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Chapter 3 The Coast Range Episode New Lands Along an Evolving Margin This document is designed to be viewed in book format on the Adobe Reader platform.To format this properly, open the “view” category and select the “page display” option. From this, select the “two page view” option.This will display the document properly. 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 85 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. 86 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. 87 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. 88 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. 89 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- 90 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”. 91 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. 92 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 93 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. 94 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. 95 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 96 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. 99 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 101 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 139 140