In this series of articles, Utah artist J. Brad Holt talks about what artists are seeing as they look at the landscape. Holt studied geology in college and is attentive to what the rocks suggest in the scenes he paints.
Lead Image: “Cedar Canyon 9AUG15,” by J. Brad Holt, 2015, oil, 12 x 16 in.
With the advent of the theory of plate tectonics came the explanation for the orogenesis of most of the world’s mountain ranges. Subduction zones along convergent plate boundaries have given rise to the Andes and similar chains, or have produced island arc volcanic ranges, such as the Aleutians. Continental collisions have produced the Himalaya and the European Alps, just as more ancient collisions produced the Appalachians and the Urals.
But the Rocky Mountains of North America don’t seem to make tectonic sense. What is all of this topographic relief doing in the middle of a continent? There is no subduction zone off the West Coast, save for a small bit off the coast of Cascadia, where the last remnants of the Farallon Plate are being consumed. Nevertheless, since the end of the Cretaceous Period, the North American Cordillera has risen, eroded, and risen again, seemingly in defiance of conventional theory.
One thing to bear in mind is that the western fifth of North America consists of exotic terranes that have accreted to the continent since the Mesozoic Era. In the Jurassic Period, the West Coast was somewhere on the Utah-Nevada border. Much of the American West consisted of low-lying sedimentary basins and desert country, underlain by crystalline metamorphic basement rock. As the North American plate overrode the Farallon Plate, getting closer to the spreading center of the East Pacific rise, a couple of things happened.
First, a lot of new land was accreted to the continent, which caused compression, uplift, and over-thrust of older strata. Second, the fresher and more buoyant oceanic crust near the spreading center subducted at a much more shallow angle than usual. This increased compression and uplift. Finally, as the North American plate moved slowly to the west over the zone of uplift, the land to the west of the former spreading center experienced tensional forces, pulling it apart, and dropping the land in a series of fault blocks. These are the north-south trending ranges of the Basin and Range province.
Sixty-five million years ago, as the dinosaurs exited along with the Mesozoic Era, the Cenozoic Era arose, and with it, the mountains of the American interior west. In Idaho, Wyoming, and Utah, crustal plates detached and over-thrust younger sediments for many miles, piling into jumbled ranges. Eastward, the underlying basement rock began to fold and rear skyward. The young sharp mountains soon began to bury themselves in their own alluvium. They were aided in this by the addition of vast quantities of volcanic debris from all the tectonic activity to the west, as one volcanic island arc after another plowed into the West Coast. This period of mountain building, beginning in the early Tertiary Period, is called the Laramide Orogeny. The mountains rose swiftly, divested themselves of sediments to their crystalline cores, and became almost buried by the Miocene Epoch.
In the late Miocene Epoch, the fill material began to erode away as the country was again uplifted in an epeirogeny known as the Exhumation of the Rockies. The removal of all this debris happened fairly quickly, aided by increased precipitation from the glacial periods. Re-exposed mountains were sectioned by existing rivers down-cutting along their courses. To the north and west, vast outpourings of basalt continued to occur. The motion of the North American plate over an active mantle plume can be traced from southern Idaho, across the Snake River Plain, to its present location under Yellowstone. The Tetons and the Wind River Range seem to have resulted from fault blocks, which might be considered the easternmost expression of the Basin and Range province. Igneous intrusions in Utah and Colorado, and extrusive events throughout the Colorado Plateau region, have occurred until very recently.
Elsewhere in the American West the mountains appear to be a mixture of tectonic types. The Cascades are conventional stratovolcanos created by the Cascadia subduction zone. The Sierra Nevada are comprised of batholithic intrusions emplaced prior to the subduction of the East Pacific rise. The Sierra Madre seem to be largely compressional mountains, similar to the Rockies in the north. In the south they become more conventional stratovolcanos as a subduction zone reappears, and continues down the coast of Central America. The Canadian Rockies are similar to the U.S. Rockies. The subduction zone of Cascadia disappears north of Vancouver Island, to be replaced by the Queen Charlotte transform fault, north to the south coast of Alaska, where another subduction trench begins.
Note that there are active volcanos in Alaska, the Cascades, and Southern Mexico/Central America. In British Columbia and California/Northern Mexico, which are bounded by transform faults, stratovolcanos are absent. However, tectonically active mountain belts are still present, which shows that the riddle of the Rockies persists to the north and south.
The period of uplift continues, making much of the Rocky Mountain region topographically rugged. This tectonically unique orogeny is far from finished.