Magma Loading in the Southern Coast Plutonic Complex , British Columbia and Washington

1.1. Setting and Statement of the Problem. The Coast Plutonic Complex (CPC) of western North America is a coast margin orogenic welt comprised of granitic plutons ranging in age from ~170 to 45Ma, intruded into metamorphosed country rock (e.g., [1, 2]). The orogen, some 1800 km long and variable in width from ~50 to 150 km, extends from southeast Alaska to northwest Washington (Figure 1). This belt is one of the Earth’s great mountain chains, ranking with the Alps, Himalayas, Andes, and Appalachians. How did it form? The growth of this mountain belt is related to the convergence and subduction of the Farallon oceanic plate against the western margin of the Laurentian-North American plate. The CPC is a magmatic arc, but its history involves great crustal thickening, up to 40 km, suggesting a thrust loading component during its development as many geologists have concluded (e.g., [3]). However, for the southern CPC, a case can be made that magmatism is by far the dominant loading process, proposed by Brown and Walker [4] and further addressed by Brown and McClelland [5] and Brown et al. [6]. This report is an update, based on field and laboratory data carried out by numerous workers since publication of the previous papers. The earlier work is summarized here, but the reader looking for more background detail is directed to these studies. The geologic history of the southern segment of the CPC potentially applies to the 1700 km northerly part of this great orogenic belt and may be a significant crustal thickening process of magmatic arcs elsewhere on Earth. An example is the northern Andean orogen for which both tectonic and magmatic processes have been invoked as the major crustal growth mechanism, but the relative significance of these two processes is debated ([7], and references within). Good access and intriguing geology of the CPC in the Pacific Northwest have attracted many geologists in recent years. Besides mapping and other outcrop study, this work in the past 20 years has yielded a considerable number of radiometric ages and pressure-temperature values providing further constraints on the evolution of the orogen. GeoScienceWorld Lithosphere Volume 2020, Article ID 8856566, 17 pages https://doi.org/10.2113/2020/8856566

In the Pacific Northwest, the CPC is split by the Fraser River-Straight Creek strike-slip fault active at~45 Ma (Figures 1 and 2). Thus, we have two domains to work on: the "Crystalline Core" of the North Cascades in Washington and the Harrison Lake area in British Columbia. The CPC geology is mid-to Late Cretaceous in age and was mutually continuous in the two regions prior to displacement along the fault.
The orogen during the time frame of plutonism and country rock metamorphism (Figures 3-6) was outboard from the western edge of the Quesnellia terrane and inboard of the accreted Insular Belt [2]. The southern Insular Belt, which includes the western part of the southern Coast Plutonic Complex as well as basement rocks of Vancouver Island, is interpreted to have been displaced southward some 800 km during Late Jurassic-Early Cretaceous time based primarily on matching of age-distinctive magmatic belts [1,8]. The precise location of this fault zone in the southern Coast Plutonic Complex is not identified but lies somewhere in the region west oHarrison Lake where relatively older pluton ages (>100 Ma) are found.
Plutons ranging up to 600 km 2 in an aerial extent occur in clusters and as separate masses encased in country rock. The plutons being more resistant to erosion than country rock stand out in alpine exposures. Country rock along the western side of the Harrison Lake area is the mostly Jurassic to mid-Cretaceous metamorphosed volcanic and sedimentary rock of the Harrison-Gambier assemblage considered to be part of the CPC magmatic suite. Country rock units along the eastern side of Harrison Lake include the Harrison-Gambier assemblage and also the Late Jurassic-Early Cretaceous Cogburn mafic schists and Settler pelitic schists repre-senting ocean crust and overlying continent-derived sediment, respectively [9]. In the Cascades, mafic schists of ocean floor origin together with ocean arc deposits, named the Chelan Mountains terrane, correlate with the Cogburn schists. A structurally overlying unit of pelitic schists is the Nason terrane in the Cascades correlative with the Settler terrane in British Columbia ( Figure 2). Equivalents of the Harrison-Gambier unit are not found in the Cascades, nor is the 70-45 Ma Skagit Gneiss of the Cascades found in the Harrison Lake area. Great depth of burial and subsequent exhumation of the pluton-country rock complex are documented by mineralogy in pluton aureoles. A shallow initial contact metamorphism caused by plutons, followed by burial, is evidenced by relict andalusite+biotite assemblages in pluton aureoles that are overprinted with high-pressure assemblages including kyanite. The andalusite in pluton aureoles is not isotopically dated but is reasonably interpreted to be close in age (less than a million years) to the dated plutons considering that andalusite occurs in narrow bands following the pluton  Adapted from Monger and Journeay [58].  2 Lithosphere margins [5] and that the plutons themselves are the only igneous rock in the vicinity to provide the high T/P conditions for crystallization of andalusite (discussed below). Pressuresensitive mineralogy in the plutons, especially Al-rich hornblende, indicates pressures up to 10-11 kb, equivalent to 40 km burial during pluton emplacement (Figures 4 and 7) [5,6]. The high pressures are a consequence of deep burial, of which the mechanism is the problem of interest here. One interpretation is burial by thrusting or other regional crustal contraction (e.g., [10,11]). Supporting this concept, the CPC arc is in a convergent zone between the continental North American plate and the oceanic Farallon plate. Thrust faults have been mapped in northern parts of the CPC (e.g., [12]).

