca ridge over the last 40 million years.
1108 Journal of Geophysical Research 93, 11863-11876.
1109
1110 Wright, N.M., Seton, M., Williams, S.E. and Muller, R.D. (2016) The Late Cretaceous to recent
1111 tectonic history of the Pacific Ocean basin. Earth Science Reviews 154, 138-173.
1112
1113 Woodsworth, G.J. (1977) Pemberton (92J) map area, Geological Survey of Canada Open File

1114 Report 482, scale 1:250,000.

1115
1116 Woodsworth, G.J., Anderson, R.G., and Armstrong, R.L. (1991) Plutonic regimes, Chapter 15 in

1117 Geology of the Cordilleran Orogen in Canada, H. Gabrielse and C.J. Yorath (eds.); Geological Survey of Canada, Geology of Canada, No. 4, p. 491-531.

1119
1120 Zelt, B. C., Ellis, R. M. and Clowes, R. M. (1993) Crustal velocity structure in the eastern Insular

1121 and southernmost Coast Belts, Canadian Cordillera. Canadian Journal of Earth Science 30, 1014-
1122 1027.
<table><caption>Table 1. Samples and Dating Results</caption><thead><tr><th><sup>1</sup>Rock unit</th><th>Sample #</th><th>Latitude (°N)</th><th>Longitude (°W)</th><th>Rock type</th><th><sup>2</sup>Previous dates (Ma)</th><th>Previous dating method</th><th><sup>3</sup>Reference</th><th>Date (Ma) from this study</th></tr></thead><tbody><tr><td colspan="9"><strong>Chilliwack-Mt. Baker magmatic focus</strong></td></tr><tr><td>Kulshan caldera</td><td>08-MB-124</td><td>47°12.90'</td><td>122°13.13'</td><td>rhyodacite tephra</td><td>1.149 ± 0.010</td><td><sup>40</sup>Ar/<sup>39</sup>Ar plag</td><td>6</td><td><strong>1.165 ± 0.013</strong></td></tr><tr><td rowspan="2">Lake Ann: central upper</td><td>06-MB-70</td><td>48°49.794'</td><td>121°39.261'</td><td>bio-hbl grdi</td><td rowspan="2">2.75 ± 0.13</td><td rowspan="2"><sup>40</sup>Ar/<sup>39</sup>Ar bio</td><td rowspan="2">6</td><td><strong>2.344 ± 0.021</strong></td></tr><tr><td>06-MB-68</td><td>48°49.823'</td><td>121°38.700'</td><td>bio-hbl grdi</td><td><strong>2.430 ± 0.016</strong></td></tr><tr><td>Mineral Mountain</td><td>RWT269-87</td><td>48°50.2'</td><td>121°27.2'</td><td>bio granite</td><td>~7.3 to 8.1</td><td>multi-grain U-Pb zircon</td><td>5</td><td><strong>7.83 ± 0.06</strong></td></tr><tr><td>Whatcom Camp</td><td>CB87-50</td><td>48°52.2'</td><td>121°22.3'</td><td>hbl-bio granite</td><td>--</td><td></td><td></td><td><strong>9.538 ± 0.065</strong></td></tr><tr><td>Mt. Sefrit</td><td>CB82-103</td><td>48°53.7'</td><td>121°35.1'</td><td>hbl qz diorite</td><td>22.86 ± 0.05</td><td>Rb-Sr isochron</td><td>3</td><td><strong>10.27 ± 0.06</strong></td></tr><tr><td>Indian