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Chapter 3
The Coast Range Episode
New Lands Along an Evolving Margin
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Chapter 3
The Coast Range Episode
New Lands Along an Evolving Margin
120 - 58 Million Years Ago
Introduction
The Coast Range Episode, the second episode in the assembly of the Pacific Northwest, was initiated with the accretion of the Insular Belt megaterrane in mid-Cretaceous
time. The accretion of that terrane belt moved the western
edge of the continent another several hundred kilometers
to the west, establishing what would eventually become
the modern continental margin of British Columbia.
With the addition of the Insular Belt, a new continental
arc regime developed across this newly-accreted terrane
belt. That continental arc was known as the Coast Range
Arc, and it was the largest arc complex to ever develop
on the North American continent. The North Cacades, the
Okanogan Region and Northeastern Washington all bear the imprint of this orogenic regime. In British Columbia,
these rocks are historically known as the Coast Plutonic Complex. Magmatism and orogeny associated with the Coast
Range Arc are the dominant features of this episode, to which it lends its name.
This episode spans the latter half of the Cretaceous Period, a very dramatic period in the evolution of western North
America. Along the west coast, this was a period of aggressive mountain-building. Across much of the continent,
it is marked by extensive transgressive and regressive sequences, as sea levels rose and fell. In addition to geologic
events, this represents the heyday of the Dinosaur species, up until their extinction some 65 million years ago.
The Coast Range Episode persisted for almost 60 million years, into earliest Tertiary time. In addition to the Coast
Range Orogeny, it also included the accretion of several belts of “mélange” terranes across the southern end of the
province, and likely involved a certain degree of northward displacement on regional fault systems. These events developed under a convergent but evolving marginal setting. Over time, oceanic plate motion here developed an increasingly northern direction. By earliest Tertiary time, that setting evolved into a transform margin along the western edge
of the continent. This development would mark the end of the Coast Range Episode.
Figure 3-1 (Cover) Mount Shuksan from Austin Pass, North Cascades. Rocks of the Northwest Cascades System
Figure 3-2 (Left) Black Peak, North Cascades: A Coast Range Arc intrusion.
Figure 3-3 (Above Right) Big Four Mountain, Stilliguamish Valley: Rocks of the Eastern and Western Melange Belts
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Alexander
Terrane
Bridge River
Terrane
Cadwallader
Terrane
Wrangell
Terrane
Harrison Lake
Terrane
Figure 3(Left) Map illustrating the regional extent of the various terranes of the
southern Insular Belt. This is a diagramatic representation, as much of the central
portion of this belt has been intruded by
large volumes of granitic magma. This illustrates the likely extent of the various terranes prior to that intrusive episode. The
southern extent of this belt is uncertain, as
it lies buried beneath younger rocks.
The Wrangell, Alexander and Chilliwack
Terranes evolved as Paleozoic island-arc
groups. The Wrangell and Alexander terranes were amalgamated by Late Triassic time, and were probably joined by the
Chilliwack Terrane in Jurassic time. The
Jurassic Harrison Lake Terrane appears
to have developed on that amalgamated
arc-terrane unit.
Chilliwack
Terrane
The Insular Belt:
Anatomy of a Megaterrane
Like the Intermontane Belt, the Insular Belt has its origins as a composite island-arc archipelago, delivered to the
continental margin by the combined motions of the continental and oceanic plates. In contrast to the Intermontane Belt
however, the Insular Belt is considerably more complex. Along its southern end it consists of at least five different
island-arc groupings, and at least one ocean basin that goes by as many names. The fundamental terrane units along
the southern end of the Insular Belt include the Wrangell, Alexander, Chilliwack, Harrison Lake and Cadwallader
island-arc terranes, and the ocean-basin Bridge River terrane. The belt takes its name from the Insular Mountains of
Vancouver Island and the Queen Charlotte Islands.
The oldest island-arc terrane in the Insular Belt appears to be the Alexander Terrane (“Alexandria”), which is built of
Late Proterozoic to Mid-Paleozoic arc-type volcanic, plutonic and sedimentary rocks. These are topped by a Devonian
to Permian section of limestone and marine clastic rocks, an assemblage which also includes a pluton of Late Carboniferous age (309 ma). That pluton also appears in the Devonian to Permian arc-type section of the adjacent Wrangell
Terrane (“Wrangellia”) to the south. Thus the Wrangell and Alexander Terranes appear to have been amalgamated by
that date. Late Paleozoic rocks of Wrangellia also include considerable sections of limestone, suggesting extensive
periods of relative stability. Both of these terranes take their names from features (the town of Wrangell and the Alexander Archipelago) in southeast Alaska.
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Figure 3(Above) Limestone quarry in the Chilliwack Group. Clausen Quarry, Silver Lake Road north of Maple Falls.
These beds are the remains of extensive reef-type communities which developed around the Chilliwack Islands in Carboniferous times. This quarry is in the Red Mountain subgroup, and produces aggregate products for local use. The limestone products of the Chilliwack Group were a key ingredient in constructing the dams on the Skagit River.
In Late Triassic time, the amalgamated Wrangell and Alexander Terranes experienced an episode of rift volcanism,
producing an extensive series of submarine basalt flows known as the Karmutsen Basalts. These pillowed flows are
exposed locally on Vancouver Island. The significance of this rift sequence remains uncertain. Speculatively, it can
be pointed out that this is broadly the same age as the rifting episode which broke up the supercontinent of Pangea.
Neither the Wrangell or Alexander Terranes extend into Washington State. There is long-standing speculation that the
Seven Devils Terrane in the Blue Mountains of northeastern Oregon is a displaced fragment of Wrangellia, but such a
connection remains tenuous.
The Chilliwack Terrane, along the southeast end of the Wrangell Terrane, also dates from Early Paleozoic time. It
takes its name from the town of Chilliwack in southeastern British Columbia, but the terrane extends south well into
Washington State. The earliest rocks of this unit are pre-Mid-Devonian in age, known as the Sumas Mountain Subgroup. A prominent limestone section characterizes the Carboniferous Red Mountain Subgroup; while a volcanic and
volcaniclastic section of basaltic to andesitic island-arc rocks dominate the Permian Black Mountain Subgroup. The
Chilliwack is depositionally (paraconformably) overlain by the Late Triassic to Early Jurassic Cultus Formation, a
sequence of volcanic sediments which suggest an offshore island slope or deep-water setting. The boundary between
the Chilliwack and Wrangell Terranes has been obscured by later deposition, so we can’t be certain of when they were
amalgamated.
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Chilliwack Group Rocks
Figure 3-6 (above) Chilliwack Group rocks, north side of
Sumas Mountain. These are mafic to intermediate volcanic
rocks, likely Permian in age.
Figure 3-7 (Right) Chilliwack Group rocks, summit ridge of
Sauk Mountain. Here is the transition from metavolcanic to
metaclastic rocks.
Figure 3-8 Chilliwack Group rocks, from the quarry at
Concrete. Polished limestone slab reveals the cross-sections
of crinoid fossils. Fossils date this section as Mississippian in
age.
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Undifferentiated
Metamorphic
Rocks
Wrangell
Terrane
Cadwallader and
Cascade
River
Terranes
Vancouver Island
Bridge
River and
Napeequa
Terranes
Harrison Lake Terane
Chilliwack Terrane
Figure 3-9 (Above) Simplified map of the Insular Belt Terranes in Washington and southern British Columbia. The extent of the
Harrison Lake Terrane is generously illustrated here. Its actual extent is uncertain, as it grades into the metamorphic rocks to the
north.
To the (now) northeast of the Chilliwack Terrane is another volcanic island-arc group, the Cadwallader Terrane. The
Cadwallader is a Late Triassic volcanic arc sequence including over a kilometer of pillow basalt, topped by volcanic
and granitic conglomerate and sandstones. The pillow basalts represent the early volcanic edifice, while the granite
conglomerates and sandstones reflect erosion into the plutonic root of the more mature arc complex. Late Triassic
volcanic and plutonic rocks of this group are known in British Columbia as the Hurley Formation, while in Washington State the plutonic components include the Marblemount and Dumbell Mountain Plutons in the North Cascades.
The Marblemount and Dumbell Mountain plutons are quartz-diorites, reflecting a maturing of that island-arc complex.
The sedimentary unit is known in British Columbia as the Pioneer Formation, and in the North Cascades of WashFigure 3-10 (Below) The Marblemount Pluton, on the Cascade
River Road east of Marblemount.
Figure 3-11 (Below) Hand-sample of the Marblemount Meta
- Quartz Diorite. Greenish color is metamorphic epidote.
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Cobbles
Figure 3-12 (Above) The Cascade River Schist, along the Cascade River Road east of Marblemount. Arrows point to several
large cobbles of Marblemount Meta-Quartz Diorite.
Figure 3-12 (Below) Hand-sample of the Cascade River Schist. Note light-colored grains derived from weathering of the
Marblemount Pluton.
ington as the Cascade River
Unit (also known locally as
the Holden Assemblage to the
east). These once-continuous
units in British Columbia and
Washington were later offset
by about 145 km by right-lateral motion along the (Eocene)
Fraser Fault.
The Cadwallader Terrane appears to have developed on the
oceanic plate that intervened
between the Insular Belt and
the continental margin. Those
oceanic rocks are known in
British Columbia as the Bridge
River Terrane, and in Washing-
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Figure 3-13 (Right)
An outcrop of the Napeequa
(Bridge River) Terrane,
along SR 20. These rocks
are largely greenstones, the
metamorphosed equivalent of
ocean floor basalt.
The Napeequa is a highly
dismembered ophiolite unit,
consisting of (meta-) basalt,
gabbro, chert and siltstone,
along with bodies of ultramafic rock. .
ton as the Napeequa (locally, Hozameen) Terrane. The ages of these rocks range from Carboniferous to Mid-Jurassic.
Where regional metamorphism has not obscured their identities, these terranes contain the classic components of an
oceanic plate (gabbro, basalt, chert, siltstone). Included are some significant ultramafic bodies, most notably the large
Shulaps Ultramafic Complex in the Bridge River portion.
In Washington, the combined Napeequa and Cascade River units have been called the “Chelan Mountains Terrane.”
Southwest of this belt occurs another section of oceanic rock known locally as the Nason Terrane. The Nason Terrane
is broadly equivalent to the Napeequa, but contains more aluminous sediment, suggesting a closer proximity to an
island group. It has not been dismembered to the degree seen in the Napeequa, and contains fewer ultramafic sections.
A good case can be advanced (but not resolved) that the Alexander, Wrangell and Chilliwack Terranes were amalgamated by Late Triassic time. Because all of these are older than the Bridge River Terrane, they must have developed
Figure 3-14 (Left)
Polished hand-sample of the
Napeequa Schist. These are
metasedimentary rocks which
have their origins as oceanfloor mudstones. Dime provides
scale.