Lithosphere
How is this finding reconciled? Many workers over the past few decades have gathered relevant field and laboratory data. Mapping of Barrovian metamorphic zones shows a general linking of high-grade metamorphic country rock with plutons (e.g., [13]), an exception being the Swakane Gneiss ( Figure 6). Pressure-sensitive mineral assemblages reported in Whitney [14], Brown and Walker [4], and Stowell et al. [15] define depths of metamorphism and coupled with isotopic ages identify relatively specifically the time and place of burial (Figures 3 and 6). This evidence makes a connection of plutonism with country rock loading and indicates a localization and migration of the burial events. A magma loading model based on these findings was proposed by Brown and Walker [4] and followed by a later study of Brown and McClelland [5] (Figure 8).

Identifying the Burial Events.
Depth of burial is determined by calculated pressure of crystallization of mineral assemblages at a ratio of 1.0 kilobar to 3.7-kilometer depth, based on the density of average granitic crust. Pressures thus determined yield values ranging up to 11 kilobars (40.7 km) for metamorphic rocks and plutons. If crustal density was less in a relatively hot active arc, the depth for a given pressure would be greater. For metamorphic rocks, peak burial depths are derived from the garnet-biotite-muscovite-plagioclase (GAMI) and garnet-aluminum silicate-plagioclase (GASP) assemblages (both from [16]). Cooling ages (interpreted as exhumation) are determined from K/Ar, Ar-Ar, and Sm-Nd in micas and hornblende [17,18]. Andalusite together with biotite indicates pressures less than 3.0 kb and temperature greater than 600 degrees C [19], conditions indicative of crystallization in a shallow magma chamber at a considerably higher geothermal gradient than that associated with tectonic burial (~50 vs. 25 degrees/km). For plutons, pressure is based on the aluminum content of hornblende [20]. Ages of plutons are derived from U-Pb zircon ages. These data are compiled in Figures 3, 6, and 7 and represent the work of many geologists.
The burial and exhumation history across the orogen reflects a crustal thickening process by magmatism as opposed to regional thrusting. Pressures are variable in space and time (Figures 7 and 8). High-pressure mineralogy overprints low-pressure-high-temperature assemblages marked by andalusite, now pseudomorphed. Peak pressures are dated to range from~65 to 100 Ma at different localities across the orogen. Uplift ages vary also, from~55 to 80 Ma. Across the region, processes of burial and subsequent uplift were localized and shifted from one place to another.

Harrison Lake Area
Bedrock geology on the east side of Harrison Lake of interest in this analysis includes the Harrison Lake-Gambier, Settler, and Cogburn country rock terranes [2,9]. The Harrison Lake-Gambier terrane is westernmost of these terranes, occurring as part of the Coast Plutonic Complex. It is composed of arc-related Jurassic-Early Cretaceous sedimentary and volcanic rocks and associated batholithic plutons. To the east, the Harrison Lake-Gambier terrane is faulted against the Cogburn and Settler terranes. The Cogburn is an ophiolitic assemblage of chert, argillite, greywacke, mafic volcanic rocks, gabbro, and ultramafic rock [21] (Figures 9  and 10). The Settler Schist is a psammitic to pelitic assemblage of sedimentary rocks; derivation from the continental margin is suggested.
Assembly of the terranes is estimated to have occurred between the~130 Ma maximum age of terrane accretion and the age of the plutons intruded across terrane boundaries [22,23]. The 96 Ma Spuzzum pluton ( Figure 3) establishes a youngest age limit of terrane assembly. However, an argument can be made that terrane assembly occurred prior to intrusion of the Breakenridge pluton (100-107 Ma) considering that distinctive Breakenridge lithology and age are shared by both the Clear Creek and Breakenridge plutons which are intruded into different terranes ( Figure 3).
