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ОглавлениеIN BRITISH COLUMBIA, it is hard to ignore geology. Most of us may not understand the rocks around us as fully as we would like, but we are well acquainted with them—they stare at us from mountain cliffs and rugged shorelines every day.
If you were to take a flight across the province—say, from Jasper, Alberta, to Port Hardy on Vancouver Island—you would see a wide variety of geological features. First come the sedimentary rocks of the glistening Rockies, rising abruptly above the forested plains. Next, the snowy peaks of the Cariboo Mountains rise up across the Rocky Mountain Trench, their crystalline rocks tortured by the heat and pressure of unimaginable forces beneath the surface of the earth. Now the broad Interior plateau comes into view, with its flat surface of poured lava surrounding the eroded valleys and canyons of the Fraser and Chilcotin Rivers. And finally, the shining white ice of Mount Waddington rises ahead, towering over the Coast Mountains’ choppy sea of granite. The face of the province that you have seen—its two great montane belts, separated by the Intermontane plateau and fringed to the west by the Insular belt with its islands and passages—is the cover of a book in which we may read a long and fascinating story.
In more ways than one, geology is the foundation of natural history. Geological formations not only form the physical base of terrestrial life and control the climate around it but also tell the temporal history of nature. Geology tells us how things came to be the way they are. Moving continents, rising and falling mountain barriers, vast volcanic eruptions and continental ice sheets all played an essential role in creating the diversity of life in British Columbia today.
In the field of geology, an oft-quoted maxim is “The present is the key to the past,” meaning that in order to understand old rocks, we have to look at geological processes that are at work today. The reverse is also true: to understand the diversity of landscape and life as we see it now, we need to delve into the deep past to see how the land of British Columbia came to be.
The Building of British Columbia: Plate Tectonics
British Columbia is part of the North American Cordillera—the mighty set of mountain ranges that stretch from northern Alaska to southern Mexico (Map 1, overleaf). This mountainous landscape arose through plate tectonic processes. Plate tectonics is how the earth works. Its crust and underlying relatively stiff upper mantle form a carapace of plates like the bones of a baby’s skull before they suture and lock together. The plates are constantly moving—some growing, some shrinking—at about the speed a fingernail grows. The key to the Cordillera is a long history of interactions between the western edge of the continent, the plates that make up the floor of the Pacific Ocean, and the small, mobile pieces of crust in between that have been created, that have evolved and that have shifted between ocean and land.
Our planet is unique in having plate tectonics. The constant swirling and recycling of the ocean’s rocky floor requires that the planet’s interior—like Goldilocks’s bowl of porridge—must be just right, not too hot and not too cold. Plate tectonics results from a balance between subduction—the sinking of oceanic plates at trenches like the modern Cascadia subduction zone off British Columbia’s west coast—and spreading at ocean ridges, where new crust is created by the rise of hot material in the earth’s mantle, as is happening at the Juan de Fuca Ridge a little farther west (Figure 1, overleaf). The process of plate tectonics as we know it began sometime in Precambrian time. Exactly when is a matter of current discussion. But geologists agree that before then, the young earth was too hot and the plates were too buoyant to sink deep into the mantle. Eventually, the planet will cool to the point that upward rise of mantle and melting of basalt to supply the ridges will fail—but we have billions of years left before that time. Meanwhile, the plates shift constantly, slowly, inexorably, building mountains while we sleep, only rumbling their intentions with earthquakes from time to time.
MAP 1. LAURENTIA: THE CORE OF NORTH AMERICA. The Precambrian core of the North American continent is made up of the ancient continent Laurentia. Laurentia’s own core, often referred to as the Canadian Shield, contains some of the oldest rocks on earth, including the 4.0 billion-year-old Acasta gneiss in the Northwest Territories and the 4.28 billion-year-old Nuvvuagittuq belt in northern Quebec (shown by stars). Progressively younger rocks were added to the old Shield by successive mountain-building events that occurred as a result of Laurentia’s interactions with other tectonic plates. After the Atlantic Ocean began to open, new lands were added along the Pacific Coast, creating the youngest mountain belt, the North American Cordillera.
