In the last issue we gave a very general overview of the geology of North America and how Nevada fits in that complex story. Now we will begin to look in more detail at the landforms that we see every day in Nevada. This article focuses on the geologic macro terrain, known as the Basin and Range, that comprises most of our state. The origin of the name becomes clear simply by looking at a topographic map. The numerous mountain ranges and intervening valleys have a remarkably consistent north-south trend, appearing almost like crepe paper. From the map we can see that the Basin and Range also extends into parts of California, Oregon, Idaho, Utah, and Arizona. A large part of the Nevada and Utah portions include an area known as the Great Basin, so called because no water flowing over its surface ever reaches the sea.
The Ancient Farallon Plate and the Origin of Western North America
The Basin and Range is a major feature of the North American geography that been in continuous development since the mid-Jurassic period, some 165 million years ago, when the ancient Farallon Plate oceanic crust began to subduct beneath the west coast of the North American continent, which at our latitude would have been near present-day Utah. This roughly rectangular tectonic plate extended thousands of miles in the north-south direction and remained in place for over a hundred million years, acting like a conveyor belt that carried both oceanic and continental crust fragments eastward into the coast. Repeatedly, these so-called “exotic” terrains from the Pacific basin collided with the continent, slowly building its margin over 600 miles westward to its present location, western California. The rate of plate movement was roughly one centimeter per year, but in the vast scale of geologic time this was sufficient to accomplish tremendous change.
Through this process of continental crust accretion large areas of the west from Alaska to Central America, including virtually all of Nevada and California, were actually “assembled” from exotic foreign crust and incorporated into the continent. Looking at the very simplified diagrams at right, the top shows normal subduction of oceanic crust (in this case, the Farallon plate) beneath the continent, with some of that crust “scraped” off to become some accreted terrain. A modern-day example of this is the Olympic Mountains of Washington State, but some of Nevada’s land was probably added in this way long ago. The diagram also shows how magma can be formed inland as the subducted crust melts at great depth and rises through the crust. This may form volcanoes, like the Cascade Range, or solidify underground into plutons, as seen in the granites found throughout the Sierra Nevada. The lower diagram shows how blocks of exotic continental crust can also be carried into the main continent and is accreted, often forming high mountains from the resulting compression. A dramatic example of this today is the Indian subcontinent colliding with Asia and uplifting the mighty Himalayas. The same basic process created our state, but the original mountains of that era have long since eroded. Much of that crust remains but as we will discuss next, other processes have continually rearranged the geology and topography.
Plate Transformation and Regional Uplift Forms the Basin and Range
As the Farallon plate continued its steady motion under the North American continent, the Pacific plate that bordered it to the west was continuously expanding, to the point that it became the largest plate on earth. The like-named ocean above it grew to approach the huge area it occupies today. By about 30 million years ago, the growing Pacific plate had displaced large sections of the Farallon plate, which virtually disappeared beneath the continent. The dominant motion of the Pacific plate at our latitude was (and remains today) toward the northwest, so the subduction process ceased over much of the west coast. The boundary of remaining subduction migrated northward. The northern remnant of the original Farallon plate is known today as the Juan de Fuca Plate and extends offshore from Northern California through British Columbia. The subduction continues there, driving the ongoing mountain building of the Coast Ranges and Olympic Mountains, as well as feeding magma to the volcanoes of the Cascade Range, many of which are the still active. From Point Reyes, California, south through length of the Baja Peninsula in Mexico, the Pacific plate now borders the North American continent. It has a translational, shearing motion in a southeast-to-northwest direction. This boundary is the well-known San Andreas Fault. The west coast of California is actually part of the Pacific plate. It has migrated some 600 miles to the northwest over the 30 million years since this motion began to dominate. Most of this coastal strip, which includes Los Angeles and San Diego, originally lay adjacent to Mexico. Its migration to the northwest opened the Gulf of California.
Concurrent with these changes, new tectonic forces started to influence the continent’s interior. Upwelling of the Earth’s mantle led to a broad regional uplift of much of the western United States, raising even the valleys and flatlands from thousands of feet to well over a mile above sea level. This is perhaps best seen at the Grand Canyon, where the Colorado River has relentlessly carved into the rising plateau to create one of the most spectacular landforms on the planet. This uplift also resulted in an east-west stretching of the North American plate for hundreds of miles inland, extending all the way to the Colorado Plateau and Rocky Mountains. Geologists are still trying to understand all of the driving mechanisms, but the combination of plate motions and this regional uplift has fractured the terrain of the Basin and Range region along parallel faults, forming its north-south trending valleys and ranges. Among these are Death Valley, Owens Valley, the Great Salt Lake Valley. Our own Truckee Meadows is also an example, albeit smaller.
A drive along U.S. Highway 50, “The Loneliest Road in America” from Carson City to Great Basin National Park is a wonderful way to experience the unique topography of the Basin and Range region, as you traverse numerous valleys and climb over or around a number of dramatic fault-block mountain ranges, like the Toiyabe Range below.
If this very brief article has increased your interest in our regional geology and you want to learn more, I would recommend the excellent series of articles published by the U.S. National Park Service, which can be found at https://www.nps.gov/subjects/geology.
Next month, we will look at the formation of the Sierra Nevada, including Lake Tahoe.