Stretching of the Basin and Range and Lifting of the Colorado Plateau

Quick Summary:

Parashant straddles the Basin and Range and Colorado Plateau. The western portion of the monument includes the giant Grand Wash Trough (Pakoon Basin). This is part of the Basin and Range transition zone. The eastern portion from the Grand Wash Cliffs to Mt. Trumbull is part of the southwest corner of the Colorado Plateau.

Key Points in Basin and Range Extension

• Before 30 million years ago, the subduction of the Farallon Plate under California compressed and pushed up western North America. It subducted into the mantle at a shallow angle rather than diving steeply. This friction on the bottom of western North America pushed up a huge mountain range just west of Parashant as tall as the Andes called the Mogollon-Sevier Highland. Occassionally huge plumes of sticky magma rose up in the region from 50 to 20 million years ago creating some of the largest eruptions known in the world (Wah Wah Springs and Indian Peaks calderas) or injected magma near the surface, creating giant laccoliths like the Pine Valley mountains.

• When the East Pacific Rise spreading center on the ocean floor subducted under California (it is still subducting under Washington and Oregon and called the Juan de Fuca Plate now), that plate boundary turned into a transform fault called the San Andreas.
• As the west side of San Andreas slid northwest and the east side slid relatively southwest, the lack of subduction zone pressure on much of what would become the Basin and Range disappeared.
• When the pressure from the Farallon Plate went away, the huge Andes-like mountain range collapsed of its own weight and spread out to the west starting about 17 million years ago. This spreading is called crustal extension.
• Crustal extension caused massive blocks of crust to settle. Their bases slid down and to the west. This caused their tops to tilt or slump, creating the parallel north-south trending mountain ranges of Nevada. The crustal extension actually covers an even more vast area from southeast Oregon down to New Mexico and west Texas and northwest Mexico.
• Crustal extension and the settling of giant blocks of crust effectively thinned the crust in the Basin and Range. Here in Parashant Earth's crust is only about 18 miles thick. Below that is the very hot mantle.
• Crustal extension continues to this day. The Grand Wash Trough (Pakoon Basin) of Parashant is widening about a half inch per year. The many small earthquakes in the region are almost entirely related to the settling of these giant blocks. Local faults like the Hurricane fault (which created the Hurricane Cliffs) are capable of earthquakes up to 7.0 magnitude. Fault slips of 5 to 10 feet in a single earthquake are possible. Geologists are studying how much of the motion on the Hurricane fault is 'down to the west.' It may be more of an 'up motion on the east side' of the fault due to continued uplift of the Colorado Plateau, or perhaps some of both.

Key Points in Colorado Plateau Uplift

• The Colorado Plateau has acted like an island of sedimentary rock layers on top of very old (over one billion year-old) basement rocks like the Vishnu Schist. While not fully understood, it appears that the swelling mantle is lifting the plateau. However, the plateau is holding together. The plateau has risen thousands of feet over the last 6 million years. This rise started the cutting of the Grand Canyon.
• The lifting of the plateau may be associated with an upwelling plume of solid but putty-like hot mantle rock. Heat from the hot mantle causes the rocks to expand in an upward direction. Hot material is less dense than when it is cold, and it occupies more space, so this expansion of the mantle's volume and its buoyancy may be what is lifting the Colorado Plateau. Active scientific research in the region will hopefully clarify this one day. If a large earthquake happens on the Hurricane fault, sophisticated sensors and GPS technology will tell geologists more about Basin and Range extension and Colorado Plateau uplift.
• Seismic tomography of the lower crust of the Colorado Plateau and upper mantle seem to show that a huge slab of the base of the plateau is falling into the mantle. As it falls, hotter rock from the asthenosphere below is going around this slab, putting even hotter rock right under the plateau.

 

A Deeper Dive into this Story

The Colorado Plateau section of the North American continent is being pushed upwards by hot mantle rock. The reason the mantle rock can cause the Colorado Plateau to rise is due to heat. Hotter materials are less dense. At the molecular level, the mineral molecules are more energetic. This increases the space between them, making them lighter, thus buoyant. It is this hot material in the form of solid mantle rock (putty-like but not liquid), that causes it to rise, lifting the Colorado Plateau that sits on top of it. The Colorado Plateau has been pushed up thousands of feet higher in elevation than it otherwise would be, buoyed by heat from below. All this high elevation terrain created a steep gradient down to the ocean. Rainwater and melting snow cascaded off the rising plateau in the form of fast-moving streams and rivers, eroding into the plateau rapidly. This resulted in its sharp and scenic vertical topography. Many of the layers of the plateau are made of soft clays and sand, which are easily eroded. This has resulted in strange landforms, scenic landscapes, canyons, arches, and badlands. The most famous feature is the 277-mile long mile-deep Grand Canyon. The Grand Canyon was cut into the Colorado Plateau in less than 6 million years!