Orogeny in the Harrison Lake area is characterized by both metamorphism and plutonism, processes in this region that are coeval and mutually intertwined. In the region of interest, a belt some 30-40 km wide along the east side of Harrison Lake exhibits a degree of metamorphism ranging widely from greenschist to upper amphibolite facies. Recorded temperatures range from~300 to 700 degrees C, and more significantly burial ranges from less than 10 km to more than 35 km. As discussed below, the metamorphism is interpreted to be a consequence of the plutonism. External to the main mass and colored the same are satellite bodies including the large Clear Creek pluton (CC), which gives zircon ages [22] in the same range as the main pluton (100-107 Ma). Localities of measured metamorphic pressures are marked by red dots. Data in the northern area are from Lapen [29], and those in the southern area are from Mitrovic [24]. The 8.5-kilobar measurement at the south end of the pluton marks the site of the Figure 12 photograph of the intrusive contact. 4 Lithosphere Three plutons in the Harrison Lake area played a distinctive role in crustal thickening: Breakenridge, Urquhart, and Scuzzy ( Figure 3) [4-6, 22, 24]. The Breakenridge pluton is a folded sheeted body (Figures 4 and 11) interlayered with schist septa of country rock. The pluton exhibits intrusive contacts with country rock marked by migmatitic injection complexes ( Figure 12). Several satellite intrusions of the same lithology occur within a few kilometers of the main body, including the large Clear Creek pluton ( Figure 3) which also displays an intrusive margin. Sheeting is also seen in the Scuzzy pluton by way of belts of country rock outcrop exposure and by a seismic reflection study revealing a country rock layer deep within the pluton body [25] ( Figure 5).
The age of components of the Breakenridge pluton has been considered from zircon U-Pb ages to range from 103 Ma at the south end of the pluton [4] to 96 Ma along the east flank of the body [26]. More recent work by Mitrovic [24] considerably modified this analysis. The 96 Ma age is found to date a separate pluton lying along the northeast margin of the Breakenridge body ( Figure 3), named by Mitrovic as the Snowshoe pluton. The lithologies of the two bodies differ, and a country rock septum lies between the two. An additional 96 Ma age for this flanking pluton was obtained by Gibson and Monger [22]. Mitrovic [24] further clarified the age of intrusion of the Breakenridge pluton, obtaining zircon ages from six samples distributed across the pluton defining a range of 100-107 Ma.
With unfolding of the Breakenridge pluton, the sheet structure is roughly horizontal. In country rock adjacent to the plutonic complex, zoned garnets contain relict andalusite in cores, as well as low Ca garnets, indicating pressures less than 3 kb and elevated temperatures associated with initial plutonism [24,27]. Garnet rims yield pressure values of 8-9 kb ( Figure 13) [6,24]. Metamorphic minerals in schist septa between pluton sheets record pressures up to 10 kb documenting a pluton depth of~37 km (GAMI, GASP) [5,6,24,28,29]. Unloading and exhumation of country rock were well underway at 88 Ma when Ar-Ar ages of hornblende in the Breakenridge pluton were set ( [28]; Figure 3).
A question arises as to when the high P (10 kb) metamorphism of country rock screens within the Breakenridge pluton occurred ( Figure 3). Three measurements of metamorphic pressures within the outcrop limits of the Breakenridge pluton are in the range of 9.8-10.0 kb. Other pressures obtained within the pluton are 8.3 and 8.6 kb, and these values correspond with a number of other measurements 6.9-8.7 kb in country rock external to the pluton ( Figure 3). That the 9.8-10.0 kb values are constrained to the pluton indicates conditions of pluton emplacement. The 8.3 and 8.6 kb values within the pluton apparently represent an imprint of later metamorphism that also is recorded in country rock surrounding the pluton.
Notably, andalusite in country rock at the Breakenridge pluton margin is now replaced by kyanite ( Figure 14(a)), documenting a transition from the original high T/P conditions of initial pluton emplacement to high P/T conditions resulting from some 30 km of overburden of sheeted pluton growth. Andalusite in country rock away from the Breakenridge pluton, along the margin of the shallow Spuzzum pluton ( Figures 3 and 14(b)), remains unaltered and thus is consistent with the interpretation that the high-pressure metamorphism was not regional.