The Napeequa takes its name
from the Napeequa River southeast of Glacier Peak. The name
translates to “quiet waters”.
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Rocks of the Napeequa (Bridge River)
Terrane
Figure 3-15 (Right)
The Napeequa Schist, at an outcrop along the
Cascade River Road, east of Marblemount.
These are meta-pelites, metamorphosed at
amphibolite grade.
Figure 3-16 (Below left)
Chert beds in the Napeequa Schist. At Hard
Creek, along the Cascade River Road. These
are classic ocean-floor deposits, the siliceous
remains of one-celled silica-shelled organisms like radiolaria and diatoms. When they
die, their remains accumulate on the ocean
floor.
Figure 3-17 (Below Right)
Talc Schist. At an outcrop at Hard Creek, on
the Cascade River Road. Talc develops from
hydrothermal metamorphism of ultramafic
rocks. Iron dissolves in the hot water and is
carried away, leaving a magnesium silicate
behind. Also known as “soapstone”, this is
a popular carving medium. The preferred
method of mining is by chain-saw.
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Rocks of the Nason Terrane
Figure 3-18 (Right)
The Chiwaukum Schist, the
dominant rock of the Nason
Terrane. At an outcrop along
the Beckler River Road,
north of US 2. The Chiwaukum Schist is similar to the
metapelites of the Napeequa
Terrane, but is slightly more
aluminous. The Nason Terrane also lacks the ultramafic component seen in the
Napeequa.
Figure 3-19 (below)
The Chiwaukum Schist,
along the North Fork of the
Skykomish River.
on another (or more than one other) oceanic
plate. When these units attained a position
on or adjacent to the Bridge River plate is
uncertain, but the Late Triassic (Cadwallader)
island-arc sequence which developed upon
it would be a likely source terrane for the Cultus Formation on the Chilliwack Terrane. A
succession of common arc sequences across
the Insular Belt starting in Mid - Jurassic time
indicates that it was a composite feature by
this date.
Over Jurassic and Early Cretaceous time at
least four island-arc sequences overprinted
the southern end of the Insular Archipelago.
The earliest of these was an Early Jurassic
episode called the Bonanza Arc. This arc is
best known for the Bonanza Volcanics on
Vancouver Island and the Bowen Island Volcanics to the east.
To the east, a somewhat younger arc was
apparently responsible for Mid-Jurassic
volcanism in the emerging Harrison Lake
Terrane. This arc sequence appears to have
accumulated on Bridge River oceanic crust,
above a Mid-Triassic assemblage of silicified argillite and siltstone known as the Camp
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Alexander
Terrane
100
Wrangell
Terrane
Cretaceous
Chilliwack
Terrane
Cultus Fm.
Gambier GroupVolcanics
Nooksack Group
Harrison
Lake
Terrane
Cadwallader
Terrane
Brokenback
Hill
Taylor Cr. Grp.
Peninsula
Harrison Lake
Jurassic
200
Triassic
Bonanza Arc
Karmutsen Basalt
Wells Cr.Volcanics
Black Mtn.
Subgroup
Red Mountain
Subgroup
Permian
300
400
Cayoosh
Last Creek
Formation
Cadwallader
Camp Cove
Formation
Bridge River
Sumas Mtn
Subgroup
Carboniferous
Devonian
Relay Mtn.
Group
Bridge
River
Terrane
Pluton
Silurian
Ordovician
500
Cambrian
Figure 3-20 (Above) Straigraphic columns for the terranes of the Insular Belt. Dates along the left margin are ages, in million of
years ago.
Cove Formation. Above the Camp Cove Formation is the volcanic Harrison Lake Formation, which is Mid-Jurassic
in age. Within this unit are preserved distinctive clasts of Chilliwack rocks, the earliest truly compelling evidence that
these terranes were amalgamated at this date. The Harrison Lake Formation is represented south of the international
boundary by the Wells Creek Volcanics in the Mt. Baker region. These volcanics erupted on the Chilliwack Terrane.
In the Harrison Lake area, a package of Mid to Late Jurassic sedimentary rocks – the Mysterious Creek and Billhook
Formations, overlie the Harrison Lake Formation.
A third, Late Jurassic arc sequence developed across the Insular Belt between 167 and 145 million years ago. Representative plutonic bodies of this age include the Mount Jasper, Elbow Lake, Port Douglas, Horseshoe Bay, Cloudburst
and Thornborough intrusions, to name but a few. None of these extend south of the international border. Note that
this episode of vigorous arc-magmatism coincides with that experienced in the Omenica Arc, some distance to the
east. Rapid plate production was a characteristic of the Late Jurassic Period.
A fourth episode of island-arc magmatism developed in Early Cretaceous time, starting at about 135 Ma. Known as
the Gambier Arc, it persisted up until the time that the Insular Belt was finally accreted to the continent. The Gambier
Arc is best known for the extensive volcaniclastic sequence which it produced, which spans the Wrangell, Chilliwack
and Harrison Lake Terranes. The lower part of the Gambier Group is a section of marine to nonmarine volcanic rocks
of andesitic character, while the upper section is mostly sandstone and siltstone. In the Harrison Lake area, Gambier
rocks include the Fire Lake, Peninsula and Brokenback Hill Formations. South of the international border, Gambier
rocks are found on the Chilliwack Terrane as the Nooksack Group, in the Mount Baker area. The Nooksack rocks are
largely slate, and contain clam and belemnite fossils of Early Cretaceous age.
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Rocks of the Nooksack Group
Figure 3-21 (Above) Clam fossils, Nooksack Group. From an outcrop on the North Fork Nooksack River.
Figure 3-22 (Below) Nooksack Group Rocks, Finney Creek Road,
south of Rockport. These rocks are a massive slate, cut by calcite
veins. This is a typical outcrop of the Nooksack.
Figure 3-23 (Right) A large outcrop of Nooksack Slate, along the
Baker River Road on the south side of Mt. Baker.
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Figure 3-24 (Above)
Rocks of the Nooksack Group, on Skyline Divide on the north
side of Mt. Baker. The Nooksack has been interpreted as a
submarine volcanic debris fan. The thin layers here display
the classic fining-upward sequence typical of turbidite (submarine landslide) deposits.
Figure 3-25 (Right)
Clam and Belemnite fossils in the Nooksack Formation. On
Skyline Divide, just below Chowder Ridge. The cone-shaped
fossils of belemites (a Cretaceous cephlopod) are common
here. Clam species include Buccia specimens, and less common Pina varieties.
Figure 3-26 (Right)
Polished Nooksack
siltstone, showing fossil belemnite shells.
Photo by George
Mustoe
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The Chilliwack Terrane
The Insular Belt inWashington
The Chilliwack Terrane is described here as the southernmost terrane element of the island-arc components of the Insular
Belt Megaterrane. This is an interpretation often advanced in the Canadian literature, but less often adopted in American
studies. American researchers typically include the Chilliwack Terrane as part of the terranes of the Northwest Cascades
System. That “system” is defined by a structural stacking of terrane units, of which the Chilliwack terrane is a part. Large
sections of the Chilliwack have been incorporated into the accreted belt of the Northwest Cascades System. In places,
sections have actually been overturned by the cumulative effects of various tectonic events.
The Chilliwack Terrane is however markedly different than the mélange rocks of the Northwest Cascades Terranes. It is
much older, and its island-arc origins contrast with the ocean-basin character of the mélange rocks. Most significantly, the
Chilliwack shares the Late Jurassic and Early Cretaceous volcaniclastic cover which mantles much of the Insular Belt.
Known regionally as the Gambier Group (locally as the Nooksack Formation), these sediments link the Chilliwack with
the rest of the Insular Belt by Early Cretaceous time. Clasts of Chilliwack-type rocks in the Jurassic Harrison Lake Formation to the north suggest that this affiliation was established at an earlier date. The Chilliwack Terrane may have been
part of the Insular Belt by Late Triassic time, if not earlier.
In the interpretation adopted here, the mélange rocks of the Northwest Cascades Terranes were obducted north over the
southern end of the Chilliwack Terrane as they were accreted to the continental margin here. In that process, sections
of Chilliwack rock were incorporated into the structural “stack” of terranes which makes up the Northwest Cascades
System. Those sections were not however, part of the terranes of the Melange Belts. In this interpretation however, all of
these “stacks” of terrane units ultimately rest on rocks of the Chilliwack Terrane at depth. This is an interpretation consistent with the regional structure here.
The Chilliwack Terrane outcrops as far south as the town of Darrington, on the Stilliguamish River. Reversing some
~145 km of northward displacement on the Fraser (Straight Creek) fault to the east places those exposures at about the
latitude of Olympia. This suggests that NWCS rocks in the region south of Cle Elum were probably thrust north over the
Chilliwack Terrane in that area. The boundary between the Chilliwack and Nason Terranes must lie under those accreted
units.
The southern and eastern extent of the Chilliwack Terrane can only be speculated. To the south it is covered by rocks of
the Melange Belts and more recent volcanic rocks of the Cascade Arc. To the east, it is mantled by flows of the Columbia
River Basalt Group. It is likely a much more extensive unit than its present exposures suggest.
The Accretion of the Insular Belt:
The Collapse of the Bridge River Basin
Given their age and character, it seems likely that the older elements of the Insular Belt (the Alexander, Wrangell
and Chilliwack Terranes) are probably well-removed from their original locales of origin. Considering the extensive
Late Jurassic magmatism seen in the Omenica Belt, it is a reasonable conclusion that a substantial expanse of (Bridge
River) oceanic plate was subducted over that period. This places the Insular Belt at a considerable distance from the
continent in Mid-Jurassic time.
At the same time however, the amalgamated Insular Belt appears to have been an active island-arc complex from at
least Mid- Jurassic time. This combination of two active arc complexes is the same situation we faced with the accretion of the Intermontane Belt. It demands that either the intervening Bridge River Terrane is being simultaneously subducted at both ends of the basin, or that the island arc is supported by eastward subduction of an outboard plate. This
would be the same Philippine-type margin we have inferred for the Early to Mid-Jurassic Intermontane Belt. Once
again, this second scenario appears to be the most appropriate.
As noted in the previous chapter, the lack of Early Cretaceous magmatism in the Omenica Arc suggests that the
intervening Bridge River Basin was not being subducted to the east over this period. Over this ~15 million year span
however, North America continued to advance to the west, a distance of perhaps 600 kilometers. The lack of continental magmatism in this scenario suggests that the North American and Bridge River plates are locked together at
Figure 3-27 (Above) Ross Lake, North Cascades. Red line illustrates the approximate western margin of the Ross Lake Fault
Zone, the boundary between the Intermontane and Insular Belts. Rocks to the left (west) are part of the Insular Belt.
