Basin and Range Province

Alternating linear ranges and valleys characterize the Basin and Range Province of western North America. This topography resulted from crustal stretching that caused large normal faults. The crustal stretching, which occurred over the last forty million years, is superimposed on a complex record of older geologic events.

Location and Characteristics

The Basin and Range Province comprises three different but related entities: a geologic province of western North America, the alternating linear mountain ranges and valleys that characterize that province, and the fault-block bedrock structure responsible for the distinctive topography. The Basin and Range Province includes the area between the Rocky Mountains on the east and the Sierra Nevada and Cascade Mountains on the west, from southern Montana, Idaho, and Oregon south to northern Mexico. Crustal stretching in a roughly east-west direction across this area in the last forty million years is responsible for the distinctive topography and crustal structure. The stretching is the most recent chapter of a long, complex geologic history.

Alternating elongated mountain ranges and valleys (basins) define Basin and Range topography, which occurs not only in the Basin and Range Province but also in other areas of young continental stretching, such as southern Greece and the Yunnan Province in China. Ranges and basins are usually similar in extent, commonly 50 to 200 kilometers long and twenty to thirty kilometers wide. The climate throughout the Basin and Range Province is arid or semiarid. As a result, the ranges are generally rugged, sparsely vegetated, and drained by ephemeral and occasional perennial streams. Topographic relief from range tops to basin bottoms is usually one to three kilometers. Internal drainage is common in an individual basin or with several basins linked in a single internal drainage system. An example of the latter is the Great Salt Lake in Utah, the terminus of a drainage system that includes most of the basins and ranges of northwestern Utah.

Ranges and basins are defined on one or both sides by young, and in some instances seismically active, normal faults. The range side of the fault is the upthrown block, and the basin side the downthrown block, so the fault movements are expressed directly in the topography. However, erosion of the ranges and sedimentation in the basins cause topographic relief much less than the total displacement across the faults, which can exceed ten kilometers. Typical Basin and Range faults dip from thirty to seventy degrees. Internal drainage results because subsidence along active normal faults ponds the water running off in streams. The common fault pattern is one in which basins and ranges are faulted on one side only to form a series of half-grabens, or asymmetrical structural depressions. Between the half-grabens are asymmetrical tilted-block ranges that are straight and steep on the faulted side and more gently sloping on the other side, which, downslope, forms the bedrock floor of the next basin. Series of tilted-block ranges and half-graben basins are sometimes called “domino-style” fault systems, because the fault-bounded blocks resemble a series of dominos that have been stood on end next to each other and then toppled together. Strike-slip faults also are common in the Basin and Range Province, commonly acting to link together the ends of normal faults. Some of these strike-slip faults are very large. For example, the Furnace Creek fault zone that runs along northern Death Valley in California, linking normal faults in Death Valley to others farther north, may have accommodated 70 kilometers of strike slip.

Metamorphic Core Complexes

Much study of the Basin and Range has focused on ranges called metamorphic core complexes (MCCs) that are not domino-style fault blocks. There are more than twenty MCCs in the Basin and Range, and more may exist but are yet unrecognized. The structure and the topography of MCCs are controlled by a detachment fault that separates an upper level, usually composed of sedimentary and volcanic rocks, from a lower level made up of metamorphic and plutonic rocks formed deeper in the crust. Rocks of the upper level generally have been intensely stretched along numerous normal faults, such that they have increased in width by several hundred percent. Rocks beneath the detachment fault commonly have flowed at high temperatures and great depth.

Analysis of the flow structures indicates that much or all of this flow also represents a large amount of horizontal stretching. Upper-level faulting and lower-level flow are, therefore, believed to be related. The detachment fault in an MCC is usually dome-shaped, so the fault dips gently off the flanks of the range in every direction. Differences in erosion resistance between rock types above and below the detachment fault resulted in the domelike shape of the ranges, which is quite different from that of the more typical fault-block ranges. It is widely accepted that MCCs record extreme horizontal stretching, but since their recognition in the 1970s, their origin and significance have been intensely debated and remain controversial. There are two main issues: the relationship between MCCs and “normal” Basin and Range domino-style faults, and the significance of detachment faults.