The Snowshoe pluton together with the Ascent Creek and Hornet Creek plutons comprises a band of plutons of similar age (96-98 Ma) that show a degree of metamorphic overprint marked by foliation and northwest-southeast lineations indicating northwest displacement. The band of plutons continues to the southeast including the Hut Creek and Spuzzum plutons, but the metamorphic overprint dies out farther to the southeast within the Hornet Creek pluton and does not affect the Spuzzum pluton which lies within the end of this string of age-related plutons. As noted above,   Figure 6: Geology of the North Cascades. Sources: (a) [46], (b) [65], (d) [54], (e) [42], (f) [30], (g) [43], (h) [66], (j) [33], (l) [48], (m) [50], (n) [67], (p) [36], (q) [68], (r) [49], (s) [15], (t) [55], and (u) [39]. WPT = Windy Pass thrust; WRSZ = White River Shear Zone; RLFZ = Ross Lake fault zone; GPTB = Gabriel Peak tectonic belt; A = pseudomorphed andalusite. Napeequa Schist: 89f and 90f are zircon ages in small plutons, 125q is a detrital zircon age in the Tonga Formation component of the Nason terrane, and 149izq is a zircon age from volcanic strata interlayered with sedimentary rocks in the Nason terrane (Brown, unpublished data). The Diablo Lake inset is described in the text. Note that the view northwest from Lookout Mountain (LM) is seen in the photograph of Figure 18. 6 Lithosphere country rock pressures in the vicinity of the south end of the Breakenridge pluton and the Hut Creek pluton are in the range of 6-8 kb; further southeast pressure diminishes into the stability range of unaltered andalusite at the margin of the Spuzzum pluton in contrast to the pseudomorphed andalusite in the higher-grade zones (Figures 3 and 14). The 91-92 Ma Urquhart pluton is not affected. The tectonic implications of this metamorphism occurring in the time frame after 96 Ma and before 92 Ma are not obvious. Significant regional thrusting and related metamorphism seem unlikely considering that the metamorphism dies out not far south of the Breakenridge pluton and that the metamorphic mineral assemblages of the Breakenridge screens, interpreted to be coeval with plutonism (100-107 Ma), are not reset to a younger metamorphic event.
Contractional folding of the igneous layering and metamorphic foliation in the Breakenridge pluton is defined by a doubly plunging antiformal structure [28]. Related folding in nearby rock extends from the Breakenridge pluton to the edge of the Urquhart pluton. This structural event is coincident with the 96-92 Ma late fabric imprint described above.
Separating the Breakenridge pluton and associated 96-98 Ma plutons from the Urquhart and Scuzzy plutons is the Butter Creek fault (BCF, Figure 3) of dip-slip displacement. Judging from inferred ages and depths of these bodies, initial uplift of the Breakenridge complex and other rocks west of the BCF was on the order of 10 km occurring in the time range of~93-100 Ma, whereas terranes on the east side of the fault at that time were not much displaced.
The Urquhart pluton bears a prominent inward dipping magmatic foliation which is largely concentric following the external shape of the pluton. Steep topography along the south side of the pluton reveals the gently inward dipping pluton floor overlying Settler Schist across hundreds of meters into the pluton body ( Figure 15). Pseudomorphed andalusite is abundant in the pluton aureole, indicating shallow initial intrusion [5]. Measured pressures of crystallization in this pluton based on Al-in-hornblende barometry range from 4 kb at the rim to 6 kb in the interior (Figure 3). In view of the data for low pressures in bounding country rock (<3 kb, pseudomorphed andalusite coexisting with biotite), initial emplacement occurred at depths less than 10 km; subsequently, the pluton grew to a thickness of 20 km over the exposed parts ( Figure 3). Zircon ages dating pluton emplacement are 91 Ma in the core and 92 Ma in the rim [5,30].
The Scuzzy pluton is a relatively large batholith (Figure 3): 20 km wide and >40 km long (northern extent not mapped). Textures throughout the body are igneous, marked by a weak foliation following the ovoid pluton shape. At the margins, pluton sheets are interlaminated with country rock schist ( Figure 16). This sheeted margin is~0.5 km wide on the west side but is much thicker along its eastern margin where seismic reflection study of the pluton ( Figure 5) [25] indicates a basal sheeted structure~2 km thick and a floor dipping 30°inward that is measured across a third of the pluton width. The pluton apparently grew by stacking of horizontal sheets along planes of weakness; the older sheets subsequently sagged as more magma piled on top (Figure 8). Country rock in the pluton aureole hosts pseudomorphed andalusite. Maximum pressures measured in the pluton are 8 kb, equating to 30 km of crustal thickness (Figure 7). Pressures recorded in the aureole are 6 kb. Zircon ages are 86 Ma for the pluton rim [22] and 84 Ma in the pluton core [26].