98
this time. This would either drive the Bridge River plate west underneath the Insular Belt, or would drive the Insular
Belt (on the Bridge River Plate) to the west over the outboard subduction zone. This latter arrangement is known as
a Japanese-type margin, and it seems the best fit for the conditions observed here. In this arrangement, vigorous arc
magmatism of the Gambier Arc reflects a combination of both east-directed ocean plate movement and west-directed
Bridge River Plate movement, driven by the westward movement of the North American Plate.
If this interpretation is accurate, then the Early Cretaceous Bridge River Basin held a position much like the Sea of
Japan does today. Certain elements of the sedimentary record support such an interpretation. The oceanic rocks of the
Bridge River Group in British Columbia are overlain by the sediments of the Brew Group. The lower portions of this
group, including the lower part of a section called the Cayoosh Assemblage, bear a strong resemblance to the Early
Cretaceous Buck Mountain Formation in the Methow region. Upper parts of the Brew Group are quartz-rich clastic
rocks, similar to the upper formations of the Methow Group. Of equal significance, the rocks and fossils of the Early
Cretaceous Nooksack Group have long been suspected as being correlative with the upper part of the Buck Mountain Formation in the Methow region. If this is the case, then the Nooksack (Gambier), lower Brew Group and Buck
Mountain Formations may be a basin-wide feature of Bridge River Ocean. To the degree that these represent sediments deposited in a restricted marine environment, it suggests that the Bridge River Ocean was a relatively modest
feature by that date. Speculatively, it may have had a width of something like 600 km (400 miles).
The Insular Belt may have held a Japanese-type position with respect to the continental margin for perhaps 10 - 15
million years before the Bridge River Basin began its final collapse. As that collapse began, the basin lost its structural
integrity, meaning that the North American Plate could no longer drive it westward. Instead, the continent appears to
Insular Islands
Bridge River Basin
North
America
Gambier Arc
Bridge River Plate
Intermontane Belt
Figure 3-28 (Above) The Japan-type margin setting inferred for the Insular Islands in Early Cretaceous time, just prior to the onset of accretionary tectonics. By this date, the Omenica Arc had ceased to be active. Within the Insular Archipelago, the Gambier
Arc was an active island-arc complex. Intervening between the continent and these offshore islands was the Bridge River Basin.
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Bridge River Basin
North America
(Intermontane Belt)
Insular Belt
Islands
Okanogan Spences
Bridge
Arc
Gambier
Arc
Figure 3-29 (Above) Onset of the collapse of the Bridge River Basin, and the inception of the Okanogan - Spences Bridge Arc.
Figure 3-30 (Below) Map showing location of the Okanogan - Spences Bridge Arc, and the Gambier Group Volcanics
have resumed overthrusting the Bridge River
Plate. This time however, it appears that the
oceanic plate was subducted at a much steeper
angle, as the resulting magmatic arc lies well
west of most of the older Omenica Arc rocks.
This arc, known as the Okanogan-Spences
Bridge Arc, was a short-lived feature ranging
from 115-105 million years ago.
Yalakom
Fault
Fraser
Fault
Spences Bridge
Group
Gambier Group
Volcanics
Pasayten
Fault
Rocks in the northern end of this complex
include the Spences Bridge Group, consisting
of some 2400 m of andesitic to dacitic flows,
breccia and sediments (Pimainus Formation)
overlain by 1000 m of basalt flows (Spius Formation). To the south in the Methow region,
the large Okanogan Batholith was intruded as
part of this regime. It is largely a quartz-diorite
in composition. The Okanogan Batholith intruded the Quesnel Terrane, while the Spences
Bridge Group erupted on the Stikine Terrane.
Their common role in this arc complex sug-
Okanogan
Batholith
100
Figure 3-31 (Right) A
roadcut in the Okanogan
Batholith, along SR 20 east
of Twisp.
These are coarse-grained
granitic rocks, the product
of a short-lived arc complex
(the Okanogan -Spences
Bridge Arc) which developed
in the final collapse of the
Bridge River Basin.
Figure 3-32 (Below)
Diagram ilustrating the accreted Insular Belt, and the
preservation of the intervening rocks of the Bridge River
Basin. Note paleoscope reversal in the Methow Basin.
gests that the Stikine Terrane was part of the continental margin at this date. This argues against an interpretation of
the Stikine Terrane as part of the Insular Belt.
The earliest evidence for the accretion of the Insular Belt comes from Barremian-age (~120 Ma) rocks in Stikina.
Accretion of the Insular Belt initiated an episode of widespread contraction within the Insular Belt, the Intermontane
Belt, and within the ancestral continental margin. To the south, rocks of the Bridge River Basin show a similar pattern
of deformation dating from Aptian time, about 115 million years ago. Large-scale west-vergent thrusting served to
thicken the sequence and probably to uplift the large Shulaps Ultramafic Complex. Regional contraction culminated in
a thickened belt extending along the boundary between the island-arc rocks of the Insular Belt and the oceanic rocks
of the Bridge River Terrane to the east. That belt is known as the Coast Range Thrust Belt. The fabrics in the earliest
plutons of the Coast Range Arc (from 114 – 95 Ma) evidence that large-scale contraction was underway across the
entire accreted belt between these dates.
Insular Belt
Insular Belt
Oceanic
(Kula?)
Plate
Bridge River Basin
Methow Basin
Virginian Ridge
Formation
Cadwallader
Terrane
Bridge River
Plate
Intermontane
Belt
North
America
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Figure 3-33 (Above) Marble Creek Divide, North Cascades. Steeply dipping rocks of the Napeequa (Bridge River) and Cascade
River (Cadwallader) Terranes, stacked together during the accretion of the Insular Belt. This pattern of stacking resulted in a
thickening of this accreted belt. View is looking to the north.
Evidence for the final accretion of the Methow Group dates from Albian time, about 110 Ma. To the west, in the
Skagit region of the North Cascades, Bridge River (Napeequa) and Cadwallader (Cascade River) rocks were structurally stacked along west-verging thrust faults prior to intrusion by ~95 million year old plutons. Throughout the region,
Bridge River rocks have been deformed (and metamorphosed) to the degree that the original lithologies are all but
obscured. Locally, the only place that these rocks (along with those of the Cadwallader (Cascade River) Terrane) are
preserved is along the southwestern margin of the Skagit region.
Deformation associated with the accretion of the Insular Belt was substantially more extensive than that seen in the
Intermontane Belt, despite its smaller size. Deformation associated with this episode extended east across the Northern Rocky Mountain Trench and well into the Rocky Mountain Belt. The pattern of deformation is one of overthrusting to the east, thickening by contraction. The eastern edge of the thrust front is known as the Alberta Thrust Belt in
Canada. South of the border, that belt extends as the Idaho-Wyoming Thrust Belt, and eventually as the Sevier Thrust
Belt in Utah and Nevada. Large-scale contraction and overthrusting extends in a belt from northern Canada to the
desert southwest of the United States. South of our region, this contraction is largely interpreted as reflecting a steepening of the subducting slab along the western margin of the continent. In our area, it was clearly consequent to the
accretion of the Insular Belt. As the Insular Belt was accreted, the entire western margin of the continent became an
Andean-type marginal setting.
102
Hungry Valley
Fault
Cadwallader
Terrane
Tyaughton
Bralorne-Kwoik
Fault System
Bridge River
Terrane
Pasayten
Fault
Future
Fraser
Fault
Coast Range
Megalineament
Harrison
Lake
Terrane
Wrangell Terrane
Chilliwack
Terrane
Methow
Ross
Lake
Fault
Napeequa
White River
Fault
Figure 3-34 (Above) Map illustrating the major (accretionary) faults in the southern Coast Range belt. The fundamental “suture”
zone is the Ross Lake - Bralorne /Kwoik Fault system, which separates rocks of the Intermontane Belt (Methow-Tyaughton) and
those of the Insular Belt (Bridge River / Napeequa). Also shown are the Caost Range Megalineament and the Pasayten - Hungry
Valley Faults. Note the location of the Fraser Fault, which offsets these belts by some 145 km in Eocene time.
As the Insular Belt was accreted, the tectonic boundary between the Insular (Bridge River) Terranes and the rocks of
the Intermontane margin (the Tyaughton – Methow Group) became a feature known in Washington as the Ross Lake
Fault Zone, and in British Columbia as the Bralorne-Kwoik Fault System. This system represents the fundamental
“suture” zone along the old continental margin. Within the accreted Insular Belt, the major structural element is a
~2000 km-long feature known as the Coast Range Megalineament. This fault is a structural break between the Island
Arc terranes of the Insular Belt and the Bridge River Terrane to the east. It includes (from north to south) the Work
Channel Megalineament, the Central Coast Belt Fault, and in Washington State, the White River Shear Zone. That
latter feature divides the Nason Terrane to the south from the Chelan Mountains Terrane to the north. This entire feature holds a very important role in the orogenic development of the Coast Range Belt.
Inboard of Ross Lake-Bralorne System, a fault along the eastern margin of the Methow-Tyaughton Belt is known as
the Pasayten – Yalakom Fault. On the Pasayten section in Washington State, movement accompanied intrusion of the
Okanogan Batholith of the Okanogan-Spences Bridge Arc. These faults were all later offset along the (Eocene) Fraser
Fault.
103
The Early Coast Range Arc:
The Great Coast Range Batholith of North America
The accretion of the Insular Belt resulted in the development of a new arc regime along this portion of the continental margin. That regime is known as the (Canadian) Coast Range Arc. The development of this arc appears to have
completed an Andean-type setting which extended from southern Alaska all the way south to Baja California. The
southern end of this arc, as noted earlier, dates from Late Jurassic time. With the addition of the Coast Range Arc, this
system represents a continuous belt of magmatism along the western edge of North America over much of the Late
Cretaceous.
In our area, this continental-scale arc was responsible for the accumulation of the composite Coast Range Batholith
along the west coast of Canada, into the North Cascades of Washington, east through the Okanogan Highlands and
into the Idaho Batholith in Idaho. This magmatic trend reflects the Late Cretaceous continental margin in this area.
Northern Washington and British Columbia were assembled of the Intermontane and Insular Belts, which extended
the continent westward by hundreds of kilometers. No such terranes were accreted immediately to the south, leaving
a large oceanic “embayment” in the coast. This is known as the “Columbia Embayment”, and the trace of the Coast
Range Arc paralleled that ancient coastline as it headed east to the Idaho Batholith.
Figure 3-35 (Above) Eldorado Peak, North Cascades. Rocks of the ~90 Ma Eldorado Pluton, an early Coast Range pluton in the
Skagit region. Photo by E. Andersen.
104
Intermontane
Coast Range
Batholith
Belt
Insular
Belt
Idaho
Batholith
Columbia Embayment
Sierra Nevada
Batholith
Figure 3-36 (Above) Terrane map showing location of the
Columbia Embayment, which is reflected in the geographic
distribution of the Coast Range and Idaho Batholith rocks.
Figure 3-37 (Right) Map showing extent of batholithic intrusion
associated with the Andean-type regime which developed along
the coast here in Late Cretaceous time.