The modern North American continent is constructed like a chocolate-covered nut (Map 1). At its core is an ancient continent, or craton, called Laurentia. Laurentia holds the record for the oldest rocks yet dated on earth, announced by Quebec researchers in 2008 as 4.28 billion years old. Compared with the craton, the rocks that make up the outer continent margin—including those of the Cordillera—are much younger, generally less than 700 million years. All of them have been added to the original continent. There are piles of sedimentary rock that once lay at its outskirts but rode up over it during periodic collisions. There are also continental fragments that had split from the continent but were later pushed back against it, parts of the margin that were dragged sideways by the motion of offshore oceanic plates. Some of the added pieces are actually crustal wanderers that crossed oceans to reach the western reach of the growing continent, and they play their role in building mountains there.
Not that Laurentia simply sat there, waiting for all this to happen. Its story, too, is that of a wanderer. It has been part of two supercontinents, and probably others before them, in the endless flamenco of approach and spurn, touch and turn away that has marked the earth’s rocky carapace ever since it formed. The breaking up of the Precambrian supercontinent Rodinia 750 to 550 million years ago did not create our Cordillera—that was many eons later—but it made the Cordillera possible. Without that breakup, Laurentia would have lain serenely within a vast continental interior: a prairie, perhaps, or a vast plain of lakes and wetlands, its smooth, low surface unbroken by even a dream of mountains.
FIGURE 1: THE PLATE TECTONIC SETTING OFF THE COAST OF BRITISH COLUMBIA TODAY. Magma from the earth’s mantle rises upward along the Juan de Fuca Ridge, cooling to form new ocean floor. The walls of the ridge are pulled apart by the same convection currents, and the Pacific Plate and the Juan de Fuca Plate grow symmetrically on either side of the ridge. Where the latter plate encounters westward-moving North America, it slides beneath the continental shelf and descends into the mantle. When it reaches depths of 150 to 100 kilo-metres, the plate partly melts again, and the resulting magma rises up to reappear as the volcanoes of the Cascade-Garibaldi Arc—among them Mount Meager, Mount Garibaldi, Mount Baker, Mount Rainier and Mount St. Helens. Adapted from C.J. Yorath, Where Terranes Collide, p. 123.
But as it happened, towards the end of Precambrian time, a rift formed in what is now southern British Columbia, one of the many that fragmented the world continent Rodinia into many pieces. Whatever was on the other side of that rift—Australia, Antarctica and Siberia each has its advocates—moved slowly and stately away to the west. The Pacific Ocean was born, and the whole tectonic drama of Cordilleran evolution could begin.
The Cordilleran terranes are pieces of once-mobile crust that make up much of the Cordillera, extending west to the Pacific Ocean from an eastern edge in the Omineca Mountains. On Map 2 (page 16) you see them divided into realms, according to their origins. The peri-Laurentian terranes lie between the Omineca Mountains and the western Coast Mountains and underlie the Intermontane region in between. They once were the bedrock of arc-shaped chains of volcanic islands and small oceans that lay west of the old continent, in a complex and evolving geography comparable with the other side of the Pacific Ocean basin today. Think of Japan, perhaps, or the Philippines. One of the ancient island arcs is named Quesnellia, after the town of Quesnel. It runs from there north to the Yukon border east of Teslin Lake and south past Princeton. The other old island arc, Stikinia, spans western British Columbia from Bella Coola to Atlin. Island arcs form above subduction zones. Their volcanoes build from lavas and explosive volcanic deposits that originate as melts of the subducted plate as it plunges down into hotter and hotter mantle.
Parts of these volcanic island chains were founded on rifted fragments of Laurentia (to imagine a rifted fragment, think of California west of the San Andreas Fault—this piece of the continent is being pushed inexorably north and will eventually sail past the west coast of British Columbia). The Slide Mountain terrane is the Late Paleozoic seafloor of a minor ocean that grew between one of these rifted chunks and the mother continent. It is spectacularly exposed in the Cassiar Mountains of far northern British Columbia, forming dark peaks of basalt and deep-water sediments where it now rests atop the pearl-grey limestones of western Laurentia. It is as if the floor of today’s Sea of Japan, a small ocean that for the last 20 million years or so has been widening between Japan and mainland Asia, were to be shoved back up on top of Korea, and then the whole pile uplifted and carved into mountains.