Lithosphere
Uplift of the Scuzzy pluton on its western flank was apparently to a degree coincident with uplift of the older neighboring plutons of the Ascent Creek-Spuzzum belt, as illustrated in the plot in Figure 7. In this scenario, the pseudomorphed andalusite occurring in the band of rock between the west side of the Scuzzy and the various neighboring 91-95 Ma plutons developed during initial intrusion and along the margin of these older plutons. Subsequent crustal thickening in this area occurred related to ongoing intrusion of the older plutons. The Scuzzy pluton was initially intruded at moderate depth on its western flank at pressure exceeding andalusite stability and at shallow level on the eastern flank where andalusite crystallized in the aureole. In this scenario, if the pluton formed as even sheets, the west side experienced greater uplift than the east.
The Scuzzy and Urquhart plutons are in close contact; the space between them is small, <100 meters wide; and the bedrock is not exposed. Whether this association is a consequence of thrusting of the Urquhart against and over the edge of the Scuzzy or intrusive emplacement is unclear.
For the three plutons discussed here, Breakenridge, Urquhart, and Scuzzy, the internal pluton structure is characterized by an assemblage of originally horizontal pluton sheets, pointing to the origin of these plutons by vertical stacking of igneous layers. An alternative interpretation of horizontal shouldering aside of country rock, as in emplacement models invoking ballooning or diapirism, is not supported.

North Cascades
Orogeny in the Crystalline Core of the North Cascades is, with reconstruction of the Straight Creek-Fraser River fault, an extension of that in the Harrison Lake area. Similarities and differences between the two areas are apparent in the reconstruction of the Cascade geology onto the south end of the Harrison Lake geology. Here, the discussion concerns the Nason terrane, Chelan Mountains terrane, Skagit Gneiss, Swakane Gneiss, and Methow sequence.  (Figures 2 and 6). With restoration of the Fraser River-Straight Creek fault, the Nason terrane is seen to have been coextensive with the Settler Schist in British Columbia [32].
The Nason terrane at the northern edge of the Mt. Stuart Batholith bears andalusite overprinted by kyanite, noted by Evans and Berti [32] who recognized that shallow intrusion of this part of the batholith was followed by high-pressure metamorphism. This metamorphism extends throughout the length of the Nason terrane. In more recent years, other workers have further documented the pressure-time history (Figure 7). Zoned garnets in the Nason terrane at the northern edge of the Mt. Stuart Batholith record a pressure range of <2 to 6 kb from the core to the rim, interpreted to  9 Lithosphere represent contact metamorphism by the batholith followed by regional loading, up to 8 kb in places [4]. Peak metamorphic ages in the Nason terrane in the vicinity of the batholith, determined by garnet Sm-Nd analysis, are 86-88 Ma [15] ( Figure 6, lower left inset). K/Ar cooling ages, marking uplift, are 81-84 Ma for biotite and 83-86 Ma for muscovite [33]. Zircon ages of sills at dispersed localities in the Nason terrane are 91 Ma, 90 Ma, and 89 Ma ( Figure 6) [15,30]. These small plutons are coeval with the high-pressure metamorphic event in the Nason terrane [4].
What was the cause of this high-pressure metamorphism extensive throughout the Nason terrane? Regional thrusting coeval with the metamorphism is not evident. A terrane bounding thrust fault at the south end placed the Ingalls ophiolite over the Nason terrane, the Windy Pass thrust [34] (Figure 6). But this structure is intruded by the Mt. Stu-art Batholith and thus predates the Nason loading event. Another contractional fault in the region is the White River Shear Zone, a reverse fault located along the western margin of the Tenpeak pluton [35]. This structure, steeply dipping and of limited extent along the margin of the pluton, is not a likely candidate for burial of the broadly extensive Nason terrane. At the north margin of the Mt. Stuart Batholith (Figure 6, inset in the Nason terrane), neither the pluton, which is 96 Ma in this region [36], nor the country rock shows tectonite fabric that could point to burial by contraction in this southern end of the terrane. Elsewhere in the Nason terrane, metamorphic fabric is observed, including folded foliation and orogen-parallel mineral lineations [37], but these features are not useful in deducing orogen-normal contraction as the cause of some 20 km of crustal thickening.  Figure 13: Zoned garnet in schist at the margin of the Breakenridge pluton, data new to this report. The sample site is located (Figure 4) at the south end of the pluton and is marked by a notation of pressure of 8.5 kb. Zoning records garnet growth during increasing pressure, interpreted to be caused by inflation of the overlying pluton. The garnet image was recorded with a microprobe at the University of Otago, New Zealand. P-T plot based on GAMI and GASP calculations.