Creek</td><td>CB87-37</td><td>48°56.4'</td><td>121°18.2'</td><td>hbl qz mzdi</td><td>12.1 ± 0.8</td><td>K-Ar bio</td><td>7</td><td><strong>11.33 ± 0.08</strong></td></tr><tr><td>Whatcom Arm</td><td>CB87-33</td><td>48°51.7'</td><td>121°22.3'</td><td>hbl grdi</td><td>--</td><td></td><td></td><td><strong>22.75 ± 0.17</strong></td></tr><tr><td>Radium Peak stock</td><td>15-PB-01</td><td>49°02.436'</td><td>121°24.133'</td><td>bio-hbl grdi</td><td>27 ± 2</td><td>K-Ar bio</td><td>4</td><td><strong>27.38 ± 0.12</strong></td></tr><tr><td>Paleface stock</td><td>15-PB-02</td><td>49°02.919'</td><td>121°24.273'</td><td>bio-hbl grdi</td><td>--</td><td></td><td></td><td><strong>28.28 ± 0.15</strong></td></tr><tr><td>Western tonalite</td><td>15-PB-10</td><td>49°04.944'</td><td>121°31.736'</td><td>tonalite</td><td>--</td><td></td><td></td><td><strong>28.76 ± 0.14</strong></td></tr><tr><td rowspan="2">Eastern tonalite</td><td>CB89-1</td><td>49°03.2'</td><td>121°17.2'</td><td>bio-hbl grdi</td><td rowspan="2">30, 29 ± 2</td><td rowspan="2">K-Ar bio</td><td rowspan="2">4</td><td><strong>29.43 ± 0.16</strong></td></tr><tr><td>15-PB-06</td><td>49°05.062'</td><td>121°25.666'</td><td>bio-hbl tonalite</td><td><strong>29.41 ± 0.13</strong></td></tr><tr><td>Eastern tonalite: Maselpanik lobe</td><td>15-PB-15</td><td>49°07.201'</td><td>121°15.017'</td><td>hbl-bio tonalite</td><td></td><td></td><td></td><td><strong>31.93 ± 0.16</strong></td></tr><tr><td>Hannegan Trail</td><td>CB82-151</td><td>48°53.8'</td><td>121°33.6'</td><td>porph hbl-bio tonalite</td><td>8.7</td><td>zircon FT</td><td>3</td><td><strong>31.95 ± 0.24</strong></td></tr><tr><td>Thornton Creek</td><td>TH-20</td><td>48°37.98'</td><td>121°19.50'</td><td>qz diorite</td><td>~29-35</td><td>K-Ar, multi-grain U-Pb and FT zircon</td><td>5</td><td><strong>32.02 ± 0.16</strong></td></tr><tr><td>Rexford stock</td><td>15-PB-11</td><td>49°03.089'</td><td>121°34.423'</td><td>bio-hbl diorite</td><td>27 ± 2</td><td>K-Ar bio</td><td>4</td><td><strong>32.34 ± 0.17</strong></td></tr></tbody></table>
<table><tr><td>Copper Lake</td><td>CB87-112</td><td>48°54.8'</td><td>121°27.3'</td><td>hbl qz diorite</td><td>34.03 ± 0.91</td><td>Rb-Sr isochron</td><td>3</td><td><b>33.96 ± 0.18</b></td></tr><tr><td>Post Creek stock</td><td>CB87-10</td><td>49°05.7'</td><td>121°26.3'</td><td>hbl-bio qz monzonite</td><td>27 ± 2</td><td>K-Ar bio</td><td>4</td><td><b>34.74 ± 0.24</b></td></tr><tr><td>Copper Ridge Lookout</td><td>CB87-116</td><td>48°54.5'</td><td>121°27.8'</td><td>hbl-bio tonalite</td><td>--</td><td></td><td></td><td><b>86.91 ± 0.43</b></td></tr></table>