One area of research by Lavender, et al., using new seismic tomography technology from the groundbreaking US Transportable Array, seems to show that a large chunk of the bottom of the plateau (lower crust and upper lithosphere) may have detached from the layer cake rocks above, sinking into the mantle. Rising hot asthenosphere rock (which is still solid but moves like putty) is slowly replacing the cooler lithosphere and lower crust as they fall into the mantle. The layers that are delaminating may be due to rising plumes of liquid magma that stalled and refroze back into rock in the base of the plateau. These magma plumes never made it to the surface. This increased the density of the rock at that level, adding weight, which may have caused these layers to sink and pull away (delaminate) from the bottom of the plateau and sink down (a lithosphere ‘drip’) into the mantle. If you have ever had a shoe where the rubber sole came unglued in front and flopped down, this is basically what the bottom of the Colorado Plateau is doing as the bottom flap of the plateau is peeling off. As asthenospheric rock rises and replaces the sinking slab, much hotter rock is sitting right below the plateau. This is just one of several hypotheses being researched by geologists. It is still controversial and heavily debated. Others propose a mantle plume welling up from the deep pushing the plateau higher. Seismic tomography is a new technology, but it is the best way scientists have to reveal processes deep in the earth where temperatures over 2,000 degrees Fahrenheit (1,100 Celsius) and pressures in the hundreds of thousands of pounds per square inch range (1-3 gigapascals) make exploration otherwise impossible. A final answer may be decades away that fully explains the process of Colorado Plateau rise.

Basin and Range Province
The Basin and Range is an arc of land that starts far south in the Mexican states of Chihuahua and Sonora and runs northwest through Big Bend National Park in West Texas, over to New Mexico and its Rio Grande Rift Valley, on to Arizona (including Parashant), western Utah, through all of Nevada, eastern California, and finally the high lava plains of southern Oregon and Idaho. It is characterized by a mix of rocks that were accreted through subduction of the Farallon plate under the western edge of the North American continent. Rock layers include seafloor sediments, ocean floor basalt (lava) rock, and ancient offshore islands arcs. The Basin and Range region also contains large bodies of magma (plutons) that were injected up into the crust forming huge laccoliths like the Pine Valley Mountains, Navajo Mountain, the La Sal Mountains, and the Henry Mountains. Some of these plutons were rich in iron, depositing huge amounts of iron north of St. George Utah. Other plutons were so rich in water and other volatiles they caused massive eruptions such as Wah Wah Springs or Indian Peaks, which were some of the largest eruptions in earth's history.

The Basin and Range is best known for being on the move. The B&R region is widening extensively. Originally pushed up by the Farallon plate, the ancient Nevadaplano/Sevier-Mogollon Highland (the Andes-like mountain range in eastern California and Nevada) collapsed starting 30 million years ago when the East Pacific Rise subducted under California and stopped pushing up the crust. It was further stretched by the northwestward motion of the San Andreas Fault. This stretching has resulting in a doubling of the surface distance from the state line of Arizona and Nevada out to the California coast in only 17 million years. See this animation to visualize crustal extension. The mountain ranges of Nevada and the rest of the Basin and Range are actually just massive slumped and tilted crustal blocks from extension. The base of the blocks slid down into the lithosphere toward the west. This tilted the tops of most slumping blocks 20-60 degrees. Other famous tilted blocks are Mt. Charleston west of Las Vegas, Wheeler Peak at Great Basin National Park, and Mt. Lemmon and Kitt Peak in southern Arizona. Some of these upended blocks expose very ancient crustal rocks that had been part of offshore island arcs accreted to North America. For instance, uplifted and exposed rock layers in the Virgin Mountains are up to 1.7 billion years old.

As time has passed, erosion has filled the deep V-shaped valleys between the peaks, creating flat valley floors with shallow lakes called playas. The Bonneville Salt Flats is part of the largest of these playas. Hidden deep beneath the salt flats is a deep valley thousands of feet filled with sediments and salt. In Mesquite, NV, gravity tests have determined there is a valley 26,000 feet deep, all filled with sand and gravel. This crustal extension is even visible in the Grand Canyon. Most of the north-south side canyons are fractures caused by extension. Water, ice, and wind have eroded open these cracks into giant side canyons in the Grand Canyon.