Lithosphere
The magma loading explanation for metamorphism of the Nason terrane is based on conjecture that these schists and gneisses comprise the floor of large plutons now eroded. Support for this interpretation comes from reconstruction of the Fraser River-Straight Creek fault which places the Nason terrane on strike with the lithologically similar Napeequa Schist and the large Scuzzy and Urquhart plutons (84-92 Ma) of the Harrison Lake area (Figures 2 and 3). Consistent with this hypothesis, the Nason terrane hosts relatively small plutons (noted above) in the age range of plutons of the Harrison Lake area. These small plutons and regional migmatite of the Nason terrane are interpreted to mark the upward passage of magmas that fed now-eroded overlying plutons, creating 20 km of crustal thickening.

Chelan Mountains Terrane.
Northeast of and adjacent to the Nason terrane is the Chelan Mountains terrane (Figure 6) [31], consisting of the Napeequa Schist, originating as ocean floor sediments, and the Cascade River Schist of arc origin (Figures 17 and 18). The Chelan Mountains terrane extends into the Skagit Gneiss, comprising country rock to the igneous injection complex ( Figure 19) that characterizes the gneiss. In the Cascade River area, the Chelan Mountains terrane is intruded by the Eldorado and Marble Creek plutons and other smaller plutons (Figure 6(a)).
The metamorphic history displayed in the Cascade River valley is complicated by numerous high P-T events and more so by displacement on the Entiat fault that runs along the center of the valley (Figure 6(a); [13,38]). The earliest metamorphism affected rocks on the southwest side of the fault where a strongly southeast increasing metamorphic grade is documented ranging over a distance of~60 km from greenschist facies to upper amphibolite facies. The metamorphism at the low-grade end is dated by a K-Ar age of 94 Ma for muscovite and 3-4 kb pressure for actinolite ( Figure 6(a)), and a zircon age of 91 ± 2 Ma was obtained by Sauer et al. [39] in siliceous schist. In higher-grade schists, an age~95 Ma and 9 kb are measured [40,41]. How this relatively high P and old-age metamorphic zone fits into the regional orogenic processes remains to be explained.
On the northeast site of the Entiat fault, low-pressure metamorphism is marked by pseudomorphed contact metamorphic andalusite (P < 3 kb) at the margin of the Eldorado pluton. The Eldorado pluton was fully intruded at 88 Ma [42], crystallizing at pressures of 4-5 kb (Al-in-hornblende barometry). Further crustal thickening occurred in this area northeast of the Entiat fault to the extent that the Marble Creek pluton (75 Ma, [42]) intruded at a depth equal to a pressure of~8 kb (Al-in-hornblende barometry, igneous epidote) [40]. Zoned garnet in the Eldorado aureole records a pressure range from the core to the rim of 2 kb to 8 kb; the rim pressure of 8 kb is a consequence of metamorphism. Finally, pressure increased to 9-10 kb in the area of the Marble Creek pluton and to the northeast into the domain of the Skagit Gneiss. In this final phase of stepwise pressure increases, the Marble Creek pluton developed metamorphic fabric.

Lithosphere
The contact between the Napeequa Schist and Skagit Gneiss can be observed north of the Marble Creek pluton (Figure 6(a)) and is described by McShane [43] as transitional: "… the schists [Napeequa] grade into the Skagit paragneiss and become progressively more migmatitic toward the northeast. … The bedding [layering] that is observed gradually becomes less distinctive to the northeast …". McShane suggests that country rock screens in the Skagit Gneiss hosting the injection complex are components of the Napeequa Schist, a conclusion made also by Tabor et al. [44].
This transition from high-pressure metamorphism of the Skagit Gneiss into the Chelan Mountains terrane to the southwest is marked by a gradient from >9 to 6 kb in 5 km (Figure 6(a)), steeper than a simple depth gradient, indicating a metamorphic overprint of the Skagit Gneiss pressure regime onto older metamorphic rocks of the Chelan Mountains terrane formed at lower pressures.     [14,48]. Radiometric dating (see text) indicates construction of the gneiss in a period from about 60 to 70 Ma. The rock here is exposed in a fresh roadcut along the North Cascades highway shortly after its completion in 1972. 12 Lithosphere and volcanic rocks of the Chelan Mountains terrane. The Skagit Gneiss and Chelan Mountains terrane lie northeast of the Nason terrane (Figures 2 and 6), southwest of the Black Peak and Oval Peak batholiths, and southwest also of the Methow sequence [31,45]. The igneous intrusive component comprises about 75% of the Skagit Gneiss complex [42]. From zircon ages, most of the igneous rocks are estimated by Miller et al. [45] to have been intruded in the period from 59 to 73 Ma; lesser intrusions continued until 45 Ma ( [42,[45][46][47][48]). The host rocks of the Chelan Mountains terrane range in age from Triassic to Cretaceous [38,39].