Pacific Ocean
Baja
California
Batholith
The transition from the Early Cretaceous island-arc of the Gambier Group to the continental regime of the Coast
Range Arc was evidently a relatively seamless and continuous process. Notably, this was a development which appears to have been unaffected by the concurrent development of the (transient) Okanogan-Spences Bridge Arc. The
earliest plutons of the Coast Range Arc date from about 115 Ma, with early representatives including the Chelan
Migmatite Complex at what is now the southeast end of the Chelan Mountains Terrane. Widespread and vigorous
continental-arc magmatism was underway by 110 Ma, largely concentrated within the accreted Insular Belt.
In general, the trend of Coast Range magmatism is that early (110 – 85 Ma) magmatism is largely concentrated along
the west side of the boundary between the Island-arc rocks of the Insular Belt and the oceanic Bridge River Terrane
to the east. It then heads east across the southern end of the North Cascades region, across the southern end of the Intermontane Belt, and then southeast into the ~90 ma Idaho Batholith. Later (80 – 70 Ma) magmatism is concentrated
in the central belt of the Bridge River – type rocks, while the latest stage (70-60 Ma) appears largely along the eastern
half of the Bridge River Terrane and along the Ross Lake – Bralorne Kwoik Fault System. This eastward-younging
trend is seen along the length of the cordillera, as far south as the Sierra Belt. It likely reflects a shallowing of the
angle of subduction, consequent to over-riding by the North American plate. The southern Intermontane Belt and continental margin to the east do not appear to have shared in the latest stage of magmatism.
105
Late (80 - 60 Ma)
Coast Range Arc
Early
(110 - 90 Ma)
Coast Range
Arc
British Columbia
Alberta
Washington
Montana
Pacific Ocean
Idaho
Figure 3-38 (Above) Diagramatic map illustrating the extent of the early and late Coast Range Arc intrusions. In British Columbia, these are known as the Western and Central Belt episodes of magmatism. The Skagit region in the modern North Cascades
(ruled) experienced some early Coast Range magmatism, in a region dominated by later (80-60 Ma) intrusion.
Figure 3-39 (Left)
Mount Stuart, an early Coast
Range pluton in the north central Cascades. This tonalite
body was intruded between 96
and 90 million years ago, and
is a dominant feature in the
Nason Terrane.
While Mt. Stuart is the southernmost Coast Range pluton in
Washington State, there is the
possibility that other plutons
were intruded at the same
latitude to the east, but remain
concealed beneath the (Miocene) Columbia River Basalts.
106
Figure 3-40 (Left)
Orthogneiss and paragneiss
of the Tenas Mary Creek unit
in the north-central Okanogan
region. These were older rocks
of the Quesnel Terrane, along
with younger plutons, which
were metamorphosed at amphibolite facies as part of the
(early) Coast Range orogen.
Along the Kettle River, west of
Curlew.
Figure 3-41 (Below)
Biotite tonalite of the Spirit
Pluton, north of Colville. The
Spirit Pluton is a large ~90
Ma Coast Range intrusive
in northeastern Washington.
Along with other local plutons
of this age (e.g. the Starvation
Flat Pluton), it illustrates the
continuity of magmatism east
into the Idaho Batholith.
The result is that the Coast Range Arc developed in two
distinct magmatic / orogenic belts. These include an early
(110-85 Ma) southwestern belt concentrated in the western
Insular Belt, the Nason Terrane of Washington, and east into
the southern Intermontane Belt and Idaho; and a later (80-60
Ma) northeastern belt in the Bridge River Terrane and the
equivalent Chelan Mountains Terrane of Washington.
Plutons of 100-95 Ma in British Columbia are centered
around the Breckenridge area, and included the Ascent
Creek, Mount Mason and Breckenridge Plutons. In the North
Cascades of Washington, they are concentrated in the Nason Terrane, including the Sulpher Mountain, Bench Lake,
107
Figure 3-42 (Right)
The Chelan Migmatites, an
early Coast Range intrusive
complex. These are spectacular migmatites, here showing
inclusions of earlier, more
mafic phases. At a location
below Lake Chelan.
Figure 3-43 (Below)
The Chiwaukum Schist,
the dominant rock of the
Nason Terrane. These were
originally pelites, oceanfloor mudstones. They were
metamorphosed at amphibolite facies by intrusions of the
early Coast Range Arc. At a
location on the Beckler River
Road, northwest of Stevens
Pass.
108
Cheval and Mount Stuart Plutons, to name but a few. As we will develop later, these likely represent the bottom of a substantial “pile” of plutons which intruded over this period.
Plutons of this age typically have a NE-SW mineralogic “fabric” which reflects intrusion and subsequent
metamorphism under conditions of high-angle compression, the conditions which accompanied contraction
of the margin as the Insular Belt was accreted. Younger plutons of the Coast Range episode however, do not
display this contractional lineation. Instead, they display a NW-SE trending mineral lineation (fabric), the
result of shear tectonics associated with oblique (lower angle) compression. This happened as the generally
easterly motion of the oceanic plate evolved into a more northeasterly direction. The result is that early (>95
Ma) rocks reflect E-W compression, while later (<95 Ma) rocks reflect NE-SW “transpression.”
The Coast Range Arc reached its greatest extent between 100 and 85 Ma (early Late Cretaceous time).
Along the western margin, magmatism was largely concentrated in the Alexander, Wrangell, Northern
Chilliwack and Nason Terranes. Notably however, magmatism of this age also extends east across the northern margin of the Columbia Embayment. A few plutons intruded the Chelan Mountains Terrane (Eldorado
Pluton, Black Peak Pluton), volcanics erupted in the Methow region (Midnight Peak Formation) and voluminous magmatism continued east into the InHozameen Fault
termontane Belt (e.g. Conconully, Aenas Creek,
Tonasket and Evans Lake Plutons). Plutons of
this age are abundant in northeastern Washington
(e.g. Spirit, Lake Phillips, Galena Point, Blue
EL
Foggy
Grouse, Starvation Flat, etc) and southeast into
Dew
the ~90 Ma Idaho Batholith.
BP
HL
Fault
The scale of this orogenic regime was vast, and
had profound impacts on the regional geology.
We will consider it in greater detail shortly.
Before we do however, we need to consider an
important mid-orogenic accretionary event, the
accretion of what are called the Melange Belt
Terranes.
JL
CL
CH
BL
DC
Ross Lake
Fault Zone
SM
OP
BR
CP
Figure 3-44 (Right)
TP
Map Area
Map of the North Cascades region, east of the Fraser
Fault, illustrating the distribution of Coast Range plutons.
Plutons in red date from the early Coast Range Arc
(104-90 Ma), while those in yellow are from the late (80
- 60 Ma) Coast Range Arc.
The area shaded light red is the Chiwaukum Schist of the
Nason Terrane, metamorphosed and locally migmatized
in the early Coast Range episode. The area shaded light
yellow is the Skagit Gneiss of the Napeequa Terrane,
which was extensively intruded, metamorphosed and
migmatized in the late Coast Range episode.
Plutons: BL: Bench Lake, BP: Black Peak, BR: Bearcat
Ridge, CH: Cheval, CL: Cyclone Lake, CP: Cardinal
Peak, DC: Downey Creek EL: Eldorado, EN: Entiat,
HL: Hidden Lake, JL: Jordan Lake, MS: Mount Stuart,
OP: Oval Peak, SM: Sulpher Mountain, TP: Tenpeak
EN
White River
Fault
Leavenworth
Fault
Fraser Fault
109
MS
Entiat Fault
Figure 3-45 (Left)
An outcrop of the Conconully
Pluton, southwest of Conconully. This is an early Coast
Range Pluton, dating from
about 90 Ma. Other local
plutons of this age include the
Tonasket, Evans Lake, and
Aeneas Creek plutons,
Figure 3-46 (Right)
A boulder of the Starvation Flat Pluton, northeast of Spokane. This is
a biotite-granodiorite
pluton, about 90 million
years in age. It is one
of several early Coast
Range plutons in northeastern Washington.
North of this locale, the
large Spirit Pluton is of
broadly similar character,
110
The North Cascades Region
and the Coast Range Orogen
Few subjects have proven more contentious than the causes of the Cretaceous (Coast Range) orogeny in the North Cascades region of Washington. While this subject can (and should) be considered in the larger context of the Coast Range
orogen, American researchers have a longstanding disposition to consider this province as a unique setting.
As described in the body of this text, the region of the modern North Cascades Range is a plutonic and metamorphic province which developed as part of the Coast Range Orogen in the southeastern end of the Insular Belt and associated Bridge
River Terrane. American researchers back in the 1960’s first noticed some peculiar characteristics of metamorphism here.
Further studies in the 1970’s confirmed that some the rocks here reflected at least two metamorphic / plutonic events. In
particular, earlier plutons displayed mineral assemblages characteristic of relatively shallow (10-12 km) levels of emplacement, while younger adjoining plutons displayed assemblages characteristic of deep (20-30 km) levels of crystallization.
These relationships indicate that this region was deeply buried as part of this orogenic cycle. The mechanism by which this
happened has been a subject of longstanding debate.
At the same time, researchers were trying to resolve the origins of the Northwest Cascades System, which is characterized
by large-scale low-angle thrust faults. Suspecting that there was a relationship between these two regional enigmas, some
researchers suggested that perhaps the deep burial of the North Cascade region happened as the large thrust sheets of the
Northwest Cascades System overthrust the orogen. In this scenario, the rocks of the NWCS were an outboard element of
the Methow sequence, and were subsequently thrust westward over the southern end of the Insular Belt as it was accreted.
This would have accomplished tectonic burial of the orogen, which was interpreted as the precipitating condition for highpressure metamorphism here. This is known as the “thrust loading” model, as burial is interpreted as the consequence of
tectonic “loading” of the crust. This interpretation remains a popular one, citing evidence for west-directed backthrusting
in the Coast Range to the north.
In the North Cascades region (as throughout the orogen), this interpretation faces a number of problems. These include the
lack of Methow rocks in the NWCS, the lack of a master thrust fault on which they could have been displaced, and certain
aspects of the geochronometry of the NWCS rocks. Most importantly, as described in the main body of text, the development of the Coast Range Orogen here (and elsewhere) was a diachronous process. Intrusion, deep burial and uplift of the
western coast belt (here, the Nason Terrane) was largely accomplished prior to the onset of this process in the eastern belt
(here, the Chelan Mountains Terrane). These developments are difficult to reconcile with the overthrust model.
There is evidence for overthrusting (backthrusting) within the Coast Belt Thrust Zone, which likely contributed to crustal
thickening to some degree. There is no evidence, however, for a large-scale overthrusting of the entire orogen by great
(10-15 km thick) thrust sheets for hundreds of kilometers. The thrust nappes (as such sheets are known) of the North
Cascades are unique to this region, and are not a characteristic (much less a cause) of the larger Coast Range Orogen.