Before the Basin and Range
Earth's crust may seem thick to humans, but in fact it is extremely thin relative to the thickness of earth's mantle and core. From the surface to the center of the earth is just shy of 4,000 miles. The crust here is 18 miles thick. Basically the crust is less than one percent of the diameter of earth, comparable to the shell of an egg. Meanwhile, the putty-like mantle is fully 84% of Earth's total volume. The mantle is the egg white, and the core is the yolk. The solid rock of the mantle acts like putty and is slowly circulating about the speed of a growing fingernail. Meanwhile, Earth's crust is brittle and full of fractures (also known as faults). These are places where pieces of crust move against each other to relieve tension that builds up by mantle forces that push or pull on them. This has created a patchwork of giant plates that constantly jostle against each other as convecting rock circulates in the mantle.

While hard to believe, ocean waves once crashed on beaches at approximately the Utah/Nevada State line, just on Parashant's doorstep. The coastline moved west over time because of the subduction of Farallon Plate under North America's west coast. The subducting Farallon Plate was covered in deep layers of sediments and giant island arcs. Some of these islands were the size of Hawaii. One was as big as California! As the oceanic Farallon plate subducted under North America, these islands, thick layers of metamorphosed seafloor basalt (called serpentine), limestones, and deep sediment on the seafloor were all added to the western edge of North America. Geologists call this the North American Cordillera. It stretches from Mexico to Alaska. The materials that had been scraped off are what are known as accreted terranes. This added new land to North America which can be seen in the differences between these paleogeographic maps of the western US 245 Million Years Ago and 215 Million Years Ago as material was added.

Unlike most subduction zones where the oceanic crust dives steeply back into Earth's mantle and melts away, the Farallon did something unusual. It subducted but stayed up against the bottom of the continent instead of diving at a sharp angle down into the mantle. This had a major impact on the landscapes of western North America. As the Farallon slid eastward, friction between the top of the Farallon and the bottom of the North American continent pushed up surface rocks from eastern California to central Colorado. This caused two massive mountain-building events, known as orogenies. The first was the Sevier, followed by the Laramide.

Almost unknown by the general public the Sevier Orogeny is named for the Sevier River of west-central Utah between Zion and Bryce Canyon National Parks. Unlike the Laramide Orogeny that came next and built the Rocky Mountains, the Sevier included two types of mountains. The first was a typical mountain range in what is today eastern California and Nevada. Called by some the Nevadaplano and others the Sevier-Mogollon Highlands, it was a mountain range estimated to be as high as the Andes Mountains. Further east the friction between the Farallon and North American plates pushed brittle platy layers of surface sedimentary rocks into an overlapping pile called the Sevier Thrust Belt in Nevada and western Utah. The brittle sedimentary rocks in the thrust belt were broken into flat pieces that start to ride up and stack on top of each other, similar to how broken pieces of glass pushed by a broom start to stack on top of each other. The next mountain building event, the Laramide, happened further east, pushing up the Rocky Mountains, which reached their full height around 50 million years ago.

Here you can see the southwestern US 85 million years ago and again at 50 million years ago. Scroll down each image to see the events labeled. You can see how the Nevadaplano/Mogollon Highlands and Sevier Thrust Belt came up during the time the interior seaway was still in place 85Ma. By 50Ma the seaway was gone and the Rocky Mountains were uplifted. Keep in mind that at the time the Sevier Orogeny happened, the surface distance between the ocean and the Utah/Arizona border with Nevada was much less than it is today due to crustal compression from the subducting Farallon plate. Why it collapsed will be explained shortly.

The Sevier left clues for geologists to find in the slumped blocks of the Basin and Range as well as the Colorado Plateau. One of the most telling were sediments and rocks deposited by ancient rivers, which show water direction. What puzzled geologists long ago was why the angles of the rocks showed streams and rivers flowing to the east, but uphill! That isn't possible. To understand how a river creates deposits of rocks, gravels, and clay, the process is called imbrication. When a river flows off a mountain the water flows very fast down those steep slopes. Fast water has the kinetic energy to tilt the tops of rocks in a downstream direction. Sand bars grow in the downstream direction as well, creating directional crossbeds.