MAP 2. TERRANES. The pieces of the earth’s crust that have come together to form British Columbia, grouped here according to their origins. Outboard: Recently added terranes that have travelled up the west coast to their present locations. Arctic: Terranes that originated in what is now northern Europe and Russia and travelled across the Arctic to the northwest coast. Tethyan: Terranes that formed in or near the ancient Tethys Sea. Peri-Laurentian: Terranes that either rifted from ancestral North America and subsequently returned or were formed as island arcs close to its west coast. Ancestral North America: Terranes or regions that are part of the ancient continental margin. Rocks of Ancestral North America are divided between those deposited along the continental shelf (NAp—mainly limestone and sandstone as seen in the Rockies) and those deposited in deeper water settings along the continental slope and rise (NAb). The Cassiar and Kootenay terranes never left the continent’s edge but have been displaced relative to it. The dark lines on the map denote the major faults of the Cordillera.
Compared with the relatively local peri-Laurentian terranes, those of the Tethyan and Arctic realms have travelled astounding distances to arrive in their present Cordilleran berths. Among the many lines of evidence for their exotic origins, fossils are one of the most compelling. The Cache Creek terrane forms a discontinuous strip in the British Columbia Interior, surrounded to the east, north and west by more local peri-Laurentian terranes. Its southern exposures can be seen around Cache Creek and Clinton and as far west as the white limestone bluffs of Marble Canyon. On the drive north of Cache Creek on Highway 97, some of the nearby low hills are made of curiously bare, crumbly, dark green to blue-green scree. This is serpentinite, a rock that once made up the deep mantle underpinnings of oceanic crust. Serpentine is a stone that grows little moss, and still less complex forms of vegetation. Compared with continental crust, which has benefited from the distillation of nutrients in generations of magmas and of sedimentary cycles, mantle is a poverty-stricken substrate composed of silica, magnesium, iron, nickel, cobalt and precious little else. Few plants can survive in its nutrient-poor soils. But its presence here delights the geologist, because its exhumation from deep mantle to grassland demonstrates a powerful process of planet-scale plate motion and, more specifically, a dramatic collision of an oceanic plate with the continent.
YABEINA
If you were to look closely at the limestones around Marble Canyon you would find, along with corals, some unassuming little fossils that look like fat grains of wheat. They are fusulinids, a now-extinct family of foraminifera (shelled amoeboid organisms) that flourished in warm Late Paleozoic seas. The youngest Marble Canyon fusulinids are Late Permian, and some are of the genus Yabeina. These small foreign creatures have no known relatives in or near Laurentia, but they and all their cousins can be found in their billions in the Permian limestones of China and Japan. In Permian time, long before the continental collisions that drove the Alps and Himalayas skyward, a bend of ocean called the Tethys lay surrounded by Europe, Siberia, Africa, India and Antarctica, with the continental fragments that now make up China on its east. Yabeina grew prolifically there. The Marble Canyon limestones are thought to have been reefs built on an ocean island somewhere on that side of the Pacific. After that, the island must have become entrained in an eastward-moving oceanic plate, reeled towards the Laurentian margin by rapid subduction under its fringing island arcs, Stikinia and Quesnellia.
Marble Canyon aerial perspective: Permian-Triassic limestone from a tropical southwest Pacific island, now lodged in central B.C.