More likely, as described in the main body of this text, the NWCS developed elsewhere, and was accreted as a terrane belt
thrust northward across the southern end of the Insular Belt. There is little to suggest that it had any significant bearing on
the course of orogenic development here.
Throughout the Coast Range Orogen, in both a temporal and physical context, deep burial and regional metamorphism
have been accompanied by the voluminous intrusion of the Coast Range magmas. The quantities of plutonic rock accumulated here are difficult to overstate. In the North Cascades, over 80% of the rocks in the Skagit “core” region are Coast
Range plutons. It seems a more likely proposition that burial and metamorphism were in fact a product of Coast Range
magmatism. The high-grade plutonic-metamorphic belts of the North Cascades (the Chiwaukum and Skagit regions) likely
represent the bases of thick piles of plutons accumulated in these episodes, most of which were subsequently eroded away
as the deep “core” rocks were uplifted late in the orogenic cycle. This interpretation is known as the “magma loading”
hypothesis, as burial is interpreted as the result “loading” the overlying crust with thick accumulations of plutonic rock.
111
The Accretion of the Melange Belt Terranes
Great Thrust Sheets of Oceanic Rocks
Sometime in the middle of the Coast Range Episode, probably between 95 and 85 million years ago, another “package” of accreted terranes was emplaced along the continental margin. This belt of terranes was accreted across the
southern end of the Insular (and probably Intermontane) Belt, along the northern margin of the Columbia Embayment.
They appear to have been emplaced by ocean plate motion which had a significant northward sense of movement.
In contrast to the Intermontane and Insular Belts, these terranes do not have a significant island-arc component. Instead, they are largely a highly dismembered collection of ophiolitic (ocean crust) rocks, including mantle rocks and
the exhumed components of an Early Cretaceous subduction zone. These elements are juxtaposed in a largely random
manner, with little or no coherent sense of stratigraphy. Such a mumbled “mix” of lithologies is known by the French
term “mélange,” meaning a mix. These are known as the Melange Belt Terranes
Figure 3- 47 (Above)
Mount Shuksan, North Cascades. This 9,127’ (2782m) peak is comprised almost entirely of greenschist and phyllite of
the Shuksan suite. Near the summit, one can distinguish relict pillow structures in the greenschist. Photograph is from the
Mount Baker Ski Area.
112
Figure 3-48 (Left)
Northwest
Cascades
Belt
Eastern
Melange
Belt
Western
Melange Belt
Generalized diagram of the
Melange Belt Terranes. These
belts have overthrust the
southern end of the Chilliwack
Terrane (see figure 3-4) The
southern extent of these rocks
is uncertain, as they lie buried
beneath more recent volcanic
strata. A small inlier of these
rocks outcrops at Rimrock Lake,
south of Mt. Rainier. Similarly,
the eastern extent of these rocks
is uncertain, as they disappear
underneath the (Miocene) Columbia River Basalts. They are
likely far more extensive than
their map exposure suggests.
Melange lithologies usually accumulate as an “accretionary wedge” which sometimes develops underneath the leading edge of the continent as the oceanic plate is subducted underneath it. This wedge consists of oceanic sediments
and sections of oceanic crust which have been scraped off of the subducting plate, and which accumulate underneath
the edge of the continent in tectonic slices. The Franciscan Melange which makes up much of the coast of California
is a classic example of this, accumulating to as much as 7 km in thickness.
The Melange Belts which were accreted across the southern end of the Insular Belt did not, in all likelihood, accumulate here. These belts in part consist of an Early Cretaceous subduction complex which has been uplifted from considerable depth, and which appears to have been thoroughly dismembered in the process of transport. The point is that
here, magmatism of the Early Cretaceous Gambier Arc was continuous into the Coast Range Arc, so we are lacking in
a “disposable” Early Cretaceous subduction complex which could have undergone this process. Moreover, there is no
evidence of the kind of rift-tectonics which would produce such an assemblage in this area.
Figure 3 -49 (Right)
Illustration showing the development
of an accretionary wedge on the
leading edge of the
continent, scraped
off of the descending
oceanic plate. This
is the origin of most
“melange” -type
rock units.
Accretionary Wedge
Continent
Oceanic Lithosphere
113
Instead, it appears that the mélange belts which were accreted here in Late Cretaceous time were probably rifted off of
the continental margin to the south, perhaps along modern-day coast of southern Oregon or northern California. This
happened about 120 Ma, an event probably related to the development of the Kula Plate in the northern Pacific Basin.
It is thought that the Kula- Farallon- North American triple junction originally was located just north of Cape Mendicino, California. These rifted rocks were apparently transported northward by transform motion, arriving here in
Late Cretaceous time. Three distinct belts are recognized. The northern-most belt is known as the Northwest Cascades
System, while the southern belts are divided as the Western and Eastern Melange Belts.
The Northwest Cascades System is the only belt which contains any coherent lithologies. These include a suite of
oceanic crust (the Shuksan Greenschist) and its pelitic (oceanic mud) cover (the Darrington Phyllite). The Shuksan
rocks, despite metamorphism, still in places show the “pillow” structure characteristic of ocean-plate basalts. The Darrington rocks have a high carbon content (they are a graphitic phyllite), a legacy of organic material. They probably
accumulated in a deep marginal basin close to the continent, receiving abundant organic material from that source.
Known as the Shuksan Suite, these rocks have undergone the unique high-pressure, low-temperature metamorphism
(blueschist facies metamorphism) which develops in the upper part of a subduction zone. They are known on the east
side of the Cascades (east of the Fraser Fault) as the Easton Suite in the Cle Elum area.
Accreted
Terranes
Kula Plate
Melange Belts are obducted
across the southern end of
the Chilliwack Terrane,
along low-angle thrust
faults.
Northward Transport
Columbia Embayment
New Kula-Farallon
Ridge
Earlier Subduction Zone
Terranes are rifted off
of the (now) Northern
California Coast
Figure 3 -50 (Above) Diagram illustrating the probable origins and history of the Melange Belts, as displaced fragments of the
southern Oregon and northern California coast. Diagram depicts conditions at ~120 Ma, with the development of the new KulaFarallon Ridge.
114
Figure 3-51 (Above)
The Shuksan Greenschist. This rock was originally oceanfloor basalt, in part metamorphosed at blueschist facies in
a subduction zone setting. The degree of metamorphism
varies locally. At this outcrop, along the Middle Fork
Nooksack River Road, one can still distinguish relict pillow
structures. These rocks are at lower greenschist grade.
Figure 3 - 52 (Right)
The Shuksan Greenschist, at an outcrop along the Finney
Creek Road south of Concrete. These rocks are blueschist
varieties, distinguished by the presence of crossite. The
rocks of the Shuksan suite are the only large continuous
terrane fragments in the Melange Belts. They extend east
of the (Eocene) Fraser Fault as the Easton Suite. The
Easton name is actually older, but the “Shuksan” name is
firmly rooted in the literature.
115
Figure 3 -53 (Above)
Deformed Darrington Phyllite, along the base of the
Shuksan Thrust Fault. This
is a meta-pelite, originally
ocean-floor sediments. Note
white knobs of quartz. On
Yellow Aster Butte.
Figure 3- 54 (Left)
Hand-sample of the Darrington Phyllite, from a location along the South Skagit
Road. This is a graphitic
phyllite. Note phyllitic texture, deformed quartz lenses.
116
Methow
Region
Bellingham
Figure 3 - 55
(Right)
Map of the
Melange Belts in
Washington, along
with rocks of the
Chilliwack Terrane. All contacts
are thrust faults,
with the overthrust
block on the south
side.
The affinities of
the rocks of the
Rimrock Lake
Inlier remain
somewhat uncertain, but they are
clearly part of
the Melange Belt
terranes. South of
Snoqualmie Pass,
more recent volcanics cover most
of the basement
rocks.
Igneous and
Metamorphic
“Core” Region
Fraser
Fault
Seattle
Rimrock Lake Inlier
*Cle Elum
Eastern Melange Belt
Western Melange Belt
Northwest Cascades Belt
Chilliwack Terrane
Of particular note in the Northwest Cascades System is a large ultramafic body, the Twin Sisters Dunite. This 15
km-long tectonic fragment, exhumed from the upper mantle, is the largest ultramafic body in the western hemisphere.
These rocks reflect the depth from which this subduction complex was exhumed. We will touch again on the tectonics of this process in the next chapter. The remainder of the Northwest Cascades rocks include at least a half-dozen
oceanic varieties, and a minor component of (Precambrian) continental gneiss. They are all tectonically- dismembered
units, largely devoid of any original sense of structure. These are more typical mélange components.
When this assemblage of rocks arrived along the southern end of the Insular Belt, it appears that these rocks were essentially scraped off of the oceanic plate, and were thrust to the north over the Chilliwack Terrane, in a process known
as obduction. In the process, sections of Chilliwack Terrane were incorporated into the mix. Along with the more
coherent Shuksan /Darrington lithologies, these rocks were thrust across the southern end of the Insular Belt as great
thrust sheets, along low-angle thrust faults. Total displacement on these faults may be as much as several hundred
kilometers. These sheets overlap in a distinctive set of “thrust nappes” in the San Juan Islands, Mt. Baker and western
Skagit regions of Washington State. This “stacking” of units is an arrangement described as a “system” (here, The
Northwest Cascades System).
117
Figure 3-56 (Above)
The Twin Sisters Range,
from the west, along SR 9
south of Acme. This is the
largest body of ultramafic
rock in North America.
It is a leading source of
dunite, used for refractory
purposes. Mount Baker to
the left.
Figure 3-57 (Right)
A hand-sample of the Twin
Sisters Dunite - widely
regarded as a world standard for this rock type.
The rock is largely olivine,
and is green on a fresh
surface. On exposure,
it quickly weathers to a
rusty red color.
118
Figure 3- 58 (Right)
Schematic diagram illustrating the general stacking
order of the Melange Belt
Terranes in the North Cascades. All of the solid lines
are low-angle thrust faults
From Tabor, 1994.
Figure 3- 59 (Below)
The Shuksan Thrust Fault,
on the side of Yellow Aster
Butte, North Cascades. The
fault here places the Darrington Phyllite over the Yellow Aster Gneiss. The fault
zone is marked by a section
of tectonized ultramafic
rocks, dipping to the right
(north).
East
West
Western and Eastern Melange Belts
Helena-Haystack
Melange
Shuksan Suite
Bell Pass
Melange
Yellow
Aster
Olympic
Coast
Belt
Nooksack
Group
Shuksan Thrust
Fault
Elbow Lake
Formation Cultus
Formation
Chilliwack
Terrane
Wells Creek
Volcanics
Darrington
Phyllite
Yellow Aster
Gneiss
119
WEMB
Helena Haystack
Melange
Northwest
Cascades
System
Figure 3- 60 (Right)
Thrust faults of the Northwest Cascades System, on
the south side of Yellow
Aster Butte. The view here is
looking north. These faults
are marked by sections of
ultramafic rocks, deliniated
by the yellow lines.