Crustal thickening and magmatism in the Skagit Gneiss were coeval processes. In the northern part of the Skagit Gneiss, along the North Cascade Highway in the vicinity of Diablo Lake (inset in the Skagit Gneiss in Figure 6), geothermobarometers indicate metamorphic conditions of up to 700°C and 10 kb [14]. The age of the high-pressure metamorphism in this area based on U/Pb in zircon and monazite in leucosomes in migmatite covers the range of 61-71 Ma ( [49], locality 2). Samarium-neodymium ages in garnet are 60 Ma ( [48], locality 1) marking the age of exhumation and cooling to closure temperatures less than 600°C (pressure inferred~5-6 kb, based on the assumption of a normal geothermal gradient, less if rocks in the inferred arc are hotter than the normal gradient).
A degree of contraction in the Skagit Gneiss is displayed by mountain-scale folds across the orogen [45,50]. These are mostly open folds that postdate the foliation and metamorphism and thus are not useful as a basis for explaining the deep burial (35-40 km) and associated metamorphism of the Skagit Gneiss. Metamorphic fabric in the gneiss is characterized by orogen-parallel lineations [37] suggesting orogenparallel displacements during metamorphism. Potential crustal shortening structures pervasive through the gneiss and synchronous with metamorphism are not known. Magmatism as the dominant loading process is indicated by structural and metamorphic features of the geology marginal to the Skagit Gneiss in the Chelan Mountains terrane on the southwest flank that are discussed above, and the history of rocks on the northeast side as presented below.
3.4. Northeast Margin of the Skagit Gneiss. The northeastern flank of the Skagit Gneiss is marked by northwest striking steeply dipping faults. Along the more northerly segment of the gneiss margin is the 20 kilometers long dextral strikeslip Ross Lake fault separating Skagit and Methow rocks ( Figure 6) [13]. The estimated vertical fault displacement is 6-12 km down-dip of the Methow side of the Ross Lake fault. The magnitude of strike-slip displacement is uncertain. Locally observed along the Ross Lake fault system is a wellexposed steep metamorphic gradient from the Skagit Gneiss into the flanking rocks of the Methow Group, ranging from the sillimanite zone to prehnite-pumpellyite facies [44]. No evidence is found along this fault system of a major thrust emplacing Methow rocks over the Skagit Gneiss.
Extending some 80 km south from the end of the Ross Lake fault is a complex zone of plutons and faults that separates the Skagit Gneiss from the Methow Group ( Figure 6). Within this domain, the "Gabriel Peak tectonic belt" (GPTB), ranging up to a few km wide, borders the flank of the Skagit Gneiss and separates the gneiss from the Oval Peak pluton discussed below [51,52]. The GPTB is a mylonitic shear zone dominated by reverse-slip steeply dipping fabric.
From these observations of the Skagit Gneiss and its surroundings, findings are that the Skagit Gneiss is an injection complex of sills and dikes intruded into the Napeequa Schist of the Chelan Mountains terrane. The high-grade metamorphism of the Skagit Gneiss, marking crustal thickening, is broadly synchronous with the magmatism. The contact of the Skagit Gneiss with the Chelan Mountains terrane on the southwest flank is intrusive, not a thrust fault. On the northeast flank, the Skagit Gneiss is separated from the Methow sequence by steeply dipping strike-slip and reverse faults. The possibility of deep burial of the Skagit Gneiss by emplacement of a nappe from either flank of the gneiss is inconsistent with these findings.
Although thrust sheets in the North Cascades are mostly not evident, one does occur along the eastern edge of the Gneiss where Triassic ocean floor basalt and associated oceanic sedimentary rocks of the Hozomeen Group structurally overlie Cretaceous sedimentary rocks of the Methow Group ( Figure 6). This thrusting is discerned [31] as east-vergent, occurring in the time frame of 105-90 Ma, before the origin of the Skagit Gneiss complex. Other contractional faults in the North Cascades described earlier and deemed unlikely as the cause of Skagit loading are (1) the White River Shear Zone, a steeply dipping reverse fault at the southwest margin of the Tenpeak batholith ( Figure 6) [34,35], and (2) the Windy Pass thrust where the Ingalls Complex is thrust over the south end of the Nason terrane and is intruded by the 96 Ma Mt. Stuart Batholith [53].