Outside and west of the peri-Laurentian terranes in British Columbia lie the Insular terranes, Wrangellia and the Alexander terrane—the bedrock of Vancouver Island, Haida Gwaii (the Queen Charlotte Islands) and the islands of the Inside Passage. These rocks are also exotic but probably with an entirely different origin than that of the Cache Creek terrane: they once were part of the Arctic realm. Their older parts formed and evolved somewhere near northern Scandinavia and eastern Siberia until in mid-Paleozoic time, when they were propelled westward through the Arctic Ocean and into the Pacific. Again, some of the key evidence is fossils. For instance, some unusual early Paleozoic sponges (480 to 420 million years ago) are found in the Alexander terrane on Prince of Wales Island in southeastern Alaska just north of Prince Rupert. Other than the Alexander terrane, these particular sponges are found only in terranes of northwestern Alaska and Oregon, and in the southern Ural Mountains.
Silurian sphinctozoan sponges, from Alaska Prince of Wales Island—natives of the Ural Mountains region, brought to these distant shores by plate tectonics.
The transport of the Arctic and Insular crustal fragments westward across the Arctic seaway left its traces in glancing mid-Paleozoic collisions recorded in the rocks of the Canadian Arctic Islands and the Brooks Range of northern Alaska. Unlike the ocean floor that ferried the Cache Creek oceanic islands towards the Laurentian margin under traction from its subduction zones, the Arctic terranes were fragments of volcanic island arc and continental origin that might have transited between northern Laurentia and Siberia by a mechanism like the recent history of the Caribbean ocean (Maps 3–8). In the Caribbean, an island arc that once lay next to the Pacific Ocean reformed into a giant, bulging loop that surged over a thousand kilometres across to the Atlantic side, its ends colliding with the Bahama Banks to the north and Venezuela to the south. This incredible journey is well documented by geological observations. It has taken about 60 million years to accomplish, and is still happening, with the eastward migration of the Lesser Antilles island chain. The tragic earthquake in Haiti in 2010 was a catastrophic release of pent-up strain on the Enriquillo-Plaintain Fault, one of the great faults that separates the eastward-moving Caribbean plate from westward-moving North America.
The “loopiness” of island arc chains in general—think of the graceful festoons of the Aleutians, Kuriles and Marianas around the north and west of the Pacific—is caused by the oceanward advance of island arcs towards their subduction zones. The shorter the total length of the arc, the faster its advance because the easier it is for mantle to flow around its ends and into the gap behind it, where a new little ocean opens wider with time. Short arcs clock high rates of forward migration—1.8 centimetres a year for the Lesser Antilles, 5.7 centimetres a year for the Scotia arc southeast of Tierra del Fuego and 6.8 centimetres a year for the Calabrian arc, a tiny obscure feature of the Mediterranean Sea. By contrast, the centre of the 4000-kilometre-long Andean arc is thought to be actually retreating at 0.7 centimetres per year. With this in mind, it is easy to imagine that the short arc segment between Laurentia and Siberia would have been a prime bet as a fast forward traveller.
The evolution of marine faunas in the Insular terranes attests to the terranes’ westward migration. By Late Paleozoic time, instead of eastern Arctic forms, fossils in them are typical of northern Pacific waters. They were not yet interacting directly with anything on the western Laurentian margin, but they were getting close enough to play their part in the events to come.
Collisions and Upheavals: the Continent Grows West
The mid-Jurassic, about 185 to 170 million years ago, was a time of crisis and profound change in the Cordillera. Before then, the peri-Laurentian terranes formed a dynamic, shape-shifting zone west of Laurentia. Farther west, the Insular terranes shifted and rifted, still all on their own. After the mid-Jurassic, all of these massive crustal blocks came together to collide and coalesce, heave and pile, thrust and thicken, creating the Cordilleran mountains that we know now.
What happened?
The key is in the timing. The Insular terranes collided with the outer margin of the peri-Laurentian terranes, in what is now the western Coast Mountains, in the mid-Jurassic. In southeastern British Columbia, in the Goat Range near New Denver, the peri-Laurentian terranes were first thrust up on the sedimentary apron of the continent—in the mid-Jurassic. The youngest ocean-bottom deposits in the Cache Creek terrane that represent the end of the terrane’s existence as an open ocean are from the late Early Jurassic. The volcanoes of Stikinia and Quesnellia all shut down in the mid-Jurassic, signifying the death of the subduction zones that had fed them. Whatever triggered these sweeping and simultaneous changes must have been at a scale vaster than all the terranes taken together.