Chilliwack
Group
Metasedimentary
rocks
These faults separate rocks
of the Chilliwack Group from
those of the Yellow Aster
Complex (here, Yellow Aster
Gneiss). The main Shuksan
Thrust lies above this stack,
to the west (see Figure 3-59).
Yellow Aster
Gneiss
There is some controversy
about the direction in which
these rocks have been transported. One theory proposes
that they are overthrust from
the east. Fabrics in the ultramafic rocks here reflect northsouth movement, as described
in the text.
In what are now the San Juan Islands, those thrust sheets are collectively known as the San Juan Thrust Nappes.
Along the western side of the Cascades, they are stacked along thrust faults known as the Shuksan and Church Mountain Faults. At the leading edge of these sheets, along the international border, the Northwest Cascades, Chilliwack
and Nooksack rocks are stacked, interleaved and locally, overturned. On the east side of the modern Cascades, the
Windy Pass Thrust Fault places rocks of the Ingalls Ophiolite over the Nason Terrane (Mt. Stuart Pluton). South of
here, rocks of the Shuksan (here Easton) Suite are (presumably) thrust over the Chilliwack Terrane. Due to the later
cover of the Columbia River Basalts, we don’t know how much further east this belt may extend.
The Western and Eastern Melange Belts lie to the south of the Northwest Cascades System. They are of similar oceanic character, but neither contain significant continuous lithologies. The two belts display some contrasting faunal
assemblages (North American and Tethyn types), and are a somewhat different mix of lithologies. These appear to be
true mélange units, compared to the mixed character of the Northwest Cascades System rocks.
The Eastern Melange Belt overlies the Northwest Cascades System along a tectonic zone which contains rocks of the
Helena – Haystack Melange. The southern margin of that tectonic zone is known as the Darrington-Devils Mountain
Fault Zone. The Western Melange Belt lies to the south and west of the Eastern Belt, and structurally underlies it
along a thrust fault. The western belt contains some large blocks, the largest being a 4-km long block of metagabbro
120
Figure 3- 61 (Above) Rocks of the Helena-Haystack
Melange, 0n the Deer Creek Pass Road, northwest of
Darrington.
Figure 3 - 62 (Right) Orcas Chert overthrusting slate on
Fidalgo Island, San Juan Islands. These are parts of the
San Juan thrust nappes. View is to the east.
Figure 3 -63 (Below Right) Phyllitic slate of the Western
Melange Belt, on Green Mountain, north of Verlot. These
are typical Western Melange Belt rocks.
which makes up Mount Si, above North
Bend.
The southern extent of these rocks
is uncertain, but a window through
the later volcanic cover has exposed
similar rocks in the Mt. Rainier area
(the Rimrock Lake inlier). Their eastern
extent is unknown, being completely
obscured by more recent rocks. The age
of accretion for these rocks is poorly
constrained. It seems likely that they
were accreted either with or shortly after the rocks of the Northwest Cascades
System.
121
Figure 3 - 64 (Left)
Rocks of the Western
Melange Belt. These
are typical rocks of
this unit, a mix of
low-grade metasedimentary and metavolcanic rocks of broadly
oceanic origins.
The strata here are
almost vertical. A
metavolcanic layer
highlights a prominent fold in the rocks.
Most of these are
metasedimentary
varieties.
This locale is on
Meadow Mountain,
northeast of Granite
Falls
The Melange Belt rocks were obducted across the southern exposure of the Chilliwack and Nason Terranes during the
Early Coast Range Orogeny. Significantly, the obduction of these relatively minor terrane belts had no apparent effect
on the course of Coast Range orogeny, nor did it substantively change the location of the continental margin. By this
measure, their accretion does not appear to be an event of significant regional consequence beyond the southern end of
the accreted terranes.
122
Ultramafic Rocks of the Pacific Northwest:
Fragments of the Earth’s Mantle
Prior to the advent of modern plate-tectonic theories, geophysicists had a pretty good notion about what kind of material
makes up the Earth’s mantle. Based on its density gauged by seismic studies and other geophysical measurements, they
figured it was a iron and magnesium silicate rock, an ultramafic rock consisting primarily of the minerals of the olivine
family. What they were lacking, of course, was a sample to examine. Even underneath the thin oceanic crust, the mantle
is still 5-7 km deep. Several ambitious projects were undertaken to drill down through the crust and retrieve samples,
none of which were successful.
With the advent of plate-tectonic theories, it became evident that mantle rocks could be brought up to the surface along
deep fault zones in oceanic plates – although usually only as small tectonic slices, typically metamorphosed to serpentinite. Indeed the presence of ultramafic rocks or their equivalents are important diagnositic elements in identifying the
remains of ancient oceanic plates in the Pacific Northwest “collage” of terranes.
Owing to the number of oceanic sections which have been accreted to the continent here, the Pacific Northwest has a
unique abundance of ultramafic rocks. They can be seen in the rocks of the Slide Mountain Terrane, in the Cache Creek
Terrane, the Bridge River Terrane, and in the mélange-belt terranes across the southern end of the province. In the
Bridge River Terrane of British Columbia, the large Shulaps Ultramafic Complex is a classic example of that setting,
where ultramafic rocks have been juxtaposed against other characteristic elements of the ophiolite suite. Small slices
and pods of ultramafic rocks are common throughout the Bridge River (=Napeequa, Hozameen) Terrane, illustrating the
depth of the faults which finally dismembered the Bridge River Basin in mid-Cretaceous time.
The Shulaps Complex would be considered a world-class exposure were it not overshadowed by an even larger body
to the west. That body is an element in the Northwest Cascades System in the Mélange Belt Terranes, and makes up the
~16 km-long Twin Sisters Mountain just east of Bellingham. It is the largest exposure in the western hemisphere. This
oblong body is made up of a rock called dunite, which is composed of nearly pure olivine. It contains black specks of
chromite in the mix, which weather out to produce a very abrasive rock. It is a green rock, but quickly weathers to a
rusty red color as the surface iron oxidizes. The mineral composition of this rock weathers to produce a rather toxic soil,
one which depresses the local timberline around this range of 2000+ m peaks.
Olivine is mined for a number of uses, most notably as a high-temperature refractory material. It is used to line furnaces
and other in such settings, because it handles heat particularly well (remember the conditions under which it crystallizes). The largest exposures of this rock are in the Oman Ophiolite.
Image (Above) The Twin Sisters Mountain, from the east. See also figure 3-56
123
The Late Coast Range Arc:
Magmatism in the Bridge River Belt
The Coast Range Arc, as noted earlier, migrated northeast over the course of Late Cretaceous time. Early (110-85 Ma)
plutons were concentrated in the western arc terranes of the Insular Belt, and spread east into the Intermontane Belt
and all the way to Idaho. As noted, this reflects the shape of the Late Cretaceous shoreline here, the outline of the Columbia Embayment. The fact that the arc turns east here also implies a significant oblique component to the movement
of the oceanic plate, relative to the continent.
Between 85 and 70 Ma, magmatism of the Coast Range Arc moved northeast into the Bridge River Terrane and the
Insular-North American suture zone. Over most of the Canadian Cordillera, magmatism was apparently continuous
over the period of 110-65 Ma, although later intrusions were less voluminous. A robust suite of plutons dating from
85 to 70 Ma mark Canadian maps, notably the large Scuzzy Pluton. Most Canadian plutons of this age have romantic
names like GSC 78-83 or GSC 81-24. There are an abundance of them
In the southeast end of the Coast Range Belt, in the present-day North Cascades region, there is a well-defined break
Figure 3 - 65 (Above) Rocks of the Coast Range Arc. Peak to the rear (Eldorado Peak) is an early Coast Range intrusion. Rocks
in middle are the Cascade River Schist. Rocks in the foreground are the ~75 Ma Hidden Lakes Pluton, a Late Coast Range Arc
intrusion. Note: This is the cover photograph.
124
Late Coast
Range Belt
Map Area
Early Coast Range
Belt
Ross Lake
Fault
Vancouver
Vancouver
Island
Georgia Strait
Bellingham
Future Fraser Fault
Entiat Fault
Figure 3 - 66 (Above) Map illustrating the extent of the early and late Coast Range magmatic belts. The early (114-80 Ma)
belt is in pink, the late (80-60Ma) belt is in yellow. This reflects the eastward migration of the magmatic front over time. Map
is restored to pre-Fraser configuration.
between the early and late Coast Range Orogens, marked by a lack of plutons dated between 87 and 80 Ma. Over this
period, magmatism migrated from the Nason Terrane, across the White River Shear Zone (Coast Range Megalineament) and into the Chelan Mountains Terrane. This happened over a period of very rapid uplift and erosion in the
Nason Terrane. Rocks which were at deep (20-25 km) crustal levels under intrusion at ~95 Ma had been uplifted 1015 km by 80 Ma. This pattern of voluminous plutonism followed by rapid uplift and erosion appears to be a regional
characteristic of the Coast Range Orogen.
Between 75 and 60 Ma (Latest Cretaceous through earliest Paleocene) a large number of Coast Range Plutons intruded the Chelan Mountains Terrane. Representative members of this suite include the Riddle Peaks Pluton, the Marble
Creek and Hidden Lake Plutons, the Cardinal Peak and Entiat Plutons. These bodies date from 78 – 70 Ma, and tend
to concentrate along the SW side of the Chelan Mountains Terrane. Between 70 and 60 Ma, younger plutons like the
Bendor, Diablo, Custer and Oval Peak intrusions were largely concentrated along the NE side of the Chelan Mountains Terrane. This progressive northeasterly migration of the arc is a characteristic of the entire Coast Range Belt.
The course of magmatism in the North Cascades region has another curious twist, one that it shares with rocks of the
southern Intermontane Belt to the east. In both cases, early and relatively shallow plutons of the Coast Range Arc
are found to have been metamorphosed at considerable depths at a later date. In the Chelan Mountains Terrane for
example, the (88-90 Ma) Eldorado Pluton originally crystallized at depths of 10-12 km, but was later metamorphosed
at a depth of 20-25 km between 88 and 70 Ma. The long-standing question is: by what mechanism were these early
plutons buried to such great depths?
125
Figure 3 -67 (Above) Peaks of the Skagit region, part of the Bridge River terrane intruded by the Coast Range magmas. The
rocks here are over 75% orthogneiss, largely from the Late Coast Range arc.
Figure 3 -68 (Below Left) Migmatitic paragneiss along the Skagit River. (at John Pierce Falls). This is part of a “raft” of
paragneiss derived from rocks of the Napeequa (Bridge River) Terrane, in a region dominated by orthogneiss.
Figure 3 -69 (Below Right) Detail of migmatites at location to the left.