Geologists study these exposed river deposits and map river flow direction. But why were they all facing east when that direction is uphill? The geologists were left with no other possible conclusion than there used to be a massive mountain range to the west of the Colorado Plateau tens of millions of years ago, and a downwarped lowland to the east. This is opposite of today where the Colorado Plateau is much higher than in the Basin and Range to the west. The North American interior seaway filled the downwarp with water during the Age of Dinosaurs. West and east of that seaway the elevation was higher and ancient rivers of Nevada and western Utah flowed to the east into the interior seaway. One place to see this up close is the riverbed deposits of the Morrison Formation at Dinosaur National Monument. The Morrison is about 149 million years old. If you visit the famous fossil wall at that park, you will see that the bone wall is made of Morrison age riverbed sands and small gravels (as well as over 1,500 dinosaur fossils!). These dinosaurs were washed downstream and sank to the bottom of a large but slow eastward flowing river, similar perhaps to the Missouri River.

So why does the Colorado River today flow to the southwest? It's simple, the topography has tilted to the west due to the rise of the Colorado Plateau and the collapse of the Basin and Range to the west!

What happened to the ancient Nevadaplano/Mogollon Highland then? And what does this have to do with Parashant? Let's return to our subducting Farallon plate for that answer. Until around 30 million years ago all along the entire west coast from Mexico to British Columbia, the Farallon was subducting under North America. The Farallon and the Pacific oceanic plates were being made by a tear in the ocean floor called the East Pacific Rise. Magma rising at the tear created new seafloor that went both east and west. Right around 30 million years ago the piece of the East Pacific Rise subducted under California and made no more new seafloor. The subduction zone was replaced by a transform fault known as the San Andreas. Meanwhile to the north there was still a lot of the Farallon to subduct from very northern California up to British Columbia. It is called the Juan de Fuca Plate, as seen here. The place where the San Andreas fault meets the still subducting Farallon plate is at the Cape Mendocino Triple Junction where three plates interact. South of the triple junction, the land on the west side of the San Andreas fault is moving quickly to the northwest, while the east side of the San Andreas fault is moving relatively slowly to the southwest with the rest of North America.

Here is where things get interesting and explain the Basin and Range landscape and what destroyed the Nevadaplano/Mogollon Highland.

The Death of the Nevadaplano/Mogollon Highland
There is a difference in speed between the San Andreas Fault where the Pacific Plate is moving northwest and the North American continent that is moving southwest. The San Andreas is moving up to twice as fast as North America. The Pacific Plate is moving northwest about 3-4 inches per year, while North America is moving southwest at 1.5-2 inches per year. In the area between them is essentially a complex broken zone of low pressure.

The development of the northwest-moving San Andreas fault doomed the Nevadaplano/Sevier-Mogollon Highland and the Sevier Thrust Belt. Geologists use a term called isostasy to explain how pieces of crust always seek elevation and density equilibrium with the rest of the surface of the earth. If something is heavy and at a high elevation, unless there are forces to keep it there, that high topography will sink until it is at equilibrium with everything around it. Only about the top 7 miles of Earth's crust is brittle. Everything under it is soft and plastic, which is the sort of gooey material that a mountain range can slump down into. The Himalayas are a good modern example of this as they are being pushed up by the collision of the Indian subcontinent against the Asian Plate. If the pressure of the Indian subcontinent against Asia and the Himalayas stopped, the Himalayas would collapse.

To achieve equilibrium in the Basin and Range, something had to fill the growing low-pressure zone between the San Andreas fault and North America. The high Nevadaplano and Thrust Belt would do nicely. In a process called post-orogenic collapse, the Nevadaplano/Sevier-Mogollon Highland collapsed and sank into this zone of low pressure. Blocks of crust that had been 20,000 feet above sea level dropped over ten thousand feet and spread out into the low pressure zone of the newly formed Basin and Range.

As you can see, many forces contributed to the battered Basin and Range landscape. The Farallon is almost entirely subducted now. The remaining pieces of it will disappear under North America over the next few 10s of millions years. This includes the Juan de Fuca, the Nazca, and the Cocos Plates.

While still the subject of research, it appears that pieces of the Farallon plate still sit under the continent. This graphic from Berkeley University shows in red where pieces of the broken up Farallon Plate still sit close to the surface today. This will be important in the next section on magma melts.

Parashant sits on the boundary of two surface features known as the Colorado Plateau and the Basin and Range Province of the west-central North American continent. This has been an area of incredible deformation over the last 1.7 billion years. Geologists come from around the world to study its features

Colorado Plateau

The Colorado Plateau, famous for its dozens of colorful rock layers in places like the Grand Canyon, Zion, Arches, and Parashant, occupies western Colorado, most of Utah, northern New Mexico, and northern Arizona. It is made of colorful sedimentary and metamorphosed rocks that have been deposited over 1.7 billion years. The lowest levels, such as Vishnu Schist, were scraped off a subducting plate and scrunched onto (accreted) to North America from ocean plate subduction. The most colorful layers above were deposited in seas, river deltas, and deserts that came and went. The Colorado Plateau is often described as a layer cake as it has many distinct and visually impressive rock layers stacked one on top of the next. It also includes a variety of volcanic features that worked their way up through the plateau much later.