North of Chilliwack batholith

<table><tr><td>Fall Creek stock</td><td>FCS</td><td>50°40.91'</td><td>123°30.67'</td><td>qz monzonite</td><td>10.1 ± 1.2</td><td>K-Ar bio</td><td>9</td><td><b>6.646 ± 0.046</b></td></tr><tr><td>Mount Barr batholith: Conway phase</td><td>15-PB-13</td><td>49°15.462'</td><td>121°40.760'</td><td>hbl-bio qz monzonite</td><td>18.5 ± 1.8</td><td>K-Ar bio</td><td>11</td><td><b>18.13 ± 0.10</b></td></tr><tr><td>Silver Creek stock</td><td>15-PB-14</td><td>49°14.920'</td><td>121°36.175'</td><td>bio-hbl qz diorite</td><td>36 ± 4</td><td>K-Ar hbl</td><td>4</td><td><b>23.77 ± 0.10</b></td></tr><tr><td>Hicks Lake pluton</td><td>15-PB-12</td><td>49°21.722'</td><td>121°42.683'</td><td>hbl granite</td><td>25 ± 1.4</td><td>K-Ar bio</td><td>4</td><td><b>26.23 ± 0.16</b></td></tr></table>

South of Chilliwack batholith

<table><tr><td>Cascade Pass dike</td><td>15-PB-16</td><td>48°28.642'</td><td>121°04.198'</td><td>porph hbl-bio qz diorite</td><td>18.5 ± 3, 18.8 ± 2</td><td>K-Ar hbl, K-Ar bio</td><td>8</td><td><b>32.78 ± 0.28</b></td></tr><tr><td>Index batholith</td><td>ISM-IX<br>CC11-26</td><td>47°48.281'</td><td>121°31.249'</td><td>hbl-bio grdi</td><td>~33-36</td><td>K-Ar, Rb-Sr, multi-grain U-Pb and FT zircon</td><td>8</td><td><b>33.26 ± 0.19</b></td></tr><tr><td>Squire Creek stock</td><td>15-PB-19b</td><td>47°47.878'</td><td>121°30.875'</td><td>tonalite</td><td>~33-36</td><td>K-Ar, multi-grain U-Pb and FT zircon</td><td>8</td><td><b>33.53 ± 0.15</b></td></tr><tr><td>Grotto batholith</td><td>16-EM-01</td><td>47°43.031'</td><td>121°23.365'</td><td>sulfide-bearing bio-hbl tonalite</td><td>~33-36</td><td>K-Ar hbl</td><td>12</td><td><b>35.84 ± 0.16</b></td></tr><tr><td>Snoqualmie batholith, northern sector</td><td>16-EM-03</td><td>47°39.677'</td><td>121°22.889'</td><td>bio tonalite</td><td>25.7 ± 3 Ma</td><td>K-Ar hbl</td><td>12</td><td><b>24.14 ± 0.45</b></td></tr><tr><td></td><td></td><td></td><td></td><td></td><td>23.0 ± 1.2 Ma</td><td>K-Ar hbl</td><td>12</td><td><b>23.87 ± 0.15</b></td></tr></table>

1. Chilliwack batholith nomenclature follows the usage of Tepper et al. (1993) and Tepper (1996, 1991) in Washington and Richards and McTaggart (1976) in British Columbia.

2. Most recent best age estimate or measured age (or range), with 2σ errors listed on individual measurements (where reported in the original reference). Where necessary, K-Ar dates have been corrected to 1976 IUGS constants following Dalrymple (1979).

3. Tepper (1991), 4. Richards and McTaggart (1976), 5. Tabor et al. (2003), 6. Hildreth et al. (2003), 7. Mathews et al. (1981), 8. Tabor et al. (2002), 9. Stevens et al. (1982), 10. Ray (1986), 11. Baadsgaard et al. (1961), 12. Tabor et al. (1993)

Abbreviations: porph – porphvritic, αz – quartz, bio – biotite, hbl – hornblende, plag = plagioclase, grdi – granodiorite, mzdi – monzodiorite, FT – fission track.
1123 FIGURE CAPTIONS

1124

**Figure 1.** (a) Geological elements of the Cascade Arc. The area in the gray box is enlarged in (b). Map showing location of magmatic rocks in the Northern Cascade Arc (Mt. Rainier to the north). The area enclosed by the red box (the Chilliwack-Mt. Baker magmatic focus) is shown on an expanded scale in Figure 2. Juan de Fuca and Gorda plate configurations are from McCrory et al. (2014); Explorer plate from Rohr and Tryon (2010) and Rohr (2015).

1130

**Figure 2.** Map of the Chilliwack-Mount Baker magmatic focus, based on the maps of Tabor et al. (2003), Hildreth et al. (2003), Monger (1989), Richards and McTaggart (1976), and Tepper (1991). Magmatic rocks older than 1 Ma are color-coded in three phases based on the new dates, augmented by dates from previous studies (Tabor et al., 2003 and references therein). Note that the sample dated from the Kulshan caldera was collected farther south near Lake Tapps, as discussed in the main text (Fig. 1b). The 31.95 Ma Hannegan Trail unit is too small to depict on the map but the sample locality is indicated. Southwesterly migration of the magmatic focus from ~11 Ma to present is shown schematically by the heavy black arrow extending from the Indian Creek pluton through the Hannegan caldera to Mt. Baker. Basement rocks of the NW Cascades thrust system are shown in light purple, the Cascade Core in light orange, and the Methow-Pasayten Belt in light green.