The likely cause lies in global plate tectonics. In Middle to Late Paleozoic time, Laurentia had become incorporated into the supercontinent Pangaea, by collisions with Europe and South America that built the Appalachians. But supercontinents, like empires, carry the seeds of their own demise. Like Rodinia before it, Pangaea began to break up in the Early Jurassic. The North Atlantic began to open about 180 million years ago—first a crack, then a seaway, and then, by the mid-Jurassic, a nascent ocean. A new continent, North America, with old Laurentia in its core, started to move ponderously westward. The once-independent terranes of the Cordillera simply got in the way.
MAPS 3 TO 8. THE TECTONIC EVOLUTION OF WESTERN NORTH AMERICA (following pages). The assembling of the west coast of North America is a complex story, and this series of maps serves as a visual guide to the wanderings of terranes. An approximate outline of the present continent is in blue, and the inferred extent of continent through time is shown by grey shading. Orange shading shows active mountain belts.
MAP 3. SILURIAN (425 million years ago). The Arctic terranes (yellow) that now occupy coastal B.C. and part of Alaska probably originated near the northern end of the Caledonian mountain belt, between the continents of Laurentia, Siberia and Baltica.
MAP 4. EARLY DEVONIAN (395 million years ago). The westward travel of the Arctic terranes towards Panthalassa (the precursor to the Pacific Ocean) is believed to have been propelled by a Caribbean-style subduction zone. This subduction zone, its island arcs and continental fragments travelled rapidly along a Paleozoic Northwest Passage.
MAP 5. LATE DEVONIAN–MISSISSIPPIAN (360 million years ago). The northward shift of Euramerica (the now-combined Laurentia and Baltica), during its collision with Gondwana, results in the formation of a subduction zone along the west coast. This subduction begins from the small Caribbean-type zone and moves some of the Arctic terranes southward. The hot upwelling beneath the subduction zone causes a rift in Laurentia, giving birth to the first of the peri-Laurentian arc terranes along the west coast, including the Yukon-Tanana terrane (blue-green).
MAP 6. PENNSYLVANIAN–EARLY PERMIAN (300 to 285 million years ago). Westward retreat of the subduction zone leads to widespread island arc volcanism (green) that develops on fragments of western Laurentia (blue-green) and on some of the Arctic terranes (yellow) that had recently arrived. These new arc terranes include the early expressions of Stikinia and Quesnellia. The Slide Mountain Ocean develops between the island arc and the continental margin in the wake of the westward migration of the arc, mirroring what is happening in the present-day Sea of Japan. The Insular terranes of coastal B.C. (Alexander and Wrangellia) come together far out in Panthalassa. Alexander is one of the Arctic terranes with northern European origins.
The result was our mountains—low ones at first, with the initial collisions, but as the continent continued its inexorable course, sedimentary strata at its margin piled up like snow in front of a vast, majestic snowplow, riding up and over eastward to make the shingled stack that later would be sculpted into the modern Rockies. The physiography of British Columbia—its twin backbones of the Coast Mountains and the Ominecas and Rockies separated by the more subdued Intermontane belt—is the result of the two, slow-motion, simultaneous collisions. Where the Intermontane terranes piled up onto the old continental margin, the Omineca and Rocky Mountains rose. Where the Insular terranes collided with the outer edge of the Intermontane terranes, the Coast Mountains were born.
MAP 7. LATE PERMIAN–EARLY TRIASSIC (250 million years ago). All major continental masses have converged to form the supercontinent Pangaea. Along its west coast, subduction has reversed to consume the Slide Mountain Ocean, returning the peri-Laurentian arc terranes (green) to near the continental margin. Later in Triassic time, subduction flips once more (dashed grey line), and arc volcanism flourishes again on Quesnellia and Stikinia. Alexander and Wrangellia remain at large in Panthalassa.
