The basin & range province of North America

The long, thin, northerly-trending mountain ranges and broad valleys of the eastern Rocky Mountains represent a geologic area unique on this planet: the Basin & Range province. The Basin & Range extends from Sonora, Mexico, through Arizona, eastern California, Nevada and Utah, into the southern plains of Idaho. This region contains three of the four North American deserts, most of our western copper deposits, and abundant ground water. The area was born from geologic violence, which tore apart a continent.

The earth’s crust is divided into many mobile plates, which move away from spreading centers, places where the deep mantle wells up to the surface. The Mid-Atlantic ridge is a spreading center which tore apart a super-continent and separated North and South America from Europe and Africa. A similar spreading center, called the East Pacific Rise, occurs in the Pacific Ocean. The northern part of the Rise extends beneath the Gulf of California and is separating the Baja California Peninsula from mainland Mexico.

As the North American continent was being rafted westward, it collided with oceanic crust traveling southeast. The collision produced intense compressive forces which built the ancestral Rocky Mountains. The heavier oceanic crust dived under the lighter rocks of the continent. When this oceanic crust got deep enough, it began to melt and the lighter magma rose like a bubble through the crust to produce extensive volcanism and finally, to cool into the core of mountain ranges such as the Sierra Nevada. From about 70- to 55 million years ago, this melting oceanic crust was producing the type of magma that would become our major copper deposits.

About 40 million years ago, things changed. By that time, the westward moving North American plate had overridden the spreading center of the East Pacific Rise. The western part of the continent, which is now California, Oregon and Washington, became attached to a northwestward moving plate while the rest of the continent was attached to the southeastward moving plate. The heat of the spreading center softened the lower crust allowing it to stretch. At first, the cooler upper crust, riding on the hot, elasticized rocks of the lower crust, broke along low angle structures inherited from the earlier period of mountain building.

By about 12 million years ago, however, stretching reached its limit, and the cool upper crust shattered along steep faults into long, narrow blocks. Alternating blocks rotated and sank to produce the ranges and valleys we see today. Some of these valleys are more than 10,000 feet from surface to bedrock. As the valley blocks sank, the basins filled with sediments from the eroding mountains. Copper deposits were uncovered, exposed to weathering and became enriched; other deposits were buried. The basins were filled, not only with sediments but with water. Many lakes were formed, only to evaporate and leave their mineral content as great salt deposits within the sinking sediments. The sediments under Phoenix, for example, contain salt deposits up to one mile thick. Only the upper valley-fill sediments received the fresh water of the glacial periods; water which we mine today. By 6-to 8 million years ago, most continental extensional rending ended, and the lower crust cooled. The plate motion was transferred to tearing faults, such as the San Andreas system which, among other things, ripped Baja California away from the Mexican mainland and formed the Gulf of California. Except for a few basaltic volcanoes erupting from about 4 million years ago to about 700 years ago, the Basin & Range has been quiet, awaiting the next geologic adventure.

The cross-section below is an interpretation from the Arizona Geological Survey of the faulting that has occurred in the Santa Catalina Mountains, Tucson Valley, and San Pedro Valley. This interpretation shows several periods of faulting in response to crustal stretching, which made the geology quite complex.