1142

**Figure 3.** Tera-Wasserburg $^{207}$Pb/$^{206}$Pb vs. $^{238}$U/$^{206}$Pb diagrams for samples from the Chilliwack-Mt. Baker magmatic focus. Each ellipse represents the 2σ error associated with a single zircon spot analysis. Data are not corrected for common Pb. Lower intercept ages and Concordia ages (also listed in Table 1) were calculated using the filled gray ellipses ("n" is the
1147 number of analyses). MSWD is the mean square of weighted deviates; for Concordia ages, it
1148 includes both equivalence and concordance (indicated by subscript “C+E”). Data points lying
1149 outside the main cluster of data are shown as open dashed ellipses and were excluded from age
1150 calculations. Filled red data points are inherited grains. Red points in (a) are from two inherited
1151 grains; those in (b) are from a single inherited grain.

1152
1153 **Figure 4.** Tera-Wasserburg $^{207}$Pb/$^{206}$Pb vs. $^{238}$U/$^{206}$Pb diagrams for intrusions north of the
1154 Chilliwack batholith. Symbols and abbreviations are the same as in Figure 3 caption.

1155
1156 **Figure 5.** Tera-Wasserburg $^{207}$Pb/$^{206}$Pb vs. $^{238}$U/$^{206}$Pb diagrams for intrusions south of the
1157 Chilliwack batholith. Symbols and abbreviations are the same as in Figure 3 caption.

1158
1159 **Figure 6.** Cathodoluminescence images of representative dated zircon grains from (a, b) the
1160 Lake Ann stock (sample #06-MB-70) and (c-f) the Fall Creek stock (sample #FCS). Circles
1161 indicate analyzed spots and corresponding numbers refer to analytical points in Table S2.

1162
1163 **Figure 7.** Dates for the Chilliwack-Mt. Baker magmatic focus versus (a) longitude, (b) latitude.
1164 Dates from this study are shown as red filled circles, except for inherited zircons shown as red
1165 asterisks. 2σ error bars are smaller than symbols. Previous age determinations are shown as
1166 filled gray circles with corresponding 2σ error bars (Tabor et al., 2003 and references therein;
1167 Hildreth et al., 2003; Tucker et al., 2007).

1168
1169 **Figure 8.** (a) Stacked histogram of the areas (km²) defined by Northern Cascades intrusive
1170 rocks, binned into 1 million year intervals. Pluton areas are tabulated in Table S3. Gray line is
1171 the convergence rate of the Farallon plate system (cm/yr) from Verplanck and Duncan (1987)

1172 with scale on the right-side y axis. (b-d) Same data for pluton areas shown in (a) but subdivided

1173 into three regions: (b) Chilliwack-Mt. Baker magmatic focus, (c) South of Chilliwack, and (d)

1174 North of Chilliwack. The area covered by <1 Ma volcanic rocks of Mt. Baker is shown for

1175 reference in yellow in (b). Note the different y-axis scales associated with each panel. Although

1176 few intrusive rocks south of the Chilliwack batholith date between ~25 and 32 Ma (panel c), a

1177 number of volcanic and volcaniclastic rock units lie in this range, most notably the Ohanapecosh

1178 Formation, as indicated by the horizontal black arrow (Jutzeler et al., 2014; Tabor et al., 2000;

1179 Schasse, 1987; Evarts et al., 1987).

1180
1181 **Figure 9.** New dates from this study (red circles) vs. latitude; 2σ error bars are smaller than

1182 symbols. Vertical dashed line at 4 Ma distinguishes the modern from the ancestral Cascades.

1183 Other radiometric age determinations for Cascade Arc volcanic rocks (yellow diamonds) and

1184 plutonic rocks (pink circles) are shown with 2σ error bars where reported and are from Berman

1185 and Armstrong (1980), du Bray et al. (2011a), Green et al. (1988), Hildreth et al. (2003),

1186 Hildreth and Lanphere (1994), Jicha et al. (2009), Jutzeler et al. (2014), Mathews et al. (1981),

1187 Mattinson (1977), Ray (1986), Reiners et al. (2000), Richards and McTaggart (1976), Tucker et

1188 al. (2007), Wanless et al. (1978), Evarts et al. (1987), Tabor et al. (2003, 2002, 2000, 1993),

