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 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