126
Figure 3 -70 (Above) and 3 - 71 (Right)
Skagit Paragneiss along the Skagit River, above the town of Diablo. These are remants of the Napeequa (Bridge River) Terrane,
metamorphosed by adjacent plutons. These are amphibolitegrade rocks.
Figure 3 -72
(Right)
A typical gneiss of the Coast
Range Orogen, reflecting
high-grade metamorphism of
older plutonic rocks. Rocks of
this character can be found
throughout the Coast Range
belt.
This outcrop is part of the
gneiss of Tiffany Mountain,
northwest of Conconully.
127
3
Scuzzy
Pluton
7
4
5
6
7
Entiat
Fault
7
4
6
8
5
3
5
4
7
6
9
Figure 3 -73
(Left) Reconstruction across ~145 km of (Eocene) displacement on the Fraser Fault places the Scuzzy Pluton
opposite the Nason Terrane and the Chiwaukum Schist. Thermobarometric estimates (as illustrated) suggest that the original
Scuzzy Pluton extended over the Chiwaukum region. Suprajacent
plutons probably accumulated much of the overburden which
resulted in the deep burial of these rocks. A similar setting is interpreted for the Skagit region to the north. Diagram adapted from
Brown and Walker, 1993.
Migmatitic Rocks
Schists
Mt. Stuart
Isobars in Kilobars
Probably the best hypothesis here is that large batholiths of granitic rock, now since eroded away, were intruded over
existing plutons, burying them to the requisite depth. This was followed by rapid uplift and erosion of the belt. A
good candidate for this in the Nason Terrane is the large Scuzzy Pluton, now offset to the north in British Columbia.
It would appear that this pluton once capped the currently exposed rocks in the Nason Terrane. It has been suggested
that plutons like this may have intruded above the areas of current exposure, accumulating to as much as 10 km in
thickness. In this interpretation, current exposures in this part of the orogen represent rocks which were at the very
bottom of this plutonic “pile.” In this interpretation, most of the rocks of the Coast Range Batholith have long since
been eroded away.
This is a very contentious issue which reflects on the very nature of orogeny in the Coast Range Belt. Other theories
for deep burial of this region have suggested that the province was overthrust from the east, or effectively “sank” due
to density differences with the adjoining rocks. In both of these cases, orogeny and regional metamorphism is attributed to tectonic burial. By contrast, the position adopted here is that burial and regional metamorphism were in large
part the product of voluminous plutonic intrusion.
The highest degree of pressures and temperatures appear to have been reached in the Skagit “core” region of the
Chelan Mountains Terrane. These were pressures approaching 9 kilobars (thousands of atmospheres of pressure) and
temperatures in excess of 700oC, reflecting burial at depths of 20 – 25 km. At these conditions, the rocks acquired a
migmatitic appearance, a swirled mix of light and dark colored rocks which look like they have been melted together.
The Coast Range Belt as a whole is noted for having some spectacular migmatites.
Most of the rocks (>75%) preserved in the Skagit “core” region are plutonic intrusives of the Coast Range Arc. Residing at the bottom of the plutonic “pile,” they have largely been metamorphosed to orthogneiss. Along with most
128
Figure 3-74 (Above) Gneiss of the Skagit Gneiss Complex. Most of the Skagit region is orthogneiss, the metamorphosed equivalent of granite-type intrusive rocks. Pictured is a classic banded gneiss, typical of the rocks here. Light-colored cross-cutting
dike is an Eocene feature. Image courtesy of the USGS.
of the original rocks of the Napeequa and Cascade River units, they have been metamorphosed at amphibolite facies.
Much of this region is a “sea” of orthogneiss with “rafts” of paragneiss derived from the older Chelan Mountains
Terrane. A common rock is a “banded gneiss” of mixed character. This high-grade belt goes by the name of the Skagit
Gneiss Complex. In the southern end of this block less voluminous intrusion has left the Napeequa and Cascade
River units more intact. Common rock species here include amphibolite (meta-basalt, meta-gabbro), schists of various
types (talc, mica, garnet), marble (meta-limestone) and quartzite (meta-chert). These units are known as the Napeequa
and Cascade River Schists.
Similar conditions prevailed in the Nason Terrane over the early portion of the Coast Range Orogen. Here however, it
would appear that greater uplift has resulted in erosion of more of the plutonic overburden. While still dominantly a
meta-plutonic (orthogneiss) belt, a greater proportion of metasedimentary rocks are preserved in this area. The dominant variety of these is known as the Chiwaukum Schist. It is largely a pelitic to semi-pelitic schist at amphibolite
metamorphic facies. Also common are sections of amphibolite and banded gneisses similar to those seen in the Chelan
Mountains Terrane. Its correlative formation on the other side of the Fraser Fault is the Settler Schist.
Coast Range magmatism started to wane after about ~70 Ma. In the North Cascades region, plutons dating from 7060 Ma are largely concentrated along the eastern boundary of the Chelan Mountains Terrane, particularly along the
Ross Lake Fault zone. By this date, the angle of plate interaction along the western margin was decreasing rapidly.
By about 60 Ma this low-angle interaction led to a fundamental change in plate relationships here, ending the Coast
Range Episode. The circumstances surrounding that change will be considered in the next chapter.
129
The Coast Range Megalineament:
Master Fault of the Coast Range Orogen
The Coast Range Megalineament is a continental-scale feature which appears to extend the entire length of the
accreted Insular Belt. It is known by this name in southeast Alaska, and as the “Work Channel Megalineament”
along the Canadian coast. The combined length of these features is over 1200 km. Along the southeast end of
the Coast Belt, this feature appears as the Central Coast Belt Fault, as far east as the Fraser Fault. In the North
Cascades of Washington, its southeastern continuation is apparently the White River Shear Zone. This brings
the total length of this feature to some 1900 km. To the north, it may extend into the Denali Fault System,
which would bring its total length to about 2500 km. It is a continental-scale feature.
In Canada, this feature is the tectonic zone between the largely island-arc terranes of Alexandria, Wrangellia
and the Harrison Lake Terrane, and the oceanic rocks of the Bridge River Terrane to the east. It likely had its
origins in the accretionary collapse of the Bridge River Basin, and may have held a role in the early contractional phase of the orogen. As a product of that orogeny, rocks on both side of this feature have been heavily intruded, deformed and metamorphosed. In Canada, the region southwest of this feature is known as the Western
Coast Range Belt, while that to the northeast is known as the Central Gneiss Belt.
This feature held a critical role in the evolution of the Coast Range Orogen. That orogen developed on the southwest side of the Megalineament
in the Western Belt between 110 and 85 Ma, and then on the northeast
side of that feature in the Central Gneiss Belt between 80 and 65 Ma. The
western belt was intruded, buried to depths of ~25 km, metamorphosed
and uplifted back to relatively shallow levels by the time that orogeny had
reached its peak in the Central Gneiss Belt to the northeast. Large-scale
uplift, cooling and erosion of the Central Gneiss Belt was accomplished
by 60 Ma. The different schedules of burial and uplift in these two parallel
belts must have been accommodated along this feature.
In the later phase of the Coast
Range Orogen, a large section of
the Megalineament in southeast
Alaska and northwest British Columbia was intruded by a series of
tonalite (quartz diorite) sills. This
has produced a composite batholith
over 1000 km in length, but less
than 25 km in thickness. These
repeated intrusions reflect the
deep-seated nature of this major
tectonic feature.
Mid - K Coast Range
Mountains
Naniamo
Sea
Pacific Ocean
Overthrusting reflects
contraction in the accreted
Insular Belt
Wrangell
Terrane
Deposition of the
Naniamo Formation
Subduction of the oceanic Kula Plate
Deposition in the Coast Range Episode:
Reflections on the Regional Cretaceous Paleogeography
Across the southern end of the Pacific Northwest, the record of Late Cretaceous sedimentation is restricted to the
Methow-Tyaughton trough, as introduced in the last chapter, and along the western margin of the continent, on what is
now Vancouver Island. Between these locales, the uplifting Coast Range Orogen shed debris to the east and west.
The Naniamo Group on Vancouver Island started to accumulate about 90 Ma, receiving sediments largely derived
from the rapid uplift and erosion of the western phase of the Coast Range Orogen. Those sediments also include varieties derived from the rocks of the Northwest Cascades System. This suggests that these rocks may have overthrust
the orogen before the major western uplift episode, and that significant amounts were likely eroded away in that event.
Along the southern end of the province, these rocks may have been part of the “roof” of the western orogen.
The Nanaimo Group offers our most complete fossil record for this period. It includes mosasaurs, elasmosaurs and
crocodiles which swam in the Late Cretaceous seas, along with ammonites, crabs, and a host of invertebrate species.
Parts of the Nanaimo contain significant coal beds, accumulated in swampy estuaries. Plant fossils from these sections reveal a decidedly warmer climate than exists today, reflecting a paratropical setting along the shores of ancestral
Vancouver Island. The Nanaimo offers a rich fossil record for this period, with important new discoveries being made
each year.
Figure 3 -75 (Above)
Setting of the Nanaimo Basin during Late Cretaceous time. Based on an illustration by NRC Canada. Based on a wide
variety of fossil evidence, it would appear that this was a paratropical setting over this period.
131
Figure 3 -76 (Left)
Fossil of an elasmosaur, from
the Nanaimo Group. Photo
from the Courtenay Museum.
Figure 3 - 77 (Below Left)
Map showing location of the
Nanaimo Group.
Figure 3 -78 (Below Right)
A collection of Ammonite
fossils from the Nanaimo
Group. Photo courtesy of the
Vancouver Island Paleontological Society.
To the east in the Methow-Tyaughton basin, early Late Cretaceous deposition included the Harts Pass, Virginian
Ridge and Winthrop Formations, as discussed in the last chapter. Deposition in this basin continued into Late Cretaceous (Cenomanian) time, largely as a collection of volcanic and conglomeratic sediments. In the Methow area these
include the Midnight Peak volcanics, as discussed in the last chapter. These are a mix of subaerial volcanic and clastic
rocks, of early Late Cretaceous age. In British Columbia,slightly younger rocks include the Silverquick Conglomerate. Overlying the Silverquick Conglomerate are the Powell Creek Volcanics, dated at about 79 Ma. In part, these
deposits may have accumulated in a back-arc basin behind the early western orogen. In the Methow region, a possible
candidate for Late Cretaceous deposition is the Pipestone Canyon Formation. This is a largely conglomerate unit,
including pumice clasts likely representing Coast Range volcanics. These deposits may have also been deposited in a
132
60
Vancouver Is.
Tyaughton
Pipestone
Canyon (?)
70
Figure 3 - 80 (Right)
Stratigraphic columns for early Late
Cretaceous sedimentary formations on
the flanks of the Coast
Range Orogen.
Methow
Powell Cr. Vol.
80
Silverquick Fm
90
100
Midnight
Peak Fm.