MAP 8. EARLY JURASSIC (190 million years ago). The Atlantic Ocean is born, growing in the rift between North America, Africa and part of Europe, and propelling North America westward. Buckling of the peri-Laurentian (Intermontane) terranes traps part of the ancient Pacific Ocean floor that was brought to North America by the subduction conveyor belt from far reaches—the Cache Creek terrane. At the same time, the westward-moving North America is on a collision course with the Insular terranes lying offshore in the Pacific. The ultimate collision will build the mountains of western North America and shape the final terrane patchwork (Map 1, page 10).
The Omineca–Rocky Mountain Collision Zone
As North America drove under its western neighbours during the Middle Jurassic, large pieces of Quesnellia and the Slide Mountain terrane began to peel off the oceanic plate. Some slices up to 25 kilometres thick overrode the continental margin, becoming stacked like pancakes on top of it. This stacking makes it difficult to say precisely where the old edge of North America lies today. The rocks of the terranes and the old continental shelf were squeezed and folded to form the Columbia, Omineca and Cassiar Mountains. In some areas the intense compression and consequent heating recrystallized the rocks into the metamorphic rocks of the Omineca and Monashee Mountains and the Quesnel and Shuswap Highlands. Partial melting in some regions gave rise to local intrusive igneous rocks.
A river of golden volcanic rock flows down a steep slope in the Ilgachuz Mountains, north of Anahim Lake.
Compression continued, and the thick layers of sedimentary rocks covering the continental core were pushed ever eastward in front of the colliding wedge and were squeezed, folded and telescoped (Figure 2). The sedimentary layers first were deformed into waves like those in a carpet being pushed. But the strong, resistant limestone layers broke when folded and became stacked up one on top of another in gently sloping piles. These breaks are called thrust faults, and the blocks of rocks above the break are called thrust sheets. By 120 million years ago, the western ranges of the Rockies were stacking up. A deep depression, the Rocky Mountain Trough (not Trench) formed east of the mountain-building wave, the result of the tremendous weight building up on the edge of the continent. The rapid uplift caused massive erosion of the new mountains, and sediments soon piled up in the trough’s inland sea, forming thick deposits of mudstone and shale.
The mountain-building wave in the Rockies continued to move eastward. The main ranges were rising about 100 million years ago and, by the time the pushing stopped about 60 million years ago, the eastern ranges and foothills had been created. When all was said and done, the thrust sheets (Figure 2) had been telescoped and shoved up to 250 kilometres eastward from their original position—the rocks of Mount Rundle, at Banff, were originally laid down somewhere around Revelstoke. As the thrust sheets moved to the east and stacked on top of one another, the Rocky Mountain Trough moved eastward ahead of them. But the thrust sheets overtook the shales that had been deposited in the trough’s earlier position, and layers of these soft shales were caught between the sheets. The shales erode much more easily than the resistant limestones, and this difference results in a pattern that is seen over and over again in the Rockies—hard, limestone cliffs towering over soft, shale-bottomed valleys (Figure 2, page 34).
FIGURE 2: THE FORMATION OF THRUST FAULTS AND THRUST-FAULTED MOUNTAINS. Stage 1: Compression from the left bends and finally breaks the rock layers. Stage 2: The upper sheet of rocks, the “thrust sheet,” is pushed over the lower sheet. Stage 3: The face of the mountain after erosion. Some of the ancient limestones at the bottom of the sedimentary pile (e.g., layer D) end up on top of the younger shales (e.g., layer B). Adapted from C.J. Yorath, Where Terranes Collide, p. 9.
Treadmill Ridge, looking south along the Continental Divide between Jasper National Park and Mount Robson Provincial Park. These gently sloping mountains end in abrupt cliffs to the east, which mark today’s eroded edge of a thrust sheet.
The Coast Mountains Collision Zone
As the Insular terranes ran into the Intermontane terranes, a new subduction zone formed to the west near the present continental margin, and a new belt of continental magmatism replaced the old island arcs along the line of the present Coast Mountains. Many separate but coalescing igneous intrusions rose up in a succession of pulses from 170 to 50 million years ago, creating the Coast Mountains batholith, one of the largest bodies of granite and granitoid rocks on the planet.
