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.
Конец ознакомительного фрагмента.
Текст