Nanaimo
Formation
Jackass Mtn
Group
Winthrop
FM
110
Figure 3 - 79 (Below)
Rocks of the Pipestone Canyon Formation, near Campbell Lake east of Twisp. The rock is a conglomerate, with clasts of sedimentary and volcanic rocks. It has been suggested that these rocks may have accumulated in a back--arc basin behind the Late Coast
Range Orogen. An important field note: these rocks are a favored residence for rattlesnakes
133
Figure 3 -80 (Above)
Rocks of the Midnight Peak Formation. These are the volcanic rocks which make up most of this unit. At an outcrop above
the town of Mazama.
back-arc basin setting, in this case behind the late (eastern) Coast Range Orogen. It may be something on the order of
70 million years old.
We do know that the Early Late Cretaceous was a period of marine transgression, one of the most extensive on record.
We suspect that this is tied to very rapid rates of ocean plate production which characterized this period, resulting in
the expansion of the mid-ocean ridges. A great interior seaway formed across the center of the continent, east of what
are now the Rocky Mountains. A long north-south belt, consisting of the uplifted Sierra-Klamath belt, north through
the northern Rocky Mountains of Idaho, west to the Coast Range belt and north along the modern Pacific Coast, intervened as an upland province between that interior sea and the Pacific Ocean. The northwestern shoreline of that province probably lay not far from where it does today in southeast Alaska and British Columbia, as the very substantial
mountain range raised by the early Coast Range Orogen probably rose directly from the coastline. Along the southern
end of that portion, material eroded off of these mountains was shed westward, and collected in the Naniamo Basin.
South of the international border, that mountain range probably did not extend much further south than present-day
Snoqualmie Pass, east of Seattle. That range then extended east into the southern Intermontane Belt and north-central
Idaho, marking the trace of the magmatic arc along the southern end of the Insular and Intermontane Belts. Speculatively, the northern coastline of the Columbia Embayment may have extended from modern-day Seattle to Yakima to
the Tri Cities, before heading south along the eastern end of the embayment.
134
Rocky
Mountains
Canadian
Coastal Range
Pacific Ocean
Chelan Mtns
Region
Figure 3 - 81 (Above) Late Cretaceous paleogeography. This was a time of very high sea levels, which covered much of the continent. The shorelines in this area are somewhat speculative, but likely looked something like this. Note the late K Coast Range,
well to the west of the mid-K orogen. Locally, magmatism here was concentrated in the Skagit - Chelan Mtns Region.
The scale of the mountains which must have developed along the western edge of British Columbia would have likely
presented a fairly hostile environment to the Cretaceous fauna. Elsewhere however, it was an accomodating setting.
Fossils from the Nanaimo Formation include elasmosaurs and mosasaurs, reflecting what must have been a rich marine environment along the coast. The climate at that time was warm and moist, and the coastal areas probably enjoyed a paratropical climate. One of the more accommodating regions might have been that along the northern shores
of the Columbia Embayment, in what is now eastern Washington. Completely sealed in a thick layer of younger basalt
flows, we don’t really know what underlies them, but we expect that this is the southern end of the Insular and Intermontane Belts. Here, on the coastal plain and highlands which rose to the north, dinosaurs and their kin may well have
roamed this region.
135
The Case for Large-Scale Lateral Displacement
The “Baja British Columbia” Hypothesis
Over the Coast Range Episode, transpressional forces established a system of northwest-southeast trending strike-slip faults
across the region. These included earlier accretionary features like the Ross Lake – Bralorne /Kwoik Fault Zone and the Coast
Range Megalineament, other terrane-bounding faults (e.g. the Pasayten – Yalakom and the Hozameen-Hungry Valley Faults),
along with younger, similarly oriented features such as the Entiat – Harrison Lake Fault. The amount of lateral displacement on
these faults has been a long-standing subject of debate.
Starting in the 1970’s, researchers began using a new tool, paleomagnatism, to plot the origins of displaced terranes. This technique uses the fact that iron-bearing minerals in plutonic rocks record the orientation of the Earth’s magnetic field when they
crystallize. The angle of that orientation differs depending on one’s latitude. When the observed angle differs from the current
declination, then the difference might reflect a component of latitudinal (north-south) displacement since that time.
Measuring a number of Coast Range plutons in this area, including the ~95 Ma Mount Stuart Pluton in the North Cascades,
researchers concluded that large-scale displacements may have affected rocks of these ages. In the case of the Mt. Stuart Pluton,
results indicated that it may have crystallized over 2,000 km to the south. Out of these observations has evolved a hypothesis that
the pre-Tertiary evolution of this region has occurred as the constituent elements have been progressively displaced northward for
distances up to 3,000 km, arriving at their present location in early Tertiary time. In its various forms, this is known as the “Baja
British Columbia” hypothesis.
The problem, as demonstrated over the past two chapters, is that the course of regional geologic evolution here seems to follow
a “normal” course of events as would be expected along an accumulating convergent margin. Terranes have been accreted and
sutured, magmatic arc regimes have developed across different accreted units, and large-scale orogenic episodes appear to have
developed along well-defined zones. In short, the geologic record here seems to reflect an “in suitu” development of the province,
contrary the interpreted paleomagnetic record.
Paleomagnetic study is predicated on an assumption that the rock being measured has not been appreciably tilted or rotated from
its original orientation. Most of the plutons of the Omenica and Coast Range episodes have been significantly deformed by postintrusive activity. The few relatively undeformed plutons have almost certainly been reoriented within the surrounding matrix of
the orogen. There is a strong suspicion that the more extreme paleomagnetic discrepancies are probably owed to these factors.
The notion that these rocks were intruded, rifted off a southern margin, transported thousands of kilometers up the coast along
fault systems, and were then accreted to the continental margin here - all the while still retaining their original orientation, is a
difficult premise to accept.
There is however, evidence for more modest displacements. To the east, about 450 km of northward displacement can be demonstrated on the Tintina – Northern Rocky Mountain Fault. Within the orogen, there may be a much more modest degree of lateral
displacement on the Coast Range Megalineament. The other northwest-southeast trending faults, as described earlier, probably
only record tens of kilometers of lateral movement. Some sections of these faults were “locked” at an early date by plutons which
intruded them. At a later date, Eocene displacement on the Fraser Fault added about 150 km to this total. In the end, it is not unreasonable to suggest a cumulative northward displacement of perhaps 750 to 1000 km between Mid-Cretaceous and Late Eocene
time (120 – 40 Ma).
Most of this displacement however, would have been well to the east on the Northern Rocky Mountain Fault, and on the (Eocene)
Fraser Fault. Modest displacements on the order of a few tens of kilometers probably characterize many of the internal faults of
the region, displacements which have not radically disturbed the patterns of accretion, magmatism, orogeny and sedimentation. It
is very important to note that this is not simply another issue of academic debate. The large-scale integrity of this province over
time is a fundamental premise of the model adopted here, and the interpretations which are drawn from these observations are
136
Porcupine
Trevor
Teslin
Fairweather
Chatham
Strait
Queen
Charlotte
Northern
Rocky
Mountain
Trench
Fraser
Coast Range
Megalineament
Yalakom
Pinchi
Pacific Ocean
Pasayten
West Coast
Ross Lake
137
predicated on this assumption. Having entertained a considerable body of evidence on the subject, this is a premise we adopt with
a strong measure of confidence.
Summary:
New Lands Along an Evolving Margin.
The Coast Range and Omenica Episodes were both precipitated by the accretion of large composite island-arc terrane
belts, were at least initially characterized by contractional thickening along the margin of the continent, and supported
regimes of continental-arc magmatism which developed as large-scale orogenic belts. Regimes of broadly similar
character appear to have characterized much of the west coast of North American over the Jura-Cretaceous, some of
which were continuous with the events described here. It appears that both terrane belts may have occupied a Philip-
Figure 3-81 (Previous Page)
Principal faults of the Canadian Cordillera. Adapted from Geological Survey Canada
Figure 3 -82 (Above)
The Twin Sisters Mountain, from the northeast. The rock here is almost entirely dunite, an ultramafic variety.
138
pine-type setting with the continent during transport, and a Japan-Type setting for a period prior to their accretion.
With the accretion of the Insular Belt, that margin became a classic Andean-type setting.
These circumstances may account for the differences in the relative intensity of tectonic, magmatic and orogenic
events between the two regimes. When the Intermontane Belt was accreted, the Insular Belt may have lay ~1500 km
to the west. In that position, it was able to absorb much of the east-directed ocean plate movement from the outboard
plate. As a result, the scale of deformation associated with accretion, as well as the scale of Omenica magmatism,
were reduced compared to what would have developed on a true Andean-Type margin.
That true Andean-Type margin was first established when the Insular Belt was accreted. The scale of the deformation
which accompanied accretion, along with the scale of Coast Range Arc magmatism and orogeny, reflect the full-force
interaction between the oceanic and continental plates along this margin. The results of this interaction included
deformation extending all the way east into the Alberta Fold Belt, and the intrusion of the largest composite batholith
in the continent.
The Andean-type margin of the Coast Range Episode started out as a contractional regime, but evolved into a
transpressional setting as the angle of incidence between the oceanic and continental plates decreased over time. That
northerly component to ocean plate motion was apparently responsible for the transport and accretion of the Melange
Belt Terranes, which were obducted across the southern end of the region mid to late Coast Range episode.
The Coast Range Orogen largely developed as two parallel belts of diachronous activity. This consisted of a southwestern belt developing in the island-arc terranes of the Insular Belt between 110 and 80 Ma, and a northeastern belt
developing in the oceanic rocks of the Bridge River Terrane between 80 and 60 Ma. Both belts were characterized by
voluminous intrusion of magma, accomplishing large-scale crustal thickening through intrusion, followed by rapid uplift and erosion. The main tectonic feature accommodating this diachronous pattern of burial and uplift was the Coast
Range Megalineament. Locally, the Entiat Fault may have its origins in this process.
Over the course of the Coast Range Episode, the lateral angle of interaction between the continental and oceanic plate
became progressively less as the oceanic plate assumed a more northerly direction. The episode was initiated by a
contractional event starting at about 120 Ma, and developed into a transpressional regime by ~90 Ma. By about 60
Ma, the angle of interaction between the two plates had diminished to a point where subduction apparently ceased
along the western margin. Magmatism of the Coast Range Arc ceased, and a new tectonic regime was established
along the margin. With these events, The Coast Range Episode ended at about 58 Ma.
That new regime was a transform margin, similar to that which exists along the San Andreas System in southern California. This tectonic regime is the signature of the next episode in regional geologic evolution, known as the Challis
Episode. The Challis episode developed just 5-7 million years after the great extinction event of the Cretaceous- Ter-
Chapter 3: The Coast Range Episode
Evolution of the Pacific Northwest, © J. Figge 2009
Published by the Northwest Geological Institute, Seattle
Available on-line at www.northwestgeology.com
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