Friday, August 8, 2014

Northern Convergence: A Confusion of Orogens, Belts, and Terranes

The first thing to remember about Canada as we start our geological journey is that it is big. Really, really big. It's larger than the United States (including Alaska). And there are fewer people living there than live in California. I'm used to driving long distances across the wilds of the California desert and the American Southwest, but there were regions on our recent journey that made the southwest feel crowded. And even at that, we were in a crowded part of the country. There were actual paved roads and villages every so often. In some parts of Canada, the empty lands extend for thousands of miles.
The Olympic Mountains from Vancouver Island across the Strait of Juan de Fuca
The second thing to know is that the western part of Canada is mountainous and geologically active. As noted in the previous post, this mountain belt is called the Cordilleran Orogen or North American Cordillera. An orogen is a word constructed from the Greek oros for "mountain" plus genesis for "creation" or "origin". The mountain system extends from the southern tip of South America to Alaska.
The Belts of British Columbia and Alberta, Canada. Source: Wikipedia and Black Tusk
It doesn't take long to realize that as one crosses the Cordillera the appearance and topography of the mountains changes. There are definable belts across the mountain system in which a series of mountain ranges that share a similar tectonic origin can be distinguished from adjacent ranges of different topography. Five such belts have been defined in western Canada: the Insular, Coast, Intermontane, Omineca, and Foreland. The Interior Plains are the stable part of the continent that haven't undergone mountain-building activity in any kind of recent geologic time.

And finally, there are the terranes. The age of a mountain range bears little relationship to the age of the rocks exposed in the mountain range. There are youthful mountain ranges around the world that have existed for no more than 3-4 million years, but contain rocks that formed billions of years ago. The Black Mountains of Death Valley National Park in California are an excellent example. The tectonic belts of British Columbia contain rocks that not only are not the same age as the physical mountains, but which formed in an entirely different part of the world! A terrane (or tectonostratigraphic terrane) can be defined as a section of the Earth's crust that has been transported by tectonic processes from its place of origin. Terranes are usually bounded by faults. Some of the terranes in western Canada have traveled thousands of miles, while others originated fairly close to the Pacific Coast.

The Insular Belt is the tectonically active edge of the continent, characterized by far-traveled exotic terranes exposed on Vancouver and other coastal islands. We explored parts of Vancouver Island on the second day of our journey.
The granitic dome Stawamus Chief from Shannon Falls Provincial Park
The Coastal Belt also includes far-traveled metamorphic terranes, but the province mainly includes vast amounts of intrusive granitic rock as well as the northernmost volcanoes of the Cascades Range (Mt. Garibaldi being the most prominent volcano). Examples of the terrain (landscape) can be seen in the picture above of Stawamus Chief, a granite dome on the edge of Howe Sound, and below in a picture of a part of the Chilcotin Range near Lillooet.
Part of the Chilcotin Mountains near Lillooet, British Columbia
The Intermontane Belt consists of somewhat more muted topography with eroded plateaus and lower elevation mountain ranges containing several prominent metamorphic terranes. Some parts of the region experienced volcanic activity as well, mostly of horizontal flows of basalt. We spent a night at Kamloops in the midst of the province.
Eroded plateaus in the vicinity of Kamloops, B.C. Photo by Mrs. Geotripper
The Omineca Belt is a region of highly metamorphosed rocks that connected exotic terranes of the Pacific Ocean basin with the rocks of the original North American Continent. The Monashee and Selkirk Mountains were spectacular. Have you ever been at a concert where the warm-up band played as well as the featured headliners? That's how I felt about the mountains of the Omineca; they are as stunning as the adjacent Rocky Mountains. We spent time at Mount Revelstoke National Park during the journey, and spent a night in Golden at the foot of the Selkirks.
The Monashee Mountains from Mount Revelstoke.
The Foreland Belt is the region normally recognized as the Rocky Mountains of British Columbia and Alberta. The mountains are composed mostly of sediments deposited along the margins of the North American Continent which were subsequently pushed up and over the continental margin, forming a series of thrust faults. Thrusts have the effect of pushing older rocks up and over younger rocks, and in Banff, Yoho, and Jasper National Parks one sees a series of thrust sheets that repeat the sequence of rocks over and over. It's a complicated mess!
Emerald Lake in Yoho National Park
Because of the high elevations and plentiful precipitation, the Omineca and Foreland Belts are among the best places in southern Canada to see active glaciers. Literally all of Canada was covered by glacial ice sheets as recently as 12,000 years ago, but the ice retreated to the highest peaks. Active glaciers can still be easily accessed along the Icefields Parkway in Banff and Jasper National Parks.
Peyto Lake in Banff National Park

Our journey begins in the next post as we gathered in Seattle to start the class.
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