1189 Schasse (1987), Phillips (1987), and du Bray and John (2011). K-Ar dates are corrected to 1976

1190 IUGS constants (following Dalrymple, 1979) as necessary.

1191
1192 **Figure 10.** Map of northern Washington (WA) and southern British Columbia (B.C.) showing

1193 intrusive bodies (pink) and modern volcanic centers (red triangles) with a superimposed slab
1194 migration model. Dashed red contours (labeled 35, 25, 15 and 5 Ma) are based on the maximum
1195 ages of intrusive bodies in each sector (see Figs. 3-5 for age details) and record the temporal
1196 location of the northern edge of the subducted slab relative to the North American plate (NAM).
1197 Major intrusions and volcanic complexes are numbered: 1. Tatoosh, 2. Snoqualmie, 3. Index, 4.
1198 Grotto, 5. Squire Creek, 6. Cloudy Pass, 7. Cascade Pass, 8. Chilliwack, 9. Mt. Barr, 10. Silver
1199 Creek, 11. Harrison Lake, 12. Hicks Lake, 13. Podunk Creek, 14. Coquihalla complex, 15.
1200 Doctors Point, 16. Rogers Creek, 17. Fall Creek, 18. Salal Creek. Spreading centers, transform
1201 faults, and trenches are based on Botros and Johnson (1988), Rohr (2015), and McCrory et al.
1202 (2014). In this model (after Rohr, 2015), the Explorer-North American-Pacific triple junction is
1203 a zone of distributed shear between the strike-slip Queen Charlotte fault (QCF) and Dellwood-
1204 Revere fault (DRF). Seismic tomography places the present-day northern edge of the Explorer
1205 plate at the Brooks Peninsula Fault Zone (BPFZ), which extends inland from the southern
1206 boundary of the Winona basin (WB) (Audet et al., 2008) and beneath the Alert Bay volcanic
1207 field (AB). Modern volcanic centers: ST: Silverthrone, FG: Franklin Glacier, BR: Bridge River,
1208 SG: Salal Glacier, MM: Mt. Meager, MC: Mt. Cayley, MG: Mt. Garibaldi, MB: Mt. Baker, GP:
1209 Glacier Peak, MR: Mt. Rainier. Other abbreviations: V: Vancouver, S: Seattle, NFZ: Nootka
1210 Fault Zone, SFZ: Sovanco Fracture Zone, JDF: Juan de Fuca plate, PAC: Pacific plate, EXP:
1211 Explorer plate.
<figure><img src="image_2.png" /><figcaption>Figure 1</figcaption></figure>

139x107mm (300 x 300 DPI)
<figure><img src="image_3.png" /><figcaption>Figure 2</figcaption></figure>

180x176mm (300 x 300 DPI)
<figure><img src="image_4.png" /><figcaption>Kulshan caldera
rhyodacite tephra #08-MB-124</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Lake Ann stock
#06-MB-70</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>(d)</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Mineral Mountain
#RWT269-87</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Whatcom Camp granite
#CB87-50</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Mt. Sefrit diorite
#CB82-103</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Indian Creek monzodiorite
#CB87-37</figcaption></figure>

<figure><img src="image_11.png" /></figure>
Thornton Creek diorite
#TH-20

Canadian Journal of Earth Science

Copper Lake diorite
#CB87-112

207Pb/206Pb

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

206Pb/207Pb

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

238U/206Pb

204Pb/206Pb

<figure><img src="image_16.png" /></figure>
<figure><img src="image_17.png" /><figcaption>Figure 4
124x99mm (300 x 300 DPI)</figcaption></figure>
<figure><img src="image_18.png" /><figcaption>Figure 5</figcaption></figure>

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<figure><img src="image_19.png" /><figcaption>Figure 6
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<figure><img src="image_20.png" /><figcaption>Figure 7</figcaption></figure>

78x33mm (300 x 300 DPI)

<figure><img src="image_21.png" /></figure>
<figure><img src="image_22.png" /><figcaption>Figure 8</figcaption></figure>

95x50mm (300 x 300 DPI)
<figure><img src="image_23.png" /><figcaption>Figure 9</figcaption></figure>

170x310mm (300 x 300 DPI)
<figure><img src="image_24.png" /><figcaption>Figure 10</figcaption></figure>

132x95mm (300 x 300 DPI)