 

The two main types of Basin and Range earth movement.

USGS

To help readers understand how the giant cube-like blocks of Basin and Range crust can slump, rise, or tip to the side see the graphic at left. As movement of the San Andreas fault moves toward the northwest, this creates a zone of low pressure on the Basin and Range region. The crustal blocks of the ancient Nevadaplano and Sevier Thrust Belt are incredibly heavy and unevenly weighted. Once the pressure began to lessen 17 million years ago, these blocks started to drop and slump. Their upturned corners form the parallel tilted block mountain ranges of the Basin and Range.

Where blocks have tilted there should be deep valleys. What we see instead are broad flat basins. Ground-penetrating radar has determined that in fact the valleys are down there. We just do not see them because over millions of years the valleys filled up with sand, silt, and water from erosion of the uptilted blocks. This sediment is called 'basin fill.' One valley in Arizona is filled with 30,000 feet of sediment.

Basin and Range deformation continues to this day, but it isn't as rapid as it was between 17 and 15 million years ago when it first started. Still, these blocks continue to slowly jostle and grind against each other as they are acted on by the forces of plate tectonics hundreds of miles away and heat from the mantle just below the surface. The Basin and Range region is also alive with small earthquakes up to 5.0 magnitude. Scientists monitor the movements of mountain blocks that cause these quakes, including the stretching of the Pakoon Basin. This stretching is measured by lasers at permanent monitoring stations between the Grand Wash Cliffs and the Virgin Mountains. Right now the Pakoon Basin is widening about one centimeter each year.

So what will happen millions of years from now? Scientific investigations are ongoing to help us understand the forces at work in Parashant. This area will continue to change. As the San Andreas Fault continues its movement, it will set the stage for the Gulf of California to slowly move into the United States and creep toward Parashant. Eventually the ocean will again be on Parashant's doorstep!

 

Magma Trivia:

When you think about plate tectonics and how the continents slide over, under, or past each other, have you ever wondered how they can do that? Flux melting is what greases plate tectonics all around the globe. Under great pressure supercritical water and carbon dioxide in the mantle create melts at plate boundaries. This makes the boundaries slippery so continents can move past each other. You may ask though what about those giant subduction zone earthquakes, like the one that hit Japan in 2011 and is predicted to hit off the Oregon and Washington coast in the next 100 years. The thing is, sometimes the plates are hung up at subduction zones. These areas are too close to Earth's surface where the rock is too cold and brittle to benefit from slippery flux melted magma. The tension is then released through a catastrophic megathrust earthquake which can last 5 minutes and may exceed magnitude 9.0. This creates catastrophic damage to buildings and has killed millions of people over time, but these quakes don't hurt Earth's plates as they move around.

At Pakoon and Tassi Springs air bubbles can be seen rising out with the water. These bubbles contain ancient gases. 1% of the gas content of the bubbles is helium. Helium is produced by the decay of uranium in Earth's mantle. It is this radioactive decay that keeps earth's core and mantle so hot. Basically, the presence of helium bubbles in Parashant's springs is a reminder that magma is relatively close to the surface.

The supercritical property of H2O and CO2 under great pressure is a challenge for geothermal electricity generating stations. There are several such plants in the southwest near volcanically active areas such as the Blundell geothermal plant near Milford, Utah. H2O and CO2 is under so much pressure that its super-critical nature actually can deteriorate the various pipes and drilling equipment deep underground. The geothermal industry is experimenting with different materials that can withstand the pressure and corrosive effects of super-critical water to take better advantage of the vast amount of energy available just a few miles below ground.

In Section 2 we will look at how magma causes the different kinds of eruptions we see around the world and why Parashant has the kind of eruptions it does.

 

View from the Pearce Ferry airstrip north to the widening Pakoon Basin and Grand Wash Cliffs. The opening to the Grand Canyon is where the dark ground in the center of the photo disappears into the cliffs to the right. Those are the lowlands of the Colorado River and part of Lake Mead National Recreation Area.The vertical displacement from the river to the top of the highest mesa in the photo (Snap Point) is over 5,000 feet.

J. Axel - NPS