The heat of all that intrusion softened and weakened the earth’s crust and created a second, outboard zone of crustal thickening between the advancing new continental margin and the subduction zone. If you drive from Terrace west along the Skeena River towards Prince Rupert, you can see the gneisses that make up the roots of these mountains. Some of them have experienced conditions of pressure and temperature that could only have occurred at 25 kilometres below the surface, showing the amount of uplift that has made, and made again and over again, this maritime mountain range over the years.
Marble Peak on the mainland, east of Princess Royal Island. The Coast Shear Zone passes through here, as shown by the highly sheared metamorphic rocks of the main peak.
The huge volume of granites in the Coast Mountains, as well as the intensity of deformation at later stages in their geological evolution, has made the early history of this mountain range particularly hard to decipher. Crustal thickening and metamorphism 100 to 80 million years ago produced such profound changes that evidence for the initial collision between the Insular and Intermontane terranes has been nearly wiped off the record. Another key structural event that was mostly overwritten by the frenzied later Cretaceous is a series of older faults that can help us understand the geological puzzles posed by the southernmost Coast Mountains and North Cascades.
Terranes of the Southern Coastal Belt
The southern Coast Mountains and the North Cascades, so accessible to hikers and skiers from Vancouver, actually contain some of the most perplexing geology to be found anywhere in the province. Instead of a few big terranes, they are made up of a whole structural stack of little ones (Map 9). Some of these, the Bridge River, Methow and Cadwallader terranes, represent an ocean like the Cache Creek, except that instead of closing in mid-Jurassic time, it did no such thing until halfway through the Cretaceous. Other terranes, like the Chilliwack and Harrison Lake, resemble parts of Stikinia. Then there are piles of little terranes in northern Washington State that represent nothing else in British Columbia and in fact have no known equivalents north of the Klamath Mountains of California. A satisfying solution to this puzzle is finally emerging, thanks to Jim Monger and his colleagues. They point out that the late closure of the Bridge River ocean means that, somehow, Stikinia and the Insular terrane were not even there until about 100 million years ago, unlike farther north where they were well in place 70 million years earlier. Also, the stack of little terranes in Washington were thrust up from the south—neither from the northeast nor southwest, as is the usual case in the main thrust belts of the Coast Mountains or Rockies. These geological anomalies can be explained if you imagine that the outer part of the Coast Mountains, along with the Insular belt, moved southward between mid-Jurassic and mid-Cretaceous time, closing off the Bridge River ocean as it went, and eventually rammed into the western Klamaths of northern California. The faults that this happened along have only recently been found. One of them lies under Grenville Channel, that long, narrow straight stretch of water that marks the Inside Passage south of Prince Rupert.
Nowadays, we take for granted northward motion of the Pacific plate relative to North America. This movement is what gives us great modern faults like the San Andreas and Denali, and older ones like the Tintina, the Fraser and the Cassiar. But oceanic plates are fickle and evanescent compared with the long-term existence of continents. It seems that in Jurassic up to mid-Cretaceous time, some plate was out there, charging south with respect to North America and dragging the outer part of British Columbia along with it. It only vanished about 100 million years ago, and other north-travelling plates coupled with the Cordilleran margin and dragged the outer parts of it back up— some might say—where it belongs.
MAP 9. TERRANES OF THE SOUTHERN COAST. A detailed look at the terranes of the Cascade Range, southern Coast Mountains and southern Vancouver Island. For the location of this map, see MAP 2.
Slipping and Sliding
About 85 million years ago, the Farallon Plate under the Pacific Ocean rifted in two (Figure 3). The northern plate, named the Kula Plate, began spreading in a much more northerly direction than before. Because the North American Plate was still moving west, the new continental margin was now not only squeezed and foreshortened but smeared to the northwest. The crust had to give, and it slid north along faults such as the Northern Rocky Mountain Trench and the Fraser and Queen Charlotte–Fairweather Faults. Along the Northern Rocky Mountain Trench, the land to the west moved certainly 450 kilometres, and possibly up to 750 kilometres northward relative to the Rockies to the east. Faults that separate laterally moving surfaces are called strike-slip faults; the San Andreas Fault in California is probably the best known example of such a fault. The resulting pattern from all this faulting and sliding is one of elongate, northwestward-trending terranes, as shown in Map 2, page 16. But the strike-slip faults do not necessarily mark the edges of foreign terranes—the Northern Rocky Mountain Trench, for example, is 50 to 100 kilometres east of the continental margin. The land displaced to the west of it, although originally part of North America, is called the Cassiar terrane.