3.5. Oval Peak Batholith. The Oval Peak batholith, in the Chelan Mountains terrane on the eastern flank of the Skagit Gneiss ( Figure 6), was intruded in the time frame of loading of the Skagit Gneiss and is thus of special importance in understanding the regional tectonics. Much is known about this pluton from the study of Miller and Bowring [54], as summarized here. The zircon age of the pluton is~65 Ma; country rock in the pluton aureole yields K-Ar cooling ages of 54-56 Ma. Importantly, mineralogy of the pluton aureole records an increase in pressure during pluton emplacement from early conditions of andalusite stability (<3 kb) to later high-pressure crystallization of kyanite. Final pressures under which the pluton crystallized are in the range of 5-7 kb, recorded by magmatic epidote and Al-in-hornblende barometry. Miller and Bowring describe the formation of the batholith as an "expanding diapir." They conclude that "the inferred metamorphic history may indicate that the batholith was intruded during crustal thickening," a finding supportive of the magma loading interpretation.
Emplacement of the Oval Peak pluton occurred along the margin of the Gabriel Peak tectonic belt ( Figure 6). Tectonism along this fault was apparently ongoing during intrusion of the Oval Peak pluton (65 Ma) evidenced by magmatic foliation that developed coincident with active tectonic fabric of the country rock along the outer rim of the 13 Lithosphere pluton where it extends into the fault zone [54]. The faulting was of reverse-slip nature along steeply dipping shear zones. Thus, there is no apparent evidence of deep burial of the Oval Peak pluton by stacking of low-dip thrust sheets. The Oval Peak pluton is coeval with the neighboring Skagit Gneiss, and for both plutonic domains, magma loading is the likely burial mechanism.
The occurrence within the Oval pluton aureole of early andalusite (now pseudomorphed) and a later crystallization of high-pressure magmatic assemblages is similar to the history of batholithic aureoles of the Coast Plutonic Complex in the Harrison Lake area.
3.6. Swakane Gneiss. The Swakane Gneiss was affected by high-pressure metamorphism at the same time as the Skagit Gneiss ( Figure 6). Could these two rock assemblages be related? The Swakane Gneiss occurs in two tectonic blocks in the Eastern Cascades Core separated by displacement on the Fraser River-Straight Creek fault ( Figure 6). This rock is a high-grade quartzofeldspathic gneiss developed from clastic sediment, likely arkose. Zircon and garnet ages are interpreted to indicate sedimentation from 75 to 91 Ma and loading accompanied by high-grade metamorphism (11 kb) from 63 to 75 Ma [55].
The Swakane Gneiss is overthrust by schist of oceanic origin similar to the Napeequa Schists. High-grade metamorphism is imprinted on both the schist and Swakane units (burial depth 35-40 km) and also on the thrust fault that separates the two units [56]. Aside from felsic sills and dikes scattered through the Swakane metasediments, significant plutons are absent and thus burial by magma loading is not evident. Tectonic loading is the apparent cause of burial. Detrital zircon ages in the Swakane Gneiss include Precambrian populations of 1400 and 1600-1800 Ma, a cratonal signature that distinguishes this unit from the Napeequa and other North Cascades formations [57]. Apparently, sediments of the Swakane Gneiss accumulated in a locality with exposure to the North American craton and are relatively exotic to the ancestral Cascades. The similarity of ages of the Swakane and Skagit gneisses is thus apparently coincidental. The tectonics of how the Swakane Gneiss became embedded in the geology of the North Cascades remains to be explained.

Summary
From 100 to 60 Ma, great crustal thickening, 20-40 km, followed by uplift, occurred episodically in varying places across the southern Coast Plutonic Complex in the Harrison Lake area and in the Crystalline Core of the North Cascades, the two terranes being contiguous along strike prior to displacement on the Fraser River-Straight Creek fault. Four individual episodes of crustal thickening localized in different areas associated with plutons and each lasting a few million years are documented (Figure 7). The areas characterized by the dominant lithology involved are the (1) Breakenridge, (2) Nason and Urquhart, (3) Scuzzy, and (4) Skagit and Oval Peak. That these crustal thickening events are tied to the pluton emplacement process is evidenced by mineralogy and structures in the pluton aureoles and within the plutons. Regional structures, mineral isograds, and mineral ages do not support significant burial by tectonic shortening. However, considerable ductile strain occurred, well-marked by deformed elongate clasts in metaconglomerate (Figure 9). This fabric together with mineral lineations is aligned parallel to the length of the orogen [6,37] indicating strike-slip orogen-parallel displacements. The great crustal thickening is best interpreted to have occurred by intrusion and vertical stacking of horizontal pluton sheets (Figure 8). This finding for the southern part of the Coast Plutonic Complex may be applicable throughout the parts of the 1800 km long Coast Plutonic Complex and potentially to other arc plutons marginal to the Pacific basin and elsewhere.

Conflicts of Interest
This manuscript presents no conflicts of interest.