One view is that MCCs and domino-style Basin and Range faults represent distinct extensional processes and that an MCC forms instead of large domino-style faults when the crust in an area is especially hot and susceptible to flow. Others consider MCCs to be exposures of what underlies the domino-style fault systems at depth; that is, the domino-style faults end downward at a detachment fault like those exposed in MCCs. In this view, greater than normal stretching in some areas caused the upper crust to be stretched so thin that deeper rocks and structures were exposed as MCCs.

Researchers are similarly split into two camps regarding the significance of the detachment faults. One group considers the detachment faults to represent a horizontal zone along which the cooler upper crust that deforms by faulting is mechanically separated (detached) from the hotter lower crust that deforms by flow. In this case, the detachment fault would act to accommodate differences in the movement patterns of rocks above and below it; it need not accommodate large displacement, but merely local adjustments. The other view is that the detachment fault represents a large, gently dipping normal fault that cuts down through the crust. In this view, as much as fifty to sixty kilometers of horizontal displacement has occurred along detachment faults in MCCs, thus bringing together originally widely separated upper- and deeper-crustal rocks.

Consensus tends to favor the latter view of each issue—that is, that MCCs represent the deeper parts of domino-fault systems and that detachment faults are major dislocations that cut through the crust and accommodate tens of kilometers of slip. Much future work will be needed before either of these issues will be considered resolved.

Formation

The structure summarized earlier in this article has been superimposed in the Basin and Range upon a long and complex previous geologic history. Indeed, much of the knowledge of that history can be attributed to the Basin and Range faulting that has uplifted and exposed crustal sections many kilometers thick to allow the geologic record to be read. The oldest rocks in the Basin and Range are scattered occurrences of ancient, deep-seated metamorphic and plutonic rocks as much as 2.5 billion years old. These rocks are exposures of the ancient crust of the North American continent. In the Late Precambrian eon, part of the west side of the North American continent was faulted away and carried by plate tectonics to some other part of the globe. This event formed a new western continental margin facing a new ocean basin. This ancient continental margin presently runs southwestward across southern Idaho and central Nevada, so the eastern and southern parts of the Basin and Range formed from crust of the ancient continent, but the western Basin and Range was part of a deep ocean basin. From this time until the middle of the Paleozoic era, this continental margin resembled the modern East Coast of North America: a broad, gently sloping shelf, generally tectonically stable and largely covered by shallow seawater.

During the Late Paleozoic and Early Mesozoic eras, lithospheric plate movements caused two major lithospheric blocks to collide successively with this continental margin. The collisions resulted in periods of mountain building along the North American continental margin, known as the Antler and Sonoman orogenies. Other results of these plate collisions included the building outward of the North American continent, such that by 250 million years ago, all of what now lies in the Basin and Range was part of continental North America, and strike-slip faulting that shifted some crustal blocks hundreds of kilometers along the continental margin, substantially changing the shape of the coastline in the process.

Following the Sonoman orogeny, the western continental margin of North America changed completely. By 200 million years ago, the continental margin was the site of underthrusting of oceanic lithosphere (subduction), causing the construction of a high mountain chain along western North America similar to the modern Andes in South America. What is now the Basin and Range Province covered an area analogous to modern Peru, Bolivia, and northern Argentina, including the eastern parts of the Andes, its foothills, and some of the adjacent plains. The mountain range was an area of volcanism and magmatic intrusion, now represented by large masses of granite in the Sierra Nevada and much of the Basin and Range. Thrust faulting occurred along the east side of the mountain chain, as it does today on the east side of the Andes. Such thrust faults are especially well known around Las Vegas, Nevada, and Salt Lake City, Utah.

The formation of the Basin and Range is part of a comparatively recent change in the tectonic pattern of western North America. From about thirty million years ago to the present, the Andes-like plate boundary (still active in the Pacific Northwest) has been changing into a strike-slip plate boundary (transform fault) that includes the San Andreas fault of Southern California. Somewhat earlier, large-scale stretching of the lithosphere began in the Basin and Range. Most of the MCCs formed between twenty and forty million years ago. At the same time, numerous large volcanic centers appeared in and around the Basin and Range. These volcanic centers were the sources of enormous explosive eruptions that blanketed much of the western United States with hot ash. Volume estimates of these eruptions reach and even exceed 3,000 cubic kilometers, greatly exceeding the largest known historic eruptions on the earth. Both extension and explosive volcanism continue to the present, as evidenced by modern earthquakes along Basin and Range faults and by a few major volcanic centers that are still active. Most geologists, however, believe that the rate of extension and the intensity of volcanism have decreased.