This squeezing and slipping along the coast of North America continues today—Baja California and all of California west of the San Andreas Fault are sliding slowly northward and will probably collide with Alaska in 50 million years or so. Off the British Columbia coast, the Queen Charlotte–Fairweather Fault separates similarly sliding chunks of crust.
FIGURE 3: SPECULATED PLATE HISTORY IN THE PACIFIC OCEAN. Successive plates are born, grow and are then consumed by succeeding plates. North America is presented as a fixed entity to give a stable reference point, and arrows give relative directions of oceanic plate movement. The lengths of the arrows are proportional to the plates’ velocities. In (A), the Farallon Plate dominates the floor of the eastern Pacific Ocean 100 million years ago. At 65 million years ago (B), the Farallon Plate has rifted in two, creating the Kula Plate to the north. By 37 million years ago (C), the Pacific Plate dominates the ocean floor; the Kula Plate has gone and the Farallon Plate is fragmented, creating the small northern Juan de Fuca Plate. Adapted from H. Gabrielse and C.J. Yorath, eds., Geology of the Cordilleran Orogen in Canada, Fig. 3.3.
Relaxation
By 60 million years ago, the Rocky Mountains were a wide band of magnificent high plateaus and towering mountains probably over 4000 metres in elevation. But then the pushing stopped. The Kula Plate found a new subduction route beneath the new continental margin and the tectonic pressure eased. The compressed crust relaxed and pieces of it began to slide off the thick pile. Along the western wall of the Rockies from the Robson Valley south, the Southern Rocky Mountain Trench formed. There, the crust foundered and the western block fell as much as 1000 metres relative to the mountains on the east. This same faulting process has created valleys such as the Elk, Flathead and Okanagan.
In the south Okanagan, beginning about 50 million years ago, a large piece of Quesnellia slid off about 90 kilometres to the west, exposing the basement rocks of the old continental margin. These ancient rocks can be seen along the east side of Skaha Lake, on both sides of Vaseux Lake and, most spectacularly, in the vertical wall of McIntyre Bluff, just south of Vaseux Lake.
The Latest Collisions
About 55 million years ago, the relative direction of the Kula Plate changed, and it began to slide more northward along a fault similar to today’s San Andreas Fault in California. Since the Olympic Peninsula did not yet exist, Vancouver Island projected out into the Pacific and formed a trap for northwesterly slipping terranes. During this period, two terranes were brought up the coast on the Kula Plate and pushed into southern Vancouver Island. The Pacific Rim terrane, which arrived about 55 million years ago, consists of sedimentary and volcanic rocks that now form the southwest coast of the island (Map 9, page 41). After it made contact, the fault between the Kula Plate and North America jammed and a new fault formed slightly farther out in the Pacific. The Crescent terrane, made up of former marine volcanoes that had formed along the fault, was brought alongside sometime later, but before 40 million years ago. In British Columbia, the Crescent terrane’s oceanic lavas form the rocks of Victoria’s Western Communities; to the south, they make up the Coast Mountains of Washington and Oregon (the rocks of the Olympic Mountains came later). The Crescent terrane is separated from the Pacific Rim terrane by a fault along Loss Creek. The force of this collision did not create big mountain ranges in British Columbia, but it did fold and thrust-fault the sedimentary rocks along the Strait of Georgia to form the ridges and bays of the Gulf Islands.
The Southern Rocky Mountain Trench—seen here at Columbia Lake—is a big crack in the earth’s crust formed as the crust stretched after the compression of continent-terrane collisions ended.
