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.
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