Study of the Basin and Range Province

The Basin and Range Province is one of the foremost areas worldwide for study of the processes by which the continental crust stretches. It is a natural laboratory in which the earth has done an experiment, and the geologists' job is to analyze the results and to determine just what the experiment was. Three main types of techniques are brought to bear in such studies: field studies of surface geology, laboratory analysis of samples collected in the field, and geophysical field studies. The first step is basic fieldwork, which involves mapping the rock bodies and structures exposed at the surface. The geologist uses this information to make inferences about the spatial relationships of rock bodies and the geologic history that led to those arrangements. Despite being one of the best-studied continental rifts in the world, at least half of the Basin and Range Province's surface geology has yet to be studied in detail. Fundamental questions remain about continental extension processes. Therefore, the surface geology of the Basin and Range will continue to be an important source of new insights for years to come.

Laboratory analytical techniques provide more precise and detailed information about rocks than can be gathered in the field. A complete summary cannot be attempted here because relevant techniques span all of earth science. However, isotopic geochronology and thermobarometry are particularly important. Isotopic geochronology is based on measuring the progressive decay of radioactive elements and the consequent buildup of the products of that decay. This is the principal method of determining numerical ages of ancient rocks, and, therefore, it is vital in determining when ancient events occurred, for comparing ages from one area to another to look for spatial patterns, and for determining the rates at which processes have operated. Thermobarometry uses chemical compositions of minerals in rocks to estimate the temperature and pressure conditions under which the minerals grew. Mineral compositions are generally determined using the electron microprobe on a polished chip of the rock. Thermobarometry is used to find where in the crust deep-seated igneous and metamorphic rocks formed.

Important geophysical field techniques include seismology and studies of potential fields such as gravity and magnetism. Seismologists study the transmission of sound waves through the earth, including sound waves from earthquakes and artificial sources such as explosions. The variations in the time it takes for sound waves to travel from their sources to various receivers (seismometers) and variations in the characteristics of the waves recorded at different seismometers allow seismologists to map out the internal structure of the earth in terms of the rocks' sound-transmitting properties. For example, it is mainly through seismological studies that researchers know the thickness of the earth's crust (about thirty kilometers in most of the Basin and Range). Variations in the earth's gravity field from one place to another can be used to investigate variations in the densities of rocks at depth, and similar variations in the magnetic field indicate changes in the magnetic properties of rocks at depth. It is by combining such geophysical studies with observations of the surface geology that the present understanding of the structure and evolution of the Basin and Range has been achieved.

Principal Terms

detachment fault: a horizontal or gently dipping, regionally extensive fault; the hanging wall usually contains numerous smaller, steeper normal faults that end at the detachment fault

dip: the angle between a sloping surface, such as a fault, and a horizontal plane

ductile shear zone: a planar zone of rock that accommodates relative movement like a fault, but the movement has occurred by processes of solid-state flow rather than by fracture

fault: a fracture or system of fractures across which relative movement of rock bodies has occurred

fault slip: the direction and amount of relative movement between the two blocks of rock separated by a fault, respectively

half-graben: an asymmetrical structural depression formed along a normal fault where the downthrown block is tilted toward the fault

hanging wall and footwall: the rock bodies located above and below a fault, respectively

internal drainage: the condition in which a river system has no outlet; instead, it drains into a saline lake or playa (a lake basin that contains water only immediately after a rainstorm)

lithospheric plates: rigid blocks that make up the outer shell of the earth, 100 to 200 kilometers thick and generally similar in size to the continents, forming a mosaic that covers the earth's surface

normal fault: a fault across which slip caused the hanging wall to move downward relative to the footwall

plate tectonics: the theory that earth movements reflect the relative movements between a small number of rigid lithospheric plates along narrow zones of deformation called plate boundaries

strike-slip fault: a fault across which the relative movement is mainly lateral

thrust fault: a fault, usually dipping less than thirty degrees, across which the hanging wall moved upward relative to the footwall

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